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 modulate haemocyte behaviour such that effective encapsulation and the generation of lethal concentrations of ROIs are prevented.

In order to demonstrate the involvement of nitric oxide in the defence systems of marine bivalves, we investigated the production of superoxide and nitrite, following in vitro phorbol myristate acetate stimulation of Mytilus galloprovincialis, Crassostrea gigas and Ruditapes decussatus haemocytes. Whereas M. galloprovincialis and C. gigas haemocytes were found to produce superoxide and nitrite, R. decussatus haemocytes were found to be unable to generate either of these mediators. Nitrite is a stable end product of nitric oxide and peroxynitrite as well; it appeared therefore that some marine bivalves, to kill microbial pathogens, use NADPH-oxidase and nitric oxide-synthase pathways. This was confirmed at an experimental level where inhibitors of both enzymatic pathways blocked the production of nitrite. Moreover, this notion was strengthened by the inability of the haemocytes from R. decussatus, which cannot produce superoxide, to release nitrite when stimulated.