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Depression of Hydrogen Peroxide Dependent Killing of Schistosomula in Vitro by Peritoneal Exudate Cells from Schistosoma Mansoni Infected Mice

Peritoneal exudate cells from mice infected with Schistosoma mansoni (S-PEC) can kill a small proportion of schistosomula in vitro in the presence of immune serum. S-PEC produce a low level of respiratory burst. However, schistosomula mortality in their presence is not reduced when exogenous antioxidants are added, suggesting that with S-PEC, oxidative killing may not be important. Hydrogen peroxide (H₂O₂) and superoxide production by S-PEC, and cells from Bacillus Calmette-Guerin (BCG) and thioglycollate (THGL) injected mice, nonspecifically stimulated with opsonized zymosan, were measured. Levels of H₂O₂ produced by S-PEC were significantly lower than BCG or THGL PEC, and were below the threshold for schistosomula killing. This correlated with lower levels of cell-mediated killing of schistosomula in vitro by S-PEC than by BCG-PEC. Superoxide levels, however, were similar between the 3 cell populations. It therefore appears that the efficiency of PEC to kill schistosomules in vitro correlates with H₂O₂ rather than superoxide levels. It was found that there was a sharp concentration threshold in H₂O₂ mediated killing of schistosomula. A depression in the levels of H₂O₂ produced may be a mechanism by which the parasite can partially evade the host immune system.