Immunity to fish rhabdoviruses

Isolation and identification of hybrid snakehead rhabdovirus (HSHRV) and its immune response in the hybrid Snakehead((male Channa argus × female Channa maculata)

Hybrid snakehead is an emerging aquaculture species obtained from the mating of Channa argus (♂) and Channa maculate (♀). It has the advantages of fast growth and strong disease resistance. Viral diseases caused by hybrid snakehead rhabdovirus (HSHRV) critically affect the hybrid snakehead industry. We isolated and identified a highly virulent strain of HSHRV from a naturally occurring hybrid snakehead, namely HSHRV-GZ22. It showed clinical signs of sinking, superficial blackening, spinning, acute internal congestion, and hemorrhage, along with blackening and enlargement of the liver, spleen, and kidneys. Histopathological analysis showed multiple tissue lesions in the liver, spleen, and kidneys, characterized mainly by massive inflammatory cell infiltration, interstitial hemorrhage, and partial cell necrosis. Pathogen analysis identified the virus as HSHRV. Immunofluorescence analysis (IFA) with HSHRV-specific antibodies confirmed the virus and electron microscopic observation showed that the bullet-like virus particles had a size of approximately 150 nm. The replication efficiency of HSHRV was 107.33 TCID50/mL. The glycoproteins of the isolates were cloned and sequenced, and a phylogenetic tree was constructed. The HSHRV-GZ22 isolates clustered into a single branch with the reported HSHRV-C1207, and it had a high degree of homology with Siniperca chuatsi rhabdovirus (SCRV). HSHRV-GZ22 was regressively infected, clinical and pathological symptoms were similar to naturally occurring fish, with a fatality rate of about 85 %. qRT-PCR was performed to determine the viral replication in different tissues of hybrid snakehead, and the viral copies were found to be highly expressed in the liver, spleen, kidney, and intestine. HSHRV-GZ22 activated the antiviral immune pathway in hybrid snakeheads during infection, and the expressions of IgM, IRF7, ISG12, and IFNγ were significantly altered. In this study, we isolated a strong virulent strain of HSHRV and characterized it; in addition, it provided insights into the pathogenesis of HSHRV and immune response in hybrid snakehead, while also advancing the methods for diagnosing and preventing diseases caused by HSHRV.

The current status and development forecasts of vaccines for aquaculture and its effects on bacterial and viral diseases

The aquaculture sector predicts protein-rich meals by 2040 and has experienced significant economic shifts since 2000. However, challenges emanating from disease control measures, brood stock improvement, feed advancements, hatchery technology, and water quality management due to environmental fluctuations have been taken as major causative agents for hindering the sector's growth. For the past years, aquatic disease prevention and control have principally depended on the use of various antibiotics, ecologically integrated control, other immunoprophylaxis mechanisms, and chemical drugs, but the long-term use of chemicals such as antibiotics not only escalates antibiotic-resistant bacteria and genes but also harms the fish and the environments, resulting in drug residues in aquatic products, severely obstructing the growth of the aquaculture sector. The field of science has opened new avenues in basic and applied research for creating and producing innovative and effective vaccines and the enhancement of current vaccines to protect against numerous infectious diseases. Recent advances in vaccines and vaccinology could lead to novel vaccine candidates that can tackle fish diseases, including parasitic organism agents, for which the current vaccinations are inadequate. In this review, we study and evaluate the growing aquaculture production by focusing on the current knowledge, recent progress, and prospects related to vaccinations and immunizations in the aquaculture industry and their effects on treating bacterial and viral diseases. The subject matter covers a variety of vaccines, such as conventional inactivated and attenuated vaccines as well as advanced vaccines, and examines their importance in real-world aquaculture scenarios. To encourage enhanced importation of vaccines for aquaculture sustainability and profitability and also help in dealing with challenges emanating from diseases, national and international scientific and policy initiatives need to be informed about the fundamental understanding of vaccines.

Antiviral effect of miR-206 in rainbow trout (Oncorhynchus mykiss) against infectious hematopoietic necrosis virus infection

Infectious hematopoietic necrosis (IHN), caused by IHN virus, is a highly contagious and lethal disease that seriously hampers the development of rainbow trout (Oncorhynchus mykiss) aquaculture. However, the immune response mechanism of rainbow trout underlying IHNV infection remains largely unknown. MicroRNAs act as post-transcriptional regulators of gene expression and perform a crucial role in fish immune response. Herein, the regulatory mechanism and function of miR-206 in rainbow trout resistance to IHNV were investigated by overexpression and silencing. The expression analysis showed that miR-206 and its potential target receptor-interacting serine/threonine-protein kinase 2 (RIP2) exhibited significant time-dependent changes in headkidney, spleen and rainbow trout primary liver cells infected with IHNV and their expression displayed a negative correlation. In vitro, the interaction between miR-206 and RIP2 was verified by luciferase reporter assay, and miR-206 silencing in rainbow trout primary liver cells markedly increased RIP2 and interferon (IFN) expression but significantly decreased IHNV copies, and opposite results were obtained after miR-206 overexpression or RIP2 knockdown. In vivo, overexpressed miR-206 with agomiR resulted in a decrease in the expression of RIP2 and IFN in liver, headkidney and spleen. This study revealed the key role of miR-206 in anti-IHNV, which provided potential for anti-viral drug screening in rainbow trout.

Expression characteristics of non-virion protein of Hirame novirhabdovirus and its transfection induced response in hirame natural embryo cells

The non-virion (NV) protein is the signature of genus Novirhabdovirus, which has been of considerable concern due to its potential role in viral pathogenicity. However, its expression characteristics and induced immune response remain limited. In the present work, it was demonstrated that Hirame novirhabdovirus (HIRRV) NV protein was only detected in the viral infected hirame natural embryo (HINAE) cells, but absent in the purified virions. Results showed that the transcription of NV gene could be stably detected in HIRRV-infected HINAE cells at 12 h post infection (hpi) and then reached the peak at 72 hpi. A similar expression trend of NV gene was also found in HIRRV-infected flounders. Subcellular localization analysis further exhibited that HIRRV-NV protein was predominantly localized in the cytoplasm. To elucidate the biological function of HIRRV-NV protein, NV eukaryotic plasmid was transfected into HINAE cells for RNA-seq. Compared to empty plasmid group, some key genes in RLR signaling pathway were significantly downregulated in NV-overexpressed HINAE cells, indicating that RLR signaling pathway was inhibited by HIRRV-NV protein. The interferon-associated genes were also significantly suppressed upon transfection of NV gene. This research would improve our understanding of expression characteristics and biological function of NV protein during HIRRV infection process.

Microplastics exacerbate virus-mediated mortality in fish

Microplastics are a persistent and increasing environmental hazard. They have been reported to interact with a variety of biotic and abiotic environmental stressors, but the ramifications of such interactions are largely unknown. We investigated virus-induced mortalities in a commercially important salmonid following exposure to microplastics, plastic microfibers, and natural (non-plastic) microparticles. Microplastics or microparticles alone were not lethal. Mortality increased significantly when fish were co-exposed to virus and microplastics, particularly microfibers, compared to virus alone. This presents the unique finding that microplastics (not natural microparticulate matter) may have a significant impact on population health when presented with another stressor. Further, we found that mortality correlated with host viral load, mild gill inflammation, immune responses, and transmission potential. We hypothesize that microplastics can compromise host tissues, allowing pathogens to bypass defenses. Further research regarding this mechanism and the interplay between microplastics and infectious disease are paramount, considering microplastics increasing environmental burden.

Integrated analysis of immune parameters, miRNA-mRNA interaction, and immune genes expression in the liver of rainbow trout following infectious hematopoietic necrosis virus infection

Rainbow trout (Oncorhynchus mykiss) is an important economical cold-water fish worldwide. However, infection with infectious hematopoietic necrosis virus (IHNV) has severely restricted the development of aquaculture and caused huge economic losses. Currently, little is known about the immune defense mechanisms of rainbow trout against IHNV. In this study, we detected the changes of immune parameters over different post-infection periods (6-, 12-, 24-, 48-, 72-, 96-, 120-, and 144 hours post-infection (hpi)), mRNA and miRNA expression profiles under 48 hpi (T48L) compared to control (C48L), and key immune-related genes expression patterns in rainbow trout liver following IHNV challenge through biochemical methods, RNA-seq, and qRT-PCR, and the function of miR-330-y was verified by overexpression and silencing in vitro and in vivo. The results revealed that alkaline phosphatase (AKP), alanine aminotransferase (ALT), catalase (CAT), and total superoxide dismutase (T-SOD) activities, and lysozyme (LZM) content showed significant peaks at 48 hpi, whereas malondialdehyde (MDA) content and aspartate aminotransferase (AST) activity decreased continuously during infection, and acid phosphatase (ACP) activity varied slightly. From RNA-seq, a total of 6844 genes and 86 miRNAs were differentially expressed, and numerous immune-related differentially expressed genes (DEGs) involved in RIG-I-like receptor signaling pathway, Toll-like receptor signaling pathway, NOD-like receptor signaling pathway, cytokine-cytokine receptor interaction, and antigen processing and presentation were significantly upregulated in T48Lm group, including IFIH1, DHX58, MAVS, TRAF3, IRF3, IRF7, MX1, TLR3, TLR8, MYD88, NOD1, NOD2, IL-8, CXCR1, CD209, CD83, and TAP1. Integrated analysis identified seven miRNAs (miR-425-x, miR-185-x, miR-338-x, miR-330-y, miR-361-x, miR-505-y, and miR-191-x) that target at least three key immune-related DEGs. Expression analysis showed that IFIH1, DHX58, IRF3, IRF7, MX1, TLR3, TLR8, and MYD88 showed a marked increase after 24 hpi during infection. Further research confirmed TAP1 as one of the targets of miR-330-y, overexpression of miR-330-y with mimics or agomir significantly reduced the expression levels of TAP1, IRF3, and IFN, and the opposite effects were obtained by inhibitor. These results facilitate in-depth understanding of the immune mechanisms in rainbow trout against IHNV.

