Immunogenicity Modulation of VHSV-IVb upon Natural Changes in the Great Lakes and by Matrix Protein Mutation

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For more than a decade the Laurentian Great Lakes region of North America has hosted a unique Viral Hemorrhagic Septicemia virus (VHSV) genotype -IVb. Mortality outbreaks affected up to 32 fish species between 2005-2009, when several new haplotypes emerged approximating a “quasi-species” distribution pattern. Thirty-seven sites were investigated between 2015-2016, screening 2561 individuals of 55 species for VHSV-IVb detection. Seven species and 21 individuals were tested positive from Lake Erie (76%) and Lake Michigan (24%), but all specimens lacked evident clinical sings and the original VHSV haplotypes appear eradicated. Three new selected Lake Erie isolates, respectively from round goby (Neogobius melanostomus), Gizzard shad (Dorosoma cepedianum) and largemouth bass (Micropterus salmoides), were compared to the original muskellunge (Esox masquinongy) - IVb isolate. Upon inoculation on fish cell lines, BF-2, EPC, RTG-2 and RTgill-W1, those new isolates showed differential pathogenicity, kinetics of replication and profiles of antiviral gene transcription (focusing on the Type I IFN pathway). Despite decades of effort worldwide, commercial vaccines against VHSV are not yet available for use in aquaculture. Infective viral strains attenuated by reverse genetic could provide a valuable platform towards vaccination strategies. Novirhabdoviruses express 6 viral genes encoding structural and nonstructural proteins, some of which remain poorly characterized. Matrix protein (M) acts as a potent inhibitor of the host transcription. Using a reverse genetic system, single (D62A) or double (D62/E181A) amino acid changes to M were incorporated into a recombinant wild type (rWT) VHSV-IVb. Their replicative capacity, pathogenicity and immunogenicity were assessed upon infection of fish cell lines. VHSV-IVb harboring double M-mutations showed a reduced anti-transcriptional activity, but also decreased replicative abilities as compared to rWT. VHSV-IVb harboring a single M-mutation showed a slight decreased antitranscriptional action, but retained rWT replicative capacity. Interestingly, both M-mutants exhibit reduced cytopathicity and allowed higher antiviral host responsiveness when compared to rWT.


Poster presentation at the the 18th International Conference of Diseases of Fish and Shellfish, Belfast, UK.