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

Age- and dose-dependent susceptibility of axolotls (Ambystoma mexicanum) by bath exposure to Ambystoma tigrinum virus (ATV)

Ranaviruses are large, dsDNA viruses that have significant ecological and economic impact on cold-blooded vertebrates. However, our understanding of the viral proteins and subsequent host immune response(s) that impact susceptibility to infection and disease is not clear. The ranavirus Ambystoma tigrinum virus (ATV), originally isolated from the Sonoran tiger salamander (Ambystoma mavortium stebbinsi), is highly pathogenic at low doses of ATV at all tiger salamander life stages and this model has been used to explore the host-pathogen interactions of ATV infection. However, inconsistencies in the availability of laboratory reared larval tiger salamanders required us to look at the well characterized axolotl (A. mexicanum) as a model for ATV infection. Data obtained from five infection experiments over different developmental timepoints suggest that axolotls are susceptible to ATV in an age- and dose-dependent manner. These data support the use of the ATV-axolotl model to further explore the host-pathogen interactions of ranavirus infections.

Genome Characteristics of Two Ranavirus Isolates from Mandarin Fish and Largemouth Bass

Ranaviruses are promiscuous pathogens that threaten lower vertebrates globally. In the present study, two ranaviruses (SCRaV and MSRaV) were isolated from two fishes of the order Perciformes: mandarin fish (Siniperca chuatsi) and largemouth bass (Micropterus salmoides). The two ranaviruses both induced cytopathic effects in cultured cells from fish and amphibians and have the typical morphologic characteristics of ranaviruses. Complete genomes of the two ranaviruses were then sequenced and analyzed. Genomes of SCRaV and MSRaV have a length of 99, 405, and 99, 171 bp, respectively, and both contain 105 predicted open reading frames (ORFs). Eleven of the predicted proteins have differences between SCRaV and MSRaV, in which only one (79L) possessed a relatively large difference. A comparison of the sequenced six ranaviruses from the two fish species worldwide revealed that sequence identities of the six proteins (11R, 19R, 34L, 68L, 77L, and 103R) were related to the place where the virus was isolated. However, there were obvious differences in protein sequence identities between the two viruses and iridoviruses from other hosts, with more than half lower than 55%. Especially, 12 proteins of the two isolates had no homologs in viruses from other hosts. Phylogenetic analysis revealed that ranaviruses from the two fishes clustered in one clade. Further genome alignment showed five groups of genome arrangements of ranaviruses based on the locally collinear blocks, in which the ranaviruses, including SCRaV and MSRaV, constitute the fifth group. These results provide new information on the ranaviruses infecting fishes of Perciformes and also are useful for further research of functional genomics of the type of ranaviruses.

Deleting ORF71L of infectious spleen and kidney necrosis virus (ISKNV) resulted in virulence attenuation in Mandarin fish

Infectious spleen and kidney necrosis virus (ISKNV), the type species of the genus Megalocytivirus, infects a variety of teleost fish species and causes substantial losses in the aquaculture industry worldwide. ISKNV ORF71L is 1611 bp in length, encodes a 537-amino-acid peptide and was previously identified as a viral structural protein in the ISKNV virion. In this study, the ORF71L deletion mutant virus strain ISKNV-Δ71 was obtained through a homologous recombination approach. The multistep growth curves showed that ISKNV-Δ71 replication was faster than ISKNV-WT replication in mandarin fish fry cells (MFF-1 cells) before 48 h post-infection (hpi). The cumulative mortality of ISKNV-Δ71-infected mandarin fish (Siniperca chuatsi) was lower than that of fish infected with ISKNV-WT. The copy numbers of viral genome equivalents (GEs) in ISKNV-Δ71-infected mandarin fish spleens were also lower than those in ISKNV-WT-infected spleens. Deletion of ORF71L resulted in ISKNV virulence attenuation in mandarin fish. Furthermore, we found that the number of melanomacrophage centers (MMCs) in ISKNV-Δ71-infected mandarin fish spleens was higher than that in ISKNV-WT-infected mandarin fish spleens. Transcriptomic analysis showed that the cytokine-cytokine receptor interaction pathway had the most significant change between ISKNV-Δ71- and ISKNV-WT-infected MFF-1 cells. These results indicated ORF71L is a virulence-related gene of ISKNV. ORF71L could be considered as a potential target for the development of engineered attenuated live vaccines via multigene deletion or as a potential insertion site for exogenous protein expression.