Immunological Effects of Recombinant Lactobacillus casei Expressing IHNV G Protein and Rainbow Trout (Oncorhynchus mykiss) Chemokine CK6 as an Oral Vaccine

IHNV is a virus that infects salmonids and causes serious economic damage to the salmonid farming industry. There is no specific treatment for the disease caused by this pathogen and the main preventive measure is vaccination, but this is only possible for small groups of individuals. Therefore, it is important to investigate new oral vaccines to prevent IHNV. In this study, the CK6 chemokine protein of rainbow trout and the truncated G protein of IHNV were used to construct a secretory expression recombinant L.casei vaccine for rainbow trout. The results showed that the levels of IgM and IgT antibodies in rainbow trout reached the highest level on the 15th day after the secondary immunization, and the antibodies exhibited high inhibitory activity against viral infection. Furthermore, the expression of relevant cytokines in different tissues was detected and found to be significantly higher in the oral vaccine group than in the control group. It was also found that pPG-612-CK6-G/L.casei 393 could stimulate splenic lymphocyte proliferation and improve mucosal immunity with significant differences between the immunized and control groups. When infected with IHNV, the protection rate of pPG-612-CK6-G/L.casei 393 was 66.67% higher than that of the control group. We found that pPG-612-CK6-G/L.casei 393 expressed and secreted the rainbow trout chemokine CK6 protein and IHNV truncated G protein, retaining the original immunogenicity of rainbow trout while enhancing their survival rate. This indicates that recombinant L.casei provides a theoretical basis and rationale for the development of an oral vaccine against IHNV and has important practical implications for the protection of rainbow trout from IHNV infection.

Molecular characterization, antiviral activity, and UV-B damage responses of Caspase-9 from Amphiprion clarkii

Apoptosis plays a vital role in maintaining cellular homeostasis in multicellular organisms. Caspase-9 (casp-9) is one of the major initiator caspases that induces apoptosis by activating downstream intrinsic apoptosis pathway genes. Here, we isolated the cDNA sequence (1992 bp) of caspase-9 from Amphiprion clarkii (Accasp-9) that consists of a 1305 bp coding region and encodes a 434 aa protein. In silico analysis showed that Accasp-9 has a theoretical isoelectric point of 5.81 and a molecular weight of 48.45 kDa. Multiple sequence alignment revealed that the CARD domain is located at the N-terminus, whereas the large P-20 and small P-10 domains are located at the C-terminus. Moreover, a highly conserved pentapeptide active site (²⁹⁶QACGG³⁰¹), as well as histidine and cysteine active sites, are also retained at the C-terminus. In phylogenetic analysis, Accasp-9 formed a clade with casp-9 from different species, distinct from other caspases. Accasp-9 was highly expressed in the gill and intestine compared with other tissues analyzed in healthy A. clarkii. Accasp-9 expression was significantly elevated in the blood after stimulation with Vibrio harveyi and polyinosinic:polycytidylic acid (poly I:C; 12-48 h), but not with lipopolysaccharide. The nucleoprotein expression of the viral hemorrhagic septicemia virus was significantly reduced in Accasp-9 overexpressed fathead minnow (FHM) cells compared with that in the control. In addition, other in vitro assays revealed that cell apoptosis was significantly elevated in poly I:C and UV-B-treated Accasp-9 transfected FHM cells. However, H^248P or C^298S mutated Accasp-9 significantly reduced apoptosis in UV-B irradiated cells. Collectively, our results show that Accasp-9 might play a defensive role against invading pathogens and UV-B radiation and H²⁴⁸ and C²⁹⁸ active residues are significantly involved in apoptosis in teleosts.

Isolations of the Spring Viremia of Carp Virus in the Upper Mississippi River (USA), Including a New Host, the Quillback

In July of 2018 and 2019, wild fish health surveys were conducted along the Wisconsin and Minnesota portions of the upper Mississippi River. Spring viremia of carp virus (SVCV) was isolated from Common Carp Cyprinus carpio as well as a newly identified host species, the Quillback Carpiodes cyprinus. Sanger sequencing of the gene encoding for the G protein revealed a high similarity of the Quillback isolate to various SVCV isolates identified from Common Carp that were collected during earlier wild fish health surveys and mortality events in the USA. Despite annual monitoring, this virus has been infrequently identified. The speculative role of native fish and invertebrates in allowing the virus to persist for long periods without detection is discussed.

Multi-tissue RNAseq reveals genetic and temporal differences in acute response to viral (IHNV) infection among three selected lines of rainbow trout with varying resistance

Utilizing RNA-seq, this study compared the transcriptomic responses of three improved strains (VSel, PSel, and CSel) of rainbow trout fry during acute stages of challenge with infectious hematopoietic necrosis virus (IHNV). The VSel strain has been selected for resistance against the specific strain of IHNV used in our challenge, PSel has undergone selection for utilization of plant-protein based feeds and previously has shown elevated non-specific disease resistance despite no disease related selection pressures, and the final strain, CSel, is a commercial strain that has been domesticated for several years but has not been selected for specific viral disease resistance. Following a 21-day IHNV challenge, Kaplan-Meier survival estimator curves and cumulative percent mortality (CPM) showed significant differences in IHNV resistance across strains: VSel - 19.3 ± 5.0%, PSel - 67. ± 3.03%, CSel - 94.6 ± 4.1% CPM. To evaluate acute responses to IHNV infection, whole blood, as well as samples from the kidney, liver, and intestine, were collected at 0, 4, 12, 24, and 48 h post infection (hpi). Serum lysozyme activity, a marker of non-specific innate immunity, showed strain and temporal effects during the acute infection phase with PSel showing the highest activity at 0 and 48 hpi. Differential gene expression responses were detected, with varying degrees, in all tissues, both between strains, as well as across acute timepoints within strains. The VSel strain showed upregulation for a particular subset of viral recognition genes during early infection timepoints and rather limited upregulation of immune genes later, while maintaining and reactivating metabolic pathways. The CSel strain showed a downregulation of metabolic related genes and a limited upregulation of immune genes, while the PSel strain showed similar downregulation of metabolic genes during acute infection, yet when compared to the CSel strain, showed a more robust innate immune response. Evaluation of upregulated immune response genes, as well as interferon-related genes showed the PSel strain to have the greatest number of uniquely upregulated immune genes in both the kidney and intestine, with CSel and PSel showing a similar number of such genes upregulated in liver. A moderate number of immune response genes were shared between PSel and CSel in all tissues, though both PSel and VSel showed a high number of uniquely overexpressed immune response genes in the kidney, and PSel showed the highest number of uniquely upregulated interferon related genes in the intestine. Overall, the VSel response was unique from the CSel with very little overlap in activated immune responses. Findings from this study highlight the disparity in IHNV resistance among genetic strains of rainbow trout, while identifying molecular mechanisms underlying differences in disease phenotypes. Furthermore, our results on trout strains with distinct selection backgrounds yields comparative insights into the adaptive gains brought about by selection programs for pathogen-specific disease resistance, as well as the non-specific immune enhancement associated with selection for utilization of plant-based diets.

Pathogenic Mechanism of a Highly Virulent Infectious Hematopoietic Necrosis Virus in Head Kidney of Rainbow Trout (Oncorhynchus mykiss) Analyzed by RNA-Seq Transcriptome Profiling

Infectious hematopoietic necrosis virus (IHNV) is a pathogen that causes high rates of mortality in salmonid fishes. Therefore, an RNA-seq-based transcriptome analysis was performed in the head kidney of rainbow trout infected with a highly virulent IHNV strain to understand the pathogenesis of and defense strategies for IHNV infection in rainbow trout. The results showed that the numbers of DEGs were 618, 2626, and 774 (control vs. IHNV) on days 1, 3, and 5, respectively. Furthermore, the enrichment analysis of gene ontology (GO) annotations to classify DEGs showed that GO terms considerably associated with DEGs were gluconeogenesis, inflammatory response, and cell adhesion in the Biological Process (BP) category, apical plasma membrane, extracellular matrix (ECM) in the Cellular Component category, and transporter activity, integrin binding, and protein homodimerization activity in the Molecular Function category, on days 1, 3, and 5, respectively. Notably, GO terms in the BP category, including the negative regulation of type I interferon production and positive regulation of interleukin-1β secretion, were commonly identified at all time points. In the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis, complement and coagulation cascades were commonly identified at all time points. Importantly, the widely recognized GO terms and KEGG pathways extensively linked to DEGs were related to energy metabolism on day 1, the immune response on day 3, and cell proliferation on day 5. Furthermore, protein–protein interaction networks and centrality analysis showed that the metabolism and signaling transduction pathways were majorly upregulated. Conclusively, the virulent IHNV infection drives pathogenesis by activating the metabolic energy pathway for energy use for viral replication, facilitating necrosis through autophagy, and causing a shutoff response of the host immune system through the downregulation of type I IFN at the initial stage of infection.

