Perkinsus marinus secretory 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 dead Perkinsus marinus was 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-killed P. marinus triggered a rapid CL response that peaked at a value 5-7-fold greater than the CL level of the untreated cells. However, phagocytosis of viable P. marinus was associated with minor, short-lived CL stimulation followed by a period (>2 h) of sustained inhibition of O-2(-) generation. 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-killed P. marinus produced higher levels of O-2(-) than those receiving zymosan alone. Cells similarly exposed to both zymosan and living P. marinus produced significantly lower levels of O-2(-) than the zymosan-stimulated haemocytes. The stimulatory and inhibitory effects of killed and living P. marinus on CL were apparently both dose-dependent. The data suggest that both living and dead P. marinus are avidly phagocytosed, but only live P. marinus cells produce excretory/secretory products that scavenge O-2(-) and/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. (C) 1999 Academic Press.