Identification and functional characterization of an interferon regulatory factor 7-like (IRF7-like) gene from orange-spotted grouper, Epinephelus coioides

Grouper FTR14 negatively regulates inflammatory response by targeting TRAF4 and TRAF6

Accumulated evidence indicates that tripartite motif (TRIM) family proteins play crucial roles in host antiviral defense. Notably, the functions of members of fish-specific TRIM (FTR, also known as finTRIM) subfamily remained largely unknown. Our recent results revealed that grouper FTR14 (EcFTR14) could negatively regulate interferon response by targeting TBK1-IRF3 axis. In this study, the regulatory mechanism underlying the action of EcFTR14 on host inflammatory response was elucidated. Using yeast two-hybrid screening, we found that EcFTR14 interacted with EcTRAF4 and EcTRAF6 which was further confirmed by Co-immunoprecipitation (Co-IP) assay. EcFTR14 could recruit the interacting proteins for their co-localization in vitro. Moreover, EcFTR14 hindered the activation of the NF-κB promoter induced by EcTRAF6, but enhanced the inhibitory effect of EcTRAF4. Consistently, overexpression of EcFTR14 significantly reduced the transcriptional levels of pro-inflammatory factors induced by EcTRAF6. Additionally, EcFTR14 degraded EcTRAF6 via the ubiquitin-proteasome pathway, leading to the abatement of EcTRAF6-induced antiviral effects. Conversely, overexpression of EcFTR14 enhanced the pro-viral effect of EcTRAF4. Together, our results suggested that EcFTR14 could negatively regulate the inflammatory response by targeting TRAF4/6, which might be an alternative mechanism for its function as a pro-viral factor.

SGIV VP82 inhibits the interferon response by degradation of IRF3 and IRF7

During virus-host co-evolution, viruses have developed multiple strategies to dampen IFN response and prevent its antiviral activity in host cells. To date, the interactions between host IFN response and the immune evasion strategies exploited by fish iridoviruses still remain largely uncertain. Here, a potential immune evasion protein candidate of Singapore grouper iridovirus (SGIV), VP82 (encoded by SGIV ORF82) was screened and its roles during viral replication were investigated in detail. Firstly, VP82 overexpression dramatically decreased IFN or ISRE promoter activity and the transcription levels of IFN stimulated genes (ISGs) stimulated by grouper cyclic GMP-AMP synthase (EccGAS)/stimulator of interferon genes (EcSTING), TANK-binding kinase 1 (EcTBK1), IFN regulatory factor 3 (EcIRF3)and EcIRF7. Secondly, Co-IP assays indicated that VP82 interacted with EcIRF3 and EcIRF7, but not EcSTING and EcTBK1, which was consistent with the co-localization between VP82 and EcIRF3 or EcIRF7. Furthermore, VP82 promoted the degradation of EcIRF3 and EcIRF7 in a dose-dependent manner via the autophagy pathway. Finally, VP82 overexpression accelerated SGIV replication, evidenced by the increased transcriptions of viral core genes and viral production. Moreover, the antiviral action of EcIRF3 or EcIRF7 was significantly depressed in VP82 overexpressed cells. Together, VP82 was speculated to exert crucial roles for SGIV replication by inhibiting the IFN response via the degradation of IRF3 and IRF7. Our findings provided new insights into understanding the immune evasion strategies utilized by fish iridovirus through IFN regulation.

The antiviral role of largemouth bass STING against iridovirus infection

Stimulator of interferon gene (STING) plays a crucial role in the innate immune response against viral and bacterial pathogens. However, its function in largemouth bass iridovirus (LMBV) infection remains uncertain. Here, a STING homolog (MsSTING) from largemouth bass (Micropterus salmoides) was cloned and characterized. MsSTING encoded a 407-amino-acid polypeptide, which shared 84.08% and 41.45% identity with golden perch (Perca flavescens) and human (Homo sapiens) homologs, respectively. MsSTING contained four transmembrane domains and a conserved C-terminal domain. The mRNA level of MsSTING was significantly increased in response to LMBV infection in vitro. Subcellular localization observation indicated that MsSTING encoded a cytoplasmic protein, which co-localized predominantly with endoplasmic reticulum (ER) and partially with mitochondria. Moreover, its accurate localization was dependent on the N-terminal transmembrane motif (TM) domains. MsSTING was able to activate interferon (IFN) response, evidenced by the activation of IFN1, IFN3 and ISRE promoters by its overexpression in vitro. Mutant analysis showed that both the N-terminal and C-terminal domain of MsSTING were essential for its activation on IFN response. In addition, overexpression of MsSTING inhibited the transcription and protein levels of viral core genes, indicating that MsSTING exerted antiviral action against LMBV. Consistently, the inhibitory effects were significantly attenuated when the N-terminal or C-terminal domains of MsSTING was deleted. Furthermore, MsSTING overexpression upregulated the transcriptions of interferon-related genes and pro-inflammatory factors, including TANK-binding kinase 1(TBK1), interferon regulatory factor 3 (IRF3), interferon regulatory factor 7 (IRF7), interferon stimulated exonuclease gene 20 (ISG20), interferon-induced transmembrane protein 1(IFITM1), interferon γ (IFN-γ), tumor necrosis factor α (TNF-α), interleukin 1β (IL-1β), and interleukin 6 (IL-6). Together, MsSTING exerted antiviral action upon LMBV infection through positive regulation the innate immune response.

Functional characterization of Malabar grouper (Epinephelus malabaricus) interferon regulatory factor 9 involved in antiviral response

IRF9 is a crucial component in the JAK-STAT pathway. IRF9 interacts with STAT1 and STAT2 to form IFN-I-stimulated gene factor 3 (ISGF3) in response to type I IFN stimulation, which promotes ISG transcription. However, the mechanism by which IFN signaling regulates Malabar grouper (Epinephelus malabaricus) IRF9 is still elusive. Here, we explored the nd tissue-specific mRNA distribution of the MgIRF9 gene, as well as its antiviral function in E. malabaricus. MgIRF9 encodes a protein of 438 amino acids with an open reading frame of 1317 base pairs. MgIRF9 mRNA was detected in all tissues of a healthy M. grouper, with the highest concentrations in the muscle, gills, and brain. It was significantly up-regulated by nervous necrosis virus infection and poly (I:C) stimulation. The gel mobility shift test demonstrated a high-affinity association between MgIRF9 and the promoter of zfIFN in vitro. In GK cells, grouper recombinant IFN-treated samples showed a significant response in ISGs and exhibited antiviral function. Subsequently, overexpression of MgIRF9 resulted in a considerable increase in IFN and ISGs mRNA expression (ADAR1, ADAR1-Like, and ADAR2). Co-immunoprecipitation studies demonstrated that MgIRF9 and STAT2 can interact in vivo. According to the findings, M. grouper IRF9 may play a role in how IFN signaling induces ISG gene expression in grouper species.

Identification and characterization of a highly virulent Citrobacter freundii isolate and its activation on immune responses in largemouth bass (Micropterus salmoides)

Citrobacter freundii, a common pathogen of freshwater fish, causes significant commercial losses to the global fish farming industry. In the present study, a highly pathogenic C. freundii strain was isolated and identified from largemouth bass (Micropterus salmoides). The pathogenicity and antibiotic sensitivity of the C. freundii strain were evaluated, and the histopathology and host immune response of largemouth bass infected with C. freundii were investigated. The results showed that C. freundii was the pathogen causing disease outbreaks in largemouth bass, and the infected fish showed typical signs of acute hemorrhages and visceral enlargement. Antimicrobial susceptibility testing showed that the C. freundii strain was resistant to Kanamycin, Medimycin, Clindamycin, Penicillin, Oxacillin, Ampicillin, Cephalexin, Cefazolin, Cefradine and Vancomycin. Histopathological analysis showed different pathological changes in major tissues of diseased fish. In addition, humoral immune factors such as superoxide dismutase (SOD), catalase (CAT) and lysozyme (LZM) were used as serum indicators to evaluate the immune response of largemouth bass after infection. Quantitative real-time PCR (qRT-PCR) was performed to investigate the expression pattern of immune-related genes (CXCR1, IL-8, IRF7, IgM, CD40, IFN-γ, IL-1β, Hep1, and Hep2) in liver, spleen, and head kidney tissues, which demonstrated a strong immune response induced by C. freundii infection in largemouth bass. The present study provides insights into the pathogenic mechanism of C. freundii and immune response in largemouth bass, promoting the prevention and treatment of diseases caused by C. freundii infection.

The regulatory mechanisms of IRF7 mediated by the type I IFN signalling pathway against Streptococcus iniae in yellowfin seabream, Acanthopagrus latus (Hottuyn, 1782)

Interferon regulatory factor 7 (IRF7) regulates type I interferon (IFN) genes via combining to the ISRE region in the immune response against bacteria. Streptococcus iniae is one of the dominant pathogenic bacteria of yellowfin seabream, Acanthopagrus latus. However, the regulatory mechanisms of A. latus IRF7 (AlIRF7) mediated by the type I IFN signalling pathway against S. iniae was ambiguously. In the present study, IRF7, and two IFNa3s (IFNa3 and IFNa3-like) were authenticated from A. latus. The total length of AlIRF7 cDNA is 2142 bp, containing a 1314 bp open reading frame (ORF) encoding an inferred 437 amino acids (aa). Three typical regions, a serine-rich domain (SRD), a DNA-binding domain (DBD), and an IRF association domain (IAD), are conserved in AlIRF7. Furthermore, AlIRF7 is fundamentally expressed in various kinds of organs, with high levels in the spleen and liver. Additionally, S. iniae challenge promoted AlIRF7 expression in the spleen, liver, kidney, and brain. AlIRF7 is confirmed to be located at the nucleus and cytoplasm by overexpression of AlIRF7. Moreover, truncation mutation analyses shows that the regions, -821 bp to +192 bp and -928 bp to +196 bp, were known as core promoters from AlIFNa3 and AlIFNa3-like, respectively. The point mutation analyses and electrophoretic mobile shift assay (EMSA) verified that AlIFNa3 and AlIFNa3-like transcriptions are depended on the M2/5 and M2/3/4 binding sites with AlIRF7 regulation, respectively. Additionally, an overexpression experiment showed that AlIRF7 can dramatically decrease the mRNA levels of two AlIFNa3s and interferon signalling molecules. These results suggest that two IFNa3s may mediate the regulation of AlIRF7 in the immune responses of A. latus against S. iniae infection.

