Virucidal effects of various agents-including protease-against koi herpesvirus and viral haemorrhagic septicaemia virus

In a search for alternative, environmentally friendly and effective disinfecting agents, a commercially available protease-Neutrase^® -was tested in this work for inactivation of koi herpesvirus (KHV) and of viral haemorrhagic septicaemia virus (VHSV). For comparison, the stability of these viral pathogens in similar configurations at various pH values and concentrations of peracetic acid or quicklime, typically used for disinfection, was tested. Therefore, virus suspensions were incubated with various concentrations of different agents for 24 hr and the titre of the remaining infectious particles was determined by virus titration. Furthermore, the treatment of both viruses, with the agents at concentrations that were previously appointed as effective, was also examined in the presence of solid material (quartz sand). All procedures investigated in this study, including the protease treatment, were able to reduce the titre of KHV and VHSV below the detection limit of the titration. Although further studies are necessary, this is the first report of the application of a protease for the inactivation of the selected fish pathogens, demonstrating the great potential of the latter for disinfection.

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

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

Effects of a broad-spectrum caspase inhibitor, Z-VAD(OMe)-FMK, on viral hemorrhagic septicemia virus (VHSV) infection-mediated apoptosis and viral replication

In the development of inactivated or attenuated viral vaccines for cultured fish, viral titers harvested from the cultured cells would be the most important factor for the determination of vaccine's cost effectiveness. In this study, we hypothesized that the lengthening of cell survival time by the inhibition of apoptosis can lead to an increase of the final titer of viral hemorrhagic septicemia virus (VHSV). To test the hypothesis, we investigated the effects of a broad-spectrum caspase inhibitor, Z-VAD(OMe)-FMK, on VHSV infection-mediated apoptosis in Epithelioma papulosum cyprini (EPC) cells and on the VHSV titers. VHSV infection induced the DNA laddering in EPC cells, and the progression of DNA fragmentation was in proportion to the CPE extension. The progression of DNA fragmentation in EPC cells infected with VHSV was clearly inhibited by exposure to Z-VAD(OMe)-FMK, and the inhibition was intensified according to the increase of the inhibitor concentration. These results confirmed the previous reports that the death of host cells by VHSV infection is through apoptosis. Cells infected with a recombinant VHSV, rVHSV-ΔNV-eGFP, that was generated from our previous study by replacement of the NV gene ORF with the enhanced green fluorescent protein (eGFP) gene ORF, showed earlier and more distinct DNA fragmentations compared to the cells infected with wild-type VHSV, suggesting the inhibitory role of the NV protein in VHSV-mediated apoptosis that was previously reported. The final viral titers in the supernatant isolated from Z-VAD(OMe)-FMK treated cells after showing an extensive CPE were significantly higher than the viral titers from cells infected with virus alone, indicating that the delay of apoptosis by Z-VAD(OMe)-FMK extended the survival time of EPC cells, which lengthen the time for VHSV replication in the cells. In conclusion, Z-VAD(OMe)-FMK-mediated inhibition of apoptosis significantly increased the final titers of both wild-type VHSV and rVHSV-ΔNV-eGFP, indicating that apoptosis inhibition can be a way to get higher titers of VHSV.

Identification of regulators of the early stage of viral hemorrhagic septicemia virus infection during curcumin treatment

The effect of curcumin pretreatment (15-240 μM) in fathead minnow cells infected with viral hemorrhagic septicemia virus (VHSV) was evaluated. Cell viability, apoptosis and viral copy number were analyzed using Cell Counting Kit-8 assay, Annexin V staining, and reverse transcription-PCR, respectively. Pretreatment with 120 μM curcumin showed an increase in viability (>90% of mock) of VHSV-infected cells and reduction in the copy number (0.2-log reduction in VHSV N gene expression), reactive oxygen species and apoptosis in the cells without cytotoxic effects. To understand the mechanisms underlaying the antiviral effects of curcumin pretreatment, a comparative proteomic analysis was performed in four samples (M, mock; C, curcumin-treated; V, VHSV-infected; and CV, curcumin-treated VHSV-infected) in triplicate. In total, 185 proteins were detected. The analysis showed that three proteins, including heat shock cognate 71 (HSC71), actin, alpha cardiac muscle (ACTC1) and elongation factor 1 (EEF1) were differentially expressed between V and CV samples. Network analysis performed by Ingenuity Pathways Analysis (IPA) showed that HSC71 was the primary protein interacting with fibronectin (FN) 1, actins (ACTB, ACTG, F-actin) and gelsolin (GSN) in both V and CV samples and thus is a strong target candidate for the protection from VHSV infection at the viral entry stage. Our proteomics data suggest that curcumin pretreatment inhibits entry of VHSV in cells by downregulating FN1 or upregulating F-actin. For both proteins, HSC71 acts as a binding protein that modulates their functions. Furthermore, consistent with the effect of a heat shock protein inhibitor (KNK437), curcumin downregulated HSC71 expression with increasing viability of VHSV-infected cells and inhibited VHSV replication, suggesting that the downregulation of HSC71 could be responsible for the antiviral activity of curcumin. In conclusion, this study indicates that the suppression of viral entry by rearrangement of the F-actin/G-actin ratio via downregulating HSC71 is a plausible mechanism by which curcumin pretreatment controls the early stages of VHSV infection.

