Cloning of an Mx cDNA from Atlantic halibut (Hippoglossus hippoglossus) and characterization of Mx mRNA expression in response to double-stranded RNA or infectious pancreatic necrosis virus

Characterization, Expression Pattern and Antiviral Activities of Mx Gene in Chinese Giant Salamander, Andrias davidianus

Mx, Myxovirus resistance is an important interferon-stimulated protein that mediates antiviral responses. In this study, the expression and activities of Chinese giant salamander, Andrias davidianus Mx gene, AdMx, were investigated. The AdMx cDNA sequence contains an open reading frame (ORF) of 2112 nucleotides, encoding a putative protein of 703 aa. Meanwhile, AdMx possesses the conserved tripartite GTP binding motif and a dynamin family signature. qRT-PCR analysis revealed a broad expression of AdMx in vivo, with the highest expression levels in brain, kidney and spleen. The AdMx expression level in kidney, spleen and muscle significantly increased at 6 h after Chinese giant salamander iridovirus (GSIV) infection and peaked at 48 h, while that in muscle cell line (GSM) was not noticeably up-regulated until 72 h post infection. Additionally, a plasmid expressing AdMx was constructed and transfected into the Chinese giant salamander GSM cells. The virus load and gene copies in AdMx over-expressed cells were significantly reduced compared with those in the control cells. Moreover, compared to the control cells, a lower level of virus major capsid protein (MCP) synthesis in AdMx over-expressed cells was confirmed by Western blot. These results collectively suggest that Mx plays an important antiviral role in the immune responses against GSIV in Chinese giant salamander.

Lineage/species-specific expansion of the Mx gene family in teleosts: Differential expression and modulation of nine Mx genes in rainbow trout Oncorhynchus mykiss

Myxovirus resistance (Mx) proteins are interferon (IFN)-inducible Dynamin-like GTPases, which play an important role in antiviral immunity. Three Mx genes (Mx1-3) have been cloned previously in rainbow trout. In this study, an additional six Mx genes were cloned that reside in four chromosomal loci. Further bioinformatics analysis suggests the presence of three teleost Mx groups (TMG) each with a characteristic gene organisation. Salmonid Mx belong to TMG1 and TMG2. The increased salmonid Mx gene copies are due mainly to local gene duplications that happened before and after salmonid speciation, in a lineage/species specific manner. Trout Mx molecules have been diversified in the loop 1 and 4 regions, and in the nuclear localisation signal in loop 4. The trout Mx genes were shown to be differentially expressed in tissues, with high levels of expression of TMG1 (Mx1-4) in blood and TMG2 (Mx5-9) in intestine. The expression of the majority of the trout Mx genes was induced by poly IC in vitro and in vivo, and increased during development. In addition, induction by antiviral (IFN) and proinflammatory cytokines was studied, and showed that type I IFN, IFNγ and IL-1β can induce Mx gene expression in an Mx gene-, cytokine- and cell line-dependent manner. These results show that salmonids possess a large number Mx genes as well as complex regulatory pathways, which may contribute to their success in an anadromous life style.

Molecular characterization, constitutive expression and GTP binding mechanism of Cirrhinus mrigala (Hamilton, 1822) Myxovirus resistance (Mx) protein

Myxovirus resistance (Mx) proteins represents the subclass of the dynamin superfamily of large Guanosine triphosphates (GTPases), play esential role in intracellular vesicle trafficking, endocytosis, organelle homeostasis and mitochondria distribution. These proteins are key players of the vertebrate immune system, induced by type-I and type-III interferons (IFN) of infected host and inhibit viral replication by sequestering its nucleoprotein. In the present study, we report the sequencing and characterization of Cirrhinus mrigala Mx protein (CmMx) for the first time and observed its constitutive expression in different tissues for a period of fourteen days. The synthetic peptide, LSGVALPRGTGI, was dissolved in PBS and injected into a rabbit and the antibody raised against CmMx was used to study the level of its expression. The full length of the CmMx cDNA is 2244 bp with a molecular mass of 70.9 kDa and a predicted isoelectric point of 8.25. The 627 amino acids polypeptide formed of three main functional domains: N-terminal GTPase domain (GD), a middle domain (MD) and GTPase effector domain (GED) with carboxy terminal leucine zipper motif. The 3D models of CmMx protein was modeled based on available close structural homologs and further validated through molecular dynamics (MD) simulations. MD study revealed the importance of G-domain responsible for recognition of GTP, which perfectly corroborate with earlier studies. MM/PBSA binding free energy analysis displayed that van der Waals and electrostatic energy were the key driving force behind molecular recognition of GTP by CmMx protein. The results from this study will illuminate more lights into the ongoing research on myxovirus resistance protein and its role in inhibition of viral replication in other eukaryotic system as well.

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.

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.

Identification of four type I IFNs from Japanese eel with differential expression properties and Mx promoter inducibility

Type I IFNs are a family of cytokines with antiviral, anti-proliferative and immune-modulatory functions. In this study, four type I IFNs (termed AjIFN1-4) have been cloned from the Japanese eel, Anguilla japonica. The open reading frames of AjIFN1-4 are 552, 534, 546 and 561 bp in length, encoding 183, 177, 181, and 186 amino acids (aa), respectively. Sequence comparison and phylogenetic analysis results revealed that AjIFN1 and AjIFN2 belong to group one (2C-containing) IFNs, while AjIFN3 and AjIFN4 belong to group two (4C-containing) IFNs. Syntenic comparison showed that chromosome block duplication and rearrangement events might have occurred at IFN loci in different teleost lineages. Expression analysis revealed the rapid induction of AjIFNl and AjIFN2 in response to poly I:C stimulation, while AjIFN3 and AjIFN4 were predominantly expressed at later time points. Two Mx promoter reporter assays were conducted to assess the Mx-inducing capability of AjIFN1-4. It is shown that the overexpression of AjIFN1-4 all promoted the luciferase activity of MxB reporter, but the activity of MxC reporter increased only in cells transfected with AjIFN1. Collectively, it is suggested that teleost IFNs were evolved independently in different lineages of fish and may function differently in teleost antiviral immunity.

