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

Establishment and identification of spleen cell line from tiger puffer fish (Takifugu rubripes) and immune transcriptome response to IHNV

The tiger puffer fish (Takifugu rubripes) is highly valued both economically and for its culinary appeal. However, it currently faces a significant risk of disease. This study aimed to establish a spleen cell line to aid in disease prevention and control for this species. We established a new stable cell line (TRSC) from the spleen tissue of the tiger puffer fish. The TRSC cells exhibited stable growth over more than 90 generations in L-15 medium supplemented with 4 ng/mL recombinant human basic fibroblast growth factor (bFGF) and 2 ng/mL recombinant human leukemia inhibitory factor (LIF) at 24 °C. Chromosomal analysis revealed a diploid number of 2n = 44. The TRSC cells were susceptibly infected by both viral hemorrhagic septicemia virus (VHSV) and infectious hematopoietic necrosis virus (IHNV), exhibiting a clear cytopathic effect (CPE). After 48 h of infection, alterations in cell morphology and disintegration were observed. Quantitative reverse-transcription polymerase chain reaction (qRT-PCR) analysis revealed differential replication of the two viruses in TRSC cells. Transcriptome sequencing identified 1308 differentially expressed genes (DEGs) following IHNV infection, with 668 genes upregulated and 640 downregulated. These DEGs were significantly enriched in MAPK signaling pathway, p53 signaling pathway, and DNA replication and so on. The establishment of the TRSC cell line provides valuable insights into viral infection mechanisms, aids in the identification of key immune genes, and offers a more effective approach to disease control in aquaculture, supporting the sustainable development of the industry.

Utilizing shRNA-expressing lentivectors for viral hemorrhagic septicemia virus suppression via NV gene targeting

Background

Viral hemorrhagic septicemia virus or VHSV, is a single-stranded negative-sense RNA virus that is a member of the Rhabdoviridae family's genus Novirhabdovirus. Its major host is rainbow trout. Severe clinical symptoms and a higher mortality rate in fish populations are caused by this virus. Regretfully, there is currently no medication or vaccination available to treat it. Recently, there has been a lot of interest in developing antiviral therapies employing interfering RNA (RNAi), particularly shRNA. This study used shRNAs targeting the NV gene of VHSV to test its effectiveness in preventing VHSV proliferation in cell culture. Using the VHSV-Fil3 strain, the appropriate oligonucleotide sequence for NV gene coding was chosen for this purpose. Subsequently, shRNA molecules were designed and synthesized with the aid of shRNA design tools. The shRNAs were transfected into HEK293T cells after being cloned into the suitable vectors using the third generation of lentiviral packaging system. The CS2-2 cell line was subsequently transduced with these shRNA-expressing lentiviruses in order to challenge the VHS virus. Finally, TCID50 was employed to calculate the viral infectious titer in order to assess the effectiveness of shRNAs.

Results

According to the final calculations, all shRNAs exhibited antiviral activity. When compared to the control groups, the shRNAs 1, 2, and 3 considerably lowered VHSV output in the TCID50 test (nearly 99.99, 99.99, and 99.99%, respectively, compared to cells with VHSV inoculation and nearly 99.98, 99.98, and 99.97%, respectively, compared to cells with VHSV and scrambled vector inoculation).

Conclusion

Thus, it can be declared that RNA interference (RNAi) has the potential to be an exceptionally effective therapeutic option against viruses like VHSV.

Dual RNA-seq analyses of viral hemorrhagic septicemia virus and olive flounder (Paralichthys olivaceus) interactions at low and high water temperatures

Transcriptomic studies in fish have considerably contributed to understanding the host responses under viral hemorrhagic septicemia virus (VHSV) infection. However, changes from the perspective of the pathogen and host-pathogen interactions have been relatively underestimated. Given that VHSV genes can be poly-adenylated during replication, this study investigated the global changes from both VHSV and host perspectives using the host transcriptomic data. To achieve this, we utilized transcriptomic data from VHSV infected flounder obtained at different water temperatures (13 °C and 20 °C). We collected the sequence reads belonging to VHSV through an bioinformatic pipeline developed especially for this study. The VHSV reads were used to construct the consensus reference genome and to investigate the expression of VHSV genes and the frequency of variants under different water temperatures. In addition, both linear and logarithmic scales of VHSV transcription levels, along with host transcriptomes, were used to understand pathogen-host interaction through weighted correlation network analysis (WGCNA). The results revealed that VHSV transcription can exceed 5% of host transcriptome during the infection. Single nucleotide variants (SNVs) appeared more frequently in 13 °C groups than in 20 °C groups. While VHSV can replicate at both 13 °C and 20 °C, host transcriptomic responses were notably different, with stronger immune responses and more frequent VHSV genetic changes observed at a lower temperature. This suggests that VHSV infection at low water temperatures significantly influences both the host transcriptional changes and pathogen. Through WGCNA, numerous genes in the module that correlated with VHSV reads on a linear scale were found to be related to cytoskeleton modulation and viral activity. By contrast, the gene module (black) correlated with VHSV reads on a logarithmic scale was strongly associated with host immune responses, such as TNF signaling, necroptosis, and the NF-kappa B signaling pathway. The dual RNA-seq approaches developed in this study will immensely enhance our understanding of host-pathogen interactions across different temperatures.

