Rational Design of a Safe and Efficacious Attenuated Recombinant Vaccine Against Cyprinid Herpesvirus 3 Using Prokaryotic Mutagenesis and In Vivo Imaging System

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 safe and efficacious vaccine. With this goal in mind, recombinant strains deleted for single gene were produced using prokaryotic recombination technologies. While producing such a recombinant for ORF134, we unexpectedly obtained a clone deleted for ORF56 and ORF57 in addition to the expected deletion of ORF134. Interestingly, this triple deleted recombinant replicated efficiently in vitro, exhibited an attenuated profile and induced an immune protection against a lethal challenge in vivo.

To determine the role of each deleted locus in the phenotype of the triple deleted recombinant, a series of recombinant were produced and tested in vivo. Firstly, to investigate the role of ORF134 deletion, a double ORF56-57 deleted recombinant and an independent triple ORF56-57-ORF134 deleted recombinant were produced. These experiments revealed that ORF134 deletion did contribute significantly neither to the attenuation nor to the immune protection observed for the triple deleted recombinant. Secondly, to determine the relative importance of ORF56 and ORF57 deletion in the attenuation observed, two single deleted recombinants were produced for ORF56 and ORF57. These experiments demonstrated that ORF57 deletion, in contrast to ORF56 deletion, correlates with an attenuated phenotype.

Based on its safety-efficacy profile, the ORF56-57 double deleted recombinant was selected as a candidate vaccine. Complementary experiments were performed to further investigate the potential of this strain as a safe and efficacious attenuated recombinant vaccine. The results obtained can be summarized as follows: (i) In vivo imaging (IVIS) of vaccinated fish challenged with a wild type strain expressing luciferase as a reporter gene, suggested that vaccination induces a sterile immunity at the portal of entry. (ii) Study of viral tropism by qPCR, IVIS, and histopathological analyses, demonstrated that in comparison to the wild type parental strain, the vaccine strain replicates at lower level, at later time post-infection and for a shorter period of time. (iii) Transmission studies demonstrated that there is no detectable spread of the vaccine from freshly vaccinated fish to naive fish located immediately down stream of vaccinated fish (water sharing).

All together, the results of the present study demonstrate the potential of ORF56-ORF57 double deleted CyHV-3 recombinant strain for safe and efficacious mass vaccination of carp.