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

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

Comparison of treatments to inactivate viral hemorrhagic septicemia virus (VHSV-IVb) in frozen baitfish

Current US state and federal fish health regulations target the spread of viral hemorrhagic septicemia virus-IVb (VHSV-IVb) through movement restrictions of live fish; however, they largely ignore the potential for the virus to be spread through commercial distribution and use of frozen baitfish from VHSV-IVb-positive regions. Some state laws do require treatment of frozen baitfish to inactivate VHSV, and additional methods have been proposed, but few scientific studies have examined the efficacy of these treatments. In this study, bluegills Lepomis macrochirus were challenged with VHSV-IVb and frozen to represent standard industry methods, disinfected by various treatments, and tested for infectious VHSV-IVb using virus isolation. The virus was isolated from 70% of fish subjected to 3 freeze/thaw cycles. All other treatment methods were effective in inactivating the virus, including treatment with isopropyl alcohol, mineral oil, salt and borax, and dehydration. Dehydration followed by rehydration is rapid and effective, and therefore, seems to be the best option for inactivating VHSV-IVb present in frozen baitfish while maintaining their usefulness as bait.

RNA aptamers inhibit the growth of the fish pathogen viral hemorrhagic septicemia virus (VHSV)

Viral hemorrhagic septicemia virus (VHSV) is a serious disease impacting wild and cultured fish worldwide. Hence, an effective therapeutic method against VHSV infection needs to be developed. Aptamer technology is a new and promising method for diagnostics and therapeutics. It revolves around the use of an aptamer molecule, an artificial ligand (nucleic acid or protein), which has the capacity to recognize target molecules with high affinity and specificity. Here, we aimed at selecting RNA aptamers that can specifically bind to and inhibit the growth of a strain of fish VHSV both in vitro and in vivo. Three VHSV-specific RNA aptamers (F1, F2, and C6) were selected from a pool of artificially and randomly produced oligonucleotides using systematic evolution of ligands by exponential enrichment. The three RNA aptamers showed obvious binding to VHSV in an electrophoretic mobility shift assay but not to other tested viruses. The RNA aptamers were tested for their ability to inhibit VHSV in vitro using hirame natural embryo (HINAE) cells. Cytopathic effect and plaque assays showed that all aptamers inhibited the growth of VHSV in HINAE cells. In vivo tests using RNA aptamers produced by Rhodovulum sulfidophilum showed that extracellular RNA aptamers inhibited VHSV infection in Japanese flounder. These results suggest that the RNA aptamers are a useful tool for protection against VHSV infection in Japanese flounder.

Antiviral protection mechanisms mediated by ginbuna crucian carp interferon gamma isoforms 1 and 2 through two distinct interferon gamma-receptors

Fish genomes possess three type II interferon (IFN) genes, ifnγ1, ifnγ2 and ifnγ-related (ifnγrel). The IFNγ-dependent STAT signalling pathway found in humans and mice had not been characterized in fish previously. To identify the antiviral functions and signalling pathways of the type II IFN system in fish, we purified the ifnγ1, ifnγ2 and ifnγrel proteins of ginbuna crucian carp expressed in bacteria and found them to elicit high antiviral activities against crucian carp hematopoietic necrosis virus. We also cloned two distinct ifnγ receptor alpha chain (ifngr1) isoforms, 1 and 2, and stably expressed them in HeLa cells by transfecting the cells with ifngr1-1 or ifngr1-2 cDNA. When receptor transfectants were treated with the ligands in a one-ligand-one-receptor manner (ifnγ1 and ifngr1-2 or ifnγ2 and ifngr1-1), the stat1 protein was phosphorylated at both serine-727 and tyrosine-701 residues. Gel shift mobility analysis and reporter assay clearly showed that the specific ligand-receptor interaction resulted in the binding of the stat1 protein to the GAS element and enhanced transcription. Therefore, the actions of ifnγ1 and ifnγ2 were found to be mediated by a specific receptor for each signalling pathway via a stat1-dependent mechanism.

