The reverse genetics applied to fish RNA viruses

Effect of mutations in GDNV motif of viral hemorrhagic septicemia virus (VHSV) L protein on polymerase activity, viral growth, and in vivo virulence

Most Mononegavirales viruses have a GDNQ motif within the L protein, whereas Novirhabdovirus species feature a GDNV motif. This study examined the function of the GDNV motif within the L protein of viral hemorrhagic septicemia virus (VHSV) by modifying its amino acid composition. Substituting the aspartic acid (D) with valine (V) completely abolished polymerase activity in a minigenome assay. Replacing GDNV with GDNQ showed no significant difference in luciferase activity. Further characterization using reverse genetically engineered recombinant viruses revealed that rVHSV-LGDNQ exhibited an accelerated replication rate and higher virus titer in EPC cells than rVHSV-wild. Olive flounder infected with rVHSV-LGDNQ experienced higher early-stage mortality but lower overall mortality than those infected with rVHSV-wild. These findings suggest that while the GDNQ motif may positively influence VHSV replication speed, it may not confer an overall advantage for the ultimate viral pathogenicity.

RNA Interference Applied to Crustacean Aquaculture

RNA interference (RNAi) is a powerful tool that can be used to specifically knock-down gene expression using double-stranded RNA (dsRNA) effector molecules. This approach can be used in aquaculture as an investigation instrument and to improve the immune responses against viral pathogens, among other applications. Although this method was first described in shrimp in the mid-2000s, at present, no practical approach has been developed for the use of dsRNA in shrimp farms, as the limiting factor for farm-scale usage in the aquaculture sector is the lack of cost-effective and simple dsRNA synthesis and administration procedures. Despite these limitations, different RNAi-based approaches have been successfully tested at the laboratory level, with a particular focus on shrimp. The use of RNAi technology is particularly attractive for the shrimp industry because crustaceans do not have an adaptive immune system, making traditional vaccination methods unfeasible. This review summarizes recent studies and the state-of-the-art on the mechanism of action, design, use, and administration methods of dsRNA, as applied to shrimp. In addition, potential constraints that may hinder the deployment of RNAi-based methods in the crustacean aquaculture sector are considered.

In vivo multiscale analyses of spring viremia of carp virus (SVCV) infection: From model organism to target species

Spring viremia of carp virus (SVCV) has a broad fish host spectrum and is responsible for a disease that generally affects juvenile fishes with a mortality rate of up to 90%. In the absence of treatments or vaccines against SVCV, the search for prophylactic or therapeutic solutions is thus relevant, particularly to identify solutions compatible with mass vaccination. In addition to being a threat to aquaculture and ecosystems, SVCV is a unique pathogen to study virus-host interactions in the zebrafish model. Establishing the first reverse genetics system for SVCV and the design of recombinant SVCV (rSVCV) expressing fluorescent or bioluminescent proteins adds a new dimension for the study of these interactions using innovative imaging techniques. The infection by bath immersion of zebrafish larvae with rSVCV expressing mCherry allows us to define the first SVCV replication sites and the host innate immune responses using different transgenic lines of zebrafish. The fins were found as the main initial sites of infection in both zebrafish and carp, its natural host. Hence, new insights into the physiopathology of SVCV infection have been described. We report that neutrophils are recruited at the sites of infection and persist up to the death of the animal leading to an uncontrolled inflammation correlated with the expression of the pro-inflammatory cytokine IL1β. Tissue damage was observed at the site of initial replication, a likely consequence of virus-induced injury or the pro-inflammatory response. Interestingly, SVCV infection by bath immersion triggers a persistent pro-inflammatory response rather than activation of the antiviral IFN signaling pathway as observed following intravenous injection, highlighting the importance of the route of infection on the progression of pathogenicity. Thus, this model of zebrafish larvae infection by rSVCV offers new perspectives to study in detail virus-host interactions and to discover new prophylactic or therapeutic solutions.

Heterologous Exchanges of Glycoprotein and Non-Virion Protein in Novirhabdoviruses: Assessment of Virulence in Yellow Perch (Perca flavescens) and Rainbow Trout (Oncorhynchus mykiss)

Infectious hematopoietic necrosis virus (IHNV) and viral hemorrhagic septicemia virus (VHSV) are rhabdoviruses in two different species belonging to the Novirhabdovirus genus. IHNV has a narrow host range restricted to trout and salmon species, and viruses in the M genogroup of IHNV have high virulence in rainbow trout (Oncorhynchus mykiss). In contrast, the VHSV genotype IVb that invaded the Great Lakes in the United States has a broad host range, with high virulence in yellow perch (Perca flavescens), but not in rainbow trout. By using reverse-genetic systems of IHNV-M and VHSV-IVb strains, we generated six IHNV:VHSV chimeric viruses in which the glycoprotein (G), non-virion-protein (NV), or both G and NV genes of IHNV-M were replaced with the analogous genes from VHSV-IVb, and vice versa. These chimeric viruses were used to challenge groups of rainbow trout and yellow perch. The parental recombinants rIHNV-M and rVHSV-IVb were highly virulent in rainbow trout and yellow perch, respectively. Parental rIHNV-M was avirulent in yellow perch, and chimeric rIHNV carrying G, NV, or G and NV genes from VHSV-IVb remained low in virulence in yellow perch. Similarly, the parental rVHSV-IVb exhibited low virulence in rainbow trout, and chimeric rVHSV with substituted G, NV, or G and NV genes from IHNV-M remained avirulent in rainbow trout. Thus, the G and NV genes of either virus were not sufficient to confer high host-specific virulence when exchanged into a heterologous species genome. Some exchanges of G and/or NV genes caused a loss of host-specific virulence, providing insights into possible roles in viral virulence or fitness, and interactions between viral proteins.

Novel herpesvirus in the critically endangered Galapagos pink land iguana

Virus surveillance in wildlife is important to understanding ecosystem health, taxonomy, and evolution. Nevertheless, viruses in reptiles, and specifically in squamates, continue to be understudied. Herein, we conducted a health assessment on the critically endangered Galapagos pink land iguana (Conolophus marthae) and the vulnerable Galapagos land iguana (Conolophus subcristatus). We collected oral and/or cloacal swabs from 47 clinically healthy iguanas and tested for adenovirus (cloacal swabs, n = 47) and herpesvirus (oral swabs, n = 45) using broad-spectrum PCRs. Two out of 38 (5.3 %) Galapagos pink land iguanas tested positive for herpesvirus, while no herpesvirus was detected in all Galapagos land iguanas (n = 7). Both herpesviral sequences were identical between them and divergent (61.9 % amino acid identity) when compared to the closest herpesvirus sequences available in GenBank/EMBL/DDBJ. The genetic distance between this and other herpesviruses is consistent with its classification as a novel virus species. All iguanas were negative for adenovirus. This is the first description of a herpesvirus in iguanas of the Galapagos islands, and the first report of a potential pathogen for the iconic Galapagos pink land iguana. Further research is needed to understand the implications of this virus in the conservation and management of one of the most endangered iguana species in the world.

The antiviral effects of TRIM23 and TRIM32 proteins in rainbow trout (Oncorhynchus mykiss)

TRIM proteins play a crucial antiviral effector role in the innate immune system of vertebrates. In this study, we found that TRIM proteins exhibited the highest expression levels in immune organs such as spleen and kidney during IHNV infection in rainbow trout, meanwhile, we successfully amplified TRIM23 and TRIM32 from diseased rainbow trout and analyzed their gene sequences, revealing that rainbow trout TRIM23 and TRIM32 proteins are closely related to Atlantic salmon and Chinook salmon; In this experiment, the TRIM23 and TRIM32 protein genes were resoundingly constructed as a recombinant plasmids and expressed in CHSE-214 cells. Upon transfected with the recombinant plasmid, followed by viral infection, significant decreasion in the copy numbers of the virus was observed, indicating that the TRIM23 and TRIM32 proteins of rainbow trout play an important role in inhibiting virus replication, with the TRIM32 role being the most pronounced. These results provide a basis for subsequent in-depth study of the antiviral effects of TRIM proteins, and provide new ideas for immune enhancers.

