Development of an attenuated recombinant vaccine against Cyprinid herpesvirus 3

The cyprinid herpesvirus 3 (CyHV-3, genus Cyprinivirus, family Alloherpesviridae, order Herpesvirales) is the aetiological agent of a highly contagious and lethal disease affecting koi and common carp (Cyprinus carpio). Common carp is currently the third most important fish species of aquaculture with a world production between 3 and 5 million tons per year. A safe and efficacious attenuated vaccine compatible with mass vaccination is needed to control CyHV-3. In this thesis, we produced an ORF56-57 double deleted (Δ56-57) attenuated recombinant vaccine candidate using prokaryotic mutagenesis. The safety of this vaccine candidate was demonstrated using in vivo bioluminescent imaging system (IVIS), qPCR, and histopathological examination. In comparison to the parental wild type strain, the vaccine candidate replicated at lower levels and spread less efficiently to secondary sites of infection. Transmission experiments allowing contamination through water with or without additional physical contact between fish demonstrated that the vaccine candidate has a reduced ability to spread from vaccinated fish to naïve sentinel cohabitants. Finally, challenge experiments demonstrated that the vaccine candidate induces a protective mucosal immune response at the portal of entry. The first part of this thesis led to the rational development of a recombinant attenuated vaccine against CyHV-3 compatible with mass vaccination of carp.
In the second part of this thesis, the relative contributions of ORF56 and ORF57 to the safety and efficacy profile of the Δ56-57 vaccine candidate have been assessed by producing and phenotyping viruses individually deleted for ORF56 or ORF57. Inoculation of these viruses to carp demonstrated that the deletion of ORF56 did not affect virulence, whereas the absence of ORF57 led to an attenuation comparable to, though slightly less than, that of the Δ56-57 vaccine candidate. To demonstrate further the role of ORF57 as a key virulence factor, a mutant retaining the ORF57 region but unable to express the ORF57 protein was produced by inserting multiple in-frame stop codons into the coding region. Analysis of this virus in vivo revealed a safety and efficacy profile comparable to that of the double deleted virus. These findings show that CyHV-3 ORF57 encodes an essential virulence factor.
CyHV-3 is considered as the prototype of the genus Cyprinivirus which encompasses a growing list of phylogenetically related viruses causing massive economical losses to the aquaculture sector. This thesis led for the first time to the identification of an essential virulence factor of CyHV-3 and to the demonstration that recombinant viruses deleted for this gene are potential vaccine candidates. All cypriniviruses described to date encode orthologues of CyHV-3 ORF57. Consequently, the present work opens new perspectives for the development of recombinant attenuated vaccines against this economically important viral genus.