Inhibitory effect of a nucleotide analog on infectious salmon anemia virus infection

Modulation of infectious Salmon Anaemia virus infection by clathrin-mediated endocytosis and macropinocytosis inhibitors

Infectious salmon anaemia virus (ISAV) is a pathogen that causes disease and large mortality in farm-raised Salmo salar L., being considered as a major problem in the salmon industry. However, despite its relevance, there are still numerous knowledge gaps on virus entry and early stages of infection. Previous studies suggested that virus entry into cells occurs via endocytosis, with no description of specific mechanisms. However, it remains unknown if the endocytosis induced by ISAV is a clathrin-dependent or clathrin-independent process. This study aimed to identify cellular mechanisms allowing ISAV entry into Atlantic Salmon head kidney (ASK) cells. Our results showed that ISAV can be found in coated pits and membrane ruffles, the latter being induced by a rearrangement of actin filaments promoted by ISAV infection. Additionally, it was determined that ISAV stimulate the uptake of extracellular fluid in a multiplicity of infection (MOI)-dependent manner. When the clathrin-mediated endocytic pathway was pharmacologically inhibited, ISAV infection was significantly reduced but not entirely inhibited. Similarly, when the Na+/H+ exchanger (NHE), a key component of macropinocytosis, was inhibited, ISAV infection was negatively affected. Our results suggest that ISAV enters cells via both clathrin-mediated endocytosis and most likely macropinocytosis.

The RNA-dependent RNA polymerase of the infectious pancreatic necrosis virus is linked to viral mRNA acting as a cap substitute

The infectious pancreatic necrosis virus (IPNV) is responsible for significant economic losses in the aquaculture industry. It is an unenveloped virus with an icosahedral capsid. Its viral genome comprises two dsRNA segments, A and B. Segment A contains a small ORF, which encodes VP5, and a large ORF, which encodes a polyprotein that generates the structural proteins and the viral protease. Segment B encodes the RNA-dependent RNA polymerase (RdRp), called VP1 in this free form, or Vpg when it covalently attaches to the viral RNA. The viral genome does not have cap or poly(A). Instead, each 5′ end is linked to the Vpg. Recently, we demonstrated that mRNA-A contains an internal ribosome entry site (IRES) to command polyprotein synthesis. However, the presence of Vpg on IPNV mRNAs and its impact on cellular translation has not been investigated. This research demonstrates that IPNV mRNAs are linked to Vpg and that this protein inhibits cap-dependent translation on infected cells. Also, it is demonstrated that Vpg interacts with eIF4E and that rapamycin treatment partially diminishes the viral protein synthesis. In addition, we determined that an IRES does not command translation of IPNV mRNA-B. We show that VPg serves as a cap substitute during the initiation of IPNV translation, contributing to understanding the replicative cycle of Birnaviruses. Our results indicate that the viral protein VP1/Vpg is multifunctional, having a significant role during IPNV RNA synthesis as the RdRp and the primer for IPNV RNA synthesis and translation as the viral protein genome, acting as a cap substitute.

Infectious Salmon Anemia Virus Infectivity Is Determined by Multiple Segments with an Important Contribution from Segment 5

Infectious salmon anemia virus (ISAV) is the etiological agent of infectious salmon anemia. It belongs to the genus isavirus, one of the genera of the Orthomyxoviridae family, as does Influenzavirus A. The ISAV genome comprises eight negative-sense single-stranded RNA segments that code for at least 10 proteins. Although some ISAV strains can reach 100% mortality rates, the factors that determine isavirus infectivity remain unknown. However, some studies suggest that segments 5 and 6 are responsible for the different degrees of virulence and infectivity among ISAV subtypes, unlike the influenza A virus, where most segments are involved in the virus infectivity. In this work, synthetic reassortant viruses for the eight segments of ISAV were generated by reverse genetics, combining a highly virulent virus, ISAV 752_09 (HPR7b), and an avirulent strain, SK779/06 (HPR0). We characterized the rescued viruses and their capacity to replicate and infect different cell lines, produce plaques in ASK cells, and their ability to induce and modulate the cellular immune response in vitro. Our results show that the majority of ISAV segments are involved in at least one of the analyzed characteristics, segment 5 being one of the most important, allowing HPR0 viruses, among other things, to produce plaques and replicate in CHSE-214 cells. We determined that segments 5 and 6 participate in different stages of the viral cycle, and their compatibility is critical for viral infection. Additionally, we demonstrated that segment 2 can modulate the cellular immune response. Our results indicate a high degree of genetic compatibility between the genomic segments of HPR7b and HPR0, representing a latent risk of reassortant that would give rise to a new virus with an unknown phenotype.

