Monique Leberre

Mitochondrial Antiviral Signaling Protein Plays a Major Role in Induction of the Fish Innate Immune Response against RNA and DNA Viruses

ABSTRACT Viral infection triggers host innate immune responses through cellular sensor molecules which activate multiple signaling cascades that induce the production of interferons (IFN) and other cytokines. The recent identification of mammalian cytoplasmic viral RNA sensors, such as retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs) and their mitochondrial adaptor, the mitochondrial antiviral signaling protein (MAVS), also called IPS-1, VISA, and Cardif, highlights the significance of these molecules in the induction of IFN. Teleost fish also possess a strong IFN system, but nothing is known concerning the RLRs and their downstream adaptor. In this study, we cloned MAVS cDNAs from several fish species (including salmon and zebrafish) and showed that they were orthologs of mammalian MAVS. We demonstrated that overexpression of these mitochondrial proteins in fish cells led to a constitutive induction of IFN and IFN-stimulated genes (ISGs). MAVS-overexpressing cells were almost fully protected against RNA virus infection, with a strong inhibition of both DNA and RNA virus replication (1,000- and 10,000-fold decreases, respectively). Analyses of MAVS deletion mutants showed that both the N-terminal CARD-like and C-terminal transmembrane domains, but not the central proline-rich region, were indispensable for MAVS signaling function. In addition, we cloned the cDNAs encoding a RIG-I-like molecule from salmonid and cyprinid cell lines. Like the case with MAVS, overexpression of RIG-I CARDs in fish cells led to a strong induction of both IFN and ISGs, conferring on fish cells full protection against RNA virus infection. This report provides the first demonstration that teleost fish possess a functional RLR pathway in which MAVS may play a central role in the induction of the innate immune response.

Bioluminescence Imaging of Live Infected Salmonids Reveals that the Fin Bases Are the Major Portal of Entry for Novirhabdovirus

ABSTRACT Although Novirhabdovirus viruses, like the Infectious hematopietic necrosis virus (IHNV), have been extensively studied, limited knowledge exists on the route of IHNV entry during natural infection. A recombinant IHNV (rIHNV) expressing the Renilla luciferase gene was generated and used to infect trout. A noninvasive bioluminescence assay was developed so that virus replication in live fish could be followed hours after infection. We provide here evidence that the fin bases are the portal of entry into the fish. Confirmation was brought by the use of a nonpathogenic rIHNV, which was shown to persist in fins for 3 weeks postinfection.

Recovery of a Recombinant Salmonid Alphavirus Fully Attenuated and Protective for Rainbow Trout

ABSTRACT Sleeping disease virus (SDV) is a member of the new Salmonid alphavirus genus within the Togaviridae family. The single-stranded RNA genome of SDV is 11,894 nucleotides long, excluding the 3′ poly(A) tail. A full-length cDNA has been generated; the cDNA was fused to a hammerhead ribozyme sequence at the 5′ end and inserted into a transcription plasmid (pcDNA3) backbone, yielding pSDV. By transfection of pSDV into fish cells, recombinant SDV (rSDV) was successfully recovered. Demonstration of the recovery of rSDV was provided by immunofluorescence assay on rSDV-infected cells and by the presence of a genetic tag, a BlpI restriction enzyme site, introduced into the rSDV RNA genome. SDV infectious cDNA was used for two kinds of experiments (i) to evaluate the impact of various targeted mutations in nsP2 on viral replication and (ii) to study the virulence of rSDV in trout. For the latter aspect, when juvenile trout were infected by immersion in a water bath with the wild-type virus-like rSDV, no deaths or signs of disease appeared in fish, although they were readily infected. In contrast, cumulative mortality reached 80% in fish infected with the wild-type SDV (wtSDV). When rSDV-infected fish were challenged with wtSDV 3 and 5 months postinfection, a long-lasting protection was demonstrated. Interestingly, a variant rSDV (rSDV14) adapted to grow at a higher temperature, 14°C instead of 10°C, was shown to become pathogenic for trout. Comparison of the nucleotide sequences of wtSDV, rSDV, and rSDV14 genomes evidenced several amino acid changes, and some changes may be linked to the pathogenicity of SDV in trout.