Review: Recent Applications of Gene Editing in Fish Species and Aquatic Medicine

Gene editing and gene silencing techniques have the potential to revolutionize our knowledge of biology and diseases of fish and other aquatic animals. By using such techniques, it is feasible to change the phenotype and modify cells, tissues and organs of animals in order to cure abnormalities and dysfunctions in the organisms. Gene editing is currently experimental in wide fields of aquaculture, including growth, controlled reproduction, sterility and disease resistance. Zink finger nucleases, TALENs and CRISPR/Cas9 targeted cleavage of the DNA induce favorable changes to site-specific locations. Moreover, gene silencing can be used to inhibit the translation of RNA, namely, to regulate gene expression. This methodology is widely used by researchers to investigate genes involved in different disorders. It is a promising tool in biotechnology and in medicine for investigating gene function and diseases. The production of food fish has increased markedly, making fish and seafood globally more popular. Consequently, the incidence of associated problems and disease outbreaks has also increased. A greater investment in new technologies is therefore needed to overcome such problems in this industry. To put it concisely, the modification of genomic DNA and gene silencing can comprehensively influence aquatic animal medicine in the future. On the ethical side, these precise genetic modifications make it more complicated to recognize genetically modified organisms in nature and can cause several side effects through created mutations. The aim of this review is to summarize the current state of applications of gene modifications and genome editing in fish medicine.

Recombinant viral hemorrhagic septicemia virus with rearranged genomes as vaccine vectors to protect against lethal betanodavirus infection

The outbreaks of viral hemorrhagic septicemia (VHS) and viral encephalopathy and retinopathy (VER) caused by the enveloped novirhabdovirus VHSV, and the non-enveloped betanodavirus nervous necrosis virus (NNV), respectively, represent two of the main viral infectious threats for aquaculture worldwide. Non-segmented negative-strand RNA viruses such as VHSV are subject to a transcription gradient dictated by the order of the genes in their genomes. With the goal of developing a bivalent vaccine against VHSV and NNV infection, the genome of VHSV has been engineered to modify the gene order and to introduce an expression cassette encoding the major protective antigen domain of NNV capsid protein. The NNV Linker-P specific domain was duplicated and fused to the signal peptide (SP) and the transmembrane domain (TM) derived from novirhabdovirus glycoprotein to obtain expression of antigen at the surface of infected cells and its incorporation into viral particles. By reverse genetics, eight recombinant VHSVs (rVHSV), termed NxGyCz according to the respective positions of the genes encoding the nucleoprotein (N) and glycoprotein (G) as well as the expression cassette (C) along the genome, have been successfully recovered. All rVHSVs have been fully characterized in vitro for NNV epitope expression in fish cells and incorporation into VHSV virions. Safety, immunogenicity and protective efficacy of rVHSVs has been tested in vivo in trout (Oncorhynchus mykiss) and sole (Solea senegalensis). Following bath immersion administration of the various rVHSVs to juvenile trout, some of the rVHSVs were attenuated and protective against a lethal VHSV challenge. Results indicate that rVHSV N2G1C4 is safe and protective against VHSV challenge in trout. In parallel, juvenile sole were injected with rVHSVs and challenged with NNV. The rVHSV N2G1C4 is also safe, immunogenic and efficiently protects sole against a lethal NNV challenge, thus presenting a promising starting point for the development of a bivalent live attenuated vaccine candidate for the protection of these two commercially valuable fish species against two major diseases in aquaculture.

Protection of rainbow trout (Oncorhynchus mykiss) against VHSV genotype Ia and IHNV by immunization with VHSV genotype IVa backbone-based single-cycle viruses

To evaluate the protective effect of viral hemorrhagic septicemia virus genotype IVa (VHSV IVa) genome-based single-cycle viruses against VHSV genotype Ia (VHSV Ia) and infectious hematopoietic necrosis virus (IHNV) in rainbow trout, three kinds of single-cycle VHSVs were rescued using reverse genetic technology: i) rVHSV-IaGΔTM^-IVaG containing the transmembrane and cytoplasmic region-deleted G protein (GΔTM) of VHSV Ia instead of VHSV IVa full G gene ORF and having VHSV IVa G proteins on the envelope; ii) rVHSV-IaGΔTM^-IaG containing VHSV Ia GΔTM instead of VHSV IVa full G gene ORF and having VHSV Ia G proteins on the envelope; iii) rVHSV-IaGΔTM-ihnvGΔTM^-IVaG containing not only VHSV Ia GΔTM instead of full G gene but also IHNV GΔTM instead of NV gene and having VHSV IVa G proteins on the envelope. Rainbow trout immunized with rVHSV-IaGΔTM^-IaG and rVHSV-IaGΔTM^-IVaG showed significantly higher serum antibody titers against both VHSV Ia and VHSV IVa, and showed no mortality against VHSV Ia infection, while fish in the control groups showed 100% mortalities. Fish immunized with rVHSV-IaGΔTM-ihnvGΔTM^-IVaG showed significantly higher serum antibody titers against VHSV IVa, VHSV Ia, and IHNV compared to fish in the control group. Immunization with rVHSV-IaGΔTM-ihnvGΔTM^-IVaG induced significantly higher protection against not only VHSV Ia but also IHNV. These results suggest that the present single-cycle rVHSV-based system can be used as a platform to produce combined vaccines that can protect fish from multiple pathogenic species. However, the mechanism of the high protection against IHNV despite comparatively low antibody titer remains to be investigated.

Infectious hematopoietic necrosis virus (IHNV) nucleoprotein amino acid residues affect viral virulence and immunogenicity in rainbow trout (Oncorhynchus mykiss)

This study compared the N protein sequences of genotype J with other genotypes of IHNV to select amino acid residues that may be related to the change in viral virulence. The recombinant viruses containing different mutation sites were rescued by alanine scanning mutagenesis and the reverse genetic system. The nine recombinant virus strains obtained in this work were named rIHNV-N₈₅, rIHNV-N₁₀₂, rIHNV-N₁₄₆, rIHNV-N₃₈₀, rIHNV-N_85-102-146, rIHNV-N_85-102-380, rIHNV-N_85-146-380, rIHNV-N_102-146-380, and rIHNV-N_{85-102-146-380}. Pathogenicity and immunity assays were performed to determine the role of virulence sites. The result of the pathogenicity test showed that the survival rates of rIHNV-N₈₅, rIHNV-N₁₀₂, rIHNV-N_85-102-146, and rIHNV-N_85-102-380 groups were 52.5%, 55%, 67.5%, and 57.5%, while the survival rate of wild-type (wt) IHNV HLJ-09 group was only 10%. The replication ability of recombinant viruses with substitutions at positions 85 and 102 was significantly inhibited in vivo and in vitro. The qRT-PCR result indicated that the cytokines of IFN1, IL-8, and IL-1β expression levels were increased in rIHNV-N₈₅, rIHNV-N₁₀₂, rIHNV-N_85-102-146, and rIHNV-N_85-102-380 groups. In addition, these four recombinant viruses could cause the rainbow trout to produce anti-IHNV-specific antibodies immunoglobulin M (IgM) earlier, confirming that 85 and 102 amino acid residues of N protein affected the virulence and immunogenicity of IHNV. All these results suggest that mutations of the N protein virulence sites reduce virulence while retaining immunogenicity. This also provides a new idea for studying the virulence mechanism of rhabdoviruses and preparing attenuated vaccines.

Fundamentals of Fish Vaccination

Fish health management has become a critical component of disease control and is invaluable for improved harvests and sustainable aquaculture. Vaccination is generally accepted as the most effective prophylactic measure for fish disease prevention, on environmental, social, and economic grounds. Although the historical approach for developing fish vaccines was based on the principle of Louis Pasteur's "isolate, inactivate and inject," but their weak immunogenicity and low efficacies in many cases, have shifted the focus of fish vaccine development from traditional to next-generation technologies. However, before any fish vaccine can be successfully commercialized, several hurdles need to be overcome regarding the production cost, immunogenicity, effectiveness, mode of administration, environmental safety, and associated regulatory concerns. In this context, the chapter summarises the basic aspects of fish vaccination such as type of vaccine, modalities of vaccine delivery, the immunological basis of fish immunization as well as different challenges associated with the development process and future opportunities.