Transcriptomic and biometric parameters analysis in rainbow trout (Oncorhynchus mykiss) challenged with viral hemorrhagic septicemia virus (VHSV)

BACKGROUND

Viral hemorrhagic septicemia virus (VHSV) is a highly pathogenic virus that poses a significant threat to the health of diverse marine species. Among these, trout species, particularly rainbow trout (Oncorhynchus mykiss), are highly susceptible. This study evaluated the effects of VHSV infection on the biometric traits of rainbow trout and investigated the molecular mechanisms associated with the disease.

RESULTS

Biometric traits of fish were collected and documented weekly during the fourth and fifth weeks of the experiment. A statistically significant difference in body weight was observed in the fifth week, particularly between the control group and the groups injected with either physiological saline or the virus. Additionally, body length-related attributes showed significant variation across all treatment groups within the designated timeframe. RNA was extracted from spleen tissue of the group injected with high doses of physiological saline and the group injected with high doses of the virus using the TRIzol protocol. Differential gene expression analysis revealed 1,726 genes with significant differences between the two groups. Several key immune-related genes were identified, including TLR2, TLR7, TLR8, TLR22, IRF5, IRF6, IRF7, IRF8, IRF10, IL11a, IL12B, IL1b, IL7R, ILR1 II, HSP90B1, HSP47, TNF-α, TRF3, SPRY1, CASP3, FN1, GAPDH, and IgGFc-binding proteins. Network-based analysis of differentially expressed genes was conducted using the GeneMANIA module in Cytoscape, and metabolic pathways were identified through the DAVID database. The results highlighted the involvement of key pathways, including the Toll-like receptor pathway, p53 signaling pathway, PPAR signaling pathway, and the cell cycle, in the infected group. Validation tests for selected upregulated (EPCAM, APOC2 and XDD4) and downregulated (TLR7, XDH, and TSPAN36) candidate genes, were conducted using qRT-PCR. The qPCR results showed a strong and statistically significant correlation with the RNA-seq data, confirming the reliability of the findings.

CONCLUSIONS

VHSV significantly impacts the growth of rainbow trout, affecting both body length and gene expression. This study underscores the substantial economic risks posed by the virus and the absence of an effective cure, highlighting the importance of preventative measures. Additionally, potential resistance genes and pathways were identified through RNA sequencing, providing valuable insights for improving trout breeding programs.

The genetics of resistance to infectious pancreatic necrosis virus in rainbow trout unveiled through survival and virus load data

Infectious Pancreatic Necrosis virus (IPNV) is one of the major threats to the animal welfare and economy of the rainbow trout farming industry. Previous research has demonstrated significant genetic variation for resistance against IPNV. The main objective of the study was to investigate the genetic architecture of resistance against IPNV in rainbow trout fry. To achieve this, 610 rainbow trout fry, from a full factorial mating between 5 sires and 5 dams, were bath challenged with the IPNV isolate (IPNV-AS) from Atlantic salmon reared at a commercial farm. The resistance against IPNV was accessed using three different phenotypes; binary survival (BS), total days survived (TDS) and virus load (VL) recorded on the fish throughout the 40-day challenge test. All fish were genotyped using a 57K Affymetrix SNP array. The IPNV-AS isolate resulted in an overall mortality of 62.1%. The heritability estimates for survival (BS h2 = 0.21 ± 0.06, TDS h2 = 0.25 ± 0.07) and VL traits (h2 = 0.23 ± 0.08) were moderate and indicative of potential use of selection for increased resistance to IPNV in rainbow trout selective breeding programs. The unity estimated genetic correlation between the two survival traits (BS and TDS) indicates that the traits can be considered the same trait. In contrast, a moderate favourable negative genetic correlation was found between VL and the two survival traits (-0.61 ± 0.22 to -0.70 ± 0.19). The GWAS of the traits with many QTLs crossing the chromosome-wide Bonferroni corrected threshold indicates the polygenic nature of the studied traits. Most of the 10 possible identified genes were found to be linked with immunity or viral pathogenesis, which could be potentially responsible for the significant genetic variation in survival against the IPNV-AS. The QTL validation analysis revealed no significant difference in the mortalities and VL among the three genotypes of the detected QTL. The VL trait showed larger variation among the dead fry and with a concordant pattern with the two survival phenotypes, but with no significant difference in the proportion of IPNV VL positive samples in the dead and the survived fry. Overall, the results indicate the polygenic nature of the studied traits and support the use of genomic selection to improve resistance against IPNV in rainbow trout breeding companies.

