Induction of Mx protein by interferon and double-stranded RNA in salmonid cells

Structural insights of Labeo catla (catla) myxovirus resistance protein,GTP binding recognition and constitutive expression induced with Poly I:C

The Myxovirus resistance (Mx) proteins are critical effectors belonging to the super-family of guanidine triphosphatase, often stimulated by type I interferon (IFN) and mediates antiviral responses to restrict the replication of numerous viral genes in fishes. In teleosts, Mx proteins display diverse and complicated antiviral activity in different species. The present investigation seeks to characterize the Mx gene from Labeo catla upon induction by double-stranded (ds) RNA, polyinosinic-polycytidylic acid, (poly I: C). Molecular modeling and all-atoms molecular dynamics (MD) simulations were employed to understand the architecture of the GTPase domain and its plausible mode of GTP recognition in Mx protein. The full-length L. catla Mx (LcMx) gene sequence (1821 bp nucleotides) encodes an open reading frame of 606 amino acids. Domain search indicated conserved tripartite domain architecture of LcMx and forms a major cluster with the Mx from other teleosts. The positively charged Arginine and polar Glutamine residues from helix 3 and 4 of stalk region LcMx aid in homo-oligomerization. MD simulation portrayed the role of conserved critical residues aid in GTP recognition by the GTPase domain which perfectly corroborates with experimental findings and prior MD studies. After injection of poly I:C, the temporal mRNA profile showed that LcMx expression was significantly elevated in the spleen, brain, kidney, liver, muscle, heart, intestine, and gill tissues. Collectively, these results suggest that the elevated expression of the major innate immune defense gene Mx was able to inhibit the poly I: C mediated virulence in fish.Communicated by Ramaswamy H. Sarma.

Myxovirus resistance (Mx) gene and its differential expression regulated by three type I and two type II IFNs in mandarin fish, Siniperca chuatsi

Interferons (IFNs) can induce the expression of IFN-stimulated genes (ISGs), such as myxovirus resistance (Mx) protein, to inhibit virus replication. In this study, the expression of Mx gene in mandarin fish, and the IFN-sensitive response elements (ISREs) and gamma-interferon activated sites (GASs) in the promoter of Mx gene were analyzed in relation to the stimulation of three distinct type I IFNs, IFNc, IFNd and IFNh, and two type II IFNs, IFN-γ and IFN-γ related molecule (IFN-γrel). A single Mx gene was found in mandarin fish, and its expression was highly and constitutively observed in all organs/tissues examined. The Mx gene was significantly induced in vivo for 120 h following infectious spleen and kidney necrosis virus (ISKNV) infection. Furthermore, the overexpression and recombinant of IFNh, IFNc, as well as IFN-γ can significantly induce Mx expression in MFF-1 cells at transcript and protein levels, although all the three type I IFNs and the two type II IFNs can activate the Mx promoter. In addition, ISRE1 which is the proximal one among the three predicted ISREs seems to be the important ISRE for the higher and efficient activation of the Mx promoter. However, the possible interaction between the GASs and type II IFN signalling molecules require further study.

Gene expression analysis of the innate immune system during early rearing and weaning of meagre (Argyrosomus regius)

The present study is the first report of some representative innate immune genes in meagre (Argyrosomus regius) larvae. This study has specifically focused on the growth period from hatching to the juvenile stage, a critical time in marine fish development when reliance on innate immune mechanisms are required for survival. We report molecular cloning of partial open reading frames and expression patterns for some innate immune genes (c3, cox2, met, lyzc, mxp, myd88, nod2, nod3). In addition, phylogenetic analyses of some of the sequences obtained was performed where confusion among closely allied isoforms may have existed. These results show the met isoform from meagre is met II, an isoform more similar to a homolog described in Larimichthys crocea; lysozyme (lyzc) corresponds to the c-type and NOD isoforms (nod2, nod3) separate into different clades confirming their distinctness within a common evolutionary history. Gene expression profiles of innate genes were investigated, for nine developmental stages, from 8 days post-hatching (dph) to 120 dph. Present results demonstrated that c3, cox2, met II, lyzc, mxp, myd88, nod2, and nod3 were expressed in all stages of larval development and displayed distinct expression profiles in separate tissues (kidney, spleen gut and gill). Moreover, expression patterns suggested theses innate immune genes may be influenced by feeding practices, i.e. switching from live prey (rotifer and Artemia) and weaning onto an inert commercial diet. In addition to evaluating changes in gene expression during early development, this study evaluated the modulation of gene expression by means of in vivo trials in juveniles that were stimulated with PAMPs (LPS, poly I:C, β-glucan). These results revealed significant changes in mRNA levels of target genes in the kidney, spleen, gut and gills. However, expression profiles differed in magnitude depending on the stimulant and/or tissue. These results are discussed in terms of their relevance and potential application in aquaculture practices.

Transcriptome sequencing of hybrid bester sturgeon: Responses to poly (I:C) in the context of comparative immunogenomics

Sturgeons represent a substantial scientific interest due to their high economic value, endangered status and also as the most primitive group of ray-finned fishes. Rapid progress in knowledge of sturgeon immunity was achieved recently with use of RNA sequencing. We report transcriptome sequencing of gill, head kidney, and spleen of bester sturgeon (a hybrid of beluga Huso huso and sterlet Acipenser ruthenus) injected with synthetic double-stranded RNA (polyI:C). The composition of transcriptome and responses to treatment were examined in the context of comparative genomics with focus on immune genes. Sturgeon transcripts matched to 21.5 k different proteins (blastx). With reference to Atlantic salmon, the functional groups and pathways of the immune system were uniformly represented: at average 36.5 ± 0.8% genes were found. Immune genes comprise a significant fraction of transcriptome. Among twenty genes with highest transcription levels, five are specialized immune genes and two encode heme and iron binding proteins (serotransferrin and hemopexin) also known as acute phase proteins. Challenge induced multiple functional groups including apoptosis, cell cycle and a number of metabolic pathways. Treatment stimulated innate antiviral immunity, which is well conserved between sturgeon and salmon, the most responsive genes were mx, rsad2 (viperin), interferon induced protein 44 and protein with tetratricopeptide repeats 5, cd87 and receptor transporting protein 3. Results added to knowledge of immune phylogeny. Gain and loss of genes was assessed by comparison with genomes from different phylogenetic groups. Among differentially expressed genes, percentage of acquired and lost genes was much lower in comparison with genes present in all vertebrates. Innate antiviral immunity was subject to the greatest changes in evolution of jawed vertebrates. A significant fraction of genes (15%) was lost in mammals and only half of genes is annotated in public databases as involved in antiviral responses. Change of function may have an important role in evolution of immunity together with gain and loss of genes.

