IL-7 enhances the protective efficacy of inactivated grass carp reovirus vaccine as adjuvant in grass carp (Ctenopharyngodon idella).

Grass carp reovirus (GCRV) causes severe hemorrhagic disease and significant threat to grass carp (Ctenopharyngodon idella) aquaculture. Although inactivated vaccines are commonly used, their limited protective efficacy necessitates effective adjuvants. In this study, recombinant grass carp interleukin-7 (rIL-7) and its enterokinase-cleaved form (EK rIL-7), obtained by removing the fusion tag using enterokinase, were successfully expressed, purified, and evaluated as adjuvants for an inactivated GCRV vaccine. Following GCRV challenge, both rIL-7 adjuvanted and EK rIL-7 adjuvanted vaccines significantly increased the survival of grass carp and effectively suppressed viral replication in the head kidney and spleen. Histopathological examination further confirmed that fish in the IL-7 adjuvant exhibited significantly reduced tissue damage compared to other treatments. Additionally, IL-7 enhanced both innate and adaptive immune responses in grass carp. The activities of serum enzymes (SOD, LDH and LZM) in the groups receiving IL-7 in combination with the vaccine increased significantly, reflecting the enhancement of antioxidant capacity and non-specific immune defense ability. In parallel, IL-7 treatment promoted IgM synthesis at both the transcriptional and serum antibody levels and increased the expression of immune-related genes such as CD4(-1, -2), MHCII (α, β), IFN-γ2, and IL-1β, indicating the activation of both humoral and cellular immune pathways. However, the difference between EK rIl-7 and rIl-7 is not significant. Collectively, these findings demonstrate that IL-7 acts as a potent adjuvant to enhance the protective efficacy of inactivated GCRV vaccines, and provide valuable insights for exploring other cytokines as immune adjuvants in the prevention of viral diseases in aquaculture.