Antiviral effects of umbelliferone against viral hemorrhagic septicemia virus in olive flounder (Paralichthys olivaceus)

Viral hemorrhagic septicemia virus (VHSV) poses a significant threat to global aquaculture, prompting ongoing efforts to identify potential drug candidates for disease prevention. Coumarin derivatives have recently emerged as a promising class of compounds effective against rhabdoviruses, which severely impact the aquaculture industry. In this study, we assessed the anti-VHSV activity of umbelliferone (7-hydroxycoumarin) in fathead minnow (FHM) cells and olive flounder Paralichthys olivaceus. Umbelliferone exhibited an EC50 of 100 μg/mL by reducing cytopathic effect, with a maximum cytotoxicity of 30.9 % at 750 μg/mL. Mechanistic analyses via a time-course plaque reduction assay revealed that direct incubation with the virus for 1 h resulted in 97.0 ± 1.8 % plaque reduction, showing excellent direct virucidal activity. Pretreatment for 4 h resulted in a 33.5 ± 7.8 % plaque reduction, which increased with longer incubation times. Cotreatment led to a 33.5 ± 2.9 % plaque reduction, suggesting interference with viral binding, whereas postinfection treatment proved less effective. Umbelliferone was prophylactically administered to the olive flounder through short-term (3 days) and long-term (14 days) medicated feeding, followed by a 4-day postinfection period. Short-term administration at 100 mg/kg body weight (bw)/day resulted in the highest relative percent survival (RPS) of 56 %, whereas long-term administration achieved a maximum RPS of 44 % at 30 mg/kg bw/day. Umbelliferone administration delayed mortality at these doses. Additionally, umbelliferone significantly inhibited the expression of the VHSV N gene during viral challenge, with no observed toxic effects in fish up to an administration dose of 30 mg/kg bw/day for 28 days. Our findings suggest that the protective mechanism of short-term administration of 100 mg umbelliferone against VHSV infection may involve the overexpression of TLR2, MDA5, STAT1, and NF-κB at 24 h postinfection (hpi). IL-8 and IFN II expression was upregulated, whereas TNF-α, IL-1β, and IFN I expression was suppressed at 24 hpi. The upregulation of ISG15 at 48 hpi may contribute to the inhibition of VHSV replication, whereas the downregulation of Caspase 3 expression at 96 hpi suggests a possible inhibition of virus-induced apoptosis at later infection stages. Overall, umbelliferone exhibited anti-VHSV activity through multiple mechanisms, with the added advantage of convenient administration via medicated feed.

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.

Transcriptome Profiling of Oncorhynchus mykiss Infected with Low or Highly Pathogenic Viral Hemorrhagic Septicemia Virus (VHSV)

The rainbow trout (Oncorhynchus mykiss) is the most important produced species in freshwater within the European Union, usually reared in intensive farming systems. This species is highly susceptible to viral hemorrhagic septicemia (VHS), a severe systemic disease widespread globally throughout the world. Viral hemorrhagic septicemia virus (VHSV) is the etiological agent and, recently, three classes of VHSV virulence (high, moderate, and low) have been proposed based on the mortality rates, which are strictly dependent on the viral strain. The molecular mechanisms that regulate VHSV virulence and the stimulated gene responses in the host during infection are not completely unveiled. While some preliminary transcriptomic studies have been reported in other fish species, to date there are no publications on rainbow trout. Herein, we report the first time-course RNA sequencing analysis on rainbow trout juveniles experimentally infected with high and low VHSV pathogenic Italian strains. Transcriptome analysis was performed on head kidney samples collected at different time points (1, 2, and 5 days post infection). A large set of notable genes were found to be differentially expressed (DEGs) in all the challenged groups (e.s. trim63a, acod1, cox-2, skia, hipk1, cx35.4, ins, mtnr1a, tlr3, tlr7, mda5, lgp2). Moreover, the number of DEGs progressively increased especially during time with a greater amount found in the group infected with the high VHSV virulent strain. The gene ontology (GO) enrichment analysis highlighted that functions related to inflammation were modulated in rainbow trout during the first days of VHSV infection, regardless of the pathogenicity of the strain. While some functions showed slight differences in enrichments between the two infected groups, others appeared more exclusively modulated in the group challenged with the highly pathogenic strain.

