The present invention relates to a method for modulating the activity of cells of the immune system. Natural killer cells and T cells are key effectors of immune responses. A panel of receptors and costimulatory molecules modulates their activity. T-cells become activated by engagement of their clonotypic T-cell antigen receptor (TCR)-CD3 complexes by specific MHC class I or II-peptide complexes and by engagement of costimulatory receptors by their ligands expressed on professional antigen-presenting cells. The antigen activation threshold can be modulated by NK receptors expressed on their cell surface. As opposed to T cells, NKl cell recognition of targets is not antigen specific and NK cells are capable of killing target cells without prior sensitization. NK cell activity is regulated by a balance between activating and inhibitory signals triggered by engagement of NK cell receptors -with their ligands. NK cells generally bind and kill target cells which are deficient in the expression of MHC class I proteins ("missing self hypothesis) (Llunggren et al, Immunol. Today. 11:237-244, 1990). This is because NK cells express NK cell inhibitory receptors that recognizes MHC class I molecules. They include Killer cell Ig-like Receptors (KIR) interacting with groups of classical MHC class I molecules, CD94/NKG2A recognizing non classical HLA-E molecule and Ig-like transcript 2 (ILT -2) interacting with most if not all MHC class I proteins (Moretta et al, Annu. Rev. Immunol. 19:197-223, 2001). Activation and lysis of target cells is triggered mainly by Natural Cytotoxicity Receptors (NCR) and NKG2D. But a number of other activating receptors or costimulatory molecules also modulate NK cell activation. Only when all receptors and costimulatory molecules and their respective ligands are identified, will it be possible to fully understand the regulation of NK cell activity and their function. Human Lectin-like transcript 1 (LLTl) has an extracellular domain with similarity to the C-type lectin like domains shared with other NK cell receptors including CD 161 (also named NKR-PlA). LLTl was cloned in 1999 (Genebank accession number AF133299) and is localized to the NK gene complex on human chromosome 12 in between NKR-PlA and CD69 (Boles et al, Immunogenetics. 50:1-7, 1999). Recently, a monoclonal antibody L9.7 (mAb) has been described to specifically recognize LLTl and to induce IFN-γ cytokine production but not cytotoxicity in YT cells, a human NK cell line but also in resting NK cells freshly isolated from peripheral blood and IL-2 activated NK cells (Mathew PA et al, MoI. Immunol. 40:1157, 2004). L9.7 mAb recognizes most cells from peripheral blood mononuclear cells including NK cells (4-10%), T cells (5-14%), B cells (64-73%) and monocytes (100%). LLTl is expressed on the cell surface as a homodimer. CD161 (also named NKR-PlA) is also a C-type lectin expressed by natural killer cells and subsets of T cells, including TCR αβ and γδ T cells, invariant CD Id-restricted YaIA+ NKT and CD2"CD3/TCR" thymocytes (Lanier et al, J. Immunol. 153:2417-2428,1994; Poggi et al, Eur. J. Immunol. 26:1266-1272,1996; Exley et al, J. Exp. Med. 188:867-876,1998). CDl 61 is a disulfide-bond homodimer. Anti-CD161 mAbs have been reported to inhibit the NK. cell-mediated lysis of some Fcγ receptor"1" tumour target cells (Lanier et al, J. Immunol. 153:2417-2428, 1994; Poggi et al, Eur. J. Immunol. 28:1611-1616, 1998). CD161 was also shown to act as a costimulatory molecule on CD Id-restricted Vα24+ NKT cells, mediating both proliferation and cytokine secretion (Exley et al, J. Exp. Med., 188:867-876,1998). So far, no ligand has been identified for CD161. It was reported that CDl 61 binds carbohydrates but the authors later retracted the article. Expression of CD 161 on the cell surface is upregulated by IL-12 cytokine (Poggi et al, Eur. J. Immunol. 28:1611-1616, 1998). CD161 has also been reported to be involved in transendothelial migration (Poggi et al, Eur. J. Immuol. 27:2345-2350, 1997). It may upregulate LFAl Mg2+ binding site and βl and β2 (CD29) integrin at the cell surface. The present invention arises from the unexpected finding made by the Inventors, that CD 161 expressing cells could be specifically linked to LLT 1. Thus, an object of the present invention is to provide a new interaction between two surface receptors of cells of the immune system, namely CD161 and LLTl. Another object of the invention is to use this interaction to modulate the activity of cells of the immune system. A further object of the invention is to provide pharmaceutical compositions comprising compounds liable to mimic and/or to block the above-defined interaction. The present invention relates to a method for modulating in vivo or in vitro the activity of cells expressing CD 161 and/or LLTl, characterized in that said cells are contacted respectively with LLTl or fragments thereof, and/or with CD161 or fragments thereof. As intended herein "modulating the activity of cells" relates to the activation or to the inhibition of the activity of said cells. The activity of given cells can be measured according to methods well known to the man skilled in the art. Such methods notably involve measuring the migration, the proliferation, the cytokine production, or the cytotoxicity of said cells. The present invention also relates to a method as defined above, for modulating in vivo or in vitro the activity of cells expressing CDl 61 and/or LLTl, characterized in that said cells are contacted respectively with LLTl or fragments thereof, and/or with CD 161 or fragments thereof, the contacting of said cells with LLTl or fragments thereof, and/or with CD 161 or fragments thereof comprising respectively an interaction between LLTl, or fragments thereof, with CD161 expressed by said cells, or an interaction between CD161, or fragments thereof, with LLTl expressed by said cells. As intended herein, the expression "comprising an interaction" means that the contacting of cells expressing CD 161 and/or LLTl respectively with LLTl or fragments thereof, and/or with CD161 or fragments thereof, entails the mutual binding of CD161, or fragments thereof, and of LLTl, or fragments thereof. According to a preferred embodiment of the above-defined method, the CDl 61 expressing cells are NK cells, CDId restricted Vα24+ NKT cells, or T cells, such as aβ T cells or γδ T cells. According to another preferred embodiment of the above-defined method, when the CDl 61 expressing cells are NK cells, the contacting of said cells with LLTl or fragments thereof leads to the inhibition of said cells. Under certain conditions, the contacting of CDl 61 expressing NK cells with LLTl or fragments thereof also leads to the activation of said cells. In another preferred embodiment of the above-defined method, when the CDl 61 expressing cells are T cells or CDId restricted Vα24+ NKT cells, the contacting of said cells with LLTl or fragments thereof leads to the activation of said cells. Under certain conditions, the contacting of CDl 61 expressing T cells or Vα24+ NKT cells with LLTl or fragments thereof also leads to the inhibition of said cells. In yet another preferred embodiment of the above-defined method, the CD 161 expressing cells are contacted with: - a human LLTl, in particular as represented by SEQ ID NO: 2, or - a fragment of said human LLTl corresponding to the extracellular domains of LLTl, in particular as represented by SEQ ID NO: 4. Advantageously, extracellular domains of LLTl are soluble by themselves. According to a particular embodiment of the above-defined method, LLTl3 or fragments thereof, are presented at the surface of biological membranes, and in particular at the surface of cells, are used as soluble molecules, or are immobilized on a support. By "biological membrane" is meant any membrane constituted of amphiphatic lipids, such as glycerophospholipids. By "support", is meant any solid material onto which proteins can be attached, either directly or by means of a suitable linker molecule, according to methods well-known to the man skilled in the art. According to another particular embodiment of the above-defined method, the LLTl fragment is part of a compound comprising one or more LLTl fragments. Such LLTl comprising compounds are defined hereafter. Advantageously, the presence of more than one LLTl fragments in said compounds increases the potency of said compounds to modulate the activity of CDl 61 expressing cells. In a particularly preferred embodiment of the above-defined method, CD161 expressing cells express the human CDl 61 receptor, in particular as represented by SEQ ID NO: 6. The present invention also relates to a method as defined above, characterized in that the LLTl expressing cells are NK cells, T cells, dendritic cells, macrophages, monocytes or B cells. According to a preferred embodiment of the above-defined method the contacting of LLTl expressing cells with CD161 or fragments thereof leads to the activation of said cells. Under certain conditions the contacting of LLTl expressing cells with CDl 61 or fragments thereof also leads to the inhibition of said cells. According to another preferred embodiment of the above-defined method the LLTl expressing cells are contacted with: - a human CDl 61, in particular as represented by SEQ ID NO: 6, or - a fragment of said human CDl 61 corresponding to the extracellular domains of CDl 61, in particular as represented by SEQ ID NO: 8. According to another preferred embodiment of the above-defined method CDl 61, or fragments thereof, are presented at the surface of biological membranes, and in particular at the surface of cells, are used as soluble molecules, or are immobilized on a support. In another preferred embodiment of the above-defined method the CD161 fragment is part of a compound comprising one or more CDl 61 fragments. Advantageously, the presence of more than one CD161 fragments in said compound increases the potency of said compound to modulate the activity of LLTl expressing cells. In yet another preferred embodiment of the above-defined method, LLTl expressing cells express the human LLTl receptor, in particular as represented by SEQ ID NO: 2. According to a particularly preferred embodiment of the above-defined method, LLTl or fragments thereof, or CD 161 or fragments thereof, may be modified by the insertion, the deletion or the substitution of at least one amino acid, provided that said modifications result in an improved CD161-LLT1 interaction, such as an enhanced binding capability or an improved stability. In particular, modified LLTl or CD161 having an improved interaction stability at elevated temperatures, such as temperatures over 4°C, preferably over 2O0C, and more preferably at approximately 37°C, are particularly preferred in the frame of the present invention. The present invention also relates to a method for