Identification of unique binding interactions between certain antibodies and the human b7.1 and b7.2 co-stimulatory antigens

This application is a continuation-in-part of U.S. application Ser. No. 08/746,361, filed Nov. 8, 1996, in turn a continuation-in-part of U.S. application Ser. No. 08/487,550, filed Jun. 7, 1995, both of which are incorporated by reference in their entirety herein.

The present invention relates to the identification and use of monoclonal antibodies which are specific to B7.1 antigens (CD80). More specifically, the present invention relates to the identification and use of monoclonal antibodies or primatized forms thereof which are capable of inhibiting the binding of human B7.1 antigen to a CD28 receptor and which are not capable of inhibiting the binding of B7.1 to a CTLA-4 receptor. Thus, the invention relates to the identification and use of monoclonal antibodies and primatized forms thereof which recognize specific sites on the B7.1 antigen which are exclusive of CTLA-4 receptor binding.

The invention further relates to monoclonal antibodies or primatized forms thereof which recognize specific sites on the human B7.1 antigen and are capable of inhibiting IL-2 production.

Also, the present invention relates to pharmaceutical compositions containing monoclonal or primatized antibodies specific to human B7.1 and their use as immunosuppressants by modulating the B7:CD28 pathway, e.g., for the treatment of autoimmune disorders, and the prevention of organ rejection.

The clinical interface between immunology, hematology, and oncology has long been appreciated. Many conditions treated by the hematologist or oncologist have either an autoimmune or immuno-deficient component to their pathophysiology that has led to the widespread adoption of immunosuppressive medications by hematologists, whereas oncologists have sought immunologic adjuvants that might enhance endogenous immunity to tumors. To date, these interventions have generally consisted of nonspecific modes of immunosuppression and immune stimulation. In addition to the limited efficacy of these interventions, toxicities secondary to their nonspecificity have also limited their overall success. Therefore, alternative strategies have been sought.

Elucidation of the functional role of a rapidly increasing number of cell surface molecules has contributed greatly to the integration of immunology with clinical hematology and oncology. Nearly 200 cell surface antigens have been identified on cells of the immune and hematopoietic systems (Schlossman S F, Boumsell L, Gilks J M, Harlan T, Kishimoto, C Morimoto C, Ritz J., Shaw S, Silverstein R L, Springer T A, Tedder T F, Todd R F:CD antigens (1993), Blood 83:879, 1994). These antigens represent both lineage-restricted and more widely distributed molecules involved in a variety of processes, including cellular recognition, adhesion, induction and maintenance of proliferation, cytokine secretion, effector function, and even cell death. Recognition of the functional attributes of these molecules has fostered novel attempts to manipulate the immune response. Although molecules involved in cellular adhesion and antigen-specific recognition have previously been evaluated as targets of therapeutic immunologic intervention, recent attention has focused on a subgroup of cell surface molecules termed co-stimulatory molecules (Bretscher P: “The two-signal model of lymphocyte activation twenty-one years later.” Immunol. Today 13:73 (1992); Jenkins M K, Johnson J G: “Molecules involved in T-cell co-stimulation.” Curr Opin Immunol 5:351 (1993); Geppert T, Davis L. Gur H. Wacholtz M. Lipsky P: “Accessory cell signals involved in T-cell activation.” Immunol Rev 117:5 (1990); Weaver C T, Unanue E R: “The co-stimulatory function of antigen-presenting cells.” Immunol Today 11:49 (1990); Stennam R M, Young J W: “Signals arising from antigen-presenting cells.” Curr Opin Immunol 3:361 (1991)). Co-stimulatory molecules do not initiate but rather enable the generation and amplification of antigen-specific T-cell responses and effector function (Bretscher P: “The two-signal model of lymphocyte activation twenty-one years later.” Immunol. Today 13:73 (1992); Jenkins M K, Johnson J G: “Molecules involved in T-cell co-stimulation.” Curr Opin Immunol 5:351 (1993); Geppert T, Davis L. Gur H. Wacholtz M. Lipsky P: “Accessory cell signals involved in T-cell activation.” Immunol Rev 117:5 (1990); Weaver C T, Unanue E R: “The co-stimulatory function of antigen-presenting cells.” Immunol Today 11:49, (1990); Stennam R M, Young J W: “Signals arising from antigen-presenting cells.” Curr Opin Immunol 3:361 (1991); June C H, Bluestone J A, Linsley P S, Thompson C D: “Role of the CD28 receptor in T-cell activation.” Immunol Today 15:321 (1994)).