The Immune System and the Antiviral Responses in Chinese Giant Salamander, Andrias davidianus

The Chinese giant salamander, belonging to an ancient amphibian lineage, is the largest amphibian existing in the world, and is also an important animal for artificial cultivation in China. However, some aspects of the innate and adaptive immune system of the Chinese giant salamander are still unknown. The Chinese giant salamander iridovirus (GSIV), a member of the Ranavirus genus (family Iridoviridae), is a prominent pathogen causing high mortality and severe economic losses in Chinese giant salamander aquaculture. As a serious threat to amphibians worldwide, the etiology of ranaviruses has been mainly studied in model organisms, such as the Ambystoma tigrinum and Xenopus. Nevertheless, the immunity to ranavirus in Chinese giant salamander is distinct from other amphibians and less known. We review the unique immune system and antiviral responses of the Chinese giant salamander, in order to establish effective management of virus disease in Chinese giant salamander artificial cultivation.

Development of a gene-deleted live attenuated candidate vaccine against fish virus (ISKNV) with low pathogenicity and high protection

Aquaculture provides important food, nutrition, and income sources for humans. However, aquaculture industry is seriously threatened by viral diseases. Infectious spleen and kidney necrosis virus (ISKNV) disease causes high mortality and economic losses to the fish culture industry in Asia and has been listed as a certifiable disease by the International Epizootic Office. Vaccine development is urgent to control this disease. Here, a gene-deleted live attenuated candidate vaccine (ΔORF022L) against ISKNV with low pathogenicity and high protection was developed. ΔORF022L replicated well in mandarin fish fry-1 cells and showed similar structure with wild-type ISKNV. However, the pathogenicity was significantly lower as 98% of the mandarin fish infected with ΔORF022L survived, whereas all those infected with wild-type ISKNV died. Of importance, 100% of the ΔORF022L-infected fish survived the ISKNV challenge. ΔORF022L induced anti-ISKNV specific antibody response and upregulation of immune-related genes. This work could be beneficial to the control of fish diseases.

Low intraspecific variation of Frog virus 3 with evidence for novel FV3-like isolates in central and northwestern Canada

Frog virus 3 (FV3) and FV3-like ranaviruses can infect a variety of cold-blooded aquatic species and present a primary threat to amphibians across the globe. Previous studies of FV3-like viruses have largely investigated higher-level phylogenetic distinctions of these pathogens via portions of the conserved major capsid protein (MCP), and the putative virulence gene vIF-2α. Few studies, however, have investigated the spatial distribution of FV3 variants at the population level3-data that can be used to further understand the spatial epidemiology of this disease. In this study, we sequenced the MCP and vIF-2α of 127 FV3-positive amphibians sampled from Canadian water bodies in Ontario, northeastern Alberta, and southern Northwest Territories to explore whether intraspecific genetic variation exists within FV3. There was a lack of variation at the 2 markers across these regions, suggesting that there is a lack of FV3 sequence diversity in Canada, which may hint at a single source of infection that has spread. However, an undocumented variant termed Wood Buffalo ranavirus (WBRV) was detected in samples from 3 sites in Alberta and Northwest Territories that clustered within the FV3-like lineage with 99.3% sequence homology for MCP. For vIF-2α, all sequences were the expected truncated variant except for 6 samples in Ontario. These latter sequences were suggestive of recombination with common midwife toad virus (CMTV). The lack of variation suggests that higher-resolution genome analyses will be required to further explore the spatial spread and intraspecific variation of the disease.

Development of reverse-transcriptase quantitative PCR assays for detection of the cytokines IL-1β, TNF-α, and IL-10 in chelonians

In response to viral pathogens, a host releases pro-inflammatory cytokines such as interleukin-1 beta (IL-1β) and tumor necrosis factor alpha (TNF-α) and anti-inflammatory cytokines such as interleukin-10 (IL-10). While several approaches exist to measure cytokine responses, evaluating gene transcription through reverse transcription quantitative polymerase chain reaction (RT-qPCR) provides a fast, reproducible, and sensitive method for quantifying this response. The objective of this study was to develop an effective and sensitive RT-qPCR assay for the quantification of red-eared slider (Trachemys scripta elegans) and eastern box turtle (Terrapene carolina carolina) cytokines: IL-1β, TNFα, IL-10 and the reference gene β-actin. RNA was isolated from the buffy coat layer of whole blood, comprised mainly of circulating leukocytes, and complimentary DNA (cDNA) was produced. Conventional PCR was performed to obtain cytokine mRNA sequences, products were sequenced, and a hydrolysis probe-based RT-qPCR assay was designed for each cytokine. Standard curves were generated using the target gene sequences cloned within a plasmid. Efficiencies for each assay were between of 85-110%, R² > 0.98, and limits of detection of 10-100 copies per reaction. The initial samples used to identify the novel target sequences were then used to evaluate the performance of the qPCR assays. Consistent transcription of beta actin across individuals in both species and measurable transcription of IL-1β, TNF-α, and IL-10 transcript targets in individuals of both species were observed. The assays are a novel technique in chelonians to evaluate host innate immune response.

