Comparative study of the immune effect of an Edwardsiella tarda antigen in two forms: Subunit vaccine vs DNA vaccine

Edwardsiella tarda is a Gram-negative bacterial pathogen and the etiological agent of a systematic fish disease called edwardsiellosis, which affects a wide range of marine and freshwater fish. E. tarda vaccines in various forms have been reported by a number of research groups; however, comparative studies on the immune mechanisms of these vaccines are lacking. In this report, we identified a new E. tarda vaccine candidate, Eta2, and analyzed in a comparative manner the immune response induced by Eta2 in two different forms: purified recombinant subunit vaccine and DNA vaccine. Eta2 is a protein of 178 residues and shares high levels of sequence identities with the OmpH family of outer membrane protein chaperones of several bacterial species. Recombinant Eta2 (rEta2) purified from Escherichia coli was highly protective against E. tarda challenge in a Japanese flounder (Paralichthys olivaceus) model and produced relative percent of survival rates of 83% and 78%, respectively, at 4- and 8-week post-vaccination (p.v.). Eta2 as a DNA vaccine in the form of plasmid pCEta2 also induced strong protective immunity at 4- and 8-week p.v. Immunological analysis indicated that (i) rEta2 and pCEta2 enhanced head kidney macrophage activation at 1- and, for pCEta2, 7-day p.v.; (ii) rEta2 and pCEta2 induced similar patterns of serum antibody production, however, the antibodies induced by rEta2 were of much higher levels and afforded stronger passive immunoprotection upon naïve flounder than those induced by pCEta2; (iii) both rEta2 and pCEta2 upregulated the expression of specific and nonspecific immune factors which include, in the case of pCEta2 but not rEta2, interferon, interferon-induced Mx protein, and CD8α; however, the induction patterns effected by rEta2 and pCEta2 were different. While high levels of interleukin 1β (IL-1β), natural killer cell enhancing factor, Mx, MHC Iα, and IgM inductions were observed in pCEta2-vaccinated fish, only IL-1β, complement C3, and IgM inductions were highly induced in rEta2-vaccinated fish. Taken together, these results indicate that both rEta2 and pCEta2 induce specific and nonspecific immunities, however, pCEta2 induces both B cell and T cell responses, whereas rEta2 induces mainly humoral response.