Interferon-inducible Mx proteins in fish

An antiviral optimized extract from Sanguisorba officinalis L. roots using response surface methodology, and its efficacy in controlling viral hemorrhagic septicemia of olive flounder (Paralichthys olivaceus)

Viral hemorrhagic septicemia causes considerable economic losses for Korea's olive flounder (Paralichthys olivaceus) aquaculture farms; therefore, effective antiviral agents for controlling viral hemorrhagic septicemia virus (VHSV) infection are imperative. The present study implemented a Box-Behnken design and cytopathic reduction assay to derive an optimized extract of Sanguisorba officinalis L. roots (OE-SOR) with maximum antiviral activity against VHSV. OE-SOR prepared under optimized extraction conditions (55% ethanol concentration at 50 °C for 5 h) exhibited potent antiviral activity against VHSV, with a 50% effective 0.21 μg/mL concentration and a 340 selective index. OE-SOR also showed direct virucidal activity in the plaque reduction assay. Administering OE-SOR to olive flounder exhibited substantial efficacies against VHSV infection. Fish receiving 100 mg/kg body weight/day of OE-SOR as a preventive (40.0%; p < 0.05) or therapeutic (44.4%; p < 0.05) exhibited a higher relative survival than the untreated VHSV-infected control group (mortalities of 100% and 90%, respectively). In addition, fish fed with OE-SOR (100 mg/kg body weight/day) for two weeks conveyed a significantly higher inflammatory cytokine expression (nuclear factor kappa-light-chain-enhancer of activated B cells [NF-κB], interleukin-1 beta [IL-1β], and tumor necrosis factor-alpha [TNF-α]) than the control group one to two days post-administration. Moreover, no hematological or histological changes were observed in olive flounder treated with OE-SOR over four weeks. Liquid chromatography-quadrupole-time of flight tandem mass spectrometry and -triple quadrupole tandem mass spectrometry analyses identified ziyuglycoside I as a prominent OE-SOR constituent and marker compound (content: 14.5%). This study verifies that OE-SOR is an effective alternative for controlling viral hemorrhagic septicemia in olive flounder farms as it exhibits efficient in vivo anti-VHSV activity and increases innate immune responses.

Differential response of RTGUTGC and RTGILL-W1 rainbow trout epithelial cell lines to viral stimulation

Mucosal surfaces constitute the main route of entry of pathogens into the host. In fish, these mucosal tissues include, among others, the gastrointestinal tract, the gills and the skin. However, knowledge about the mechanisms of regulation of immunity in these tissues is still scarce, being essential to generate a solid base that allows the development of prevention strategies against these infectious agents. In this work, we have used the RTgutGC and RTgill-W1 epithelial-like cell lines, derived from the gastrointestinal tract and the gill of rainbow trout (Oncorhynchus mykiss), respectively, to investigate the transcriptional response of mucosal epithelial cells to a viral mimic, the dsRNA poly I:C, as well as to two important viral rainbow trout pathogens, namely viral haemorrhagic septicaemia virus (VHSV) and infectious pancreatic necrosis virus (IPNV). Additionally, we have established how the exposure to poly I:C affected the susceptibility of RTgutGC and RTgill-W1 cells to both viruses. Our results reveal important differences in the way these two cell lines respond to viral stimuli, providing interesting information on these cell lines that have emerged in the past years as useful tools to study mucosal responses in fish.

Host–Pathogen Interactions of Marine Gram-Positive Bacteria

Simple Summary

Complex interactions between marine Gram-positive pathogens and fish hosts in the marine environment can result in diseases of economically important finfish, which cause economic losses in the aquaculture industry. Understanding how these pathogens interact with the fish host and generate disease will contribute to efficient prophylactic measures and treatments. To our knowledge, there are no systematic reviews on marine Gram-positive pathogens. Therefore, here we reviewed the host–pathogen interactions of marine Gram-positive pathogens from the pathogen-centric and host-centric points of view.

Abstract

Marine Gram-positive bacterial pathogens, including Renibacterium salmoninarum, Mycobacterium marinum, Nocardia seriolae, Lactococcus garvieae, and Streptococcus spp. cause economic losses in marine fish aquaculture worldwide. Comprehensive information on these pathogens and their dynamic interactions with their respective fish–host systems are critical to developing effective prophylactic measures and treatments. While much is known about bacterial virulence and fish immune response, it is necessary to synthesize the knowledge in terms of host–pathogen interactions as a centerpiece to establish a crucial connection between the intricate details of marine Gram-positive pathogens and their fish hosts. Therefore, this review provides a holistic view and discusses the different stages of the host–pathogen interactions of marine Gram-positive pathogens. Gram-positive pathogens can invade fish tissues, evade the fish defenses, proliferate in the host system, and modulate the fish immune response. Marine Gram-positive pathogens have a unique set of virulence factors that facilitate adhesion (e.g., adhesins, hemagglutination activity, sortase, and capsules), invasion (e.g., toxins, hemolysins/cytolysins, the type VII secretion system, and immune-suppressive proteins), evasion (e.g., free radical quenching, actin-based motility, and the inhibition of phagolysosomal fusion), and proliferation and survival (e.g., heme utilization and siderophore-mediated iron acquisition systems) in the fish host. After infection, the fish host initiates specific innate and adaptive immune responses according to the extracellular or intracellular mechanism of infection. Although efforts have continued to be made in understanding the complex interplay at the host–pathogen interface, integrated omics-based investigations targeting host–pathogen–marine environment interactions hold promise for future research.

Fish Innate Immune Response to Viral Infection-An Overview of Five Major Antiviral Genes

Fish viral diseases represent a constant threat to aquaculture production. Thus, a better understanding of the cellular mechanisms involved in establishing an antiviral state associated with protection against virus replication and pathogenesis is paramount for a sustainable aquaculture industry. This review summarizes the current state of knowledge on five selected host innate immune-related genes in response to the most relevant viral pathogens in fish farming. Viruses have been classified as ssRNA, dsRNA, and dsDNA according to their genomes, in order to shed light on what those viruses may share in common and what response may be virus-specific, both in vitro (cell culture) as well as in vivo. Special emphasis has been put on trying to identify markers of resistance to viral pathogenesis. That is, those genes more often associated with protection against viral disease, a key issue bearing in mind potential applications into the aquaculture industry.

Differential virulence of infectious hematopoietic necrosis virus (IHNV) isolated from salmonid fish in Gangwon Province, Korea

The present study investigated the virulence and expression of innate immunity genes in isolates of infectious hematopoietic necrosis virus (IHNV) in Gangwon province, South Korea, by challenging rainbow trout, Atlantic salmon, and coho salmon. Eight IHNV isolates were used to infect RTG-2 cells for viral replication using plaque assays. Three isolates with the highest replication rates, the RtPc0314g and RtPc0314c isolates of the JRt-Shizuoka type and the RtPc0816g isolate of the JRt-Nagano type, were experimentally infected into the fish. In rainbow trout, both RtPc0314c and RtPc0314g isolates showed 100% cumulative mortality while the RtPc0816g isolate showed 60% cumulative mortality for 14 days. In contrast, all three isolates showed <60% cumulative mortality in Atlantic salmon and coho salmon. The expression of G genes in the kidney was higher than that in the spleen-infected fish, with the highest expression observed in the kidneys of rainbow trout. The relative expression levels of innate immunity genes were higher in rainbow trout than in Atlantic salmon and coho salmon. The expression level of immunoglobulin M increased until day 7, and the expression of type I interferon was higher in the spleen than in other tissues. The expression of Mx-1 was higher in the kidney and liver than other tissues. These results indicate that IHNV isolates from Gangwon province show host-specific virulence in rainbow trout and that their virulence and replication were higher in JRt-Shizuoka type than in JRt-Nagano type isolates.

