Virus infection induces the assembly of coordinately activated transcription factors on the IFN-beta enhancer in vivo

IRF3 function and immunological gaps in sepsis

Lipopolysaccharide (LPS) induces potent cell activation via Toll-like receptor 4/myeloid differentiation protein 2 (TLR4/MD-2), often leading to septic death and cytokine storm. TLR4 signaling is diverted to the classical acute innate immune, inflammation-driving pathway in conjunction with the classical NF-κB pivot of MyD88, leading to epigenetic linkage shifts in nuclear pro-inflammatory transcription and chromatin structure-function; in addition, TLR4 signaling to the TIR domain-containing adapter-induced IFN-β (TRIF) apparatus and to nuclear pivots that signal the association of interferons alpha and beta (IFN-α and IFN-β) with acute inflammation, often coupled with oxidants favor inhibition or resistance to tissue injury. Although the immune response to LPS, which causes sepsis, has been clarified in this manner, there are still many current gaps in sepsis immunology to reduce mortality. Recently, selective agonists and inhibitors of LPS signals have been reported, and there are scattered reports on LPS tolerance and control of sepsis development. In particular, IRF3 signaling has been reported to be involved not only in sepsis but also in increased pathogen clearance associated with changes in the gut microbiota. Here, we summarize the LPS recognition system, main findings related to the IRF3, and finally immunological gaps in sepsis.

Epigenetic Control of Innate Immunity: Consequences of Acute Respiratory Virus Infection

Infections caused by acute respiratory viruses induce a systemic innate immune response, which can be measured by the increased levels of expression of inflammatory genes in immune cells. There is growing evidence that these acute viral infections, alongside transient transcriptomic responses, induce epigenetic remodeling as part of the immune response, such as DNA methylation and histone modifications, which might persist after the infection is cleared. In this article, we first review the primary mechanisms of epigenetic remodeling in the context of innate immunity and inflammation, which are crucial for the regulation of the immune response to viral infections. Next, we delve into the existing knowledge concerning the impact of respiratory virus infections on the epigenome, focusing on Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), Influenza A Virus (IAV), and Respiratory Syncytial Virus (RSV). Finally, we offer perspectives on the potential consequences of virus-induced epigenetic remodeling and open questions in the field that are currently under investigation.

The SARS-CoV-2 protein ORF3c is a mitochondrial modulator of innate immunity

The SARS-CoV-2 genome encodes a multitude of accessory proteins. Using comparative genomic approaches, an additional accessory protein, ORF3c, has been predicted to be encoded within the ORF3a sgmRNA. Expression of ORF3c during infection has been confirmed independently by ribosome profiling. Despite ORF3c also being present in the 2002-2003 SARS-CoV, its function has remained unexplored. Here we show that ORF3c localizes to mitochondria, where it inhibits innate immunity by restricting IFN-β production, but not NF-κB activation or JAK-STAT signaling downstream of type I IFN stimulation. We find that ORF3c is inhibitory after stimulation with cytoplasmic RNA helicases RIG-I or MDA5 or adaptor protein MAVS, but not after TRIF, TBK1 or phospho-IRF3 stimulation. ORF3c co-immunoprecipitates with the antiviral proteins MAVS and PGAM5 and induces MAVS cleavage by caspase-3. Together, these data provide insight into an uncharacterized mechanism of innate immune evasion by this important human pathogen.

Antagonism between viral infection and innate immunity at the single-cell level

When infected with a virus, cells may secrete interferons (IFNs) that prompt nearby cells to prepare for upcoming infection. Reciprocally, viral proteins often interfere with IFN synthesis and IFN-induced signaling. We modeled the crosstalk between the propagating virus and the innate immune response using an agent-based stochastic approach. By analyzing immunofluorescence microscopy images we observed that the mutual antagonism between the respiratory syncytial virus (RSV) and infected A549 cells leads to dichotomous responses at the single-cell level and complex spatial patterns of cell signaling states. Our analysis indicates that RSV blocks innate responses at three levels: by inhibition of IRF3 activation, inhibition of IFN synthesis, and inhibition of STAT1/2 activation. In turn, proteins coded by IFN-stimulated (STAT1/2-activated) genes inhibit the synthesis of viral RNA and viral proteins. The striking consequence of these inhibitions is a lack of coincidence of viral proteins and IFN expression within single cells. The model enables investigation of the impact of immunostimulatory defective viral particles and signaling network perturbations that could potentially facilitate containment or clearance of the viral infection.

Lidocaine inhibits influenza a virus replication by up-regulating IFNα4 via TBK1-IRF7 and JNK-AP1 signaling pathways

Influenza A viruses (IAV), significant respiratory pathogenic agents, cause seasonal epidemics and global pandemics in intra- and interannual cycles. Despite effective therapies targeting viral proteins, the continuous generation of drug-resistant IAV strains is challenging. Therefore, exploring novel host-specific antiviral treatment strategies is urgently needed. Here, we found that lidocaine, widely used for local anesthesia and sedation, significantly inhibited H1N1(PR8) replication in macrophages. Interestingly, its antiviral effect did not depend on the inhibition of voltage-gated sodium channels (VGSC), the main target of lidocaine for anesthesia. Lidocaine significantly upregulated early IFN-I, interferon α4 (IFNα4) mRNA, and protein levels, but not those of early IFNβ in mouse RAW 264.7 cell line and human THP-1 derived macrophages. Knocking out IFNα4 by CRISPR-Cas9 partly reversed lidocaine's inhibition of PR8 replication in macrophages. Mechanistically, lidocaine upregulated IFNα4 by activating TANK-binding kinase 1 (TBK1)-IRF7 and JNK-AP1 signaling pathways. These findings indicate that lidocaine has an incredible antiviral potential by enhancing IFN-I signaling in macrophages. In conclusion, our results indicate the potential auxiliary role of lidocaine for anti-influenza A virus therapy and even for anti-SARS-CoV-2 virus therapy, especially in the absence of a specific medicine.

IL-33 promotes increased replication of Theiler's Murine Encephalomyelitis Virus in RAW264.7 macrophage cells with an IRF3-dependent response

Interleukin-33 (IL-33), which promotes M2 macrophage development, may influence the control of viruses, such as Theiler's Murine Encephalomyelitis Virus (TMEV) that infect macrophages. Because Interferon Regulatory Factor-3 (IRF3) is also critical to control of TMEV infection in macrophages, information on the relationship between IL-33 and IRF3 is important. Thus, RAW264.7 Lucia murine macrophage lineage cells with an endogenous IRF3-ISRE promoter driving secreted luciferase and IRF3KO RAW Lucia, a subline deficient in IRF3, were challenged with TMEV. After the challenge, considerable TMEV RNA detected at 18 and 24 h in RAW cells was significantly elevated in IRF3KO RAW cells. TMEV induction of ISRE-IRF3 promoter activity, IFN-β and IL-33 gene expression, and IL-6 and IL-10 protein production, which was strong in RAW cells, was less in IRF3KO RAW cells. In contrast, expression of CD206 and ARG1, classical M2 macrophage markers, was significantly elevated in IRF3KO RAW cells. Moreover, RAW and IRF3KO RAW cells produced extracellular IL-33 prior to and after infection with TMEV and antibody blockade of the IL-33 receptor, ST2, reduced CD206 and ARG1 expression, but increased IL-6 gene expression. Pre-treating both RAW and IRF3KO RAW cells with IL-33 prior to challenge significantly increased TMEV infection, but also increased IL-33, IL-10, IL-6 mRNA expression, and NO production without increasing IFN-β. Notably, IL-33 induction of IL-33, IRF3-ISRE promoter activity, and IL-10 by TMEV or poly I:C/IFN-γ was significantly dependent upon IRF3. The results show that the expression of IL-33 and the repression of M2 macrophage phenotypic markers are dependent on IRF3 and that IL-33 decreases the ability of macrophages to control infection with macrophage-tropic viruses.

