Background Homing of B lymphocytes into specialized microenvironments within secondary lymphoid tissues is essential for normal immune function, yet the molecular cues guiding this cellular traffic are not well defined. Evidence suggests the involvement of chemokines (1-5), but no chemokine has been shown to have the required expression pattern or chemoattractant activity (6). Here we describe a chemokine, B Lymphocyte Chemoattractant (BLC), that is highly expressed in the follicles of Peyer's patches, spleen and lymph nodes. BLC strongly attracts B lymphocytes while promoting migration of only small numbers of T cells and macrophages and therefore is the first chemokine identifie with selectivity for B cells.

Recently an orphan chemokine receptor, Burkitt's Lymphoma Receptor 1 (BLR1) was found to be required for B cell migration into lymphoid follicles (4). We also disclose that BLC stimulates calcium influx and chemotaxis in cells transfected with BLR1, indicating that BLC functions as a BLRI ligand and guides B lymphocytes to follicles in secondary lymphoid organs. BLRI: BLC interactions provide a valable target for phamaceutical development and therapeuticintervention.

RelevantLiterature Förster et al, 1996, Cell 87,1037-1047, describe the functions of BLRI as inferred from a knock-out mouse. Guegler et al., 1997, US Patent No. 5,633,149 describe a gene specific to inflamed adenoid tissue inferred to encode a protein, ADEC, with sequence similarity to a native BLC.

SUMMARY OF THE INVENTION The invention provides methods and compositions for modulating and identifying agents which modulate the interaction of BLRI and BLC polypeptides. The methods for

identifying BLRI: BLC modulators find particular application in commercial drug screens.

These methods generally comprise combining BLR1 and BLC polypeptides with a candidate agent under conditions whereby, but for the presence of the agent, the polypeptides engage in a first interaction, and determining a second interaction of the polypeptides in the presence of the agent, wherein a difference between the first and second interactions indicates that the agent modulates the interaction of the polypeptides. The subject methods of modulating the interaction of BLRI and BLC polypeptides involve combining BLR1 and BLC polypeptides expressed in other than adenoid tissue with a modulator, under conditions whereby, but for the presence of the modulator, the polypeptides engage in a first interaction, and whereby the polypeptides engage in a second interaction different from the first interaction. In a particular embodiment, the modulator is an antagonistic, esp. dominant negative, form of the BLC polypeptide. The invention also provides compositions useful in the subject methods, such as in vitro mixtures comprising BLR1 and BLC polypeptides.

BRIEF DESCRIPTION OF THE FIGURES Figure 1 (a)- (g). Chemotactic activity of BLC on leukocyte subtypes. Results are expressed as the percentage of input cells of each subtype migrating to the lower chamber of a Transwell filter. Panels show migration of a, B cells; b, CD4+ T cells; c, CD8+ T cells; d, granulocytes; and e, monocytes/macrophages to BLC. Positive controls are SDF I a (a, b, c, e) and IL8 (d). f, Failure of B cells to migrate in the absence of a BLC gradient. BLC was added to the upper or lower chamber of the apparats as indicated. g, Inhibition of BLC-induced migration by pretreatment of cells with pertussis toxin (PTX). Data points with error bars represent the mean 1 s. d. for triplicates; individual data points are shown for duplicates. Each expriment was performed a minimum of two times.

Figure 2 (a)-(f). BLRl-mediated calcium mobilization and chemotaxis in response to BLC.

HEK 293 cells, stably transfected with the indicated chemokine receptors (a-d), were loaded with the calcium probe Indo-l and assayed by spectrofluorimetry for changes in intracellular calcium in response to BLC. a, Calcium flux as a function of BLC concentration (nM). b, Specificity of the response of BLR1 to BLC. c, Lack of response to BLC in CCR1-transfected cells. d, Lack of response to BLC in CXCR2-transfected cells. e, Percentage of maximal calcium flux as a function of BLC concentration in BLR1-transfected 300-19 cells. f,

Chemotactic response of BLRI-transfected Jurkat cells to BLC. Results of chemotaxis are expressed as in Fig. 1.

DETAILED DESCRIPTION OF THE INVENTION The invention provides efficient methods of identifying agents, compound or lead compound for agents active at the level of a BLRI: BLC modulatable cellular function, particularly in vitro assays for agents, including agonists and antagonists, which alter the receptor: ligand binding of BLR1 and BLC polypeptides. A wide variety of in vitro assays for binding agents are provided including labeled protein-protein binding assays, immunoassays, cell based assays, etc. In one aspect, the methods involve forcing a mixture of BLR1 and BLC polypeptides and a candidate agent, and determining the effect of the agent on the interactions of the BLR1 and BLC polypeptides. The methods are amenable to automate, cost-effective high throughput screening of chemical libraries for lead compound. Identifie reagents find use in the pharmaceutical industries for animal and human trials; for example, the reagents may be derivatized and rescreened in in vitro and/or in situ (animal) assays to optimize activity and minimize toxicity for pharmaceutical development.

The BLRI polypeptides of the assays, which may be part of a fusion product with another peptide or polypeptide, e. g. a tag for detection or anchoring, etc., are generally provided as transmembrane proteins, on liposomes, cells, isolated phospholipid membranes, etc. A wide variety of molecular and biochemical methods for biochemical synthesis, molecular expression and purification of the subject compositions, including the expression of heterologous recombinant proteins in cells, including bacterial cells (e. g. E. coli), yeast (e. g. S.