Polystyrene nanoplastics alter virus replication in orange-spotted grouper (Epinephelus coioides) spleen and brain tissues and spleen cells

Polystyrene nanoplastics (PS-NPs) are known to impair the function of the digestive system, intestinal flora, immune system, and nervous system of marine organisms. We tested whether PS-NPs influence viral infection of orange-spotted grouper (Epinephelus coioides). We found that grouper spleen (GS) cells took up PS-NPs at exposure concentrations of 5, 50, and 500 μg/mL and experienced cytotoxicity at 50 and 500 μg/mL concentrations. At 12 h after exposure to 50 μg/mL of PS-NPs, the replication of Singapore grouper iridovirus (SGIV) and red-spotted grouper nervous necrosis virus (RGNNV) increased in GS cells after their invasion. Juvenile fish exposed to 300 and 3000 μg/L of PS-NPs for 7 d showed PS-NPs uptake to the spleen and vacuole formation in brain tissue. Moreover, PS-NPs exposure accelerated SGIV replication in the spleen and RGNNV replication in the brain. PS-NP exposure also decreased the expression of toll-like receptor genes and interferon-related genes before and after virus invasion in vitro and in vivo, thus reducing the resistance of cells and tissues to viral replication. This is the first report that PS-NPs have toxic effects on GS cells and spleen and brain tissues, and it provides new insights into assessing the impact of PS-NPs on marine fish.

Screening for the Proteins That Can Interact with Grouper Nervous Necrosis Virus Capsid Protein

Nervous necrosis virus (NNV) can infect many species of fish and has an 80-100% mortality rate. NNV capsid protein (NNVCP) is the only structural protein of NNV, but there are few studies on the protein-protein interaction between NNVCP and the host cell. To investigate NNV morphogenesis, native NNV capsid protein (NNVCP) was used to screen for protein-protein interactions in this study. The results identified that 49 grouper optic nerve proteins can interact with NNVCP and may function as putative receptor or co-receptor, cytoskeleton, glucose metabolism and ATP generation, immunity, mitochondrial ion regulation, and ribosomal proteins. Creatine kinase B-type (CKB) is one of those 49 optic nerve proteins. CKB, a kind of enzyme of ATP generation, was confirmed to interact with NNVCP by far-Western blot and showed to colocalize with NNVCP in GF-1 cells. Compared to the control, the expression of CKB was significantly induced in the brain and eyes infected with NNV. Moreover, the amount of replication of NNV is relatively high in cells expressing CKB. In addition to providing the database of proteins that can interact with NNVCP for subsequent analysis, the results of this research also verified that CKB plays an important role in the morphogenesis of NNV.

Functional nutrition modulates the early immune response against viral haemorrhagic septicaemia virus (VHSV) in rainbow trout

Although viruses represent a major threat for cultured fish worldwide, the commercialization of vaccines capable of providing effective and long-lasting protection is still lacking for most of these viral diseases. In this situation, the use of supplemented diets could be a suitable strategy to increase the immune status of the fish and reduce the impact of viral pathogens. Among possible immunostimulants that could be included in these functional feeds, some studies have previously shown that certain β-glucans can significantly increase certain immune parameters of fish and reduce the impact of viral diseases. However, the mechanisms through which β-glucans exert their activity have not been fully elucidated yet. In the current study, we have studied the immune response of different tissues to viral haemorrhagic septicaemia virus (VHSV) in rainbow trout fed with a non-supplemented control diet as well as in fish fed a commercial functional aquafeed (Protec™, Skretting) containing β-glucans, vitamin C, vitamin E and zinc. For this, after 30 days of feeding the fish with one of the two diets, they were subsequently infected with VHSV by bath or mock-infected. After 2 or 6 days post-infection, fish were sacrificed and the levels of transcription of different immune genes such as IgM, IgT, IgD, Mx, interferon γ (IFN γ) and perforin studied in different tissues (kidney, gut and gills). Additionally, the levels of natural IgMs in serum were also determined. Our results demonstrate that fish fed the functional diet were capable of mounting an increased IgM, IgT, IgD and Mx transcriptional response to the virus. Additionally, these fish also showed increased levels of natural IgMs in serum. These results reveal a previously undescribed effect of functional diets on fish Ig production and point to Protec™ as an adequate diet to be incorporated in holistic programs aimed at mitigating the effect of viral diseases.

Evaluation of zebrafish (Danio rerio) as an animal model for the viral infections of fish

Zebrafish (Danio rerio) is a laboratory model organism used in different areas of biological research including studies of immune response and host-pathogen interactions. Thanks to many biological tools available, zebrafish becomes also an important model in aquaculture research since several fish viral infection models have been developed for zebrafish. Here, we have evaluated the possible use of zebrafish to study infections with fish viruses that have not yet been tested on this model organism. In vitro studies demonstrated that chum salmon reovirus (CSV; aquareovirus A) and two alloherpesviruses cyprinid herpesvirus 1 (CyHV-1) and cyprinid herpesvirus 3 (CyHV-3) are able to replicate in zebrafish cell lines ZF4 and SJD.1. Moreover, CSV induced a clear cytopathic effect and up-regulated the expression of antiviral genes vig-1 and mxa in both cell lines. In vivo studies demonstrated that both CSV and CyHV-3 induce up-regulation of vig-1 and mxa expression in kidney and spleen of adult zebrafish after infection by i.p. injection but not in larvae after infection by immersion. CyHV-3 is eliminated quickly from fish; therefore, virus clearing process could be evaluated, and in CSV-infected fish, a prolonged confrontation of the host with the pathogen could be studied.

Piscine orthoreovirus demonstrates high infectivity but low virulence in Atlantic salmon of Pacific Canada

Piscine orthoreovirus (PRV) is ubiquitous in farmed Atlantic salmon and sometimes associated with disease - most notably, Heart and Skeletal Muscle Inflammation (HSMI). However, PRV is also widespread in non-diseased fish, particularly in Pacific Canada, where few cases of severe heart inflammation have been documented. To better understand the mechanisms behind PRV-associated disease, this study investigated the infection dynamics of PRV from Pacific Canada and the potential for experimental passage of putatively associated heart inflammation in Pacific-adapted Mowi-McConnell Atlantic salmon. Regardless of the PRV source (fish with or without HSMI-like heart inflammation), infections led to high-load viremia that induced only minor focal heart inflammation without significant transcriptional induction of inflammatory cytokines. Repeated screening of PRV dsRNA/ssRNA along with histopathology and gene expression analysis of host blood and heart tissues identified three distinct phases of infection: (1) early systemic dissemination and replication without host recognition; (2) peak replication, erythrocyte inclusion body formation and load-dependent host recognition; (3) long-term, high-load viral persistence with limited replication or host recognition sometimes accompanied by minor heart inflammation. These findings contrast previous challenge trials with PRV from Norway that induced severe heart inflammation and indicate that strain and/or host specific factors are necessary to initiate PRV-associated disease.