A novel low-molecular-weight chitosan/gamma-polyglutamic acid polyplexes for nucleic acid delivery into zebrafish larvae

Many challenges, such as virus infection, extreme weather and long cultivation periods, during the development of fish larvae have been observed, especially in aquaculture. Gene delivery is a useful method to express functional genes to defend against these challengers. However, the methods for fish larvae are insufficient. In our earlier report, low-molecular-weight chitosan (LMWCS) showed a strong positive charge and may be useful for polyplex formulation. Herein, we present a simple self-assembly of LMWCS polyplexes (LMWCSrNPs) for gene delivery into zebrafish larvae. Different weight ratios of LMWCS/gamma-polyglutamic acid (γ-PGA)/plasmid DNA were analyzed by gel mobility assay. Delivery efficiency determined by green fluorescent protein (GFP) expression in zebrafish liver (ZFL) cells showed that delivery efficiency at a weight ratio of 20:8:1 was higher than others. Zeta potential and transmission electron microscopy (TEM) analysis showed that the round shape of the particle size varied. In our earlier reports, IRF9S2C could induce interferon-stimulated gene (ISG) expression to induce innate immunity in zebrafish and pufferfish. Further delivery of pcDNA3-IRF9S2C-HA plasmid DNA into ZFL cells and zebrafish larvae by LMWCSrNP successfully induced ISG expression. Collectively, LMWCSrNP could be a novel gene delivery system for zebrafish larvae and might be used to improve applications in aquaculture.

Chitosan Treatment of E-11 Cells Modulates Transcription of Nonspecific Immune Genes and Reduces Nodavirus Capsid Protein Gene Expression

This study explores whether crustacean products inhibit viral infections in aquaculture. Chitosan (CHT) was extracted from waste products of Parapenaeus longirostris. Biochemical composition, viscosity measurement, molecular weight, structure and cytotoxicity tests were used to characterize the extracted chitosan. Cultures of E-11 cells derived from snakehead Ophicephalus striatus were inoculated with 106.74 TCID50 of an isolate of betanodavirus genotype RGNNV (redspotted grouper nervous necrosis virus) after being treated with solutions of 0.3% CHT for 1 h at room temperature. The antiviral effect of CHT was assessed by comparing the ability of RGNVV to replicate and produce cytopathic effects on CHT-treated cell cultures. The change in RNA expression levels of the nodavirus capsid protein gene and three mediator genes in infected cells with or without CHT treatment was evaluated by qPCR. Changes in gene expression compared to control groups were monitored at 6, 24, 48 and 71 h post treatment in all target gene transcripts. The CCR3 expression in CHT treated cells showed a significant increase (p < 0.05) until day 3. On the other hand, the expression of TNF-α decreased significantly (p < 0.05) in CHT treated cells throughout the experimental period. Likewise, the expression of the IL-10 gene showed a significant downregulation in CHT treated cells at all time points (p ≤ 0.05). As further evidence of an antiviral effect, CHT treatment of cells produced a reduction in virus load as measured by a reduced expression of the viral capsid gene and the increase in RQ values from 406 ± 1.9 at hour 1 to 695 ± 3.27 at 72 h post inoculation. Statistical analysis showed that the expression of the viral capsid gene was significantly lower in cells treated with chitosan (p ≤ 0.05). These results improve our knowledge about the antiviral activity of this bioactive molecule and highlight its potential use in fish feed industry.

HSC70 Inhibits Spring Viremia of Carp Virus Replication by Inducing MARCH8-Mediated Lysosomal Degradation of G Protein

As a fierce pathogen, spring viremia of carp virus (SVCV) can cause high mortality in the common carp, and its glycoprotein (G protein) is a component of the viral structure on the surface of virion, which is crucial in viral life cycle. This report adopted tandem affinity purification (TAP), mass spectrometry analysis (LC-MS/MS), immunoprecipitation, and confocal microscopy assays to identify Heat shock cognate protein 70 (HSC70) as an interaction partner of SVCV G protein. It was found that HSC70 overexpression dramatically inhibited SVCV replication, whereas its loss of functions elicited opposing effects on SVCV replication. Mechanistic studies indicate that HSC70 induces lysosomal degradation of ubiquitinated-SVCV G protein. This study further demonstrates that Membrane-associated RING-CH 8 (MARCH8), an E3 ubiquitin ligase, is critical for SVCV G protein ubiquitylation and leads to its lysosomal degradation. Furthermore, the MARCH8 mediated ubiquitylation of SVCV G protein required the participation of HSC70 through forming a multicomponent complex. Taken together, these results demonstrate that HSC70 serves as a scaffold for MARCH8 and SVCV G, which leads to the ubiquitylation and degradation of SVCV G protein and thus inhibits viral replication. These findings have established a novel host defense mechanism against SVCV.

Viperin_sv1 promotes RIG-I expression and suppresses SVCV replication through its radical SAM domain

SVCV infection is known to activate the host's innate immune responses, including the production of interferon (IFN) and interferon-stimulated genes (ISGs). Viperin_sv1 is a novel splice variant of viperin, which is induced during SVCV infection and proves to positively regulate the IFN activation and production. However, the underlying mechanism remains unsolved. In this study, the P protein of SVCV was identified to be the key to induce the mRNA modification and production of viperin_sv1 during the virus infection. Besides, Viperin_sv1 was able to trigger the RLR signaling cascades to activate type-1 interferon response. Additional analysis revealed that viperin_sv1 promoted the stability and function of RIG-I, which result in the production of IFN and ISGs. Moreover, the central SAM domain of viperin_sv1 was demonstrated to be essential for regulating RIG-I protein expression and inducing IFN production. Furthermore, this study also showed that SVCV replication could be inhibited by the viperin_sv1 SAM domain. In conclusion, our study demonstrates that viperin_sv1 reduces the replication of SVCV by promoting the RIG-I protein expression. Our findings identified the antiviral function played by the SAM domain of viperin_sv1 and suggested an antiviral mechanism that is conserved among different species.

α-Lipoic Acid Exerts Its Antiviral Effect against Viral Hemorrhagic Septicemia Virus (VHSV) by Promoting Upregulation of Antiviral Genes and Suppressing VHSV-Induced Oxidative Stress

Viral hemorrhagic septicemia virus (VHSV), belonging to the genus Novirhabdovirus, Rhabdoviridae family, is a causative agent of high mortality in fish and has caused significant losses to the aquaculture industry. Currently, no effective vaccines, Food and Drug Administration-approved inhibitors, or other therapeutic intervention options are available against VHSV. α-Lipoic Acid (LA), a potent antioxidant, has been proposed to have antiviral effects against different viruses. In this study, LA (CC₅₀ = 472.6 μmol/L) was repurposed to exhibit antiviral activity against VHSV. In fathead minnow cells, LA significantly increased the cell viability post-VHSV infection (EC₅₀ = 42.7 μmol/L), and exerted a dose-dependent inhibitory effect on VHSV induced-plaque, cytopathic effects, and VHSV glycoprotein expression. The time-of-addition assay suggested that the antiviral activity of LA occurred at viral replication stage. Survival assay revealed that LA could significantly upregulated the survival rate of VHSV-infected largemouth bass in both co-injection (38.095% vs. 1.887%, P < 0.01) and post-injection manner (38.813% vs. 8.696%, P < 0.01) compared with the control group. Additional comparative transcriptome and qRT-PCR analysis revealed LA treatment upregulated the expression of several antiviral genes, such as IRF7, Viperin, and ISG15. Moreover, LA treatment reduced VHSV-induced reactive oxygen species production in addition to Nrf2 and SOD1 expression. Taken together, these data demonstrated that LA suppressed VHSV replication by inducing antiviral genes expression and reducing VHSV-induced oxidative stress. These results suggest a new direction in the development of potential antiviral candidate drugs against VHSV infection.

Novirhabdoviruses versus fish innate immunity: A review

Novirhabdoviruses belong to the Rhabdoviridae family of RNA viruses. All of the four members are pathogenic for bony fish. Particularly, Infectious hematopoietic necrosis virus (IHNV) and Viral hemorrhagic septicemia virus (VHSV) often cause mass animal deaths and huge economic losses, representing major obstacles to fish farming industry worldwide. The interactions between fish and novirhabdoviruses are becoming better understood. In this review, we will present our current knowledge of fish innate immunity, particularly type I interferon (IFN-I) response, against novirhabdoviral infection, and the evasion strategies exploited by novirhabdoviruses. Members of Toll-like receptors (TLRs) and RIG-I-like receptors (RLRs) appear to be involved in novirhabdovirus surveillance. NF-κB activation and IFN-I induction are primarily triggered for antiviral defense. Autophagy can also be induced by viral glycoprotein (G). Although sensitive to IFN-I, novirhabdoviruses have nucleoprotein (N), matrix protein (M), and non-virion protein (NV) to interfere with host signal transduction and gene expression steps toward antiviral state establishment. Moreover, novirhabdoviruses may exploit some microRNAs for immunosuppression.

Temperature Variation and Host Immunity Regulate Viral Persistence in a Salmonid Host

Environmental variation has important effects on host-pathogen interactions, affecting large-scale ecological processes such as the severity and frequency of epidemics. However, less is known about how the environment interacts with host immunity to modulate virus fitness within hosts. Here, we studied the interaction between host immune responses and water temperature on the long-term persistence of a model vertebrate virus, infectious hematopoietic necrosis virus (IHNV) in steelhead trout (Oncorhynchus mykiss). We first used cell culture methods to factor out strong host immune responses, allowing us to test the effect of temperature on viral replication. We found that 15 ∘C water temperature accelerated IHNV replication compared to the colder 10 and 8 ∘C temperatures. We then conducted in vivo experiments to quantify the effect of 6, 10, and 15 ∘C water temperatures on IHNV persistence over 8 months. Fish held at 15 and 10 ∘C were found to have higher prevalence of neutralizing antibodies compared to fish held at 6 ∘C. We found that IHNV persisted for a shorter time at warmer temperatures and resulted in an overall lower fish mortality compared to colder temperatures. These results support the hypothesis that temperature and host immune responses interact to modulate virus persistence within hosts. When immune responses were minimized (i.e., in vitro) virus replication was higher at warmer temperatures. However, with a full potential for host immune responses (i.e., in vivo experiments) longer virus persistence and higher long-term virulence was favored in colder temperatures. We also found that the viral RNA that persisted at later time points (179 and 270 days post-exposure) was mostly localized in the kidney and spleen tissues. These tissues are composed of hematopoietic cells that are favored targets of the virus. By partitioning the effect of temperature on host and pathogen responses, our results help to better understand environmental drivers of host-pathogen interactions within hosts, providing insights into potential host-pathogen responses to climate change.

