Expression of interferon and interferon--induced genes in Atlantic salmon Salmo salar cell lines SHK-1 and TO following infection with Salmon AlphaVirus SAV

In Situ Detection of Salmonid Alphavirus 3 (SAV3) in Tissues of Atlantic Salmon in a Cohabitation Challenge Model with a Special Focus on the Immune Response to the Virus in the Pseudobranch

Salmonid alphavirus strain 3 is responsible for outbreaks of pancreas disease in salmon and rainbow trout in Norway. Although the extensive amount of research on SAV3 focused mainly on the heart and pancreas (of clinical importance), tropism and pathogenesis studies of the virus in other salmon tissues are limited. Here, we used a combination of RT-qPCR (Q_nsp1 gene) and in situ hybridization (RNAscope®) to demonstrate the tropism of SAV3 in situ in tissues of Atlantic salmon, employing a challenge model (by cohabitation). In addition, as previous results suggested that the pseudobranch may harbor the virus, the change in the expression of different immune genes upon SAV3 infection (RT-qPCR) was focused on the pseudobranch in this study. In situ hybridization detected SAV3 in different tissues of Atlantic salmon during the acute phase of the infection, with the heart ventricle showing the most extensive infection. Furthermore, the detection of the virus in different adipose tissues associated with the internal organs of the salmon suggests a specific affinity of SAV3 to adipocyte components. The inconsistent immune response to SAV3 in the pseudobranch after infection did not mitigate the infection in that tissue and is probably responsible for the persistent low infection at 4 weeks post-challenge. The early detection of SAV3 in the pseudobranch after infection, along with the persistent low infection over the experimental infection course, suggests a pivotal role of the pseudobranch in SAV3 pathogenesis in Atlantic salmon.

CRISPR-Cas- induced IRF3 and MAVS knockouts in a salmonid cell line disrupt PRR signaling and affect viral replication

Background

Interferon (IFN) responses are critical in the resolution of viral infections and are actively targeted by many viruses. They also play a role in inducing protective responses after vaccination and have been successfully tested as vaccine adjuvants. IFN responses are well conserved and function very similar in teleosts and mammals. Like in mammals, IFN responses in piscine cells are initiated by intracellular detection of the viral infection by different pattern recognition receptors. Upon the recognition of viral components, IFN responses are rapidly induced to combat the infection. However, many viruses may still replicate and be able to inhibit or circumvent the IFN response by different means.

Methods

By employing CRISPR Cas9 technology, we have disrupted proteins that are central for IFN signaling in the salmonid cell line CHSE-214. We successfully generated KO clones for the mitochondrial antiviral signaling protein MAVS, the transcription factors IRF3 and IRF7-1, as well as a double KO for IRF7-1/3 using an optimized protocol for delivery of CRISPR-Cas ribonucleoproteins through nucleofection.

Results

We found that MAVS and IRF3 KOs inhibited IFN and IFN-stimulated gene induction after intracellular poly I:C stimulation as determined through gene expression and promoter activation assays. In contrast, the IRF7-1 KO had no clear effect. This shows that MAVS and IRF3 are essential for initiation of intracellular RNA-induced IFN responses in CHSE-214 cells. To elucidate viral interference with IFN induction pathways, the KOs were infected with Salmon alphavirus 3 (SAV3) and infectious pancreatic necrosis virus (IPNV). SAV3 infection in control and IRF7-1 KO cells yielded similar titers and no cytopathic effect, while IRF3 and MAVS KOs presented with severe cytopathic effect and increased titers 6 days after SAV 3 infection. In contrast, IPNV yields were reduced in IRF3 and MAVS KOs, suggesting a dependency on interactions between viral proteins and pattern recognition receptor signaling components during viral replication.

Conclusion

Aside from more insight in this signaling in salmonids, our results indicate a possible method to increase viral titers in salmonid cells.