The immune evasion strategies of fish viruses

Viral infection of a host rapidly triggers intracellular signaling events that induce interferon production and a cellular antiviral state. Viral diseases are important concerns in fish aquaculture. The major mechanisms of the fish antiviral immune response are suggested to be similar to those of mammals, although the specific details of the process require further studies. Throughout the process of pathogen-host coevolution, fish viruses have developed a battery of distinct strategies to overcome the biochemical and immunological defenses of the host. Such strategies include signaling interference, effector modulation, and manipulation of host apoptosis. This review provide an overview of the different mechanisms that fish viruses use to evade host immune responses. The basic mechanisms of immune evasion of fish virus are discussed, and some examples are provided to illustrate particular points.

Aquatic animal viruses mediated immune evasion in their host

Viruses are important and lethal pathogens that hamper aquatic animals. The result of the battle between host and virus would determine the occurrence of diseases. The host will fight against virus infection with various responses such as innate immunity, adaptive immunity, apoptosis, and so on. On the other hand, the virus also develops numerous strategies such as immune evasion to antagonize host antiviral responses. Here, We review the research advances on virus mediated immune evasions to host responses containing interferon response, NF-κB signaling, apoptosis, and adaptive response, which are executed by viral genes, proteins, and miRNAs from different aquatic animal viruses including Alloherpesviridae, Iridoviridae, Nimaviridae, Birnaviridae, Reoviridae, and Rhabdoviridae. Thus, it will facilitate the understanding of aquatic animal virus mediated immune evasion and potentially benefit the development of novel antiviral applications.

Characterization of a PKR inhibitor from the pathogenic ranavirus, Ambystoma tigrinum virus, using a heterologous vaccinia virus system

Ambystoma tigrinum virus (ATV) (family Iridoviridae, genus Ranavirus) was isolated from diseased tiger salamanders (Ambystoma tigrinum stebbinsi) from the San Rafael Valley in southern Arizona, USA in 1996. Genomic sequencing of ATV, as well as other members of the genus, identified an open reading frame that has homology to the eukaryotic translation initiation factor, eIF2α (ATV eIF2α homologue, vIF2αH). Therefore, we asked if the ATV vIF2αH could also inhibit PKR. To test this hypothesis, the ATV vIF2αH was cloned into vaccinia virus (VACV) in place of the well-characterized VACV PKR inhibitor, E3L. Recombinant VACV expressing ATV vIF2αH partially rescued deletion of the VACV E3L gene. Rescue coincided with rapid degradation of PKR in infected cells. These data suggest that the salamander virus, ATV, contains a novel gene that may counteract host defenses, and this gene product may be involved in the presentation of disease caused by this environmentally important pathogen.

The Ambystoma tigrinum virus (ATV) RNase III gene can modulate host PKR activation and interferon production

The iridovirus RNase III gene is one of 26 conserved core genes among the family Iridoviridae. Initial studies suggest this viral protein functions to suppress RNA interference pathways that may attack viral RNA during infection. Therefore, to determine if the Ambystoma tigrinum virus (ATV) RNase III-like gene (ORF 25R) can modulate the host innate immune response fish and human cells ectopically expressing 25R were treated with polyI:C and monitored for interferon synthesis and phosphorylation of eIF2α and PKR. We found a decrease in cellular IFN production and modulation of the PKR pathway. In addition, ATV deleted of the RNase III gene (ATVΔ25R) shows reduced pathogenicity in tiger salamanders. Collectively our data suggest that the ATV 25R protein is a pathogenesis factor that may function to help evade the host's immune response by masking activators of the IFN pathway.