Virulence and Infectivity of UC, MD, and L Strains of Infectious Hematopoietic Necrosis Virus (IHNV) in Four Populations of Columbia River Basin Chinook Salmon

Infectious Hematopoietic Necrosis Virus (IHNV) infects juvenile salmonid fish in conservation hatcheries and aquaculture facilities, and in some cases, causes lethal disease. This study assesses intra-specific variation in the IHNV susceptibility of Chinook salmon (Oncorhynchus tshawytscha) in the Columbia River Basin (CRB), in the northwestern United States. The virulence and infectivity of IHNV strains from three divergent virus genogroups are measured in four Chinook salmon populations, including spring-run and fall-run fish from the lower or upper regions of the CRB. Following controlled laboratory exposures, our results show that the positive control L strain had significantly higher virulence, and the UC and MD strains that predominate in the CRB had equivalently low virulence, consistent with field observations. By several experimental measures, there was little variation in host susceptibility to infection or disease. However, a small number of exceptions suggested that the lower CRB spring-run Chinook salmon population may be less susceptible than other populations tested. The UC and MD viruses did not differ in infectivity, indicating that the observed asymmetric field prevalence in which IHNV detected in CRB Chinook salmon is 83% UC and 17% MD is not due to the UC virus being more infectious. Overall, we report little intra-species variation in CRB Chinook salmon susceptibility to UC or MD IHNV infection or disease, and suggest that other factors may instead influence the ecology of IHNV in the CRB.

Replication of heterologous glycoprotein-expressing chimeric recombinant snakehead rhabdoviruses (rSHRVs) and viral hemorrhagic septicemia viruses (rVHSVs) at different temperatures

Water temperature is an important environmental factor for the outbreaks of fish rhabdovirus diseases. In the present study, to know the role of piscine rhabdoviral glycoproteins in the determination of replication temperature, several chimeric snakehead rhabdoviruses (SHRVs) and viral hemorrhagic septicemia viruses (VHSVs) expressing heterologous glycoproteins (rSHRV-Gvhsv, SHRV expressing VHSV G protein; rSHRV-Gsvcv, SHRV expressing spring viremia of carp virus G protein; rVHSV-Gshrv, VHSV expressing SHRV G protein; rVHSV-Gsvcv, VHSV expressing SVCV G protein) were generated using reverse genetics, and their replication characteristics at different temperatures were investigated. Furthermore, based on SHRV minigenome containing a reporter gene, the role of VHSV N, P, and L proteins in the determination of VHSV's low-temperature replication was investigated. In Epithelioma papulosum cyprini (EPC) cells, rSHRV-Gvhsv could replicate only at low temperatures (15 and 20 °C) but not at 25 and 28 °C, while rSHRV-Gsvcv could replicate both low and high temperatures, indicating that VHSV G protein is a critical factor that determines the limit of replication-possible temperatures in VHSV. The range of replication-possible temperature of chimeric VHSVs (rVHSV-Gshrv and rVHSV-Gsvcv) was not different from that of rVHSV-wild (replicated only at 15 and 20 °C) in spite of having the G protein of high temperature-replicating viruses, suggesting that not only G protein but also other viral protein(s) would be involved in the determination of replication-possible temperature limit in VHSV. Cells harboring SHRV minigenome that expressing eGFP as a reporter protein were co-transfected with heterologous combinations of helper plasmids of SHRV and VHSV, through which we could exclude VHSV N and P proteins for the low-temperature replication of VHSV, because cells harboring SHRV minigenome showed fluorescence at high temperatures when VHSV N or P protein encoding plasmids were supplied. However, no fluorescence was observed in cells co-transfected with plasmids encoding SHRV N, SHRV P and VHSV L protein at all tested temperatures, suggesting that the combination of SHRV N, P and VHSV L proteins could not form a functional ribonucleoprotein (RNP) complex. Although we could not directly demonstrate the involvement of VHSV L protein in the temperature limit of VHSV replication, it is highly probable that not only VHSV G protein but also VHSV L protein may participate in the determination of VHSV replication temperature.

Transcriptome profiling in head kidney of rainbow trout (Oncorhynchus mykiss) after infection with the low-virulent Nagano genotype of infectious hematopoietic necrosis virus

Infectious hematopoietic necrosis virus (IHNV) causes clinical diseases and mortality in a wide variety of salmonid species. Here, we studied transcriptional responses in rainbow trout infected by the IHNV-Nagano strain isolated in Korea. RNA-seq-based transcriptome analysis of head kidney tissues cataloged differentially expressed genes. Enrichment analysis of gene ontology annotations was performed, and a total of fifteen biological process terms were commonly identified at all time points. In the Kyoto Encyclopedia of Genes and Genomes pathway analysis, pathogen recognition receptor (PRR) signaling pathways such as the retinoic-acid-inducible gene-I-like receptor signaling pathway and the Toll-like receptor signaling pathway were identified at all time points. The nucleotide-binding oligomerization-domain-like receptor signaling pathway and cytosolic DNA-sensing pathway were identified at days 1 and 3. Protein-protein interaction network and centrality analyses revealed that the immune system, signaling molecules, and interaction pathways were upregulated at days 1 and 3, with the highest centrality of tumor necrosis factor. Cancer, cellular community, and endocrine system pathways were downregulated, with the highest centrality of fibronectin 1 at day 5. STAT1 was upregulated from days 1 to 5 with a high centrality. The reproducibility and repeatability of the transcriptome analysis were validated by RT-qPCR. IHNV-Nagano infection dynamically changed the transcriptome profiles in the head kidney of rainbow trout and induced a defense mechanism by regulating the immune and inflammatory pathways through PRR signaling at an early stage. Downregulated pathways involved in extracellular matrix formation and focal adhesion at day 5 indicated the possible failure of wound healing, which is important in the pathogenesis of IHNV infection.

Naturally occurring substitution in one amino acid in VHSV phosphoprotein enhances viral virulence in flounder

Viral hemorrhagic septicemia virus (VHSV) is a rhabdovirus that causes high mortality in cultured flounder. Naturally occurring VHSV strains vary greatly in virulence. Until now, little has been known about genetic alterations that affect the virulence of VHSV in flounder. We recently reported the full-genome sequences of 18 VHSV strains. In this study, we determined the virulence of these 18 VHSV strains in flounder and then the assessed relationships between differences in the amino acid sequences of the 18 VHSV strains and their virulence to flounder. We identified one amino acid substitution in the phosphoprotein (P) (Pro55-to-Leu substitution in the P protein; P^P55L) that is specific to highly virulent strains. This P^P55L substitution was maintained stably after 30 cell passages. To investigate the effects of the P^P55L substitution on VHSV virulence in flounder, we generated a recombinant VHSV carrying P^P55L (rVHSV-P) from rVHSV carrying P55 in the P protein (rVHSV-wild). The rVHSV-P produced high level of viral RNA in cells and showed increased growth in cultured cells and virulence in flounder compared to the rVHSV-wild. In addition, rVHSV-P significantly inhibited the induction of the IFN1 gene in both cells and fish at 6 h post-infection. An RNA-seq analysis confirmed that rVHSV-P infection blocked the induction of several IFN-related genes in virus-infected cells at 6 h post-infection compared to rVHSV-wild. Ectopic expression of P^P55L protein resulted in a decrease in IFN induction and an increase in viral RNA synthesis in rVHSV-wild-infected cells. Taken together, our results are the first to identify that the P55L substitution in the P protein enhances VHSV virulence in flounder. The data from this study add to the knowledge of VHSV virulence in flounder and could benefit VHSV surveillance efforts and the generation of a VHSV vaccine.

Viral hemorrhagic septicemia virus (VHSV) is a rhabdovirus that causes huge economic losses to the fish culture industry throughout the world. Virulence among naturally occurring VHSV strains varies widely. However, little is known about the viral factors that determine VHSV virulence. Here, we identify a naturally-occurring, single-amino-acid substitution in the VHSV P protein that enhances VHSV virulence in flounder. This amino acid substitution in the P protein was detected only in highly virulent VHSV strains, and it enhances viral RNA synthesis and inhibits the interferon response of host cells early after virus infection. Recombinant VHSV containing this amino acid substitution caused increased mortality in flounder compared with the wild type. This is the first study to identify a naturally occurring amino acid substitution in VHSV that determines its virulence in flounder. We expect that our result can be applied to other fish species, and this finding will provide new opportunities to generate an effective VHSV vaccine.

