International audienceThe impact of diets upon the fatty acid composition of haemocyte polar lipids and consequently upon immune parameters has been tested in the oyster Crassostrea gigas and the clam Ruditapes philippinarum. Oysters and clams were fed each of three cultured algae: Chaetoceros calcitrans, which is rich in 20:5(n-3) and 20:4(n-6) and poor in 22:6(n-3) fatty acids; T-Iso (Isochrysis sp.), which is rich in 22:6(n-3) and deficient in 20:5(n-3) and 20:4(n-6); and Tetraselmis suecica, which is deficient in 22:6(n-3) and contains only small amounts of 20:5(n-3) and 20:4(n-6). Fatty acid composition of haemocyte polar lipids was greatly affected by the diet. Oysters and clams fed C. calcitrans maintained a higher proportion of 20:5(n-3) and 20:4(n-6) in their haemocyte polar lipids, while these polyunsaturated fatty acids decreased drastically for animals fed T-Iso. However, the T-Iso diet maintained 22:6(n-3) in haemocyte polar lipids of both species. Higher 20:5(n-3) and 20:4(n-6) contents in diets appeared to have a positive effect upon total haemocyte count, granulocyte percentage, phagocytic rate and oxidative activity of clam haemocytes. Similarly, a positive effect of 20:5(n-3) on oxidative activity of oyster haemocytes was observed but to a lesser extent than in clams. Interestingly, when oyster haemocytes are submitted to a stressful condition, a positive effect of a higher dietary 22:6(n-3) content on the phagocytic rate was noticed
A flow cytometric method to measure the production of oxidative metabolism products was adapted for use with Crassostrea gigas hemocytes. The method is based upon the oxidation, by hydrogen peroxide (H2O2), of intracellular 2',7'-dichlorofluorescin (DCFH) to green-fluorescent dichlorofluorescein. Activation of the respiratory burst (RB) was tested using phorbol myristate acetate with no success. By contrast, activation by zymosan particles increased oxidation of DCFH in C. gigas hemocytes, mainly granulocytes, and optimization tests showed a good response with 20 zymosan particles per hemocyte. Anti-aggregant solution, used to prevent hemocytes from clumping during bleeding, inhibited the RB activity measured by DCFH oxidation. The flow cytometric method developed during this work was used to evaluate the DCFH oxidation-inhibiting capacity of four strains of vibrio bacteria, known or suspected to be pathogenic for bivalves.
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