Construction and analysis of the immune effect of an Edwardsiella tarda DNA vaccine encoding a D15-like surface antigen

Edwardsiella tarda is a severe aquaculture pathogen with a broad host range that includes both humans and animal. In a previous study, we have identified in E. tarda a D15-like surface antigen, Esa1, which, when used as a recombinant subunit vaccine, is able to induce protective immunity in Japanese flounder (Paralichthys olivaceus) against E. tarda challenge. In this study, we examined further the immunoprotective potential of Esa1 as a DNA vaccine. For this purpose, the DNA vaccine plasmid pCEsa1 was constructed, which expresses esa1 under the cytomegalovirus immediate-early promoter. The vaccine potential of pCEsa1 was analyzed in the Japanese flounder model. The results showed that following vaccination, pCEsa1 and esa1 transcripts were detected in the muscle, liver, spleen, and kidney of the fish at 7, 21, and 49 days post-vaccination (p.v.). Production of Esa1 protein was also detected in the muscle tissue of pCEsa1-vaccinated fish. Compared to control fish, fish vaccinated with pCEsa1 exhibited significantly increased survival rates following E. tarda challenge at one and two months p.v.. Immunological analysis showed that vaccination with pCEsa1 (i) enhanced the respiratory burst activity, acid phosphatase activity, and bactericidal activity of head kidney macrophages; (ii) increased serum bactericidal activity in a Ca2+-independent manner; (iii) induced the production of specific serum antibodies, which became detectable at 3-week p.v. and afforded 57% protection (in terms of relative percent survival) upon naïve fish as determined by passive immunization; (iv) upregulated the expression of a broad spectrum of genes possibly involved in both innate and adaptive immunity. These results indicate that pCEsa1 is an effective vaccine candidate against E. tarda and also provide insights to the immune mechanism of bacterial DNA vaccine.