Grass Carp Reovirus (GCRV) Giving Its All to Suppress IFN Production by Countering MAVS Signaling Transduction

Viruses typically target host RIG-I-like receptors (RLRs), a group of key factors involved in interferon (IFN) production, to enhance viral infection. To date, though immune evasion methods to contradict IFN production have been characterized for a series of terrestrial viruses, the strategies employed by fish viruses remain unclear. Here, we report that all grass carp reovirus (GCRV) proteins encoded by segments S1 to S11 suppress mitochondrial antiviral signaling protein (MAVS)-mediated IFN expression. First, the GCRV viral proteins blunted the MAVS-induced expression of IFN, and impair MAVS antiviral capacity significantly. Interestingly, subsequent co-immunoprecipitation experiments demonstrated that all GCRV viral proteins interacted with several RLR cascades, especially with TANK-binding kinase 1 (TBK1) which was the downstream factor of MAVS. To further illustrate the mechanisms of these interactions between GCRV viral proteins and host RLRs, two of the viral proteins, NS79 (S4) and VP3 (S3), were selected as representative proteins for two distinguished mechanisms. The obtained data demonstrated that NS79 was phosphorylated by gcTBK1, leading to the reduction of host substrate gcIRF3/7 phosphorylation. On the other hand, VP3 degraded gcMAVS and the degradation was significantly reversed by 3-MA. The biological effects of both NS79 and VP3 were consistently found to be related to the suppression of IFN expression and the promotion of viral evasion. Our findings shed light on the special evasion mechanism utilized by fish virus through IFN regulation, which might differ between fish and mammals.

Natural killer cell responses to emerging viruses of zoonotic origin

Emerging viral diseases pose a major threat to public health worldwide. Nearly all emerging viruses, including Ebola, Dengue, Nipah, West Nile, Zika, and coronaviruses (including SARS-Cov2, the causative agent of the current COVID-19 pandemic), have zoonotic origins, indicating that animal-to-human transmission constitutes a primary mode of acquisition of novel infectious diseases. Why these viruses can cause profound pathologies in humans, while natural reservoir hosts often show little evidence of disease is not completely understood. Differences in the host immune response, especially within the innate compartment, have been suggested to be involved in this divergence. Natural killer (NK) cells are innate lymphocytes that play a critical role in the early antiviral response, secreting effector cytokines and clearing infected cells. In this review, we will discuss the mechanisms through which NK cells interact with viruses, their contribution towards maintaining equilibrium between the virus and its natural host, and their role in disease progression in humans and other non-natural hosts.

Transcriptome analysis based on RNA-seq of common innate immune responses of flounder cells to IHNV, VHSV, and HIRRV

Viral hemorrhagic septicemia virus (VHSV) and hirame rhabdovirus (HIRRV) belong to the genus Novirhabdovirus and are the causative agents of a serious disease in cultured flounder. However, infectious hematopoietic necrosis virus (IHNV), a prototype of the genus Novirhabdovirus, does not cause disease in flounder. To determine whether IHNV growth is restricted in flounder cells, we compared the growth of IHNV with that of VHSV and HIRRV in hirame natural embryo (HINAE) cells infected with novirhabdoviruses at 1 multiplicity of infection. Unexpectedly, we found that IHNV grew as well as VHSV and HIRRV. For successful growth in host cells, viruses modulate innate immune responses exerted by virus-infected cells. Our results suggest that IHNV, like VHSV and HIRRV, has evolved the ability to overcome the innate immune response of flounder cells. To determine the innate immune response genes of virus-infected HINAE cells which are commonly modulated by the three novirhabdoviruses, we infected HINAE cells with novirhabdoviruses at multiplicity of infection (MOI) 1 and performed an RNA sequencing-based transcriptome analysis at 24 h post-infection. We discovered ~12,500 unigenes altered by novirhabdovirus infection and found that many of these were involved in multiple cellular pathways. After novirhabdovirus infection, 170 genes involved in the innate immune response were differentially expressed compared to uninfected cells. Among them, 9 genes changed expression by more than 2-fold and were commonly modulated by all three novirhabdoviruses. Interferon regulatory factor 8 (IRF8), C-X-C motif chemokine receptor 1 (CXCR1), Toll/interleukin-1 receptor domain-containing adapter protein (TIRAP), cholesterol 25-hydroxylase (CH25H), C-X-C motif chemokine ligand 11, duplicate 5 (CXCL11.5), and Toll-like receptor 2 (TLR2) were up-regulated, whereas C-C motif chemokine receptor 6a (CCR6a), interleukin-12a (IL12a), and Toll-like receptor 1 (TLR1) were down-regulated. These genes have been reported to be involved in antiviral responses and, thus, their modulation may be critical for the growth of novirhabdovirus in flounder cells. This is the first report to identify innate immune response genes in flounder that are commonly modulated by IHNV, VHSV, and HIRRV. These data will provide new insights into how novirhabdoviruses survive the innate immune response of flounder cells.

Different immune responses of flounder (Paralichthys olivaceus) towards the full-length and N-terminal or C-terminal portion of hirame novirhabdovirus glycoprotein

Glycoprotein is an important immunogenic protein of Hirame novirhabdovirus (HIRRV). In this study, the full-length and N-/C-terminal portions of glycoprotein were recombinantly expressed (rG, rGn and rGc protein), and the induced immune responses were investigated in flounder (Paralichthys olivaceus) model. The results showed that compared to PBS control, rG, rGn and rGc proteins and inactivated HIRRV suspension (iVS) could all stimulate significant increases of flounder CD4-1⁺, CD4-2⁺ T lymphocytes and surface IgM positive (sIgM⁺) B lymphocytes in peripheral blood, spleen and head kidney (p < 0.05). However, no significant differences of the percentages of CD4-1⁺ or CD4-2⁺ T lymphocytes were observed among three protein vaccination groups (p > 0.05). iVS could induce the highest mean levels of CD4⁺ T lymphocytes in peripheral blood and spleen. For sIgM⁺ B lymphocytes, the average peak percentages in rG and rGc groups were higher than rGn group. Moreover, significant increases of specific serum IgM against HIRRV or rG protein were observed in iVS, rG, rGn and rGc groups, but rG group exhibited the highest mean level. Furthermore, rG protein induced the highest titer of neutralizing antibodies against HIRRV, followed by iVS. Meanwhile, the challenge test showed that the relative percent survival (RPS) of rG, rGn, rGc and iVS groups were 75.0%, 35.7%, 53.6% and 60.7%, respectively. These results revealed that the full-length G protein would be a more effective subunit vaccine candidate against HIRRV infection.

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.

RNA-Seq analysis reveals that spring viraemia of carp virus induces a broad spectrum of PIM kinases in zebrafish kidney that promote viral entry

PIM kinases are a family of serine/threonine protein kinases that potentiate the progression of the cell cycle and inhibit apoptosis. Because of this, they are considered to be proto-oncogenes, and they represent an interesting target for the development of anticancer drugs. In mammals, three PIM kinases exist (PIM-1, PIM-2 and PIM-3), and different inhibitors have been developed to block their activity. In addition to their involvement in cancer, some publications have reported that the PIM kinases have pro-viral activity, and different mechanisms where PIM kinases favour viral infections have been proposed. Zebrafish possess more than 300 Pim kinase members in their genome, and by using RNA-Seq analysis, we found a high number of Pim kinase genes that were significantly induced after infection with spring viraemia of carp virus (SVCV). Moreover, analysis of the miRNAs modulated by this infection revealed that some of them could be involved in the post-transcriptional regulation of Pim kinase abundance. To elucidate the potential role of the 16 overexpressed Pim kinases in the infectivity of SVCV, we used three different pan-PIM kinase inhibitors (SGI-1776, INCB053914 and AZD1208), and different experiments were conducted both in vitro and in vivo. We observed that the PIM kinase inhibitors had a protective effect against SVCV, indicating that, similar to what is observed in mammals, PIM kinases are beneficial for the virus in zebrafish. Moreover, zebrafish Pim kinases seem to facilitate viral entry into the host cells because when ZF4 cells were pre-incubated with the virus and then were treated with the inhibitors, the protective effect of the inhibitors was abrogated. Although more investigation is necessary, these results show that pan-PIM kinase inhibitors could serve as a useful treatment for preventing the spread of viral diseases.

Proteomic Profiling of Zebrafish Challenged by Spring Viremia of Carp Virus Provides Insight into Skin Antiviral Response

Spring viremia of carp virus (SVCV) causes the skin hemorrhagic disease in cyprinid species, but its molecular mechanism of skin immune response remains unclear at the protein level. In the present study, the differential proteomics of the zebrafish (Danio rerio) skin in response to SVCV infection were examined by isobaric tags for relative and absolute quantitation and quantitative polymerase chain reaction (qPCR) assays. A total of 3999 proteins were identified, of which 320 and 181 proteins were differentially expressed at 24 and 96 h postinfection, respectively. The expression levels of 16 selected immune-related differentially expressed proteins (DEPs) were confirmed by qPCR analysis. Furthermore, Gene Ontology and Kyoto Encyclopedia of Genes and Genomes enrichment analyses revealed that DEPs were significantly associated with complement, inflammation, and antiviral response. The protein-protein interaction network of cytoskeleton-associated proteins, ATPase-related proteins, and parvalbumins from DEPs was shown to be involved in skin immune response. This is first report on the skin proteome profiling of zebrafish against SVCV infection, which will contribute to understand the molecular mechanism of local mucosal immunity in fish.

Effect of suboptimal temperature on the regulation of endogenous antigen presentation in a rainbow trout hypodermal fibroblast cell line

Rainbow trout (Oncorhynchus mykiss) face low environmental temperatures over winter months and during extreme low temperature events. Suboptimal temperatures are known to negatively impact the teleost immune system, although there is mixed evidence in rainbow trout as to the effect on the endogenous antigen processing and presentation pathway (EAPP). The EAPP is an important pathway for antiviral defense that involves the presentation of endogenous peptides on the cell surface for recognition by cytotoxic T cells. Using a rainbow trout hypodermal fibroblast (RTHDF) cell line as an in vitro model, we determined that constitutive EAPP transcript levels are not impaired at low temperature, but induction of up-regulation of these transcripts is delayed at the suboptimal temperature following exposure to poly(I:C) or viral haemorrhagic septicaemia virus IVb, which was still able to enter and replicate in the cell line at 4 °C, albeit with reduced efficiency. The delay in the induction of EAPP mRNA level up-regulation following poly(I:C) stimulation coincided with a delay in ifn1 transcript levels and secretion, which is important since interferon-stimulated response elements were identified in the promoter regions of the EAPP-specific members of the pathway, implying that IFN1 is involved in the regulation of these genes. Our results suggest that the ability of rainbow trout to mount an effective immune response to viral pathogens may be lessened at suboptimal temperatures.

