Transcriptomics analysis reveals candidate genes and pathways for susceptibility or resistance to Singapore grouper iridovirus in orange-spotted grouper (Epinephelus coioides)

Fish Iridoviridae: infection, vaccination and immune response

Each year, due to climate change, an increasing number of new pathogens are being discovered and studied, leading to an increase in the number of known diseases affecting various fish species in different regions of the world. Viruses from the family Iridoviridae, which consist of the genera Megalocytivirus, Lymphocystivirus, and Ranavirus, cause epizootic outbreaks in farmed and wild, marine, and freshwater fish species (including ornamental fish). Diseases caused by fish viruses of the family Iridoviridae have a significant economic impact, especially in the aquaculture sector. Consequently, vaccines have been developed in recent decades, and their administration methods have improved. To date, various types of vaccines are available to control and prevent Iridoviridae infections in fish populations. Notably, two vaccines, specifically targeting Red Sea bream iridoviral disease and iridoviruses (formalin-killed vaccine and AQUAVAC® IridoV, respectively), are commercially available. In addition to exploring these themes, this review examines the immune responses in fish following viral infections or vaccination procedures. In general, the evasion mechanisms observed in iridovirus infections are characterised by a systemic absence of inflammatory responses and a reduction in the expression of genes associated with the adaptive immune response. Finally, this review also explores prophylactic procedure trends in fish vaccination strategies, focusing on future advances in the field.

Rab32, a novel Rab small GTPase from orange-spotted grouper, Epinephelus coioides involved in SGIV infection

Rab32 is a member of the Rab GTPase family that is involved in membrane trafficking and immune response, which are crucial for controlling pathogen infection. However, the role of Rab32 in virus infection is not well understood. In this study, we focused on the regulation of Rab32 on virus infection and the host immunity in orange-spotted grouper, Epinephelus coioides. EcRab32 encoded a 213-amino acid polypeptide, which shared a high sequence identity with other Rab32 proteins from fishes to mammals. In healthy orange-spotted grouper, the mRNA of EcRab32 was expressed in all the detected tissues, with the more expression levels in the head kidney, liver and gill. Upon SGIV infection, the expression of EcRab32 was significantly up-regulated in vitro, indicating its potential role in viral infection. EcRab32 was observed to be distributed in the cytoplasm as punctate and vesicle-like structures. EcRab32 overexpression was found to notably inhibit SGIV infection, while the interruption of EcRab32 significantly promoted SGIV infection. In addition, using single particle imaging analysis, we found that EcRab32 overexpression prominently reduced the attachment and internalization of SGIV particles. Furthermore, the results demonstrated that EcRab32 played a positive role in regulating the interferon immune and inflammatory responses. Taken together, these findings indicated that EcRab32 influenced SGIV infection by regulating the host immune response, providing an overall understanding of the interplay between the Rab32 and innate immunity.

Grouper Rab1 inhibits nodovirus infection by affecting virus entry and host immune response

Rab1, a GTPase, is present in all eukaryotes, and is mainly involved in vesicle trafficking between the endoplasmic reticulum and Golgi, thereby regulating many cellular activities and pathogenic infections. However, little is known of how Rab1 functions in fish during virus infection. Groupers (Epinephelus spp.) are high in economic value and widely cultivated in China and Southeast Asia, although they often suffer from diseases. Red-spotted grouper nervous necrosis virus (RGNNV), a highly pathogenic RNA virus, is a major pathogen in cultured groupers, and causes huge economic losses. A series of host cellular proteins involved in RGNNV infection was identified. However, the impact of Rab1 on RGNNV infection has not yet been reported. In this study, a novel Rab1 homolog (EcRab1) from Epinephelus coioides was cloned, and its roles during virus infection and host immune responses were investigated. EcRab1 encoded a 202 amino acid polypeptide, showing 98% and 78% identity to Epinephelus lanceolatus and Homo sapiens, respectively. After challenge with RGNNV or poly(I:C), the transcription of EcRab1 was altered both in vitro and in vivo, implying that EcRab1 was involved in virus infection. Subcellular localization showed that EcRab1 was displayed as punctate structures in the cytoplasm, which was affected by EcRab1 mutants. The dominant negative (DN) EcRab1, enabling EcRab1 to remain in the GDP-binding state, caused EcRab1 to be diffusely distributed in the cytoplasm. Constitutively active (CA) EcRab1, enabling EcRab1 to remain in the GTP-binding state, induced larger cluster structures of EcRab1. During the late stage of RGNNV infection, some EcRab1 co-localized with RGNNV, and the size of EcRab1 clusters was enlarged. Importantly, overexpression of EcRab1 significantly inhibited RGNNV infection, and knockdown of EcRab1 promoted RGNNV infection. Furthermore, EcRab1 inhibited the entry of RGNNV to host cells. Compared with EcRab1, overexpression of DN EcRab1 or CA EcRab1 also promoted RGNNV infection, suggesting that EcRab1 regulated RGNNV infection, depending on the cycles of GTP- and GDP-binding states. In addition, EcRab1 positively regulated interferon (IFN) immune and inflammatory responses. Taken together, these results suggest that EcRab1 affects RGNNV infection, possibly by regulating host immunity. Our study furthers the understanding of Rab1 function during virus infection, thus helping to design new antiviral strategies.