Manipulation of innate immune signaling pathways by SARS-CoV-2 non-structural proteins

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of the current coronavirus disease 2019 (COVID-19) pandemic, induces an unbalanced immune response in the host. For instance, the production of type I interferon (IFN) and the response to it, which act as a front-line defense against virus invasion, are inhibited during SARS-CoV-2 infection. In addition, tumor necrosis factor alpha (TNF-α), a proinflammatory cytokine, is upregulated in COVID-19 patients with severe symptoms. Studies on the closely related betacoronavirus, SARS-CoV, showed that viral proteins such as Nsp1, Orf6 and nucleocapsid protein inhibit IFN-β production and responses at multiple steps. Given the conservation of these proteins between SARS-CoV and SARS-CoV-2, it is not surprising that SARS-CoV-2 deploys similar immune evasion strategies. Here, we carried out a screen to examine the role of individual SARS-CoV-2 proteins in regulating innate immune signaling, such as the activation of transcription factors IRF3 and NF-κB and the response to type I and type II IFN. In addition to established roles of SARS-CoV-2 proteins, we report that SARS-CoV-2 proteins Nsp6 and Orf8 inhibit the type I IFN response but at different stages. Orf6 blocks the translocation of STAT1 and STAT2 into the nucleus, whereas ORF8 inhibits the pathway in the nucleus after STAT1/2 translocation. SARS-CoV-2 Orf6 also suppresses IRF3 activation and TNF-α-induced NF-κB activation.

eIF4A3 Promotes RNA Viruses’ Replication by Inhibiting Innate Immune Responses

Production of type I IFN is pivotal for the cellular antiviral immunity. Virus infection leads to the activation of transcription factor IRF3 and subsequent production of type I IFN to eliminate viral infection.

An Overview of the Infectious Cycle of Bunyaviruses

Bunyaviruses represent the largest group of RNA viruses and are the causative agent of a variety of febrile and hemorrhagic illnesses. Originally characterized as a single serotype in Africa, the number of described bunyaviruses now exceeds over 500, with its presence detected around the world. These predominantly tri-segmented, single-stranded RNA viruses are transmitted primarily through arthropod and rodent vectors and can infect a wide variety of animals and plants. Although encoding for a small number of proteins, these viruses can inflict potentially fatal disease outcomes and have even developed strategies to suppress the innate antiviral immune mechanisms of the infected host. This short review will attempt to provide an overall description of the order Bunyavirales, describing the mechanisms behind their infection, replication, and their evasion of the host immune response. Furthermore, the historical context of these viruses will be presented, starting from their original discovery almost 80 years ago to the most recent research pertaining to viral replication and host immune response.

The RNA helicase DDX3 promotes IFNB transcription via enhancing IRF-3/p300 holocomplex binding to the IFNB promoter

The human DEAD-box protein 3 (DDX3) has been reported as a positive regulator and functions in the induction of type I interferon signaling. We elucidated the function of DDX3 in the positive regulation of IFNB production in non-pDC cells. We found that DDX3 regulates virus-induced activation of IFNB at the level of IRF-3. However, it does not affect conventional innate signaling, including IRF-3 phosphorylation, dimerization, or nuclear translocation of IRF-3, but has some downstream events after IRF-3 phosphorylation. Co-immunoprecipitation analyses revealed that DDX3 interacts with IRF-3 through its DNA-binding domain and promotes IRF-3-mediated IFNB promoter activation. DDX3 does not affect the formation of the IRF-3/p300/CBP complex. Instead, ChIP and EMSA assay revealed that DDX3 promotes the recruitment of IRF-3 and transcriptional co-activator p300/CBP to the IFNB promoter. The ATP binding pocket of DDX3 is involved in this association and is essential for the transcriptional activation. Taken together, our study demonstrates that DDX3 plays an important role in guiding a transcription factor complex formed by antiviral signaling to the target gene promoter.

The Molecular Mechanism of Herpes Simplex Virus 1 UL31 in Antagonizing the Activity of IFN-β

Virus infection triggers intricate signal cascade reactions to activate the host innate immunity, which leads to the production of type I interferon (IFN-I). Herpes simplex virus 1 (HSV-1), a human-restricted pathogen, is capable of encoding over 80 viral proteins, and several of them are involved in immune evasion to resist the host antiviral response through the IFN-I signaling pathway. Here, we determined that HSV-1 UL31, which is associated with nuclear matrix and is essential for the formation of viral nuclear egress complex, could inhibit retinoic acid-inducible gene I (RIG-I)-like receptor pathway-mediated interferon beta (IFN-β)-luciferase (Luc) and (PRDIII-I)4-Luc (an expression plasmid of IFN-β positive regulatory elements III and I) promoter activation, as well as the mRNA transcription of IFN-β and downstream interferon-stimulated genes (ISGs), such as ISG15, ISG54, ISG56, etc., to promote viral infection. UL31 was shown to restrain IFN-β activation at the interferon regulatory factor 3 (IRF3)/IRF7 level. Mechanically, UL31 was demonstrated to interact with TANK binding kinase 1 (TBK1), inducible IκB kinase (IKKi), and IRF3 to impede the formation of the IKKi-IRF3 complex but not the formation of the IRF7-related complex. UL31 could constrain the dimerization and nuclear translocation of IRF3. Although UL31 was associated with the CREB binding protein (CBP)/p300 coactivators, it could not efficiently hamper the formation of the CBP/p300-IRF3 complex. In addition, UL31 could facilitate the degradation of IKKi and IRF3 by mediating their K48-linked polyubiquitination. Taken together, these results illustrated that UL31 was able to suppress IFN-β activity by inhibiting the activation of IKKi and IRF3, which may contribute to the knowledge of a new immune evasion mechanism during HSV-1 infection. IMPORTANCE The innate immune system is the first line of host defense against the invasion of pathogens. Among its mechanisms, IFN-I is an essential cytokine in the antiviral response, which can help the host eliminate a virus. HSV-1 is a double-stranded DNA virus that can cause herpes and establish a lifelong latent infection, due to its possession of multiple mechanisms to escape host innate immunity. In this study, we illustrate for the first time that the HSV-1-encoded UL31 protein has a negative regulatory effect on IFN-β production by blocking the dimerization and nuclear translocation of IRF3, as well as promoting the K48-linked polyubiquitination and degradation of both IKKi and IRF3. This study may be helpful for fully understanding the pathogenesis of HSV-1.

Single-Cell Monitoring of Activated Innate Immune Signaling by a d2eGFP-Based Reporter Mimicking Time-Restricted Activation of IFNB1 Expression

The innate immune system represents a balanced first line of defense against infection. Type I interferons (IFNs) are key regulators of the response to viral infections with an essential early wave of IFN-β expression, which is conditional, time-restricted, and stochastic in its nature. The possibility to precisely monitor individual cells with active IFNB1 transcription during innate signaling requires a robust reporter system that mimics the endogenous IFN-β signal. Here, we present a reporter system based on expression of a destabilized version of eGFP (d2eGFP) from a stably integrated reporter cassette containing the IFNB1 promoter and 3'-untranslated region, enabling both spatial and temporal detection of regulated IFNB1 expression. Specifically, this reporter permits detection, quantification, and isolation of cells actively producing d2eGFP in a manner that fully mimics IFN-β production allowing tracking of IFNB1 gene activation and repression in monocytic cells and keratinocytes. Using induced d2eGFP expression as a readout for activated immune signaling at the single-cell level, we demonstrate the application of the reporter for FACS-based selection of cells with genotypes supporting cGAS-STING signaling. Our studies provide a novel approach for monitoring on/off-switching of innate immune signaling and form the basis for investigating genotypes affecting immune regulation at the single-cell level.

