Roles of RIG-I N-terminal tandem CARD and splice variant in TRIM25-mediated antiviral signal transduction

Genome-wide analysis of differentially expressed mRNAs, lncRNAs, and circRNAs in chicken bursae of Fabricius during infection with very virulent infectious bursal disease virus

Background

Infectious bursal disease virus (IBDV) causes acute, highly contagious, immunosuppressive, and lethal infectious disease in young chickens and mainly infects the bursa of Fabricius (BF). To investigate interactions between IBDV and its host, RNA sequencing was applied to analyze the responses of the differentially expressed transcriptional profiles of BF infected by very virulent IBDV (vvIBDV).

Results

In total, 317 upregulated and 94 downregulated mRNAs were found to be significantly differentially expressed in infected chickens, compared to controls. Long non-coding RNA (lncRNA) and circular RNA (circRNA) alterations were identified in IBDV-infected chickens, and significantly different expression was observed in 272 lncRNAs and 143 circRNAs. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analyses were performed to assess the functions of significantly dysregulated genes, which showed that the JAK-STAT signaling pathway, the NOD-like receptor signaling pathway, and apoptosis may be activated by IBDV infection. We predicted interactions between differentially expressed genes and produced lncRNA-mRNA and circRNA-miRNA-mRNA regulator network.

Conclusions

The present study identified the expression profiles of mRNAs, lncRNAs, and circRNAs during vvIBDV infection and provides new insights into the pathogenesis of IBDV and antiviral immunity of the host.

Transcriptome analysis reveals seven key immune pathways of Japanese flounder (Paralichthys olivaceus) involved in megalocytivirus infection

Megalocytivirus is a serious viral pathogen to many farmed fish including Japanese flounder (Paralichthys olivaceus). In this study, in order to systematically identify host immune genes induced by megalocytivirus infection, we examined the transcription profiles of flounder infected by megalocytivirus for 2, 6, and 8 days. Compared with uninfected fish, virus-infected fish exhibited 1242 differentially expressed genes (DEGs), with 225, 275, and 877 DEGs occurring at 2, 6, and 8 days post infection, respectively. Of these DEGs, 728 were upregulated and 659 were downregulated. The majority of DEGs were time-specific and formed four distinct expression profiles well correlated with the time of infection. The DEGs were classified into diverse Gene Ontology (GO) functional terms and enriched in 27 Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways, approximately one third of which were related to immunity. Weighted co-expression network analysis (WGCNA) was used to identify 16 key immune DEGs belonging to seven immune pathways (RIG-I-like receptor signaling pathway, JAK-STAT signaling pathway, TLR signaling pathway, cytokine-cytokine receptor interaction, phagosome, apoptosis, and p53 signaling pathway). These pathways interacted extensively and formed complicated networks. This study provided a global picture of megalocytivirus-induced gene expression profiles of flounder at the transcriptome level and uncovered a set of key immune genes and pathways closely linked to megalocytivirus infection. These results provided a set of targets for future delineation of the key factors implicated in the anti-megalocytivirus immunity of flounder.

Viral Ubiquitin and Ubiquitin-Like Deconjugases-Swiss Army Knives for Infection

Posttranslational modifications of cellular proteins by covalent conjugation of ubiquitin and ubiquitin-like polypeptides regulate numerous cellular processes that are captured by viruses to promote infection, replication, and spreading. The importance of these protein modifications for the viral life cycle is underscored by the discovery that many viruses encode deconjugases that reverse their functions. The structural and functional characterization of these viral enzymes and the identification of their viral and cellular substrates is providing valuable insights into the biology of viral infections and the host's antiviral defense. Given the growing body of evidence demonstrating their key contribution to pathogenesis, the viral deconjugases are now recognized as attractive targets for the design of novel antiviral therapeutics.

Distinct and Orchestrated Functions of RNA Sensors in Innate Immunity

Faithful maintenance of immune homeostasis relies on the capacity of the cellular immune surveillance machinery to recognize "nonself", such as the presence of pathogenic RNA. Several families of pattern-recognition receptors exist that detect immunostimulatory RNA and then induce cytokine-mediated antiviral and proinflammatory responses. Here, we review the distinct features of bona fide RNA sensors, Toll-like receptors and retinoic-acid inducible gene-I (RIG-I)-like receptors in particular, with a focus on their functional specificity imposed by cell-type-dependent expression, subcellular localization, and ligand preference. Furthermore, we highlight recent advances on the roles of nucleotide-binding oligomerization domain (NOD)-like receptors and DEAD-box or DEAH-box RNA helicases in an orchestrated RNA-sensing network and also discuss the relevance of RNA sensor polymorphisms in human disease.

A RIG-I-like receptor directs antiviral responses to a bunyavirus and is antagonized by virus-induced blockade of TRIM25-mediated ubiquitination

The RIG-I-like receptors (RLRs) retinoic acid-inducible gene I protein (RIG-I) and melanoma differentiation-associated protein 5 (MDA5) are cytosolic pattern recognition receptors that recognize specific viral RNA products and initiate antiviral innate immunity. Severe fever with thrombocytopenia syndrome virus (SFTSV) is a highly pathogenic member of the Bunyavirales RIG-I, but not MDA5, has been suggested to sense some bunyavirus infections; however, the roles of RLRs in anti-SFTSV immune responses remain unclear. Here, we show that SFTSV infection induces an antiviral response accompanied by significant induction of antiviral and inflammatory cytokines and that RIG-I plays a main role in this induction by recognizing viral 5'-triphosphorylated RNAs and by signaling via the adaptor mitochondrial antiviral signaling protein. Moreover, MDA5 may also sense SFTSV infection and contribute to IFN induction, but to a lesser extent. We further demonstrate that the RLR-mediated anti-SFTSV signaling can be antagonized by SFTSV nonstructural protein (NSs) at the level of RIG-I activation. Protein interaction and MS-based analyses revealed that NSs interacts with the host protein tripartite motif-containing 25 (TRIM25), a critical RIG-I-activating ubiquitin E3 ligase, but not with RIG-I or Riplet, another E3 ligase required for RIG-I ubiquitination. NSs specifically trapped TRIM25 into viral inclusion bodies and inhibited TRIM25-mediated RIG-I-Lys-63-linked ubiquitination/activation, contributing to suppression of RLR-mediated antiviral signaling at its initial stage. These results provide insights into immune responses to SFTSV infection and clarify a mechanism of the viral immune evasion, which may help inform the development of antiviral therapeutics.

Comparative characterization of flavivirus production in two cell lines: Human hepatoma-derived Huh7.5.1-8 and African green monkey kidney-derived Vero

The Flaviviridae is a family of enveloped viruses with a positive-sense single-stranded RNA genome. It contains many viruses that threaten human health, such as Japanese encephalitis virus (JEV) and yellow fever virus (YFV) of the genus Flavivirus as well as hepatitis C virus of the genus Hepacivirus. Cell culture systems highly permissive for the Flaviviridae viruses are very useful for their isolation, propagation, and diagnosis, an understanding of their biology, and the development of vaccines and antiviral agents. Previously, we isolated a human hepatoma HuH-7-derived cell clone, Huh7.5.1-8, which is highly permissive to hepatitis C virus infection. Here, we have characterized flavivirus infection in the Huh7.5.1-8 cell line by comparing with that in the African green monkey kidney-derived Vero cell line, which is permissive for a wide spectrum of viruses. Upon infection with JEV, Huh7.5.1-8 cells produced a higher amount of virus particles early in infection and were more susceptible to virus-induced cell death than Vero cells. Similar outcomes were obtained when the cells were infected with another flavivirus, YFV (17D-204 strain). Quantification of cellular and extracellular viral RNA revealed that high JEV production in Huh7.5.1-8 cells can be attributed to rapid viral replication kinetics and efficient virus release early in infection. In a plaque assay, Huh7.5.1-8 cells developed JEV plaques more rapidly than Vero cells. Although this was not the case with YFV plaques, Huh7.5.1-8 cells developed higher numbers of YFV plaques than Vero cells. Sequence analysis of cDNA encoding an antiviral RNA helicase, RIG-I, showed that Huh7.5.1-8 cells expressed not only a full-length RIG-I mRNA with a known dominant-negative missense mutation but also variants without the mutation. However, the latter mRNAs lacked exon 5/6-12, indicating functional loss of RIG-I in the cells. These characteristics of the Huh7.5.1-8 cell line are helpful for flavivirus detection, titration, and propagation.