Recently, one specific co-stimulatory pathway termed B7:CD28 has been studied by different research groups because of its significant role in B- and T-cell activation (June C H, Bluestone J A, Linsley P S, Thompson C D: “Role of the CD28 receptor in T-cell activation.” Immunol Today 15:321 (1994); June C H, Ledbetter J A: “The role of the CD28 receptor during T-cell responses to antigen.” Annu Rev Immunol 11:191 (1993); Schwartz R H: “Co-stimulation of T lymphocytes: The role of CD28, CTLA-4, and B7/BB1 in interleukin-2 production and immunotherapy.” Cell 71:1065-1068 (1992); Jenkins M K, Taylor P S, Norton S D, Urdahl K B: “CD28 delivers a co-stimulatory signal involved in antigen-specific IL-2 production by human T cells.” Journal of Immunology 147:2461-2466 (1991)). Since this ligand:receptor pathway was discovered four years ago, a large body of evidence has accumulated suggesting that B7:CD28 interactions represent one of the critical junctures in determining immune reactivity versus anergy (June C H, Bluestone J A, Linsley P S, Thompson C D: “Role of the CD28 receptor in T-cell activation.” Immunol Today 15:321 (1994); June C H, Ledbetter J A: “The role of the CD28 receptor during T-cell responses to antigen.” Annu Rev Immunol 11:191 (1993); Schwartz R H: “Co-stimulation of T lymphocytes: The role of CD28, CTLA-4, and B7/BB1 in interleukin-2 production and immunotherapy.” Cell 71:1065-1068 (1992); Cohen J: “Mounting a targeted strike on unwanted immune responses” (news; comment). Science 257:751 (1992); Cohen J: “New protein steals the show as ‘co-stimulator’ of T cells” (news; comment). Science 262:844 (1993)).

In particular, the role of the human B7 antigens, i.e., human B7.1 (CD80) and B7.2 (CD86), has been reported to play a co-stimulatory role in T-cell activation. See, e.g., Gimmi C D, Freeman, G J, Gribben J G, Sugita K, Freedman A S, Morimoto C, Nadler L M: “B-cell surface antigen B7 provides a costimulatory signal that induces T cells to proliferate and secrete interleukin 2.” Proc. Natl. Acad. Sci. (USA) 88:6575-6579 (1991).

1. B7.1 and B7.2 Co-stimulatory Role in T Cell Activation

The elaboration of a successful immune response depends on a series of specific interactions between a T cell and an antigen presenting cell. Although the essential first step in this process depends upon the binding of antigen to the T cell receptor, in the context of the MHC class II molecule (Lane, P. J. L., F. M. McConnell, G. L. Schieven, E. A. Clark, and J. A. Ledbetter, (1990), “The Role of Class II Molecules in Human B Cell Activation.” The Journal of Immunology 144:3684-3692), this interaction alone is not sufficient to induce all the events necessary for a sustained response to a given antigen (Schwartz, R.H. (1990), “A Cell Culture Model for T Lymphocyte Clonal Anergy.” Science 248:1349; Jenkins, M.K. (1992), “The Role of Cell Division in the Induction of Clonal Anergy.” Immunology Today 13:69; Azuma, M., M. Cayabyab, D. Buck, J. H. Phillips, and L. L. Lanier (1992), “Involvement of CD28 in MHC-unrestricted Cytotoxicity Mediated by a Human Natural Killer Leukemia Cell Line.” The Journal of Immunology 149:1115-1123; Azuma, M., M. Cayabyab, D. Buck, J. H. Phillips, and L. L. Lanier (1992), “CD28 Interaction with B7 Costimulates Primary Allogeneic Proliferative Responses and Cytotoxicity Mediated by Small Resting T Lymphocytes.” J. Exp. Med. 175:353-360); S.D. Norton, L. Zuckerman, K. B. Urdahl, R. Shefner, J. Miller, and M. K. Jenkins (1992), “The CD28 Ligand, B7, Enhances IL-2 Production by Providing a Costimulatory Signal to T Cells.” The Journal of Immunology 149:1556-1561; R. H. Schwartz (1992), “Costimulation of T Lymphocytes: The Role of CD28, CTLA-4, and B7/BB1 in Interleukin-2 Production and Immunotherapy.” Cell 71:1065-1068).