Establishment of an In Vitro Model to Study Viral Infections of the Fish Intestinal Epithelium

Viral infections are still a major concern for the aquaculture industry. For salmonid fish, even though breeding strategies and vaccine development have reduced disease outbreaks, viral diseases remain among the main challenges having a negative impact on the welfare of fish and causing massive economic losses for the industry. The main entry port for viruses into the fish is through mucosal surfaces including that of the gastrointestinal tract. The contradictory functions of this surface, both creating a barrier towards the external environment and at the same time being responsible for the uptake of nutrients and ion/water regulation make it particularly vulnerable. The connection between dietary components and viral infections in fish has been poorly investigated and until now, a fish intestinal in vitro model to investigate virus-host interactions has been lacking. Here, we established the permissiveness of the rainbow trout intestinal cell line RTgutGC towards the important salmonid viruses-infectious pancreatic necrosis virus (IPNV), salmonid alphavirus (subtype 3, SAV3) and infectious salmon anemia virus (ISAV)-and explored the infection mechanisms of the three different viruses in these cells at different virus to cell ratios. Cytopathic effect (CPE), virus replication in the RTgutGC cells, antiviral cell responses and viral effects on the barrier permeability of polarized cells were investigated. We found that all virus species infected and replicated in RTgutGC cells, although with different replication kinetics and ability to induce CPE and host responses. The onset and progression of CPE was more rapid at high multiplicity of infection (MOI) for IPNV and SAV3 while the opposite was true of ISAV. A positive correlation between the MOI used and the induction of antiviral responses was observed for IPNV while a negative correlation was detected for SAV3. Viral infections compromised barrier integrity at early time points prior to observations of CPE microscopically. Further, the replication of IPNV and ISAV had a more pronounced effect on barrier function than SAV3. The in vitro infection model established herein can thus provide a novel tool to generate knowledge about the infection pathways and mechanisms used to surpass the intestinal epithelium in salmonid fish, and to study how a virus can potentially compromise gut epithelial barrier functions.

CXCL9-11 chemokines and CXCR3 receptor in teleost fish species

The coordinated migration of immune cells from lymphoid organs to in or out of the bloodstream, and towards the site of infection or tissue damage is fundamental for an efficient innate and adaptive immune response. Interestingly, an essential part of this movement is mediated by chemoattractant cytokines called chemokines. Although the nature and function of chemokines and their receptors are well documented in mammals, much research is needed to accomplish a similar level of understanding of the role of chemokines in fish immunity. The first chemokine gene identified in teleosts (rainbow trout, Oncorhynchus mykiss) was CK1 in 1998. Since then, the identification of fish chemokine orthologue genes and characterization of their role has been more complex than expected, primarily because of the whole genome duplication processes occurring in fish, and because chemokines evolve faster than other immune genes. Some of the most studied chemokines are CXCL9, CXCL10, CXCL11, and the CXCR3 receptor, all involved in T cell migration and in the induction of the T helper 1 (Th1) immune response. Data from the zebrafish and rainbow trout CXCL9-11/CXCR3 axis suggest that these chemokines and the receptor arose early in evolution and must be present in most teleost fish. However, the pieces of knowledge also indicate that different numbers of gene copies can be present in different species, with distinct regulatory expression mechanisms and probably, also with different roles, as the differential expression in fish tissues suggest. Here, we revised the current knowledge of the CXCL9-11/CXCR3 axis in teleost fishes, identifying the gaps in knowledge, and raising some hypotheses for the role of CXCL9, CXCL10 CXCL11, and CXCR3 receptor axis in fish, which can encourage further studies in the field.

Atlantic salmon kidney (ASK) cells are an effective model to characterise interferon (IFN) and IFN-induced gene expression following salmonid alphavirus infection

Salmonid alphavirus (SAV), the causative agent of pancreas disease, is a serious pathogen of farmed Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss). Given the economic impact of SAV outbreaks, much effort is focussed upon understanding the fish immune response following infection and the exploitation of this knowledge to reduce disease impact. Herein we examine the utility of the long-term Atlantic salmon kidney (ASK) cell line as a tool to study antiviral responses upon infection with SAV. Following infection with SAV subtype 1 (isolate V4640) we examined the kinetics and magnitude of induction of IFNa, IFN-regulatory factor (IRF) genes IRF1, IRF3, and IRF7b, as well as the antiviral effector Mx by RT-qPCR. SAV-1 non-structural protein (nsp1) transcript levels increased continuously over the experimental period, indicating viral replication, but cytopathic effect (CPE) was not observed. All the immune genes studied showed an increase in transcript levels over the 96-h study period following SAV infection, with strongest induction of Mx. Our data confirm that ASK cells are a suitable model to study the virus-associated immune responses of salmonids and may be a useful tool when assaying the effectiveness of potential prophylactic or antiviral treatments.

