Perkinsus marinussecretory products modulate superoxide anion production by oyster (Crassostrea virginica) haemocytes

Superoxide anion (O−2) generation by Eastern oyster haemocytes before and after phagocytosis of living or deadPerkinsus marinuswas quantified by lucigenin-augmented chemiluminescence (CL). Superoxide is the initial product formed during the respiratory burst; this radical and its more cytotoxic metabolites are thought to play roles in the oxygen-dependent defence mechanisms available to the oyster. The haemocytes, in the absence of added particles, produced a constant, very low-level CL activity. Phagocytosis of osmotically-killedP. marinustriggered a rapid CL response that peaked at a value 5–7-fold greater than the CL level of the untreated cells. However, phagocytosis of viableP.marinus was associated with minor, short-lived CL stimulation followed by a period (>2 h) of sustained inhibition of O−2gener-ation. Phagocytosis of zymosan produced an immediate CL response, which was maintained for the >2 h course of the experiment. Haemocytes simultaneously exposed to zymosan and osmotically-killedP. marinusproduced higher levels of O−2than those receiving zymosan alone. Cells similarly exposed to both zymosan and livingP. marinusproduced significantly lower levels of O−2than the zymosan-stimulated haemocytes. The stimulatory and inhibitory effects of killed and livingP.marinus on CL were apparently both dose-dependent. Thedata suggest that both living and deadP. marinusare avidly phagocytosed, but only liveP. marinuscells produce excretory/secretory products that scavenge O−2and/or suppress aspects of the oxygen-dependent immune system. In this fashion the parasites can survive within the haemocytes, multiply in the haemolymph, and eventually produce lethal infections.