Antiviral Activity of Ribavirin against Tilapia tilapinevirus in Fish Cells

The outbreak of the novel Tilapia tilapinevirus or Tilapia lake virus (TiLV) is having a severe economic impact on global tilapia aquaculture. Effective treatments and vaccines for TiLV are lacking. In this study, we demonstrated the antiviral activity of ribavirin against TiLV in E-11 cells. Our findings revealed that at concentrations above 100 μg/mL, ribavirin efficiently attenuates the cytopathic effect of the TiLV infection in fish cells. When administered in a dose-dependent manner, ribavirin significantly improved cell survival compared to the untreated control cells. Further investigation revealed that the cells exposed to ribavirin and TiLV had a lower viral load (p < 0.05) than the untreated cells. However, at concentrations above 1000 μg/mL, ribavirin led to cell toxicity. Taken together, our results demonstrate the efficacy of this antiviral drug against TiLV and could be a useful tool for future research on the pathogenesis and replication mechanism of TiLV as well as other piscine viruses.

Acyclovir inhibits white spot syndrome virus replication in crayfish Procambarus clarkii

White spot syndrome virus (WSSV) is a fatal pathogen threatening global crustacean industry with no commercially available drugs to control WSSV. To address the urgent need for finding effective antiviral agents against WSSV, we examined the anti-WSSV activities of 11 common antiviral agents in crayfish Procambarus clarkia. The results showed that acyclovir displayed the highest inhibition on WSSV replication in vivo (92.59%, 50 mg/kg). Acyclovir repressed WSSV proliferation followed a dose-dependent fashion and pre- or post-treatment of acyclovir exerted strong inhibition on the viral loads. Further, we observed a markedly reduced expression levels of WSSV genes (immediate-early IE gene ie1, DNA polymerase gene DNApol and envelope protein gene Vp28) that are crucial in viral life cycle with the acyclovir treatment during the early infection. Meantime, we also found a significantly increased expressions of anti-oxidative as well as apoptosis related genes, suggesting that acyclovir could effectively suppress WSSV replication in vivo. Finally, acyclovir treatment could significantly improve the survival rate of WSSV-challenged crayfish by 56%. Taken together, acyclovir has the potential to be developed as a promising preventive or therapeutic agent against WSSV infection, and this finding may provide a reference for rapid discovery anti-WSSV agent in crustacean aquaculture.

Identification of Inhibitory Compounds Against Singapore Grouper Iridovirus Infection by Cell Viability-Based Screening Assay and Droplet Digital PCR

Singapore grouper iridovirus (SGIV) is one of the major causative agents of fish diseases and has caused significant economic losses in the aquaculture industry. There is currently no commercial vaccine or effective antiviral treatment against SGIV infection. Annually, an increasing number of small molecule compounds from various sources have been produced, and many are proved to be potential inhibitors against viruses. Here, a high-throughput in vitro cell viability-based screening assay was developed to identify antiviral compounds against SGIV using the luminescent-based CellTiter-Glo reagent in cultured grouper spleen cells by quantificational measurement of the cytopathic effects induced by SGIV infection. This assay was utilized to screen for potential SGIV inhibitors from five customized compounds which had been reported to be capable of inhibiting other viruses and 30 compounds isolated from various marine organisms, and three of them [ribavirin, harringtonine, and 2-hydroxytetradecanoic acid (2-HOM)] were identified to be effective on inhibiting SGIV infection, which was further confirmed with droplet digital PCR (ddPCR). In addition, the ddPCR results revealed that ribavirin and 2-HOM inhibited SGIV replication and entry in a dose-dependent manner, and harringtonine could reduce SGIV replication rather than entry at the working concentration without significant toxicity. These findings provided an easy and reliable cell viability-based screening assay to identify compounds with anti-SGIV effect and a way of studying the anti-SGIV mechanism of compounds.

