DNA vaccine encoding molecular chaperone GroEL of Edwardsiella tarda confers protective efficacy against edwardsiellosis

In the present study, molecular chaperone GroEL of Edwardsiella tarda was found both react with flounder (Paralichthys olivaceus) anti-recombinant GroEL (rGroEL) and anti-E. tarda antibodies, and it was both detected in the outer membrane and the secretome. To evaluate the vaccine potential of GroEL, pCI-neo-GFP-GroEL (pCG-GroEL) was prepared and used to vaccinate healthy flounder, and immune protective effects were investigated. The results showed that pCG-GroEL produced an RPS of 60% following E. tarda challenge at week 5 after immunization. Moreover, GroEL transcripts and GFP-tagged GroEL could be detected in vaccinated flounder after immunization with pCG-GroEL, and the GFP-tagged GroEL could be also detected in HINAE cells after transfection with pCG-GroEL. Meanwhile, the immune response of flounder induced by pCG-GroEL was investigated, and the results showed that: (1) the levels of specific serum antibodies against E. tarda induced by pCG-GroEL were significantly higher than other groups at 3-5 week post vaccination; (2) pCG-GroEL could induce the proliferation of sIg+ lymphocytes, and the levels of sIg+ lymphocytes in blood, spleen and pronephros of pCG-GroEL vaccinated fish were significantly increased compared with the PBS vaccinated fish at 3 w, 2 w and 2 w after immunization, respectively; (3) MHCIα, MHCIIα, CD4-1, CD8α, IL-1β and TNF-α were significantly induced by immunization with pCG-GroEL, and the mRNA levels of CD4-1 and CD8α rapidly increased in pCG-GroEL vaccinated fish after challenge with E. tarda. Taking together, pCG-GroEL could elicit highly protective effects against E. tarda and induce strong immune response of flounder, suggesting that GroEL was a promising vaccine candidate.