Replacement of dietary fish oil by vegetable oils affects humoral immunity and expression of pro-inflammatory cytokines genes in gilthead sea bream Sparus aurata

Abstract

Commercial gilthead sea bream feeds are highly energetic, fish oil traditionally being the main lipid source. But the decreased fish oil production together with the increased prices of this oil encourages its substitution by vegetable oils, imposing new nutritional habits to aquaculture species. Partial replacement of fish oil by vegetable oils in diets for marine species allows good feed utilization and growth but may affect fish health, since imbalances in dietary fatty acids may alter fish immunological status. The effect of dietary oils on different aspects of fish immune system has been reported for some species, but very little is known about the effect of dietary oils on immune-related genes expression in fish. Thus, the objective of this study was to elucidate the role of dietary oils on the expression of two pro-inflammatory cytokines, Tumor Necrosis Factor-α (TNF-α) and Interleukine 1β (IL-1β) on intestine and head kidney after exposure to the bacterial pathogen Photobacterium damselae sp. piscicida.

For that purpose, 5 iso-nitrogenous and iso-lipidic diets (45% crude protein, 22% crude lipid content) were formulated. Anchovy oil was the only lipid source used in the control diet (FO), but in the other diets, fish oil was totally (100%) or partially (70%) substituted by linseed (rich in n-3 fatty acids) or soybean (rich in n-6 fatty acids) (100L, 100S, 70L, 70S). Fish were fed experimental diets during 80 days and after this period were exposed to an experimental intestinal infection with the pathogen. Serum and tissue samples were obtained at pre-infection and after 1, 3 and 7 days of infection. RNA was extracted and cDNA was synthesized by reverse transcription from intestine and head kidney and the level expression of TNF-α and IL-1β were assayed by using quantitative real time PCR. The expression level of genes analysed was represented as relative value, using the comparative Ct method (2−ΔΔCt). Serum anti-bacterial activity was measured as serum bactericidal capacity and lysozyme activity.

Reduction of FO tends to reduce basal (pre-infection) genetic expression of both cytokines. However, complete FO replacement caused an over expression of both pro-inflammatory cytokines, particularly after 3 days of induced infection in fish fed soybean oil based diets. On the other hand, fish fed diets with low content of n-6 fatty acids showed better serum bactericidal capacity after infection, suggesting that the substitution of fish oil by vegetable oils containing high levels of n-6 fatty acids may induce imbalances on fish immune response, leading to a lower potential response against infections.