ANTIBODIES SPECIFICALLY RECOGNIZING B- AND T-LYMPHOCYTE ATTENUATOR (BTLA) AND USES THEREOF CROSS-REFERENCE TO RELATED APPLICATION [0001] This application claims priority to U.S. Application 63/368,870, filed July 19, 2022, which is incorporated herein by reference in its entirety. REFERENCE TO AN ELECTRONIC SEQUENCE LISTING [0002] The contents of the electronic sequence listing (710262000940SEQLIST.xml; Size: 49,713 bytes; and Date of Creation: July 14, 2023) is herein incorporated by reference in its entirety. FIELD OF THE APPLICATION [0003] This application pertains to antibodies that specifically recognize B- and T- lymphocyte attenuator (BTLA), and methods of manufacture and uses thereof, including methods of treating cancers or infectious diseases. BACKGROUND OF THE APPLICATION [0004] BTLA (CD272), a member of the CD28 family, induces immunosuppression by inhibiting the function and proliferation of B cells and T cells. Structure-wise, BTLA is a type I transmembrane glycoprotein comprising 289 amino acids and belongs to the immunoglobulin family (Ning, et al.2021). Notably, Herpesvirus Entry Mediator (HVEM, also known as tumor necrosis factor receptor superfamily 14, TNFRSF14) which was identified as the specific ligand of BTLA belongs to the TNFR family (Compaan, D.M, et al, 2005; Gonzalez, L.C, et al.2005; Sedy, J.R, et al.2005). In addition to BTLA, HVEM also binds to LIGHT (also known as TNFSF14) and CD160. Most publications support that BTLA and CD160 binding to CRD1/CRD2 domain on HVEM mediate inhibitory signaling, while LIGHT transduces stimulatory signaling by binding to CRD2/CRD3 region (Cai G, et al.2008: Rodriguez-Barbosa JI, et al.2019; Sedy, J.R, et al.2005). However, there are also reports suggesting that BTLA and CD160 can induce bidirectional signals in some cases (Gavrieli M and Murphy KM, 2006; Tan CL, et al.2018). [0005] HVEM is highly expressed in resting T cells, immature B cells, and memory B cells, but downregulated in activated T and B cells. It is also widely expressed in all other immune cells, certain types of cancer cells, intestinal epithelial cells, etc. Interestingly, HVEM functions as either a ligand or a receptor and plays multiple different roles in diverse physiological and pathological conditions (Demerlé C, et al.2021). Although HVEM/BTLA/CD160/LIGHT mediate complicated signaling pathways, HVEM knockout mice showed stronger CD4+ T cell response to stimulation indicating the overall function mediated by HVEM is suppressive (Wang Y, et al.2005). Additionally, HVEM and BTLA in naive T-cells form a cis-heterodimeric complex, blocking the external CD160 and other co- signaling molecules from binding to HVEM, thereby maintaining T-cell tolerance (Cheung TC, et al.2009). [0006] In contrast to the broad expression profile of HVEM, expression of BTLA is limited in lymph organs, with little or no expression detected in other organs, such as the heart, kidney, brain, and liver (Yu, et al.2019). Among immune cells, BTLA is mainly expressed in B and T cells, although its expression can also be detected in innate immune cells, such as dendritic cells (DCs) and monocytes (Ning, et al.2021). BTLA consists of an Ig-like ectodomain, a single transmembrane domain (TMD), and an intracellular tail. The cytoplasmic domain of BTLA contains immunoreceptor tyrosine-based inhibitory motif (ITIM), immunoreceptor tyrosine-based switch motif (ITSM), and growth factor receptor- bound protein-2 (Grb-2) association motif. Activation of BTLA signaling pathway by HVEM leads to the tyrosine phosphorylation of the ITIM which allows the binding of the Src homology domain 2 (SH2)-containing protein tyrosine phosphatases, SHP-1 and SHP-2, which suppress the activating signaling in the immune cells (Gavrieli M and Murphy KM, 2006; Gavrieli M, et al.2003; Watanabe N, et al.2003). It was recently demonstrated that in contrast to PD-1 which inhibits CD28 signaling through SHP-2, BTLA suppresses both TCR and CD28 signaling mainly through SHP-1 (Celis-Gutierrez J, et al.2019; Xu X, et al.2020). It was also found that binding of the Grb-2 association motif with Grb-2 leads to the recruitment of PI3K protein subunit p85 and T cell activation. Thus, the BTLA molecule could exert bidirectional regulatory effects (Gavrieli M and Murphy KM, 2006). [0007] BTLA plays a key role in inducing and maintaining T cell immune tolerance. Krieg et al. reported that BTLA contributes to the regulation of CD8+ T cell intrinsic homeostasis and memory cell generation (Krieg C, et al.2007). It was also shown that BTLA deficient CD8+ T cells are more aggressive in inducing type 1 diabetes. In addition, immune tolerance could not be induced in BTLA knockout mice even with high doses of ovalbumin (Liu X, et al. 2009). Moreover, BTLA inhibits IL-21 production by follicular helper T cells and thereby reduces IgG serum level (Kashiwakuma D, et al.2010). In addition to T cell function, BTLA may also affect B cell, dendritic cell and NKT cell function (Vendel AC, et al.2009; Jones A, et al.2016; Miller ML, et al.2009). [0008] BTLA is upregulated in tumor-infiltrating lymphocytes (TILs) while HVEM can be expressed on tumor cells. There is an abundance of data showing that the HVEM/BTLA interaction impairs the anti-tumor immunity (Demerlé C, et al.2021). In some reports, the expression level of BTLA is increased in circulating CD4+ cells but not in CD8+ T cells, and blocking the BTLA/HVEM interaction can enhance IFN-γ production by circulating CD4+ and CD8+ T cells in hepatocellular carcinoma patients (Liu J, et al.2018; Zhao Q, et al. 2016). In other publications, BTLA upregulation was found in CD8+ TILs which correlates with poor outcome in gallbladder cancer patients (Oguro S, et al.2015). BTLA upregulation was also identified in tumor antigen specific T cells (Fourcade J, et al.2012; Derre L, et al. 2010). BTLA+ tumor antigen specific T cells show increased levels of other inhibitory molecules, such as PD-1, lymphocyte-activation gene-3 (LAG3), T cell immunoglobulin and mucin-domain-containing molecule-3 (TIM-3) (Quan L, et al.2018; Fourcade J, et al.2012). In addition, blocking both BTLA and PD-1 with antibodies synergistically enhanced the expansion, proliferation, and cytokine production of NY-ESO-1-specific CD8+ T cells from melanoma patients compared to PD-1 blockade alone (Fourcade J, et al.2012). [0009] Thus, there remains a need in the art for therapeutic antibodies that effectively inhibit or otherwise antagonize BTLA, and related methods of treating diseases or conditions mediated through BTLA, such as cancer or infectious diseases. [0010] The disclosures of all publications, patents, patent applications and published patent applications referred to herein are hereby incorporated herein by reference in their entirety. BRIEF SUMMARY OF THE APPLICATION [0011] In some embodiments, there is provided an isolated anti-BTLA antibody comprising: a heavy chain variable domain (V _H ) comprising a heavy chain complementarity determining region (HC-CDR) 1 comprising TFGMGVS (SEQ ID NO: 1); an HC-CDR2 comprising HIYWDDDKRFNPSLKS (SEQ ID NO: 4); and an HC-CDR3 comprising GNWDGETYFDY (SEQ ID NO: 7); and a light chain variable domain (V _L ) comprising a light chain complementarity determining region (LC-CDR) 1 comprising KSTQSLLDSDGKTYLN (SEQ ID NO: 10); an LC-CDR2 comprising LVSKLDS (SEQ ID NO: 13); and an LC-CDR3 comprising WQGTHFPWT (SEQ ID NO: 15). [0012] In some embodiments, there is provided an isolated anti-BTLA antibody comprising: a V _H comprising an HC-CDR1 comprising the amino acid sequence of SEQ ID NO: 1, or a variant thereof comprising up to about 3 amino acid substitutions; an HC-CDR2 comprising the amino acid sequence of SEQ ID NO: 4, or a variant thereof comprising up to about 3 amino acid substitutions; and an HC-CDR3 comprising the amino acid sequence of SEQ ID NO: 7, or a variant thereof comprising up to about 3 amino acid substitutions; and a V _L comprising an LC-CDR1 comprising the amino acid sequence of SEQ ID NO: 10, or a variant thereof comprising up to about 3 amino acid substitutions; an LC-CDR2 comprising the amino acid sequence of SEQ ID NO: 13, or a variant thereof comprising up to about 3 amino acid substitutions; and an LC-CDR3 comprising the amino acid sequence of SEQ ID NO: 15, or a variant thereof comprising up to about 3 amino acid substitutions. [0013] In some embodiments, there is provided an isolated anti-BTLA antibody comprising a V _H comprising an HC-CDR1, an HC-CDR2, and an HC-CDR3 of the V _H comprising the amino acid sequence of any one of SEQ ID NOs: 18-22; and a V _L comprising an LC-CDR1, an LC-CDR2, and an LC-CDR3 of the V _L comprising the amino acid sequence of any one of SEQ ID NOs: 25-29. [0014] In some embodiments, there is provided an isolated anti-BTLA antibody comprising: (i) a V _H comprising an HC-CDR1, an HC-CDR2, and an HC-CDR3 of a V _H comprising the amino acid sequence of SEQ ID NO: 18; and a V _L comprising an LC-CDR1, an LC-CDR2, and an LC-CDR3 of a V _L comprising the amino acid sequence of SEQ ID NO: 25; (ii) a V _H comprising an HC-CDR1, an HC-CDR2, and an HC-CDR3 of a V _H comprising the amino acid sequence of SEQ ID NO: 19; and a V _L comprising an LC-CDR1, an LC-CDR2, and an LC-CDR3 of a V _L comprising the amino acid sequence of SEQ ID NO: 26; (iii) a V _H comprising an HC-CDR1, an HC-CDR2, and an HC-CDR3 of a V _H comprising the amino acid sequence of SEQ ID NO: 19; and a V _L comprising an LC-CDR1, an LC-CDR2, and an LC-CDR3 of a V _L comprising the amino acid sequence of SEQ ID NO: 27; (iv) a V _H comprising an HC-CDR1, an HC-CDR2, and an HC-CDR3 of a V _H comprising the amino acid sequence of SEQ ID NO: 19; and a V _L comprising an LC-CDR1, an LC-CDR2, and an LC-CDR3 of a V _L comprising the amino acid sequence of SEQ ID NO: 28; (v) a V _H comprising an HC-CDR1, an HC-CDR2, and an HC-CDR3 of a V _H comprising the amino acid sequence of SEQ ID NO: 19; and a V _L comprising an LC-CDR1, an LC-CDR2, and an LC-CDR3 of a V _L comprising the amino acid sequence of SEQ ID NO: 29; (vi) a V _H comprising an HC-CDR1, an HC-CDR2, and an HC-CDR3 of a V _H comprising the amino acid sequence of SEQ ID NO: 20; and a V _L comprising an LC-CDR1, an LC-CDR2, and an LC-CDR3 of a V _L comprising the amino acid sequence of SEQ ID NO: 26; (vii) a V _H comprising an HC-CDR1, an HC-CDR2, and an HC-CDR3 of a V _H comprising the amino acid sequence of SEQ ID NO: 20; and a V _L comprising an LC-CDR1, an LC-CDR2, and an LC-CDR3 of a V _L comprising the amino acid sequence of SEQ ID NO: 27; (viii) a V _H comprising an HC-CDR1, an HC-CDR2, and an HC-CDR3 of a V _H comprising the amino acid sequence of SEQ ID NO: 20; and a V _L comprising an LC-CDR1, an LC-CDR2, and an LC-CDR3 of a V _L comprising the amino acid sequence of SEQ ID NO: 28; (ix) a V _H comprising an HC-CDR1, an HC-CDR2, and an HC-CDR3 of a V _H comprising the amino acid sequence of SEQ ID NO: 20; and a V _L comprising an LC-CDR1, an LC-CDR2, and an LC-CDR3 of a V _L comprising the amino acid sequence of SEQ ID NO: 29; (x) a V _H comprising an HC-CDR1, an HC-CDR2, and an HC-CDR3 of a V _H comprising the amino acid sequence of SEQ ID NO: 21; and a V _L comprising an LC-CDR1, an LC-CDR2, and an LC-CDR3 of a V _L comprising the amino acid sequence of SEQ ID NO: 28; (xi) a V _H comprising an HC-CDR1, an HC-CDR2, and an HC-CDR3 of a V _H comprising the amino acid sequence of SEQ ID NO: 21; and a V _L comprising an LC-CDR1, an LC-CDR2, and an LC-CDR3 of a V _L comprising the amino acid sequence of SEQ ID NO: 29; (xii) a V _H comprising an HC-CDR1, an HC-CDR2, and an HC-CDR3 of a V _H comprising the amino acid sequence of SEQ ID NO: 22; and a V _L comprising an LC-CDR1, an LC-CDR2, and an LC-CDR3 of a V _L comprising the amino acid sequence of SEQ ID NO: 28; (xiii) a V _H comprising an HC-CDR1, an HC-CDR2, and an HC-CDR3 of a V _H comprising the amino acid sequence of SEQ ID NO: 22; and a V _L comprising an LC-CDR1, an LC-CDR2, and an LC-CDR3 of a V _L comprising the amino acid sequence of SEQ ID NO: 29. [0015] In some embodiments, there is provided an isolated anti-BTLA antibody comprising: (i) a V _H comprising an HC-CDR1 comprising the amino acid sequence of SEQ ID NO: 1, an HC-CDR2 comprising the amino acid sequence of SEQ ID NO: 4, and an HC-CDR3 comprising the amino acid sequence of SEQ ID NO: 7, or a variant thereof comprising up to about 5 amino acid substitutions in the HC-CDRs; and a V _L comprising an LC-CDR1 comprising the amino acid sequence of SEQ ID NO: 10, an LC-CDR2 comprising the amino acid sequence of SEQ ID NO: 13, and an LC-CDR3 comprising the amino acid sequence of SEQ ID NO: 15, or a variant thereof comprising up to about 5 amino acid substitutions in the LC-CDRs. [0016] In some embodiments, according to any one of the isolated anti-BTLA antibodies described above, the isolated anti-BTLA antibody comprises: a V _H comprising the amino acid sequence of any one of SEQ ID NOs: 18-22, or a variant thereof having at least about 80% sequence identity to the amino acid sequence of any one of SEQ ID NOs: 18-22; and a V _L comprising the amino acid sequence of any one of SEQ ID NOs: 25-29, or a variant thereof having at least about 80% sequence identity to the amino acid sequence of any one of SEQ ID NOs: 25-29. [0017] In some embodiments, the isolated anti-BTLA antibody comprises: (i) a V _H comprising the amino acid sequence of SEQ ID NO: 18, or a variant thereof having at least about 80% sequence identity to the amino acid sequence of SEQ ID NO: 18; and a V _L comprising the amino acid sequence of SEQ ID NO: 25, or a variant thereof having at least about 80% sequence identity to the amino acid sequence of SEQ ID NO: 25; (ii) a V _H comprising the amino acid sequence of SEQ ID NO: 19, or a variant thereof having at least about 80% sequence identity to the amino acid sequence of SEQ ID NO: 19; and a V _L comprising the amino acid sequence of SEQ ID NO: 26, or a variant thereof having at least about 80% sequence identity to the amino acid sequence of SEQ ID NO: 26; (iii) a V _H comprising the amino acid sequence of SEQ ID NO: 19, or a variant thereof having at least about 80% sequence identity to the amino acid sequence of SEQ ID NO: 19; and a V _L comprising the amino acid sequence of SEQ ID NO: 27, or a variant thereof having at least about 80% sequence identity to the amino acid sequence of SEQ ID NO: 27; (iv) a V _H comprising the amino acid sequence of SEQ ID NO: 19, or a variant thereof having at least about 80% sequence identity to the amino acid sequence of SEQ ID NO: 19; and a V _L comprising the amino acid sequence of SEQ ID NO: 28, or a variant thereof having at least about 80% sequence identity to the amino acid sequence of SEQ ID NO: 28; (v) a V _H comprising the amino acid sequence of SEQ ID NO: 19, or a variant thereof having at least about 80% sequence identity to the amino acid sequence of SEQ ID NO: 19; and a V _L comprising the amino acid sequence of SEQ ID NO: 29, or a variant thereof having at least about 80% sequence identity to the amino acid sequence of SEQ ID NO: 29; (vi) a V _H comprising the amino acid sequence of SEQ ID NO: 20, or a variant thereof having at least about 80% sequence identity to the amino acid sequence of SEQ ID NO: 20; and a V _L comprising the amino acid sequence of SEQ ID NO: 26, or a variant thereof having at least about 80% sequence identity to the amino acid sequence of SEQ ID NO: 26; (vii) a V _H comprising the amino acid sequence of SEQ ID NO: 20, or a variant thereof having at least about 80% sequence identity to the amino acid sequence of SEQ ID NO: 20; and a V _L comprising the amino acid sequence of SEQ ID NO: 27, or a variant thereof having at least about 80% sequence identity to the amino acid sequence of SEQ ID NO: 27; (viii) a V _H comprising the amino acid sequence of SEQ ID NO: 20, or a variant thereof having at least about 80% sequence identity to the amino acid sequence of SEQ ID NO: 20; and a V _L comprising the amino acid sequence of SEQ ID NO: 28, or a variant thereof having at least about 80% sequence identity to the amino acid sequence of SEQ ID NO: 28; (ix) a V _H comprising the amino acid sequence of SEQ ID NO: 20, or a variant thereof having at least about 80% sequence identity to the amino acid sequence of SEQ ID NO: 20; and a V _L comprising the amino acid sequence of SEQ ID NO: 29, or a variant thereof having at least about 80% sequence identity to the amino acid sequence of SEQ ID NO: 29; (x) a V _H comprising the amino acid sequence of SEQ ID NO: 21, or a variant thereof having at least about 80% sequence identity to the amino acid sequence of SEQ ID NO: 21; and a V _L comprising the amino acid sequence of SEQ ID NO: 28, or a variant thereof having at least about 80% sequence identity to the amino acid sequence of SEQ ID NO: 28; (xi) a V _H comprising the amino acid sequence of SEQ ID NO: 21, or a variant thereof having at least about 80% sequence identity to the amino acid sequence of SEQ ID NO: 21; and a V _L comprising the amino acid sequence of SEQ ID NO: 29, or a variant thereof having at least about 80% sequence identity to the amino acid sequence of SEQ ID NO: 29; (xii) a V _H comprising the amino acid sequence of SEQ ID NO: 22, or a variant thereof having at least about 80% sequence identity to the amino acid sequence of SEQ ID NO: 22; and a V _L comprising the amino acid sequence of SEQ ID NO: 28, or a variant thereof having at least about 80% sequence identity to the amino acid sequence of SEQ ID NO: 28; (xiii) a V _H comprising the amino acid sequence of SEQ ID NO: 22, or a variant thereof having at least about 80% sequence identity to the amino acid sequence of SEQ ID NO: 22; and a V _L comprising the amino acid sequence of SEQ ID NO: 29, or a variant thereof having at least about 80% sequence identity to the amino acid sequence of SEQ ID NO: 29. [0018] In some embodiments, there is provided an isolated anti-BTLA antibody comprising: (i) a V _H comprising an HC-CDR1, an HC-CDR2, and an HC-CDR3 of a V _H comprising the amino acid sequence of SEQ ID NO: 23; and a V _L comprising an LC-CDR1, an LC-CDR2, and an LC-CDR3 of a V _L comprising the amino acid sequence of SEQ ID NO: 30. [0019] In some embodiments, there is provided an isolated anti-BTLA antibody comprising: (i) a V _H comprising an HC-CDR1 comprising the amino acid sequence of SEQ ID NO: 2, an HC-CDR2 comprising the amino acid sequence of SEQ ID NO: 5, and an HC-CDR3 comprising the amino acid sequence of SEQ ID NO: 8, or a variant thereof comprising up to about 5 amino acid substitutions in the HC-CDRs; and a V _L comprising an LC-CDR1 comprising the amino acid sequence of SEQ ID NO: 11, an LC-CDR2 comprising the amino acid sequence of SEQ ID NO: 14, and an LC-CDR3 comprising the amino acid sequence of SEQ ID NO: 16, or a variant thereof comprising up to about 5 amino acid substitutions in the LC-CDRs. [0020] In some embodiments, according to any one of the isolated anti-BTLA antibodies described above, the isolated anti-BTLA antibody comprises: a V _H comprising the amino acid sequence of SEQ ID NO: 23, or a variant thereof having at least about 80% sequence identity to the amino acid sequence of SEQ ID NO: 23; and a V _L comprising the amino acid sequence of SEQ ID NO: 30, or a variant thereof having at least about 80% sequence identity to the amino acid sequence of SEQ ID NO: 30. [0021] In some embodiments, there is provided an isolated anti-BTLA antibody comprising: (i) a V _H comprising an HC-CDR1, an HC-CDR2, and an HC-CDR3 of a V _H comprising the amino acid sequence of SEQ ID NO: 24; and a V _L comprising an LC-CDR1, an LC-CDR2, and an LC-CDR3 of a V _L comprising the amino acid sequence of SEQ ID NO: 31. [0022] In some embodiments, there is provided an isolated anti-BTLA antibody comprising: (i) a V _H comprising an HC-CDR1 comprising the amino acid sequence of SEQ ID NO: 3, an HC-CDR2 comprising the amino acid sequence of SEQ ID NO: 6, and an HC-CDR3 comprising the amino acid sequence of SEQ ID NO: 9, or a variant thereof comprising up to about 5 amino acid substitutions in the HC-CDRs; and a V _L comprising an LC-CDR1 comprising the amino acid sequence of SEQ ID NO: 12, an LC-CDR2 comprising the amino acid sequence of SEQ ID NO: 13, and an LC-CDR3 comprising the amino acid sequence of SEQ ID NO: 17, or a variant thereof comprising up to about 5 amino acid substitutions in the LC-CDRs. [0023] In some embodiments, according to any one of the isolated anti-BTLA antibodies described above, the isolated anti-BTLA antibody comprises: a V _H comprising the amino acid sequence of SEQ ID NO: 24, or a variant thereof having at least about 80% sequence identity to the amino acid sequence of SEQ ID NO: 24; and a V _L comprising the amino acid sequence of SEQ ID NO: 31, or a variant thereof having at least about 80% sequence identity to the amino acid sequence of SEQ ID NO: 31. [0024] In some embodiments, there is provided an isolated anti-BTLA antibody that specifically binds to the human BTLA with a Kd from about 0.1 pM to about 10 nM. [0025] In some embodiments, there is provided an isolated anti-BTLA antibody that specifically binds to BTLA competitively with any one of the isolated anti-BTLA antibodies described above. In some embodiments, there is provided an isolated anti-BTLA antibody that specifically binds to the same epitope as any one of isolated anti-BTLA antibodies described above. [0026] In some embodiments according to any of the isolated anti-BTLA antibodies described above, the isolated anti-BTLA antibody comprises an Fc fragment. In some embodiments, the isolated anti-BTLA antibody is a full-length IgG antibody. In some embodiments, the isolated anti-BTLA antibody is a full-length IgG1, IgG2, IgG3, or IgG4 antibody. In some embodiments, the anti-BTLA antibody is a chimeric, human, or humanized antibody. In some embodiments, the anti-BTLA antibody is an antigen binding fragment selected from the group consisting of a Fab, a Fab', a F(ab)' ₂ , a Fab'-SH, a single-chain Fv (scFv), an Fv fragment, a dAb, a Fd, a nanobody, a diabody, and a linear antibody. [0027] In some embodiments, there is provided isolated nucleic acid molecule(s) that encodes any one of the anti-BTLA antibodies described above. In some embodiments, there is provided a vector comprising any one of the nucleic acid molecules described above. In some embodiments, there is provided a host cell comprising any one of the anti-BTLA antibodies described above, any one of the nucleic acid molecules described above, or any one of the vectors described above. In some embodiments, there is provided a method of producing an anti-BTLA antibody, comprising: a) culturing any one of the host cells described above under conditions effective to express the anti-BTLA antibody; and b) obtaining the expressed anti-BTLA antibody from the host cell. [0028] In some embodiments, there is provided a pharmaceutical composition comprising any one of the anti-BTLA antibodies described above, any one of the nucleic acid molecules described above, any one of the vectors described above, or any one of the isolated host cells as described above, and a pharmaceutically acceptable carrier. In some embodiments, the pharmaceutical composition further comprises an antigen-binding protein specifically recognizing PD-1. [0029] In some embodiments, there is provided a method of treating a disease or condition in an individual in need thereof, comprising administering to the individual an effective amount of any one of the anti-BTLA antibodies described above. In some embodiments, the method further comprises administering to the individual an effective amount of an antigen-binding protein specifically recognizing PD-1. In some embodiments, any one of the anti-BTLA antibodies described above and the antigen-binding protein specifically recognizing PD-1 are administered concurrently or consecutively. In some embodiments, there is provided the use of any one of the anti-BTLA antibodies described herein for the preparation of pharmaceutical compositions for treating a disease or condition in an individual in need. In some embodiments, provided is the use of any one of the anti-BTLA antibodies described above, or a pharmaceutical composition comprising any one of anti-BTLA antibodies described above in the manufacture of a medicament for treating a disease or condition. In some embodiments, the disease or condition is associated with BTLA, comprising cancer or infectious diseases or condition. In some embodiments, the disease or condition is selected from the group consisting of non-small cell lung cancer, adrenal gland cancer, bladder cancer, brain cancer, pancreatic adenocarcinoma, breast cancer, colorectal cancer, melanoma, esophageal cancer, gastric cancer, cervical cancer, head and neck cancer, hepatocellular carcinoma, kidney cancer, liver cancer, ovarian cancer, pancreatic cancer, prostate cancer, small cell lung cancer, testicular cancer, thyroid cancer, uterine cancer, and any types of leukemia, lymphoma and myeloma, and infectious diseases, including, but not limited to infectious diseases associated with Human Papilloma Virus (HPV), Human Immunodeficiency Virus (HIV), Herpes Simplex Virus (HSV), Varicella Zoster Virus (VSV), Cytomegalovirus (CMV), Epstein Barr Virus (EBV), chlamydozoan, rickettsia bacterium, mycobacterium, staphylococci, streptococci, pneumonococci, meningococci and conococci, klebsiella, proteus, serratia, pseudomonas, legionella, diphtheria, salmonella, bacilli, cholera, tetanus, botulism, anthrax, plague, leptospirosis, and Lymes disease bacteria. [0030] Also provided are pharmaceutical compositions, kits and articles of manufacture comprising any one of the anti-BTLA antibodies described above. BRIEF DESCRIPTION OF THE DRAWINGS [0031] FIGS.1A-1B show the binding affinity of chimeric anti-BTLA antibodies 83F2, 86B7 and 96F11 to human BTLA or rhesus monkey BTLA as analyzed by ELISA. FIG.1A shows the binding affinity of 83F2, 86B7 and 96F11 to human BTLA. FIG.1B shows the binding affinity of 83F2, 86B7 and 96F11 to rhesus monkey BTLA. [0032] FIG.2 shows the blocking activity of chimeric anti-BTLA antibodies 83F2, 86B7, and 96F11 that inhibit human HVEM binding to human BTLA as analyzed by ELISA. [0033] FIG.3A shows the Expi293 cells stably overexpressing huBTLA as analyzed by FACS. FIG.3B shows the binding affinity of exemplary chimeric anti-BTLA antibodies 83F2 and 86B7 to huBTLA-overexpressing Expi293 cells as analyzed by FACS. FIG.3C shows the blocking activity of exemplary chimeric anti-BTLA antibodies 83F2 and 86B7 that inhibit soluble HVEM binding to huBTLA-overexpressing Expi293 cells as analyzed by FACS. [0034] FIG.4A shows a schematic diagram of Raji-HVEM/Jurkat-BTLA coculture system comprising HVEM-overexpressing Raji cells cocultured with BTLA-overexpressing Jurkat cells upon the stimulation of anti-αCD3/CD19 bispecific antibody in the presence or absence of anti-BTLA antibodies. FIG.4B shows the recovery of IL-2 production in T cells by chimeric anti-BTLA antibodies 83F2, 86B7, and 96F11 through inhibiting HVEM-BTLA signaling pathway in Raji-HVEM/Jurkat-BTLA coculture system. [0035] FIG.5A shows the binding affinity of exemplary humanized anti-BTLA antibodies SB2003-3, SB2003-4, SB2003-11 and SB2003-12 to huBTLA-overexpressing Expi293 cells as analyzed by FACS. FIG.5B shows the blocking activity of exemplary humanized anti- BTLA antibodies SB2003-3, SB2003-4, SB2003-11 and SB2003-12 that inhibit soluble HVEM binding to huBTLA-overexpressing Expi293 cells as analyzed by FACS. [0036] FIG.6 shows that the inhibition of huHVEM-Fc-induced cell apoptosis by exemplary humanized anti-BTLA antibodies SB2003-11 and SB2003-12 analyzed in Jurkat-BTLA-FAS cell system. [0037] FIG.7A shows the recovery of IL-2 production in T cells by exemplary humanized anti-BTLA antibody SB2003-12 through inhibiting HVEM-BTLA signaling pathway in Raji- HVEM/Jurkat-BTLA coculture system. FIG.7B shows the recovery of IL-2 production in T cells by exemplary humanized anti-BTLA antibodies SB2003-11 and SB2003-12 through inhibiting HVEM-BTLA signaling pathway in Raji-HVEM/primary human T cell coculture system. [0038] FIG.8A shows that the mouse HVEM binds to human BTLA and the exemplary humanized anti-BTLA antibody SB2003-12 blocked this interaction. FIG.8B shows the average tumor volume of individual mouse after the treatment of exemplary humanized anti- BTLA antibody SB2003-12 and isotype control antibody. DETAILED DESCRIPTION OF THE APPLICATION^ [0039] The present application in one aspect provides an isolated anti-BTLA antibody. By using a combination of hybridoma technology, humanization of antibody, affinity maturation and appropriately designed biochemical and biological assays, we have identified highly potent antibody molecules that bind to human BTLA and inhibit the interaction of human BTLA with its receptor. The results presented herein indicate that the antibodies of the present application bind potently to BTLA. Moreover, when compared with the lead anti- BTLA antibody in the field, e.g., Icatolimab (Shanghai Junshi, currently in clinical trial), the antibodies in the present application surprisingly are even more potent as demonstrated in a variety of biological assays. See e.g., Table 12 and FIG.7A. [0040] The anti-BTLA antibodies provided by the present application include, for example, full-length anti-BTLA antibodies, anti-BTLA scFvs, anti-BTLA Fc fusion proteins, multi- specific (such as bispecific) anti-BTLA antibodies, anti-BTLA immunoconjugates, and the like. [0041] In some embodiments, the isolated anti-BTLA antibody comprises: a heavy chain variable domain (V _H ) comprising a heavy chain complementarity determining region (HC- CDR) 1 comprising TFGMGVS (SEQ ID NO: 1); an HC-CDR2 comprising HIYWDDDKRFNPSLKS (SEQ ID NO: 4); and an HC-CDR3 comprising GNWDGETYFDY (SEQ ID NO: 7); and a light chain variable domain (V _L ) comprising a light chain complementarity determining region (LC-CDR) 1 comprising KSTQSLLDSDGKTYLN (SEQ ID NO: 10); an LC-CDR2 comprising LVSKLDS (SEQ ID NO: 13); and an LC-CDR3 comprising WQGTHFPWT (SEQ ID NO: 15). [0042] In some embodiments, the isolated anti-BTLA antibody comprises: a heavy chain variable domain (V _H ) comprising a heavy chain complementarity determining region (HC- CDR) 1 comprising DFWIQ (SEQ ID NO: 2); an HC-CDR2 comprising TIYPGDGDTRENQKFKG (SEQ ID NO: 5); and an HC-CDR3 comprising GNGNSWFAY (SEQ ID NO: 8); and a light chain variable domain (V _L ) comprising a light chain complementarity determining region (LC-CDR) 1 comprising RASESVDDYGISFIN (SEQ ID NO: 11); an LC-CDR2 comprising AASNQGS (SEQ ID NO: 14); and an LC-CDR3 comprising LQSREIPYT (SEQ ID NO: 16). [0043] In some embodiments, the isolated anti-BTLA antibody comprises: a heavy chain variable domain (V _H ) comprising a heavy chain complementarity determining region (HC- CDR) 1 comprising DTYIY (SEQ ID NO: 3); an HC-CDR2 comprising RIDPANGHTKYDPRFQD (SEQ ID NO: 6); and an HC-CDR3 comprising GGDHPYYVMDW (SEQ ID NO: 9); and a light chain variable domain (V _L ) comprising a light chain complementarity determining region (LC-CDR) 1 comprising KSSQNLLDSDGKTYLI (SEQ ID NO: 12); an LC-CDR2 comprising LVSKLDS (SEQ ID NO: 13); and an LC-CDR3 comprising WQGTHFPRT (SEQ ID NO: 17). [0044] Also provided are nucleic acids encoding the anti-BTLA antibodies, compositions comprising the anti-BTLA antibodies, and methods of making and using the anti-BTLA antibodies. Definitions [0045] As used herein, "treatment" or "treating" is an approach for obtaining beneficial or desired results, including clinical results. For purposes of this application, beneficial or desired clinical results include, but are not limited to, one or more of the following: alleviating one or more symptoms resulting from the disease, diminishing the extent of the disease, stabilizing the disease (e.g., preventing or delaying the worsening of the disease), preventing or delaying the spread (e.g., metastasis) of the disease, preventing or delaying the recurrence of the disease, delaying or slowing the progression of the disease, ameliorating the disease state, providing a remission (partial or total) of the disease, decreasing the dose of one or more of other medications required to treat the disease, delaying the progression of the disease, increasing or improving the quality of life, increasing weight gain, and/or prolonging survival. Also encompassed by "treatment" is a reduction of pathological consequence of the disease (such as, for example, tumor volume for cancer). The methods of the application contemplate any one or more of these aspects of treatment. [0046] The term "antibody" includes full-length antibodies and antigen-binding fragments thereof. A full-length antibody comprises two heavy chains and two light chains. The variable regions of the light and heavy chains are responsible for antigen binding. The variable regions in both chains generally contain three highly variable loops called the complementarity determining regions (CDRs) (light chain (LC) CDRs including LC-CDR1, LC-CDR2, and LC-CDR3, heavy chain (HC) CDRs including HC-CDR1, HC-CDR2, and HC-CDR3). CDR boundaries for the antibodies and antigen-binding fragments disclosed herein may be defined or identified by the conventions of Kabat, Chothia, or Al-Lazikani (Al- Lazikani 1997; Chothia 1985; Chothia 1987; Chothia 1989; Kabat 1987; Kabat 1991). The three CDRs of the heavy or light chains are interposed between flanking stretches known as framework regions (FRs), which are more highly conserved than the CDRs and form a scaffold to support the hypervariable loops. The constant regions of the heavy