Singapore grouper iridovirus (SGIV) TNFR homolog VP51 functions as a virulence factor via modulating host inflammation response

Virus encoded tumor necrosis factor receptor (TNFR) homologues are usually involved in immune evasion by regulating host immune response or cell death. Singapore grouper iridovirus (SGIV) is a novel ranavirus which causes great economic losses in aquaculture industry. Previous studies demonstrated that SGIV VP51, a TNFR-like protein regulated apoptotic process in VP51 overexpression cells. Here, we developed a VP51-deleted recombinant virus Δ51-SGIV by replacing VP51 with puro^R-GFP. Deletion of VP51 resulted in the decrease of SGIV virulence, evidenced by the reduced replication in vitro and the decreased cumulative mortalities in Δ51-SGIV challenged grouper compared to WT-SGIV. Moreover, VP51 deletion significantly increased virus induced apoptosis, and reduced the expression of pro-inflammatory cytokines in vitro. In addition, the expression of several pro-inflammatory genes were decreased in Δ51-SGIV infected grouper compared to WT-SGIV. Thus, we speculate that SGIV VP51 functions as a critical virulence factor via regulating host cell apoptosis and inflammation response.

Xenopus-FV3 host-pathogen interactions and immune evasion

We first review fundamental insights into anti-ranavirus immunity learned with the Xenopus laevis/ranavirus FV3 model that are generally applicable to ectothermic vertebrates. We then further investigate FV3 genes involved in immune evasion. Focusing on FV3 knockout (KO) mutants defective for a putative viral caspase activation and recruitment domain-containing (CARD)-like protein (Δ64R-FV3), a β-hydroxysteroid dehydrogenase homolog (Δ52L-FV3), and an immediate-early18kDa protein (FV3-Δ18K), we assessed the involvement of these viral genes in replication, dissemination and interaction with peritoneal macrophages in tadpole and adult frogs. Our results substantiate the role of 64R and 52L as critical immune evasion genes, promoting persistence and dissemination in the host by counteracting type III IFN in tadpoles and type I IFN in adult frogs. Comparably, the substantial accumulation of genome copy numbers and exacerbation of type I and III IFN gene expression responses but deficient release of infectious virus suggests that 18K is a viral regulatory gene.

Recombinant Ranaviruses for Studying Evolution of Host-Pathogen Interactions in Ectothermic Vertebrates

Ranaviruses (Iridoviridae) are large DNA viruses that are causing emerging infectious diseases at an alarming rate in both wild and captive cold blood vertebrate species all over the world. Although the general biology of these viruses that presents some similarities with poxvirus is characterized, many aspects of their replication cycles, host cell interactions and evolution still remain largely unclear, especially in vivo. Over several years, strategies to generate site-specific ranavirus recombinant, either expressing fluorescent reporter genes or deficient for particular viral genes, have been developed. We review here these strategies, the main ranavirus recombinants characterized and their usefulness for in vitro and in vivo studies.

Identification of essential and non-essential genes in Ambystoma tigrinum virus

Members of the genus Ranavirus (family Iridoviridae) are large double-stranded (ds) DNA viruses that are found world-wide infecting fish, amphibian and reptile ectothermic hosts. Ranavirus genomes range from 105 to 155kbp in length and they are predicted to encode around 90-125 genes. Currently, our knowledge of the function of ∼50% of these genes is known or inferred based on homology to orthologous genes characterized in other systems; however, the function of the remaining open reading frames (ORFS) is unknown. Therefore, in order to begin to uncover the function of unknown ORFs in ranaviruses we developed a standardized approach to generate a recombination cassette for any ORF in Ambystoma tigrinum virus (ATV). Our standardized approach quickly and efficiently assembles recombination cassettes and recombinant ATV. We have used this approach to identify two essential, one semi-essential and two non-essential genes in ATV.

Rana grylio virus TK and DUT gene locus could be simultaneously used for foreign gene expression

Ranaviruses (family Iridoviridae, genus Ranavirus) have been recognized as emerging infectious pathogens and caused a great loss to the global biodiversity. Thymidine kinase (TK) and deoxyuridine triphosphatase (dUTPase, DUT, encoded by ORF 67R) are ubiquitous, existing in iridoviruses and other organisms. Previous studies showed that TK and DUT could be individually knocked out without impeding viral replication. In this study, we tried to insert two fluorescence genes into the above loci. We started with Δ67R-RGV, a recently generated recombinant Rana grylio virus (RGV) with the whole DUT replaced by enhanced green fluorescence protein (EGFP) gene. Then, a red fluorescence protein (RFP) gene initiated by RGV immediate-early (IE) ICP18 gene promoter was inserted into TK locus through homologous recombination. A novel recombinant virus, ΔDUT, TK-RGV, was generated by nine successive rounds of plaque isolation using RFP selection. All of the plaques produced by this recombinant virus could emit both green and red fluorescence. Furthermore, one-step and multiple-step growth curves of ΔDUT, TK-RGV were similar to those of wt-RGV and Δ67R-RGV. In conclusion, a novel dual-fluorescence labeled recombinant iridovirus in which DUT and TK gene locus were simultaneously used for foreign gene expression was constructed.