Expression of Mx Gene in Cirrhinus mrigala (Hamilton, 1822) to OmpC Protein of Aeromonas hydrophila and Bacterial Infection

The aims of this study were to identify alternative myxovirus (Mx) stimulatory compounds in Cirrhinus mrigala and to characterize the kinetics and intensity of their stimulated responses by semi-quantitative RT-PCR. Mx transcripts were measured in C. mrigala injected with Aeromonas OmpC (outer membrane protein) at a dose 0.4 mg/fish. At day 1, day 2, day 3, day 5, day 10, day 20 and day 30, samples were collected from kidney, spleen, liver, heart brain, gill, intestine and muscle for the study of Mx transcript and housekeeping gene β-actin. Similarly, Mx gene expression was also studied in Aeromonas hydrophila-infected fish for a period of 10 days. Mx/β-actin ratio was constitutively expressed from all the organs of OmpC-vaccinated fish. The expression was significantly highest (P ≤ 0.05) in spleen, followed by liver, kidney, intestine, gill, heart, muscle and brain. The expression was highest in day 2 except spleen (on day 3) and subsequently reduced up to day 30. Control fish also showed Mx expression. Similarly, A. hydrophila-infected fish showed Mx/β-actin ratio upregulated significantly in the spleen and kidney on day 5, liver on day 2 and intestine on day 3. This study revealed that OmpC of A. hydrophila and its infection could stimulate the antiviral Mx gene of C. mrigala.

Immunization of sea bream (Sparus aurata) juveniles against Photobacterium damselae subsp. piscicida by short bath: Effect on some pro-inflammatory molecules and the Mx gene expression

Cytokines are a family of proteins derived from macrophages, lymphocytes, granulocytes, mast cells and epithelial cells and can be divided into interferons (IFNs), Interleukins (ILs) and Tumor Necrosis factors (TNFs) among others. The presence of cytokines in a wide number of fish species has been proved and several molecules types have been already cloned and sequenced. In this work some proinflamatory molecules and Mx gene were detected in the liver of vaccinated sea bream juveniles with an average body weight of 5 g. The method of immunization was by short bath and three different bacterins against the marine pathogen Photobacterium damselae subsp. piscicida were designed and used to immunize fish. Five genes encoding for five different molecules were analyzed by real time PCR: IL-1β, IL Ir-2, Cox-2, Mx and TNFα. Gene expression was quantified along four days after fish immunization and results were compared among groups. Results show that the heat-inactivated vaccine stimulates the up-regulation of IL-1β, IL Ir-2, Cox-2 and TNFα genes whereas the UV-light inactivated vaccine was the unique vaccine which stimulates the expression of Mx gene. The present is a novel study that shows by the first time the effect of the inactivation process of vaccines on the expression levels of genes involved in the defense against Photobacterium damselae subsp piscicida.

Insights into the antiviral immunity against grass carp (Ctenopharyngodon idella) reovirus (GCRV) in grass carp

Global fish production from aquaculture has rapidly grown over the past decades, and grass carp shares the largest portion. However, hemorrhagic disease caused by grass carp reovirus (GCRV) results in tremendous loss of grass carp (Ctenopharyngodon idella) industry. During the past years, development of molecular biology and cellular biology technologies has promoted significant advances in the understanding of the pathogen and the immune system. Immunoprophylaxis based on stimulation of the immune system of fish has also got some achievements. In this review, authors summarize the recent progresses in basic researches on GCRV; viral nucleic acid sensors, high-mobility group box proteins (HMGBs); pattern recognition receptors (PRRs), Toll-like receptors (TLRs) and retinoic acid inducible gene I- (RIG-I-) like receptors (RLRs); antiviral immune responses induced by PRRs-mediated signaling cascades of type I interferon (IFN-I) and IFN-stimulated genes (ISGs) activation. The present review also notices the potential applications of molecule genetic markers. Additionally, authors discuss the current preventive and therapeutic strategies (vaccines, RNAi, and prevention medicine) and highlight the importance of innate immunity in long term control for grass carp hemorrhagic disease.

The first characterization of two type I interferons in turbot (Scophthalmus maximus) reveals their differential role, expression pattern and gene induction

Type I interferons (IFNs) are considered the main cytokines directing the antiviral immune response in vertebrates. These molecules are able to induce the transcription of interferon-stimulated genes (ISGs) which, using different blocking mechanisms, reduce the viral proliferation in the host. In addition, a contradictory role of these IFNs in the protection against bacterial challenges using murine models has been observed, increasing the survival or having a detrimental effect depending on the bacteria species. In teleosts, a variable number of type I IFNs has been described with different expression patterns, protective capabilities or gene induction profiles even for the different IFNs belonging to the same species. In this work, two type I IFNs (ifn1 and ifn2) have been characterized for the first time in turbot (Scophthalmus maximus), showing different properties. Whereas Ifn1 reflected a clear antiviral activity (over-expression of ISGs and protection against viral haemorrhagic septicaemia virus), Ifn2 was not able to induce this response, although both transcripts were up-regulated after viral challenge. On the other hand, turbot IFNs did not show any protective effect against the bacteria Aeromonas salmonicida, although they were induced after bacterial challenge. Both IFNs induced the expression of several immune genes, but the effect of Ifn2 was mainly limited to the site of administration (intramuscular injection). Interestingly, Ifn2 but not Ifn1 induced an increase in the expression level of interleukin-1 beta (il1b). Therefore, the role of Ifn2 could be more related with the immune regulation, being involved mainly in the inflammation process.