Identification of genetic loci associated with higher resistance to pancreas disease (PD) in Atlantic salmon (Salmo salar L.)

BACKGROUND

Pancreas disease (PD) is a contagious disease caused by salmonid alphavirus (SAV) with significant economic and welfare impacts on salmon farming. Previous work has shown that higher resistance against PD has underlying additive genetic components and can potentially be improved through selective breeding. To better understand the genetic basis of PD resistance in Atlantic salmon, we challenged 4506 smolts from 296 families of the SalmoBreed strain. Fish were challenged through intraperitoneal injection with the most virulent form of the virus found in Norway (i.e., SAV3). Mortalities were recorded, and more than 900 fish were further genotyped on a 55 K SNP array.

RESULTS

The estimated heritability for PD resistance was 0.41 ± 0.017. The genetic markers on two chromosomes, ssa03 and ssa07, showed significant associations with higher disease resistance. Collectively, markers on these two QTL regions explained about 60% of the additive genetic variance. We also sequenced and compared the cardiac transcriptomics of moribund fish and animals that survived the challenge with a focus on candidate genes within the chromosomal segments harbouring QTL. Approximately 200 genes, within the QTL regions, were found to be differentially expressed. Of particular interest, we identified various components of immunoglobulin-heavy-chain locus B (IGH-B) on ssa03 and immunoglobulin-light-chain on ssa07 with markedly higher levels of transcription in the resistant animals. These genes are closely linked to the most strongly QTL associated SNPs, making them likely candidates for further investigation.

CONCLUSIONS

The findings presented here provide supporting evidence that breeding is an efficient tool for increasing PD resistance in Atlantic salmon populations. The estimated heritability is one of the largest reported for any disease resistance in this species, where the majority of the genetic variation is explained by two major QTL. The transcriptomic analysis has revealed the activation of essential components of the innate and the adaptive immune responses following infection with SAV3. Furthermore, the complementation of the genomic with the transcriptomic data has highlighted the possible critical role of the immunoglobulin loci in combating PD virus.

Advances in salmonid fish immunology: A review of methods and techniques for lymphoid tissue and peripheral blood leucocyte isolation and application

Evaluating studies over the past almost 40 years, this review outlines the current knowledge and research gaps in the use of isolated leucocytes in salmonid immunology understanding. This contribution focuses on the techniques used to isolate salmonid immune cells and popular immunological assays. The paper also analyses the use of leucocytes to demonstrate immunomodulation following dietary manipulation, exposure to physical and chemical stressors, effects of pathogens and parasites, vaccine design and application strategies assessment. We also present findings on development of fish immune cell lines and their potential uses in aquaculture immunology. The review recovered 114 studies, where discontinuous density gradient centrifugation (DDGC) with Percoll density gradient was the most popular leucocyte isolation method. Fish head kidney (HK) and peripheral blood (PB) were the main sources of leucocytes, from rainbow trout (Oncorhynchus mykiss) and Atlantic salmon (Salmo salar). Phagocytosis and respiratory burst were the most popular immunological assays. Studies used isolated leucocytes to demonstrate that dietary manipulations enhance fish immunity, while chemical and physical stressors suppress immunity. In addition, parasites, and microbial pathogens depress fish innate immunity and induce pro-inflammatory cytokine gene transcripts production, while vaccines enhance immunity. This review found 10 developed salmonid cell lines, mainly from S. salar and O. mykiss HK tissue, which require fish euthanisation to isolate. In the face of high costs involved with density gradient reagents, the application of hypotonic lysis in conjunction with mico-volume blood methods can potentially reduce research costs, time, and using nonlethal and ethically flexible approaches. Since the targeted literature review for this study retrieved no metabolomics study of leucocytes, indicates that this approach, together with traditional technics and novel flow cytometry could help open new opportunities for in vitro studies in aquaculture immunology and vaccinology.