Molecular cloning, GTP recognition mechanism and tissue-specific expression profiling of myxovirus resistance (Mx) protein in Labeo rohita (Hamilton) after Poly I:C induction

The myxovirus resistance (Mx) proteins belong to interferon-induced dynamin GTPase and play pivotal role in the inhibition of replication of numerous viruses. These antiviral proteins are released in usual or diseased condition to prevent the viral attack and to carry regular cellular activities like endocytosis and trafficking of nucleoproteins into the nucleus. The invasion of virus up-regulates the expression of Mx transcripts and double-stranded RNA mimic like polyinosinic polycytidyilic acid (Poly I:C). To understand the tissue-specific expression profiling and mechanism of GTP recognition of Mx protein from Labeo rohita (rohu), the full-length gene was cloned, sequenced and characterized through various Bioinformatics tools for the first time. The Mx cDNA was comprised of 2297 bp, and the open reading frame of 1938 bp encodes polypeptide of 631 amino acids. The coding sequence of Mx protein possess the signature motif of dynamin superfamily, LPRG(S/K)GIVTR, the tripartite guanosine-5/triphosphate (GTP)-binding motif (GXXXSGKS/T, DXXG and T/NKXD) and the leucine zipper motifs at the C-terminal end, well conserved in all interferon-induced Mx protein in vertebrates. Western blotting confirmed the molecular weight of Mx protein to be 72 kDa. After the intraperitoneal challenge of L. rohita with a Poly I:C, up-regulation of Mx protein was observed in brain, spleen, liver, kidney, intestine, heart, muscle, and gill. Ontogeny study displayed pronounced expression of Mx protein in all stages of the developmental of Rohu after Poly I:C induction. However a persistent expression of Mx transcript was also observed in Rohu egg as well as milt without induction with Poly I:C. Higher expression of Mx gene was observed on 96 h where it was 6.4 folds higher than the control. The computational modelling of Mx protein portrayed the tripartite N-terminal G-domain that binds to GTP, the bundle-signaling element (BSE) which interconnects the G-domain to the elongated stalk domain and C-terminal helical stalk domain. In agreement with the experimental studies, a series of conserved residues viz., Gln52, Ser53, Ser54, Leu68, Pro69, Gly71, Gly73, Thr76, Asp151, Gly154, Thr220, Lys221, Val251, Cys253, Arg254, and Gly255 were computed to be indispensable for tight anchoring of GTP within binding cavity of G-domain. The binding free energy calculation study depicted that the van der Waals and electrostatic terms contributs significantly to molecular recognition of GTP. Collectively, our study provides mechanistic insights into the tissue-specific expression profiling and GTP binding mechanism of Mx protein from Labeo rohita, which is expected to drive further research on several cellular events including viral resistance and endocytosis in the near future.

Extracellular dsRNA induces a type I interferon response mediated via class A scavenger receptors in a novel Chinook salmon derived spleen cell line

Despite increased global interest in Chinook salmon aquaculture, little is known of their viral immune defenses. This study describes the establishment and characterization of a continuous cell line derived from Chinook salmon spleen, CHSS, and its use in innate immune studies. Optimal growth was seen at 14-18 °C when grown in Leibovitz's L-15 media with 20% fetal bovine serum. DNA analyses confirmed that CHSS was Chinook salmon and genetically different from the only other available Chinook salmon cell line, CHSE-214. Unlike CHSE-214, CHSS could bind extracellular dsRNA, resulting in the rapid and robust expression of antiviral genes. Receptor/ligand blocking assays confirmed that class A scavenger receptors (SR-A) facilitated dsRNA binding and subsequent gene expression. Although both cell lines expressed three SR-A genes: SCARA3, SCARA4, and SCARA5, only CHSS appeared to have functional cell-surface SR-As for dsRNA. Collectively, CHSS is an excellent cell model to study dsRNA-mediated innate immunity in Chinook salmon.

Interferon induced Mx protein from Indian snow trout Schizothorax richardsonii (Gray) lacks critical functional features unlike its mammalian homologues

Viral attack within host cells triggers the production of type I interferons and leads to the induction of interferon stimulated genes (ISGs). One of the ISG Mx, encodes type I interferon inducible GTPase that is responsible for the establishment of an anti-viral state within cells. Intriguingly, several isoforms of Mx have been reported in fish, but the structural analysis of fish Mx proteins remains unexplored. For the first time, we have identified and unraveled the molecular structure of Mx protein from Indian snow trout, Schizothorax richardsonii (Gray) a Coldwater fish that inhabits the water bodies in the sub-Himalayan region. The snow trout Mx coding region consists of 2518 nucleotides with an open reading frame (ORF) of 1854 nucleotides. It codes for a polypeptide of 617 amino acids with a predicted molecular weight of 70 kDa. In silico analysis of snow trout Mx protein revealed signature of dynamin family (LPRGTGIVTR) along with a tripartite GTP-binding domain (GDQSSGKS, DLPG, and TKPD). Homology modelling established that the Mx protein is an elongated structure with a G domain, bundle signaling element (BSE) and a GTPase effector domain (GED). Moreover, the GED of Mx contains two highly conserved leucine zippers at the COOH-terminal of the protein suggesting its structural similarity with human homologues. However, snow trout Mx lacks the essential features of its mammalian homologues questioning its functional characteristics. Further, a ligand binding site in the said protein has also been predicted adjacent to the GTPase switch within the G domain.

Integrated transcriptomic and proteomic analyses reveal potential mechanisms linking thermal stress and depressed disease resistance in the turbot Scophthalmus maximus

A worldwide increase in the reports of diseases affecting marine organisms has paralleled the climate warming over the past few decades. In this study, we applied omics to explore the mechanisms underlying thermo-linked epizootics, by comparing both the transcriptome- and proteome-wide response of turbots to a mimic pathogen (poly I:C) between high temperature and low temperature using a time-course approach. Our results showed that myeloperoxidase (MPO) and insulin were differentially expressed transcripts shared by all five time-points post poly I:C-injection between high and low temperature and also had a consistent expression trend as differentially expressed proteins at 24 h post injection. Combined with other data, it was suggested that the elevated temperature enhanced neutrophil-mediated immunity and the resultant MPO-mediated oxidative stress, which lasted for at least 5 days. The contents of malondialdehyde and protein carbonyls, markers of oxidative damage for lipids and proteins, respectively, were compared between different temperature groups, and the results further implied the emergence of oxidative damage under high temperature. It was also suggested that metabolism disorder likely occur considering the sustained expression changes of insulin. Hence, prolonged MPO-mediated oxidative stress and metabolic disorder might be involved in the thermo-linked epizootic.

Atlantic salmon endothelial cells from the heart were more susceptible than fibroblasts from the bulbus arteriosus to four RNA viruses but protected from two viruses by dsRNA pretreatment

Heart diseases caused by viruses are major causes of Atlantic salmon aquaculture loss. Two Atlantic salmon cardiovascular cell lines, an endothelial cell line (ASHe) from the heart and a fibroblast cell line (BAASf) from the bulbus arteriosus, were evaluated for their response to four fish viruses, CSV, IPNV, VHSV IVa and VHSV IVb, and the innate immune agonist, double-stranded RNA mimic poly IC. All four viruses caused cytopathic effects in ASHe and BAASf. However, ASHe was more susceptible to all four viruses than BAASf. When comparing between the viruses, ASHe cells were found to be moderately susceptible to CSV and VHSV IVb, but highly susceptible to IPNV and VHSV IVa induced cell death. All four viruses were capable of propagating in the ASHe cell line, leading to increases in virus titre over time. In BAASf, CSV and IPNV produced more than one log increase in titre from initial infection, but VHSV IVb and IVa did not. When looking at the antiviral response of both cell lines, Mx proteins were induced in ASHe and BAASf by poly IC. All four viruses induced Mx proteins in BAASf, while only CSV and VHSV IVb induced Mx proteins in ASHe. IPNV and VHSV IVa suppressed Mx proteins expression in ASHe. Pretreatment of ASHe with poly IC to allow for Mx proteins accumulation protected the culture from subsequent infections with IPNV and VHSV IVa, resulting in delayed cell death, reduced virus titres and reduced viral proteins expression. These data suggest that endothelial cells potentially can serve as points of infections for viruses in the heart and that two of the four viruses, IPNV and VHSV IVa, have mechanisms to avoid or downregulate antiviral responses in ASHe cells. Furthermore, the high susceptibility of the ASHe cell line to IPNV and VHSV IVa can make it a useful tool for studying antiviral compounds against these viruses and for general detection of fish viruses.