Expression kinetics of key genes in the early innate immune response to Great Lakes viral hemorrhagic septicemia virus IVb infection in yellow perch (Perca flavescens)

The recently discovered strain of viral hemorrhagic septicemia virus, VHSV-IVb, represents an example of the introduction of an extremely pathogenic rhabdovirus capable of infecting a wide variety of new fish species in a new host-environment. The goal of the present study was to delineate the expression kinetics of key genes in the innate immune response relative to the very early stages of VHSV-IVb infection using the yellow perch (Perca flavescens) as a model. Administration of VHSV-IVb by IP-injection into juvenile yellow perch resulted in 84% cumulative mortality, indicating their high susceptibility to this disease. In fish sampled in the very early stages of infection, a significant up-regulation of Mx gene expression in the liver, as well as IL-1β and SAA activation in the head kidney, spleen, and liver was directly correlated to viral load. The potential down-regulation of Mx in the hematopoietic tissues, head kidney and spleen, may represent a strategy utilized by the virus to increase replication.

Characterization of four Mx isoforms in the European eel, Anguilla anguilla

Mx protein is known to play an important role in vertebrate immune response to viral infection. In this study, cDNA sequences of four Mx isoforms, designated as MxA, B, C and D were characterized in the European eel, Anguilla anguilla. These sequences contained an open reading frame of 1899, 1896, 1866, 1779 bp, flanked by 95, 53, 138, 69 bp of 5' untranslated region and 389, 241, 136, 124 bp of 3' untranslated region, respectively. A phylogenetic tree constructed with Mx peptide sequences from vertebrates revealed that MxA, C and D in the European eel formed into a clade containing zebrafish MxA and MxB and Mx proteins in other teleosts, whereas MxB in the eel was clustered together with zebrafish MxD, MxG and MxF. The transcription level of all Mx isoforms increased in a poly I:C dose-dependent manner in peripheral blood leukocytes of eels, as revealed by real-time PCR. A further experiment was conducted to reveal the temporal change in expression of these isoforms in various organs/tissues following poly I:C stimulation, and significant increase in expression was observed at various degrees in different organs or in different sampling occasions within the 12 h experimental period. In particular, MxA had the highest level of increase, while MxB had the lowest; and three isoforms, MxA, MxB and MxD had the highest increase in intestine, while the highest increase of MxC expression was observed in liver. These four isoforms of eel Mx are thus expressed differentially, and further work is certainly required to clarify the activity of promoter elements and antiviral activity of these Mx isoforms.

Pathogen-driven adaptive evolution of myxovirus resistance (Mx) genes in fishes

Myxovirus resistance (Mx) proteins, which belong to the dynamin super-family, are known to inhibit RNA viral replication in a wide range of taxonomic groups, including fishes. Given their crucial role in host immune defense, the key amino acid residues in the GTP effector domain (GED) near the C-terminus are expected to evolve adaptively in order to protect the host against invading viral pathogens. The present study reveals the role of recombination and positive selection in the evolution of Mx proteins in fishes. While the GTP-binding domain in the N-terminal domain has experienced purifying selection, several amino acid residues in GED have evolved under positive selection, thus indicating adaptive evolution. Given the antiviral activity of GED, the adaptive evolutionary changes that were observed in this region are therefore predicted to be pathogen-driven.

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.

Effect of lipopolysaccharides from Vibrio alginolyticus on the Mx gene expression and virus recovery from gilthead sea bream (Sparus aurata L.) experimentally infected with Nodavirus

Infections with nodavirus affect a wild and farmed fish species throughout the world, mostly from the marine environment. The aim of this work was to determine the immune status of gilthead sea bream that comes as a result of a Nodavirus infection, induced by activation of the interferon response pathway by lipopolysaccharides from Vibrio alginolyticus and the expression of interferoninduced Mx protein in liver samples. The enhancement of Mx protein gene expression was detected in liver samples of experimentally nodavirus infected fish and, furthermore, the immunostimulant LPS of V. alginolyticus decreased almost three times the virus titration with respect to no-immunized or infected with nodavirus group of fish.

Protective roles of grass carp Ctenopharyngodon idella Mx isoforms against grass carp reovirus

Background

Myxovirus resistance (Mx) proteins are crucial effectors of the innate antiviral response against a wide range of viruses, mediated by the type I interferon (IFN-I) signaling pathway. However, the antiviral activity of Mx proteins is diverse and complicated in different species.

Methodology/Principal Findings

In the current study, two novel Mx genes (CiMx1 and CiMx3) were identified in grass carp (Ctenopharyngodon idella). CiMx1 and CiMx3 proteins exhibit high sequence identity (92.1%), and low identity with CiMx2 (49.2% and 49.5%, respectively) from the GenBank database. The predicted three-dimensional (3D) structures are distinct among the three isoforms. mRNA instability motifs also display significant differences in the three genes. The spatial and temporal expression profiles of three C. idella Mx genes and the IFN-I gene were investigated by real-time fluorescence quantitative RT-PCR (qRT-PCR) following infection with grass carp reovirus (GCRV) in vivo and in vitro. The results demonstrated that all the four genes were implicated in the anti-GCRV immune response, that mRNA expression of Mx genes might be independent of IFN-I, and that CIK cells are suitable for antiviral studies. By comparing expression patterns following GCRV challenge or poly(I:C) treatment, it was observed that GCRV blocks mRNA expression of the four genes. To determine the functions of Mx genes, three CiMx cDNAs were cloned into expression vectors and utilized for transfection of CIK cells. The protection conferred by each recombinant CiMx protein against GCRV infection was evaluated. Antiviral activity against GCRV was demonstrated by reduced cytopathic effect, lower virus titer and lower levels of expressed viral transcripts. The transcription of IFN-I gene was also monitored.

Conclusions/Significance

The results indicate all three Mx genes can suppress replication of grass carp reovirus and over-expression of Mx genes mediate feedback inhibition of the IFN-I gene.