Grouper annexin A2 affects RGNNV by regulating the host immune response

Annexin A2 (AnxA2) is ubiquitous in vertebrates and has been identified as a multifunctional protein participating in a series of biological processes, such as endocytosis, exocytosis, signal transduction, transcription regulation, and immune responses. However, the function of AnxA2 in fish during virus infection still remains unknown. In this study, we identified and characterized AnxA2 (EcAnxA2) in Epinephelus coioides. EcAnxA2 encoded a 338 amino acids protein with four identical annexin superfamily conserved domains, which shared high identity with other AnxA2 of different species. EcAnxA2 was widely expressed in different tissues of healthy groupers, and its expression was significantly increased in grouper spleen cells infected with red-spotted grouper nervous necrosis virus (RGNNV). Subcellular locatio n analyses showed that EcAnxA2 diffusely distributed in the cytoplasm. After RGNNV infection, the spatial distribution of EcAnxA2 was unaltered, and a few EcAnxA2 co-localized with RGNNV during the late stage of infection. Furthermore, overexpression of EcAnxA2 significantly increased RGNNV infection, and knockdown of EcAnxA2 reduced RGNNV infection. In addition, overexpressed EcAnxA2 reduced the transcription of interferon (IFN)-related and inflammatory factors, including IFN regulatory factor 7 (IRF7), IFN stimulating gene 15 (ISG15), melanoma differentiation related gene 5 (MDA5), MAX interactor 1 (Mxi1) laboratory of genetics and physiology 2 (LGP2), IFN induced 35 kDa protein (IFP35), tumor necrosis factor receptor-associated factor 6 (TRAF6) and interleukin 6 (IL-6). The transcription of these genes was up-regulated when EcAnxA2 was inhibited by siRNA. Taken together, our results showed that EcAnxA2 affected RGNNV infection by down-regulating the host immune response in groupers, which provided new insights into the roles of AnxA2 in fish during virus infection.

Transcriptome analysis reveals the host immune response upon LMBV infection in largemouth bass (Micropterus salmoides)

Largemouth bass (Micropterus salmoides) is one of the important economical freshwater aquaculture species in China. However, the outbreak of viral diseases always caused great economic losses in the largemouth bass aquaculture industry. Largemouth bass virus (LMBV), a double-stranded DNA (dsDNA) virus belonging to genus Ranavirus, family Iridoviridae causes high mortality in cultivated largemouth bass. However, host responses, especially the molecular events involved in LMBV infection still remained largely uncertain. Here, we established an in vivo model of LMBV infection, and systematically investigated the mRNA expression profiles of host genes in liver and spleen from infected largemouth bass using RNA sequencing (RNA-seq). Histopathological analysis indicated that necrotic cells and the formed necrotic focus were present in spleen, while numerous basophilic cells, hepatocytes volume shrinkage, nucleus pyknosis, and the disappeared boundary of hepatocytes were observed in the liver of infected largemouth bass. Transcriptomic analysis showed that transcription levels of 5128 genes (2804 up-regulated genes and 2324 down-regulated) in liver and 7008 genes (2603 up-regulated and 4405 down-regulated) in spleen were altered significantly. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis indicated that numerous co-regulated differentially expressed genes (DEGs) in liver and spleen were enriched in the pathways related to cell death and immune signaling, such as apoptosis, necroptosis, cytokine-cytokine receptor interaction and JAK-STAT signaling. Moreover, the DEGs specially regulated by LMBV infection in liver were significantly enriched in the KEGG pathways related to metabolism and cell death, while those in spleen were enriched in the immune related pathways. In addition, the expression changes of several randomly selected genes, such as SOCS1, IL-6, CXCL2, CASP8, CYC and TNF from qPCR were consistent with the transcriptomic data. Taken together, our findings will provide new insights into the fundamental patterns of molecular responses induced by LMBV in vivo, but also contribute greatly to understanding the host defense mechanisms against iridoviral pathogens.