Viperin mutation is linked to immunity, immune cell dynamics, and metabolic alteration during VHSV infection in zebrafish

Viperin is a prominent antiviral protein found in animals. The primary function of Viperin is the production of 3'-deoxy-3',4'-didehydro-cytidine triphosphate (ddhCTP), an inhibitory nucleotide involved in viral RNA synthesis. Studies in mammalian models have suggested that ddhCTP interferes with metabolic proteins. However, this hypothesis has yet to be tested in teleost. In this study, the role of Viperin in regulating metabolic alterations during viral hemorrhagic septicemia virus (VHSV) infection was tested. When infected with VHSV, viperin -/- fish showed considerably higher mortality rates. VHSV copy number and the expression of the NP gene were significantly increased in viperin -/- fish. Metabolic gene analysis revealed significant differences in soda, hif1a, fasn, and acc expression, indicating their impact on metabolism. Cholesterol analysis in zebrafish larvae during VHSV infection showed significant upregulation of cholesterol production without Viperin. In vitro analysis of ZF4 cells suggested a considerable reduction in lipid production and a significant upregulation of reactive oxygen species (ROS) generation with the overexpression of viperin. Neutrophil and macrophage recruitment were significantly modulated in viperin -/- fish compared to the wild-type (WT) fish. Thus, we have demonstrated that Viperin plays a role in interfering with metabolic alterations during VHSV infection.

Eukaryotic translation elongation factor 1 alpha (eEF1A) inhibits Siniperca chuatsi rhabdovirus (SCRV) infection through two distinct mechanisms

IMPORTANCE

Although a virus can regulate many cellular responses to facilitate its replication by interacting with host proteins, the host can also restrict virus infection through these interactions. In the present study, we showed that the host eukaryotic translation elongation factor 1 alpha (eEF1A), an essential protein in the translation machinery, interacted with two proteins of a fish rhabdovirus, Siniperca chuatsi rhabdovirus (SCRV), and inhibited virus infection via two different mechanisms: (i) inhibiting the formation of crucial viral protein complexes required for virus transcription and replication and (ii) promoting the ubiquitin-proteasome degradation of viral protein. We also revealed the functional regions of eEF1A that are involved in the two processes. Such a host protein inhibiting a rhabdovirus infection in two ways is rarely reported. These findings provided new information for the interactions between host and fish rhabdovirus.

Genotyping tool for salmonid gill pox virus (SGPV) obtained from farmed and wild Atlantic salmon (Salmo salar)

Poxviruses are common viruses found in vertebrate species. In 2006, the first poxvirus associated with salmon, salmonid gill poxvirus (SGPV), was identified during an outbreak of gill disease at a smolt production site in northern Norway and at two marine farms in western Norway. Poxviruses had previously been detected in ayu (Plecoglossus altivelis) and koi carp (Cyprinus carpio). In all three fish species, poxviruses are associated with gill disease. It has not been possible to culture SGPV from Norway, and little is known about its virulence. However, the association between SGPV and gill disease in salmon has shown the need for molecular tools to identify reservoirs and transmission routes. Sequencing the genome of a second isolate of SGPV has made it possible to compare variable regions between two strains of the virus, showing the presence of a large number of variable regions that exhibit both variable numbers of tandem repeats and intra-ORF variation. We present eight regions that are suitable for distinguishing strains of SGPV and determining their phylogenetic relationship, and these were used to compare SGPV isolates obtained from both farmed and wild salmon in fresh and sea water. The prevalence of the virus was found to be higher in wild salmon in rivers than in returning wild salmon collected from traps in Norwegian fjords. Genotyping based on the eight selected variable regions, suggests the presence of geographically distinct isolates in freshwater among both farmed and wild salmon, while SGPV from marine farms shows high local diversity and a wide geographical distribution of similar strains of the virus.

