Phagocytosis inhibition of clam and mussel haemocytes by Perkinsus atlanticus secretion products.

The in vitro effects of the secretion products (SP) released by cultured Perkinsus atlanticus on the phagocytic activity of two shellfish species, Ruditapes decussatus and Mytilus galloprovincialis were determined. Haemocytes from clam and mussel were incubated with Perkinsus SP, and the number of phagocytosed particles were counted. The particles assayed were zymosan, Escherichia coli and Vibrio tapetis. Phagocytosis of zymosan was measured at 15 and 21° C, and different incubation times in SP were tested. In all cases, phagocytosis was inhibited in SP-treated haemocytes. Phagocytosis of zymosan was inhibited to a greater extent in haemocytes from mussel than from clam. No differences were found between phagocytosis at 15 and 21° C, and the decrease in the phagocytic activity was evident within 30 min of haemocyte incubation in SP. Clam haemocyte phagocytosis of V. tapetis was inhibited to a greater extent than for zymosan and E. coli, while phagocytosis by the mussel haemocytes was inhibited to the same extent for the two bacterial species tested. Otherwise, the phagocytic index was higher for clam haemocytes than for mussel haemocytes in all the experiments. SP contained high protein concentration and acid phosphatase and protease activity. These results suggest that the products secreted by Perkinsus atlanticus may have a negative effect on the cellular defence response of these bivalve molluscs.