Construction of KHV-CJ ORF25 DNA vaccine and immune challenge test

KHV ORF25 fragments were cloned from Koi Herpes Virus-CJ (KHV-CJ) strains isolated in our laboratory. The amplified products were inserted into the eukaryotic expression vector pIRES-neo, forming recombinant plasmid pIRES-ORF25. The recombinant plasmid pIRES-ORF25 at 1 μg per koi, 10 μg per koi and 50 μg per koi was intramuscularly injected into healthy kois, respectively. The results showed that the recombinant pIRES-ORF25 could induce the production of specific antibodies in koi determined by indirect ELISA. The differences of immune effect between three doses were not significant (P > 0.05), but all of them could induce the production of neutralizing antibodies. The immune challenge test showed that the mortality of koi injected with PBS, blank pIRES-neo vector and nothing was 90%, 92.5% and 85% at 25 days. While the mortalities of koi injected with eukaryotic expression plasmid pIRES-ORF25 were 20%, 17.5% and 12.5%. Differences in comparison with the control group were highly significant (P < 0.01). Histopathological staining revealed that the tissues of the immunized koi did not change apparently. In conclusion, the DNA vaccine pIRES-ORF25 construct could well protect koi against KHV and had the potential to be applied in practice.