The V protein of human parainfluenza virus 2 antagonizes type I interferon responses by destabilizing signal transducer and activator of transcription 2

Differential TLR-ERK1/2 activity promotes viral ssRNA and dsRNA mimic-induced dysregulated immunity in macrophages

RNA virus induced excessive inflammation and impaired antiviral interferon (IFN-I) responses are associated with severe disease. This innate immune response, also referred to as 'dysregulated immunity,' is caused by viral single-stranded RNA (ssRNA) and double-stranded-RNA (dsRNA) mediated exuberant inflammation and viral protein-induced IFN antagonism. However, key host factors and the underlying mechanism driving viral RNA-mediated dysregulated immunity are poorly defined. Here, using viral ssRNA and dsRNA mimics, which activate toll-like receptor 7 (TLR7) and TLR3, respectively, we evaluated the role of viral RNAs in causing dysregulated immunity. We show that murine bone marrow-derived macrophages (BMDMs) stimulated with TLR3 and TLR7 agonists induce differential inflammatory and antiviral cytokine response. TLR7 activation triggered a robust inflammatory cytokine/chemokine induction compared to TLR3 activation, whereas TLR3 stimulation induced significantly increased IFN/IFN stimulated gene (ISG) response relative to TLR7 activation. To define the mechanistic basis for dysregulated immunity, we examined cell-surface and endosomal TLR levels and downstream mitogen-activated protein kinase (MAPK) and nuclear factor kappa B (NF-kB) activation. We identified a significantly higher cell-surface and endosomal TLR7 expression compared to TLR3, which further correlated with early and robust MAPK (pERK1/2 and p-P38) and NF-kB activation in TLR7-stimulated macrophages. Furthermore, blocking EKR1/2, p38, and NF-kB activity reduced TLR3/7-induced inflammatory cytokine/chemokine levels, whereas only ERK1/2 inhibition enhanced viral RNA-mimic-induced IFN/ISG responses. Collectively, our results illustrate that high cell surface and endosomal TLR7 expression and robust ERK1/2 activation drive viral ssRNA mimic-induced excessive inflammatory and reduced IFN/ISG responses, and blocking ERK1/2 activity would mitigate viral-RNA/TLR-induced dysregulated immunity.

Physiological functions of RIG-I-like receptors

RIG-I-like receptors (RLRs) are crucial for pathogen detection and triggering immune responses and have immense physiological importance. In this review, we first summarize the interferon system and innate immunity, which constitute primary and secondary responses. Next, the molecular structure of RLRs and the mechanism of sensing non-self RNA are described. Usually, self RNA is refractory to the RLR; however, there are underlying host mechanisms that prevent immune reactions. Studies have revealed that the regulatory mechanisms of RLRs involve covalent molecular modifications, association with regulatory factors, and subcellular localization. Viruses have evolved to acquire antagonistic RLR functions to escape the host immune reactions. Finally, the pathologies caused by the malfunction of RLR signaling are described.

Structural mechanism of CRL4-instructed STAT2 degradation via a novel cytomegaloviral DCAF receptor

Human cytomegalovirus (CMV) is a ubiquitously distributed pathogen whose rodent counterparts such as mouse and rat CMV serve as common infection models. Here, we conducted global proteome profiling of rat CMV-infected cells and uncovered a pronounced loss of the transcription factor STAT2, which is crucial for antiviral interferon signalling. Via deletion mutagenesis, we found that the viral protein E27 is required for CMV-induced STAT2 depletion. Cellular and in vitro analyses showed that E27 exploits host-cell Cullin4-RING ubiquitin ligase (CRL4) complexes to induce poly-ubiquitylation and proteasomal degradation of STAT2. Cryo-electron microscopy revealed how E27 mimics molecular surface properties of cellular CRL4 substrate receptors called DCAFs (DDB1- and Cullin4-associated factors), thereby displacing them from the catalytic core of CRL4. Moreover, structural analyses showed that E27 recruits STAT2 through a bipartite binding interface, which partially overlaps with the IRF9 binding site. Structure-based mutations in M27, the murine CMV homologue of E27, impair the interferon-suppressing capacity and virus replication in mouse models, supporting the conserved importance of DCAF mimicry for CMV immune evasion.

Leishmania intercepts IFN-γR signaling at multiple levels in macrophages

IFN-γ, a type 2 interferon and a cytokine, is critical for both innate and adaptive immunity. IFN-γ binds to the IFN-γRs on the cell membrane of macrophages, signals through JAK1-STAT-1 pathway and induces IFN-γ-stimulated genes (ISGs). As Leishmania amastigotes reside and replicate within macrophages, IFN-γ mediated macrophage activation eventuate in Leishmania elimination. As befits the principle of parasitism, the impaired IFN-γ responsiveness in macrophages ensures Leishmania survival. IFN-γ responsiveness is a function of integrated molecular events at multiple levels in the cells that express IFN-γ receptors. In Leishmania-infected macrophages, reduced IFN-γRα expression, impaired IFN-γRα and IFN-γRβ hetero-dimerization due to altered membrane lipid composition, reduced JAK-1 and STAT-1 phosphorylation but increased STAT-1 degradation and impaired ISGs induction collectively determine the IFN-γ responsiveness and the efficacy of IFN-γ induced antileishmanial function of macrophages. Therefore, parasite load is not only decided by the levels of IFN-γ produced but also by the IFN-γ responsiveness. Indeed, in Leishmania-infected patients, IFN-γ is produced but IFN-γ signalling is downregulated. However, the molecular mechanisms of IFN-γ responsiveness remain unclear. Therefore, we review the current understanding of IFN-γ responsiveness of Leishmania-infected macrophages.

Type I and Type II Interferon Antagonism Strategies Used by Paramyxoviridae: Previous and New Discoveries, in Comparison

Paramyxoviridae is a viral family within the order of Mononegavirales; they are negative single-strand RNA viruses that can cause significant diseases in both humans and animals. In order to replicate, paramyxoviruses–as any other viruses–have to bypass an important protective mechanism developed by the host’s cells: the defensive line driven by interferon. Once the viruses are recognized, the cells start the production of type I and type III interferons, which leads to the activation of hundreds of genes, many of which encode proteins with the specific function to reduce viral replication. Type II interferon is produced by active immune cells through a different signaling pathway, and activates a diverse range of genes with the same objective to block viral replication. As a result of this selective pressure, viruses have evolved different strategies to avoid the defensive function of interferons. The strategies employed by the different viral species to fight the interferon system include a number of sophisticated mechanisms. Here we analyzed the current status of the various strategies used by paramyxoviruses to subvert type I, II, and III interferon responses.

Evasion of Host Antiviral Innate Immunity by Paramyxovirus Accessory Proteins

For efficient replication, viruses have developed multiple strategies to evade host antiviral innate immunity. Paramyxoviruses are a large family of enveloped RNA viruses that comprises diverse human and animal pathogens which jeopardize global public health and the economy. The accessory proteins expressed from the P gene by RNA editing or overlapping open reading frames (ORFs) are major viral immune evasion factors antagonizing type I interferon (IFN-I) production and other antiviral innate immune responses. However, the antagonistic mechanisms against antiviral innate immunity by accessory proteins differ among viruses. Here, we summarize the current understandings of immune evasion mechanisms by paramyxovirus accessory proteins, specifically how accessory proteins directly or indirectly target the adaptors in the antiviral innate immune signaling pathway to facilitate virus replication. Additionally, some cellular responses, which are also involved in viral replication, will be briefly summarized.

