Experimental transmission of sleeping disease in one-year-old rainbow trout, Oncorhynchus mykiss (Walbaum), induced by sleeping disease virus

The C-Terminal Domain of Salmonid Alphavirus Nonstructural Protein 2 (nsP2) Is Essential and Sufficient To Block RIG-I Pathway Induction and Interferon-Mediated Antiviral Response

Salmonid alphavirus (SAV) is an atypical alphavirus that has a considerable impact on salmon and trout farms. Unlike other alphaviruses, such as the chikungunya virus, SAV is transmitted without an arthropod vector, and it does not cause cell shutoff during infection. The mechanisms by which SAV escapes the host immune system remain unknown. By studying the role of SAV proteins on the RIG-I signaling cascade, the first line of defense of the immune system during infection, we demonstrated that nonstructural protein 2 (nsP2) effectively blocks the induction of type I interferon (IFN). This inhibition, independent of the protease activity carried by nsP2, occurs downstream of IRF3, which is the transcription factor allowing the activation of the IFN promoter and its expression. The inhibitory effect of nsP2 on the RIG-I pathway depends on the localization of nsP2 in the host cell nucleus, which is linked to two nuclear localization sequences (NLS) located in its C-terminal part. The C-terminal domain of nsP2 by itself is sufficient and necessary to block IFN induction. Mutation of the NLS of nsP2 is deleterious to the virus. Finally, nsP2 does not interact with IRF3, indicating that its action is possible through a targeted interaction within discrete areas of chromatin, as suggested by its punctate distribution observed in the nucleus. These results therefore demonstrate a major role for nsP2 in the control by SAV of the host cell's innate immune response. IMPORTANCE The global consumption of fish continues to rise, and the future demand cannot be met by capture fisheries alone due to limited stocks of wild fish. Aquaculture is currently the world's fastest-growing food production sector, with an annual growth rate of 6 to 8%. Recurrent outbreaks of SAV result in significant economic losses with serious environmental consequences for wild stocks. While the clinical and pathological signs of SAV infection are fairly well known, the molecular mechanisms involved are poorly described. In the present study, we focus on the nonstructural protein nsP2 and characterize a specific domain containing nuclear localization sequences that are critical for the inhibition of the host innate immune response mediated by the RIG-I pathway.

Fine mapping of a salmonid E2 alphavirus neutralizing epitope

In this study, we aimed to characterize the epitope recognized by the neutralizing 17H23 mAb directed against the E2 glycoprotein of most of salmonid alphavirus (SAV) subtypes and widely used in several laboratories to routinely diagnose SAV. We hypothesized that the 17H23 epitope was located in the major domain B, previously identified in the E2 of mammalian alphaviruses as the domain recognized by most of the E2 neutralizing mAbs. Indeed, the SAV E2 domain B counterpart is contained in the protein domain previously characterized as being recognized by mAb 17H23. Thus, to precisely characterize the 17H23 epitope, we developed an alanine scanning mutagenesis approach coupled with the generation of the respective recombinant SAV (rSAV) by using the available infectious cDNA. Ten mutant rSAVs termed A-J from E2 aa 223-236 were produced and characterized in vitro using indirect immunofluorescence assays on virus-infected cells with mAbs 17H23, 51B8 (another non-neutralizing anti-E2 mAb) and 19F3 directed against the non-structural protein nsp1. Two of the mutant rSAVs (G and H) escaped neutralization by mAb 17H23. In addition, we showed that when juvenile trout were infected by bath immersion with the rSAV mutants, some of them were either totally (D, E and G) or partially (H) attenuated. Together, the data from the in vitro and in vivo experiments indicated that the putative 17H23 amino acid sequence epitope comprised the short amino acid sequence (227)FTSDS(231).

Rainbow trout (Oncorhynchus mykiss) muscle satellite cells are targets of salmonid alphavirus infection

