T cell receptor-induced Fas ligand expression in cytotoxic T lymphocyte clones is blocked by protein tyrosine kinase inhibitors and cyclosporin A

Fas/APO-1 is a member of the tumor necrosis factor receptor family of proteins, that induces apoptosis when cross-linked with monoclonal antibody (mAb) or with its physiological ligand. Recently, both a perforin-based and a Fas-based mechanism have been proposed to account for T cell-mediated cytotoxicity. In the present study we used a murine CD8⁺ cytotoxic T lymphocyte (CTL) clone (KB5.C20) specific for H-2K^b and a T cell receptor (TcR)-negative variant of the same clone (2005^?D4) to test (i) whether the same cell can exert both cytotoxic effector mechanisms and (ii) the role of TcR engagement in the induction of Fas-based cytotoxicity. We demonstrate that both the TcR⁺ and TcR^? clones were able to express the Fas ligand after stimulation with phorbol 12-myristate 13-acetate (PMA)/ionomycin, and that TcR engagement of the KB5.C20 clone by means of antigen-bearing cells or of its anticlonotypic mAb (Désiré-1), which leads to Ca²⁺-dependent, presumably perforin-based, cytotoxicity, was also able to induce Fas-based cytotoxicity. In addition, using inhibitors we investigated the signal transduction pathway(s) involved in the induction of Fas-based cytotoxicity and expression of the Fas ligand mRNA in the CTL clones. The involvement of src-like protein tyrosine kinases (PTK) in Fas ligand induction through TcR engagement, was strongly suggested by inhibition with the src-like PTK inhibitor herbimycin A. Inhibition of Fas ligand induction by genistein, a more general TPK inhibitor, even upon stimulation by PMA plus ionomycin, suggested the possible involvement of PTK activities downstream of protein kinase C (PKC) in Fas ligand induction in CTL. Finally, the implication of the Ca²⁺/calmodulin-dependent protein phosphatase calcineurin in Fas ligand induction was demonstrated by the partial inhibition of Fas ligand induction with cyclosporin A. Thus, in CTL clones, Fas ligand expression is inducible by TcR engagement through a pathway similar to that involved in expression of some lymphokine genes.