Cerevisiae), animal cells (e. g. CHO, 3T3, BHK, baculovirus-compatible insect cells, etc.) see e. g. Molecular Cloning, A Laboratory Manual (Sambrook, et al. Cold Spring Harbor Laboratory), Current Protocols in Molecular Biology (Eds. Ausubel, et al., Greene Publ.

Assoc., Wiley-Interscience, NY) and incorporation of polypeptides into liposomes, are described or referenced herein or are otherwise known in the art. The nucleotide sequences of exemplary natural cDNAs encoding mouse and human BLRI polypeptides are shown as SEQ ID NOS: 5 and 7, respectively, and the full conceptual translates are shown as SEQ ID NOS: 6 and 8. The BLR1 polypeptides may be deletion mutants of SEQ ID NOS: 6 or 8 which retain BLC specific binding activity. BLC-specific binding is readily determined by convenient in

vitro binding assays, in vitro cell-based assays, or in vivo assays in animals (e. g. transgenics, etc.), etc. In one embodiment, BLRI polypeptide-encoding constructs comprising SEQ ID NO: 5 or 7 are expressed in COS cells and assayed for binding to radiolabeled BLC ligands (Table 1).

Table 1. BLC-specific BLR1 polypeptides. BLR1 polypeptide-encoding constructs are expressed in COS cells and assayed for binding to radiolabeled BLC ligand.

BLR1 Polvpeptide Sequence BLC Bindin <BR> <BR> <BR> <BR> <BR> SEQ ID NO: 6, residues 5-371 +++<BR> <BR> <BR> <BR> <BR> <BR> <BR> SEQ ID NO: 6, residues 4-366 +++ SEQ ID NO : 6, residues 3-361 +++ SEQ ID NO : 6, residues 2-356 +++ SEQ ID NO : 6, residues 1-351 +++ SEQ ID NO : 8, residues 5-371 +++ SEQ ID NO : 8, residues 4-366 +++ SEQ ID NO : 8, residues 3-361 +++ SEQ ID NO : 8, residues 2-356 +++ SEQ ID NO : 8, residues 1-351 +++ The BLC polypeptides are generally provided in soluble form. The nucleotide sequences of exemplary natural cDNAs encoding mouse and human BLC polypeptides are shown as SEQ ID NOS: l and 3, respectively, and the full conceptual translates are shown as SEQ ID NOS: 2 and 4. The BLC polypeptides may be deletion mutants of SEQ ID :NOS 2 or 4 which retain BLR1 specific binding activity. Molecular and biochemical methods for biochemical synthesis, molecular expression and purification of the subject compositions are known in the art (supra). BLR1-specific binding is readily determined by convenient in vitro, cell-based, or in vivo assays: e. g. in vitro binding assays, cell culture assays, in animals (e. g. transgenics, etc.), etc. In one embodiment, radiolabled BLC polypeptides are assayed for binding to BLRI polypeptides expressed on COS cells (Table 2).

Table 2. BLR1-specific BLC polypeptides. Radiolabled BLC polypeptides are assayed for

binding to BLRI polypeptides expressed on COS cells.

BLC Polypeptide Sequence BLR1 Binding SEQ ID 1-103+++residues SEQ ID NO : 2, residues 1-98 i i i SEQ ID 1-93+++residues SEQ ID NO : 2, residues 5-108 +++ SEQ ID NO : 2, residues 10-108 +++ SEQ ID 1-104+++residues SEQ ID 1-99+++residues SEQ ID 1-94+++residues SEQ ID NO : 4, residues 5-109 +++ SEQ ID NO : 4, residues 10-109 +++ In other embodiments, BLC polypeptides are screened for chemotactic activity.

Methods for measuring chemotactic activity of BLC polypeptides are well known in the art, see e. g. US Pat No. 5,633,149. For example, activity may be measured in 48-well microchemotaxis chambers according to Falk W. R. et al (1980) J Immunol Methods 33: 239.

In each well, two compartments are separated by a filter that allows the passage of cells in response to a chemical gradient. Cell culture medium such as RPMI 1640 containing the BLC polypeptide is placed on one side of a filter, usually polycarbonate, and cells suspende in the same media are placed on the opposite side of the filter. Sufficient incubation time is allowed for the cells to traverse the filter in response to the concentration gradient across the filter.

Filters are recovered from each well, and cells adhering to the side of the filter facing the BLC polypeptides are typed and quantified.

The specificity of the chemoattraction may be determined by performing the chemotaxis assay on specific populations of cells. In one example, blood cells obtained from venipuncture are fractionated by density gradient centrifugation and the chemotactic activity of BLC polypeptides is tested on enriched populations of neutrophils, peripheral blood mononuclear cells, monocytes and lymphocytes. Optionally, such enriched cell populations are further fractionated using CD8+ and CD4+ specific antibodies for negative selection of CD4+ and CD8+ enriched T-cell populations, respectively. Another assay elucidates the chemotactic effect of BLC polypeptides on activated T-cells. For example, unfractionated T-cells or

fractionated T-cell subsets may be cultured for 6 to 8 hours in tissue culture vessels coated with CD-3 antibody. After this CD-3 activation, the chemotactic activity of the BLC polypeptides are tested as described above. Other methods for obtaining enriched cell populations are known in the art.

