Mechanism of BCG-activated macrophage-induced tumor cell cytotoxicity: evidence for both oxygen-dependent and independent mechanisms.

Activated peritoneal macrophages have been shown to be cytotoxic to cancer cells. Bacillus Calmette-Guerin (BCG)-activated rat peritoneal macrophages have a basal cytolytic potential for 3H-thymidine-labelled Walker 256 cancer cells in vitro that can be markedly enhanced by digitonin. This stimulation of cytotoxicity can be partially inhibited by catalase and the combination of superoxide dismutase plus catalase. This suggests that digitonin stimulates activated macrophages to produce superoxide, hydrogen peroxide and possibly other free radicals which can augment macrophage-induced tumor cell cytotoxicity. After a 2-hour incubation with digitonin, macrophages are no longer stimulated by digitonin. However, after a 2-hour drug preincubation period, inhibitors of serine protease activity (DFP, TLCK, SBTI) and inhibitors of protein synthesis (cycloheximide) are potent inhibitors of basal macrophage-induced tumor cytotoxicity. We suggest that BCG-activated macrophages have two mechanisms for destroying cancer cells: one mediated by proteolytic activity, and a second mechanism dependent on the generation of oxygen-derived free radicals.