Functional analysis of a novel MHC-Iα genotype in orange-spotted grouper: Effects on Singapore grouper iridovirus (SGIV) replication and apoptosis

The classical major histocompatibility complex class I (MHC-Ⅰ) molecule plays a key role in vertebrate immune response for its important functions in antigen presentation and immune regulation. MHC pathway is closely related to many diseases involving autoimmunity, antigen intrusion and inflammation. However, rare literatures about the effect of MHC-I on fish cells apoptosis were reported. In this study, a novel type of MHC-Ⅰα genotype from orange-spotted grouper (named EcMHC-ⅠA*01) were cloned and characterized. It shared a 77% identity to its Epinephelus coioides MHC-Iα homology that has been uploaded to NCBI (ACZ97571.1). Molecular characterization analysis showed that EcMHC-ⅠA*01 encodes a 357-amino-acid protein, containing a signal peptide，α1，α2，α3, Cytoplasmic (Cyt) and Transmembrane (TM) domains. Tissue expression pattern showed that EcMHC-ⅠA*01 was extensively distributed in twelve selected tissues, with higher expression in the gill, intestine and skin. The expression of EcMHC-ⅠA*01 in grouper liver and spleen tissues were significantly induced by different stimuli (Zymosan A, LPS, Ploy I:C, RGNNV and SGIV). Comparing with the EcMHC-ⅠA*01 expression levels induced by Zymosan A, Ploy I:C and RGNNV, the effects induced by SGIV and LPS were more significant. Subcellular localization analysis showed that EcMHC-ⅠA*01 localizes throughout the cytoplasm appeared both diffuse and focal intracellular expression pattern. Overexpression of EcMHC-ⅠA*01 inhibited the CPE progression, the mRNA expression of the SGIV related genes (MCP, LITAF, ICP-18 and VP19) and the protein expression of MCP. Meanwhile, qRT-PCR result showed that EcMHC-ⅠA*01 overexpression upregulated the expression of interferon signaling molecules (IFN-γ, ISG56, MDA5 and MXI) and inflammatory cytokines (IL-1β, IL-6, TNF-α and TRAF6). In addition, our results showed that overexpression of EcMHC-ⅠA*01 promoted the apoptosis of normal fathead minnow (FHM) cells as well as the apoptosis of FHM cells induced by SGIV. However, there was no significant change in the activity of caspase 3 between control group and EcMHC-ⅠA*01 overexpression group, suggesting that EcMHC-ⅠA*01-induced apoptosis may not depend on the caspase 3 pathway. Taken together, these data in our study provide new insights into the role of MHC-I in antiviral immune response and apoptosis in fish.

Comparative Transcriptome Analysis Reveals the Molecular Immunopathogenesis of Chinese Soft-Shelled Turtle (Trionyx sinensis) Infected with Aeromonas hydrophila

Simple Summary

The Chinese soft-shelled turtle (Trionyx sinensis) is an important cultured reptile in East Asia. Hemorrhagic sepsis caused by Aeromonas hydrophila infection is the dominant disease in the aquaculture of Chinese soft-shelled turtles, while the molecular pathology is far from clear due to the lag of research on turtle immunology. It has been reported in mammals and fish that the dysfunction of immune responses to pathogen infections causes host tissue hemorrhagic sepsis. In this study, two groups of turtles with different susceptibility to A. hydrophila infection are found. A comparative transcriptome strategy is adopted to examine the gene expression profiles in liver and spleen for these two phenotypes of turtles post A. hydrophila infection, for the first time revealing the full picture of immune mechanisms against A. hydrophila, which provides new insight into the molecular pathology during A. hydrophila infection in T. sinensis. The findings will promote further investigations on pathogenic mechanisms of hemorrhagic sepsis caused by A. hydrophila infection in T. sinensis, and also will benefit their culture industry.

