TBK1-like transcript negatively regulates the production of IFN and IFN-stimulated genes through RLRs-MAVS-TBK1 pathway

Nile tilapia TBK1 interacts with STING and TRAF3 and is involved in the IFN-β pathway in the immune response

Nile tilapia (Oreochromis niloticus) occupies an important position in the culture of economic fish in China. However, the high mortality caused by streptococcal disease has had a significant impact on the tilapia farming industry. Therefore, it is necessary to clarify the immune mechanism of tilapia in response to Streptococcus agalactiae. As a hub in the natural immune signaling pathway, the junction molecule can help the organism defend against and clear pathogens and is crucial in the signaling pathway. In this study, the cDNA sequence of Nile tilapia TBK1 was cloned, and the expression profile was examined in normal fish and challenged fish. The cDNA sequence of the TBK1 gene was 3378 bp, and its open reading frame (ORF) was 2172 bp, encoding 723 amino acids. The deduced TBK1 protein contained an S_TKc domain, a coiled coil domain and a ubiquitin-like domain (ULD). TBK1 had the highest homology with zebra mbuna (Maylandia zebra) and Lake Malawi cichlid fish (Astatotilapia calliptera), both at 97.59%. In the phylogenetic tree, TBK1 forms a large branch with other scleractinian fish. TBK1 expression was highest in the brain and lowest in the liver. LPS, Poly I:C, and S. agalactiae challenge resulted in significant changes in TBK1 expression in the tissues examined. The subcellular localization showed that TBK1-GFP was distributed in the cytoplasm and could significantly increase IFN-β activation. Pull-down results showed that there was an interaction between TBK1 and TRAF3 and an interaction between STING protein and TBK1 protein. The above results provide a basis for further investigation into the mechanism of TBK1 involvement in the signaling pathway.

Non-coding RNAs targeting NF-κB pathways in aquatic animals: A review

Nuclear factor-kappa B (NF-κB) pathways have a close relationship with many diseases, especially in terms of the regulation of inflammation and the immune response. Non-coding RNAs (ncRNAs) are a heterogeneous subset of endogenous RNAs that directly affect cellular function in the absence of proteins or peptide products; these include microRNAs (miRNAs), long noncoding RNAs (lncRNAs), circular RNAs (circRNAs), etc. Studies on the roles of ncRNAs in targeting the NF-κB pathways in aquatic animals are scarce. A few research studies have confirmed detailed regulatory mechanisms among ncRNAs and the NF-κB pathways in aquatic animals. This comprehensive review is presented concerning ncRNAs targeting the NF-κB pathway in aquatic animals and provides new insights into NF-κB pathways regulatory mechanisms of aquatic animals. The review discusses new possibilities for developing non-coding-RNA-based antiviral applications in fisheries.

Histone H2A Nuclear/Cytoplasmic Trafficking Is Essential for Negative Regulation of Antiviral Immune Response and Lysosomal Degradation of TBK1 and IRF3

Histone H2A is a nuclear molecule tightly associated in the form of the nucleosome. Our previous studies have demonstrated the antibacterial property of piscine H2A variants against gram-negative bacteria Edwardsiella piscicida and Gram-positive bacteria Streptococcus agalactiae. In this study, we show the function and mechanism of piscine H2A in the negative regulation of RLR signaling pathway and host innate immune response against spring viremia of carp virus (SVCV) infection. SVCV infection significantly inhibits the expression of histone H2A during an early stage of infection, but induces the expression of histone H2A during the late stage of infection such as at 48 and 72 hpi. Under normal physiological conditions, histone H2A is nuclear-localized. However, SVCV infection promotes the migration of histone H2A from the nucleus to the cytoplasm. The in vivo studies revealed that histone H2A overexpression led to the increased expression of SVCV gene and decreased survival rate. The overexpression of histone H2A also significantly impaired the expression levels of those genes involved in RLR antiviral signaling pathway. Furthermore, histone H2A targeted TBK1 and IRF3 to promote their protein degradation via the lysosomal pathway and impair the formation of TBK1-IRF3 functional complex. Importantly, histone H2A completely abolished TBK1-mediated antiviral activity and enormously impaired the protein expression of IRF3, especially nuclear IRF3. Further analysis demonstrated that the inhibition of histone H2A nuclear/cytoplasmic trafficking could relieve the protein degradation of TBK1 and IRF3, and blocked the negative regulation of histone H2A on the SVCV infection. Collectively, our results suggest that histone H2A nuclear/cytoplasmic trafficking is essential for negative regulation of RLR signaling pathway and antiviral immune response in response to SVCV infection.