Influenza PB1-F2 Inhibits Avian MAVS Signaling

RIG-I plays an essential role in the duck innate immune response to influenza infection. RIG-I engages the critical adaptor protein mitochondrial antiviral signaling (MAVS) to activate the downstream signaling pathway. The influenza A virus non-structural protein PB1-F2 interacts with MAVS in human cells to inhibit interferon production. As duck and human MAVS share only 28% amino acid similarity, it is not known whether the influenza virus can similarly inhibit MAVS signaling in avian cells. Using confocal microscopy we show that MAVS and the constitutively active N-terminal end of duck RIG-I (2CARD) co-localize in DF-1 cells, and duck MAVS is pulled down with GST-2CARD. We establish that either GST-2CARD, or duck MAVS can initiate innate signaling in chicken cells and their co-transfection augments interferon-beta promoter activity. Demonstrating the limits of cross-species interactions, duck RIG-I 2CARD initiates MAVS signaling in chicken cells, but works poorly in human cells. The D122A mutation of human 2CARD abrogates signaling by affecting MAVS engagement, and the reciprocal A120D mutation in duck 2CARD improves signaling in human cells. We show mitochondrial localization of PB1-F2 from influenza A virus strain A/Puerto Rico/8/1934 (H1N1; PR8), and its co-localization and co-immunoprecipitation with duck MAVS. PB1-F2 inhibits interferon-beta promoter activity induced by overexpression of either duck RIG-I 2CARD, full-length duck RIG-I, or duck MAVS. Finally, we show that the effect of PB1-F2 on mitochondria abrogates TRIM25-mediated ubiquitination of RIG-I CARD in both human and avian cells, while an NS1 variant from the PR8 influenza virus strain does not.

NOD1 Promotes Antiviral Signaling by Binding Viral RNA and Regulating the Interaction of MDA5 and MAVS

Nucleotide oligomerization domain-like receptors (NLRs) and RIG-I-like receptors (RLRs) detect diverse pathogen-associated molecular patterns to activate the innate immune response. The role of mammalian NLR NOD1 in sensing bacteria is well established. Although several studies suggest NOD1 also plays a role in sensing viruses, the mechanisms behind this are still largely unknown. In this study, we report on the synergism and antagonism between NOD1 and MDA5 isoforms in teleost. In zebrafish, the overexpression of NOD1 enhances the antiviral response and mRNA abundances of key antiviral genes involved in RLR-mediated signaling, whereas the loss of NOD1 has the opposite effect. Notably, spring viremia of carp virus-infected NOD1^-/- zebrafish exhibit reduced survival compared with wild-type counterparts. Mechanistically, NOD1 targets MDA5 isoforms and TRAF3 to modulate the formation of MDA5-MAVS and TRAF3-MAVS complexes. The cumulative effects of NOD1 and MDA5a (MDA5 normal form) were observed for the binding with poly(I:C) and the formation of the MDA5a-MAVS complex, which led to increased transcription of type I IFNs and ISGs. However, the antagonism between NOD1 and MDA5b (MDA5 truncated form) was clearly observed during proteasomal degradation of NOD1 by MDA5b. In humans, the interactions between NOD1-MDA5 and NOD1-TRAF3 were confirmed. Furthermore, the roles that NOD1 plays in enhancing the binding of MDA5 to MAVS and poly(I:C) are also evolutionarily conserved across species. Taken together, our findings suggest that mutual regulation between NOD1 and MDA5 isoforms may play a crucial role in the innate immune response and that NOD1 acts as a positive regulator of MDA5/MAVS normal form-mediated immune signaling in vertebrates.

Dual targeting of RIG-I and MAVS by MARCH5 mitochondria ubiquitin ligase in innate immunity

The mitochondrial antiviral signaling (MAVS) protein on the mitochondrial outer membrane acts as a central signaling molecule in the RIG-I-like receptor (RLR) signaling pathway by linking upstream viral RNA recognition to downstream signal activation. We previously reported that mitochondrial E3 ubiquitin ligase, MARCH5, degrades the MAVS protein aggregate and prevents persistent downstream signaling. Since the activated RIG-I oligomer interacts and nucleates the MAVS aggregate, MARCH5 might also target this oligomer. Here, we report that MARCH5 targets and degrades RIG-I, but not its inactive phosphomimetic form (RIG-IS8E). The MARCH5-mediated reduction of RIG-I is restored in the presence of MG132, a proteasome inhibitor. Upon poly(I:C) stimulation, RIG-I forms an oligomer and co-expression of MARCH5 reduces the expression of this oligomer. The RING domain of MARCH5 is necessary for binding to the CARD domain of RIG-I. In an in vivo ubiquitination assay, MARCH5 transfers the Lys 48-linked polyubiquitin to Lys 193 and 203 residues of RIG-I. Thus, dual targeting of active RIG-I and MAVS protein oligomers by MARCH5 is an efficient way to switch-off RLR signaling. We propose that modulation of MARCH5 activity might be beneficial for the treatment of chronic immune diseases.

TRIM25 and its emerging RNA-binding roles in antiviral defense

The innate immune system is the body's first line of defense against viruses, with pattern recognition receptors (PRRs) recognizing molecules unique to viruses and triggering the expression of interferons and other anti-viral cytokines, leading to the formation of an anti-viral state. The tripartite motif containing 25 (TRIM25) is an E3 ubiquitin ligase thought to be a key component in the activation of signaling by the PRR retinoic acid-inducible gene I protein (RIG-I). TRIM25 has recently been identified as an RNA-binding protein, raising the question of whether its RNA-binding activity is important for its role in innate immunity. Here, we review TRIM25's mechanisms and pathways in noninfected and infected cells. We also introduce models that explain how TRIM25 binding to RNA could modulate its functions and play part in the antiviral response. These findings have opened new lines of investigations into functional and molecular roles of TRIM25 and other E3 ubiquitin ligases in cell biology and control of pathogenic infections. This article is categorized under: RNA in Disease and Development > RNA in Disease RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications RNA Interactions with Proteins and Other Molecules > Protein-RNA Recognition.

To TRIM the Immunity: From Innate to Adaptive Immunity

The tripartite motif (TRIM) proteins have been intensively studied as essential modulators in various biological processes, especially in regulating a wide range of signaling pathways involved in immune responses. Most TRIM proteins have E3 ubiquitin ligase activity, mediating polyubiquitination of target proteins. Emerging evidence demonstrates that TRIM proteins play important roles in innate immunity by regulating pattern recognition receptors, vital adaptor proteins, kinases, and transcription factors in innate immune signaling pathways. Additionally, the critical roles of TRIM proteins in adaptive immunity, especially in T cell development and activation, are increasingly appreciated. In this review, we aim to summarize the studies on TRIMs in both innate and adaptive immunity, focusing on their E3 ubiquitin ligase functions in pattern recognition receptor signaling pathways and T cell functions, shedding light on the developing new strategies for modulating innate and adaptive immune responses against invading pathogens and avoiding autoimmunity.

