A conserved protein of cypriniviruses is a major virulence factor of Cyprinid herpesvirus 3

Cyprinid herpesvirus 3 (CyHV-3) is the causative agent of a lethal disease in common and koi carp. Since its emergence, in the late 1990s, CyHV 3 has caused severe economic losses worldwide creating a need for a vaccine. In a previous study, we showed that the double deletion of ORF56 and ORF57 is associated with a safe and efficacious profile in vivo. This attenuated vaccine candidate is currently under development.
In the present study, we investigated the contribution of each gene to the attenuated phenotype observed. To do so, a series of recombinant single deleted for ORF56 or ORF57 were produced. The deleted sequences were removed in between predicted eukaryotic promoters for neighbored ORFs and replaced by a galactokinase cassette. These recombinants were characterized in vitro for their correct molecular structure. In addition, immunofluorescence staining showed that the deletion of ORF56 did not abrogate the expression of ORF57, and vice versa. These recombinants were further tested in vivo and revealed that the attenuation resulted mostly from the single deletion of ORF57.
In conclusion, this study demonstrates the role ORF57 as a major virulence factor of CyHV-3, responsible for most of the attenuation observed in the double deleted ORF56-57 vaccine candidate. Interestingly, ORF57 is conserved in cypriniviruses and could therefore represent a target for attenuated vaccine development in several other major fish pathogens.