TBK1 regulates the induction of innate immune response against GCRV by phosphorylating IRF3 in rare minnow (Gobiocypris rarus)

Rare minnow (Gobiocypris rarus), a small cyprinid species that is highly sensitive to the grass carp reovirus (GCRV), is regarded as an ideal model to study the mechanisms of innate immunity in fish. In the present study, a TBK1 homologue from rare minnow (GrTBK1) was identified and its roles in defence against viral infection were investigated. Sequence analysis showed that GrTBK1 encoded a 727-amino acid peptide which shared 98% and 72% identity to the black carp (Mylopharyngodon piceus) and human (Homo sapiens) orthologues, respectively. The amino acid sequence analysis demonstrated that GrTBK1 contains a conserved Serine/Threonine protein kinases catalytic domain (S_TKc) at the N-terminus. Furthermore, cellular distribution proved that GrTBK1 was located in the cytoplasm region. Quantitative real-time PCR analysis revealed that GrTBK1 was ubiquitously expressed in all examined organs, but especially highly in liver. Temporal expression analysis in vivo showed that the expression levels of GrTBK1 were obviously up-regulated in response to GCRV infection. Meanwhile, qRT-PCR assay revealed that the levels of S7 RNA, an important segment of GCRV genome, were higher in the liver than in other tissues. This indicates that GrTBK1 might play a crucial role in responses to GCRV infection in fish. In addition, GrTBK1 activated several type I interferon (IFN) promoters and induced the expression of downstream type I IFN-stimulated genes (ISGs). Furthermore, GrTBK1 obviously phosphorylated the interferon regulatory factor 3 (IRF3). Furthermore, overexpression of GrTBK1 remarkably decreased the GCRV proliferation. In summary, we systematically characterized GrTBK1 and illustrated its role in the innate immune response to GCRV infections.

The Sp1-Responsive microRNA-15b Negatively Regulates Rhabdovirus-Triggered Innate Immune Responses in Lower Vertebrates by Targeting TBK1

As is known to all, the production of type I interferon (IFN) plays pivotal roles in host innate antiviral immunity, and its moderate production play a positive role in promoting the activation of host innate antiviral immune response. However, the virus will establish a persistent infection model by interfering with the production of IFN, thereby evading the organism inherent antiviral immune response. Therefore, it is of great necessity to research the underlying regulatory mechanisms of type I IFN appropriate production under viral invasion. In this study, we report that a Sp1–responsive miR-15b plays a negative role in siniperca chuatsi rhabdovirus (SCRV)-triggered antiviral response in teleost fish. We found that SCRV could dramatically upregulate miiuy croaker miR-15b expression. Enhanced miR-15b could negatively regulate SCRV-triggered antiviral genes and inflammatory cytokines production by targeting TANK-binding kinase 1 (TBK1), thereby accelerating viral replication. Importantly, we found that miR-15b feedback regulates antiviral innate immune response through NF-κB and IRF3 signaling pathways. These findings highlight that miR-15b plays a crucial role in regulating virus–host interactions, which outlines a new regulation mechanism of fish’s innate immune responses.

Immunogene expression analysis in betanodavirus infected-Senegalese sole using an OpenArray® platform