Inhibition of nervous necrosis virus by ribavirin in a zebrafish larvae model

The guanosine analog ribavirin is a broad-spectrum antiviral drug, mostly used in human clinical practice. It has in vitro and in vivo activity against a broad range of RNA and DNA viruses. Here, we report that treatment of zebrafish larvae with ribavirin prior to infection with nervous necrosis virus (NNV) significantly reduces the mortality caused by the virus during the first 10 days post-infection. The RNA genome of NNV harvested from ribavirin-treated infected larvae contains three synonymous and one single non-synonymous mutation, resulting in the replacement of a serine codon with a glycine codon in the RNA-dependent RNA polymerase gene. Adding increasing amounts of guanosine to ribavirin prior to larvae infection did not impede the antiviral activity. Ribavirin treatment of uninfected larvae reduces the basal level of IFNγ, but increases the level of IL-1β mRNA expression. Furthermore, infecting larvae with NNV following ribavirin treatment reduces the expression levels of IFNγ, IFN-I, Mx, and TNF-α genes, while the expression of IL-1β is increased. These results suggest that cytokine modulation plays an important role in the activity of ribavirin against NNV. Mortality of more than 40 species of teleost fish, mostly larvae and juveniles, from NNV is a major obstacle in hatcheries, and impedes the supply of young fish to farms. Hence, cost-effective ribavirin treatment should be considered as an efficient means to reduce the peril of NNV.

Production of infectious salmon anaemia virus (ISAV) ribonucleoprotein complexes using a mammalian cell based minigenome system

Developments in recombinant virus techniques have been crucial to understand the mechanisms of virulence acquisition and study the replication of many different negatively stranded RNA viruses. However, such technology has been lacking for infectious salmon anaemia virus (ISAV) until recently. This was due in part to the lack of a Polymerase I promoter in Atlantic salmon to drive the production of recombinant vRNA. Therefore, the present study investigated a different alternative to produce ISAV recombinant vRNA, based on Mouse Pol I promoter/terminator sequences and expression in baby hamster kidney (BHK-21) cells. As a first step, a pathogenic ISAV was demonstrated to replicate and produce viable virions in BHK-21 cells. This indicated that the virus could use the mammalian cellular and nuclear machinery to produce vRNA segments and viral proteins, albeit in a limited capacity. Co-transfection of vRNA expressing plasmids with cytomegalovirus (CMV) promoter constructs coding for the three viral polymerase and nucleoprotein led to the generation of functional ribonucleoproteins (RNPs) which expressed either, green fluorescence protein (GFP) or firefly luciferase (FF). Further experiments demonstrated that a 21h incubation at 37°C was optimal for RNPs production. Inhibition by ribavirin confirmed that FF expression was linked to specific RNPs polymerase transcription. The present minigenome system provides a novel and alternative approach to investigate various aspects of ISAV replication and potentially those of other negatively stranded RNA viruses. Expression of RNPs in mammalian cells could also provide a method for the rapid screening of anti-viral compounds targeting ISAV replication.

Pharmacokinetics, efficacy prediction indexes, and residue depletion of ribavirin in Atlantic salmon's (Salmo salar) muscle after oral administration in feed

Ribavirin is an antiviral used in human medicine, but it has not been authorized for use in veterinary medicine although it is effective against infectious salmon anemia (ISA) virus, between others. In this study, we present a pharmacokinetic profile of ribavirin in Atlantic salmon (Salmo salar), efficacy prediction indexes, and the measure of its withdrawal time. To determine the pharmacokinetic profile, fishes were orally administered with a single ribavirin dose of 1.6 mg/kg bw, and then, plasma concentrations were measured at different times. From the time-vs.-concentration curve, Cmax = 413.57 ng/mL, Tmax  = 6.96 h, AUC = 21394.01 μg·h/mL, t1/2  = 81.61 h, and K10  = 0.0421/h were obtained. Ribavirin reached adequate concentrations during the pharmacokinetic study, with prediction indexes of Cmax /IC50  = 20.7, AUC/IC50  = 1069.7, and T>IC50  = 71 h, where IC is the inhibitory concentration 50%. For ribavirin depletion study, fishes were orally administered with a dairy dose of 1.6 mg/kg bw during 10 days. Concentrations were measured on edible tissue on different days post-treatment. A linear regression of the time vs. concentration was conducted, obtaining a withdrawal time of 1966 °C days. Results obtained reveal that the dose of 1.6 mg/kg bw orally administered is effective for ISA virus, originating a reasonable withdrawal period within the productive schedules of Atlantic salmon.