Atlantic halibut experimentally infected with nodavirus shows increased levels of T-cell marker and IFNγ transcripts

The transcript levels of viral RNAs, selected T-cell marker and cytokine genes, toll like receptor (TLR) 7, and two interferon stimulated genes (ISG) were analysed in sexually immature adult Atlantic halibut (Hippoglossus hippoglossus L.) experimentally infected with nodavirus. The expression of the T-cell markers, TLR7 and the cytokine genes was further explored in in vitro stimulated anterior kidney leucocytes (AK leucocytes) isolated from the experiment fish and from additional untreated non-injected fish. The levels of viral RNA1 and RNA2 were increasing in brain and eye at around 4 and 8weeks post injection (wpi), respectively, and still increasing at the end of the experiment, especially in eye. Immuno-positive cells and signs of vacuolisation in both brain and eye were seen at 14wpi. Increased transcript levels of TCRβ, CD4-2, CD4, CD8α, and Lck in brain and eye of the experimentally infected halibut suggested an involvement of halibut T-cells in the immune response against nodavirus. Interestingly, a similar expression pattern of TCRβ, CD4 and Lck was seen in both brain and eye. However, compared to brain that showed elevated transcript levels of TCRβ, CD4 and Lck mainly at 10 and 14wpi, the increase appeared earlier between 3 and 4wpi in the eye. Yet, an increase in the transcript level of IFNγ was seen at 10 and 14wpi in both organs. Moreover, elevated levels of TLR7, IL-1β, IL-6, ISG15 and Mx were detected in vivo. The in vitro experiments, stimulating AK leucocytes with ConA-PMA, imiquimod or nodavirus, further supported an involvement of IL-6 and IFNγ in the immune response against nodavirus and the involvement of CD8β(+) cells. Results from the present study thus indicate an importance of T-cells, IFNγ and the analysed ISGs in the immune response against nodavirus in Atlantic halibut, and would be of great help in future vaccination trials giving the possibility to monitor the immune response rather than mortality during post-vaccination challenge experiments.

Early antiviral response and virus-induced genes in fish

In fish as in mammals, virus infections induce changes in the expression of many host genes. Studies conducted during the last fifteen years revealed a major contribution of the interferon system in fish antiviral response. This review describes the screening methods applied to compare the impact of virus infections on the transcriptome in different fish species. These approaches identified a "core" set of genes that are strongly induced in most viral infections. The "core" interferon-induced genes (ISGs) are generally conserved in vertebrates, some of them inhibiting a wide range of viruses in mammals. A selection of ISGs -PKR, vig-1/viperin, Mx, ISG15 and finTRIMs - is further analyzed here to illustrate the diversity and complexity of the mechanisms involved in establishing an antiviral state. Most of the ISG-based pathways remain to be directly determined in fish. Fish ISGs are often duplicated and the functional specialization of multigenic families will be of particular interest for future studies.

Euthanization methods influence cytokine mRNA expression levels in age 0 year Oncorhynchus mykiss

Significant differences in cytokine transcription were found between Oncorhynchus mykiss euthanized using the pharmacological agents MS-222 v. benzocaine and also when contrasting death induced by carbon dioxide asphyxiation v. physical methods (cervical dislocation). This study highlights the need to consider the potentially confounding effect of euthanization method on gene expression data.

Evaluation of the level of Mx3 protein synthesis induced by infectious pancreatic necrosis virus (IPNV) strains of different infectivity

The in vitro infectivity and genotype of three IPNV strains (V70, V112 and V98) was linked to the level of transcript synthesis for the Mx3 protein in RTG-2 (Rainbow trout gonad) cells and in Salmo salar. The V70 and V98 strains corresponded to the Sp genotype, whilst the V112 corresponded to VR-299; the presence of Pro-217 and Ala-221 in VP2 identified V70 as a strain of medium virulence level whilst V112 (Ala-217 and Thr-221) and V98 (Pro-217 and Thr-221) were of low virulence. This is concurrent with several in vitro tests which showed V70 to be a strain with highly infectivity (P<0.05). In both the in vitro and in vivo trials, the strains demonstrated the induction of the Mx transcript, although no differences were detected, and the level always were significantly lesser that observed in poly I:C samples. The results suggest that the infectivity observed is related to the presence of certain specific residues in VP2, and that neither the infectivity nor the genotype appears to bear any relation to Mx induction capacity.

The fight between the teleost fish immune response and aquatic viruses

Teleost fish represent a transition point on the phylogenetic spectrum between invertebrates that depend only on innate immunity and mammals that heavily depend on adaptive immunity. The major mechanisms of the teleost fish innate immune response are suggested to be similar to mammals, although fine details of the process require further studies. Within the innate immune response the type I interferon (IFN) system is an essential innate antiviral component that protects fish from some virus infections. The current progress of cloning and functional characterization of fish antiviral genes is promising in further elucidation of the fish antiviral response. The adaptive immune system of fish utilizes cellular components more or less similar to mammals. Teleost fish produce IgM as a primary antibody response and lack isotype switching to mount virus-specific antibodies during the infection process. Despite this, the development of successful fish rhabdoviral vaccines suggest that vaccination may prove to be an effective way of promoting fish adaptive immune responses to viruses. This paper reviews the bony fish antiviral response with specific discussion on the evolutionary mechanisms that allow aquatic viruses to co-exist with their host. Detailed aspects of the teleost type I IFN system are also addressed.

Rapid antiviral assay using QD-tagged fish virus as imaging nanoprobe

Development of rapid antiviral assays can expedite the process of screening potential agents against viral pathogens. In the present study, fluorescent quantum dots (QDs) incorporated with infectious pancreatic necrosis virus (IPNV) were used as imaging nanoprobes to detect the threshold amount of poly I:C (an interferon inducer) required to induce zebrafish cells into an antiviral state against IPNV. QD-IPNV hybrids were formed by colloidal clustering of negatively charged QDs and IPNV, using the cationic polymer polybrene (50 μg/mL). To test the screening potential of the QD-IPNV hybrids for anti-IPNV drug candidates, zebrafish ZF4 cells primed with the immunostimulant poly I:C at concentrations of 1, 5, and 10 μg/mL for 6h were used as a model system. After poly I:C treatment, cells were exposed to the QD-IPNV hybrids for 6h at a multiplicity of infection (MOI) of 5. The anti-IPNV effectiveness of poly I:C was assessed via fluorescence intensity of the QDs. Our results showed that ZF4 cells primed with poly I:C at 10 μg/mL were highly protected from IPNV challenge (i.e., no detection of QD fluorescence). In summary, a rapid and efficient cell-based imaging platform has been developed for assessing the anti-IPNV activity of poly I:C on ZF4 cells using QD-IPNV hybrids. This approach may be applied to a wider range of fish species and fish pathogenic viruses.