The transcriptomic response of Senegalese sole (Solea senegalensis) triggered by two betanodaviruses with different virulence to that fish species has been assessed using an OpenArray® platform based on TaqMan™ quantitative PCR. The transcription of 112 genes per sample has been evaluated at two sampling times in two organs (head kidney and eye/brain-pooled samples). Those genes were involved in several roles or pathways, such as viral recognition, regulation of type I (IFN-1)-dependent immune responses, JAK-STAT cascade, interferon stimulated genes, protein ubiquitination, virus responsive genes, complement system, inflammatory response, other immune system effectors, regulation of T-cell proliferation, and proteolysis and apoptosis. The highly virulent isolate, wSs160.3, a wild type reassortant containing a RGNNV-type RNA1 and a SJNNV-type RNA2 segments, induced the expression of a higher number of genes in both tested organs than the moderately virulent strain, a recombinant harbouring mutations in the protruding domain of the capsid protein. The number of differentially expressed genes was higher 2 days after the infection with the wild type isolate than at 3 days post-inoculation. The wild type isolate also elicited an exacerbated interferon 1 response, which, instead of protecting sole against the infection, increases the disease severity by the induction of apoptosis and inflammation-derived immunopathology, although inflammation seems to be modulated by the complement system. Furthermore, results derived from this study suggest a potential important role for some genes with high expression after infection with the highly virulent virus, such as rtp3, sacs and isg15. On the other hand, the infection with the mutant does not induce immune response, probably due to an altered recognition by the host, which is supported by a different viral recognition pathway, involving myd88 and tbkbp1.

Grass Carp Reovirus (GCRV) Giving Its All to Suppress IFN Production by Countering MAVS Signaling Transduction

Viruses typically target host RIG-I-like receptors (RLRs), a group of key factors involved in interferon (IFN) production, to enhance viral infection. To date, though immune evasion methods to contradict IFN production have been characterized for a series of terrestrial viruses, the strategies employed by fish viruses remain unclear. Here, we report that all grass carp reovirus (GCRV) proteins encoded by segments S1 to S11 suppress mitochondrial antiviral signaling protein (MAVS)-mediated IFN expression. First, the GCRV viral proteins blunted the MAVS-induced expression of IFN, and impair MAVS antiviral capacity significantly. Interestingly, subsequent co-immunoprecipitation experiments demonstrated that all GCRV viral proteins interacted with several RLR cascades, especially with TANK-binding kinase 1 (TBK1) which was the downstream factor of MAVS. To further illustrate the mechanisms of these interactions between GCRV viral proteins and host RLRs, two of the viral proteins, NS79 (S4) and VP3 (S3), were selected as representative proteins for two distinguished mechanisms. The obtained data demonstrated that NS79 was phosphorylated by gcTBK1, leading to the reduction of host substrate gcIRF3/7 phosphorylation. On the other hand, VP3 degraded gcMAVS and the degradation was significantly reversed by 3-MA. The biological effects of both NS79 and VP3 were consistently found to be related to the suppression of IFN expression and the promotion of viral evasion. Our findings shed light on the special evasion mechanism utilized by fish virus through IFN regulation, which might differ between fish and mammals.

GCRV hijacks TBK1 to evade IRF7-mediated antiviral immune responses in grass carp Ctenopharyngodon idella

TANK-binding kinase 1 (TBK1) is an important kinase that regulates the activation of interferon regulatory factor 3/7 (IRF3/7) to induce type I interferon (IFN-I) production in antiviral immune responses. However, in long-term virus-host crosstalk, viruses have evolved elaborate strategies to evade host immune defense mechanisms. In the present study, we found that grass carp (Ctenopharyngodon idella) reovirus (GCRV) hijacks TBK1 to escape IRF7-IFN-Is signaling activation. In brief, GCRV inhibited TBK1 activation by restaining K63-linked ubiquitination of TBK1 and promoting its K48-linked ubiquitination. This regulation resulted in that under low titer of GCRV infection, TBK1 overexpression specifically supressed promoter activity and phosphorylation of IRF7 and induction of downstream IFN1and IFN3. qRT-PCR data uncovered that TBK1 negatively regulated IRF7, IFN1 and IFN3 transcription levels under low viral titer infection. Along with enhancement of GCRV titers, TBK1 swiched its function to up-regulate IRF7, IFN1 and IFN3 mRNA levels. Accordingly, TBK1 promoted GCRV replication at low infected titer, but inhibited GCRV replication at high infected titer. All these results revealed a viral evasion strategy that GCRV utilizes TBK1 to block cellular IFN responses at low titers or early stages in fish species, which will lay a foundation for further researching on host-virus interactions and developing novel antiviral strategies in lower vertebrates.