identifying, purifying, inactivating or destroying CDl 61 and/or LLTl expressing cells, characterized in that said cells are contacted respectively with LLTl or fragments thereof, and/or with CDl 61 or fragments thereof. Accordingly, for identifying CD 161 and/or LLTl expressing cells, respectively LLTl or CDl 61 will be advantageously linked to marker moieties such as fluorophores or haptens. For purifying methods, LLTl or fragments thereof, or CDl 61 or fragments thereof will be advantageously linked to a solid support or to marker moieties, so that the respective association of CD161 or LLTl expressing cells leads to a complex which can be easily retrieved from a medium containing unpurified CD161 or LLTl expressing cells. For inactivating or destroying CD 161 and/or LLTl expressing cells, LLTl or fragments thereof, or CDl 61 or fragments thereof will be advantageously linked to antibodies, or to toxic or cytolytic moieties, such as toxins or radioactive molecules. For destroying CD161 and/or LLTl expressing cells, LLTl or fragments thereof, or CD161 or fragments thereof will be advantageously linked to marker moieties such as fluorophores to visualize the cells and then destroy them by use of a laser. The present invention also relates to a method for screening compounds liable to be used for the treatment of cancers, infectious diseases, such as viral, bacterial, or parasitic diseases, autoimmune diseases, inflammatory diseases, allergic reactions, pregnancy failures, or organ and bone marrow transplantation rejection, characterized in that the compounds liable to be used for the treatment of the above mentioned pathologies are selected on their •^properties of inhibiting the binding between CDl 61 or fragments thereof, and LLTl or J fragments thereof. As intended herein cancers notably comprise leukaemia, lymphoproliferative cancers, cervical cancers, prostate cancers, lung cancers, breast cancers, and melanoma. Infectious diseases comprise in particular HIV infection, human Cytomegalovirus infection, Hepatitis B and. C infection, Ebola virus infection, Dengue, Yellow fever, Listeriosis, Tuberculosis, Cholera, Malaria, Leishmaniasis, and Trypanosoma infection. Autoimmune, inflammatory and allergic diseases notably comprise type I diabetes, rheumatoid arthritis, Lupus, inflammatory bowel diseases, such as celiac disease or Crohn disease, psoriasis, and multiple sclerosis. Pregnancy failures notably relate to implantation disorders. Organ transplantation in particular relates to xenotransplantations. Several studies show that CD161 expression is either increased or reduced in cancerous contexts: • in NK-type lymphoproliferative disease of granular lymphocytes (NK-LDGL), the frequency of CDl 61 expressing NK cells in PBMCs is significantly reduced compared to healthy donors (p< 0.0001) (Pascal et al. (2004) Eur J Immunol 34:2930-2940); • T lymphocytes infiltrating human prostate carcinoma express high level of CDl 61 (Bronte et al (2005) J Exp Med 201:1257-1268); • Tumor-infiltrating lymphocytes (TILs) in human cervical cancer express up-regulated CD94/NKG2A inhibitory receptor and concomitantly lower levels of CD161.(Sheu et al. (2005) Cancer Res 65:2921-2929). Thus, LLT1/CD161 interaction is probably modulating anti-tumor immune responses. As such blocking the interaction between LLTl and CD161 should enhance NK cell mediated tumor rejection, whereas promoting the interaction should enhance CD8+ T cell mediated rejection. Further, recent data have shown the efficiency of alloreactive NK cells in the cure of leukaemia. A particularly favourable outcome was observed in a group of patients who received a haploidentical bone-marrow transplant, that is when the bone-marrow donor and recipient are identical for one MHC class I haplotype and fully mismatched for the other. In this allogeneic setting, a fraction of NK cells of the donor express KIR that fail to recognize one or more MHC class I of the host. As KIR interaction with MHC class I induces an inhibitory signal, these cells are not inhibited and can kill residual leukemic cells. At the same time, a markedly reduced incidence of graft- versus-host disease is observed due to the killing of recipient dendritic cells by these alloreactive NK cells (Ruggeri et al. (1999) Blood 94:333- 9; Ruggeri et al. (2002) Science 295:2097-100). So blocking interaction of LLTl with inhibitory CDl 61 on NK cells should have the same effect. As regards infections, it is clear that NK cells play an important role in immune responses to infections, either directly acting during the initial phase called innate immune response, but also in the second phase or at least in the initiation of the second phase called adaptive immune responses. In this second phase, it is also important to consider the manipulation of LLTl or CD161 not only on NK cells but also on T cells. Thus, an impairment of CDl 61 expression on NK and T cells in patients infected with HIV-I has been reported (Alter et al. (2004) J Immunol 173:5305-5311; Jacobs et al. (2004) Clin Immunol 24:281-286). Further, the modulation of LLTl and CD 161 interaction should also have an impact in human Cytomegalovirus infection, Hepatitis B and C virus infection, Ebola virus infection, Dengue, Yellow fever, Listeriosis , Tuberculosis, Cholera, Malaria, Leishmaniasis, and Trypanosoma infection. NK cells and T cells also play a major role in autoimmunity: • In diabetes, for example, there is evidence for a role of NK cells and NK receptors on T cells regulating diabetes. Autoimmune (type 1) diabetes results from a loss of beta cells that is mediated by self-reactive T cells. It has been reported that NK cells mediate the protective effects of CFA (Complete Freund's Adjuvant) possibly through the down-regulation of autoreactive CTL and stimulation of NK cells, which represents a novel approach to the prevention of autoimmune diabetes (Lee et al. (2004) J. Immunol. 172:937-42). Further, the NK cell NKG2D receptor is an activating receptor and represents a new therapeutic target for autoimmune type I diabetes (Ogasawara et al. (2004) Immunity 20:757-67). It thus can be postulated that modulating LLT1/CD161 interaction should improve the outcome by inhibiting autoagressive T cells or modulating the activity of NK cells. • In rheumatoid arthritis (RA), a subset of NK cells is expanded within inflamed joints. The functional properties of these NK cells renders them good candidates for a role in interacting ■ with the macrophage/monocyte population within the joint, thus amplifying the production of proinflammatory cytokines (Dalbeth & Callan (2002) Arthritis Rheum 46:1763-72). Further, expansion of an unusual subset of CD4+ CD28" T cells that infiltrates the tissue lesions has been shown to occur in RA. These cells are KIR/KAR+, CD94-, CDl 61+. CD4+ CD 161+ T cells are present in follicular microstructures typical of rheumatoid synovitis, which links them to involvement in the disease.(Warrington et al. (2001) Arthritis Rheum 44:13-20). So it -can be postulated that modulating LLT1/CD161 interaction should improve the outcome by inhibiting autoagressive T cells and NK cells. Celiac disease (an inflammatory bowel disease) is a gluten-induced enteropathy characterized by the presence of gliadin-specific CD4(+) T cells in the lamina propria and by a prominent intraepithelial T -cell infiltration of unknown mechanism. In active celiac disease, ' there is a specific and selective increase of IELs (IntraEpithelial Lymphocytes) expressing CD94, but IELs also express CDl 61 both in the disease and in healthy controls. CD 161+ T cells are abundant in human intestinal epithelium and most likely play a role in local immunity. Modulation of the activity of these CD161-expressing cells should therefore provide for a new therapeutic strategy against this pathology. • Psoriasis is a ThI disease. It is triggered when activated immunocytes infiltrate the skin, subsequently inducing prominent epidermal thickening and angiogenesis. Engrafting human skin onto SCID mice is a good animal model for psoriasis. When activated CD4+ T cells but not CD8+ T cells were injected into engrafted pre-psoriasis skin, typical psoriatic plaques were created (Nickoloff and Wrone-Smith (1999) Am J Pathol 155:145-158). The blood- derived psoriatic immunocytes in the skin graft expressed CD 161. In this case modulation of the activity of these CD161 -expressing cells should also provide for a new therapeutic strategy against this pathology. CDl 61 expressing cells seem also to be involved in Crohn disease, in Lupus and in multiple sclerosis. It has been shown that CD 161 expression by CD56+ T cells was significantly increased in women with implantation failures when compared with normal controls (Ntrivalas et al. (2005) Am J Reprod Immunol 53:215-221; Yamada et al. (2004) Am J Reprod Immunol 51:241-247). It is therefore likely that a dysregulation of this mechanism could be responsible for major pregnancy failures. In xenotransplantation, inhibition of NK-mediated immunity through the modulation of LLT1-CD161 interaction should restrict graft rejection. Further, expression of LLTl in graft cells, such as porcine endothelial cells in case of porcine xenotransplantation, may directly protect sensitive graft cells from human NK cell-mediated xenogeneic cytotoxicity. Accordingly, a method for screening compounds with biological activity comprises providing cells expressing CDl 61 and/or LLTl receptors at their cell surface, contacting the cells respectively with LLTl or fragments thereof, and/or with CD161 or fragments thereof in the presence of the test compound, and determining whether the presence of the compound affects the binding of LLTl or fragments thereof, and/or with CDl 61 or fragments thereof to CDl 61 and/or LLTl expressing cells respectively. Preferably the cells expressing CD 161 and/or LLTl receptors in the method according to this aspect of the invention do not naturally express the receptors, and most preferably they are non-human cells. The cells are preferably stable transfectants, that is to say they contain nucleic acid material expressing CDl 61 and/or LLTl stably integrated into their genome. • Alternatively, CD 161 and LLTl, or fragments thereof, need not necessarily be expressed at the surface of cells. They can be linked to a solid support or both be in solution. The present invention also relates to a method for screening compounds liable to be used for the treatment of cancers, infectious diseases, such as viral, bacterial, or parasitic diseases, autoimmune diseases, inflammatory diseases, allergic reactions, pregnancy failures, or organ and bone marrow transplantation rejection, characterized in that the compounds liable to be used for the treatment of the above mentioned pathologies are selected on their properties of inducing CD161 internalization from CD161-expressing