The involvement of certain other co-stimulatory molecules is necessary (Norton, S. D., L. Zuckerman, K. B. Urdahl, R. Shefner, J. Miller, and M. K. Jenkins (1992), “The CD28 Ligand, B7, Enhances IL-2 Production by Providing A Costimulatory Signal to T Cells.” The Journal of Immunology 149:1556-1561)). “The homodimers CD28 and CTLA-4 expressed on T cells” (June, C. H., J. A. Ledbetter, P. S. Linsley, and C. B. Thompson (1990), “Role of the CD28 Receptor in T-Cell Activation.” Immunology Today 11:211-216; Linsley, P. S., W. Brady, M. Umes, L. S. Grosmaire, N. K. Damle, and J. A. Ledbetter (1991), “CTLA-4 is a Second Receptor for the B Cell Activation Antigen B7.” J. Exp. Med. 174:561)), together with B7.1 (CD80) and B7.2 (CD86) expressed on antigen presenting cells, are major pairs of co-stimulatory molecules necessary for a sustained immune response (Azuma, M., H. Yssel, J. H. Phillips, H. Spits, and L. L. Lanier (1993), “Functional Expression of B7/BB1 on Activated T Lymphocytes.” J. Exp. Med. 177:845-850; Freeman, G. J., A. S. Freedman, J. M. Segil, G. Lee, J. F. Whitman, and L M. Nadler (1989), “B7, A New Member of the Ig Superfamily with Unique Expression on Activated and Neoplastic B Cells.” The Journal of Immunology 143:2714-2722; Hathcock, K. S., G. Laslo, H. B. Dickler, J. Bradshaw, P. Linsley, and R. J. Hodes (1993), “Identification of an Alternative CTLA-4 Ligand Costimulatory for T Cell Activation.” Science 262:905-911; Hart, D. N. J., G. C. Starling, V. L. Calder, and N. S. Fernando (1993), “B7/BB-1 is a Leucocyte Differentiation Antigen on Human Dendritic Cells Induced by Activation.” Immunology 79:616-620). It can be shown in vitro that the absence of these co-stimulatory signals leads to an aborted T cell activation pathway and the development of unresponsiveness to the specific antigen, or anergy. (See, e.g., Harding, F. A., J. G. McArthur, J. A. Gross, D. M. Raulet, and J. P. Allison (1992), “CD28 Mediated Signaling Co-stimulates Murine T Cells and Prevents Induction of Anergy in T Cell Clones.” Nature 356:607-609; Gimmi, C. D., G. J. Freeman, J. G. Gribben, G. Gray, and L. M. Nadler (1993); “Human T-Cell Clonal Anergy is Induced by Antigen Presentation in the Absence of B7 Costimulation.”, Proc. Natl. Acad. Sci. 90:6586-6590; Tan, P., C. Anasefti, J. A. Hansen, J. Melrose, M. Brunvand, J. Bradshaw, J. A. Ledbetter, and P. S. Linsley (1993), “Induction of Alloantigen-specific Hyporesponsiveness in Human T Lymphocytes by Blocking Interaction of CD28 with Its Natural Ligand B7/BB1.” J. Exp. Med. 177:165-173). Achievement of in vivo tolerance constitutes a mechanism for immunosuppression and a viable therapy for organ transplant rejection and for the treatment of autoimmune diseases. This has been achieved in experimental models following the administration of CTLA-4Ig (Lenschow, D. J., Y. Zeng, R. J. Thistlethwaite, A. Montag, W. Brady, M. G. Gibson, P. S. Linsley, and J. A. Bluestone (1992), “Long-Term Survival of Xenogeneic Pancreatic Islet Grafts Induced by CTLA-4Ig.” Science 257:789-795).