Mechanisms of Viral Degradation of Cellular Signal Transducer and Activator of Transcription 2

Virus infection of eukaryotes triggers cellular innate immune response, a major arm of which is the type I interferon (IFN) family of cytokines. Binding of IFN to cell surface receptors triggers a signaling cascade in which the signal transducer and activator of transcription 2 (STAT2) plays a key role, ultimately leading to an antiviral state of the cell. In retaliation, many viruses counteract the immune response, often by the destruction and/or inactivation of STAT2, promoted by specific viral proteins that do not possess protease activities of their own. This review offers a summary of viral mechanisms of STAT2 subversion with emphasis on degradation. Some viruses also destroy STAT1, another major member of the STAT family, but most viruses are selective in targeting either STAT2 or STAT1. Interestingly, degradation of STAT2 by a few viruses requires the presence of both STAT proteins. Available evidence suggests a mechanism in which multiple sites and domains of STAT2 are required for engagement and degradation by a multi-subunit degradative complex, comprising viral and cellular proteins, including the ubiquitin–proteasomal system. However, the exact molecular nature of this complex and the alternative degradation mechanisms remain largely unknown, as critically presented here with prospective directions of future study.

Coronavirus disease 2019, allergic diseases, and allergen immunotherapy: Possible favorable mechanisms of interaction

Background: Both, allergen immunotherapy (AIT) and SARS-COV-2 infection cause a set of immunologic changes that respectively vary during the course of the treatment or the disease. Objective: To review immune changes brought along by each of these entities and how they might interrelate. Methods: We start presenting a brief review of the structure of the new coronavirus and how it alters the functioning of the human immune system. Subsequently, we describe the immune changes induced by AIT and how these changes could be favorable or unfavorable in the allergic patient infected with SARS-CoV-2 at a particular point of time during the evolving infection. Results: We describe how a healthy immune response against SARS-CoV-2 develops, versus an immune response that is initially suppressed by the virus, but ultimately overactivated, leading to an excessive production of cytokines (cytokine-storm-like). These changes are then linked to the clinical manifestations and outcomes of the patient. Reviewing the immune changes secondary to AIT, it becomes clear how AIT is capable of restoring a healthy innate immunity. Investigators have previously shown that the frequency of respiratory infections is reduced in allergic patients treated with AIT. On the other hand it also increases immunoregulation. Conclusion: As there are many variables involved, it is hard to predict how AIT could influence the allergic patient's reaction to a SARS-CoV-2 infection. In any case, AIT is likely to be beneficial for the patient with allergic rhinitis and/or allergic asthma in the context of the SARS-CoV-2 pandemic as controlling allergic diseases leads to a reduced need for contact with healthcare professionals. The authors remind the reader that everything in this article is still theoretical, since at the moment, there are no published clinical trials on the outcome of COVID-19 in allergic patients under AIT.

The Human Cytomegalovirus pUL145 Isoforms Act as Viral DDB1-Cullin-Associated Factors to Instruct Host Protein Degradation to Impede Innate Immunity

Human cytomegalovirus (HCMV) causes diseases in individuals with immature or compromised immunity. To evade immune control, HCMV evolved numerous antagonists targeting the interferon system at multiple levels. By comparative analysis of naturally arising variants of the most widely studied HCMV strain, AD169, and a panel of targeted mutants, we uncover the UL145 gene as indispensable for STAT2 downregulation. Ribosome profiling confirms the translation of the canonical pUL145 protein (pUL145-Long) and newly identifies a shorter isoform (pUL145-Short). Both isoforms recruit DDB1-containing ubiquitin ligases to induce proteasomal degradation of STAT2. An alanine-scanning mutagenesis discloses the DDB1 interaction motif of pUL145 that resembles the DDB1-binding interface of cellular substrate receptors of DDB1-containing ubiquitin ligases. Thus, pUL145 constitutes a viral DDB1-cullin-associated factor (vDCAF), which mimics cellular DCAFs to exploit the ubiquitin-proteasome system to impede antiviral immunity. Notably, the viral exploitation of the cullins can be targeted to restore the efficacy of the host immune response.

Computationally predicted SARS-COV-2 encoded microRNAs target NFKB, JAK/STAT and TGFB signaling pathways

Recently an outbreak that emerged in Wuhan, China in December 2019, spread to the whole world in a short time and killed >1,410,000 people. It was determined that a new type of beta coronavirus called severe acute respiratory disease coronavirus type 2 (SARS-CoV-2) was causative agent of this outbreak and the disease caused by the virus was named as coronavirus disease 19 (COVID19). Despite the information obtained from the viral genome structure, many aspects of the virus-host interactions during infection is still unknown. In this study we aimed to identify SARS-CoV-2 encoded microRNAs and their cellular targets. We applied a computational method to predict miRNAs encoded by SARS-CoV-2 along with their putative targets in humans. Targets of predicted miRNAs were clustered into groups based on their biological processes, molecular function, and cellular compartments using GO and PANTHER. By using KEGG pathway enrichment analysis top pathways were identified. Finally, we have constructed an integrative pathway network analysis with target genes. We identified 40 SARS-CoV-2 miRNAs and their regulated targets. Our analysis showed that targeted genes including NFKB1, NFKBIE, JAK1-2, STAT3-4, STAT5B, STAT6, SOCS1-6, IL2, IL8, IL10, IL17, TGFBR1-2, SMAD2-4, HDAC1-6 and JARID1A-C, JARID2 play important roles in NFKB, JAK/STAT and TGFB signaling pathways as well as cells' epigenetic regulation pathways. Our results may help to understand virus-host interaction and the role of viral miRNAs during SARS-CoV-2 infection. As there is no current drug and effective treatment available for COVID19, it may also help to develop new treatment strategies.

The Dynamic Interface of Viruses with STATs

Viruses commonly antagonize the antiviral type I interferon response by targeting signal transducer and activator of transcription 1 (STAT1) and STAT2, key mediators of interferon signaling. Other STAT family members mediate signaling by diverse cytokines important to infection, but their relationship with viruses is more complex. Importantly, virus-STAT interaction can be antagonistic or stimulatory depending on diverse viral and cellular factors. While STAT antagonism can suppress immune pathways, many viruses promote activation of specific STATs to support viral gene expression and/or produce cellular conditions conducive to infection. It is also becoming increasingly clear that viruses can hijack noncanonical STAT functions to benefit infection. For a number of viruses, STAT function is dynamically modulated through infection as requirements for replication change. Given the critical role of STATs in infection by diverse viruses, the virus-STAT interface is an attractive target for the development of antivirals and live-attenuated viral vaccines. Here, we review current understanding of the complex and dynamic virus-STAT interface and discuss how this relationship might be harnessed for medical applications.

The Intrinsically Disordered W Protein Is Multifunctional during Henipavirus Infection, Disrupting Host Signalling Pathways and Nuclear Import

Nipah and Hendra viruses are highly pathogenic, zoonotic henipaviruses that encode proteins that inhibit the host’s innate immune response. The W protein is one of four products encoded from the P gene and binds a number of host proteins to regulate signalling pathways. The W protein is intrinsically disordered, a structural attribute that contributes to its diverse host protein interactions. Here, we review the role of W in innate immune suppression through inhibition of both pattern recognition receptor (PRR) pathways and interferon (IFN)-responsive signalling. PRR stimulation leading to activation of IRF-3 and IFN release is blocked by henipavirus W, and unphosphorylated STAT proteins are sequestered within the nucleus of host cells by W, thereby inhibiting the induction of IFN stimulated genes. We examine the critical role of nuclear transport in multiple functions of W and how specific binding of importin-alpha (Impα) isoforms, and the 14-3-3 group of regulatory proteins suggests further modulation of these processes. Overall, the disordered nature and multiple functions of W warrant further investigation to understand henipavirus pathogenesis and may reveal insights aiding the development of novel therapeutics.

Respirovirus C protein inhibits activation of type I interferon receptor-associated kinases to block JAK-STAT signaling

Respirovirus C protein blocks the type I interferon (IFN)-stimulated activation of the JAK-STAT pathway. It has been reported that C protein inhibits IFN-α-stimulated tyrosine phosphorylation of STATs, but the underlying mechanism is poorly understood. Here, we show that the C protein of Sendai virus (SeV), a member of the Respirovirus genus, binds to the IFN receptor subunit IFN-α/β receptor subunit (IFNAR)2 and inhibits IFN-α-stimulated tyrosine phosphorylation of the upstream receptor-associated kinases, JAK1 and TYK2. Analysis of various SeV C mutant (Cm) proteins demonstrates the importance of the inhibitory effect on receptor-associated kinase phosphorylation for blockade of JAK-STAT signaling. Furthermore, this inhibitory effect and the IFNAR2 binding capacity are observed for all the respirovirus C proteins examined. Our results suggest that respirovirus C protein inhibits activation of the receptor-associated kinases JAK1 and TYK2 possibly through interaction with IFNAR2.