Comparative modulation of lncRNAs in wild-type and rag1-heterozygous mutant zebrafish exposed to immune challenge with spring viraemia of carp virus (SVCV)

Although the modulation of immune-related genes after viral infection has been widely described in vertebrates, the potential implications of non-coding RNAs (ncRNAs), especially long non-coding RNAs (lncRNAs), in immunity are still a nascent research field. The model species zebrafish could serve as a useful organism for studying the functionality of lncRNAs due to the numerous advantages of this teleost, including the existence of numerous mutant lines. In this work, we conducted a whole-transcriptome analysis of wild-type (WT) and heterozygous rag1 mutant (rag1^+/-) zebrafish after infection with the pathogen spring viraemia of carp virus (SVCV). WT and rag1^+/- zebrafish were infected with SVCV for 24 h. Kidney samples were sampled from infected and uninfected fish for transcriptome sequencing. From a total of 198,540 contigs, 12,165 putative lncRNAs were identified in zebrafish. Most of the putative lncRNAs were shared by the two zebrafish lines. However, by comparing the lncRNA profiles induced after SVCV infection in WT and rag1^+/- fish, most of the lncRNAs that were significantly induced after viral challenge were exclusive to each line, reflecting a highly differential response to the virus. Analysis of the neighboring genes of lncRNAs that were exclusively modulated in WT revealed high representation of metabolism-related terms, whereas those from rag1^+/- fish showed enrichment in terms related to the adaptive immune response, among others. On the other hand, genes involved in numerous antiviral processes surrounded commonly modulated lncRNAs, as expected. These results clearly indicate that after SVCV infection in zebrafish, the expression of an array of lncRNAs with functions in different aspects of immunity is induced.

Differences of Viral Hemorrhagic Septicemia Virus Loads among Organs of Dead and Surviving Olive Flounder Infected by Intramuscular Injection and Immersion Challenge

Viral hemorrhagic septicemia virus (VHSV) is an important viral pathogen in the culture of Olive Flounder Paralichthys olivaceus. Based on cumulative mortality, the virulence of VHSV was found to be highly different depending on challenge routes and exposure doses (using tissue culture infectious dose with 50% endpoint [TCID50]). Olive Flounder were injected with VHSV at 10^2.5 , 10^4.5 , 10^6.5 , and 10^8.5 TCID50/100 μL/fish. A second group of fish was immersed at 10^3.5 , 10^5.5 , and 10^7.5 TCID50/mL at 10°C for 1 h in this study. The cumulative mortality was observed at 15 d postinfection. Immersion challenge at 10^3.5 TCID50/mL caused no mortality, while intramuscular injection challenge resulted in high levels of mortality with all VHSV exposure doses. Overall, Olive Flounder was susceptible to VHSV, with cumulative mortality of 90% or 100% in fish intramuscularly injected with high or low doses of VHSV. The cumulative mortality was 40% and 70% at 10^5.5 and 10^7.5 TCID50/mL, respectively, in the immersion challenge group. The VHSV titration and copy numbers were estimated by TCID50 and quantitative reverse transcription PCR methods. From dead Olive Flounder, VHSV titration was consistently detected in all tested organs, ranging from 10⁵ to 10⁹ TCID50/mL. The VHSV titration was under the detection limit from surviving Olive Flounder, but the VHSV N gene was detected.

Synergistic osmoregulatory dysfunction during salmon lice (Lepeophtheirus salmonis) and infectious hematopoietic necrosis virus co-infection in sockeye salmon (Oncorhynchus nerka) smolts

While co-infections are common in both wild and cultured fish, knowledge of the interactive effects of multiple pathogens on host physiology, gene expression and immune response is limited. To evaluate the impact of co-infection on host survival, physiology and gene expression, sockeye salmon Oncorhynchus nerka smolts were infected with the salmon louse Lepeophtheirus salmonis (V-/SL+), infectious hematopoietic necrosis virus (IHNV; V+/SL-), both (V+/SL+), or neither (V-/SL-). Survival in the V+/SL+ group was significantly lower than the V-/SL- and V-/SL+ groups (p = 0.024). Co-infected salmon had elevated osmoregulatory indicators and lowered haematocrit values as compared to the uninfected control. Expression of 12 genes associated with the host immune response was analysed in anterior kidney and skin. The only evidence of L. salmonis-induced modulation of the host antiviral response was down-regulation of mhc I although the possibility of modulation cannot be ruled out for mx-1 and rsad2. Co-infection did not influence the expression of genes associated with the host response to L. salmonis. Therefore, we conclude that the reduced survival in co-infected sockeye salmon resulted from the osmoregulatory consequences of the sea lice infections which were amplified due to infection with IHNV.

Elucidation of mechanism for host response to VHSV infection at varying temperatures in vitro and in vivo through proteomic analysis

Seasonal temperature has a major influence on the infectivity of pathogens and the host immune system. Viral hemorrhagic septicemia virus (VHSV) is one such pathogen that only causes the mortality of fish at low temperatures. This study aims to discover the host defense mechanism and pathway for resistance to VHSV at higher temperatures. We first observed the VHSV infection patterns at low and higher temperatures in fathead minnow (FHM) cells (20 °C and 28 °C) and zebrafish (15 °C and 25 °C). In comparison to the 20 °C infection, FHM cells infected at 28 °C showed decreased apoptosis, increased cell viability, and reduced VHSV N gene expression. In zebrafish, infection at 25 °C caused no mortality and significantly reduced the N gene copy number in comparison to infection at 15 °C. To explore the antiviral infection mechanisms induced by high temperature in vitro and in vivo, the changes in the proteomic profile were measured through UPLC-MS^E analysis. ACADL, PTPN6, TLR1, F7, A2M, and GLI2 were selected as high temperature-specific biomarkers in the FHM cell proteome; and MYH9, HPX, ANTXR1, APOA1, HBZ, and MYH7 were selected in zebrafish. Increased immune response, anticoagulation effects, and the formation of lymphocytes from hematopoietic stem cells were analyzed as functions that were commonly induced by high temperature in vitro and in vivo. Among these biomarkers, GLI2 was predicted as an upstream regulator. When treated with GANT58, a GLI-specific inhibitor, cell viability was further reduced due to GLI2 inhibition during VHSV infection at varying temperatures in FHM cells, and the mortality in zebrafish was induced earlier at the low temperature. Overall, this study discovered a new mechanism for VHSV infection in vitro and in vivo that is regulated by GLI2 protein.

Addicted to sugar: roles of glycans in the order Mononegavirales

Glycosylation is a biologically important protein modification process by which a carbohydrate chain is enzymatically added to a protein at a specific amino acid residue. This process plays roles in many cellular functions, including intracellular trafficking, cell-cell signaling, protein folding and receptor binding. While glycosylation is a common host cell process, it is utilized by many pathogens as well. Protein glycosylation is widely employed by viruses for both host invasion and evasion of host immune responses. Thus better understanding of viral glycosylation functions has potential applications for improved antiviral therapeutic and vaccine development. Here, we summarize our current knowledge on the broad biological functions of glycans for the Mononegavirales, an order of enveloped negative-sense single-stranded RNA viruses of high medical importance that includes Ebola, rabies, measles and Nipah viruses. We discuss glycobiological findings by genera in alphabetical order within each of eight Mononegavirales families, namely, the bornaviruses, filoviruses, mymonaviruses, nyamiviruses, paramyxoviruses, pneumoviruses, rhabdoviruses and sunviruses.

Increase of viral hemorrhagic septicemia virus growth by knockout of IRF9 gene in Epithelioma papulosum cyprini cells

Viral hemorrhagic septicemia virus (VHSV) has been a notorious pathogen in freshwater and marine fish. Due to the lack of effective treatment measures against VHSV disease, the development of prophylactic vaccines has been required, and methods that can produce high-titered viruses would be advantageous in producing cost-effective vaccines. Type I interferon (IFN) responses are the key elements of vertebrates' antiviral activities, and IFN-stimulated gene factor 3 (ISGF3) complex formed through type I IFNs up-regulates the expression of IFN-stimulated genes (ISGs). IFN regulatory factor 9 (IRF9) is a key component of ISGF3, so the inhibition of IRF9 would compromise host's type I IFN responses, which would weaken host antiviral activity. In this study, to increase the replication of VHSV, we generated IRF9 knockout Epithelioma papulosum cyprini (EPC) cells using a CRISPR/Cas9 vector that contains an EPC cell's U6 promoter-driven guide RNA cassette (targeting IRF9 gene) and a Cas9 expressing cassette. In the clones of IRF9 knockout EPC cells, there were no increase in ISG15 gene by poly I:C, and in Mx1 gene by both poly I:C and VHSV. Interestingly, although the increased folds were conspicuously lower than control EPC cells, the expression of ISG 15 gene in all the IRF9 knockout clones was significantly increased by VHSV infection. Control EPC cells pre-treated with poly I:C did not show any CPE when infected with VHSV, however, IRF9 knockout EPC cells showed CPE by VHSV infection in spite of being pretreated with poly I:C. The replication of VHSV in IRF9 knockout EPC cells was significantly faster and higher than that in control EPC cells indicating that the IRF9 knockout-mediated decrease of type I IFN responses allowed VHSV to replicate efficiently. Considering an economical aspect for the production of fish vaccines, the present IRF9 knockout EPC cells can be used to get higher-titered VHSV.

