Comparative study of the vaccine potential of six outer membrane proteins of Edwardsiella tarda and the immune responses of flounder (Paralichthys olivaceus) after vaccination

To obtain effective vaccine candidates for Edwardsiella tarda, six outer membrane proteins (OMPs) of E. tarda were selected and recombinantly expressed. Western blotting analysis showed that flounder anti-E. tarda serum could recognize rOmpV, rOmpK, rOmpX and rOmpI, but not rOmpH and rOmpR. Immunization of flounder with different rOMPs resulted in high levels of protection following the E. tarda challenge with relative percent survivals (RPS) of 92.1% (rOmpI), 84.2% (rOmpX), 68.4% (rOmpK), 60.5% (rOmpV), 57.9% (rOmpH) and 39.5% (rOmpR). Meanwhile, the immune response of flounder induced by rOMPs was investigated, and the results showed that: (1) The levels of specific antibodies against E. tarda induced by rOmpI and rOmpX were significantly higher at weeks 3 and 4 post-vaccination than those of the other rOMPs; (2) all six rOMPs could induce the proliferation of sIg+ lymphocytes, and the peaks of sIg+ lymphocytes in the blood and spleen of rOmpI-, rOmpX- and rOmpH-vaccinated fish were significantly higher than those of other rOMPs; (3) all six rOMPs could induce up-regulation of the MHC IIα, CD4-1, CD4-2, CD8α, CD8β, IL-1β, TNF-α and IFN-γ genes. Among them, the mRNA levels of the CD4-1 and CD4-2 genes induced by rOmpI and rOmpX were generally higher than those induced by the other rOMPs, while there was no significant difference in the expression of TLR2, TLR5M and MHC Iα between the rOMPs-vaccinated group and control group. These results demonstrate that rOmpI and rOmpX could produce a RPS of over 80% against the challenge of E. tarda and induce stronger immune responses in flounder, which indicates that OmpI and OmpX are promising vaccine candidates against E. tarda infection.