Abstract:Ctenopharyngodon idella is one of the main freshwater aquaculture fishes in China, with an annual output of more than 5 million tons, which is widely distributed throughout China. Grass carp reovirus (GCRV) is a member of the Aquareovirus genus of the family Reoviridae, a large family of double-stranded RNA (dsRNA) viruses infecting plants, insects, fishes and mammals. Proteasome subunit beta type-7 (PSMB7) is one of the major components of the 20S core proteasome complex and displays a trypsin-like activity. Capsid protein VP7 of GCRV has been shown to interact with PSMB7 both in vitro and in vivo. To investigate the interaction between GCRV non-structural protein 12 (NS12) and C. idella host protein PSMB7 and the effect of PSMB7 on the expression of ns12 during GCRV infection, yeast two-hybrid experiment, GST pull-down experiment and Real time PCR detection of ns12 transcription level after GCRV infection of grass carp ovarian cells (GCO) overexpressing PSMB7 were conducted in this research. The results of yeast two-hybrid experiments showed that PSMB7 and GCRV-encoded membrane-associated non-structural protein NS12 also had potential interactions; The results of Western blot showed that PSMB7 interacted with the membrane-related non-structural protein NS12 encoded by GCRV; The overexpression of PSMB7 by about 60 times can up-regulate the expression level of ns12 during viral infection by about three times; Western blot verified that NS12 was not sensitive to protein degradation. Previous studies in our laboratory have verified that PSMB7 expression is constant during viral infection. In summary, these results revealed that GCRV NS12 had an intermolecular interaction with PSMB7, but it did not act as a substrate for PSMB7. This indicates that the function of NS12 induced by PSMB7 may be positively correlated with the replication of GCRV in host cells, revealing that NS12 may participate in the early replication cycle of GCRV virus in host cells and avoid degradation of viral proteins by competitively recruiting PSMB7. The interference of viral proteins with the accumulation of intracellular PSMB7 of the proteasome complex may be a viral strategy to escape from proteasome-mediated innate immunity.

Abstract:With the development of the aquaculture industry, the stocking density has increased and the outbreaks of bacterial diseases have become more and more frequent. Nowadays, antibiotics are still the main control tood against bacterial infection. However its massive and irregular use makes the problem of drug resistance more serious. In order to effectively prevent and control aquatic bacterial diseases, accelerate the sustainable development of the aquaculture industry, and promote industrial transformation, it is urgently needed to seek new antibacterial methods to alleviate the current situation. Bacteriophages (phages) are viruses that specifically infect bacteria. A bacteriophage, also known informally as a phage, is a virus that infects and replicates within bacteria and archaea. They have the advantages of strong specificity, less resistance, easy to be metabolized, easy development, and low cost. Phages have been widely studied worldwide, and also have been applied to the prevention and control of various diseases. The related products have also been well- recognized, but the limitations cannot be ignored. This article first comprehensively introduces the principles and advantages of phage therapy, and then reviews the research progress of phage therapy in bacterial diseases of aquaculture animals. Subsequently, the existing dilemmas of phage therapy and coping strategies were discussed. Finally, a prospect is made on this basis, hoping to provide references for the follow-up research on the application of bacteriophages in aquaculture.

Abstract:Snakehead vesiculovirus (SHVV) is a kind of fish rhabdovirus that has caused great economic losses in snakehead fish culture in China. However, the mechanism on SHVV’s pathogenicity is still unclear. The phosphoprotein (P) isomers have been identified in mammalian rhabdovirus rabies virus and played important roles in the proliferation of rabies virus, but the function of P isomers in fish rhabdovirus has not been investigated. In order to identify the SHVV P isomers and study their roles in virus proliferation, the P gene of SHVV was amplified, and a prokaryotic expression plasmid pET32a-P was constructed. The recombinant His-P protein was purified by Ni-NTA affinity chromatography column, and polyclonal antibody against the His-P protein was acquired through immunization of New Zealand white rabbits. Using the P protein polyclonal antibody, the SHVV P isomers were determined. Furthermore, the subcellular localization of P isomers was assessed using enhanced green fluorescent protein (EGFP), and the effect of overexpression of P isomers on SHVV proliferation was determined using qRT-PCR, Western Blot, and TCID₅₀. The results showed that three P protein bands were detected during SHVV-infected channel catfish ovary (CCO) cells. Further verification determined that two of the three bands were P protein (also called P1) and its isomer P2. Subcellular localization experiment showed that both P1 and P2 localized mainly in cytoplasm, and P1 and P2 could shuttle between nucleus and cytoplasm. Overexpression of P1 or P2 in CCO cells increased G mRNA level about four times and G protein level around two times. The viral titer of SHVV was also significantly promoted via overexpression of P1 and P2, indicating that P1 and P2 promoted SHVV proliferation. Overall, SHVV can produce P protein (P1) and its isomer P2 during its infection, and P1 and P2 play an important role in SHVV proliferation. Our results will be helpful to understand SHVV pathogenicity and the function of P isomers of rhabdoviruses.