14-3-3β/α-A interacts with glycoprotein of spring viremia of carp virus and positively affects viral entry

Spring viremia of carp virus (SVCV) is a fierce pathogen causing high mortality in the common carp. The glycoprotein (G protein) of SVCV is a pivotal component of the viral structure, located in the surface of the virion, and plays a key role in viral endocytosis. In this study, tandem affinity purification (TAP) followed by mass spectrometry analysis (LC-MS/MS) was carried out to search for novel host molecules that interact with SVCV G protein and a 14-3-3β/α-A protein was identified. The level of 14-3-3β/α-A mRNA expression was dramatically down regulated by SVCV infection. Furthermore, over expression of 14-3-3β/α-A results in a significantly increased SVCV attachment and entry in FHM cells. This study reveals an important role of 14-3-3 protein in regulating the early stage of SVCV infection, which offers a potential target for development of anti-SVCV therapies.

Broad-spectrum antiviral JL122 blocks infection and inhibits transmission of aquatic rhabdoviruses

The aquaculture industry is growing rapidly to meet the needs for global protein consumption. Viral diseases in aquaculture are quite challenging due to lack of treatment options as well as limited injection-delivery vaccines, which are costly. Thus, water-immersion antiviral treatments are highly desirable. This study focused on broad-spectrum, light-activated antivirals that target the viral membrane (envelope) of viruses to prevent viral-cell membrane fusion, ultimately blocking viral entry into cells. Of the tested small-molecules, JL122, a new broad-spectrum antiviral previously unexplored against aquatic viruses, blocked infection of three aquatic rhabdoviruses (IHNV, VHSV and SVCV) in cell culture and in two live fish challenge models. Importantly, JL122 inhibited transmission of IHNV from infected to uninfected rainbow trout. Further, the effective antiviral concentrations were not toxic to cells or susceptible fish. These results show promise for JL122 to become an immersion treatment option for outbreaks of aquatic enveloped viral infections.

Detection and characterization of a rhabdovirus causing mortality in black bullhead catfish, Ameiurus melas

This study fully describes a severe disease outbreak occurred in 2016 in black bullhead catfish farmed in Italy. Affected fish showed nervous clinical signs as well as emaciations and haemorrhagic petechiae on the skin at the fin bases, abdomen and gills. Viral isolation in cell culture allowed the subsequent identification of a rhabdovirus, tentatively named ictalurid rhabdovirus (IcRV), through electron microscopy, immunofluorescence and whole genome sequencing (WGS). The newly isolated virus, together with 14 additional viral strains stored in our repository and detected during similar mortality episodes in the period 1993-2016, was phylogenetically analysed on the basis of the nucleoprotein and the glycoprotein nucleotide and amino acid sequences. The genetic distances among Italian IcRV strains were also estimated. Our results show that all the IcRV strains belong to the genus Sprivivirus and are closely related to the tench rhabdovirus (TenRV). Italian catfish production is constantly decreasing, mainly due to viral infections, which include the newly characterized IcRV. Data presented in this work will assist to investigate the molecular epidemiology and the diffusive dynamics of this virus and to develop adequate surveillance activities.

Comparative susceptibilities and immune reactions of wild and cultured populations of Caspian trout Salmo trutta caspius to VHSV

Caspian trout Salmo trutta caspius is an endangered subspecies of brown trout Salmo trutta which is native to the Caspian Sea. Restocking programmes have been established, but recent introduction of the rhabdovirus viral haemorrhagic septicaemia virus (VHSV) into Iranian rainbow trout farms connected to waterbodies supporting wild Caspian trout may represent an additional threat to the declining stock. The susceptibility of wild and cultured populations of this endemic subspecies was demonstrated by performing controlled VHSV infection experiments (both by bath and injection challenges). Subsequently, VHSV infection in exposed fish was confirmed (CPE and quantitative PCR), virus levels were measured, and regulation of immune genes in exposed fish was investigated with a focus on the genes encoding IL-8, IFNγ, TGFβ, TNFα, SAA, C3-4, CD8α, IgM, MHC I, MHC II, iNOS and IGF-1. The presence of IgM-, CD8α- and MHC II-positive cells in host organs was visualized by immunohistochemistry. Both wild and cultured trout strains proved to be VHSV-susceptible following experimental challenge, but the mortality curves and associated regulation of immune-related genes differed between the 2 trout types. Implications of the results for future management of Caspian trout populations are discussed.

Understanding Viral dsRNA-Mediated Innate Immune Responses at the Cellular Level Using a Rainbow Trout Model

Viruses across genome types produce long dsRNA molecules during replication [viral (v-) dsRNA]. dsRNA is a potent signaling molecule and inducer of type I interferon (IFN), leading to the production of interferon-stimulated genes (ISGs), and a protective antiviral state within the cell. Research on dsRNA-induced immune responses has relied heavily on a commercially available, and biologically irrelevant dsRNA, polyinosinic:polycytidylic acid (poly I:C). Alternatively, dsRNA can be produced by in vitro transcription (ivt-) dsRNA, with a defined sequence and length. We hypothesized that ivt-dsRNA, containing legitimate viral sequence and length, would be a more appropriate proxy for v-dsRNA, compared with poly I:C. This is the first study to investigate the effects of v-dsRNA on the innate antiviral response and to compare v-dsRNA to ivt-dsRNA-induced responses in fish cells, specifically rainbow trout. Previously, class A scavenger receptors (SR-As) were found to be surface receptors for poly I:C in rainbow trout cells. In this study, ivt-dsRNA binding was blocked by poly I:C and v-dsRNA, as well as SR-A competitive ligands, suggesting all three dsRNA molecules are recognized by SR-As. Downstream innate antiviral effects were determined by measuring IFN and ISG transcript levels using qRT-PCR and antiviral assays. Similar to what has been shown previously with ivt-dsRNA, v-dsRNA was able to induce IFN and ISG transcript production between 3 and 24 h, and its effects were length dependent (i.e., longer v-dsRNA produced a stronger response). Interestingly, when v-dsRNA and ivt-dsRNA were length and sequence matched both molecules induced statistically similar IFN and ISG transcript levels, which resulted in similar antiviral states against two aquatic viruses. To pursue sequence effects further, three ivt-dsRNA molecules of the same length but different sequences (including host and viral sequences) were tested for their ability to induce IFN/ISG transcripts and an antiviral state. All three induced responses similarly. This study is the first of its kind to look at the effects v-dsRNA in fish cells as well as to compare ivt-dsRNA to v-dsRNA, and suggests that ivt-dsRNA may be a good surrogate for v-dsRNA in the study of dsRNA-induced responses and potential future antiviral therapies.

DNA vaccines for fish: Review and perspectives on correlates of protection

Recently in 2016, the European Medicines Agency (EMA) recommended granting a marketing authorization in the EU for "Clynav," a DNA vaccine against salmon pancreas disease (salmonid alphavirus-3). Generally, DNA vaccines induce both early and late immune responses in fish that may be protective against disease. Several transcriptomic approaches have been performed to map immunome profiles following DNA vaccination, but the precise immune mechanism(s) that is responsible for protection is not known, although reasonable suggestions have been made. The current review includes an overview on main transcriptomic findings from microarray experiments after DNA vaccination against VHSV, IHNV, HIRRV and IPNV-with considerations of what can be considered as correlates of protection (CoP) or merely a surrogate of protection. Identification and use of correlates of protection (COPs) may be a strategic tool for accelerated and targeted vaccine design, testing and licensure. General rules on what can be considered as CoPs can be extracted from past knowledge on protective immune responses following vaccination that induced protection. Lastly, there will be an overview on non-viral molecular adjuvants that have been exploited to obtain higher vaccine potencies and efficacies.

Effects of different cytokines on immune responses of rainbow trout in a virus DNA vaccination model

Seven rainbow trout cytokine genes (interleukin (IL)-2, IL-8, IL-15, IL-17, IL-1β, intracellular interferon (iIFN) 1a, and IFN-γ2) were evaluated for their adjuvant effects on a DNA vaccine, called pG, containing the glycoprotein gene of infectious hematopoietic necrosis virus (IHNV). Distinct DNA constructs in expression plasmid pcDNA3.1 encoding a cytokine gene were generated. Immunofluorescence assays in rainbow trout gonadal cells demonstrated successful protein expression from all these constructs. Subsequently, fish were immunized with pG alone or together with a cytokine expression plasmid. Results showed that each cytokine plasmids at an appropriate dose showed notable effects on immune gene expression. IL-17 and IFN-γ2 can enhance early specific IgM response. All cytokines, except IL-8, can benefit initial neutralizing antibody (NAb) titers. At 35 days post immunization (dpi), NAb titers of fish immunized with pG and IL-2, iIFN1a, or IFN-γ2 plasmids remained at high levels (1:160). NAb titers of fish immunized with pG alone decreased to 1:40. IL-8 or IL-1β can enhance antigen-specific proliferative T-cell responses at 14 dpi. At 28 dpi, coinjection of pG with IL-2, IL-8, IL-15, or IL-17 plasmids induced considerably stronger lymphocyte proliferation than that with injection of pG alone. All cytokine plasmids delivered with pG plasmid enhanced protection of trout against IHNV-mediated mortality. These results indicate that the type and dose of trout cytokine genes injected into fish affect quality of immune response to DNA vaccination.