Singapore grouper iridovirus infection counteracts poly I:C induced antiviral immune response in vitro

Groupers are important mariculture fish in South China and Southeast Asian countries. However, the increasing frequency of infectious disease outbreaks has caused great economic losses in the grouper industry. Among these pathogens, Singapore grouper iridovirus (SGIV) infection causes high mortality in larval and juvenile stages of grouper. However, the mechanism underlying the action of viral manipulation on cellular immune response still remained largely uncertain. Here, using RNA-seq technology, we investigated the regulatory roles of SGIV infection on synthetic RNA duplex poly I:C induced immune response in vitro. Using reporter gene assays, we found that SGIV infection decreased poly I:C induced interferon promoter activation. Transcriptomic analysis showed that the mRNA expression levels of 2238 genes were up-regulated, while 1247 genes were down-regulated in poly I:C transfected grouper spleen (GS) cells. Interestingly, SGIV infection decreased the expression of 1479 up-regulated genes and increased the expression of 297 down-regulated genes in poly I:C transfected cells. The differentially expressed genes (DEGs) down-regulated by SGIV were directly related to immune, inflammation and viral infection, and JUN, STAT1, NFKB1, MAPK14A, TGFB1 and MX were the 6 top hub genes in the down-regulated DEGs' protein-protein interaction (PPI) network. Furthermore, quantitative real-time PCR (qPCR) analysis confirmed that the interferon signaling and inflammatory-related genes, including cGAS, STING, TBK1, MAVS, TNF, IRAK4 and NOD2 were up-regulated by poly I:C stimulation, but all significantly down-regulated after SGIV infection. Thus, we speculated that SGIV infection counteracted poly I:C induced antiviral immune response and this ability helped itself to escape host immune surveillance. Together, our data will contribute greatly to understanding the potential immune evasion mechanism of iridovirus infection in vitro.

Red-grouper nervous necrosis virus B1 protein inhibits fish IFN response by targeting Ser5-phosphorylated RNA polymerase II to promote viral replication

Nervous necrosis virus (NNV) could infect more than 200 fish species worldwide, with almost 100% mortality in affected larvae and juvenile fish. Among different genotypes of NNV, the red-grouper nervous necrosis virus (RGNNV) genotype is the most widely reported with the highest number of susceptible species. Interferon (IFN) is a crucial antiviral cytokine and RGNNV needs to develop some efficient strategies to resist host IFN-stimulated antiviral immune. Although considerable researches on RGNNV, whether RGNNV B1 protein participates in regulating the host's IFN response remains unknown. Here, we reported that B1 protein acted as a transcript inhibition factor to suppress fish IFN production. We firstly found that ectopic expression of B1 protein significantly decreased IFN and IFN-stimulated genes (ISGs) mRNA levels and IFNφ1 promoter activity induced by polyinosinic:polycytidylic acid [poly (I:C)]. Further studies showed that B1 protein inhibited the IFNφ1 promoter activity stimulated by the key RIG-I-like receptors (RLRs) factors, including MDA5, MAVS, TBK1, IRF3, and IRF7 and decreased their protein levels. Moreover, B1 protein significantly inhibited the activity of constitutively active cytomegalovirus (CMV) promoter, which suggested that B1 protein was a transcription inhibitor. Western blot indicated that B1 protein decreased the Ser5 phosphorylation of RNA polymerase II (RNAP II) C-terminal domain (CTD). Together, our data demonstrated that RGNNV B1 protein was a host transcript antagonist, which intervened RNAP II Ser5-phosphorylation, inhibiting host IFN response and facilitating RGNNV replication.

The origin and loss of interferon regulatory factor 10 (IRF10) in different lineages of vertebrates

The vertebrate IFN regulatory factor (IRF) family consists of 11 members that exert distinct roles in a variety of biological processes, including antiviral defense, regulation of cell proliferation, differentiation and apoptosis. Of these, IRF10 is widely present in different vertebrate lineages, but appears to have been lost in primates and rodents. To understand the evolutionary occurrence of IRF10, we performed comparative analyses of currently available genomic data in a taxonomically diverse set of vertebrates, and found that IRF10 originated after the divergence of chondrichthyans from gnathostomes. Phylogenetically, vertebrate IRF10 is much more closely related to IRF4 than to IRF8 or IRF9, although these four IRFs may have a common ancestor. In addition, the loss of IRF10 in Euarchontoglires might be resulted from mutation accumulation, and the rate of mutations in rodents appears to be higher than in the primate lineage. In primates, the gene-disruptive mutations may have occurred at least prior to the separation of new world monkey and old world primates, roughly 40 million years ago. Overall, we propose a detailed evolutionary scenario for IRF10, which may help us understand the evolutionary mechanisms in the expansion and contraction of the IRF family.

Grouper TRIM23 exerts antiviral activity against iridovirus and nodavirus

TRIM (tripartite motif) proteins have been demonstrated to exert critical roles in host defense against different microbial pathogens. Among them, TRIM23 acts as an important regulatory factor in antiviral immune and inflammatory responses, but the roles of fish TRIM23 against virus infection still remain largely unknown. Here, we investigated the characteristics of TRIM23 homolog from orange spotted grouper (Epinephelus coioides) (EcTRIM23). EcTRIM23 encoded a 580 amino acid peptide, which shared 93.1%, 89.73% and 86.36% identity with golden perch (Perca flavescens), zebrafish (Danio rerio) and human (Homo sapiens), respectively. The transcription levels of EcTRIM23 were significantly up-regulated in response to Singapore grouper iridovirus (SGIV) and red-spotted grouper nervous necrosis virus (RGNNV) infection. EcTRIM23 overexpression in vitro significantly inhibited RGNNV and SGIV replication, evidenced by the delayed cytopathic effect (CPE) progression and the decreased expression of viral core genes. EcTRIM23 significantly increased the expression levels of interferon (IFN) related signaling molecules and pro-inflammatory cytokines, as well as the promoter activities of IFN and NF-κB, suggesting that EcTRIM23 exerted antiviral function by positively regulating host IFN response. Exogenous EcTRIM23 exhibited either diffuse or aggregated localization in grouper cells. After co-transfection, TANK binding kinase 1 (TBK1), TNF receptor associated factor (TRAF) 3 and TRAF4, TRAF5 and TRAF6 were found to interact with EcTRIM23 in grouper cells. Moreover, these proteins could be recruited and co-localized with EcTRIM23 in vitro. Together, our results demonstrated that fish TRIM23 exerted antiviral activity against fish viruses by interacting with multiple host proteins to regulate immune responses.

Fish TRIM21 exhibits antiviral activity against grouper iridovirus and nodavirus infection

Growing evidences have demonstrated that multiple TRIM (tripartite motif) proteins exert critical roles in host defense against different microbial pathogens. Although mammalian TRIM21 has been reported to function as an important regulatory factor in antiviral immune and inflammatory response, the role of fish TRIM21 against virus infection still remains largely unknown. In the present study, we investigated the characteristics of TRIM21 gene (EcTRIM21) from orange spotted grouper (Epinephelus coioides). The full-length EcTRIM21 cDNA encoded a 557 amino acid peptide with 92.1% and 31.14% identity with giant grouper (Epinephelus lanceolatus) and human (Homo sapiens), respectively. EcTRIM21 contained four conserved domains, including RING, B-Box, PRY and SPRY domain. EcTRIM21 expression was significantly up-regulated in response to Singapore grouper iridovirus (SGIV) and red-spotted grouper nervous necrosis virus (RGNNV) infection, suggesting that EcTRIM21 might be involved in host defense against fish virus infections. Subcellular localization showed that EcTRIM21 were distributed in the cytoplasm in a punctate manner. Overexpression of EcTRIM21 in vitro significantly inhibited RGNNV and SGIV replication, as evidenced by the decreased severity of cytopathic effect (CPE) and the reduced expression levels of viral core genes. Consistently, knockdown of EcTRIM21 by small interfering RNA (siRNA) promoted the replication of RGNNV and SGIV in vitro. Furthermore, EcTRIM21 overexpression increased both interferon (IFN) and interferon stimulated response element (ISRE) promoter activities. In addition, the transcription levels of IFN signaling related molecules were positively regulated by EcTRIM21 overexpression. Together, our data demonstrated that fish TRIM21 exerted antiviral activity against fish viruses through positive regulation of host interferon response.

The roles of grouper TAK1 in regulating the infection of Singapore grouper iridovirus

Transforming growth factor-β activated kinase 1 (TAK1) is a member of the mitogen-activated protein kinase family. It is an upstream factor of the IκB kinase, which activates IKKα and IKKβ. TAK1 is a key factor in the induction of nuclear factor κB (NF-κB) and plays a crucial role in the activation of inflammatory responses. However, the roles of TAK1 during viral infection in teleost fish are largely unknown. In this study, we cloned a TAK1 homolog (HgTAK1) from the hybrid grouper (Epinephelus fuscoguttatus♂ × Epinephelus lanceolatus♀). The open reading frame of HgTAK1 consists of 1728 nucleotides encoding 575 amino acids, and the predicted molecular weight is 64.32 kDa HgTAK1 has an S_TKc domain, which consists of a serine/threonine protein kinase and a catalytic domain. Expression pattern analysis showed that HgTAK1 was distributed in all tested tissues, with abundant contents in the heart, head kidney, and blood. Additionally, HgTAK1 was distributed in the cytoplasm of grouper spleen (GS) cells. After Singapore grouper iridovirus (SGIV) infection, the expression of HgTAK1 increased in GS cells. Overexpression of HgTAK1 could promote the replication of SGIV in GS cells and inhibit the activation of NF-κB and IFN stimulated response elements (ISRE) in reporter assay. When co-expressed with IRF3 or HgIRF7 in GS cells, HgTAK1 obviously down-regulated IRF3- or IRF7-mediated the NF-κB and ISRE promoter induction. The interaction between HgTAK1 and IRF3 or IRF7 has been identified by co-immunoprecipitation assay. These findings provide a basis for understanding the innate immune mechanism of the grouper response to viral infection.

