During the interaction of T cells with antigen presenting cells (APCs), receptors and intracellular proteins translocate into the contact region known as the Immunological Synapse (IS). In CD4 T cells IS formation can readily be rendered independent of CD4 by increasing the amount of antigenic peptide whereas the same does not hold true for CD8 T cells. In addition, CD4 rapidly moves out of the center of the IS whereas we do not observe any such clearance of CD8. In this proposal we shall examine the IS of CD8 T cells interacting with APCs. In particular we shall focus on the requirement of the CD4/8 co-receptor and on the movement of both antigenic and non-antigenic MHC class I molecules. We shall also examine SMAC formation in immature thymocytes during a CD8 mediated, TCR independent interaction with APCs. It has been demonstrated that MHC I tetramers can bind to CD8 on DP thymocytes in a TCR independent manner. This binding event has been attributed to a developmentally regulated difference in the glycosylation of CD8 on DP thymocytes compared to CD8 single positive thymocytes and peripheral T cells. In contrast there is no evidence for TCR independent binding of MHC class II tetramers to CD4 on DP thymocytes. The implications and/or biological relevance of this event have not been determined. Recently we have found that antibody mediated cross-linking of CDS, but not CD4, results in rapid apoptosis of CD4 CD8 (DP) thymocytes. Treatment with the phorbol ester, PMA, prevents the induction of apoptosis resulting from CD8 cross-linking. We hypothesize that the ligation of CD8 by physiological ligands in the absence of TCR engagement leads to the death of DP thymocytes during T cell development. In this application we shall further characterize the induction of apoptosis in DP thymocytes by CD8 antibodies and also investigate whether classical or non-classical MHC class I molecules can induce apoptosis in DP thymocytes in a TCR independent manner. These studies will incorporate 4 specific aims: (1) To determine whether CD8 behaves as a co-receptor within the immunological synapse, for the activation of peripheral T cells (2) To determine the biochemical events leading to the CD8 mediated death of DP thymocytes; (3) To determine the mechanism by which treatment with PMA prevents CD8 mediated death in DP thymocytes; (4) To determine whether CD8 mediated death has a physiological function.