cells. The expression "CDl 61 internalization from CDl 61 -expressing cells" means that compounds are selected on their capacity of inducing downregulation of CDl 61 molecules which are expressed at the surface, i.e. in the plasma membrane, of said CD161-expressing cells. Downregulation of CD 161 molecules can be evidenced according to numerous methods well known to the man skilled in the art, such as those described in the Examples. In a preferred embodiment of the above mentioned screening method, the compounds are selected: - on their properties of inhibiting the binding between CD161 or fragments thereof, and LLTl or fragments thereof, and - on their properties of inducing CD 161 internalization from CD 161 -expressing cells. In a preferred embodiment of the above-mentioned screening method, the compounds to screen are antibodies, such as anti-CD161 or anti-LLTl antibodies, or fragments thereof such as Fab, F(ab)'2 or scFv fragments. The antibodies to screen are in particular monoclonal antibodies and/or humanized antibodies. In another preferred embodiment of the above-mentioned screening method the compounds to screen are aptamers. The present invention also relates to an isolated compound comprising at least two LLTl molecules or fragments thereof. The expression "isolated" relates to a compound comprising at least two LLTl molecules or fragments thereof which is not part of a biological membrane. In particular, the isolated compound according to the invention is soluble. In such a compound, the LLTl molecules or fragments thereof are linked together, either directly or indirectly, via linker molecules, through covalent or non-covalent bonds. According to a preferred embodiment of the isolated LLTl comprising compound, said compound comprises LLTl fragments as represented by SEQ ID NO: 4. According to a another preferred embodiment of the isolated LLTl comprising compound, the LLTl fragments are linked to anchoring moieties, such as a γ-immuno globulin Fc domain or biotin, and associated together via a linker molecule which binds to said anchoring molecules, such as protein A or an avidin protein. Anchoring molecules may also be made of, or covered with a magnetic material or a biologically inert material, such as silicon, and be in the form of beads. According to yet another preferred embodiment of the isolated LLTl comprising compound, said compound also comprises: - toxic or cytolytic moieties, such as toxins, radioactive molecules, recombinant active molecules or chemicals, - marker moieties, such as fluorophores, magnetic beads, or haptens. The present invention also relates to a pharmaceutical composition comprising as active substance, a compound liable to bind to CD161 or a compound liable to bind to LLTl, in association with a pharmaceutically acceptable carrier. According to a preferred embodiment of the invention, the above-defined pharmaceutical composition comprises as active substance, a mammalian LLTl or fragments thereof, as compounds liable to bind to CD 161, or a mammalian CD 161 or fragments thereof, as compounds liable to bind to LLTl, in association with a pharmaceutically acceptable carrier. According to another preferred embodiment of the invention, the above-defined pharmaceutical composition comprises as active substance, - a human LLTl, in particular as represented by SEQ ID NO: 2, or - a fragment of said human LLTl corresponding to the extracellular domain of LLTl, in particular as represented by SEQ ID NO: 4 According to another preferred embodiment of the invention, the above-defined pharmaceutical composition comprises as active substance an above-defined LLTl comprising compound. In another preferred embodiment, the present invention relates to an above-defined „ pharmaceutical composition comprising as active substance: - a human CDl 61, in particular as represented by SEQ ID NO: 6, or - a fragment of said human CDl 61 corresponding to the extracellular domain of CDl 61, in particular as represented by SEQ ID NO: 8. In a particularly preferred embodiment, the present invention relates to an above- defined pharmaceutical composition, comprising as active substance an anti-CD161 or anti- LLTl antibody, in particular a humanized anti-CD161 or anti-LLTl antibody. Such antibodies may be monoclonal or polyclonal antibodies, which can be easily obtained according to methods well-known to the man skilled in the art. In particular, monoclonal antibodies can be produced from hybridomas. Such hybridomas can be obtained by fusion of an antibody secreting cell, such as a B cell, with an immortalized cell, according to Kohler and Milstein. Nature. 256:495-497, 1975 or to Buttin et al. GLUT, Top. Microbiol. Immun. 81 :27-36, 1978, for example. As intended herein, the fragments of such antibodies, such as Fab, F(ab)'2 or scFv fragments, are also considered as antibodies. Humanized antibodies can be for example chimeric antibodies, in which, if appropriate, constant parts of animal antibodies, such as mouse or rabbit antibodies, are replaced by the corresponding parts of human antibodies, such as the Fc fragment for instance (Sharon et al, Nature. 309:364-367, 1984; Neuberger et al, Nature. 314:268-271, 1985). Alternatively, the complex determining region (CDR) of the animal antibodies can be grafted onto human antibodies, such as described for instance in US patent number 5,824,307, a process called "antibody reshaping" (Jones et al. Nature, 321:522-525, 1986). Another alternative technique is to produce human antibodies in mice using transgenic and transomic animals (Bruggemann et al, PNAS. 86:6709-6713, 1989; Richa et al, Science. 245:175-177, 1989; Capecchi, Trends in Genetics. 5:70-76, 1989). The present invention also relates to a pharmaceutical composition comprising as active substance, single or multiple stranded nucleic acids encoding mammalian, in particular human, LLTl or fragments thereof, or their complementary strands in the case of single stranded nucleic acids, in particular as represented respectively by SEQ ID NO: 1 and SEQ ID NO: 3, optionally comprised in a mammalian expression vector, in association with a pharmaceutically acceptable carrier. The present invention also relates to a pharmaceutical composition comprising as active substance, single or multiple stranded nucleic acids encoding mammalian, in particular ' human, CD 161 or fragments thereof, or their complementary strands in the case of single stranded nucleic acids, in particular as represented respectively by SEQ ID NO: 5 and SEQ ID NO: 7, optionally comprised in a mammalian expression vector, in association with a pharmaceutically acceptable carrier. Such nucleic acids or vectors are useful for various purposes, such as: - the in vivo, ex vivo or in vitro production of the proteins they encode, i.e. LLTl or fragments thereof and CDl 61 or fragments thereof; - the in vivo, ex vivo or in vitro production or delivery of antisens nucleic acids, which are liable to inhibit the expression of CD 161 or LLTl; - the in vivo, ex vivo or in vitro production or delivery of interference nucleic acids, such as siRNA, which are liable to inhibit the expression of CDl 61 or LLTl. The present invention also relates to the use of a compound liable to bind to CD161 or a compound liable to bind to LLTl, for the manufacture of a medicament intended for the prevention or the treatment of cancers, infectious diseases, such as viral, bacterial, or parasitic diseases, autoimmune diseases, inflammatory diseases, allergic reactions, pregnancy failures, or organ and bone marrow transplantation rejection. In a preferred embodiment, the present invention relates to the above-defined use of a mammalian LLTl or fragments thereof, as compounds liable to bind to CD161, or of a mammalian CDl 61 or fragments thereof, as compounds liable to bind to LLTl. In another preferred embodiment, the invention relates to the above-defined use, of - a human LLTl, in particular as represented by SEQ ID NO: 2, or - a fragment of said human LLTl corresponding to the extracellular domain of LLTl, in particular as represented by SEQ ID NO: 4. In a particularly preferred embodiment the present invention relates to the above- defined use, of a LLTl comprising compound as defined above. In another embodiment, the present invention relates to the above-defined use, of: - a human CD161, in particular as represented by SEQ ID NO: 6, or - a fragment of said human CDl 61 corresponding to the extracellular domain of CDl 61, in particular as represented by SEQ ID NO: 8. In a preferred embodiment, the invention also relates to the above-defined use, of an anti-CD161 or anti-LLTl antibody, in particular a humanized anti-CD 161 or anti-LLTl antibody. The present invention also relates to the use of single or multiple stranded nucleic acids encoding mammalian, in particular human, LLTl or fragments thereof, or their ^complementary strands in the case of single stranded nucleic acids, in particular as represented respectively by SEQ ID NO: 1 and SEQ ID NO: 3, optionally comprised in a mammalian expression vector, for the manufacture of a medicament intended for the prevention or the treatment of cancers, infectious diseases, such as viral, bacterial, or parasitic diseases, autoimmune diseases, inflammatory diseases, allergic reactions, pregnancy failures, or organ and bone marrow transplantation rejection. The present invention also relates to the use of single or multiple stranded nucleic acids encoding mammalian, in particular human, CD 161 or fragments thereof, or their complementary strands in the case of single stranded nucleic acids, in particular as represented respectively by SEQ ID NO: 5 and SEQ ID NO: 7, optionally comprised in a mammalian expression vector, in association with a pharmaceutically acceptable carrier, for the manufacture of a medicament intended for the prevention or the treatment of cancers, infectious diseases, such as viral, bacterial, or parasitic diseases, autoimmune diseases, inflammatory diseases, allergic reactions, pregnancy failures, or organ and bone marrow transplantation rej ection. As intended herein cancers notably comprise leukaemia, lymphoproliferative cancers, cervical cancers, prostate cancers, lung cancers, breast cancers, and melanoma. Infectious diseases comprise in particular HIV infection, human Cytomegalovirus infection, Hepatitis B and C infection, Ebola virus infection, Dengue, Yellow fever, Listeriosis, Tuberculosis, Cholera, Malaria, Leishmaniasis, and Trypanosoma infection. Autoimmune, inflammatory and allergic diseases notably comprise type I diabetes, rheumatoid arthritis, Lupus, inflammatory bowel diseases, such as celiac disease or Crohn disease, psoriasis, and multiple sclerosis. Pregnancy failures notably relate to implantation disorders. Organ transplantation in particular relates to xenotransplantations. DESCRIPTION OF THE FIGURES