Some chemokines also produce a non-chemotactic cell activation of neutrophils and monocytes. This may be tested via standard mesures of neutrophil activation such as actin polymerization, increase in respiratory burst activity, alegranulation of the azurophilic granule and mobilization of Cavas part of the signal transduction pathway. An assay for mobilization of Ca2+ involves preloading neutrophils with a fluorescent probe whose emission characteristics have been altered by Ca2+ binding. When the cells are exposed to an activating stimulus, Ca2+ flux is determined by observation of the cells in a fluorometer. The measurement of Ca2+ mobilization has been described in Gpynkievicz G. et al. (1985) J Biol Chem 260: 3440, and McColl S. et al. (1993) J Immunol 150: 4550-4555. Degranulation and respiratory burst responses are also measured in monocytes (Zachariae C. O. C. et al. (1990) J Exp Med 171: 2177-82). Further mesures of monocyte activation are regulation of adhesion molecule expression and cytokine production (Jiang Y. et al. (1992) J Immunol 148: 2423-8).

Expression of adhesion molecules also varies with lymphocyte activation (Taub. D. et al.

(1993) Science 260: 355-358).

In certain embodiments, the BLC and BLRI polypeptides are encoded by nucleic acids comprising SEQ ID NO: 1 or 3, and SEQ ID NO: 5 or 7, respectively, or nucleic acids which hybridize with full-length strands thereof, preferably under stringent conditions. The invention also provides nucleic acid hybridization probes and replication/amplification primers having a BLC cDNA specific sequence comprising SEQ ID NO: 1,3,5 or 7 and sufficient to effect specific hybridization thereto (i. e. specifically hybridize with SEQ ID NO: 1,3,5 or 7, respectively). These probes/primers find diagnostic uses for detecting BLC expression in nonadenoid tissue. Such nucleic acids are at least 36, preferably at least 72, more preferably at least 144 and most preferably at least 288 bases in length. Demonstrating specific hybridization generally requires stringent conditions, for example, hybridizing in a buffer comprising 30% formamide in 5 x SSPE (0.18 M NaCl, 0.01 M NaPO4, pH7.7,0.001 M EDTA) buffer at a temperature of 42°C and remaining bound when subject to washing at 42°C with 0.2 x SSPE (Conditions I); preferably hybridizing in a buffer comprising 50% formamide in 5 x SSPE buffer at a temperature of 42°C and remaining bound when subject to washing at

42°C with 0.2 x SSPE buffer at 42°C (Conditions Ici).

The assay mixture also comprises a candidate pharmacological agent. Candidate agents encompass numerus chemical classes, though typically they are organic compound; preferably small organic compound and are obtained from a wide variety of sources including libraries of synthetic or natural compound. A variety of other reagents may also be included in the mixture. These include reagents like salts, buffers, neutral proteins, e. g. albumin, detergents, protase inhibitors, nuclease inhibitors, antimicrobial agents, etc. may be used.

The resultant mixture is incubated under conditions whereby, but for the presence of the candidate pharmacological agent, the BLRl and BLC polypeptides interact or bind with a reference binding affinity. The mixture components can be added in any order that provides for the requisite bindings and incubations may be performed at any temperature which facilitates optimal binding. Incubation periods are likewise selected for optimal binding but also minimized to facilitate rapid, high-throughput screening.

After incubation, the agent-biased binding between the BLR1 and BLC polypeptides is detected by any convenient way. Where at least one of the polypeptides comprises a label, the label may provide for direct detection as radioactivity, luminescence, fluorescence, optical or electron density, etc. or indirect detection such as an epitope tag, etc. A variety of methods may be used to detect the label depending on the nature of the label and other assay components, e. g. through optical or electron density, radiative emissions, nonradiative energy transfers, etc. or indirectly detected with antibody conjugates, etc. A difference in the binding affinity of the polypeptides in the absence of the agent as compare with the binding affinity in the presence of the agent indicates that the agent modulates the binding of the BLR1 and BLC polypeptides. A difference, as used herein, is statistically significant and preferably represents at least a 10%, more preferably at least a 5 0%, most preferably at least a 90% difference.

The invention also provides methods for modulating the interaction of BLRI and BLC polypeptides. In a particular embodiment, the BLRl is expressed on the surface of a cell, which may reside in culture or in situ, i. e. within the natural host. The methods involve combining the BLR1 and BLC polypeptides with a modulator which alters their interaction.

In preferred in situ applications, the BLR1 and BLC polypeptides are endogenous (naturally expressed by cells at the target site), the modulator is exogenous (not naturally present at the target site) and the target site is other than adenoid tissue. Exemplary modulators include BLC-specific antibodies, antagonistic or dominant negative BLC deletion mutants, antisense

nucleic acids and ribozymes derived from SEQ ID NOS: I and 3, agents identifie in the foregoing screens, etc. The invention provides a wide variety of approches to modulate, especially inhibit, BLC function in situ.

