Two signaling pathways can lead to Fas ligand expression in CDS' cytotoxic T lymphocyte clones
As shown previously, a given cytotoxic T lymphocyte (CTL) clone (KB5.C20) could be induced to express the Fas ligand (FasL) by either T cell receptor (TCR) engagement or phorbol 12-myristate 13-acetate (PMA)/ionomycin stimulation. In contrast, another CTL clone (BM3.3) has now been found to exert Fas-based cytotoxicity only after TCR engagement, but not after PMNionomycin stimulation. This suggested the existence of a PMA-insensitive, antigeninduced pathway leading to FasL expression. The inability of PMA to promote Fas-based cytotoxicity in BM3.3 cells was correlated with a defect in expression of the classical protein kinase C (PKC) isoforms a and fiI. In KB5.C20 cells depleted of PMA-sensitive PKC isoforms and thus no longer responsive to PMA, Fas-based cytotoxicity could still be induced via the TClUCD3 pathway. On the other hand, a requirement for phosphatidylinositol-3 kinase (PI3K) selectively in this TCWCD3-induced pathway was demonstrated by specific inhibition with wortmannin. These results suggest that FasL expression when induced via the TCR/CD3 involves PI3K, and when induced by PMNionomycin requires the expression of PMA-sensitive PKC isoforms absent in clone BM3.3. Additional data suggest that in neither case was NF-xB activation implicated in FasL expression.