The role of type I interferons in innate and adaptive immunity against viruses in Atlantic salmon

Type I IFNs (IFN-I) are cytokines, which play a crucial role in innate and adaptive immunity against viruses of vertebrates. In essence, IFN-I are induced and secreted upon host cell recognition of viral nucleic acids and protect other cells against infection by inducing antiviral proteins. Atlantic salmon possesses an extraordinary repertoire of IFN-I genes encompassing at least six different classes (IFNa, IFNb, IFNc, IFNd, IFNe and IFNf) most of which are encoded by several genes. This review describes recent research on the functions of salmon IFNa, IFNb, IFNc and IFNd. As in mammals, expression of different salmon IFN-I in response to virus infection is dependent on their promoters, properties of the virus and the cell's expression of nucleic acid receptors and interferon regulatory factors (IRFs). While IFNa mainly display local antiviral activity, IFNb and IFNc show systemic antiviral activity. In addition, salmon appears to possess several IFN-I receptors, which show selectivity in binding different IFN-I. This complexity in IFN-I and receptors allows for a large variation in functions of the salmon IFN-I. Studies with intramuscular injection of IFN expression plasmids have recently provided surprising results, which may be of relevance for application of IFN-I in prophylaxis against virus infection. Firstly, injection of IFNc plasmid protected salmon presmolts against virus infection for at least 10 weeks. Secondly, IFN plasmids showed potent adjuvant activity when injected together with a DNA vaccine against infectious salmon anemia virus (ISAV).

Use of Salmon Cardiac Primary Cultures (SCPCs) of different genotypes for comparative kinetics of mx expression, viral load and ultrastructure pathology, after infection with Salmon Pancreas Disease Virus (SPDV)

In vitro fish based models have been extensively applied in human biomedical research but, paradoxically, less frequently in the research of fish health issues. Farmed Atlantic salmon can suffer from several viral conditions affecting the heart. Therefore, species-specific, cardiac in vitro models may represent a useful tool to help further understanding and management of these diseases. The mechanisms underlying genotype based resistance are complex and usually rely on a combined effect of elements from both the innate and adaptive immune response, which are further complicated by external environmental factors. Here we propose that Salmon Cardiac Primary Cultures (SCPCs) are a useful tool to investigate these mechanisms as the basis for genotypic differences between Atlantic salmon families in susceptibility to cardiotropic viral disease. Using SCPCs produced from two different commercially available Atlantic salmon embryonated ova (Atlantic Ova IPN sensitive" (S) and "Atlantic QTL-innOva^® IPN/PD" (R)), the influence of host genotype on the viral load and mx expression following Salmon Pancreas Disease Virus infection was assessed over a 15 day period. Both R and S SCPCs groups were successfully infected. A measurable difference between groups of viral nsP1 and host antiviral mx gene expression was observed (i.e. a later, but larger onset of mx expression in the R group). Mx expression peaks were followed by a decrease in viral nsP1 in both groups. Additionally, ultrastructural examination of infected SCPCs allowed the description of degenerative changes at the individual cell level. The SCPC model presents some advantages, over current fish cell culture monolayers and in vivo material, such as the presence of different cell components normally present in the target organ, as well as the removal of a layer of functional complexity (acquired immunity), making it possible to focus on tissue specific, early innate immune mechanisms. These preliminary results highlight the importance of considering genetic origin when selecting the fish source for the production of SCPCs, as well as their usefulness as screening tools for assessment of genotypic differences in disease resistance.

Atlantic salmon cardiac primary cultures: An in vitro model to study viral host pathogen interactions and pathogenesis