Efficacy of algal Ecklonia cava extract against viral hemorrhagic septicemia virus (VHSV)

The inhibition efficacy of an extract from Ecklonia cava (E. cava) was studied to determine whether the extract and compounds exhibited inhibitory activity against VHSV in the fathead minnow (FHM) cell line and following oral administration to the olive flounder. Based on its low toxicity and effective concentration, the E. cava extract (Ext) and compounds (eckol and phlorofucofuroeckol A) were selected for further analysis. In the plaque reduction assay, simultaneous co-exposure of VHSV to Ext, eckol and phlorofucofuroeckol A showed a higher level of inhibition than the pre- and post-exposure groups. The antiviral activity in the FHM cell line was time-dependent and increased with the exposure time with the virus and Ext or the compounds. In the in vivo experiments, different Ext concentrations were orally administered to the olive flounder. In trial I, the relative percent survival (RPS) following oral administration of 500 and 50 μg/g/day of Ext was 31.25% and 12.50%, respectively. In trial II, the RPS for 1000, 500 and 50 μg/g/day of Ext was 31.57%, 0% and 0%, respectively. In trial III, the RPS after 1 and 2 weeks (1000 μg/g/day) of exposure to Ext was 26.31% and 31.57%, respectively. Oral administration of Ext (1000 μg/g/day) significantly induced inflammatory cytokine responses (IL-1β, IL-6 and IFN-γ) at 1 and 2 days post-oral administration (dpa). Additionally, IFN-α/β (7-12 dpa), ISG15 (2, 7 and 10 dpa) and Mx (7-12 dpa) were significantly activated in the olive flounder. In conclusion, we demonstrated an inhibitory ability of the E. cava extract and compounds against VHSV in the FHM cell line. Moreover, oral administration of the E. cava extract to the olive flounder enhanced antiviral immune responses and the efficacy of protection against VHSV, resulting in an anti-viral status in the olive flounder.

Fish interferon-stimulated genes: The antiviral effectors

Type I interferons (IFN) are the cornerstone cytokine of innate antiviral immunity. In response to a viral infection, IFN signaling results in the expression of a diverse group of genes known as interferon-stimulated genes (ISGs). These ISGs are responsible for interfering with viral replication and infectivity, helping to limit viral infection within a cell. In mammals, many antiviral effector ISGs have been identified and the antiviral mechanisms are at least partially elucidated. In fish fewer ISGs have been identified and while there is evidence they limit viral infection, almost nothing is known of their respective antiviral mechanisms. This review discusses seven ISGs common to mammals and fish and three ISGs that are unique to fish. The lack of understanding regarding fish ISG's antiviral effector functions is highlighted and draws attention to the need for research in this aspect of aquatic innate immunity.

Understanding the interaction between Betanodavirus and its host for the development of prophylactic measures for viral encephalopathy and retinopathy

Over the last three decades, the causative agent of viral encephalopathy and retinopathy (VER) disease has become a serious problem of marine finfish aquaculture, and more recently the disease has also been associated with farmed freshwater fish. The virus has been classified as a Betanodavirus within the family Nodaviridae, and the fact that Betanodaviruses are known to affect more than 120 different farmed and wild fish and invertebrate species, highlights the risk that Betanodaviruses pose to global aquaculture production. Betanodaviruses have been clustered into four genotypes, based on the RNA sequence of the T4 variable region of their capsid protein, and are named after the fish species from which they were first derived i.e. Striped Jack nervous necrosis virus (SJNNV), Tiger puffer nervous necrosis virus (TPNNV), Barfin flounder nervous necrosis virus (BFNNV) and Red-spotted grouper nervous necrosis virus (RGNNV), while an additional genotype turbot betanodavirus strain (TNV) has also been proposed. However, these genotypes tend to be associated with a particular water temperature range rather than being species-specific. Larvae and juvenile fish are especially susceptible to VER, with up to 100% mortality resulting in these age groups during disease episodes, with vertical transmission of the virus increasing the disease problem in smaller fish. A number of vaccine preparations have been tested in the laboratory and in the field e.g. inactivated virus, recombinant proteins, virus-like particles and DNA based vaccines, and their efficacy, based on relative percentage survival, has ranged from medium to high levels of protection to little or no protection. Ultimately a combination of effective prophylactic measures, including vaccination, is needed to control VER, and should also target larvae and broodstock stages of production to help the industry deal with the problem of vertical transmission. As yet there are no commercial vaccines for VER and the aquaculture industry eagerly awaits such a product. In this review we provide an overview on the current state of knowledge of the disease, the pathogen, and interactions between betanodavirus and its host, to provide a greater understanding of the multiple factors involved in the disease process. Such knowledge is needed to develop effective methods for controlling VER in the field, to protect the various aquaculture species farmed globally from the different Betanodavirus genotypes to which they are susceptible.

A functional genomics tool for the Pacific bluefin tuna: Development of a 44K oligonucleotide microarray from whole-genome sequencing data for global transcriptome analysis

Bluefin tunas are one of the most important fishery resources worldwide. Because of high market values, bluefin tuna farming has been rapidly growing during recent years. At present, the most common form of the tuna farming is based on the stocking of wild-caught fish. Therefore, concerns have been raised about the negative impact of the tuna farming on wild stocks. Recently, the Pacific bluefin tuna (PBT), Thunnus orientalis, has succeeded in completing the reproduction cycle under aquaculture conditions, but production bottlenecks remain to be solved because of very little biological information on bluefin tunas. Functional genomics approaches promise to rapidly increase our knowledge on biological processes in the bluefin tuna. Here, we describe the development of the first 44K PBT oligonucleotide microarray (oligo-array), based on whole-genome shotgun (WGS) sequencing and large-scale expressed sequence tags (ESTs) data. In addition, we also introduce an initial 44K PBT oligo-array experiment using in vitro grown peripheral blood leukocytes (PBLs) stimulated with immunostimulants such as lipopolysaccharide (LPS: a cell wall component of Gram-negative bacteria) or polyinosinic:polycytidylic acid (poly I:C: a synthetic mimic of viral infection). This pilot 44K PBT oligo-array analysis successfully addressed distinct immune processes between LPS- and poly I:C- stimulated PBLs. Thus, we expect that this oligo-array will provide an excellent opportunity to analyze global gene expression profiles for a better understanding of diseases and stress, as well as for reproduction, development and influence of nutrition on tuna aquaculture production.