Figure IA and Figure IB Production of chimeric LLTl-Fc fusion protein Figure IA: Purified LLTl-Fc fusion protein was visualized on a 10% SDS-PAGE gel using reducing and nonreducing conditions yielding a ~55kDa and -110 kDa protein respectively. Figure IB: Treatment of purified LLTl-Fc with peptide N-glycosidase F (PNGase F) yielded a ~48kDa protein.

Figure 2 LLTl interacts with CDl 61 receptor LLTl multimer were generated using LLTl-Fc dimer conjugated to protein A-biotin and streptavidin-APC. Saturating amount of purified human IgG were added to block the remaining free protein A sites. A multimer control was generated by incubating ProteinA- bio tin with saturing amount of purified human IgG. LLTl and Ctrl multimers were used to stain HEK293T cell untransfected or transfected with CD161-pIRES2-EGFP or LLTl-pIRES2-EGFP vectors. LLTl multimer bound to HEK293T cells transfected with CD161 but not to HEK293T expressing LLTl or untransfected cells. Control multimer did not bind to any of these cells.

Figure 3A and Figure 3B LLTl multimer staining to CD161+ cells is inhibited by anti-CD161 antibodies Figure 3 A: LLTl multimer was used to stain HEK293T cells transfected with CD161- pIRES2-EGFP or LLTl-pIRES2-EGFP vectors in the absence or presence of 3μg of irrelevant isotype IgG control (DX22) or anti-CD161 antibody (DX12). Similar blocking was seen using 191B8 anti-CD161 antibody. Figure 3B: LLTl multimer was used to stain HEK293T cells transfected with CD161- ρIRES2-EGFP (squares) or LLTl-pIRES2-EGFP (circles) vectors in the presence of increasing concentration of irrelevant isotype IgG control (DX22) (open symbols) or anti- CD161 antibody (DX12) (closed symbols). Figure 4A and Figure 4B Interaction of polyclonal CDIoI+NK and T cells with target cells expressing LLTl results in a selective downregulation of CD161 on NK and T cells HEK293T cells and HeIa cells transfected or not with LLTl were incubated with a polyclonal human NK and T cell population for 2 and 4 hours in complete medium at 37°C (ratio 1:1). Cells were fixed and stained with anti-CD 161 antibodies to monitor CDl 61 level of expression at the cell surface of NK and T cells. Figure 4A: % of CD161+ NK cells incubated with the indicated targets for 2 and 4 hours. Figure 4B: level of CD161 expression (mean fluorescence) on NK cells incubated with the indicated targets for 2 and 4 hours. Similar downregulation of CDl 61 on T cells is observed.