Molecular cloning and expression analysis of three Mx isoforms of rock bream, Oplegnathus fasciatus

Complementary DNAs (cDNAs) corresponding to three isoforms of rock bream (Oplegnathus fasciatus) Mx (RbMx1, RbMx2 and RbMx3) were cloned using RACE reactions. Analysis of deduced amino acid sequences revealed that the tripartite GTP-binding domain, the dynamine family signature and the leucine zipper repeat were present in all three rock bream Mx isoforms. Cloning of genomic DNA sequence and Southern blot analysis showed that three rock bream Mx isoforms were encoded by different genomic loci, and they were not alternative splicing variants, although some alternative splicing variants were found in RbMx1 and RbMx2. When comparing amino acid sequence identity, RbMx1 shares about 60-70% identities with other fish Mx proteins, whereas both RbMx2 and RbMx3 share slightly high identity of 70-90%. As a result of expression analysis using RT-PCR, RbMx1 was constitutively expressed in the spleen and kidney of rock bream yearling, but RbMx2 and RbMx3 were rarely detected in both organs. When injected with synthetic double-stranded RNA polyinosinic:polycytidylic acid (poly I:C), expression of all rock bream Mx isoforms was up-regulated in spleen and head kidney. RbMx1 was continuously up-regulated throughout experimental period of 72 h but RbMx2 and RbMx3 were down-regulated to almost non-detectable level at 48 h post-injection.

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.

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.

Nodavirus increases the expression of Mx and inflammatory cytokines in fish brain

Nodavirus has become a serious pathogen for a wide range of cultured marine fish species. In the present work, the expression of genes related to immune and inflammatory responses of sea bream (Sparus aurata L.), considered as non susceptible species, was studied both in vitro and in vivo. No replication of the virus was observed in head kidney macrophages and blood leukocytes. Moreover, the enhancement of expression of several immune genes (tumor necrosis factor alpha (TNFalpha), interleukin-1-beta (IL-1beta), interferon-induced Mx protein) was not detected in both head kidney macrophages and blood leucocytes in response to an in vitro infection with nodavirus. However, in vivo, nodavirus was detected 1 day post-infection (p.i.) by a reverse transcription-polymerase chain reaction (RT-PCR) in blood, liver, head kidney and brain of experimentally infected sea bream, while its presence clearly decreased in blood after 3 days p.i. Also, a transitory increment of the expression of TNFalpha and IL-1beta was detected in the brain of intramuscular (i.m.) infected sea bream 3 days p.i. In head kidney, the over expression of TNFalpha was only observed 1 day p.i. The expression of Mx, an interferon induced gene, was increased in brain and head kidney of infected sea bream, reaching values of 1300-fold compared to controls in brain three days post-infection. For comparative purposes, we analyzed the expression of the same genes on a susceptible species, such as sea bass (Dicentrarchus labrax) and, although the same pattern of expression was observed both in brain and kidney, the magnitude was different mainly in the case of brain, the key organ of the infection, where higher expression of TNFalpha and lower expression of Mx compared with control was observed.

Expression of Mx protein in tissues of Atlantic salmon post-smolts--an immunohistochemical study

A rabbit antiserum was produced from a 12-amino acid long peptide common to the 3 known isoforms of Atlantic salmon Mx proteins. The antibody stained ASK-1 cells 48h after stimulation with poly I:C. In Western blots of these cells, the antibody stained a doublet with MW about 75kDa and another band at about 65kDa, typical of the MW of Atlantic salmon Mx. Western blots of kidney from IPNV-injected salmon showed a similar staining pattern. In immunohistochemistry, the antibody stained the gill, kidney and liver tissue of a fish infected with IPNV by cohabitation. These tissues also expressed high levels of interferon (IFN) and Mx transcripts as determined by real-time qRT-PCR. Normal healthy salmon post-smolts sampled at 4-8 weeks after transfer to sea water had very low-level expression of IFN and Mx transcripts. However, at 4 and 5 weeks after sea water transfer the gill, kidney and liver of these fish stained strongly for Mx protein. Thereafter, immunostaining of Mx markedly diminished in all tissues, persisting weakly in the gill. It has been reported that Atlantic salmon smolts constitutively express IFN and Mx transcripts around the time of smolting. Presumably the Mx protein detected in the tissues for about 6 weeks after transfer to sea water resulted from such a transcriptional event. As Mx is known to provide protection against IPNV infections it is tempting to associate the duration of persistence of Mx protein with the outbreaks of IPN-related mortalities in post-smolts, 6-8 weeks after transfer to sea water.

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.

Expression analysis of Mx protein and evaluation of its antiviral activity against sole aquabirnavirus in SAF-1 and TV-1 cell lines

The transcription of Mx mRNA after poly I:C induction and sole aquabirnavirus infection has been analysed in SAF-1 and TV-1 cells (derived from gilt-head seabream and turbot, respectively). Both cell lines were stimulated with 10 microg ml(-1) poly I:C and Mx mRNA was analysed by a specific RT-PCR at several times post-induction. The results showed a high level of Mx expression from 12 to 120 h after induction in SAF-1 cells, whereas in TV-1 cells Mx mRNA was only detected at 12 and 24h. The treatment with different concentrations of poly I:C showed that TV-1 cells are less sensitive to this inductor than the SAF-1 cell line. The antiviral activity derived from poly I:C induction has been clearly demonstrated against sole aquabirnavirus on both cell lines. The inoculation of sole aquabirnavirus resulted in the Mx mRNA transcription at 48, 72, and 96 h post-infection (p.i.) in SAF-1 cells. On the contrary, inoculated TV-1 cells only showed a faint Mx mRNA band at 24 and 48 h p.i. This study has established different patterns of Mx expression in both cells under study as a consequence of the poly I:C induction and sole aquabirnavirus infection, and it shows that gilt-head seabream and turbot Mx inhibit sole aquabirnavirus replication.

