Adjuvant activity of fish type I interferon shown in a virus DNA vaccination model

There is a need for more efficient vaccines to combat viral diseases of Atlantic salmon and other farmed fish. DNA vaccines are highly effective against salmonid rhabdoviruses, but have shown less effect against other viruses. In the present work we have studied if type I IFNs might be used as adjuvants in fish DNA vaccines. For this purpose we chose a DNA vaccine model based on the hemagglutinin-esterase (HE) gene of infectious salmon anemia virus (ISAV) as antigen. Salmon presmolts were injected with a plasmid encoding HE alone or together with a plasmid encoding Atlantic salmon type I IFN (IFNa1, IFNb or IFNc). Sera were harvested after 7-10 weeks for measurements of antibody against ISAV and the fish were challenged with ISAV to measure protective effects of the vaccines. The results showed that all three IFN plasmids delivered together with HE plasmid potently enhanced protection of salmon against ISAV mediated mortality and stimulated an increase in IgM antibodies against the virus. In contrast, HE plasmid alone gave low antibody titers and a minor protection against ISAV. This demonstrates that type I IFNs stimulate adaptive immune responses in fish, which may be a benefit also in other fish DNA vaccines. Quantitative RT-PCR studies showed that the salmon IFNs caused an increased influx of B-cells and cytotoxic T-cells at the muscle injection site, which may in part explain the adjuvant effect of the IFNs.