Establishment of a Zebrafish Infection Model for the Study of Wild-Type and Recombinant European Sheatfish Virus

Amphibian-like ranaviruses include pathogens of fish, amphibians, and reptiles that have recently evolved from a fish-infecting ancestor. The molecular determinants of host range and virulence in this group are largely unknown, and currently fish infection models are lacking. We show that European sheatfish virus (ESV) can productively infect zebrafish, causing a lethal pathology, and describe a method for the generation of recombinant ESV, establishing a useful model for the study of fish ranavirus infections.

Characterization of Frog Virus 3 knockout mutants lacking putative virulence genes

To identify ranavirus virulence genes, we engineered Frog Virus 3 (FV3) knockout (KO) mutants defective for a putative viral caspase activation and recruitment domain-containing (CARD) protein (Δ64R-FV3) and a β-hydroxysteroid dehydrogenase homolog (Δ52L-FV3). Compared to wild type (WT) FV3, infection of Xenopus tadpoles with Δ64R- or Δ52L-FV3 resulted in significantly lower levels of mortality and viral replication. We further characterized these and two earlier KO mutants lacking the immediate-early18kDa protein (FV3-Δ18K) or the truncated viral homolog of eIF-2α (FV3-ΔvIF-2α). All KO mutants replicated as well as WT-FV3 in non-amphibian cell lines, whereas in Xenopus A6 kidney cells replication of ΔvCARD-, ΔvβHSD- and ΔvIF-2α-FV3 was markedly reduced. Furthermore, Δ64R- and ΔvIF-2α-FV3 were more sensitive to interferon than WT and Δ18-FV3. Notably, Δ64R-, Δ18K- and ΔvIF-2α- but not Δ52L-FV3 triggered more apoptosis than WT FV3. These data suggest that vCARD (64R) and vβ-HSD (52L) genes contribute to viral pathogenesis.

Prominent amphibian (Xenopus laevis) tadpole type III interferon response to the frog virus 3 ranavirus

Ranaviruses (Iridoviridae) are posing an increasing threat to amphibian populations, with anuran tadpoles being particularly susceptible to these viral infections. Moreover, amphibians are the most basal phylogenetic class of vertebrates known to possess both type I and type III interferon (IFN)-mediated immunity. Moreover, little is known regarding the respective roles of the IFN mediators in amphibian antiviral defenses. Accordingly, we transcriptionally and functionally compared the amphibian Xenopus laevis type I (IFN) and III (IFN-λ) IFNs in the context of infections by the ranavirus frog virus 3 (FV3). X. laevis IFN and IFN-λ displayed distinct tissue expression profiles. In contrast to our previous findings that X. laevis tadpoles exhibit delayed and modest type I IFN responses to FV3 infections compared to the responses of adults, here we report that tadpoles mount timely and robust type III IFN gene responses. Recombinant forms of these cytokines (recombinant X. laevis IFN [rXlIFN] and rXlIFN-λ) elicited antiviral gene expression in the kidney-derived A6 cell line as well as in tadpole leukocytes and tissues. However, rXlIFN-λ was less effective than rXlIFN in preventing FV3 replication in A6 cells and tadpoles and inferior at promoting tadpole survival. Intriguingly, FV3 impaired A6 cell and tadpole kidney type III IFN receptor gene expression. Furthermore, in A6 cultures rXlIFN-λ conferred equal or greater protection than rXlIFN against recombinant viruses deficient for the putative immune evasion genes, the viral caspase activation and recruitment domain (vCARD) or a truncated vIF-2α gene. Thus, in contrast to previous assumptions, tadpoles possess intact antiviral defenses reliant on type III IFNs, which are overcome by FV3 pathogens.