Difference in Japanese flounder, Paralichthys olivaceus gene expression profile following hirame rhabdovirus (HIRRV) G and N protein DNA vaccination

The glycoprotein (G protein) gene, but not other genes, of fish rhabdoviruses, when used as a DNA vaccine was previously shown to be highly effective in inducing a protective immune response. In this study we used a DNA microarray to examine differential gene expression in Japanese flounder (Paralichthys olivaceus) in response to a DNA vaccine made from the genes of hirame rhabdovirus (Rhabdovirus olivaceus) (HIRRV) G protein (pHRV-G) and nucleocapsid (N) protein (pHRV-N). A high level of protection against HIRRV infection was observed following vaccination with the pHRV-G but no protection was observed following vaccination with the pHRV-N. Microarray analyses showed that the set of genes induced by pHRV-G was different from the set induced by pHRV-N. Specifically, five genes (Interferon-stimulated gene, 15kDa (ISG15), Interferon-stimulated gene, 56kDa (ISG56), Mx and two unknown genes) were strongly induced after injection by the pHRV-G but not pHRV-N and three of these genes are known as type I IFN-inducible genes. Poly I:C, a known inducer of type I interferon that elicits immune response similar to that elicited by a virus infection, also induced these five genes in kidney cells. These results suggest that in order to be effective and confer protection, vaccines against HIRRV and probably fish rhabdoviruses may need to stimulate the type I IFN system.

Effects of salmonid fish viruses on Mx gene expression and resistance to single or dual viral infections

We examined the ability of several fish viruses to induce protection against homologous or heterologous viruses in single or double infections, and assessed whether such protection is correlated with innate immunity or expression of the Mx gene. Monolayers of BF2 cells pre-treated with supernatants of brown trout (Salmo trutta L.) macrophage cultures that had been stimulated with either polyinosinic polycytidylic acid (poly I:C) or viruses, such as infectious pancreatic necrosis virus (IPNV), infectious haematopoietic necrosis virus (IHNV) or a mixture of the two, showed varying degrees of protection against viral infections. The virus showing the strongest induction was IPNV, and the antiviral activity against IHNV was also high: around 6 log(10) reduction of virus yield. Consequently, the IPNV-IHNV co-infection yield was also reduced by varying amounts. In vivo, the cumulative mortality observed in the IPNV-IHNV co-infected fish was always less than that in those with a single infection. Stimulation with poly I:C for 7 days significantly reduced cumulative mortality in single-infected fish, but not in the double-infected, in which the IPNV was the only virus isolated from moribund animals. By RT-PCR, Mx was expressed in all the organ samples tested (kidney, liver and spleen) from virus-stimulated fish at 1, 2 and 3 days. By qRT-PCR the extent and timing of Mx expression was shown to differ in the poly I:C and the single or dual viral infections. The highest increase in Mx expression (21.6-fold above basal levels) occurred (after 24 h) in fish infected with the IHNV, and expression remained high until day 7. Mx expression in fish infected with IPNV peaked later, at 2 days post infection, and also remained high until day 7. The dual infection with IPNV-IHNV induced high Mx expression on day 1, which peaked on day 2 and remained high until day 7. These results indicate that activation of the immune system could explain the interference and loss of IHNV in the IPNV-IHNV co-infections.

Gene structure and transcription of IRF-1 and IRF-7 in the mandarin fish Siniperca chuatsi

The genes of IRF-1 and IRF-7 have been cloned from the mandarin fish (Siniperca chuatsi). The IRF-1 gene has 4919 nucleotides (nt) and contains 10 exons and 9 introns, with an open reading frame (ORF) of 903nt encoding 301aa. The IRF-7 gene has 6057nt and also contains 10 exons and 9 introns, with an ORF of 1308nt encoding 436aa. The IRF-1 and IRF-7 genes have only one copy each in the genome. The transcription of IRF-1 and IRF-7 in different organs was analyzed by real-time PCR, and both molecules were constitutively expressed. The IRF-1 and IRF-7 mRNAs were abundant in gill, spleen, kidney and pronephros. The temporal transcriptional changes for IRF-1, IRF-7 and Mx were investigated within 48h after poly I: C stimulation in liver, gill, spleen and pronephros. An increased transcription was detected for IRF-1 and IRF-7 12h post-stimulation, being earlier than the transcription of Mx protein; however, IRF-1 and IRF-7 transcription decreased while the Mx protein was stable at 48h post-stimulation.

Infectious pancreatic necrosis virus suppresses type I interferon signalling in rainbow trout gonad cell line but not in Atlantic salmon macrophages

RTG-P1 cells are a rainbow trout fibroblastic cell line permanently transfected with the luciferase gene under the control of the Mx promoter. On exposure to interferon (IFN) or IFN inducing agents, the cells produce luciferase. IPNV did not induce luciferase production up to 24h post-infection but did not suppress constitutive luciferase production. Furthermore, IPNV suppressed luciferase production induced by poly I:C. RT-PCR analysis of IPNV infected cells showed IFN gene transcription from 6h post-infection with increasing expression up to 24h. Housekeeping genes beta-actin and GAPDH were also expressed along with upregulation of IRF1 and slight upregulation of STAT1. When RTG-P1 cells were stimulated with IFN, Mx transcripts, measured by qRT-PCR, peaked at 3-6h and thereafter fell to low levels, but in the presence of IPNV, Mx transcription at this time was significantly suppressed but continued to rise gradually. Luciferase production was lower in infected cells at 12h post-infection but not significantly after 24h. These results indicate that, in non-stimulated RTG-P1 cells, while IPNV induces IFN transcription, activation of Mx expression is suppressed. Furthermore, when stimulated by IFN, the rate of Mx transcription is significantly suppressed by the virus. This would probably give time for the virus to replicate rapidly in the early phases of infection. Contrary to the fibroblastic cell line, IPNV stimulated IFN production by salmon macrophages in vitro at least as strongly as poly I:C, with no suppression of the IFN response to poly I:C, and the virus persisted for up to 9 days without causing CPE.