Effect of NV gene deletion in the genome of hirame rhabdovirus (HIRRV) on viral replication and the type I interferon response of the host cell

Hirame rhabdovirus (HIRRV), a member of the genus Novirhabdovirus, causes morbidity and mortality in farmed olive flounder (Paralichthys olivaceus). As no information is available on the role of the NV gene of HIRRV, we produced a recombinant HIRRV with the NV gene deleted (rHIRRV-ΔNV) using reverse genetic technology and investigated whether the NV gene knockout affected HIRRV replication and the type I interferon response of the host cell. The rescue of rHIRRV-ΔNV was successful only when IRF9-gene-knockout Epithelioma papulosum cyprini (ΔIRF9-EPC) cells were used, suggesting that the NV protein of HIRRV might be involved in inhibition of the type I interferon response of the host cell. This conclusion was also supported by the significantly higher level of Mx gene induction in EPC cells infected with rHIRRV-ΔNV than in cells infected with recombinant HIRRV without the deletion. When cells were coinfected with rHIRRV-ΔNV and either wild-type HIRRV or wild-type viral hemorrhagic septicemia virus (VHSV), there was a decrease in the growth rate of not only wild-type HIRRV but also wild-type VHSV in a concentration-dependent manner. Further studies are required to investigate the role of HIRRV NV in virulence and its possible importance for the development of attenuated vaccines.

Technical challenges in the development of reverse genetics for a viral haemorrhagic septicaemia virus (VHSV) genotype Ib isolate: Alternative cell lines and general troubleshooting

Several reverse genetics systems for viral haemorrhagic septicaemia virus (VHSV) have been developed over the last decade. These systems have been based on genotype Ia, IVa and IVb isolates and have used the fish cell line EPC, which is less susceptible to some VHSV isolates belonging to genotype I and genotypes II and III. While developing a reverse genetics system in our laboratories for VHSV genotype Ib, we realized that the isolate in interest (SE SVA 1033 9C) did not grow in EPC cells and it was necessary to adapt the reverse genetics protocols to the BF-2 fish cell line. This cell line is very sensitive to high temperatures and is therefore not compatible with the original protocols based on the use of recombinant vaccinia virus (vTF7-3) as a provider of the T7 RNA polymerase (T7-RNAP) to the system, which includes incubation periods at 37 °C. Transfection efficiency was assessed in BF-2 cells using a reporter plasmid and it showed to be highest when using Lipofectamine™ 3000 compared to other transfection reagents. A luciferase assay was performed to determine the optimal activity of T7-RNAP in BF-2 cells with different amounts of vTF7-3. We successfully recovered recombinant VHSV (rVHSV) in BF-2 cells by reducing the incubation time at 37 °C after transfection to both 3 and 6 h. Another strategy we attempted successfully was to transfect mammalian BHK-21 cells, which are routinely used to propagate vTF7-3, and after the 37 °C incubation period, a BF-2 cell suspension was added hypothesizing that the virions formed in the transfected mammalian cells would infect the subsequently added fish cells at 15 °C incubation over the following days. We have successfully recovered rVHSV from both BHK-21 with a BF-2 cells suspension as well as a new protocol for VHSV reverse genetics in BF-2 cells has been established.

Antiviral Actions of 25-Hydroxycholesterol in Fish Vary With the Virus-Host Combination

Cholesterol is essential for building and maintaining cell membranes and is critical for several steps in the replication cycle of viruses, especially for enveloped viruses. In mammalian cells virus infections lead to the accumulation of the oxysterol 25-hydroxycholesterol (25HC), an antiviral factor, which is produced from cholesterol by the cholesterol 25 hydroxylase (CH25H). Antiviral responses based on CH25H are not well studied in fish. Therefore, in the present study putative genes encoding for CH25H were identified and amplified in common carp and rainbow trout cells and an HPLC-MS method was applied for determination of oxysterol concentrations in these cells under virus infection. Our results give some evidence that the activation of CH25H could be a part of the antiviral response against a broad spectrum of viruses infecting fish, in both common carp and rainbow trout cells in vitro. Quantification of oxysterols showed that fibroblastic cells are capable of producing 25HC and its metabolite 7α,25diHC. The oxysterol 25HC showed an antiviral activity by blocking the entry of cyprinid herpesvirus 3 (CyHV-3) into KFC cells, but not spring viremia of carp virus (SVCV) or common carp paramyxovirus (Para) in the same cells, or viral haemorrhagic septicaemia virus (VHSV) and infectious pancreatic necrosis virus (IPNV) into RTG-2 cells. Despite the fact that the CH25H based antiviral response coincides with type I IFN responses, the stimulation of salmonid cells with recombinant type I IFN proteins from rainbow trout could not induce ch25h_b gene expression. This provided further evidence, that the CH25H-response is not type I IFN dependent. Interestingly, the susceptibility of CyHV-3 to 25HC is counteracted by a downregulation of the expression of the ch25h_b gene in carp fibroblasts during CyHV-3 infection. This shows a unique interplay between oxysterol based immune responses and immunomodulatory abilities of certain viruses.

The Viral Hemorrhagic Septicemia Virus (VHSV) Markers of Virulence in Rainbow Trout (Oncorhynchus mykiss)

Viral hemorrhagic septicemia virus (VHSV) is a highly contagious virus leading to high mortality in a large panel of freshwater and marine fish species. VHSV isolates originating from marine fish show low pathogenicity in rainbow trout. The analysis of several nearly complete genome sequences from marine and freshwater isolates displaying varying levels of virulence in rainbow trout suggested that only a limited number of amino acid residues might be involved in regulating the level of virulence. Based on a recent analysis of 55 VHSV strains, which were entirely sequenced and phenotyped in vivo in rainbow trout, several amino acid changes putatively involved in virulence were identified. In the present study, these amino acid changes were introduced, alone or in combination, in a highly-virulent VHSV 23–75 genome backbone by reverse genetics. A total of 35 recombinant VHSV variants were recovered and characterized for virulence in trout by bath immersion. Results confirmed the important role of the NV protein (R116S) and highlighted a major contribution of the nucleoprotein N (K46G and A241E) in regulating virulence. Single amino acid changes in these two proteins drastically affect virus pathogenicity in rainbow trout. This is particularly intriguing for the N variant (K46G) which is unable to establish an active infection in the fins of infected trout, the main portal of entry of VHSV in this species, allowing further spread in its host. In addition, salmonid cell lines were selected to assess the kinetics of replication and cytopathic effect of recombinant VHSV and discriminate virulent and avirulent variants. In conclusion, three major virulence markers were identified in the NV and N proteins. These markers explain almost all phenotypes (92.7%) observed in trout for the 55 VHSV strains analyzed in the present study and herein used for the backward validation of virulence markers. The identification of VHSV specific virulence markers in this species is of importance both to predict the in vivo phenotype of viral isolates with targeted diagnostic tests and to improve prophylactic methods such as the development of safer live-attenuated vaccines.

Evolutionary trajectory of fish Piscine novirhabdovirus (=Viral Hemorrhagic Septicemia Virus) across its Laurentian Great Lakes history: Spatial and temporal diversification

Piscine novirhabdovirus = Viral Hemorrhagic Septicemia Virus (VHSV) first appeared in the Laurentian Great Lakes with large outbreaks from 2005 to 2006, as a new and novel RNA rhabdovirus subgenogroup (IVb) that killed >30 fish species. Interlude periods punctuated smaller more localized outbreaks in 2007, 2010, and 2017, although some fishes tested positive in the intervals. There have not been reports of outbreaks or positives from 2018, 2019, or 2020. Here, we employ a combined population genetics and phylogenetic approach to evaluate spatial and temporal evolutionary trajectory on its G-gene sequence variation, in comparison with whole-genome sequences (11,083 bp) from a subset of 44 individual isolates (including 40 newly sequenced ones). Our results show that IVb (N = 184 individual fish isolates) diversified into 36 G-gene haplotypes from 2003 to 2017, stemming from two originals ("a" and "b"). G-gene haplotypes "a" and "b" differed by just one synonymous single-nucleotide polymorphism (SNP) substitution, remained the most abundant until 2011, then disappeared. Group "a" descendants (14 haplotypes) remained most prevalent in the Upper and Central Great Lakes, with eight (51%) having nonsynonymous substitutions. Group "b" descendants primarily have occurred in the Lower Great Lakes, including 22 haplotypes, of which 15 (68%) contained nonsynonymous changes. Evolutionary patterns of the whole-genome sequences (which had 34 haplotypes among 44 isolates) appear congruent with those from the G-gene. Virus populations significantly diverged among the Upper, Central, and Lower Great Lakes, diversifying over time. Spatial divergence was apparent in the overall patterns of nucleotide substitutions, while amino acid changes increased temporally. VHSV-IVb thus significantly differentiated across its less than two decades in the Great Lakes, accompanied by declining outbreaks and virulence. Continuing diversification likely allowed the virus to persist at low levels in resident fish populations, and may facilitate its potential for further and future spread to new habitats and nonacclimated hosts.

