Macrophage oxygen-dependent antimicrobial activity. I. Susceptibility of Toxoplasma gondii to oxygen intermediates.

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Abstract
A sensitive method for evaluating extracellular parasite viability was used to determine the in vitro susceptibility of virulent T. gondii to selected oxygen intermediates. By acridine orange fluorescent staining criteria, toxoplasmas were resistant to up to 10-3 M reagent H2O2 or H2O2 generated by glucose-glucose oxidase. In keeping with a lack of sensitivity to H2O2, toxoplasmas contained endogenous catalase (5.7 .times. 10-4 Baudhuin units/106 organisms). The addition of a peroxidase and halide markedly accelerated killing and lowered the H2O2 requirement by 1000-fold. Toxoplasmas were promptly killed after exposure to products generated by xanthine (1.5 .times. 10-4 M) and xanthine oxidase (50 .mu.g). The inhibition of this system''s microbicidal activity by scavengers of .**GRAPHIC**. (superoxide dismutase) and H2O2 (catalase) indicated that although neither .**GRAPHIC**. nor H2O2 were toxoplasmacidal, their interaction was required for parasite killing. Quenching OH.cntdot. and 1O2, presumed products of .**GRAPHIC**. interaction, by mannitol, benzoate, diazabicyclooctane and histidine inhibited Toxoplasma killing by xanthine-xanthine oxidase. .**GRAPHIC**. and H2O2 apparently functioned in precursor roles and that OH.cntdot. and 1O2 were toxoplasmacidal. The capacity of normal peritoneal macrophages to pinocytose an oxygen intermediate scavenger, soluble catalase, was demonstrated. Appreciable extraphagosomal concentrations of catalase were achieved by exposing [mouse] macrophages to 1 mg/ml of the enzyme for 3 h. Maintenance of high intracellular levels required constant exposure because interiorized catalase was rapidly degraded.