A Single Point Mutation in the Mumps V Protein Alters Targeting of the Cellular STAT Pathways Resulting in Virus Attenuation

Mumps virus (MuV) is a neurotropic non-segmented, negative-stranded, enveloped RNA virus in the Paramyxovirus family. The 15.4 kb genome encodes seven genes, including the V/P, which encodes, among other proteins, the V protein. The MuV V protein has been shown to target the cellular signal transducer and activator of transcription proteins STAT1 and STAT3 for proteasome-mediated degradation. While MuV V protein targeting of STAT1 is generally accepted as a means of limiting innate antiviral responses, the consequence of V protein targeting of STAT3 is less clear. Further, since the MuV V protein targets both STAT1 and STAT3, specifically investigating viral antagonism of STAT3 targeting is challenging. However, a previous study reported that a single amino acid substitution in the MuV V protein (E95D) inhibits targeting of STAT3, but not STAT1. This provided us with a unique opportunity to examine the specific role of STAT 3 in MuV virulence in an in vivo model. Here, using a clone of a wild type MuV strain expressing the E95D mutant V protein, we present data linking inhibition of STAT3 targeting with the accelerated clearance of the virus and reduced neurovirulence in vivo, suggesting its role in promoting antiviral responses. These data suggest a rational approach to virus attenuation that could be exploited for future vaccine development.

Peste des Petits Ruminants Virus Nucleocapsid Protein Inhibits Beta Interferon Production by Interacting with IRF3 To Block Its Activation

Peste des petits ruminants is a highly contagious animal disease affecting small ruminants, which threatens both small livestock and endangered susceptible wildlife populations in many countries. The causative agent, peste des petits ruminants virus (PPRV), often causes acute immunosuppression in its natural hosts during infection. Here, for the first time, we demonstrate that N protein, the most abundant protein of PPRV, plays an extremely important role in suppression of interferon regulatory factor 3 (IRF3) function and type I interferon (IFN) production by interfering with the formation of the TBK1-IRF3 complex. This study explored a novel antagonistic mechanism of PPRV.

Peste des petits ruminants virus (PPRV) is the etiological agent of peste des petits ruminants, causing acute immunosuppression in its natural hosts. However, the molecular mechanisms by which PPRV antagonizes the host immune responses have not been fully characterized. In particular, how PPRV suppresses the activation of the host RIG-I-like receptor (RLR) pathway has yet to be clarified. In this study, we demonstrated that PPRV infection significantly suppresses RLR pathway activation and type I interferon (IFN) production and identified PPRV N protein as an extremely important antagonistic viral factor that suppresses beta interferon (IFN-β) and IFN-stimulated gene (ISG) expression. A detailed analysis showed that PPRV N protein inhibited type I IFN production by targeting interferon regulatory factor 3 (IRF3), a key molecule in the RLR pathway required for type I IFN induction. PPRV N protein interacted with IRF3 (but not with other components of the RLR pathway, including MDA5, RIG-I, VISA, TBK1, and MITA) and abrogated the phosphorylation of IRF3. As expected, PPRV N protein also considerably impaired the nuclear translocation of IRF3. The TBK1-IRF3 interaction was involved significantly in IRF3 phosphorylation, and we showed that PPRV N protein inhibits the association between TBK1 and IRF3, which in turn inhibits IRF3 phosphorylation. The amino acid region 106 to 210 of PPRV N protein was determined to be essential for suppressing the nuclear translocation of IRF3 and IFN-β production, and the 140 to 400 region of IRF3 was identified as the crucial region for the N-IRF3 interaction. Together, our findings demonstrate a new mechanism evolved by PPRV to inhibit type I IFN production and provide structural insights into the immunosuppression caused by PPRV.

IMPORTANCE Peste des petits ruminants is a highly contagious animal disease affecting small ruminants, which threatens both small livestock and endangered susceptible wildlife populations in many countries. The causative agent, peste des petits ruminants virus (PPRV), often causes acute immunosuppression in its natural hosts during infection. Here, for the first time, we demonstrate that N protein, the most abundant protein of PPRV, plays an extremely important role in suppression of interferon regulatory factor 3 (IRF3) function and type I interferon (IFN) production by interfering with the formation of the TBK1-IRF3 complex. This study explored a novel antagonistic mechanism of PPRV.

Decoding type I and III interferon signalling during viral infection

Interferon (IFN)-mediated antiviral responses are central to host defence against viral infection. Despite the existence of at least 20 IFNs, there are only three known cell surface receptors. IFN signalling and viral evasion mechanisms form an immensely complex network that differs across species. In this Review, we begin by highlighting some of the advances that have been made towards understanding the complexity of differential IFN signalling inputs and outputs that contribute to antiviral defences. Next, we explore some of the ways viruses can interfere with, or circumvent, these defences. Lastly, we address the largely under-reviewed impact of IFN signalling on host tropism, and we offer perspectives on the future of research into IFN signalling complexity and viral evasion across species.

The Molecular Basis of Viral Inhibition of IRF- and STAT-Dependent Immune Responses

The antiviral innate immunity is the first line of host defense against virus infections. In mammalian cells, viral infections initiate the expression of interferons (IFNs) in the host that in turn activate an antiviral defense program to restrict viral replications by induction of IFN stimulated genes (ISGs), which are largely regulated by the IFN-regulatory factor (IRF) family and signal transducer and activator of transcription (STAT) family transcription factors. The mechanisms of action of IRFs and STATs involve several post-translational modifications, complex formation, and nuclear translocation of these transcription factors. However, many viruses, including human immunodeficiency virus (HIV), Zika virus (ZIKV), and herpes simplex virus (HSV), have evolved strategies to evade host defense, including alteration in IRF and STAT post-translational modifications, disturbing the formation and nuclear translocation of the transcription complexes as well as proteolysis/degradation of IRFs and STATs. In this review, we discuss and summarize the molecular mechanisms by which how viral components may target IRFs and STATs to antagonize the establishment of antiviral host defense. The underlying host-viral interactions determine the outcome of viral infection. Gaining mechanistic insight into these processes will be crucial in understanding how viral replication can be more effectively controlled and in developing approaches to improve virus infection outcomes.

STAT2-Dependent Immune Responses Ensure Host Survival despite the Presence of a Potent Viral Antagonist

A pathogen encounter induces interferons, which signal via Janus kinases and STAT transcription factors to establish an antiviral state. However, the host and pathogens are situated in a continuous arms race which shapes host evolution toward optimized immune responses and the pathogens toward enhanced immune-evasive properties. Mouse cytomegalovirus (MCMV) counteracts interferon responses by pM27-mediated degradation of STAT2, which directly affects the signaling of type I as well as type III interferons. Using MCMV mutants lacking M27 and mice lacking STAT2, we studied the opposing relationship between antiviral activities and viral antagonism in a natural host-pathogen pair in vitro and in vivo In contrast to wild-type (wt) MCMV, ΔM27 mutant MCMV was efficiently cleared from all organs within a few days in BALB/c, C57BL/6, and 129 mice, highlighting the general importance of STAT2 antagonism for MCMV replication. Despite this effective and relevant STAT2 antagonism, wt and STAT2-deficient mice exhibited fundamentally different susceptibilities to MCMV infections. MCMV replication was increased in all assessed organs (e.g., liver, spleen, lungs, and salivary glands) of STAT2-deficient mice, resulting in mortality during the first week after infection. Taken together, the results of our study reveal the importance of cytomegaloviral interferon antagonism for viral replication as well as a pivotal role of the remaining STAT2 activity for host survival. This mutual influence establishes a stable evolutionary standoff situation with fatal consequences when the equilibrium is disturbed.IMPORTANCE The host limits viral replication by the use of interferons (IFNs), which signal via STAT proteins. Several viruses evolved antagonists targeting STATs to antagonize IFNs (e.g., cytomegaloviruses, Zika virus, dengue virus, and several paramyxoviruses). We analyzed infections caused by MCMV expressing or lacking the STAT2 antagonist pM27 in STAT2-deficient and control mice to evaluate its importance for the host and the virus in vitro and in vivo The inability to counteract STAT2 directly translates into exaggerated IFN susceptibility in vitro and pronounced attenuation in vivo Thus, the antiviral activity mediated by IFNs via STAT2-dependent signaling drove the development of a potent MCMV-encoded STAT2 antagonist which became indispensable for efficient virus replication and spread to organs required for dissemination. Despite this clear impact of viral STAT2 antagonism, the host critically required the remaining STAT2 activity to prevent overt disease and mortality upon MCMV infection. Our findings highlight a remarkably delicate balance between host and virus.