Chromatin Immunoprecipitation Assay in Primary Mouse Hepatocytes and Mouse Liver

Non-alcoholic steatohepatitis (NASH) is an advanced form of non-alcoholic fatty liver disease characterized by hepatosteatosis, liver cell injury, and inflammation. The pathogenesis of NASH involves dysregulated transcription of genes involved in critical processes in the liver, including metabolic homeostasis and inflammation. Chromatin immunoprecipitation (ChIP) utilizes antibody-mediated immunoprecipitation followed by the detection of associated DNA fragments via real-time PCR or high-throughput sequencing to quantitatively profile the interactions of proteins of interest with functional chromatin elements. Here, we present a detailed protocol to study the interactions of DNA and chromatin-associated proteins (e.g., transcription factors, co-activators, co-repressors, and chromatin modifiers) and modified histones (e.g., acetylated and methylated) in isolated primary mouse hepatocytes and mouse liver. The application of these methods can enable the identification of molecular mechanisms that underpin dysregulated hepatic processes in NASH.

The transcription factor activity gradient (TAG) model: contemplating a contact-independent mechanism for enhancer-promoter communication

How distal cis-regulatory elements (e.g., enhancers) communicate with promoters remains an unresolved question of fundamental importance. Although transcription factors and cofactors are known to mediate this communication, the mechanism by which diffusible molecules relay regulatory information from one position to another along the chromosome is a biophysical puzzle-one that needs to be revisited in light of recent data that cannot easily fit into previous solutions. Here we propose a new model that diverges from the textbook enhancer-promoter looping paradigm and offer a synthesis of the literature to make a case for its plausibility, focusing on the coactivator p300.

Viral Induced Effects on a Vulnerable Epithelium; Lessons Learned From Paediatric Asthma and Eosinophilic Oesophagitis

The epithelium is integral to the protection of many different biological systems and for the maintenance of biochemical homeostasis. Emerging evidence suggests that particular children have epithelial vulnerabilities leading to dysregulated barrier function and integrity, that resultantly contributes to disease pathogenesis. These epithelial vulnerabilities likely develop in utero or in early life due to various genetic, epigenetic and environmental factors. Although various epithelia are uniquely structured with specific function, prevalent allergic-type epithelial diseases in children potentially have common or parallel disease processes. These include inflammation and immune response dysregulation stemming from atypical epithelial barrier function and integrity. Two diseases where aetiology and pathogenesis are potentially linked to epithelial vulnerabilities include Paediatric Asthma and Eosinophilic Oesophagitis (EoE). For example, rhinovirus C (RV-C) is a known risk factor for paediatric asthma development and is known to disrupt respiratory epithelial barrier function causing acute inflammation. In addition, EoE, a prevalent atopic condition of the oesophageal epithelium, is characterised by similar innate immune and epithelial responses to viral injury. This review examines the current literature and identifies the gaps in the field defining viral-induced effects on a vulnerable respiratory epithelium and resulting chronic inflammation, drawing from knowledge generated in acute wheezing illness, paediatric asthma and EoE. Besides highlighting the importance of epithelial structure and barrier function in allergic disease pathogenesis regardless of specific epithelial sub-types, this review focuses on the importance of examining other parallel allergic-type disease processes that may uncover commonalities driving disease pathogenesis. This in turn may be beneficial in the development of common therapeutics for current clinical management and disease prevention in the future.

The Impact of Rubella Virus Infection on a Secondary Inflammatory Response in Polarized Human Macrophages

Macrophages (MΦ) are known to exhibit distinct responses to viral and bacterial infection, but how they react when exposed to the pathogens in succession is less well understood. Accordingly, we determined the effect of a rubella virus (RV)-induced infection followed by an LPS-induced challenge on cytokine production, signal transduction and metabolic pathways in human GM (M1-like)- and M (M2-like)-MΦ. We found that infection of both subsets with RV resulted in a low TNF-α and a high interferon (IFN, type I and type III) release whereby M-MΦ produced far more IFNs than GM-MΦ. Thus, TNF-α production in contrast to IFN production is not a dominant feature of RV infection in these cells. Upon addition of LPS to RV-infected MΦ compared to the addition of LPS to the uninfected cells the TNF-α response only slightly increased, whereas the IFN-response of both subtypes was greatly enhanced. The subset specific cytokine expression pattern remained unchanged under these assay conditions. The priming effect of RV was also observed when replacing RV by IFN-β one putative priming stimulus induced by RV. Small amounts of IFN-β were sufficient for phosphorylation of Stat1 and to induce IFN-production in response to LPS. Analysis of signal transduction pathways activated by successive exposure of MΦ to RV and LPS revealed an increased phosphorylation of NFκB (M-MΦ), but different to uninfected MΦ a reduced phosphorylation of ERK1/2 (both subtypes). Furthermore, metabolic pathways were affected; the LPS-induced increase in glycolysis was dampened in both subtypes after RV infection. In conclusion, we show that RV infection and exogenously added IFN-β can prime MΦ to produce high amounts of IFNs in response to LPS and that changes in glycolysis and signal transduction are associated with the priming effect. These findings will help to understand to what extent MΦ defense to viral infection is modulated by a following exposure to a bacterial infection.

STING nuclear partners contribute to innate immune signaling responses

STimulator of INterferon Genes (STING) is an adaptor for cytoplasmic DNA sensing by cGAMP/cGAS that helps trigger innate immune responses (IIRs). Although STING is mostly localized in the ER, we find a separate inner nuclear membrane pool of STING that increases mobility and redistributes to the outer nuclear membrane upon IIR stimulation by transfected dsDNA or dsRNA mimic poly(I:C). Immunoprecipitation of STING from isolated nuclear envelopes coupled with mass spectrometry revealed a distinct nuclear envelope-STING proteome consisting of known nuclear membrane proteins and enriched in DNA- and RNA-binding proteins. Seventeen of these nuclear envelope STING partners are known to bind direct interactors of IRF3/7 transcription factors, and testing a subset of these revealed STING partners SYNCRIP, MEN1, DDX5, snRNP70, RPS27a, and AATF as novel modulators of dsDNA-triggered IIRs. Moreover, we find that SYNCRIP is a novel antagonist of the RNA virus, influenza A, potentially shedding light on reports of STING inhibition of RNA viruses.

Myeloid neddylation targets IRF7 and promotes host innate immunity against RNA viruses

Neddylation, an important type of post-translational modification, has been implicated in innate and adapted immunity. But the role of neddylation in innate immune response against RNA viruses remains elusive. Here we report that neddylation promotes RNA virus-induced type I IFN production, especially IFN-α. More importantly, myeloid deficiency of UBA3 or NEDD8 renders mice less resistant to RNA virus infection. Neddylation is essential for RNA virus-triggered activation of Ifna gene promoters. Further exploration has revealed that mammalian IRF7undergoes neddylation, which is enhanced after RNA virus infection. Even though neddylation blockade does not hinder RNA virus-triggered IRF7 expression, IRF7 mutant defective in neddylation exhibits reduced ability to activate Ifna gene promoters. Neddylation blockade impedes RNA virus-induced IRF7 nuclear translocation without hindering its phosphorylation and dimerization with IRF3. By contrast, IRF7 mutant defective in neddylation shows enhanced dimerization with IRF5, an Ifna repressor when interacting with IRF7. In conclusion, our data demonstrate that myeloid neddylation contributes to host anti-viral innate immunity through targeting IRF7 and promoting its transcriptional activity.