Salmonid fish viruses and cell interactions at early steps of the infective cycle

A flow cytometric virus-binding assay that directly visualizes the binding and entry of infectious pancreatic necrosis virus (IPNV), infectious haematopoietic necrosis virus (IHNV) and virus haemorrhagic septicaemia virus (VHSV) to several cell lines was established. The highest efficiency of binding was shown by the BF-2 cell line and this was used to study, at the attachment level, the interactions of these cells with salmonid fish viruses in coinfections, and to further determine if the earliest stage of the viral growth cycle could explain the previously described loss of infectivity of IHNV when IPNV is present. Our results demonstrated that IPNV binds to around 88% of cells either in single or dual infections, whereas IHNV attachment always decreased in the presence of any of the other viruses. VHSV binding was not affected by IPNV, but coinfection with IHNV reduced the percentage of virus-binding cells, which suggests competition for viral receptors or co-receptors. Internalization of the adsorbed IHNV was not decreased by coinfection with IPNV, so the hypothetical competence could be restricted to the binding step. Treatment of the cells with antiviral agents, such as amantadine or chloroquine, did not affect the binding of IPNV and VHSV, but reduced IHNV binding by more than 30%. Tributylamine affected viral binding of the three viruses to different degrees and inhibited IPNV or IHNV entry in a large percentage of cells treated for 30 min. Tributylamine also inhibited IHNV cytopathic effects in a dose-dependent manner, decreasing the virus yield by 4 log of the 50% endpoint titre, at 10 mm concentration. IPNV was also inhibited, but at a lower level. The results of this study support the hypothesis that IHNV, in contrast to VHSV or IPNV, is less efficient at completing its growth cycle in cells with a simultaneous infection with IPNV. It can be affected at several stages of viral infection and is more sensitive to the action of antiviral compounds.

Dual antiviral activity of human alpha-defensin-1 against viral haemorrhagic septicaemia rhabdovirus (VHSV): inactivation of virus particles and induction of a type I interferon-related response

It is well known that human alpha-defensin-1, also designated as human neutrophil peptide 1 (HNP1), is a potent inhibitor towards several enveloped virus infecting mammals. In this report, we analyzed the mechanism of the antiviral action of this antimicrobial peptide (AMP) on viral haemorrhagic septicaemia virus (VHSV), a salmonid rhabdovirus. Against VHSV, synthetic HNP1 possesses two antiviral activities. The inactivation of VHSV particles probably through interfering with VHSV-G protein-dependent fusion and the inhibition of VHSV replication in target cells by up-regulating genes related to the type I interferon (IFN) response, such as Mx. Neither induction of IFN-stimulated genes (ISGs) by HNP1 nor their antiviral activity against fish rhabovirus has been previously reported. Therefore, we can conclude that besides to acting as direct effector, HNP1 acts across species and can elicit one of the strongest antiviral responses mediated by innate immune system. Since the application of vaccine-based immunization strategies is very limited, the used of chemicals is restricted because of their potential harmful impact on the environment and no antimicrobial peptides from fish that exhibit both antiviral and immunoenhancing capabilities have been described so far, HNP1 could be a model molecule for the development of antiviral agents for fish. In addition, these results further confirm that molecules that mediate the innate resistance of animals to virus may prove useful as templates for new antivirals in both human and animal health.

Assessment of the inhibitory effect of ribavirin on the rainbow trout rhabdovirus VHSV by real-time reverse-transcription PCR

Viral hemorrhagic septicemia virus (VHSV) is one of the most ubiquitous viruses in salmonid aquaculture in Europe. This infectious disease results in significant losses in the farming industry and therefore effective therapeutic agents are needed to control outbreaks caused by this pathogen. Thus, accurate methods to test new antiviral compounds need to be developed. Our goal was to establish a model system for testing novel antivirals with potential applications to aquaculture. In a previous study, a TaqMan real-time RT-PCR assay was designed to detect and quantitate VHSV in rainbow trout tissues [Chico, V., Gomez, N., Estepa, A., Perez, L., 2006. Rapid detection and quantitation of viral hemorrhagic septicemia virus in experimentally challenged rainbow trout by real-time RT-PCR. J. Virol. Methods 132, 154-159]. In this report, we applied the real-time RT-PCR assay to the evaluation of the inhibitory effect of ribavirin, a well-known broad spectrum antiviral drug, in a cell culture system. When added from the beginning of the infection, ribavirin caused a dose-dependent reduction of VHSV RNA accumulation. Real-time RT-PCR measurements showed 99.8% inhibition at 25 microg/ml ribavirin, with an IC50 of 0.43 microg/ml. Ribavirin maintained its inhibitory activity against VHSV when added at 6 h post-infection. Quantitation of N protein messenger RNA and plus-stranded RNA showed a substantial decrease of viral transcription in ribavirin-treated cells. Partial reversion of the effect of ribavirin by addition of GTP was observed, confirming that ribavirin targets the synthesis of guanidine nucleotides in the cells. This is the first report of a real-time PCR-based assay for addressing the efficacy and mechanism of action of an antiviral agent for rainbow trout.