Nonstructural protein of severe fever with thrombocytopenia syndrome phlebovirus targets STAT2 and not STAT1 to inhibit type I interferon-stimulated JAK-STAT signaling

The nonstructural protein NSs of severe fever with thrombocytopenia syndrome phlebovirus blocks type I interferon (IFN)-stimulated JAK-STAT signaling. However, there is continuing controversy as to whether NSs targets STAT1 or STAT2 or both for this blockade. The present study was designed to gain a further understanding of the blockade mechanism. Immunoprecipitation experiments revealed a stronger interaction of NSs with STAT2 than with any other component constituting the JAK-STAT pathway. Expression of NSs resulted in the formation of cytoplasmic inclusion bodies (IBs), and affected cytoplasmic distribution of STAT2. STAT2 was relocated to NSs-induced IBs. Consequently, NSs inhibited IFN-α-stimulated tyrosine phosphorylation and nuclear translocation of STAT2. These inhibitory effects as well as the signaling blockade activity were not observed in NSs mutant proteins lacking the STAT2-binding ability. In contrast, NSs affected neither subcellular distribution nor phosphorylation of STAT1 in response to IFN-α and IFN-γ, demonstrating that NSs has little physical and functional interactions with STAT1. Taken together, these results suggest that NSs sequesters STAT2 into NSs-induced IBs, thereby blocking type I IFN JAK-STAT signaling.

Possible role of the Nipah virus V protein in the regulation of the interferon beta induction by interacting with UBX domain-containing protein1

Nipah virus (NiV) is a highly pathogenic paramyxovirus that causes lethal encephalitis in humans. We previously reported that the V protein, one of the three accessory proteins encoded by the P gene, is one of the key determinants of the pathogenesis of NiV in a hamster infection model. Satterfield B.A. et al. have also revealed that V protein is required for the pathogenicity of henipavirus in a ferret infection model. However, the complete functions of NiV V have not been clarified. In this study, we identified UBX domain-containing protein 1 (UBXN1), a negative regulator of RIG-I-like receptor signaling, as a host protein that interacts with NiV V. NiV V interacted with the UBX domain of UBXN1 via its proximal zinc-finger motif in the C-terminal domain. NiV V increased the level of UBXN1 protein by suppressing its proteolysis. Furthermore, NiV V suppressed RIG-I and MDA5-dependent interferon signaling by stabilizing UBXN1 and increasing the interaction between MAVS and UBXN1 in addition to directly interrupting the activation of MDA5. Our results suggest a novel molecular mechanism by which the induction of interferon is potentially suppressed by NiV V protein via UBXN1.

Parainfluenza Virus Infection Sensitizes Cancer Cells to DNA-Damaging Agents: Implications for Oncolytic Virus Therapy

A parainfluenza virus 5 (PIV5) with mutations in the P/V gene (P/V-CPI^-) is restricted for spread in normal cells but not in cancer cells in vitro and is effective at reducing tumor burdens in mouse model systems. Here we show that P/V-CPI^- infection of HEp-2 human laryngeal cancer cells results in the majority of the cells dying, but unexpectedly, over time, there is an emergence of a population of cells that survive as P/V-CPI^- persistently infected (PI) cells. P/V-CPI^- PI cells had elevated levels of basal caspase activation, and viability was highly dependent on the activity of cellular inhibitor-of-apoptosis proteins (IAPs) such as Survivin and XIAP. In challenge experiments with external inducers of apoptosis, PI cells were more sensitive to cisplatin-induced DNA damage and cell death. This increased cisplatin sensitivity correlated with defects in DNA damage signaling pathways such as phosphorylation of Chk1 and translocation of damage-specific DNA binding protein 1 (DDB1) to the nucleus. Cisplatin-induced killing of PI cells was sensitive to the inhibition of wild-type (WT) p53-inducible protein 1 (WIP1), a phosphatase which acts to terminate DNA damage signaling pathways. A similar sensitivity to cisplatin was seen with cells during acute infection with P/V-CPI^- as well as during acute infections with WT PIV5 and the related virus human parainfluenza virus type 2 (hPIV2). Our results have general implications for the design of safer paramyxovirus-based vectors that cannot establish PI as well as the potential for combining chemotherapy with oncolytic RNA virus vectors.IMPORTANCE There is intense interest in developing oncolytic viral vectors with increased potency against cancer cells, particularly those cancer cells that have gained resistance to chemotherapies. We have found that infection with cytoplasmically replicating parainfluenza virus can result in increases in the killing of cancer cells by agents that induce DNA damage, and this is linked to alterations to DNA damage signaling pathways that balance cell survival versus death. Our results have general implications for the design of safer paramyxovirus-based vectors that cannot establish persistent infection, the repurposing of drugs that target cellular IAPs as antivirals, and the combined use of DNA-damaging chemotherapy agents in conjunction with oncolytic RNA virus vectors.

Interplay between Janus Kinase/Signal Transducer and Activator of Transcription Signaling Activated by Type I Interferons and Viral Antagonism

Interferons (IFNs), which were discovered a half century ago, are a group of secreted proteins that play key roles in innate immunity against viral infection. The major signaling pathway activated by IFNs is the Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway, which leads to the expression of IFN-stimulated genes (ISGs), including many antiviral effectors. Viruses have evolved various strategies with which to antagonize the JAK/STAT pathway to influence viral virulence and pathogenesis. In recent years, notable progress has been made to better understand the JAK/STAT pathway activated by IFNs and antagonized by viruses. In this review, recent progress in research of the JAK/STAT pathway activated by type I IFNs, non-canonical STAT activation, viral antagonism of the JAK/STAT pathway, removing of the JAK/STAT antagonist from viral genome for attenuation, and the potential pathogenesis roles of tyrosine phosphorylation-independent non-canonical STATs activation during virus infection are discussed in detail. We expect that this review will provide new insight into the understanding the complexity of the interplay between JAK/STAT signaling and viral antagonism.

Tetherin antagonism by V proteins is a common trait among the genus Rubulavirus

Tetherin (BST-2/CD317/HM1.24) is an anti-viral factor that restricts the budding of several enveloped viruses. Most of these viruses have evolved to encode tetherin antagonists. Our previous study demonstrated that the growth of human parainfluenza virus type 2 (hPIV-2), a member of the genus Rubulavirus in the family Paramyxoviridae, was inhibited by tetherin, and its V protein decreases the amount of cell surface tetherin by the interaction. In the present study, we investigated whether tetherin inhibits the growth of other rubulaviruses including PIV-5, mumps virus (MuV), simian virus 41, and hPIV-4, and whether their V proteins act as tetherin antagonists. Plaque assay demonstrated that the growth of PIV-5 and MuV was inhibited by tetherin. Flow cytometry and immunoblot analyses revealed that the infection of PIV-5 and MuV caused reduction of cell surface tetherin without affecting total amount of tetherin. Immunoprecipitation analysis showed that all V proteins of rubulaviruses tested bound to tetherin. These results suggest that tetherin antagonism by V proteins is common among the genus Rubulavirus.

