Oxidative mechanisms of monocyte - mediated cytotoxicity ( phorbol myristate acetate / leukocytes / hydrogen peroxide / superoxide anion / superoxide dismutase ) STEPHEN

Human monocytes stimulated with phorbol myristate acetate were able to rapidly destroy autologous erythrocte targets. Monocyte-mediated cytotoxicity was related to phorbol myristate acetate concentration and monocyte number. Purified preparations of lymphocytes were incapable of mediating erytlrocyte lysis in this system. The ability of phorbol myristate acetate-stimulated monocytes to lyse erythrocyte targets was markedly impaired by catalase or superoxide dismutase but not by heat-inactivated enzymes or albumin. Despite a simultaneous requirement for superoxide anion and hydrogen peroxide in the cytotoxic event, a variety of hydroxyl radical and singlet oxygen scavengers did not effect cytolysis. However, tryptophan significantly inhibited cytotoxicity. The myeloperoxidase inhibitor cyanide enhanced erythrocyte destruction, whereas azide reduced it modestly. The inability of cyanide to reduce cytotoxicity coupled with the protective effect of superoxide dismutase suggests that cytotoxicity is independent of the classic myeloperoxidase system. We conclude that monocytes, stimulated with phorbol myristate acetate, generate superoxide anion and hydrogen peroxide, which together play an integral role in this cytotoxic mechanism. Human monocytes are capable of destroying bacteria, fungi, intracellular parasites, and mammalian cell targets (1, 2). Various cytotoxic mechanisms have been hypothesized to play a role in monocyte-mediated target cell destruction (1-3). Recently, increased attention has focused on the role of leukocyte oxidative metabolism in cytotoxic events (3). The monocyte can mediate the sequential univalent reduction of oxygen to a variety of potentially toxic agents, including superoxide anion (02--, hydrogen peroxide (H202), the hydroxyl radical (OH-), and possibly singlet oxygen (021) (3). The ultimate role of these and similar species in monocyte-dependent cytotoxicity is not known. In this study, we demonstrate the ability of purified human monocytes exposed to the potent stimulator of oxidative metabolism, phorbol myristate acetate (PMA) (4), to destroy erythrocyte (RBC) targets. It appears that monocytes mediate maximal RBC destruction by a mechanism dependent on the simultaneous generation of 02'and H202.