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Nocardia seriolae has been identified as the causative agent of fish nocardiosis, resulting in serious economic losses in aquaculture. With an aim to screen potential candidates for vaccine development against N. seriolae , the in vivo -induced genes of N. seriolae in hybrid snakehead (Channa maculate ♀ × Channa argus ♂) model were profiled via in vivo -induced antigen technology (IVIAT) in the present study, and 6 in vivo -induced genes were identified as follows: IS701 family transposase (is701), membrane protein insertase YidC (yidC), ergothioneine biosynthesis glutamate-cysteine ligase (egtA), molybdopterin respectively-dependent oxidoreductase (mol), phosphoketolase family protein (Ppl), hypothetical protein 6747 (hp6747). Additionally, the yidC was inserted into eukaryotic expression vector pcDNA3.1-myc-his-A to construct a DNA vaccine named as pcDNA-YidC to evaluate immunoprotection in hybrid snakehead after artificial challenge with N. serioale. Results showed that the transcription of yidC was detected in spleen, trunk kidney, muscle and liver in vaccinated fish, suggesting that this antigenic gene can be recombinantly expressed in fish. Meanwhile, indexes of humoral immunity were evaluated in the vaccinated fish through assessing specific-antibody IgM and serum enzyme activities, including lysozyme (LZM), superoxide dismutase (SOD), acid phosphatase (ACP) and alkaline phosphatase (AKP). Quantitative real-time PCR analysis indicated that pcDNA-YidC DNA vaccine could notably enhance the expression of immune-related genes (CD4 、 CD8α 、 MHCIIα 、 TNFα 、 IL-1β and MHCIα) in 4 tissues (spleen, trunk kidney, muscle and liver) of the vaccinated fish. Finally, an immuno-protection with a relative survival rate of 65.71 % was displayed in vaccinated fish in comparison to the control groups. Taken together, these results indicate that pcDNA-YidC DNA vaccine could boost strong immune responses in hybrid snakehead and show preferably protective efficacy against N. seriolae , indicating that IVIAT is a helpful strategy to screen the highly immunogenic antigens for vaccine development against fish nocardiosis. • Six antigenic genes of Nocardia seriolae were screened via IVIAT technology. • The pcDNA-yidC was constructed and used as DNA vaccine to induce immune responses. • The trials of pcDNA-yidC DNA vaccine could provide 65.71 % RPS. • IVIAT can be used as a strategy to study the antigenic genes for vaccine development against N. seriolae. [ABSTRACT FROM AUTHOR]

Fish nocardiosis is a widespread chronic granulomatous disease in aquatic environment, which was particularly caused by Nocardia seriolae. The phage shock protein A (PspA) and tellurium resistance protein D (TerD) were identified to be the immunodominant antigens of the wild-type N. seriolae strain ZJ0503 in our previous study. In an attempt to develop effective DNA vaccines against this pathogen, PspA and TerD were used as candidates to ligate with pcDNA3.1-Flag plasmids, respectively. In addition, the abilities of these two DNA vaccines to elicit various immune responses in hybrid snakehead and supply protective efficacy against artificial challenge with N. seriolae were determined in the present study. The results showed that intramuscular injection with pcDNA-PspA and pcDNA-TerD did not exhibit cytotoxic activities in hybrid snakehead via histopathological examination. Besides, hybrid snakehead immunization with pcDNA-PspA and pcDNA-TerD could increase several non-specific immune paraments in serum, including LYZ, POD, ACP, AKP and SOD activities. Meanwhile, the pcDNA-TerD DNA vaccine could induce strongly specific antibody (IgM) titer in hybrid snakehead with a relative percent of survival (RPS) value of 83.14% against N. seriolae , while that of pcDNA-PspA DNA vaccine was displayed comparably low IgM titer with RPS value of 57.83%. Furthermore, quantitative real-time PCR assays presented that the expression of immune-related genes (MHCIα , MHCIIα , CD4 , CD8α , IL-1β and TNFα) were up-regulated to various degrees after vaccination with pcDNA-PspA or pcDNA-TerD, indicating that these two DNA vaccines were able to boost humoral and cell-mediated immune responses in hybrid snakehead. Taken together, both the pcDNA-PspA and pcDNA-TerD DNA vaccines were proved to be safe, immunogenic and effective in protecting hybrid snakehead against N. seriolae infection, which can promote the development and application of DNA vaccines to control fish nocardiosis in aquaculture. • Two DNA vaccines were prepared by utilizing the ribosomal proteins (PspA and TerD) as antigens for treatment of fish nocardiosis. • Both DNA vaccines not only did not exhibit cytotoxic activities but also could boost the innate, humoral and cellular immune response in hybrid snakehead. • Two DNA vaccines show highly protective efficacy against N. seriolae infection in hybrid snakehead with relative percentage survival (RPS) of 57.83% and 83.14%, respectively. [ABSTRACT FROM AUTHOR]