Abstract:Signal transducer and activator of transcription 3 (STAT3) is a member of the STAT protein family that controls cell proliferation, differentiation and inflammation. Recent studies have found that STAT3 also plays a key role in regulating host antiviral immune response. In order to study the roles of STAT3 in teleost antiviral response, two cDNA sequences encoding STAT3 (AjSTAT3-L and AjSTAT3-S) were obtained from Anguilla japonica, by reverse transcription polymerase chain reactions (RT-PCR) and rapid amplification of cDNA ends PCR (RACE-PCR) experiments. The open reading frames of AjSTAT3-L and AjSTAT3-S are 2349 bp and 1470 bp in length, coding 782 and 489 amino acids (aa), respectively. Gene structure analysis revealed that AjSTAT3-L consists of 23 exons and 22 introns, while AjSTAT3-S was generated by alternative splicing lacking the exons 2-7 and exon 21. Similar to its mammalian counterpart, the predicted protein sequence of AjSTAT3-L possesses an N terminal domain (NTD, 1-120 aa), a coiled coil domain (CCD, 104-313 aa), a DNA binding domain (DBD, 325-462 aa) and a Src homology 2 domain (SH2, 577-672 aa). Whereas, AjSTAT3-S lacks the entire CCD and C-terminal tail sequence of NTD (69 aa). Phylogenetic analysis showed that AjSTAT3-L and AjSTAT3-S formed a basal clade to teleost STAT3, and vertebrate STAT3 genes were grouped together to form a clade that was sister to the clade of STAT1 and STAT4. Subcellular localization experiments showed that AjSTAT3-L and AjSTAT3-S are predominantly localized in cytoplasm. Luciferase reporter assays were performed to study the effect of AjSTAT3-L and AjSTAT3-S on the transcriptional activity of antiviral genes, showing that the overexpression of AjSTAT3-L or AjSTAT3-S suppressed the Poly I∶C-induced activation of IFN and Mx promoters. Taken together, we have identified AjSTAT3 and its transcript variant, a novel isoform of STAT3 generated by alternative splicing. Both isoforms are functionally relevant and played a negative regulatory role in fish antiviral immune response.

Abstract:In order to study the role of hemojuvelin (hjv) in the process of resisting pathogen infection and maintaining its iron homeostasis in teleost fish, the present study amplified the open reading frame (ORF) of Oreochromis niloticus hjv (Onhjv), and analyzed its distribution pattern in various tissues of healthy tilapia and its related role in resisting pathogen infection and regulating iron homeostasis. The ORF of Onhjv was 1 248 bp, encoding 415 amino acids, which was conserved among different species. The results of quantitative real-time PCR (qRT-PCR) showed that Onhjv was widely distributed in all detected tissues of O. niloticus, and the highest expression level was found in liver. The expression of Onhjv was significantly up-regulated in the liver, spleen, intestine and gills after being challenged by Streptococcus agalactiae or Aeromonas hydrophila. The Onhjv expression in monocytes/macrophages (MO/Mø) and hepatocytes was also up-regulated after being stimulated by these two pathogens. In addition, the expression of Onhjv also showed an up-regulation trend in the liver, spleen, intestine, and gills, and head kidney MO/Mø and hepatocytes after stimulation with 1 μmol/L or 10 μmol/L FeCl₃ solution. After stimulation with recombinant O. niloticus IL-6 protein [(r)OnIL-6)], the expression of Onhjv in head kidney MO/Mø was significantly up-regulated, indicating that inflammatory factors could promote the expression of Onhjv. In this study, the results indicate that the hemojuvelin in O. niloticus plays an important role in host resistance to pathogen infection and maintenance of iron homeostasis.