A Safe and Efficient Double-Gene-Deleted Live Attenuated Immersion Vaccine to Prevent the Disease Caused by the Infectious Spleen and Kidney Necrosis Virus

Infectious diseases seriously threaten sustainable aquaculture development, resulting in more than $10 billion in economic losses annually. Immersion vaccines are emerging as the key technology for aquatic disease prevention and control. Here, a safe and efficacious candidate immersion vaccine strain (Δorf103r/tk) of infectious spleen and kidney necrosis virus (ISKNV), in which the orf103r and tk genes were knocked out by homologous recombination, is described. Δorf103r/tk was severely attenuated in mandarin fish (Siniperca chuatsi), inducing mild histological lesions, a mortality rate of only 3%, and eliminated within 21 days. A single Δorf103r/tk immersion-administered dose provided long-lasting protection rates over 95% against lethal ISKNV challenge. Δorf103r/tk also robustly stimulated the innate and adaptive immune responses. For example, interferon expression was significantly upregulated, and the production of specific neutralizing antibodies against ISKNV was markedly induced postimmunization. This work provides proof-of-principle evidence for orf103r- and tk-deficient ISKNV for immersion vaccine development to prevent ISKNV disease in aquaculture production. IMPORTANCE Global aquaculture production reached a record of 122.6 million tons in 2020, with a total value of 281.5 billion U.S. dollars (USD). However, approximately 10% of farmed aquatic animal production is lost due to various infectious diseases, resulting in more than 10 billion USD of economic waste every year. Therefore, the development of vaccines to prevent and control aquatic infectious diseases is of great significance. Infectious spleen and kidney necrosis virus (ISKNV) infection occurs in more than 50 species of freshwater and marine fish and has caused great economic losses to the mandarin fish farming industry in China during the past few decades. Thus, it is listed as a certifiable disease by the World Organization for Animal Health (OIE). Herein, a safe and efficient double-gene-deleted live attenuated immersion vaccine against ISKNV was developed, providing an example for the development of aquatic gene-deleted live attenuated immersion vaccine.

Conserved and divergent arms of the antiviral response in the duplicated genomes of salmonid fishes

Antiviral innate immunity is orchestrated by the interferon system, which appeared in ancestors of jawed vertebrates. Interferon upregulation induces hundreds of interferon-stimulated-genes (ISGs) with effector or regulatory functions. Here we investigated the evolutionary diversification of ISG responses through comparison of two salmonid fishes, accounting for the impact of sequential whole genome duplications ancestral to teleosts and salmonids. We analysed the transcriptomic response of the IFN pathway in the head kidney of rainbow trout and Atlantic salmon, which separated 25-30 Mya. We identified a large set of ISGs conserved in both species and cross-referenced them with zebrafish and human ISGs. In contrast, around one-third of salmonid ISG lacked orthologs in human, mouse, chicken or frog, and often between rainbow trout and Atlantic salmon, revealing a fast-evolving, lineage-specific arm of the antiviral response. This study also provides a key resource for in-depth functional analysis of ISGs in salmonids of commercial significance.

Tilapia lake virus (TiLV) causes severe anaemia and systemic disease in tilapia

Tilapia lake virus disease (TiLVD) is an emerging disease in tilapia that is associated with mass mortality affecting global tilapia aquaculture. In this study, red hybrid tilapias (Oreochromis spp.) were experimentally infected by intracoelomic injection with Tilapia lake virus (TiLV) to gain a better understanding of the clinicopathological changes during infection. Pale bodies and gill were observed in infected fish after 7 days of post-challenge (dpc) associated with severe anaemia. Further haematological analysis in TiLV-infected fish revealed decreased levels of haemoglobin and haematocrit at 3 dpc. Common pathological findings included pale and friable liver, pale intestine with catarrhal content, and dark and shrunken spleen in TiLV-infected fish at 7 dpc and 14 dpc. Histologically, reduced numbers of red blood cells and accumulation of melano-macrophage centre in the spleen were found in infected fish at 3 dpc, and severe lesions were more commonly observed at 7 and 14 dpc. Lymphocyte infiltration, syncytial cell formation and multifocal necrotic hepatitis were the prominent pathological findings in the liver of infected fish. The severity of pathological changes was associated with TiLV-infection with higher viral loads and with the expression pattern of pro-inflammatory cytokines and antiviral genes, including interferon regulatory factor 1 (irf1), interleukin (il-8), radical s-adenosyl methionine domain containing 2 (rsad2) and mx. Our study provides a comprehensive analysis of the haematological profile and pathological changes in tilapia during TiLV infection. Overall, lesions present in various organs, together with alteration of host immune response in TiLV-infected fish, indicate the systemic infection of this virus. The knowledge gained from this study improves our understanding of how TiLV causes pathological and haematological changes in tilapia.

