Peptide modification or blocking of CD8, resulting in weak TCR signaling, can activate CTL for Fas- but not perforin-dependent cytotoxicity or cytokine production

This study describes a form of partial agonism for a CD8(+)CTL clone, S15, in which perforin-dependent killing and IFN-gamma production were lost but Fas (APO1 or CD95)-dependent cytotoxicity preserved. Cloned S15 CTL are H-2 K(d) restricted and specific for a photoreactive derivative of the Plasmodi um berghei circumsporozoite peptide PbCS 252-260 (SYIPSAEKI), The presence of a photoactivatable group in the epitope permitted assessment of TCR-liga nd binding by TCR photoaffinity labeling. Selective activation of Pas-depen dent killing was observed for a peptide-derivative variant containing a mod ified photoreactive group. A similar functional response was obtained after binding of the wild-type peptide derivative upon blocking of CD8 participa tion in TCR-ligand binding. The epitope modification or blocking of CD8 res ulted in an greater than or equal to 8-fold decrease in TCR-ligand binding. In both cases, phosphorylation of zeta-chain and ZAP-70, as well as calciu m mobilization were reduced close to background levels, indicating that act ivation of Pas-dependent cytotoxicity required weaker TCR signaling than ac tivation of perforin-dependent killing or IFN-gamma production. Consistent with this, we observed that depletion of the protein tyrosine kinase p56(lc k) by preincubation of S15 CTL with herbimycin A severely impaired perforin - but not Pas-dependent cytotoxicity. Together with the observation that S1 5 CTL constitutively express Pas ligand, these results indicate that TCR si gnaling too weak to elicit perforin-dependent cytotoxicity or cytokine prod uction can induce Fas-dependent cytotoxicity, possibly by translocation of preformed Fas ligand to the cell surface.