As disclosed herein, the chemotactic function of BLC depends on BLC gradients within lymphoid and other tissues, and treatment with BLC polypeptides is shown to disrupt in vivo BLC gradients. The invention provides a wide variety of BLC polypeptides. For example, in one embodiment, the invention provides BLC polypeptides with enhanced in vivo half-life isolated from mutagenesis screens for decreased binding to the Duffy antigen, a chemokine clearance receptor expressed on red blood cells, and attachent to immunoglobulin In another embodiment, the invention provides N-terminal truncated BLC deletion mutants having antagonistic function. Deletion of 8 residues from the amino terminus of the CC chemokine RANTES established a molecule with potent antagonistic activity (J. Biol.

Chem. 271,10521). Antagonistic properties of some viral chemokines have also been related to truncations of the amino terminus (Proc. Natl. Acad. Sci. 94,9875). Similarly, N-terminal deletion mutants of BLC lacking from 1-10 amino terminal amino acids demonstrate antagonistic activity in chemotaxis assays performed with lymphocytes from mouse spleen and baculovirus-expressed BLC, Table 3.

Table 3. N-terminal deletion mutant BLC antagonists.

Deletion Mutant Activity A1 BLC (N-terminus 3 residue truncation) + A2 BLC (N-terminus 4 residue truncation) ++ A3 BLC (N-terminus 5 residue truncation) +++ A4 BLC (N-terminus 6 residue truncation) +++ A5 BLC (N-terminus 7 residue truncation) +++ A6 BLC (N-terminus 8 residue truncation) +++ A7 BLC (N-terminus 9 residue truncation) +++ A8 BLC (N-terminus 10 residue truncation) +++ A9 BLC (C-terminus 3 residue truncation) O10 BLC (C-terminus 5 residue truncation)

In another embodiment, antagonistic BLC polypeptides are generated by substitution screens of selected BLC residues. Antagonists of IL8 and CINC have been made by replacing the ELR sequence preceding the CXC motif with the sequence AAR and simultaneously truncating the amino terminal 5 amino acids (J. Biol. Chem. 268,7125; J. Immunol. 159, 1059). Similarly, a BLC antagonist will be created by replacing the NLK sequence preceding the CXC motif with AAR or AAK and truncating the 5 amino terminal amino acids. In addition, appending additional residues, especially N-terminal residues can generate antagonists, as shown with recombinant human RANTES retaining the initiating methionine (Proudfoot AE, et al, J Biol Chem 1996 Feb 2; 271 (5): 2599-2603). Substitutions made based on comparison of BLC with viral chemokine antagonists (Science 277,1656; Proc. Natl.

Acad. Sci. 94,9875) also produce BLC antagonists. Accordingly, differences in amino acid sequence between viral antagonists and conserve residues in the CXC chemokine family, especially amino acids that lead to charge inversions, hydrophobicity changes or structural changes are introduced into BLC to generate antagonists.

BLC antagonists are also generated by chemical modifications. A derivative of RANTES created by chemical modification of the amino terminus, aminooxypentane (AOP)- RANTES, is a potent RANTES antagonist (Simmons G, et al., 1997, Science 276,276-279).

Similarly, modification of BLC by attachment of this or other small organic molecules generates structures with antagonistic activity.

Modifie BLC polypeptides may also be used to retain BLC receptors within cells and so reduce the responsiveness of the cell to BLC. For example, BLC may be expressed as a fusion with a terminal KDEL or related endoplasmic reticulum (ER) retention signal, with or without a linker sequence. Such modification of the CXC chemokine SDF 1 a leads to intracellular retention of receptor CXCR4 and reduced cell responsiveness to the chemokine (Nat. Med. 3,1110). A related strategy involves attachment of BLC to a transmembrane and cytoplasmic sequence from an ER retained transmembrane protein, such as the adenovirus E 19 protein.

For in situ applications, the subject compositions, including agents and modulators (e. g. BLC antagonists), may be administered in any convenient way, wherein the dosage, route and site of administration are determined by the targeted disease, generally involving mammalian host having a lymphoid follicle in need of BLC-responsive lymphoid traffic alteration. For example, antagonist proteins generated by mutation or chemical modification

may be inoculated intravenously or subcutaneously, by inhalation or ingestion, or applied topically. Intravenous administration is a preferred route of treatment for systemic diseases such as AIDS and lymphomas/leukemias. Local administration is preferred when only one or a small number of sites are affecte, such as treatment of certain joints in rheumatoid arthritis patients. Gene therapy approches are used for the introduction of intracellulary retained chemokines, e. g. peripheral blood T cells or hematopoietic stem cells removed from AIDS patients are transfected or retrovirally infecte with the BLC construct and then refuse.

In one embodiment, the subject polypeptides are amenable to direct injection or infusion, topical, intratracheal/nasal administration e. g. through aerosol, intraocularly, or within/on implants e. g. fibers e. g. collagen, osmotic pumps, grafts comprising appropriately transformed cells, etc. A particular method of administration involves coating, embedding or derivatizing fibers, such as collagen fibers, protein polymers, etc. with therapeutic polypeptides. Generally, the amount administered will be empirically determined, typically in the range of about 10 to 1000 Fg/kg of the recipient and the concentration will generally be in the range of about 50 to 500 llg/ml in the dose administered. Other additives may be included, such as stabilizers, bactericides, etc. will be present in conventional amonts.

