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

Immunoanalysis of antiviral Mx protein expression in Japanese flounder (Paralichthys olivaceus) cells

Mx proteins are interferon-inducible GTPases that possess antiviral properties in vertebrates. Japanese flounder (Paralichthys olivaceus) Mx protein has previously been shown to possess some antiviral activity against rhabdoviruses. A polyclonal antibody was generated against a purified peptide fragment of Japanese flounder Mx protein that had been produced in an Escherichia coli expression system. The PAb detected the approximately 71 kDa Mx protein from Japanese flounder (hirame) natural embryo (HINAE) cells that had been cultured with poly I:C, an interferon inducer, but not in unstimulated cells. The polyclonal antibody did not cross react with Mx protein from carp epithelial, grouper fin and zebrafish embryo cell lines that had been similarly induced or transfected with poly I:C. By immunofluorescence cytochemistry, Japanese flounder Mx protein was localized to the cell cytoplasm. Hirame rhabdovirus stimulated expression of Mx protein in the infected and surrounding HINAE cells. Within virus-infected cells, there was some indication of Mx protein colocalizing with viral proteins. Poly I:C stimulation of HINAE cells induced an early increase in Mx protein mRNA transcripts, but maximum Mx mRNA transcript and protein expression was reached after 48 h. Both Mx mRNA transcripts and protein levels were maintained till at least 72 h.

Mx expression in Atlantic salmon (Salmo salar L.) parr in response to Listonella anguillarum bacterin, lipopolysaccharide and chromosomal DNA

Mx genes are inducible by Type I interferons and are involved in antiviral defences. A commercially available vibrio bacterin, intended for immersion vaccination, was shown to be a potent inducer of Mx gene expression in Atlantic salmon parr following intraperitoneal injection. The response was dose and temperature dependent. At 10 degrees C and 10 times concentration the bacterin induced Mx response kinetics similar to that induced by poly I:C. At 10 degrees C, enhanced Mx responses were detected from days 1 to 9 with both 1 times (1x) and 10 times (10x) concentrated bacterin, with a tendency for a higher response to the concentrated bacterin on days 1 and 3. Basal levels of Mx mRNA were detected on day 12 after injection to both concentrations. The response induced by poly I:C was higher on day 1 and it was still present at day 12, with basal levels being reached on day 18. At 6 degrees C, there was a more definitive dose effect of the vibrio bacterin and the Mx response was delayed in comparison to that at 10 degrees C. Increased Mx expression did not appear until day 6 and with the 1x dose it had disappeared by day 9. However, the 10x dose continued to induce Mx at day 12, disappearing by day 18. The Mx response to the purified Listonella anguillarum lipopolysaccharide (LPS) and DNA in fish held at 10 degrees C showed some differences in the rate of onset. The response to DNA was faster, beginning on day 1 compared with day 3 for the LPS. The response to DNA peaked on day 3 while for LPS the peak was on day 9. However, the response to both components had disappeared by day 12. The response kinetics to the L. anguillarum DNA was essentially similar to the 10x dose of the vibrio bacterin and to poly I:C at 10 degrees C.

Inhibition of infectious pancreatic necrosis virus replication by atlantic salmon Mx1 protein

Mx proteins form a family of interferon (IFN)-induced GTPases with potent antiviral activity against various single-stranded RNA viruses in mammals and chickens. In fish, alpha/beta IFN has been reported to inhibit the replication of infectious pancreatic necrosis virus (IPNV), but the mode of action has not been elucidated. A correlation between the inhibition of IPNV and Mx protein expression has, however, been observed. To examine whether Atlantic salmon Mx1 protein (ASMx1) possesses antiviral activity against IPNV, CHSE-214 cells constitutively expressing ASMx1 were established. ASMx1 appeared to be localized in the cytoplasm. The ASMx1-expressing clone selected showed a severely reduced IPNV-induced cytopathic effect, which was confirmed by a 500-fold reduction in virus yield. The antiviral activity against IPNV was further confirmed by the inhibition of virus protein synthesis and the reduced accumulation of virus transcripts. The present work further adds to the body of evidence which suggests that antiviral activity is a major functional role of vertebrate Mx proteins. Moreover, the list of viruses inhibited by Mx proteins is extended to include double-stranded RNA viruses.