With the features of high mutation rates and fast propagation, RNA viruses remain a great challenge for the control and prevention of epidemic. Better understanding of the molecular mechanisms involved in host innate immunity against RNA viruses will facilitate the development of anti-viral drugs and vaccines. Neddylation has been implicated in innate and adapted immunity. But the role of neddylation in RNA virus-triggered type I IFN production remains elusive. Here, using mouse models with myeloid deficiency of UBA3 or NEDD8, we report for the first time that neddylation contributes to innate immunity against RNA viruses in mammals. Neddylation is indispensable for RNA virus-induced IFN-α production although its role in IFN-β production is much blunted in macrophages. In mechanism, neddylation directly targets IRF7 and enhances its transcriptional activity through, at least partially, promoting its nuclear translocation and preventing its dimerization with IRF5, an Ifna repressor when interacting with IRF7. Our study provides insight into the regulation of IRF7 and innate immune signaling.

Grass carp (Ctenopharyngodon idella) interferon regulatory factor 8 down-regulates interferon1 expression via interaction with interferon regulatory factor 2 in vitro

Interferon regulatory factor 8 (IRF8), also known as interferon consensus sequence-binding protein (ICSBP), is a negative regulatory factor of interferon (IFN) and plays an important role in cell differentiation and innate immunity in mammals. In recent years, some irf8 homologous genes have been cloned and confirmed to take part in innate immune response in fish, but the mechanism still remains unclear. In this paper, a grass carp (Ctenopharyngodon idella) irf8 gene (Ciirf8) was cloned and characterized. The deduced protein (CiIRF8) possesses a highly conserved N-terminal DNA binding domain but a less well-conserved C-terminal IRF association domain (IAD). Ciirf8 was widely expressed in all tested tissues of grass carp and up-regulated following poly(I:C) stimulation. Ciirf8 expression was also up-regulated in CIK cells upon treatment with poly(I:C). To explore the molecular mechanism of how fish IRF8 regulates ifn1 expression, the similarities and differences of grass carp IRF8 and IRF2 were compared and contrasted. Subcellular localization analysis showed that CiIRF8 is located both in the cytoplasm and nucleus; however, CiIRF2 is only located in the nucleus. The nuclear-cytoplasmic translocation of CiIRF8 was observed in CIK cells under stimulation with poly(I:C). The interaction of CiIRF8 and CiIRF2 was further confirmed by a co-immunoprecipitation assay in the nucleus. Dual-luciferase reporter assays showed that the promoter activity of Ciifn1 was significantly inhibited by co-transfection with CiIRF2 and CiIRF8. The transcription inhibition of Ciifn1 was alleviated by competitive binding of CiIRF2 and CiIRF8 to CiIRF1. In conclusion, CiIRF8 down-regulates Ciifn1 expression via interaction with CiIRF2 in cells.

A Novel Intronic Circular RNA Antagonizes Influenza Virus by Absorbing a microRNA That Degrades CREBBP and Accelerating IFN-β Production

Virus-host interactions are complicated processes, and multiple cellular proteins promote or inhibit viral replication through different mechanisms. Recent progress has implicated circular RNAs (circRNAs) in cancer biology and progression; however, the role of circRNAs in viral infection remains largely unclear. Here, we detected 11,620 circRNAs in A549 cells and found that 411 of them were differentially expressed in influenza virus-infected A549 cells. We characterized a novel intronic circRNA, AIVR, that was upregulated in influenza virus-infected A549 cells and found that silencing of AIVR significantly promoted influenza virus replication in A549 cells. We further found that AIVR predominantly localizes in the cytoplasm and works as a microRNA (miRNA) sponge. One of the miRNAs absorbed by AIVR binds the mRNA of CREBBP, which is an important component of the large nucleoprotein complex interferon beta (IFN-β) enhanceosome that accelerates IFN-β production. AIVR overexpression significantly increased the mRNA and protein levels of IFN-β in the influenza virus-infected A549 cells. Therefore, the upregulation of AIVR is a cellular antiviral strategy, with AIVR exerting its antiviral effect by absorbing miRNA and promoting the expression of CREBBP to facilitate IFN-β production. Our study provides new insights into the roles of circRNAs in the cellular innate antiviral response.

ETV7 limits antiviral gene expression and control of influenza viruses

The type I interferon (IFN) response is an important component of the innate immune response to viral infection. Precise control of IFN responses is critical because insufficient expression of IFN-stimulated genes (ISGs) can lead to a failure to restrict viral spread, whereas excessive ISG activation can result in IFN-related pathologies. Although both positive and negative regulatory factors control the magnitude and duration of IFN signaling, it is also appreciated that several ISGs regulate aspects of the IFN response themselves. In this study, we performed a CRISPR activation screen to identify previously unknown regulators of the type I IFN response. We identified the strongly induced ISG encoding ETS variant transcription factor 7 (ETV7) as a negative regulator of the type I IFN response. However, ETV7 did not uniformly suppress ISG transcription. Instead, ETV7 preferentially targeted a subset of antiviral ISGs that were particularly important for IFN-mediated control of influenza viruses. Together, our data assign a function for ETV7 as an IFN response regulator and also identify ETV7 as a potential therapeutic target to increase innate antiviral responses and enhance IFN-based antiviral therapies.

Activation of cGAS/STING pathway upon paramyxovirus infection

During inflammatory diseases, cancer, and infection, the cGAS/STING pathway is known to recognize foreign or self-DNA in the cytosol and activate an innate immune response. Here, we report that negative-strand RNA paramyxoviruses, Nipah virus (NiV), and measles virus (MeV), can also trigger the cGAS/STING axis. Although mice deficient for MyD88, TRIF, and MAVS still moderately control NiV infection when compared with wild-type mice, additional STING deficiency resulted in 100% lethality, suggesting synergistic roles of these pathways in host protection. Moreover, deletion of cGAS or STING resulted in decreased type I interferon production with enhanced paramyxoviral infection in both human and murine cells. Finally, the phosphorylation and ubiquitination of STING, observed during viral infections, confirmed the activation of cGAS/STING pathway by NiV and MeV. Our data suggest that cGAS/STING activation is critical in controlling paramyxovirus infection and possibly represents attractive targets to develop countermeasures against severe disease induced by these pathogens.

Interferon regulatory factor 7 impairs cellular metabolism in aging adipose-derived stromal cells

Dysregulated immunity and widespread metabolic dysfunctions are the most relevant hallmarks of the passing of time over the course of adult life, and their combination at midlife is strongly related to increased vulnerability to diseases; however, the causal connection between them remains largely unclear. By combining multi-omics and functional analyses of adipose-derived stromal cells established from young (1 month) and midlife (12 months) mice, we show that an increase in expression of interferon regulatory factor 7 (IRF7) during adult life drives major metabolic changes, which include impaired mitochondrial function, altered amino acid biogenesis and reduced expression of genes involved in branched-chain amino acid (BCAA) degradation. Our results draw a new paradigm of aging as the 'sterile' activation of a cell-autonomous pathway of self-defense and identify a crucial mediator of this pathway, IRF7, as driver of metabolic dysfunction with age.

Protective effects of SP600125 on mice infected with H1N1 influenza A virus

Influenza A virus (IAV) can cause high morbidity and mortality globally every year. Myriad host kinases and their related signaling pathways are involved in IAV infection, and the important role of the c-Jun N-terminal kinase signaling pathway during infection has been demonstrated. SP600125, an inhibitor of c-Jun N-terminal kinase, was found in our previous study to suppress IAV replication in vitro. In this study, we established a mouse model of H1N1 IAV infection and treated the mice with SP600125 to study its protective effect. The results showed that SP600125 treatment reduced the pulmonary inflammatory response, lung injury, and pulmonary viral load and increased the survival rate of H1N1-infected mice. Our data confirm the crucial role of c-Jun N terminal kinase in H1N1 virus replication and inflammatory responses in vivo. Hence, we speculate that SP600125 has a potential antiviral therapeutic benefit against IAV infection.

