Vibrio harveyi Hsp70: Immunogenicity and application in the development of an experimental vaccine against V. harveyi and Streptococcus iniae

Vibrio harveyi and Streptococcus iniae are severe bacterial pathogens of farmed fish. In a previous study, we have identified a secretory antigen, Sia10, from S. iniae. In this study, we cloned and analyzed an hsp70 gene from a pathogenic V. harveyi strain. We found that purified recombinant Hsp70 possesses ATPase activity and, when used as a subunit vaccine, could induce protection in Japanese flounder (Paralichthys olivaceus) against V. harveyi infection. Based on this observation, we fused Hsp70 to Sia10 and thus obtained a chimeric antigen Sia10-Hsp70, which was expressed from a plasmid pTSH contained in Escherichia coli DH5α. Western blot analysis showed that DH5α/pTSH was able to secret Sia10-Hsp70 into the extracellular milieu. The potential of DH5α/pTSH as a live vector vaccine was examined in a flounder model, which showed that DH5α/pTSH induced effective protection against not only V. harveyi infection but also S. iniae infection. ELISA analysis showed that fish vaccinated with DH5α/pTSH produced specific serum antibodies against both Hsp70 and Sia10. These results indicate that (i) V. harveyi Hsp70 is a biologically active protein with immunoprotective property; (ii) the chimeric antigen Sia10-Hsp70 delivered by a live bacterial host is an effective vaccine candidate against V. harveyi and S. iniae infection in aquaculture; (iii) using recombinant bacteria as vaccine hosts not only enables easier vaccine preparation but also easy construction of vaccines with cross protective potentials.