IFN-adjuvanted DNA vaccine against infectious salmon anemia virus: Antibody kinetics and longevity of IFN expression

• IFN plasmids increased the antibody response induced by the ISAV hemagglutinin-esterase (HE) plasmid through week 22.
• The anti-ISAV antibody kinetics was different using IFNa- and IFNc plasmids as adjuvants.
• Fish injected with IFNc-plasmid alone showed systemic expression of Mx and ISG15 during 22 weeks.
• Vaccination with IFNc- and HE-plasmids together abolished systemic expression of Mx and ISG15 after 16–22 weeks.
• This suggests that the adaptive immune response against HE causes elimination of cells expressing HE.

Plasmids expressing interferon (IFN) have recently been shown to function as adjuvants in Atlantic salmon when co-injected with a DNA vaccine encoding hemagglutinin-esterase (HE) from infectious salmon anemia virus (ISAV). In this work we have compared the antibody kinetics and the systemic Mx/ISG15 response of fish vaccinated with HE-plasmid using either IFNa plasmid (pIFNa) or pIFNc as adjuvants over a longer time period, i.e. 22 weeks post vaccination (pv). The results showed that the antibody response against ISAV with pIFNa as adjuvant arose earlier (7 weeks pv) than with pIFNc as adjuvant (10 weeks pv), peaked at week 10 and declined at week 22. The antibody response with pIFNc as adjuvant peaked at 16 weeks and kept at this level 22 weeks pv. Fish injected with pIFNc alone expressed high levels of Mx and ISG15 in liver throughout the 22 week period. In contrast, fish injected with pIFNc together with HE-plasmid expressed high levels of Mx and ISG15 in liver for the first 10 weeks, but at week 16 this response was absent in two of three fish at week 16 and was absent in all tested fish at week 22 pv. This suggests that cells expressing HE and IFNc are intact at week 10 pv, but are eliminated by adaptive immune responses after week 10 due to recognition of HE. The longevity of the Mx/ISG15 response in pIFNc treated fish is likely due to the fact that IFNc is a self-antigen of salmon and is not attacked by the adaptive immune system.