Schisandrin A inhibits dengue viral replication via upregulating antiviral interferon responses through STAT signaling pathway

Dengue virus (DENV) infects 400 million people worldwide annually. Infection of more than one serotype of DENV highly corresponds to dengue hemorrhagic fever and dengue shock syndrome, which are the leading causes of high mortality. Due to lack of effective vaccines and unavailable therapies against DENV, discovery of anti-DENV agents is urgently needed. We first characterize that Schisandrin A can inhibit the replication of four serotypes of DENV in a concentration- and time-dependent manner, with an effective half-maximal effective concentration 50% (EC₅₀) value of 28.1 ± 0.42 μM against DENV serotype type 2 without significant cytotoxicity. Furthermore, schisandrin A can effectively protect mice from DENV infection by reducing disease symptoms and mortality of DENV-infected mice. We demonstrate that STAT1/2-mediated antiviral interferon responses contribute to the action of schisandrin A against DENV replication. Schisandrin A represents a potential antiviral agent to block DENV replication in vitro and in vivo. In conclusion, stimulation of STAT1/2-mediated antiviral interferon responses is a promising strategy to develop antiviral drug.

Activation of c-Abl Kinase Potentiates the Anti-myeloma Drug Lenalidomide by Promoting DDA1 Protein Recruitment to the CRL4 Ubiquitin Ligase

Cullin-RING ligase 4 (CRL4), a complex of Cul4 and DDB1, regulates the cell cycle, DNA damage repair, and chromatin replication by targeting a variety of substrates for ubiquitination. CRL4 is also hijacked by viral proteins or thalidomide-derived compounds to degrade host restriction factors. Here we report that the c-Abl non-receptor kinase phosphorylates DDB1 at residue Tyr-316 to recruit a small regulatory protein, DDA1, leading to increased substrate ubiquitination. Pharmacological inhibition or genetic ablation of the Abl-DDB1-DDA1 axis decreases the ubiquitination of CRL4 substrates, including IKZF1 and IKZF3, in lenalidomide-treated multiple myeloma cells. Importantly, panobinostat, a recently approved anti-myeloma drug, and dexamethasone enhance lenalidomide-induced substrate degradation and cytotoxicity by activating c-Abl, therefore providing a mechanism underlying their combination with lenalidomide to treat multiple myeloma.

Foot-and-mouth disease virus structural protein VP3 degrades Janus kinase 1 to inhibit IFN-γ signal transduction pathways

Foot-and-mouth disease is a highly contagious viral disease of cloven-hoofed animals that is caused by foot-and-mouth disease virus (FMDV). To replicate efficiently in vivo, FMDV has evolved methods to circumvent host antiviral defense mechanisms, including those induced by interferons (IFNs). Previous research has focused on the effect of FMDV L(pro) and 3C(pro) on type I IFNs. In this study, FMDV VP3 was found to inhibit type II IFN signaling pathways. The overexpression of FMDV VP3 inhibited the IFN-γ-triggered phosphorylation of STAT1 at Tyr701 and the subsequent expression of downstream genes. Mechanistically, FMDV VP3 interacted with JAK1/2 and inhibited the tyrosine phosphorylation, dimerization and nuclear accumulation of STAT1. FMDV VP3 also disrupted the assembly of the JAK1 complex and degraded JAK1 but not JAK2 via a lysosomal pathway. Taken together, the results reveal a novel mechanism used by which FMDV VP3 counteracts the type II IFN signaling pathways.

Vaccinia Virus Protein C6 Inhibits Type I IFN Signalling in the Nucleus and Binds to the Transactivation Domain of STAT2

The type I interferon (IFN) response is a crucial innate immune signalling pathway required for defense against viral infection. Accordingly, the great majority of mammalian viruses possess means to inhibit this important host immune response. Here we show that vaccinia virus (VACV) strain Western Reserve protein C6, is a dual function protein that inhibits the cellular response to type I IFNs in addition to its published function as an inhibitor of IRF-3 activation, thereby restricting type I IFN production from infected cells. Ectopic expression of C6 inhibits the induction of interferon stimulated genes (ISGs) in response to IFNα treatment at both the mRNA and protein level. C6 inhibits the IFNα-induced Janus kinase/signal transducer and activator of transcription (JAK/STAT) signalling pathway at a late stage, downstream of STAT1 and STAT2 phosphorylation, nuclear translocation and binding of the interferon stimulated gene factor 3 (ISGF3) complex to the interferon stimulated response element (ISRE). Mechanistically, C6 associates with the transactivation domain of STAT2 and this might explain how C6 inhibits the type I IFN signalling very late in the pathway. During virus infection C6 reduces ISRE-dependent gene expression despite the presence of the viral protein phosphatase VH1 that dephosphorylates STAT1 and STAT2. The ability of a cytoplasmic replicating virus to dampen the immune response within the nucleus, and the ability of viral immunomodulators such as C6 to inhibit multiple stages of the innate immune response by distinct mechanisms, emphasizes the intricacies of host-pathogen interactions and viral immune evasion.

In response to a viral infection, infected host cells mount an early, innate immune response to limit viral replication and spread. Type I interferons (IFNs) are produced by a cell when a viral infection is detected and are a crucial aspect of this early immune response. IFNs are released from the infected cell and can act on the infected cell itself or neighbouring cells to initiate a signalling pathway that results in the production of hundreds of anti-viral proteins. In this work we investigated a vaccinia virus protein called C6, a known inhibitor of type I IFN production. Here we show that C6 also inhibits signalling initiated in response to type I IFNs, therefore providing a dual defence against this essential immune response. The results show that, unlike the majority of viral inhibitors of IFN signalling, C6 inhibits the signalling pathway at a late stage once the proteins required for IFN-stimulated gene transcription have reached the nucleus and bound to the DNA. This work illustrates the complex relationship between infecting viruses and the host immune response and further investigation of the mechanism by which C6 inhibits this important immune pathway will likely increase our knowledge of the pathway itself.

The Matrix Protein of Nipah Virus Targets the E3-Ubiquitin Ligase TRIM6 to Inhibit the IKKε Kinase-Mediated Type-I IFN Antiviral Response

For efficient replication, viruses have developed mechanisms to evade innate immune responses, including the antiviral type-I interferon (IFN-I) system. Nipah virus (NiV), a highly pathogenic member of the Paramyxoviridae family (genus Henipavirus), is known to encode for four P gene-derived viral proteins (P/C/W/V) with IFN-I antagonist functions. Here we report that NiV matrix protein (NiV-M), which is important for virus assembly and budding, can also inhibit IFN-I responses. IFN-I production requires activation of multiple signaling components including the IκB kinase epsilon (IKKε). We previously showed that the E3-ubiquitin ligase TRIM6 catalyzes the synthesis of unanchored K48-linked polyubiquitin chains, which are not covalently attached to any protein, and activate IKKε for induction of IFN-I mediated antiviral responses. Using co-immunoprecipitation assays and confocal microscopy we show here that the NiV-M protein interacts with TRIM6 and promotes TRIM6 degradation. Consequently, NiV-M expression results in reduced levels of unanchored K48-linked polyubiquitin chains associated with IKKε leading to impaired IKKε oligomerization, IKKε autophosphorylation and reduced IFN-mediated responses. This IFN antagonist function of NiV-M requires a conserved lysine residue (K258) in the bipartite nuclear localization signal that is found in divergent henipaviruses. Consistent with this, the matrix proteins of Ghana, Hendra and Cedar viruses were also able to inhibit IFNβ induction. Live NiV infection, but not a recombinant NiV lacking the M protein, reduced the levels of endogenous TRIM6 protein expression. To our knowledge, matrix proteins of paramyxoviruses have never been reported to be involved in innate immune antagonism. We report here a novel mechanism of viral innate immune evasion by targeting TRIM6, IKKε and unanchored polyubiquitin chains. These findings expand the universe of viral IFN antagonism strategies and provide a new potential target for development of therapeutic interventions against NiV infections.