Sleeping disease in rainbow trout is characterized by an abnormal swimming behaviour of the fish which stay on their side at the bottom of the tanks. This sign is due to extensive necrosis and atrophy of red skeletal muscle induced by the sleeping disease virus (SDV), also called salmonid alphavirus 2. Infections of humans with arthritogenic alphaviruses, such as Chikungunya virus (CHIKV), are global causes of debilitating musculoskeletal diseases. The mechanisms by which the virus causes these pathologies are poorly understood due to the restrictive availability of animal models capable of reproducing the full spectrum of the disease. Nevertheless, it has been shown that CHIKV exhibits a particular tropism for muscle stem cells also known as satellite cells. Thus, SDV and its host constitute a relevant model to study in details the virus-induced muscle atrophy, the pathophysiological consequences of the infection of a particular cell-type in the skeletal muscle, and the regeneration of the muscle tissue in survivors together with the possible virus persistence. To study a putative SDV tropism for that particular cell type, we established an in vivo and ex vivo rainbow trout model of SDV-induced atrophy of the skeletal muscle. This experimental model allows reproducing the full panel of clinical signs observed during a natural infection since the transmission of the virus is arthropod-borne independent. The virus tropism in the muscle tissue was studied by immunohistochemistry together with the kinetics of the muscle atrophy, and the muscle regeneration post-infection was observed. In parallel, an ex vivo model of SDV infection of rainbow trout satellite cells was developed and virus replication and persistence in that particular cell type was followed up to 73 days post-infection. These results constitute the first observation of a specific SDV tropism for the muscle satellite cells.

First outbreak of sleeping disease in Switzerland: disease signs and virus characterization

Sleeping disease is a contagious disease mainly of freshwater farmed rainbow trout, caused by salmonid alphavirus (SAV) Subtype 2. Here we describe the first case in Switzerland. Pathological changes ranged from acute pancreas necrosis to more chronic lesions with complete loss of exocrine pancreas and simultaneous degenerative, inflammatory and regenerative heart and muscle lesions. The partial sequencing of SAV E2 and nsp3 genes placed the Swiss SAV variant within the Subtype 2 clustering together with freshwater isolates from UK and continental Europe. Although mortality stayed low, growth rates were significantly reduced, making the disease economically relevant.

Development of infectious cDNA clones of Salmonid alphavirus subtype 3

Background

Salmonid alphavirus (SAV) is a widespread pathogen in European aquaculture of salmonid fish. Distinct viral subtypes have been suggested based on sequence comparisons and some of these have different geographical distributions. In Norway, only SAV subtype 3 have so far been identified. Little is known about viral mechanisms important for pathogenesis and transmission. Tools for detailed exploration of SAV genomes are therefore needed.

Results

Infectious cDNA clones in which a genome of subtype 3 SAV is under the control of a CMV promoter were constructed. The clones were designed to express proteins that are putatively identical to those previously reported for the SAVH20/03 strain. A polyclonal antiserum was raised against a part of the E2 glycoprotein in order to detect expression of the subgenomic open reading frame (ORF) encoding structural viral proteins. Transfection of the cDNA clone revealed the expression of the E2 protein by IFAT, and in serial passages of the supernatant the presence of infectious recombinant virus was confirmed through RT-PCR, IFAT and the development of a cytopathic effect similar to that seen during infection with wild type SAV. Confirmation that the recovered virus originated from the infectious plasmid was done by sequence identification of an introduced genetic tag. The recombinant virus was infectious also when an additional ORF encoding an EGFP reporter gene under the control of a second subgenomic alphavirus promoter was added. Finally, we used the system to study the effect of selected point mutations on infectivity in Chinook salmon embryo cells. While introduced mutations in nsP2₁₉₇, nsP3₂₆₃ and nsP3₃₂₃ severely reduced infectivity, a serine to proline mutation in E2₂₀₆ appeared to enhance the virus titer production.

Conclusion

We have constructed infectious clones for SAV based on a subtype 3 genome. The clones may serve as a platform for further functional studies.

Alphavirus infections in salmonids--a review

The first alphavirus to be isolated from fish was recorded in 1995 with the isolation of salmon pancreas disease virus from Atlantic salmon, Salmo salar L., in Ireland. Subsequently, the closely related sleeping disease virus was isolated from rainbow trout, Oncorhynchus mykiss (Walbaum), in France. More recently Norwegian salmonid alphavirus (SAV) has been isolated from marine phase production of Atlantic salmon and rainbow trout in Norway. These three viruses are closely related and are now considered to represent three subtypes of SAV, a new member of the genus Alphavirus within the family Togaviridae. SAVs are recognized as serious pathogens of farmed Atlantic salmon and rainbow trout in Europe. This paper aims to draw together both historical and current knowledge of the diseases caused by SAVs, the viruses, their diagnosis and control, and to discuss the differential diagnosis of similar pathologies seen in cardiomyopathy syndrome and heart and skeletal muscle inflammation of Atlantic salmon.