The compositions are frequently administered in combination with a pharmaceutically acceptable excipient, carrier, diluent, etc., such as sterile saline or other medium, gelatin, an oil, etc. to form pharmaceutically acceptable compositions, and such administration may be provided in single or multiple dosages. Useful carriers include solid, semi-solid or liquid media including water and non-toxic organic solvents. In another embodiment, the invention provides the subject compound in the form of a pro-drug, which can be metabolically converted to the subject compound by the recipient host. A wide variety of pro-dru formulations for polypeptide-based therapeutics is known in the art. The compositions may be provided in any convenient form and/or pharmaceutically acceptable dosage units/unit containers, including tables, pills, capsules, troches, powders, sprays, creams, etc. The compositions may be advantageously combine and/or used in combination with other therapeutic or prophylactic agents, different from the subject compound. In many instances, administration in conjunction with the subject compositions enhances the efficacy of such agents, see e. g. Goodman & Gilman's The Pharmacological Basis of Therapeutics, 9th Ed., 1996, McGraw-Hill.

Exemplary indications and therapeutic strategies are described below:

Infection: Natural BLC functions to attract cells into lymphoid follicles, sites where large amounts of HIV are trapped in infecte patients. Blocking the response of T cells to BLC reduces or inhibits entry of T cells into follicles and so reduces or inhibits access to these viral reservoirs. Accordingly, treatment of individuals infecte with HIV (or other T tropic viruses) inclues the systemic inoculation of BLC and BLC antagonists or introduction of ER- retained BLC by gene therapy. Such treatment may be performed in combination with other anti-HIV therapies. In another example, anti-BLC agents offer novel therapeutic approches to viruses that have exploite the strong chemoattractant property of BLC-like chemokines to attract target cells for infection, e. g. Marek's Disease Virus, which is transmitted by inhalation but rapidly infects B cells recruited into the lung.

Lymphoma: Many lymphomas occupy specific niches within lymphoid tissues for long periods before apparently becoming independent of these zones and spreading to other sites.

Progression of lymphomas that position in B cell zones of secondary lymphoid tissues, such as mantle cell lymphoma, follicular center lymphoma and Burkitt's lymphoma may be blocked if cell localization is disrupted by BLC or BLC antagonists, administered i. v. or s. c. Such treatment may be performed in combination with other anti-tumor therapies such as chemotherapy or radiotherapy. Treatment with BLC or BLC antagonists also disrupts recirculation and survival of some leukemias, especially B lineage leukemias.

Autoimmune Disease: In several autoimmune diseases, including rheumatoid arthritis, thyroiditis and diabetes, lymphoid follicles form in the inflamed tissue and contribute to the autoimmune pathology. Treatment with BLC or BLC antagonists, either locally or systemically, inhibits formation or persistence of these structures in some cases and thereby reduces the severity of disease. In another example, persistent production of autoantibodies in patients with autoimmune diseases such as Systemic Lupus Erythrematosis and Myasthenia Gravis depends on appropriate positioning of B lymphocytes within lymphoid or other tissues.

The BLC-BLR1 interaction is essential for mounting normal antibody responses (in mice) and is also needed for autoantibody production. Treatment with BLC or BLC antagonists inhibits or reduces production of autoantibodies. In yet another example, local accumulation of B lymphocytes in the lungs of patients with lung diseases such as asthma often contributes to the disease process. Treatment with BLC antagonists by any of the above routes, especially by inhalation, reduces B cell recruitment and local antibody production and thereby ameliorates the disease.

The following experimental section and examples are offered by way of illustration and not by way of limitation: EXAMPLES BLC Identification and Functional Characterization as BLR1 Ligand To identify novel chemokines that might play a role in lymphocyte homing we hybridized mouse tissues in situ with anti-sense transcripts of expressed sequence tags (ESTs) having homology to chemokines. One such EST (I. M. A. G. E. Consortium Clone 596050) hybridized strongly to spleen, Peyer's patches and lymph nodes but weakly or not at all to multiple non-lymphoid tissues. We refer to this transcrit and the protein it encodes as BLC.

In the spleen BLC hybridized to the B cell rich zones, or follicles, present in the outer region of the white pulp cords. A strong signal was detected in a reticular pattern within the follicle and at the outer boundary where the follicle meets the surrounding marginal zone. In Peyer's patches expression of BLC was strongest within germinal centers, sites where B cells undergo somatic mutation and affinity maturation (7), and extended into the surrounding mantle zone.

Expression in lymph nodes was again concentrated in a reticular pattern within the follicles although the hybridization signal was variable and was not seen in all follicles. Northern blotting revealed a 1.2 kb transcrit in wildtype spleen, Peyer's patches and lymph nodes but not in resting B or T cells. BLC expression was reduced 85% in spleens of lymphocyte-deficient RAGI-knockout mice, suggesting that lymphocytes provide a stimulus that promotes BLC expression in non-lymphoid splenic cells. Accumulation of follicular dendritic cells (FDC) in lymphoid tissues is known to depend on the presence of B and T lymphocytes (8). Furthermore, FDC have extensive processes that extend throughout lymphoid follicles in a pattern similar to the BLC in situ hybridization pattern (9). These findings indicate that FDC may be a source of this novel chemokine.