Innate Multigene Family Memories Are Implicated in the Viral-Survivor Zebrafish Phenotype

Since adaptive features such as memory were discovered in mammalian innate immunity, interest in the immunological status of primitive vertebrates after infections has grown. In this context, we used zebrafish (Danio rerio), a primitive vertebrate species suited to molecular and genetic studies to explore transcriptional memories of the immune system in long-term survivors of viral haemorrhagic septicemia virus infections. Immune-gene targeted microarrays designed in-house, multipath genes, gene set enrichment, and leading-edge analysis, reveal unexpected consistent correlations between the viral-survivor phenotype and several innate multigene families. Thus, here we describe in survivors of infections the upregulation of the multigene family of proteasome subunit macropains, zebrafish-specific novel gene sets, mitogen activated protein kinases, and epidermal growth factor. We also describe the downregulation of the multigene families of c-reactive proteins, myxovirus-induced proteins and novel immunoglobulin-type receptors. The strength of those immunological memories was reflected by the exceptional similarity of the transcriptional profiles of survivors before and after re-infection compared with primary infected fish. On the other hand, the high levels of neutralizing antibodies in the blood plasma of survivors contrasted with the depletion of transcripts specific for most cell types present in lymphoid organs. Therefore, long-term survivors maintained unexpected molecular/cellular memories of previous viral encounters by modulating the expression levels of innate multigene families as well as having specific adaptive antibodies. The implications of the so-called "trained immunity" for future research in this field are also discussed.

Sensors of Infection: Viral Nucleic Acid PRRs in Fish

Viruses produce nucleic acids during their replication, either during genomic replication or transcription. These nucleic acids are present in the cytoplasm or endosome of an infected cell, or in the extracellular space to be sensed by neighboring cells during lytic infections. Cells have mechanisms of sensing virus-generated nucleic acids; these nucleic acids act as flags to the cell, indicating an infection requiring defense mechanisms. The viral nucleic acids are called pathogen-associated molecular patterns (PAMPs) and the sensors that bind them are called pattern recognition receptors (PRRs). This review article focuses on the most recent findings regarding nucleic acids PRRs in fish, including: Toll-like receptors (TLRs), RIG-I-like receptors (RLRs), cytoplasmic DNA sensors (CDSs) and class A scavenger receptors (SR-As). It also discusses what is currently known of the downstream signaling molecules for each PRR family and the resulting antiviral response, either type I interferons (IFNs) or pro-inflammatory cytokine production. The review highlights what is known but also defines what still requires elucidation in this economically important animal. Understanding innate immune systems to virus infections will aid in the development of better antiviral therapies and vaccines for the future.

Mx proteins: antiviral gatekeepers that restrain the uninvited

Fifty years after the discovery of the mouse Mx1 gene, researchers are still trying to understand the molecular details of the antiviral mechanisms mediated by Mx proteins. Mx proteins are evolutionarily conserved dynamin-like large GTPases, and GTPase activity is required for their antiviral activity. The expression of Mx genes is controlled by type I and type III interferons. A phylogenetic analysis revealed that Mx genes are present in almost all vertebrates, usually in one to three copies. Mx proteins are best known for inhibiting negative-stranded RNA viruses, but they also inhibit other virus families. Recent structural analyses provide hints about the antiviral mechanisms of Mx proteins, but it is not known how they can suppress such a wide variety of viruses lacking an obvious common molecular pattern. Perhaps they interact with a (partially) symmetrical invading oligomeric structure, such as a viral ribonucleoprotein complex. Such an interaction may be of a fairly low affinity, in line with the broad target specificity of Mx proteins, yet it would be strong enough to instigate Mx oligomerization and ring assembly. Such a model is compatible with the broad "substrate" specificity of Mx proteins: depending on the size of the invading viral ribonucleoprotein complexes that need to be wrapped, the assembly process would consume the necessary amount of Mx precursor molecules. These Mx ring structures might then act as energy-consuming wrenches to disassemble the viral target structure.

Innate immune responses of salmonid fish to viral infections

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

Immunity to betanodavirus infections of marine fish

Betanodaviruses cause viral nervous necrosis in numerous fish species, but some species are resistant to infection by these viruses. It is essential to fully characterize the immune responses that underlie this protective response. Complete characterization of the immune responses against nodaviruses may allow the development of methods that stimulate fish immunity and of an effective betanodavirus vaccine. Such strategies could include stimulation of specific immune system responses or blockage of factors that decrease the immune response. The innate immune system clearly provides a front-line defense, and this includes the production of interferons and other cytokines. Interferons that are released inside infected cells and that suppress viral replication may be the most ancient form of innate immunity. This review focuses on the immune responses of fish to betanodavirus infection.

Antiviral specificity of the Solea senegalensis Mx protein constitutively expressed in CHSE-214 cells

Interferons play a key role in fish resistance to viral infections by inducing the expression of antiviral proteins, such as Mx. The aim of the present study was to test the antiviral activity of the Senegalese sole Mx protein (SsMx) against RNA and DNA viruses pathogenic to fish, i.e. the infectious pancreatic necrosis virus (IPNV, dsRNA), the viral haemorrhagic septicaemia virus (VHSV, ssRNA), and the European sheatfish virus (ESV, dsDNA), using a CHSE-214 cell clone expressing this antiviral protein. A strong inhibition of IPNV and VHSV replication was recorded in SsMx-expressing cells, as has been shown by the virus yield reduction and the decrease in the synthesis of the viral RNA encoding the polyprotein (for IPNV) and the nucleoprotein (for VHSV). The titres of these viruses replicating on SsMx-expressing cells were 100 times lower than those recorded on non-transfected cells. In contrast, SsMx did not inhibit ESV replication since no significant differences were observed regarding the virus yield or the major capsid protein gene transcription in transfected and non-transfected cells.

Ontogeny of anti-viral hemorrhagic septicemia virus (VHSV) immunity in developing Japanese flounder

We examined the ability of developing Japanese flounder (Paralichthys olivaceus) to acquire protective immunity after exposure to viral hemorrhagic septicemia virus (VHSV). Juveniles measuring 9.8 cm average body length were not susceptible to infection with VHSV at 20 °C, while the smaller fish were susceptible. Mortality was not observed after secondary infection at 15 °C in the 9.8 cm cohort that had previously been exposed to the virus at 20 °C, while the smaller fish were susceptible to secondary infection. The expression of interferon (IFN)-related genes was shown to be better developed in larger fish upon virus infection and basal expression levels of the virus recognition proteins were higher in larger fish. Virus-specific antibody was detected in the larger fish, but not in smaller fish. These data indicate that the largest juvenile (9.8 cm) acquired immunity against VHSV infection at the first virus challenge, but smaller fish did not. The anti-viral immune system in the Japanese flounder matures when juveniles reach approximately 10 cm.

Nervous necrosis virus replicates following the embryo development and dual infection with iridovirus at juvenile stage in grouper

Infection of virus (such as nodavirus and iridovirus) and bacteria (such as Vibrio anguillarum) in farmed grouper has been widely reported and caused large economic losses to Taiwanese fish aquaculture industry since 1979. The multiplex assay was used to detect dual viral infection and showed that only nervous necrosis virus (NNV) can be detected till the end of experiments (100% mortality) once it appeared. In addition, iridovirus can be detected in a certain period of rearing. The results of real-time PCR and in situ PCR indicated that NNV, in fact, was not on the surface of the eggs but present in the embryo, which can continue to replicate during the embryo development. The virus may be vertically transmitted by packing into eggs during egg development (formation) or delivering into eggs by sperm during fertilization. The ozone treatment of eggs may fail to remove the virus, so a new strategy to prevent NNV is needed.

