Protective efficacies and immune responses induced by recombinant HCD, atpD and gdhA against bacterial cold-water disease in ayu (Plecoglossus altivelis)

• Protective antigens against Flavobacterium psychrophilum infection were identified.
• Ayu was vaccinated with 3 proteins and challenged with F. psychrophilum.
• Vaccinated fish had a significantly longer survival time after the challenge.
• Serum from vaccinated fish recognized surface protein of F. psychrophilum.
• These 3 surface proteins are good vaccine candidates against F. psychrophilum.

Protective efficacies of three antigenic proteins (3-hydroxyacyl-CoA dehydrogenase (HCD), ATP synthase beta subunit (atpD), and glutamate dehydrogenase (gdhA)) against Flavobacterium psychrophilum were investigated in ayu (Plecoglossus altivelis). Recombinant proteins of HCD, atpD, and gdhA were expressed in Escherichia coli BL21 cells. Ayu were then vaccinated with inactivated cells via the intraperitoneal route. Compared with the empty BL21- and PBS-injected groups, the vaccinated group had a significantly longer survival time after challenge with F. psychrophilum. The antibody titers against each recombinant protein were significantly higher in serum from vaccinated fish, compared with serum from control fish. Results of indirect immunofluorescence assays using serum indicated that the HCD, atpD, and gdhA proteins are located on the surface of F. psychrophilum. These results suggest that these three surface proteins are protective antigens and are good candidates for development of vaccines against bacterial cold-water disease in ayu.