To identify the full length cDNA for BLC, we searched for ESTs contiguous to the clone used for hybridization. Sequence analysis of four overlapping clones revealed a 1112 bp cDNA (SEQ ID NO: 1) containing an open reading that encoded a putative protein of 109 amino acids (SEQ ID NO: 2) with a predicted 21 amino acid leader peptide. This sequence contained four cystines in a pattern typical of the CXC family of chemokines (10) and BLC was found to have strongest similarity to GROS. We also identifie a cluster of six human EST clones encoding a protein with 64% amino acid similarity to murine BLC that is human

BLC (SEQ ID NOS: 3 and 4). A sequence tagged site (STS) derived from this sequence (Genbank #G14456) had been mapped to chromosome segment 4q21 (11), placing the BLC gene in proximity to most known CXC chemokines including IL-8, GRO, IP-10, and PF4 (12).

Interestingly, the protein with the greatest similarity to mouse and human BLC is Meq-sp, a product of the Marek's Disease Virus (MDV) Eco Q gene. MDV is a lymphotropic avian herpesvirus that causes a disease common to almost all commercial chicken stocks characterized by the development of lymphomas in multiple organs (13). Meq-sp was identifie in MDV infecte cells and was not previously recognized to contain a consensus chemokine motif (14).

The above findings suggested that BLC may be a B lymphocyte chemoattractant. To test this possibility, chemotaxis assays were performed with lymphocytes from mouse spleen and baculovirus-expressed BLC estimated to be greater than 95% pure by silver staining.

BLC induced a strong chemotactic response in B cells (Fig. 1 a) while showing limited activity towards CD4 and CD8 T cells (Fig. lb, c). The response was chemotactic rather than chemokinetic since cells incubated with BLC in the absence of a gradient failed to migrate (Fig. 1 f). SDF 1 a, previously described as the most efficacious chemokine for resting lymphocytes (15), attracted fewer B cells than BLC and lacked any B cell specificity (Fig. la-c), highlighting the unique properties of the novel chemokine and leading us to name it BLC for B-Lymphocyte Chemoattractant. BLC had weak but reproducible chemotactic activity for spleen monocytes/macrophages (Fig. 1 e) but in contrast to many CXC chemokines, showed no chemotactic activity towards granulocytes (Fig. lad). Despite its efficacy as a B cell attractant, BLC had a potency less than that of most chemokines, possibly because the baculovirus expressed protein is not fully active. An alternative possibility is that high potency is not required for a chemokine that is expressed constitutively within lymphoid tissues.

All chemokines studied thus far signal via pertussis toxin sensitive G-protein-coupled receptors (12) and this was also found to be the case for BLC as pertussis toxin pretreated B cells failed to migrate (Fig. lg). Recent experiments in mice with targeted disruption of the orphan chemokine receptor BLR1, have indicated that this receptor is required for B cell homing to follicles in spleen and Peyer's patches (4). We therefore tested whether BLC could signal through BLRI. Human embryonic kidney 293 cells stably transfected with mouse BLRI showed a dose dependent calcium flux in response to BLC (Fig. 2a) whereas several

other chemokines did not stimulate a response and did not desensitize these cells to BLC (Fig.

2b). BLC failed to stimulate a calcium flux in 293 cells transfected with CCR1, CCR2 or CXCR2 demonstrating that the response of BLR1 transfected cells was specific (Fig. 2c, d).

Using BLRI transfected 300-19 pre-B cells a more complete dose response curve was obtained showing that the response to BLC is saturable (Fig. 2e). Control transfectants of either cell line did not respond to BLC. We next tested the ability of BLC to stimulate chemotaxis through BLR1. Jurkat T cells transfected with BLRI showed a chemotactic response toward BLC whereas BLR1-negative cells failed to respond (Fig. 2e). Our findings demonstrate that BLR1 confers cells with responsiveness to BLC and that BLC-responsiveness of cells from mouse Iymphoid tissues correlates with the reporte expression of BLR1 in all B cells and in subsets of T cells and monocytes (4,16,17).

In summary, we disclose a novel CXC chemokine, BLC, expressed in the follicles of spleen, Peyer's patches and lymph nodes that is a strong B cell chemoattractant. BLC's expression pattern, chemotactic activity, and ability to stimulate cells expressing BLRI indicate that it is a physiological BLR1 ligand, acting to direct the migration of B lymphocytes to follicles in secondary lymphoid organs. Although BLR1 is required for B cell migration into splenic and Peyer's patch follicles, it is not needed for B cell localization in lymph node follicles. Taking the results presented here together with the recent identification of chemokines expressed in lymphoid T cell areas that strongly attract resting T cells (18,19), indicates that chemokines are the major cues promoting cell compartmentalization within lymphoid tissues.

Sequence Analysis. Pattern searches of the NCBI EST database using TFASTA (20) with human MCP-1 as a template retrieved human and mouse EST's for BLC. BLAST (21) searches with these sequences identifie contiguous ESTs. I. M. A. G. E. Consortium [LLNL] CANA clones 596050,598232,617961, and 749241 (22) were obtained from Genome Systems Inc (St. Louis, MO) as EcoRI-NotI inserts in the pT7T3-Pac vector and sequenced.