Characterization of Atlantic halibut (Hippoglossus hippoglossus) Mx protein expression

Mx proteins are antiviral GTPases that are induced by type I interferons in vertebrates. An Atlantic halibut Mx cDNA (HHMx) was recently cloned. In this work, a polyclonal antiserum against HHMx protein was generated that detected a 71 kDa protein in the nuclei of Chinook salmon embryo cells transfected with the HHMx cDNA. Mx protein expression in organs of halibut was studied by immunoblot analysis after injection with the double-stranded RNA poly I:C or infectious pancreatic necrosis virus. Poly I:C stimulated increased Mx protein expression in liver, kidney, heart, spleen, gills and intestine. The Mx protein level in liver reached a maximum after 3 days and remained elevated for 14 days after treatment. IPNV infection resulted in increased Mx protein in liver from 4 to at least 35 days. Immunocytochemical detection of Mx proteins in blood smears from poly I:C treated halibut indicated that a cytoplasmic Mx form might exist in this species. Detection of Mx proteins in blood leukocytes could thus work as an early non-lethal test for viral infections.

In vivo and in vitro analysis of the resistance against viral haemorrhagic septicaemia virus in Japanese flounder (Paralichthys olivaceus) precedingly infected with aquabirnavirus

The resistance of Japanese flounder (Paralichthys olivaceus Temminck et Schlegel) against a viral haemorrhagic septicaemia virus (VHSV) challenge induced by a preceding non-lethal aquabirnavirus (ABV) challenge was investigated through experimental dual-infections with different intervals between the two challenges. The non-specific protection conferred by the primary ABV infection against the secondary VHSV infection commenced at Day 3 and persisted up to Day 14 but vanished at Day 21 post-ABV challenge. The in vitro assay using HINAE (hirame natural embryo) cells demonstrated anti-VHSV activity in the serum of ABV-challenged flounder from Day 1 to Day 14 but not at Day 21 post-ABV challenge. A high expression of a Mx gene, a molecular marker of type I interferon(s) (IFN) occurred in the head kidneys of ABV-challenged flounder from Day 1 to Day 7. These results suggest that the non-specific protection against the secondary VHSV infection in flounder was due to IFN(s) induced by the primary ABV infection.

Cloning and characterization of an Mx gene and its corresponding promoter from the zebrafish, Danio rerio

Type I interferons (IFNs) represent a crucial component of the innate immune response to viruses. An important downstream effector of IFN is the Mx gene, which is activated solely through this pathway. Mx proteins are characterized by a tripartite GTP-binding domain, dynamin family signature, and leucine zipper motif. Mx genes are transcribed upon activation of an interferon-stimulated response element (ISRE) located in the Mx promoter region. In this article, we describe the cloning and analysis of an Mx gene and its corresponding promoter from the zebrafish (Danio rerio). The deduced amino acid sequence of zebrafish Mx contains the conserved GTP-binding domain, dynamin family signature, and leucine zipper motif common to Mx proteins, and shows a 50% identity to human MxA and 69% identity both to rainbow trout and to Atlantic salmon. Zebrafish liver cells produced high levels of Mx mRNA in response to induction by the known IFN-inducer polyinosinic-polycytidylic acid (Poly[I:C]). The zebrafish Mx promoter contains two ISREs homologous to those found in the promoter regions of many IFN-inducible genes, and was able to drive transcription of a luciferase reporter gene when induced by either purified zebrafish IFN or Poly[I:C].