Cerebrospinal fluid type I interferon and cytokine profiles in enteroviral meningitis according to the presence or absence of pleocytosis

BACKGROUND

Enteroviral meningitis is typically diagnosed as the presence of pleocytosis and of viral RNA in cerebrospinal fluid. However, it was recently reported that more than 50% of infants with enteroviral meningitis diagnosed by polymerase chain reaction had no cerebrospinal fluid pleocytosis. This study investigated type I interferon (IFN) and cytokine profiles in the cerebrospinal fluid based on the presence or absence of cerebrospinal fluid pleocytosis in children with enteroviral meningitis.

METHODS

We included 51 enteroviral meningitis patients showing cerebrospinal fluid pleocytosis (pleocytosis group), 31 enteroviral meningitis patients without cerebrospinal fluid pleocytosis (non-pleocytosis group), and 52 controls (control group) and compared cerebrospinal fluid interleukin 6 (IL-6), IL-8, chemokine (C-X-C motif) ligand 10 (CXCL-10), IFN-α, and IFN-β levels.

RESULTS

A significant difference was observed in IL-6, IL-8, and CXCL-10 levels across the three groups, with highest values in the pleocytosis patients, followed by those in the non-pleocytosis and control subjects. IFN-α level was higher in the pleocytosis group than in the non-pleocytosis and control groups. Meanwhile, the IFN-β level was higher in the pleocytosis and non-pleocytosis groups than in the control group (34.54 [31.23-38.59] pg/mL vs. 33.21 [31.23-35.21] pg/mL vs. 0.00 [0.00-0.00] pg/mL, respectively; P < 0.001). Furthermore, cerebrospinal fluid IFN-β was detected in all patients with enteroviral meningitis, except one (98.8%) regardless of pleocytosis, whereas it was detected in only two (3.8%) control subjects (P < 0.001).

CONCLUSION

The cerebrospinal fluid cytokine profiles remarkably differed based on the presence or absence of cerebrospinal fluid pleocytosis. Further investigations are required to determine whether cerebrospinal fluid IFN-β could be used as a surrogate marker of viral meningitis instead of cerebrospinal fluid pleocytosis.

Viral infections in humans and mice with genetic deficiencies of the type I IFN response pathway

Type I IFNs are so-named because they interfere with viral infection in vertebrate cells. The study of cellular responses to type I IFNs led to the discovery of the JAK-STAT signaling pathway, which also governs the response to other cytokine families. We review here the outcome of viral infections in mice and humans with engineered and inborn deficiencies, respectively, of (i) IFNAR1 or IFNAR2, selectively disrupting responses to type I IFNs, (ii) STAT1, STAT2, and IRF9, also impairing cellular responses to type II (for STAT1) and/or III (for STAT1, STAT2, IRF9) IFNs, and (iii) JAK1 and TYK2, also impairing cellular responses to cytokines other than IFNs. A picture is emerging of greater redundancy of human type I IFNs for protective immunity to viruses in natural conditions than was initially anticipated. Mouse type I IFNs are essential for protection against a broad range of viruses in experimental conditions. These findings suggest that various type I IFN-independent mechanisms of human cell-intrinsic immunity to viruses have yet to be discovered.

Type I Interferon-Mediated Regulation of Antiviral Capabilities of Neutrophils

Interferons (IFNs) are induced by viruses and are the main regulators of the host antiviral response. They balance tissue tolerance and immune resistance against viral challenges. Like all cells in the human body, neutrophils possess the receptors for IFNs and contribute to antiviral host defense. To combat viruses, neutrophils utilize various mechanisms, such as viral sensing, neutrophil extracellular trap formation, and antigen presentation. These mechanisms have also been linked to tissue damage during viral infection and inflammation. In this review, we presented evidence that a complex cross-regulatory talk between IFNs and neutrophils initiates appropriate antiviral immune responses and regulates them to minimize tissue damage. We also explored recent exciting research elucidating the interactions between IFNs, neutrophils, and severe acute respiratory syndrome-coronavirus-2, as an example of neutrophil and IFN cross-regulatory talk. Dissecting the IFN-neutrophil paradigm is needed for well-balanced antiviral therapeutics and development of novel treatments against many major epidemic or pandemic viral infections, including the ongoing pandemic of the coronavirus disease that emerged in 2019.

Functional Interfaces, Biological Pathways, and Regulations of Interferon-Related DNA Damage Resistance Signature (IRDS) Genes

Interferon (IFN)-related DNA damage resistant signature (IRDS) genes are a subgroup of interferon-stimulated genes (ISGs) found upregulated in different cancer types, which promotes resistance to DNA damaging chemotherapy and radiotherapy. Along with briefly discussing IFNs and signalling in this review, we highlighted how different IRDS genes are affected by viruses. On the contrary, different strategies adopted to suppress a set of IRDS genes (STAT1, IRF7, OAS family, and BST2) to induce (chemo- and radiotherapy) sensitivity were deliberated. Significant biological pathways that comprise these genes were classified, along with their frequently associated genes (IFIT1/3, IFITM1, IRF7, ISG15, MX1/2 and OAS1/3/L). Major upstream regulators from the IRDS genes were identified, and different IFN types regulating these genes were outlined. Functional interfaces of IRDS proteins with DNA/RNA/ATP/GTP/NADP biomolecules featured a well-defined pharmacophore model for STAT1/IRF7-dsDNA and OAS1/OAS3/IFIH1-dsRNA complexes, as well as for the genes binding to GDP or NADP+. The Lys amino acid was found commonly interacting with the ATP phosphate group from OAS1/EIF2AK2/IFIH1 genes. Considering the premise that targeting IRDS genes mediated resistance offers an efficient strategy to resensitize tumour cells and enhances the outcome of anti-cancer treatment, this review can add some novel insights to the field.

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

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

IFN Regulatory Factor 3 in Health and Disease

Immunity to viruses requires an array of critical cellular proteins that include IFN regulatory factor 3 (IRF3). Consequently, most viruses that infect vertebrates encode proteins that interfere with IRF3 activation. This review describes the cellular pathways linked to IRF3 activation and where those pathways are targeted by human viral pathogens. Moreover, key regulatory pathways that control IRF3 are discussed. Besides viral infections, IRF3 is also involved in resistance to some bacterial infections, in anticancer immunity, and in anticancer therapies involving DNA damage agents. A recent finding shows that IRF3 is needed for T cell effector functions that are involved in anticancer immunity and also in T cell autoimmune diseases. In contrast, unregulated IRF3 activity is clearly not beneficial, considering it is implicated in certain interferonopathies, in which heightened IRF3 activity leads to IFN-β-induced disease. Therefore, IRF3 is involved largely in maintaining health but sometimes contributing to disease.

Induction of interferon-β and interferon signaling by TRAIL and Smac mimetics via caspase-8 in breast cancer cells

Breast cancer prognosis is frequently good but a substantial number of patients suffer from relapse. The death receptor ligand TRAIL can in combination with Smac mimetics induce apoptosis in some luminal-like ER-positive breast cancer cell lines, such as CAMA-1, but not in MCF-7 cells. Here we show that TRAIL and the Smac mimetic LCL161 induce non-canonical NF-κB and IFN signaling in ER-positive MCF-7 cells and in CAMA-1 breast cancer cells when apoptosis is blocked by caspase inhibition. Levels of p52 are increased and STAT1 gets phosphorylated. STAT1 phosphorylation is induced by TRAIL alone in MCF-7 cells and is independent of non-canonical NF-κB since downregulation of NIK has no effect. The phosphorylation of STAT1 is a rather late event, appearing after 24 hours of TRAIL stimulation. It is preceded by an increase in IFNB1 mRNA levels and can be blocked by siRNA targeting the type I IFN receptor IFNAR1 and by inhibition of Janus kinases by Ruxolitinib. Moreover, downregulation of caspase-8, but not inhibition of caspase activity, blocks TRAIL-mediated STAT1 phosphorylation and induction of IFN-related genes. The data suggest that TRAIL-induced IFNB1 expression in MCF-7 cells is dependent on a non-apoptotic role of caspase-8 and leads to autocrine interferon-β signaling.