Figure 5 LLTl -induced downregulation of CD161 is blocked by anti-CD161 antibodies HeIa cells transfected or not with LLTl were incubated with a polyclonal human NK and T cell population for 2 hours in complete medium at 37°C (ratio 1:1) in the presence of anti- CD161 (DX12) mAb or isotype IgG control (5μg/ml). Cells were fixed and stained with anti- CD161 antibody 191B8 (which recognizes a different epitope to DX12) to monitor CD161 level of expression at the cell surface of NK and T cells. % of CD161+ NK cells is shown and similar blocking is observed on T cells (data not shown). Identical blocking of LLTl -induced downregulation of CD161 is detected when anti-CD161 mAb 191B8 is used to block and anti-CD161 mAb DX12 is used to stain cells (data not shown).

Figure 6 LLTl-induced downregulation of CD161 on NK and T cells needs cell-to-cell contacts HEK293T cells transfected or not with LLTl were incubated either in normal wells or transwells with a polyclonal human NK and T cell population for 4 hours in complete medium at 370C (ratio 1 :1). CDl 61 level of expression on NK and T cells was monitored using anti-CD161 mAb. Figure 7 Expression of LLTl in HEK293T cells partially inhibits polyclonal IL-2 activated NK cell- mediated cytotoxicity.

293T cells transfected or not with LLTl were labelled with 51chromium and incubated with polyclonal IL-2 activated NK cells for 18 hours at an E:T ratio of 20:1. Release of 51chromium was measured in the supernatant and % of specific lysis of the targets calculated.

