Role for endogenous and acquired peroxidase in the toxoplasmacidal activity of murine and human mononuclear phagocytes.

Abstract

Oxygen products generated by the respiratory burst of mononuclear phagocytes are microbicidal to intracellular pathogens including Toxoplasma gondii. The toxicity of one of these products, H(2)O(2), is markedly amplified by the granule peroxidase of circulating phagocytes in the presence of a halide. Eosinophil peroxidase (EPO) binds firmly to the surface of T. gondii and such organisms remain viable as determined by vital staining, uptake of 2-deoxyglucose, and survival and replication in human fibroblasts. They are, however, rapidly killed by the addition of H(2)O(2) and iodide under conditions in which control organisms are unaffected. We have used EPO bound to T. gondii to explore the role of peroxidase in the toxoplasmacidal activity of mononuclear phagocytes. Resident mouse peritoneal macrophages lack a granule peroxidase and have a weak respiratory burst; toxoplasma survive and replicate within these cells. However, these cells acquire significant toxoplasmacidal activity, as assessed microscopically and by the inhibition of uracil uptake, when organisms are coated with EPO before ingestion, an effect which is decreased by the hemeprotein inhibitors, aminotriazole and azide. EPO on the surface of Toxoplasma does not increase their ingestion by macrophages or the associated respiratory burst. Monocytes from patients with hereditary myeloperoxidase deficiency have a significant toxoplasmacidal defect that is abolished when EPO-coated organisms are used. In contrast, the toxoplasmacidal defect of monocytes from chronic granulomatous disease patients is unaffected by surface-bound EPO. In these studies, replication of surviving intracellular organisms varied inversely with the magnitude of the respiratory burst: replication was greatest in fibroblasts, slightly less in resident macrophages, and least in monocytes; it was significantly greater in chronic granulomotous disease than in normal or myeloperoxidase-deficient monocytes. These studies support a role for oxygen products and endogenous peroxidase in the optimal killing of T. gondii by monocytes and demonstrate that peroxidase-negative phagocytes can utilize peroxidase on the surface of ingested organisms to augment microbicidal activity.