Cloning and characterisation of a channel catfish (Ictalurus punctatus) Mx gene

A 2.5 kb full-length cDNA clone of a channel catfish (Ictalurus punctatus) Mx gene was obtained using RACE (rapid amplification of cDNA ends) polymerase chain reaction (PCR) from RNA extracted from the liver of poly I:C stimulated channel catfish. The gene consists of an open reading frame of 1905 nucleotides encoding a 635 amino acid protein. The predicted protein is 72.5 kDa and contains the dynamin family signature, a tripartite GTP binding motif and a leucine zipper, characteristic of all known Mx proteins. The catfish Mx protein exhibited 79% identity with perch Mx and between 71% and 74% identity with the three Atlantic salmon and the three rainbow trout Mx proteins. Mx mRNA was constitutively expressed in channel catfish ovary (CCO) cells, but in higher quantities in response to poly I:C treatment. Mx was induced in channel catfish following injection with channel catfish virus (CCV) and poly I:C.

A DNA vaccine directed against a rainbow trout rhabdovirus induces early protection against a nodavirus challenge in turbot

A DNA vaccine encoding the envelope glycoprotein from a fish rhabdovirus, viral hemorrhagic septicemia virus (VHSV), has previously been shown to induce both early and long time protection against the virus in rainbow trout. Challenge experiments have revealed that the immunity established shortly after vaccination is cross-protective against heterologous fish rhabdoviruses. In this study, we show that the DNA vaccine encoding the VHSV glycoprotein also induces early protection against a non-enveloped, positive-sense RNA virus belonging to the Nodavirus family, the Atlantic halibut nodavirus (AHNV). In a vaccine efficacy test using juvenile turbot as model fish, the fish injected with the VHSV vaccine were completely protected against a nodavirus challenge performed 8 days post vaccination, while the cumulative mortality in the control group reached 54%. A DNA vaccine carrying the gene encoding the capsid protein of AHNV revealed no protective properties against the nodavirus challenge. Histological examination of muscle tissue sections from the vaccine injection site showed that the DNA vaccine against VHSV triggered a pronounced inflammatory response in turbot similar to what has earlier been observed in rainbow trout.

Molecular cloning of the pufferfish (Takifugu rubripes) Mx gene and functional characterization of its promoter

Mx proteins are members of a family of interferon-inducible genes that are expressed by cells in response to viral infection. They are important determinants of innate immunity against viral infection in vertebrates. We cloned the pufferfish ( Takifugu rubripes) Mx gene and sequenced 80 kb from the Mx locus. The Fugu Mx gene spans 3.4 kb from the transcription start site to the polyadenylation signal, and is made up of 12 exons and 11 introns. The protein sequence encoded by the Fugu Mx gene is 77%, 48%, and 51% identical to that of trout Mx1, chicken Mx, and mouse Mx1 genes, respectively. The Fugu Mx gene is expressed in a variety of tissues, with high expression detected in the heart, gill, kidney, intestine, and brain. Analysis of the 5'-flanking sequence of the gene showed the presence of two interferon-stimulated response elements (ISRE) at positions -51 to 38 and -97 to 85, relative to the transcription start site. The Fugu Mx promoter was inducible by human IFN-beta in the human hepatoma (Huh7) cells and by polyinosinic: polycytidilic acid in the top minnow hepatoma (PLHC-1) cells. Deletion analysis of the promoter showed that both ISREs contributed to inducibility. These results demonstrate that the molecular mechanisms involved in Mx gene regulation are conserved between fish and mammals.

Polymorphisms and the antiviral property of porcine Mx1 protein

We determined the cDNA sequences of the type I interferon-inducible proteins, pig Mx1 from PK(15) and LLC-PK1 cells, and compared the antiviral activities of both Mx proteins, including Mx1 polymorphisms against vesicular stomatitis virus (VSV). Mx1 cDNA derived from PK(15) cells had an 11 bp-deletion in the 3' end of the coding region, and was estimated to encode 8 amino acid substitutions and a 23 amino acid extension compared to that from LLC-PK1 cells. VSV replication was inhibited in the 3T3 cells expressing Mx1 mRNA after the cDNA was transfected. However, the efficiency of this inhibition was not different between the cells expressing Mx1 mRNA from both PK and LLC. These results indicate that pig Mx1 protein confers resistance to VSV.