Development of Salmon Cardiac Primary Cultures (SCPCs) from Atlantic salmon pre-hatch embryos and their application as in vitro model for cardiotropic viral infection research are described. Producing SCPCs requires plating of trypsin dissociated embryos with subsequent targeted harvest from 24h up to 3 weeks, of relevant tissues after visual identification. SCPCs are then transferred individually to chambered wells for culture in isolation, with incubation at 15-22°. SCPCs production efficiency was not influenced by embryo's origin (0.75/ farmed or wild embryo), but mildly influenced by embryonic developmental stage (0.3 decline between 380 and 445 accumulated thermal units), and strongly influenced by time of harvest post-plating (0.6 decline if harvested after 72 hours). Beating rate was not significantly influenced by temperature (15-22°) or age (2-4 weeks), but was significantly lower on SCPCs originated from farmed embryos with a disease resistant genotype (F = 5.3, p<0.05). Two distinct morphologies suggestive of an ex vivo embryonic heart and a de novo formation were observed sub-grossly, histologically, ultra-structurally and with confocal microscopy. Both types contained cells consistent with cardiomyocytes, endothelium, and fibroblasts. Ageing of SCPCs in culture was observed with increased auto fluorescence in live imaging, and as myelin figures and cellular degeneration ultra-structurally. The SCPCs model was challenged with cardiotropic viruses and both the viral load and the mx gene expression were measurable along time by qPCR. In summary, SCPCs represent a step forward in salmon cardiac disease research as an in vitro model that partially incorporates the functional complexity of the fish heart.

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.

Antiviral and metabolic gene expression responses to viral infection in Atlantic salmon (Salmo salar)

Salmonid alphavirus (SAV), the aetiological agent of pancreas disease, is recognized as a serious pathogen of farmed Atlantic salmon. This disease results in loss of weight followed by poor growth of surviving fish, as such it is viewed as a wasting disease. SAV and other chronic disease causing viruses affect the heart and skeletal muscle tissues, at present the mechanisms by which pathology occurs is unknown. The relationship between antiviral activity and other physiological parameters especially in skeletal muscle are currently not examined in depth in fish. An experimental SAV (isotype 3) infection was carried out using a cohabitation approach, from which samples were collected at 0, 4, 8 & 12 week post challenge. Maximum viral load in the muscle tissue was 4 weeks post infection which was reduced at 8 weeks and undetectable by 12 weeks. Histopathology score peaked at 4 weeks post infection in pancreas and heart whereas there was maximum damage in skeletal muscle at 8 weeks. The peak expression of antiviral immune genes coincided with the viral load. Several genes involved in protein degradation were increased following infection including atrogin-1 and cathepsin D, at 4 weeks post challenge suggesting reallocation of amino acid reserves. Taken together, these observations increase our understanding of salmon poor growth during viral infection, and will serve as a basis to develop strategies to manage this viral wasting disease.

A hemagglutinin-esterase-expressing salmonid alphavirus replicon protects Atlantic salmon (Salmo salar) against infectious salmon anemia (ISA)

A replicon expression system based on the salmonid alphavirus (SAV) that encodes the infectious salmon anemia virus (ISAV) hemagglutinin-esterase (HE) was constructed and found to be an efficacious vaccine against infectious salmon anemia (ISA). Following a single intramuscular immunization, Atlantic salmon (Salmo salar) were effectively protected against subsequent ISAV challenge. Additional replicons coding for the ISAV fusion glycoprotein (F) or the ISAV matrix protein (M) were created and tested in combination with the replicon that encodes the HE. The ISAV HE was confirmed as a potent antigen, but neither the F nor the M proteins were found to be essential for immunization-induced protection. Innate immune response induced at the site of vaccination illustrated the immunogenicity of the SAV-based replicon and its ability to activate antiviral responses in Atlantic salmon. The successful testing of the SAV-based replicon as a vaccine model against ISA showed that the replicon approach may represent a novel immunization technology for the aquaculture industry. It offers potential benefits in terms of safety, efficacy, flexibility, and vaccine production complexity.

Gene expression studies of host response to Salmonid alphavirus subtype 3 experimental infections in Atlantic salmon

