Regulation of the Fas lytic pathway in cloned CTL.

Cloned murine CTL activated via the TCR or by PMA and ionomycin up-regulate surface Fas ligand and show an increased ability to kill non-Ag-specific Fas+ target cells. This up-regulation starts after 45 to 60 min and has a t1/2 for reversal of about 90 min. Up-regulation of lytic function is accompanied by up-regulation of Fas ligand on the CTL surface, which can be blocked by protein synthesis inhibitors. When up-regulation of Fas lytic function was induced by PMA and ionomycin, EGTA blocked both activation of lytic function and expression of Fas ligand detected by FACS analysis. However, when up-regulation was induced by specific Ag, EGTA blocked activation of lytic function, but not up-regulation of Fas ligand. Moreover, EL-4 cells have very high levels of surface Fas ligand, although they are not cytotoxic. Thus, expression of surface Fas ligand may be required, but not sufficient, for Fas-mediated lysis.