Possible role of reactive forms of oxygen in in vitro killing of Schistosoma mansoni sporocysts by hemocytes of Lymnaea stagnalis.

The interactions between sporocysts of Schistosoma mansoni and hemocytes of the pond snail Lymnaea stagnalis were studied in an in vitro system. All sporocysts were encapsulated and killed in the hemolymph of juvenile or adult snails within 24 h; the killing patterns of both age groups were not very different. The separation of hemolymph into hemocytes and plasma and subsequent incubation of sporocysts showed that the cells, not the plasma, were responsible for parasite killing. The possible involvement of reactive oxygen intermediates and the enzyme peroxidase in the cytotoxic mechanism was studied. Chemiluminescence (CL) could not be detected during the hemocyte-parasite encounter. Histochemical staining with nitroblue tetrazolium (indicative for superoxide) and diaminobenzidine (indicative for hydrogen peroxide) showed that oxygen intermediates were produced by snail hemocytes that encapsulated sporocysts. Histochemistry also suggested the presence of peroxidase activity at the hemocyte-sporocyst interface. The cytotoxic mechanism underlying parasite killing by hemocytes and the role of reactive forms of oxygen and peroxidase are discussed.