Nipah virus (NiV) is a zoonotic paramyxovirus causing severe respiratory and encephalitic illness with case fatality rates of 40 to 90%. The host type-I interferon (IFN-I) system protects against viral infections; however, to establish productive infection NiV has developed mechanisms to evade these host antiviral responses. An important component of the IFN system is the IKKε kinase, which is directly involved in IFN-I production and IFN-I signaling. The activity of the IKKε kinase is regulated by unanchored K48-linked polyubiquitin chains, a novel form of ubiquitin that is not covalently attached to any protein and can induce activation of kinases by promoting protein oligomerization. These unanchored polyubiquitin chains that activate IKKε are generated by the E3-ubiquitin ligase TRIM6. Here we demonstrate that the matrix structural protein (M) of NiV, which is important for virus assembly and budding, also has IFN-I antagonist functions and interferes with the host antiviral response. We found that NiV-M interacts with TRIM6 and promotes its degradation. Consequently, association of unanchored polyubiquitin chains with IKKε is reduced leading to impaired IKKε activation and ineffective IFN responses. Since the matrix protein is present in the virions and is released immediately after virus entry into the cell, this provides an efficient mechanism to escape the host antiviral response. These data may help explain the highly pathogenic potential of these viruses.

Porcine Epidemic Diarrhea Virus Infection Inhibits Interferon Signaling by Targeted Degradation of STAT1

Porcine epidemic diarrhea virus (PEDV) is a worldwide-distributed alphacoronavirus, but the pathogenesis of PEDV infection is not fully characterized. During virus infection, type I interferon (IFN) is a key mediator of innate antiviral responses. Most coronaviruses develop some strategy for at least partially circumventing the IFN response by limiting the production of IFN and by delaying the activation of the IFN response. However, the molecular mechanisms by which PEDV antagonizes the antiviral effects of interferon have not been fully characterized. Especially, how PEDV impacts IFN signaling components has yet to be elucidated. In this study, we observed that PEDV was relatively resistant to treatment with type I IFN. Western blot analysis showed that STAT1 expression was markedly reduced in PEDV-infected cells and that this reduction was not due to inhibition of STAT1 transcription. STAT1 downregulation was blocked by a proteasome inhibitor but not by an autophagy inhibitor, strongly implicating the ubiquitin-proteasome targeting degradation system. Since PEDV infection-induced STAT1 degradation was evident in cells pretreated with the general tyrosine kinase inhibitor, we conclude that STAT1 degradation is independent of the IFN signaling pathway. Furthermore, we report that PEDV-induced STAT1 degradation inhibits IFN-α signal transduction pathways. Pharmacological inhibition of STAT1 degradation rescued the ability of the host to suppress virus replication. Collectively, these data show that PEDV is capable of subverting the type I interferon response by inducing STAT1 degradation.

IMPORTANCE

In this study, we show that PEDV is resistant to the antiviral effect of IFN. The molecular mechanism is the degradation of STAT1 by PEDV infection in a proteasome-dependent manner. This PEDV infection-induced STAT1 degradation contributes to PEDV replication. Our findings reveal a new mechanism evolved by PEDV to circumvent the host antiviral response.

Viral Inhibition of the IFN-Induced JAK/STAT Signalling Pathway: Development of Live Attenuated Vaccines by Mutation of Viral-Encoded IFN-Antagonists

The interferon (IFN) induced anti-viral response is amongst the earliest and most potent of the innate responses to fight viral infection. The induction of the Janus kinase/signal transducer and activation of transcription (JAK/STAT) signalling pathway by IFNs leads to the upregulation of hundreds of interferon stimulated genes (ISGs) for which, many have the ability to rapidly kill viruses within infected cells. During the long course of evolution, viruses have evolved an extraordinary range of strategies to counteract the host immune responses in particular by targeting the JAK/STAT signalling pathway. Understanding how the IFN system is inhibited has provided critical insights into viral virulence and pathogenesis. Moreover, identification of factors encoded by viruses that modulate the JAK/STAT pathway has opened up opportunities to create new anti-viral drugs and rationally attenuated new generation vaccines, particularly for RNA viruses, by reverse genetics.

Newcastle Disease Virus V Protein Targets Phosphorylated STAT1 to Block IFN-I Signaling

Newcastle disease virus (NDV) V protein is considered as an effector for IFN antagonism, however, the mechanism remains unknown. In this study, the expression of STAT1 and phospho-STAT1 in cells infected with NDV or transfected with V protein-expressing plasmids were analyzed. Our results showed that NDV V protein targets phospho-STAT1 reduction in the cells depends on the stimulation of IFN-α. In addition, a V-deficient genotype VII recombinant NDV strain rZJ1-VS was constructed using reverse genetic technique to confirm the results. The rZJ1-VS lost the ability to reduce phospho-STAT1 and induced higher expression of IFN-responsive genes in infected cells. Furthermore, treatment with an ubiquitin E1 inhibitor PYR-41 demonstrated that phospho-STAT1 reduction was caused by degradation, but not de-phosphorylation. We conclude that NDV V protein targets phospho-STAT1 degradation to block IFN-α signaling, which adds novel knowledge to the strategies used by paramyxoviruses to evade IFN.

Viral Anticytokine Strategies

Viruses have evolved numerous mechanisms to evade the immune response, including proteins that target the function of cytokines. This article provides an overview of the different strategies used by viruses to block the induction of cytokines and immune signals triggered by cytokines. Examples of virus evasion proteins are presented, such as intracellular proteins that block signal transduction and immune activation mechanisms, secreted proteins that mimic cytokines, or viral decoy receptors that inhibit the binding of cytokines to their cognate receptors. Virus-encoded proteins that target cytokines play a major role in immune modulation, and their contributions to viral pathogenesis, promoting virus replication or preventing immunopathology, are discussed.

Human parainfluenza virus type 2 V protein inhibits and antagonizes tetherin

Tetherin (BST-2/CD317/HM1.24) is an antiviral membrane protein that prevents the release of enveloped viruses from the cell surface. We found that the growth of human parainfluenza virus type 2 (hPIV-2), but not that of V protein-deficient recombinant hPIV-2, was inhibited by tetherin. V protein immunoprecipitates with tetherin, and this interaction requires its C-terminal Trp residues. The glycosyl phosphatidylinositol attachment signal of tetherin, but not its cytoplasmic tail, was necessary for its binding with V. The distribution of the V protein clearly changed when co-expressed with tetherin in plasmid-transfected cells. hPIV-2 infection of HeLa cells reduced cell surface tetherin without affecting total cellular tetherin. This reduction also occurred in HeLa cells constitutively expressing V, whereas mutated V protein did not affect the cell surface tetherin. Our results suggest that hPIV-2 V protein antagonizes tetherin by binding it and reducing its presence at the cell surface.

Enhanced growth of influenza A virus by coinfection with human parainfluenza virus type 2

It has been reported that dual or multiple viruses can coinfect epithelial cells of the respiratory tract. However, little has been reported on in vitro interactions of coinfected viruses. To explore how coinfection of different viruses affects their biological property, we examined growth of influenza A virus (IAV) and human parainfluenza virus type 2 (hPIV2) during coinfection of Vero cells. We found that IAV growth was enhanced by coinfection with hPIV2. The enhanced growth of IAV was not reproduced by coinfection with an hPIV2 mutant with reduced cell fusion activity, or by ectopic expression of the V protein of hPIV2. In contrast, induction of cell fusion by ectopic expression of the hPIV2 HN and F proteins augments IAV growth. hPIV2 coinfection supported IAV growth in cells originated from the respiratory epithelium. The enhancement correlated closely with cell fusion ability of hPIV2 in those cells. These results indicate that cell fusion induced by hPIV2 infection is beneficial to IAV replication and that enhanced viral replication by coinfection with different viruses can modify their pathological consequences.