and light chains are not involved in antigen binding, but exhibit various effector functions. Antibodies are assigned to classes based on the amino acid sequence of the constant region of their heavy chain. The five major classes or isotypes of antibodies are IgA, IgD, IgE, IgG, and IgM, which are characterized by the presence of α, δ, ε, γ, and μ heavy chains, respectively. Several of the major antibody classes are divided into subclasses such as IgG1 (γ1 heavy chain), IgG2 (γ2 heavy chain), IgG3 (γ3 heavy chain), IgG4 (γ4 heavy chain), IgA1 (α1 heavy chain), or IgA2 (α2 heavy chain). [0047] The term "antigen-binding fragment" as used herein includes an antibody fragment including, for example, a diabody, a Fab, a Fab', a F(ab') ₂ , an Fv fragment, a disulfide stabilized Fv fragment (dsFv), a (dsFv) ₂ , a bispecific dsFv (dsFv-dsFv'), a disulfide stabilized diabody (ds diabody), a single-chain Fv (scFv), an scFv dimer (bivalent diabody), a multispecific antibody formed from a portion of an antibody comprising one or more CDRs, a single domain antibody, a nanobody, a domain antibody, a bivalent domain antibody, or any other antibody fragments that bind to an antigen but do not comprise a complete antibody structure. An antigen-binding fragment also includes a fusion protein comprising the antibody fragment described above. An antigen-binding fragment is capable of binding to the same antigen to which the parent antibody or a parent antibody fragment (e.g., a parent scFv) binds. In some embodiments, an antigen-binding fragment may comprise one or more CDRs from a particular human antibody grafted to a framework region from one or more different human antibodies. [0048] The term "epitope" as used herein refers to the specific group of atoms or amino acids on an antigen to which an antibody or antibody moiety binds. Two antibodies or antibody moieties may bind the same epitope within an antigen if they exhibit competitive binding for the antigen. [0049] As used herein, a first antibody "competes" for binding to a target BTLA with a second antibody when the first antibody inhibits target BTLA binding of the second antibody by at least about 50% (such as at least about any of 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 98% or 99%) in the presence of an equimolar concentration of the first antibody, or vice versa. A high throughput process for "binning" antibodies based upon their cross- competition is described in PCT Publication No. WO 03/48731. [0050] As used herein, the term "specifically binds", "specifically recognizing", or "is specific for" refers to measurable and reproducible interactions, such as binding between a target and an antibody that is determinative of the presence of the target in the presence of a heterogeneous population of molecules, including biological molecules. For example, an antibody that specifically recognizes a target (which can be an epitope) is an antibody that binds to this target with greater affinity, avidity, more readily, and/or with greater duration than its binding to other targets. In some embodiments, an antibody that specifically recognizes an antigen reacts with one or more antigenic determinants of the antigen with a binding affinity that is at least about 10 times its binding affinity for other targets. [0051] An "isolated" anti-BTLA antibody as used herein refers to an anti-BTLA antibody that (1) is not associated with proteins found in nature, (2) is free of other proteins from the same source, (3) is expressed by a cell from a different species, or, (4) does not occur in nature. [0052] The term "isolated nucleic acid" as used herein is intended to mean a nucleic acid of genomic, cDNA, or synthetic origin or some combination thereof, which by virtue of its origin the "isolated nucleic acid" (1) is not associated with all or a portion of a polynucleotide in which the "isolated nucleic acid" is found in nature, (2) is operably linked to a polynucleotide which it is not linked to in nature, or (3) does not occur in nature as part of a larger sequence. [0053] As used herein, the term "CDR" or "complementarity determining region" is intended to mean the non-contiguous antigen combining sites found within the variable region of both heavy and light chain polypeptides. These particular regions have been described by Kabat et al., J. Biol. Chem.252:6609-6616 (1977); Kabat et al., U.S. Dept. of Health and Human Services, "Sequences of proteins of immunological interest" (1991); Chothia et al., J. Mol. Biol.196:901-917 (1987); Al-Lazikani B. et al., J. Mol. Biol., 273: 927-948 (1997); MacCallum et al., J. Mol. Biol.262:732-745 (1996); Abhinandan and Martin, Mol. Immunol., 45: 3832-3839 (2008); Lefranc M.P. et al., Dev. Comp. Immunol., 27: 55-77 (2003); and Honegger and Plückthun, J. Mol. Biol., 309:657-670 (2001), where the definitions include overlapping or subsets of amino acid residues when compared against each other. Nevertheless, application of either definition to refer to a CDR of an antibody or grafted antibodies or variants thereof is intended to be within the scope of the term as defined and used herein. The amino acid residues which encompass the CDRs as defined by each of the above cited references are set forth below in Table 1 as a comparison. CDR prediction algorithms and interfaces are known in the art, including, for example, Abhinandan and Martin, Mol. Immunol., 45: 3832-3839 (2008); Ehrenmann F. et al., Nucleic Acids Res., 38: D301-D307 (2010); and Adolf-Bryfogle J. et al., Nucleic Acids Res., 43: D432-D438 (2015). The contents of the references cited in this paragraph are incorporated herein by reference in their entireties for use in the present application and for possible inclusion in one or more claims herein. TABLE 1: CDR DEFINITIONS [0054] The term "chimeric antibody" refers to an antibody in which a portion of the heavy and/or light chain is identical with or homologous to corresponding sequences in antibodies derived from a particular species or belonging to a particular antibody class or subclass, while the remainder of the chain(s) is identical with or homologous to corresponding sequences in antibodies derived from another species or belonging to another antibody class or subclass, as well as fragments of such antibodies, so long as they exhibit a biological activity of this application (see U.S. Patent No.4,816,567; and Morrison et al., Proc. Natl. Acad. Sci. USA, 81:6851-6855 (1984)). [0055] "Fv" is the minimum antibody fragment which contains a complete antigen- recognition and binding site. This fragment consists of a dimer of one heavy- and one light- chain variable region domain in tight, non-covalent association. From the folding of these two domains emanate six hypervariable loops (3 loops each from the heavy and light chain) that contribute the amino acid residues for antigen binding and confer antigen binding specificity to the antibody. However, even a single variable domain (or half of an Fv comprising only three CDRs specific for an antigen) has the ability to recognize and bind antigen, although at a lower affinity than the entire binding site. [0056] "Single-chain Fv", also abbreviated as "sFv" or "scFv", are antibody fragments that comprise the V _H and V _L antibody domains connected into a single polypeptide chain. In some embodiments, the scFv polypeptide further comprises a polypeptide linker between the V _H and V _L domains which enables the scFv to form the desired structure for antigen binding. For a review of scFv, see Pluckthun in The Pharmacology of Monoclonal Antibodies, vol. 113, Rosenburg and Moore eds., Springer-Verlag, New York, pp.269-315 (1994). [0057] The term "diabodies" refers to small antibody fragments prepared by constructing scFv fragments (see preceding paragraph) typically with short linkers (such as about 5 to about 10 residues) between the V _H and V _L domains such that inter-chain but not intra-chain pairing of the V domains is achieved, resulting in a bivalent fragment, i.e., fragment having two antigen-binding sites. Bispecific diabodies are heterodimers of two "crossover" scFv fragments in which the V _H and V _L domains of the two antibodies are present on different polypeptide chains. Diabodies are described more fully in, for example, EP 404,097; WO 93/11161; and Hollinger et al., Proc. Natl. Acad. Sci. USA, 90:6444-6448 (1993). [0058] "Humanized" forms of non-human (e.g., rodent) antibodies are chimeric antibodies that contain minimal sequence derived from the non-human antibody. For the most part, humanized antibodies are human immunoglobulins (recipient antibody) in which residues from a hypervariable region (HVR) of the recipient are replaced by residues from a hypervariable region of a non-human species (donor antibody) such as mouse, rat, rabbit or non-human primate having the desired antibody specificity, affinity, and capability. In some instances, framework region (FR) residues of the human immunoglobulin are replaced by corresponding non-human residues. Furthermore, humanized antibodies can comprise residues that are not found in the recipient antibody or in the donor antibody. These modifications are made to further refine antibody performance. In general, the humanized antibody will comprise substantially at least one, and typically two, variable domains, in which all or substantially all of the hypervariable loops correspond to those of a non-human immunoglobulin and all or substantially all of the FRs are those of a human immunoglobulin sequence. The humanized antibody optionally also will comprise at least a portion of an immunoglobulin constant region (Fc), typically that of a human immunoglobulin. For further details, see Jones et al., Nature 321:522-525 (1986); Riechmann et al., Nature 332:323-329 (1988); and Presta, Curr. Op. Struct. Biol.2:593-596 (1992). [0059] "Percent (%) amino acid sequence identity" or "homology" with respect to the polypeptide and antibody sequences identified herein is defined as the percentage of amino acid residues in a candidate sequence that are identical with the amino acid residues in the polypeptide being compared, after aligning the sequences considering any conservative substitutions as part of the sequence identity. Alignment for purposes of determining percent amino acid sequence identity can be achieved in various ways that are within the skilled in the art, for instance, using publicly available computer software such as BLAST, BLAST-2, ALIGN, Megalign (DNASTAR), or MUSCLE software. Those skilled in the art can determine appropriate parameters for measuring alignment, including any algorithms needed to achieve maximal alignment over the full-length of the sequences being compared. For purposes herein, however, % amino acid sequence identity values are generated using the sequence comparison computer program MUSCLE (Edgar, R.C., Nucleic Acids Research 32(5):1792-1797, 2004; Edgar, R.C., BMC Bioinformatics 5(1):113, 2004). [0060] The terms "Fc receptor" or "FcR" are used to describe a receptor that binds to the Fc region of an antibody. In some embodiments, an FcR of this application is one that binds to an IgG antibody (a γ receptor) and includes receptors of the FcγRI, FcγRII, and FcγRIII subclasses, including allelic variants and alternatively spliced forms of these receptors. FcγRII receptors include FcγRIIA (an "activating receptor") and FcγRIIB (an "inhibiting receptor"), which have similar amino acid sequences that differ primarily in the cytoplasmic domains thereof. Activating receptor FcγRIIA contains an immunoreceptor tyrosine-based activation motif (ITAM) in its cytoplasmic domain. Inhibiting receptor FcγRIIB contains an immunoreceptor tyrosine-based inhibition motif (ITIM) in its cytoplasmic domain (see review M. in Daëron, Annu. Rev. Immunol.15:203-234 (1997)). The term includes allotypes, such as FcγRIIIA allotypes: FcγRIIIA-Phe158, FcγRIIIA-Val158, FcγRIIA-R131 and/or FcγRIIA-H131. FcRs are reviewed in Ravetch and Kinet, Annu. Rev. Immunol 9:457-92 (1991); Capel et al., Immunomethods 4:25-34 (1994); and de Haas et al., J. Lab. Clin. Med. 126:330-41 (1995). Other FcRs, including those to be identified in the future, are encompassed by the term "FcR" herein. The term also includes the neonatal receptor, FcRn, which is responsible for the transfer of maternal IgGs to the fetus (Guyer et al., J. Immunol. 117:587 (1976) and Kim et al., J. Immunol.24:249 (1994)). [0061] The term "FcRn" refers to the neonatal Fc receptor (FcRn). FcRn is structurally similar to major histocompatibility complex (MHC) and consists of an α-chain noncovalently bound to β2-microglobulin. The multiple functions of the neonatal Fc receptor FcRn are reviewed in Ghetie and Ward (2000) Annu. Rev. Immunol.18, 739-766. FcRn plays a role in the passive delivery of immunoglobulin IgGs from mother to young and the regulation of serum IgG levels. FcRn can act as a salvage receptor, binding and transporting pinocytosed IgGs in intact form both within and across cells, and rescuing them from a default degradative pathway. [0062] The "CH1 domain" of a human IgG Fc region usually extends from about amino acid 118 to about amino acid 215 (EU numbering system). [0063] "Hinge region" is generally defined as stretching from Glu216 to Pro230 of human IgG1 (Burton, Molec. Immunol.22:161-206 (1985)). Hinge regions of other IgG isotypes may be aligned with the IgG1 sequence by placing the first and last cysteine residues forming inter-heavy chain S-S bonds in the same positions. [0064] The "CH2 domain" of a human IgG Fc region usually extends from about amino acid 231 to about amino acid 340. The CH2 domain is unique in that it is not closely paired with another domain. Rather, two N-linked branched carbohydrate chains are interposed between the two CH2 domains of an intact native IgG molecule. It has been speculated that the carbohydrate may provide a substitute for the domain-domain pairing and help stabilize the CH2 domain. Burton, Molec Immunol.22:161-206 (1985). [0065] The "CH3 domain" comprises the stretch of residues of C-terminal to a CH2 domain in an Fc region (i.e. from about amino acid residue 341 to the C-terminal end of an antibody sequence, typically at amino acid residue 446 or 447 of an IgG). [0066] A "functional Fc fragment" possesses an "effector function" of a native sequence Fc region. Exemplary "effector functions" include C1q binding; complement dependent cytotoxicity (CDC); Fc receptor binding; antibody-dependent cell-mediated cytotoxicity (ADCC); phagocytosis; down regulation of cell surface receptors (e.g., B cell receptor; BCR), etc. Such effector functions generally require the Fc region to be combined with a binding domain (e.g., an antibody variable domain) and can be assessed using various assays known in the art. [0067] An antibody with a variant IgG Fc with "altered" FcR binding affinity or ADCC activity is one which has either enhanced or diminished FcR binding activity (e.g., FcγR or FcRn) and/or ADCC activity compared to a parent polypeptide or to a polypeptide comprising a native sequence Fc region. The variant Fc which "exhibits increased binding" to an FcR binds at least one FcR with higher affinity (e.g., lower apparent Kd or IC ₅₀ value) than the parent polypeptide or a native sequence IgG Fc. According to some embodiments, the improvement in binding compared to a parent polypeptide is about 3-fold, such as about any of 5, 10, 25, 50, 60, 100, 150, 200, or up to 500-fold, or about 25% to 1000% improvement in binding. The polypeptide variant which "exhibits decreased binding" to an FcR, binds at least one FcR with lower affinity (e.g., higher apparent Kd or IC ₅₀ value) than a parent polypeptide. The decrease in binding compared to a parent polypeptide may be about 40% or more decrease in binding. [0068] "Antibody-dependent cell-mediated cytotoxicity" or "ADCC" refers to a form of cytotoxicity in which secreted Ig bound to Fc receptors (FcRs) present on certain cytotoxic cells (e.g., Natural Killer (NK) cells, neutrophils, and macrophages) enable these cytotoxic effector cells to bind specifically to an antigen-bearing target cell and subsequently kill the target cell with cytotoxins. The antibodies "arm" the cytotoxic cells and are required for such killing. The primary cells for mediating ADCC, NK cells, express FcγRIII only, whereas monocytes express FcγRI, FcγRII and FcγRIII. FcR expression on hematopoietic cells is summarized in Table 3 on page 464 of Ravetch and Kinet, Annu. Rev. Immunol 9:457-92 (1991). To assess ADCC activity of a molecule of interest, an in vitro ADCC assay, such as that described in US Patent No.5,500,362 or 5,821,337 may be performed. Useful effector cells for such assays include peripheral blood mononuclear cells (PBMC) and Natural Killer (NK) cells. Alternatively, or additionally, ADCC activity of the molecule of interest may be assessed in vivo, e.g., in an animal model such as that disclosed in Clynes et al. PNAS (USA) 95:652-656 (1998). [0069] The polypeptide comprising a variant Fc region which "exhibits increased ADCC" or mediates ADCC in the presence of human effector cells more effectively than a polypeptide having wild type IgG Fc or a parent polypeptide is one which in vitro or in vivo is substantially more effective at mediating ADCC, when the amounts of polypeptide with variant Fc region and the polypeptide with wild type Fc region (or the parent polypeptide) in the assay are essentially the same. Generally, such variants will be identified using any in vitro ADCC assay known in the art, such as assays or methods for determining ADCC activity, e.g., in an animal model etc. In some embodiments, the variant is from about 5-fold to about 100-fold, e.g. from about 25 to about 50-fold, more effective at mediating ADCC than the wild type Fc (or parent polypeptide). [0070] "Complement dependent cytotoxicity" or "CDC" refers to the lysis of a target cell in the presence of complement. Activation of the classical complement pathway is initiated by the binding of the first component of the complement system (C1q) to antibodies (of the appropriate subclass) which are bound to their cognate antigen. To assess complement activation, a CDC assay, e.g. as described in Gazzano-Santoro et al., J. Immunol. Methods 202:163 (1996), may be performed. Polypeptide variants with altered Fc region amino acid sequences and increased or decreased C1q binding capability are described in US patent No. 6,194,551B1 and WO99/51642. The contents of those patent publications are specifically incorporated herein by reference. See also, Idusogie et al. J. Immunol.164: 4178-4184 (2000). [0071] Unless otherwise specified, a "nucleotide sequence encoding an amino acid sequence" includes all nucleotide sequences that are degenerate versions of each other and that encode the same amino acid sequence. The phrase nucleotide sequence that encodes a protein or a RNA may also include introns to the extent that the nucleotide sequence encoding the protein may in some version contain an intron(s). [0072] The term "operably linked" refers to functional linkage between a regulatory sequence and a heterologous nucleic acid sequence resulting in expression of the latter. For example, a first nucleic acid sequence is operably linked with a second nucleic acid sequence when the first nucleic acid sequence is placed in a functional relationship with the second nucleic acid sequence. For instance, a promoter is operably linked to a coding sequence if the promoter affects the transcription or expression of the coding sequence. Generally, operably linked DNA sequences are contiguous and, where necessary to join two protein coding regions, in the same reading frame. [0073] "Homologous" refers to the sequence similarity or sequence identity between two polypeptides or between two nucleic acid molecules. When a position in both of the two compared sequences is occupied by the same base or amino acid monomer subunit, e.g., if a position in each of two DNA molecules is occupied by adenine, then the molecules are homologous at that position. The percent of homology between two sequences is a function of the number of matching or homologous positions shared by the two sequences divided by the number of positions compared times 100. For example, if 6 of 10 of the positions in two sequences are matched or homologous then the two sequences are 60% homologous. By way of example, the DNA sequences ATTGCC and TATGGC share 50% homology. Generally, a comparison is made when two sequences are aligned to give maximum homology. [0074] An "effective amount" of an anti-BTLA antibody or composition as disclosed herein, is an amount sufficient to carry out a specifically stated purpose. An "effective amount" can be determined empirically and by known methods relating to the stated purpose. [0075] The term "therapeutically effective amount" refers to an amount of an anti-BTLA antibody or composition as disclosed herein, effective to "treat" a disease or disorder in an individual. In the case of cancer, the therapeutically effective amount of the anti-BTLA antibody or composition as disclosed herein can reduce the number of cancer cells; reduce the tumor size or weight; inhibit (i.e., slow to some extent and preferably stop) cancer cell infiltration into peripheral organs; inhibit (i.e., slow to some extent and preferably stop) tumor metastasis; inhibit, to some extent, tumor growth; and/or relieve to some extent one or more of the symptoms associated with the cancer. To the extent the anti-BTLA antibody or composition as disclosed herein can prevent growth and/or kill existing cancer cells, it can be cytostatic and/or cytotoxic. In some embodiments, the therapeutically effective amount is a growth inhibitory amount. In some embodiments, the therapeutically effective amount is an amount that extends the survival of a patient. In some embodiments, the therapeutically effective amount is an amount that improves progression free survival of a patient. [0076] As used herein, by "pharmaceutically acceptable" or "pharmacologically compatible" is meant a material that is not biological or otherwise undesirable, e.g., the material may be incorporated into a pharmaceutical composition administered to a patient without causing any significant undesirable biological effects or interacting in a deleterious manner with any of the other components of the composition in which it is contained. Pharmaceutically acceptable carriers or excipients have preferably met the required standards of toxicological and manufacturing testing and/or are included on the Inactive Ingredient Guide prepared by the U.S. Food and Drug Administration. [0077] It is understood that embodiments of the application described herein include "consisting of" and/or "consisting essentially of" embodiments. [0078] Reference to "about" a value or parameter herein includes (and describes) variations that are directed to that value or parameter per se. For example, description referring to "about X" includes description of "X". [0079] As used herein, reference to "not" a value or parameter generally means and describes "other than" a value or parameter. For example, the method is not used to treat cancer of type X means the method is used to treat cancer of types other than X. [0080] As used herein and in the appended claims, the singular forms "a", "an", and "the" include plural referents unless the context clearly dictates otherwise. Anti-BTLA antibodies [0081] In one aspect, the present application provides anti-BTLA antibodies that specifically bind to human and/or rhesus monkey BTLA. Anti-BTLA antibodies include, but are not limited to, humanized antibodies, chimeric antibodies, mouse antibodies, human antibodies, and antibodies comprising the heavy chain and/or light chain CDRs discussed herein. In one aspect, the present application provides isolated antibodies that bind to BTLA. Contemplated anti-BTLA antibodies include, for example, full-length anti-BTLA antibodies (e.g., full- length IgG1 or IgG4), anti-BTLA scFvs, anti-BTLA Fc fusion proteins, multi-specific (such as bispecific) anti-BTLA antibodies, anti-BTLA immunoconjugates, and the like. In some embodiments, the anti-BTLA antibody is a full-length antibody (e.g., full-length IgG1 or IgG4) or antigen-binding fragment thereof, which specifically binds to BTLA. In some embodiments, the anti-BTLA antibody is a Fab, a Fab', a F(ab)' ₂ , a Fab'-SH, a single-chain Fv (scFv), an Fv fragment, a dAb, a Fd, a nanobody, a diabody, or a linear antibody. In some embodiments, reference to an antibody that specifically binds to BTLA means that the antibody binds to BTLA with an affinity that is at least about 10 times (including for example at least about any one of 10, 10 ² , 10 ³ , 10 ⁴ , 10 ⁵ , 10 ⁶ , or 10 ⁷ times) more tightly than its binding affinity for a non-target. In some embodiments, the non-target is an antigen that is not BTLA. Binding affinity can be determined by methods known in the art, such as ELISA, fluorescence activated cell sorting (FACS) analysis, or radioimmunoprecipitation assay (RIA). Kd can be determined by methods known in the art, such as surface plasmon resonance (SPR) assay or biolayer interferometry (BLI). [0082] Although anti-BTLA antibodies containing human sequences (e.g., human heavy and light chain variable domain sequences comprising human CDR sequences) are extensively discussed herein, non-human anti-BTLA antibodies are also contemplated. In some embodiments, non-human anti-BTLA antibodies comprise human CDR sequences from an anti-BTLA antibody as described herein and non-human framework sequences. Non-human framework sequences include, in some embodiments, any sequence that can be used for generating synthetic heavy and/or light chain variable domains using one or more human CDR sequences as described herein, including, e.g., mammals, e.g., mouse, rat, rabbit, pig, bovine (e.g., cow, bull, buffalo), deer, sheep, goat, chicken, cat, dog, ferret, primate (e.g., marmoset, rhesus monkey), etc. In some embodiments, a non-human anti-BTLA antibody includes an anti-BTLA antibody generated by grafting one or more human CDR sequences as described herein onto a non-human framework sequence (e.g., a mouse or chicken framework sequence). [0083] The amino acid sequence of an exemplary extracellular domain (ECD) of human BTLA comprises or consists of the amino acid sequence of SEQ ID NO: 36. The amino acid sequence of an exemplary ECD of musculus BTLA comprises or consists of the amino acid sequence of SEQ ID NO:37. [0084] In some embodiments, the anti-BTLA antibody described herein specifically recognizes an epitope within human BTLA. In some embodiments, the anti-BTLA antibody cross-reacts with BTLA from species other than human. In some embodiments, the anti- BTLA antibody is completely specific for human BTLA and does not exhibit cross-reactivity with BTLA from other non-human species. [0085] In some embodiments, the anti-BTLA antibody cross-reacts with at least one allelic variant of the BTLA protein (or fragments thereof). In some embodiments, the allelic variant has up to about 30 (such as about any of 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, or 30) amino acid substitutions (such as a conservative substitution) when compared to the naturally occurring BTLA (or fragments thereof). In some embodiments, the anti-BTLA antibody does not cross-react with any allelic variants of the BTLA protein (or fragments thereof). [0086] In some embodiments, the anti-BTLA antibody cross-reacts with at least one interspecies variant of the BTLA protein. In some embodiments, for example, the BTLA protein (or fragments thereof) is human BTLA and the interspecies variant of the BTLA protein (or fragments thereof) is a rhesus monkey variant thereof. In some embodiments, the anti-BTLA antibody does not cross-react with any interspecies variants of the BTLA protein. [0087] In some embodiments, according to any of the anti-BTLA antibodies described herein, the anti-BTLA antibody comprises an antibody heavy chain constant region and an antibody light chain constant region. In some embodiments, the anti-BTLA antibody comprises an IgG1 heavy chain constant region. In some embodiments, the anti-BTLA antibody comprises an IgG2 heavy chain constant region. In some embodiments, the anti- BTLA antibody comprises an IgG3 heavy chain constant region. In some embodiments, the anti-BTLA antibody comprises an IgG4 heavy chain constant region. In some embodiments, the heavy chain constant region comprises (including consisting of or consisting essentially of) the amino acid sequence of SEQ ID NO: 32. In some embodiments, the heavy chain constant region comprises (including consisting of or consisting essentially of) the amino acid sequence of SEQ ID NO: 33. In some embodiments, the anti-BTLA antibody comprises a kappa light chain constant region. In some embodiments, the light chain constant region comprises (including consisting of or consisting essentially of) the amino acid sequence of SEQ ID NO: 34. In some embodiments, the anti-BTLA antibody comprises a lambda light chain constant region. In some embodiments, the light chain constant region comprises (including consisting of or consisting essentially of) the amino acid sequence of SEQ ID NO: 35. In some embodiments, the anti-BTLA antibody comprises an antibody heavy chain variable domain and an antibody light chain variable domain. [0088] In some embodiments, the isolated anti-BTLA antibody comprises: a heavy chain variable domain (V _H ) comprising a heavy chain complementarity determining region (HC- CDR) 1 comprising TFGMGVS (SEQ ID NO: 1); an HC-CDR2 comprising HIYWDDDKRFNPSLKS (SEQ ID NO: 4); and an HC-CDR3 comprising GNWDGETYFDY (SEQ ID NO: 7); and a light chain variable domain (V _L ) comprising a light chain complementarity determining region (LC-CDR) 1 comprising KSTQSLLDSDGKTYLN (SEQ ID NO: 10); an LC-CDR2 comprising LVSKLDS (SEQ ID NO: 13); and an LC-CDR3 comprising WQGTHFPWT (SEQ ID NO: 15) [0089] In some embodiments, the anti-BTLA antibody comprises a V _H comprising: an HC- CDR1 comprising the amino acid sequence of SEQ ID NO: 1, or a variant thereof comprising up to about 3 (such as about any of 1, 2, or 3) amino acid substitutions, an HC-CDR2 comprising the amino acid sequence of SEQ ID NO: 4, or a variant thereof comprising up to about 3 (such as about any of 1, 2, or 3) amino acid substitutions, and an HC-CDR3 comprising the amino acid sequence of SEQ ID NO: 7, or a variant thereof comprising up to about 3 (such as about any of 1, 2, or 3) amino acid substitutions. [0090] In some embodiments, the anti-BTLA antibody comprises a V _H comprising: an HC- CDR1 comprising the amino acid sequence of SEQ ID NO: 1, an HC-CDR2 comprising the amino acid sequence of SEQ ID NO: 4, and an HC-CDR3 comprising the amino acid sequence of SEQ ID NO: 7. [0091] In some embodiments, the anti-BTLA antibody comprises a V _L comprising: an LC- CDR1 comprising the amino acid sequence of SEQ ID NO: 10, or a variant thereof comprising up to about 3 (such as about any of 1, 2, or 3) amino acid substitutions, an LC- CDR2 comprising the amino acid sequence of SEQ ID NO: 13, or a variant thereof comprising up to about 3 (such as about any of 1, 2, or 3) amino acid substitutions, and an LC-CDR3 comprising the amino acid sequence of SEQ ID NO: 15, or a variant thereof comprising up to about 3 (such as about any of 1, 2, or 3) amino acid substitutions. [0092] In some embodiments, the anti-BTLA antibody comprises a V _L comprising: an LC- CDR1 comprising the amino acid sequence of SEQ ID NO: 10, an LC-CDR2 comprising the amino acid sequence of SEQ ID NO: 13, and an LC-CDR3 comprising the amino acid sequence of SEQ ID NO: 15. [0093] In some embodiments, the anti-BTLA antibody comprises a V _H comprising: an HC- CDR1 comprising the amino acid sequence of SEQ ID NO: 1, or a variant thereof comprising up to about 3 (such as about any of 1, 2, or 3) amino acid substitutions, an HC-CDR2 comprising the amino acid sequence of SEQ ID NO: 4, or a variant thereof comprising up to about 3 (such as about any of 1, 2, or 3) amino acid substitutions, and an HC-CDR3 comprising the amino acid sequence of SEQ ID NO: 7, or a variant thereof comprising up to about 3 (such as about any of 1, 2, or 3) amino acid substitutions; and a V _L comprising: an LC-CDR1 comprising the amino acid sequence of SEQ ID NO: 10, or a variant thereof comprising up to about 3 (such as about any of 1, 2, or 3) amino acid substitutions, an LC- CDR2 comprising the amino acid sequence of SEQ ID NO: 13, or a variant thereof comprising up to about 3 (such as about any of 1, 2, or 3) amino acid substitutions, and an LC-CDR3 comprising the amino acid sequence of SEQ ID NO: 15, or a variant thereof comprising up to about 3 (such as about any of 1, 2, or 3) amino acid substitutions. [0094] In some embodiments, the anti-BTLA antibody comprises a V _H comprising: an HC- CDR1 comprising the amino acid sequence of SEQ ID NO: 1, an HC-CDR2 comprising the amino acid sequence of SEQ ID NO: 4, and an HC-CDR3 comprising the amino acid sequence of SEQ ID NO: 7; and a V _L comprising: an LC-CDR1 comprising the amino acid sequence of SEQ ID NO: 10, an LC-CDR2 comprising the amino acid sequence of SEQ ID NO: 13, and an LC-CDR3 comprising the amino acid sequence of SEQ ID NO: 15. [0095] In some embodiments, the anti-BTLA antibody comprises a V _H comprising: an HC- CDR1 comprising the amino acid sequence of SEQ ID NO: 1, an HC-CDR2 comprising the amino acid sequence of SEQ ID NO: 4, and an HC-CDR3 comprising the amino acid sequence of SEQ ID NO: 7, or a variant thereof comprising up to about 