Protective immunity against rock bream iridovirus (RBIV) infection and TLR3-mediated type I interferon signaling pathway in rock bream (Oplegnathus fasciatus) following poly (I:C) administration

In this study, we evaluated the potential of poly (I:C) to induce antiviral status for protecting rock bream from RBIV infection. Rock bream injected with poly (I:C) at 2 days before infection (1.1 × 10⁴) at 20 °C had significantly higher protection with RPS 13.4% and 33.4% at 100 and 200 μg/fish, respectively, through 100 days post infection (dpi). The addition of boost immunization with poly (I:C) at before/post infection at 20 °C clearly enhanced the level of protection showing 33.4% and 60.0% at 100 and 200 μg/fish, respectively. To investigate the development of a protective immune response, rock bream were re-infected with RBIV (1.1 × 10⁷) at 200 dpi. While 100% of the previously unexposed fish died, 100% of the previously infected fish survived. Poly (I:C) induced TLR3 and Mx responses were observed at several sampling time points in the spleen, kidney and blood. Moreover, significantly high expression levels of IRF3 (2.9- and 3.1-fold at 1 d and 2 days post administration (dpa), respectively), ISG15 and PKR expression (5.4- and 10.2-fold at 2 dpa, respectively) were observed in the blood, but the expression levels were low in the spleen and kidney after poly (I:C) administration. Our results showed the induction of antiviral immune responses and indicate the possibility of developing long term preventive measures against RBIV using poly (I:C).

Sexual maturation in Atlantic salmon induces a constitutive Mx protein production and influences the infectious pancreatic necrosis virus carrier-status

The aim of this study was to demonstrate for the first time that sexual maturation induces a constitutive increase in Mx gene expression and protein production in Atlantic salmon. This could explain the reduction in IPNV prevalence previously observed in broodfish at the time of ova/milt stripping. For this purpose, Mx transcript and protein levels were analysed in different tissues/samples and compared between mature broodfish (female and male) and immature parr.

CpG ODN mimicking CpG rich region of myxosporean Myxobolus supamattayai stimulates innate immunity in Asian sea bass (Lates calcarifer) and defense against Streptococcus iniae

Oligodeoxynucleotides (ODNs) containing unmethylated cytosine-phosphate-guanine CpG dinucleotides within specific sequence contexts (CpG motifs) have been reported as pathogen-associated molecular patterns (PAMPs). Its immunostimulatory effects have been demonstrated in diverse vertebrate models. CpG ODN is typically found in bacterial or viral genome and recognized by a non-self recognition receptor Toll-like receptor9 (TLR9). Here, a new CpG ODN 1013 which mimics sequence of SSU rDNA of early eukaryotic organism myxosporidia, Myxobolus supamattayai, was employed to stimulate the immune responses of Asian sea bass Lates calcarifer. Its immunostimulant potentiality was comparatively compared with that of CpG ODN 1668, a widely used as functional immunostimulant. Both unmethylated CpG ODNs with some modified phosphorothioated positions were intraperitoneally injection (5 μg/fish). Hematological examination, immunological assays and immune-related genes expression were evaluated 12 h, 1, 3 and 5 d after post CpG ODN challenge. The immunosimulatory effect of these CpG ODNs on fish immunity to protect the bacterial pathogen Streptococcus iniae was also determined. The results demonstrated that these two CpG ODNs could induce immune responses in Asian sea bass including the significant (P < 0.05) increase level of WBC, peroxidase activity and oxidative radicals in head kidney (HK) leukocyte, serum innate immune parameters and up-regulation of four immune responsive genes compared with the control group. Most of immune responses induced by ODN 1668 were strong within 1 d but lesser extended while ODN 1013 prolonged the stimulatory effects during the whole experimental period. After challenge with S. iniae, the survival proportion in ODN 1013-treated fish was apparently higher than that treated with ODN 1668 and PBS, respectively. The results together suggested that CpG ODN 1013 enhanced innate immune responses, including humoral and cellular responses, through TLR9 mediated signaling pathway which is mainly contribute to the protective immunity in Asian sea bass against S. iniae infection. These findings can lead to a new approach in immunostimulant development by using the novel CpG ODN originating from the parasite M. supamattayai, besides those from bacterial and viral genomes, for disease control in fish host.

The impact of co-infections on fish: a review

Co-infections are very common in nature and occur when hosts are infected by two or more different pathogens either by simultaneous or secondary infections so that two or more infectious agents are active together in the same host. Co-infections have a fundamental effect and can alter the course and the severity of different fish diseases. However, co-infection effect has still received limited scrutiny in aquatic animals like fish and available data on this subject is still scarce. The susceptibility of fish to different pathogens could be changed during mixed infections causing the appearance of sudden fish outbreaks. In this review, we focus on the synergistic and antagonistic interactions occurring during co-infections by homologous or heterologous pathogens. We present a concise summary about the present knowledge regarding co-infections in fish. More research is needed to better understand the immune response of fish during mixed infections as these could have an important impact on the development of new strategies for disease control programs and vaccination in fish.

Protective oral vaccination against infectious salmon anaemia virus in Salmo salar

Infectious salmon anemia (ISA) is a systemic disease caused by an orthomyxovirus, which has a significant economic impact on the production of Atlantic salmon (Salmo salar). Currently, there are several commercial ISA vaccines available, however, those products are applied through injection, causing stress in the fish and leaving them susceptible to infectious diseases due to the injection process and associated handling. In this study, we evaluated an oral vaccine against ISA containing a recombinant viral hemagglutinin-esterase and a fusion protein as antigens. Our findings indicated that oral vaccination is able to protect Atlantic salmon against challenge with a high-virulence Chilean isolate. The oral vaccination was also correlated with the induction of IgM-specific antibodies. On the other hand, the vaccine was unable to modulate expression of the antiviral related gene Mx, showing the importance of the humoral response to the disease survival. This study provides new insights into fish protection and immune response induced by an oral vaccine against ISA, but also promises future development of preventive solutions or validation of the current existing therapies.

Persistent infections with infectious pancreatic necrosis virus (IPNV) of different virulence in Atlantic salmon, Salmo salar L

Infectious pancreatic necrosis virus (IPNV) is a prevalent pathogen in fish worldwide. The virus causes substantial mortality in Atlantic salmon juveniles and smolts when transferred to sea water and persistent infection in surviving fish after disease outbreaks. Here, we have investigated the occurrence of the virus as well as the innate immune marker Mx in the head kidney (HK) of Atlantic salmon throughout an experimental challenge covering both a fresh and a seawater phase. The fish were challenged with a high (HV) and low virulence (LV) IPNV. Both isolates caused mortality due to reactivation of the virus after transfer to sea water. In the freshwater phase, higher levels of virus transcripts were detected in the HK of fish infected with LV IPNV compared to HV, suggesting that the HV isolate is able to limit its own replication to a level where the innate immune system is not alerted. Further, ex vivoHK leucocytes derived from fish infected with the two isolates were stimulated with CpG DNA. Significantly, higher IFN levels were found in the LV compared to the HV group in the freshwater phase. This suggests that the viruses attenuate the antiviral host immune response at different levels which may contribute to the observed differences in disease outcome.