The molecules B7.1 and B7.2 can bind to either CD28 or CTLA-4, although B7.1 binds to CD28 with a Kd of 200 Nm and to CTLA-4 with a 20-fold higher affinity (Linsley, P. S., E. A. Clark, and J. A. Ledbetter (1990), “T-Cell Antigen CD28 Mediates Adhesion with B Cells by Interacting with Activation Antigen B7/BB-1.” Proc. Natl. Acad. Sci. 87:5031-5035; Linsley et al (1993), “The Role of the CD28 receptor during T cell responses to antigen,” Annu. Rev. Immunol. 11:191-192; Linesley et al (1993), “CD28 Engagement by B7/BB-1 Induces Transient Down-Regulation of CD28 Synthesis and Prolonged Unresponsiveness to CD28 Signaling,” The Journal of Immunology 150:3151-3169). B7.1 is expressed on activated B cells and interferon induced monocytes, but not resting B cells (Freeman, G. J., G. S. Gray, C. D. Gimmi, D. B. Lomarrd, L-J. Zhou, M. White, J. D. Fingeroth, J. G. Gribben, and L M. Nadler (1991). “Structure, Expression and T Cell Costimulatory Activity of the Murine Homologue of the Human B Lymphocyte Activation Antigen B7,” J. Exp. Med., 174:625-631). B7.2, on the other hand, is constitutively expressed at very low levels on resting monocytes, dendritic cells and B cells, and its expression is enhanced on activated T cells, NK cells and B lymphocytes (Azuma, M. D. Ito, H. Yagita, K. Okumura, J. H. Phillips, L. L. Lanier, and C. Somoza 1993, “B70 Antigen is a Second Ligand for CTLA-4 and CD28,” Nature, 366:76-79). Although B7.1 and B7.2 can be expressed on the same cell type, their expression on B cells occurs with different kinetics (Lenschow, D. J., G. H. Su, L. A. Zuckerman, N. Nabavi, C. L. Jellis, G. S. Gray, J. Miller, and J. A. Bluestone (1993), “Expression and Functional Significance of an Additional Ligand for CTLA-4,” Proc. Natl. Acad. Sci., USA, 90:11054-11058; Boussiotis, V. A., G. J. Freeman, J. G. Gribben, J. Daley, G. Gray, and L. M. Nadler (1993), “Activated Human B Lymphocytes Express Three CTLA-4 Counter-receptors that Co-stimulate T-Cell Activation.” Proc. Natl. Acad. Sci., USA, 90:11059-11063). Further analysis at the RNA level has demonstrated that B7.2 mRNA is constitutively expressed, whereas B7.1 mRNA is detected 4 hours after activation and initial low levels of B7.1 protein are not detectable until 24 hours after stimulation (Boussiotis, V. A., G. J. Freeman, J. G. Gribben, J. Daley, G. Gray, and L. M. Nadler (1993), “Activated Human B Lymphocytes Express Three CTLA-4 Counter-receptors that Co-stimulate T-Cell Activation,” Proc. Natl. Acad. Sci., USA, 90:11059-11063). CTLA-4/CD28 counter receptors, therefore, may be expressed at various times after B Cell activation.

More recently, it has been suggested that the second T cell associated co-receptor CTLA-4 apparently functions as a negative modulator to override and prevent a runaway immune system (Krummel M, Allison J: “CD28 and CTLA-4 have opposing effects on the response of T cells to stimulation.” J. Exp. Med. 182:459-466 (1995)). The CTLA-4 receptor plays a critical role in down regulating the immune response, as evidenced in CTLA-4 knockout mice. Knockout mice born without the ability to express the CTLA-4 gene die within 3-4 weeks of severe lymphoproliferative disorder (Tivol E A, Borriello G, Schweitzer A N, Lynch W P, Bluestone J A, Sharpe A H: “Loss of CTLA-4 leads to massive lymphoproliferation and fatal multiorgan tissue destruction, revealing a critical negative regulatory role of CTLA-4.” Immunity 3:541-547 (1995)). CTLA-4 is thought to function through signaling mechanisms linked to induction of apoptosis (Gribben J G, Freeman G J, Boussiotis V A, Rennert P, Jellis C L, Greenfield E, Barber M, Restivo Jr. VA, Ke X, Gray G S, Nadler L M: “CTLA-4 mediates antigen specific apoptosis of human T cells.” Proc. Natl. Acad. Sci. USA 92:811-815 (1995)), triggered through as yet undefined ligand binding to specific cites on the receptor. It has been shown in vitro that the blocking of the B7.1/B7.2 dependent co-stimulatory signals in various ways leads to an aborted T cell activating pathway and the development of unresponsiveness to the specific antigen (Lederman S, Chess L, Yellin M J: “Murine monoclonal antibody (5c8) recognizes a human glycoprotein on the surface of T-lymphocytes, compositions containing same.” U.S. Pat. No. 5,474,771 (Dec. 12, 1995); Linsley P S, Ledbetter J A, Damle N K, Brady W: “Chimeric CTLA4 receptor and methods for its use.” U.S. Pat. No. 5,434,131 (Jul. 18, 1995); Harding, 1992; Gimmi C D, Freeman G J, Bribben J G, Gray G, Nadler L M: “Human T-cell clonal anergy is induced by antigen presentation in the absence of B7 costimulation.” Proc. Natl. Acad. Sci. (USA) 90:6586-6590 (1993); Tan P, Anasetti C, Hansen J A, Melrose J, Brunvand M, Bradshaw J, Ledbetter J A, Linsley P S: “Induction of alloantigen-specific hyporesponsiveness in human T lymphocytes by blocking interaction of CD28 with its natural ligand B7/BB1.” J. Exp. Med. 177:165-173 (1993)). Achievement of in vivo tolerance, anergy, or depleting of antigen-specific T cells would constitute a mechanism for immunosuppression and a viable therapy for organ transplant rejection or plausible treatment for autoimmune diseases.