Figure 8 LLTl interaction with CD161 results in a downregulation of CD161 on NK cells Figure 8 represents the CDl 61 cell surface expression (mean fluorescence intensity (MFI)) upon time on NK cells either untreated (squares) or fixed (circles) and incubated with untransfected ClR cells (white), or with CRl cells expressing LLTl (black). Cell surface CDl 61 decreased upon time on NK cells incubated with ClR-LLTl transfectants but not when NK cells were fixed prior exposure or when they were incubated with ClR cells.

Figure 9 LLTl -induced downregulation of CD161 on NK cells required cell-to-cell contacts Figure 9 represents CDl 61 cell surface expression (vertical axis, mean fluorescence in %) on NK cells incubated with untransfected 293T cells (white squares) or with LLTl -expressing 293T cells (black squares) in settings allowing intercellular contacts or forbidding them (transwells). NK cells were incubated with 293T or 293T-LLT1 cells for 4 hr at 37°C. The mean fluorescence intensities obtained after exposure to negative control 293T cells were arbitrarily set as 100%.

Figure 10 LLTl -induced downregulation of CD 161 on NK cells is not affected by bafilomycin and chloroquine treatment. Figure 10 represents CDl 61 cell surface expression (hatched bar) and total CD161 expression (black bar) (vertical axis, mean fluorescence in %) on NK cells incubated with untransfected ClR cells or LLTl expressing ClR transfectants untreated or treated with two agents blocking intracellular degradation (chloroquine and bafilomycin Al). Treatment with DMSO 1% is the control for bafilomycin diluted in DMSO. Surface and total CD161 expression decreased after exposure of NK cells to ClR-LLTl transfectants in the presence or absence of bafilomycin Al and chloroquine. NK cells were preincubated with bafilomycin Al (0.5 μM) and chloroquine (33μM) for lhr at 37°C, followed by incubation for 4hr with ClR or ClR- LLTl cells. The mean fluorescence intensities obtained after exposure to negative control ClR cells were arbitrarily set as 100%. CD56 surface expression (white bar) and total expression (grey bar) were monitored and used as a negative control marker.

Figure HA and Figure HB LLTl transcripts Figures HA and HB represent an electrophoresis gel showing the results of RT-PCR experiments carried on various cell lines (on top) for detecting the presence of LLTl transcripts. The GAPDH transcript is used as an internal control.

Figure 12 LLTl interaction with CD 161 results in an inhibition of NK cell-mediated cytotoxicity NK cell-mediated cytotoxicity was measured in a chromium release assay against ClR and ClR-LLTl target cells. Figure 12 represents the percentage of untransfected ClR cells (squares) or LLTl -expressing ClR cells (circles) lysed by NK cells, in the absence of monoclonal antibody (black squares and circles), in the presence of an irrelevant antibody (mIgG2a, grey quares and circles) or in the presence of an anti-CD161 mAb (αCDlβl, white squares and circles). Effector (NK cells) to Target (ClR or ClR-LLTl) ratio used were 10 :1, 5 :1, 1 :1. Isotype control mIgG2a and anti-CD161 mAb (191B8) were used at lOμg/ml. LLTl expression significantly inhibited NK cell-mediated cytotoxicity which was restored by blocking anti-CD161 mAb.

Figure 13 LLTl interaction with CDl 61 results in an inhibition of NK cell-mediated cytotoxicity: redirected killing Figure 13 represents the percentage of untransfected FcR-bearing P815 cells (squares) or LLTl -expressing P815 cells (circles) lysed by NK cells in the presence of an irrelevant antibody (mIgG2a, dark grey quares and circles), of an anti-CD161 mAb (αCD161, light grey squares and circles), of an irrelevant antibody and an activating anti-CD 16 antibody (αCD16 + mIgG2a, black squares and circles), or of an anti-CD161 mAb and an activating anti-CD 16 antibody (αCD16 + αCD161, white squares and circles). Effector (NK cells) to Target (ClR or ClR-LLTl) ratio used were 5 :1, 1 :1, 0.5:1. Redirected killing could be inhibited by expression of LLTl on the FcR expressing cell line P815 or by addition of anti-CD161 mAb together with anti-CD 16 used to induce NK cell cytotoxicity.

Figure 14 LLTl interaction with CD161 results in an inhibition of NK cell-mediated cytotoxicity

LAMP-I or CD107a is a marker of activation induced degranulation, a necessary precursor of cyto lysis. Figure 14 represents CDl 61 surface expression (vertical axis) versus CD 107a surface expression (horizontal axis) on polyclonal NK cells incubated alone (medium), in the presence of ClR cells or in the presence of LLTl-expressingClR cells, as measured by flow cytometry. The percentage of CDl 61+/CD 107 a+ cells is indicated. NK cells were incubated with ClR or ClR-LLTl cells for lhr in the presence of anti-CD107a mAb followed by 4hr in the presence of Brefeldin A (10 μg/ml). Effector to target ratio was 1: 5. Cells were fixed and stained for surface expression of CD 161.

Figure 15 LLTl interaction with CD161 inhibits IFN-γ production by NK cells Figure 15 represents IFNγ production (vertical axis) versus CDl 61 surface expression (horizontal axis) in polyclonal NK cells incubated alone (medium), in the presence of ClR cells, in the presence of LLTl -expressing ClR cells, or in the presence of PMA/ionomycin (positive control), as measured by flow cytometry. The percentage of IFNγ+/CD161+ cells is indicated. The polyclonal NK cell population was incubated for 4hr at 37°C with ClR or ClR-LLTl target cells in the presence of brefeldin A (lOμg/ml). Effector to target ratio was 1: 5. NK cells were identified by cell surface staining using CDl 61 and CD56 markers and IFN-γ producing cells by intracellular staining.

Figure 16 LLTl interaction with CD161 inhibits IFN-γ production by NK cells Figure 16 represents IFNγ production (vertical axis) versus CDl 61 surface expression (horizontal axis) in polyclonal NK cells incubated with ClR cells (upper panels) or LLTl- expressing ClR cells (lower panels) in the presence of a control antibody (+Ctrl Ig2a at 10 ~*μg/mi) or an anti-CD161 antibody (191B8) at 10 μg/ml, 1 μg/ml or 0.1 μg/ml. The percentage of IFN7+ NK cells is indicated.

Figure 17 Ligation of CD 161 enhances CD3 triggered-IFN-γ production by T cells A polyclonal T cell population was incubated for 4hr at 37°C with P815 or P815-LLT1 target cells in the presence of increasing concentrations of anti-CD3 and brefeldin A (lOμg/ml). Figure 17 represents IFNγ production (vertical axis) versus CD161 surface expression (horizontal axis) in polyclonal T cells incubated with P815 cells (upper panels) or LLTl- expressing P815 cells (lower panels) either without any antibody (no mAb), or in the presence of an anti-CD3 antibody at 5 μg/ml, 500 ng/ml or 50 ng/ml. Effector to target ratio was 1 : 2.5. T cells were identified by cell surface staining using CD3 and CDl 61 markers and IFN-γ producing cells by intracellular staining. The percentage of IFNγf/ T cells is indicated.