Identification of selective inhibitors of VHSV from biased combinatorial libraries of N,N′-disubstituted 2,5-piperazinediones

A combinatorial strategy has been used to design and identify inhibitors of viral haemorrhagic septicemia virus (VHSV), a salmonid rhabdovirus of economic importance. Two libraries of N,N'-disubstituted 2,5-piperazinediones (DKP), DKP-I and DKP-II were screened for inhibition of VHSV infectivity. Among the 98 DKP-derivatives (R1-DKP-R2) screened, a novel class of VHSV in vitro inhibitors was identified. Evidences are presented showing that the selected DKP-derivatives cause dose-dependent inhibition of VHSV infectivity in the absence of cellular toxicity. Preliminary characterization of its inhibition mechanism ruled out direct inactivation of the virus (virucidal effect) or interference with early viral replication steps. Furthermore, analysis of infection foci sizes, virus titers, viral protein accumulation and presence of cell free virus derived from VHSV-infected cell cultures in the presence of DKP-derivates suggested that virus assembly/release was impaired leading to a reduced virus spread in cell culture. New DKP-derivatives with a significant higher specific activity need to be developed to start testing its possible practical use but the selected DKP-derivatives described here may contribute to their further development as well as being tools to improve our knowledge on the fish rhabdovirus infection cycle.

Induction of antiviral state in fish cells by Japanese flounder, Paralichthys olivaceus, interferon regulatory factor-1

Interferon regulatory factor-1 (IRF-1) mediates an antiviral state in cells by regulating the expression of the interferon (IFN-alpha/beta) system. To elucidate the role of IRF-1 in fish during virus infections, we constructed a recombinant plasmid of the Japanese flounder, Paralichthys olivaceus IRF-1 (JF IRF-1) under the control of the cytomegalovirus (CMV) immediate/early enhancer promoter. The antiviral mechanism of JF IRF-1 was studied using transfection experiments in a homologous cell line. Here, we show that cell supernatants obtained from transiently transfected cells enhanced cell viability of a heterologous cell line upon incubation, reduced the titers of hirame rhabdovirus (HIRRV) and viral hemorrhagic septicemia virus (VHSV), and possessed cytokine-like activity, as shown by their ability to protect cells against virus infections. The supernatants also inhibited the replication of the rhabdoviruses during the early stages of infection as indicated by the reduction of viral titers in the presence of the supernatants obtained from the transfected cells. Further analysis showed that the cell culture supernatants contain cytokine-like substances that possess acid-labile and temperature-resistant properties. These results indicate that JF IRF-1 induces an antiviral state in cells by mediating the production of cytokine-like substances. Thus, JF IRF-1 might be useful as an adjuvant in the development of DNA vaccines against commercially important viral pathogens in Japanese flounder aquaculture.

The olive leaf extract exhibits antiviral activity against viral haemorrhagic septicaemia rhabdovirus (VHSV)

A commercial plant extract derived from olive tree leaf (Olea europaea) (LExt) and its major compound, oleuropein (Ole), inhibited the in vitro infectivity of the viral haemorrhagic septicaemia virus (VHSV), a salmonid rhabdovirus. Incubation of virus with LExt or Ole before infection reduced the viral infectivity to 10 and 30%, respectively. Furthermore, LExt drastically decreased VHSV titers and viral protein accumulation (virucidal effect) in a dose dependent manner when added to cell monolayers 36 h post-infection. On the other hand, both the LExt and Ole were able to inhibit cell-to-cell membrane fusion induced by VHSV in uninfected cells, suggesting interactions with viral envelope. Therefore, we propose that O. europaea could be used as a potential source of promising natural antivirals, which have demonstrated to lack impact on health and environment. In addition, Ole could be used to design other related antiviral agents.