Interaction With 14-3-3 Correlates With Inactivation of the RIG-I Signalosome by Herpesvirus Ubiquitin Deconjugases

The hijacking of cellular function through expression of proteins that interfere with the activity of cellular enzymes and regulatory complexes is a common strategy used by viruses to remodel the cell environment in favor of their own replication and spread. Here we report that the ubiquitin deconjugases encoded in the N-terminal domain of the large tegument proteins of Epstein-Barr virus (EBV), Kaposi Sarcoma herpesvirus (KSHV) and human cytomegalovirus (HCMV), but not herpes simplex virus-1 (HSV-1), target an early step of the IFN signaling cascade that involves the formation of a trimolecular complex with the ubiquitin ligase TRIM25 and the 14-3-3 molecular scaffold. Different from other homologs, the HSV-1 encoded enzyme fails to interact with 14-3-3, which correlates with failure to promote the autoubiquitination and sequestration of TRIM25 in cytoplasmic aggregates, and inability to block the activation and nuclear translocation of the IRF3 transcription factor. These findings highlight a key role for 14-3-3 molecular scaffolds in the regulation of innate immune response to herpesvirus infections and points to a possible target for the development of a new type of antivirals with applications in a broad spectrum of human diseases.

The influenza NS1 protein modulates RIG-I activation via a strain-specific direct interaction with the second CARD of RIG-I

A critical role of influenza A virus nonstructural protein 1 (NS1) is to antagonize the host cellular antiviral response. NS1 accomplishes this role through numerous interactions with host proteins, including the cytoplasmic pathogen recognition receptor, retinoic acid-inducible gene I (RIG-I). Although the consequences of this interaction have been studied, the complete mechanism by which NS1 antagonizes RIG-I signaling remains unclear. We demonstrated previously that the NS1 RNA-binding domain (NS1^RBD) interacts directly with the second caspase activation and recruitment domain (CARD) of RIG-I. We also identified that a single strain-specific polymorphism in the NS1^RBD (R21Q) completely abrogates this interaction. Here we investigate the functional consequences of an R21Q mutation on NS1's ability to antagonize RIG-I signaling. We observed that an influenza virus harboring the R21Q mutation in NS1 results in significant up-regulation of RIG-I signaling. In support of this, we determined that an R21Q mutation in NS1 results in a marked deficit in NS1's ability to antagonize TRIM25-mediated ubiquitination of the RIG-I CARDs, a critical step in RIG-I activation. We also observed that WT NS1 is capable of binding directly to the tandem RIG-I CARDs, whereas the R21Q mutation in NS1 significantly inhibits this interaction. Furthermore, we determined that the R21Q mutation does not impede the interaction between NS1 and TRIM25 or NS1^RBD's ability to bind RNA. The data presented here offer significant insights into NS1 antagonism of RIG-I and illustrate the importance of understanding the role of strain-specific polymorphisms in the context of this specific NS1 function.

The role of ZAP and OAS3/RNAseL pathways in the attenuation of an RNA virus with elevated frequencies of CpG and UpA dinucleotides

Zinc finger antiviral protein (ZAP) is a powerful restriction factor for viruses with elevated CpG dinucleotide frequencies. We report that ZAP similarly mediates antiviral restriction against echovirus 7 (E7) mutants with elevated frequencies of UpA dinucleotides. Attenuation of both CpG- and UpA-high viruses and replicon mutants was reversed in ZAP k/o cell lines, and restored by plasmid-derived reconstitution of expression in k/o cells. In pull-down assays, ZAP bound to viral RNA transcripts with either CpG- and UpA-high sequences inserted in the R2 region. We found no evidence that attenuation of CpG- or UpA-high mutants was mediated through either translation inhibition or accelerated RNA degradation. Reversal of the attenuation of CpG-high, and UpA-high E7 viruses and replicons was also achieved through knockout of RNAseL and oligodenylate synthetase 3 (OAS3), but not OAS1. WT levels of replication of CpG- and UpA-high mutants were observed in OAS3 k/o cells despite abundant expression of ZAP, indicative of synergy or complementation of these hitherto unconnected pathways. The dependence on expression of ZAP, OAS3 and RNAseL for CpG/UpA-mediated attenuation and the variable and often low level expression of these pathway proteins in certain cell types, such as those of the central nervous system, has implications for the use of CpG-elevated mutants as attenuated live vaccines against neurotropic viruses.

Know Thyself: RIG-I-Like Receptor Sensing of DNA Virus Infection

The RIG-I-like receptors (RLRs) are double-stranded RNA-binding proteins that play a role in initiating and modulating cell intrinsic immunity through the recognition of RNA features typically absent from the host transcriptome. While they are initially characterized in the context of RNA virus infection, evidence has now accumulated establishing the role of RLRs in DNA virus infection. Here, we review recent advances in the RLR-mediated restriction of DNA virus infection with an emphasis on the RLR ligands sensed.

Identification of a novel RIG-I isoform and its truncating variant in Japanese eel, Anguilla japonica

Retinoic acid-inducible gene-I (RIG-I) is a cytoplasmic viral RNA sensor that triggers the production of type I interferons (IFNs) and proinflammatory cytokines during viral infection. RIG-I gene has been identified previously in Japanese eel, Anguilla japonica. In the present study, we have characterized a novel isoform of RIG-I (designated as AjRIG-Ib) and its truncated variant (AjRIG-Ibv). The AjRIG-Ib encodes 940 amino acids (aa) consisting of two N-terminal caspase activation and recruitment domains (CARDs), a DEX(D/H) box RNA helicase domain, and a C-terminal regulatory domain (CTD). The AjRIG-Ibv encodes a protein of 843 aa, that shares similar structural organization with AjRIG-Ib, but lacking CTD. The gene expression analyses showed that AjRIG-Ib and AjRIG-Ibv were detectable in all tissues/organs examined, and AjRIG-Ib was the predominant form. The mRNA level of AjRIG-Ibv was upregulated rapidly at 8 h after the Poly I:C injection, and the significant increase of AjRIG-Ib was observed at 16 and 24 h post-injection (hpi). Laser confocal microscopy showed that AjRIG-Ib and AjRIG-Ibv were both located in cytoplasm. In addition, the overexpression of AjRIG-Ib or AjRIG-Ibv led to the increased activity of IFN promoter in transient transfection assay. Taken together, our results indicated that AjRIG-Ib and AjRIG-Ibv may play cooperative or somewhat complementary roles in coordinating the antiviral response in fish.

Recovery of TRIM25-Mediated RIG-I Ubiquitination through Suppression of NS1 by RNA Aptamers

Non-structural protein 1 (NS1) of influenza virus has been shown to inhibit the innate immune response by blocking the induction of interferon (IFN). In this study, we isolated two single-stranded RNA aptamers specific to NS1 with K d values of 1.62 ± 0.30 nM and 1.97 ± 0.27 nM, respectively, using a systematic evolution of ligand by exponential enrichment (SELEX) procedure. The selected aptamers were able to inhibit the interaction of NS1 with tripartite motif-containing protein 25 (TRIM25), and suppression of NS1 enabled retinoic acid inducible gene I (RIG-I) to be ubiquitinated regularly by TRIM25. Additional luciferase reporter assay and quantitative real-time PCR (RT-PCR) experiments demonstrated that suppression of NS1 by the selected aptamers induced IFN production. It is noted that viral replication was also inhibited through IFN induction in the presence of the selected aptamers. These results suggest that the isolated aptamers are strongly expected to be new therapeutic agents against influenza infection.