Figure 18 Ligation of CDl 61 enhances CD3 triggered-IFN-γ production by T cells A polyclonal T cell population was incubated for 4hr at 37°C with plate-bound anti-CD3 (500 ng/ml) alone or together with isotype control mIgG2a (5μg/ml) or anti-CD161 191B8 (5μg/ml) and in the presence of brefeldin A (lOμg/ml). Figure 18 represents IFN7 production (vertical axis) versus CDl 61 surface expression (horizontal axis) in polyclonal T cells incubated on plate-bound anti-CD3 antibody (+αCD3), anti-CD3 antibody and a control antibody (+αCD3 +oϋgG2a) and anti-CD3 antibody and a an anti-CD161 antibody (+oCD3 + αCD161). T cells were identified by cell surface staining using CD3 and CDl 61 markers and IFN-γ producing cells by intracellular staining. The percentage of IFN-γ + T cells is indicated.

Figure 19 CD161 cell surface level of expression is upregulated with increasing concentrations of IL-12 Figure 19 represents CDl 61 surface expression (vertical axis, mean fluorescence intensity (MFI)) on polyclonal NK cells, incubated for 48hr at 37°C in medium supplemented with the indicated concentrations of recombinant human IL- 12. EXAMPLES

Example 1 Construction of LLTl multimeric complexes

Extracellular domains (residues 60-191) (SEQ ID NO: 4) of LLTl (AF133299, Q9UHP7) (SEQ ID NO: 2) were fused to the human IgGl FC portion and the chimeric protein produced in transfected eukaryotic cells. The soluble fusion LLTl-Fc protein secreted in the media by the transfected cells was purified on protein-A-Sepharose column and conjugated to proteinA-biotin to generate a multimer. This multimer was labelled with a fluorescent marker using Streptavidin-APC. This multimer can then be used to visualise interactions between LLTl and cell surface receptors.

Method

LLTl AA sequence (191 amino acid residues) (AFl 33299, Q9UHP7): MHDSNNVEKDITPSELPANPGCLHSKEHSIKATLIWRLFFLIM FLTΠVCGMVAALSAIRANCHOEPSVCLOAACPESWIGFORK CFYFSDDTKNWTSSQRFCDSQDADLAQVESFQELNFLLRYK GPSDHWIGLSREQGQPWKWINGTEWTRQFPILGAGECAYLN DKGASSARHYTERKWICSKSDIHV (SEQ ID NO: 2) Amino acid residues underlined were removed in the LLTl-Fc chimeric molecule.

Full length LLTl cDNA was amplified by reverse transcription (RT)-PCR from polyclonal NK total RNA extracted with TRIzol Reagent (15596-26, Invitrogen) using an oligo(dT) primer for the RT and the primers LLTl-F (5'- ATGCATGAC AGTAACAATGTGG-3') (SEQ ID NO: 9) and LLTl-R (5'- TAGTTGGGGCTTTGCTGTAA-3') (SEQ ID NO: 10) for the PCR (95°C for 2min, 35 cycles with 95°C for 45 sec, 60°C for 45 sec, 720C for 1 min and a final elongation at 72°C for 10 min). The PCR product was cloned into pGEMT-easy vector (A1360, Promega). Codons corresponding to the extracellular domains (60-191) of LLTl were amplified by PCR using full length LLTl cDNA as a template and the following primers: FLLTIBamHI- 5 '-AAGGGGATCCGAGAGCTAACTGCCATCAAGAGCC-S' (SEQ ID NO: 11) RLLTlNotl- 5'-CTCGAGCGGCCGCCTAGACATGTATATCTGATTTGG-S' (SEQ ID NO: 12) The PCR product was subcloned into pcDNA3 vector in frame with the signal sequence of human CD5 and the Fc portion of human IgGl. The final construct was sequenced. LLTl-Fc dimers were generated by transiently transfecting eukaryotic HEK293T cells. The chimeric protein was released in the media and purified onto a ProteinA-Sepharose column. Purity was assessed by SDS-PAGE electrophoresis (Figure IAV Peptide N- glycosidase F (PNGase F) treatment was performed according to the manufactor instruction (New England Bio labs) (Figure IB). Briefly, 5 μg of purified LLTl-Fc was denatured in 0.5 % SDS, 1% β-mercaptoethanol denaturing buffer at 1000C for 10 minutes, and digested with 2000 Unit of PNGase F in 1% NP40, 50 mM sodium phosphate pH 7.5 for 1 hour at 37°C.

A multimeric complex was generated by incubating the dimers with ProteinA-biotin (Pierce) at a ratio of 10:1 for 30 minutes at room temperature. Saturating amount of purified human IgG were added to block the remaining free protein A sites. These complexes were conjugated with Streptavidin-APC to generate a fluorescent reagent that can be used in flow cytometry analysis. A multimer control was generated by incubating ProteinA-biotin with saturing amount of purified human IgG.

Example 2 Binding of LLTl multimer

Flow cytometry using the LLTl-Fc-ProteinA-biotin-Streptavidin-APC multimer (LLTl -multimer) was performed to identify the receptor of LLTl. LLTl -multimer stained HEK293T cells stably transfected with human CD161 (Ul 1276, Q12918) cloned into pIRES2-EGFP (clontech) while it did not stain untransfected HEK293T or HEK293T stably transfected with LLTl-pIRES2-EGFP (Figure 2). A control multimer did not stain any of the cells. The specificity of the binding of LLTl -multimer to CDl 61 was further confirmed by ■^addition of anti-CD161 antibodies which abolished the staining while irrelevant isotype control antibodies did not (Figures 3 A and 3B).

