SCHISTOSOMICIDAL ACTIVITIES OF LYMNAEA-STAGNALIS HEMOCYTES - THE ROLEOF OXYGEN RADICALS

Macrophage-like defence cells (haemocytes) of the pond snail Lymnaea s tagnalis mediate cytotoxicity through reactive oxygen intermediates (R OIs). This activity is NADPH-oxidase dependent, as in mammalian phagoc ytes during the respiratory burst. In this study, mother sporocysts of schistosomes, the compatible Trichobilharzia ocellata and the incompa tible Schistosoma mansoni evoke in vitro ROI activities (detected by l uminol dependent chemiluminescence, LDCL) from L. stagnalis haemocytes . S. mansoni is encapsulated by haemocytes and eliminated, whereas T. ocellata escapes encapsulation and survives. Both schistosomes were eq ually susceptible to in vitro oxidative damage from exposure to hydrog en peroxide and to ROIs generated by a xanthine/xanthine oxidase syste m. Protocatechuic acid, a specific antagonist of NADPH-oxidase, delaye d the killing of T. ocellata and S. mansoni sporocysts by haemocytes o f resistant snails (Biomphalaria glabrata and L. stagnalis, respective ly). We conclude that ROIs take part in haemocyte-mediated cytotoxicit y. However, neither a snail's capability to generate ROIs, nor a schis tosome's susceptibility to ROIs, determine snail/schistosome incompati bility. Snail/schistosome compatibility is rather determined by the pa rasite's ability to modulate haemocyte behaviour such that effective e ncapsulation and the generation of lethal concentrations of ROIs are p revented.