Poly (I:C) and imiquimod induced immune responses and their effects on the survival of olive flounder (Paralichthys olivaceus) from viral haemorrhagic septicaemia

The stimulation of immune genes by polyinosinic:polycytidylic acid (poly (I:C)) and imiquimod in olive flounder (Paralichthys olivaceus) and their role in control of viral haemorrhagic septicaemia virus (VHSV) infection were examined. Poly (I:C) (100 μg/fish) treated olive flounder had very low mortality (5%) post VHSV infection, while the imiquimod treated group had 65% and 85% mortality at a dose of 100 μg/fish and 50 μg/fish, respectively. Though the imiquimod treated group had high mortality, it was lower than the untreated group, which had 90% mortality. In vivo experiments were conducted to determine effect of the two ligands on immune modulation in the head kidney of olive flounder. Poly (I:C) activated the immune genes (TLR-3, TLR-7, MDA-5, LGP-2, IRF-3, IRF-7, IL-1β type I IFN and Mx) very early, within 1 d post stimulation, faster and stronger than imiquimod. Though Mx levels were enhanced by imiquimod, the host was still susceptible to VHSV. The poly (I:C) treated group had a high immune response at the time of infection and 1 dpi, though it decreased at later stages. The imiquimod treated group and the unstimulated group had a higher immune response to VHSV compared to the poly (I:C) treated group. The nucleoprotein copies of VHSV were very low in the poly (I:C) treated group but interestingly, were high in both untreated and imiquimod treated fish. Thus, host survival from a viral infection does not only depend on the quantity of immune response but also the time of response. Although imiquimod enhanced immune gene expression in olive flounder, a delayed response could be the reason for high mortality to VHS compared with poly (I:C), which induced the immune system effectively and efficiently to protect the host.

Development and use of an Arctic charr cell line to study antiviral responses at extremely low temperatures

Arctic charr (Salvelinus alpinus) are the northernmost distributed freshwater fish and can grow at water temperatures as low as 0.2 °C. Other teleost species have impaired immune function at temperatures that Arctic charr thrive in, and thus, charr may maintain immune function at these temperatures. In this study, a fibroblastic cell line, named ACBA, derived from the bulbus arteriosus (BA) of Arctic charr was developed for use in immune studies at various temperatures. ACBA has undergone more than forty passages at 18 °C over 3 years, while showing no signs of senescence-associated β-galactosidase activity and producing nitric oxide. Remarkably, ACBA cells survived and maintained some mitotic activity even at 1 °C for over 3 months. At these low temperatures, ACBA also continued to produce MH class I proteins. After challenge with poly I:C, only antiviral Mx proteins were induced while MH proteins remained constant. When exposed to live viruses, ACBA was shown to permit viral infection and replication of IPNV, VHSV IVa and CSV at 14 °C. Yet at the preferred temperature of 4 °C, only VHSV IVa was shown to replicate within ACBA. This study provides evidence that Arctic charr cells can maintain immune function while also resisting infection with intracellular pathogens at low temperatures.

Generation of a recombinant viral hemorrhagic septicemia virus (VHSV) expressing olive flounder (Paralichthys olivaceus) interferon-γ and its effects on type I interferon response and virulence

Rhabdoviruses including viral hemorrhagic septicemia virus (VHSV) are highly susceptible to type I interferon (IFN) responses, and IFN-γ that is belonging to the type II IFN has been known to enhance type I IFN responses in mammals. In this study, we generated a recombinant VHSV that can express olive flounder IFN-γ (rVHSV-A-IFNγ) using reverse genetics technology, and analyzed the effect of rVHSV-A-IFNγ infection on type I IFN response in Epithelioma papulosum cyprini (EPC) cells. Furthermore, the virulence of rVHSV-A-IFNγ was evaluated by infection to olive flounder (Paralichthys olivaceus). Using a recombinant VHSV full genome vector in which the olive flounder IFN-γ ORF was inserted between N and P genes, rVHSV-A-IFNγ was successfully rescued, and the recombinant virus was grown well in EPC cells. On the other hand, the growth of rVHSV-A-IFNγ rescued from EPC cells was severely retarded when infected into hirame natural embryo (HINAE) cells that were originated from olive flounder. These results indicate that the EPC cell's IFN-γ receptor could not bind to olive flounder IFN-γ, but the species-specific binding of IFN-γ in HINAE cells induced antiviral responses. The expression of Mx1 gene in EPC cells infected with rVHSV-A-IFNγ was not greatly different from cells infected with rVHSV-Arfp (a recombinant VHSV harboring red fluorescent protein gene between N and P genes of the genome), however, in HINAE cells, rVHSV-A-IFNγ infection induced distinctively higher Mx1 gene expression compared to other recombinant viruses. These results suggest that olive flounder IFN-γ produced from rVHSV-A-IFNγ effectively enhanced type I IFN response in HINAE cells. In the present study, the lowest mortality of olive flounder fingerlings was recorded in the group of fish challenged with rVHSV-A-IFNγ, suggesting that the recombinant VHSV was attenuated by production of IFN-γ by itself. However, although rVHSV-A-IFNγ induced significantly lower mortality, the mortality still reached to 40%. Therefore, to be safely used in the aquaculture farms as prophylactic vaccines or immunostmulators, further manipulations that can guarantee safety are needed.

Potential Reservoirs and Risk Factors for VHSV IVb in an Enzootic System: Budd Lake, Michigan

Viral hemorrhagic septicemia virus genotype IVb (VHSV IVb) has caused major, sporadic fish die-offs in the Laurentian Great Lakes region of North America since 2005. Presently, factors affecting VHSV IVb persistence in enzootic systems are not well understood. Even with annual surveillance, the virus can go undetected for several years after an outbreak before again re-emerging, which suggests that the virus is maintained in the system either below detectable levels or in untested reservoirs. The aim of this study was to identify potential reservoirs of VHSV IVb in Budd Lake, Michigan; VHSV IVb was first detected in Budd Lake in 2007 but remained undetected until 2011. Additionally, we explored the susceptibility of naive fish introduced into a water body enzootic for VHSV IVb by stocking age-0 Largemouth Bass Micropterus salmoides at varying densities into enclosures in the lake. The virus was not detected among samples of the fishes Notropis spp. and Lepomis spp., cylindrical papershell mussels Anodontoides ferussacianus, leeches (subclass Hirudinea), sediment, or water. However, the virus was successfully isolated from amphipods (family Hyalellidae) and Largemouth Bass held in the enclosures. Our finding of VHSV IVb in Hyalellidae amphipods in combination with other research that has detected the virus in Diporeia spp., a large benthic amphipod important as a food resource to Great Lake fishes, suggests that benthic macroinvertebrates are a reservoir for VHSV IVb in infected systems. If there are environmental reservoirs for VHSV IVb in infected systems, they are likely unevenly distributed. Findings of this study add to our understanding of the seemingly complex ecology of this deadly and economically detrimental virus. Received February 22, 2016; accepted October 16, 2016.

DNA Vaccination Partially Protects Muskellunge against Viral Hemorrhagic Septicemia Virus (VHSV-IVb)

A DNA vaccine containing the glycoprotein (G) gene of the North American viral hemorrhagic septicemia virus (VHSV) genotype IVb was developed to evaluate the immune response of fish following vaccination and evaluate its efficacy in protecting a susceptible species, the Muskellunge Esox masquinongy, against VHSV-IVb challenge. Seven weeks (539 degree-days) following vaccination with 10 μg of either pVHSivb-G or a control plasmid, Muskellunge were challenged by immersion with 10⁵ plaque-forming units (pfu)/mL of VHSV-IVb. Fish vaccinated with pVHSivb-G had a relative percent survival (RPS) of 45%. Vaccinated fish also had significantly lower mean viral titers in tissues (4.2 × 10² pfu/g) and viral prevalence (4%) than fish receiving the plasmid control vaccine (3.3 × 10⁵ pfu/g; 82%). Neutralizing antibodies were detected 28 d (308 degree-days) postchallenge (11 weeks postvaccination) in 100% of Muskellunge vaccinated with pVHSivb-G compared with only 12% of plasmid-control-vaccinated Muskellunge, suggesting robust induction of a secondary, adaptive immune response. In addition, pVHSivb-G-vaccinated Rainbow Trout Oncorhynchus mykiss challenged 7 d (100 degree-days) postvaccination with the heterologous novirhabdovirus, infectious hematopoietic necrosis virus (IHNV), experienced an RPS of 61%, compared to control fish, suggesting induction of an early and transient nonspecific antiviral immune response. This study provides an important starting point for VHSV-IVb vaccine development and useful information about the antiviral immune response elicited by DNA vaccination in a nondomesticated fish species. Received May 1, 2016; accepted September 1, 2016.

Immune gene profiles in Atlantic salmon (salmo salar L.) post-smolts infected with SAV3 by bath-challenge show a delayed response and lower levels of gene transcription compared to injected fish

Salmonid alphavirus (SAV) causes pancreatic disease (PD) in salmonids in Northern Europe which results in large economic losses within the aquaculture industry. In order to better understand the underlying immune mechanisms during a SAV3 infection Atlantic salmon post-smolts were infected by either i.m.-injection or bath immersion and their immune responses compared. Analysis of viral loads showed that by 14 dpi i.m.-injected and bath immersion groups had 95.6% and 100% prevalence respectively and that both groups had developed the severe pathology typical of PD. The immune response was evaluated by using RT-qPCR to measure the transcription of innate immune genes involved in the interferon (IFN) response as well as genes associated with inflammation. Our results showed that IFNa transcription was only weakly upregulated, especially in the bath immersion group. Despite this, high levels of the IFN-stimulated genes (ISGs) such as Mx and viperin were observed. The immune response in the i.m.-injected group as measured by immune gene transcription was generally faster, and more pronounced than the response in the bath immersion group, especially at earlier time-points. The response in the bath immersion group started later as expected and appeared to last longer often exceeding the response in the i.m-injected fish at later time-points. High levels of transcription of many genes indicative of an active innate immune response were present in both groups.