PARTICIPATION OF TARGET FAS PROTEIN IN APOPTOSIS PATHWAY INDUCED BY CD4(+) TH1 AND CD8(+) CYTOTOXIC T-CELLS

The results presented here provide evidence that the presence of Pas protein in target cells is essential to permit cytotoxicity (resulting in apoptosis) mediated by cloned CD4(+) Th1 cells. Using mitogen-activated B cells as targets, antigen-dependent lysis by CD4(+) Th1 effecters was observed with MRL/MpJ+ but not with MRL/MpJ-lpr targets. The congenic MRL/MpJ-lpr strain is defective in Pas expression. Target cells from various lymphoid tissues of C3H.MRL-lpr mice were also resistant to the lectin-dependent cytotoxicity of Th1 effecters, whereas C3H/HeJ targets were sensitive. Moreover, a rapid DNA fragmentation prior to Cr-51 release was induced only in C3H/HeJ targets. Thus, cytotoxicity induced by Th1 effecters correlates with target Fas expression. In contrast to Th1 effectors, CD8(+) cytotoxic T lymphocytes (CTLs) killed C3H.MRL-lpr targets. When cytotoxicity was assayed in the presence of EGTA and MgCl2, which chelates extracellular Ca2+ [(Ca2+)(ext)], only C3H.MRL-lpr targets became resistant to CD8(+) CTLs. This (Ca2+)(ext)-independent cytotoxicity of both Th1 and CD8(+) effecters could be inhibited with unlabeled C3H/HeJ thymocytes or with a transfectoma carrying a murine Fas-human mu gene construct. In comparison, C3H.MRL-lpr thymocytes and the nontransfected parental cell line were poor inhibitors. Our study demonstrates that CD4(+) Th1 cells and CD8(+) CTLs differ in their (Ca2+)(ext)-dependent cytotoxicity but share a (Ca2+)(ext)-independent cytotoxicity that requires participation of Fas molecules for cytotoxic signal transduction leading to target apoptosis.