The differential temporal expression of B7.1 and B7.2 suggests that the interaction of these two molecules with CTLA-4 and/or CD28 deliver distinct but related signals to the T cell (LaSalle, J. M., P. J. Tolentino, G. J. Freeman, L. M. Nadler, and D. A. Hafler, (1992), “CD28 and T Cell Antigen Receptor Signal Transduction Coordinately Regulate Interleukin 2 Gene Expression In Response to Superantigen Stimulation,” J. Exp. Med., 176:177-186; Vandenberghe, P., G. J. Freeman, L. M. Nadler, M. C. Fletcher, M. Kamoun, L. A. Turka, J. A. Ledbetter, C. B. Thompson, and C. H. June (1992), “Antibody and B7/BB1-mediated Ligation of the CD28 Receptor Induces Tyrosine Phosphorylation in Human T Cells,” The Journal of Experimental Medicine 175:951-960)). The exact signaling functions of CTLA-4 and CD28 on the T cell are currently unknown (Janeway, C. A., Jr. and K. Bottomly, (1994), “Signals and Signs for Lymphocyte Responses,” Cell 76.275285). However, it is possible that one set of receptors could provide the initial stimulus for T cell activation and the second, a sustained signal to allow further elaboration of the pathway and clonal expansion to take place (Linsley, P. S., J. L. Greene, P. Tan, J. Bradshaw, J. A. Ledbetter, C. Anasetti, and N. K. Damle, (1992), “Coexpression and Functional Cooperation of CTLA-4 and CD28 on Activated T Lymphocytes,” J. Exp. Med. 176:1595-1604). The current data supports the two-signal hypothesis proposed by Jenkins and Schwartz (Schwartz, R. H. (1990), “A Cell Culture Model for T Lymphocyte Clonal Anergy,” Science 248:1349; Jenkins, M.K., (1992), “The Role of Cell Division in the Induction of Clonal Anergy,” Immunology Today 13:69)) that both a TCR and co-stimulatory signal are necessary for T cell expansion, lymphokine secretion and the full development of effector function (Greenan, V. and G. Kroemer (1993), “Multiple Ways to Cellular Immune Tolerance,” Immunology Today 14:573). The failure to deliver the second signal results in the inability of T cells to secrete IL-2 and renders the cell unresponsive to antigen.

