Recent work in our lab has demonstrated that Fas ligand (FasL or CD95L), best known for delivering a death signal through its interaction with Fas, is also capable of positive reverse signaling. Thus, CD8+ T cells lacking functional FasL exhibit depressed antigen-specific proliferation relative to their FasL+ counterparts, and FasIgG fusion proteins can inhibit the response of wildtype CD8+ cells. These findings place FasL among the growing number of tumor necrosis factor superfamily members capable of bipolar signaling -- both of delivering signals through their receptors and of transducing signals inward upon ligand binding. The first goal of the proposed experiments is to explore the nature of FasL-mediated costimulation by investigating what regions of the FasL molecule are required and whether binding of FasL to molecules other than Fas can initiate reverse signaling. The second overall goal is to determine the in vivo relevance of this reverse signaling, both for mature peripheral T cells and for developing thymocytes. The Specific Aims are as follows: Specific Aim 1: To determine whether the cytoplasmic tail of FasL is required for reverse signaling, to pinpoint critical residues in this domain, and to identify associated molecules. Specific Aim 2: To explore the in vivo role of FasL signaling in the regulation of CD8+ T cell responses and intrathymic T cell differentiation. Specific Aim 3: To continue to investigate FasL-mediated costimulation induced by a molecule other than Fas, and to identify and characterize this molecule by expression cloning.