Infectious hematopoietic necrosis virus truncated G protein effect on survival, immune response, and disease resistance in rainbow trout

The major antigenic protein of infectious hematopoietic necrosis virus (IHNV) is the surface glycoprotein G, which contains neutralizing epitopes that induce the production of immune neutralizing antibodies. In this study, the IHNV G gene sequence was truncated according to bioinformatics principles and then recombinantly expressed via an E. coli expression system. We then assessed the specific antibody immunoglobin M (IgM) levels of rainbow trout immunized with recombinant truncated G protein (emulsified with Freund's incomplete adjuvant), and showed that antibody IgM levels of immunized fish were significantly higher than in the control group (p < 0.01). The mRNA expression levels of interferon 1 (IFN1) and interleukin-8 (IL-8) were also up-regulated significantly (p < 0.01) in head kidneys and spleens of rainbow trout immunized with recombinant truncated G protein. Also, after challenge with wild-type IHNV HLJ-09 virus on Day 28, rainbow trout immunized with recombinant truncated G protein showed cumulative survival rates of 60%. These results indicate that the truncated G protein of IHNV expressed by the E. coli prokaryotic expression system can be used as a candidate immunogen for an IHNV subunit vaccine, which lays a theoretical foundation for the study of further potential IHNV subunit vaccines.

The L-domains in M and G proteins of infectious hematopoietic necrosis virus (IHNV) affect viral budding and pathogenicity

RNA viruses including many retroviruses encode "late-domain" motifs that can interact with host proteins to mediate viral assembly and affect viral budding and pathogenicity. For IHNV, our previous studies demonstrated that the respective interactions of the L domains of IHNV with host proteins could mediate viral assembly and budding. To our knowledge, the role of L domains of the IHNV in the budding and pathogenicity has not investigated yet. In this study, we generated two recombinant IHNV strains rIHNV-M_(PH>A4) and rIHNV-G_(PS>A4) with mutations in the L domains (PPPH to AAAA or PSAP to AARA) of IHNV by reverse genetics and explored the effect of the mutations on budding and pathogenicity of the two recombinant viruses. The RT-qPCR results showed that the production levels of the extracellular particles of rIHNV-M_(PH>A4) or rIHNV-G_(PS>A4) declined significantly, compared with those of wild-type (wt) IHNV HLJ-09. Furthermore, the challenge test showed that the survival rates of juvenile rainbow trout challenged with rIHNV-M_(PH>A4) or rIHNV-G_(PS>A4) were 90% or 87%, respectively; however, the survivability was zero in groups challenged with wtIHNV HLJ-09 or rIHNV HLJ-09 (recombinant IHNV). Additionally, the RT-qPCR results showed that the recombinant viruses induced higher expression levels of IFN1, IL-1β, and IL-8 compared with those induced by wtIHNV HLJ-09 as well as the ELISA results showed that fish vaccinated with recombinant viruses produced high levels of specific IgM antibodies, demonstrating that the two recombinant viruses may induce immune responses to resist infection by IHNV. Also, these results demonstrated for the first time that the L domains of the M and G proteins of IHNV could affect the budding and pathogenicity of IHNV, which may be beneficial in the prevention and control of IHNV infections in fish. Taken together, our study as the first research provides the foundation for effect of rhabdovirus L domains on viral budding and pathogenicity.

Genetically engineered viral hemorrhagic septicemia virus (VHSV) vaccines

Viral hemorrhagic septicemia virus (VHSV) has been one of the major causes of mortality in a wide range of freshwater and marine fishes worldwide. Although various types of vaccines have been tried to prevent VHSV disease in cultured fishes, there are still no commercial vaccines. Reverse genetics have made it possible to change a certain regions on viral genome in accordance with the requirements of a research. Various types of VHSV mutants have been generated through the reverse genetic method, and most of them were recovered to investigate the virulence mechanisms of VHSV. In the reverse genetically generated VHSV mutants-based vaccines, high protective efficacies of attenuated VHSVs and single-cycle VHSV particles have been reported. Furthermore, the application of VHSV for the delivery tools of heterologous antigens including not only fish pathogens but also mammalian pathogens has been studied. As not much research has been conducted on VHSV mutants-based vaccines, more studies on the enhancement of immunogenicity, vaccine administration routes, safety to environments are needed for the practical use in aquaculture farms.

Capsid amino acids at positions 247 and 270 are involved in the virulence of betanodaviruses to European sea bass

European sea bass (Dicentrarchus labrax) is severely affected by nervous necrosis disease, caused by nervous necrosis virus (NNV). Two out of the four genotypes of this virus (red-spotted grouper nervous necrosis virus, RGNNV; and striped jack nervous necrosis virus, SJNNV) have been detected in sea bass, although showing different levels of virulence to this fish species. Thus, sea bass is highly susceptible to RGNNV, whereas outbreaks caused by SJNNV have not been reported in this fish species. The role of the capsid protein (Cp) amino acids 247 and 270 in the virulence of a RGNNV isolate to sea bass has been evaluated by the generation of recombinant RGNNV viruses harbouring SJNNV-type amino acids in the above mentioned positions (Mut247Dl965, Mut270Dl965 and Mut247 + 270Dl965). Viral in vitro and in vivo replication, virus virulence and fish immune response triggered by these viruses have been analysed. Mutated viruses replicated on E-11 cells, although showing some differences compared to the wild type virus, suggesting that the mutations can affect the viral cell recognition and entry. In vivo, fish mortality caused by mutated viruses was 75% lower, and viral replication in sea bass brain was altered compared to non-mutated virus. Regarding sea bass immune response, mutated viruses triggered a lower induction of IFN I system and inflammatory response-related genes. Furthermore, mutations caused changes in viral serological properties (especially the mutation in amino acid 270), inducing higher seroconversion and changing antigen recognition.

The role of infectious hematopoietic necrosis virus (IHNV) proteins in recruiting the ESCRT pathway through three ways in the host cells of fish during IHNV budding

In cytokinetic abscission, phagophore formation, and enveloped virus budding are mediated by the endosomal sorting complex required for transport (ESCRT). Many retroviruses and RNA viruses encode "late-domain" motifs that can interact with the components of the ESCRT pathway to mediate the viral assembly and budding. However, the rhabdovirus in fish has been rarely investigated. In this study, inhibition the protein expression of the ESCRT components reduces the extracellular virion production, which preliminarily indicates that the ESCRT pathway is involved in IHNV release. The respective interactions of IHNV proteins including M, G, L protein with Nedd4, Tsg101, and Alix suggest the underlying molecular mechanism by which IHNV gets access to the ESCRT pathway. These results are the first observation that rhabdovirus in fish gains access to the ESCRT pathway through three ways of interactions between viral proteins and host proteins. In addition, the results show that IHNV is released from host cells through the ESCRT pathway. Taken together, our study provides a theoretical basis for studying the budding mechanism of IHNV.

Immunity induced by recombinant attenuated IHNV (infectious hematopoietic necrosis virus)-GN438A expresses VP2 gene-encoded IPNV (infectious pancreatic necrosis virus) against both pathogens in rainbow trout

Infectious hematopoietic necrosis virus (IHNV) and infectious pancreatic necrosis virus (IPNV) are important pathogens in rainbow trout farming worldwide. Their co-infection is also common, which causes great economic loss in juvenile salmon species. Development of a universal virus vaccine providing broadly cross-protective immunity will be of great importance. In this study, we generated two recombinant (r) virus (rIHNV-N438A-ΔNV-EGFP and rIHNV-N438A-ΔNV-VP2) replacing the NV gene of the backbone of rIHNV at the single point mutation at residue 438 with an efficient green fluorescent protein (EGFP) reporter gene and antigenic VP2 gene of IPNV. Meanwhile, we tested their efficacy against the wild-type (wt) IHNV HLJ-09 virus and IPNV serotype Sp virus challenge. The relative per cent survival rates of two recombinant viruses against (wt) IHNV HLJ-09 virus challenge were 84.6% and 81.5%, respectively. Simultaneously, the relative per cent survival rate of rIHNV-N438A-ΔNV-VP2 against IPNV serotype Sp virus challenge was 88.9%. It showed the two recombinant viruses had high protection rates and induced a high level of antibodies against IHNV or IPNV. Taken together, these results suggest the VP2 gene of IPNV can act as candidate gene for vaccine and attenuated multivalent live vaccines and molecular marker vaccines have potential application for viral vaccine.