IMPORTANCE

Anuran tadpoles, including those of Xenopus laevis, are particularly susceptible to infection by ranavirus such as FV3. We investigated the respective roles of X. laevis type I and type III interferons (IFN and IFN-λ, respectively) during FV3 infections. Notably, tadpoles mounted timely and more robust IFN-λ gene expression responses to FV3 than adults, contrasting with the poorer tadpole type I IFN responses. However, a recombinant X. laevis IFN-λ (rXlIFN-λ) conferred less protection to tadpoles and the A6 cell line than rXlIFN, which may be explained by the FV3 impairment of IFN-λ receptor gene expression. The importance of IFN-λ in tadpole anti-FV3 defenses is underlined by the critical involvement of two putative immune evasion genes in FV3 resistance to IFN- and IFN-λ-mediated responses. These findings challenge the view that tadpoles have defective antiviral immunity and suggest, rather, that their antiviral responses are predominated by IFN-λ responses, which are overcome by FV3.

Phylogeny and differentiation of reptilian and amphibian ranaviruses detected in Europe

Ranaviruses in amphibians and fish are considered emerging pathogens and several isolates have been extensively characterized in different studies. Ranaviruses have also been detected in reptiles with increasing frequency, but the role of reptilian hosts is still unclear and only limited sequence data has been provided. In this study, we characterized a number of ranaviruses detected in wild and captive animals in Europe based on sequence data from six genomic regions (major capsid protein (MCP), DNA polymerase (DNApol), ribonucleoside diphosphate reductase alpha and beta subunit-like proteins (RNR-α and -β), viral homolog of the alpha subunit of eukaryotic initiation factor 2, eIF-2α (vIF-2α) genes and microsatellite region). A total of ten different isolates from reptiles (tortoises, lizards, and a snake) and four ranaviruses from amphibians (anurans, urodeles) were included in the study. Furthermore, the complete genome sequences of three reptilian isolates were determined and a new PCR for rapid classification of the different variants of the genomic arrangement was developed. All ranaviruses showed slight variations on the partial nucleotide sequences from the different genomic regions (92.6–100%). Some very similar isolates could be distinguished by the size of the band from the microsatellite region. Three of the lizard isolates had a truncated vIF-2α gene; the other ranaviruses had full-length genes. In the phylogenetic analyses of concatenated sequences from different genes (3223 nt/10287 aa), the reptilian ranaviruses were often more closely related to amphibian ranaviruses than to each other, and most clustered together with previously detected ranaviruses from the same geographic region of origin. Comparative analyses show that among the closely related amphibian-like ranaviruses (ALRVs) described to date, three recently split and independently evolving distinct genetic groups can be distinguished. These findings underline the wide host range of ranaviruses and the emergence of pathogen pollution via animal trade of ectothermic vertebrates.

Differential transcription of fathead minnow immune-related genes following infection with frog virus 3, an emerging pathogen of ectothermic vertebrates

Frog virus 3 (FV3) and other ranaviruses are responsible for die-offs involving wild, farmed, and captive amphibians, fish, and reptiles. To ascertain which elements of the immune system respond to infection, we explored transcriptional responses following infection of fathead minnow cells with either wild type (wt) FV3 or a knock out (KO) mutant targeting the 18 kDa immediate early gene (18K). At 8h post infection we observed marked upregulation of multiple transcripts encoding proteins affecting innate and acquired immunity. Sequences expressed 4-fold or higher in wt-infected cells included transcripts encoding interferon (IFN), IFN regulatory factors (IRFs), IFN stimulated genes (ISGs) such as Mx and MHC class I, and interleukins IL-1β, IL-8, IL-17C and IL-12. Cells infected with the 18K KO mutant (∆18K) showed qualitative differences and lower levels of induction. Collectively, these results indicate that ranavirus infection induced expression of multiple cellular genes affecting both innate and acquired immunity.

Recent advances in understanding viral evasion of type I interferon

The type I interferon (IFN) system mediates a wide variety of antiviral effects and represents an important first barrier to virus infection. Consequently, viruses have developed an impressive diversity of tactics to circumvent IFN responses. Evasion strategies can involve preventing initial virus detection, via the disruption of the Toll‐like receptors or the retinoic acid inducible gene I (RIG‐I) ‐like receptors, or by avoiding the initial production of the ligands recognized by these receptors. An alternative approach is to preclude IFN production by disarming or degrading the transcription factors involved in the expression of IFN, such as interferon regulatory factor 3 (IRF3)/IRF7, nuclear factor‐κB (NF‐κB), or ATF‐2/c‐jun, or by inducing a general block on host cell transcription. Viruses also oppose IFN signalling, both by disturbing the type I IFN receptor and by impeding JAK/STAT signal transduction upon IFN receptor engagement. In addition, the global expression of IFN‐stimulated genes (ISGs) can be obstructed via interference with epigenetic signalling, and specific ISGs can also be selectively targeted for inhibition. Finally, some viruses disrupt IFN responses by co‐opting negative regulatory systems, whereas others use antiviral mechanisms to their own advantage. Here, we review recent developments in this field.

