FUNCTIONAL-ANALYSIS OF FAS SIGNALING IN-VIVO USING SYNTHETIC INDUCERSOF DIMERIZATION

Background: Genetic abnormalities in the Fas receptor or its trimeric ligand, Fast, result in massive T-cell proliferation and a lupus-like autoimmune syndrome, which was initially attributed to excessive lymph oproliferation but is now ascribed to the absence of Fas-mediated cell death. Although Fas is normally expressed on most thymocyte, negative selection seems to be unperturbed in Fas-deficient (lpr) mice, This s uggests that Pas has an important function in peripheral, but not thym ic, T cells. Results: To explore the Pas-mediated cell death pathway b oth in vitro and in vivo, we used conditional alleles of the Fas recep tor that can be triggered by an intracellularly active chemical induce r of dimerization known as FK1012. We found that membrane attachment i s important for Fas function and, unlike previous results with anti-fa s monoclonal antibodies, we show that dimerization is sufficient to tr igger apoptosis. Finally, the administration of FK1012 in vivo to tran sgenic animals expressing the conditional Fas receptor in thymocytes d emonstrates that sensitivity to Fas-mediated apoptosis is restricted t o CD4(+) CD8(+) thymocytes. Conclusions: Here, we describe the first i n vivo application of non-toxic, cell-permeable synthetic ligands to r egulate signal transduction in transgenic mice expressing a conditiona l receptor, Using this system, we show that the Fas pathway is restric ted to double-positive thymocytes in vivo, consistent with recent in v itro findings with thymocytes. This method promises to be useful not o nly for developmental studies involving cell ablation, but also for st udies involving the regulation of a wide variety of signaling molecule s.