IN-VITRO THYMOCYTE 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-mediated apoptosis can be modulated. Thymocyte susc eptibility to Fas decreases under in vitro culture conditions that pro mote aspects of thymocyte maturation. The hyporesponsive state is spec ific for the Fas pathway, since cellular susceptibility to other apopt otic stimuli is not reduced. Hyporesponsiveness is not associated with alterations in the thymocyte subset distribution, decreased expressio n of full length Fas protein, or alterations in FADD, Bcl-2, or Bcl-X- L expression. Hyporesponsiveness is overcome by increasing the strengt h of the Fas cross-linking stimulus, leading us to propose that reduce d thymocyte susceptibility to apoptosis results from altered Fas signa ling. The block in Fas signaling resides proximal to ceramide generati on, since Fas-hyporesponsive thymocytes are susceptible to ceramide-in duced apoptosis. Further characterization of Fas signaling in these in vitro cultured thymocytes may facilitate the identification of factor s regulating the susceptibility of wild-type lymphocytes to Fas. (C) 1 998 Academic Press.