Identification, expression profiling, and functional characterization of cystatin C from big-belly seahorse (Hippocampus abdominalis)

Cystatins are natural inhibitors of lysosomal cysteine proteases, including cathepsins B, L, H, and S. Cystatin C (CSTC) is a member of the type 2 cystatin family and is an essential biomarker in the prognosis of several diseases. Emerging evidence suggests the immune regulatory roles of CSTC in antigen presentation, the release of different inflammatory mediators, and apoptosis in various pathophysiologies. In this study, the 390-bp cystatin C (HaCSTC) cDNA from big-belly seahorse (Hippocampus abdominalis) was cloned and characterized by screening the pre-established cDNA library. Based on similarities in sequence, HaCSTC is a homolog of the teleost type 2 cystatin family with putative catalytic cystatin domains, signal peptides, and disulfide bonds. HaCSTC transcripts were ubiquitously expressed in all tested big-belly seahorse tissues, with the highest expression in ovaries. Immune challenge with lipopolysaccharides, polyinosinic:polycytidylic acid, Edwardsiella tarda, and Streptococcus iniae caused significant upregulation in HaCSTC transcript levels. Using a pMAL-c5X expression vector, the 14.29-kDa protein of recombinant HaCSTC (rHaCSTC) was expressed in Escherichia coli BL21 (DE3), and its protease inhibitory activity against papain cysteine protease was determined with the aid of a protease substrate. Papain was competitively blocked by rHaCSTC in a dose-dependent manner. In response to viral hemorrhagic septicemia virus (VHSV) infection, HaCSTC overexpression strongly decreased the expression of VHSV transcripts, pro-inflammatory cytokines, and pro-apoptotic genes; while increasing the expression of anti-apoptotic genes in fathead minnow (FHM) cells. Furthermore, HaCSTC overexpression protected VHSV-infected FHM cells against VHSV-induced apoptosis and increased cell viability. Our findings imply the profound role of HaCSTC against pathogen infections by modulating fish immune responses.

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.

Generation of viperin-knockout zebrafish by CRISPR/Cas9-mediated genome engineering and the effect of this mutation under VHSV infection

Viperin is an important virus-induced protein in animals that negatively participates in RNA viral replication and transcription. The reactive machinery of viperin suggests that it produces a regulatory molecule ddhCTP, which may affect immune regulation. In this study, we investigated the expression pattern of viperin in larval and adult stages of zebrafish by whole-mount in situ hybridization and reverse transcription-quantitative PCR (RT-qPCR). To elucidate the function of viperin, we generated a zebrafish knockout model using the CRISPR/Cas9 method and evaluated the mutation's effects under viral hemorrhagic septicemia virus (VHSV) infections. In zebrafish larvae, viperin was expressed in the brain region, eye, and pharynx, which was confirmed by cryosectioning. In adult zebrafish, blood cells showed the highest levels of viperin expression. In 5 dpf fish challenged with VHSV, the expression of the viral NP protein was significantly enhanced in viperin^-/- compared to wild-type fish. In vitro VHSV propagation analysis indicated comparatively higher levels of virus propagation in viperin^-/- fish. Mortality analysis confirmed higher mortality rates, and interferon gene expression analysis showed a strong upregulation of interferon (ifn)φ1 and 3 gene in viperin^-/- fish infected with VHSV. This study describes the successful generation of a viperin-knockout model and the role of viperin during VHSV infections.