SGIV Induced and Exploited Cellular De Novo Fatty Acid Synthesis for Virus Entry and Replication

Considerable attention has been paid to the roles of lipid metabolism in virus infection due to its regulatory effects on virus replication and host antiviral immune response. However, few literature has focused on whether lipid metabolism is involved in the life cycle of lower vertebrate viruses. Singapore grouper iridovirus (SGIV) is the causative aquatic virus that extensively causes fry and adult groupers death. Here, the potential roles of cellular de novo fatty acid synthesis in SGIV infection was investigated. SGIV infection not only increased the expression levels of key enzymes in fatty acid synthesis in vivo/vitro, including acetyl-Coenzyme A carboxylase alpha (ACC1), fatty acid synthase (FASN), medium-chain acyl-CoA dehydrogenase (MCAD), adipose triglyceride lipase (ATGL), lipoprotein lipase (LPL) and sterol regulatory element-binding protein-1 (SREBP1), but it also induced the formation of lipid droplets (LDs), suggesting that SGIV altered de novo fatty acid synthesis in host cells. Using the inhibitor and specific siRNA of ACC1 and FASN, we found that fatty acid synthesis was essential for SGIV replication, evidenced by their inhibitory effects on CPE progression, viral gene transcription, protein expression and virus production. Moreover, the inhibitor of fatty acid β-oxidation could also reduce SGIV replication. Inhibition of fatty acid synthesis but not β-oxidation markedly blocked virus entry during the life cycle of SGIV infection. In addition, we also found that inhibition of ACC1 and FASN increased the IFN immune and inflammatory response during SGIV infection. Together, our data demonstrated that SGIV infection in vitro regulated host lipid metabolism and, in that process, cellular fatty acid synthesis might exert crucial roles during SGIV infection via regulating virus entry and host immune response.

Epinephelus coioides Hsp27 negatively regulates innate immune response and apoptosis induced by Singapore grouper iridovirus (SGIV) infection

Heat shock proteins (Hsps) are important for maintaining protein homeostasis and cell survival. In this study, Hsp27 of Epinephelus coioides, an economically important marine fish in China and Southeast Asian countries, was characterized. E. coioides Hsp27 contains the consered ACD_HspB1_like domain and three p38 MAPK phosphorylation sites, located at Thr-13, Thr-60 and Ser-167. E. coioides Hsp27 was distributed in both the cytoplasm and nucleus, its mRNA was detected in all 14 tissues examined, and its expression was up-regulated after challenge with Singapore grouper iridovirus (SGIV), an important E. coioides pathogen. Over-expression of E. coioides Hsp27 significantly upregulated the expressions of the key SGIV genes (VP19, LITAF, MCP, and ICP18), downgraded the expressions of the E. coioides immune factors (IRF3, IRF7, ISG15, and TRAF6) and proinflammatory factors (TNF-α, IL-8), downgraded the activation of nuclear factor kappa-B (NF-κB) and activator protein-1 (AP-1), and substantially inhibited the cell apoptosis induced by SGIV infection. These data illustrated that E. coioides Hsp27 might be involved in SGIV infection by negatively regulating the innate immune response.

α-Lipoic Acid Exerts Its Antiviral Effect against Viral Hemorrhagic Septicemia Virus (VHSV) by Promoting Upregulation of Antiviral Genes and Suppressing VHSV-Induced Oxidative Stress

Viral hemorrhagic septicemia virus (VHSV), belonging to the genus Novirhabdovirus, Rhabdoviridae family, is a causative agent of high mortality in fish and has caused significant losses to the aquaculture industry. Currently, no effective vaccines, Food and Drug Administration-approved inhibitors, or other therapeutic intervention options are available against VHSV. α-Lipoic Acid (LA), a potent antioxidant, has been proposed to have antiviral effects against different viruses. In this study, LA (CC₅₀ = 472.6 μmol/L) was repurposed to exhibit antiviral activity against VHSV. In fathead minnow cells, LA significantly increased the cell viability post-VHSV infection (EC₅₀ = 42.7 μmol/L), and exerted a dose-dependent inhibitory effect on VHSV induced-plaque, cytopathic effects, and VHSV glycoprotein expression. The time-of-addition assay suggested that the antiviral activity of LA occurred at viral replication stage. Survival assay revealed that LA could significantly upregulated the survival rate of VHSV-infected largemouth bass in both co-injection (38.095% vs. 1.887%, P < 0.01) and post-injection manner (38.813% vs. 8.696%, P < 0.01) compared with the control group. Additional comparative transcriptome and qRT-PCR analysis revealed LA treatment upregulated the expression of several antiviral genes, such as IRF7, Viperin, and ISG15. Moreover, LA treatment reduced VHSV-induced reactive oxygen species production in addition to Nrf2 and SOD1 expression. Taken together, these data demonstrated that LA suppressed VHSV replication by inducing antiviral genes expression and reducing VHSV-induced oxidative stress. These results suggest a new direction in the development of potential antiviral candidate drugs against VHSV infection.

Identification and functional characterization of three irf7 transcript variants in obscure puffer (Takifugu obscurus)

Interferon regulatory factor 7 (IRF7) is a key mediator in regulating the type Ι IFN response. Although irf7 has been identified in more than twenty fish species, alternative splicing has not been found in teleost irf7. Alternative splicing is an important mechanism expanding the transcriptomic and proteomic diversity, and has been found in several IRF family members. Here, three alternative splicing variants of irf7 were identified and characterized in obscure puffer. The first splicing transcript (Toirf7v1) was predicted to encode 428 amino acids with a DNA-binding domain (DBD), an interaction-associated domain (IAD) and a serine-rich domain (SRD). Toirf7v2 encoded 430 amino acids caused by the intron retention, and contained the whole conserved domains. Toirf7v3 encoded a truncated protein with 337 amino acids resulting from the alternative 5' splice-site selection, and lacked part of IAD domain and the entire SRD domain. Functional studies demonstrated that all of the three isoforms could activate the expression of type I IFN and IFN-stimulated genes (ISGs). Nevertheless, the two variants (Toirf7v2 and Toirf7v3) exhibited much less ability to induce transcription of IFN and ISGs compared to the Toirf7v1. Our findings suggest that these splicing variants may have distinct roles in the regulation of immune response. These results will be beneficial to understand the functional characteristics of irf7 variants in fish.

Interferon regulatory factor 7 contributes to the host response during Vibrio harveyi infection in the golden pompano Trachinotus ovatus

Vibrio harveyi is regarded as serious pathogen for marine fishes. However, host defense mechanisms involved in V. harveyi infection remain incompletely defined. The transcription factor IFN regulatory factor 7 (IRF7) is largely associated with host defense against viral infections, and the role of IRF7 during V. harveyi infection in fish has not been well illuminated previously. In this study, IRF7 from golden pompano (Trachinotus ovatus) was characterized (TroIRF7). The TroIRF7 gene is 1323 bp, which encodes 440 amino acid residues. Multiple amino acid alignments of TroIRF7 shows 30.37%-80.18% identity with other fish IRF7s, including Epinephelus coioides (80.18%), Larimichthys crocea (79.72%), Collichthys lucidus (79.26%), Miichthys miiuy (79.26%), Channa argus (78.77%), Cynoglossus semilaevis (72.67%), and Gadus morhua (65.23%). Like other IRF7s, TroIRF7 also contains 3 conserved domains: an N-terminal DNA-binding domain (DBD), an IRF association domain (IAD), and a C-terminal serine-rich domain (SRD). In the DBD, 4-5 conserved tryptophans were observed, which is a characteristic unique to all fish IRF7 members. TroIRF7 was constitutively expressed, with high levels in gill, head kidney, spleen, skin, and intestine. V. harveyi infection-induced TroIRF7 transcripts significantly up-regulation and translocation to the nucleus. TroIRF7 overexpression promote the fish to inhibit the replication of V. harveyi. And TroIRF7 knockdown led to decreased bacterial clearance in fish tissue. Furthermore, over-expression of TroIRF7 resulted in an increased production of interferon a3 and IFN signaling molecule in the spleen, suggesting that V. harveyi activates the IRF7- IFN pathway. These results suggest that TroIRF7 is an important component of immune responses against V. harveyi infection.

Grouper Interferon-Induced Transmembrane Protein 1 Inhibits Iridovirus and Nodavirus Replication by Regulating Virus Entry and Host Lipid Metabolism

Interferon-induced transmembrane proteins (IFITMs) are novel viral restriction factors which inhibit numerous virus infections by impeding viral entry into target cells. To investigate the roles of IFITMs during fish virus infection, we cloned and characterized an IFITM1 homolog from orange spotted grouper (Epinephelus coioides) (EcIFITM1) in this study. EcIFITM1 encodes a 131-amino-acid polypeptide, which shares 64 and 43% identity with Seriola dumerili and Homo sapiens, respectively. The multiple sequence alignment showed that EcIFITM1 contained five domains, including NTD (aa 1–45), IMD (aa 46–67), CIL (aa 68–93), TMD (aa 94–119), and CTD (aa 120–131). In vitro, the level of EcIFITM1 mRNA expression was significantly up-regulated in response to Singapore grouper iridovirus (SGIV), or red-spotted grouper nervous necrosis virus (RGNNV) infection. EcIFITM1 encoded a cytoplasmic protein, which was partly colocalized with early endosomes, late endosomes, and lysosomes. The ectopic expression of EcIFITM1 significantly inhibited the replication of SGIV or RGNNV, which was demonstrated by the reduced virus production, as well as the levels of viral gene transcription and protein expression. In contrast, knockdown of EcIFITM1 using small interfering RNAs (siRNAs) promoted the replication of both viruses. Notably, EcIFITM1 exerted its antiviral activity in the step of viral entry into the host cells. Furthermore, the results of non-targeted lipometabolomics showed that EcIFITM1 overexpression induced lipid metabolism remodeling in vitro. All of the detected ceramides were significantly increased following EcIFITM1 overexpression, suggesting that EcIFITM1 may suppress SGIV entry by regulating the level of ceramide in the lysosomal system. In addition, EcIFITM1 overexpression positively regulated both interferon-related molecules and ceramide synthesis-related genes. Taken together, our results demonstrated that EcIFITM1 exerted a bi-functional role, including immune regulation and lipid metabolism in response to fish virus infections.