CHID1 positively regulates RLR antiviral signaling by targeting the RIG-I/VISA signalosome

Retinoic acid-inducible gene-I (RIG-I) belongs to the RIGI-like receptors (RLRs), a class of primary pattern recognition receptors. It senses viral double-strand RNA in the cytoplasm and delivers the activated signal to its adaptor virus-induced signaling adapter (VISA), which then recruits the downstream TNF receptor-associated factors and kinases, triggering a downstream signal cascade that leads to the production of proinflammatory cytokines and antiviral interferons (IFNs). However, the mechanism of RIG-I-mediated antiviral signaling is not fully understood. Here, we demonstrate that chitinase domain-containing 1 (CHID1), a member of the chitinase family, positively regulates the RLR antiviral signaling pathway by targeting the RIG-I/VISA signalosome. CHID1 overexpression enhances the activation of nuclear factor κB (NF-кB) and interferon regulatory factor 3 (IRF3) triggered by Sendai virus (SeV) by promoting the polyubiquitination of RIG-I and VISA, thereby potentiating IFN-β production. CHID1 knockdown in human 239T cells inhibits SeV-induced activation of IRF3 and NF-κB and the induction of IFN-β. These results indicate that CHID1 positively regulates RLR antiviral signal, revealing the novel mechanism of the RIG-I antiviral signaling pathway.

Comparative Structure and Function Analysis of the RIG-I-Like Receptors: RIG-I and MDA5

RIG-I (Retinoic acid-inducible gene I) and MDA5 (Melanoma Differentiation-Associated protein 5), collectively known as the RIG-I-like receptors (RLRs), are key protein sensors of the pathogen-associated molecular patterns (PAMPs) in the form of viral double-stranded RNA (dsRNA) motifs to induce expression of type 1 interferons (IFN1) (IFNα and IFNβ) and other pro-inflammatory cytokines during the early stage of viral infection. While RIG-I and MDA5 share many genetic, structural and functional similarities, there is increasing evidence that they can have significantly different strategies to recognize different pathogens, PAMPs, and in different host species. This review article discusses the similarities and differences between RIG-I and MDA5 from multiple perspectives, including their structures, evolution and functional relationships with other cellular proteins, their differential mechanisms of distinguishing between host and viral dsRNAs and interactions with host and viral protein factors, and their immunogenic signaling. A comprehensive comparative analysis can help inform future studies of RIG-I and MDA5 in order to fully understand their functions in order to optimize potential therapeutic approaches targeting them.

Disease classification via gene network integrating modules and pathways

Disease classification based on gene information has been of significance as the foundation for achieving precision medicine. Previous works focus on classifying diseases according to the gene expression data of patient samples, and constructing disease network based on the overlap of disease genes, as many genes have been confirmed to be associated with diseases. In this work, the effects of diseases on human biological functions are assessed from the perspective of gene network modules and pathways, and the distances between diseases are defined to carry out the classification models. In total, 1728 diseases are divided into 12 and 14 categories by the intensity and scope of effects on pathways, respectively. Each category is a mix of several types of diseases identified based on congenital and acquired factors as well as diseased tissues and organs. The disease classification models on the basis of gene network are parallel with traditional pathology classification based on anatomic and clinical manifestations, and enable us to look at diseases in the viewpoint of commonalities in etiology and pathology. Our models provide a foundation for exploring combination therapy of diseases, which in turn may inform strategies for future gene-targeted therapy.

Mechanisms of Non-segmented Negative Sense RNA Viral Antagonism of Host RIG-I-Like Receptors

The pattern recognition receptors RIG-I-like receptors (RLRs) are critical molecules for cytosolic viral recognition and for subsequent activation of type I interferon production. The interferon signaling pathway plays a key role in viral detection and generating antiviral responses. Among the many pathogens, the non-segmented negative sense RNA viruses target the RLR pathway using a variety of mechanisms. Here, I review the current state of knowledge on the molecular mechanisms that allow non-segmented negative sense RNA virus recognition and antagonism of RLRs.

The long noncoding RNA Lnczc3h7a promotes a TRIM25-mediated RIG-I antiviral innate immune response

The helicase RIG-I initiates an antiviral immune response after recognition of pathogenic RNA. TRIM25, an E3 ubiquitin ligase, mediates K63-linked ubiquitination of RIG-I, which is crucial for RIG-I downstream signaling and the antiviral innate immune response. The components and mode of the RIG-I-initiated innate signaling remain to be fully understood. Here we identify a novel long noncoding RNA (Lnczc3h7a) that binds to TRIM25 and promotes RIG-I-mediated antiviral innate immune responses. Depletion of Lnczc3h7a impairs RIG-I signaling and the antiviral innate response to RNA viruses in vitro and in vivo. Mechanistically, Lnczc3h7a binds to both TRIM25 and activated RIG-I, serving as a molecular scaffold for stabilization of the RIG-I-TRIM25 complex at the early stage of viral infection. Lnczc3h7a facilitates TRIM25-mediated K63-linked ubiquitination of RIG-I and thus promotes downstream signaling transduction. Our findings reveal that host RNAs can enhance the response of innate immune sensors to foreign RNAs, ensuring effective antiviral defense.

The two TRIM25 isoforms were differentially induced in Larimichthys crocea post poly (I:C) stimulation

In this study, we identified and characterized a tripartite motif containing 25 (TRIM25) gene homologue, LcTRIM25, from large yellow croaker (Larimichthys crocea). Two isoforms of LcTRIM25, which were generated via alternative splicing, were identified via a molecular analysis of cDNA clones. The long isoform of LcTRIM25 (termed as LcTRIM25-L) contained the full open reading frame of the gene, encoded a protein of 698 amino acid residues, and possessed 11 exons. The short isoform of LcTRIM25 (termed as LcTRIM25-S) contained 9 exons and encoded a protein of 665 amino acid residues. The two LcTRIM25 isoforms contained a conserved Really Interesting New Gene (RING) domain, a B-box2 domain, a Coiled-coil domain (CCD), and variable C-terminal PRY/SPRY domains. Phylogenetic analysis showed that the two LcTRIM25 isoforms of the large yellow croaker was clustered together with their counterparts from other teleost fish. The Real-time PCR analysis showed that the LcTRIM25-L and LcTRIM25-S isoforms were both ubiquitously expressed in nine examined tissues in the large yellow croaker, with predominant expressions in the liver. The expression levels of the two isoforms of LcTRIM25 were rapidly and significantly upregulated in vivo after poly (I:C) stimulation in peripheral blood, head kidney, spleen and liver. Moreover, LcTRIM25-L and LcTRIM25-S showed differential expression post poly(I:C) stimulation. LcTRIM25 may have a dual role in innate immunity via alternative gene splicing. These results indicated that LcTRIM25 is likely to be involved in antiviral immune responses.

Nature of selection varies on different domains of IFI16-like PYHIN genes in ruminants

Background

ALRs (AIM2-like Receptors) are germline encoded PRRs that belong to PYHIN gene family of cytokines, which are having signature N-terminal PYD (Pyrin, PAAD or DAPIN) domain and C-terminal HIN-200 (hematopoietic, interferon-inducible nuclear protein with 200 amino acid repeat) domain joined by a linker region. The positively charged HIN-200 domain senses and binds with negatively charged phosphate groups of single-stranded DNA (ssDNA) and double-stranded DNA (dsDNA) purely through electrostatic attractions. On the other hand, PYD domain interacts homotypically with a PYD domain of other mediators to pass the signals to effector molecules downwards the pathways for inflammatory responses. There is remarkable inter-specific diversity in the numbers of functional PYHIN genes e.g. one in cow, five in human, thirteen in mice etc., while there is a unique loss of PYHIN genes in the bat genomes which was revealed by Ahn et al. (2016) by studying genomes of ten different bat species belonging to sub-orders yinpterochiroptera and yangochiroptera. The conflicts between host and pathogen interfaces are compared with “Red queen’s arms race” which is also described as binding seeking dynamics and binding avoidance dynamics. As a result of this never-ending rivalry, eukaryotes developed PRRs as antiviral mechanism while viruses developed counter mechanisms to evade host immune defense. The PYHIN receptors are directly engaged with pathogenic molecules, so these should have evolved under the influence of selection pressures. In the current study, we investigated the nature of selection pressure on different domain types of IFI16-like (IFI16-L) PYHIN genes in ruminants.