Threats to UK freshwaters under climate change: Commonly traded aquatic ornamental species and their potential pathogens and parasites

The aquatic ornamental industry, whilst providing socio-economic benefits, is a known introduction pathway for non-native species, which if invasive, can cause direct impacts to native species and ecosystems and also drive disease emergence by extending the geographic range of associated parasites and pathogens and by facilitating host-switching, spillover and spill-back. Although current UK temperatures are typically below those necessary for the survival and establishment of commonly-traded tropical, and some sub-tropical, non-native ornamental species, the higher water temperatures predicted under climate-change scenarios are likely to increase the probability of survival and establishment. Our study aimed primarily to identify which of the commonly-traded non-native ornamental aquatic species (fish and invertebrates), and their pathogens and parasites, are likely to benefit in terms of survival and establishment in UK waters under predicted future climate conditions. Out of 233 ornamental species identified as traded in the UK, 24 were screened, via literature search, for potential parasites and pathogens (PPPs) due to their increased risk of survival and establishment under climate change. We found a total of 155 PPPs, the majority of which were platyhelminths, viruses and bacteria. While many of the identified PPPs were already known to occur in UK waters, PPPs currently absent from UK waters and with zoonotic potential were also identified. Results are discussed in the context of understanding potential impact, in addition to provision of evidence to inform risk assessment and mitigation approaches.

Immunostimulant properties of full-length and truncated Marinobacter algicola flagellins, and their effects against viral hemorrhagic septicemia virus (VHSV) in trout

Adjuvants that would help optimize fish vaccines against bacterial and viral pathogens are highly demanded by the aquaculture sector. Flagellin has been proposed as an immunostimulant and an adjuvant for more than a decade. However, the adjuvant ability of flagellins with hypervariable region deleted is still unclear in fish. In this study, we evaluated the immune-stimulating capacity of two recombinant flagellins, the wild-type flagellin F from Marinobacter algicola and a version with the hypervariable region deleted (FredV2), to induce the transcription of a wide range of immune genes using two rainbow trout cell lines: a monocyte/macrophage-cell line (RTS-11) and an epithelial cell line from intestine (RTgutGC). Additionally, we studied the capacity of both flagellins to limit the replication of viral hemorrhagic septicemia virus (VHSV) on the RTgutGC cell line. Our results demonstrated that both recombinant flagellins can significantly increase the transcription of IL-1β1, IL-6, and IL-8 in both cell lines. However, other cytokines such as IFNγ1, and TNFα or antimicrobial peptides such as hepcidin were induced by both flagellins in RTgutGC but not in RTS-11 cells. Furthermore, both flagellins were capable of reducing the replication of VHSV in RTgutGC cells. Although the immunostimulatory and the antiviral capacities exerted by F were slightly more potent than those obtained with FredV2, the effects were retained after losing the hypervariable region. Our results provide new information on the immunostimulating and antiviral capacities of flagellins that point to their potential as suitable adjuvants for the future optimization of vaccines in aquaculture.

Development of a recombinase polymerase amplification assay for viral haemorrhagic septicemia virus

Viral diseases of fish cause significant economic losses in the aquaculture industry. Viral haemorrhagic septicemia virus (VHSV) is one of the most important viral diseases that affects more than 80 fish species. Detection of the disease, especially in the field, is critical to managing disease prevention and control programmes. Recombinase polymerase amplification (RPA) is an isothermal method with a very short amplification period and a single incubation temperature ranging from 37 to 42°C, which is a good alternative to the polymerase chain reaction (PCR). This study aimed to develop an RPA assay as sensitive as a real-time RT-PCR to detect VHSV. For this purpose, primers and probes are designed for the same targeted region of gG of VHSV. The ssRNA standards were prepared to find the detection limits of the assay. Detection limits were found ten-fold differences between real-time RT-PCR and real-time RT-RPA. While the detection limit of the RT-PCR was found as 95.5 viral RNA molecules/reaction in 95% probit value, the detection limit of RT-RPA was found as 943.75 viral RNA molecules/reaction in 95% probit value using ssRNA standards. These results show that RPA is a suitable test for VHSV Ie detection.

Genome-wide association study of VHSV-resistance trait in Paralichthys olivaceus

In flounder aquaculture, selective breeding plays a vital role in the development of disease-resistant traits and animals with high growth rates. Moreover, superior animals are required to achieve high profits. Unlike growth-related traits, disease-resistant experiments need to be conducted in a controlled environment, as the improper measurement of traits often leads to low genetic correlation and incorrect estimation of breeding values. In this study, viral hemorrhagic septicemia virus (VHSV) resistance was studied using a genome-wide association study (GWAS), and the genetic parameters were estimated. Genotyping was performed using a high-quality 70 K single nucleotide polymorphism (SNP) Affymetrix® Axiom® myDesign™ Genotyping Array of olive flounder. A heritability of ∼0.18 for resistance to VHSV was estimated using genomic information of the fish. According to the GWAS, significant SNPs were detected in chromosomes 21, 24, and contig AGQT02032065.1. Three SNPs showed significance at the genome-wide level (p < 1 × 10^-6), while others showed significance above the suggestive cutoff (p < 1 × 10^-4). The 3% phenotypic variation was explained by the highest significant SNP, named AX-419319631. Of the important genes for disease resistance, SNPs were associated with plcg1, epha4, clstn2, pik3cb, hes6, meis3, prx6, cep164, siae, and kirrel3b. Most of the genes associated with these SNPs have been previously reported with respect to viral entry, propagation, and immune mechanisms. Therefore, our study provides helpful information regarding VHSV resistance in olive flounder, which can be used for breeding applications.