Differential expression profiling of orange-spotted grouper larvae, Epinephelus coioides (Hamilton), that survived a betanodavirus outbreak

Nervous necrosis virus (NNV), a piscine nodavirus, has caused serious viral nervous necrosis and viral encephalopathy and retinopathy in hatchery-reared larvae and juveniles of a wide range of marine teleost species worldwide in the last two decades. Although the mortality of NNV-infected larvae is nearly 100%, there are still some larvae that survive this catastrophe. To comprehensively understand the variations of these survivors at the molecular level, we collected orange-spotted grouper larvae that survived an NNV outbreak in an indoor hatchery in southern Taiwan to study differential gene expression. Healthy larvae with high, medium and low levels of detected NNV were compared with morbid larvae using a 9600-clone-containing grouper larva cDNA microarray, and differential gene expression was further confirmed by a quantitative real-time polymerase chain reaction. Significant variation exists in healthy larvae. The following genes were upregulated: adenylate kinase 1-2, myosin binding protein H-like, myosin light chain 2, myosin light chain 3, tropomyosin, fast/white muscle troponin T embryonic isoform, and parvalbumin 1 and 2 genes. The following genes were downregulated: apolipoprotein A-I, trypsinogen, pyruvate kinase and astacin-like metalloprotease. Moreover, immunoglobulin M heavy chain gene transcription was significantly higher in healthy larvae that had high virus levels, indicating that humoral immunity might protect organisms from viral infection. These results suggest that some non-immune-related genes may have played important roles in survival during the larval metamorphosis stage, after betanodavirus infection.

Immune-regulatory transcriptional responses in multiple organs of Atlantic salmon after tributyltin exposure, alone or in combination with forskolin

Tributyltin (TBT) is a widespread marine pollutant that influences physiological conditions of fish and other aquatic organisms. In addition to effects on reproduction, the immune system has been proposed as a possible target for TBT effects. In the present study, the effects of TBT exposure were examined on the expression of genes involved in immune system compentence in liver and head kidney of Atlantic salmon, in the presence and absence of a second-messenger activator (forskolin). Juvenile salmon were force-fed a diet containing TBT (0-solvent control, 0.1, 1, or 10 mg/kg fish) for 72 h. Consequently, fish from the control group and 10-mg/kg TBT group were exposed to the adenylate cyclase (AC) activator forskolin (200 μg/L) for 2 or 4 h. Forskolin was selected for this study because it is known to exhibit potent immune system enhancement by activating macrophages and lymphocytes. After sacrifice, liver and head kidney were sampled and transcript changes for interleukin (IL)-1β, IL-10, transforming growth factor (TGF) β, interferon (INF) α, INFγ, tumor necrosis factor (TNF) α, Mx3, and insulin-like growth factor (IGF)-1 were determined in both tissues by quantitative polymerase chain reaction (qPCR) using gene-specific primers. TBT, when given alone and also in combination with forskolin, decreased IL-1β, TNFα, IFNγ, IFNα, Mx3, and IGF-1 gene expression. In contrast, IL-10 and TGFβ transcripts were increased after TBT exposure alone and also in combination with forskolin. Generally, these effects were largely dependent on TBT dose and time of exposure when given in combination with forskolin. Overall, our findings suggest a possible immunomodulatory effect of TBT, possibly involving cAMP activation.

Evolution of duplicated IgH loci in Atlantic salmon, Salmo salar

BACKGROUND

The Atlantic salmon (Salmo salar) immunoglobulin heavy chain (IgH) locus possesses two parallel IgH isoloci (IGH-A and IGH-B), that are related to the genomic duplication event in the family Salmonidae. These duplicated IgH loci in Atlantic salmon provide a unique opportunity to examine the mechanisms of genome diversity and genome evolution of the IgH loci in vertebrates. In this study, we defined the structure of these loci in Atlantic salmon, and sequenced 24 bacterial artificial chromosome (BAC) clones that were assembled into the IGH-A (1.1 Mb) and IGH-B (0.9 Mb) loci. In addition, over 7,000 cDNA clones from the IgH variable (VH) region have been sequenced and analyzed.

RESULTS

The present study shows that the genomic organization of the duplicated IgH loci in Atlantic salmon differs from that in other teleosts and other vertebrates. The loci possess multiple Cτ genes upstream of the Cμ region, with three of the Cτ genes being functional. Moreover, the duplicated loci possess over 300 VH segments which could be classified into 18 families. This is the largest number of VH families currently defined in any vertebrate. There were significant structural differences between the two loci, indicating that both IGH-A and -B loci have evolved independently in the short time after the recent genome duplication approximately 60 mya.

CONCLUSIONS

Our results indicate that the duplication of the IgH loci in Atlantic salmon significantly contributes to the increased diversity of the antibody repertoire, as compared with the single IgH locus in other vertebrates.

Cellular and molecular immune responses of the sea bass (Dicentrarchus labrax) experimentally infected with betanodavirus

Naïve sea bass juveniles (38.4 + or - 4.5 g) were intramuscularly infected with a sublethal dose of betanodavirus isolate 378/I03, followed after 43 days by a similar boosting. This infection resulted in an overall mortality of 7.6%. At various intervals, sampling of fish tissues was performed to investigate: i) B and T lymphocyte content in organs and tissues; ii), proliferation of leucocytes re-stimulated in vitro with inactivated virus; iii) presence of serum antibody specific for betanodavirus; iv) expression of genes coding for the following immunoregulatory molecules involved in innate and acquired responses: type I IFN, Mx, IL-1, Cox-2; IL-10, TGF-beta, TCRbeta, CD4, CD8alpha, IgM, by using a quantitative PCR array system developed for sea bass. The obtained results showed a detectable increase of T cells and B cells in PBL during betanodavirus infection. Furthermore, leucocytes obtained from blood, head kidney, and gills showed a detectable "in vitro" increase in viability upon addition of inactivated viral particles, as determined by measuring intracellular ATP concentration. ELISA analysis of sera showed that exposure to nodavirus induced a low, but specific antibody titer measured 43 days after infection, despite the presence of measurable levels of natural antibody. Finally, a strong upregulation of genes coding for type I IFN, Mx, and IgM was identified after both infection and boosting. Interestingly, an upregulation of Cox-2 until boosting, and of TGF-beta and IL-10 after boosting was also observed, while the other tested genes did not show any significant variations with respect to mock-treated fish. Overall, our work represents a first comprehensive analysis of cellular and molecular immune parameters in a fish species exposed to a pathogenic virus.

Cell markers and determinants in fish immunology

Despite the impressive increase in the cloning and expression of genes encoding fish immunoregulatory molecules, the knowledge on "in vivo" and "in vitro" functional immunology of the corresponding peptide products is still at an initial stage. This is partly due to the lacking of specific markers for immunoregulatory peptides, that represent an indispensible tool to dissect immune reactions and to trace the fate of cellular events downstream of the activation. In this review we summarise the available information on functional immune activities of some teleost species and discuss the obtained data in an evolutionary and applied context.