Abstract:Streptococcus agalactiae infectious disease outbreaks have occurred continuously in Oreochromis spp. farms in China since 2009. In order to further explore the regulation mechanism of S. agalactiae virulence, the function of a putative two-component system (TCS) RscSR was investigated in a highly virulent strain GD201008-001, which was isolated from moribund cultured tilapia with meningoencephalitis in Guangdong Province of China in 2010. The 3-D structures and conserved domains of histidine kinase RscS and response regulator RscR were predicted using the I-TASSER server (https://zhanglab.ccmb.med.umich.edu/I-TASSER) and SMART (https://smart. embl.de). The rscSR deletion mutant strain ΔrscSR was constructed using homologous recombination. Bacterial tolerances to acid or oxidative stress were investigated by pre-exposing to pH 5.0 or 20 mmol/L H₂O₂. RAW264.7 macrophages were used as cell models to evaluate the bacterial anti-phagocytosis ability, intracellular survival ability and cytotoxicity. The bacterial pathogenicity was then tested in adult mice. The bioinformatics prediction showed that RscS and RscR were a typical histidine kinase and a response regulator, respectively. The stimuli sensed by RscS might be membrane-associated or occur directly at membrane interface; RscR was predicted to bind DNA to directly affect the transcription of the target gene. Compared with the wild-type and complemented strains, the resistance to acid (pH 5.0) of ΔrscSR was significantly decreased (2.83 times and 1.65 times), and furthermore, its resistance to H₂O₂ was also significantly decreased (1.93 times and 1.77 times). The ΔrscSR mutant displayed significantly decreased resistance to being ingested by macrophages (2.81 times and 1.56 times) compared to that of the wild-type and complemented strains. Moreover, at 3 h after being taken up by the macrophages, the intracellular survival rate of the ΔrscSR mutant was only approximately 44%; in contrast, it was 60% both for the wild type and complemented strains. After 6 h of phagocytosis, the intracellular survival rates of the wild type, ΔrscSR and complemented strains were decreased to about 46%, 28% and 40%, respectively. The cytotoxicity assay indicated that the relative number of lysed RAW 264.7 cells infected with the ΔrscSR mutant was significantly lower, exhibiting 1.8-fold and 2.5-fold decreases, respectively, compared to the wild-type and CΔrscSR strains. The mice infected with the wild strain all died within 19 hours, while mice infected with the ΔrscSR strain all died within 48 hours. This study suggests RscSR plays an important role in stress response, intracellular survival and virulence of S. agalactiae, which will provide useful information for further exploration of pathogenic mechanism of this bacterium.

Abstract:In recent years, RNA-seq technology has been used in fish researches. The transcriptome analysis in Ctenopharyngodon idella kidney cell lines (CIK) is focused on virus, and that based on bacteria or lipopolysaccharide (LPS) is rarely reported. In order to elucidate the mechanism of mammalian sterile20-like kinase 2 (mst2) in C. idella during immune response, we analyzed and verified the transcriptome sequence of CIK incubated with LPS after being interfered with mst2 by small interfering RNA technology (siRNA). Firstly, a total of 22 374 unigenes were obtained from the original sequence data by De novo assembly, of which 21 199 genes were known and 1 175 new genes were predicted. Secondly, the analysis of unigenes expression showed that there were 38 differential genes (differentially expressed genes, DEGs) including 16 up-regulated genes and 22 down-regulated genes. Thirdly, 38 DEGs were verified by quantitative real-time PCR (qRT-PCR). The results showed that the qRT-PCR analysis was consistent with transcriptome sequencing, indicating that the transcriptome sequencing was reliable. 38 DEGs in CIK cells were mainly involved in immune metabolism pathway, containing MAPK signal pathway, endocytosis pathway, autophagy pathway and cytokine receptor interaction pathway. Moreover, after being interfered with mst2 and treated with LPS, the detection of apoptosis-related genes showed that the transcriptional levels of pro-apoptotic genes (fas, bad1, bad2, caspase-3, caspase-8 and caspase-9) were up-regulated, while the anti-apoptotic genes (bcl2) were down-regulated. Therefore, it was proved that interfering mst2 could induce cell apoptosis after LPS treatment. To sum up, mst2 can participate in the body's immune response by regulating apoptosis-related processes. The present results preliminarily clarified the molecular mechanism of mst2 in grass carp during immune response, and may provide some basic theoretical reference for the prevention and control of grass carp bacterial diseases.