An antiviral state induced in Chinook salmon embryo cells (CHSE-214) by transfection with the double-stranded RNA poly I:C

Type I interferons (IFN alpha and beta) convert vertebrate cells into an antiviral state by inducing expression of proteins that inhibit virus replication. In humans and mice, Mx proteins constitute one family of interferon-induced antiviral proteins. Mx genes have recently been cloned from Atlantic salmon and rainbow trout. Moreover, double-stranded RNA (dsRNA) and type I IFN-like activity have been shown to induce Mx protein in salmonid cells. Chinook salmon embryo cells (CHSE-214 cells) have been suggested to have a defect in the IFN-system because the dsRNA polyinosinic polycytidylic acid (poly I:C) failed to induce an antiviral state in the cells. We have studied this phenomenon more closely in the present work. CHSE-214 cells were either transfected with poly I:C or incubated with poly I:C without transfection reagent. The cells were then studied for Mx protein expression and protection against infectious pancreatic necrosis virus (IPNV) infection. The results showed that cells transfected with poly I:C were protected from IPNV infection, whilst cells incubated with poly I:C were not protected. Cells transfected with the double-stranded DNA poly dI:dC were also not protected against IPNV. Mx protein was expressed in CHSE-214 cells upon transfection with poly I:C, but not after incubation with poly I:C alone. Stimulation of CHSE-214 cells with supernatants from cells transfected with poly I:C, induced protection against IPNV, indicating production of type I IFN-like activity. These results suggest that CHSE-214 cells in fact are able to produce type I IFN, but may have defects in the mechanisms mediating uptake of poly I:C or may degrade unprotected poly I:C.

Effect of poly I:C on the expression of Mx proteins and resistance against infection by infectious salmon anaemia virus in Atlantic salmon

Mx proteins are induced by type I interferons (IFN alpha and beta) in mice and humans and inhibit the replication of orthomyxoviruses and some other single-stranded RNA viruses. Recently, Mx genes have been cloned from Atlantic salmon. Mx transcripts were shown to be induced in head-kidney, liver and gills of the fish by the synthetic double-stranded RNA polyinosinic polycytidylic acid (poly I:C). In the present work we have studied expression of Mx protein in organs of Atlantic salmon treated with poly I:C. A quantitative immunoblot method was established to monitor expression of Mx protein and to compare relative amounts of Mx protein in different organs. Treatment of Atlantic salmon with poly I:C increased the relative amount of Mx protein in liver, stomach, hindgut, head-kidney and spleen. In gills the levels of Mx protein were similar in control fish and poly I:C treated fish. Immunohistochemistry of tissue sections from liver, head-kidney and gills from poly I:C treated fish was in accordance with the immunoblotting data and showed staining for Mx protein in several different cell types. Classification of infectious salmon anaemia virus as an orthomyxovirus makes it a putative target for Atlantic salmon Mx protein. Atlantic salmon treated with poly I:C showed reduced cumulative mortality compared to the control fish when challenged with infectious salmon anaemia virus (ISAV) by intraperitoneal injection. This demonstrates that poly I:C has some protective effect against ISAV in vivo.