Similarity scores were calculated using the Blossum 30 matrix.

RNA Expression studies. For Northern analysis, MARNA from mouse tissues or purifie cells was subjected to gel electrophoresis, transferred to Hybond-N+ membranes (Amersham), and probed using randomly primed mouse BLC EST 596050, which spans bases 10-532 of the BLC cDNA. For in situ hybridizations, paraffin sections (5 mm) from C57BL/6 mice were deparaffinized, fixed in 4% paraformaldehyde, and treated with

proteinase K. After washing in 0.5 x SSC, the sections were covered with hybridization solution, prehybridized for 1 to 3 hrs. at 55 °C, and hybridized overnight with sense or antisense S35-labeled riboprobe transcribed from the mouse BLC EST 596050. After hybridization, sections were washed at high stringency, dehydrated, dipped in photographic mulsion NTB2 (Kodak), stored at 4 ° C for 2-8 weeks, developed, and counterstained with hematoxylin and eosin. In some experiments, frozen sections were hybridized with sense or antisense digoxygenin-labeled riboprobes, immunostained with alkaline phosphatase coupled anti-digoxygenin antibody and developed with NBT/BCIP as described (http://www. cco. caltech. edu/mercer/htmls/Bigjn-Situ. html). Immuno-histochemistry with anti-B220 antibody was as described (1).

Production of Recombinant Proteins. The mouse BLC EST 596050 was cloned into the pVL1393 baculovirus transfer vector and co-transfected with BaculoGold (Pharmingen) into SF9 cells according to the manufacturer's instructions. For protein production, SF21 cells were infecte at an MOI of 10-20 and cultured in serum-free media for 60 hrs. Conditioned media was cleared, loaded onto a HiTrap heparin affinity column (Pharmacia), and eluted with a 0.2-1M NaCI gradient in 50mM HEPES (pH 7.9). Fractions containing BLC were pooled, run on a C-18 reverse phase HPLC column (Vydac), and eluted with an acetonitrile gradient.

SDS PAGE and silver staining of this preparation revealed a single protein band of the expected molecular weight for BLC (I OkD) that represented more than 95% of the total protein. Protein concentration was measured using the Bio-Rad protein assay. Protein sequence analysis identifie the isoleucine at position 22 as the amino terminus of the mature recombinant protein.

Chemotaxis. Lymphocytes and macrophages were obtained from spleens of C57BL/6 mice. For macrophage chemotaxis, B cells were depleted by passage over a MACS column (Milteny Biotec, Auburn, CA) after incubation with biotinylated anti-B220 antibodies and streptavidin-coated magnetic beads. Granulocytes were obtained from mouse bone marrow suspensions. Mouse BLR1 transfected Jurkat cells were obtained by transfection with pREP4 containing the mouse BLRI coding region (16), isolated by RT-PCR from mouse spleen RNA, and an amino terminal prolactin leader sequence and FLAG epitope (23). Positive clones were identifie using the anti-FLAG antibody M1 (Kodak). Chemotaxis assays were performed as previously described (15) and subsets of migrating cells were identifie by flow cytometry using antibodies specific for B220, CD4, CD8 (Pharmingen, San Diego, CA) and

Mac-1 (Caltag, South San Francisco, CA). Granulocytes were identifie by their characteristic large side scatter profile. In some experiments, cells were preincubated with 100 ng/ml pertussis toxin (List Biol. Labs, Campbell, CA) for 2 hrs at 37o C. IL-8 (R&D Systems, Minneapolis, MN) and synthetic human SDF I a (N33A) synthesized by native chemical ligation (Gryphon Sciences, South San Francisco) were used as positive controls. SDFla (N33A) has identical activity to native human and mouse SDFla (24,25).

Calcium fluorimetry on transfected 293 and 300-19 cells. Native mouse BLRI was subcloned into pBK-CMV (Stratagene) and used to transfect HEK 293 cells and 300-19 pre-B cells. G418-resistant clones were tested for BLR1 expression using an affinity purifie rabbit anti-mouse antiserum that is specific for the BLR1 amino terminus. HEK293 cells expressing CCR1, CCR2 and CXCR2 were from the Cardiovascular Research Institute, UCSF.

Ca2+-mobilization studies were performed as described (26) using a Hitachi 4500 spectrometer. Intracellular calcium concentrations were calculated using the Hitachi 4500 Intracellular Cation Measurement program.

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Protocol for Ligand Screening of Transfected COS cells.

I. Prepare the Ligand Expression Construct: cDNAs encoding targeted BLC polypeptides are tagged with alkaline phosphatase (AP) and suboloned into a 293 expression vector (pCEP4: In Vitrogen).

Transfection: 293 EBNA cells are transfected (CaP04 method) with the BLC expression constructs. After 24 h recovery, transfected cells are selected with G418 (geneticin, 250 ug/ml, Gibco) and hygromycin (200 ug/ml). Once the selection process is complete, cells are maintained in Dulbecco's Modifie Eagles medium (DME)/10% FCS under selection.

