Nonspecific resistance to the multicellular organism Schistosoma mansoni can be induced in mice by several infectious agents. We utilized the observed genetic restriction of such acquired resistance to study the mediators of killing of the larval stage of S. mansoni in vitro. Adherent peritoneal cell monolayers from Corynebacterium parvum-treated C57BL/6J but not from C. parvum-treated BALB/cJ mice killed an increased proportion of schistosomula in 24 h. Activated macrophages (Mφ) from both strains exhibited enhanced H(2)0(2) production after incubation with the parasites or phorbol myristate acetate. Thus H(2)0(2) production was not associated with schistosomula killing. Moreover, schistosomula killing was unaffected by catalase or superoxide dismutase. In contrast, activated C57BL/6J (but not BALB/cJ) Mφ released fourfold more arginase into supernates than control Mφ. Schistosomula killing by these Mφ correlated with arginase content of the supernates, was exaggerated in arginine-poor medium, and could be blocked by the addition of arginine. Exogenous bovine arginase added to Fischer's medium without macrophages produced comparable parasite mortality. Our data suggest that arginase is a critical mediator of in vitro killing of this multicellular organism by activated macrophages.

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