The glycoprotein, non-virion protein, and polymerase of viral hemorrhagic septicemia virus are not determinants of host-specific virulence in rainbow trout

Background

Viral hemorrhagic septicemia virus (VHSV), a fish rhabdovirus belonging to the Novirhabdovirus genus, causes severe disease and mortality in many marine and freshwater fish species worldwide. VHSV isolates are classified into four genotypes and each group is endemic to specific geographic regions in the north Atlantic and Pacific Oceans. Most viruses in the European VHSV genotype Ia are highly virulent for rainbow trout (Oncorhynchus mykiss), whereas, VHSV genotype IVb viruses from the Great Lakes region in the United States, which caused high mortality in wild freshwater fish species, are avirulent for trout. This study describes molecular characterization and construction of an infectious clone of the virulent VHSV-Ia strain DK-3592B from Denmark, and application of the clone in reverse genetics to investigate the role of selected VHSV protein(s) in host-specific virulence in rainbow trout (referred to as trout-virulence).

Methods

Overlapping cDNA fragments of the DK-3592B genome were cloned after RT-PCR amplification, and their DNA sequenced by the di-deoxy chain termination method. A full-length cDNA copy (pVHSVdk) of the DK-3592B strain genome was constructed by assembling six overlapping cDNA fragments by using natural or artificially created unique restriction sites in the overlapping regions of the clones. Using an existing clone of the trout-avirulent VHSV-IVb strain MI03 (pVHSVmi), eight chimeric VHSV clones were constructed in which the coding region(s) of the glycoprotein (G), non-virion protein (NV), G and NV, or G, NV and L (polymerase) genes together, were exchanged between the two clones. Ten recombinant VHSVs (rVHSVs) were generated, including two parental rVHSVs, by transfecting fish cells with ten individual full-length plasmid constructs along with supporting plasmids using the established protocol. Recovered rVHSVs were characterized for viability and growth in vitro and used to challenge groups of juvenile rainbow trout by intraperitoneal injection.

Results

Complete sequence of the VHSV DK-3592B genome was determined from the cloned cDNA and deposited in GenBank under the accession no. KC778774. The trout-virulent DK-3592B genome (genotype Ia) is 11,159 nt in length and differs from the trout-avirulent MI03 genome (pVHSVmi) by 13% at the nucleotide level. When the rVHSVs were assessed for the trout-virulence phenotype in vivo, the parental rVHSVdk and rVHSVmi were virulent and avirulent, respectively, as expected. Four chimeric rVHSVdk viruses with the substitutions of the G, NV, G and NV, or G, NV and L genes from the avirulent pVHSVmi constructs were still highly virulent (100% mortality), while the reciprocal four chimeric rVHSVmi viruses with genes from pVHSVdk remained avirulent (0–10% mortality).

Conclusions

When chimeric rVHSVs, containing all the G, NV, and L gene substitutions, were tested in vivo, they did not exhibit any change in trout-virulence relative to the background clones. These results demonstrate that the G, NV and L genes of VHSV are not, by themselves or in combination, major determinants of host-specific virulence in trout.

Development of a reverse genetics system for snakehead vesiculovirus (SHVV)

Snakehead vesiculovirus (SHVV) is a new rhabdovirus isolated from diseased hybrid snakehead fish (Channa maculate ♀ x Channa argus ♂) and has caused serious economic losses in snakehead fish culture in China. To better understand the pathogenicity of SHVV, we developed a reverse genetics system for SHVV by using human and fish cells. In detail, human 293T cells were co-transfected with four plasmids encoding the full-length SHVV antigenomic RNA or the supporting proteins including nucleoprotein (N), phosphoprotein (P), and large polymerase (L), followed by the cultivation in Channel catfish ovary (CCO) cells. We also rescued a recombinant SHVV expressing enhanced green fluorescent protein (EGFP), which was inserted into the 3' non-coding region (NCR) of the glycoprotein (G) gene of SHVV. Our study provides a potential tool for unveiling the pathogenicity of SHVV and a template for the rescue of other fish viruses by using both human 293T and fish cells.

Isolation, identification, and classification of a novel rhabdovirus from diseased Chinese rice-field eels (Monopterus albus)

In 2017, a clinical disease outbreak resulted in substantial mortality of adults and larvae of cultured Chinese rice-field eels (Monopterus albus) on a farm in Hubei, Central China. A rhabdovirus was isolated from moribund specimens, and typical clinical symptoms associated with an outbreak included an enlarged and swollen head. This differed from previous observations. Histological changes included necrosis and cavities of various sizes within the brain and kidney. Homogenized tissues of diseased Chinese rice-field eels were screened for viral isolation using six different fish cell lines. A rhabdovirus was isolated following observation of cytopathic effect (CPE) in a gibel carp brain (GiCB) cell line and confirmed by RT-PCR. Electron microscopy showed large numbers of rhabdovirus-shaped particles in the cytoplasm of the brain cells of the diseased Chinese rice-field eels and in the infected GiCB cell line. This virus has been named "Chinese rice-field eel rhabdovirus" (CrERV), and the complete nucleotide sequence of CrERV was cloned. This rhabdovirus is composed of 11,545 nucleotides with the following genomic organization: 3'-N-P-M-G-L-5'. The genes are separated by conserved gene junctions, and phylogenetic analysis of the L sequence revealed that CrERV forms a separate branch with Siniperca chuatsi rhabdovirus (SCRV) and hybrid snakehead rhabdovirus C1207 (HSHRV-C1207). This is the first report of the complete sequence of CrERV from the Chinese rice-field eel in China.

Identification of amino acid residues in infectious hematopoietic necrosis virus (IHNV) NV protein necessary for viral replication and pathogenicity

Our previous studies demonstrated that the nonstructural NV protein of infectious hematopoietic necrosis virus (IHNV) was essential for efficient viral replication and pathogenicity, and that the amino acid residues ³²EGDL³⁵ of the NV protein were responsible for nuclear localization, and played important roles in suppressing IFN and inhibiting NF-κB activity. However, little is known about the influence of ³²EGDL³⁵ on IHNV replication and pathogenicity. In the present study, two recombinant IHNV strains with deletions of NV ³²EGDL³⁵ were generated and the effect on IHNV replication and pathogenicity was explored. Our results showed that both mutants stably replicated in Chinook salmon embryo cells for 15 consecutive passages, and had similar host-tropism as wild-type (wt) IHNV; however, titers of the mutants were lower than those of wt IHNV in CHSE-214 cells. Infection of rainbow trout showed wt IHNV produced 90% cumulative mortality, while the mutants produced 55% and 60% cumulative mortality, respectively. Histopathological evaluation showed that tissues from the liver, brain, kidney, and heart of fish infected with wt IHNV exhibited pathological changes, but significant lesions were found only in the liver and heart of fish infected with the recombinant viruses. In addition, the recombinant viruses induced higher expression levels of IFN1, Mx-1, and IL-6 compared with those induced by wt IHNV. These results indicated that the ³²EGDL³⁵ residues were essential for the efficient anti-IFN and NF-κB-inhibiting activity of NV. Our results provide a basis for understanding the roles of ³²EGDL³⁵ in IHNV replication and pathogenicity, and may prove beneficial in the prevention and control of IHNV infections of fish.

A new age in AquaMedicine: unconventional approach in studying aquatic diseases

BACKGROUND

Marine and aquaculture industries are important sectors of the food production and global trade. Unfortunately, the fish food industry is challenged with a plethora of infectious pathogens. The freshwater and marine fish communities are rapidly incorporating novel and most up to date techniques for detection, characterization and treatment strategies. Rapid detection of infectious diseases is important in preventing large disease outbreaks.

MAIN TEXT

One hundred forty-six articles including reviews papers were analyzed and their conclusions evaluated in the present paper. This allowed us to describe the most recent development research regarding the control of diseases in the aquatic environment as well as promising avenues that may result in beneficial developments. For the characterization of diseases, traditional sequencing and histological based methods have been augmented with transcriptional and proteomic studies. Recent studies have demonstrated that transcriptional based approaches using qPCR are often synergistic to expression based studies that rely on proteomic-based techniques to better understand pathogen-host interactions. Preventative therapies that rely on prophylactics such as vaccination with protein antigens or attenuated viruses are not always feasible and therefore, the development of therapies based on small nucleotide based medicine is on the horizon. Of those, RNAi or CRISPR/Cas- based therapies show great promise in combating various types of diseases caused by viral and parasitic agents that effect aquatic and fish medicine.

CONCLUSIONS

In our modern times, when the marine industry has become so vital for feed and economic stability, even the most extreme alternative treatment strategies such as the use of small molecules or even the use of disease to control invasive species populations should be considered.

