Synergistic effect of extracellular adenosine 5'-triphosphate and tumor necrosis factor on DNA degradation

Extracellular ATP (ATPo) was recently considered a possible mediator of cell-mediated cytotoxicity since it is secreted by effector cells following appropriate stimulus and causes lysis as well as DNA degradation of susceptible target cells. This hypothesis however is not readily reconciled with the finding that ATPo-resistant cells are fully susceptible to intact effector cells, which instead suggests that a necessary step in cell-mediated cytotoxicity is the interaction between different molecules released by a cytotoxic cell. By combining ATPo with TNF or lymphotoxin (LT), cytokines which induce late DNA damage, we observed a synergistic effect on target cell death. Under these conditions, the phenomenon of DNA degradation also appeared early, with a kinetics reminiscent of that observed during target cell incubation with intact effector cells. Target cells which are resistant to one of the two molecules exhibited an enhanced rate of cell death when exposed to their association. Target cell lysis and DNA degradation were also Ca(2+)-independent events as they took place following external Ca2+ chelation by EGTA addition and under experimental conditions in which little or no Ca2+ was released from target cell intracellular stores. These findings suggest that ATPo might represent a further mediator which is responsible for the alternative Ca(2+)-independent cytolytic pathway in association with other molecules released by effector cytotoxic lymphocytes.