5 amino acid substitutions in the HC-CDRs; and a V _L comprising an LC-CDR1 comprising the amino acid sequence of SEQ ID NO: 10, an LC-CDR2 comprising the amino acid sequence of SEQ ID NO: 13, and an LC-CDR3 comprising the amino acid sequence of SEQ ID NO: 15, or a variant thereof comprising up to about 5 amino acid substitutions in the LC-CDRs. [0096] In some embodiments, the anti-BTLA antibody comprises a V _H comprising: an HC- CDR1 comprising the amino acid sequence of SEQ ID NO: 1, an HC-CDR2 comprising the amino acid sequence of SEQ ID NO: 4, and an HC-CDR3 comprising the amino acid sequence of SEQ ID NO: 7; and a V _L comprising an LC-CDR1 comprising the amino acid sequence of SEQ ID NO: 10, an LC-CDR2 comprising the amino acid sequence of SEQ ID NO: 13, and an LC-CDR3 comprising the amino acid sequence of SEQ ID NO: 15. [0097] In some embodiments, the anti-BTLA antibody comprises a V _H comprising an HC- CDR1, an HC-CDR2 and an HC-CDR3 of the V _H comprising the amino acid sequence of any one of SEQ ID NOs: 18-22; and a V _L comprising an LC-CDR1, an LC-CDR2, and an LC- CDR3 of the V _L comprising the amino acid sequence of any one of SEQ ID NOs: 25-29. [0098] In some embodiments, the anti-BTLA antibody comprises a V _H comprising an HC- CDR1, an HC-CDR2 and an HC-CDR3 of the V _H comprising the amino acid sequence of SEQ ID NO: 18, and a V _L comprising an LC-CDR1, an LC-CDR2 and an LC-CDR3 of the V _L comprising the amino acid sequence of SEQ ID NO: 25. [0099] In some embodiments, the anti-BTLA antibody comprises a V _H comprising an HC- CDR1, an HC-CDR2 and an HC-CDR3 of the V _H comprising the amino acid sequence of SEQ ID NO: 19, and a V _L comprising an LC-CDR1, an LC-CDR2 and an LC-CDR3 of the V _L comprising the amino acid sequence of SEQ ID NO: 26. [0100] In some embodiments, the anti-BTLA antibody comprises a V _H comprising an HC- CDR1, an HC-CDR2 and an HC-CDR3 of the V _H comprising the amino acid sequence of SEQ ID NO: 19, and a V _L comprising an LC-CDR1, an LC-CDR2 and an LC-CDR3 of the V _L comprising the amino acid sequence of SEQ ID NO: 27. [0101] In some embodiments, the anti-BTLA antibody comprises a V _H comprising an HC- CDR1, an HC-CDR2 and an HC-CDR3 of the V _H comprising the amino acid sequence of SEQ ID NO: 19, and a V _L comprising an LC-CDR1, an LC-CDR2 and an LC-CDR3 of the V _L comprising the amino acid sequence of SEQ ID NO: 28. [0102] In some embodiments, the anti-BTLA antibody comprises a V _H comprising an HC- CDR1, an HC-CDR2 and an HC-CDR3 of the V _H comprising the amino acid sequence of SEQ ID NO: 19, and a V _L comprising an LC-CDR1, an LC-CDR2 and an LC-CDR3 of the V _L comprising the amino acid sequence of SEQ ID NO: 29. [0103] In some embodiments, the anti-BTLA antibody comprises a V _H comprising an HC- CDR1, an HC-CDR2 and an HC-CDR3 of the V _H comprising the amino acid sequence of SEQ ID NO: 20, and a V _L comprising an LC-CDR1, an LC-CDR2 and an LC-CDR3 of the V _L comprising the amino acid sequence of SEQ ID NO: 26. [0104] In some embodiments, the anti-BTLA antibody comprises a V _H comprising an HC- CDR1, an HC-CDR2 and an HC-CDR3 of the V _H comprising the amino acid sequence of SEQ ID NO: 20, and a V _L comprising an LC-CDR1, an LC-CDR2 and an LC-CDR3 of the V _L comprising the amino acid sequence of SEQ ID NO: 27. [0105] In some embodiments, the anti-BTLA antibody comprises a V _H comprising an HC- CDR1, an HC-CDR2 and an HC-CDR3 of the V _H comprising the amino acid sequence of SEQ ID NO: 20, and a V _L comprising an LC-CDR1, an LC-CDR2 and an LC-CDR3 of the V _L comprising the amino acid sequence of SEQ ID NO: 28. [0106] In some embodiments, the anti-BTLA antibody comprises a V _H comprising an HC- CDR1, an HC-CDR2 and an HC-CDR3 of the V _H comprising the amino acid sequence of SEQ ID NO: 20, and a V _L comprising an LC-CDR1, an LC-CDR2 and an LC-CDR3 of the V _L comprising the amino acid sequence of SEQ ID NO: 29. [0107] In some embodiments, the anti-BTLA antibody comprises a V _H comprising an HC- CDR1, an HC-CDR2 and an HC-CDR3 of the V _H comprising the amino acid sequence of SEQ ID NO: 21, and a V _L comprising an LC-CDR1, an LC-CDR2 and an LC-CDR3 of the V _L comprising the amino acid sequence of SEQ ID NO: 28. [0108] In some embodiments, the anti-BTLA antibody comprises a V _H comprising an HC- CDR1, an HC-CDR2 and an HC-CDR3 of the V _H comprising the amino acid sequence of SEQ ID NO: 21, and a V _L comprising an LC-CDR1, an LC-CDR2 and an LC-CDR3 of the V _L comprising the amino acid sequence of SEQ ID NO: 29. [0109] In some embodiments, the anti-BTLA antibody comprises a V _H comprising an HC- CDR1, an HC-CDR2 and an HC-CDR3 of the V _H comprising the amino acid sequence of SEQ ID NO: 22, and a V _L comprising an LC-CDR1, an LC-CDR2 and an LC-CDR3 of the V _L comprising the amino acid sequence of SEQ ID NO: 28. [0110] In some embodiments, the anti-BTLA antibody comprises a V _H comprising an HC- CDR1, an HC-CDR2 and an HC-CDR3 of the V _H comprising the amino acid sequence of SEQ ID NO: 22, and a V _L comprising an LC-CDR1, an LC-CDR2 and an LC-CDR3 of the V _L comprising the amino acid sequence of SEQ ID NO: 29. [0111] In some embodiments, the anti-BTLA antibody comprises a V _H comprising the amino acid sequence of any one of SEQ ID NOs: 18-22, or a variant thereof having at least about 80% (such as at least about any of 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%) sequence identity, and a V _L comprising the amino acid sequence of any one of SEQ ID NOs: 25-29, or a variant thereof having at least about 80% (such as at least about any of 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%) sequence identity. In some embodiments, the anti- BTLA antibody comprises a V _H comprising the amino acid sequence of any one of SEQ ID NOs: 18-22, and a V _L comprising the amino acid sequence of any one of SEQ ID NOs: 25- 29. [0112] In some embodiments, the anti-BTLA antibody comprises a V _H comprising the amino acid sequence of SEQ ID NO: 18, or a variant thereof having at least about 80% (such as at least about any of 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%) sequence identity, and a V _L comprising the amino acid sequence of SEQ ID NO: 25, or a variant thereof having at least about 80% (such as at least about any of 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%) sequence identity. In some embodiments, the anti-BTLA antibody comprises a V _H comprising the amino acid sequence of SEQ ID NO: 18 and a V _L comprising the amino acid sequence of SEQ ID NO: 25. [0113] In some embodiments, the anti-BTLA antibody comprises a V _H comprising the amino acid sequence of SEQ ID NO: 19, or a variant thereof having at least about 80% (such as at least about any of 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%) sequence identity, and a V _L comprising the amino acid sequence of SEQ ID NO: 26, or a variant thereof having at least about 80% (such as at least about any of 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%) sequence identity. In some embodiments, the anti-BTLA antibody comprises a V _H comprising the amino acid sequence of SEQ ID NO: 19 and a V _L comprising the amino acid sequence of SEQ ID NO: 26. [0114] In some embodiments, the anti-BTLA antibody comprises a V _H comprising the amino acid sequence of SEQ ID NO: 19, or a variant thereof having at least about 80% (such as at least about any of 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%) sequence identity, and a V _L comprising the amino acid sequence of SEQ ID NO: 27, or a variant thereof having at least about 80% (such as at least about any of 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%) sequence identity. In some embodiments, the anti-BTLA antibody comprises a V _H comprising the amino acid sequence of SEQ ID NO: 19 and a V _L comprising the amino acid sequence of SEQ ID NO: 27. [0115] In some embodiments, the anti-BTLA antibody comprises a V _H comprising the amino acid sequence of SEQ ID NO: 19, or a variant thereof having at least about 80% (such as at least about any of 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%) sequence identity, and a V _L comprising the amino acid sequence of SEQ ID NO: 28, or a variant thereof having at least about 80% (such as at least about any of 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%) sequence identity. In some embodiments, the anti-BTLA antibody comprises a V _H comprising the amino acid sequence of SEQ ID NO: 19 and a V _L comprising the amino acid sequence of SEQ ID NO: 28. [0116] In some embodiments, the anti-BTLA antibody comprises a V _H comprising the amino acid sequence of SEQ ID NO: 19, or a variant thereof having at least about 80% (such as at least about any of 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%) sequence identity, and a V _L comprising the amino acid sequence of SEQ ID NO: 29, or a variant thereof having at least about 80% (such as at least about any of 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%) sequence identity. In some embodiments, the anti-BTLA antibody comprises a V _H comprising the amino acid sequence of SEQ ID NO: 19 and a V _L comprising the amino acid sequence of SEQ ID NO: 29. [0117] In some embodiments, the anti-BTLA antibody comprises a V _H comprising the amino acid sequence of SEQ ID NO: 20, or a variant thereof having at least about 80% (such as at least about any of 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%) sequence identity, and a V _L comprising the amino acid sequence of SEQ ID NO: 26, or a variant thereof having at least about 80% (such as at least about any of 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%) sequence identity. In some embodiments, the anti-BTLA antibody comprises a V _H comprising the amino acid sequence of SEQ ID NO: 20 and a V _L comprising the amino acid sequence of SEQ ID NO: 26. [0118] In some embodiments, the anti-BTLA antibody comprises a V _H comprising the amino acid sequence of SEQ ID NO: 20, or a variant thereof having at least about 80% (such as at least about any of 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%) sequence identity, and a V _L comprising the amino acid sequence of SEQ ID NO: 27, or a variant thereof having at least about 80% (such as at least about any of 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%) sequence identity. In some embodiments, the anti-BTLA antibody comprises a V _H comprising the amino acid sequence of SEQ ID NO: 20 and a V _L comprising the amino acid sequence of SEQ ID NO: 27. [0119] In some embodiments, the anti-BTLA antibody comprises a V _H comprising the amino acid sequence of SEQ ID NO: 20, or a variant thereof having at least about 80% (such as at least about any of 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%) sequence identity, and a V _L comprising the amino acid sequence of SEQ ID NO: 28, or a variant thereof having at least about 80% (such as at least about any of 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%) sequence identity. In some embodiments, the anti-BTLA antibody comprises a V _H comprising the amino acid sequence of SEQ ID NO: 20 and a V _L comprising the amino acid sequence of SEQ ID NO: 28. [0120] In some embodiments, the anti-BTLA antibody comprises a V _H comprising the amino acid sequence of SEQ ID NO: 20, or a variant thereof having at least about 80% (such as at least about any of 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%) sequence identity, and a V _L comprising the amino acid sequence of SEQ ID NO: 29, or a variant thereof having at least about 80% (such as at least about any of 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%) sequence identity. In some embodiments, the anti-BTLA antibody comprises a V _H comprising the amino acid sequence of SEQ ID NO: 20 and a V _L comprising the amino acid sequence of SEQ ID NO: 29. [0121] In some embodiments, the anti-BTLA antibody comprises a V _H comprising the amino acid sequence of SEQ ID NO: 21, or a variant thereof having at least about 80% (such as at least about any of 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%) sequence identity, and a V _L comprising the amino acid sequence of SEQ ID NO: 28, or a variant thereof having at least about 80% (such as at least about any of 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%) sequence identity. In some embodiments, the anti-BTLA antibody comprises a V _H comprising the amino acid sequence of SEQ ID NO: 21 and a V _L comprising the amino acid sequence of SEQ ID NO: 28. [0122] In some embodiments, the anti-BTLA antibody comprises a V _H comprising the amino acid sequence of SEQ ID NO: 21, or a variant thereof having at least about 80% (such as at least about any of 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%) sequence identity, and a V _L comprising the amino acid sequence of SEQ ID NO: 29, or a variant thereof having at least about 80% (such as at least about any of 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%) sequence identity. In some embodiments, the anti-BTLA antibody comprises a V _H comprising the amino acid sequence of SEQ ID NO: 21 and a V _L comprising the amino acid sequence of SEQ ID NO: 29. [0123] In some embodiments, the anti-BTLA antibody comprises a V _H comprising the amino acid sequence of SEQ ID NO: 22, or a variant thereof having at least about 80% (such as at least about any of 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%) sequence identity, and a V _L comprising the amino acid sequence of SEQ ID NO: 28, or a variant thereof having at least about 80% (such as at least about any of 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%) sequence identity. In some embodiments, the anti-BTLA antibody comprises a V _H comprising the amino acid sequence of SEQ ID NO: 22 and a V _L comprising the amino acid sequence of SEQ ID NO: 28. [0124] In some embodiments, the anti-BTLA antibody comprises a V _H comprising the amino acid sequence of SEQ ID NO: 22, or a variant thereof having at least about 80% (such as at least about any of 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%) sequence identity, and a V _L comprising the amino acid sequence of SEQ ID NO: 29, or a variant thereof having at least about 80% (such as at least about any of 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%) sequence identity. In some embodiments, the anti-BTLA antibody comprises a V _H comprising the amino acid sequence of SEQ ID NO: 22 and a V _L comprising the amino acid sequence of SEQ ID NO: 29. [0125] In some embodiments, the anti-BTLA antibody comprises a V _H comprising: an HC- CDR1 comprising the amino acid sequence of SEQ ID NO: 2, an HC-CDR2 comprising the amino acid sequence of SEQ ID NO: 5, and an HC-CDR3 comprising the amino acid sequence of SEQ ID NO: 8, or a variant thereof comprising up to about 5 amino acid substitutions in the HC-CDRs; and a V _L comprising an LC-CDR1 comprising the amino acid sequence of SEQ ID NO: 11, an LC-CDR2 comprising the amino acid sequence of SEQ ID NO: 14, and an LC-CDR3 comprising the amino acid sequence of SEQ ID NO: 16, or a variant thereof comprising up to about 5 amino acid substitutions in the LC-CDRs. [0126] In some embodiments, the anti-BTLA antibody comprises a V _H comprising: an HC- CDR1 comprising the amino acid sequence of SEQ ID NO: 2, an HC-CDR2 comprising the amino acid sequence of SEQ ID NO: 5, and an HC-CDR3 comprising the amino acid sequence of SEQ ID NO: 8; and a V _L comprising an LC-CDR1 comprising the amino acid sequence of SEQ ID NO: 11, an LC-CDR2 comprising the amino acid sequence of SEQ ID NO: 14, and an LC-CDR3 comprising the amino acid sequence of SEQ ID NO: 16. [0127] In some embodiments, the anti-BTLA antibody comprises a V _H comprising an HC- CDR1, an HC-CDR2 and an HC-CDR3 of the V _H comprising the amino acid sequence of SEQ ID NO: 23, and a V _L comprising an LC-CDR1, an LC-CDR2 and an LC-CDR3 of the V _L comprising the amino acid sequence of SEQ ID NO: 30. [0128] In some embodiments, the anti-BTLA antibody comprises a V _H comprising the amino acid sequence of SEQ ID NO: 23, or a variant thereof having at least about 80% (such as at least about any of 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%) sequence identity, and a V _L comprising the amino acid sequence of SEQ ID NO: 30, or a variant thereof having at least about 80% (such as at least about any of 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%) sequence identity. In some embodiments, the anti-BTLA antibody comprises a V _H comprising the amino acid sequence of SEQ ID NO: 23 and a V _L comprising the amino acid sequence of SEQ ID NO: 30. [0129] In some embodiments, the anti-BTLA antibody comprises a V _H comprising: an HC- CDR1 comprising the amino acid sequence of SEQ ID NO: 3, an HC-CDR2 comprising the amino acid sequence of SEQ ID NO: 6, and an HC-CDR3 comprising the amino acid sequence of SEQ ID NO: 9, or a variant thereof comprising up to about 5 amino acid substitutions in the HC-CDRs; and a V _L comprising an LC-CDR1 comprising the amino acid sequence of SEQ ID NO: 12, an LC-CDR2 comprising the amino acid sequence of SEQ ID NO: 13, and an LC-CDR3 comprising the amino acid sequence of SEQ ID NO: 17, or a variant thereof comprising up to about 5 amino acid substitutions in the LC-CDRs. [0130] In some embodiments, the anti-BTLA antibody comprises a V _H comprising: an HC- CDR1 comprising the amino acid sequence of SEQ ID NO: 3, an HC-CDR2 comprising the amino acid sequence of SEQ ID NO: 6, and an HC-CDR3 comprising the amino acid sequence of SEQ ID NO: 9; and a V _L comprising an LC-CDR1 comprising the amino acid sequence of SEQ ID NO: 12, an LC-CDR2 comprising the amino acid sequence of SEQ ID NO: 13, and an LC-CDR3 comprising the amino acid sequence of SEQ ID NO: 17. [0131] In some embodiments, the anti-BTLA antibody comprises a V _H comprising an HC- CDR1, an HC-CDR2 and an HC-CDR3 of the V _H comprising the amino acid sequence of SEQ ID NO: 24, and a V _L comprising an LC-CDR1, an LC-CDR2 and an LC-CDR3 of the V _L comprising the amino acid sequence of SEQ ID NO: 31. [0132] In some embodiments, the anti-BTLA antibody comprises a V _H comprising the amino acid sequence of SEQ ID NO: 24, or a variant thereof having at least about 80% (such as at least about any of 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%) sequence identity, and a V _L comprising the amino acid sequence of SEQ ID NO: 31, or a variant thereof having at least about 80% (such as at least about any of 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%) sequence identity. In some embodiments, the anti-BTLA antibody comprises a V _H comprising the amino acid sequence of SEQ ID NO: 24 and a V _L comprising the amino acid sequence of SEQ ID NO: 31. [0133] In some embodiments, the amino acid substitutions described above are limited to "exemplary substitutions" shown in Table 4 of this application. In some embodiments, the amino acid substitutions are limited to “preferred substitutions” shown in Table 4 of this application. [0134] In some embodiments, functional epitopes can be mapped by combinatorial alanine scanning. In this process, a combinatorial alanine-scanning strategy can be used to identify amino acids in the BTLA protein that are necessary for interaction with BTLA antibodies. In some embodiments, the epitope is conformational and crystal structure of anti-BTLA antibodies bound to BTLA may be employed to identify the epitopes. [0135] In some embodiments, the present application provides antibodies which compete with any one of the BTLA antibodies described herein for binding to BTLA. In some embodiments, the present application provides antibodies which compete with any one of the anti-BTLA antibodies provided herein for binding to an epitope on the BTLA. In some embodiments, an anti-BTLA antibody is provided that binds to the same epitope as an anti- BTLA antibody comprising a V _H comprising the amino acid sequence of any one of SEQ ID NOs: 18-24, and a V _L comprising the amino acid sequence of any one of SEQ ID NOs: 25- 31. In some embodiments, an anti-BTLA antibody is provided that specifically binds to BTLA competitively with an anti-BTLA antibody comprising a V _H comprising the amino acid sequence of any one of SEQ ID NOs: 18-24 and a V _L comprising the amino acid sequence of any one of SEQ ID NOs: 25-31. [0136] In some embodiments, competition assays may be used to identify a monoclonal antibody that competes with an anti-BTLA antibody described herein for binding to BTLA. Competition assays can be used to determine whether two antibodies bind to the same epitope by recognizing identical or sterically overlapping epitopes or one antibody competitively inhibits binding of another antibody to the antigen. In certain embodiments, such a competing antibody binds to the same epitope that is bound by an antibody described herein. Exemplary competition assays include, but are not limited to, routine assays such as those provided in Harlow and Lane (1988) Antibodies: A Laboratory Manual ch.14 (Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y.). Detailed exemplary methods for mapping an epitope to which an antibody binds are provided in Morris (1996) "Epitope Mapping Protocols", in Methods in Molecular Biology vol.66 (Humana Press, Totowa, N.J.). In some embodiments, two antibodies are said to bind to the same epitope if each blocks binding of the other by 50% or more. In some embodiments, the antibody that competes with an anti-BTLA antibody described herein is a chimeric, humanized or human antibody. [0137] Exemplary anti-BTLA antibody sequences are shown in Tables 2 and 3, wherein the CDR numbering is according to the EU index of Kabat. Those skilled in the art will recognize that many algorithms are known for prediction of CDR positions and for delimitation of antibody heavy chain and light chain variable regions. Anti-BTLA antibodies comprising CDRs, V _H and/or V _L sequences from antibodies described herein, but based on prediction algorithms other than those exemplified in the tables below, are within the scope of this invention. Table 2. Exemplary anti-BTLA antibody CDR sequences. Table 3. Exemplary sequences. BTLA [0138] BTLA (B and T lymphocyte attenuator) is a member of the CD28 family of receptors which also includes CD28, CTLA-4, ICOS, and PD-1. The initial members of the family, CD28 and ICOS, were discovered by functional effects on augmenting T cell proliferation following the addition of monoclonal antibodies (Hutloff et al., Nature, 1999.). BTLA was discovered through screening for differential expression in TH1 cells. In addition, BTLA has been described as providing negative inhibitory signals, analogous to CTLA-4. In the presence of agonist anti-BTLA antibodies, anti-CD3 and anti-CD28 activated T-cells show reduced IL-2 production and proliferation (Kreig et al., J. Immunol., 2005). Mice lacking an intact BTLA gene show higher titers to DNP-KLH post- immunization and an increased sensitivity to EAEA (Watanabe rt al., Nat. Immunol, 2003). [0139] The protein structure of BTLA is similar to programmed cell death-1 (PD-1) and cytotoxic T lymphocyte associated antigen-4 (CTLA-4), which includes extracellular domain, transmembrane domain and cytoplasmic domain (Sedy JR et al., Nat Immunol., 2005; Gonzalez et al., Proc Natl Acad., 2005). The cytoplasmic domain contains growth factor receptor-bound protein-2 (Grb-2) association motif, immunoreceptor tyrosine-based switch motif (ITSM), and immunoreceptor tyrosine-based inhibitory motif (ITIM). HVEM binding activates tyrosine phosphorylation of the ITIM in BTLA and leads to the recruitment of the Src homology domain 2 (SH2)-containing protein tyrosine phosphatases, SHP-1 and SHP-2, which generally mediate immunosuppressive effects (Gavrieli et al., Biochem Biophys Res Commmun, 2003; Watanabe N et al., Nat Immunol, 2003). BTLA can be extensively expressed in lymph nodes, thymus, and spleen, but little or no expression is detected in organs, such as the heart, kidney, brain, and liver (Yu et al., Front Immunol, 2019). Among immune cells, BTLA is mainly expressed in B and T cells. In mouse spleen, the expression of BTLA is higher in B cells than T cells. With regards to T cells, BTLA expression can be detected on both CD4+ and CD8+ T cells, whereas CD4+ T cells express more BTLA than CD8+ T cells (Rio ML et al., Immunobiology.2010). Besides, its expression can also be detected in innate immune cells, such as dendritic cells (DCs) and monocytes (De Sousa LA et al., Font Immunol, 2018). HVEM binding to BTLA exerts direct negative effects on the proliferation and activation of B and T cells (Cai et al., Immuno Rev, 2009). HVEM [0140] HVEM (TNFRSF14) is a member of the TNFR superfamily that serves a shared ligand for the costimulatory and coinhibitory receptors. Human and murine HVEM are type I cell surface proteins of 283 and 276 amino acids, respectively, with an extracellular domain composed of four cysteine-rich domains (CRDs), CRD1, CRD2, CRD3, CRD4 (Hsu et al. J. Biol. Chem, 1997). CRD2 and CRD3 domains of HVEM interact with LIGHT (Rooney et al, J. Biol. Chem, 2000.), and BTLA and CD160 bind to CRD1 and CRD2 of HVEM (Gonzalez et al., Proc Natl. Acad. Sci, 2005. Cheung et al. Proc. Natl. Acad.2005.). CRD1 is essential for inhibitory signaling induced by the recombinant HVEM-Ig fusion protein, as deletion of this domain results in costimulation by HVEM-Ig (Admas, et al. Am. J. Transplant, 2002.). HVEM displays dual functional activity by binding to coinhibitory receptors, such as BTLA or CD160, and attenuating TCR-mediated signaling; or acting as a receptor of LIGHT and costimulating T cells (Admas, et al. Am. J. Transplant, 2002; Watanabe et al.2003, Nat. Immunol.). [0141] HVEM expression is distributed widely on hematopoietic and nonhematopoietic cells. Interestingly, the expression of LIGHT and HVEM is regulated reciprocally on the same cell (Tamada et al., J. Immunol., 2000). HVEM expression is high in naive and memory B cells but is not present on activated B cells in the germinal center (Duhen et al., J. Immunol., 2004). Similarly to T cells, HVEM engagement on naive B cells by LIGHT expressed on DC and T cells costimulates B cell proliferation and Ig secretion, thereby enhancing humoral immune responses. Besides T and B cells, HVEM is expressed on a wide range of other hematopoietic (DC, Tregs, monocytes, neutrophils, and NK cells) and nonhematopoietic cells (parenchymal cells). Triggering HVEM on these cell types leads to activation of their effector functions, increasing bactericidal activity and promoting NK cell activation (Marsters et al. J. Biol. Chem, 1997; Garrieli et al. Adv. Immunol.2006.; Fan et al. Blood, 2006.). Full-length anti-BTLA antibody [0142] The anti-BTLA antibody in some embodiments is a full-length anti-BTLA antibody. In some embodiments, the full-length anti-BTLA antibody is an IgA, IgD, IgE, IgG, or IgM. In some embodiments, the full-length anti-BTLA antibody comprises IgG constant domains, such as constant domains of any one of IgG1, IgG2, IgG3, and IgG4 including variants thereof. In some embodiments, the full-length anti-BTLA antibody comprises a lambda light chain constant region. In some embodiments, the full-length anti-BTLA antibody comprises a kappa light chain constant region. In some embodiments, the full-length anti-BTLA antibody is a full-length human anti-BTLA antibody. In some embodiments, the full-length anti-BTLA antibody comprises an Fc sequence of a mouse immunoglobulin. In some embodiments, the full-length anti-BTLA antibody comprises an Fc sequence that has been altered or otherwise changed so that it has enhanced antibody dependent cellular cytotoxicity (ADCC) or complement dependent cytotoxicity (CDC) effector function. [0143] Thus, for example, in some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG1 constant domains, wherein the anti-BTLA antibody specifically binds to BTLA. In some embodiments, the IgG1 is human IgG1. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0144] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG2 constant domains, wherein the anti-BTLA antibody specifically binds to BTLA. In some embodiments, the IgG2 is human IgG2. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0145] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG3 constant domains, wherein the anti-BTLA antibody specifically binds to BTLA. In some embodiments, the IgG3 is human IgG3. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0146] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG4 constant domains, wherein the anti-BTLA antibody specifically binds to BTLA. In some embodiments, the IgG4 is human IgG4. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0147] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG1 constant domains, wherein the anti-BTLA antibody comprises a) a heavy chain variable domain comprising an HC-CDR1 comprising the amino acid sequence of any one of SEQ ID NOs: 1-3, or a variant thereof comprising up to about 3 (such as about any of 1, 2, or 3) amino acid substitutions, an HC-CDR2 comprising the amino acid sequence of any one of SEQ ID NOs: 4-6, or a variant thereof comprising up to about 3 (such as about any of 1, 2, or 3) amino acid substitutions, and an HC-CDR3 comprising the amino acid sequence of any one of SEQ ID NOs: 7-9, or a variant thereof comprising up to about 3 (such as about any of 1, 2, or 3) amino acid substitutions; and b) a light chain variable domain comprising an LC-CDR1 comprising the amino acid sequence of any one of SEQ ID NOs: 10-12, or a variant thereof comprising up to about 3 (such as about any of 1, 2, or 3) amino acid substitutions, an LC-CDR2 comprising the amino acid sequence of any one of SEQ ID NOs: 13-14, or a variant thereof comprising up to about 3 (such as about any of 1, 2, or 3) amino acid substitutions, and an LC-CDR3 comprising the amino acid sequence of any one of SEQ ID NOs: 15-17, or a variant thereof comprising up to about 3 (such as about any of 1, 2, or 3) amino acid substitutions. In some embodiments, the IgG1 is human IgG1. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0148] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG2 constant domains, wherein the anti-BTLA antibody comprises a) a heavy chain variable domain comprising an HC-CDR1 comprising the amino acid sequence of any one of SEQ ID NOs: 1-3, or a variant thereof comprising up to about 3 (such as about any of 1, 2, or 3) amino acid substitutions, an HC-CDR2 comprising the amino acid sequence of any one of SEQ ID NOs: 4-6, or a variant thereof comprising up to about 3 (such as about any of 1, 2, or 3) amino acid substitutions, and an HC-CDR3 comprising the amino acid sequence of any one of SEQ ID NOs: 7-9, or a variant thereof comprising up to about 3 (such as about any of 1, 2, or 3) amino acid substitutions; and b) a light chain variable domain comprising an LC-CDR1 comprising the amino acid sequence of any one of SEQ ID NOs: 10-12, or a variant thereof comprising up to about 3 (such as about any of 1, 2, or 3) amino acid substitutions, an LC-CDR2 comprising the amino acid sequence of any one of SEQ ID NOs: 13-14, or a variant thereof comprising up to about 3 (such as about any of 1, 2, or 3) amino acid substitutions, and an LC-CDR3 comprising the amino acid sequence of any one of SEQ ID NOs: 15-17, or a variant thereof comprising up to about 3 (such as about any of 1, 2, or 3) amino acid substitutions. In some embodiments, the IgG2 is human IgG2. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0149] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG3 constant domains, wherein the anti-BTLA antibody comprises a) a heavy chain variable domain comprising an HC-CDR1 comprising the amino acid sequence of any one of SEQ ID NOs: 1-3, or a variant thereof comprising up to about 3 (such as about any of 1, 2, or 3) amino acid substitutions, an HC-CDR2 comprising the amino acid sequence of any one of SEQ ID NOs: 4-6, or a variant thereof comprising up to about 3 (such as about any of 1, 2, or 3) amino acid substitutions, and an HC-CDR3 comprising the amino acid sequence of any one of SEQ ID NOs: 7-9, or a variant thereof comprising up to about 3 (such as about any of 1, 2, or 3) amino acid substitutions; and b) a light chain variable domain comprising an LC-CDR1 comprising the amino acid sequence of any one of SEQ ID NOs: 10-12, or a variant thereof comprising up to about 3 (such as about any of 1, 2, or 3) amino acid substitutions, an LC-CDR2 comprising the amino acid sequence of any one of SEQ ID NOs: 13-14, or a variant thereof comprising up to about 3 (such as about any of 1, 2, or 3) amino acid substitutions, and an LC-CDR3 comprising the amino acid sequence of any one of SEQ ID NOs: 15-17, or a variant thereof comprising up to about 3 (such as about any of 1, 2, or 3) amino acid substitutions. In some embodiments, the IgG3 is human IgG3. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0150] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG4 constant domains, wherein the anti-BTLA antibody comprises a) a heavy chain variable domain comprising an HC-CDR1 comprising the amino acid sequence of any one of SEQ ID NOs: 1-3, or a variant thereof comprising up to about 3 (such as about any of 1, 2, or 3) amino acid substitutions, an HC-CDR2 comprising the amino acid sequence of any one of SEQ ID NOs: 4-6, or a variant thereof comprising up to about 3 (such as about any of 1, 2, or 3) amino acid substitutions, and an HC-CDR3 comprising the amino acid sequence of any one of SEQ ID NOs: 7-9, or a variant thereof comprising up to about 3 (such as about any of 1, 2, or 3) amino acid substitutions; and b) a light chain variable domain comprising an LC-CDR1 comprising the amino acid sequence of any one of SEQ ID NOs: 10-12, or a variant thereof comprising up to about 3 (such as about any of 1, 2, or 3) amino acid substitutions, an LC-CDR2 comprising the amino acid sequence of any one of SEQ ID NOs: 13-14, or a variant thereof comprising up to about 3 (such as about any of 1, 2, or 3) amino acid substitutions, and an LC-CDR3 comprising the amino acid sequence of any one of SEQ ID NOs: 15-17, or a variant thereof comprising up to about 3 (such as about any of 1, 2, or 3) amino acid substitutions. In some embodiments, the IgG4 is human IgG4. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0151] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG1 constant domains, wherein the anti-BTLA antibody comprises a) a heavy chain variable domain comprising an HC-CDR1 comprising the amino acid sequence of any one of SEQ ID NOs: 1-3, an HC-CDR2 comprising the amino acid sequence of any one of SEQ ID NOs: 4-6, and an HC-CDR3 comprising the amino acid sequence of any one of SEQ ID NOs: 7-9, or a variant thereof comprising up to about 5 (such as about any of 1, 2, 3, 4, or 5) amino acid substitutions in the HC-CDR sequences; and b) a light chain variable domain comprising an LC-CDR1 comprising the amino acid sequence of any one of SEQ ID NOs: 10-12, an LC-CDR2 comprising the amino acid sequence of any one of SEQ ID NOs: 13-14, and an LC-CDR3 comprising the amino acid sequence of any one of SEQ ID NOs: 15-17, or a variant thereof comprising up to about 5 (such as about any of 1, 2, 3, 4, or 5) amino acid substitutions in the LC-CDR sequences. In some embodiments, the IgG1 is human IgG1. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0152] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG4 constant domains, wherein the anti-BTLA antibody comprises a) a heavy chain variable domain comprising an HC-CDR1 comprising the amino acid sequence of any one of SEQ ID NOs: 1-3, an HC-CDR2 comprising the amino acid sequence of any one of SEQ ID NOs: 4-6, and an HC-CDR3 comprising the amino acid sequence of any one of SEQ ID NOs: 7-9, or a variant thereof comprising up to about 5 (such as about any of 1, 2, 3, 4, or 5) amino acid substitutions in the HC-CDR sequences; and b) a light chain variable domain comprising an LC-CDR1 comprising the amino acid sequence of any one of SEQ ID NOs: 10-12, an LC-CDR2 comprising the amino acid sequence of any one of SEQ ID NOs: 13-14, and an LC-CDR3 comprising the amino acid sequence of any one of SEQ ID NOs: 15-17, or a variant thereof comprising up to about 5 (such as about any of 1, 2, 3, 4, or 5) amino acid substitutions in the LC-CDR sequences. In some embodiments, the IgG4 is human IgG4. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0153] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG1 constant domains, wherein the anti-BTLA antibody comprises a) a heavy chain variable domain comprising an HC-CDR1 comprising the amino acid sequence of any one of SEQ ID NOs: 1-3, an HC-CDR2 comprising the amino acid sequence of any one of SEQ ID NOs: 4-6, and an HC-CDR3 comprising the amino acid sequence of any one of SEQ ID NOs: 7-9; and b) a light chain variable domain comprising an LC-CDR1 comprising the amino acid sequence of any one of SEQ ID NOs: 10-12, an LC-CDR2 comprising the amino acid sequence of any one of SEQ ID NOs: 13-14, and an LC-CDR3 comprising the amino acid sequence of any one of SEQ ID NOs: 15-17. In some embodiments, the IgG1 is human IgG1. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0154] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG4 constant domains, wherein the anti-BTLA antibody comprises a) a heavy chain variable domain comprising an HC-CDR1 comprising the amino acid sequence of any one of SEQ ID NOs: 1-3, an HC-CDR2 comprising the amino acid sequence of any one of SEQ ID NOs: 4-6, and an HC-CDR3 comprising the amino acid sequence of any one of SEQ ID NOs: 7-9; and b) a light chain variable domain comprising an LC-CDR1 comprising the amino acid sequence of any one of SEQ ID NOs: 10-12, an LC-CDR2 comprising the amino acid sequence of any one of SEQ ID NOs: 13-14, and an LC-CDR3 comprising the amino acid sequence of any one of SEQ ID NOs: 15-17. In some embodiments, the IgG4 is human IgG4. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0155] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG1 constant domains, wherein the anti-BTLA antibody comprises a) a heavy chain variable domain comprising an HC-CDR1 comprising the amino acid sequence of SEQ ID NO: 1, an HC-CDR2 comprising the amino acid sequence of SEQ ID NO: 4, and an HC- CDR3 comprising the amino acid sequence of SEQ ID NO: 7; and b) a light chain variable domain comprising an LC-CDR1 comprising the amino acid sequence of SEQ ID NO: 10, an LC-CDR2 comprising the amino acid sequence of SEQ ID NO: 13, and an LC-CDR3 comprising the amino acid sequence of SEQ ID NO: 15. In some embodiments, the IgG1 is human IgG1. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0156] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG1 constant domains, wherein the anti-BTLA antibody comprises a) a heavy chain variable domain comprising an HC-CDR1 comprising the amino acid sequence of SEQ ID NO: 2, an HC-CDR2 comprising the amino acid sequence of SEQ ID NO: 5, and an HC- CDR3 comprising the amino acid sequence of SEQ ID NO: 8; and b) a light chain variable domain comprising an LC-CDR1 comprising the amino acid sequence of SEQ ID NO: 11, an LC-CDR2 comprising the amino acid sequence of SEQ ID NO: 14, and an LC-CDR3 comprising the amino acid sequence of SEQ ID NO: 16. In some embodiments, the IgG1 is human IgG1. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0157] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG1 constant domains, wherein the anti-BTLA antibody comprises a) a heavy chain variable domain comprising an HC-CDR1 comprising the amino acid sequence of SEQ ID NO: 3, an HC-CDR2 comprising the amino acid sequence of SEQ ID NO: 6, and an HC- CDR3 comprising the amino acid sequence of SEQ ID NO: 9; and b) a light chain variable domain comprising an LC-CDR1 comprising the amino acid sequence of SEQ ID NO: 12, an LC-CDR2 comprising the amino acid sequence of SEQ ID NO: 13, and an LC-CDR3 comprising the amino acid sequence of SEQ ID NO: 17. In some embodiments, the IgG1 is human IgG1. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0158] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG4 constant domains, wherein the anti-BTLA antibody comprises a) a heavy chain variable domain comprising an HC-CDR1 comprising the amino acid sequence of SEQ ID NO: 1, an HC-CDR2 comprising the amino acid sequence of SEQ ID NO: 4, and an HC- CDR3 comprising the amino acid sequence of SEQ ID NO: 7; and b) a light chain variable domain comprising an LC-CDR1 comprising the amino acid sequence of SEQ ID NO: 10, an LC-CDR2 comprising the amino acid sequence of SEQ ID NO: 13, and an LC-CDR3 comprising the amino acid sequence of SEQ ID NO: 15. In some embodiments, the IgG4 is human IgG4. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0159] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG4 constant domains, wherein the anti-BTLA antibody comprises a) a heavy chain variable domain comprising an HC-CDR1 comprising the amino acid sequence of SEQ ID NO: 2, an HC-CDR2 comprising the amino acid sequence of SEQ ID NO: 5, and an HC- CDR3 comprising the amino acid sequence of SEQ ID NO: 8; and b) a light chain variable domain comprising an LC-CDR1 comprising the amino acid sequence of SEQ ID NO: 11, an LC-CDR2 comprising the amino acid sequence of SEQ ID NO: 14, and an LC-CDR3 comprising the amino acid sequence of SEQ ID NO: 16. In some embodiments, the IgG4 is human IgG4. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0160] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG4 constant domains, wherein the anti-BTLA antibody comprises a) a heavy chain variable domain comprising an HC-CDR1 comprising the amino acid sequence of SEQ ID NO: 3, an HC-CDR2 comprising the amino acid sequence of SEQ ID NO: 6, and an HC- CDR3 comprising the amino acid sequence of SEQ ID NO: 9; and b) a light chain variable domain comprising an LC-CDR1 comprising the amino acid sequence of SEQ ID NO: 12, an LC-CDR2 comprising the amino acid sequence of SEQ ID NO: 13, and an LC-CDR3 comprising the amino acid sequence of SEQ ID NO: 17. In some embodiments, the IgG4 is human IgG4. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0161] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG1 constant domains, wherein the anti-BTLA antibody comprises a heavy chain variable domain comprising the amino acid sequence of any one of SEQ ID NOs: 18- 24, or a variant thereof having at least about 80% (such as at least about any of 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%) sequence identity, and a light chain variable domain comprising the amino acid sequence of any one of SEQ ID NOs: 25-31, or a variant thereof having at least about 80% (such as at least about any of 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%) sequence identity. In some embodiments, the IgG1 is human IgG1. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0162] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG2 constant domains, wherein the anti-BTLA antibody comprises a heavy chain variable domain comprising the amino acid sequence of any one of SEQ ID NOs: 18- 24, or a variant thereof having at least about 80% (such as at least about any of 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%) sequence identity, and a light chain variable domain comprising the amino acid sequence of any one of SEQ ID NOs: 25-31, or a variant thereof having at least about 80% (such as at least about any of 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%) sequence identity. In some embodiments, the IgG2 is human IgG2. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0163] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG3 constant domains, wherein the anti-BTLA antibody comprises a heavy chain variable domain comprising the amino acid sequence of any one of SEQ ID NOs: 18- 24, or a variant thereof having at least about 80% (such as at least about any of 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%) sequence identity, and a light chain variable domain comprising the amino acid sequence of any one of SEQ ID NOs: 25-31, or a variant thereof having at least about 80% (such as at least about any of 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%) sequence identity. In some embodiments, the IgG3 is human IgG3. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0164] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG4 constant domains, wherein the anti-BTLA antibody comprises a heavy chain variable domain comprising the amino acid sequence of any one of SEQ ID NOs: 18- 24, or a variant thereof having at least about 80% (such as at least about any of 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%) sequence identity, and a light chain variable domain comprising the amino acid sequence of any one of SEQ ID NOs: 25-31, or a variant thereof having at least about 80% (such as at least about any of 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%) sequence identity. In some embodiments, the IgG4 is human IgG4. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0165] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG1 constant domains, wherein the anti-BTLA antibody comprises a heavy chain variable domain comprising the amino acid sequence of any one of SEQ ID NOs: 18- 24, and a light chain variable domain comprising the amino acid sequence of any one of SEQ ID NOs: 25-31. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0166] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG4 constant domains, wherein the anti-BTLA antibody comprises a heavy chain variable domain comprising the amino acid sequence of any one of SEQ ID NOs: 18- 24, and a light chain variable domain comprising the amino acid sequence of any one of SEQ ID NOs: 25-31. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0167] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG1 constant domains, wherein the anti-BTLA antibody comprises a heavy chain variable domain comprising the amino acid sequence of SEQ ID NO: 18 and a light chain variable domain comprising the amino acid sequence of SEQ ID NO: 25. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0168] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG1 constant domains, wherein the anti-BTLA antibody comprises a heavy chain variable domain comprising the amino acid sequence of SEQ ID NO: 19 and a light chain variable domain comprising the amino acid sequence of SEQ ID NO: 26. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0169] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG1 constant domains, wherein the anti-BTLA antibody comprises a heavy chain variable domain comprising the amino acid sequence of SEQ ID NO: 19 and a light chain variable domain comprising the amino acid sequence of SEQ ID NO: 27. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0170] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG1 constant domains, wherein the anti-BTLA antibody comprises a heavy chain variable domain comprising the amino acid sequence of SEQ ID NO: 19 and a light chain variable domain comprising the amino acid sequence of SEQ ID NO: 28. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0171] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG1 constant domains, wherein the anti-BTLA antibody comprises a heavy chain variable domain comprising the amino acid sequence of SEQ ID NO: 19 and a light chain variable domain comprising the amino acid sequence of SEQ ID NO: 29. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0172] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG1 constant domains, wherein the anti-BTLA antibody comprises a heavy chain variable domain comprising the amino acid sequence of SEQ ID NO: 20 and a light chain variable domain comprising the amino acid sequence of SEQ ID NO: 26. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0173] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG1 constant domains, wherein the anti-BTLA antibody comprises a heavy chain variable domain comprising the amino acid sequence of SEQ ID NO: 20 and a light chain variable domain comprising the amino acid sequence of SEQ ID NO: 27. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0174] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG1 constant domains, wherein the anti-BTLA antibody comprises a heavy chain variable domain comprising the amino acid sequence of SEQ ID NO: 20 and a light chain variable domain comprising the amino acid sequence of SEQ ID NO: 28. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0175] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG1 constant domains, wherein the anti-BTLA antibody comprises a heavy chain variable domain comprising the amino acid sequence of SEQ ID NO: 20 and a light chain variable domain comprising the amino acid sequence of SEQ ID NO: 29. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0176] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG1 constant domains, wherein the anti-BTLA antibody comprises a heavy chain variable domain comprising the amino acid sequence of SEQ ID NO: 21 and a light chain variable domain comprising the amino acid sequence of SEQ ID NO: 28. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0177] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG1 constant domains, wherein the anti-BTLA antibody comprises a heavy chain variable domain comprising the amino acid sequence of SEQ ID NO: 21 and a light chain variable domain comprising the amino acid sequence of SEQ ID NO: 29. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0178] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG1 constant domains, wherein the anti-BTLA antibody comprises a heavy chain variable domain comprising the amino acid sequence of SEQ ID NO: 22 and a light chain variable domain comprising the amino acid sequence of SEQ ID NO: 28. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0179] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG1 constant domains, wherein the anti-BTLA antibody comprises a heavy chain variable domain comprising the amino acid sequence of SEQ ID NO: 22 and a light chain variable domain comprising the amino acid sequence of SEQ ID NO: 29. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0180] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG1 constant domains, wherein the anti-BTLA antibody comprises a heavy chain variable domain comprising the amino acid sequence of SEQ ID NO: 23 and a light chain variable domain comprising the amino acid sequence of SEQ ID NO: 30. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0181] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG1 constant domains, wherein the anti-BTLA antibody comprises a heavy chain variable domain comprising the amino acid sequence of SEQ ID NO: 24 and a light chain variable domain comprising the amino acid sequence of SEQ ID NO: 31. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0182] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG4 constant domains, wherein the anti-BTLA antibody comprises a heavy chain variable domain comprising the amino acid sequence of SEQ ID NO: 18 and a light chain variable domain comprising the amino acid sequence of SEQ ID NO: 25. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0183] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG4 constant domains, wherein the anti-BTLA antibody comprises a heavy chain variable domain comprising the amino acid sequence of SEQ ID NO: 19 and a light chain variable domain comprising the amino acid sequence of SEQ ID NO: 26. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0184] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG4 constant domains, wherein the anti-BTLA antibody comprises a heavy chain variable domain comprising the amino acid sequence of SEQ ID NO: 19 and a light chain variable domain comprising the amino acid sequence of SEQ ID NO: 27. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0185] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG4 constant domains, wherein the anti-BTLA antibody comprises a heavy chain variable domain comprising the amino acid sequence of SEQ ID NO: 19 and a light chain variable domain comprising the amino acid sequence of SEQ ID NO: 28. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0186] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG4 constant domains, wherein the anti-BTLA antibody comprises a heavy chain variable domain comprising the amino acid sequence of SEQ ID NO: 19 and a light chain variable domain comprising the amino acid sequence of SEQ ID NO: 29. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0187] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG4 constant domains, wherein the anti-BTLA antibody comprises a heavy chain variable domain comprising the amino acid sequence of SEQ ID NO: 20 and a light chain variable domain comprising the amino acid sequence of SEQ ID NO: 26. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0188] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG4 constant domains, wherein the anti-BTLA antibody comprises a heavy chain variable domain comprising the amino acid sequence of SEQ ID NO: 20 and a light chain variable domain comprising the amino acid sequence of SEQ ID NO: 27. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0189] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG4 constant domains, wherein the anti-BTLA antibody comprises a heavy chain variable domain comprising the amino acid sequence of SEQ ID NO: 20 and a light chain variable domain comprising the amino acid sequence of SEQ ID NO: 28. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0190] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG4 constant domains, wherein the anti-BTLA antibody comprises a heavy chain variable domain comprising the amino acid sequence of SEQ ID NO: 20 and a light chain variable domain comprising the amino acid sequence of SEQ ID NO: 29. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0191] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG4 constant domains, wherein the anti-BTLA antibody comprises a heavy chain variable domain comprising the amino acid sequence of SEQ ID NO: 21 and a light chain variable domain comprising the amino acid sequence of SEQ ID NO: 28. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0192] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG4 constant domains, wherein the anti-BTLA antibody comprises a heavy chain variable domain comprising the amino acid sequence of SEQ ID NO: 21 and a light chain variable domain comprising the amino acid sequence of SEQ ID NO: 29. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0193] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG4 constant domains, wherein the anti-BTLA antibody comprises a heavy chain variable domain comprising the amino acid sequence of SEQ ID NO: 22 and a light chain variable domain comprising the amino acid sequence of SEQ ID NO: 28. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0194] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG4 constant domains, wherein the anti-BTLA antibody comprises a heavy chain variable domain comprising the amino acid sequence of SEQ ID NO: 22 and a light chain variable domain comprising the amino acid sequence of SEQ ID NO: 29. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0195] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG4 constant domains, wherein the anti-BTLA antibody comprises a heavy chain variable domain comprising the amino acid sequence of SEQ ID NO: 23 and a light chain variable domain comprising the amino acid sequence of SEQ ID NO: 30. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0196] In some embodiments, there is provided a full-length anti-BTLA antibody comprising IgG4 constant domains, wherein the anti-BTLA antibody comprises a heavy chain variable domain comprising the amino acid sequence of SEQ ID NO: 24 and a light chain variable domain comprising the amino acid sequence of SEQ ID NO: 31. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33 and the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. Binding affinity [0197] Binding affinity can be indicated by K _d , K _off , K _on , or K _a . The term "K _off ", as used herein, is intended to refer to the off-rate constant for dissociation of an antibody from the antibody /antigen complex, as determined from a kinetic selection set up. The term "K _on ", as used herein, is intended to refer to the on-rate constant for association of an antibody to the antigen to form the antibody/antigen complex. The term dissociation constant "K _d ", as used herein, refers to the dissociation constant of a particular antibody- antigen interaction, and describes the concentration of antigen required to occupy one half of all of the antibody-binding domains present in a solution of antibody molecules at equilibrium, and is equal to K _off /K _on . The measurement of K _d presupposes that all binding agents are in solution. In the case where the antibody is tethered to a cell wall, e.g., in a yeast expression system, the corresponding equilibrium rate constant is expressed as EC ₅₀ , which gives a good approximation of K _d . The affinity constant, K _a , is the inverse of the dissociation constant, K _d . [0198] The dissociation constant (K _d ) is used as an indicator showing affinity of antibody moieties to antigens. For example, easy analysis is possible by the Scatchard method using antibodies marked with a variety of marker agents, as well as by using Biacore (made by Amersham Biosciences), analysis of biomolecular interactions by surface plasmon resonance, according to the user's manual and attached kit. The K _d value that can be derived using these methods is expressed in units of M. An antibody that specifically binds to a target may have a K _d of, for example, ≤ 10 ^-7 M, ≤ 10 ^-8 M, ≤ 10 ^-9 M, ≤ 10 ^-10 M, ≤ 10 ^-11 M, ≤ 10 ^-12 M, or ≤ 10 ^-13 M. [0199] Binding specificity of the antibody can be determined experimentally by methods known in the art. Such methods comprise, but are not limited to, Western blots, ELISA-, RIA-, ECL-, IRMA-, EIA-, BIAcore-tests and peptide scans. [0200] In some embodiments, the anti-BTLA antibody specifically binds to a target BTLA with a K _d of about 10 ^-7 M to about 10 ^-13 M (such as about 10 ^-7 M to about 10 ^-13 M, about 10- 8 M to about 10 ^-13 M, about 10 ^-9 M to about 10 ^-13 M, or about 10 ^-10 M to about 10 ^-12 M). Thus in some embodiments, the K _d of the binding between the anti-BTLA antibody and BTLA, is about 10 ^-7 M to about 10 ^-13 M, about 1×10 ^-7 M to about 5×10 ^-13 M, about 10 ^-7 M to about 10 ^-12 M, about 10 ^-7 M to about 10 ^-11 M, about 10 ^-7 M to about 10 ^-10 M, about 10 ^-7 M to about 10 ^-9 M, about 10 ^-8 M to about 10 ^-13 M, about 1×10 ^-8 M to about 5×10 ^-13 M, about 10 ^-8 M to about 10 ^-12 M, about 10 ^-8 M to about 10 ^-11 M, about 10 ^-8 M to about 10 ^-10 M, about 10 ^-8 M to about 10 ^-9 M, about 5×10 ^-9 M to about 1×10 ^-13 M, about 5×10 ^-9 M to about 1×10- 12 M, about 5×10 ^-9 M to about 1×10 ^-11 M, about 5×10 ^-9 M to about 1×10 ^-10 M, about 10 ^-9 M to about 10 ^-13 M, about 10 ^-9 M to about 10 ^-12 M, about 10 ^-9 M to about 10 ^-11 M, about 10 ^-9 M to about 10 ^-10 M, about 5×10 ^-10 M to about 1×10 ^-13 M, about 5×10 ^-10 M to about 1×10 ^-12 M, about 5×10 ^-10 M to about 1×10 ^-11 M, about 10 ^-10 M to about10 ^-13 M, about 1×10 ^-10 M to about 5×10 ^-13 M, about 1×10 ^-10 M to about 1×10 ^-12 M, about 1×10 ^-10 M to about 5×10 ^-12 M, about 1×10 ^-10 M to about 1×10 ^-11 M, about 10 ^-11 M to about 10 ^-13 M, about 1×10 ^-11 M to about 5×10 ^-13 M, about 10 ^-11 M to about 10 ^-12 M, or about 10 ^-12 M to about 10 ^-13 M. In some embodiments, the K _d of the binding between the anti-BTLA antibody and a BTLA is about 10 ^-7 M to about 10 ^-13 M. [0201] In some embodiments, the K _d of the binding between the anti-BTLA antibody and a non-target is more than the K _d of the binding between the anti-BTLA antibody and the target, and is herein referred to in some embodiments as the binding affinity, of the anti- BTLA antibody to the target (e.g., BTLA) is higher than that to a non-target. In some embodiments, the non-target is an antigen that is not BTLA. In some embodiments, the K _d of the binding between the anti-BTLA antibody (against BTLA) and a non-BTLA target can be at least about 10 times, such as about 10-100 times, about 100-1000 times, about 10 ³ -10 ⁴ times, about 10 ⁴ -10 ⁵ times, about 10 ⁵ -10 ⁶ times, about 10 ⁶ -10 ⁷ times, about 10 ⁷ -10 ⁸ times, about 10 ⁸ -10 ⁹ times, about 10 ⁹ -10 ¹⁰ times, about 10 ¹⁰ -10 ¹¹ times, or about 10 ¹¹ -10 ¹² times of the K _d of the binding between the anti-BTLA antibody and a target BTLA. [0202] In some embodiments, the anti-BTLA antibody binds to a non-target with a K _d of about 10 ^-1 M to about 10 ^-6 M (such as about 10 ^-1 M to about 10 ^-6 M, about 10 ^-1 M to about 10 ^-5 M, or about 10 ^-2 M to about 10 ^-4 M). In some embodiments, the non-target is an antigen that is not BTLA. Thus in some embodiments, the K _d of the binding between the anti-BTLA antibody and a non-BTLA target is about 10 ^-1 M to about 10 ^-6 M, about 1×10 ^-1 M to about 5×10 ^-6 M, about 10 ^-1 M to about 10 ^-5 M, about 1×10 ^-1 M to about 5×10 ^-5 M, about 10 ^-1 M to about 10 ^-4 M, about 1×10 ^-1 M to about 5×10 ^-4 M, about 10 ^-1 M to about 10 ^-3 M, about 1×10 ^-1 M to about 5×10 ^-3 M, about 10 ^-1 M to about 10 ^-2 M, about 10 ^-2 M to about 10 ^-6 M, about 1×10 ^-2 M to about 5×10 ^-6 M, about 10 ^-2 M to about 10 ^-5 M, about 1×10 ^-2 M to about 5×10 ^-5 M, about 10 ^-2 M to about 10 ^-4 M, about 1×10 ^-2 M to about 5×10 ^-4 M, about 10 ^-2 M to about 10 ^-3 M, about 10 ^-3 M to about 10 ^-6 M, about 1×10 ^-3 M to about 5×10 ^-6 M, about 10 ^-3 M to about 10 ^-5 M, about 1×10 ^-3 M to about 5×10 ^-5 M, about 10 ^-3 M to about 10 ^-4 M, about 10 ^-4 M to about 10 ^-6 M, about 1×10 ^-4 M to about 5×10 ^-6 M, about 10 ^-4 M to about 10 ^-5 M, or about 10 ^-5 M to about 10 ^-6 M. [0203] In some embodiments, when referring to that the anti-BTLA antibody specifically recognizes a target BTLA at a high binding affinity, and binds to a non-target at a low binding affinity, the anti-BTLA antibody will bind to the target BTLA with a K _d of about 10- 7 M to about 10 ^-13 M (such as about 10 ^-7 M to about 10 ^-13 M, about 10 ^-8 M to about 10 ^-13 M, about 10 ^-9 M to about 10 ^-13 M, or about 10 ^-10 M to about 10 ^-12 M), and will bind to the non- target with a K _d of about 10 ^-1 M to about 10 ^-6 M (such as about 10 ^-1 M to about 10 ^-6 M, about 10 ^-1 M to about 10 ^-5 M, or about 10 ^-3 M to about 10 ^-4 M). [0204] In some embodiments, when referring to that the anti-BTLA antibody specifically recognizes BTLA, the binding affinity of the anti-BTLA antibody is compared to that of a control anti-BTLA antibody. In some embodiments, the K _d of the binding between the control anti-BTLA antibody and BTLA can be at least about 2 times, such as about 2 times, about 3 times, about 4 times, about 5 times, about 6 times, about 7 times, about 8 times, about 9 times, about 10 times, about 10-100 times, about 100-1000 times, about 10 ³ -10 ⁴ times of the K _d of the binding between the anti-BTLA antibody described herein and BTLA. Nucleic Acids [0205] Nucleic acid molecules encoding the anti-BTLA antibodies are also contemplated. In some embodiments, there is provided a nucleic acid (or a set of nucleic acids) encoding a full-length anti-BTLA antibody, including any of the full-length anti-BTLA antibodies described herein. In some embodiments, the nucleic acid (or a set of nucleic acids) encoding the anti-BTLA antibody described herein may further comprise a nucleic acid sequence encoding a peptide tag (such as protein purification tag, e.g., His-tag, HA tag). [0206] Also contemplated here are isolated host cells comprising an anti-BTLA antibody, an isolated nucleic acid encoding the polypeptide components of the anti-BTLA antibody, or a vector comprising a nucleic acid encoding the polypeptide components of the anti-BTLA antibody described herein. [0207] The present application also includes variants to these nucleic acid sequences. For example, the variants include nucleotide sequences that hybridize to the nucleic acid sequences encoding the anti-BTLA antibodies of the present application under at least moderately stringent hybridization conditions. [0208] The present application also provides vectors in which a nucleic acid of the present application is inserted. [0209] In brief summary, the expression of an anti-BTLA antibody (e.g., full-length anti- BTLA antibody) by a natural or synthetic nucleic acid encoding the anti-BTLA antibody can be achieved by inserting the nucleic acid into an appropriate expression vector, such that the nucleic acid is operably linked to 5' and 3' regulatory elements, including for example a promoter (e.g., a lymphocyte-specific promoter) and a 3' untranslated region (UTR). The vectors can be suitable for replication and integration in eukaryotic host cells. Typical cloning and expression vectors contain transcription and translation terminators, initiation sequences, and promoters useful for regulation of the expression of the desired nucleic acid sequences. [0210] The nucleic acids of the present application may also be used for nucleic acid immunization and gene therapy, using standard gene delivery protocols. Methods for gene delivery are known in the art. See, e.g., U.S. Pat. Nos.5,399,346, 5,580,859, 5,589,466, incorporated by reference herein in their entireties. In some embodiments, the application provides a gene therapy vector. [0211] The nucleic acid can be cloned into a number of types of vectors. For example, the nucleic acid can be cloned into a vector including, but not limited to a plasmid, a phagemid, a phage derivative, an animal virus, and a cosmid. Vectors of particular interest include expression vectors, replication vectors, probe generation vectors, and sequencing vectors. [0212] Further, the expression vector may be provided to a cell in the form of a viral vector. Viral vector technology is well known in the art and is described, for example, in Green and Sambrook (2013, Molecular Cloning: A Laboratory Manual, Cold Spring Harbor Laboratory, New York), and in other virology and molecular biology manuals. Viruses which are useful as vectors include, but are not limited to, retroviruses, adenoviruses, adeno- associated viruses, herpes viruses, and lentiviruses. In general, a suitable vector contains an origin of replication functional in at least one organism, a promoter sequence, convenient restriction endonuclease sites, and one or more selectable markers (see, e.g., WO 01/96584; WO 01/29058; and U.S. Pat. No.6,326,193). [0213] A number of viral based systems have been developed for gene transfer into mammalian cells. For example, retroviruses provide a convenient platform for gene delivery systems. A selected gene can be inserted into a vector and packaged in retroviral particles using techniques known in the art. The recombinant virus can then be isolated and delivered to cells of the subject either in vivo or ex vivo. A number of retroviral systems are known in the art. In some embodiments, adenovirus vectors are used. A number of adenovirus vectors are