Use of Poly(I:C) Stabilized with Chitosan As a Vaccine-Adjuvant Against Viral Hemorrhagic Septicemia Virus Infection in Zebrafish

There is an urgent need for more efficient viral vaccines in finfish aquaculture worldwide. Here, we report the use of poly(I:C) stabilized with chitosan as an adjuvant for development of better finfish vaccines. The adjuvant was co-injected with inactivated viral hemorrhagic septicemia virus (VHSV) (CSpIC+iV vaccine) in adult zebrafish and its efficiency in protection against VHSV infection was compared to a live, attenuated VHS virus vaccine (aV). Both free and stabilized poly(I:C) were strong inducers of an antiviral state, measured by transcriptional activation of the genes of viral sensors: toll-like receptors, interferons, and interferon-stimulated genes, such as MXa within 48 h after injection. Both the CSpIC+iV and the aV formulations provided a significant protection against VHSV-induced mortality. However, when plasma from survivors was tested for neutralizing antibodies in an in vitro protection assay, we could not demonstrate any protective effect. On the contrary, plasma from aV vaccinated fish enhanced cytopathic effects, indicating that antibody-dependent entry may play a role in this system. Our results show that poly(I:C) is a promising candidate as an adjuvant for fish vaccination against viral pathogens, and that the zebrafish is a promising model for aquaculture-relevant vaccination studies.

Infectious hematopoietic necrosis virus (IHNV) persistence in Sockeye Salmon: influence on brain transcriptome and subsequent response to the viral mimic poly(I:C)

Background

Sockeye Salmon are an iconic species widely distributed throughout the North Pacific. A devastating pathogen of Sockeye Salmon is infectious hematopoietic necrosis virus (IHNV, genus Novirhabdovirus, family Rhabdoviridae). It has been postulated that IHNV is maintained in salmon populations by persisting over the life of its host and/or by residing in natural reservoirs other than its susceptible hosts. Herein we demonstrate the presence of IHNV in the brain of Sockeye Salmon that survived an experimentally-induced outbreak, suggesting the presence of viral persistence in this susceptible species. To understand the viral persistent state in Sockeye Salmon we profiled the transcriptome to evaluate the host response in asymptomatic carriers and to determine what effects (if any) IHNV exposure may have on subsequent virus challenges.

Results

A laboratory disease model to simulate a natural IHNV outbreak in Sockeye Salmon resulted in over a third of the population incurring acute IHN disease and mortality during the first four months after initial exposure. Nine months post IHNV exposure, despite the absence of disease and mortality, a small percentage (<4 %) of the surviving population contained IHNV in brain. Transcriptome analysis in brain of asymptomatic virus carriers and survivors without virus exhibited distinct transcriptional profiles in comparison to naïve fish. Characteristic for carriers was the up-regulation of genes involved in antibody production and antigen presentation. In both carriers and survivors a down-regulation of genes related to cholesterol biosynthesis, resembling an antiviral mechanism observed in higher vertebrates was revealed along with differences in nervous system development. Moreover, following challenge with poly(I:C), survivors and carriers displayed an elevated antiviral immune response in comparison to naïve fish.

Conclusions

IHN virus persistence was identified in Sockeye Salmon where it elicited a unique brain transcriptome profile suggesting an ongoing adaptive immune response. IHNV carriers remained uncompromised in mounting efficient innate antiviral responses when exposed to a viral mimic. The capacity of IHNV to reside in asymptomatic hosts supports a virus carrier hypothesis and if proven infectious, could have significant epidemiological consequences towards maintaining and spreading IHNV among susceptible host populations.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1759-y) contains supplementary material, which is available to authorized users.

Development of a walleye spleen stromal cell line sensitive to viral hemorrhagic septicemia virus (VHSV IVb) and to protection by synthetic dsRNA

A cell line, WE-spleen6, has been developed from the stromal layer of primary spleen cell cultures. On conventional plastic, WE-spleen6 cells had a spindle-shaped morphology at low cell density but grew to become epithelial-like at confluency. On the commercial extracellular matrix (ECM), Matrigel, the cells remained spindle-shaped and formed lumen-like structures. WE-spleen6 cells had intermediate filament protein, vimentin and the ECM protein, collagen I, but not smooth muscle α-actin (SMA) and von Willebrand factor (vWF) and lacked alkaline phosphatase and phagocytic activities. WE-spleen6 was more susceptible to infection with VHSV IVb than a fibroblast and epithelial cell lines from the walleye caudal fin, WE-cfin11f and WE-cfin11e, respectively. Viral transcripts and proteins appeared earlier in WE-spleen6 cultures as did cytopathic effect (CPE) and significant virus production. The synthetic double-stranded RNA (dsRNA), polyinosinic: polycytidylic acid (pIC), induced the antiviral protein Mx in both cell lines. Treating WE-spleen6 cultures with pIC prior to infection with VHSV IVb inhibited the early accumulation of viral transcripts and proteins and delayed the appearance of CPE and significant viral production. Of particular note, pIC caused the disappearance of viral P protein 2 days post infection. WE-spleen6 should be useful for investigating the impact of VHSV IVb on hematopoietic organs and the actions of pIC on the rhabdovirus life cycle.

VHSV G glycoprotein major determinants implicated in triggering the host type I IFN antiviral response as DNA vaccine molecular adjuvants

We have recently identified the two major determinants of the glycoprotein G of the viral hemorrhagic septicaemia rhabdovirus (gpGVHSV), peptides p31 and p33 implicated in triggering the host type I IFN antiviral response associated to these rhabdoviral antigens. With the aim to investigate the properties of these viral glycoprotein regions as DNA molecular adjuvants, their corresponding cDNA sequences were cloned into a plasmid (pMCV1.4) flanked by the signal peptide and transmembrane sequences of gpGVHSV. In addition, a plasmid construct encoding both sequences p31 and p33 (pMCV1.4-p31+p33) was also designed. In vitro transitory cell transfection assays showed that these VHSV gpG regions were able to induce the expression of type I IFN stimulated genes as well as to confer resistance to the infection with a different fish rhabdovirus, the spring viremia of carp virus (SVCV). In vivo, zebrafish intramuscular injection of only 1μg of the construct pMCV1.4-p31+p33 conferred fish protection against SVCV lethal challenge up to 45 days post-immunization. Moreover, pMCV1.4-p31+p33 construct was assayed for molecular adjuvantcity's for a DNA vaccine against SVCV based in the surface antigen of this virus (pAE6-GSVCV). The results showed that the co-injection of the SVCV DNA vaccine and the molecular adjuvant allowed (i) a ten-fold reduction in the dose of pAE6-Gsvcv without compromising its efficacy (ii) an increase in the duration of protection, and (iii) an increase in the survival rate. To our knowledge, this is the first report in which specific IFN-inducing regions from a viral gpG are used to design more-efficient and cost-effective viral vaccines, as well as to improve our knowledge on how to stimulate the innate immune system.

