Schistosomicidal activities of Lymnaea stagnalis haemocytes: the role of oxygen radicals.

Macrophage-like defence cells (haemocytes) of the pond snail Lymnaea stagnalis mediate cytotoxicity through reactive oxygen intermediates (ROIs). This activity is NADPH-oxidase dependent, as in mammalian phagocytes during the respiratory burst. In this study, mother sporocysts of schistosomes, the compatible Trichobilharzia ocellata and the incompatible Schistosoma mansoni evoke in vitro ROI activities (detected by luminol 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 equally susceptible to in vitro oxidative damage from exposure to hydrogen peroxide and to ROIs generated by a xanthine/xanthine oxidase system. Protocatechuic acid, a specific antagonist of NADPH-oxidase, delayed the killing of T. ocellata and S. mansoni sporocysts by haemocytes of resistant snails (Biomphalaria glabrata and L. stagnalis, respectively). We conclude that ROIs take part in haemocyte-mediated cytotoxicity. However, neither a snail's capability to generate ROIs, nor a schistosome's susceptibility to ROIs, determine snail/schistosome incompatibility. Snail/schistosome compatibility is rather determined by the parasite's ability to modulate haemocyte behaviour such that effective encapsulation and the generation of lethal concentrations of ROIs are prevented.