Macrophage cytotoxicity against schistosomula of Schistosoma mansoni involves arginine-dependent production of reactive nitrogen intermediates.

Lymphokine (LK)-activated macrophages are cytotoxic for multicellular larvae of the helminth parasite Schistosoma mansoni. Macrophage-mediated larval killing was found to be arginine dependent, as indicated by inhibition in the presence of exogenous arginase or the competitive inhibitor NG-monomethyl-L-arginine. Culture supernatant fluids from the larvicidal LK-activated macrophages contained nitrite, a product of activated macrophages derived by oxidation of arginine and implicated in the antitumor and antimicrobial effector function of these cells. Nitrite was not detectable in supernatant fluids obtained from nonactivated macrophages or from macrophages stimulated with LK in the presence of arginase or NG-monomethyl-L-arginine. Addition of excess iron or the reductant sodium dithionite to LK-activated macrophage cultures also inhibited larval killing in vitro, under conditions that have been shown by others to stabilize the activity of iron-containing enzymes involved in respiration. Nitrite production was not decreased under these conditions. These observations are consistent with the hypothesis that macrophage-mediated schistosomulum killing is caused, at least in part, by a mechanism proposed for tumor cytotoxicity, whereby production of reactive nitrogen intermediates triggers iron loss from critical target cell enzymes leading to lethal metabolic inhibition. In accordance, schistosomula were shown to be killed by inhibitors of mitochondrial respiration.