Innate immune responses of salmonid fish to viral infections

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

A short history of research on immunity to infectious diseases in fish

This review describes the history of research on immunity to infectious diseases of fish in the period between 1965 and today. Special attention is paid to those studies, which are dealing with the interaction between immune system and invading pathogens in bony fish. Moreover, additional biographic information will be provided of people involved. In the 1960s and 1970s the focus of most studies was on humoral (Ig, B-cell) responses. Thorough studies on specific cellular (T-cell) responses and innate immunity (lectins, lysozyme, interferon, phagocytic cells) became available later. In the period between 1980 and today an overwhelming amount of data on regulation (e.g. cell cooperation, cytokines) and cell surface receptors (e.g. T-cell receptor; MHC) was published. It became also clear, that innate responses were often interacting with the acquired immune responses. Fish turned out to be vertebrates like all others with a sophisticated immune system showing specificity and memory. These basic data on the immune system could be applied in vaccination or in selection of disease resistant fish. Successful vaccines against bacterial diseases became available in the 1970s and 1980s. Effective anti-viral vaccines appeared from the 1980s onwards. There is no doubt, that Fish Immunology has become a flourishing science by the end of the 20th century and has contributed to our understanding of fish diseases as well as the success of aquaculture.

IFN-γ in turtle: conservation in sequence and signalling and role in inhibiting iridovirus replication in Chinese soft-shelled turtle Pelodiscus sinensis

The IFN-γ gene was identified in a turtle, the Chinese soft-shelled turtle, Pelodiscus sinensis, with its genome consisting of 4 exons and 3 introns. The deduced amino acid sequence of this gene contains a signal peptide, an IFN-γ family signature motif (130)IQRKAVNELFPT, an NLS motif (155)KRKR and three potential N-glycosylation sites. As revealed by real-time quantitative PCR, the gene was constitutively expressed in all tested organs/tissues, with higher level observed in blood, intestine and thymus. An induced expression of IFN-γ at mRNA level was observed in peripheral blood leucocytes (PBLs) in response to in vitro stimulation of LPS and PolyI:C. The overexpression of IFN-γ in the Chinese soft-shelled turtle artery (STA) cell line resulted in the increase in the expression of transcriptional regulators, such as IRF1, IRF7 and STAT1, and antiviral genes, such as Mx, PKR, implying possibly the existence of a conserved signalling network and role for IFN-γ in the turtle. Furthermore, the infection of soft-shelled turtle iridovirus (STIV) in the cell line transfected with IFN-γ may cause the cell death as demonstrated with the elevated lactate dehydrogenase (LDH) level and cell mortality. However, the mechanism involved in the antiviral activity may require further investigation.

Transgene and immune gene expression following intramuscular injection of Atlantic salmon (Salmo salar L.) with DNA-releasing PLGA nano- and microparticles

The use of poly-(D,L-lactic-co-glycolic) acid (PLGA) particles as carriers for DNA delivery has received considerable attention in mammalian studies. DNA vaccination of fish has been shown to elicit durable transgene expression, but no reports exist on intramuscular administration of PLGA-encapsulated plasmid DNA (pDNA). We injected Atlantic salmon (Salmo salar L.) intramuscularly with a plasmid vector containing a luciferase (Photinus pyralis) reporter gene as a) naked pDNA, b) encapsulated into PLGA nano- (~320 nm) (NP) or microparticles (~4 μm) (MP), c) in an oil-based formulation, or with empty particles of both sizes. The ability of the different pDNA-treatments to induce transgene expression was analyzed through a 70-day experimental period. Anatomical distribution patterns and depot effects were determined by tracking isotope labeled pDNA. Muscle, head kidney and spleen from all treatment groups were analyzed for proinflammatory cytokines (TNF-α, IL-1β), antiviral genes (IFN-α, Mx) and cytotoxic T-cell markers (CD8, Eomes) at mRNA transcription levels at days 1, 2, 4 and 7. Histopathological examinations were performed on injection site samples from days 2, 7 and 30. Injection of either naked pDNA or the oil-formulation was superior to particle treatments for inducing transgene expression at early time-points. Empty particles of both sizes were able to induce proinflammatory immune responses as well as degenerative and inflammatory pathology at the injection site. Microparticles demonstrated injection site depots and an inflammatory pathology comparable to the oil-based formulation. In comparison, the distribution of NP-encapsulated pDNA resembled that of naked pDNA, although encapsulation into NPs significantly elevated the expression of antiviral genes in all tissues. Together the results indicate that while naked pDNA is most efficient for inducing transgene expression, the encapsulation of pDNA into NPs up-regulates antiviral responses that could be of benefit to DNA vaccination.

Cloning and preliminary functional studies of the JAM-A gene in grass carp (Ctenopharyngodon idellus)

Grass carp (Ctenopharyngodon idellus) is a very important aquaculture species in China and other South-East Asian countries; however, disease outbreaks in this species are frequent, resulting in huge economic losses. Grass carp hemorrhage caused by grass carp reovirus (GCRV) is one of the most serious diseases. Junction adhesion molecule A (JAM-A) is the mammalian receptor for reovirus, and has been well studied. However, the JAM-A gene in grass carp has not been studied so far. In this study, we cloned and elucidated the structure of the JAM-A gene in grass carp (GcJAM-A) and then studied its functions during grass carp hemorrhage. GcJAM-A is composed of 10 exons and 9 introns, and its full-length cDNA is 1833 bp long, with an 888 bp open reading frame (ORF) that encodes a 295 amino acid protein. The GcJAM-A protein is predicted to contain a typical transmembrane domain. Maternal expression pattern of GcJAM-A is observed during early embryogenesis, while zygote expression occurs at 8 h after hatching. GcJAM-A is expressed strongly in the gill, liver, intestine and kidney, while it is expressed poorly in the blood, brain, spleen and head kidney. Moreover, lower expression is observed in the gill, liver, intestine, brain, spleen and kidney of 30-month-old individuals, compared with 6-month-old. In a GcJAM-A-knockdown cell line (CIK) infected with GCRV, the expression of genes involved in the interferon and apoptosis pathways was significantly inhibited. These results suggest that GcJAM-A could be a receptor for GCRV. We have therefore managed to characterize the GcJAM-A gene and provide evidence for its role as a receptor for GCRV.

Immune modulatory effects of Aloe arborescens extract on the piscine SAF-1 cell line

The pharmacological potential of Aloe arborescens Miller leaf components was investigated, with special attention deserved to immune modulatory effects on the Sparus aurata fibroblast cell line SAF-1. The cells were treated with Aloe extract at different concentrations (1.2-4.8 mg ml(-1)) for various times (24-72 h). The lowest concentration did not provoke any cellular damage observable by SEM and did not affect ATP amounts after 24 and 48 h, while even induced a significant increase over controls after 72 h. We next examined the transcription kinetics of different immune-related genes (IL-1β, TGF-β, TNF-α, COX-2, IFN-I, Mx and MHCI-α) in SAF-1 cells stimulated with LPS or poly I:C. The Aloe extract (1.2 mg ml(-1)) acted as a powerful immune stimulant in LPS- or poly I:C-activated SAF-1 cells, inducing a synergic effect on interconnected genes that are expected to be involved in different aspects of the immune responses. These reports provide a new perspective for the use of A. arborescens to prevent or oppose bacterial and viral fish diseases and to face, as an alternative strategy based on natural plant extracts, the growing unwillingness to rely upon standard solutions involving antibiotics or antimicrobial chemicals.