Results

Three transcript variants of the IFI16-like gene were found in PBMCs of ruminant animals-water buffalo, zebu cattle, goat, and sheep. The IFI16-like gene has one N-terminal PYD domain and one C-terminal HIN-200 domain, separated by an inter-domain linker region. HIN domain and inter-domain region are positively selected while the PYD domain is under the influence of purifying selection.

Conclusion

Herein, we conclude that the nature of selection pressure varies on different parts (PYD domain, HIN domain, and inter-domain linker region) of IFI16-like PYHIN genes in the ruminants. This data can be useful to predict the molecular determinants of pathogen interactions.

Electronic supplementary material

The online version of this article (10.1186/s12862-018-1334-7) contains supplementary material, which is available to authorized users.

Advances in Analyzing Virus-Induced Alterations of Host Cell Splicing

Alteration of host cell splicing is a common feature of many viral infections which is underappreciated because of the complexity and technical difficulty of studying alternative splicing (AS) regulation. Recent advances in RNA sequencing technologies revealed that up to several hundreds of host genes can show altered mRNA splicing upon viral infection. The observed changes in AS events can be either a direct consequence of viral manipulation of the host splicing machinery or result indirectly from the virus-induced innate immune response or cellular damage. Analysis at a higher resolution with single-cell RNAseq, and at a higher scale with the integration of multiple omics data sets in a systems biology perspective, will be needed to further comprehend this complex facet of virus-host interactions.

Human Respiratory Syncytial Virus NS 1 Targets TRIM25 to Suppress RIG-I Ubiquitination and Subsequent RIG-I-Mediated Antiviral Signaling

Respiratory syncytial virus (RSV) causes severe acute lower respiratory tract disease. Retinoic acid-inducible gene-I (RIG-I) serves as an innate immune sensor and triggers antiviral responses upon recognizing viral infections including RSV. Since tripartite motif-containing protein 25 (TRIM25)-mediated K63-polyubiquitination is crucial for RIG-I activation, several viruses target initial RIG-I activation through ubiquitination. RSV NS1 and NS2 have been shown to interfere with RIG-I-mediated antiviral signaling. In this study, we explored the possibility that NS1 suppresses RIG-I-mediated antiviral signaling by targeting TRIM25. Ubiquitination of ectopically expressed RIG-I-2Cards domain was decreased by RSV infection, indicating that RSV possesses ability to inhibit TRIM25-mediated RIG-I ubiquitination. Similarly, ectopic expression of NS1 sufficiently suppressed TRIM25-mediated RIG-I ubiquitination. Furthermore, interaction between NS1 and TRIM25 was detected by a co-immunoprecipitation assay. Further biochemical assays showed that the SPRY domain of TRIM25, which is responsible for interaction with RIG-I, interacted sufficiently with NS1. Suppression of RIG-I ubiquitination by NS1 resulted in decreased interaction between RIG-I and its downstream molecule, MAVS. The suppressive effect of NS1 on RIG-I signaling could be abrogated by overexpression of TRIM25. Collectively, this study suggests that RSV NS1 interacts with TRIM25 and interferes with RIG-I ubiquitination to suppress type-I interferon signaling.

TRIM25 Binds RNA to Modulate Cellular Anti-viral Defense

TRIM25 is a multi-domain, RING-type E3 ubiquitin ligase of the tripartite motif family that has important roles in multiple RNA-dependent processes. In particular, TRIM25 functions as an effector of RIG-I and ZAP, which are innate immune sensors that recognize viral RNA and induce ubiquitin-dependent anti-viral response mechanisms. TRIM25 is reported to also bind RNA, but the molecular details of this interaction or its relevance to anti-viral defense have not been elucidated. Here, we characterize the RNA-binding activity of TRIM25 and find that the protein binds both single-stranded and double-stranded RNA. Multiple regions of TRIM25 contribute to this functionality, including the C-terminal SPRY domain and a lysine-rich motif in the linker segment connecting the SPRY and coiled-coil domains. RNA binding modulates TRIM25's ubiquitination activity in vitro, its localization in cells, and its anti-viral activity. Taken together with other studies, our results indicate that RNA binding by TRIM25 has at least three important functional consequences: by enhancing ubiquitination activity, either through allosteric effects or through clustering of multiple TRIM25 molecules; by modulating the multi-domain structure of the TRIM25 dimer, and thereby structural coupling of the SPRY and RBCC elements during the ubiquitination reaction; and by facilitating subcellular localization of the E3 ligase during virus infection.

How Dengue Virus Circumvents Innate Immunity

In the battle between a virus and its host, innate immunity serves as the first line of defense protecting the host against pathogens. The antiviral actions start with the recognition of pathogen-associated molecular patterns derived from the virus, then ultimately turning on particular transcription factors to generate antiviral interferons (IFNs) or proinflammatory cytokines via fine-tuned signaling cascades. With dengue virus (DENV) infection, its viral RNA is recognized by the host RNA sensors, mainly retinoic acid inducible gene-I (RIG-I)-like receptors (RLRs) and toll-like receptors. DENV infection also activates the cyclic GMP-AMP synthase–stimulator of interferon genes (cGAS–STING)-mediated DNA-sensing pathway despite the absence of a DNA stage in the DENV lifecycle. In the last decade, DENV has been considered a weak IFN-inducing pathogen with the evidence that DENV has evolved multiple strategies antagonizing the host IFN system. DENV passively escapes from innate immunity surveillance and also actively subverts the innate immune system at multiple steps. DENV targets both RNA-triggered RLR–mitochondrial antiviral signaling protein (RLR–MAVS) and DNA-triggered cGAS–STING signaling to reduce IFN production in infected cells. It also blocks IFN action by inhibiting IFN regulatory factor- and signal transducer and activator of transcription-mediated signaling. This review explores the current understanding of how DENV escapes the control of the innate immune system by modifying viral RNA and viral protein and by post-translational modification of cellular factors. The roles of the DNA-sensing pathway in DENV infection, and how mitochondrial dynamics participates in innate immunity are also discussed.

Cloning, expression, and bioinformatics analysis of a putative pigeon retinoid acid-inducible gene-I

Retinoid acid-inducible gene-I (RIG-I) is a major cytoplasmic RNA sensor, playing an essential role in detecting viral RNA and triggering antiviral innate immune responses. The objective of the present study was to characterize the structure and expression of the RIG-I gene in pigeons. The pigeon RIG-I (piRIG-I) was cloned from splenic lymphocytes of pigeons using reverse transcription polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends. The cDNA of piRIG-I contains a 147 bp 5′-untranslated regions (UTRs), a 2787 bp open reading frame, and 2962 bp 3′-UTRs. Based on this sequence, the encoded piRIG-I protein is predicted to consist of 928 amino acids, and it has conserved domains typical of RIG-I-like receptors (RLRs) including two tandem arranged N-terminal caspase recruitment domains, a domain with the signature of DExD/H box helicase (helicase domain), and a C-terminal repression domain similar to finch RIG-I, duck RIG-I, goose RIG-I, human RIG-I, and mouse RIG-I. The piRIG-I shows 82.1%, 78.6%, and 78.2% amino acid sequence identity with previously described finch RIG-I, duck RIG-I, and goose RIG-I, respectively, and 49.7%–53.8% sequence identity with mammalian homologs. Quantitative RT-PCR (qRT-PCR) analysis indicated that the piRIG-I mRNA is scarcely detected in healthy tissues, and it is strongly expressed in the thymus gland, kidney, spleen, and bursa of Fabricius. These findings lay the foundation for further research on the function and mechanism of avian RIG-I in innate immune response related to vaccinations and infectious diseases in the pigeon.