Abstract:Myxobolus including 979 species, is the largest genus in Myxozoa, which can parasitize different tissues in fish and bring about myxosporosis to result in various degrees of economic losses in the fishery. To identify the pathogen infecting the gill of common carp and analyze the pathogeny of its gill seriously damaged, those methods on morphometry, molecular biology, phylogeny and histopathology were employed in the research. The results showed that the myxospore of the pathogen was elliptic or nearly round in valvular view, lemon shape in sutural view, olive shaped in apical view. The myxospore was 9.2-11.8 μm long, 8.3-10.5 μm wide and 4.4 μm thick. Two water-drop polar capsules were unequal in size. The anterior ends of the two polar capsules were slightly close together. The larger polar capsule was 3.5-4.7 μm long and 2.4-3.1 μm wide, while the smaller one was 2.8-3.8 μm long and 1.8-2.6 μm wide. Polar filaments within the large and small polar capsules were coiled with 5 and 3 turns, respectively. V-shaped folds were obvious in the posteriors of some spores. However, intercapsular appendix as well as mucous envelope and aiodinophilous vacuole was not observed. The similarity of 18S rDNA between M. basilamellaris reported and the present species was 99.69%. Thus, morphological and molecular data of the parasite as well as host species proved that the present species is M. basilamellaris, and this is a new record in China. The cysts developed between the afferent and efferent arteries of the diseased carp’s gills. Due to the increase of cyst volume, the afferent and efferent arteries were compressed to be deformed and blocked, and the bases of filaments were also mechanically fractured. Some connective tissues around the cysts were destroyed. Moreover, the myxospores infiltrated into the surrounding tissues after the rupture of the cysts. Therefore, the main cause of death for diseased carp is that the cysts of M. basilamellaris oppressed the main vessels and fractured the gills, which resulted in poor blood flow and insufficient blood supply, and finally led to the loss of respiratory function of gills. The morphological, molecular and pathological data on M. basilamellaris from this study can provide a theoretical basis for its prevention and control in aquaculture.

Abstract:The shrimp postlarva vitrified syndrome broke out in spring of 2020 and spread explosively along the coastal areas from south to north of China. In order to find out the main pathogenic agents of shrimp postlarva vitrified syndrome, in this study, the pathogen was isolated and identified, and the histopathology was investigated. The postlarva symptoms included emaciation, dark cloud, decreased activity, anorexia, empty intestinal tract and stomach. The hepatopancreas showed atrophy, blurring of contour, paleness and even vitrified syndrome. Histopathological analysis showed that the epithelial cells of liver tubule were necrotic and exfoliated, liver tubule was filled with a large amount of debris tissue, leaving continuous glassy homogeneous areas. The intestinal tract was filled with tissue fragments, chorionic membrane fell off and even disappears. Ultrastructural pathological examination showed that the membranes of epithelial cells were ablated, organelles disintegrated, and nuclei were solidified. Subsequently, the cells disintegrated, fell off, and even the hepatic tubular tissue structure was ablated. The bacteria were found in the hepatopancreas, intestinal tract and gastric mucosa. The dominant strain was rod-shaped and curved, and no virions were found. Two dominant bacteria (Lv-A, Lv-B) were isolated from the diseased shrimp postlarvae. Artificial infection experiments illustrated that the strains of Lv-A and Lv-B were the causative pathogens with a median lethal dose of 1.62×10³ CFU/mL and 5.38×10³ CFU/mL, respectively. Based on molecular analyses (16S rDNA and gyrB), Lv-A and Lv-B were highly similar to Vibrio alginolyticus, V. neocaledonicus and Vibrio parahaemolyticus, preliminarily named shrimp postlarva bacterial vitrified syndrome (BVS). The chemotherapeutant sensitivity tests illustrated that Lv-A and Lv-B were sensitive to minocycline, doxycycline, nalidixic acid, etc. and resistant to neomycin, pipemidic, rifampicin, etc. This study provides theoretical basis and technical support for the effective prevention of BVS and the healthy development of shrimp industry.