Structurally, both B7.1 and B7.2 contain extracellular immunoglobulin superfamily V and C-like domains, a hydrophobic transmembrane region and a cytoplasmic tail (Freeman, G. J., J. G. Gribben, V. A. Boussiotis, J. W. Ng, V. Restivo, Jr., L. A. Lombard, G. S. Gray, and L. M. Nadler (1993), “Cloning of B7.2: A CTLA-4 Counter-receptor that Co-stimulates Human T Cell Proliferation,” Science 262:909). Both B7.1 and B7.2 are heavily glycosylated. B7.1 is a 44-54 kD glycoprotein comprised of a 223 amino acid extracellular domain, a 23 amino acid transmembrane domain, and a 61 amino acid cytoplasmic tail. B7.1 contains 3 potential protein kinase phosphorylation sites. (Azuma, M., H. Yssel, J. H. Phillips, H. Spits, and L. L. Lanier, (1993), “Functional Expression of B7/BB1 on Activated T Lymphocytes,” J. Exp. Med. 177:845-850). B7.2 is a 306 amino acid membrane glycoprotein. It consists of a 220 amino acid extracellular region, a 23 amino acid hydrophobic transmembrane domain and a 60 amino acid cytoplasmic tail (Freeman, G. J., A. S. Freedman, J. M. Segil, G. Lee, J. F. Whitman, and L M. Nadler (1989), “B7, A New Member of the Ig Superfamily with Unique Expression on Activated and Neoplastic B Cells,” The Journal of Immunology 143:2714-2722). Although both B7.1 and B7.2 genes are localized in the same chromosomal region (Freeman, G. J., D. B. Lombard, C. D. Gimmi, S. A. Brod, L Lee, J. C. Laning, D. A. Hafler, M. E. Dorf, G. S. Gray, H. Reiser, C. H. June, C. B. Thompson, and L. M. Nadler (1992), “CTLA-4 and CD28 mRNA are Coexpressed in Most T Cells After Activation,” The Journal of Immunology 149:3795-3801; Schwartz, R. H. (1992), “Costimulation of T Lymphocytes: The Role of CD28, CTLA-4, and B7/BB1” in Selvakumar, A., B. K. Mohanraj, R. L. Eddy, T. B. Shows, P. C. White, C. Perrin, and B. Dupont (1992), “Genomic Organization and Chromosomal Location of the Human Gene Encoding the B-Lymphocyte Activation Antigen B7,” Immunogenetics 36:175-181), these antigens do not share a high level of homology. The overall homology between B7.1 and B7.2 is 26% and between murine B7.1 and human B7.1 is 27% (Azuma, M., H. Yssel, J. H. Phillips, H. Spits, and L. L. Lanier (1993), “Functional Expression of B7/BB1 on Activated T Lymphocytes,” J. Exp. Med. 177:845-850; Freeman, G. J., A. S. Freedman, J. M. Segil, G. Lee, J. F. Whitman, and L M. Nadler (1989), “B7, A New Member of the Ig Superfamily with Unique Expression on Activated and Neoplastic B Cells,” The Journal of Immunology 143:2714-2722). Although alignment of human B7.1 human B7.2 and murine B7.1 sequences shows few stretches of lengthy homology, it is known that all three molecules bind to human CTLA-4 and CD28. Thus, there is most likely a common, or closely homologous region shared by the three molecules that may be either contiguous or conformational. This region may constitute the binding site of the B7.1 and B7.2 molecules to their counter-receptors. Antibodies raised against these epitopes could potentially inhibit the interaction of B7 with its counter-receptor on the T cell. Furthermore, antibodies that cross-reacted with this region on both B7.1 and B7.2 molecules would potentially have practical advantages over antibodies directed against B7.1 or B7.2 separately.

2. Blockade of the B7/CD28 Interaction

Blocking of the B7/CD28 interaction offers the possibility of inducing specific immunosuppression, with potential for generating long lasting antigen-specific therapeutic effects. Antibodies or agents that temporarily prevent this interaction may be useful, specific and safe clinical immunosuppressive agents, with potential for generating long term antigen-specific therapeutic effects.

Antibodies to either B7.1 or B7.2 have been shown to block T cell activation, as measured by the inhibition of IL-2 production in vitro (DeBoer, M., P. Parren, J. Dove, F. Ossendorp, G. van der Horst, and J. Reeder (1992), “Functional Characterization of a Novel Anti-B7 Monoclonal Antibody,” Eur. Journal of Immunology 22:3071-3075; Azuma, M., H. Yssel, J. H. Phillips, H. Spits, and L. L. Lanier (1993), “Functional Expression of B7/BB1 on Activated T Lymphocytes,” J. Exp. Med. 177:845-850)). However, different antibodies have been shown to vary in their immunosuppressive potency, which may reflect either their affinity or epitope specificity. A possible explanation for this may reside in the ability of some antibodies to block only the binding of B7 to CD28, while promoting apoptosis or some other form of negative signaling through the CTLA-4 receptor in activated T cells. Some antibodies to B7.1 or B7.2 may, in fact, hinder the activity of CTLA-4 by cross-reacting with the CTLA-4 binding domain. CTLA-4Ig fusion protein and anti-CD28 Fabs were shown to have similar effects on the down regulation of IL-2 production.