Modifications of the nucleoprotein of viral haemorrhagic septicaemia virus showed gain of virulence in intraperitoneally infected rainbow trout

Viral haemorrhagic septicaemia virus (VHSV) is the cause of an important listed disease in European rainbow trout (Oncorhynchus mykiss) aquaculture and can be present in a wide range of fish species, including marine fish, which can act as viral reservoir. Recent studies revealed putative genetic virulence markers of VHSV to rainbow trout highlighting the roles of the nucleoprotein, phosphoprotein and non-virion protein. Using reverse genetics, we produced recombinant viruses by introducing parts of or the entire nucleoprotein from a high-virulent isolate VHSV into a low-virulent backbone. Furthermore, we also made recombinant viruses by introducing residue modifications in the nucleoprotein that seem to play a role in virulence. Rainbow trout challenged with these recombinant viruses (rVHSVs) by intraperitoneal injection (IP) developed clinical signs and showed lower survival when compared to the parental rVHSV whereas fish challenged by immersion did not show clinical signs except for the high-virulent control. The mutations did not influence the viral growth in cell culture. The recombinant viruses and parental recombinant were unable to replicate and show cytopathic effect in EPC cells whereas the high-virulent control was well adapted in all the fish cell lines tested. We showed evidence that corroborates with the hypothesis that the nucleoprotein has virulence motifs associated with VHSV virulence in rainbow trout.

Genomic and immunogenic changes of Piscine novirhabdovirus (Viral Hemorrhagic Septicemia Virus) over its evolutionary history in the Laurentian Great Lakes

A unique and highly virulent subgenogroup (-IVb) of Piscine novirhabdovirus, also known as Viral Hemorrhagic Septicemia Virus (VHSV), suddenly appeared in the Laurentian Great Lakes, causing large mortality outbreaks in 2005 and 2006, and affecting >32 freshwater fish species. Periods of apparent dormancy have punctuated smaller and more geographically-restricted outbreaks in 2007, 2008, and 2017. In this study, we conduct the largest whole genome sequencing analysis of VHSV-IVb to date, evaluating its evolutionary changes from 48 isolates in relation to immunogenicity in cell culture. Our investigation compares genomic and genetic variation, selection, and rates of sequence changes in VHSV-IVb, in relation to other VHSV genogroups (VHSV-I, VHSV-II, VHSV-III, and VHSV-IVa) and with other Novirhabdoviruses. Results show that the VHSV-IVb isolates we sequenced contain 253 SNPs (2.3% of the total 11,158 nucleotides) across their entire genomes, with 85 (33.6%) of them being non-synonymous. The most substitutions occurred in the non-coding region (NCDS; 4.3%), followed by the Nv- (3.8%), and M- (2.8%) genes. Proportionally more M-gene substitutions encoded amino acid changes (52.9%), followed by the Nv- (50.0%), G- (48.6%), N- (35.7%) and L- (23.1%) genes. Among VHSV genogroups and subgenogroups, VHSV-IVa from the northeastern Pacific Ocean has shown the fastest substitution rate (2.01x10^-3), followed by VHSV-IVb (6.64x10^-5) and by the VHSV-I, -II and-III genogroups from Europe (4.09x10^-5). A 2016 gizzard shad (Dorosoma cepedianum) from Lake Erie possessed the most divergent VHSV-IVb sequence. The in vitro immunogenicity analysis of that sample displayed reduced virulence (as did the other samples from 2016), in comparison to the original VHSV-IVb isolate (which had been traced back to 2003, as an origin date). The 2016 isolates that we tested induced milder impacts on fish host cell innate antiviral responses, suggesting altered phenotypic effects. In conclusion, our overall findings indicate that VHSV-IVb has undergone continued sequence change and a trend to lower virulence over its evolutionary history (2003 through present-day), which may facilitate its long-term persistence in fish host populations.