Abstract:Interferon (IFN)-mediated antiviral signaling pathway is a very important part of fish innate immune response, and interferon regulatory factor 2 (IRF2) can affect fish immunity by regulating the expression of IFN. Therefore, this experiment studied the role of medaka irf2 (Olirf2) in the antiviral signal pathway, which may provide a theoretical basis for antiviral fish through gene editing or genetic modification. In this study, the Olirf2 was cloned, and RNA expression was detected in adult tissues. Moreover, the plasmid pTol2/CMV-IRF2/IE1-pr was constructed by cloning the coding sequence of Olirf2 for eukaryotic expression. In fish cell line FHM, transient expression of Olirf2 promoted the replication of spring viremia of carp virus (SVCV) and reduced the expression of mx1, ifn and irf3 significantly. Further studies by luciferase reporter assay showed that Olirf2 inhibited promoter activities of SVCV-induced NF-κB and ISRE, indicating that Olirf2 may promote the replication of SVCV by inhibiting cellular innate immune response. However, constant overexpression of Olirf2 enhanced the antiviral ability in FHM and increased the expression levels of mx1, ifn and irf3. Therefore, the current research suggested that the Olirf2 has double effect to regulate the antiviral effect based on the duration of expression.

Abstract:Scophthalmus maximus is one of the important species of mariculture in China, and occupies an important position in China's mariculture industry. At present, S. maximus farming industry shows a rapid growth trend, but as S. maximus farming intensification and scale continue to increase, bacterial diseases continue to occur. As pathogens of scabies, Aeromonas salmonicida has caused great economic losses to aquaculture. In order to prepare the inactivated vaccine of A. salmonicida and evaluate its immune effect on S. maximus, HHSM1905 strains were killed by formalin to prepare the inactivated vaccine. The experimental animals were randomly divided into two groups, i.e. immune group and control group. S. maximus was immunized by intraperitoneal injection. The indexes such as serum antibody titer, serum lysozyme activity, acid phosphatase activity and vaccine protection rate were determined. The expression of related immune genes (IL-1β, TLR-5, MHC I, MHC Ⅱ-α and CD4) was studied with β-actin gene as internal reference. The results showed that 2 weeks after vaccination, the antibody titer of immune group was significantly higher than that of control group and reaching the peak value. The lysozyme activity of the immune group reached the peak value 2 weeks after immunization, and remained significantly different from that of the control group at 4 weeks; the acid phosphatase activity of the immune group reached the peak value at 2 weeks after immunization; the relative percentage survival of the combined vaccine was 72.72%. Compared with the control group, IL-1β, TLR-5, MHC I, MHC Ⅱ-α and CD4 immune genes in the immune group had an upward trend, and most of them reached the maximum value at 1-2 weeks, which had significant difference with the control group. In conclusion, the inactivated vaccines of A. salmonicida for S. maximus were successfully prepared in this study. The A. salmonicida inactivated vaccine was effective for preventing A. salmonicida disease, and may provide a novel method for disease prevention in S. maximus culture.

Abstract:No.2 of Xiangyun crucian carp possesses the advantages of sterility, rapid growth and improved disease and stress resistance. In order to explore the molecular mechanism of its disease resistance, triploid hybrid interferon a3 (3nIFNa3) has been cloned and characterized. The CDS of 3nIFNa3 comprised 552 nucleotides, encoding 184 amino acids. The first 23 amino acids of N-terminal were signal peptides. There are 2 cysteine residues involved in the formation of disulfide bonds in mature peptides of 3nIFNa3, which indicates that 3nIFNa3 belongs to group I type I interferon. qPCR results showed that the mRNA levels of 3nIFNa3 in the host cells reached the peak at 8 h after poly (I: C) stimulation and reached the highest at 48 h after GCRV or SVCV infection. Both Western blot and immunofluorescence results showed that 3nIFNa3 was a secretory protein and its molecular weight was around 21.8 ku, which was mainly distributed in the cytoplasm before secretion. Further studies showed that the transcription of endogenous ISG genes were significantly up-regulated after the host cells overexpressed 3nIFNa3 or were incubated with the 3nIFNa3-containing conditioned media. The transcription levels of 3nSTAT1, 3nVIPERIN or 3nPKR reached the highest at 2, 2 or 4 h after the host cells were incubated with the 3nIFNa3-containing conditioned media. Besides, the results of the classic plaque assay and the crystal violet staining showed that EPC cells presented significantly enhanced antiviral ability against GCRV and SVCV after incubation with 3nIFNa3 or overexpression of 3nIFNa3. The above results show that 3nIFNa3 is a secretory cytokine and plays an important role in the host antiviral innate immune response.

