Dietary supplementation of yeast mannan enhances antiviral immunity of zebrafish (Danio rerio)

Spring viraemia of carp virus (SVCV) is a critical pathogen of cyprinids. In this study, we investigated the effects of mannan on the antiviral resistance of zebrafish and elucidated the underlying mechanisms. Zebrafish were fed with a control diet or diet supplemented with 0.2%, 0.4%, 0.6% and 0.8% of mannan for 2 weeks, and were challenged by SVCV. The intestinal microbiota was evaluated by 16S rRNA gene pyrosequencing. Zebrafish embryonic fibroblast (ZF4) cells were treated with mannan and were infected by SVCV, and the expression of antiviral genes was investigated. In addition, the effect of the in vitro culture supernatant of mannan-associated microbiota on SVCV infection was evaluated in ZF4 cells model. Results showed that dietary 0.2%, 0.4% and 0.6% mannan significantly improved the survival rate of zebrafish compared with the control group (P < 0.05). Consistent with in vivo results, mannan significantly inhibited the replication of SVCV in ZF4 cells (P < 0.05). Moreover, mannan inhibited the adsorption of SVCV to ZF4 cells (P < 0.05). The expression of type I interferon (IFN) signaling pathway related genes, including IRF3, IFNφ1, IFNφ2, IFNφ3, and PKZ, was significantly increased in mannan treated cells versus control (P < 0.05). Dietary mannan modulated gut microbiota, with enrichment of Proteobacteria and reduced abundance of Firmicutes, Actinobacteriota, and Fusobacteriota. Furthermore, the culture supernatant of mannan-associated microbiota significantly inhibited SVCV replication in ZF4 cells versus control microbiota counterpart (P < 0.05). Initial evaluation suggests that the antiviral effect of culture supernatant of mannan-associated microbiota involves a heat stable component derived from the commensal bacteria. To sum up, our results indicate that the mannan enhanced the resistance of zebrafish against SVCV infection. The mechanism underlying the antiviral effect of mannan include enhancement of IFN signaling and inhibition of viral adsorption. Dietary addition of Mannan altered the structure of the intestinal microbiota of zebrafish, and the intestinal microbiota also contributed to the anti-viral effect of mannan.