Modulation of innate immune responses in Atlantic salmon by chronic hypoxia-induced stress

Atlantic salmon post-smolts were exposed to either chronic hypoxic (Hy) or normal oxygen (No) conditions in seawater tanks for 58 days, mimicking conditions typical of sea cages for farmed salmon at some periods of the year. By day 29 head kidney macrophages were isolated and subjected to in vitro poly I:C stimulation to simulate viral infection, and samples were collected over 48 h. By day 58 fish were subjected to in vivo stimulation using poly I:C or a Vibrio water-based vaccine to simulate viral or bacterial infection, respectively. The fish were monitored for stress responses and expression of several pro-inflammatory genes in head kidney and intestinal tissue up to five days post-injection. Stress load was monitored by plasma cortisol estimation at days 29 and 58, and on days 1, 2, 3 and 5 post-injection in the in vivo trial. Hy exposure resulted in elevated plasma cortisol levels on day 29 compared to No, while on day 58 cortisol levels were higher in the control group. Additionally, both poly I:C and the Vibrio vaccine gave significantly increased cortisol levels one day post-injection compared to PBS treated controls, irrespective of previous oxygen exposure. In vitro stimulation of macrophages with poly I:C revealed higher IFNα mRNA levels at 6, 12 and 24 h and for Mx at 12 and 24 h post-stimulation, for both No and Hy individuals. Moreover, IFNα levels were higher in No than in Hy individuals at all time points, and a similar difference was seen in Mx at 48 h. In vivo stimulation with poly I:C elicited strong elevation of the IL-1β, IFNγ, Mx and IP10 mRNA transcripts in head kidney, while TNFα1 and IFNα were found unaffected. The Vibrio vaccine elicited a strong up regulation of IL-1β, IFNγ and IP10 mRNA, whereas Mx, TNFα1 and IFNα appeared unchanged. Significant differences in expression between different oxygen exposure groups were found for all genes and both stimuli. The overall trend suggests that long-term hypoxia either reduces or delays the expression of these genes in head kidney. Expression of IFNγ and Mx in intestinal tissues also showed a strong up regulation of the genes following poly I:C stimulation, and also here the overall trend suggests that chronic hypoxia results in a lower or delayed expression of the measured genes. In summary, our results indicate that chronic hypoxia modulates the expression of important immune related genes putatively altering the immune response. As the effect is present in isolated macrophages as well as head kidney and intestinal tissue the modulation appears to be affecting local as well as systemic responses.

Slow release cortisol implants result in impaired innate immune responses and higher infection prevalence following experimental challenge with infectious pancreatic necrosis virus in Atlantic salmon (Salmo salar) parr

Stress can affect the immune system and increase susceptibility to various diseases but knowledge of the underlying mechanisms is scarce. There is a complex interaction between the immune system and the endocrine system of vertebrates. In fish, cortisol is a key hormone regulating stress response and recent studies have also suggested that this hormone can affect the immune system, where cortisol is mainly regarded as an immunosuppressive factor. The aim of the present study was to examine the impact of chronically elevated levels of cortisol on the immune response and susceptibility to experimental infection with infectious pancreatic necrosis virus (IPNV). Further, the effect of IPNV challenge on circulating levels of cortisol was investigated. Atlantic salmon parr were implanted intraperitoneally with sustained-release implants of bovine of cortisol (50 μg cortisol g(-1) body weight in an implant based on vegetable lipids). Vehicle implants were used as control (sham-injected). At 45 days after implantation (DAI), fish were challenged with a low virulent isolate of IPNV (by immersion). Samples of plasma, liver and head kidney was taken from fish before and 24 h, 48 h, 7 days week and 21 days post infection (DPI). Cortisol level in plasma was measured using radioimmunoassay and gene expression in liver and head kidney was analyzed with real-time PCR (RT-PCR). Infection prevalence in infected fish was assessed by virus culture and RT-PCR of head kidney samples. Cortisol implantation compared with sham-implanted fish had increased levels of plasma cortisol at 45 DAI. The relative expression of Interferon alpha-1 (IFNα-1), Myxo virus-1 Mx, Heat-shock protein 70 (HSP70), Serum amyloid A (SAA), Glucocorticoid receptor (GR) and Heat-shock protein 90 (HSP90) tends to be down-regulated by cortisol implantation. There was a higher prevalence of fish with detectable levels of IPNV, as measured by cell culture and RT-PCR, in the cortisol-implanted group challenged with IPNV (0 = 0.0305) relative to the group that received a sham implantation. Further, cortisol seems to delay the induction of the antiviral IFNα-1 pathway and Mx mRNA expression. This study shows that elevated plasma cortisol level leads to an impaired innate immune response, and higher virus (IPNV) prevalence in Atlantic salmon parr.

Induction of anti-viral genes during acute infection with Viral hemorrhagic septicemia virus (VHSV) genogroup IVa in Pacific herring (Clupea pallasii)

Infection with the aquatic rhabdovirus Viral hemorrhagic septicemia virus (VHSV) genogroup IVa results in high mortality in Pacific herring (Clupea pallasii) and is hypothesized to be a potential limiting factor for herring recovery. To investigate anti-viral immunity in the Pacific herring, four immune response genes were identified: the myxovirus resistance (Clpa-Mx), a major histocompatibility complex IB (named Clpa-UAA.001), the inducible immunoproteosome subunit 9 (Clpa-PSMB9) and the neutrophil chemotactic factor (Clpa-LECT2). Reverse transcriptase quantitative PCR (RT-qPCR) assays were developed based on these gene sequences to investigate the host immune response to acute VHSV infection following both injection and immersion challenge. Virus levels were measured by both plaque assay and RT-qPCR and peaked at day 6 during the 10-day exposure period for both groups of fish. The interferon stimulated genes (Clpa-Mx, -UAA.001, and -PSMB9) were significantly up-regulated in response to VHSV infection at both 6 and 10 days post-infection in both spleen and fin. Results from this study indicate that Pacific herring mount a robust, early antiviral response in both fin and spleen tissues. The immunological tools developed in this study will be useful for future studies to investigate antiviral immunity in Pacific herring.

Characterization of a TnMAVS protein from Tetraodon nigroviridis

A growing family of cellular proteins encoding for caspase activation and the recruitment domain (CARD) plays a crucial role in immunity by sensing viral infections and signaling antiviral immune defenses. We obtained a MAVS-like protein (named TnMAVS) from Tetradon nigroviridis, which contains a CARD domain, a pro-rich domain, and a TM domain similar to human MAVS. A fluorescence assay showed that TnMAVS was located in the cytoplasm and near by the membrane, and not the mitochondria in FHM cells. As such, it was considered as a new member of MAVS. The TnMAVS was highly expressed in the liver and muscle of T. nigroviridis. In the spleen, TnMAVS was down-regulated when the fish was treated with polyinosinic:polycytidylic acid or challenged with ISKNV, but was not affected by PGN or LPS. The dual luciferase reporter assay revealed that TnMAVS overexpression resulted in the activation of the interferon-sensitive response element and NF-κB signal pathways. In addition, a characteristic TRAF3-associated peptide PVQD was found in the TnMAVS sequence. Co-immunoprecipitation assays indicated that TnMAVS could interact with zfTRAF3 in eukaryotic cells.