Development of a reverse genetic system for infectious salmon anemia virus: rescue of recombinant fluorescent virus by using salmon internal transcribed spacer region 1 as a novel promoter

Infectious salmon anemia (ISA) is a serious disease of marine-farmed Atlantic salmon (Salmo salar) caused by ISA virus (ISAV), belonging to the genus Isavirus, family Orthomyxoviridae. There is an urgent need to understand the virulence factors and pathogenic mechanisms of ISAV and to develop new vaccine approaches. Using a recombinant molecular biology approach, we report the development of a plasmid-based reverse genetic system for ISAV, which includes the use of a novel fish promoter, the Atlantic salmon internal transcribed spacer region 1 (ITS-1). Salmon cells cotransfected with pSS-URG-based vectors expressing the eight viral RNA segments and four cytomegalovirus (CMV)-based vectors that express the four proteins of the ISAV ribonucleoprotein complex allowed the generation of infectious recombinant ISAV (rISAV). We generated three recombinant viruses, wild-type rISAV(901_09) and rISAVr(S6-NotI-HPR) containing a NotI restriction site and rISAV(S6/EGFP-HPR) harboring the open reading frame of enhanced green fluorescent protein (EGFP), both within the highly polymorphic region (HPR) of segment 6. All rescued viruses showed replication activity and cytopathic effect in Atlantic salmon kidney-infected cells. The fluorescent recombinant viruses also showed a characteristic cytopathic effect in salmon cells, and the viruses replicated to a titer of 6.5105 PFU/ml,similar to that of the wild-type virus. This novel reverse genetics system offers a powerful tool to study the molecular biology of ISAV and to develop a new generation of ISAV vaccines to prevent and mitigate ISAV infection, which has had a profound effect on the salmon industry.

Differential immune gene expression profiles in susceptible and resistant full-sibling families of Atlantic salmon (Salmo salar) challenged with infectious pancreatic necrosis virus (IPNV)

This study aims to identify at the expression level the immune-related genes associated with IPN-susceptible and resistant phenotypes in Atlantic salmon full-sibling families. We have analyzed thirty full-sibling families infected by immersion with IPNV and then classified as resistant or susceptible using a multivariate survival analysis based on a gamma-Cox frailty model and the Kaplan-Meier mortality curves. In four families within each group head kidneys were pooled for real-time PCR and one-color salmon-specific oligonucleotide microarray (21K) analysis at day 1 and 5 post-infection. Transcripts involved in innate response (IL-6, IFN-α), antigen presentation (HSP-70, HSP-90, MHC-I), TH1 response (IL-12, IFN-γ, CRFB6), immunosuppression (IL-10, TGF-β1) and leukocyte activation and migration (CCL-19, CD18) showed a differential expression pattern between both phenotypes, except in IL-6. In susceptible families, except for IFN-γ, the expressions dropped to basal values at day 5 post-infection. In resistant families, unlike susceptible families, levels remained high or increased (except for IL-6) at day 5. Transcriptomic analysis showed that both families have a clear differential expression pattern, resulting in a marked down-regulation in immune related genes involved in innate response, complement system, antigen recognition and activation of immune response in IPN-resistant. Down-regulation of genes, mainly related to tissue differentiation and protein degradation metabolism, was also observed in resistant families. We have identified an immune-related gene patterns associated with susceptibility and resistance to IPNV infection of Atlantic salmon. This suggests that a limited immune response is associated with resistant fish phenotype to IPNV challenge while a highly inflammatory but short response is associated with susceptibility.

Ribavirin Inhibits Parrot Bornavirus 4 Replication in Cell Culture

Parrot bornavirus 4 is an etiological agent of proventricular dilatation disease, a fatal neurologic and gastrointestinal disease of psittacines and other birds. We tested the ability of ribavirin, an antiviral nucleoside analog with antiviral activity against a range of RNA and DNA viruses, to inhibit parrot bornavirus 4 replication in duck embryonic fibroblast cells. Two analytical methods that evaluate different products of viral replication, indirect immunocytochemistry for viral specific nucleoprotein and qRT-PCR for viral specific phosphoprotein gene mRNA, were used. Ribavirin at concentrations between 2.5 and 25 μg/mL inhibited parrot bornavirus 4 replication, decreasing viral mRNA and viral protein load, in infected duck embryonic fibroblast cells. The addition of guanosine diminished the antiviral activity of ribavirin suggesting that one possible mechanism of action against parrot bornavirus 4 may likely be through inosine monophosphate dehydrogenase inhibition. This study demonstrates parrot bornavirus 4 susceptibility to ribavirin in cell culture.