Abstract

Although hemorrhagic sepsis caused by Aeromonas hydrophila infection is the dominant disease in the aquaculture of Chinese soft-shelled turtle, information on its molecular pathology is seriously limited. In this study, ninety turtles intraperitoneally injected with A. hydrophila exhibited two different phenotypes based on the pathological symptoms, referred to as active and inactive turtles. Comparative transcriptomes of liver and spleen from these two groups at 6, 24, and 72 h post-injection (hpi) were further analyzed. The results showed that cytokine–cytokine receptor interaction, PRRs mediated signaling pathway, apoptosis, and phagocytosis enriched in active and inactive turtles were significantly different. Pro-inflammatory cytokines, the TLR signaling pathway, NLR signaling pathway, and RLR signaling pathway mediating cytokine expression, and apoptosis-related genes, were significantly up-regulated in inactive turtles at the early stage (6 hpi). The significant up-regulation of phagocytosis-related genes occurred at 24 hpi in inactive turtles and relatively lagged behind those in active turtles. The anti-inflammatory cytokine, IL10, was significantly up-regulated during the tested periods (6, 24, and 72 hpi) in active turtles. These findings offer valuable information for the understanding of molecular immunopathogenesis after A. hydrophila infection, and facilitate further investigations on strategies against hemorrhagic sepsis in Chinese soft-shelled turtle T. sinensis.

Combining Multiple Approaches and Models to Dissect the Genetic Architecture of Resistance to Infections in Fish

Infectious diseases represent a major threat for the sustainable development of fish farming. Efficient vaccines are not available against all diseases, and growing antibiotics resistance limits the use of antimicrobial drugs in aquaculture. It is therefore important to understand the basis of fish natural resistance to infections to help genetic selection and to develop new approaches against infectious diseases. However, the identification of the main mechanisms determining the resistance or susceptibility of a host to a pathogenic microbe is challenging, integrating the complexity of the variation of host genetics, the variability of pathogens, and their capacity of fast evolution and adaptation. Multiple approaches have been used for this purpose: (i) genetic approaches, QTL (quantitative trait loci) mapping or GWAS (genome-wide association study) analysis, to dissect the genetic architecture of disease resistance, and (ii) transcriptomics and functional assays to link the genetic constitution of a fish to the molecular mechanisms involved in its interactions with pathogens. To date, many studies in a wide range of fish species have investigated the genetic determinism of resistance to many diseases using QTL mapping or GWAS analyses. A few of these studies pointed mainly toward adaptive mechanisms of resistance/susceptibility to infections; others pointed toward innate or intrinsic mechanisms. However, in the majority of studies, underlying mechanisms remain unknown. By comparing gene expression profiles between resistant and susceptible genetic backgrounds, transcriptomics studies have contributed to build a framework of gene pathways determining fish responsiveness to a number of pathogens. Adding functional assays to expression and genetic approaches has led to a better understanding of resistance mechanisms in some cases. The development of knock-out approaches will complement these analyses and help to validate putative candidate genes critical for resistance to infections. In this review, we highlight fish isogenic lines as a unique biological material to unravel the complexity of host response to different pathogens. In the future, combining multiple approaches will lead to a better understanding of the dynamics of interaction between the pathogen and the host immune response, and contribute to the identification of potential targets of selection for improved resistance.

Functional analysis of Epinephelus coioides peroxisome proliferative-activated receptor α (PPARα): Involvement in response to viral infection

Peroxisome proliferative-activated receptor α (PPARα) belongs to the superfamily of nuclear receptors (NR). Studies have demonstrated that PPARα functions in energy metabolism, hepatic function, immune response, cell cycle, and apoptosis. In teleost fish, few studies have investigated the role of PPARα in the immune response. In this study, the grouper PPARα gene (EcPPARα) was investigated for its role in viral infection. The open reading frame of EcPPARα encoded a protein of 469 amino acids and contained an N-terminal domain (NTD), a DNA-binding domain (DBD), a hinge region, and a C-terminal ligand-binding domain (LBD). Phylogenetic analysis revealed that EcPPARα was most closely related to homologous genes in Sander lucioperca and Perca flavescens. Upon challenge with SGIV (Singapore grouper iridovirus) and RGNNV (Red-spotted grouper nervous necrosis virus), EcPPARα expression levels were significantly upregulated in different tissues. Subcellular localization analysis showed that the EcPPARα protein localized throughout the cytoplasm and nucleus with diffuse intracellular expression patterns, which is consistent with the localization pattern of mammalian PPARs. Based on morphological observation of cytopathic effect (CPEs), viral gene expression mRNAs, and virus titer assays, the results presented here showed that an overexpression of EcPPARα promoted SGIV production in grouper spleen cells. Overexpression of EcPPARα significantly inhibited the expression of several cytokines, including interferon-related genes (IFN-γ, ISG15, MXI, MXII, MAVS and MDA5), inflammatory cytokines (IL-1β, IL-6, IL-8, TNF-α) and Toll like receptor adaptors (TRAF6 and MyD88). Luciferase activity of IFN-α, IFN-γ, ISRE and NF-κB promoters was also significantly decreased in EcPPARα overexpression cells. Due to these detected interferon-related genes and inflammatory cytokines play important antiviral effect against SGIV in grouper, we speculated that the promotion effect of EcPPARα on SGIV replication may be caused by down-regulation of interferon and inflammatory response. In addition, through apoptotic body observation, capspase-3 activity detection, and flow cytometry analysis, it was found that overexpression of EcPPARα promoted SGIV-induced apoptosis in fathead minnow (FHM) cells. These data may increase an understanding of the role of PPARα in fish antiviral immune responses and apoptosis.

