Flagellin enhances the immunoprotection of formalin-inactivated Edwardsiella tarda vaccine in turbot

Edwardsiella tarda is an important fish pathogen that causes extensive losses in farmed fish around the world. Various types of vaccines have been developed against E. tarda infection, such as inactivated vaccine, live attenuated vaccine and subunit vaccine. However, inactivated vaccine exhibits poor immune protection against edwardsiellosis in some reports. In this study, we aimed to evaluate the protective effect of formalin-killed cells (FKC) against E. tarda with flagellin, FlgD, as an adjuvant in turbot. As a result, the relative percent survival (RPS) of turbot vaccinated with FKC reached 70% in the presence of FlgD adjuvant. Moreover, specific serum antibodies reached the highest level at 3 weeks post vaccination. Furthermore, the transcription of some immune response-related genes, such as MHC-I, IgM, IL-1β, TCR, and TNFα were up-regulated in turbot after vaccination, indicating that both humoral and cellular immune responses were induced by this vaccine. In summary, flagellin can enhance immunoprotection of the formalin-inactivated E. tarda vaccine, which can be used as a potential adjuvant in fish vaccine.