ACTIVATION OF T-CELLS THROUGH AN ANTIGEN-INDEPENDENT ALTERNATIVE PATHWAY INDUCES PRECOCIOUS SENSITIVITY TO FAS-INDUCED APOPTOSIS

Autoreactive T-cells can be activated inadvertently during immune resp onses through antigen-independent pathways. It has been suggested that Fas/Fas-ligand interactions may play a role in eliminating these cell s, but the extent that cells activated through such alternative pathwa ys are sensitive to Fas-induced apoptosis has not been extensively eva luated. Proliferation of peripheral blood T-cells from normal individu als activated for 4 days with PHA or PMA + ionophore was not influence d by the presence of anti-Fas antibody. When the same cells were activ ated with soluble factors produced by previously activated T-cells (ly mphostimulatory activity), anti-Fas antibodies inhibited thymidine inc orporation by 74+/-4%. The presence of typical morphological changes a nd oligonucleosomal fragmentation of DNA indicated that the reduced pr oliferation resulted from apoptotic death of the lymphoblasts. Fas-sen sitivity of T-cells activated by lymphostimulatory activity was first detectable 4 days after activation, and at 5 days the majority of lymp hoblasts bad become sensitive to Fas, whereas no evidence of sensitivi ty to Fas was observed for lymphoblasts generated by PHA or PMA + iono phore during the first 5 days of culture. Incubation of cells activate d with PHA or PMA+ionophore in the presence of IL-2 at concentrations 10-fold higher than that present in lymphostimulatory activity did not induce early sensitivity to Fas, indicating that exposure to IL-2 cou ld not explain the precocious development of sensitivity to Fas seen f ollowing activation by lymphostimulatory activity. These studies demon strate that T-cells activated through an antigen-independent 'alternat ive' pathway develop precocious sensitivity to Fas-induced apoptosis, which may be important in permitting the elimination of autoreactive b ystander cells activated in the course of immune responses. (C) 1997 E lsevier Science B.V.