Mapping and Sequencing of a Significant Quantitative Trait Locus Affecting Resistance to Koi Herpesvirus in Common Carp

Cyprinids are the most highly produced group of fishes globally, with common carp being one of the most valuable species of the group. Koi herpesvirus (KHV) infections can result in high levels of mortality, causing major economic losses, and is listed as a notifiable disease by the World Organization for Animal Health. Selective breeding for host resistance has the potential to reduce morbidity and losses due to KHV. Therefore, improving knowledge about host resistance and methods of incorporating genomic data into breeding for resistance may contribute to a decrease in economic losses in carp farming. In the current study, a population of 1,425 carp juveniles, originating from a factorial cross between 40 sires and 20 dams was challenged with KHV. Mortalities and survivors were recorded and sampled for genotyping by sequencing using Restriction Site-Associated DNA sequencing (RADseq). Genome-wide association analyses were performed to investigate the genetic architecture of resistance to KHV. A genome-wide significant QTL affecting resistance to KHV was identified on linkage group 44, explaining approximately 7% of the additive genetic variance. Pooled whole genome resequencing of a subset of resistant (n = 60) and susceptible animals (n = 60) was performed to characterize QTL regions, including identification of putative candidate genes and functional annotation of associated polymorphisms. The TRIM25 gene was identified as a promising positional and functional candidate within the QTL region of LG 44, and a putative premature stop mutation in this gene was discovered.

Phosphatase Cdc25A Negatively Regulates the Antiviral Immune Response by Inhibiting TBK1 Activity

The phosphatase Cdc25A plays an important role in cell cycle regulation by dephosphorylating its substrates, such as cyclin-dependent kinases. In this study, we demonstrate that Cdc25A negatively regulates RIG-I-mediated antiviral signaling. We found that ectopic expression of Cdc25A in 293T cells inhibits the activation of beta interferon (IFN-β) induced by Sendai virus and poly(I·C), while knockdown of Cdc25A enhances the transcription of IFN-β stimulated by RNA virus infection. The inhibitory effect of Cdc25A on the antiviral immune response is mainly dependent on its phosphatase activity. Data from a luciferase assay indicated that Cdc25A can inhibit TBK1-mediated activation of IFN-β. Further analysis indicated that Cdc25A can interact with TBK1 and reduce the phosphorylation of TBK1 at S172, which in turn decreases the phosphorylation of its downstream substrate IRF3. Consistently, knockdown of Cdc25A upregulates the phosphorylation of both TBK1-S172 and IRF3 in Sendai virus-infected or TBK1-transfected 293T cells. In addition, we confirmed that Cdc25A can directly dephosphorylate TBK1-S172-p. These results demonstrate that Cdc25A inhibits the antiviral immune response by reducing the active form of TBK1. Using herpes simplex virus 1 (HSV-1) infection, an IFN-β reporter assay, and reverse transcription-quantitative PCR (RT-qPCR), we demonstrated that Cdc25A can also inhibit DNA virus-induced activation of IFN-β. Using a vesicular stomatitis virus (VSV) infection assay, we confirmed that Cdc25A can repress the RIG-I-like receptor (RLR)-mediated antiviral immune response and influence the antiviral status of cells. In conclusion, we demonstrate that Cdc25A negatively regulates the antiviral immune response by inhibiting TBK1 activity.IMPORTANCE The RLR-mediated antiviral immune response is critical for host defense against RNA virus infection. However, the detailed mechanism for balancing the RLR signaling pathway in host cells is not well understood. We found that the phosphatase Cdc25A negatively regulates the RNA virus-induced innate immune response. Our studies indicate that Cdc25A inhibits the RLR signaling pathway via its phosphatase activity. We demonstrated that Cdc25A reduces TBK1 activity and consequently restrains the activation of IFN-β transcription as well as the antiviral status of nearby cells. We showed that Cdc25A can also inhibit DNA virus-induced activation of IFN-β. Taken together, our findings uncover a novel function and mechanism for Cdc25A in regulating antiviral immune signaling. These findings reveal Cdc25A as an important negative regulator of antiviral immunity and demonstrate its role in maintaining host cell homeostasis following viral infection.

Positive regulatory role of c-Src-mediated TRIM25 tyrosine phosphorylation on RIG-I ubiquitination and RIG-I-mediated antiviral signaling pathway

Retinoic acid-inducible gene I (RIG-I) detects viral RNAs and induces antiviral responses. During viral RNA recognition by RIG-I, tripartite motif protein 25 (TRIM25) plays a critical regulatory role by inducing K63-linked RIG-I polyubiquitination. Previous proteomics analysis revealed several phosphorylation sites on TRIM25, including tyrosine 278 (Y278), yet the roles of these modifications remain elusive. Here, we demonstrated that TRIM25 interacted with c-Src and underwent tyrosine phosphorylation by c-Src kinase upon viral infection and the phosphorylation is required for the complete activation of RIG-I signaling. Analysis using a c-Src inhibitor and TRIM25 mutant, in which tyrosine 278 is substituted by phenylalanine (Y278F), suggested that the phosphorylation positively regulates K63-linked polyubiquitination of RIG-I and subsequent antiviral signaling. The TRIM25 Y278F mutant displayed decreased E3-ubiquitin ligase activity in vitro, suggesting that this phosphorylation event affects the E3-ligase activity of TRIM25. Thus, we provide a molecular mechanism of c-Src-mediated positive regulation of RIG-I signaling.

Functional Evolution of Avian RIG-I-Like Receptors

RIG-I-like receptors (retinoic acid-inducible gene-I-like receptors, or RLRs) are family of pattern-recognition receptors for RNA viruses, consisting of three members: retinoic acid-inducible gene I (RIG-I), melanoma differentiation-associated gene 5 (MDA5) and laboratory of genetics and physiology 2 (LGP2). To understand the role of RLRs in bird evolution, we performed molecular evolutionary analyses on the coding genes of avian RLRs using filtered predicted coding sequences from 62 bird species. Among the three RLRs, conservation score and dN/dS (ratio of nonsynonymous substitution rate over synonymous substitution rate) analyses indicate that avian MDA5 has the highest conservation level in the helicase domain but a lower level in the caspase recruitment domains (CARDs) region, which differs from mammals; LGP2, as a whole gene, has a lower conservation level than RIG-I or MDA5. We found evidence of positive selection across all bird lineages in RIG-I and MDA5 but only on the stem lineage of Galliformes in LGP2, which could be related to the loss of RIG-I in Galliformes. Analyses also suggest that selection relaxation may have occurred in LGP2 during the middle of bird evolution and the CARDs region of MDA5 contains many positively selected sites, which might explain its conservation level. Spearman’s correlation test indicates that species-to-ancestor dN/dS of RIG-I shows a negative correlation with endogenous retroviral abundance in bird genomes, suggesting the possibility of interaction between immunity and endogenous retroviruses during bird evolution.

Nuclear-resident RIG-I senses viral replication inducing antiviral immunity

The nucleus represents a cellular compartment where the discrimination of self from non-self nucleic acids is vital. While emerging evidence establishes a nuclear non-self DNA sensing paradigm, the nuclear sensing of non-self RNA, such as that from nuclear-replicating RNA viruses, remains unexplored. Here, we report the identification of nuclear-resident RIG-I actively involved in nuclear viral RNA sensing. The nuclear RIG-I, along with its cytoplasmic counterpart, senses influenza A virus (IAV) nuclear replication leading to a cooperative induction of type I interferon response. Its activation signals through the canonical signaling axis and establishes an effective antiviral state restricting IAV replication. The exclusive signaling specificity conferred by nuclear RIG-I is reinforced by its inability to sense cytoplasmic-replicating Sendai virus and appreciable sensing of hepatitis B virus pregenomic RNA in the nucleus. These results refine the RNA sensing paradigm for nuclear-replicating viruses and reveal a previously unrecognized subcellular milieu for RIG-I-like receptor sensing.