An Overview of Current Knowledge of Deadly CoVs and Their Interface with Innate Immunity

Coronaviruses are a large family of zoonotic RNA viruses, whose infection can lead to mild or lethal respiratory tract disease. Severe Acute Respiratory Syndrome-Coronavirus-1 (SARS-CoV-1) first emerged in Guangdong, China in 2002 and spread to 29 countries, infecting 8089 individuals and causing 774 deaths. In 2012, Middle East Respiratory Syndrome-Coronavirus (MERS-CoV) emerged in Saudi Arabia and has spread to 27 countries, with a mortality rate of ~34%. In 2019, SARS-CoV-2 emerged and has spread to 220 countries, infecting over 100,000,000 people and causing more than 2,000,000 deaths to date. These three human coronaviruses cause diseases of varying severity. Most people develop mild, common cold-like symptoms, while some develop acute respiratory distress syndrome (ARDS). The success of all viruses, including coronaviruses, relies on their evolved abilities to evade and modulate the host anti-viral and pro-inflammatory immune responses. However, we still do not fully understand the transmission, phylogeny, epidemiology, and pathogenesis of MERS-CoV and SARS-CoV-1 and -2. Despite the rapid application of a range of therapies for SARS-CoV-2, such as convalescent plasma, remdesivir, hydroxychloroquine and type I interferon, no fully effective treatment has been determined. Remarkably, COVID-19 vaccine research and development have produced several offerings that are now been administered worldwide. Here, we summarise an up-to-date understanding of epidemiology, immunomodulation and ongoing anti-viral and immunosuppressive treatment strategies. Indeed, understanding the interplay between coronaviruses and the anti-viral immune response is crucial to identifying novel targets for therapeutic intervention, which may even prove invaluable for the control of future emerging coronavirus.

IRF1 governs the differential interferon-stimulated gene responses in human monocytes and macrophages by regulating chromatin accessibility

Myeloid lineage cells use TLRs to recognize and respond to diverse microbial ligands. Although unique transcription factors dictate the outcome of specific TLR signaling, whether lineage-specific differences exist to further modulate the quality of TLR-induced inflammation remains unclear. Comprehensive analysis of global gene transcription in human monocytes, monocyte-derived macrophages, and monocyte-derived dendritic cells stimulated with various TLR ligands identifies multiple lineage-specific, TLR-responsive gene programs. Monocytes are hyperresponsive to TLR7/8 stimulation that correlates with the higher expression of the receptors. While macrophages and monocytes express similar levels of TLR4, macrophages, but not monocytes, upregulate interferon-stimulated genes (ISGs) in response to TLR4 stimulation. We find that TLR4 signaling in macrophages uniquely engages transcription factor IRF1, which facilitates the opening of ISG loci for transcription. This study provides a critical mechanistic basis for lineage-specific TLR responses and uncovers IRF1 as a master regulator for the ISG transcriptional program in human macrophages.

Inhibition of the DNA-Sensing pathway by pseudorabies virus UL24 protein via degradation of interferon regulatory factor 7

The cyclic guanosine monophosphate-adenosine monophosphate synthase (cGAS)-stimulator of interferon genes (STING) signaling pathway plays an important role in the innate immune response by the production of type I interferon (IFN) against DNA virus infection. However, viruses have evolved a variety of strategies to antagonize the host antiviral response to facilitate infection and replication. Pseudorabies virus (PRV), a DNA virus that causes great economic losses to the swine industry, encodes approximate 70 proteins, including some that are involved in evasion of host immunity. However, the mechanism employed by PRV to regulate type I IFN remains unclear. The results of the present study showed that the transcription levels of type I IFN were significantly upregulated by a UL24-deleted PRV strain. Furthermore, IFN-β activation induced by poly(dA:dT) or stimulated by cGAS-STING was inhibited by UL24 overexpression in PK15 cells. Co-immunoprecipitation analysis demonstrated that UL24 interacts with and can degrade interferon regulatory factor 7 (IRF7) through the proteasome pathway in a dose-dependent manner. Together, these results showed that PRV UL24 interacted with IRF7 via the proteasome pathway and antagonized cGAS-STING-mediated activation of IFN-β.

Innate Immune Sensing of Viruses and Its Consequences for the Central Nervous System

Viral infections remain a global public health concern and cause a severe societal and economic burden. At the organismal level, the innate immune system is essential for the detection of viruses and constitutes the first line of defense. Viral components are sensed by host pattern recognition receptors (PRRs). PRRs can be further classified based on their localization into Toll-like receptors (TLRs), C-type lectin receptors (CLR), retinoic acid-inducible gene-I (RIG-I)-like receptors (RLRs), NOD-like receptors (NLRs) and cytosolic DNA sensors (CDS). TLR and RLR signaling results in production of type I interferons (IFNα and -β) and pro-inflammatory cytokines in a cell-specific manner, whereas NLR signaling leads to the production of interleukin-1 family proteins. On the other hand, CLRs are capable of sensing glycans present in viral pathogens, which can induce phagocytic, endocytic, antimicrobial, and pro- inflammatory responses. Peripheral immune sensing of viruses and the ensuing cytokine response can significantly affect the central nervous system (CNS). But viruses can also directly enter the CNS via a multitude of routes, such as the nasal epithelium, along nerve fibers connecting to the periphery and as cargo of infiltrating infected cells passing through the blood brain barrier, triggering innate immune sensing and cytokine responses directly in the CNS. Here, we review mechanisms of viral immune sensing and currently recognized consequences for the CNS of innate immune responses to viruses.

Molecular determinants of disease severity in urinary tract infection

The most common and lethal bacterial pathogens have co-evolved with the host. Pathogens are the aggressors, and the host immune system is responsible for the defence. However, immune responses can also become destructive, and excessive innate immune activation is a major cause of infection-associated morbidity, exemplified by symptomatic urinary tract infections (UTIs), which are caused, in part, by excessive innate immune activation. Severe kidney infections (acute pyelonephritis) are a major cause of morbidity and mortality, and painful infections of the urinary bladder (acute cystitis) can become debilitating in susceptible patients. Disease severity is controlled at specific innate immune checkpoints, and a detailed understanding of their functions is crucial for strategies to counter microbial aggression with novel treatment and prevention measures. One approach is the use of bacterial molecules that reprogramme the innate immune system, accelerating or inhibiting disease processes. A very different outcome is asymptomatic bacteriuria, defined by low host immune responsiveness to bacteria with attenuated virulence. This observation provides the rationale for immunomodulation as a new therapeutic tool to deliberately modify host susceptibility, control the host response and avoid severe disease. The power of innate immunity as an arbitrator of health and disease is also highly relevant for emerging pathogens, including the current COVID-19 pandemic.

The emerging role of probiotics as a mitigation strategy against coronavirus disease 2019 (COVID-19)

COVID-19 is an acute respiratory infection accompanied by pneumonia caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which has affected millions of people globally. To date, there are no highly efficient therapies for this infection. Probiotic bacteria can interact with the gut microbiome to strengthen the immune system, enhance immune responses, and induce appropriate immune signaling pathways. Several probiotics have been confirmed to reduce the duration of bacterial or viral infections. Immune fitness may be one of the approaches by which protection against viral infections can be reinforced. In general, prevention is more efficient than therapy in fighting viral infections. Thus, probiotics have emerged as suitable candidates for controlling these infections. During the COVID-19 pandemic, any approach with the capacity to induce mucosal and systemic reactions could potentially be useful. Here, we summarize findings regarding the effectiveness of various probiotics for preventing virus-induced respiratory infectious diseases, especially those that could be employed for COVID-19 patients. However, the benefits of probiotics are strain-specific, and it is necessary to identify the bacterial strains that are scientifically established to be beneficial.

