Immunomodulatory effect of different dietary ARA/EPA/DHA ratios in gilthead sea bream ( Sparus aurata ) juveniles after infection with Photobacterium damselae subsp. piscicida

Abstract

The immunomodulatory effect of different dietary arachidonic acid (ARA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA) ratios was evaluated in gilthead sea bream juveniles after intraperitoneal injection (i.p.) with formalin-killed Photobacterium damselae subsp. piscicida (Phdp). Fish were fed with four isoproteic (47% crude protein) and isolipidic (18% crude lipids) diets with the following long-chain polyunsaturated fatty acids (LC-PUFA) ratios: Diet A (2.0% ARA: 0.2% EPA: 0.1% DHA); Diet B (1.0% ARA: 0.4% EPA: 0.4% DHA); Diet C (0% ARA: 0.6% EPA: 0.6% DHA); Diet D (0% ARA: 0.3% EPA: 1.5% DHA). After being fed for 56 days with the experimental diets, fish from each dietary treatment were randomly selected and i.p. injected with 0.2 ml of killed Phdp (1 × 108 CFU ml−1) or phosphate-buffered saline (control group). Four and twenty-four hours (i.e., twice) after the i.p., the haematological profile, plasma humoral innate immune parameters, and the intestine expression of immune-related genes were assessed. Results showed that fish fed diet D (high DHA content) had increased complement activity and higher intestinal expression of cyclooxygenase 2 (cox2) and interleukin 1β (il-1β) at 4 and 24 hours after i.p., as well as increased intestinal tumour necrosis factor α (tnfα) expression at 4 hours. On the other hand, fish fed with diet A (high ARA content) showed higher plasma bactericidal activity against Phdp than fish fed with diet D. Overall, the results of this study suggest a positive modulatory effect of DHA in response to the pathogenic agent.