Ctenopharyngodon idella TBK1 activates innate immune response via IRF7

In mammals, IFN regulatory factor (IRF) 7 is a central regulator of IFN-α expression in response to variable pathogenic infections. There are several pathogenic sensors involved in monitoring pathogen intrusion in mammals. These sensors trigger IRF7-mediated responses through different pathways. TANK-binding kinase 1 (TBK1) is a critical mediator of IRF7 activation upon pathogen infection. In fish, there are many reports on TBK1, IRF3 and IRF7, especially on TBK1-IRF3 signaling pathway. However, it is not very clear how TBK1-IRF7 works in innate immune signaling pathway. In this study, we explored how TBK1 up-regulates IFN, ISG expression, and how TBK1 initiates innate immune response through IRF7 in fish under lipopolysaccharides (LPS) stimulation. After stimulation with LPS, grass carp IRF3 and IRF7 transcriptions were up-regulated, indicating they participate in TLR-mediated antiviral signaling pathway. It is interesting that the response time of grass carp IRF3 to LPS was earlier than that of IRF7. In addition, IRF7 rather than IRF3 acted as a stronger positive regulator of IFN and ISG transcription in Ctenopharyngodon idella kidney cells (CIKs). It is suggested the potential function differentiation between IRF3 and IRF7 upon LPS infection in fish. Dual luciferase assays also showed that overexpression of grass carp IRF7 and TBK1 up-regulated the transcription level of IFN and PKR. However, knockdown of IRF7 inhibits ISG expression, suggesting that grass carp TBK1 regulates the transcription via IRF7. Co-immunoprecipitation and GST pull-down assays proved the binding of grass carp IRF7 to TBK1. Furthermore, grass carp TBK1 can promote the nuclear translocation of IRF7. The results indicated that grass carp TBK1 can bind directly to and activate IRF7.

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.

TANK-Binding Kinase 1 (TBK1) Isoforms Negatively Regulate Type I Interferon Induction by Inhibiting TBK1-IRF3 Interaction and IRF3 Phosphorylation

TANK-binding kinase 1 (TBK1) is an important serine/threonine-protein kinase that mediates phosphorylation and nuclear translocation of IRF3, which contributes to induction of type I interferons (IFNs) in the innate antiviral response. In mammals, TBK1 spliced isoform negatively regulates the virus-triggered IFN-β signaling pathway by disrupting the interaction between retinoic acid-inducible gene I (RIG-I) and mitochondria antiviral-signaling protein (MAVS). However, it is still unclear whether alternative splicing patterns and the function of TBK1 isoform(s) exist in teleost fish. In this study, we identify two alternatively spliced isoforms of TBK1 from zebrafish, termed TBK1_tv1 and TBK1_tv2. Both TBK1_tv1 and TBK1_tv2 contain an incomplete STKc_TBK1 domain. Moreover, the UBL_TBK1_like domain is also missing for TBK1_tv2. TBK1_tv1 and TBK1_tv2 are expressed in zebrafish larvae. Overexpression of TBK1_tv1 and TBK1_tv2 inhibits RIG-I-, MAVS-, TBK1-, and IRF3-mediated activation of IFN promoters in response to spring viremia of carp virus infection. Also, TBK1_tv1 and TBK1_tv2 inhibit expression of IFNs and IFN-stimulated genes induced by MAVS and TBK1. Mechanistically, TBK1_tv1 and TBK1_tv2 competitively associate with TBK1 and IRF3 to disrupt the formation of a functional TBK1-IRF3 complex, impeding the phosphorylation of IRF3 mediated by TBK1. Collectively, these results demonstrate that TBK1 spliced isoforms are dominant negative regulators in the RIG-I/MAVS/TBK1/IRF3 antiviral pathway by targeting the functional TBK1-IRF3 complex formation. Identification and functional characterization of piscine TBK1 spliced isoforms may contribute to understanding the role of TBK1 expression in innate antiviral response.

