SummaryFunctional interactions between T and B lymphocytes are necessary for optimal activation of an immune response . Recently, the T lymphocyte receptor CD28 was shown to bind the B7 counter-receptor on activated B lymphocytes, and subsequently to costimulate interleukin 2 production and T cell proliferation . CTLA-4 is a predicted membrane receptor from cytotoxic T cells that is homologous to CD28 and whose gene maps to the same chromosomal band as the gene for CD28 . It is not known, however, ifCD28 and CTLA-4 also share functional properties . To investigate functional properties of CTLA4, we have produced a soluble genetic fusion between the extracellular domain of CTLA-4 and an immunoglobulin Cy chain . Here, we show that the fusion protein encoded by this construct, CTLA4Ig, bound specifically to B7-transfected Chinese hamster ovary cells and to lymphoblastoid cells. CTLA4Ig also immunoprecipitated B7 from cell surface 125 1-labeled extracts of these cells . The avidity of 1251-labeled B7Ig fusion protein for immobilized CTLA4Ig was estimated (Kd -12 nM) . Finally, we show that CTLA4Ig was a potent inhibitor of in vitro immune responses dependent upon cellular interactions between T and B lymphocytes . These findings provide direct evidence that, like its structural homologue CD28, CTLA4 is able to bind the 137-counter-receptor on activated B cells. Lymphocyte interactions involving the B7 counter-receptor are functionally important for alloantigen responses in vitro .

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SummaryA successful immune response requires intercellular contact between T and B lymphocytes . We recently showed that CD28, a T cell surface protein that regulates an activation pathway, could mediate intercellular adhesion with activated B cells by interaction with the B7 antigen . Here we show that CD28 is the primary receptor for B7 on activated peripheral blood T cells, that CD28 binds to B7 in the absence of other accessory molecules, and that interaction between CD28 and B7 is costimulatory for T cell activation . To characterize the binding of CD28 to B7, we have produced genetic fusions of the extracellular portions of B7 and CD28, and immunoglobulin (Ig) Cy1 chains . 121 I-labeled B7 Ig bound to CD28-transfected Chinese hamster ovary (CHO) cells, and to immobilized CD28 Ig with a Kd -200 nM . B7 Ig also inhibited CD28-mediated cellular adhesion . The function of CD28-B7 interactions during T cell activation was investigated with soluble fusion proteins and with B7-transfected CHO cells. Immobilized B7 Ig and B7+ CHO cells costimulated T cell proliferation . Stimulation of T cells with B7+ CHO cells also specifically increased levels of interleukin 2 transcripts. These results demonstrate that the CD28 signaling pathway could be activated by B7, resulting in increased T cell cytokine production and T cell proliferation . Cellular interactions mediated by B7 and CD28 may represent an important component of the functional interactions between T and B lymphoid cells. I t has long been known that interactions between T and B lymphocytes play a central role in regulating an immune response (1, 2). More recent studies have shown that activation and differentiation of both T and B lymphocytes is dependent upon direct intercellular (cognate) interactions between these cell types. While the specificity of T. cell-B cell interactions is determined by interaction between the TCR/ CD3 complex (3-5) and antigen associated with class II MHC molecules on B cells (6), interactions between other (accessory) cell adhesion molecules are also necessary for a full immune response (7-9). Interactions between accessory receptors and their counter-receptors may increase the avidity of cellular interactions (7) ; control lymphocyte localization and migration (10); and have direct signaling functions (7,11,12) during lymphocyte activation. Accessory receptors and their counter-receptors involved in T-B cell interactions include (reviewed in reference 7): CD2 and LFA 3, CD4 and class II MHC molecules, LFA1, and ICAM-1 and ICAM-2 .The T cell homodimer, CD28, a member of the Ig superfamily (.13), has been shown in studies using mAbs to have an accessory function during T cell activation (14). Anti-CD28 mAbs have been shown to costimulate T cell proliferation induced by a number of polyclonal stimuli (reviewed in reference 14) . These mAbs also inhibited alloantigen and soluble antigen-specific T cell responses (15, 16), indicating that CD28-mediated signalling may be crucial during these responses. CD28-mediated T cell activation...

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Antigen-specific T cell activation depends on T cell receptor-ligand interaction and costimulatory signals generated when accessory molecules bind to their ligands, such as CD28 to the B7 (also called BB1) molecule. A soluble fusion protein of human CTLA-4 (a protein homologous to CD28) and the immunoglobulin (lg) G1 Fc region (CTLA4lg) binds to human and murine B7 with high avidity and blocks T cell activation in vitro. CTLA4lg therapy blocked human pancreatic islet rejection in mice by directly affecting T cell recognition of B7+ antigen-presenting cells. In addition, CTLA4lg induced long-term, donor-specific tolerance, which may have applications to human organ transplantation.