Innate immune evasion mediated by the Ambystoma tigrinum virus eukaryotic translation initiation factor 2alpha homologue

Ranaviruses (family Iridoviridae, genus Ranavirus) are large, double-stranded DNA (dsDNA) viruses whose replication is restricted to ectothermic vertebrates. Many highly pathogenic members of the genus Ranavirus encode a homologue of the eukaryotic translation initiation factor 2α (eIF2α). Data in a heterologous vaccinia virus system suggest that the Ambystoma tigrinum virus (ATV) eIF2α homologue (vIF2αH; open reading frame [ORF] 57R) is involved in evading the host innate immune response by degrading the interferon-inducible, dsRNA-activated protein kinase, PKR. To test this hypothesis directly, the ATV vIF2αH gene (ORF 57R) was deleted by homologous recombination, and a selectable marker was inserted in its place. The ATVΔ57R virus has a small plaque phenotype and is 8-fold more sensitive to interferon than wild-type ATV (wtATV). Infection of fish cells with the ATVΔ57R virus leads to eIF2α phosphorylation, in contrast to infection with wtATV, which actively inhibits eIF2α phosphorylation. The inability of ATVΔ57R to prevent phosphorylation of eIF2α correlates with degradation of fish PKZ, an interferon-inducible enzyme that is closely related to mammalian PKR. In addition, salamanders infected with ATVΔ57R displayed an increased time to death compared to that of wtATV-infected salamanders. Therefore, in a biologically relevant system, the ATV vIF2αH gene acts as an innate immune evasion factor, thereby enhancing virus pathogenesis.

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.

Liver involvement in post-smolt Atlantic salmon, Salmo salar L., infected with infectious pancreatic necrosis virus (IPNV): a retrospective histopathological study

Histological changes associated with infectious pancreatic necrosis virus (IPNV) infection have historically been described for the pancreas and gut, but any involvement of the liver was poorly acknowledged or described. The aims of this study were to find robust evidence that the reported increase in liver pathology in Atlantic salmon post-smolts in natural outbreaks was effectively related to IPNV infection and retrospectively to report when such a shift in the involvement of the liver had taken place, supported by a histopathological description for a differential diagnosis. The study reports new findings concerning the dynamics of liver pathology development, with apoptosis, demonstrated by histological and immunological techniques, described as the most relevant and particular feature. Immunohistochemical examination of affected liver suggests apoptosis is not only the result of the virus infection itself but triggered through the action of the host's innate immune response. Liver involvement contributes to the nature of infection and becomes an important factor in the disease process. Additionally, it was established that the increase in infectious pancreatic necrosis prevalence is correlated with a new distinct pattern of outbreak distribution throughout the year. The role of smolt category (i.e. S1, S1/2 or S0), hence timing of seawater transfer as a strong correlating factor, is discussed.

The persistence of infectious pancreatic necrosis virus and its influence on the early immune response

Persistent infection by IPNV was induced in RTG-2 and RTG-P1 cells in vitro and the influence of this phenomenon on viral infectivity, viral antigen expression and interference with homologous and heterologous viruses was characterized over successive passages. The induction of IFN was also assessed, as was the sequence of the VP2 viral capsid protein, the region believed to be responsible for virulence, attenuation or persistence. Viral antigen expression was recorded in cells with no evidence of cytopathic effects and in these conditions, flow cytometry was more sensitive than RT-PCR to demonstrate the presence of a non-lytic virus. Interference of homologous viral infection could be detected in cross-infection experiments and in RTG-P1 cells persistently infected with IPNV, the Mx1 promoter could still be activated for at least 5 successive passages. Indeed, although over-induction of luciferase was not observed by re-infection with homologous or heterologous viruses, a significant increase in luciferase was induced by poly I:C. IFN transcripts could be quantified by qRT-PCR in the persistent cells at several passages, suggesting that IFN plays a role in maintaining IPNV persistence. In addition, we observed the same determinants in the VP2 sequences from the persistent virus as those described previously for IPNV adaptation and persistence in cell culture.

The effect of sub-culturing on the basal level of type I interferon (IFN) gene expression in the Salmon Head Kidney (SHK-1) cell line

Over sub-culturing a cell line generates a selective pressure which can result in key cellular functions being altered such as gene and protein expression. The present study set out to determine whether serial sub-culturing affects the antiviral state of the Salmon Head Kidney (SHK-1) cell line. Cells were cultured under constant conditions and real-time PCR was performed to measure the level of interferon (IFN) and Mx gene expression over different passage numbers. A significant increase in the basal level of IFN and Mx gene expression was recorded at passage number 58 (3 and 14-fold increase versus passage number 53), suggesting a sub-culturing effect on the type I IFN response in SHK-1 cells. Passage dependent variations in morphology and cell sub-populations have been previously observed in SHK-1 cells. Such variations in cell sub-types were suspected to be responsible for the fluctuations in IFN and Mx gene expression recorded in this study.

Identification and expression analyses of poly [I:C]-stimulated genes in channel catfish (Ictalurus punctatus)

Channel catfish (Ictalurus punctatus) have proven to be an excellent model with which to study immune responses of lower vertebrates. Identification of anti-viral antibodies and cytotoxic cells, as well as both type I and II interferon (IFN), demonstrates that catfish likely mount a vigorous anti-viral immune response. In this report, we focus on other elements of the anti-viral response, and identify more than two dozen genes that are induced following treatment of catfish cells with poly [I:C]. We showed that poly [I:C] induced type I interferon within 2 h of treatment, and that characteristic interferon stimulated genes (ISGs) appeared 6-12 h after exposure. Among the ISGs detected by RT-PCR assay were homologs of ISG15, Mx1, IFN regulatory factor 1 (IRF-1), inhibitor of apoptosis protein-1 (IAP-1) and the chemokine CXCL10. Microarray analyses showed that 13 and 24 cellular genes, respectively, were upregulated in poly [I:C]-treated B cell and fibroblast cultures. Although many of these genes were novel and did not fit the profile of mammalian ISGs, there were several (ISG-15, ubiquitin-conjugating enzyme E2G1, integrin-linked kinase, and clathrin-associated protein 47) that were identified as ISGs in mammalian systems. Taken together, these results suggest that dsRNA, either directly or through the prior induction of IFN, upregulates catfish gene products that function individually and/or collectively to inhibit virus replication.

An antiserum against Atlantic salmon IFNa1 detects IFN and neutralizes antiviral activity produced by poly I:C stimulated cells

Type I interferons (IFNs) play a crucial role in innate immune responses against virus infections in vertebrates. Two IFNs (IFNa1 and IFNa2) have previously been cloned from Atlantic salmon. In the present work a polyclonal antiserum, which was generated against salmon IFNa1 was used to study its production in cells by immunoblot detection and neutralization of antiviral activity. The antiserum was first confirmed to detect and neutralize the antiviral activity of recombinant salmon IFNa1 produced in HEK293 cells. The antiserum also detected IFNa1 and neutralized 95-98% of the antiviral activity in supernatants of poly I:C stimulated salmon TO cells. This suggests that IFNa1/IFNa2 are the major IFNs produced by poly I:C stimulated TO cells. The antiserum neutralized most of the IFN activity in poly I:C stimulated head kidney leucocytes from three of five individuals, but in stimulated leucocytes from the other two individuals only 75% of the antiviral activity was neutralized. This shows that although IFNa1/IFNa2 are major IFNs secreted by poly I:C stimulated leucocytes, these cells can also produce additional molecules with IFN-like activity.

