A bivalent Vibrio harveyi DNA vaccine induces strong protection in Japanese flounder (Paralichthys olivaceus).

Vibrio harveyi is a severe aquaculture pathogen that can infect a large number of fish species. In previous studies, we have identified two protective immunogens, DegQ and Vhp1, from pathogenic V. harveyi strains. In this study, we examined the potentials of DegQ and Vhp1 as monovalent and bivalent DNA vaccines in a Japanese flounder (Paralichthys olivaceus) model. For this purpose, the DNA vaccine plasmids pDegQ, pVhp1, and pDV were constructed. pDegQ and pVhp1 express, respectively, degQ and vhp1, while pDV expresses degQ and vhp1 as a bicistronic mRNA transcript (degQ-vhp1). Following vaccination, all vaccine plasmids were detected in muscle, spleen, and kidney at 7 and 14 days post-vaccination. At the same time points and in the same tissues, degQ, vhp1, and degQ-vhp1 mRNA transcripts were detected in, respectively, pDegQ-, pVhp1-, and pDV-vaccinated fish. Production of DegQ and Vhp1 proteins was found in the muscle tissues of pDegQ- and pVhp1-vaccinated fish respectively, while both DegQ and Vhp1 proteins were detected in pDV-vaccinated fish. Compared to control fish, fish vaccinated with pDegQ and pVhp1 exhibited, respectively, 64.1% and 56.5% relative percent survival rates following lethal V. harveyi challenge, while fish vaccinated with pDV exhibited a survival rate that is more than 20% higher than those induced by pDegQ and pVhp1. Furthermore, in addition to V. harveyi, pDV also elicited dramatic cross protection against Vibrio parahaemolyticus. Immunological analysis showed that all vaccine plasmids induced the production of specific serum antibodies and the expression of a wide range of immune genes; however, the induction folds effected by pDV were in general higher than those effected by pDegQ and pVhp1. These results indicate that DNA vaccines, such as that represented by pDV, is a good vaccination option to combat V. harveyi-related vibriosis.