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.