Virulence of a chimeric recombinant infectious haematopoietic necrosis virus expressing the spring viraemia of carp virus glycoprotein in salmonid and cyprinid fish

Infectious haematopoietic necrosis virus (IHNV) and spring viraemia of carp virus (SVCV) are both rhabdoviruses of fish, listed as notifiable disease agents by the World Organization for Animal Health. Recombinant rhabdoviruses with heterologous gene substitutions have been engineered to study genetic determinants and assess the potential of these recombinant viruses for vaccine development. A recombinant IHNV (rIHNV), containing the full-length genome of a European IHNV strain, was modified by deleting the glycoprotein (G) gene and replacing it with a European SVCV G-gene to make the rIHNV-Gsvcv. The chimeric rIHNV-Gsvcv level of virulence in rainbow trout, common carp and koi was assessed, and its ability to induce a protective immune response in surviving koi against wild-type SVCV infection was tested. The rIHNV-Gsvcv infection of trout led to high mortality, ranging from 78% to 92.5%, after immersion. In contrast, no deaths occurred in juvenile common carp after infection with rIHNV-Gsvcv by either immersion or intraperitoneal (IP) injection. Similarly, koi infected with rIHNV-Gsvcv via IP injection had little to no mortality (≤9%). Koi that survived initial infection with a high dose of recombinant virus rIHNV-Gsvcv were protected against a virulent SVCV challenge resulting in a high relative per cent survival of 82.5%.

The role of infectious hematopoietic necrosis virus (IHNV) proteins in the modulation of NF-κB pathway during IHNV infection

Viral infections frequently lead to the activation of host innate immune signaling pathways involved in the defense against invading pathogens. To ensure their survival, viruses have evolved sophisticated mechanisms to overcome the host immune responses. The present study demonstrated for the first time that infectious hematopoietic necrosis virus (IHNV) activated NF-κB pathway in fish cells. We further identified that the IHNV L protein could activate the NF-κB signaling pathway and that IHNV NV functioned as an inhibitor of NF-κB activation. Further results demonstrated that the NV protein blocked the degradation of the inhibitor of NF-κB (IκBα) and suppressed the SeV-induced NF-κB nuclear translocation. In conclusion, our study explored the functions of different IHNV proteins on NF-κB activation, and revealed a potential mechanism by which IHNV evades innate immune responses.

NV Proteins of Fish Novirhabdovirus Recruit Cellular PPM1Bb Protein Phosphatase and Antagonize RIG-I-Mediated IFN Induction

Non virion (NV) protein expression is critical for fish Novirhabdovirus, viral hemorrhagic septicemia virus (VHSV) and infectious hematopoietic necrosis virus (IHNV), in vivo pathogenesis. However, the mechanism by which NV promotes the viral replication is still unclear. We developed an approach based on reverse genetics and interactomic and identified several NV-associated cellular partners underlying cellular pathways as potential viral targets. Among these cell partners, we showed that NV proteins specifically interact with a protein phosphatase, Mg²⁺/Mn²⁺-dependent, 1Bb (PPM1Bb) and recruit it in the close vicinity of mitochondria, a subcellular compartment important for retinoic acid-inducible gene-I- (RIG-I)-mediated interferon induction pathway. PPM1B proteins belong to the PP2C family of serine/threonine (Ser/Thr) protein phosphatase and have recently been shown to negatively regulate the host antiviral response via dephosphorylating Traf family member-associated NF-κB activator (TANK)-binding kinase 1 (TBK1). We demonstrated that NV proteins and PPM1Bb counteract RIG-I- and TBK1-dependent interferon (IFN) and IFN-stimulated gene promoter induction in fish cells and, hence, the establishment of an antiviral state. Furthermore, the expression of VHSV NV strongly reduced TBK1 phosphorylation and thus its activation. Our findings provide evidence for a previously undescribed mechanism by which a viral protein recruits PPM1Bb protein phosphatase to subvert innate immune recognition.

The family Rhabdoviridae: mono- and bipartite negative-sense RNA viruses with diverse genome organization and common evolutionary origins

The family Rhabdoviridae consists of mostly enveloped, bullet-shaped or bacilliform viruses with a negative-sense, single-stranded RNA genome that infect vertebrates, invertebrates or plants. This ecological diversity is reflected by the diversity and complexity of their genomes. Five canonical structural protein genes are conserved in all rhabdoviruses, but may be overprinted, overlapped or interspersed with several novel and diverse accessory genes. This review gives an overview of the characteristics and diversity of rhabdoviruses, their taxonomic classification, replication mechanism, properties of classical rhabdoviruses such as rabies virus and rhabdoviruses with complex genomes, rhabdoviruses infecting aquatic species, and plant rhabdoviruses with both mono- and bipartite genomes.

Complete Protection against Influenza Virus H1N1 Strain A/PR/8/34 Challenge in Mice Immunized with Non-Adjuvanted Novirhabdovirus Vaccines

Novirhabdoviruses like Viral Hemorrhagic Septicemia Virus (VHSV) and Infectious Hematopoietic Necrosis Virus (IHNV) are fish-infecting Rhabdoviruses belonging to the Mononegavirales order. By reverse genetics, we previously showed that a recombinant VHSV expressing the West Nile Virus (WNV) E glycoprotein could serve as a vaccine platform against WNV. In the current study, we aimed to evaluate the potential of the Novirhabdovirus platform as a vaccine against influenza virus. Recombinant Novirhabdoviruses, rVHSV-HA and rIHNV-HA, expressing at the viral surface the hemagglutinin HA ectodomain were generated and used to immunized mice. We showed that mice immunized with either, rVHSV-HA or rIHNV-HA, elicited a strong neutralizing antibody response against influenza virus. A complete protection was conferred to the immunized mice when challenged with a lethal dose of influenza H1N1 A/PR/8/34 virus. Furthermore we showed that although acting as inert antigen in mice, since naturally inactivated over 20°C, mice immunized with rVHSV-HA or rIHNV-HA in the absence of adjuvant were also completely protected from a lethal challenge. Novirhabdoviruses platform are of particular interest as vaccines for mammals since they are cost effective to produce, relatively easy to generate and very effective to protect immunized animals.

Aquatic viruses induce host cell death pathways and its application

Virus infections of mammalian and animal cells consist of a series of events. As intracellular parasites, viruses rely on the use of host cellular machinery. Through the use of cell culture and molecular approaches over the past decade, our knowledge of the biology of aquatic viruses has grown exponentially. The increase in aquaculture operations worldwide has provided new approaches for the transmission of aquatic viruses that include RNA and DNA viruses. Therefore, the struggle between the virus and the host for control of the cell's death machinery is crucial for survival. Viruses are obligatory intracellular parasites and, as such, must modulate apoptotic pathways to control the lifespan of their host to complete their replication cycle. This paper updates the discussion on the detailed mechanisms of action that various aquatic viruses use to induce cell death pathways in the host, such as Bad-mediated, mitochondria-mediated, ROS-mediated and Fas-mediated cell death circuits. Understanding how viruses exploit the apoptotic pathways of their hosts may provide great opportunities for the development of future potential therapeutic strategies and pathogenic insights into different aquatic viral diseases.

In vitro and in vivo characterization of molecular determinants of virulence in reassortant betanodavirus

We previously reported that betanodavirus reassortant strains [redspotted grouper nervous necrosis virus/striped jack nervous necrosis virus (SJNNV)] isolated from Senegalese sole (Solea senegalensis) exhibited a modified SJNNV capsid amino acid sequence, with changes at aa 247 and 270. In the current study, we investigated the possible role of both residues as putative virulence determinants. Three recombinant viruses harbouring site-specific mutations in the capsid protein sequence, rSs160.03247 (S247A), rSs160.03270 (S270N) and rSs160.03247+270 (S247A/S270N), were generated using a reverse genetics system. These recombinant viruses were studied in cell culture and in vivo in the natural fish host. The three mutant viruses were shown to be infectious and able to replicate in E-11 cells, reaching final titres similar to the WT virus, although with a somewhat slower kinetics of replication. When the effect of the amino acid substitutions on virus pathogenicity was evaluated in Senegalese sole, typical clinical signs of betanodavirus infection were observed in all groups. However, fish mortality induced by all three mutant viruses was clearly affected. Roughly 40 % of the fish survived in these three groups in contrast with the WT virus which killed 100 % of the fish. These data demonstrated that aa 247 and 270 play a major role in betanodavirus virulence although when both mutated aa 247 and 270 are present, corresponding recombinant virus was not further attenuated.