Context Is Key: Delineating the Unique Functions of IFNα and IFNβ in Disease

Type I interferons (IFNs) are critical effector cytokines of the immune system and were originally known for their important role in protecting against viral infections; however, they have more recently been shown to play protective or detrimental roles in many disease states. Type I IFNs consist of IFNα, IFNβ, IFNϵ, IFNκ, IFNω, and a few others, and they all signal through a shared receptor to exert a wide range of biological activities, including antiviral, antiproliferative, proapoptotic, and immunomodulatory effects. Though the individual type I IFN subtypes possess overlapping functions, there is growing appreciation that they also have unique properties. In this review, we summarize some of the mechanisms underlying differential expression of and signaling by type I IFNs, and we discuss examples of differential functions of IFNα and IFNβ in models of infectious disease, cancer, and autoimmunity.

Interleukin-17A Facilitates Chikungunya Virus Infection by Inhibiting IFN-α2 Expression

Interferons (IFNs) are the key components of innate immunity and are crucial for host defense against viral infections. Here, we report a novel role of interleukin-17A (IL-17A) in inhibiting IFN-α2 expression thus promoting chikungunya virus (CHIKV) infection. CHIKV infected IL-17A deficient (Il17a^-/- ) mice expressed a higher level of IFN-α2 and developed diminished viremia and milder footpad swelling in comparison to wild-type (WT) control mice, which was also recapitulated in IL-17A receptor-deficient (Il17ra^-/- ) mice. Interestingly, IL-17A selectively blocked IFN-α2 production during CHIKV, but not West Nile virus (WNV) or Zika virus (ZIKV), infections. Recombinant IL-17A treatment inhibited CHIKV-induced IFN-α2 expression and enhanced CHIKV replication in both human and mouse cells. We further found that IL-17A inhibited IFN-α2 production by modulating the expression of Interferon Regulatory Factor-5 (IRF-5), IRF-7, IFN-stimulated gene 49 (ISG-49), and Mx1 expression during CHIKV infection. Neutralization of IL-17A in vitro leads to the increase of the expression of these antiviral molecules and decrease of CHIKV replication. Collectively, these results suggest a novel function of IL-17A in inhibiting IFN-α2-mediated antiviral responses during CHIKV infection, which may have broad implications in viral infections and other inflammatory diseases.

PA-X protein of H5N1 avian influenza virus inhibits NF-kappaB activity, a potential mechanism for PA-X counteracting the host innate immune responses

PA-X is a fusion protein of influenza virus which plays a crucial role in modulating influenza virus-induced host innate immune response and subsequent pathogenicity. However, the potential mechanism of PA-X regulation of the host innate immune response remains largely unknown. It is well known that NF-κB signal pathway is crucial for the immediate early step of immune responses activation, while the specific role of PA-X in NF-κB transcriptional activity is totally unknown. In this study, we initially showed that PA-X inhibits NF-κB transcription that stimulated by poly(I:C). We then further determined that the inhibitory effect on NF-κB activation mediated by PA-X was characterized by restricting NF-κB p65 nuclear translocation and nuclear NF-κB p65 activity but not by impeding the phosphorylation of NF-κB p65. Correspondingly, PA-X decreases the amount of NF-κB signaling pathway-associated genes, including TNF-α, Nos2, IL-6 and IL-2. Moreover, PA-X also suppresses both the mRNA and protein expression level of IFN-β, suggesting the direct contribution of PA-X to the inhibition of NF-κB-regulated IFN-β expression. Together, our study sheds light on the potential molecular mechanisms underlying the regulation of host NF-κB activity by PA-X and also identifies a novel functional role for PA-X in counteracting the host innate immune response. However, further exploration of the more elaborate mechanism of PA-X-mediated inhibition of NF-κB activity and the associated signaling pathway may help to elucidate its precise mechanism of evading and subverting the host immune response.

Of Keeping and Tipping the Balance: Host Regulation and Viral Modulation of IRF3-Dependent IFNB1 Expression

The type I interferon (IFN) response is a principal component of our immune system that allows to counter a viral attack immediately upon viral entry into host cells. Upon engagement of aberrantly localised nucleic acids, germline-encoded pattern recognition receptors convey their find via a signalling cascade to prompt kinase-mediated activation of a specific set of five transcription factors. Within the nucleus, the coordinated interaction of these dimeric transcription factors with coactivators and the basal RNA transcription machinery is required to access the gene encoding the type I IFN IFNβ (IFNB1). Virus-induced release of IFNβ then induces the antiviral state of the system and mediates further mechanisms for defence. Due to its key role during the induction of the initial IFN response, the activity of the transcription factor interferon regulatory factor 3 (IRF3) is tightly regulated by the host and fiercely targeted by viral proteins at all conceivable levels. In this review, we will revisit the steps enabling the trans-activating potential of IRF3 after its activation and the subsequent assembly of the multi-protein complex at the IFNβ enhancer that controls gene expression. Further, we will inspect the regulatory mechanisms of these steps imposed by the host cell and present the manifold strategies viruses have evolved to intervene with IFNβ transcription downstream of IRF3 activation in order to secure establishment of a productive infection.

Small Heterodimer Partner Controls the Virus-Mediated Antiviral Immune Response by Targeting CREB-Binding Protein in the Nucleus

Small heterodimer partner (SHP) is an orphan nuclear receptor that acts as a transcriptional co-repressor by interacting with nuclear receptors and transcription factors. Although SHP plays a negative regulatory function in various signaling pathways, its role in virus infection has not been studied. Here, we report that SHP is a potent negative regulator of the virus-mediated type I IFN signaling that maintains homeostasis within the antiviral innate immune system. Upon virus infection, SHP interacts specifically with CREB-binding protein (CBP) in the nucleus, thereby obstructing CBP/β-catenin interaction competitively. Consequently, SHP-deficient cells enhance antiviral responses, including transcription of the type I IFN gene, upon virus infection. Furthermore, SHP-deficient mice show higher levels of IFN production and are more resistant to influenza A virus infection. Our results suggest that SHP is a nuclear regulator that blocks transcription of the type I IFN gene to inhibit excessive innate immune responses.

Interferon regulatory factor 7 mediates obesity-associated MCP-1 transcription

Hypertrophy, associated with adipocyte dysfunction, causes increased pro-inflammatory adipokine, and abnormal glucose and lipid metabolism, leading to insulin resistance and obesity-related-health problems. By combining DNA microarray and genomic data analyses to predict DNA binding motifs, we identified the transcription factor Interferon Regulatory Factor 7 (IRF7) as a possible regulator of genes related to adipocyte hypertrophy. To investigate the role of IRF7 in adipocytes, we examined gene expression patterns in 3T3-L1 cells infected with a retrovirus carrying the IRF7 gene and found that enforced IRF7 expression induced the expression of monocyte chemoattractant protein-1 (MCP-1), a key initial adipokine in the chronic inflammation of obesity. CRISPR/Cas9 mediated-suppression of IRF7 significantly reduced MCP-1 mRNA. Luciferase assays, chromatin immunoprecipitation PCR analysis and gel shift assay showed that IRF7 transactivates the MCP-1 gene by binding to its proximal Interferon Stimulation Response Element (ISRE), a putative IRF7 binding motif. IRF7 knockout mice exhibited lower expression of MCP-1 in epidydimal white adipose tissue under high-fat feeding conditions, suggesting the transcription factor is physiologically important for inducing MCP-1. Taken together, our results suggest that IRF7 transactivates MCP-1 mRNA in adipocytes, and it may be involved in the adipose tissue inflammation associated with obesity.