TBK1 of black carp plays an important role in host innate immune response against SVCV and GCRV

Tank-binding kinase 1 (TBK1) plays a pivotal role in the induction of type I IFNs in higher vertebrates. To explore the function of TBK1 in teleost, TBK1 of black carp (Mylopharyngodon Piceus) was cloned and characterized in this paper. The full-length cDNA of black carp TBK1 (bcTBK1) consists of 2857 nucleotides and the predicted bcTBK1 protein contains 727 amino acids, which includes an N-terminal kinase domain (KD), an ubiquitin-like domain (ULD) and two C-terminal coiled-coils. The transcription of bcTBK1 was constitutively detected in all the selected tissues and bcTBK1 mRNA level was increased in all selected tissues in response to SVCV or GCRV infection except that in muscle post GCRV invasion. The transcription of bcTBK1 in Mylopharyngodon Piceus fin (MPF) cells was up-regulated by the stimulation of SVCV, GCRV or poly (I:C) but not by LPS treatment. bcTBK1 migrated around 80 kDa in immunoblot assay and was identified as a cytosolic protein by immunofluorescence staining. bcTBK1 showed strong IFN-inducing ability in reporter assay and presented strong antiviral activity against both GCRV and SVCV in EPC cells. The reporter assay demonstrated that TRAF6 of black carp (bcTRAF6) up-regulated bcTBK1-induced IFN expression and the subcellular distribution of bcTBK1 overlapped with that of bcTRAF6 when these two proteins were co-expressed in EPC cells. Taken together, our study support the conclusion that bcTBK1 plays an important role in the antiviral innate immune response of black carp against SVCV and GCRV, in which its activity was positively regulated by bcTRAF6.

Fish TRIM35 negatively regulates the interferon signaling pathway in response to grouper nodavirus infection

Tripartite motif-containing protein 35 (TRIM35) has been demonstrated to exert critical roles in cancer, cell death and other multiple cell processes. However, the precisely roles of TRIM35 during virus infection still remained largely unknown. In the current study, we cloned a TRIM35 gene from orange spotted grouper (EcTRIM35) and uncovered its roles in response to nodavirus infection. EcTRIM35 encoded a 456-aa protein which showed 65% and 32% identity to large yellow croaker (Larimichthys crocea) and human (Homo sapiens), respectively. Structure prediction and amino acid alignment analysis indicated that EcTRIM35 contained three conserved domains, including RING domain, B-BOX and SPRY domain. In healthy grouper, the high expression level of EcTRIM35 could be detected in liver, spleen and intestine. After infection with red-spotted grouper nervous necrosis (RGNNV) and Singapore grouper iridovirus (SGIV) in GS cells, the transcript of EcTRIM35 was significantly up-regulated with the infection time increased. Under fluorescence microscopy, the bright fluorescence aggregates were observed in EcTRIM35 transfected cells, but the fluorescence distribution was obviously altered in the EcTRIM35-ΔRING transfected cells. After incubation with RGNNV, the overexpression of EcTRIM35 in vitro significantly enhanced the viral replication, evidenced by the enhancement of cytopathic effect (CPE) severity and the up-regulation of the viral gene transcription. Moreover, the ectopic expression of EcTRIM35 significantly decreased the expression of interferon signaling molecules or effectors. Further studies elucidated that EcTRIM35 overexpression significantly weakened the MAVS-, MITA- or TBK1-induced interferon immune response, but showed no effects on MDA5-induced immune response. Thus, our results will shed new lights on the roles of fish TRIM35 in innate immune response against grouper virus infection.

Comparative transcriptome analysis of zebrafish (Danio rerio) brain and spleen infected with spring viremia of carp virus (SVCV)

Spring viremia of carp virus (SVCV) is the pathogen of spring viremia of carp (SVC) and often causes acute hemorrhagic symptoms in various kinds of cyprinids and induces serious environmental and economic losses. However, the molecular mechanisms of infection remain poorly understood, especially at the individual level. In this study, zebrafish was employed as the infection model to explore the pathogenesis of SVCV. 4 groups of zebrafish tissues were set and RNA sequencing (RNA-Seq) technology was employed to analyze the differentially expressed genes (DEGs) after SVCV-infection. A total of 360,971,498 clean reads were obtained from 12 samples, 382 DEGs in the brain and 926 DEGs in the spleen were identified. These DEGs were annotated into three ontologies after gene ontology (GO) enrichment analysis. The Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis showed that these DEGs were primarily related to Influenza A pathway and Herpes simplex infection pathway in brain and Tuberculosis and Toxoplasmosis pathways in spleen, and all of these pathways may be involved in response to pathogen invasion. At the same time, 3' and 5' alternative splicing (AS) events were significantly up-regulated in the spleen. The transcriptome analysis results demonstrated changes and tissue-specific influences caused by SVCV in vivo, which provided us with more information to understand the complex relationships between SVCV and its host.