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.

Promoters of type I interferon genes from Atlantic salmon contain two main regulatory regions

Recognition of viral nucleic acids by vertebrate host cells results in the synthesis and secretion of type I interferons (IFN-alpha/beta), which induce an antiviral state in neighboring cells. We have cloned the genes and promoters of two type I IFNs from Atlantic salmon. Both genes have the potential to encode IFN transcripts with either a short or a long 5'-untranslated region, apparently controlled by two distinct promoter regions, PR-I and PR-II, respectively. PR-I is located within 116 nucleotides upstream of the short transcript and contains a TATA-box, two interferon regulatory factor (IRF)-binding motifs, and a putative nuclear factor kappa B (NFkappaB)-binding motif. PR-II is located 469-677 nucleotides upstream of the short transcript and contains three or four IRF-binding motifs and a putative ATF-2/c-Jun element. Complete and truncated versions of the promoters were cloned in front of a luciferase reporter gene and analyzed for promoter activity in salmonid cells. Constructs containing PR-I were highly induced after treatment with the dsRNA poly(I:C), and promoter activity appeared to be dependent on NFkappaB. In contrast, constructs containing exclusively PR-II showed poor poly(I:C)-inducible activity. PR-I is thus the main control region for IFN-alpha/beta synthesis in salmon. Two pathogenic RNA viruses, infectious pancreatic necrosis virus and infectious salmon anemia virus, were tested for their ability to stimulate the minimal PR-I, but only the latter was able to induce promoter activity. The established IFN promoter-luciferase assay will be useful in studies of host-virus interactions in Atlantic salmon, as many viruses are known to encode proteins that prevent IFN synthesis by inhibition of promoter activation.

The interferon system of teleost fish

Interferons (IFNs) are secreted proteins, which induce vertebrate cells into an antiviral state. In mammals, three families of IFNs (type I IFN, type II IFN and IFN-lambda) can be distinguished on the basis of gene structure, protein structure and functional properties. Type I IFNs, which include IFN-alpha and IFN-beta, are encoded by intron lacking genes and have a major role in the first line of defense against viruses. The human IFN-lambdas have similar biological properties as type I IFNs, but are encoded by intron containing genes. Type II IFN is identical to IFN-gamma, which is produced by T helper 1 cells in response to mitogens and antigens and has a key role in adaptive cell mediated immunity. IFNs, which show structural and functional properties similar to mammalian type I IFNs, have recently been cloned from Atlantic salmon, channel catfish, pufferfish, and zebrafish. Teleost fish appear to have at least two type I IFN genes. Phylogenetic sequence analysis shows that the fish type I IFNs form a group separated from the avian type I IFNs and the mammalian IFN-alpha, -beta and -lambda groups. Interestingly, the fish IFNs possess the same exon/intron structure as the IFN-lambdas, but show most sequence similarity to IFN-alpha. Recently, IFN-gamma genes have also been cloned from several fish species and shown to have the same exon/intron structure as mammalian IFN-gamma genes. The antiviral effect of mammalian type I IFN is exerted through binding to the IFN-alpha/beta-receptor, which triggers signal transduction through the JAK-STAT signal transduction pathway resulting in expression of Mx and other antiviral proteins. Putative IFN receptor genes have been identified in pufferfish. Several interferon regulatory factors and members of the JAK-STAT pathway have also been identified in various fish species. Moreover, Mx and several other interferon stimulated genes have been cloned and studied in fish. Furthermore, antiviral activity of Mx protein from Atlantic salmon and Japanese flounder has recently been demonstrated.

Cytotoxic activities of fish leucocytes

Like mammalian leucocytes, white blood cells of fish are able to kill altered (e.g. virus-infected) and foreign (allogeneic or xenogeneic) cells. The existence of natural killer (NK)-like and specific cytotoxic cells in fish was first shown using allogeneic and xenogeneic effector/target cell systems. In addition to in vivo and ex vivo studies, very important contributions were made by in vitro analysis using a number of different long-term cytotoxic cell lines established from channel catfish. In mammals, specific cell-mediated cytotoxicity (CMC) as part of the adaptive immune response requires a number of key molecules expressed on effector leucocytes and target cells. CD8+ T lymphocytes kill infected cells only, if their antigen receptor (TCR) matches the MHC class I with bound peptide of the target cell. Expression patterns of the fish gene homologues for TCR, CD8 and MHC class I, as well as related genes, are in agreement with similar function. Convenient systems for the analysis of specific CMC have only recently become available for fish with the combination of clonal fish with syngeneic or allogeneic but MHC class I matching cell lines. It was demonstrated that both, NK- and cytotoxic T (Tc) cells are involved in the killing of virus infected MHC class I matching and mismatching target cells. Analysis of these lymphocyte subsets is only starting for fish. There is also evidence that the different viral proteins trigger different subsets of killer cells. This review further discusses findings on fish CMC with regard to temperature/seasons and ontogeny.

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.

Interferon mediated antiviral activity against salmonid fish viruses in BF-2 and other cell lines

Double-stranded RNA and type I interferon-like activity induce an antiviral state in vertebrate cells and in several fish cell lines by increasing the expression of proteins that inhibit virus replication. We compared the protection induced by the polyinosinic:polycytidylic acid (poly I:C) or poly I:C plus transfection agents against the infectious pancreatic necrosis virus (IPNV) and the infectious hematopoietic necrosis virus (IHNV) in BF-2 cells, with that induced in RTG-2, CHSE-214, or SAF cells. In addition, we examined the reduction in the infective titers of these viruses and the correlation with Mx protein expression as IFN marker. Furthermore, the suitability of BF-2 cells for the evaluation and optimization of immune responses in an IPNV-IHNV co-infection was assessed. The results demonstrated strong anti-IPNV and anti-IHNV activity (around 90% of infected cells surviving) in BF-2 cells transfected with poly I:C, in which a loss of 1log(10) or 3log(10) of the IPNV or IHNV infective titers, respectively, was observed. No antiviral activity was evident in the cells incubated with poly I:C alone. The protection recorded in the co-infection experiments was comparable with those of the single infections. The SAF cell line exhibited the lowest antiviral capacity (45%), which was also increased after transfection with poly I:C. In addition, medium from transfected BF-2 provided protection against IPNV (1log(10) loss of infective titer) and IHNV (2log(10) loss of infective titer) in new monolayers, indicating that these cells secreted the factors that induce antiviral activity. A correlation between antiviral activity and Mx protein expression was observed in all the cells. These results indicate that poly I:C transfection could improve IFN-like production in these cell lines. However, the antiviral effectiveness of poly I:C differed between cell lines. On the basis of our findings, we conclude that the BF-2 cell line is a useful model in which to study the role of IFN-induced cytokines in resistance against single or double infections with salmonid fish viruses.

