FUNCTIONAL EXPRESSION OF FAS AND FAS LIGAND ON HUMAN GUT LAMINA PROPRIA T-LYMPHOCYTES - A POTENTIAL ROLE FOR THE ACIDIC SPHINGOMYELINASE PATHWAY IN NORMAL IMMUNOREGULATION

The expression and function of Fas (CD95/APO-1), a cell surface recept or directly responsible for triggering cell death by apoptosis, was in vestigated on human T lymphocytes resident within the intestinal lamin a propria, a major site of antigen challenge and persistent lymphocyte activation. Three color immunofluorescence and FACS analysis indicate d that virtually all freshly isolated human gut lamina propria T lymph ocytes (T-LPL) express Fas, together with the marker of pregress activ ation CD45R0. A discrete fraction of freshly isolated T-LPL also const itutively expressed Fas ligand (FasL), perhaps as a result of recent i n vivo activation. Importantly, whereas Fas cross-linking did not resu lt in apoptosis induction in peripheral blood T lymphocytes (T-PBL), F as was found to be fully effective in generating the apoptotic signal in T-LPL. This was associated with the activation of an acidic sphingo myelinase and with ceramide generation, early events known to be invol ved in Fas-mediated apoptotic signaling. By contrast, acidic sphingomy elinase activation and ceramide production were not detectable in T-PB L after Fas cross-linking. However C-2-ceramide, a cell permeant synth etic analog of ceramide, could efficiently induce apoptosis in T-LPL a nd T-PBL when added exogenously. These data indicate that T-LPL consti tutively express both Fas and Fast and that Fas cross-linking generate s signals resulting in sphingomyelin hydrolysis and apoptosis, outlini ng a potential mechanism involved in intestinal tolerance. Moreover, t hey provide the first evidence of a role for ceramide-mediated pathway s in normal immunoregulation.