La Piedad Michoacán Mexico Virus V protein antagonizes type I interferon response by binding STAT2 protein and preventing STATs nuclear translocation

La Piedad Michoacán Mexico Virus (LPMV) is a member of the Rubulavirus genus within the Paramyxoviridae family. LPMV is the etiologic agent of "blue eye disease", causing a significant disease burden in swine in Mexico with long-term implications for the agricultural industry. This virus mainly affects piglets and is characterized by meningoencephalitis and respiratory distress. It also affects adult pigs, causing reduced fertility and abortions in females, and orchitis and epididymitis in males. Viruses of the Paramyxoviridae family evade the innate immune response by targeting components of the interferon (IFN) signaling pathway. The V protein, expressed by most paramyxoviruses, is a well-characterized IFN signaling antagonist. Until now, there were no reports on the role of the LPMV-V protein in inhibiting the IFN response. In this study we demonstrate that LPMV-V protein antagonizes type I but not type II IFN signaling by binding STAT2, a component of the type I IFN cascade. Our results indicate that the last 18 amino acids of LPMV-V protein are required for binding to STAT2 in human and swine cells. While LPMV-V protein does not affect the protein levels of STAT1 or STAT2, it does prevent the IFN-induced phosphorylation and nuclear translocation of STAT1 and STAT2 thereby inhibiting cellular responses to IFN α/β.

Interplay between the virus and the ubiquitin-proteasome system: molecular mechanism of viral pathogenesis

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Many viruses have evolved to utilize the host UPS for their own benefit.

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Viruses subvert the UPS to maintain optimal level/function of viral proteins.

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Viruses exploit the UPS to degrade host proteins which impede viral growth.

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The UPS serves as an important host anti-viral defense mechanism.

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The UPS is inhibited by some viruses to prevent viral clearance.

The ubiquitin–proteasome system (UPS) plays a central role in a wide range of fundamental cellular functions by ensuring protein quality control and through maintaining a critical level of important regulatory proteins. Viruses subvert or manipulate this cellular machinery to favor viral propagation and to evade host immune response. The UPS serves as a double-edged sword in viral pathogenesis: on the one hand, the UPS is utilized by many viruses to maintain proper function and level of viral proteins; while on the other hand, the UPS constitutes a host defense mechanism to eliminate viral components. To combat this host anti-viral machinery, viruses have evolved to employ the UPS to degrade or inactivate cellular proteins that limit viral growth. This review will highlight our current knowledge pertaining to the different roles for the UPS in viral pathogenesis.

Interferons and viruses: an evolutionary arms race of molecular interactions

Over half a century has passed since interferons (IFNs) were discovered and shown to inhibit virus infection in cultured cells. Since then, researchers have steadily brought to light the molecular details of IFN signaling, catalogued their pleiotropic effects on cells, and harnessed their therapeutic potential for a variety of maladies. While advances have been plentiful, several fundamental questions have yet to be answered and much complexity remains to be unraveled. We explore the current knowledge surrounding four main questions: are type I IFN subtypes differentially produced in response to distinct pathogens? How are IFN subtypes distinguished by cells? What are the mechanisms and consequences of viral antagonism? Lastly, how can the IFN response be harnessed to improve vaccine efficacy?

STAT2: A shape-shifting anti-viral super STAT

STAT2 is unique among the STAT family of transcription factors in that its activation is driven predominantly by only two classes of cell surface receptors: Type I and III interferon receptors. As such, STAT2 plays a critical role in host defenses against viral infections. Viruses have evolved to target STAT2 by either inhibiting its expression, blocking its activity, or by targeting it for degradation. Consequently, these viral onslaughts have driven remarkable divergence in the STAT2 gene across species that is not observed in other STAT family members. Thus, the evolution of STAT2 may preserve its activity and protect each species in the face of an ever-changing viral community.

Human parainfluenza virus type 2 vector induces dendritic cell maturation without viral RNA replication/transcription

The dendritic cell (DC), a most potent antigen-presenting cell, plays a key role in vaccine therapy against infectious diseases and malignant tumors. Although advantages of viral vectors for vaccine therapy have been reported, potential risks for adverse effects prevent them from being licensed for clinical use. Human parainfluenza virus type 2 (hPIV2), one of the members of the Paramyxoviridae family, is a nonsegmented and negative-stranded RNA virus. We have developed a reverse genetics system for the production of infectious hPIV2 lacking the F gene (hPIV2ΔF), wherein various advantages for vaccine therapy exist, such as cytoplasmic replication/transcription, nontransmissible infectivity, and extremely high transduction efficacy in various types of target cells. Here we demonstrate that hPIV2ΔF shows high transduction efficiency in human DCs, while not so high in mouse DCs. In addition, hPIV2ΔF sufficiently induces maturation of both human and murine DCs, and the maturation state of both human and murine DCs is almost equivalent to that induced by lipopolysaccharide. Moreover, alkylating agent β-propiolactone-inactivated hPIV2ΔF (BPL-hPIV2ΔF) elicits DC maturation without viral replication/transcription. These results suggest that hPIV2ΔF may be a useful tool for vaccine therapy as a novel type of paramyxoviral vector, which is single-round infectious vector and has potential adjuvant activity.

Mechanisms underlying the inhibition of interferon signaling by viruses

A hallmark of the antiviral response is the induction of interferons. First discovered in 1957 by Issac and Lindeman, interferons are noted for their ability to interfere with viral replication. Interferons act via autocrine and paracrine pathways to induce an antiviral state in infected cells and in neighboring cells containing interferon receptors. Interferons are the frontline defenders against viral infection and their primary function is to locally restrict viral propagation. Viruses have evolved mechanisms to escape the host interferon response, thus gaining a replicative advantage in host cells. This review will discuss recent findings on the mechanisms viruses use to evade the host interferon response. This knowledge is important because the treatment of viral infections is a challenge of global proportions and a better understanding of the mechanisms viruses use to persist in the host may uncover valuable insights applicable to the discovery of novel drug targets.

STAT2 signaling and dengue virus infection

Dengue virus (DENV) is an important human pathogen whose byzantine relationship with the immune response is poorly understood. DENV causes dengue fever and dengue hemorrhagic fever/dengue shock syndrome, diseases for which palliative care is the only treatment. DENV immunopathogenesis studies are complicated by the lack of an immunocompetent small-animal model, and this has hindered anti-DENV drug and vaccine development. This review describes strategies that DENV uses to evade the type I interferon response and focuses on how data gained from the study of DENV NS5-mediated STAT2 degradation may be used to create immunocompetent DENV mouse models and design anti-DENV therapeutics.

Paramyxovirus activation and inhibition of innate immune responses

Paramyxoviruses represent a remarkably diverse family of enveloped nonsegmented negative-strand RNA viruses, some of which are the most ubiquitous disease-causing viruses of humans and animals. This review focuses on paramyxovirus activation of innate immune pathways, the mechanisms by which these RNA viruses counteract these pathways, and the innate response to paramyxovirus infection of dendritic cells (DC). Paramyxoviruses are potent activators of extracellular complement pathways, a first line of defense that viruses must face during natural infections. We discuss mechanisms by which these viruses activate and combat complement to delay neutralization. Once cells are infected, virus replication drives type I interferon (IFN) synthesis that has the potential to induce a large number of antiviral genes. Here we describe four approaches by which paramyxoviruses limit IFN induction: by limiting synthesis of IFN-inducing aberrant viral RNAs, through targeted inhibition of RNA sensors, by providing viral decoy substrates for cellular kinase complexes, and through direct blocking of the IFN promoter. In addition, paramyxoviruses have evolved diverse mechanisms to disrupt IFN signaling pathways. We describe three general mechanisms, including targeted proteolysis of signaling factors, sequestering cellular factors, and upregulation of cellular inhibitors. DC are exceptional cells with the capacity to generate adaptive immunity through the coupling of innate immune signals and T cell activation. We discuss the importance of innate responses in DC following paramyxovirus infection and their consequences for the ability to mount and maintain antiviral T cells.

