The influence of dietary polyunsaturated fatty acids on the immune response of Atlantic salmon (Salmo salar L.)

Abstract

The present work examined the effects of dietary (n-3)/(n-6) PUFA ratios in Atlantic salmon parr on, firstly, the fatty acid composition of lipids in peripheral blood cells and immunocompetent tissues and secondly, disease resistance. No notable differences in physiological and immunological parameters were observed between fish fed different (n-3)/(n-6) PUFA diets, except for a significantly higher number of responding B cells in kidney and spleen of the fish fed high (n-3)/(n-6) PUFA diet. The protective vaccination of the groups of fish on the different (n-3)/(n-6) PUFA ratio diets was inconclusive, but significantly more salmon died in the low (n-3)/(n-6) group when non-vaccinated fish were challenged with Vibrio anguillarum. Lipid class composition of lipids from leucocytes, erythrocytes and .serum were all found to be independent of diet, while component fatty acids were definitely influenced by dietary PUFA. Total fatty acids of the erythrocyte lipid were always high in (n-3) PUFA. Leucocyte lipid, by comparison, contained higher levels of saturated and monocnoic fatty acids, particularly 18:1 (n-9). The overall (n-6) PUFA unsaturation was higher in the lipid of leucocytes than the erythrocytes and leucocytes incorporated greater proportions of dietary 18:2(n-6) into their lipid than erythrocytes. Levels of 18:2(n-6) in the lipid of .serum and leucocytes from (n-6) PUFA fed fish began to rise after four weeks on the diet, and increased steadily until a (n-3)/(n-6) PUFA ratio of 0.8 was maintained after 16 weeks. Erythrocytes did not show diet induced modification until 8-16 weeks, at which point 18;2(n-6) fatty acid levels were observed to plateau. Influences of dietary fatty acid composition were most evident in the PC and PE fractions. Dietary 18:2(n-6) fatty acid incorporation was greater in the lipids of the lymphoid organ tissues than in leucocytes isolated from these tissues. Lymphoid tissues from (n-3) dietary group fish possessed higher (n-3)/(n-6) PUFA ratios than their leucocytes, while (n-3)/(n-6) PUFA ratios were found to be similar between the lymphoid tissues and their corresponding leucocytes of the (n-6) PUFA dietary group. Lipid from headkidney macrophages of Atlantic salmon was higher in phospholipids than headkidney leucocytes and T and B-cells, while their fatty acid profiles were similar. Comparisons of salmon parr were made with a warm water species of fish and a mammalian species. The fatty acid composition of rabbit cell lipid reflected their dietary 18:2(n-6) intake, whereas the fish had high levels of (n-3) PUFA. Erythrocytes and leucocytes of African catfish had similar fatty acid compositions, while Atlantic salmon erythrocytes contained a higher degree of 22:6(n-3) fatty acid than did the leucocytes. T3ic leucocyte lipid of salmon contained higher levels of saturated and monoenoic fatty acids than the catfish leucocytes. The lipid class and fatty acid composition of Atlantic salmon erythrocytes and leucocytes were unaffected by water temperature, except for a higher PE level in the erythrocytes of cold water acclimated fish. When salmon leucocytes were cultured in vino, the lipid composition of the leucocytes was unaffected by FCS or fish serum when these were used as medium supplements. The rate of incoiporation of exogenous fatty acids into the peripheral blood leucocytes was influenced by metabolic temperature. Greatest incorporation of exogenous fatty acids by fish peripheral blood leucocytes occurred within the first day of incubation, but peaked around day 2 when cells were cultured at 15 C, and day 5 when incubated at 4°C. Kidney and blood leucocytes incorporated greater amounts of 20:4(n-6) and 18:l(n-9) fatty acid into their lipid than those from thymus and spleen, but all leucocytes displayed a preference for 20:4(n-6) and 18:l(n-9) fatty acids over 20:S(n-S), 18:2(n-6) and 18:3(n-3). The study suggests that the lymphoid system of salmon is predisposed to (n-6) PUFA despite the natural abundance of (n-3) PUFA in the lipids of salmon diets and tissues.