Poly I:C induces Mx transcription and promotes an antiviral state against sole aquabirnavirus in the flatfish Senegalese sole (Solea senegalensis Kaup)

Mx is an interferon-induced protein that protects against viral infections. In this study the absolute number of Mx transcripts after poly I:C injection (a synthetic dsRNA) or sole aquabirnavirus (solevirus) inoculation in Senegalese sole (Solea senegalensis Kaup) has been quantified. Mx expression profiles differed clearly in both experimental conditions; the induction response was faster and more intense after poly I:C injection than after solevirus inoculation. Moreover, pre-injection of soles with poly I:C prior to solevirus infection eliminated the induction of Mx expression associated with this virus. To evaluate the possible interference of poly I:C treatments on solevirus replication, the mRNA levels of the virus capsid protein (VP2) were determined by RT-PCR. VP2 transcripts were hardly detected in poly I:C pre-injected animals from 12 to 72 h after solevirus inoculation. All these data suggest that poly I:C is able to induce an antiviral state that interferes with solevirus replication, and support the suitability of Mx expression analysis as a marker to study the defensive response against solevirus.

Total substitution of fish oil by vegetable oils in gilthead sea bream (Sparus aurata) diets: effects on hepatic Mx expression and some immune parameters

The use of vegetable oils in fish nutrition has been extensively studied; and recent work has focused attention on replacing fish oil with alternative fatty acid sources and their effect on the immune system. However, little is known about the effect of these oils on immune parameters such as the fish interferon system. In this study we evaluate the effect of two vegetable oils (linseed and soybean) on gilthead sea bream Mx expression and other innate immune parameters. Experimental diets were formulated where fish oil was totally replaced by vegetable oils or for a mixture of them (50% linseed and 50% soybean). Another diet prepared with pure fish oil was used as a control. Two experiments were carried out in order to evaluate growth, feed utilization, serum alternative complement pathway activity, serum lysozyme and phagocytic activity of head kidney leucocytes as well as Mx expression in the liver. In the first experiment fish were fed with experimental diets for 6 months and then, growth and feed utilization as well as immune parameters were analyzed. In the second experiment, fish from the previous feeding trial were injected with either a sub-lethal dose of Photobacterium damselae subsp. piscicida (94/99) or a synthetic dsRNA (Poly I:C) in order to stimulate an Mx response. The results show that total substitution of fish oil by vegetable oils decreased the growth of gilthead sea bream juveniles. Furthermore, both phagocytic activity and serum alternative complement pathway activity were significantly reduced by the inclusion of either vegetable oil individually in the sea bream diets, but the diet with mixed vegetable oils had no significant effect. There was no effect on serum lysozyme levels but the basal constitutive levels of Mx transcript expression in the liver were elevated in the fish fed the vegetable oil diets. The time-course of the Mx response to injection of Poly I:C was shorter in the fish fed the fish oil diet and the fish fed the diet based on a mixture of both vegetable oils showed a faster Mx response to bacterial injection. Following stimulation with Poly I:C or PDP the fish fed the vegetable oil based diets still maintained higher basal levels of hepatic Mx expression than the fish fed the fish oil diet which returned to undetectable levels.

Grouper Mx confers resistance to nodavirus and interacts with coat protein

Over-expression of grouper Mx negatively regulated nodavirus activity through direct interaction, likely via the binding and perturbation of the intracellular localization of nodavirus coat protein. Deletion analysis of grouper Mx indicated that the coat protein binds to the effector domain of Mx. The presence of grouper Mx in a poly [I:C] interferon system inhibited nodavirus infection, demonstrating that grouper Mx over-expression has an inhibitory effect on both coat protein and RNA-dependent RNA polymerase of nodavirus antigens, which results in reduced viral yields. We conclude that grouper Mx has a key role in cellular resistance to nodavirus infection.

The innate immune response of finfish--a review of current knowledge

The decline in the fisheries of traditional marine species has been an incentive for the diversification of today's aquaculture sector into the intensive rearing of many finfish species. The increasing interest in commercial farming of different finfish species is expected to result in similar environmental and husbandry-related problems as have been experienced in the development of the salmonid farming industry. An understanding of the biology of the fish species being cultured, in particular the immune response is important for improved husbandry and health management of the species. The innate immune system of fish has generated increasing interest in recent years and is now thought to be of key importance in primary defence and in driving adaptive immunity. This review focuses on key components (cellular and humoral) of the innate immune responses of different fish species of commercial importance.

Induction of antiviral genes, Mx and vig-1, by dsRNA and Chum salmon reovirus in rainbow trout monocyte/macrophage and fibroblast cell lines

The expression of potential antiviral genes, Mx1, Mx2, Mx3 and vig-1, was studied in two rainbow trout cell lines: monocyte/macrophage RTS11 and fibroblast-like RTG-2. Transcripts were monitored by RT-PCR; Mx protein by Western blotting. In unstimulated cultures Mx1 and vig-1 transcripts were seen occasionally in RTS11 but rarely in RTG-2. A low level of Mx protein was seen in unstimulated RTS11 but not in RTG-2. In both cell lines, Mx and vig-1 transcripts were induced by a dsRNA, poly inosinic: poly cytidylic acid (poly IC), and by Chum salmon reovirus (CSV). Medium conditioned by cells previously exposed to poly IC or CSV and assumed to contain interferon (IFN) induced the antiviral genes in RTS11. However, RTG-2 responded only to medium conditioned by RTG-2 exposed previously to CSV. In both cell lines, poly IC and CSV induced Mx transcripts in the presence of cycloheximide, suggesting a direct induction mechanism, independent of IFN, was also possible. For CSV, ribavirin blocked induction in RTS11 but not in RTG-2, suggesting viral RNA synthesis was required for induction only in RTS11. In both RTS11 and RTG-2 cultures, Mx protein showed enhanced accumulation by 24h after exposure to poly IC and CSV, but subsequently Mx protein levels declined back to control levels in RTS11 but not in RTG-2. These results suggest that Mx can be regulated differently in macrophages and fibroblasts.

Salmonid microarrays identify intestinal genes that reliably monitor P deficiency in rainbow trout aquaculture

Nutrient-responsive genes can identify important metabolic pathways and evaluate optimal dietary levels. Using a 16K Salmo salar microarray, we identified in rainbow trout (Oncorhynchus mykiss) 21 potential phosphorus (P)-responsive genes, mainly involved in immune response, proteolysis or transport, whose expression levels changed in the intestine after 5 days of feeding a low-P (LP) diet. Diet-induced changes in the expression levels of several genes in each fish were tightly correlated with changes in serum P, and the changes persisted for an additional 15 days after dietary P deficiency. We then evaluated these and previously identified P-responsive genes under simulated farm conditions, and monitored the intestinal gene expression from 6 h to 7 days after the trout were switched from a sufficient-P (SP) diet to a LP diet (SP-->LP), and from a LP diet to a SP diet (LP-->SP). After 7 days, mean serum P decreased 0.14 mM/day for SP-->LP and increased 0.10 mm/day for LP-->SP. The mRNA abundance of the metalloendopeptidase meprin 1alpha (MEP1alpha), the Na(+)-dependent phosphate co-transporter (NaPi2b,SLC34A2), the sulfotransferase SULT2beta1 and carbonic anhydrase XIII genes all increased after SP-->LP and decreased after LP-->SP, suggesting that adaptive expression is reversible and correlated with dietary P. The duration of change in gene expression in response to SP-->LP was generally shorter than that of LP-->SP, suggesting potentially different mechanisms of adaptation to deficiency as opposed to excess. Diet-induced changes in mRNA abundance of other genes were either transient or modest. We identified, by heterologous microarray hybridization, new genes sensitive to perturbations in dietary P, and then showed that these genes can reliably monitor P deficiency under field conditions. Simultaneous changes in the expression of these P biomarkers could predict either P deficiency (to prevent economic losses to the farmers) or P excess (to prevent inadvertent pollution of nearby waters).

