A tyrosine phosphorylation requirement for cytotoxic T lymphocyte degranulation

Phorbol myristate acetate (PMA) plus ionomycin induces the tyrosine phosphorylation of several cytotoxic T lymphocyte (CTL) substrates, including one with an apparent molecular weight of 100,000 (pp100) in cloned murine CTL. cis-Unsaturated fatty acids and low concentrations of phenylarsine oxide specifically inhibit the tyrosine phosphorylation of pp100. Genistein also inhibits tyrosine phosphorylation of pp100, but not with the same specificity as cis-fatty acids or low concentrations of phenylarsine oxide. Degranulation triggered by PMA plus ionomycin is inhibited by cis-fatty acids, low concentrations of phenylarsine oxide, and genistein, under the same conditions that these agents inhibit tyrosine phosphorylation of pp100. Depleting CTL of protein kinase C (PKC) activity by prolonged exposure to PMA eliminates the increase in tyrosine phosphorylation when challenged by PMA plus ionomycin, but not when these PKC-depleted CTL are activated by cognate target cells, immobilized anti-T cell receptor (TCR) antibodies, or concanavalin A. Tyrosine phosphorylation of pp100 triggered by TCR engagement in PKC-depleted cells is inhibited by cis-fatty acids and phenylarsine oxide, indicating that the inhibitory mechanism of the tyrosine phosphorylation of pp100 is independent of PKC. Furthermore, because all three tyrosine phosphorylation inhibitors are unlikely to inhibit PKC, these results suggest that, in addition to PKC activation and a rise in intracellular Ca2+, CTL degranulation requires the tyrosine phosphorylation of a CTL substrate(s), in addition to phospholipase C, and the present results are consistent with pp100 as that substrate. Taken together with previous studies, these results suggest that tyrosine phosphorylation of pp100 may play a central role in CTL function.