Molecular characterization of yellow catfish (Pelteobagrus fulvidraco) IRF7 suggests involvement in innate immune response

Interferon regulatory factor 7 (IRF7) serves as a critical mediator in the regulation of type Ι interferon (IFN) response to invading pathogens. Here, an ortholog of IRF7 was characterized in yellow catfish (Pelteobagrus fulvidraco). The full-length cDNA of PfIRF7 consisted of 1516 bp encoding a polypeptide of 425 amino acids. PfIRF7 protein comprised a typical IRF structural architecture, including a DNA binding domain (DBD), an IRF association domain (IAD) and a serine-rich domain (SRD). PfIRF7 was expressed predominantly in the immune-related tissues and transcriptionally upregulated by PolyI:C, LPS, and Edwardsiella ictaluri. Ectopic expression of PfIRF7 led to activation of fish type I IFN promoters and induction of IFN and Vig1, thereby conferring a strong antiviral effect against spring viremia of carp virus (SVCV). Overall, the present data suggest that PfIRF7 may play an essential role in type I IFN response of yellow catfish.

The Innate Immune Response of Atlantic Salmon (Salmo salar) Is Not Negatively Affected by High Temperature and Moderate Hypoxia

Climate change is predicted to increase water temperatures and decrease oxygen levels in freshwater and marine environments, however, there is conflicting information regarding the extent to which these conditions may impact the immune defenses of fish. In this study, Atlantic salmon were exposed to: (1) normoxia (100–110% air saturation) at 12°C; (2) an incremental temperature increase (1°C per week from 12 to 20°C), and then held at 20°C for an additional 4 weeks; and (3) “2” with the addition of moderate hypoxia (~65–75% air saturation). These conditions realistically reflect what farmed salmon in some locations are currently facing, and future conditions in Atlantic Canada and Europe, during the summer months. The salmon were sampled for the measurement of head kidney constitutive anti-bacterial and anti-viral transcript expression levels, and blood parameters of humoral immune function. Thereafter, they were injected with either the multi-valent vaccine Forte V II (contains both bacterial and viral antigens) or PBS (phosphate-buffer-saline), and the head kidney and blood of these fish were sampled at 6, 12, 24, and 48 h post-injection (HPI). Our results showed that: (1) neither high temperature, nor high temperature + moderate hypoxia, adversely affected respiratory burst, complement activity or lysozyme concentration; (2) the constitutive transcript expression levels of the anti-bacterial genes il1β, il8-a, cox2, hamp-a, stlr5-a, and irf7-b were up-regulated by high temperature; (3) while high temperature hastened the peak in transcript expression levels of most anti-bacterial genes by 6–12 h following V II injection, it did not affect the magnitude of changes in transcript expression; (4) anti-viral (viperin-b, mx-b, and isg15-a) transcript expression levels were either unaffected, or downregulated, by acclimation temperature or V II injection over the 48 HPI; and (5) hypoxia, in addition to high temperature, did not impact immune transcript expression. In conclusion, temperatures up to 20°C, and moderate hypoxia, do not impair the capacity of the Atlantic salmon's innate immune system to respond to bacterial antigens. These findings are surprising, and highlight the salmon's capacity to mount robust innate immune responses (i.e., similar to control fish under optimal conditions) under conditions approaching their upper thermal limit.

Grouper IFIT1 inhibits iridovirus and nodavirus infection by positively regulating interferon response

Interferon-induced protein with tetratricopeptide repeats 1 (IFIT1), one of the interferon stimulated genes (ISGs), is strongly induced by type I interferon (IFN), double-stranded RNAs and virus infection. To investigate the actions of fish IFIT1 in response to virus infection, we cloned an IFIT1 homolog from orange spotted grouper (EcIFIT1) and clarified its function in this study. The full-length cDNA of EcIFIT1 is 1839 bp, which is composed of 436 amino acid (aa) residues, with 77.8% and 22.8% identity to IFIT1 homolog of yellow perch (Perca flavescens) and humans (homo sapiens), respectively. Sequence alignment analysis showed that EcIFIT1 contained three tetratricopeptide repeats (TPRs). Tissue distribution analysis indicated that EcIFIT1 was abundant in intestine, spleen, liver, and heart. Moreover, EcIFIT1 was significantly up-regulated by Singapore grouper iridovirus (SGIV) or red-spotted grouper nervous necrosis virus (RGNNV) infection, and polyinosinic-polycytidylic acid (poly I:C) or lipopolysaccharide (LPS) treatment in vitro. Under fluorescence microscopy, EcIFIT1 was found to localize throughout the cytoplasm in transfected cells. EcIFIT1 overexpression significantly suppressed the replication of SGIV and RGNNV, demonstrated by decreasing the cytopathic effect (CPE) severity, viral gene transcription and the virus titers. Further studies showed that the ectopic expression of EcIFIT1 increased the transcription level of IFN related molecules, including IFN regulatory factor (IRF) 3, IRF7, IFN stimulated gene (ISG) 15 and myxovirus resistance gene (MX) I. Meanwhile, the expression levels of pro-inflammation cytokines were differently regulated by the ectopic expression of EcIFIT1. In addition, flow cytometry analysis suggested that EcIFIT1 overexpression affected cell cycle progression by mediating S/G2 transition. Taken together, our results indicated that EcIFIT1 might exert antiviral function against fish virus by up-regulating interferon response or affecting cell cycle.

Molecular cloning, expression and functional analysis of Atg16L1 from orange-spotted grouper (Epinephelus coioides)

Autophagy related gene 16 (Atg16), which encodes a core protein for autophagosome formation, participates in autophagy activity, the ubiquitin proteasome system and inflammatory response in mammals. In this study, we cloned and characterized an Atg16 homolog from orange-spotted grouper (Epinephelus coioides) (EcAtg16L1). EcAtg16L1 encodes a 656-amino acid polypeptide, which shares 94.22% and 72.65% homology with large yellow croakers (Larimichthys crocea) and humans (Homo sapiens), respectively. EcAtg16L1 contains a conserved Atg16 domain and a WD-repeat-containing domain. Subcellular localization showed that EcAtg16L1 was distributed in the cytoplasm of grouper cells with a dot-like pattern. EcAtg16L1 overexpression promoted Singapore grouper iridovirus (SGIV) and red-spotted grouper nervous necrosis virus (RGNNV) replication, as evidenced by the increase in viral gene transcription and viral coat protein. Furthermore, EcAtg16L1 overexpression negatively regulated interferon (IFN)-related molecules and proinflammatory cytokines, and decreased IFN, IFN-stimulated response element, and nuclear factor κB promoter activities. Taken together, aside from its function in autophagosome formation, EcAtg16L1 also plays role in promoting SGIV and RGNNV replication and the pro-viral effect might involve its down regulation to interferon and inflammatory responses.

Identification of Beclin-1 from orange-spotted grouper (Epinephelus coioides) involved in viral infection

Beclin-1 is an essential autophagic regulator that plays diverse roles in physiology and disease. However, reports about the function of fish Beclin-1 during pathogen infection are still very limited. In this study, a Beclin-1 homolog (EcBeclin-1) from orange-spotted grouper (Epinephelus coioides) was identified and its roles in viral infection were investigated. EcBeclin-1 encoded 447amino acids protein with a BH3 domain, a CCD domain and an ECD domain, which shared high identities (97%-82%) with reported Beclin-1 proteins from mammal to fish. Quantitative real-time PCR (qRT-PCR) analysis revealed that EcBeclin-1 was predominantly expressed in brain and muscle of healthy grouper. Using fluorescence microscopy, we found that EcBeclin-1 was co-localized with endoplasmic reticulum (ER) in grouper spleen cells (EAGS). After red-spotted grouper nervous necrosis virus (RGNNV) infection in vitro, EcBeclin-1 transcript was significantly up-regulated, implying that EcBeclin-1 might be involved in viral infection. Furthermore, the in vitro studies of EcBeclin-1 overexpression promoted RGNNV induced autophagy, as well as the expression of coat protein (CP) and RNA-dependent RNA polymerase (RdRp). The overexpression of EcBeclin-1 suppressed the expressions of interferon pathway-related factors, inflammatory-related factors and activities of NF-κB and ISRE. Additionally, EcBeclin-1 could interact with EcBcl-xL in vitro. These data suggest that EcBeclin-1 affect viral replication through modulating IFN and inflammatory responses, as well as virus-induced cell death, which will help us to further explore the immune response of fish during viral infection.

Grouper Atg12 negatively regulates the antiviral immune response against Singapore grouper iridovirus (SGIV) infection

Autophagy is an evolutionarily conserved, multi-step lysosomal degradation process used to maintain cell survival and homeostasis. A series of autophagy-related genes (Atgs) are involved in the autophagic pathway. In mammals, a growing number of studies have attributed functions to some Atgs that are distinct from their classical role in autophagosome biogenesis, such as resistance to pathogens. However, little is known about the functions of fish Atgs. In this study, we cloned and characterized an atg12 homolog from orange spotted grouper (Epinephelus coioides) (Ecatg12). Ecatg12 encodes a 117 amino acid protein that shares 94.0% and 76.8% identity with gourami (Anabas_testudineus) and humans (Homo sapiens), respectively. The transcription level of Ecatg12 was lower in cells infected with Singapore grouper iridovirus (SGIV) than in non-infected cells. Fluorescence microscopy revealed that EcAtg12 localized in the cytoplasm and nucleus in grouper spleen cells. Overexpression of EcAtg12 significantly increased the replication of SGIV, as evidenced by increased severity of the cytopathic effect, transcription levels of viral genes, levels of viral proteins, and progeny virus yield. Further studies showed that EcAtg12 overexpression decreased the expression levels of interferon (IFN) related molecules and pro-inflammatory factors and inhibited the promoter activity of IFN-3, interferon-stimulated response element, and nuclear factor-κB. Together, these results demonstrate that EcAtg12 plays crucial roles in SGIV replication by downregulating antiviral immune responses.