known in the art. In some embodiments, lentivirus vectors are used. Vectors derived from retroviruses such as the lentivirus are suitable tools to achieve long-term gene transfer since they allow long-term, stable integration of a transgene and its propagation in daughter cells. Lentiviral vectors have the added advantage over vectors derived from onco-retroviruses such as murine leukemia viruses in that they can transduce non-proliferating cells, such as hepatocytes. They also have the added advantage of low immunogenicity. [0214] Additional promoter elements, e.g., enhancers, regulate the frequency of transcriptional initiation. Typically, these are located in the region 30-110 bp upstream of the start site, although a number of promoters have recently been shown to contain functional elements downstream of the start site as well. The spacing between promoter elements frequently is flexible, so that promoter function is preserved when elements are inverted or moved relative to one another. In the thymidine kinase (tk) promoter, the spacing between promoter elements can be increased to 50 bp apart before activity begins to decline. [0215] One example of a suitable promoter is the immediate early cytomegalovirus (CMV) promoter sequence. This promoter sequence is a strong constitutive promoter sequence capable of driving high levels of expression of any polynucleotide sequence operatively linked thereto. Another example of a suitable promoter is Elongation Factor-1α (EF-1α). However, other constitutive promoter sequences may also be used, including, but not limited to the simian virus 40 (SV40) early promoter, mouse mammary tumor virus (MMTV), human immunodeficiency virus (HIV) long terminal repeat (LTR) promoter, MoMuLV promoter, an avian leukemia virus promoter, an Epstein-Barr virus immediate early promoter, a Rous sarcoma virus promoter, as well as human gene promoters such as, but not limited to, the actin promoter, the myosin promoter, the hemoglobin promoter, and the creatine kinase promoter. Further, the application should not be limited to the use of constitutive promoters. Inducible promoters are also contemplated as part of the application. The use of an inducible promoter provides a molecular switch capable of turning on expression of the polynucleotide sequence to which it is operatively linked when such expression is desired, or turning off the expression when expression is not desired. Examples of inducible promoters include, but are not limited to a metallothionine promoter, a glucocorticoid promoter, a progesterone promoter, and a tetracycline promoter. [0216] In some embodiments, the expression of the anti-BTLA antibody is inducible. In some embodiments, a nucleic acid sequence encoding the anti-BTLA antibody is operably linked to an inducible promoter, including any inducible promoter described herein. Inducible promoters [0217] The use of an inducible promoter provides a molecular switch capable of turning on expression of the polynucleotide sequence to which it is operatively linked when such expression is desired, or turning off the expression when expression is not desired. Exemplary inducible promoter systems for use in eukaryotic cells include, but are not limited to, hormone-regulated elements (e.g., see Mader, S. and White, J. H. (1993) Proc. Natl. Acad. Sci. USA 90:5603-5607), synthetic ligand-regulated elements (see, e.g., Spencer, D. M. et al. (1993) Science 262: 1019-1024) and ionizing radiation-regulated elements (e.g., see Manome, Y. et al. (1993) Biochemistry 32: 10607-10613; Datta, R. et al. (1992) Proc. Natl. Acad. Sci. USA 89: 1014- 10153). Further exemplary inducible promoter systems for use in in vitro or in vivo mammalian systems are reviewed in Gingrich et al. (1998) Annual Rev. Neurosci 21:377-405. In some embodiments, the inducible promoter system for use to express the anti-BTLA antibody is the Tet system. In some embodiments, the inducible promoter system for use to express the anti-BTLA antibody is the lac repressor system from E. coli. [0218] An exemplary inducible promoter system for use in the present application is the Tet system. Such systems are based on the Tet system described by Gossen et al. (1993). In an exemplary embodiment, a polynucleotide of interest is under the control of a promoter that comprises one or more Tet operator (TetO) sites. In the inactive state, Tet repressor (TetR) will bind to the TetO sites and repress transcription from the promoter. In the active state, e.g., in the presence of an inducing agent such as tetracycline (Tc), anhydrotetracycline, doxycycline (Dox), or an active analog thereof, the inducing agent causes release of TetR from TetO, thereby allowing transcription to take place. Doxycycline is a member of the tetracycline family of antibiotics having the chemical name of 1-dimethylamino-2,4a,5,7,12- pentahydroxy-11-methyl-4,6-dioxo-1,4a,11,11a,12,12a-hexahydr otetracene-3-carboxamide. [0219] In one embodiment, a TetR is codon-optimized for expression in mammalian cells, e.g., murine or human cells. Most amino acids are encoded by more than one codon due to the degeneracy of the genetic code, allowing for substantial variations in the nucleotide sequence of a given nucleic acid without any alteration in the amino acid sequence encoded by the nucleic acid. However, many organisms display differences in codon usage, also known as "codon bias" (i.e., bias for use of a particular codon(s) for a given amino acid). Codon bias often correlates with the presence of a predominant species of tRNA for a particular codon, which in turn increases efficiency of mRNA translation. Accordingly, a coding sequence derived from a particular organism (e.g., a prokaryote) may be tailored for improved expression in a different organism (e.g., a eukaryote) through codon optimization. [0220] Other specific variations of the Tet system include the following "Tet-Off" and "Tet-On" systems. In the Tet-Off system, transcription is inactive in the presence of Tc or Dox. In that system, a tetracycline-controlled transactivator protein (tTA), which is composed of TetR fused to the strong transactivating domain of VP16 from Herpes simplex virus, regulates expression of a target nucleic acid that is under transcriptional control of a tetracycline-responsive promoter element (TRE). The TRE is made up of TetO sequence concatamers fused to a promoter (commonly the minimal promoter sequence derived from the human cytomegalovirus (hCMV) immediate-early promoter). In the absence of Tc or Dox, tTA binds to the TRE and activates transcription of the target gene. In the presence of Tc or Dox, tTA cannot bind to the TRE, and expression from the target gene remains inactive. [0221] Conversely, in the Tet-On system, transcription is active in the presence of Tc or Dox. The Tet-On system is based on a reverse tetracycline-controlled transactivator, rtTA. Like tTA, rtTA is a fusion protein comprised of the TetR repressor and the VP16 transactivation domain. However, a four amino acid change in the TetR DNA binding moiety alters rtTA's binding characteristics such that it can only recognize the tetO sequences in the TRE of the target transgene in the presence of Dox. Thus, in the Tet-On system, transcription of the TRE-regulated target gene is stimulated by rtTA only in the presence of Dox. [0222] Another inducible promoter system is the lac repressor system from E. coli (See Brown et al., Cell 49:603-612 (1987)). The lac repressor system functions by regulating transcription of a polynucleotide of interest operably linked to a promoter comprising the lac operator (lacO). The lac repressor (lacR) binds to LacO, thus preventing transcription of the polynucleotide of interest. Expression of the polynucleotide of interest is induced by a suitable inducing agent, e.g., isopropyl-β-D-thiogalactopyranoside (IPTG). [0223] In order to assess the expression of a polypeptide or portions thereof, the expression vector to be introduced into a cell can also contain either a selectable marker gene or a reporter gene or both to facilitate identification and selection of expressing cells from the population of cells sought to be transfected or infected through viral vectors. In other aspects, the selectable marker may be carried on a separate piece of DNA and used in a co- transfection procedure. Both selectable markers and reporter genes may be flanked with appropriate regulatory sequences to enable expression in the host cells. Useful selectable markers include, for example, antibiotic-resistance genes, such as neo and the like. [0224] Reporter genes are used for identifying potentially transfected cells and for evaluating the functionality of regulatory sequences. In general, a reporter gene is a gene that is not present in or expressed by the recipient organism or tissue and that encodes a polypeptide whose expression is manifested by some easily detectable property, e.g., enzymatic activity. Expression of the reporter gene is assayed at a suitable time after the DNA has been introduced into the recipient cells. Suitable reporter genes may include genes encoding luciferase, β-galactosidase, chloramphenicol acetyl transferase, secreted alkaline phosphatase, or the green fluorescent protein gene (e.g., Ui-Tel et al., 2000 FEBS Letters 479: 79-82). Suitable expression systems are well known and may be prepared using known techniques or obtained commercially. In general, the construct with the minimal 5' flanking region showing the highest level of expression of reporter gene is identified as the promoter. Such promoter regions may be linked to a reporter gene and used to evaluate agents for the ability to modulate promoter-driven transcription. [0225] In some embodiments, there is provided nucleic acid encoding a full-length anti- BTLA antibody according to any of the full-length anti-BTLA antibodies described herein. In some embodiments, the nucleic acid comprises one or more nucleic acid sequences encoding the heavy and light chains of the full-length anti-BTLA antibody. In some embodiments, each of the one or more nucleic acid sequences are contained in separate vectors. In some embodiments, at least some of the nucleic acid sequences are contained in the same vector. In some embodiments, all of the nucleic acid sequences are contained in the same vector. Vectors may be selected, for example, from the group consisting of mammalian expression vectors and viral vectors (such as those derived from retroviruses, adenoviruses, adeno- associated viruses, herpes viruses, and lentiviruses). [0226] Methods of introducing and expressing genes into a cell are known in the art. In the context of an expression vector, the vector can be readily introduced into a host cell, e.g., mammalian, bacterial, yeast, or insect cell by any method in the art. For example, the expression vector can be transferred into a host cell by physical, chemical, or biological means. [0227] Physical methods for introducing a polynucleotide into a host cell include calcium phosphate precipitation, lipofection, particle bombardment, microinjection, electroporation, and the like. Methods for producing cells comprising vectors and/or exogenous nucleic acids are well-known in the art. See, for example, Green and Sambrook (2013, Molecular Cloning: A Laboratory Manual, Cold Spring Harbor Laboratory, New York). In some embodiments, the introduction of a polynucleotide into a host cell is carried out by calcium phosphate transfection. [0228] Biological methods for introducing a polynucleotide of interest into a host cell include the use of DNA and RNA vectors. Viral vectors, and especially retroviral vectors, have become the most widely used method of inserting genes into mammalian, e.g., human cells. Other viral vectors can be derived from lentivirus, poxviruses, herpes simplex virus 1, adenoviruses and adeno-associated viruses, and the like. See, for example, U.S. Pat. Nos. 5,350,674 and 5,585,362. [0229] Chemical means for introducing a polynucleotide into a host cell include colloidal dispersion systems, such as macromolecule complexes, nanocapsules, microspheres, beads, and lipid-based systems including oil-in-water emulsions, micelles, mixed micelles, and liposomes. An exemplary colloidal system for use as a delivery vehicle in vitro and in vivo is a liposome (e.g., an artificial membrane vesicle). [0230] In the case where a non-viral delivery system is utilized, an exemplary delivery vehicle is a liposome. The use of lipid formulations is contemplated for the introduction of the nucleic acids into a host cell (in vitro, ex vivo or in vivo). In another aspect, the nucleic acid may be associated with a lipid. The nucleic acid associated with a lipid may be encapsulated in the aqueous interior of a liposome, interspersed within the lipid bilayer of a liposome, attached to a liposome via a linking molecule that is associated with both the liposome and the oligonucleotide, entrapped in a liposome, complexed with a liposome, dispersed in a solution containing a lipid, mixed with a lipid, combined with a lipid, contained as a suspension in a lipid, contained or complexed with a micelle, or otherwise associated with a lipid. Lipid, lipid/DNA or lipid/expression vector associated compositions are not limited to any particular structure in solution. For example, they may be present in a bilayer structure, as micelles, or with a "collapsed" structure. They may also simply be interspersed in a solution, possibly forming aggregates that are not uniform in size or shape. Lipids are fatty substances which may be naturally occurring or synthetic lipids. For example, lipids include the fatty droplets that naturally occur in the cytoplasm as well as the class of compounds which contain long-chain aliphatic hydrocarbons and their derivatives, such as fatty acids, alcohols, amines, amino alcohols, and aldehydes. [0231] Regardless of the method used to introduce exogenous nucleic acids into a host cell or otherwise expose a cell to the inhibitor of the present application, in order to confirm the presence of the recombinant DNA sequence in the host cell, a variety of assays may be performed. Such assays include, for example, "molecular biological" assays well known to those of skill in the art, such as Southern and Northern blotting, RT-PCR and PCR; "biochemical" assays, such as detecting the presence or absence of a particular peptide, e.g., by immunological means (ELISAs and Western blots) or by assays described herein to identify agents falling within the scope of the application. Preparation of anti-BTLA antibodies^ [0232] In some embodiments, the anti-BTLA antibody is a monoclonal antibody or derived from a monoclonal antibody. In some embodiments, the anti-BTLA antibody comprises V _H and V _L domains, or variants thereof, from the monoclonal antibody. In some embodiments, the anti-BTLA antibody further comprises C _H 1 and C _L domains, or variants thereof, from the monoclonal antibody. Monoclonal antibodies can be prepared, e.g., using known methods in the art, including hybridoma methods, phage display methods, or using recombinant DNA methods. Additionally, exemplary phage display methods are described herein and in the Examples below. [0233] In a hybridoma method, a hamster, mouse, or other appropriate host animal is typically immunized with an immunizing agent to elicit lymphocytes that produce or are capable of producing antibodies that will specifically bind to the immunizing agent. Alternatively, the lymphocytes can be immunized in vitro. The immunizing agent can include a polypeptide or a fusion protein of the protein of interest. Generally, peripheral blood lymphocytes ("PBLs") are used if cells of human origin are desired, or spleen cells or lymph node cells are used if non-human mammalian sources are desired. The lymphocytes are then fused with an immortalized cell line using a suitable fusing agent, such as polyethylene glycol, to form a hybridoma cell. Immortalized cell lines are usually transformed mammalian cells, particularly myeloma cells of rodent, bovine, and human origin. Usually, rat or mouse myeloma cell lines are employed. The hybridoma cells can be cultured in a suitable culture medium that preferably contains one or more substances that inhibit the growth or survival of the unfused, immortalized cells. For example, if the parental cells lack the enzyme hypoxanthine guanine phosphoribosyl transferase (HGPRT or HPRT), the culture medium for the hybridomas typically will include hypoxanthine, aminopterin, and thymidine ("HAT medium"), which prevents the growth of HGPRT-deficient cells. [0234] In some embodiments, the immortalized cell lines fuse efficiently, support stable high-level expression of antibody by the selected antibody-producing cells, and are sensitive to a medium such as HAT medium. In some embodiments, the immortalized cell lines are murine myeloma lines, which can be obtained, for instance, from the Salk Institute Cell Distribution Center, San Diego, California and the American Type Culture Collection, Manassas, Virginia. Human myeloma and mouse-human heteromyeloma cell lines also have been described for the production of human monoclonal antibodies. [0235] The culture medium in which the hybridoma cells are cultured can then be assayed for the presence of monoclonal antibodies directed against the polypeptide. The binding specificity of monoclonal antibodies produced by the hybridoma cells can be determined by immunoprecipitation or by an in vitro binding assay, such as radioimmunoassay (RIA) or enzyme-linked immunosorbent assay (ELISA). Such techniques and assays are known in the art. The binding affinity of the monoclonal antibody can, for example, be determined by the Scatchard analysis of Munson and Pollard, Anal. Biochem., 107:220 (1980). [0236] After the desired hybridoma cells are identified, the clones can be sub-cloned by limiting dilution procedures and grown by standard methods. Goding, supra. Suitable culture media for this purpose include, for example, Dulbecco's Modified Eagle's Medium and RPMI-1640 medium. Alternatively, the hybridoma cells can be grown in vivo as ascites in a mammal. [0237] The monoclonal antibodies secreted by the sub-clones can be isolated or purified from the culture medium or ascites fluid by conventional immunoglobulin purification procedures such as, for example, protein A-Sepharose, hydroxylapatite chromatography, gel electrophoresis, dialysis, or affinity chromatography. [0238] In some embodiments, according to any of the anti-BTLA antibodies described herein, the anti-BTLA antibody comprises sequences from a clone selected from an antibody library (such as a phage library presenting scFv or Fab fragments). The clone may be identified by screening combinatorial libraries for antibody fragments with the desired activity or activities. For example, a variety of methods are known in the art for generating phage display libraries and screening such libraries for antibodies possessing the desired binding characteristics. Such methods are reviewed, e.g., in Hoogenboom et al., Methods in Molecular Biology 178:1-37 (O'Brien et al., ed., Human Press, Totowa, N.J., 2001) and further described, e.g., in McCafferty et al., Nature 348:552-554; Clackson et al., Nature 352: 624-628 (1991); Marks et al., J. Mol. Biol.222: 581-597 (1992); Marks and Bradbury, Methods in Molecular Biology 248:161-175 (Lo, ed., Human Press, Totowa, N.J., 2003); Sidhu et al., J. Mol. Biol.338(2): 299-310 (2004); Lee et al., J. Mol. Biol.340(5): 1073-1093 (2004); Fellouse, Proc. Natl. Acad. Sci. USA 101(34): 12467-12472 (2004); and Lee et al., J. Immunol. Methods 284(1-2): 119-132(2004). [0239] In certain phage display methods, repertoires of V _H and V _L genes are separately cloned by polymerase chain reaction (PCR) and recombined randomly in phage libraries, which can then be screened for antigen-binding phage as described in Winter et al., Ann. Rev. Immunol., 12: 433-455 (1994). Phage typically display antibody fragments, either as scFv fragments or as Fab fragments. Libraries from immunized sources provide high-affinity antibodies to the immunogen without the requirement of constructing hybridomas. Alternatively, the naive repertoire can be cloned (e.g., from human) to provide a single source of antibodies to a wide range of non-self and also self-antigens without any immunization as described by Griffiths et al., EMBO J, 12: 725-734 (1993). Finally, naive libraries can also be made synthetically by cloning unrearranged V-gene segments from stem cells, and using PCR primers containing random sequence to encode the highly variable CDR3 regions and to accomplish rearrangement in vitro, as described by Hoogenboom and Winter, J. Mol. Biol., 227: 381-388 (1992). Patent publications describing human antibody phage libraries include, for example: U.S. Pat. No.5,750,373, and US Patent Publication Nos.2005/0079574, 2005/0119455, 2005/0266000, 2007/0117126, 2007/0160598, 2007/0237764, 2007/0292936, and 2009/0002360. [0240] The anti-BTLA antibodies can be prepared using phage display to screen libraries for anti-BTLA antibody moieties specific to the target BTLA. The library can be a human scFv phage display library having a diversity of at least 1 × 10 ⁹ (such as at least about any of 1 × 10 ⁹ , 2.5 × 10 ⁹ , 5 × 10 ⁹ , 7.5 × 10 ⁹ , 1 × 10 ¹⁰ , 2.5 × 10 ¹⁰ , 5 × 10 ¹⁰ , 7.5 × 10 ¹⁰ , or 1 × 10 ¹¹ ) unique human antibody fragments. In some embodiments, the library is a naïve human library constructed from DNA extracted from human PMBCs and spleens from healthy donors, encompassing all human heavy and light chain subfamilies. In some embodiments, the library is a naïve human library constructed from DNA extracted from PBMCs isolated from patients with various diseases, such as patients with autoimmune diseases, cancer patients, and patients with infectious diseases. In some embodiments, the library is a semi-synthetic human library, wherein heavy chain CDR3 is completely randomized, with all amino acids (with the exception of cysteine) equally likely to be present at any given position (see, e.g., Hoet, R.M. et al., Nat. Biotechnol.23(3):344-348, 2005). In some embodiments, the heavy chain CDR3 of the semi-synthetic human library has a length from about 5 to about 24 (such as about any of 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, or 24) amino acids. In some embodiments, the library is a fully-synthetic phage display library. In some embodiments, the library is a non-human phage display library. [0241] Phage clones that bind to the target BTLA with high affinity can be selected by iterative binding of phage to the target BTLA, which is bound to a solid support (such as, for example, beads for solution panning or mammalian cells for cell panning), followed by removal of non-bound phage and by elution of specifically bound phage. The bound phage clones are then eluted and used to infect an appropriate host cell, such as E. coli XL1-Blue, for expression and purification. The panning can be performed for multiple (such as about any of 2, 3, 4, 5, 6 or more) rounds with solution panning, cell panning, or a combination of both, to enrich for phage clones binding specifically to the target BTLA. Enriched phage clones can be tested for specific binding to the target BTLA by any methods known in the art, including for example ELISA and FACS. [0242] An alternative method for screening antibody libraries is to display the protein on the surface of yeast cells. Wittrup et al. (US Patent Nos.6,699,658 and 6,696,251) have developed a method for a yeast cell display library. In this yeast display system, a component involves the yeast agglutinin protein (Aga1), which is anchored to the yeast cell wall. Another component involves a second subunit of the agglutinin protein Aga2, which can display on the surface yeast cells through disulfide bonds to Aga1 protein. The protein Aga1 is expressed from a yeast chromosome after the Aga1 gene integration. A library of single chain variable fragments (scFv) is fused genetically to Aga2 sequence in the yeast display plasmid, which, after transformation, is maintained in yeast episomally with a nutritional marker. Both of the Aga1 and Aga2 proteins were expressed under the control of the galactose-inducible promoter. [0243] Human antibody V gene repertoire (V _H and VK fragments) are obtained by PCR method using a pool of degenerate primers (Sblattero, D. & Bradbury, A. Immunotechnology 3, 271-2781998). The PCR templates are from the commercially available RNAs or cDNAs, including PBMC, spleen, lymph nodes, bone marrow and tonsils. Separate V _H and V _K PCR libraries were combined, then assembled together in the scFv format by overlap extension PCR ( Sheets, M.D. et al., Proc. Natl. Acad. Sci. USA 95, 6157-6162 1998.). To construct the yeast scFv display library, the resultant scFv PCR products are cloned into the yeast display plasmid in the yeasts by homologous recombination. (Chao, G, et al., Nat Protoc. 2006;1(2):755-68. Miller KD, et al., Current Protocols in Cytometry 4.7.1-4.7.30, 2008). [0244] The anti-BTLA antibodies can be discovered using mammalian cell display systems in which antibody moieties are displayed on the cell surface and those specific to the target BTLA are isolated by the antigen-guided screening method, as described in U.S. patent No. 7,732,195B2. A Chinese hamster ovary (CHO) cell library representing a large set of human IgG antibody genes can be established and used to discover the clones expressing high- affinity antibody genes. Another display system has been developed to enable simultaneous high-level cell surface display and secretion of the same protein through alternate splicing, where the displayed protein phenotype remains linked to genotype, allowing soluble secreted antibody to be simultaneously characterized in biophysical and cell-based functional assays. This approach overcomes many limitations of previous mammalian cell display, enabling direct selection and maturation of antibodies in the form of full-length, glycosylated IgGs (Peter M. Bowers, et al., Methods 2014,65:44-56). Transient expression systems are suitable for a single round of antigen selection before recovery of the antibody genes and therefore most useful for the selection of antibodies from smaller libraries. Stable episomal vectors offer an attractive alternative. Episomal vectors can be transfected at high efficiency and stably maintained at low copy number, permitting multiple rounds of panning and the resolution of more complex antibody libraries. [0245] The IgG library is based on germline sequence V-gene segments joined to rearranged (D)J regions isolated from a panel of human donors. RNA collected from 2000 human blood samples was reverse-transcribed into cDNA, and the V _H and VK fragments were amplified using V _H - and V _K -specific primers and purified by gel extraction. IgG libraries were generated by sub-cloning the V _H and VK fragments into the display vectors containing IgG1 or K constant regions respectively and then electroporating into or transducing 293T cells. To generate the scFv antibody display library, scFvs were generated by linking V _H and V _K , and then sub-cloned into the display vector, which were then electroporated into or transduce 293T cells. As we known, the IgG library is based on germline sequence V-gene segments joined to rearranged (D)J regions isolated from a panel of donors, the donor can be a mouse, rat, rabbit, or monkey. [0246] Monoclonal antibodies can also be made by recombinant DNA methods, such as those described in U.S. Patent No.4,816,567. DNA encoding the monoclonal antibodies of the application can be readily isolated and sequenced using conventional procedures (e.g., by using oligonucleotide probes that are capable of binding specifically to genes encoding the heavy and light chains of murine antibodies). Hybridoma cells as described above or BTLA- specific phage clones of the application can serve as a source of such DNA. Once isolated, the DNA can be placed into expression vectors, which are then transfected into host cells such as simian COS cells, Chinese hamster ovary (CHO) cells, or myeloma cells that do not otherwise produce immunoglobulin protein, to obtain the synthesis of monoclonal antibodies in the recombinant host cells. The DNA also can be modified, for example, by substituting the coding sequence for human heavy- and light-chain constant domains and/or framework regions in place of the homologous non-human sequences (U.S. Patent No.4,816,567; Morrison et al., supra) or by covalently joining to the immunoglobulin coding sequence all or part of the coding sequence for a non-immunoglobulin polypeptide. Such a non- immunoglobulin polypeptide can be substituted for the constant domains of an antibody of the application, or can be substituted for the variable domains of one antigen-combining site of an antibody of the application to create a chimeric bivalent antibody. [0247] The antibodies can be monovalent antibodies. Methods for preparing monovalent antibodies are known in the art. For example, one method involves recombinant expression of immunoglobulin light chain and modified heavy chain. The heavy chain is truncated generally at any point in the Fc region so as to prevent heavy-chain crosslinking. Alternatively, the relevant cysteine residues are substituted with another amino acid residue or are deleted so as to prevent crosslinking. [0248] In vitro methods are also suitable for preparing monovalent antibodies. Digestion of antibodies to produce fragments thereof, particularly Fab fragments, can be accomplished using any method known in the art. [0249] Antibody variable domains with the desired binding specificities (antibody-antigen combining sites) can be fused to immunoglobulin constant-domain sequences. The fusion preferably is with an immunoglobulin heavy-chain constant domain, comprising at least part of the hinge, CH2, and CH3 regions. In some embodiments, the first heavy-chain constant region (CH1) containing the site necessary for light-chain binding is present in at least one of the fusions. DNAs encoding the immunoglobulin heavy-chain fusions and, if desired, the immunoglobulin light chain, are inserted into separate expression vectors, and are co- transfected into a suitable host organism. Human and Humanized Antibodies [0250] The anti-BTLA antibodies (e.g., full-length anti-BTLA antibodies) can be humanized antibodies or human antibodies. Humanized forms of non-human (e.g., murine) antibody moieties are chimeric immunoglobulins, immunoglobulin chains, or fragments thereof (such as Fv, Fab, Fab', F(ab') ₂ , scFv, or other antigen-binding subsequences of antibodies) that typically contain minimal sequence derived from non-human immunoglobulin. Humanized antibody moieties include human immunoglobulins, immunoglobulin chains, or fragments thereof (recipient antibody) in which residues from a CDR of the recipient are replaced by residues from a CDR of a non-human species (donor antibody) such as mouse, rat, or rabbit having the desired specificity, affinity, and capacity. In some instances, Fv framework residues of the human immunoglobulin are replaced by corresponding non-human residues. Humanized antibody moieties can also comprise residues that are found neither in the recipient antibody nor in the imported CDR or framework sequences. In general, the humanized antibody can comprise substantially at least one, and typically two, variable domains, in which all or substantially all of the CDR regions correspond to those of a non-human immunoglobulin, and all or substantially all of the FR regions are those of a human immunoglobulin consensus sequence. [0251] Generally, a humanized antibody has one or more amino acid residues introduced into it from a source that is non-human. These non-human amino acid residues are often referred to as "import" residues, which are typically taken from an "import" variable domain. According to some embodiments, humanization can be essentially performed following the method of Winter and co-workers (Jones et al., Nature, 321: 522-525 (1986); Riechmann et al., Nature, 332: 323-327 (1988); Verhoeyen et al., Science, 239: 1534-1536 (1988)), by substituting rodent CDRs or CDR sequences for the corresponding sequences of a human antibody. Accordingly, such "humanized" antibody moieties are antibody moieties (U.S. Patent No.4,816,567), wherein substantially less than an intact human variable domain has been substituted by the corresponding sequence from a non-human species. In practice, humanized antibody moieties are typically human antibody moieties in which some CDR residues and possibly some FR residues are substituted by residues from analogous sites in rodent antibodies. [0252] As an alternative to humanization, human antibody moieties can be generated. For example, it is now possible to produce transgenic animals (e.g., mice) that are capable, upon immunization, of producing a full repertoire of human antibodies in the absence of endogenous immunoglobulin production. For example, it has been described that the homozygous deletion of the antibody heavy-chain joining region (JH) gene in chimeric and germ-line mutant mice results in complete inhibition of endogenous antibody production. Transfer of the human germ-line immunoglobulin gene array into such germ-line mutant mice will result in the production of human antibodies upon antigen challenge. See, e.g., Jakobovits et al., PNAS USA, 90:2551 (1993); Jakobovits et al., Nature, 362:255-258 (1993); Bruggemann et al., Year in Immunol., 7:33 (1993); U.S. Patent Nos.5,545,806, 5,569,825, 5,591,669; 5,545,807; and