Salmonid alphavirus subtype-3 (SAV-3) infection in Atlantic salmon is exclusively found in Norway. The salmonid alphaviruses have been well characterized at the genome level but there is limited information about the host-pathogen interaction phenomena. This study was undertaken to characterize the replication and spread of SAV-3 in internal organs of experimentally infected Atlantic salmon and the subsequent innate and adaptive immune responses. In addition, suitability of a cohabitation challenge model for this virus was also examined. Groups of fish were infected by intramuscular injection (IM), cohabited (CO) or kept uninfected in a separate tank. Samples of pancreas, kidney, spleen, heart and skeletal muscles were collected at 2, 4 and 8 weeks post infection (wpi). Pathological changes were assessed by histology concurrently with viral loads and mRNA expression of immune genes by real time RT-PCR. Pathological changes were only observed in the pancreas and heart (target organs) of both IM and CO groups, with changes appearing first in the pancreas (2 wpi) in the former. Lesions with increasing severity over time coincided with high viral loads despite significant induction of IFN-α, Mx and ISG15. IFN-γ and MHC-I were expressed in all tissues examined and their induction appeared in parallel with that of IL-10. Inflammatory genes TNF-α, IL-12 and IL-8 were only induced in the heart during pathology while T cell-related genes CD3ε, CD4, CD8, TCR-α and MHC-II were expressed in target organs at 8 wpi. These findings suggest that the onset of innate responses came too late to limit virus replication. Furthermore, SAV-3 infections in Atlantic salmon induce Th1/cytotoxic responses in common with other alphaviruses infecting higher vertebrates. Our findings demonstrate that SAV-3 can be transmitted via the water making it suitable for a cohabitation challenge model.

Transcriptomic analysis of the host response to early stage salmonid alphavirus (SAV-1) infection in Atlantic salmon Salmo salar L

Salmon pancreas disease, caused by salmonid alphavirus (SAV) of the family Togaviridae, is an economically important disease affecting farmed Atlantic salmon (Salmo salar L.) in Scotland, Norway, and Ireland. The virus causes characteristic lesions in the pancreas, heart, kidney and skeletal muscle of infected fish. The mechanisms responsible for the pathology and the immune responses elicited in infected Atlantic salmon are not fully understood. A microarray-based study was therefore performed to evaluate the host transcriptomic response during the early stages of an experimentally-induced SAV-1 infection. Atlantic salmon parr were injected intra-peritoneally with viral cell culture supernatant or cell culture supernatant without virus. RNA, extracted from head kidney sampled from infected and control fish at 1, 3 and 5 days post-injection (d.p.i.), was interrogated with the 17 k TRAITS/SGP cDNA microarray. The greatest number of significantly differentially expressed genes was recorded at 3 d.p.i., mainly associated with immune and defence mechanisms, including genes involved in interferon I pathways and Major Histocompatibility Complex Class I and II responses. Genes associated with apoptosis and cellular stress were also found to be differentially expressed between infected and uninfected individuals, as were genes involved in inhibiting viral attachment and replication. The microarray results were validated by follow-on analysis of eight genes by real-time PCR. The findings of the study reflect mechanisms used by the host to protect itself during the early stages of SAV-1 infection. In particular, there was evidence of rapid induction of interferon-mediated responses similar to those seen during mammalian alphavirus infections, and also early involvement of an adaptive immune response. This study provides essential knowledge to assist in the development of effective control and management strategies for SAV-1 infection.

Superior protection conferred by inactivated whole virus vaccine over subunit and DNA vaccines against salmonid alphavirus infection in Atlantic salmon (Salmo salar L.)

Salmonid alphavirus 3 (SAV-3) is an emerging pathogen in Norwegian salmon farming and causes severe annual losses. We studied the immunogenicity and protective ability of subunit and DNA vaccines based on E1 and E2 spike proteins of salmonid alphavirus subtype 3 (SAV-3), and compared these to an experimental inactivated, whole virus (IWV) vaccine in Atlantic salmon. The antigens were delivered as water-in-oil emulsions for the subunit and inactivated vaccines and non-formulated for the DNA vaccines. The IWV and the E2 subunit prime-boost groups had circulating neutralizing antibodies at challenge, correlating with high protection against lethal challenge and 3-log(10) reduction of virus titer in heart for the IWV group. Prime-boost with E1 subunit vaccine also conferred significant protection against mortality, but did not correlate with neutralizing antibody levels. Protection against pathology in internal organs was only seen for the IWV group. Prime-boost with E1 and E2 DNA vaccines showed marginal protection in terms of reduction of viral replication in target organs and protection against mortality was not different from controls. The IWV group showed significant upregulation of IFNγ and IL2 mRNA expression at 4 weeks post challenge possibly indicating that other mechanisms in addition to antibody responses play a role in mediating protection against infection. This is the first report comparing the immunogenicity and protection against mortality for IWV vaccines and spike protein subunit and DNA vaccines against salmonid alphavirus infection in Atlantic salmon. The IWV vaccine has superior immunogenicity over sub-unit and DNA vaccines.