Effects of neurotoxic insecticides on heat-shock proteins and cytokine transcription in Chinook salmon (Oncorhynchus tshawytscha)

This study investigated sublethal, molecular effects of two current-use insecticides, chlorpyrifos (CP) and esfenvalerate (EV) in juvenile Chinook salmon. Heat-shock protein (hsp60, hsp70, hsp90) expression was quantified by Western blotting in muscle, liver and gill, and transcription of four cytokines (TGF-beta, IL-1beta, IGF-1, Mx-protein) was measured by real-time TaqMan PCR in anterior kidney and spleen. Expression of hsp was increased in muscle and liver at 1.2 and 7.2 microg/L CP, and at 0.01 and 0.1 microg/L EV, respectively. Transcription of IL-1beta and TGF-beta was elevated in kidney at 1.2 microg/L CP, while EV had no effect. No changes in cytokine transcription were observed in the spleen. Our results show that these insecticides cause cellular effects at environmental concentrations, and that hsps are sensitive indicators of sublethal exposure to CP and EV. In addition, CP may exert immunotoxic effects by altering the transcription of important mediators of the fish immune system.

Expression of interferon and interferon-induced genes in salmonids in response to virus infection, interferon-inducing compounds and vaccination

Interferons (IFNs) involved in innate immunity against viruses have recently been cloned from Atlantic salmon and rainbow trout. Moreover, several IFN-stimulated genes (ISGs) have been cloned from salmonids although only Mx has been shown to possess antiviral properties. Much less in known about how viruses induce IFNs in salmonids, but synthetic ligands for some of the main mammalian viral sensors also induce IFNs and ISGs in salmonids. Analysis of the promoters of the salmon IFN-alpha1 and IFN-alpha2 genes shows that activation is dependent on both NFkappaB and IRFs similar to human IFN-beta. Furthermore, several IFN-stimulated genes (ISGs) have been cloned from salmonids although only Mx has been shown to possess antiviral properties. The synthetic compounds poly I:C, imidazoquinolines and CpG oligonucleotides induce IFNs and ISGs in salmonids, probably through the same pathways as in mammals. Salmonid viruses show potent ability to stimulate expression of IFN and ISGs in vivo. Differences between viruses in the ability to stimulate host gene expression are often more evident in cell culture, but more work is needed to pinpoint how salmonid viruses antagonize the IFN system of their host. Finally, existing data suggest that IFNs play a role in the early non-specific protection observed after vaccination of salmonids with rhabdoviral DNA vaccines and conventional polyvalent vaccines.

Engineered virus-encoded pre-microRNA (pre-miRNA) induces sequence-specific antiviral response in addition to nonspecific immunity in a fish cell line: convergence of RNAi-related pathways and IFN-related pathways in antiviral response

Transfection with synthesized virus-specific small interfering RNAs (siRNAs) efficiently inhibits viral replication in viral-infected fish cell lines, implying the involvement of RNA interference (RNAi)-related pathways in the antiviral response of fish cells. Here, we demonstrate that plasmid expressing virus-encoded pre-microRNAs (pre-miRNAs) can also inhibit viral replication through these pathways. By incorporating sequences encoding miRNAs specific to major capsid protein (MCP) gene of red sea bream iridovirus (RSIV) and a miRNA specific to hirame rhabdovirus (HIRRV) genome into a murine miR-155 pre-miRNA backbone, we were able to intracellularly express viral pre-miRNAs (miR-MCPs and miR-HIRRV) in a fish cell line. The miR-MCPs and miR-HIRRV, delivered as pre-miRNA precursors in transfected cells, inhibited viral replication when these cells were infected with the target virus. Although this may suggest sequence-specific interference, inhibitory effect on viral replication was also observed in cells transfected with a plasmid expressing pre-miRNA targeting beta-galactosidase gene (miR-LacZ) that served as a specificity control. Expression of pre-miRNAs was found to activate interferon (IFN)-related pathways, correlating with upregulation of the antiviral IFN-induced Mx protein. The antiviral effects of viral-miRNAs observed here were partly the result of the antiviral miRNA-related pathways and partly the result of the antiviral IFN-related pathways. We propose that engineered virus-encoded pre-miRNA can engage not only RNAi-related pathways but also IFN-related pathways to induce potent antiviral responses in fish cells.

Biological characterisation of a recombinant Atlantic salmon type I interferon synthesized in Escherichia coli

Type I (alpha/beta) interferons (IFNs) are a family of cytokines that stimulate the expression of numerous proteins that mediate an antiviral state in uninfected cells. Two Atlantic salmon (Salmo salar) IFN-alpha (SasaIFN-alpha1 & 2) genes have previously been cloned and both were found to contain a putative N-linked glycosylation site. Recombinant SasaIFN-alpha1 (rSasaIFN-alpha1) produced in eukaryotic systems has repeatedly been shown to confer antiviral properties. However, different IFN-alpha subtypes may exhibit differential antiviral activities and be subject to glycosylation. To evaluate the potential therapeutic impact of a rSasaIFN-alpha, the mature form of the SasaIFN-alpha2 protein was produced in a high-level Escherichia coli expression system. Expression of the rSasaIFN-alpha2 was detected by SDS-PAGE and Western blot, and its identity was confirmed by mass spectrometry. In the homologous Chinook salmon embryonic (CHSE-214) cell line, the rSasaIFN-alpha2 incited early expression of the IFN-induced Mx protein and exhibited high antiviral activity of about 2.8 x 10(6) U mg(-1) against infectious pancreatic necrosis virus (IPNV). Conversely, antiviral protection by rSasaIFN-alpha2 was not observed in a heterologous Japanese flounder embryo (HINAE) cell line. Hence, a biologically active form of rSasaIFN-alpha2 was successfully produced using a glycosylation-deficient prokaryotic system and purified to homogeneity, suggesting that glycosylation is not required for its antiviral activity.

Evolution of the zebrafish model: from development to immunity and infectious disease

The successful zebrafish developmental model has now expanded to being used as a model for the analysis of host-pathogen interactions during infectious disease. Numerous pathogens have been demonstrated to infect zebrafish and new mechanisms of virulence, as well as host defense have been uncovered using this new model. In this review we summarize the literature on how the zebrafish infectious disease model is being used to decipher virulence mechanisms used by various pathogens and the host defense mechanisms initiated to combat infection.

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.

In vitro inhibition of sole aquabirnavirus by Senegalese sole Mx

Senegalese sole, Solea senegalensis, is a flat fish of growing interest in European aquaculture. In its culture viral infections are constant threats, thus understanding antiviral defences is a key factor for a successful industry. Mx proteins are IFN-induced proteins widespread in eukaryotes; however, their antiviral activity is unclear and the results variable among species. Therefore assessment of the putative Mx antiviral activity in each species is of interest. Our group has recently cloned the Senegalese sole Mx (SsMx) cDNA and in this study its antiviral activity was assessed by infecting CHSE-214 cells expressing recombinant SsMx, with sole aquabirnavirus. The antiviral activity against this pathogen was demonstrated by reduction in induced cytopathic effects, reduction in virus yield and decrease in viral transcripts. These findings contribute to our understanding of fish antiviral mechanisms and open the possibility of using this protein as a tool for fighting viral infections in aquaculture.