The novel gene TRIM44L from orange-spotted grouper negatively regulates the interferon response

Accumulated evidence suggests that some of the tripartite motif (TRIM) -family proteins function as critical regulators of carcinogenesis, immunity, and antiviral functions. TRIM44 is an atypical TRIM family protein that lacks the entire RING domain and has been demonstrated to play a crucial role in cancer and viral infection. To our knowledge, the role of TRIM44 in fish still remains largely unknown. Here, we cloned and characterized a novel TRIM44-like gene from orange spotted grouper (EcTRIM44L). Sequence analysis indicated that EcTRIM44L encoded a 393 amino acid peptide, which shared 81.44% and 51.02% identity with large yellow croaker (Larimichthys crocea) and zebrafish (Danio rerio), respectively. However, EcTRIM44L only exhibited 24.69% identity with the TRIM44 protein of humans (Homo sapiens). Moreover, EcTRIM44L contained two conserved domains, including a B-Box domain and a coiled-coil domain, but not a RING domain. Using fluorescence microscopy, we observed green fluorescence in the cytoplasm of the EcTRIM44L-EGFP transfected grouper spleen (GS) cells. As the infection proceeded, EcTRIM44L transcription was significantly up-regulated in red-spotted grouper nervous necrosis virus (RGNNV) infection, suggesting that EcTRIM44L might be involved in fish virus infections. The in vitro overexpression of EcTRIM44L significantly enhanced RGNNV replication, as demonstrated by the accelerated cytopathic effect (CPE) progression induced by RGNNV, as well as the increased expression of coat protein (CP) and RNA-dependent RNA polymerase (RdRp). The overexpression of EcTRIM44L significantly decreased the level of interferon (IFN) related signaling molecules and pro-inflammatory cytokine expression, suggesting that EcTRIM44L affected virus replication by negatively regulating the IFN response. In addition, the melanoma differentiation-associated protein 5 (MDA5) and mitochondrial antiviral-signaling protein (MAVS), but not mediator of IRF3 activation (MITA)-evoked IFN response was negatively regulated by EcTRIM44L. Together, for the first time, our results indicate that EcTRIM44L negatively regulates the interferon response against grouper RNA virus infection.

Interferon-regulatory factors, IRF3 and IRF7 in Asian seabass, Lates calcarifer: Characterization, ontogeny and transcriptional modulation upon challenge with nervous necrosis virus

Interferon regulatory factor (IRF) 3 and IRF7 are key regulators of type I interferon (IFN) gene expression for the antiviral immune response. In the present study, interferon regulatory factor 3 and 7 from Asian seabass, namely AsIRF3 and AsIRF7 were cloned and characterized. The full-length cDNA sequence of IRF3 and IRF7 consisted of 2965 and 2343 bp respectively. AsIRF3 and AsIRF7 were true orthologes of vertebrate IRF3/7 and showed similar domain organization, with an N-terminal DBD which consisted five tryptophan residues in IRF3 and four in IRF7, a C-terminal IRF3 domain and a serine rich region. Both IRF3 and 7 constitutively expressed during the ontogenesis and in all tissues of healthy fish. The expression of both genes was up-regulated following NNV challenge with obvious transcript abundance in brain heart and kidney. Ectopic expression of AsIRF3 and AsIRF7 displayed activation of ISRE/NF-κB promoters and modulation of interferon, ISGs and pro-inflammatory cytokine gene expression. These observations indicated that IRF3 and IRF7 play an important role in Asian seabass's antiviral defense and the RIG-IRF-IFN axis is conserved in the species.

Negative regulation of the interferon response by finTRIM82 in the orange spotted grouper

Tripartite motif (TRIM) proteins have been demonstrated to exhibit critical functions in multiple cellular processes, including development, carcinogenesis, and programmed cell death, and are also widely recognized to be important antiviral restriction factors or modulators of immune and inflammatory signaling pathways. However, in teleosts, additional TRIM members have been identified and their functions remain largely unknown. Here, a novel finTRIM gene from orange spotted grouper (EcfinTRIM82) was cloned and characterized. Sequence analysis indicated that EcfinTRIM82 encoded a 575 amino acid peptide which shared 94% and 82% identity with Asian sea bass (Lates calcarifer), and zebrafish (Danio rerio) finTRIM82, respectively. EcfinTRIM82 contained three conserved domains, including a RING, B-Box, and SPRY domain. Using fluorescence microscopy, we found that green fluorescence aggregates were observed in the cytoplasm of EcfinTRIM82-EGFP transfected grouper spleen (GS) cells. As the infection proceeded, EcfinTRIM82 transcription was significantly upregulated in Singapore grouper iridovirus (SGIV) or red-spotted grouper nervous necrosis virus (RGNNV) infected GS cells. This suggests that EcfinTRIM82 might be involved in fish virus infection. The in vitro overexpression of EcfinTRIM82 in GS cells significantly enhanced the replication of SGIV and RGNNV, evidenced by increased expression of viral genes, including the SGIV major capsid protein (MCP), VP19, ICP-18, RGNNV coat protein (CP), and RNA-dependent RNA polymerase (RdRp). Furthermore, the ectopic expression of EcfinTRIM82 significantly decreased the expression of interferon (IFN)-related signaling molecules, including interferon regulatory factor 3 (IRF3), IRF7, interferon stimulated gene 15 (ISG15), ISG56, IFP35, and myxovirus resistance gene (MXI), suggesting that EcfinTRIM82 regulated viral replication via the negative regulation of the host IFN response. In addition, EcfinTRIM82 overexpression substantially decreased the level of proinflammatory cytokine transcription. Furthermore, the ectopic expression of EcfinTRIM82 significantly weakened the melanoma differentiation-associated protein 5 (MDA5), mediator of IRF3 activation (MITA) and mitochondrial antiviral-signaling (MAVS) protein-induced IFN response by detecting the transcription of interferon related cytokines and the promoter activity of IFN. Together, our results demonstrate that finTRIM82 negatively regulates the innate antiviral immune response against grouper virus infection.

Grouper viperin acts as a crucial antiviral molecule against iridovirus

Virus inhibitory protein, endoplasmic reticulum-associated, IFN-inducible (viperin), is an antiviral protein, induced by interferon (IFN), poly(I:C) and viral infection to exert antiviral function. To investigate the roles of viperin during fish virus infection, a viperin homolog from orange spotted grouper (Epinephelus coioides) (Ecviperin) was cloned and characterized in this study. Ecviperin encoded a 361-aa protein which shared 87% and 69% identity with Siniperca undulata and Homo sapiens, respectively. Amino acid alignment analysis showed that Ecviperin contained a conserved radical-SAM domain (aa73-281). Phylogenetic analysis indicated that Ecviperin showed the nearest relationship with S. undulata. In healthy grouper, Ecviperin was distributed in all tissues, and the expression of Ecviperin was the highest in kidney and spleen. In vitro, the mRNA expression of Ecviperin was significantly up-regulated in response to Singaporean grouper iridovirus (SGIV) infection. Subcellular localization analysis showed that Ecviperin was distributed in the cytoplasm and co-localized with endoplasmic reticulum (ER). The ectopic expression of Ecviperin significantly inhibited the replication of SGIV. Furthermore, overexpression of Ecviperin positively regulated the interferon related molecules, including interferon regulatory factor 3 (IRF3), IRF7, interferon stimulated gene 15 (ISG15), myxovirus resistance gene I (MXI), interferon-induced 35-kDa protein (IFP35), and TNF receptor-associated factor 6 (TRAF6). In addition, the expression of pro-inflammation cytokines was differently regulated by Ecviperin overexpression. Furthermore, reporter gene analysis showed that the overexpression of Ecviperin enhanced the activity of nuclear factor of kappa B (NF-κB), IFN-1 and interferon-stimulated response element (ISRE) promoter, suggesting that Ecviperin might restrict SGIV replication by the positive regulation of interferon and inflammatory response. Taken together, our results demonstrated that Ecviperin encoded an ER-localized protein, and exerted antiviral function against fish DNA virus by up-regulating interferon and pro-inflammatory response.

Lysine 39 of IKKε of black carp is crucial for its regulation on IRF7-mediated antiviral signaling

Interferon regulatory factor 7 (IRF7) plays a crucial role in the interferon (IFN) signaling in mammals, in which it is activated by the TBK1/IKKε complex during host antiviral innate immune response. There are few reports about the relation between IRF7 and IKKε in teleost fishes. In this study, the IRF7 homologue (bcIRF7) of black carp (Mylopharyngodon Piceus) has been cloned and characterized. The transcription of bcIRF7 gene increased in host cells in response to the stimulation of LPS, poly (I:C) and viral infection. bcIRF7 migrated around 56 KDa in immunoblot assay and was identified as a predominantly cytosolic protein by immunofluorescent staining. bcIRF7 showed IFN-inducing ability in reporter assay and EPC cells expressing bcIRF7 showed enhanced antiviral ability against both grass carp reovirus (GCRV) and spring viremia of carp virus (SVCV). IKKε of black carp (bcIKKε) was found to be recruited into host innate immune response initiated by SVCV and GCRV in the previous work; in this paper, the kinase dead mutant of bcIKKε, bcIKKε-K39A was constructed and showed no IFN-inducing activity. The data of reporter assay and plaque assay demonstrated that bcIKKε but not bcIKKε-K39A obviously enhanced bcIRF7-mediated IFN production and antiviral activity. Our data support the conclusion that bcIKKε upregulates bcIRF7-mediated antiviral signaling, which most likely depends on its kinase activity.

TBK1 from orange-spotted grouper exerts antiviral activity against fish viruses and regulates interferon response

TANK-binding kinase-1 (TBK1) has been well studied in mammals because of its importance in type I interferon induction in antiviral immunity. However, the roles of fish TBK1 in virus infection still remained largely uncertain. In the current study, a TBK1 homolog from orange-spotted grouper (Epinephelus coioides) (EcTBK1) was cloned and its roles in fish viral infections were investigated. Sequence analysis showed that EcTBK1 encoded a 723-amino acid peptide which shared 98% and 73% identity to large yellow croaker (Larimichthys crocea) and human (homo sapiens), respectively. Multiple sequence alignments indicated that EcTBK1 contained conserved domains, including N-terminal kinase domain (KD), the middle ubiquitin-like domain (ULD) and C-terminal coiled-coil (CC) domains. The tissue distribution profiles demonstrated that EcTBK1 gene was constitutively expressed in all examined tissues, with predominant expression in intestine. Temporal expression analysis in vitro showed that the expression levels of EcTBK1 were significantly up-regulated in response to both red-spotted grouper nervous necrosis virus (RGNNV) and Singapore grouper iridovirus (SGIV) infection, suggested that EcTBK1 might exert crucial roles in fish virus infection. Subcellular localization indicated that EcTBK1 expression was primarily in the cytoplasm in GS cells. The ectopic expression of EcTBK1 significantly inhibited both SGIV and RGNNV replication. Furthermore, EcTBK1 overexpression significantly increased the expression levels of interferon related cytokines and pro-inflammatory factors. In addition, the overexpression of EcTBK1 increased the IRF3- and IRF7-regulated interferon promoter ISRE and IFN activity, and the regulatory effect on interferon immune response were dependent on its kinase domain. Together, we speculated that grouper TBK1 exerted antiviral activity against iridovirus and nodavirus via regulating the interferon immune and inflammatory response.