De Novo Transcriptome Assembly and Annotation of Liver and Brain Tissues of Common Brushtail Possums (Trichosurus vulpecula) in New Zealand: Transcriptome Diversity after Decades of Population Control

The common brushtail possum (Trichosurus vulpecula), introduced from Australia in the mid-nineteenth century, is an invasive species in New Zealand where it is widespread and forms the largest self-sustained reservoir of bovine tuberculosis (Mycobacterium bovis) among wild populations. Conservation and agricultural authorities regularly apply a series of population control measures to suppress brushtail possum populations. The evolutionary consequence of more than half a century of intensive population control operations on the species’ genomic diversity and population structure is hindered by a paucity of available genomic resources. This study is the first to characterise the functional content and diversity of brushtail possum liver and brain cerebral cortex transcriptomes. Raw sequences from hepatic cells and cerebral cortex were assembled into 58,001 and 64,735 transcripts respectively. Functional annotation and polymorphism assignment of the assembled transcripts demonstrated a considerable level of variation in the core metabolic pathways that represent potential targets for selection pressure exerted by chemical toxicants. This study suggests that the brushtail possum population in New Zealand harbours considerable variation in metabolic pathways that could potentially promote the development of tolerance against chemical toxicants.

Characterization of Kruppel-like factor 6 in Epinephelus coioides: The role in viral infection and the transcriptional regulation on Peroxisome proliferator-activated receptor δ

The Kruppel-like factor 6 (KLF6) is a member of Kruppel-like factor family, which belong to the Zinc finger family of transcription factors that mediates various cellular processes, such as proliferation, differentiation, development, and programmed cell death. Peroxisome proliferator-activated receptors (PPARs) are a family of transcription factors belonging to the nuclear receptor superfamily and they regulate numerous genes through ligand-dependent transcriptional activation and repression. In this study, we focus on the role of KLF6 gene in virus infection and the regulation of KLF6 on PPAR-δ in orange-spotted grouper (Epinephelus coioides). The ORF sequence of EcKLF6 was 846 bp, encoding a polypeptide of 282 amino acids with three conserved Zinc finger (type Cys₂-His₂) domain in the C-terminal region. Basing on the detection of the mRNA levels of viral genes, western blotting of MCP protein, and morphological CPEs, we found that the overexpression of EcKLF6 suppressed the replication of Singapore grouper iridovirus (SGIV), exerting its antiviral activity against fish virus. Moreover, promoter analysis was performed to investigate whether EcKLF6 was a regulator of EcPPAR-δ. The luciferase reporter assay and real time PCR results indicated a negative regulatory role of EcKLF6 on EcPPAR-δ transcription in grouper. Further experimental analysis shows that the potential EcKLF6 binding sites may locate in the EcPPAR-δ-4-M3 (+133 to +154) and EcPPAR-δ-4-M4 (+354 to +368) region of the EcPPAR-δ promoter. Electrophoretic mobile shift assays (EMSAs) verified that EcKLF6 interacted with the binding site of the EcPPAR-δ-4-M4 promoter region. In addition, we also found that KLF6 promotes inflammatory responses in GS cells. Considering that KLF6 and PPAR-δ play opposite roles in regulating inflammatory responses, we speculated the promoting effect of KLF6 on inflammatory response may be related to its negative regulation on EcPPAR-δ. In conclusion, the present study provides the first evidence of the negative regulation of EcPPAR-δ transcription by EcKLF6 and contributes to a better understanding of the transcriptional mechanisms of EcKLF6 in fish.