Zebrafish NIK Mediates IFN Induction by Regulating Activation of IRF3 and NF-κB

Type I IFN mediates the innate immune system to provide defense against viral infections. NF-κB-inducing kinase (NIK) potentiates the basal activation of endogenous STING, which facilitates the recruitment of TBK1 with the ectopically expressed IRF3 to induce IFN production. Moreover, NIK phosphorylates IKKα and confers its ability to phosphorylate p100 (also known as NF-κB2) in mammals. Our study demonstrated that NIK plays a critical role in IFN production in teleost fish. It was found that NIK interacts with IKKα in the cytoplasm and that IKKα phosphorylates the NIK at the residue Thr432, which is different from the mammals. Overexpression of NIK caused the activation of IRF3 and NF-κB, which in turn led to the production of IFN and IFN-stimulated genes (ISGs). Furthermore, the ectopic expression of NIK was observed to be associated with a reduced replication of the fish virus, whereas silencing of endogenous NIK had an opposite effect in vitro. Furthermore, NIK knockdown significantly reduced the expression of IFN and key ISGs in zebrafish larvae after spring viremia of carp virus infection. Additionally, the replication of spring viremia of carp virus was enhanced in NIK knockdown zebrafish larvae, leading to a lower survival rate. In summary, our findings revealed a previously undescribed function of NIK in activating IFN and ISGs as a host antiviral response. These findings may facilitate the establishment of antiviral therapy to combat fish viruses.

Adaptations of Interferon Regulatory Factor 3 with Transition from Terrestrial to Aquatic Life

Interferon regulatory factor 3 (IRF3) and IRF7 are closely related IRF members and the major factors for the induction of interferons, a key component in vertebrate innate immunity. However, there is limited knowledge regarding the evolution and adaptation of those IRFs to the environments. Two unique motifs in IRF3 and 7 were identified. One motif, GASSL, is highly conserved throughout the evolution of IRF3 and 7 and located in the signal response domain. Another motif, DPHK, is in the DNA-binding domain. The ancestral protein of IRF3 and 7 seemed to possess the DPHK motif. In the ray-finned fish lineage, while the DPHK is maintained in IRF7, the motif in IRF3 is changed to NPHK with a D → N amino acid substitution. The D → N substitution are also found in amphibian IRF3 but not in amphibian IRF7. Terrestrial animals such as reptiles and mammals predominantly use DPHK sequences in both IRF3 and 7. However, the D → N substitution in IRF3 DPHK is again found in cetaceans such as whales and dolphins as well as in marsupials. These observations suggest that the D → N substitutions in the IRF3 DPHK motif is likely to be associated with vertebrate's adaptations to aquatic environments and other environmental changes.

The RNA binding protein Quaking represses host interferon response by downregulating MAVS

Quaking (QKI) is an RNA-binding protein (RBP) involved in multiple aspects of RNA metabolism and many biological processes. Despite a known immune function in regulating monocyte differentiation and inflammatory responses, the degree to which QKI regulates the host interferon (IFN) response remains poorly characterized. Here we show that QKI ablation enhances poly(I:C) and viral infection-induced IFNβ transcription. Characterization of IFN-related signalling cascades reveals that QKI knockout results in higher levels of IRF3 phosphorylation. Interestingly, complementation with QKI-5 isoform alone is sufficient to rescue this phenotype and reduce IRF3 phosphorylation. Further analysis shows that MAVS, but not RIG-I or MDA5, is robustly upregulated in the absence of QKI, suggesting that QKI downregulates MAVS and thus represses the host IFN response. As expected, MAVS depletion reduces IFNβ activation and knockout of MAVS in the QKI knockout cells completely abolishes IFNβ induction. Consistently, ectopic expression of RIG-I activates stronger IFNβ induction via MAVS-IRF3 pathway in the absence of QKI. Collectively, these findings demonstrate a novel role for QKI in negatively regulating host IFN response by reducing MAVS levels.

Distinct Roles of Interferon Alpha and Beta in Controlling Chikungunya Virus Replication and Modulating Neutrophil-Mediated Inflammation

Type I interferons (IFNs) are key mediators of the innate immune response. Although members of this family of cytokines signal through a single shared receptor, biochemical and functional variation exists in response to different IFN subtypes. While previous work has demonstrated that type I IFNs are essential to control infection by chikungunya virus (CHIKV), a globally emerging alphavirus, the contributions of individual IFN subtypes remain undefined. To address this question, we evaluated CHIKV pathogenesis in mice lacking IFN-β (IFN-β knockout [IFN-β-KO] mice or mice treated with an IFN-β-blocking antibody) or IFN-α (IFN regulatory factor 7 knockout [IRF7-KO] mice or mice treated with a pan-IFN-α-blocking antibody). Mice lacking either IFN-α or IFN-β developed severe clinical disease following infection with CHIKV, with a marked increase in foot swelling compared to wild-type mice. Virological analysis revealed that mice lacking IFN-α sustained elevated infection in the infected ankle and in distant tissues. In contrast, IFN-β-KO mice displayed minimal differences in viral burdens within the ankle or at distal sites and instead had an altered cellular immune response. Mice lacking IFN-β had increased neutrophil infiltration into musculoskeletal tissues, and depletion of neutrophils in IFN-β-KO but not IRF7-KO mice mitigated musculoskeletal disease caused by CHIKV. Our findings suggest disparate roles for the IFN subtypes during CHIKV infection, with IFN-α limiting early viral replication and dissemination and IFN-β modulating neutrophil-mediated inflammation.IMPORTANCE Type I interferons (IFNs) possess a range of biological activity and protect against a number of viruses, including alphaviruses. Despite signaling through a shared receptor, there are established biochemical and functional differences among the IFN subtypes. The significance of our research is in demonstrating that IFN-α and IFN-β both have protective roles during acute chikungunya virus (CHIKV) infection but do so by distinct mechanisms. IFN-α limits CHIKV replication and dissemination, whereas IFN-β protects from CHIKV pathogenesis by limiting inflammation mediated by neutrophils. Our findings support the premise that the IFN subtypes have distinct biological activities in the antiviral response.

Post-translational Control of Innate Immune Signaling Pathways by Herpesviruses

Herpesviruses constitute a large family of disease-causing DNA viruses. Each herpesvirus strain is capable of infecting particular organisms with a specific cell tropism. Upon infection, pattern recognition receptors (PRRs) recognize conserved viral features to trigger signaling cascades that culminate in the production of interferons and pro-inflammatory cytokines. To invoke a proper immune response while avoiding collateral tissue damage, signaling proteins involved in these cascades are tightly regulated by post-translational modifications (PTMs). Herpesviruses have developed strategies to subvert innate immune signaling pathways in order to ensure efficient viral replication and achieve persistent infection. The ability of these viruses to control the proteins involved in these signaling cascades post-translationally, either directly via virus-encoded enzymes or indirectly through the deregulation of cellular enzymes, has been widely reported. This ability provides herpesviruses with a powerful tool to shut off or restrict host antiviral and inflammatory responses. In this review, we highlight recent findings on the herpesvirus-mediated post-translational control along PRR-mediated signaling pathways.

Inhibition of Microprocessor Function during the Activation of the Type I Interferon Response

Type I interferons (IFNs) are central components of the antiviral response. Most cell types respond to viral infections by secreting IFNs, but the mechanisms that regulate correct expression of these cytokines are not completely understood. Here, we show that activation of the type I IFN response regulates the expression of miRNAs in a post-transcriptional manner. Activation of IFN expression alters the binding of the Microprocessor complex to pri-miRNAs, reducing its processing rate and thus leading to decreased levels of a subset of mature miRNAs in an IRF3-dependent manner. The rescue of Microprocessor function during the antiviral response downregulates the levels of IFN-β and IFN-stimulated genes. All these findings support a model by which the inhibition of Microprocessor activity is an essential step to induce a robust type I IFN response in mammalian cells.