Grouper STAT1a is involved in antiviral immune response against iridovirus and nodavirus infection

Signal Transducer and Activator of Transcription 1 (STAT1) has been demonstrated to function as a critical mediator in multiple cell processes, such as cell proliferation, cell death, and innate immune response. Interestingly, two orthologues of human STAT1, including STAT1a and STAT1b genes have been identified in different fish. However, the detailed roles of fish STAT1a in virus replication still remained largely uncertain. Here, we cloned a STAT1a from orange-spotted grouper Epinephelus coioides (EcSTAT1a) and characterized its roles during fish virus infection. EcSTAT1a encoded a 751-aa peptide which shared 97% and 93% identity to STAT1 from mandarin fish (Siniperca chuatsi) and Malabar grouper (Epinephelus malabaricus), respectively. Amino acid alignment analysis showed that EcSTAT1a contained a STAT-int domain, a STAT-alpha domain, a STAT-bind domain (DNA binding domain), a SH2 domain and a STAT1-TAZ2 bind domain. In examined tissues from healthy grouper, the expression of EcSTAT1a was predominant in intestine, gill and liver. In grouper cells, the relative expression levels of EcSTAT1a was significantly increased during red-spotted grouper nervous necrosis virus (RGNNV) or Singapore grouper iridovirus (SGIV) infection. Under fluorescence microscopy, we found that EcSTAT1a mainly localized in the cytoplasm. The ectopic expression of EcSTAT1a in vitro significantly delayed the cytopathic effect (CPE) progression evoked by RGNNV and SGIV. Further studies showed that the expression levels of viral genes, including SGIV major capsid protein (MCP), VP19, ICP-18, LITAF and RGNNV coat protein (CP), RNA-dependent RNA polymerase (RdRp) were all significantly reduced in EcSTAT1a overexpressing cells compared to the control vector transfected cells, suggested that EcSTAT1a exerted antiviral activity against iridovirus and nodavirus. Furthermore, overexpression of EcSTAT1a significantly increased the expression of interferon related cytokines or effectors and pro-inflammatory factors. Together, our results elucidated that EcSTAT1a might function as a critical antiviral factor by regulating the host interferon immune and inflammation response.

Molecular cloning and functional characterization of interferon regulatory factor 7 of the barbel chub, Squaliobarbus curriculus

The interferon regulatory factor 7 (IRF7) is a critical regulator of type-I interferon-dependent immune reaction that defense against virus. To investigate the antiviral function of IRF7 of barbel chub Squaliobarbus curriculus (ScIRF7), the cDNA of ScIRF7 was cloned and characterized. The full length cDNA of ScIRF7 was 1870 bp, consisted of 41 bp 5'-UTR, 560 bp 3'-UTR and a 1269 bp open reading frame (ORF). The ORF encoded 423 amino acids with a molecular weight of 49.426 KDa and a theoretical isoelectric point of 5.71. The putative ScIRF7 protein possesses typical domains of IRF family including a conserved N-terminal DBD-binding domain (DBD), a C-terminal IRF association domain and a serine-rich domain. In the DBD, four tryptophans were found to be highly conserved among all species, whilst in another conserved tryptophan site of mammals, the corresponding amino acids were methionine for fishes. The expression level of ScIRF7 was highest in the spleen and lowest in the liver. The expression level of IFN-β was highest in the gill and lowest in the liver. After GCRV infection, expression levels changes of ScIRF7 showed an overall tendency of firstly up-regulation and then down-regulation in the spleen and the gill; and expression levels of ScIRF7 in peripheral blood lymphocyte at 24 h post-infection was highest among all time points. In pEGFP-ScIRF7 overexpressing cells, the mRNA level of ScIRF7 was firstly up-regulation and then down-regulation; and the expression of IFN-β was significantly up-regulated at 12 h post-infection than that of control group (P < 0.05), which was significantly higher than those in pEGFP-N1 overexpressing cells. The results indicated that ScIRF7 may play a key role in immune responses of barbel chub Squaliobarbus curriculus against GCRV and may also functions in the Ctenopharyngodon idellus kidney cells.

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.

Expression and functional characterization of interferon regulatory factors (irf2, irf7 and irf9) in the blunt snout bream (Megalobrama amblycephala)

Interferon regulatory factors (irfs) are a family of genes that encode transcription factors with important roles in regulating the expression of Type I interferons (IFNs) and other genes associated with related pathways. irfs have multitudinous functions in growth, development and regulation of oncogenesis. In this study, three irf family members (irf2, irf7, irf9) were identified and characterized in Megalobrama amblycephala at the mRNA and amino acid levels. M. amblycephala irfs share a high sequence homology with other vertebrate irfs. Constitutive expression levels of the three genes were detected (using qPCR) in all studied tissues: low to medium in kidney, gills, heart and muscle, and high in liver, spleen, intestine and blood. qPCR was also used to analyze the dynamic expression patterns of irfs in different embryonic development stages: irf2 is not activated during the embryonic development, whereas irf9 appears to play important roles around hatching and during the larval development. Transcripts of all three studied irfs were upregulated after stimulation by Aeromonas hydrophila bacterium in liver, spleen, head kidney and trunk kidney, whereas downregulation was observed in intestine and gills. The results show that these three irfs are likely to be important factors in the blunt snout bream immune system. They also provide a foundation for studying the origin and evolution of the innate immune system in the blunt snout bream.

Fish TRIM32 functions as a critical antiviral molecule against iridovirus and nodavirus

Tripartite motif-containing 32 (TRIM32) has been demonstrated to pay vital roles in cancer, genetic disorders and antiviral immunity. However, the molecular functions of fish TRIM32 still remained largely unknown. Here, a novel TRIM32 gene from orange spotted grouper (EcTRIM32) was cloned and characterized. EcTRIM32 encoded a 685-aa protein which showed 93%, and 60% identity to large yellow croaker (Larimichthys crocea) and human (Homo sapiens), respectively. Amino acid alignment showed that EcTRIM32 contained a conserved RING-finger domain, a BBOX domain and NHL domain. In healthy grouper, the transcript of EcTRIM32 was predominantly detected in brain, liver, intestine, spleen and skin. After injection with Singapore grouper iridovirus (SGIV) and polyI:C, the relative expression of EcTRIM32 in grouper spleen was differently regulated, suggested that EcTRIM32 was involved in antiviral immune response. In transfected grouper spleen (GS) cells, EcTRIM32 displayed bright fluorescence aggregates or spots in the cytoplasm. Notably, the deletion RING domain altered its precise localization and distributed throughout the cytoplasm in GS cells. In EcTRIM32 overexpressing cells, the replication of SGIV or red-spotted grouper nervous necrosis virus (RGNNV) was significantly inhibited compared to the vector control cells. Moreover, the overexpression of EcTRIM32 positively regulated the interferon immune response, evidenced by the significant increase of the expression level of interferon related signaling molecules, including interferon regulatory factor 3 (IRF3), IRF7, interferon-stimulated gene 15 (ISG15), interferon-induced 35-kDa protein (IFP35), MXI, TIR-domain-containing adaptor-inducing interferon-β (TRIF) and melanoma differentiation-associated protein 5 (MDA5). Further studies showed that overexpression of EcTRIM32 significantly enhanced the MDA5-mediated interferon immune response, but decreased stimulator of interferon genes (STING)-mediated interferon immune response. Meanwhile, the expression levels of pro-inflammation cytokines, including TNFα, IL-6 and IL-8 were up-regulated by the ectopic expression of EcTRIM32. We speculated that the regulation of IRF7, and pro-inflammation cytokines by EcTRIM32 overexpression might contribute critical roles in SGIV infection. In addition, the deletion of RING domain not only significantly weakened the antiviral roles of EcTRIM32, but also obviously affected the regulatory effects of EcTRIM32 on interferon immune and inflammation response. Together, our results firstly demonstrated that fish TRIM32 acted as an antiviral factor against both DNA and RNA virus infection.

TRAF6 is a critical factor in fish immune response to virus infection

Tumor necrosis factor receptor-associated factor 6 (TRAF6) is one of the key adaptor molecule in Toll-like receptor signal transduction that triggers downstream cascades involved in innate immunity. In our previous study, the molecular characteristics of EtTRAF6 (TRAF6 from Epinephelus tauvina), the tissue distributions, expression patterns after challenging with bacterial and viral pathogens were investigated. Here we identified EtTRAF6 as an important regulator of virus-triggered signaling pathway. Overexpression of EtTRAF6-ORF and truncated forms of EtTRAF6, including EtTRAF6-C (delete the MATH domain), EtTRAF6-N (delete the RING domain) and EtTRAF6-MATH, inhibited IFN-β activity strongly in grouper spleen (GS) cells. Overexpression of EtTRAF6 repressed virus-induced production of type I IFNs. When EtTRAF6 cotransfected with EcIRF3 or EcIRF7, EtTRAF6 inhibited IRF-induced activation of IFN-β. Over-expressed EtTRAF6 inhibited the transcription of SGIV genes significantly in GS cells. Although TRAF6 has a role in apoptosis regulation, it is not known if EtTRAF6 has any role in apoptosis regulation. Strikingly, when over-expressed in fathead minnow (FHM) cells, EtTRAF6 protected them from cell death induced by SGIV. Therefore, these results suggest that TRAF6 may play a critical role in their response to SGIV infection, through regulation of a cell death pathway that is common to fish and humans.