Effect of double-stranded RNA and interferon on the antiviral activity of Atlantic salmon cells against infectious salmon anemia virus and infectious pancreatic necrosis virus

Type I interferons (IFN) establish an antiviral state in vertebrate cells by inducing expression of Mx and other antiviral proteins. We have studied the effect of Atlantic salmon interferon-like activity (AS-IFN) and poly I:C on the Mx protein expression and antiviral activity against infectious salmon anaemia virus (ISAV) and infectious pancreatic necrosis virus (IPNV) in the Atlantic salmon cell lines SHK-1 and TO. The double-stranded RNA poly I:C is an inducer of type I IFN in vertebrates. A cell cytotoxicity assay and measurements of virus yield were used to measure protection of cells against virus infection. Maximal induction of Mx protein in TO and SHK-1 cells occurred 48 h after poly I:C stimulation and 24 h after AS-IFN stimulation. TO cells pretreated with AS-IFN or poly I:C were protected from infection with IPNV 24 to 96 h after stimulation. Poly I:C or AS-IFN induced a minor protection against ISAV infection in SHK-1 cells, but no protection was induced against ISAV in TO cells. Western blot analysis showed that ISAV induced expression of Mx protein in TO and SHK-1 cells whereas IPNV did not induce Mx protein expression. These results suggest that ISAV and IPNV have very different sensitivities to IFN-induced antiviral activity and have developed different strategies to avoid the IFN-system of Atlantic salmon. Moreover, Atlantic salmon Mx protein appears not to inhibit replication of ISAV.

Innate host defense mechanisms of fish against viruses and bacteria

The integumental defenses provide a physical and chemical barrier to the attachment and penetration of microbes. Besides the entrapping and sloughing of microbes in the mucus, the latter contains many antibacterial substances including anti-bacterial peptides, lysozyme, lectins and proteases. The gastro-intestinal tract is a hostile environment of acids, bile salts and enzymes able to inactivate and digest many viruses and bacteria. In most cases the integumental defenses are sufficient to protect against even quite virulent organisms which often only produce disease when the integument has been physically damaged. If a microbe gains access to the tissues of the fish, it is met with an array of soluble and cellular defenses. The complement system, present in the blood plasma, plays a central role in recognising bacteria and its activated products may lyse the bacterial cells, initiate inflammation, induce the influx of phagocytes and enhance their phagocytic activity. Complement can be activated directly by bacterial products and constituents and also indirectly by other factors, principally C-reactive protein and lectins, which can also bind to the bacterial surface. Plasma also contains a number of factors which inhibit bacterial growth(e.g. transferrin and anti-proteases) or which are bactericidal e.g. lysozyme. Following the infection of fish with virus pathogens, infected cells produce interferon. This induces antiviral defenses in neighbouring cells which are then protected from becoming infected. Anti-viral cytotoxic cells are able to lyse virally infected cells and thus reduce the rate of multiplication of virus within them. Innate defenses thus provide a pre-existing and fast-acting system of protection which is non-specific and relatively temperature-independent and thus has several advantages over the slow-acting and temperature-dependent specific immune responses.

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

Mx protein is one of several antiviral proteins that are induced by the type I interferons (IFN), IFNalpha and beta, in mammals. In this work induction of a 76 kDa Mx protein by double-stranded RNA (dsRNA) or type I IFN-like activity in Atlantic salmon macrophages, Atlantic salmon fibroblast cells (AS cells) and in Chinook salmon embryo cells (CHSE-214) is reported. Type I IFN-like activity was produced by the stimulation of Atlantic salmon macrophages with the synthetic dsRNA polyinosinic polycytidylic acid (poly I:C). A correlation appeared to exist between Mx protein expression and protection against infectious pancreatic necrosis virus (IPNV) induced by IFN in CHSE-214 cells. Several observations in the present work suggest that, as in mammals, the induction of Mx protein by dsRNA in fish cells primarily occurs via induction of type I IFN. First, type I IFN-like activity but not poly I:C, induced Mx protein expression in CHSE-214 cells. These cells apparently lack the ability to produce IFN in response to poly I:C. Second, the putative IFN induced maximal Mx protein expression 48 h earlier than poly I:C in AS cells. Third, the peak expression of Mx protein in macrophages induced by poly I:C occurred after 48 h whereas peak in IFN-like activity was observed by 24 h after addition of poly I:C. The present work supports the notion of using Mx protein as a molecular marker for the production of putative type I IFN in fish.