Carbon nanotube-based nanocarrier loaded with ribavirin against grass carp reovirus

Infectious diseases of viral origin cause major aquatic production losses in different parts of the world. Because of formidable barriers for gastrointestinal tract, skin and cell, large amounts of antiviral drugs have limited therapeutic effect. In the current study, functionalized single-walled carbon nanotubes (SWCNTs) were selected as a drug carrier to carry antiviral drug for the treatment of viral diseases on fish. The results show that increasing antiviral drug (ribavirin) intake was observed by SWCNTs carrier and therapeutic dosage to kill grass carp reovirus is significantly reduced. At 12d post infection, survival rate and infection rate were 29.7% and 50.4% for naked ribavirin treatment group exposed to the highest concentration (20 mg/L); however, survival rate of 96.6% and infection rate of 9.4% were observed in 5 mg/L ribavirin-SWCNTs treatment group. In addition, the drug detention time in different organs and tissues (blood, gill, liver, muscle, kidney and intestine) was also significantly extended (about 72 h) compared with the same dosage in naked ribavirin treatment group. Moreover, the toxicity of functionalized SWCNTs in grass carp can be minimal, and physiological markers (some antioxidant enzymes activities, apoptotic factors activities and their corresponding genes expression) remained within normal ranges following treatment. Our results indicated that drug delivery with functionalized SWCNTs can improve the antiviral effect on grass carp and has a potential application value to control fish viral diseases in aquaculture.

Resveratrol inhibits enterovirus 71 replication and pro-inflammatory cytokine secretion in rhabdosarcoma cells through blocking IKKs/NF-κB signaling pathway

Polydatin and resveratrol, as major active components in Polygonum cuspidatum, have anti-inflammatory, antioxidant and antitumor functions. However, the effect and mechanism of polydatin and resveratrol on enterovirus 71 (EV71) have not been reported. In this study, resveratrol revealed strong antiviral activity on EV71, while polydatin had weak effect. Neither polydatin nor resveratrol exhibited influence on viral attachment. Resveratrol could effectively inhibit the synthesis of EV71/VP1 and the phosphorylation of IKKα, IKKβ, IKKγ, IKBα, NF-κB p50 and NF-κB p65, respectively. Meanwhile, the remarkably increased secretion of IL-6 and TNF-α in EV71-infected rhabdosarcoma (RD) cells could be blocked by resveratrol. These results demonstrated that resveratrol inhibited EV71 replication and cytokine secretion in EV71-infected RD cells through blocking IKKs/NF-κB signaling pathway. Thus, resveratrol may have potent antiviral effect on EV71 infection.

Bona fide evidence for natural vertical transmission of infectious salmon anemia virus in freshwater brood stocks of farmed Atlantic salmon (Salmo salar) in Southern Chile

Infectious salmon anemia (ISA) is a severe disease that affects farmed Atlantic salmon (Salmo salar), causing outbreaks in seawater in most salmon-producing countries worldwide, with particular aggressiveness in southern Chile. The etiological agent of this disease is a virus belonging to the Orthomyxoviridae family, named infectious salmon anemia virus (ISAV). Although it has been suggested that this virus can be vertically transmitted, even in freshwater, there is a lack of compelling experimental evidence to confirm this. Here we demonstrate significant putative viral loads in the ovarian fluid as well as in the eggs of two brood stock female adult specimens that harbored the virus systemically but without clinical signs. The target virus corresponded to a highly polymorphic region 3 (HPR-3) variant, which is known to be virulent in seawater and responsible for recent and past outbreaks of this disease in Chile. Additionally, the virus recovered from the fluid as well as from the interior of the eggs was fully infective to a susceptible fish cell line. To our knowledge, this is the first robust evidence demonstrating mother-to-offspring vertical transmission of the infective virus on the one hand and the asymptomatic transmission of a virulent form of the virus in freshwater fish on the other hand.

IMPORTANCE

The robustness of the data presented here will contribute to a better understanding of the biology of the virus but most importantly will constitute a key management tool in the control of an aggressive agent constantly threatening the sustainability of the global salmon industry.

Chlorogenic acid inhibits the replication and viability of enterovirus 71 in vitro

Enterovirus 71 (EV71) is an etiology for a number of diseases in humans. Traditional Chinese herbs have been reported to be effective for treating EV71 infection. However, there is no report about the antiviral effects of CHA against EV71. In this study, plaque reduction assay demonstrated that the inhibitory concentration 50% (IC50) of CHA on EV71 replication is 6.3 µg/ml. When both CHA (20 µg/ml) and EV71 were added, or added post-infection at different time points, CHA was able to effectively inhibit EV71 replication between 0 and 10 h. In addition, CHA inhibited EV71 2A transcription and translation in EV71-infected RD cells, but did not affect VP1, 3C, and 3D expression. Furthermore, CHA inhibited secretions of IL-6, TNF-α, IFN-γ and MCP-1 in EV71-infected RD cells. Altogether, these results revealed that CHA may have antiviral properties for treating EV71 infection.