Oxidative responses of human and murine macrophages during phagocytosis ofLeishmania chagasi

Leishmania chagasi, the cause of South American visceral leishmaniasis, mus t survive antimicrobial responses of host macrophages to establish infectio n. Macrophage oxidative responses have been shown to diminish in the presen ce of intracellular leishmania. However, using electron spin resonance we d emonstrated that murine and human macrophages produce O-2 during phagocytos is of opsonized promastigotes. Addition of the O-2(-) scavenger 4-hydroxy-2 ,2,6,6-tetramethylpiperidine-N-oxyl to cultures resulted in increased infec tion, suggesting that O-2(-) enhances macrophage leishmanicidal activity. T he importance of NO. produced by inducible NO synthase (iNOS) in controllin g murine leishmaniasis is established, but its role in human macrophages ha s been debated. We detected NO. in supernatants from murine, but not human, macrophages infected with L. chagasi. Nonetheless, the iNOS inhibitor N-G- monomethyl-L-arginine inhibited IFN-gamma-mediated intracellular killing by both murine and human macrophages. According to RNase protection assay and immunohistochemistry, iNOS mRNA and protein were expressed at higher level s in bone marrow of patients with visceral leishmaniasis than in controls. The iNOS protein also increased upon infection of human macrophages with L. chagasi promastigotes in vitro in the presence of IFN-gamma. These data su ggest that O-2(-) and NO. each contribute to intracellular killing of L. ch agasi inhuman and murine macrophages.