In VitroThymocyte Maturation Is Associated with Reduced Cellular Susceptibility to Fas-Mediated Apoptosis

We have developed a novel system in which the susceptibility of murine thymocytes to Fas-mediatedapoptosis can be modulated. Thymocyte susceptibility to Fas decreases underin vitroculture conditions that promote aspects of thymocyte maturation. The hyporesponsive state is specific for the Fas pathway, since cellular susceptibility to other apoptotic stimuli is not reduced. Hyporesponsiveness is not associated with alterations in the thymocyte subset distribution, decreased expression of full-length Fas protein, or alterations in FADD, Bcl-2, or Bcl-XLexpression. Hyporesponsiveness is overcome by increasing the strength of the Fas cross-linking stimulus, leading us to propose that reduced thymocyte susceptibility to apoptosis results from altered Fas signaling. The block in Fas signaling resides proximal to ceramide generation, since Fas-hyporesponsive thymocytes are susceptible to ceramide-induced apoptosis. Further characterization of Fas signaling in thesein vitrocultured thymocytes may facilitate the identification of factors regulating the susceptibility of wild-type lymphocytes to Fas.