The objective of the present study was to elucidate if modulation of membrane lipids after replacement of fish oil with rapeseed oil in the feed, influences pro-inflammatory cytokine expression and respiratory burst activity. As immune responses of fish show high degree of individual variation an ex vivo plasma incubation model was used.
Head kidney leukocytes isolated from salmon fed a commercial diet were incubated for 20 h in diluted plasma obtained from three groups of adult salmon fed different dietary lipid sources, fish oil (FO), rapeseed oil (RO), or their 1:1 blend (FO/RO). Plasma obtained from the feeding studies reflected the fatty acid composition of the diets with an n−3/n−6 ratio for the FO, FO/RO and RO groups of 13.6, 6.8, and 2.1 respectively. Fatty acid profiles of incubated leukocytes mirrored the plasma composition with step wise changes in major fatty acids.

Gene expression of TNF-α and IL-1β increased significantly in all groups over the 12 h study period following LPS stimulation. However, there was no significant difference in expression of the cytokines between the three lipid groups. Similarly, there were no differences in respiratory burst response between the lipid groups. A 5-fold reduction in n−3/n−6 fatty acid ratio did not change said pro-inflammatory cytokines or the respiratory burst responses following stimulation.

Despite a marked change in n−3/n−6 ratio in leukocyte membranes following plasma incubation, there were less marked alterations in arachidonic acid (AA) and eicosapentaenoic acid (EPA), key players in modulating phagocyte responses as precursors to eicosanoid synthesis. This might provide an explanation to the lack of influence of fatty acid sources on inflammatory responses in the leukocytes.

Keywords: Membrane lipid modulation; Leukocytes; Cytokine expression; Respiratory burst; Atlantic salmon