Molecular characterisation and inductive expression of a fish protein arginine methyltransferase 1 gene in response to virus infection

Protein arginine methyltransferase 1 (PRMT1) is currently thought as an effector to regulate interferon (IFN) signalling. Here Paralichthys olivaceus PRMT1 (PoPRMT1) gene was identified as a virally induced gene from UV-inactivated Scophthalmus maximus Rhabdovirus (SMRV)-infected flounder embryonic cells (FEC). PoPMRT1 encodes a 341-amino-acid protein that shares the conserved domains including post-I, motif I, II and III. Homology comparisons show that the putative PoPMRT1 protein is the closest to zebrafish PMRT1 and belongs to type I PRMT family (including PRMT1, PRMT2, PRMT3, PRMT4, PRMT6, PRMT8). Expression analyses revealed an extensive distribution of PoPMRT1 in all tested tissues of flounder. In vitro induction of PoPRMT1 was determined in UV-inactivated SMRV-infected FEC cells, and under the same conditions, flounder Mx was also transcriptionally up-regulated, indicating that an IFN response might be triggered. Additionally, live SMRV infection of flounders induced an increased expression of PoPRMT1 mRNA and protein significantly in spleen, and to a lesser extent in head kidney and intestine. Immunofluorescence analysis revealed a major cyptoplasmic distribution of PoPRMT1 in normal FEC but an obvious increase occurred in nucleus in response to UV-inactivated SMRV. This is the first report on in vitro and in vivo expression of fish PRMT1 by virus infection, suggesting that PoPRMT1 might be implicated in flounder antiviral immune response.

Characterization of three distinct CpG oligonucleotide classes which differ in ability to induce IFN alpha/beta activity and cell proliferation in Atlantic salmon (Salmo salar L.) leukocytes

Oligodeoxynucleotides (ODNs) containing unmethylated CpG dinucleotides within specific sequence contexts (CpG motifs) are in humans divided into three distinct classes (A, B and C). The CpG ODNs, like baterial DNA, are recognized by the vertebrate immune system and are known to stimulate immune responses. The characterization of the different classes is based on their structure and biological activity. In this study, we report the effects of these classes of CpG ODNs as they are defined in humans on IFN alpha/beta production and cell proliferation in Atlantic salmon spleen, head kidney and blood leukocytes. These studies revealed that CpG A together with CpG C are strong inducers of IFN alpha/beta in spleen and head kidney leukocytes, whilst CpG B and CpG C had the highest capacity to stimulate cell proliferation in all three cell populations. These findings are the first to establish that the effects of the different CpG classes are comparable between fish and man.

Inhibition of nervous necrosis virus propagation by fish Mx proteins

Mx proteins are interferon induced, antiviral proteins, expressed in response to treatment with double stranded RNA or virus infection. Here we report the cloning, sequencing, and antiviral property of three forms of Mx genes, MxI, MxII, and MxIII from grouper (Epinephelus coioides). Multiple comparison of grouper Mx amino acid sequences revealed the conservation of Mx putative GTP-binding domain, dynamin family signature, and leucine zipper motif. We have established a new cell line, grouper brain 3 (GB3), and prepared stable clones expressing Flag-epitope tagged grouper MxI, MxII, and MxIII. Immunostaining shows that all the three grouper Mx proteins are localized in the cytoplasm. To examine the antiviral activity of grouper Mx proteins, these stable clones were infected by a nodavirus, yellow grouper nervous necrosis virus (YGNNV), and the results showed that all the three Mx isoforms have the efficiency of reducing the titre of virus from 10- to 100-fold. Moreover, through immunocytochemistry we demonstrated that Mx protein can inhibit the YGNNV propagation in GB3 cells. Taken together, this study demonstrates that grouper Mx proteins have efficient inhibitory activity against nodavirus, the most endangered virus of fish, and this information would be helpful to design effective DNA vaccines that can confer an early non-specific antiviral protection.

Persistence of betanodavirus in Barramundi brain (BB) cell line involves the induction of Interferon response

The BB cell line derived from the brain tissue of a barramundi (Lates calcarifer) that survived nervous necrosis virus (NNV) infection is persistently infected with NNV. To elucidate whether interferon (IFN) plays a role in the mechanism of NNV-persistent infection in BB cell line, a virus-negative control cell line was obtained by treating BB cells with NNV-specific rabbit antiserum for 5 subcultures. After the treatment, NNV titer or RNA or capsid protein was no longer detected in the cured BB (cBB) cells. Expression of Mx gene, encoding a type I IFN-inducible antiviral protein, was found in BB cells and cBB cells following NNV infection, but not in NNV-free cBB cells. Moreover, expression of Mx gene and antiviral activity against NNV were induced in cBB cells by the treatment with MAb-neutralized BB cell supernatant. Furthermore, NNV persistent infection was induced again in cBB cell culture if multiplicity of infection (MOI) was low (< or = 1). These experimental results indicated that IFN-like cytokines existed in the culture supernatant of BB cells, and IFN-induced response played an important role in protecting the majority of cells from virus lytic infection and regulating NNV persistence in the BB cell line.

Functional characterisation of the Japanese flounder, Paralichthys olivaceus, Mx promoter

The Japanese flounder, Paralichthys olivaceus, genome appears to encode a single Mx gene based on Southern blotting and previous cDNA studies. The 5' flanking region of the Japanese flounder Mx gene was cloned and analysed for its regulatory regions. A TATA box (-24 to -30), two interferon-stimulated response elements (ISREs) (-69 to -80 and -508 to -521) and two Sp1 sites (-563 to -572 and -994 to -1003) were identified relative to the transcription start site. The effects of various stimuli, as well as the effects of various promoter mutations, were investigated in a transient expression system using Japanese flounder (hirame) natural embryo (HINAE) cells and luciferase reporter gene constructs. Although not sensitive to LPS, ConA or PMA, reporter gene expression increased more than 10-fold after stimulation by polyinosinic:polycytidilic acid (poly I:C), an established inducer of interferon. Deletion mutational analyses revealed the ISRE closest to the transcription start site to be crucial for promoter activity. The distal ISRE, despite its relatively distant location, contributed to induce maximal promoter activity, but when alone was not sufficient by itself to elicit any significant promoter activity. An electrophoretic mobility shift assay confirmed the binding of transcription factors to both ISREs. Induction of luciferase by poly I:C was inhibited by 2-Aminopurine, a protein kinase (PKR) inhibitor, in a dose-dependent (1-10 mM) manner, suggesting that PKR may be required as a signal transducer for type I IFN signaling in fish. This Mx reporter assay may be useful for quantifying the responses and elucidating the regulation pathways of IFN type I.