WO 97/17852. Alternatively, human antibodies can be made by introducing human immunoglobulin loci into transgenic animals, e.g., mice in which the endogenous immunoglobulin genes have been partially or completely inactivated. Upon challenge, human antibody production is observed that closely resembles that seen in humans in all respects, including gene rearrangement, assembly, and antibody repertoire. This approach is described, for example, in U.S. Patent Nos.5,545,807; 5,545,806; 5,569,825; 5,625,126; 5,633,425; and 5,661,016, and Marks et al., Bio/Technology, 10: 779-783 (1992); Lonberg et al., Nature, 368: 856-859 (1994); Morrison, Nature, 368: 812-813 (1994); Fishwild et al., Nature Biotechnology, 14: 845-851 (1996); Neuberger, Nature Biotechnology, 14: 826 (1996); Lonberg and Huszar, Intern. Rev. Immunol., 13: 65-93 (1995). [0253] Human antibodies may also be generated by in vitro activated B cells (see U.S. Patents 5,567,610 and 5,229,275) or by using various techniques known in the art, including phage display libraries. Hoogenboom and Winter, J. Mol. Biol., 227:381 (1991); Marks et al., J. Mol. Biol., 222:581 (1991). The techniques of Cole et al. and Boerner et al. are also available for the preparation of human monoclonal antibodies. Cole et al., Monoclonal Antibodies and Cancer Therapy, Alan R. Liss, p.77 (1985) and Boerner et al., J. Immunol., 147(1): 86-95 (1991). Anti-BTLA antibody variants [0254] In some embodiments, amino acid sequences of the anti-BTLA antibody variants (e.g., full-length anti-BTLA antibody) provided herein are contemplated. For example, it may be desirable to improve the binding affinity and/or other biological properties of the antibody. Amino acid sequences of an antibody variant may be prepared by introducing appropriate modifications into the nucleotide sequence encoding the antibody, or by peptide synthesis. Such modifications include, for example, deletions from, and/or insertions into and/or substitutions of residues within the amino acid sequences of the antibody. Any combination of deletion, insertion, and substitution can be made to arrive at the final construct, provided that the final construct possesses the desired characteristics, e.g., antigen-binding. [0255] In some embodiments, anti-BTLA antibody variants having one or more amino acid substitutions are provided. Sites of interest for substitutional mutagenesis include the HVRs and FRs. Amino acid substitutions may be introduced into an antibody of interest and the products screened for a desired activity, e.g., improved bioactivity, retained/improved antigen binding, decreased immunogenicity, or improved ADCC or CDC. [0256] Conservative substitutions are shown in Table 4 below. TABLE 4: CONSERVATIVE SUBSTITUTIONS [0257] Amino acids may be grouped into different classes according to common side-chain properties: a. hydrophobic: Norleucine, Met, Ala, Val, Leu, Ile; b. neutral hydrophilic: Cys, Ser, Thr, Asn, Gln; c. acidic: Asp, Glu; d. basic: His, Lys, Arg; e. residues that influence chain orientation: Gly, Pro; f. aromatic: Trp, Tyr, Phe. [0258] Non-conservative substitutions will entail exchanging a member of one of these classes for another class. [0259] An exemplary substitutional variant is an affinity matured antibody, which may be conveniently generated, e.g., using phage display-based affinity maturation techniques. Briefly, one or more CDR residues are mutated and the variant antibody moieties displayed on phage and screened for a particular biological activity (e.g., bioactivity based on IL-2 production mediated by T cell activation in Raji-HVEM/T cell coculture assay or binding affinity). Alterations (e.g., substitutions) may be made in HVRs, e.g., to improve bioactivity based on IL-2 production mediated by T cell activation in Raji-HVEM/T cell coculture assay or binding affinity. Such alterations may be made in HVR "hotspots", e.g., residues encoded by codons that undergo mutation at high frequency during the somatic maturation process (see, e.g., Chowdhury, Methods Mol. Biol.207:179-196 (2008)), and/or specificity determining residues (SDRs), with the resulting variant V _H and V _L being tested for binding affinity. Affinity maturation by constructing and reselecting from secondary libraries has been described, e.g., in Hoogenboom et al., in Methods in Molecular Biology 178:1-37 (O'Brien et al., ed., Human Press, Totowa, NJ, (2001)). [0260] In some embodiments of affinity maturation, diversity is introduced into the variable genes chosen for maturation by any of a variety of methods (e.g., error-prone PCR, chain shuffling, or oligonucleotide-directed mutagenesis). A secondary library is then created. The library is then screened to identify any antibody variants with the desired affinity. Another method to introduce diversity involves HVR-directed approaches, in which several HVR residues (e.g., 4-6 residues at a time) are randomized. HVR residues involved in antigen binding may be specifically identified, e.g., using alanine scanning mutagenesis or modeling. CDR-H3 and CDR-L3 in particular are often targeted. [0261] In some embodiments, substitutions, insertions, or deletions may occur within one or more HVRs so long as such alterations do not substantially reduce the ability of the antibody to bind antigen. For example, conservative alterations (e.g., conservative substitutions as provided herein) that do not substantially reduce binding affinity may be made in HVRs. Such alterations may be outside of HVR "hotspots" or SDRs. In some embodiments of the variant V _H and V _L sequences provided above, each HVR either is unaltered, or contains no more than one, two or three amino acid substitutions. [0262] A useful method for identification of residues or regions of an antibody that may be targeted for mutagenesis is called "alanine scanning mutagenesis" as described by Cunningham and Wells (1989) Science, 244:1081-1085. In this method, a residue or group of target residues (e.g., charged residues such as Arg, Asp, His, Lys, and Glu) are identified and replaced by a neutral or negatively charged amino acid (e.g., Ala or Glu) to determine whether the interaction of the antibody with antigen is affected. Further substitutions may be introduced at the amino acid locations to demonstrate functional sensitivity to the initial substitutions. Alternatively, or additionally, a crystal structure of an antigen-antibody complex can be determined to identify contact points between the antibody and antigen. Such contact residues and neighboring residues may be targeted or eliminated as candidates for substitution. Variants may be screened to determine whether they contain the desired properties. [0263] Amino acid sequence insertions include amino- and/or carboxyl-terminal fusions ranging in length from one residue to polypeptides containing a hundred or more residues, as well as intrasequence insertions of single or multiple amino acid residues. Examples of terminal insertions include an antibody with an N-terminal methionyl residue. Other insertional variants of the antibody molecule include the fusion to the N- or C-terminus of the antibody to an enzyme (e.g., for ADEPT) or a polypeptide which increases the serum half-life of the antibody. Fc Region Variants [0264] In some embodiments, one or more amino acid modifications may be introduced into the Fc region of an antibody (e.g., a full-length anti-BTLA antibody or anti-BTLA Fc fusion protein) provided herein, thereby generating an Fc region variant. In some embodiments, the Fc region variant has enhanced ADCC effector function, often related to binding to Fc receptors (FcRs). In some embodiments, the Fc region variant has decreased ADCC effector function. There are many examples of changes or mutations to Fc sequences that can alter effector function. For example, WO 00/42072 and Shields et al., J Biol. Chem. 9(2): 6591-6604 (2001) describe antibody variants with improved or diminished binding to FcRs. The contents of those publications are specifically incorporated herein by reference. [0265] Antibody-Dependent Cell-Mediated Cytotoxicity (ADCC) is a mechanism of action of therapeutic antibodies against tumor cells. ADCC is a cell-mediated immune defense whereby an effector cell of the immune system actively lyses a target cell (e.g., a cancer cell), whose membrane-surface antigens have been bound by specific antibodies (e.g., an anti- BTLA antibody). The typical ADCC involves activation of NK cells by antibodies. An NK cell expresses CD16 which is an Fc receptor. This receptor recognizes, and binds to, the Fc portion of an antibody bound to the surface of a target cell. The most common Fc receptor on the surface of an NK cell is called CD16 or FcγRIII. Binding of the Fc receptor to the Fc region of an antibody results in NK cell activation, release of cytolytic granules and consequent target cell apoptosis. The contribution of ADCC to tumor cell killing can be measured with a specific test that uses NK-92 cells that have been transfected with a high- affinity FcR. Results are compared to wild-type NK-92 cells that do not express the FcR. [0266] In some embodiments, the application contemplates an anti-BTLA antibody variant (such as a full-length anti-BTLA antibody variant) comprising an Fc region that possesses some but not all effector functions, which makes it a desirable candidate for applications in which the half-life of the anti-BTLA antibody in vivo is important yet certain effector functions (such as CDC and ADCC) are unnecessary or deleterious. In vitro and/or in vivo cytotoxicity assays can be conducted to confirm the reduction/depletion of CDC and/or ADCC activities. For example, Fc receptor (FcR) binding assays can be conducted to ensure that the antibody lacks FcγR binding (hence likely lacking ADCC activity), but retains FcRn binding ability. The primary cells for mediating ADCC, NK cells, express FcγRIII only, whereas monocytes express FcγRI, FcγRII and FcγRIII. FcR expression on hematopoietic cells is summarized in Table 3 on page 464 of Ravetch and Kinet, Annu. Rev. Immunol. 9:457-492 (1991). Non-limiting examples of in vitro assays to assess ADCC activity of a molecule of interest is described in U.S. Pat. No.5,500,362 (see, e.g., Hellstrom, I. et al., Proc. Nat'l Acad. Sci. USA 83:7059-7063 (1986)) and Hellstrom, I. et al., Proc. Nat'l Acad. Sci. USA 82:1499-1502 (1985); U.S. Pat. No.5,821,337 (see Bruggemann, M. et al., J. Exp. Med.166:1351-1361 (1987)). Alternatively, non-radioactive assay methods may be employed (see, for example, ACTI™ non-radioactive cytotoxicity assay for flow cytometry (CellTechnology, Inc. Mountain View, Calif.; and CYTOTOX 96™ non-radioactive cytotoxicity assay (Promega, Madison, Wis.). Useful effector cells for such assays include peripheral blood mononuclear cells (PBMC) and Natural Killer (NK) cells. Alternatively, or additionally, ADCC activity of the molecule of interest may be assessed in vivo, e.g., in an animal model such as that disclosed in Clynes et al., Proc. Nat'l Acad. Sci. USA 95:652-656 (1998). C1q binding assays may also be carried out to confirm that the antibody is unable to bind C1q and hence lacks CDC activity. See, e.g., C1q and C3c binding ELISA in WO 2006/029879 and WO 2005/100402. To assess complement activation, a CDC assay may be performed (see, for example, Gazzano-Santoro et al., J. Immunol. Methods 202:163 (1996); Cragg, M. S. et al., Blood 101:1045-1052 (2003); and Cragg, M. S. and M. J. Glennie, Blood 103:2738-2743 (2004)). FcRn binding and in vivo clearance/half-life determinations can also be performed using methods known in the art (see, e.g., Petkova, S. B. et al., Int'l. Immunol. 18(12):1759-1769 (2006)). [0267] Antibodies with reduced effector function include those with substitution of one or more of Fc region residues 238, 265, 269, 270, 297, 327 and 329 (U.S. Pat. No.6,737,056). Such Fc mutants include Fc mutants with substitutions at two or more of amino acid positions 265, 269, 270, 297 and 327, including the so-called "DANA" Fc mutant with substitution of residues 265 and 297 to alanine (U.S. Pat. No.7,332,581). [0268] Certain antibody variants with improved or diminished binding to FcRs are described. (See, e.g., U.S. Pat. No.6,737,056; WO 2004/056312, and Shields et al., J. Biol. Chem.9(2): 6591-6604 (2001).) [0269] In some embodiments, there is provided an anti-BTLA antibody (such as a full- length anti-BTLA antibody) variant comprising a variant Fc region comprising one or more amino acid substitutions which improve ADCC. In some embodiments, the variant Fc region comprises one or more amino acid substitutions which improve ADCC, wherein the substitutions are at positions 298, 333, and/or 334 of the variant Fc region (EU numbering of residues). In some embodiments, the anti-BTLA antibody (e.g., full-length anti-BTLA antibody) variant comprises the following amino acid substitution in its variant Fc region: S298A, E333A, and K334A. [0270] In some embodiments, alterations are made in the Fc region that result in altered (i.e., either improved or diminished) C1q binding and/or Complement Dependent Cytotoxicity (CDC), e.g., as described in U.S. Pat. No.6,194,551, WO 99/51642, and Idusogie et al., J. Immunol.164: 4178-4184 (2000). [0271] In some embodiments, there is provided an anti-BTLA antibody (such as a full- length anti-BTLA antibody) variant comprising a variant Fc region comprising one or more amino acid substitutions which increase half-life and/or improve binding to the neonatal Fc receptor (FcRn). Antibodies with increased half-lives and improved binding to FcRn are described in US2005/0014934A1 (Hinton et al.). Those antibodies comprise an Fc region with one or more substitutions therein which improve binding of the Fc region to FcRn. Such Fc variants include those with substitutions at one or more of Fc region residues: 238, 256, 265, 272, 286, 303, 305, 307, 311, 312, 317, 340, 356, 360, 362, 376, 378, 380, 382, 413, 424 or 434, e.g., substitution of Fc region residue 434 (U.S. Pat. No.7,371,826). [0272] See also Duncan & Winter, Nature 322:738-40 (1988); U.S. Pat. No.5,648,260; U.S. Pat. No.5,624,821; and WO 94/29351 concerning other examples of Fc region variants. [0273] Anti-BTLA antibodies (such as full-length anti-BTLA antibodies) comprising any of the Fc variants described herein, or combinations thereof, are contemplated. Glycosylation Variants [0274] In some embodiments, an anti-BTLA antibody (such as a full-length anti-BTLA antibody) provided herein is altered to increase or decrease the extent to which the anti- BTLA antibody is glycosylated. Addition or deletion of glycosylation sites to an anti-BTLA antibody may be conveniently accomplished by altering the amino acid sequence of the anti- BTLA antibody or polypeptide portion thereof such that one or more glycosylation sites are created or removed. [0275] Wherein the anti-BTLA antibody comprises an Fc region, the carbohydrate attached thereto may be altered. Native antibodies produced by mammalian cells typically comprise a branched, biantennary oligosaccharide that is generally attached by an N-linkage to Asn297 of the CH2 domain of the Fc region. See, e.g., Wright et al., TIBTECH 15:26-32 (1997). The oligosaccharide may include various carbohydrates, e.g., mannose, N-acetyl glucosamine (GlcNAc), galactose, and sialic acid, as well as a fucose attached to a GlcNAc in the "stem" of the biantennary oligosaccharide structure. In some embodiments, modifications of the oligosaccharide in an anti-BTLA antibody of the application may be made in order to create anti-BTLA antibody variants with certain improved properties. [0276] The N-glycans attached to the CH2 domain of Fc is heterogeneous. Antibodies or Fc fusion proteins generated in CHO cells are fucosylated by fucosyltransferase activity. See Shoji-Hosaka et al., J. Biochem.2006, 140:777- 83. Normally, a small percentage of naturally occurring afucosylated IgGs may be detected in human serum. N-glycosylation of the Fc is important for binding to FcγR; and afucosylation of the N-glycan increases Fc's binding capacity to FcγRIIIa. Increased FcγRIIIa binding can enhance ADCC, which can be advantageous in certain antibody therapeutic applications in which cytotoxicity is desirable. [0277] In some embodiments, an enhanced effector function can be detrimental when Fc- mediated cytotoxicity is undesirable. In some embodiments, the Fc fragment or CH2 domain is not glycosylated. In some embodiments, the N-glycosylation site in the CH2 domain is mutated to prevent from glycosylation. [0278] In some embodiments, anti-BTLA antibody (such as a full-length anti-BTLA antibody) variants are provided comprising an Fc region wherein a carbohydrate structure attached to the Fc region has reduced fucose or lacks fucose, which may improve ADCC function. Specifically, anti-BTLA antibodies are contemplated herein that have reduced fucose relative to the amount of fucose on the same anti-BTLA antibody produced in a wild- type CHO cell. That is, they are characterized by having a lower amount of fucose than they would otherwise have if produced by native CHO cells (e.g., a CHO cell that produce a native glycosylation pattern, such as, a CHO cell containing a native FUT8 gene). In some embodiments, the anti-BTLA antibody is one wherein less than about 50%, 40%, 30%, 20%, 10%, or 5% of the N-linked glycans thereon comprise fucose. For example, the amount of fucose in such an anti-BTLA antibody may be from 1% to 80%, from 1% to 65%, from 5% to 65% or from 20% to 40%. In some embodiments, the anti-BTLA antibody is one wherein none of the N-linked glycans thereon comprise fucose, i.e., wherein the anti-BTLA antibody is completely without fucose, or has no fucose or is afucosylated. The amount of fucose is determined by calculating the average amount of fucose within the sugar chain at Asn297, relative to the sum of all glycostructures attached to Asn 297 (e.g., complex, hybrid and high mannose structures) as measured by MALDI-TOF mass spectrometry, as described in WO 2008/077546, for example. Asn297 refers to the asparagine residue located at about position 297 in the Fc region (EU numbering of Fc region residues); however, Asn297 may also be located about ±3 amino acids upstream or downstream of position 297, i.e., between positions 294 and 300, due to minor sequence variations in antibodies. Such fucosylation variants may have improved ADCC function. See, e.g., US Patent Publication Nos. US 2003/0157108 (Presta, L.); US 2004/0093621 (Kyowa Hakko Kogyo Co., Ltd). Examples of publications related to "defucosylated" or "fucose-deficient" antibody variants include: US 2003/0157108; WO 2000/61739; WO 2001/29246; US 2003/0115614; US 2002/0164328; US 2004/0093621; US 2004/0132140; US 2004/0110704; US 2004/0110282; US 2004/0109865; WO 2003/085119; WO 2003/084570; WO 2005/035586; WO 2005/035778; WO2005/053742; WO2002/031140; Okazaki et al., J. Mol. Biol.336:1239-1249 (2004); Yamane-Ohnuki et al., Biotech. Bioeng.87: 614 (2004). Examples of cell lines capable of producing defucosylated antibodies include Lec13 CHO cells deficient in protein fucosylation (Ripka et al., Arch. Biochem. Biophys.249:533-545 (1986); US Pat Appl No US 2003/0157108 A1, Presta, L; and WO 2004/056312 A1, Adams et al., especially at Example 11), and knockout cell lines, such asα-1,6-fucosyltransferase gene, FUT8, knockout CHO cells (see, e.g., Yamane-Ohnuki et al., Biotech. Bioeng.87: 614 (2004); Kanda, Y. et al., Biotechnol. Bioeng.94(4):680-688 (2006); and WO2003/085107). [0279] Anti-BTLA antibody (such as a full-length anti-BTLA antibody) variants are further provided with bisected oligosaccharides, e.g., in which a biantennary oligosaccharide attached to the Fc region of the anti-BTLA antibody is bisected by GlcNAc. Such anti-BTLA antibody (such as a full-length anti-BTLA antibody) variants may have reduced fucosylation and/or improved ADCC function. Examples of such antibody variants are described, e.g., in WO 2003/011878 (Jean-Mairet et al.); U.S. Pat. No.6,602,684 (Umana et al.); US 2005/0123546 (Umana et al.), and Ferrara et al., Biotechnology and Bioengineering, 93(5): 851-861 (2006). Anti-BTLA antibody (such as full-length anti-BTLA antibody) variants with at least one galactose residue in the oligosaccharide attached to the Fc region are also provided. Such anti-BTLA antibody variants may have improved CDC function. Such antibody variants are described, e.g., in WO 1997/30087 (Patel et al.); WO 1998/58964 (Raju, S.); and WO 1999/22764 (Raju, S.). [0280] In some embodiments, the anti-BTLA antibody (such as a full-length anti-BTLA antibody) variants comprising an Fc region are capable of binding to an FcγRIII. In some embodiments, the anti-BTLA antibody (such as a full-length anti-BTLA antibody) variants comprising an Fc region have ADCC activity in the presence of human effector cells (e.g., T cell) or have increased ADCC activity in the presence of human effector cells compared to the otherwise same anti-BTLA antibody (such as a full-length anti-BTLA antibody) comprising a human wild-type IgG1Fc region. Cysteine Engineered Variants [0281] In some embodiments, it may be desirable to create cysteine engineered anti-BTLA antibodies (such as a full-length anti-BTLA antibody) in which one or more amino acid residues are substituted with cysteine residues. In some embodiments, the substituted residues occur at accessible sites of the anti-BTLA antibody. By substituting those residues with cysteine, reactive thiol groups are thereby positioned at accessible sites of the anti-BTLA antibody and may be used to conjugate the anti-BTLA antibody to other moieties, such as drug moieties or linker-drug moieties, to create an anti-BTLA immunoconjugate, as described further herein. Cysteine engineered anti-BTLA antibodies (e.g., full-length anti- BTLA antibodies) may be generated as described, e.g., in U.S. Pat. No.7,521,541. Derivatives [0282] In some embodiments, an anti-BTLA antibody (such as a full-length anti-BTLA antibody) provided herein may be further modified to contain additional non-proteinaceous moieties that are known in the art and readily available. The moieties suitable for derivatization of the anti-BTLA antibody include but are not limited to water soluble polymers. Non-limiting examples of water soluble polymers include, but are not limited to, polyethylene glycol (PEG), copolymers of ethylene glycol/propylene glycol, carboxymethylcellulose, dextran, polyvinyl alcohol, polyvinyl pyrrolidone, poly-1,3- dioxolane, poly-1,3,6-trioxane, ethylene/maleic anhydride copolymer, polyaminoacids (either homopolymers or random copolymers), and dextran or poly(n-vinyl pyrrolidone)polyethylene glycol, propropylene glycol homopolymers, prolypropylene oxide/ethylene oxide co- polymers, polyoxyethylated polyols (e.g., glycerol), polyvinyl alcohol, and mixtures thereof. Polyethylene glycol propionaldehyde may have advantages in manufacturing due to its stability in water. The polymer may be of any molecular weight, and may be branched or unbranched. The number of polymers attached to the anti-BTLA antibody may vary, and if more than one polymer are attached, they can be the same or different molecules. In general, the number and/or type of polymers used for derivatization can be determined based on considerations including, but not limited to, the particular properties or functions of anti- BTLA antibody to be improved, whether the anti-BTLA antibody derivative will be used in a therapy under defined conditions, etc. Pharmaceutical Compositions [0283] Also provided herein are compositions (such as pharmaceutical compositions, also referred to herein as formulations) comprising any of the anti-BTLA antibodies (such as a full-length anti-BTLA antibody), nucleic acids encoding the antibodies, vectors comprising the nucleic acids encoding the antibodies, or host cells comprising the nucleic acids or vectors described herein. In some embodiments, there is provided a pharmaceutical composition comprising any one of the anti-BTLA antibodies described herein and a pharmaceutically acceptable carrier. [0284] Suitable formulations of the anti-BTLA antibodies are obtained by mixing an anti- BTLA antibody having the desired degree of purity with optional pharmaceutically acceptable carriers, excipients or stabilizers (Remington's Pharmaceutical Sciences 16th edition, Osol, A. Ed. (1980)), in the form of lyophilized formulations or aqueous solutions. Acceptable carriers, excipients, or stabilizers are nontoxic to recipients at the dosages and concentrations employed, and include buffers such as phosphate, citrate, and other organic acids; antioxidants including ascorbic acid and methionine; preservatives (such as octadecyldimethylbenzyl ammonium chloride; hexamethonium chloride; benzalkonium chloride, benzethonium chloride; phenol, butyl or benzyl alcohol; alkyl parabens such as methyl or propylparaben; catechol; resorcinol; cyclohexanol; 3-pentanol; and m-cresol); low molecular weight (less than about 10 residues) polypeptides; proteins, such as serum albumin, gelatin, or immunoglobulins; hydrophilic polymers such as olyvinylpyrrolidone; amino acids such as glycine, glutamine, asparagine, histidine, arginine, or lysine; monosaccharides, disaccharides, and other carbohydrates including glucose, mannose, or dextrins; chelating agents such as EDTA; sugars such as sucrose, mannitol, trehalose or sorbitol; salt-forming counter-ions such as sodium; metal complexes (e.g., Zn-protein complexes); and/or non-ionic surfactants such as TWEEN™, PLURONICS™ or polyethylene glycol (PEG). Exemplary formulations are described in WO98/56418, expressly incorporated herein by reference. Lyophilized formulations adapted for subcutaneous administration are described in WO97/04801. Such lyophilized formulations may be reconstituted with a suitable diluent to a high protein concentration and the reconstituted formulation may be administered subcutaneously to the individual to be treated herein. Lipofectins or liposomes can be used to deliver the anti-BTLA antibodies of this application into cells. [0285] The formulation herein may also contain one or more active compounds in addition to the anti-BTLA antibody (such as a full-length anti-BTLA antibody) as necessary for the particular indication being treated, preferably those with complementary activities that do not adversely affect each other. For example, it may be desirable to further provide an anti- neoplastic agent, a growth inhibitory agent, a cytotoxic agent, or a chemotherapeutic agent in addition to the anti-BTLA antibody. Such molecules are suitably present in combination in amounts that are effective for the purpose intended. The effective amount of such other agents depends on the amount of anti-BTLA antibody present in the formulation, the type of disease or disorder or treatment, and other factors discussed above. These are generally used in the same dosages and with administration routes as described herein or about from 1 to 99% of the heretofore employed dosages. [0286] The anti-BTLA antibodies (e.g., full-length anti-BTLA antibodies) may also be entrapped in microcapsules prepared, for example, by coacervation techniques or by interfacial polymerization, for example, hydroxymethylcellulose or gelatin-microcapsules and poly-(methylmethacylate) microcapsules, respectively, in colloidal drug delivery systems (for example, liposomes, albumin microspheres, microemulsions, nano-particles and nanocapsules) or in macroemulsions. Sustained-release preparations may be prepared. [0287] Sustained-release preparations of the anti-BTLA antibodies (e.g., full-length anti- BTLA antibodies) can be prepared. Suitable examples of sustained-release preparations include semipermeable matrices of solid hydrophobic polymers containing the antibody (or fragment thereof), which matrices are in the form of shaped articles, e.g., films, or microcapsules. Examples of sustained-release matrices include polyesters, hydrogels (for example, poly(2-hydroxyethyl-methacrylate ), or poly(vinylalcohol)), polylactides (U.S. Pat. No.3,773,919), copolymers of L-glutamic acid and ethyl-L-glutamate, non-degradable ethylene-vinyl acetate, degradable lactic acid-glycolic acid copolymers such as the LUPRON DEPOT™ (injectable microspheres composed of lactic acid-glycolic acid copolymer and leuprolide acetate), and poly-D (-)-3-hydroxybutyric acid. While polymers such as ethylene- vinyl acetate and lactic acid-glycolic acid enable release of molecules for over 100 days, certain hydro gels release proteins for shorter time periods. When encapsulated antibody remain in the body for a long time, they can denature or aggregate as a result of exposure to moisture at 37°C, resulting in a loss of biological activity and possible changes in immunogenicity. Rational strategies can be devised for stabilization of anti-BTLA antibodies depending on the mechanism involved. For example, if the aggregation mechanism is discovered to be intermolecular S-S bond formation through thio-disulfide interchange, stabilization can be achieved by modifying sulfhydryl residues, lyophilizing from acidic solutions, controlling moisture content, using appropriate additives, and developing specific polymer matrix compositions. [0288] In some embodiments, the anti-BTLA antibody (such as a full-length anti-BTLA antibody) is formulated in a buffer comprising a citrate, NaCl, acetate, succinate, glycine, polysorbate 80 (Tween 80), or any combination of the foregoing. [0289] The formulations to be used for in vivo administration must be sterile. This is readily accomplished by, e.g., filtration through sterile filtration membranes. Methods of treatment using anti-BTLA antibodies [0290] The anti-BTLA antibodies (e.g., full-length anti-BTLA antibodies) and/or compositions of the application can be administered to individuals (e.g., mammals such as humans) to treat a disease and/or disorder associated with BTLA signaling (such as altered BTLA signaling, including elevated BTLA signaling) (e.g., cancer or infectious diseases). In some embodiments, the anti-BTLA antibody enhances immune responses by blocking the immunosuppressive actions of BTLA, for example, in the tumor microenvironment. These diseases include, but are not limited to, non-small cell lung cancer, adrenal gland cancer, bladder cancer, brain cancer, pancreatic adenocarcinoma, breast cancer, colorectal cancer, melanoma, esophageal cancer, gastric cancer, cervical cancer, head and neck cancer, hepatocellular carcinoma, kidney cancer, liver cancer, ovarian cancer, pancreatic cancer, prostate cancer, small cell lung cancer, testicular cancer, thyroid cancer, uterine cancer, and any types of leukemia, lymphoma and myeloma, and infectious diseases, including, but not limited to Human Papilloma Virus (HPV), Human Immunodeficiency Virus (HIV), Herpes Simplex Virus (HSV), Varicella Zoster Virus (VSV), Cytomegalovirus (CMV), Epstein Barr Virus (EBV), chlamydozoan, rickettsia bacterium, mycobacterium, staphylococci, streptococci, pneumonococci, meningococci and conococci, klebsiella, proteus, serratia, pseudomonas, legionella, diphtheria, salmonella, bacilli, cholera, tetanus, botulism, anthrax, plague, leptospirosis, and Lymes disease bacteria. The present application thus in some embodiments provides a method of treating a disease or condition associated with BTLA signaling (e.g., cancer or infectious diseases) in an individual comprising administering to the individual an effective amount of a composition (such as a pharmaceutical composition) comprising an anti-BTLA antibody (e.g., a full-length anti-BTLA antibody), such as any one of the anti-BTLA antibodies (e.g., full-length anti-BTLA antibodies) described herein. In some embodiments, the individual is human. In some embodiments, the disease or condition is associated with altered BTLA signaling, such as elevated BTLA signaling. [0291] For example, in some embodiments, there is provided a method of treating an individual having a disease or condition associated with BTLA signaling (e.g., cancer or infectious diseases) comprising administering to the individual an effective amount of a pharmaceutical composition comprising an anti-BTLA antibody (e.g., full-length anti-BTLA antibody) specifically binding to an epitope on human BTLA, wherein the epitope comprises amino acid residues of human BTLA. In some embodiments, the anti-BTLA antibody is a full-length antibody. In some embodiments, the full-length anti-BTLA antibody is an IgG1 or IgG4 antibody. In some embodiments, the disease or condition is selected from the group consisting of non-small cell lung cancer, adrenal gland cancer, bladder cancer, brain cancer, pancreatic adenocarcinoma, breast cancer, colorectal cancer, melanoma, esophageal cancer, gastric cancer, cervical cancer, head and neck cancer, hepatocellular carcinoma, kidney cancer, liver cancer, ovarian cancer, pancreatic cancer, prostate cancer, small cell lung cancer, testicular cancer, thyroid cancer, uterine cancer, and any types of leukemia, lymphoma and myeloma, and infectious diseases, including, but not limited to Human Papilloma Virus (HPV), Human Immunodeficiency Virus (HIV), Herpes Simplex Virus (HSV), Varicella Zoster Virus (VSV), Cytomegalovirus (CMV), Epstein Barr Virus (EBV), chlamydozoan, rickettsia bacterium, mycobacterium, staphylococci, streptococci, pneumonococci, meningococci and conococci, klebsiella, proteus, serratia, pseudomonas, legionella, diphtheria, salmonella, bacilli, cholera, tetanus, botulism, anthrax, plague, leptospirosis, and Lymes disease bacteria. In some embodiments, the individual is human. In some embodiments, the disease or condition is associated with altered BTLA signaling, such as elevated BTLA signaling. [0292] For example, in some embodiments, there is provided a method of treating an individual having a disease or condition associated with BTLA signaling (such as altered BTLA signaling, including elevated BTLA signaling) (e.g., cancer or infectious diseases) comprising administering to the individual an effective amount of a pharmaceutical composition comprising an anti-BTLA antibody (e.g., full-length anti-BTLA antibody) comprising a heavy chain variable domain (V _H ) comprising a heavy chain complementarity determining region (HC-CDR) 1 comprising TFGMGVS (SEQ ID NO: 1); an HC-CDR2 comprising HIYWDDDKRFNPSLKS (SEQ ID NO: 4); and an HC-CDR3 comprising GNWDGETYFDY (SEQ ID NO: 7); and a light chain variable domain (V _L ) comprising a light chain complementarity determining region (LC-CDR) 1 comprising KSTQSLLDSDGKTYLN (SEQ ID NO: 10); an LC-CDR2 comprising LVSKLDS (SEQ ID NO: 13); and an LC-CDR3 comprising WQGTHFPWT (SEQ ID NO: 15). In some embodiments, the anti-BTLA antibody is a full-length antibody. In some embodiments, the full-length anti-BTLA antibody is an IgG1 or IgG4 antibody. In some embodiments, the disease or condition is selected from the group consisting of non-small cell lung cancer, adrenal gland cancer, bladder cancer, brain cancer, pancreatic adenocarcinoma, breast cancer, colorectal cancer, melanoma, esophageal cancer, gastric cancer, cervical cancer, head and neck cancer, hepatocellular carcinoma, kidney cancer, liver cancer, ovarian cancer, pancreatic cancer, prostate cancer, small cell lung cancer, testicular cancer, thyroid cancer, uterine cancer, and any types of leukemia, lymphoma and myeloma, and infectious diseases, including, but not limited to Human Papilloma Virus (HPV), Human Immunodeficiency Virus (HIV), Herpes Simplex Virus (HSV), Varicella Zoster Virus (VSV), Cytomegalovirus (CMV), Epstein Barr Virus (EBV), chlamydozoan, rickettsia bacterium, mycobacterium, staphylococci, streptococci, pneumonococci, meningococci and conococci, klebsiella, proteus, serratia, pseudomonas, legionella, diphtheria, salmonella, bacilli, cholera, tetanus, botulism, anthrax, plague, leptospirosis, and Lymes disease bacteria. In some embodiments, the individual is human. In some embodiments, the disease or condition is associated with altered BTLA signaling, such as elevated BTLA signaling. [0293] In some embodiments, there is provided a method of treating an individual having a disease or condition associated with BTLA signaling (e.g., cancer or infectious diseases) comprising administering to the individual an effective amount of a pharmaceutical composition comprising an anti-BTLA antibody (e.g., full-length anti-BTLA antibody) comprising: a V _H comprising an HC-CDR1 comprising the amino acid sequence of SEQ ID NO: 1, an HC-CDR2 comprising the amino acid sequence of SEQ ID NO: 4, and an HC- CDR3 comprising the amino acid sequence of SEQ ID NO: 7, or a variant thereof comprising up to 5 amino acid substitutions in the HC-CDRs; and a V _L comprising an LC-CDR1 comprising the amino acid sequence of SEQ ID NO: 10, an LC-CDR2 comprising the amino acid sequence of SEQ ID NO: 13, and an LC-CDR3 comprising the amino acid sequence of SEQ ID NO: 15, or a variant thereof comprising up to 5 amino acid substitutions in the LC- CDRs. [0294] In some embodiments, there is provided a method of treating an individual having a disease or condition associated with BTLA signaling (e.g., cancer or infectious diseases) comprising administering to the individual an effective amount of a pharmaceutical composition comprising an anti-BTLA antibody (e.g., full-length anti-BTLA antibody) comprising a V _H comprising the amino acid sequence of any one of SEQ ID NOs: 18-22 or a variant thereof having at least about 80% sequence identity to the amino acid sequence of any one of SEQ ID NOs: 18-22, and a V _L comprising the amino acid sequence of any one of SEQ ID NOs: 25-29, or a variant thereof having at least about 80% sequence identity to the amino acid sequence of any one of SEQ ID NOs: 25-29. [0295] In some embodiments, the anti-BTLA antibody in the composition is a full-length anti-BTLA antibody comprising IgG1 or IgG4 constant domains. In some embodiments, the IgG1 is human IgG1. In some embodiments, the IgG4 is human IgG4. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0296] In some embodiments, there is provided a method of treating an individual having a disease or condition associated with BTLA signaling (e.g., cancer or infectious diseases) comprising administering to the individual an effective amount of a pharmaceutical composition comprising an anti-BTLA antibody (e.g., full-length anti-BTLA antibody) comprising: a V _H comprising an HC-CDR1 comprising the amino acid sequence of SEQ ID NO: 1, an HC-CDR2 comprising the amino acid sequence of SEQ ID NO: 4, and an HC- CDR3 comprising the amino acid sequence of SEQ ID NO: 7, or a variant thereof comprising up to 5 amino acid substitutions in the HC-CDRs; and a V _L comprising an LC-CDR1 comprising the amino acid sequence of SEQ ID NO: 10, an LC-CDR2 comprising the amino acid sequence of SEQ ID NO: 13, and an LC-CDR3 comprising the amino acid sequence of SEQ ID NO: 15, or a variant thereof comprising up to 5 amino acid substitutions in the LC- CDRs. [0297] In some embodiments, there is provided a method of treating an individual having a disease or condition associated with BTLA signaling (e.g., cancer or infectious diseases) comprising administering to the individual a) an effective amount of a pharmaceutical composition comprising an anti-BTLA antibody (e.g., full-length anti-BTLA antibody) comprising: a V _H comprising an HC-CDR1 comprising the amino acid sequence of SEQ ID NO: 1, an HC-CDR2 comprising the amino acid sequence of SEQ ID NO: 4, and an HC- CDR3 comprising the amino acid sequence of SEQ ID NO: 7, or a variant thereof comprising up to 5 amino acid substitutions in the HC-CDRs; and a V _L comprising an LC-CDR1 comprising the amino acid sequence of SEQ ID NO: 10, an LC-CDR2 comprising the amino acid sequence of SEQ ID NO: 13, and an LC-CDR3 comprising the amino acid sequence of SEQ ID NO: 15, or a variant thereof comprising up to 5 amino acid substitutions in the LC- CDRs, and b) an effective amount of a PD-1 antibody. In some embodiments, the anti-PD1 antibody is pembrolizumab. In some embodiments, the anti-PD-1 antibody is nivolumab. In some embodiments, the anti-PD-1 antibody is cemiplimab. In some embodiments, the anti- BLTA antibody and the anti-PD-1 antibody is administered concurrently. In some embodiments, the anti-BLTA antibody and the anti-PD-1 antibody is administered simultaneously. In some embodiments, the anti-BLTA antibody and the anti-PD-1 antibody is administered sequentially. In some embodiments, the anti-BLTA antibody comprises a V _H comprising the amino acid sequence of SEQ ID NO: 22 and a V _L comprising the amino acid sequence of SEQ ID NO: 29. In some embodiments, the anti-BTLA antibody in the composition is a full-length anti-BTLA antibody comprising IgG1 or IgG4 constant domains. In some embodiments, the IgG1 is human IgG1. In some embodiments, the IgG4 is human IgG4. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0298] In some embodiments, there is provided a method of treating an individual having a disease or condition associated with BTLA signaling (e.g., cancer or infectious diseases) comprising administering to the individual an effective amount of a pharmaceutical composition comprising an anti-BTLA antibody (e.g., full-length anti-BTLA antibody) comprising a V _H comprising the amino acid sequence of SEQ ID NO: 18 and a V _L comprising the amino acid sequence of SEQ ID NO: 25. In some embodiments, the anti- BTLA antibody in the composition is a full-length anti-BTLA antibody comprising IgG1 or IgG4 constant domains. In some embodiments, the IgG1 is human IgG1. In some embodiments, the IgG4 is human IgG4. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0299] In some embodiments, there is provided a method of treating an individual having a disease or condition associated with BTLA signaling (e.g., cancer or infectious diseases) comprising administering to the individual an effective amount of a pharmaceutical composition comprising an anti-BTLA antibody (e.g., full-length anti-BTLA antibody) comprising a V _H comprising the amino acid sequence of SEQ ID NO: 19 and a V _L comprising the amino acid sequence of SEQ ID NO: 26. In some embodiments, the anti- BTLA antibody in the composition is a full-length anti-BTLA antibody comprising IgG1 or IgG4 constant domains. In some embodiments, the IgG1 is human IgG1. In some embodiments, the IgG4 is human IgG4. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0300] In some embodiments, there is provided a method of treating an individual having a disease or condition associated with BTLA signaling (e.g., cancer or infectious diseases) comprising administering to the individual an effective amount of a pharmaceutical composition comprising an anti-BTLA antibody (e.g., full-length anti-BTLA antibody) comprising a V _H comprising the amino acid sequence of SEQ ID NO: 19 and a V _L comprising the amino acid sequence of SEQ ID NO: 27. In some embodiments, the anti- BTLA antibody in the composition is a full-length anti-BTLA antibody comprising IgG1 or IgG4 constant domains. In some embodiments, the IgG1 is human IgG1. In some embodiments, the IgG4 is human IgG4. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0301] In some embodiments, there is provided a method of treating an individual having a disease or condition associated with BTLA signaling (e.g., cancer or infectious diseases) comprising administering to the individual an effective amount of a pharmaceutical composition comprising an anti-BTLA antibody (e.g., full-length anti-BTLA antibody) comprising a V _H comprising the amino acid sequence of SEQ ID NO: 19 and a V _L comprising the amino acid sequence of SEQ ID NO: 28. In some embodiments, the anti- BTLA antibody in the composition is a full-length anti-BTLA antibody comprising IgG1 or IgG4 constant domains. In some embodiments, the IgG1 is human IgG1. In some embodiments, the IgG4 is human IgG4. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0302] In some embodiments, there is provided a method of treating an individual having a disease or condition associated with BTLA signaling (e.g., cancer or infectious diseases) comprising administering to the individual an effective amount of a pharmaceutical composition comprising an anti-BTLA antibody (e.g., full-length anti-BTLA antibody) comprising a V _H comprising the amino acid sequence of SEQ ID NO: 19 and a V _L comprising the amino acid sequence of SEQ ID NO: 29. In some embodiments, the anti- BTLA antibody in the composition is a full-length anti-BTLA antibody comprising IgG1 or IgG4 constant domains. In some embodiments, the IgG1 is human IgG1. In some embodiments, the IgG4 is human IgG4. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0303] In some embodiments, there is provided a method of treating an individual having a disease or condition associated with BTLA signaling (e.g., cancer or infectious diseases) comprising administering to the individual an effective amount of a pharmaceutical composition comprising an anti-BTLA antibody (e.g., full-length anti-BTLA antibody) comprising a V _H comprising the amino acid sequence of SEQ ID NO: 20 and a V _L comprising the amino acid sequence of SEQ ID NO: 26. In some embodiments, the anti- BTLA antibody in the composition is a full-length anti-BTLA antibody comprising IgG1 or IgG4 constant domains. In some embodiments, the IgG1 is human IgG1. In some embodiments, the IgG4 is human IgG4. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0304] In some embodiments, there is provided a method of treating an individual having a disease or condition associated with BTLA signaling (e.g., cancer or infectious diseases) comprising administering to the individual an effective amount of a pharmaceutical composition comprising an anti-BTLA antibody (e.g., full-length anti-BTLA antibody) comprising a V _H comprising the amino acid sequence of SEQ ID NO: 20 and a V _L comprising the amino acid sequence of SEQ ID NO: 27. In some embodiments, the anti- BTLA antibody in the composition is a full-length anti-BTLA antibody comprising IgG1 or IgG4 constant domains. In some embodiments, the IgG1 is human IgG1. In some embodiments, the IgG4 is human IgG4. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0305] In some embodiments, there is provided a method of treating an individual having a disease or condition associated with BTLA signaling (e.g., cancer or infectious diseases) comprising administering to the individual an effective amount of a pharmaceutical composition comprising an anti-BTLA antibody (e.g., full-length anti-BTLA antibody) comprising a V _H comprising the amino acid sequence of SEQ ID NO: 20 and a V _L comprising the amino acid sequence of SEQ ID NO: 28. In some embodiments, the anti- BTLA antibody in the composition is a full-length anti-BTLA antibody comprising IgG1 or IgG4 constant domains. In some embodiments, the IgG1 is human IgG1. In some embodiments, the IgG4 is human IgG4. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0306] In some embodiments, there is provided a method of treating an individual having a disease or condition associated with BTLA signaling (e.g., cancer or infectious diseases) comprising administering to the individual an effective amount of a pharmaceutical composition comprising an anti-BTLA antibody (e.g., full-length anti-BTLA antibody) comprising a V _H comprising the amino acid sequence of SEQ ID NO: 20 and a V _L comprising the amino acid sequence of SEQ ID NO: 29. In some embodiments, the anti- BTLA antibody in the composition is a full-length anti-BTLA antibody comprising IgG1 or IgG4 constant domains. In some embodiments, the IgG1 is human IgG1. In some embodiments, the IgG4 is human IgG4. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0307] In some embodiments, there is provided a method of treating an individual having a disease or condition associated with BTLA signaling (e.g., cancer or infectious diseases) comprising administering to the individual an effective amount of a pharmaceutical composition comprising an anti-BTLA antibody (e.g., full-length anti-BTLA antibody) comprising a V _H comprising the amino acid sequence of SEQ ID NO: 21 and a V _L comprising the amino acid sequence of SEQ ID NO: 28. In some embodiments, the anti- BTLA antibody in the composition is a full-length anti-BTLA antibody comprising IgG1 or IgG4 constant domains. In some embodiments, the IgG1 is human IgG1. In some embodiments, the IgG4 is human IgG4. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0308] In some embodiments, there is provided a method of treating an individual having a disease or condition associated with BTLA signaling (e.g., cancer or infectious diseases) comprising administering to the individual an effective amount of a pharmaceutical composition comprising an anti-BTLA antibody (e.g., full-length anti-BTLA antibody) comprising a V _H comprising the amino acid sequence of SEQ ID NO: 21 and a V _L comprising the amino acid sequence of SEQ ID NO: 29. In some embodiments, the anti- BTLA antibody in the composition is a full-length anti-BTLA antibody comprising IgG1 or IgG4 constant domains. In some embodiments, the IgG1 is human IgG1. In some embodiments, the IgG4 is human IgG4. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0309] In some embodiments, there is provided a method of treating an individual having a disease or condition associated with BTLA signaling (e.g., cancer or infectious diseases) comprising administering to the individual an effective amount of a pharmaceutical composition comprising an anti-BTLA antibody (e.g., full-length anti-BTLA antibody) comprising a V _H comprising the amino acid sequence of SEQ ID NO: 22 and a V _L comprising the amino acid sequence of SEQ ID NO: 28. In some embodiments, the anti- BTLA antibody in the composition is a full-length anti-BTLA antibody comprising IgG1 or IgG4 constant domains. In some embodiments, the IgG1 is human IgG1. In some embodiments, the IgG4 is human IgG4. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0310] In some embodiments, there is provided a method of treating an individual having a disease or condition associated with BTLA signaling (e.g., cancer or infectious diseases) comprising administering to the individual an effective amount of a pharmaceutical composition comprising an anti-BTLA antibody (e.g., full-length anti-BTLA antibody) comprising a V _H comprising the amino acid sequence of SEQ ID NO: 22 and a V _L comprising the amino acid sequence of SEQ ID NO: 29. In some embodiments, the anti- BTLA antibody in the composition is a full-length anti-BTLA antibody comprising IgG1 or IgG4 constant domains. In some embodiments, the IgG1 is human IgG1. In some embodiments, the IgG4 is human IgG4. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0311] In some embodiments, there is provided a method of treating an individual having a disease or condition associated with BTLA signaling (such as altered BTLA signaling, including elevated BTLA signaling) (e.g., cancer or infectious diseases) comprising administering to the individual an effective amount of a pharmaceutical composition comprising an anti-BTLA antibody (e.g., full-length anti-BTLA antibody) comprising a V _H comprising an HC-CDR1 comprising the amino acid sequence of SEQ ID NO: 2, an HC- CDR2 comprising the amino acid sequence of SEQ ID NO: 5, and an HC-CDR3 comprising the amino acid sequence of SEQ ID NO: 8, or a variant thereof comprising up to 5 amino acid substitutions in the HC-CDRs; and a V _L comprising an LC-CDR1 comprising the amino acid sequence of SEQ ID NO: 11, an LC-CDR2 comprising the amino acid sequence of SEQ ID NO: 14, and an LC-CDR3 comprising the amino acid sequence of SEQ ID NO: 16, or a variant thereof comprising up to 5 amino acid substitutions in the LC-CDRs. In some embodiments, the anti-BTLA antibody is a full-length antibody. In some embodiments, the full-length anti-BTLA antibody is an IgG1 or IgG4 antibody. In some embodiments, the disease or condition is selected from the group consisting of non-small cell lung cancer, adrenal gland cancer, bladder cancer, brain cancer, pancreatic adenocarcinoma, breast cancer, colorectal cancer, melanoma, esophageal cancer, gastric cancer, cervical cancer, head and neck cancer, hepatocellular carcinoma, kidney cancer, liver cancer, ovarian cancer, pancreatic cancer, prostate cancer, small cell lung cancer, testicular cancer, thyroid cancer, uterine cancer, and any types of leukemia, lymphoma and myeloma, and infectious diseases, including, but not limited to Human Papilloma Virus (HPV), Human Immunodeficiency Virus (HIV), Herpes Simplex Virus (HSV), Varicella Zoster Virus (VSV), Cytomegalovirus (CMV), Epstein Barr Virus (EBV), chlamydozoan, rickettsia bacterium, mycobacterium, staphylococci, streptococci, pneumonococci, meningococci and conococci, klebsiella, proteus, serratia, pseudomonas, legionella, diphtheria, salmonella, bacilli, cholera, tetanus, botulism, anthrax, plague, leptospirosis, and Lymes disease bacteria. In some embodiments, the individual is human. In some embodiments, the disease or condition is associated with altered BTLA signaling, such as elevated BTLA signaling. [0312] In some embodiments, there is provided a method of treating an individual having a disease or condition associated with BTLA signaling (e.g., cancer or infectious diseases) comprising administering to the individual an effective amount of a pharmaceutical composition comprising an anti-BTLA antibody (e.g., full-length anti-BTLA antibody) comprising a V _H comprising the amino acid sequence of SEQ ID NO: 23 or a variant thereof having at least about 80% sequence identity to the amino acid sequence of SEQ ID NO: 23, and a V _L comprising the amino acid sequence of SEQ ID NO: 30, or a variant thereof having at least about 80% sequence identity to the amino acid sequence of SEQ ID NO: 30. [0313] In some embodiments, the anti-BTLA antibody in the composition is a full-length anti-BTLA antibody comprising IgG1 or IgG4 constant domains. In some embodiments, the IgG1 is human IgG1. In some embodiments, the IgG4 is human IgG4. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0314] For example, in some embodiments, there is provided a method of treating an individual having a disease or condition associated with BTLA signaling (such as altered BTLA signaling, including elevated BTLA signaling) (e.g., cancer or infectious diseases) comprising administering to the individual an effective amount of a pharmaceutical composition comprising an anti-BTLA antibody (e.g., full-length anti-BTLA antibody) comprising a V _H comprising an HC-CDR1 comprising the amino acid sequence of SEQ ID NO: 3, an HC-CDR2 comprising the amino acid sequence of SEQ ID NO: 6, and an HC- CDR3 comprising the amino acid sequence of SEQ ID NO: 9, or a variant thereof comprising up to 5 amino acid substitutions in the HC-CDRs; and a V _L comprising an LC-CDR1 comprising the amino acid sequence of SEQ ID NO: 12, an LC-CDR2 comprising the amino acid sequence of SEQ ID NO: 13, and an LC-CDR3 comprising the amino acid sequence of SEQ ID NO: 17, or a variant thereof comprising up to 5 amino acid substitutions in the LC- CDRs.In some embodiments, the anti-BTLA antibody is a full-length antibody. In some embodiments, the full-length anti-BTLA antibody is an IgG1 or IgG4 antibody. In some embodiments, the disease or condition is selected from the group consisting of non-small cell lung cancer, adrenal gland cancer, bladder cancer, brain cancer, pancreatic adenocarcinoma, breast cancer, colorectal cancer, melanoma, esophageal cancer, gastric cancer, cervical cancer, head and neck cancer, hepatocellular carcinoma, kidney cancer, liver cancer, ovarian cancer, pancreatic cancer, prostate cancer, small cell lung cancer, testicular cancer, thyroid cancer, uterine cancer, and any types of leukemia, lymphoma and myeloma, and infectious diseases, including, but not limited to Human Papilloma Virus (HPV), Human Immunodeficiency Virus (HIV), Herpes Simplex Virus (HSV), Varicella Zoster Virus (VSV), Cytomegalovirus (CMV), Epstein Barr Virus (EBV), chlamydozoan, rickettsia bacterium, mycobacterium, staphylococci, streptococci, pneumonococci, meningococci and conococci, klebsiella, proteus, serratia, pseudomonas, legionella, diphtheria, salmonella, bacilli, cholera, tetanus, botulism, anthrax, plague, leptospirosis, and Lymes disease bacteria. In some embodiments, the individual is human. In some embodiments, the disease or condition is associated with altered BTLA signaling, such as elevated BTLA signaling. [0315] In some embodiments, there is provided a method of treating an individual having a disease or condition associated with BTLA signaling (e.g., cancer or infectious diseases) comprising administering to the individual an effective amount of a pharmaceutical composition comprising an anti-BTLA antibody (e.g., full-length anti-BTLA antibody) comprising a V _H comprising the amino acid sequence of SEQ ID NO: 24 or a variant thereof having at least about 80% sequence identity to the amino acid sequence of SEQ ID NO: 24, and a V _L comprising the amino acid sequence of SEQ ID NO: 31, or a variant thereof having at least about 80% sequence identity to the amino acid sequence of SEQ ID NO: 31. [0316] In some embodiments, the anti-BTLA antibody in the composition is a full-length anti-BTLA antibody comprising IgG1 or IgG4 constant domains. In some embodiments, the IgG1 is human IgG1. In some embodiments, the IgG4 is human IgG4. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 32. In some embodiments, the heavy chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 33. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 34. In some embodiments, the light chain constant region comprises or consists of the amino acid sequence of SEQ ID NO: 35. [0317] In some embodiments, the individual is a mammal (e.g., human, non-human primate, rat, mouse, cow, horse, pig, sheep, goat, dog, cat, etc.). In some embodiments, the individual is a human. In some embodiments, the individual is a clinical patient, a clinical trial volunteer, an experimental animal, etc. In some embodiments, the individual is younger than about 60 years old (including for example younger than about any of 50, 40, 30, 25, 20, 15, or 10 years old). In some embodiments, the individual is older than about 60 years old (including for example older than about any of 70, 80, 90, or 100 years old). In some embodiments, the individual is diagnosed with or genetically prone to one or more of the diseases or disorders described herein (such as cancer or infectious diseases). In some embodiments, the individual has one or more risk factors associated with one or more diseases or disorders described herein. [0318] The present application in some embodiments provides a method of delivering an anti-BTLA antibody (such as any one of the anti-BTLA antibodies described herein, e.g., an isolated anti-BTLA antibody) to a cell producing and/or expressing BTLA in an individual, the method comprising administering to the individual a composition comprising the anti- BTLA antibody. [0319] Many diagnostic methods for cancer or infectious diseases or any other disease associated with BTLA signaling (such as associated with altered BTLA signaling) and the clinical delineation of those diseases are known in the art. Such methods include, but are not limited to, e.g., immunohistochemistry, PCR, and fluorescent in situ hybridization (FISH). [0320] In some embodiments, the anti-BTLA antibodies (e.g., full-length anti-BTLA antibodies) and/or compositions of the application are administered in combination with a second, third, or fourth agent (including, e.g., immunogenic agent, such as purified tumor antigens; standard cancer treatments, such as chemotherapeutic regimes; antibodies which may be used to activate host immune responsiveness; cytokine treatment (e.g., interferons, GM-CSF, G-CSF, IL-2); or bispecific antibody therapy, which provides for enhanced presentation of tumor antigens, or a combination thereof) to treat diseases or disorders associated with BTLA signaling. [0321] In some embodiments, cancer treatments can be evaluated by, e.g., tumor regression, tumor weight or size shrinkage, time to progression, duration of survival, progression free survival, overall response rate, duration of response, quality of life, protein expression and/or activity. Approaches to determining efficacy of the therapy can be employed, including for example, measurement of response through radiological imaging. [0322] In some embodiments, the efficiency of treatment is measured as the percentage tumor growth inhibition (% TGI), calculated using the equation 100-(T/C × 100), where T is the mean relative tumor volume of the treated tumor, and C is the mean relative tumor volume of a non-treated tumor. In some embodiments, the %TGI is about 10%, about 20%, about 30%, about 40%, about 50%, about 60%, about 70%, about 80%, about 90%, about 91%, about 92%, about 93%, about 94%, about 95%, or more than 95%. In some embodiments, the efficacy of treatment is measured using shape change of granulocytes and/or increase in the survival of granulocytes. In some embodiments, the efficacy of treatment is measured by the increase of cytokine secretion by monocytes. Dosing and method of administering the anti-BTLA antibodies [0323] The dose of the anti-BTLA antibody (such as isolated anti-BTLA antibody) compositions administered to an individual (such as a human) may vary with the particular composition, the mode of administration, and the type of disease being treated. In some embodiments, the amount of the composition (such as composition comprising isolated anti- BTLA antibody) is effective to result in an objective response (such as a partial response or a complete response) in the treatment of cancer or infectious diseases. In some embodiments, the amount of the anti-BTLA antibody composition is sufficient to result in a complete response in the individual. In some embodiments, the amount of the anti-BTLA antibody composition is sufficient to result in a partial response in the individual. In some embodiments, the amount of the anti-BTLA antibody composition administered (for example when administered alone) is sufficient to produce an overall response rate of more than about any of 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 64%, 65%, 70%, 75%, 80%, 85%, or 90% among a population of individuals treated with the anti-BTLA antibody composition. Responses of an individual to the treatment of the methods described herein can be determined, for example, based on ACR score. [0324] In some embodiments, the amount of the composition (such as composition comprising isolated anti-BTLA antibody) is sufficient to control symptoms and reduce the risk of exacerbations of the individual. In some embodiments, the amount of the composition is sufficient to control symptoms and reduce the risk of exacerbations of the individual. In some embodiments, the amount of the composition (for example when administered along) is sufficient to produce clinical benefit of more than about any of 50%, 60%, 70%, or 77% among a population of individuals treated with the anti-BTLA antibody composition. [0325] In some embodiments, the amount of the composition (such as composition comprising isolated anti-BTLA antibody), alone or in combination with a second, third, and/or fourth agent, is an amount sufficient to control symptoms and reduce the risk of exacerbations in the same subject prior to treatment or compared to the corresponding activity in other subjects not receiving the treatment. Standard methods can be used to measure the magnitude of this effect, such as in vitro assays with purified enzyme, cell-based assays, animal models, or human testing. [0326] In some embodiments, the amount of the anti-BTLA antibody (such as a full-length anti-BTLA antibody) in the composition is below the level that induces a toxicological effect (i.e., an effect above a clinically acceptable level of toxicity) or is at a level where a potential side effect can be controlled or tolerated when the composition is administered to the individual. [0327] In some embodiments, the amount of the composition is close to a maximum tolerated dose (MTD) of the composition following the same dosing regimen. In some embodiments, the amount of the composition is more than about any of 80%, 90%, 95%, or 98% of the MTD. [0328] In some embodiments, the amount of an anti-BTLA antibody (such as a full-length anti-BTLA antibody) in the composition is included in a range of about 0.001 µg to about 1000 µg. [0329] In some embodiments of any of the above aspects, the effective amount of anti- BTLA antibody (such as a full-length anti-BTLA antibody) in the composition is in the range of about 0.1 µg/kg to about 100 mg/kg of total body weight. [0330] The anti-BTLA antibody compositions can be administered to an individual (such as human) via various routes, including, for example, intravenous, intra-arterial, intraperitoneal, intrapulmonary, oral, inhalation, intravesicular, intramuscular, intra-tracheal, subcutaneous, intraocular, intrathecal, transmucosal or transdermal. In some embodiments, sustained continuous release formulation of the composition may be used. In some embodiments, the composition is administered inhaled. In some embodiments, the composition is administered intravenously. In some embodiments, the composition is administered intraportally. In some embodiments, the composition is administered intraarterially. In some embodiments, the composition is administered intraperitoneally. In some embodiments, the composition is administered intrahepatically. In some embodiments, the composition is administered by hepatic arterial infusion. In some embodiments, the administration is to an injection site distal to a first disease site.

Articles of Manufacture and Kits

[0331] In some embodiments of the application, there is provided an article of manufacture containing materials useful for the treatment of disease or condition associated with BTLA signaling, (e.g., cancer or infectious diseases) or for delivering an anti-BTLA antibody (such as a full-length anti-BTLA antibody) to a cell producing BTLA of the individual. The article of manufacture can comprise a container and a label or package insert on or associated with the container. Suitable containers include, for example, bottles, vials, syringes, etc. The containers may be formed from a variety of materials such as glass or plastic. Generally, the container holds a composition which is effective for treating a disease or disorder described herein, and may have a sterile access port (for example the container may be an intravenous solution bag or a vial having a stopper pierceable by a hypodermic injection needle). At least one active agent in the composition is an anti-BTLA antibody of the application. The label or package insert indicates that the composition is used for treating the particular condition. The label or package insert will further comprise instructions for administering the anti-BTLA antibody composition to the patient. Articles of manufacture and kits comprising combinatorial therapies described herein are also contemplated.

