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 .

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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Signals delivered to B cells via CD40 can synergize with those provided by other B cell surface receptors to induce B cell proliferation and antibody class switching as well as modulate cytokine production and cell adhesion. Recently, it has been shown that the ligand for CD40 is a cell surface protein of approximately 39 kDa expressed by activated T cells, gp39. Here we report on the isolation and characterization of a cDNA clone encoding human gp39, a type II membrane protein with homology to TNF, and the construction and characterization of a soluble recombinant form of gp39. COS cell transfectants expressing gp39 synergized with either anti‐CD20 mAb or PMA to drive strong B cell proliferation and alone were able to drive B cells to proliferate weakly. In all cases the B cell proliferation induced by gp39‐expressing COS cells was reduced to background levels by the addition of soluble CD40. Unlike gp39‐expressing COS cells, recombinant soluble gp39 was not mitogenic alone and required co‐stimulation to drive B cell proliferation. These results suggest that B cells require a second signal besides gp39‐CD40 to drive proliferation and that soluble gp39 alone in a non‐membrane bound form is able to provide co‐stimulatory signals to B cells.

One of the P2-integrins found on hematopoietic ceUs is lymphocyte function-associated antigen 1 (LFA-1), a lymphocyte/myeloid cell-specific receptor that binds to members of the intercelular adhesion molecule (ICAM) family on antigen-presenting cels. Stimulation of LFA-1 with antibodies or purified ICAMs induces augmentation of T-cell antigen receptor (TCR)-directed T-cell responsiveness. In the present study, LFA-1 was shown to be linked to the tyrosine kinase signalig pathway that stimulates tyrosine phosphorylation and activation of phospholipase C-y1 (PLC-y1