Identification and characterization of a tumor necrosis factor receptor like protein encoded by Singapore grouper iridovirus

PIASy of orange-spotted grouper (Epinephelus coioides) negatively regulates RLRs-mediated innate antiviral immunity

During viral infection, RIG-I-like receptors (RLRs) are cytoplasmic pattern recognition receptors that recognize and bind to viral RNA components, initiating the transcription of interferon-related genes, inflammatory cytokines and other factors, thereby triggering the cellular production of an antiviral innate immune response. The protein inhibitor of activated signal transducer and activator of transcription (STAT) (PIAS) protein family has become a hot research topic due to its extensive involvement in the regulation of cytokines, inflammatory factors and innate immune signaling pathways. In the present study, we investigated the role of fish PIASy in Singapore grouper iridovirus (SGIV) and red spotted grouper nervous necrosis virus (RGNNV) infections. The homologous sequence of orange-spotted grouper (Epinephelus coioides) PIASy gene (EcPIASy) was cloned and characterized, which encoded a 498-amino acid protein with 99.20 % homology to Plectropomus leopardus. EcPIASy is expressed mainly in gills, blood, and liver. Subcellular localization showed that EcPIASy was uniformly distributed in the nucleus. Overexpression of EcPIASy promoted SGIV and RGNNV replication, and inhibited the expression of interferon related genes and pro-inflammatory factors induced by viruses. In addition, EcPIASy interacts with RLR signaling pathway-related genes EcMDA5, EcIRF3 and EcIRF7, whereas the interaction between EcPIASy and EcIRF3 does not depend on any specific structural domain of EcPIASy. The results provide a better understanding of the relationship between PIASy and viral infection in fish.

Singapore grouper iridovirus VP12 evades the host antiviral immune response by targeting the cGAS-STING signalling pathway

The emergence of Singapore grouper iridovirus (SGIV) has caused huge losses to grouper farming. SGIV is a DNA virus and belongs to the genus Ranavirus. Groupers infected with SGIV showed haemorrhaging and swelling of the spleen, with a mortality rate of more than 90% within a week. Therefore, it is of great significance to study the escape mechanism of SGIV from host innate immunity for the prevention and treatment of viral diseases in grouper. In this study, the viral proteins that interact with EccGAS were identified by mass spectrometry, and the SGIV VP12 protein that inhibits cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING)-mediated antiviral innate immunity was screened by the dual-luciferase reporter gene assay. VP12 belongs to the late gene of the virus. The immunofluorescence analysis demonstrated that VP12 was aggregated and distributed in the cytoplasm during the early stage of virus infection and translocated into the nucleus at the late stage of virus infection. VP12 inhibited the activation of IFN3, ISRE and NF-κB promoter activities mediated by cGAS-STING, EcTBK1 and EcIRF3. Quantitative real-time PCR analysis showed that VP12 inhibited the expression of interferon-related genes, including those mediated by cGAS-STING. VP12 enhanced the inhibition of IFN3, ISRE and NF-κB promoter activity by EccGAS, EccGAS-mab-21 and EccGAS-delete-mab21. The interaction between VP12 and EccGAS was found to be domain independent. The immunoprecipitation results demonstrated that VP12 interacted and co-localized with EccGAS, EcTBK1 and EcIRF3. VP12 degraded the protein levels of EcTBK1 and EcIRF3 and degraded EcIRF3 through the protease pathway. These results suggest that SGIV VP12 protein escapes the cGAS-STING signalling pathway and degrades EcIRF3 protein expression through the protease pathway.

Singapore grouper iridovirus VP146 modulates the cGAS-STING signaling pathway to escape the interferon immune response

Singapore grouper iridovirus (SGIV) is a large double-stranded DNA virus that has caused significant economic losses to the grouper aquaculture industry. So far, the structure and function of SGIV proteins have been successively reported. In the present paper, the protein of SGIV VP146 was cloned and identified. VP146 was whole-cell distributed in GS cells. VP146 promoted SGIV replication and inhibited the transcription of interferon-related genes as well as pro-inflammatory cytokines in GS cells. In addition, VP146 was involved in the regulation of the cGAS-STING signaling pathway, and decreased cGAS-STING induced the promoter of ISRE and NF-κB. VP146 interacted with the proteins of cGAS, STING, TBK1, and IRF3 from grouper, but did not affect the binding of grouper STING to grouper TBK1 and grouper IRF3. Interestingly, grouper STING was able to affect the intracellular localization of VP146. Four segment structural domains of grouper STING were constructed, and grouper STING-CTT could affect the intracellular localization of VP146. VP146 had no effect on the self-binding of EcSITNG, nor on the binding of EcSTING to EcTBK1 and EcIRF3. Together, the results demonstrated that SGIV VP146 modulated the cGAS-STING signaling pathway to escape the interferon immune response.