Immune evasion strategies of ranaviruses and innate immune responses to these emerging pathogens

Ranaviruses (RV, Iridoviridae) are large double-stranded DNA viruses that infect fish, amphibians and reptiles. For ecological and commercial reasons, considerable attention has been drawn to the increasing prevalence of ranaviral infections of wild populations and in aquacultural settings. Importantly, RVs appear to be capable of crossing species barriers of numerous poikilotherms, suggesting that these pathogens possess a broad host range and potent immune evasion mechanisms. Indeed, while some of the 95–100 predicted ranavirus genes encode putative evasion proteins (e.g., vIFα, vCARD), roughly two-thirds of them do not share significant sequence identity with known viral or eukaryotic genes. Accordingly, the investigation of ranaviral virulence and immune evasion strategies is promising for elucidating potential antiviral targets. In this regard, recombination-based technologies are being employed to knock out gene candidates in the best-characterized RV member, Frog Virus (FV3). Concurrently, by using animal infection models with extensively characterized immune systems, such as the African clawed frog, Xenopus laevis, it is becoming evident that components of innate immunity are at the forefront of virus-host interactions. For example, cells of the macrophage lineage represent important combatants of RV infections while themselves serving as targets for viral infection, maintenance and possibly dissemination. This review focuses on the recent advances in the understanding of the RV immune evasion strategies with emphasis on the roles of the innate immune system in ranaviral infections.

The genome sequence of the emerging common midwife toad virus identifies an evolutionary intermediate within ranaviruses

Worldwide amphibian population declines have been ascribed to global warming, increasing pollution levels, and other factors directly related to human activities. These factors may additionally be favoring the emergence of novel pathogens. In this report, we have determined the complete genome sequence of the emerging common midwife toad ranavirus (CMTV), which has caused fatal disease in several amphibian species across Europe. Phylogenetic and gene content analyses of the first complete genomic sequence from a ranavirus isolated in Europe show that CMTV is an amphibian-like ranavirus (ALRV). However, the CMTV genome structure is novel and represents an intermediate evolutionary stage between the two previously described ALRV groups. We find that CMTV clusters with several other ranaviruses isolated from different hosts and locations which might also be included in this novel ranavirus group. This work sheds light on the phylogenetic relationships within this complex group of emerging, disease-causing viruses.

Ecopathology of ranaviruses infecting amphibians

Ranaviruses are capable of infecting amphibians from at least 14 families and over 70 individual species. Ranaviruses infect multiple cell types, often culminating in organ necrosis and massive hemorrhaging. Subclinical infections have been documented, although their role in ranavirus persistence and emergence remains unclear. Water is an effective transmission medium for ranaviruses, and survival outside the host may be for significant duration. In aquatic communities, amphibians, reptiles and fish may serve as reservoirs. Controlled studies have shown that susceptibility to ranavirus infection and disease varies among amphibian species and developmental stages, and likely is impacted by host-pathogen coevolution, as well as, exogenous environmental factors. Field studies have demonstrated that the likelihood of epizootics is increased in areas of cattle grazing, where aquatic vegetation is sparse and water quality is poor. Translocation of infected amphibians through commercial trade (e.g., food, fish bait, pet industry) contributes to the spread of ranaviruses. Such introductions may be of particular concern, as several studies report that ranaviruses isolated from ranaculture, aquaculture, and bait facilities have greater virulence (i.e., ability to cause disease) than wild-type isolates. Future investigations should focus on the genetic basis for pathogen virulence and host susceptibility, ecological and anthropogenic mechanisms contributing to emergence, and vaccine development for use in captive populations and species reintroduction programs.

Antiviral immunity in amphibians

Although a variety of virus species can infect amphibians, diseases caused by ranaviruses ([RVs]; Iridoviridae) have become prominent, and are a major concern for biodiversity, agriculture and international trade. The relatively recent and rapid increase in prevalence of RV infections, the wide range of host species infected by RVs, the variability in host resistance among population of the same species and among different developmental stages, all suggest an important involvement of the amphibian immune system. Nevertheless, the roles of the immune system in the etiology of viral diseases in amphibians are still poorly investigated. We review here the current knowledge of antiviral immunity in amphibians, focusing on model species such as the frog Xenopus and the salamander (Ambystoma tigrinum), and on recent progress in generating tools to better understand how host immune defenses control RV infections, pathogenicity, and transmission.