IPNV modulation of pro and anti-inflammatory cytokine expression in Atlantic salmon might help the establishment of infection and persistence

IPNV is the agent of a well-characterized acute disease that produces a systemic infection and high mortality in farmed fish species and persistent infection in surviving fish after outbreaks. Because modulation of the host expression of pro and anti-inflammatory cytokines can help establish persistence, in this study, we examined the expression of IL-1β, IL-8, IFNα1 and IL-10 during acute and persistent IPNV infection of Atlantic salmon. Results showed that IPNV infection induces an increase of the IFNα1 and IL-10 mRNA levels in the spleen and head kidney (HK) of fish after acute experimental infection. Levels of the pro-inflammatory cytokines IL-1β and IL-8 did not rise in the spleen although an increase of IL-1β, but not of IL-8, was observed in head kidney. In carrier asymptomatic salmon, cytokine gene expression of IFNα1 in the spleen and IL-10 in head kidney were also significantly higher than expression in non-carrier fish. Interestingly, a decrease of IL-8 expression was also observed. IPNV infection of SHK-1, which is a macrophage-like cell line of salmon, also induced an increase of expression of the anti-inflammatory cytokine IL-10 with no effects on the expression of IL-1β and IL-8. The effects are induced by an unknown mechanism during viral infection because poly I:C and the viral genomic dsRNA showed the opposite effects on cytokine expression in SHK-1 cells. In summary, IPNV always induces up-regulation of the anti-inflammatory cytokine IL-10 in Atlantic salmon. As this is accompanied by a lack of induction of the pro-inflammatory cytokines IL-1β and IL-8, the anti-inflammatory milieu may explain the high frequency, prevalence and persistence of IPNV in salmon. Effects might be part of the viral mechanisms of immune evasion.

An IRF-3 homolog that is up-regulated by DNA virus and poly I:C in turbot, Scophthalmus maximus

In this study, we described the structure, mRNA tissue distribution and regulation of an IRF-3 gene from turbot, Scophthalmus maximus (SmIRF-3). The gene sequence of SmIRF-3 is 6077 bp long, composed of 11 exons and 10 introns similar to known IRF-3 genes of fish, and encodes a peptide of 466 amino acids. The deduced protein sequence shares the highest identity of 56.0-81.2% with fish IRF-3 and possesses a DNA-binding domain (DBD), an IRF association domain (IAD) and a serine-rich domain (SRD) known to be important for the functions of IRF-3 in vertebrates. Phylogenetic analysis grouped SmIRF-3 with other IRF3s of vertebrates. SmIRF-3 transcripts were detectable in limited tissue types of healthy fish, with higher expression observed in head, kidney, spleen and kidney,. The SmIRF-3 was transcriptionally up-regulated by turbot reddish body iridovirus (TRBIV) and polyinosinic:polycytidylic acid (poly I:C) in the head kidney, spleen and gills, with showing a two wave induced expression during a 7-day time course in all cases. The highest inducibility and the likely earliest increase of SmIRF-3 expression were observed in the spleen, and poly I:C was a stronger inducer. In addition, the maximal expression level of SmIRF-3 arose prior to that of the Mx in all the cases.

Expression and functional characterization of the RIG-I-like receptors MDA5 and LGP2 in Rainbow trout (Oncorhynchus mykiss)

