Mechanisms of lymphocyte mediated cytoxicity

Cell-mediated immunity is important for tumor rejection and clearance of pathogens. We examined the cytotoxic mechanisms used by lymphocytes to kill target cells and investigated the signalling pathways which mediate cytotoxicity. Using lymphocytes deficient for perforin and Fas ligand expression, we show that in short term assays perforin/granzymes and Fas ligand can account for all cytotoxicity. However, lymphocytes deficient in perforin and Fas ligand induce potent cytotoxicity in long term assays which is mediated by membrane-bound and soluble tumor necrosis factor. Perforin/granzyme-dependent DNA fragmentation is mediated by caspases whereas lysis, as measured by chromium release, is caspase and bcl-2 independent. Fas ligand-induced DNA fragmentation and chromium release is mediated by a pathway that is regulated by caspases and bcl-2. Tumor necrosis factor differentially signals through caspases depending on the target cell. Additionally, we show in the same lymphoid cell line that activation-induced cell death (AICD) mediated by Fas/Fas ligand and dexamethasone-induced apoptosis both utilize caspases. However, dexamethasone-induced cell death is inhibited by bcl-2 whereas AICD is not, suggesting that bcl-2 acts upstream of caspases. These studies provide important insights into the mechanisms and pathways of cytotoxicity utilized by lymphocytes