Interchange of L polymerase protein between two strains of viral hemorrhagic septicemia virus (VHSV) genotype IV alters temperature sensitivities in vitro

Viral hemorrhagic septicemia virus (VHSV) has four genotypes (I-IV) and sub-lineages within genotype I and IV. Using a reverse genetics approach, we explored the importance of the L gene for growth characteristics at different temperatures following interchange of the L gene within genotype IV (IVa and IVb) strains. VHSV strains harboring heterologous L gene were recovered and we show that the L gene determines growth characteristics at different temperatures in permissive cell lines.

RNA interference technology used for the study of aquatic virus infections

Aquaculture is one of the most important economic activities in Asia and is presently the fastest growing sector of food production in the world. Explosive increases in global fish farming have been accompanied by an increase in viral diseases. Viral infections are responsible for huge economic losses in fish farming, and control of these viral diseases in aquaculture remains a serious challenge. Recent advances in biotechnology have had a significant impact on disease reduction in aquaculture. RNAi is one of the most important technological breakthroughs in modern biology, allowing us to directly observe the effects of the loss of specific genes in living systems. RNA interference technology has emerged as a powerful tool for manipulating gene expression in the laboratory. This technology represents a new therapeutic approach for treating aquatic diseases, including viral infections. RNAi technology is based on a naturally occurring post-transcriptional gene silencing process mediated by the formation of dsRNA. RNAi has been proven widely effective for gene knockdown in mammalian cultured cells, but its utility in fish remains unexplored. This review aims to highlight the RNAi technology that has made significant contributions toward the improvement of aquatic animal health and will also summarize the current status and future strategies concerning the therapeutic applications of RNAi to combat viral disease in aquacultured organisms.

Modulation of proteome expression by F-type lectin during viral hemorrhagic septicemia virus infection in fathead minnow cells

Lectins found in fish tissues play an important role in the innate immune response against viral infection. A fucose-binding type lectin, RbFTL-3, from rock bream (Oplegnathus fasciatus) was identified using expressed sequence tag (EST) analysis. The expression of RbFTL-3 mRNA was higher in intestine than other tissues of rock bream. To determine the function of RbFTL-3, VHSV-susceptible fathead minnow (FHM) cells were transfected with pcDNA3.1(+) or pcDNA3.1(+)-RbFTL-3 and further infected with VHSV. The results show that the viability of FHM cells transfected with pcDNA3.1(+)-RbFTL-3 is higher than that of cells transfected with pcDNA3.1(+) (relative cell viability: 28.9% vs 56.2%). A comparative proteomic analysis, performed to explore the proteins related to the protective effect of RbFTL-3 in the cells during VHSV infection, identified 90 proteins differentially expressed in VHSV-infected FHM cells transfected with pcDNA3.1(+) or pcDNA3.1(+)-RbFTL-3. The expression of RbFTL-3 inhibits a vascular-sorting protein (SNF8) and diminishes the loss of prothrombin, which are closely associated with controlling viral budding and hemorrhage in fish cells, respectively. Subsequent Ingenuity Pathways Analysis enabled prediction of their biofunctional groupings and interaction networks. The results suggest RbFTL-3 modulates the expression of proteins related to viral budding (SNF8, CCT5 and TUBB) and thrombin signaling (F2) to increase the viability of VHSV infected cells.

Engineering recombinant Orsay virus directly in the metazoan host Caenorhabditis elegans

The recent identification of Orsay virus, the first virus that is capable of naturally infecting Caenorhabditis elegans, provides a unique opportunity to explore host-virus interaction studies in this invaluable model organism. A key feature of this system is the robust genetic tractability of the host, C. elegans, which would ideally be complemented by the ability to genetically manipulate Orsay virus in parallel. To this end, we developed a plasmid-based reverse genetics system for Orsay virus by creating transgenic C. elegans strains harboring Orsay virus cDNAs. Both wild-type and mutant Orsay viruses, including a FLAG epitope-tagged recombinant Orsay virus, were generated by use of the reverse genetics system. This is the first plasmid-based virus reverse genetics system in the metazoan C. elegans. The Orsay virus reverse genetics we established will serve as a fundamental tool in host-virus interaction studies in the model organism C. elegans. Importance: To date, Orsay virus is the first and the only identified virus capable of naturally infecting Caenorhabditis elegans. C. elegans is a simple multicellular model organism that mimics many fundamental features of human biology and has been used to define many biological properties conserved through evolution. Thus, the Orsay virus-C. elegans infection system provides a unique opportunity to study host-virus interactions. In order to take maximal advantage of this system, the ability to genetically engineer mutant forms of Orsay virus would be highly desirable. Most efforts to engineer viruses have been done with cultured cells. Here we describe the creation of mutant viruses directly in the multicellular organism C. elegans without the use of cell culture. We engineered a virus expressing a genetically tagged protein that could be detected in C. elegans. This provides proof of concept for modifying Orsay virus, which will greatly facilitate studies in this experimental system.

Innate immune responses of salmonid fish to viral infections

Viruses are the most serious pathogenic threat to the production of the main aquacultured salmonid species the rainbow trout Oncorhynchus mykiss and the Atlantic salmon Salmo salar. The viral diseases Infectious Pancreatic Necrosis (IPN), Pancreatic Disease (PD), Infectious Haemorrhagic Necrosis (IHN), Viral Haemorrhagic Septicaemia (VHS), and Infectious Salmon Anaemia (ISA) cause massive economic losses to the global salmonid aquaculture industry every year. To date, no solution exists to treat livestock affected by a viral disease and only a small number of efficient vaccines are available to prevent infection. As a consequence, understanding the host immune response against viruses in these fish species is critical to develop prophylactic and preventive control measures. The innate immune response represents an important part of the host defence mechanism preventing viral replication after infection. It is a fast acting response designed to inhibit virus propagation immediately within the host, allowing for the adaptive specific immunity to develop. It has cellular and humoral components which act in synergy. This review will cover inflammation responses, the cell types involved, apoptosis, antimicrobial peptides. Particular attention will be given to the type I interferon system as the major player in the innate antiviral defence mechanism of salmonids. Viral evasion strategies will also be discussed.

A recombinant novirhabdovirus presenting at the surface the E Glycoprotein from West Nile Virus (WNV) is immunogenic and provides partial protection against lethal WNV challenge in BALB/c mice

West Nile Virus (WNV) is a zoonotic mosquito-transmitted flavivirus that can infect and cause disease in mammals including humans. Our study aimed at developing a WNV vectored vaccine based on a fish Novirhabdovirus, the Viral Hemorrhagic Septicemia virus (VHSV). VHSV replicates at temperatures lower than 20°C and is naturally inactivated at higher temperatures. A reverse genetics system has recently been developed in our laboratory for VHSV allowing the addition of genes in the viral genome and the recovery of the respective recombinant viruses (rVHSV). In this study, we have generated rVHSV vectors bearing the complete WNV envelope gene (E_WNV) (rVHSV-E_WNV) or fragments encoding E subdomains (either domain III alone or domain III fused to domain II) (rVHSV-DIII_WNV and rVHSV-DII-DIII_WNV, respectively) in the VHSV genome between the N and P cistrons. With the objective to enhance the targeting of the E_WNV protein or E_WNV-derived domains to the surface of VHSV virions, Novirhadovirus G-derived signal peptide and transmembrane domain (SP_G and TM_G) were fused to E_WNV at its amino and carboxy termini, respectively. By Western-blot analysis, electron microscopy observations or inoculation experiments in mice, we demonstrated that both the E_WNV and the DIII_WNV could be expressed at the viral surface of rVHSV upon addition of SP_G. Every constructs expressing E_WNV fused to SP_G protected 40 to 50% of BALB/cJ mice against WNV lethal challenge and specifically rVHSV-SP_GE_WNV induced a neutralizing antibody response that correlated with protection. Surprisingly, rVHSV expressing E_WNV-derived domain III or II and III were unable to protect mice against WNV challenge, although these domains were highly incorporated in the virion and expressed at the viral surface. In this study we demonstrated that a heterologous glycoprotein and non membrane-anchored protein, can be efficiently expressed at the surface of rVHSV making this approach attractive to develop new vaccines against various pathogens.