The retinoic acid-inducible gene I (RIG-I)-like receptors (RLR) comprise three homologues: RIG-I, melanoma differentiation-associated gene 5 (MDA5), and laboratory of genetics and physiology 2 (LGP2). They activate the host interferon (IFN) system upon recognition of viral RNA pathogen-associated molecular patterns (PAMPs) in the cytoplasm. Bioinformatic analysis of the sequenced vertebrate genomes suggests that the cytosolic surveillance system is conserved in lower vertebrates, and recent functional studies have confirmed that RIG-I is important to fish antiviral immunity. In this study, we have identified MDA5 and LGP2 homologues from rainbow trout Oncorhynchus mykiss and an additional LGP2 variant with an incomplete C-terminal domain of RIG-I. Trout MDA5 and LGP2 were constitutively produced in fibroblast and macrophage cell lines and upregulated by poly(I:C), recombinant IFN, or infection by RNA viruses (viral hemorrhagic septicemia virus and salmon alphavirus) with a single-stranded positive or negative genome. Overexpression of MDA5 and LGP2 but not of the LGP2 variant resulted in significant accumulation of Mx transcripts in cultured cells, which correlated with a marked enhancement of protection against viral infection. These results demonstrate that both MDA5 and LGP2 are important RLRs in host surveillance against infection of both negative and positive viruses and that the LGP2 variant with a deletion of 54 amino acids at the C terminus acts as a negative regulator for LGP2-elicited antiviral signaling by competing for the viral RNA PAMPs. Interestingly, MDA5 expression was not affected by overexpressed LGP2 in transfected cells and vice versa, suggesting that they likely act in parallel as positive regulators for IFN production.

Susceptibilities of medaka (Oryzias latipes) cell lines to a betanodavirus

Background

Betanodaviruses, members of the family Nodaviridae, have bipartite, positive-sense RNA genomes and are the causal agents of viral nervous necrosis in many marine fish species. Recently, the viruses were shown to infect a few freshwater fish species including a model fish medaka (Oryzias latipes). Although virological study using cultured medaka cells would provide a lot of insight into virus-fish interactions in molecular aspects, no such cells have yet been tested for virus susceptibility.

Results

We tested ten medaka cell lines for susceptibilities to redspotted grouper nervous necrosis virus (RGNNV). Although the viral coat protein was detected in all the cell lines inoculated, the levels of cytopathic effect development and viral propagation were quite different among the cell lines. Those levels were especially high in OLHNI-1 and OLHNI-2 cells, but were extremely low in OLME-104 cells. Some cell lines entered into antiviral state after RGNNV infections probably because of inducing an antiviral system. This is the first report to examine the susceptibilities of cultured medaka cells against a virus.

Conclusion

OLHNI-1 and OLHNI-2 cells are candidates of new standard cells for betanodavirus study because of their high susceptibilities to the virus and their several advantages as model fish cells.

Alpha interferon and not gamma interferon inhibits salmonid alphavirus subtype 3 replication in vitro

Salmonid alphavirus (SAV) is an emerging virus in salmonid aquaculture, with SAV-3 being the only subtype found in Norway. Until now, there has been little focus on the alpha interferon (IFN-alpha)-induced antiviral responses during virus infection in vivo or in vitro in fish. The possible involvement of IFN-gamma in the response to SAV-3 is also not known. In this study, the two IFNs were cloned and expressed as recombinant proteins (recombinant IFN-alpha [rIFN-alpha] and rIFN-gamma) and used for in vitro studies. SAV-3 infection in a permissive salmon cell line (TO cells) results in IFN-alpha and IFN-stimulated gene (ISG) mRNA upregulation. Preinfection treatment (4 to 24 h prior to infection) with salmon rIFN-alpha induces an antiviral state that inhibits the replication of SAV-3 and protects the cells against virus-induced cytopathic effects (CPE). The antiviral state coincides with a strong expression of Mx and ISG15 mRNA and Mx protein expression. When rIFN-alpha is administered at the time of infection and up to 24 h postinfection, virus replication is not inhibited, and cells are not protected against virus-induced CPE. By 40 h postinfection, the alpha subunit of eukaryotic initiation factor 2 (eIF2alpha) is phosphorylated concomitant with the expression of the E2 protein as assessed by Western blotting. Postinfection treatment with rIFN-alpha results in a moderate reduction in E2 expression levels in accordance with a moderate downregulation of cellular protein synthesis, an approximately 65% reduction by 60 h postinfection. rIFN-gamma has only a minor inhibitory effect on SAV-3 replication in vitro. SAV-3 is sensitive to the preinfection antiviral state induced by rIFN-alpha, while postinfection antiviral responses or postinfection treatment with rIFN-alpha is not able to limit viral replication.