PPAR-δ of orange-spotted grouper exerts antiviral activity against fish virus and regulates interferon signaling and inflammatory factors

Peroxisome proliferator-activated receptor δ (PPAR-δ), also called PPAR-β or PPAR-β/δ, is a member of the peroxisome proliferator-activated receptor (PPAR) family, which belongs to the nuclear steroid receptor superfamily. Activated PPARs participate in the regulation of lipid and glucose metabolism and also affect cellular proliferation, differentiation, and apoptosis, and the immune responses. To investigate the roles of PPAR-δ in Singapore grouper iridovirus (SGIV) infection, we cloned and characterized the gene encoding a PPAR-δ homologue from the orange-spotted grouper, Epinephelus coioides (EcPPAR-δ). EcPPAR-δ encodes a 514-amino-acid polypeptide, with 95.29% and 74.76% homologue to the Seriola dumerili and human proteins, respectively. EcPPAR-δ contains a typical DNA-binding domain and a ligand-binding domain. Its expression was induced by SGIV infection in vitro. A subcellular localization analysis showed that EcPPAR-δ localizes throughout the cytoplasm and nucleus, with a diffuse intracellular expression pattern. SGIV replication was reduced by EcPPAR-δ overexpression, which was evident in the reduced severity of the cytopathic effect, reduced viral gene transcription, and the reduced expression of the viral capsid protein. The replication of SGIV increased with the knockdown of EcPPAR-δ. The overexpression and silencing of EcPPAR-δ in grouper spleen cells showed that EcPPAR-δ plays a positive role in the regulation of the interferon signaling pathway, but has an anti-inflammatory effect on the inflammatory response. The anti-inflammatory effect of EcPPAR-δ may be related to its function in maintaining cell homeostasis. Because the interferon signaling pathway plays an important role in antiviral immune responses, we speculate that the activation of the interferon signaling pathway by EcPPAR-δ overexpression underlies its inhibitory effect on SGIV replication. Together, our data greatly extend our understanding of the roles of the EcPPAR-δ family members in the pathogenesis of fish viruses.

Molecular cloning and characterization of grouper Krϋppel-like factor 9 gene: Involvement in the fish immune response to viral infection

Krϋppel-like factor 9 (KLF9) is a member of the SP/KL family, which are transcription factors implicated in several biological processes, including cell proliferation, differentiation, development and apoptosis. Studies have focused on the function of KLF9 in mammalian disease and the immune system, such as its regulatory role in the growth of tumors and its impact on interferon-related genes and inflammatory cytokines. In fish, little is known about the role of KLF9, especially its regulatory function in the innate antiviral immune response. In this study, we characterized the grouper KLF9 gene (EcKLF9) and investigated its role in viral infection. Amino acid alignment analysis showed that EcKLF9 was approximately 228 amino acids long and contained a typical three-tandem Krϋppel-like zinc fingers. Phylogenetic tree analysis revealed that EcKLF9 clustered with three fish species: Amphiprion ocellaris, Acanthochromis pollyacanthus and Stegastes partitus. Comparison analyses showed that the three Kruppel-like zinc finger domains of KLF9 were highly conserved in different fish species. Tissue expression analysis showed that EcKLF9 was constitutively expressed in all 12 tissues tested, in the healthy grouper, the highest expression being detected in the gonads. The relative expression levels of EcKLF9 in the head kidney, spleen and brain was significantly increased during red-spotted grouper nervous necrosis virus (RGNNV) and Singapore grouper iridovirus (SGIV) infections. Using fluorescence microscopy, EcKLF9 was primarily localized to the nucleus and cytoplasm. The in vitro ectopic expression of EcKLF9 significantly increased the severity of vacuoles induced by RGNNV and the cytopathic effect progression evoked by SGIV infection. Real-time PCR results showed that the transcription levels of viral genes, such as the Singapore grouper iridovirus infection genes, MCP (major capsid protein), LITAF (lipopolysaccharide induced TNF-α factor), VP19 (envelop protein) ICP-18 (infected cell protein-18) and the red-spotted grouper nervous necrosis virus genes, CP (coat protein), RdRp (RNA-dependent RNA polymerase), were all significantly increased in EcKLF9 overexpressing cells, when compared to control cells. Furthermore, western blotting analyses showed that protein levels of the RGNNV gene, CP and the SGIV gene, MCP were also increased in EcKLF9 overexpressing cells, suggesting EcKLF9 may promote viral activity against iridovirus and nodavirus, in vitro. Moreover, the overexpression of EcKLF9 significantly inhibited the expression of several interferon related cytokines and several inflammatory cytokines. Accordingly, we speculate that EcKLF9 may exert stimulatory effects on RGNNV and SGIV replication, through the negative regulation of host immune and inflammation responses.