Example 3 Interaction of LLTl with CDl 61 induces a downregulation of CD 161

Human polyclonal NK cells were isolated from peripheral mononuclear cells of healthy donors by positive selection using CD56 MicroBeads (Miltenyi Biotec). NK cells were then stimulated with PHA-P (1 ug/ml, SIGMA), irradiated PBMC and B-EBV feeder cells in Iscove's Modification- ATL medium (bio west) supplemented with L-glutamine (2mM, Gibco), Penicillin-Streptomycin (Gibco) and rIL-2 (500 U/ml). To assess the role of LLTl interaction with CD161, various NK cell targets were transfected to express LLTl. The presence of LLTl transcripts was checked by RT-PCR. No LLTl transcripts could be detected in 293T, ClR, K562, HeIa, 721.221, P815 cell lines but transcripts were detected in their respective LLTl transfectants (Figures HA, HB). Furthermore, a LLTl transcript was detected in an IL-2 activated polyclonal NK cell population and a PHA-Blast population (97% T cells, 3% NK cells (Figure HA)). HEK293T cells, HeIa cells and ClR cells, transfected or not with LLTl were incubated with a polyclonal human NK and T cell population for 5-10 minutes up to 4-5 hours in complete medium at 370C. Cells were fixed and stained with anti-CD161 antibodies to monitor CDl 61 level of expression at the cell surface of NK and T cells. A specific and significant loss of CDl 6I+ cells is observed when NK and T cells are incubated with cells expressing LLTl and not with untransfected cells (Figures 4 A, 4B, and 8). This loss is observed as early as 10 to 15 minutes (Figure 8) and is abolished by addition of anti-CD 161 mAbs (Figure S). This loss of CD 161+ NK and T cells is not due to release by shedding of a soluble form of LLTl blocking binding of anti-CD161 antibodies as no effect on CD161 expression was observed when the incubation (4 hours at 37°C) was performed using transwells (Figures 6_and_9), which means that LLTl-induced downregulation of CD161 on NK cells required cell-to-cell contact. This latter result therefore demonstrates that LLTl induces a downregulation of the CD 161 receptor. Cross-linking of CD161 with anti-CD161 mAb coated onto a plate or bound to FcR- expressing P815 cells could induce some downregulation of CD 161. Experiments using Bafilomycin Al and chloroquine, which block lysosomal degradation, then suggested that downregulation of CDl 61 was not due to degradation of the receptor (Figure 10) Briefly, surface and total CDl 61 expression decreased after exposure of NK cells to ClR-LLTl transfectants in the presence or absence of bafilomycin Al and chloroquine. NK cells were preincubated with bafilomycin Al (0.5 μM) and chloroquine (33μM) for lhr at 37°C, followed by incubation for 4hr with ClR or ClR-LLTl cells. The mean fluorescence intensities obtained after exposure to negative control ClR cells were arbitrarily set as 100%. CD56 was monitored and used as a negative control marker.

Example 4 Expression of LLTl in 293T cells partially inhibits polyclonal IL-2 activated NK cell- mediated cytotoxicity

293T cells transfected or not with LLTl were labelled with 51chromium and incubated with polyclonal IL-2 activated NK cells for 18 hours at an E:T ratio of 20:1. Release of 51 chromium was measured in the supernatant and specific lysis of the targets calculated (Figure 7). A 28.3 % lysis decrease was observed for LLTl expressing cells with respect to control cells.

Example 5 Expression of LLTl on target cells significantly inhibits polyclonal IL-2 activated NK cell- mediated cytotoxicity

NK cell-mediated cytotoxicity of an IL-2 activated polyclonal NK cell population which expressed CDl 61 on more than 90% of the cells was also measured in a chromium release assay against ClR and ClR-LLTl target cells. Killing could be restored by addition of blocking anti-CD161 mAb (DX12 or 191B8). Isotype control mIgG2a and anti-CD161 mAb (191B8) were used at lOμg/ml. LLTl expression significantly inhibited NK cell-mediated cytotoxicity which was restored with the blocking anti-CD 161 mAb (Figure 12). Similarly, redirected killing could be inhibited by expression of LLTl on the FcR expressing cell line P815, or by addition of anti-CD161 mAb to the P815 cell line (see Example 3), together with suboptimal concentration of anti-CD 16 (0.5 μg/ml) used to induce MK cell cytotoxicity. Killing could be singificantly or completely inhibited by expression of LLTl on P815 or addition of anti-CD 161 mAb (5 μg/ml). (Figure 13) In addition, anti-CD161 mAb alone was able to inhibit killing of P815 or P815-LLT1 in assays where polyclonal NK cells showed some cytotoxicity to these targets. While cytoxicity was assessed in a chromium release assay in the experiments depicted above, inhibition was also measured using a flow cytometry test monitoring CD 107a (lamp-1) induction on the NK cell surface when degranulation occurs (Figure 14). Briefly, polyclonal NK cells were incubated with ClR or ClR-LLTl cells for lhr in the presence of anti-CD107a mAb followed by 4hr in the presence of Brefeldin A (10 μg/ml) blocking intracellular secretion. Effector to target ratio was 1 : 5. Cells were fixed and stained for surface expression of CD 161.

Example 6 LLTl interaction with CD161 on NK cells inhibits IFN-γ production

The effect of LLTl interaction with CDl 61 on IFN-γ production by NK cells was assessed in a 4hr assay where polyclonal NK cells were incubated with ClR or ClR-LLTl target cells in the presence of brefeldin A (lOμg/ml). Effector to target ratio was 1: 5. NK cells were identified by cell surface staining using CD3 ,CD56 and CD161 markers and IFN-γ producing cells by intracellular staining (Figure 15). A similar assay as above was performed in the presence of isotype control mIgG2a (lOμg/ml) or anti-CD161 191B8 at increasing concentrations (0.1; 1, and lOμg/ml) (Figure 16). Expression of LLTl significantly blocked IFN-γ production by NK cells. This inhibition was specific as saturating concentrations of blocking anti-CD161 mAb 191B8 could restore IFN-γ production.

Example 7 LLTl interaction with CD161 on T cells enhances CD3 -triggered IFN-γ production

LLTl interaction with CD161 induces a downregulation of CD 161 on T cells similarly to NK cells. But as opposed to NK cells, LLTl interaction with CD161 expressed by T cells triggers a costimulatory signal resulting in an increase of IFN-γ production triggered by CD3 ligation (Figures 17 and 18). Briefly, a polyclonal T cell population was incubated for 4hr at 37°C with P815 or P815-LLT1 target cells in the presence of increasing concentrations of anti-CD3 and brefeldin A (lOμg/ml). Effector to target ratio was 1 : 2.5. T cells were identified by cell surface staining using CD3 and CD161 markers and IFN-γ producing cells by intracellular staining (Figure 17). A polyclonal T cell population was incubated for 4hr at 37°C with plate-bound anti-CD3 (500 ng/ml) alone or together with isotype control mIgG2a (5μg/ml) or anti-CD161 191B8 (5μg/ml) and in the presence of brefeldin A (lOμg/ml). IFN-γ production by T cells was measured by intracellular staining (Figure 18).

Example 8 IL-12 upregulates CD161 on NK and T cells in vitro.

Culture of polyclonal NK cells or T cells in the presence of increasing concentrations of IL-12 upregulates CD161 on their cell surface (Figure 19). Briefly, polyclonal NK cells were incubated for 48hr at 37°C in medium supplemented with the indicated concentrations of recombinant human IL-12.