Genome Characterization and Phylogenetic Analysis of Scale Drop Disease Virus Isolated from Asian Seabass (Lates calcarifer)

Scale drop disease virus (SDDV), a double-stranded DNA virus in the family Iridoviridae, has been reported widely in southeast Asian countries as a causative agent of scale drop syndrome (SDS) in Asian seabass. SDS has resulted in high mortality and significant economic losses to the aquaculture industry. This study demonstrated the use of metagenomic methods to investigate bacterial and viral communities present in infected fish tissues and recover a complete genome of the causative agent named SDDV TH7_2019. Characterization of the TH7_2019 genome revealed a genome size of 131 kb with 134 putative ORFs encoding viral proteins potentially associated with host apoptosis manipulation. A comparative genome analysis showed a high degree of amino acid identity across SDDV strains, with variations in number of repeat sequences and mutations within core genes. Phylogenetic analyses indicate a close relationship among SDDV genomes. This research enhances our understanding of the genetic diversity and evolutionary relationship of SDDV, contributing valuable insights for further development of effective control strategies of SDDV.

Singapore grouper iridovirus VP20 interacts with grouper TBK1 and IRF3 to attenuate the interferon immune response

Singapore grouper iridovirus (SGIV), belonging to genus Ranavirus, family Iridoviridae, is a highly pathogenic agent and causes heavy economic losses in the global grouper aquaculture. Recent studies demonstrated that SGIV infection attenuated antiviral immune and inflammatory response induced by poly (I:C) in vitro. However, little was known about the potential functions of the immune regulatory proteins encoded by SGIV. Here, we identified the detailed roles of VP20 and clarified the potential mechanism underlying its immune regulatory function during SGIV infection. Our results showed that VP20 was an IE gene, and partially co-localized with Golgi apparatus and lysosomes in grouper cells. Overexpression of VP20 enhanced SGIV replication, demonstrated by the increase in the transcription levels of viral core genes and the protein synthesis of MCP. Reporter gene assays showed that SGIV VP20 overexpression significantly reduced the IFN promoter activity induced by poly (I:C), grouper stimulator of interferon genes (EcSTING) and TANK-binding kinase 1 (EcTBK1). Consistently, the transcription levels of IFN related genes were significantly decreased in VP20 overexpressing cells compared to those in control cells. Co-IP assay and confocal microscopy observations indicated that VP20 co-localized and interacted with EcTBK1 and EcIRF3, but not EcSTING. In addition, VP20 was able to degrade EcIRF3 and attenuate the antiviral action of EcIRF3, while had no effect on EcTBK1. Together, SGIV VP20 was speculated to promote viral replication through attenuating the IFN response mediated by TBK1-IRF3 in vitro. Our findings provided new insights into the immune regulatory function of SGIV encoded unknown proteins.

Function analysis of fish PACT gene in response to virus infection

PACT (interferon-inducible double-stranded RNA-dependent protein kinase activator A) is a cellular protein which can activate PKR in dsRNA-independent manner. However, the role of PACT in fish virus infection remains largely unknown. In this study, a PACT homologue from grouper (Epinephelus coioides)(EcPACT) was cloned and characterized. The open reading frame of EcPACT has a full length of 924 bp and encodes a protein of 307 amino acids with a predicted molecular weight of 33.29 kDa. Similar to mammals, EcPACT contains three dsRBD domains. EcPACT shares 99.67 % homology with E. lanceolatus. Real-time fluorescence quantitative PCR results showed that EcPACT mRNA was widely expressed in all tissues and abundantly expressed in brain, blood, head kidney and kidney. In addition, SGIV and RGNNV infection significantly upregulated the transcript levels of EcPACT. Subcellular localization analysis showed that EcPACT was mainly distributed in the nucleus. Overexpression of EcPACT inhibited the replication of SGIV and RGNNV in vitro and positively regulated the expression of interferon (IFN) and pro-inflammatory factors. The results provide a better understanding of the relationship between PACT and viral infection in fish.

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.