Abstract:Early detection of pathogen is particularly important for the early warning, prevention and control of diseases. In recent years, a new viral disease occurred in farmed Micropterus salmoides in Guangdong Province and its pathogen is largemouth bass birnavirus (LBBV); In order to prevent the virus effectively, specific primers were designed for the conserved gene VP1 of LBBV to construct the recombinant plasmid pMD-LBBV-VP1 in this study. And the sensitivity and specificity of the method were tested after the PCR reaction conditions were optimized. The nested RT-PCR method for detecting the LBBV was established and the 304 clinical samples of dead fish collected from 2017 to 2020 were tested using this method; The results show that the optimal concentration of primers F1/R1 and F2/R2 were 4×10⁻¹² mol. And the optimal annealing temperatures were 64.1 ℃ and 61.5 ℃, respectively. When PCR amplification was performed for 35 cycles, a minimum plasmid concentration of 4.15 copies/μL and a minimum simulated sample concentration of 10² PFU/mL could be detected. Compared with the first round PCR, the sensitivity was increased by 10 000 times. 9 different viruses were detected at the same time and only LBBV showed bright specific bands. Among the 304 diseased fish samples, 14 positive samples were detected in the first round PCR with a detection rate of 4.60% and 28 positive samples were detected in the nested RT-PCR with a detection rate of 9.21%. The nested RT-PCR assay established in this study has high sensitivity and good specificity, which can be used for the early detection and prevention of LBBV.

Abstract:A white spot syndrome virus strain (Cambarus clarkii whispovirus, WSSV-Cc or Cc strain) was isolated from naturally infected and moribund freshwater crayfish (C. clarkii or Procambarus clarkii, P. clarkii), which is a new strain with a smaller genome. In order to find the footprints in the genome generated during virus evolution, microscopic and ultramicroscopic investigations, genome architecture and phylogenetic analysis, and specific gene amplification were performed. The products of six genes (74L, 86L, 87R, 88R, 92R and 95R) of Cc strain were selected to construct a phylogenetic tree with their homologs from eight white spot virus strains. Results showed that the strains can be divided into two clusters: one included P. clarkii nimavirus (Cc, CN02, and Pc) and the other contained marine shrimp nimaviruses (CN, CN01, CN03, CN04, TW and KR). Multiple sequence alignments of homologous proteins from different viral strains showed that Cc-87R is significantly different from its homologous membrane proteins of marine shrimp nimavirus. It lacks the transmembrane domain (TMD) and its adjacent 287 aa sequence, but still has a complete PI3K_rbd region. Further nucleic acid amplification was performed with two pairs of primers, 87R-F/87R-R and 238-F/87R-R, using the genomes of P. clarkii nimavirus Cc strain and marine shrimp nimavirus CN strain as templates respectively. A fragment with size of 709 bp containing the entire sequence of Cc-87R was amplified from the Cc genomic template. However, the fragments with size of 1 600 and 4 810 bp, only containing partial sequence of Cc-87R, were amplified from the template of the CN strain. The results provided experimental evidence that Cc-87R is a footprint with unique sequence structure of the Cc strain. This finding will benefit the detection of the P. clarkii nimavirus pathogen and the warning of its epidemic trend.

Abstract:Members of the interferon regulatory factor (IRF) family are essential transcription factors that regulate the expression of IFN genes individually and collectively in viral-infected cells. IRF family proteins are structurally conserved. The N-terminal DBD domain endows IRF proteins with the binding affinity to IFN gene promoter DNAs, while the C-terminal IAD domains mediate protein-protein interaction, e. g. interaction with other IRFs or transcription factors from other families, leading to functional specificity and diversity of IRF proteins. That is, when a given cell type is infected with a given type of virus, different signaling pathways will be triggered, where different IRF members are activated to turn on the transcription of different IFN genes. Mammalian IRF family includes 9 members, IRF1-9, and a total of 11 members are identified in fish IRF family. In addition to IRF1~9, fish genomes possess teleost-specific IRF11, and IRF10, which also exists in bird genomes. As a transcription factor, IRF proteins have to enter cellular nucleus first, followed by initiation of IFN gene transcription. In mammals, accumulated evidence has shown that IRF1/3/5/7/9, although localized to cytoplasm or nucleus in resting cells, positively regulate the transcription of IFN genes upon virus infection, but IRF2 exerts a negative role. In the past two decades, significant progresses have been made on molecular cloning, expression characterization and function analyses of fish IRF family genes, although these conclusions are often drawn from different fish species or different experiment systems. It is noted that only in vitro data have been acquired, and future work needs to focus on in vivo studies regarding the physiological function of fish IRF members. This review summarizes the recent mechanism research progresses on expression characterization, subcellular localization and molecular regulation of fish IRF family genes during IFN antiviral immune response.