Preparation of Conditioned Medium: Serum-containing media is replace with Optimem with glutamax-l (Gibco) and 300 ng/ml heparin (Sigma), and the cells are conditioned for 3 days. The media is collecte and spun at 3, 000xg for 10 minutes. The supernatant is filtered (0.45 um) and stored with 0.1 % azide at 4°C for no more than 2 weeks.

Transfection: 293 EBNA cells are transfected (CaP04 method) with the receptor expression construct. cafter 24 h recovery, transfected cells are selected with G418 (geneticin, 250 ug/ml, Gibco) and hygromycin (200 ug/ml). Once the selection process is complete, cells are maintained in Dulbecco's Modifie Eagles medium (DME)/10% FCS under selection.

Preparation of Conditioned Medium: Serum-containing media is replace with Optimem with glutamax-1 (Gibco) and 300 ng/ml heparin (Sigma), and the cells are conditioned for 3 days. The media is collecte and spun at 3, 000xg for 10 minutes. The supernatant is filtered (0.45 um) and stored with 0.1 % azide at 4°C for no more than 2 weeks.

II. Transfect COS Cells Seed COS cells (250,000) on 35 mm dishes in 2 ml DME/10% FCS.

18-24 h later, dilute 1 ug of BLR1-encoding DNA (cDNA cloned into pMT21

expression vector) into 200 ul serum-free media and add 6 ul of Lipofectamine (Gibco).

Incubate this solution at room temperature for 15-45 min.

Wash the cells 2X with PBS. Add 800 ul serum-free media to the tube containing the lipid-DNA complexes. Overlay this solution onto the washed cells.

Incubate for 6 h. Stop the rection by adding 1 ml DMA/20% FCS. Refeed cells.

Assay cells 12 hr later.

III. Ligand Binding Assay Wash plates of transfected COS cells 1X with cold PBS (plus Ca/Mg)/1% goat serum.

Add 1 ml conditioned media neat and incubate 90 min at room temp.

Wash 5X with PBS. Wash 1X alkaline phosphatase buffer (100 mM Tris-Cl, pH 9.5, 100 mM NaCl, 5 MM MgC'2)-Prepare alkaline phosphatase reagents: 4.5 ul/ml NBT and 3.5 ul/ml BCIP (Gibco) in alkaline phosphatase buffer.

Incubate 10-30 min, quench with 20 mM EDTA in PBS. Cells that have bound BLC polypeptides are visible by the presence of a dark purple rection product.

In parallel incubations, positive controls are provided by titrating BLC binding with serial dilutions of the mutant receptor conditioned medium.

IV. Results: Binding of BLC to BLRI Cell expressing mammalian BLC polypeptides were shown to bind BLRI. No reactivity was observe with control COS cells or with receptor-expressing COS cells in the presence of the conjugated AP but in the absence of the BLC-AP fusion.

Protocol for high throughput BLR1-BLC binding assav.

A. Reagents: -Neutralite Avidin: 20 µg/ml in PBS.

-Blocking buffer: 5% BSA, 0.5% Tween 20 in PBS; 1 hour at room temperature.

-Assay Buffer : 100 mM KCl, 20 mM HEPES pH 7.6,1 mM MgCl2, 1% glycerol, 0.5% NP-40,50 mM ß-mercaptoethanol, 1 mg/ml BSA, cocktail of protase inhibitors.

-33P BLC polypeptide 10x stock: 10-8 - 10-6 M "cold" BLC polypeptide supplemented with 200,000-250,000 cpm of labeled BLC (Beckman conter). Place in the 4°C microfridge during screening.

-Protease inhibitor cocktail (1000X) : 10 mg Trypsin Inhibitor (BMB # 109894), 10 mg Aprotinin (BMB # 236624), 25 mg Benzamidine (Sigma # B-6506), 25 mg Leupeptin

(BMB # 1017128), 10 mg APMSF (BMB #917575), and 2mM NaV03 (Sigma &num S-6508) in 10 ml of PBS.

-BLR1: 10-7 - 10-5 M biotinylated BLR1 expressed on COS cells suspende in PBS.

B. Preparation of assay plates: -Coat with 120 RI of stock N-Avidin per well overnight at 4°C.

-Wash 2 times with 200 ftl PBS.

-Block with 150 fil of blocking buffer.

-Wash 2 times with 200 µl PBS.

C. Assay: -Add 40 zip assay buffer/well.

-Add 10 ttl compound or extract.

-Add 10 ll133P-BLC (20-25,000 cpm/0.1-10 pmoles/well =10-9-10-'M final conc).

-Shake at 25°C for 15 minutes.

-Incubate additional 45 minutes at 25°C.

-Add 40 HM biotinylated BLRI (0.1-10 moles/40 ul in assay buffer) -Incubate 1 hour at room temperature.

-Stop the rection by washing 4 times with 200 gM PBS.

-Add 150 pM scintillation cocktail.

-Count in Topcount.

D. Controls for all assays (located on each plate): a. Non-specific binding b. Soluble (non-biotinylated BLR1) at 80% inhibition.

All publications and patent applications cited in this specification are herein incorporated by reference as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Although the foregoing invention has been described in some detail by way of illustration and example for purposes of clarity of understanding, it will be readily apparent to those of ordinary skill in the art in light of the teachings of this invention that certain changes and modifications may be made thereto without departing from the spirit or scope of the appende claims.