Phosphatidylinositol 3-kinase-dependent and -independent cytolytic effector functions

Two distinct forms of short-term cytolysis have been described for CD8⁺ CTLs, the perforin/granzyme- and Fas ligand/Fas (CD95 ligand (CD95L)/CD95)- mediated pathways. However, the difference in signal transduction events leading to these cytolytic mechanisms remains unclear. We used wortmannin, an irreversible antagonist of phosphatidylinositoi 3-kinase (PI3-K) activity, to investigate the role of PI3-K in influenza-specific CD8⁺ CTL cytolytic effector function. We found that the addition of wortmannin at concentrations as low as 1 nM significantly inhibited both the Ag/MHC-induced cytolysis of CD95^- target cells and serine esterase release. In strong contrast, W did not inhibit the Ag/MHC-induced CD95L expression or the CD95L/CD95-mediated cytolysis of CD95⁺ targets. A combination of wortmannin and blocking mAb against CD95L inhibited the cytolysis of CD95⁺ targets, indicating that the wortmannin-independent cytolysis was due to CD95L/CD95 mediated cytolysis. These findings suggest a differential role for PI3-K in mediating cytolysis and, thus far, the earliest difference between perforin/granzyme- and CD95L/CD95-dependent cytolysis. Our data reinforce the idea of a TCR with modular signal transduction pathways that can be triggered or inhibited selectively, resulting in differential effector function.