Sequestration and protection of double-stranded RNA by the betanodavirus b2 protein

Betanodavirus B2 belongs to a group of functionally related proteins from the sense-strand RNA virus family Nodaviridae that suppress cellular RNA interference. The B2 proteins of insect alphanodaviruses block RNA interference by binding to double-stranded RNA (dsRNA), thus preventing Dicer-mediated cleavage and the subsequent generation of short interfering RNAs. We show here that the fish betanodavirus B2 protein also binds dsRNA. Binding is sequence independent, and maximal binding occurs with dsRNA substrates greater than 20 bp in length. The binding of B2 to long dsRNA is sufficient to completely block Dicer cleavage of dsRNA in vitro. Protein-protein interaction studies indicated that B2 interacts with itself and with other dsRNA binding proteins, the interaction occurring through binding to shared dsRNA substrates. Induction of the dsRNA-dependent interferon response was not antagonized by B2, as the interferon-responsive Mx gene of permissive fish cells was induced by wild-type viral RNA1 but not by a B2 mutant. The induction of Mx instead relied solely on viral RNA1 accumulation, which is impaired in the B2 mutant. Hyperediting of virus dsRNA and site-specific editing of 5-HT2C mRNA were both antagonized by B2. RNA editing was not, however, observed in transfected wild-type or B2 mutant RNA1, suggesting that this pathway does not contribute to the RNA1 accumulation defect of the B2 mutant. We thus conclude that betanodavirus B2 is a dsRNA binding protein that sequesters and protects both long and short dsRNAs to protect betanodavirus from cellular RNA interference.

Zebrafish (Danio rerio) as a model for the study of vaccination against viral haemorrhagic septicemia virus (VHSV)

The rhabdovirus viral haemorrhagic septicemia virus (VHSV) is the etiological agent of one of the most important salmonid viral diseases. In the present work, the ability of VHSV to infect and replicate in zebrafish at low temperature (15 degrees C) was demonstrated. Zebrafish was also used to determine the effectiveness of the recombinant virus rIHNV-Gvhsv GFP as a live attenuated vaccine against the virulent VHSV strain. Fish intraperitoneally injected with 3 x 10(6) to 3 x 10(5)TCID50/ml of the wild type VHSV showed a 100% of cumulative mortality, meanwhile only 57% of mortality was obtained in bath infections. Infected fish showed external clinical signs and histological observations revealed the appearance of small haemorrhages in the muscle, kidney, liver and dermis. Neither mortalities nor clinical signs were recorded in fish infected with a live attenuated recombinant virus. By RT-PCR technique, VHSV was detected in all the organs as early as 24h, but the recombinant virus was not detected in all the sampled days. VHSV was able to replicate "in vitro" in head kidney cells but the replication capacity of the attenuated viral strain was limited. The recombinant virus rIHNV-Gvhsv GFP was able to protect against VHSV with a survival rate ranging from 20% to 60% depending of the vaccine dose. The increase of TLR3, IFNalphabeta, Mx, IFNgamma and TNFalpha expression at 72h post-infection in the kidney of VHSV-infected fish contrasted with the results obtained with the avirulent virus, which did not induce an increment of this expression in infected fish. Zebrafish is a suitable animal model to study VHSV infection and immune (innate and adaptive) responses and, more importantly, we demonstrate for the first time the usefulness of the zebrafish as a vaccination model to viral diseases. In addition, the high protection obtained with the live attenuated virus demonstrates that the zebrafish is able to mount an efficient antiviral immune response at 15 degrees C.

Stimulation of type I IFN activity in Atlantic salmon (Salmo salar L.) leukocytes: synergistic effects of cationic proteins and CpG ODN

Unmethylated CpG motifs in DNA are recognised by vertebrate immune cells as pathogen signatures. Consequently, oligodeoxynucleotides containing CpG motifs (CpG ODNs) are able to enhance and direct immune responses. Recent studies have demonstrated that CpG ODNs activate antiviral immune responses in Atlantic salmon (Salmo salar L.) leukocytes, and are therefore promising agents as vaccine adjuvants or immunostimulants in fish. In this work, we report synergy of CpG ODN and cationic proteins in the stimulation of type I IFN activity in Atlantic salmon leukocytes. Different fractions of cationic histone proteins derived from cod milt or poly-l-arginine and poly-l-lysine were screened for their ability to enhance CpG ODN induced type I IFN activity in Atlantic salmon leukocytes. Optimal ratio of histones to CpG ODN was identified, and effects on transcription of type I IFN and antiviral Mx genes were studied. Delivery of CpG ODN with cationic proteins enhanced the production of type I IFN and succeeding Mx transcripts after two and five days of stimulation at substimulatory concentrations of CpG ODN. These results indicate that co-delivery of CpG ODN and cationic proteins enhance antiviral mechanisms in Atlantic salmon leukocytes as compared to CpG ODN alone.

Innate immunity of fish (overview)

The innate immune system is the only defence weapon of invertebrates and a fundamental defence mechanism of fish. The innate system also plays an instructive role in the acquired immune response and homeostasis and is therefore equally important in higher vertebrates. The innate system's recognition of non-self and danger signals is served by a limited number of germ-line encoded pattern recognition receptors/proteins, which recognise pathogen associated molecular patterns like bacterial and fungal glycoproteins and lipopolysaccharides and intracellular components released through injury or infection. The innate immune system is divided into physical barriers, cellular and humoral components. Humoral parameters include growth inhibitors, various lytic enzymes and components of the complement pathways, agglutinins and precipitins (opsonins, primarily lectins), natural antibodies, cytokines, chemokines and antibacterial peptides. Several external and internal factors can influence the activity of innate immune parameters. Temperature changes, handling and crowding stress can have suppressive effects on innate parameters, whereas several food additives and immunostimulants can enhance different innate factors. There is limited data available about the ontogenic development of the innate immunological system in fish. Active phagocytes, complement components and enzyme activity, like lysozyme and cathepsins, are present early in the development, before or soon after hatching.

Cloning of an orange-spotted grouper (Epinephelus coioides) Mx cDNA and characterisation of its expression in response to nodavirus

Molecular cloning and nucleotide sequencing of cDNA encoding an orange-spotted grouper (Epinephelus coioides) homolog of Mx ("OsgMx") was conducted and its possible role in fish immunity was analysed. Similar to mammalian Mx, the OsgMx are members of a family of interferon-inducible genes that are expressed by cells in response to nodavirus and iridovirus naturally-infected. Expression of OsgMx mRNA was noticeably upregulated in all tissues by nodavirus naturally-infected grouper. The transcription of OsgMx gene increased 6 h after intramuscular injection of nodavirus experimentally-infected fish and peaked at 72 h in their brains. Analysis of the 5'-flanking sequence of the gene shows that as in pufferfish and zebrafish, the OsgMx promoter contains two potential interferon-stimulated response element (ISRE) responsible for the induction of interferon-inducer polyinosinic-polycytidylic acid (Poly[I:C]). Transient transfection of grouper cells in gfp-reporter gene assays shows that the activation of the grouper Mx promoter fragment by Poly[I:C] is sufficient to allow the expression of green fluorescent protein (GFP). These results may provide a possible regulated pathway against nodavirus.