Protective efficacy and immune responses induced by a DNA vaccine encoding codon-optimized PPA1 against Photobacterium damselae subsp. piscicida in Japanese flounder

Photobacterium damselae subsp. piscicida (Pdp) kills many cultured marine fish. As it evolves resistance to existing vaccines, new vaccines are needed. PPA1 is a major antigenic protein of Pdp. Here, DNA vaccines encoding wild-type PPA1 (pPPA1wt) and codon-optimized PPA1 (pPPA1opt) were constructed and tested against Pdp in Japanese flounder. The mRNA levels of the two antigenic genes at the vaccination site were not different, but the protein level was significantly higher in the pPPA1opt-vaccinated fish. In addition, after a bacterial challenge, the levels of interleukin (IL)-1β, IL-6 and IFN-γ mRNA significantly increased in the pPPA1opt-vaccinated fish but not in the pPPA1wt-vaccinated fish. The relative percent survival (RPS) after the challenge was higher in the pPPA1opt-vaccinated fish (90.9) than in the pPPA1wt-vaccinated fish (69.2). At the early stage of the infection after the challenge, the number of viable Pdp in the spleen was significantly lower in the pPPA1opt-vaccinated fish than in the pPPA1wt-vaccinated fish. These data show that codon-optimized DNA vaccine pPPA1opt had a strong immunogenicity and conferred protective efficacy against Pdp infection in Japanese flounder.