Protective effects of dietary Astragalus polysaccharides on large yellow croaker (Larimichthys crocea) against Vibrio alginolyticus infection

As one of the effective immunopotentiators, Astragalus polysaccharides (APS) are being used widely in prevention and control of fish bacteriosis. However, the understanding of the related immune protection mechanisms is still limited. In this study, the protective effects of dietary APS on large yellow croaker (Larimichthys crocea) following Vibrio alginolyticus infection were investigated. After continuous feeding for 42 d, fish in APS group exhibited higher survival rates against V. alginolyticus infection compared to the control group, and 500 mg/kg APS produced the highest relative percent survival (RPS) of 53.33%, therefore, 500 mg/kg (0.05%) supplementation was recommended in large yellow croaker farming from a standpoint of antibacterial infection. APS could enhance antioxidant level of fish by increasing serum T-AOC capacity and SOD activity and reducing MDA content, and improve innate immune response by increasing serum AKP and ACP activities. Head kidney differential transcriptome analysis between control group and 500 mg/kg APS group before and after V. alginolyticus infection corroborated the effects of APS on activation of antioxidant capacity and innate immunity by up-regulating the antioxidant-related genes and activating CTLR pathway, respectively. Additionally, APS could also activate fish adaptive immunity by reversing V. alginolyticus-induced inhibition on T and B cell receptor signaling pathways. All these findings suggested that APS may exert the protective effects against V. alginolyticus infection by improving antioxidant ability, innate and adaptive immune responses of large yellow croaker, thus providing insights into the protection mechanisms of APS in fish and highlighting their potential application in fish aquaculture.