Exploring RNAi as a therapeutic strategy for controlling disease in aquaculture

Aquatic animal diseases are one of the most significant constraints to the development and management of aquaculture worldwide. As a result, measures to combat diseases of fish and shellfish have assumed a high priority in many aquaculture-producing countries. RNA interference (RNAi), a natural mechanism for post-transcriptional silencing of homologous genes by double-stranded RNA (dsRNA), has emerged as a powerful tool not only to investigate the function of specific genes, but also to suppress infection or replication of many pathogens that cause severe economic losses in aquaculture. However, despite the enormous potential as a novel therapeutical approach, many obstacles must still be overcome before RNAi therapy finds practical application in aquaculture, largely due to the potential for off-target effects and the difficulties in providing safe and effective delivery of RNAi molecules in vivo. In the present review, we discuss the current knowledge of RNAi as an experimental tool, as well as the concerns and challenges ahead for the application of such technology to combat infectious disease of farmed aquatic animals.

Reverse Genetics of Mononegavirales: The Rabies Virus Paradigm

The neurotropic rabies virus (RABV) is a prototype member of the Mononegavirales order of viruses and is the most significant human pathogen of the Rhabdoviridae family. A reverse genetics system for RABV was established almost 20 years ago, providing a paradigm for other Mononegavirales members as well. The availability of engineered recombinant viruses opened a new era to study common aspects of Mononegavirales biology and specific aspects of the unique lifestyle and pathogenesis of individual members. Above all, the knowledge gained has allowed engineering of beneficial biomedical tools such as viral vectors, vaccines, and tracers. In this chapter, the development of the classical rabies virus reverse genetics approach is described, and some of the most exciting biomedical applications for recombinant RABV and other Mononegavirales are briefly addressed.

Both STING and MAVS fish orthologs contribute to the induction of interferon mediated by RIG-I

Viral infections are detected in most cases by the host innate immune system through pattern-recognition receptors (PRR), the sensors for pathogen-associated molecular patterns (PAMPs), which induce the production of cytokines, such as type I interferons (IFN). Recent identification in mammalian and teleost fish of cytoplasmic viral RNA sensors, RIG-I-like receptors (RLRs), and their mitochondrial adaptor: the mitochondrial antiviral signaling (MAVS) protein, also called IPS-1, highlight their important role in the induction of IFN at the early stage of a virus infection. More recently, an endoplasmic reticulum (ER) adaptor: the stimulator of interferon genes (STING) protein, also called MITA, ERIS and MPYS, has been shown to play a pivotal role in response to both non-self-cytosolic RNA and dsDNA. In this study, we cloned STING cDNAs from zebrafish and showed that it was an ortholog to mammalian STING. We demonstrated that overexpression of this ER protein in fish cells led to a constitutive induction of IFN and interferon-stimulated genes (ISGs). STING-overexpressing cells were almost fully protected against RNA virus infection with a strong inhibition of both DNA and RNA virus replication. In addition, we found that together with MAVS, STING was an important player in the RIG-I IFN-inducing pathway. This report provides the demonstration that teleost fish possess a functional RLR pathway in which MAVS and STING are downstream signaling molecules of RIG-I. The Sequences presented in this article have been submitted to GenBank under accession numbers: Zebrafish STING (HE856619); EPC STING (HE856620); EPC IRF3 (HE856621); EPC IFN promoter (HE856618).

Development of an improved polykaryon-based influenza virus rescue system

BACKGROUND

Virus rescue from transfected cells is an extremely useful technique that allows defined viral clones to be engineered for the purpose of rational vaccine design or fundamental reverse genetics studies. However, it is often hindered by low primary rescue success rates or yields, especially with field-derived viral strains.

APPROACH

We investigated the possibility of enhancing influenza virus rescue by eliciting cell fusion to increase the chances of having all necessary plasmids expressed within the same polykaryon. To this end we used the Maedi-Visna Virus envelope protein which has potent fusion activity in cells from a wide range of different species.

RESULTS

Co-transfecting cells with the eight plasmids necessary to rescue influenza virus plus a plasmid expressing the Maedi-Visna Virus envelope protein resulted in increased rescue efficiency. In addition, partial complements of the 8-plasmid rescue system could be transfected into two separate populations of cells, which upon fusion led to live virus rescue.

CONCLUSION

The simple modification described here has the potential to improve the efficiency of the virus rescue process and expand the potential applications for reverse genetic studies.

Antiviral properties of polymeric aziridine- and biguanide-modified core-shell magnetic nanoparticles

Polycationic superparamagnetic nanoparticles (∼150-250 nm) were evaluated as virucidal agents. The particles possess a core-shell structure, with cores consisting of magnetite clusters and shells of functional silica covalently bound to poly(hexamethylene biguanide) (PHMBG), polyethyleneimine (PEI), or PEI terminated with aziridine moieties. Aziridine was conjugated to the PEI shell through cationic ring-opening polymerization. The nanometric core-shell particles functionalized with biguanide or aziridine moieties are able to bind and inactivate bacteriophage MS2, herpes simplex virus HSV-1, nonenveloped infectious pancreatic necrosis virus (IPNV), and enveloped viral hemorrhagic septicaemia virus (VHSV). The virus-particle complexes can be efficiently removed from the aqueous milieu by simple magnetocollection.

Rhabdovirus accessory genes

The Rhabdoviridae is one of the most ecologically diverse families of RNA viruses with members infecting a wide range of organisms including placental mammals, marsupials, birds, reptiles, fish, insects and plants. The availability of complete nucleotide sequences for an increasing number of rhabdoviruses has revealed that their ecological diversity is reflected in the diversity and complexity of their genomes. The five canonical rhabdovirus structural protein genes (N, P, M, G and L) that are shared by all rhabdoviruses are overprinted, overlapped and interspersed with a multitude of novel and diverse accessory genes. Although not essential for replication in cell culture, several of these genes have been shown to have roles associated with pathogenesis and apoptosis in animals, and cell-to-cell movement in plants. Others appear to be secreted or have the characteristics of membrane-anchored glycoproteins or viroporins. However, most encode proteins of unknown function that are unrelated to any other known proteins. Understanding the roles of these accessory genes and the strategies by which rhabdoviruses use them to engage, divert and re-direct cellular processes will not only present opportunities to develop new anti-viral therapies but may also reveal aspects of cellar function that have broader significance in biology, agriculture and medicine.

Host range, host specificity and hypothesized host shift events among viruses of lower vertebrates

The successful replication of a viral agent in a host is a complex process that often leads to a species specificity of the virus and can make interspecies transmission difficult. Despite this difficulty, natural host switch seems to have been frequent among viruses of lower vertebrates, especially fish viruses, since there are several viruses known to be able to infect a wide range of species. In the present review we will focus on well documented reports of broad host range, variations in host specificity, and host shift events hypothesized for viruses within the genera Ranavirus, Novirhabdovirus, Betanodavirus, Isavirus, and some herpesvirus.

The contribution of molecular epidemiology to the understanding and control of viral diseases of salmonid aquaculture

Molecular epidemiology is a science which utilizes molecular biology to define the distribution of disease in a population (descriptive epidemiology) and relies heavily on integration of traditional (or analytical) epidemiological approaches to identify the etiological determinants of this distribution. The study of viral pathogens of aquaculture has provided many exciting opportunities to apply such tools. This review considers the extent to which molecular epidemiological studies have contributed to better understanding and control of disease in aquaculture, drawing on examples of viral diseases of salmonid fish of commercial significance including viral haemorrhagic septicaemia virus (VHSV), salmonid alphavirus (SAV) and infectious salmon anaemia virus (ISAV). Significant outcomes of molecular epidemiological studies include:

Improved taxonomic classification of viruses

A better understanding of the natural distribution of viruses

An improved understanding of the origins of viral pathogens in aquaculture

An improved understanding of the risks of translocation of pathogens outwith their natural host range

An increased ability to trace the source of new disease outbreaks

Development of a basis for ensuring development of appropriate diagnostic tools

An ability to classify isolates and thus target future research aimed at better understanding biological function

While molecular epidemiological studies have no doubt already made a significant contribution in these areas, the advent of new technologies such as pyrosequencing heralds a quantum leap in the ability to generate descriptive molecular sequence data. The ability of molecular epidemiology to fulfil its potential to translate complex disease pathways into relevant fish health policy is thus unlikely to be limited by the generation of descriptive molecular markers. More likely, full realisation of the potential to better explain viral transmission pathways will be dependent on the ability to assimilate and analyse knowledge from a range of more traditional information sources. The development of methods to systematically record and share such epidemiologically important information thus represents a major challenge for fish health professionals in making the best future use of molecular data in supporting fish health policy and disease control.