CELL-MEDIATED CYTOTOXICITY - CONTACT AND SECRETED FACTORS

The list of cells with cytotoxic potential now may include small resti ng T cells, but the exact nature of 'lethal hit delivery' by cytotoxic T lymphocytes remains elusive. Cell-mediated cytotoxicity by cytotoxi c T lymphocytes is a complex, multistep process which seems likely to be mediated by several different pathways. Recent experimental evidenc e for the functioning of a novel cytotoxic mechanism through a target cell's surface receptor illustrates and emphasizes the necessity to st udy the interactions of cytotoxic T lymphocytes and target cells as a whole. Progress is evident in the description of molecular requirement s for triggering cytotoxicity, cell-cell contacts and the regulation o f the effector responses of cytotoxic T lymphocytes by extracellular, intracellular and granular proteins. Extracellular Ca2+-dependent secr etion of perforin and protease(s) may explain several aspects of cellu lar cytotoxicity, whereas the apoptosis-mediating cell surface Fas pro tein is now implicated in Ca2+-independent cytotoxicity.