This study demonstrates that the promastigote form of virulent Leishmania donovani and Leishmania tropica are both deficient in endogenous enzymatic scavengers of H(2)0(2) (catalase, glutathione peroxidase) and susceptible to low fluxes of H(2)O(2) in a cell-free model. In addition, the killing of promastigotes by H(2)0(2) is markedly enhanced in the presence of a peroxidase and halide. Promastigotes also readily trigger the macrophage oxidative burst including the generation of H(2)0(2), and most intracellular promastigotes are killed within 18 h by unstimulated normal resident cells. Catalase, but not scavengers or quenchers of O(2)(-), OHx, or (1)O(2), protected promastigotes in a cell-free xanthine oxidase microbicidal system, and catalase also partially inhibited the leishmanicidal activity of resident macrophages. Thus, amongst various oxygen intermediates, H(2)0(2) alone appeared to be both necessary and sufficient for promastigote killing. Depriving macrophages of exogenous glucose, which inhibits the generation of oxygen intermediates, achieved effects similar to catalase treatment. These observations directly contrast with the intracellular parasite, T. gondii which is richly endowed with catalase and glutathione peroxidase, highly resistant to H(2)0(2), and requires products of O(2)(-)-H(2)0(2) interaction for effective oxidative killing. Toxoplasmas also fail to trigger the respiratory burst of normal macrophages, and readily multiply within these cells (1-5). Macrophages first activated by in vivo or in vitro immunologic stimuli, however, display an enhanced capacity to generate oxygen intermediates beyond O(2)(-) and H(2)0(2), and are able to kill toxoplasmas or inhibit their intracellular replication (1, 2).

These studies illustrate the wide spectrum of susceptibility to oxidative products which appears to exist for virulent intracellular protozoans, and indicate that such differences may be reflected in contrasting fates of parasites within cell-free oxidative environments and the cytoplasm of normal resident macrophages. In addition, these observations also demonstrate that nonactivated phagocytes may display effective microbicidal activity against certain intracellular pathogens utilizing an oxygen-dependent mechanism.

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