Signaling through CD28 family receptors: An analysis of CD80- and CD86-dependent pathways

The B7 family members CD80 (B7-1) and CD86 (B7-2) are two ligands for the CD28 receptor family proteins, CD28 and CTLA-4 (CD152). CD28 and CTLA-4 are critical regulators of T cell activation. CD28 is a major costimulatory molecule found on most T lymphocytes, while CD152 expression is induced on T cells after activation. Many in vivo animal studies demonstrate that selective blockade of these two CD28 ligands differentially alters the immune response, affecting the outcome of autoimmune diseases and the ability to fight certain types of infections. Furthermore, the binding of CD80 to CD28 differs from the binding of CD86 to CD28 and the expression of CD80 and CD86 differs both quantitatively and qualitatively on antigen presenting cells (APC). Thus, there are compelling reasons to believe that CD80 and CD86 might transmit quantitatively, if not qualitatively, different signals through CD28 into T cells. Although CTLA-4 binds the same two ligands as CD28, it appears to function as a negative regulator of T cell proliferation. In this thesis, CHO cells expressing either human CD80 (CHO-CD80) or human CD86 (CHO-CD86) were compared to each other and to anti-CD28 mAb stimulation of both purified human peripheral T cells and Jurkat T cells. In addition, cells from CTLA-4-/- mice were studied to determine the biochemical consequences of the absence of CTLA-4. I have shown that the B7 family members CD80 and CD86 are both able to induce the activation of several intracellular signaling proteins. However, I have also identified several substrates which are differentially regulated after stimulation with CD80 and CD86. Together with environmental influences such as strength of T cell receptor signal and cytokine milieu, these biochemical differences may provide a mechanism by which the two CD28 ligands differentially contribute to T cell effector functions. I have shown that cells from mice rendered genetically deficient in CTLA-4 expression have activated intracellular signaling cascades, confirming that CTLA-4 functions as a negative regulator of T cell activation.