Alternative Pre-mRNA Splicing in Mammals and Teleost Fish: A Effective Strategy for the Regulation of Immune Responses Against Pathogen Infection

Pre-mRNA splicing is the process by which introns are removed and the protein coding elements assembled into mature mRNAs. Alternative pre-mRNA splicing provides an important source of transcriptome and proteome complexity through selectively joining different coding elements to form mRNAs, which encode proteins with similar or distinct functions. In mammals, previous studies have shown the role of alternative splicing in regulating the function of the immune system, especially in the regulation of T-cell activation and function. As lower vertebrates, teleost fish mainly rely on a large family of pattern recognition receptors (PRRs) to recognize pathogen-associated molecular patterns (PAMPs) from various invading pathogens. In this review, we summarize recent advances in our understanding of alternative splicing of piscine PRRs including peptidoglycan recognition proteins (PGRPs), nucleotide binding and oligomerization domain (NOD)-like receptors (NLRs), retinoic acid-inducible gene-I (RIG-I)-like receptors (RLRs) and their downstream signaling molecules, compared to splicing in mammals. We also discuss what is known and unknown about the function of splicing isoforms in the innate immune responses against pathogens infection in mammals and teleost fish. Finally, we highlight the consequences of alternative splicing in the innate immune system and give our view of important directions for future studies.

Mitophagy and Alzheimer's Disease: Cellular and Molecular Mechanisms

Neurons affected in Alzheimer's disease (AD) experience mitochondrial dysfunction and a bioenergetic deficit that occurs early and promotes the disease-defining amyloid beta peptide (Aβ) and Tau pathologies. Emerging findings suggest that the autophagy/lysosome pathway that removes damaged mitochondria (mitophagy) is also compromised in AD, resulting in the accumulation of dysfunctional mitochondria. Results in animal and cellular models of AD and in patients with sporadic late-onset AD suggest that impaired mitophagy contributes to synaptic dysfunction and cognitive deficits by triggering Aβ and Tau accumulation through increases in oxidative damage and cellular energy deficits; these, in turn, impair mitophagy. Interventions that bolster mitochondrial health and/or stimulate mitophagy may therefore forestall the neurodegenerative process in AD.

Retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs) in fish: current knowledge and future perspectives

Retinoic acid-inducible gene I (RIG-I) -like receptors (RLRs) are found conservatively present in teleost fish. All three members, RIG-I, MDA5 and LGP2, together with the downstream molecules such as MITA, TRAF3 and TBK1, have been identified in a range of fish species. However, it is unexpected that RIG-I has not been reported in fish of Acanthopterygii, and it would be important to clarify the presence and role of the RIG-I gene in a broad range of taxa in Teleostei. RLRs in fish can be induced in vivo and in vitro by viral pathogens as well as synthetic dsRNA, poly(I:C), leading to the production of type I interferons (IFNs) and the expression of IFN-stimulated genes (ISGs). Bacterial pathogens, such as Edwardsiella tarda, and their components, such as lipopolysaccharide are also found to induce the expression of RLRs, and whether such induction was mediated through the direct recognition by RLRs or through crosstalk with other pattern recognition receptors recognizing directly bacterial pathogen-associated molecular patterns awaits to be investigated. On the other hand, RLR-activated type I IFN production can be negatively regulated in fish by molecules, such as TBK-1-like protein and IRF10, which are found to negatively regulate RIG-I and MAVS-activated type I IFN production, and to block MITA or bind ISRE motifs, respectively. It is considered that the evolutionary occurrence of RLRs in fish, and their recognized ligands, especially those from their fish pathogens, as well as the mechanisms involved in the RLR signalling pathways, are of significant interest for further investigation.