Fish TRIM16L exerts negative regulation on antiviral immune response against grouper iridoviruses

Tripartite motif 16 (TRIM16), has been demonstrated to act as a tumor suppressor through affecting cell proliferation and migration or tumorigenicity in carcigenesis. However, the roles of TRIM16 in immune response were unknown up to now. Here, we cloned a TRIM16-like gene (TRIM16L) from orange spotted grouper (EcTRIM16L) and investigated its roles in response to virus infection. EcTRIM16L encoded a 478 amino acid peptide which showed 72% and 29% identity to large yellow croaker (Larimichthys crocea) and human (Homo sapiens), respectively. Sequence alignments indicated that EcTRIM16L shared the different gene structures with human TRIM16, evidenced by the presence of RING domain, but absence of the B-box domain. In transfected grouper cells, the green fluorescence mainly distributed in cytoplasm, and the deletion of SPRY domain affected the accurate localization of EcTRIM16L. In response to different stimuli, including infection with Singapore grouper iridovirus (SGIV) or red-spotted grouper nervous necrosis (RGNNV), and transfection with b-DNA or poly I:C, the transcript of EcTRIM16L were differently regulated in grouper spleen cells. After incubation with SGIV, the ectopic expression of EcTRIM16L significantly enhanced the viral replication, demonstrated by the increase of cytopathic effect (CPE) severity and viral gene transcriptions. Simultaneously, we also found that overexpression of EcTIRM16L in vitro significantly weakened the expression of interferon related molecules, including interferon regulatory factor 3 (IRF3), IRF7, and melanoma differentiation-associated protein 5 (MDA5). Moreover, the ectopic expression of EcTRIM16L significantly decreased both MDA5-and mediator of IRF3 activation (MITA)-induced interferon immune responses. Further studies showed that the RING domain played more important roles in the molecular action of EcTIRM16L during grouper virus infection. Our data, for the first time, demonstrated that fish TRIM16L exerted negative regulation on the interferon immune response against DNA virus infection.

c-Jun N-terminal kinases 3 (JNK3) from orange-spotted grouper, Epinephelus coioides, inhibiting the replication of Singapore grouper iridovirus (SGIV) and SGIV-induced apoptosis

C-Jun N-terminal kinases (JNKs), a subgroup of serine-threonine protein kinases that activated by phosphorylation, are involve in physiological and pathophysiological processes. JNK3 is one of JNK proteins involved in JNK3 signaling transduction. In the present study, two JNK3 isoforms, Ec-JNK3 X1 and Ec-JNK3 X2, were cloned from orange-spotted grouper, Epinephelus coioides. Both Ec-JNK3 X1 and Ec-JNK3 X2 were mainly expressed in liver, gill, skin, brain and muscle of juvenile grouper. The relative expression of Ec-JNK3 X2 mRNA was much higher in muscle and gill than that of Ec-JNK3 X1. Isoform-specific immune response to challenges was revealed by the expression profiles in vivo. Immunofluorescence staining indicated that JNK3 was localized in the cytoplasm of grouper spleen (GS) cells and shown immune response to SGIV infection in vitro. Over-expressing Ec-JNK3 X1 and/or Ec-JNK3 X2 inhibited the SGIV infection and replication and the SGIV-induced apoptosis. To achieve the antiviral and anti-apoptosis activities, JNK3 promoted the activation of genes ISRE and type I IFN in the antiviral IFN signaling pathway, and inhibited the activation of transcription factors NF-κB and p53 relating to apoptosis, respectively. Ec-JNK3 X2 showed stronger activities in antivirus and anti-apoptosis than that of Ec-JNK3 X1. Our results not only define the characterization of JNK3 but also reveal new immune functions and the molecular mechanisms of JNK3 on iridoviruses infection and the virus-induced apoptosis.

JNK1 Derived from Orange-Spotted Grouper, Epinephelus coioides, Involving in the Evasion and Infection of Singapore Grouper Iridovirus (SGIV)

c-Jun N-terminal kinase (JNK) regulates cellular responses to various extracellular stimuli, environmental stresses, pathogen infections, and apoptotic agents. Here, a JNK1, Ec-JNK1, was identified from orange-spotted grouper, Epinephelus coioides. Ec-JNK1 has been found involving in the immune response to pathogen challenges in vivo, and the infection of Singapore grouper iridovirus (SGIV) and SGIV-induced apoptosis in vitro. SGIV infection activated Ec-JNK1, of which phosphorylation of motif TPY is crucial for its activity. Over-expressing Ec-JNK1 phosphorylated transcription factors c-Jun and promoted the infection and replication of SGIV, while partial inhibition of the phosphorylation of Ec-JNK1 showed the opposite effects by over-expressing the dominant-negative EcJNK1-Δ183-185 mutant. Interestingly, SGIV enhanced the viral infectivity by activating Ec-JNK1 which in turn drastically inhibited the antiviral responses of type 1 IFN, indicating that Ec-JNK1 could be involved in blocking IFN signaling during SGIV infection. In addition, Ec-JNK1 enhanced the activation of AP-1, p53, and NF-κB, and resulted in increasing the levels of SGIV-induced cell death. The caspase 3-dependent activation correlated with the phosphorylation of Ec-JNK1 and contributed to SGIV-induced apoptosis. Taken together, SGIV modulated the phosphorylation of Ec-JNK1 to inactivate the antiviral signaling, enhance the SGIV-induced apoptosis and activate transcription factors for efficient infection and replication. The “positive cooperativity” molecular mechanism mediated by Ec-JNK1 contributes to the successful evasion and infection of iridovirus pathogenesis.

Cloning and expression analysis of interferon regulatory factor 7 in the Pacific cod, Gadus macrocephalus

Interferon regulatory factor 7 (IRF7) plays an important role in regulating the response of type I interferon (IFN) to viral infection. To understand the mechanisms underlying immune reactions in the Pacific cod, Gadus macrocephalus, the gene encoding G. macrocephalus IRF7 was cloned and characterized. The cDNA of G. macrocephalus IRF7 was also cloned and sequenced. A cDNA sequence of 2032 bp was assembled using polymerase chain reaction (PCR) products. It contains an open reading frame of 1323 bp in length, which encoded a 440-amino acid polypeptide that comprised a DNA-binding domain (DBD), an IRF association domain (IAD), and a serine-rich domain (SRD). In the DBD, the tryptophan cluster consisted of only four tryptophans, which is a unique characteristic in fish IRF7. The mRNA of IRF7 was detected in various tissues, including in the spleen, thymus, kidney, intestine, and gills, using relative quantification PCR (R-qPCR). Dynamic expression of IRF7 was observed in larvae throughout post-hatching (ph) development, with the highest level detected at day of ph (dph) 25. Response to immune stimulation was examined by challenging larvae with polyriboinosinic polyribocytidylic acid (pIC) to mimic viral infection and elicit an immune reaction. R-qPCR revealed that the expression of IRF7 significantly increased in pIC-treated groups relative to that in the control groups, in a time-dependent manner, with peak responses at 48 and 72 h after pIC-treatment. These results show that IRF7 is expressed in various tissues of adult fish and larvae and is sensitive to viral infection, suggesting that it plays a role in antiviral immune defense in G. macrocephalus.

Function analysis of fish Tollip gene in response to virus infection

Toll-interacting protein (Tollip) is one of the important regulatory proteins of Toll-like receptor (TLR) signaling pathways. In previous studies, a Tollip sequence of grouper (Epinephelus coioides) was identified and the signal transduction functions of Tollip were studied. However, the response of Tollip to virus infection has not been characterized from grouper. In the present paper, the Tollip homolog (EtTollip) from grouper (Epinephelus tauvina) was cloned and its immune response to Singapore grouper iridovirus (SGIV) was investigated. EtTollip shares significant similarities to other mammalian Tollips, which contain a centrally localized protein kinase C conserved region 2 (C2) domain and a C-terminal CUE domain. After challenging with SGIV, the expression levels of EtTollip were altered in the spleen and head kidney of grouper. EtTollip mainly aggregated in the cytoplasm in a condensed state and was also distributed on the membranes of GS cells. EtTollip significantly inhibited the activities of NF-κB and IFN-β luciferase reporter when transfected into grouper spleen (GS) cells. SGIV can increase the activities of NF-κB and IFN-β luciferase reporter, especially to IFN-β. When transfected EtTollip with EcMyd88, the activity of NF-κB was increased, while transfected EtTollip with EcIRF3, the activity of IFN-β was significantly increased. Over-expressed EtTollip inhibited the transcription of SGIV genes significantly in GS cells, and silencing of EtTollip with siRNA led to increase of SGIV genes loads. Taken together, the results provide new insights in to the importance of Tollip as evolutionarily conserved molecule for grouper innate immunity against virus infection.

Antiviral role of grouper STING against iridovirus infection

Stimulator of interferon genes (STING, also known as MITA, ERIS, MPYS or TMEM173) has been identified as a central component in the innate immune response to cytosolic DNA and RNA derived from different pathogens. However, the detailed role of STING during fish iridovirus infection still remained largely unknown. Here, the STING homolog from grouper Epinephelus coioides (EcSTING) was cloned and its effects on IFN response and antiviral activity were investigated. The full-length EcSTING cDNA was composed of 1590 bp and encoded a polypeptide of 409 amino acids with 80% identity to STING homolog from large yellow croaker. Amino acid alignment analysis indicated that EcSTING contained 4 predicated transmembrane motifs (TMs) in the N terminal, and a C-terminal domain (CTD) which consisted of a dimerization domain (DD), c-di-GMP-binding domain (CBD) and a C-terminal tail (CTT). Expression profile analysis revealed that EcSTING was abundant in gill, spleen, brain, skin, and liver. Upon different stimuli in vivo, the EcSTING transcript was dramatically up-regulated after challenging with Singapore grouper iridovirus (SGIV), lipopolysaccharide (LPS) and polyinosin-polycytidylic acid (poly I:C). Reporter gene assay showed that EcSTING activated ISRE, zebrafish type I IFN and type III IFN promoter in vitro. Mutant analysis showed that IFN promoter activity was mostly mediated by the phosphorylation sites at serine residue S379 and S387. Moreover, EcSTING induced type I and III IFN promoter activity could be impaired by overexpression of EcIRF3-DN or EcIRF7-DN, suggesting that EcSTING mediated IFN response in IRF3/IRF7 dependent manner. In addition, the cytopathic effect (CPE) progression of SGIV infection and viral protein synthesis was significantly inhibited by overexpression of EcSTING, and the inhibitory effect was abolished in serine residue S379 and S387 mutant transfected cells. Together, our results demonstrated that EcSTING might be an important regulator of grouper innate immune response against iridovirus infection.

