Abstract :
[en] Cyprinid herpesvirus 3 (CyHV-3) is the etiological agent of a highly contagious and lethal disease affecting koi and common carp worldwide. Beyond its economic importance, this virus turned out to be an interesting subject for fundamental research and is currently considered as the archetype of Alloherpesviridae. The divergence of this family with the Herpesviridae is ancestral, with almost no significant homology between CyHV 3 genes and those of Herpesviridae. Consequently, the extensive knowledge acquired for the latter cannot be used to predict CyHV 3 biological features. Virion transmembrane proteins (VTPs) are well documented in Herpesviridae for their involvement in crucial processes such as entry, immune evasion, morphogenesis and egress of progeny virions from the host cells. In contrast, very little is known about the functions of these proteins in CyHV 3 or in any of the alloherpesviruses. The main objective of this thesis was to provide a first functional characterization of CyHV-3 virion transmembrane proteome. The experimental work performed is summarized in two main chapters.
The first study aimed to update the list of known CyHV-3 VTPs and to determine those that are essential to viral growth in vitro. Using mass spectrometry approaches and mutagenesis experiments, we identified 16 VTPs in the CyHV-3 FL strain, among which 8 turned out to be essential to viral growth in vitro. The non-essential VTPs were further assessed quantitatively for their relative importance in vitro and in vivo. ORF25, ORF64, ORF108, ORF132, ORF136, ORF148, and ORF149 were shown to affect viral growth in vitro; while the lack of ORF148 or ORF25 caused attenuation to a minor or major extent in vivo, respectively. Finally, we showed that a mutant lacking ORF25 was highly attenuated but induced a moderate immune protection under the conditions tested.
Bioinformatic analyses suggested that CyHV-3 ORF27 encodes a VTP. However, no protein corresponding to this ORF was detected in our proteomic analyses. Analyses of genome sequence and protein expression demonstrate that the FL strain, like several other laboratory strains, do not express ORF27 due to presence of various mutations, while it encodes a VTP in field strains. These observation led us to hypothesize that the deletion of ORF27 could confer a selective advantage to viral growth in vitro, while the expression of a functional pORF27 could confer a selective advantage in vivo. These hypotheses were addressed by producing recombinant viruses. In vitro, the CyHV 3 strain lacking ORF27 expression was shown to have a replicative advantage, especially during co-infection with a viral strain expressing this protein. In contrast, in vivo, no difference between both genotypes could be detected in the experimental conditions tested. This observation suggests that the biological functions of ORF27 cannot be revealed in the laboratory conditions used.
In conclusion, this thesis provides a first functional characterization of the virion transmembrane proteome of CyHV-3. It represents a firm basis for further research on alloherpesvirus VTPs.