Single-molecule fluorescence imaging: Generating insights into molecular interactions in virology

Single-molecule fluorescence methods remain a challenging yet information-rich set of techniques that allow one to probe the dynamics, stoichiometry and conformation of biomolecules one molecule at a time. Viruses are small (nanometers) in size, can achieve cellular infections with a small number of virions and their lifecycle is inherently heterogeneous with a large number of structural and functional intermediates. Single-molecule measurements that reveal the complete distribution of properties rather than the average can hence reveal new insights into virus infections and biology that are inaccessible otherwise. This article highlights some of the methods and recent applications of single-molecule fluorescence in the field of virology. Here, we have focused on new findings in virus-cell interaction, virus cell entry and transport, viral membrane fusion, genome release, replication, translation, assembly, genome packaging, egress and interaction with host immune proteins that underline the advantage of single-molecule approach to the question at hand. Finally, we discuss the challenges, outlook and potential areas for improvement and future use of single-molecule fluorescence that could further aid our understanding of viruses.

Harnessing RIG-I and intrinsic immunity in the tumor microenvironment for therapeutic cancer treatment

Cancer immunotherapies that remove checkpoint restraints on adaptive immunity are gaining clinical momentum. Approaches aimed at intrinsic cellular immunity in the tumor microenvironment are less understood, but are of intense interest, based on their ability to induce tumor cell apoptosis while orchestrating innate and adaptive immune responses against tumor antigens. The intrinsic immune response is initiated by ancient, highly conserved intracellular proteins that detect viral infection. For example, the RIG-I-like receptors (RLRs), a family of related RNA helicases, detect viral oligonucleotide patterns of certain RNA viruses. RLR activation induces immunogenic cell death of virally infected cells, accompanied by increased inflammatory cytokine production, antigen presentation, and antigen-directed immunity against virus antigens. Approaches aimed at non-infectious RIG-I activation in cancers are being tested as a treatment option, with the goal of inducing immunogenic tumor cell death, stimulating production of pro-inflammatory cytokines, enhancing tumor neoantigen presentation, and potently increasing cytotoxic activity of tumor infiltrating lymphocytes. These studies are finding success in several pre-clinical models, and are entering early phases of clinical trial. Here, we review pre-clinical studies of RLR agonists, including the successes and challenges currently faced RLR agonists on the path to clinical translation.

A Promising IFN-Deficient System to Manufacture IFN-Sensitive Influenza Vaccine Virus

Interferon (IFN)-sensitive and replication-incompetent influenza viruses are likely to be the alternatives to inactivated and attenuated virus vaccines. Some IFN-sensitive influenza vaccine candidates with modified non-structural protein 1 (NS1) are highly attenuated in IFN-competent hosts but induce robust antiviral immune responses. However, little research has been done on the manufacturability of these IFN-sensitive vaccine viruses. Here, RIG-I-knockout 293T cells were used to package the IFN-sensitive influenza A/WSN/33 (H1N1) virus expressing the mutant NS1 R38A/K41A. We found that the packaging efficiency of the NS1 R38A/K41A virus in RIG-I-knockout 293T cells was much higher than that in 293T cells. Moreover, the NS1 R38A/K41A virus almost lost its IFN antagonist activity and could no longer replicate in A549, MDCK, and Vero cells after 3-6 passages. This indicated that the replication of NS1 R38A/K41A virus is limited in conventional cells. Therefore, we further established a stable Vero cell line expressing the wild-type (WT) NS1 of the WSN virus, based on the Tet-On 3G system. The NS1 R38A/K41A virus was able to steadily propagate in this IFN-deficient cell line for at least 20 passages. In a mouse model, the NS1 R38A/K41A virus showed more than a 4-log reduction in lung virus titers compared to the WT virus at 3 and 5 days post infection. Furthermore, we observed that the NS1 R38A/K41A virus triggered high-level of IFN-α/β production in lung tissues and was eliminated from the host in a relatively short period of time. Additionally, this virus induced high-titer neutralizing antibodies against the WT WSN, A/Puerto Rico/8/1934 (PR8), or A/California/04/2009 (CA04) viruses and provided 100% protection against the WT WSN virus. Thus, we found that the replication of the NS1 R38A/K41A virus was limited in IFN-competent cells and mice. We also presented a promising IFN-deficient system, involving a RIG-I-knockout 293T cell line to package the IFN-sensitive vaccine virus and a stable Vero cell line expressing NS1 to propagate the IFN-sensitive vaccine virus. The IFN-deficient system is applicable for the manufacture of IFN-sensitive vaccine virus.

Viruses Seen by Our Cells: The Role of Viral RNA Sensors

The role of the innate immune response in detecting RNA viruses is crucial for the establishment of proper inflammatory and antiviral responses. Different receptors, known as pattern recognition receptors (PRRs), are present in the cytoplasm, endosomes, and on the cellular surface. These receptors have the capacity to sense the presence of viral nucleic acids as pathogen-associated molecular patterns (PAMPs). This recognition leads to the induction of type 1 interferons (IFNs) as well as inflammatory cytokines and chemokines. In this review, we provide an overview of the significant involvement of cellular RNA helicases and Toll-like receptors (TLRs) 3, 7, and 8 in antiviral immune defenses.

RIG-I and Other RNA Sensors in Antiviral Immunity

Pattern recognition receptors (PRRs) survey intra- and extracellular spaces for pathogen-associated molecular patterns (PAMPs) within microbial products of infection. Recognition and binding to cognate PAMP ligand by specific PRRs initiates signaling cascades that culminate in a coordinated intracellular innate immune response designed to control infection. In particular, our immune system has evolved specialized PRRs to discriminate viral nucleic acid from host. These are critical sensors of viral RNA to trigger innate immunity in the vertebrate host. Different families of PRRs of virus infection have been defined and reveal a diversity of PAMP specificity for wide viral pathogen coverage to recognize and extinguish virus infection. In this review, we discuss recent insights in pathogen recognition by the RIG-I-like receptors, related RNA helicases, Toll-like receptors, and other RNA sensor PRRs, to present emerging themes in innate immune signaling during virus infection.

Vibrio vulnificus quorum-sensing molecule cyclo(Phe-Pro) inhibits RIG-I-mediated antiviral innate immunity

The recognition of pathogen-derived ligands by pattern recognition receptors activates the innate immune response, but the potential interaction of quorum-sensing (QS) signaling molecules with host anti-viral defenses remains largely unknown. Here we show that the Vibrio vulnificus QS molecule cyclo(Phe-Pro) (cFP) inhibits interferon (IFN)-β production by interfering with retinoic-acid-inducible gene-I (RIG-I) activation. Binding of cFP to the RIG-I 2CARD domain induces a conformational change in RIG-I, preventing the TRIM25-mediated ubiquitination to abrogate IFN production. cFP enhances susceptibility to hepatitis C virus (HCV), as well as Sendai and influenza viruses, each known to be sensed by RIG-I but did not affect the melanoma-differentiation-associated gene 5 (MDA5)-recognition of norovirus. Our results reveal an inter-kingdom network between bacteria, viruses and host that dysregulates host innate responses via a microbial quorum-sensing molecule modulating the response to viral infection.