Identification and characterization of a tumor necrosis factor receptor like protein encoded by Cyprinid Herpesvirus 2

Virus-encoded tumor necrosis factor receptors (vTNFRs) facilitate viral escape from the host immune response during viral propagation. Cyprinid Herpesvirus-2 (CyHV-2) is a double-stranded DNA virus of alloherpesviridae family that causes great economic losses in the aquaculture industry. The present study identified and characterized a novel TNFR homolog termed ORF4 in CyHV-2. ORF4 was identified as a secreted protein and a homolog of herpesvirus entry mediator (HVEM). ORF4 localized to the cytoplasm in infected GiCF cells. ORF4 overexpression enhanced viral propagation, while downregulation of ORF4 via siRNA decreased viral propagation. ORF4 overexpression promoted GiCF proliferation, and its downregulation suppressed CyHV-2-induced apoptosis. GST-pulldown and LC-MS/MS assays identified 44 conditional binding proteins that interact with ORF4 protein, while the GST pulldown test did not support the idea that ORF4 interact with histone H3.3. Taken together, our results contribute to our understanding of the vTNFR function in alloherpesviridae pathogenesis and host immune regulation.

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.

Mechanisms of Fish Macrophage Antimicrobial Immunity

Overcrowding conditions and temperatures shifts regularly manifest in large-scale infections of farmed fish, resulting in economic losses for the global aquaculture industries. Increased understanding of the functional mechanisms of fish antimicrobial host defenses is an important step forward in prevention of pathogen-induced morbidity and mortality in aquaculture setting. Like other vertebrates, macrophage-lineage cells are integral to fish immune responses and for this reason, much of the recent fish immunology research has focused on fish macrophage biology. These studies have revealed notable similarities as well as striking differences in the molecular strategies by which fish and higher vertebrates control their respective macrophage polarization and functionality. In this review, we address the current understanding of the biological mechanisms of teleost macrophage functional heterogeneity and immunity, focusing on the key cytokine regulators that control fish macrophage development and their antimicrobial armamentarium.

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.

A tumour necrosis factor receptor-like protein encoded by Singapore grouper iridovirus modulates cell proliferation, apoptosis and viral replication

It has been demonstrated that tumour necrosis factor receptor (TNFR) homologues encoded by viruses are usually involved in virus immune evasion by regulating the host immune response or mediating apoptotic cell death. Here, a novel TNFR-like protein encoded by Singapore grouper iridovirus (SGIV VP51) was cloned and characterized. Amino acid analysis showed that VP51 contained three cysteine-rich domains (CRDs) and a transmembrane domain at its C terminus. The expression of VP51 in vitro enhanced cell proliferation, and affected cell cycle progression via altering the G1/S transition. Furthermore, VP51 overexpression improved cell viability during SGIV infection via inhibiting virus-induced apoptosis, evidenced by the reduction of apoptotic bodies and the decrease of caspase-3 activation. In addition, overexpression of VP51 increased viral titre and the expression of viral structural protein gene MCP and cell proliferation promoting gene ICP-18. In contrast, the expression of the viral apoptosis inducing gene, LITAF, was significantly decreased. Although all three CRDs were essential for the action of VP51, CRD2 and CRD3 exerted more crucial roles on virus-induced apoptosis, viral gene transcription and virus production, while CRD1 was more crucial for cell proliferation. Together, SGIV TNFR-like products not only affected cell cycle progression and enhanced cell growth by increasing the expression of the virus encoded cell proliferation gene, but also inhibited virus-induced apoptotic cell death by decreasing the expression of the viral apoptosis inducing gene. Our results provided new insights into understanding the underlying mechanism by which iridovirus regulated the apoptotic pathway to complete its life cycle.

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.

Identification and characterization of a late gene of grouper iridovirus 61l and antibody production against the protein encoded by it

In this study, a late gene encoded by grouper iridovirus, giv-61L, was identified and classified, and mouse monoclonal antibodies (mAbs) were raised against this protein. Giv-61L homologues were found only in the genus Ranavirus. Three mAbs to Giv-61L protein were produced. In drug inhibition assays, giv-61L was identified as a late gene. Finally, GIV-61L-mAb-8 was used in western blotting and immunofluorescence assays to demonstrate that Giv-61L protein was included in the GIV particle, expressed at 18 h, and localized only in the cytoplasm of GIV-infected cells. The results of this study provide insight into GIV pathogenesis and GIV-61L-mAbs will have broad applications in GIV immunodiagnostics.