Functions of the two zebrafish MAVS variants are opposite in the induction of IFN1 by targeting IRF7

IFNs create the first line of host cells to defense viral infection, however, unrestricted expression of IFN can be hazardous to the host. IRF7 is the master regulator of type I IFN expression. To our knowledge, non research about the inhibition of IFN expression by targeting IRF7 has been reported in fish. In this study, we reported that the splicing variant of wildtype MAVS (MAVS_tv1), MAVS_tv2, negatively regulated IRF7-mediated IFN production. Firstly, in vivo, the transcriptional levels of MAVS_tv2 in trunk kidney and spleen from the zebrafish infected with SVCV were monitored. Then, in vitro, the protein expression pattern of MAVS_tv2 in zebrafish cell lines was detected using anti-MAVS_tv2 antibody. Furthermore, overexpression of MAVS_tv2 decreased the activation of IFN1 promoter that induced by IRF7 in a dose-dependent manner, whereas it had little effect on IRF3, a close relative of IRF7. In addition, such inhibition was also observed in IRF7-mediated epcIFN promoter and ISRE activities, but not in the activation of the promoters of type II IFNs and NF-ĸB, due to IRF7 not regulating their expression. Lastly, overexpression of MAVS_tv2 decreased the transcriptional levels of several IFN-stimulated genes activated by IRF7. These findings suggest that MAVS_tv2 is a negative regulator of IFN1 by targeting IRF7.

CD83 is required for the induction of protective immunity by a DNA vaccine in a teleost model

In mammals, CD83 is a surface marker on mature dendritic cells and vital to lymphocyte activation. In teleost, studies on the function of CD83 are very limited. In this study, we examined the potential involvement of turbot (Scophthalmus maximus) CD83, SmCD83, in vaccine-induced immunity. For this purpose, turbot were immunized with pORF75, a DNA vaccine against megalocytivirus, in the presence or absence of pSmCD83, a plasmid that constitutively expresses SmCD83. Immune response and protection analysis showed that the presence of pSmCD83 significantly (i) enhanced the activation of head kidney macrophages (HKM) and immune gene expression, (ii) inhibited viral replication in fish tissues following megalocytivirus challenge and increased the survival of the vaccinated fish, and (iii) stimulated production of specific serum antibody and the cytotoxicity of peripheral blood leukocytes. To further examine the effect of SmCD83, pORF75 was administered into turbot in which SmCD83 was knocked down. Subsequent analysis showed that in fish with SmCD83 knockdown, vaccine-induced HKM activation and antibody production were severely reduced, and, consistently, the protectivity of pORF75 was drastically decreased. Taken together, these results indicate for the first time that teleost CD83 is required for the induction of protective immune response by DNA vaccine.

Characterization and expression analyses of five interferon regulatory factor transcripts (Irf4a, Irf4b, Irf7, Irf8, Irf10) in Atlantic cod (Gadus morhua)

The interferon regulatory factor (IRF) family of genes encodes a group of transcription factors that have important roles not only in regulating the expression of Type I interferons (IFNs) and other genes in the IFN pathway, but also in growth, development and the regulation of oncogenesis. In this study, several IRF family members (Irf4a, Irf4b, Irf7, Irf8, Irf10) in Atlantic cod (Gadus morhua) were characterized at the cDNA and putative amino acid levels, allowing for phylogenetic analysis of these proteins in teleost fish, as well as the development of gene-specific primers used in RT-PCR and quantitative PCR (QPCR) analyses. Two Atlantic cod Irf10 splice variants were identified and their presence confirmed by sequencing of the Irf10 genomic region. RT-PCR showed that Irf7, Irf8 and both Irf10 transcripts were expressed in all 15 cod tissues tested, while Irf4a and Irf4b were absent in some tissues. QPCR analysis of spleen expression expanded upon this, and upon previous work. All IRF transcripts in the study were responsive to stimulation by the viral mimic poly(I:C), and all except Irf4a were responsive to exposure to formalin-killed Aeromonas salmonicida (ASAL). These IRF genes, thus, are likely important in the cod immune response to both viral and bacterial infections. Increased temperature (10 °C to 16 °C) was also observed to modulate the antibacterial responses of all IRF transcripts, and the antiviral responses of Irf4b and Irf10-v2. This research supports earlier studies which reported that elevated temperature modulates the expression of many immune genes in Atlantic cod.

Molecular characterization and expression analysis of eleven interferon regulatory factors in half-smooth tongue sole, Cynoglossus semilaevis

Interferon regulatory factors (IRFs) act as transcription mediators in virus-, bacteria-, and interferon (IFN)-induced signaling pathways and play diverse functions in antimicrobial defense, immune modulation, hematopoietic differentiation, and cell apoptosis. In this study, we described for the first time eleven IRFs (IRF1, IRF1L, IRF2X1, IRF3, IRF4a, IRF4b, IRF5, IRF6, IRF7, IRF8, and IRF9) from half-smooth tongue sole (Cynoglossus semilaevis) and examined their tissue distributions and expression patterns under different conditions. The deduced protein sequences of these IRFs (except IRF1) share high identities (71.8-86.6%) with other corresponding IRFs in other teleosts, whereas the sequence identity of IRF1 with the corresponding IRF1 in other teleosts is only 58.1%. A conserved N-terminal DNA binding domain (DBD), which is characterized by a winged type helix-loop-helix motif with four to six tryptophan repeats, is present in all IRFs. Another conserved IRF associated domain (IAD), which mediates the interactions in the C-terminal part of the protein, is present in all IRFs except IRF1 and IRF2X1, which instead contain the IAD2 domain. Several special domains also were found, including a serine-rich domain (SRD) in IRF3, IRF4a, IRF4b, and IRF7; a proline-rich domain (PRD) in IRF9; nuclear localization signals (NLSs) in IRF5, IRF8, and IRF9; and a virus activated domain (VAD) in IRF5. Quantitative real time RT-PCR (qRT-PCR) analysis showed that expression of all IRFs occurred in multiple tissues. IRF1, IRF2X1, IRF4a, IRF5, IRF7, and IRF8 exhibited relatively high levels of expression in immune organs, whereas the other five IRFs displayed high levels of expression in non-immune organs. Infection with extracellular and intracellular bacterial pathogens and virus upregulated the expression of IRFs in a manner that depended on tissue type, pathogen, and infection stage. Specifically, IRF1 and IRF2X1 were highly induced by bacterial and viral pathogens; IRF1L and IRF6 responded mainly to extracellular and intracellular bacterial pathogens; IRF3, IRF5, IRF7, IRF8, and IRF9 were markedly induced by intracellular bacterial pathogen and virus; IRF4a and IRF4b were mainly induced by virus and intracellular bacterial pathogen respectively. These results indicate that the IRFs of C. semilaevis can be categorized into several groups which exhibit different expression patterns in response to the infection of different microbial pathogens. These results provide new insights into the roles of teleost IRFs in antimicrobial immunity.

Identification of orange-spotted grouper (Epinephelus coioides) interferon regulatory factor 3 involved in antiviral immune response against fish RNA virus

Interferon regulatory factor 3 (IRF3) is an important transcription factor which regulates the expression of interferon (IFN) and IFN-stimulated genes (ISGs) following virus recognition. In this study, a novel IRF3 gene was cloned from grouper Epinephelus coioides (EcIRF3) and its effects against Singapore grouper iridovirus (SGIV) and red spotted grouper nervous necrosis virus (RGNNV) was investigated. The full-length of EcIRF3 cDNA was composed of 2513 bp and encoded a polypeptide of 458 amino acids which shared 82% identity with European seabass (Dicentrarchus labrax). EcIRF3 contained three conserved domains including a DNA-binding domain (DBD), an IRF associated domain (IAD) and a serine-rich domain. Expression profile analysis revealed that EcIRF3 was abundant in head kidney, kidney, spleen and gill. Upon different stimuli in vitro, the transcript of EcIRF3 was significantly up-regulated after RGNNV infection or treatment with polyinosin-polycytidylic acid (poly I:C). During SGIV infection, the increase of the EcIRF3 transcription was only detected at the late stage, suggesting that EcIRF3 was differently regulated by different stimuli. Immune fluorescence assay indicated that the fluorescence signal of EcIRF3 was increased significantly after infection with RGNNV or treatment with poly I:C, but moderately at the late stage of SGIV infection. Reporter gene assay showed that EcIRF3 activated zebrafish type I IFN and type III IFN promoter in vitro. The viral gene transcription and virus production of RGNNV were significantly decreased in EcIRF3 overexpressing cells. However, the ectopic expression of EcIRF3 did not affect the gene transcription and virus production of SGIV. Moreover, the mRNA expression levels of type I IFN and IFN-inducible genes (MxI, ISG15 and ISG56) were increased in RGNNV infected EcIRF3 overexpressing cells compared to empty vector transfected cells. Together, our results demonstrated that IFN immune response mediated by grouper IRF3 was exerted crucial roles for fish RNA virus, but not for DNA virus replication.

Involvement of fish signal transducer and activator of transcription 3 (STAT3) in SGIV replication and virus induced paraptosis

Signal transducer and activator of transcription 3 (STAT3) is an important transcription factor which plays crucial roles in immune regulation, inflammation, cell proliferation, transformation, and other physiological processes of the organism. In this study, a novel STAT3 gene from orange spotted grouper (Ec-STAT3) was cloned and characterized. Bioinformatic analysis revealed that full-length of Ec-STAT3 was 3105-bp long and contained a 280-bp 5'UTR, a 470-bp 3'UTR, and a 2355-bp open reading frame (ORF) that encoded a 784-amino acid peptide. The deduced protein of Ec-STAT3 showed 98% identity to that of turbot (Scophthalmus maximus). Amino acid alignment showed that Ec-STAT3 contained four conserved domains, including a protein interaction domain, a coiled coil domain, a DNA binding domain, and an SH2 domain. Quantitative real-time PCR analysis showed that the highest expression level was detected in the liver, followed by skin and spleen. After injection with Singapore grouper iridovirus (SGIV), the transcript of Ec-STAT3 in spleen was increased significantly. To further explore the function of Ec-STAT3, we investigated the roles of Ec-STAT3 in SGIV infection in vitro. Immune fluorescence analysis indicated that SGIV infection altered the distribution of phosphorylated Ec-STAT3 in nucleus, and a small part of phosphorylated Ec-STAT3 was associated with virus assembly sites, suggesting that Ec-STAT3 might be important for SGIV infection. Using STAT3 specific inhibitor, S3I-201, we found that inhibition of Ec-STAT3 activation decreased the SGIV replication significantly. Moreover, inhibition of Ec-STAT3 activation obviously altered SGIV infection induced cell cycle arrest and the expression of pro-survival genes, including Bcl-2, Bcl-xL and Bax inhibitor. Together, our results firstly demonstrated the critical roles of fish STAT3 in DNA virus replication and virus induced paraptosis, but also provided new insights into the mechanism of iridovirus pathogenesis.