The Human Papillomavirus E6 Oncoprotein Targets USP15 and TRIM25 To Suppress RIG-I-Mediated Innate Immune Signaling

Retinoic acid-inducible gene I (RIG-I) is a key pattern recognition receptor that senses viral RNA and interacts with the mitochondrial adaptor MAVS, triggering a signaling cascade that results in the production of type I interferons (IFNs). This signaling axis is initiated by K63-linked ubiquitination of RIG-I mediated by the E3 ubiquitin ligase TRIM25, which promotes the interaction of RIG-I with MAVS. USP15 was recently identified as an upstream regulator of TRIM25, stabilizing the enzyme through removal of degradative K48-linked polyubiquitin, ultimately promoting RIG-I-dependent cytokine responses. Here, we show that the E6 oncoprotein of human papillomavirus type 16 (HPV16) as well as of other HPV types form a complex with TRIM25 and USP15 in human cells. In the presence of E6, the K48-linked ubiquitination of TRIM25 was markedly increased, and in line with this, TRIM25 degradation was enhanced. Our results further showed that E6 inhibited the TRIM25-mediated K63-linked ubiquitination of RIG-I and its CARD-dependent interaction with MAVS. HPV16 E6, but not E7, suppressed the RIG-I-mediated induction of IFN-β, chemokines, and IFN-stimulated genes (ISGs). Finally, CRISPR-Cas9 gene targeting in human keratinocytes showed that the TRIM25-RIG-I-MAVS triad is important for eliciting an antiviral immune response to HPV16 infection. Our study thus identifies a novel immune escape mechanism that is conserved among different HPV strains and further indicates that the RIG-I signaling pathway plays an important role in the innate immune response to HPV infection.IMPORTANCE Persistent infection and tumorigenesis by HPVs are known to require viral manipulation of a variety of cellular processes, including those involved in innate immune responses. Here, we show that the HPV E6 oncoprotein antagonizes the activation of the cytoplasmic innate immune sensor RIG-I by targeting its upstream regulatory enzymes TRIM25 and USP15. We further show that the RIG-I signaling cascade is important for an antiviral innate immune response to HPV16 infection, providing evidence that RIG-I, whose role in sensing RNA virus infections has been well characterized, also plays a crucial role in the antiviral host response to small DNA viruses of the Papillomaviridae family.

Host Immune Response to Influenza A Virus Infection

Influenza A viruses (IAVs) are contagious pathogens responsible for severe respiratory infection in humans and animals worldwide. Upon detection of IAV infection, host immune system aims to defend against and clear the viral infection. Innate immune system is comprised of physical barriers (mucus and collectins), various phagocytic cells, group of cytokines, interferons (IFNs), and IFN-stimulated genes, which provide first line of defense against IAV infection. The adaptive immunity is mediated by B cells and T cells, characterized with antigen-specific memory cells, capturing and neutralizing the pathogen. The humoral immune response functions through hemagglutinin-specific circulating antibodies to neutralize IAV. In addition, antibodies can bind to the surface of infected cells and induce antibody-dependent cell-mediated cytotoxicity or complement activation. Although there are neutralizing antibodies against the virus, cellular immunity also plays a crucial role in the fight against IAVs. On the other hand, IAVs have developed multiple strategies to escape from host immune surveillance for successful replication. In this review, we discuss how immune system, especially innate immune system and critical molecules are involved in the antiviral defense against IAVs. In addition, we highlight how IAVs antagonize different immune responses to achieve a successful infection.

Identification of a second binding site on the TRIM25 B30.2 domain

The retinoic acid-inducible gene-I (RIG-I) receptor recognizes short 5'-di- and triphosphate base-paired viral RNA and is a critical mediator of the innate immune response against viruses such as influenza A, Ebola, HIV and hepatitis C. This response is reported to require an orchestrated interaction with the tripartite motif 25 (TRIM25) B30.2 protein-interaction domain. Here, we present a novel second RIG-I-binding interface on the TRIM25 B30.2 domain that interacts with CARD1 and CARD2 (caspase activation and recruitment domains) of RIG-I and is revealed by the removal of an N-terminal α-helix that mimics dimerization of the full-length protein. Further characterization of the TRIM25 coiled-coil and B30.2 regions indicated that the B30.2 domains move freely on a flexible tether, facilitating RIG-I CARD recruitment. The identification of a dual binding mode for the TRIM25 B30.2 domain is a first for the SPRY/B30.2 domain family and may be a feature of other SPRY/B30.2 family members.

Viral unmasking of cellular 5S rRNA pseudogene transcripts induces RIG-I-mediated immunity

The sensor RIG-I detects double-stranded RNA derived from RNA viruses. Although RIG-I is also known to have a role in the antiviral response to DNA viruses, physiological RNA species recognized by RIG-I during infection with a DNA virus are largely unknown. Using next-generation RNA sequencing (RNAseq), we found that host-derived RNAs, most prominently 5S ribosomal RNA pseudogene 141 (RNA5SP141), bound to RIG-I during infection with herpes simplex virus 1 (HSV-1). Infection with HSV-1 induced relocalization of RNA5SP141 from the nucleus to the cytoplasm, and virus-induced shutoff of host protein synthesis downregulated the abundance of RNA5SP141-interacting proteins, which allowed RNA5SP141 to bind RIG-I and induce the expression of type I interferons. Silencing of RNA5SP141 strongly dampened the antiviral response to HSV-1 and the related virus Epstein-Barr virus (EBV), as well as influenza A virus (IAV). Our findings reveal that antiviral immunity can be triggered by host RNAs that are unshielded following depletion of their respective binding proteins by the virus.

Herpesvirus deconjugases inhibit the IFN response by promoting TRIM25 autoubiquitination and functional inactivation of the RIG-I signalosome

The N-terminal domains of the herpesvirus large tegument proteins encode a conserved cysteine protease with ubiquitin- and NEDD8-specific deconjugase activity. The proteins are expressed during the productive virus cycle and are incorporated into infectious virus particles, being delivered to the target cells upon primary infection. Members of this viral enzyme family were shown to regulate different aspects of the virus life cycle and the innate anti-viral response. However, only few substrates have been identified and the mechanisms of these effects remain largely unknown. In order to gain insights on the substrates and signaling pathways targeted by the viral enzymes, we have used co-immunoprecipitation and mass spectrometry to identify cellular proteins that interact with the Epstein-Barr virus encoded homologue BPLF1. Several members of the 14-3-3-family of scaffold proteins were found amongst the top hits of the BPLF1 interactome, suggesting that, through this interaction, BPLF1 may regulate a variety of cellular signaling pathways. Analysis of the shared protein-interaction network revealed that BPLF1 promotes the assembly of a tri-molecular complex including, in addition to 14-3-3, the ubiquitin ligase TRIM25 that participates in the innate immune response via ubiquitination of cytosolic pattern recognition receptor, RIG-I. The involvement of BPLF1 in the regulation of this signaling pathway was confirmed by inhibition of the type-I IFN responses in cells transfected with a catalytically active BPLF1 N-terminal domain or expressing the endogenous protein upon reactivation of the productive virus cycle. We found that the active viral enzyme promotes the dimerization and autoubiquitination of TRIM25. Upon triggering of the IFN response, RIG-I is recruited to the complex but ubiquitination is severely impaired, which functionally inactivates the RIG-I signalosome. The capacity to bind to and functionally inactivate the RIG-I signalosome is shared by the homologues encoded by other human herpesviruses.

Nuclear TRIM25 Specifically Targets Influenza Virus Ribonucleoproteins to Block the Onset of RNA Chain Elongation

TRIM25 is an E3 ubiquitin ligase that activates RIG-I to promote the antiviral interferon response. The NS1 protein from all strains of influenza A virus binds TRIM25, although not all virus strains block the interferon response, suggesting alternative mechanisms for TRIM25 action. Here we present a nuclear role for TRIM25 in specifically restricting influenza A virus replication. TRIM25 inhibits viral RNA synthesis through a direct mechanism that is independent of its ubiquitin ligase activity and the interferon pathway. This activity can be inhibited by the viral NS1 protein. TRIM25 inhibition of viral RNA synthesis results from its binding to viral ribonucleoproteins (vRNPs), the structures containing individual viral RNA segments, the viral polymerase, and multiple viral nucleoproteins. TRIM25 binding does not inhibit initiation of capped-RNA-primed viral mRNA synthesis by the viral polymerase. Rather, the onset of RNA chain elongation is inhibited because TRIM25 prohibits the movement of RNA into the polymerase complex.