The molecular biology of frog virus 3 and other iridoviruses infecting cold-blooded vertebrates

Frog virus 3 (FV3) is the best characterized member of the family Iridoviridae. FV3 study has provided insights into the replication of other family members, and has served as a model of viral transcription, genome replication, and virus-mediated host-shutoff. Although the broad outlines of FV3 replication have been elucidated, the precise roles of most viral proteins remain unknown. Current studies using knock down (KD) mediated by antisense morpholino oligonucleotides (asMO) and small, interfering RNAs (siRNA), knock out (KO) following replacement of the targeted gene with a selectable marker by homologous recombination, ectopic viral gene expression, and recombinant viral proteins have enabled researchers to systematically ascertain replicative- and virulence-related gene functions. In addition, the application of molecular tools to ecological studies is providing novel ways for field biologists to identify potential pathogens, quantify infections, and trace the evolution of ecologically important viral species. In this review, we summarize current studies using not only FV3, but also other iridoviruses infecting ectotherms. As described below, general principles ascertained using FV3 served as a model for the family, and studies utilizing other ranaviruses and megalocytiviruses have confirmed and extended our understanding of iridovirus replication. Collectively, these and future efforts will elucidate molecular events in viral replication, intrinsic and extrinsic factors that contribute to disease outbreaks, and the role of the host immune system in protection from disease.

Rana grylio virus as a vector for foreign gene expression in fish cells

In the present study, Rana grylio virus (RGV, an iridovirus) thymidine kinase (TK) gene and viral envelope protein 53R gene were chosen as targets for foreign gene insertion. ΔTK-RGV and Δ53R-RGV, two recombinant RGV, expressing enhanced green fluorescence protein (EGFP) were constructed and analyzed in Epithelioma papulosum cyprinid (EPC) cells. The EGFP gene which fused to the virus major capsid protein (MCP) promoter p50 was inserted into TK and 53R gene loci of RGV, respectively. Cells infected with these two recombinant viruses not only displayed plaques, but also emitted strong green fluorescence under fluorescence microscope, providing a simple method for selection and purification of recombinant viruses. ΔTK-RGV was purified by seven successive rounds of plaque isolation and could be stably propagated in EPC cells. All of the plaques produced by the purified recombinant virus emitted green fluorescence. However, Δ53R-RGV was hard to be purified even through twenty rounds of plaque isolation. The purified recombinant virus ΔTK-RGV was verified by PCR analysis and Western blotting. These results showed EGFP was expressed in ΔTK-RGV infected cells. Furthermore, one-step growth curves and electron microscopy revealed that infection with recombinant ΔTK-RGV and wild-type RGV are similar. Therefore, RGV was demonstrated could be as a viral vector for foreign gene expression in fish cells.

Improved knockout methodology reveals that frog virus 3 mutants lacking either the 18K immediate-early gene or the truncated vIF-2alpha gene are defective for replication and growth in vivo

To better assess the roles of frog virus 3 (FV3; genus Ranavirus, family Iridoviridae) genes in virulence and immune evasion, we have developed a reliable and efficient method to systematically knock out (KO) putative virulence genes by site-specific integration into the FV3 genome. Our approach utilizes a dual selection marker consisting of the puromycin resistance gene fused in frame with the enhanced green fluorescent protein (EGFP) reporter (Puro-EGFP cassette) under the control of the FV3 immediate-early (IE) 18K promoter. By successive rounds of selection for puromycin resistance and GFP expression, we have successfully constructed three recombinant viruses. In one, a "knock-in" mutant was created by inserting the Puro-EGFP cassette into a noncoding region of the FV3 genome (FV3-Puro/GFP). In the remaining two, KO mutants were constructed by replacement of the truncated viral homolog of eIF-2α (FV3-ΔvIF-2α) or the 18K IE gene (FV3-Δ18K) with the Puro-EGFP cassette. The specificity of recombination and the clonality of each mutant were confirmed by PCR, sequencing, and immunofluorescence microscopy. Viral replication of each recombinant in cell culture was similar to that of parental FV3; however, infection in Xenopus laevis tadpoles revealed that FV3-ΔvIF-2α and FV3-Δ18K replicated less and resulted in lower mortality than did GFP-FV3 and wild-type FV3. Our results suggest that 18K, which is conserved in all ranaviruses, and the truncated vIF-2α gene contribute to virulence. In addition, our study describes a powerful methodology that lays the foundation for the discovery of potentially new ranaviral genes involved in virulence and immune escape.