Intracellular localization and antiviral property of canine Mx proteins

Mx is an interferon (IFN)-induced protein that shows antiviral activities against RNA viruses. We examined an expression of mRNA, an intracellular localization of protein, and an antiviral property of canine Mx1 and Mx2. Both Mx1 and Mx2 mRNAs were induced in a canine kidney cell line Madin-Darby canine kidney (MDCK), stimulated with an IFN-inducer, poly(I) x poly(C) for 12 h, suggesting the presence of regulatory mechanisms consistent with Mx genes in other species. By immunostaining BALB/3T3 fibroblasts transiently transfected FLAG epitope-tagged canine Mx1 and Mx2 cDNAs with an anti-FLAG tag, it was revealed that both Mx1 and Mx2 proteins are localized in cytoplasm. BALB/3T3 fibroblasts expressing stably Mx2 but not Mx1 had an antiviral activity against recombinant vesicular stomatitis virus (VSV). This is the first report demonstrating the functional analysis of canine Mx proteins.

Characterization of snakehead rhabdovirus infection in zebrafish (Danio rerio)

The zebrafish, Danio rerio, has become recognized as a valuable model for the study of development, genetics, and toxicology. Recently, the zebrafish has been recognized as a useful model for infectious disease and immunity. In this study, the pathogenesis and antiviral immune response of zebrafish to experimental snakehead rhabdovirus (SHRV) infection was characterized. Zebrafish 24 h postfertilization to 30 days postfertilization were susceptible to infection by immersion in 10(6) 50% tissue culture infective doses (TCID50) of SHRV/ml, and adult zebrafish were susceptible to infection by intraperitoneal (i.p.) injection of 10(5) TCID50 of SHRV/ml. Mortalities exceeded 40% in infected fish, and clinical presentation of infection included petechial hemorrhaging, redness of the abdomen, and erratic swim behavior. Virus reisolation and reverse transcription-PCR analysis of the viral nucleocapsid gene confirmed the presence of SHRV. Histological sections of moribund embryonic and juvenile fish revealed necrosis of the pharyngeal epithelium and liver, in addition to congestion of the swim bladder by cell debris. Histopathology in adult fish injected i.p. was confined to the site of injection. The antiviral response in zebrafish was monitored by quantitative real-time PCR analysis of zebrafish interferon (IFN) and Mx expression. IFN and Mx levels were elevated in zebrafish exposed to SHRV, although expression and intensity differed with age and route of infection. This study is the first to examine the pathogenesis of SHRV infection in zebrafish. Furthermore, this study is the first to describe experimental infection of zebrafish embryos with a viral pathogen, which will be important for future experiments involving targeted gene disruption and forward genetic screens.

Poly I:C induces an antiviral state against Ictalurid Herpesvirus 1 and Mx1 transcription in the channel catfish (Ictalurus punctatus)

In vivo studies were carried out to investigate the protective effect of the interferon inducer poly I:C against channel catfish virus (CCV). Channel catfish were stimulated by intraperitoneal injection of 50 microg of poly I:C or PBS at various days prior to immersion challenge with CCV. Mortality in the poly I:C group was significantly reduced from 70% to 3% at day 1 compared to the PBS controls. Mortality increased at day 3 but was still significantly different from the PBS controls. Mx1 transcription was significantly higher only at day 1. In an additional study Mx1 transcription was monitored in the liver, kidney, gills, spleen, and intestine at various time points post-stimulation with either poly I:C or CCV. Mx1 mRNA was significantly elevated in all organs only at day 1 post-injection with poly I:C. In response to CCV, Mx1 transcription was not significantly elevated until day 3 post-challenge, but remained elevated in certain organs until day 7.

Kinetics of Mx expression in rainbow trout (Oncorhynchus mykiss) and Atlantic salmon (Salmo salar L.) parr in response to VHS-DNA vaccination

The duration of the Mx mRNA response to an intramuscular injection of the viral haemorrhagic septicaemia virus (VHSV) glycoprotein (G) gene DNA vaccine as well as to the control plasmid was determined in rainbow trout at 14 degrees C over a period of 11 weeks. The Mx response was detectable on day 7, peaked on day 14 and returned to pretreatment levels on day 21 and thereafter. No increase in Mx expression was detectable to the control plasmid. In further experiments, the kinetics of the Mx response were compared in rainbow trout and Atlantic salmon parr kept at 10 degrees C and injected with the DNA vaccine or the synthetic double-stranded RNA, poly I:C. In both species there was a rapid response to poly I:C detectable from day 1, reaching maximum from days 3 to 9 and decreasing to background level by day 12. The peak level and return to background was reached slightly later in salmon. In both species the response to the VHS/DNA vaccine was slower to begin, not being detectable on days 1 and 3, but elevated levels were found on day 6. However, in the salmon parr, the peak level was on day 6 and the signal disappeared by day 12, while in the rainbow trout, the response peaked at day 12 and lasted until day 21. The kinetics of the Mx response to the VHS/DNA vaccine in rainbow trout correlate with the early non-specific protection against VHS in this species following vaccination. It is speculated that the more transient Mx response in Atlantic salmon parr to the DNA vaccine may be related to the innate resistance of salmon to VHS.