[0332] Package insert refers to instructions customarily included in commercial packages of therapeutic products that contain information about the indications, usage, dosage, administration, contraindications and/or warnings concerning the use of such therapeutic products. In some embodiments, the package insert indicates that the composition is used for treating disease or condition associated with BTLA signaling (such as cancer or infectious diseases). In some embodiments, the package insert indicates that the composition is used for treating disease or condition selected from the group consisting of non-small cell lung cancer, adrenal gland cancer, bladder cancer, brain cancer, pancreatic adenocarcinoma, breast cancer, colorectal cancer, melanoma, esophageal cancer, gastric cancer, cervical cancer, head and neck cancer, hepatocellular carcinoma, kidney cancer, liver cancer, ovarian cancer, pancreatic cancer, prostate cancer, small cell lung cancer, testicular cancer, thyroid cancer, uterine cancer, and any types of leukemia, lymphoma and myeloma, and infectious diseases, including, but not limited to Human Papilloma Virus (HPV), Human Immunodeficiency Virus (HIV), Herpes Simplex Virus (HSV), Varicella Zoster Virus (VSV), Cytomegalovirus (CMV), Epstein Barr Virus (EBV), chlamydozoan, rickettsia bacterium, mycobacterium, staphylococci, streptococci, pneumonococci, meningococci and conococci, klebsiella, proteus, serratia, pseudomonas, legionella, diphtheria, salmonella, bacilli, cholera, tetanus, botulism, anthrax, plague, leptospirosis, and Lymes disease bacteria. [0333] Additionally, the article of manufacture may further comprise a second container comprising a pharmaceutically-acceptable buffer, such as bacteriostatic water for injection (BWFI), phosphate-buffered saline, Ringer's solution or dextrose solution. It may further include other materials desirable from a commercial and user standpoint, including other buffers, diluents, filters, needles, and syringes. [0334] Kits are also provided that are useful for various purposes, e.g., for treatment of disease or condition associated with BTLA signaling (e.g., cancer or infectious diseases), or for delivering an anti-BTLA antibody (such as a full-length anti-BTLA antibody) to a cell producing BTLA of the individual, optionally in combination with the articles of manufacture. Kits of the application include one or more containers comprising anti-BTLA antibody composition (or unit dosage form and/or article of manufacture), and in some embodiments, further comprise another agent (such as the agents described herein) and/or instructions for use in accordance with any of the methods described herein. The kit may further comprise a description of selection of individuals suitable for treatment. Instructions supplied in the kits of the application are typically written instructions on a label or package insert (e.g., a paper sheet included in the kit), but machine-readable instructions (e.g., instructions carried on a magnetic or optical storage disk) are also acceptable. [0335] For example, in some embodiments, the kit comprises a composition comprising an anti-BTLA antibody (such as a full-length anti-BTLA antibody). In some embodiments, the kit comprises a) a composition comprising any one of the anti-BTLA antibodies described herein, and b) an effective amount of at least one other agent, wherein the other agent enhances the effects (e.g., treatment effect, detecting effect) of the anti-BTLA antibody. In some embodiments, the kit comprises a) a composition comprising any one of the anti-BTLA antibodies described herein, and b) instructions for administering the anti-BTLA antibody composition to an individual for treatment of a disease or condition associated with BTLA signaling (e.g., cancer or infectious diseases). In some embodiments, the kit comprises a) a composition comprising any one of the anti-BTLA antibodies described herein, b) an effective amount of at least one other agent, wherein the other agent enhances the effect (e.g., treatment effect, detecting effect) of the anti-BTLA antibody, and c) instructions for administering the anti-BTLA antibody composition and the other agent(s) to an individual for treatment of a disease or condition associated with BTLA signaling (e.g., cancer or infectious diseases. The anti-BTLA antibody and the other agent(s) can be present in separate containers or in a single container. For example, the kit may comprise one distinct composition or two or more compositions wherein one composition comprises an anti-BTLA antibody and another composition comprises another agent. [0336] In some embodiments, the kit comprises a nucleic acid (or a set of nucleic acids) encoding an anti-BTLA antibody (such as a full-length anti-BTLA antibody). In some embodiments, the kit comprises a) a nucleic acid (or a set of nucleic acids) encoding an anti- BTLA antibody, and b) a host cell for expressing the nucleic acid (or a set of nucleic acids). In some embodiments, the kit comprises a) a nucleic acid (or a set of nucleic acids) encoding an anti-BTLA antibody, and b) instructions for i) expressing the anti-BTLA antibody in a host cell, ii) preparing a composition comprising the anti-BTLA antibody, and iii) administering the composition comprising the anti-BTLA antibody to an individual for the treatment of a disease or condition associated with BTLA signaling (e.g., cancer or infectious diseases). In some embodiments, the kit comprises a) a nucleic acid (or a set of nucleic acids) encoding an anti-BTLA antibody, b) a host cell for expressing the nucleic acid (or a set of nucleic acids), and c) instructions for i) expressing the anti-BTLA antibody in the host cell, ii) preparing a composition comprising the anti-BTLA antibody, and iii) administering the composition comprising the anti-BTLA antibody to an individual for the treatment of a disease or condition associated with BTLA signaling (e.g., cancer or infectious diseases). [0337] The kits of the application are in suitable packaging. Suitable packaging includes, but is not limited to, vials, bottles, jars, flexible packaging (e.g., sealed Mylar or plastic bags), and the like. Kits may optionally provide additional components such as buffers and interpretative information. The present application thus also provides articles of manufacture, which include vials (such as sealed vials), bottles, jars, flexible packaging, and the like. [0338] The instructions relating to the use of the anti-BTLA antibody compositions generally include information as to dosage, dosing schedule, and route of administration for the intended treatment. The containers may be unit doses, bulk packages (e.g., multi-dose packages) or sub-unit doses. For example, kits may be provided that contain sufficient dosages of an anti-BTLA antibody (such as a full-length anti-BTLA antibody) as disclosed herein to provide effective treatment of an individual for an extended period, such as any of a week, 8 days, 9 days, 10 days, 11 days, 12 days, 13 days, 2 weeks, 3 weeks, 4 weeks, 6 weeks, 8 weeks, 3 months, 4 months, 5 months, 7 months, 8 months, 9 months, or more. Kits may also include multiple unit doses of the anti-BTLA antibody and pharmaceutical compositions and instructions for use and packaged in quantities sufficient for storage and use in pharmacies, for example, hospital pharmacies and compounding pharmacies. [0339] Those skilled in the art will recognize that several embodiments are possible within the scope and spirit of this application. The application will now be described in greater detail by reference to the following non-limiting examples. The following examples further illustrate the application but, of course, should not be construed as in any way limiting its scope. EXAMPLES [0340] Various features and embodiments of the disclosure are illustrated in the following representative examples, which are intended to be illustrative, and not limiting. Those skilled in the art will readily appreciate that the specific examples are only illustrative of the invention as described more fully in the claims which follow thereafter. Every embodiment and feature described in the application should be understood to be interchangeable and combinable with every embodiment contained within Example 1: Generation of BTLA Polypeptides [0341] This example illustrates the preparation of the various BTLA polypeptide constructs used as antigens in eliciting and screening the anti-BTLA antibodies of the present disclosure. [0342] The coding sequences of the extracellular domain (ECD) of human BTLA (huBTLA) and musculus BTLA (musBTLA) were synthesized and sub-cloned into the expression vector pTTal using restriction enzyme with recognition sites HindIII and EcoRI using In-Fusion seamless cloning kit (Takara, cat# 639691). The amino acid sequences of huBTLA (ECD) and musBTLA (ECD) were provided in Table 5. All constructs had the following C-terminal human IgG1 Fc or C-terminal-mouse IgG2a Fc or C-terminal-10 × His tag sequence for purification and detection purposes. In total, six fusion proteins were expressed and purified. TABLE 5 BTLA polypeptide sequence

[0343] Fusion proteins were expressed in Expi293 cells (Thermo Fisher Scientific) according to the manufacturer’s protocol. Briefly, Expi293 cells were transfected with the expression vectors, and the cells were cultured at 37°C, under 8% CO ₂ and 120rpm for 5 days. [0344] For purification of Fc-fusion protein, after harvest, the clarified supernatant media were mixed with MabSelect protein A resin (GE Healthcare) equilibrated with PBS buffer and incubated with gentle rotation for 1.5 h at room temperature. After incubation, the slurry was loaded into a column and the resin was washed with 20 column volumes of PBS buffer containing 0.15M NaCl, then eluted with 3×column volumes of 50 mM sodium phosphate (pH 3.0). The pH of the eluent was quickly adjusted to pH 5.2 with 1 M Tris-HCl (pH 9.0) and the buffer was exchanged into PBS buffer with a PD-10 column (GE Healthcare). [0345] For purification of His-tag protein, after harvest, the clarified supernatant media were applied to a Histrap column (GE Healthcare) equilibrated with 20mM Sodium Phosphate buffer (pH7.4) containing 0.25M NaCl and 5mM imidazole (pH8.0). The column was washed with 10 column volumes of 20mM Sodium Phosphate buffer (pH7.4) containing 0.25M NaCl and 15mM imidazole (pH8.0), then eluted with 3× column volumes of 20mM Sodium Phosphate buffer (pH7.4) containing 0.25M NaCl and 100mM imidazole (pH8.0). The buffer of eluent was exchanged into PBS buffer with a PD-10 column. Example 2: Generation of Anti-BTLA Antibodies Using Hybridoma Methods, Screening and Characterization [0346] This example illustrates the methods using mouse hybridoma technology to generate anti-BTLA antibodies, and methods to screen and select antibodies for further characterization. [0347] Immunizations and fusions: Balb/c mice were immunized with recombinant ECD of human BTLA fused with His- or mouse IgG2a Fc tag produced in Expi293 cells adjuvanted with RIBI (Sigma Aldrich, cat# S6322-1VL), Titermax (Sigma Aldrich, cat# T2684-1ML), Freund’s (Freund’s adjuvant, incomplete) (Sigma Aldrich, cat# F5506-10x-10mL) or in an alternative order. Endpoint titers were determined by ELISA as described below. Three days after the last immunization, spleens and lymph nodes were harvested and processed according to standard protocols. Mouse B cells were isolated by EasySep Mouse B cell isolation Kit (StemCell, cat# 19854A) and fused with myeloma cells SP2/0-Ag14 cells (ATCC, CRL 1581) using PEG. Following standard protocols, the fused cells were plated into six-well plates in semi-solid ClonalCell-HY Cloning-Medium D (StemCell, cat# 03804). Monoclonal hybridoma clones were picked into 96 well/plate using Clone Pix 2 Machine (Molecular Devices) and cultured in low-Ig HT medium. [0348] ELISA binding assays: After 10-14 days of culture, supernatants were collected and subjected to primary screening by ELISA with 96 well round bottom ELISA plates coated with purified human His-BTLA or rhesus monkey BTLA extracellular domain proteins with His tag (Sino Biological, cat# 90250-C08H).96-well round bottom ELISA plates (Corning, cat#25381-051) were coated overnight at 4°C with 50 μL/well of human His-BTLA or rhesus monkey BTLA extracellular domain proteins with His tag at a concentration of 1 μg/mL or 0.5 μg/mL in coating buffer (1× phosphate buffered saline, PBS). After removing the coating solution, the plates were blocked by addition of 250 μL/well of blocking solution containing 1% bovine serum albumin (BSA) in phosphate buffered saline (PBS) pH 7.4 (ELISA diluent) and incubated at room temperature for 2 hours. Plates were then washed 3 times with 300 μL of PBS containing 0.05% TWEEN®-20 (wash buffer).50 μL of culture supernatant of individual hybridoma clones was added to individual wells followed by incubation at room temperature for 2 hours or at 37°C for 1 hour. Plates were washed 3 times with wash buffer, then 50 μL/well of goat anti-mouse antibody-AP (Southern Biotech, cat# 1030-04) was added at 1:2000 dilution in ELISA diluent. The plate was incubated at room temperature for 1 hour, washed 4 times with wash buffer and developed for 30 minutes by addition of 50 μL/well of Sigma Fast p-Nitrophenyl phosphate tablet (pNPP) (Sigma Aldrich, cat#N2770-50SET). Plates were analyzed with Synergy HT (Bio-TEK) at 405 nm. [0349] The parental hybridoma hits identified from the primary screen were expanded to 48 or 24-well plates and a confirmatory ELISA was run following the primary screen protocol, to further confirm and screen for anti-human and anti-rhesus monkey BTLA binders. [0350] Purification of hybridoma antibodies: Positive hybridoma clones were scaled up to 30mL cultures in serum free medium and the antibodies were purified as follows. Supernatant media were clarified by centrifugation at 300g for 10 min to remove cells and by filtration with 0.22-micron filter. Clarified supernatant media was mixed with Protein A resin (Thermo Fisher Scientific, cat# A26458) equilibrated with PBS buffer and incubated with gentle rotation for 1.5 h at room temperature. After incubation, the slurry was loaded into a column and the resin was washed with 10-fold column volumes of PBS buffer containing 0.5M NaCl, then eluted with 0.1M glycine-HCl, pH 2.8. The eluent was quickly neutralized with 1M Tris- HCl (pH 8.5) and buffer exchanged to PBS. The binding of the purified hybridoma antibodies were further validated using the protocol as described above. [0351] Sequencing and amplification of hybridoma antibody clones [0352] RNA Extraction. Monoclonal anti-human BTLA hybridoma hits were grown to a density of 1-3×10 ⁵ in standard hybridoma medium (DMEM/F12, 10% FBS, 1% Glutamax, 1% pen/strep) for 7-10 days in a T75 flask with >80% cell viability.1-3 million cells from cultures were pelleted in a 15 mL falcon tube after centrifugation at 300 g for 5 min. Pelleted cells were washed by resuspending cells in 5 mL ice cold PBS. PBS was removed and cells were resuspended in 600μL Buffer RLT Plus (Qiagen, cat# 74134). Total RNA was isolated from the lysate following the manufacturing protocol (Qiagen, cat # 74134). [0353] PCR amplification to generate cDNA. The synthesis of cDNA utilizes specific reverse PCR primers in conjunction with switch oligos for heavy and kappa chains. To generate cDNA, one microgram of RNA was used as a template followed by reverse transcription using SMART Scribe Reverse Transcriptase kit from Clontech (TAKARA, cat# 639537). Additionally, reagents include 10μM primers (Integrated DNA technologies), 10 mM deoxy nucleotide triphosphate mix (New England Biolab, cat# N0447S), H ₂ O, and an 80 U/μL RNAse inhibitor (Invitrogen, cat# 10000840). The constant region- specific reverse primers were used in conjunction with universal forward primer in 5’-RACE PCR reactions. PCR products were gel purified and cloned into TOPO TA vector (ThermoFisher, cat# 451641) and transformed into competent cells (ThermoFisher, cat# 451641). After transformation and blue/white screening, white colonies were picked and grew overnight in LB broth media containing carbenicillin. Miniprep purified plasmids were sequenced using M13 forward and T7-forward primers. The variable domain sequences of anti-human BTLA hybridomas 86B7, 83F2 and 96F11 are summarized in Table 2 and 3 and provided in the attached Sequence Listing. Example 3: In vitro Assays of Anti-BTLA Chimeric Antibodies [0354] This example illustrates cell-based assays used to characterize the functional activity of the anti-BTLA chimeric antibodies described in the previous Examples. [0355] Generation of recombinant IgG versions of anti-BTLA antibodies [0356] The heavy and light chain variable domains of mouse anti-BTLA antibodies were synthesized with a human constant region to make recombinant anti-BTLA chimeric antibody constructs. The example human heavy chain constant region and light chain constant regions were shown in Table 3. The expression of recombinant anti-BTLA chimeric antibodies was performed using Expi293 expression system in accordance with the instruction provided. The ratio of the plasmids for the heavy chain and the light chain was kept at 1:1 for the transfection reaction and the transfected cells were cultured for 6 days before harvest. Recombinant IgG molecules were purified with the following protocols. Supernatant media were clarified by centrifugation at 300 g for 10 min to remove cells and by filtration with 0.22 µm filter. Clarified supernatant media were mixed with MabSelect Protein A resin and purified as described in Example 1. [0357] Characterization of binding affinity and dissociation constant (K _d ) of the chimeric anti-BTLA antibodies [0358] Binding affinities (monovalent K _d ) of anti-BTLA antibodies were determined using Biolayer interferometry on the Octet RED96 instrument (ForteBio) at 30 °C and 1200 rpm agitation. The following kinetic assay was performed using anti-human IgG Fc capture (AHC) biosensors (ForteBio) in kinetics buffer (PBS, 0.1% Tween-20 and 1% bovine serum albumin): (a) antibody (2 µg/mL) for 300 sec, (b) baseline for 120 sec, (c) association with purified His-tag-huBTLA (2.5, 0.5 and 0 µg/mL) and His-tag-rhesus monkey BTLA (Sino Biologcal, cat# 90250-C08H) (2.5, 0.5 and 0 µg/mL) for 420 sec and (d) dissociation for 1200 sec. Data fitting and analysis was performed with Octet data analysis software 8.0 (ForteBio) using a 1:1 binding model after Savitzky–Golay filtering. The equilibrium dissociation constant (K _d ) was calculated as the ratio of K _off /K _on and summarized in Table 6 (below). TABLE 6: Binding affinity of anti-BTLA IgGs binding to HuBTLA [0359] BTLA binding of purified recombinant chimeric antibodies by ELISA: The huBTLA and rhesus monkey BTLA antigen binding ELISA was performed on the purified recombinant chimeric antibodies. Briefly, 384-well, clear flat bottom high binding plates (Corning, cat# 3700) were coated overnight at 4°C with 0.333 µg/mL of huBTLA or rhesus monkey BTLA in PBS. The following purified process was performed as described in ELISA binding assay in Example 2, except that the secondary antibody was goat anti human IgG Fc- AP (Southern Biotech, cat#2014-04). [0360] As shown in Figure 1A-1B, the chimeric anti-BTLA antibodies 83F2, 86B7, 96F11 were positive for binding to huBTLA (Figure 1A) and rhesus monkey BTLA (Figure 1B). The BTLA binding EC ₅₀ values were shown in Table 7. Table 7 [0361] HVEM blocking by purified recombinant chimeric antibodies by ELISA: Human HVEM (huHVEM) blocking ELISA was performed on purified recombinant chimeric anti- BTLA antibodies. Briefly, 384-well, clear flat bottom high binding plates (Corning, cat# 3700) were coated overnight at 4°C with 0.333 µg/mL purified huBTLA with mIgG2a Fc tag in PBS (coating solution). After removing the coating solution, the plates were blocked by adding blocking buffer and incubated at 37°C for 1 hour. Plates were then washed 3 times with wash buffer. Serial dilution of purified chimeric antibodies in PBS were added to individual well followed by incubation at 37 °C for 1 hour. Plates were washed 3 times with wash buffer. Then 30µL per well of 2µg/ml human HVEM with His tag (Sino Biological, cat#10334-H08H) in assay buffer was added into the reaction and incubated for 1 hour. Plates were washed 3 times with wash buffer. Then 30 μL per well of mouse anti-his-AP (Southern Biotech, cat: 4603-04) at 1: 500 dilution in ELISA diluent was added, and incubated at 37 °C for 30 min, then the plates were washed 5 times with wash buffer and developed for 30 minutes with 30 μL per well of pNPP substrate. Plates were analyzed with a Bio-TEK at 405 nm. The blocking IC ₅₀ value represents the antibody concentration that inhibited 50% of huHVEM binding to coated human BTLA. [0362] As shown in Table 8 and Figure 2, the chimeric anti-BTLA antibodies 83F2, 86B7, and 96F11 are capable of binding to huBTLA and blocking the interaction between huHVEM and BTLA. Table 8 [0363] Binding of chimeric anti-BTLA antibodies to huBTLA overexpressing cells by FACS [0364] To examine the binding of the chimeric anti-BTLA antibodies to huBTLA overexpressing cells, Expi293 cells stably overexpressing huBTLA were used to perform FACS analysis. Briefly, the coding sequence of huBTLA (Uniprot, Q7Z6A9) was cloned into a lentiviral vector and the recombinant virus was packaged according to the instruction of the virus packaging kit (Lenti-X™ Packaging Single Shots, Cat# 631275, Takada). The Expi293 cells were transduced with the recombinant virus. The Expi293 cells overexpressing huBTLA were selected by puromycin and were analyzed by flow cytometry (shown in Figure 3A). [0365] Expi293 cells stably overexpressing huBTLA were incubated with the chimeric anti-BTLA antibodies 83F2 or 86B7 in PBS containing 0.5% BSA, 1 mM EDTA, and 0.1% sodium azide (FACS buffer) for 30 minutes at 4°C. The cells were washed, and then incubated with 10nM phycoerythrin (PE) conjugated anti-Human Fc Ab (BioLegend, cat# 409304) for 20 minutes at 4°C. Cells were washed and then analyzed by flow cytometry with Attune (ThermoFisher Scientific). Data were analyzed with FlowJo software. Antibody binding is represented as median fluorescence intensity (MFI). [0366] The EC ₅₀ values of the exemplary chimeric anti- BTLA antibodies 83F2 and 86B7 were summarized in Table 9. The MFI values were shown in Figure 3B. As shown in Figure 3B, the exemplary chimeric anti-BTLA antibodies 83F2 and 86B7 bind to huBTLA overexpressing Expi293 cells dose-dependently and potently. Table 9 [0367] Blocking of HVEM binding to BTLA by chimeric anti-BTLA antibodies by FACS [0368] Expi293 cells stably overexpressing huBTLA were incubated with chimeric anti- BTLA antibody83F2 or 86B7 for 30 minutes at 4 °C. The cells were washed, and then incubated with human HVEM-mouse IgG2a Fc recombinant protein (Acro Biosystems, cat#HVM-H5255) at 1 μg/ml concentration for 20 minutes at 4 °C. The human HVEM bindings were detected by goat anti-mouse Alexa 488 secondary antibody (ThermoFisher, cat# A-11029). Cells were washed and acquired by flow cytometry with Attune. Data were analyzed with FlowJo software. BTLA binding is represented as MFI. [0369] The IC ₅₀ values of the exemplary chimeric anti-BTLA antibodies 83F2 and 86B7 were summarized in Table 10. The MFI values were shown in Figure 3C. As shown in Figure 3C, the exemplary chimeric anti-BTLA antibodies 83F2 and 86B7 inhibit soluble human HVEM binding to huBTLA overexpressing Expi293 cells dose-dependently and potently. Table 10 [0370] Raji-HVEM/Jurkat-BTLA coculture assay [0371] Functional activity of the anti-BTLA antagonistic antibodies on human T cells was tested using Raji-Jurkat coculture system as shown in Figure 4A. Briefly, the HVEM-BTLA signaling pathway inhibits TCR/CD28 co-stimulatory signals, which can be read out as reduced cytokine such as IL-2 production. Accordingly, inhibition of the HVEM-BTLA pathway via blocking interaction between HVEM and BTLA by the antibody against BTLA can enhance T cell activation. In Raji-Jurkat coculture assay, HVEM-overexpressing Raji cells cocultured with BTLA-overexpressing Jurkat cells upon the stimulation of anti- αCD3/CD19 bispecific antibody in the presence or absence of anti-BTLA antibodies. The ability of anti-BTLA antibodies can be observed by measuring IL-2 levels. [0372] Raji cells (ATCC, cat# CCL-86) overexpressing human HVEM and Jurkat cells (ATCC, cat#TIB-152) overexpressing human BTLA were generated by lentivirus transduction as described above. These two cell lines were incubated with 1 ng/ml αCD3/CD19 bispecific antibody (InvivoGen, CAT# bimab-hcd19cd3) in the presence or absence (as control group) of serial diluted anti-BTLA antagonistic antibodies with complete growth media in round bottom 96 well plates at 37 °C for 72 hours. Control group was treated without anti-BTLA antibody. IL-2 concentrations in supernatant were quantified by ELISA using Human IL-2 ELISA MAX Deluxe kit (BioLegend, cat# 431804). The percentage of IL-2 was calculated by comparing to Raji-HVEM/Jurkat-BTLA cells treated with αCD3/CD19 bispecific antibody in the absence of anti-BTLA antibodies. [0373] The EC ₅₀ values of the chimeric anti-BTLA antibodies 83F2, 86B7 and 96F11 were summarized in Table 11. The IL-2 levels were shown in Figure 4B. As shown in Figure 4B, the chimeric anti-BTLA antibodies 83F2, 86B7, and 96F11 dose-dependently and potently reverse HVEM-BTLA mediated inhibition of IL-2 production in T cells. Table 11 Example 4: Preparation of Humanized Versions of 86B7 [0374] This example illustrates the preparation of humanized versions of the murine anti- huBTLA antibody derived from the hybridoma clone 86B7. [0375] Humanization of murine anti-huBTLA antibody [0376] The light chain variable region (V _L ) and heavy chain variable region (V _H ) sequences of murine antibody from hybridoma 86B7 were aligned against human germline antibody sequences, the human germline kappa light chain (Gene ID–V gene: IGKV2_30*01) and the human germline heavy chain (Gene ID – V gene: IGHV2_5*01) were used as the human frameworks. [0377] The complementarity-determining regions (CDRs) of murine BTLA antibody light chain and heavy chain were grafted into the identified closest human frameworks respectively to generate humanized antibody clone. In this process, positions 24-34 in CDR-L1, 50-56 in CDR-L2 and 89-97 in CDR-L3 (residue numbering follows the EU Kabat) of murine 86B7 V _L were grafted to the human kappa light chain framework acceptor, and positions 31-35 in CDR-H1, 50-65 in CDR-H2, and 95-102 in CDR-H3 (residue numbering follows the EU Kabat) of murine 86B7 V _H were grafted to the human heavy chain framework acceptor. [0378] Position 36 in light chain framework (IGKV2_30*01) region 2 (FW-L2), position 2 in heavy chain framework (IGHV2_5*01) region 2 (FW-H2), position 94 in heavy chain framework region 3 (FW-H3) of murine BTLA antibody were also grafted into the human kappa light chain and heavy chain framework acceptors as those positions were found to be part of VH-VL interacting interface or the framework residues acting as “Vernier” zone, which may adjust CDR structure and fine-tune to fit to antigen (Foote et al., 1992). [0379] The variable domain sequences of the humanized antibodies were summarized in Table 2 and 3. [0380] Generation of recombinant IgG versions of humanized anti-BTLA antibodies [0381] The heavy and light chain variable domain of humanized BTLA antibodies were generated of full-length IgG versions as described above. Example 5.1: In vitro Assays of Humanized Anti-BTLA Antibodies [0382] Characterization of binding affinity and dissociation constant (Kd) of the humanized anti-BTLA antibodies [0383] Binding affinities (monovalent K _d ) of humanized anti-BTLA antibodies were determined using Biolayer interferometry on the Octet RED96 instrument (ForteBio) as described above in Example 3, except for (c) association with His-tag-huBTLA (1, 0.33 and 0 μg/mL) for 420 sec. Binding affinity of the exemplary humanized anti-BTLA antibodies were shown in Table 12. [0384] As shown in Table 12, the exemplary humanized anti-BTLA antibodies exhibited comparable binding activity to huBTLA as compared to their corresponding chimeric antibody 86B7, and exhibited highly better binding affinity to huBTLA as compared to the reference antibody Icatolimab. TABLE 12 Binding affinity of humanized antibodies to BTLA antigens [0385] In vitro binding assays of humanized anti-BTLA antibodies [0386] FACS based binding assays of humanized anti-BTLA antibodies were performed as described above in Example 3. [0387] As shown in Figures 5A, the exemplary humanized anti-BTLA antibodies SB2003- 3, SB2003-4, SB2003-11 and SB2003-12 exhibited comparable binding activity to huBTLA- overexpressing cells as compared to their corresponding chimeric antibody 86B7, and the humanization of anti-BTLA antibody did not affect its binding function. [0388] In vitro blocking assays of humanized anti-BTLA antibodies [0389] FACS based blocking assays of humanized anti-BTLA antibodies were performed as described above in Example 3. [0390] As shown in Figures 5B, the exemplary humanized anti-BTLA antibodies SB2003- 3, SB2003-4, SB2003-11 and SB2003-12 exhibited comparable blocking activity of the interaction between human HVEM and cell surface huBTLA as compared to their corresponding chimeric antibody 86B7, and the humanization of chimeric anti-BTLA antibody did not affect its blocking function. [0391] Apoptosis assay using Jurkat-huBTLA-FAS cells [0392] Functional activity of the humanized anti-BTLA antagonistic antibodies on human T cells was tested using huBTLA-FAS-overexpressing Jurkat cells. In this assay, the Fas mediated apoptosis signal will be activated once BTLA binds to its ligand HVEM, while anti- BTLA antibody blocks this pathway by interfering HVEM-BTLA interaction. Briefly, lentivirus containing extracellular domain of human BTLA with FAS were transduced into Jurkat cells with puromycin selection, and then the Jurkat-huBTLA-FAS cells were harvested. huHVEM-Fc proteins (Sino Biological, cat# 10334-H02H) were coated on 96 well plates at 37 °C for 2hrs. The plates were washed one time with PBS and seeded with Jurkat- huBTLA-FAS cells in the presence of serial diluted anti-BTLA antagonistic antibodies or anti-MOPC21 human IgG1 isotype control antibody (Hamlyn PH, et al.1981) which was made in-house with complete growth media at 37 °C for 18 hours. The ATP content of each sample was measured using ATPLite 1 step assay kit (PerkinElmer, cat#6016731) following the manufacture’s instruction. The ATP content was measured using luminescence and the EC ₅₀ of the humanized anti-BTLA antibodies was calculated using Prism software. [0393] The EC ₅₀ values of the exemplary humanized anti- BTLA antibodies SB2003-11 and SB2003-12 were summarized in Table 13. The luminescence values were shown in Figure 6. As shown in Figure 6, the exemplary humanized anti-BTLA antibodies SB2003-11 and SB2003-12 dose-dependently and potently reduce huHVEM-Fc-induced cellular apoptosis in Jurkat-BTLA-FAS cells, and the activity is comparable to their corresponding chimeric antibody 86B7, indicating that humanization did not alter the antibody functional activity on T cells in vitro. Table 13 [0394] Raji-HVEM/Jurkat-BTLA coculture assay of humanized antibodies [0395] Functional assays of the humanized anti-BTLA antagonistic antibodies on human T cells was performed using Raji-Jurkat coculture system as described above in Example 3. [0396] As shown in Figure 7A, the exemplary humanized anti-BTLA antibodies SB2003- 12 induce the production of IL-2 in T cells and the activity is more potent than reference antibody Icatolimab regarding the restoration of IL-2 production by T cells. [0397] Raji-HVEM/Primary T cell coculture assay [0398] Functional activity of the humanized anti-BTLA antagonistic antibodies on primary human T cells was tested using Raji-Primary T cell coculture system as shown in Figure 4A (Jurkat-BTLA cells were replaced by primary T cells). Briefly, Raji cells overexpressing human HVEM proteins were seeded on 96 well plates. Primary human T cells were isolated using EasySep Human T Cell Isolation Kit (STEMCELL, cat#17951) and incubated with 1 ng/ml αCD3/CD19 bispecific antibody in the presence or absence (as control group) of serial diluted anti-BTLA antagonistic antibodies with complete growth media in round bottom 96 well plates at 37 °C for 72 hours. Control group was treated without anti-BTLA antibody. IL- 2 concentrations in supernatant were quantified by ELISA using Human IL-2 ELISA MAX Deluxe kit (BioLegend, cat# 431804). The percentage of IL-2 was calculated by comparing to Raji-HVEM/Jurkat-BTLA cells treated with αCD3/CD19 bispecific antibody. [0399] The EC ₅₀ values of the exemplary humanized anti-BTLA antibodies SB2003-11 and SB2003-12 were summarized in Table 14. The IL-2 levels were shown in Figure 7B. As shown in Figure 7B, the exemplary humanized anti-BTLA antibodies SB2003-11 and SB2003-12 dose-dependently and potently reverse HVEM-BTLA mediated inhibition of IL-2 production in primary human T cells and the activity is comparable to their corresponding chimeric antibody 86B7, indicating that humanization did not alter the antibody function in activation of primary human T cells in vitro. Table 14 Example 5.2: In vivo Tumor Models to Evaluate Activity of anti-BTLA Antibody [0400] This example illustrates in vivo tumor model studies of the functional activity of the humanized anti-BTLA antibody. [0401] A. Validation of the mouse HVEM binding to human BTLA and the anti-BTLA antibody blocking the interaction. [0402] To test whether mouse HVEM (musHVEM) binds to human BTLA and whether the humanized antibody can block the interaction, His-mouse HVEM protein (Sino Biolobical, cat#, 10567-M03H) was incubated with Expi 293-huBTLA cell line on ice for 30 minutes. Cells were washed and stained with PE conjugated anti-His antibody (Thermofisher Scientific, CAT: TA150130). For antibody blocking assay, the exemplary humanized anti- BTLA antibody SB2003-12 was pre-incubated with the cell line followed by mouse HVEM and anti-His antibody staining. Cells were washed and fixed with 2% PFA and were analyzed by flow cytometry with Attune. MFI of HVEM staining was analyzed with FlowJo software. [0403] As shown in Figure 8A, mouse HVEM bind to human BTLA and the anti-BTLA antibody can block their interaction. [0404] B. MC38 Subcutaneous Tumor Models in Human BTLA Transgenic Mice [0405] Animals and husbandry: Female C57BL/6-Btla ^tm1(BTLA) /Bcgen (7-9 weeks of age) were used in the studies. The animals were fed breeding diet for “SPF rat and mouse growth” and water ad libitum. Animals were ear tagged for identification purposes and shaved on the left dorsal flank area in preparation of cell implantation. Animals were housed in polycarbonate cages (cage size of 320 × 200 × 135 mm). The environment was controlled to a temperature range of 20^~26^ and a humidity range of 40-70%. Animal care and use were compliant with the SOPs of JOINN LABORATORIES (Suzhou) Inc., the Guide for the Care and Use of Laboratory Animals (8th Edition, Institute of Laboratory Animal Resources, Commission on Life Sciences, National Research Council; National Academy Press; Washington, D.C., 2010), and the U.S. Department of Agriculture through the Animal Welfare Act (Public Law 99-198). [0406] Cell preparation and implantation: The mouse colon cancer cell line MC38 was obtained from Institute of Basic Medical Sciences and were cultured according to manufacture protocols known in the field. Pre-implantation cell viability was 92%-94%. The cells were suspended in Dulbecco’s Phosphate Buffered Saline (DPBS) at a concentration of 1×10 ⁷ /ml. Test animals were sterilized at the implantation site with an alcohol prep pad and MC38 cells were implanted subcutaneously in 0.2 mL using a 25-gauge needle and 1 mL syringe. [0407] Measurement and antibody treatment: Tumors were allowed to grow and mice were then randomized into study groups. Mice were distributed to ensure that the mean body weights for both groups were within 10% of the overall mean tumor burden for the study population. Mice received twice weekly i.p. injections 10 mg/kg of each antibody treatment for 3 weeks and tumor volumes were monitored (n=10 mice/group). Group 1 received anti- MOPC21 human IgG1 isotype antibody (made in-house, as control) and group 2 received SB2003-12 antibody. [0408] Assessment of side effects: All animals were observed for clinical signs of distress or toxicity at least once daily. Animals were weighed once per week. Animals were euthanized if body weight loss was in excess of 20% or other clinical signs that warranted euthanasia. Individual animals were euthanized when their tumor volume reached or exceeded 2500 mm ³ . [0409] Results [0410] As shown in Figure 8B, the exemplary anti-BTLA antibody SB2003-12 treatment dramatically inhibited tumor growth compared to the isotype control antibody in MC38 tumor model, indicating that blocking of BTLA mediated inhibitory signaling enhances anti- tumor immunity. In addition, no significant differences in body weights between groups and no abnormality in general clinical observation were noticed. Example 6: In vivo B16F10 Subcutaneous Tumor Models to Evaluate Activity of Combination of anti-BTLA Antibody and anti-PD1 Antibody [0411] This example illustrates in vivo tumor model studies of the functional activity of the combination of anti-BTLA antibody and anti-PD1 antibody. [0412] Animals and husbandry: Female C57BL/6-Btla ^tm1(BTLA) /Bcgen (7-9 weeks of age) were used in the studies. The animals were fed as described in Example 5.2 above. [0413] Cell preparation and implantation: The mouse melanoma cancer cell line B16F10 was obtained from Institute of Basic Medical Sciences and was cultured according to manufacture protocols known in the field. Pre-implantation cell viability was 92%-94%. The cells were suspended in Dulbecco’s Phosphate Buffered Saline (DPBS) at a concentration of 1×10 ⁶ /ml. Test animals were sterilized at the implantation site with an alcohol prep pad and were implanted subcutaneously with B16F10 cells in 0.2 mL using a 25-gauge needle and 1 mL syringe. [0414] Measurement and antibody treatment: Tumors were allowed to grow and mice were then randomized into study groups. Mice were distributed to ensure that the mean body weights for all groups were within 10% of the overall mean tumor burden for the study population. Mice received twice weekly i.p. injections 10 mg/kg of each treatment for 3 weeks and tumor volumes were monitored (n=10 mice/group). Group 1 received anti- MOPC21 human IgG1 isotype control antibody (made in-house), group 2 received anti-PD1 antibody (CAT# BE0146, BioCell), group 3 received SB2003-12 antibody, group 4 received both anti-PD1 antibody and SB2003-12 antibody. [0415] Assessment of side effects: The side effects was assessed as described in Example 5.2 above. [0416] Results: [0417] Combination treatment using the anti-BTLA antibody SB2003-12 and an anti-PD1 antibody showed significantly more anti-tumor efficacy in B16F10 tumor mouse model compared to either the anti-BTLA antibody or anti-PD1 antibody alone. The results indicate that anti-BTLA antibody and anti-PD1 antibody act synergistically to improve anti-tumor efficacy (data not shown).

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