Dengue virus co-opts UBR4 to degrade STAT2 and antagonize type I interferon signaling

An estimated 50 million dengue virus (DENV) infections occur annually and more than forty percent of the human population is currently at risk of developing dengue fever (DF) or dengue hemorrhagic fever (DHF). Despite the prevalence and potential severity of DF and DHF, there are no approved vaccines or antiviral therapeutics available. An improved understanding of DENV immune evasion is pivotal for the rational development of anti-DENV therapeutics. Antagonism of type I interferon (IFN-I) signaling is a crucial mechanism of DENV immune evasion. DENV NS5 protein inhibits IFN-I signaling by mediating proteasome-dependent STAT2 degradation. Only proteolytically-processed NS5 can efficiently mediate STAT2 degradation, though both unprocessed and processed NS5 bind STAT2. Here we identify UBR4, a 600-kDa member of the N-recognin family, as an interacting partner of DENV NS5 that preferentially binds to processed NS5. Our results also demonstrate that DENV NS5 bridges STAT2 and UBR4. Furthermore, we show that UBR4 promotes DENV-mediated STAT2 degradation, and most importantly, that UBR4 is necessary for efficient viral replication in IFN-I competent cells. Our data underscore the importance of NS5-mediated STAT2 degradation in DENV replication and identify UBR4 as a host protein that is specifically exploited by DENV to inhibit IFN-I signaling via STAT2 degradation.

Dengue virus (DENV) is the leading cause of mosquito-borne viral illness and death in humans. At present, there are no vaccines and no specific antiviral therapeutics to prevent or treat DENV infections. We previously described that the NS5 protein of DENV inhibits type I interferon signaling in virus-infected cells by mediating STAT2 degradation. This property allows DENV to overcome the antiviral effects of type I interferon, contributing to viral replication in the host. We have now obtained new insight into the mechanism by which DENV NS5 induces STAT2 degradation. NS5 bridges STAT2 with the cellular protein UBR4, a member of a family of predicted E3 ligases, resulting in UBR4-mediated STAT2 degradation. Elimination of UBR4 or mutations in NS5 that prevent its binding to UBR4 prevent NS5 from inducing STAT2 degradation. Importantly, UBR4 is required for optimal DENV replication in the presence of a competent type I interferon system. Our data demonstrate the requirement of a host factor, UBR4, for DENV to overcome the antiviral interferon response. This information might be important for the design of specific DENV inhibitors that prevent dengue virus from evading innate immunity.

The V protein of Tioman virus is incapable of blocking type I interferon signaling in human cells

The capacity of a virus to cross species barriers is determined by the development of bona fide interactions with cellular components of new hosts, and in particular its ability to block IFN-α/β antiviral signaling. Tioman virus (TioV), a close relative of mumps virus (MuV), has been isolated in giant fruit bats in Southeast Asia. Nipah and Hendra viruses, which are present in the same bat colonies, are highly pathogenic in human. Despite serological evidences of close contacts between TioV and human populations, whether TioV is associated to some human pathology remains undetermined. Here we show that in contrast to the V protein of MuV, the V protein of TioV (TioV-V) hardly interacts with human STAT2, does not degrade STAT1, and cannot block IFN-α/β signaling in human cells. In contrast, TioV-V properly binds to human STAT3 and MDA5, and thus interferes with IL-6 signaling and IFN-β promoter induction in human cells. Because STAT2 binding was previously identified as a host restriction factor for some Paramyxoviridae, we established STAT2 sequence from giant fruit bats, and binding to TioV-V was tested. Surprisingly, TioV-V interaction with STAT2 from giant fruit bats is also extremely weak and barely detectable. Altogether, our observations question the capacity of TioV to appropriately control IFN-α/β signaling in both human and giant fruit bats that are considered as its natural host.

HSV-2 inhibits type-I interferon signaling via multiple complementary and compensatory STAT2-associated mechanisms

Type-I interferon (IFN)-mediated responses are a crucial first line of defense against viral infections and are critical for generating both innate and adaptive immunity. Therefore, viruses have necessarily evolved mechanisms to impede the IFN response. HSV-2 was found to completely abolish type-1 IFN-mediated signaling via multiple STAT2-associated mechanisms. Although the extent and kinetics of this inactivation were indistinguishable between the various cell-lines examined, there were distinct differences in the mechanisms HSV-2 employed to subvert IFN-signaling among the cell-lines. These mechanistic differences could be segregated into two categories dependent on the phase of the HSV replicative cycle that was responsible for this inhibition: (1) early phase-inhibited cells which exhibited abrogation of IFN-signaling prior to viral DNA replication; (2) late phase-inhibited cells where early phase inhibition mechanisms were not functional, but viral functions expressed following DNA replication compensated for their ineffectiveness. In early phase-inhibited cells, HSV-2 infection targeted STAT2 protein for proteosomal degradation and prevented de novo expression of STAT2 by degrading its mRNA. In contrast, HSV-2 infected late phase-inhibited cells exhibited no apparent changes in STAT2 transcript or protein levels. However, in these cells STAT2 was not activated by phosphorylation and failed to translocate to the cell nucleus, thereby preventing transactivation of antiviral genes. In primary human fibroblasts, HSV-2 failed to fully degrade STAT2 and therefore, both early and late phase mechanisms functioned cooperatively to subvert IFN-mediated antiviral gene expression. Taken together, these results indicate the importance that HSV-2 has assigned to STAT2, investing significant genomic currency throughout its replicative lifecycle for continuous targeted destruction and inhibition of this protein.

Pathogenesis of acute respiratory illness caused by human parainfluenza viruses

Human parainfluenza viruses (HPIVs) are a common cause of acute respiratory illness throughout life. Infants, children, and the immunocompromised are the most likely to develop severe disease. HPIV1 and HPIV2 are best known to cause croup while HPIV3 is a common cause of bronchiolitis and pneumonia. HPIVs replicate productively in respiratory epithelial cells and do not spread systemically unless the host is severely immunocompromised. Molecular studies have delineated how HPIVs evade and block cellular innate immune responses to permit efficient replication, local spread, and host-to-host transmission. Studies using ex vivo human airway epithelium have focused on virus tropism, cellular pathology and the epithelial inflammatory response, elucidating how events early in infection shape the adaptive immune response and disease outcome.

The C proteins of human parainfluenza virus type 1 block IFN signaling by binding and retaining Stat1 in perinuclear aggregates at the late endosome

Interferons (IFNs) play a crucial role in the antiviral immune response. Whereas the C proteins of wild-type human parainfluenza virus type 1 (WT HPIV1) inhibit both IFN-β induction and signaling, a HPIV1 mutant encoding a single amino acid substitution (F170S) in the C proteins is unable to block either host response. Here, signaling downstream of the type 1 IFN receptor was examined in Vero cells to define at what stage WT HPIV1 can block, and F170S HPIV1 fails to block, IFN signaling. WT HPIV1 inhibited phosphorylation of both Stat1 and Stat2, and this inhibition was only slightly reduced for F170S HPIV1. Degradation of Stat1 or Stat2 was not observed. The HPIV1 C proteins were found to accumulate in the perinuclear space, often forming large granules, and co-localized with Stat1 and the cation-independent mannose 6-phosphate receptor (M6PR) that is a marker for late endosomes. Upon stimulation with IFN-β, both the WT and F170S C proteins remained in the perinuclear space, but only the WT C proteins prevented Stat1 translocation to the nucleus. In addition, WT HPIV1 C proteins, but not F170S C proteins, co-immunoprecipitated both phosphorylated and unphosphorylated Stat1. Our findings suggest that the WT HPIV1 C proteins form a stable complex with Stat1 in perinuclear granules that co-localize with M6PR, and that this direct interaction between the WT HPIV1 C proteins and Stat1 is the basis for the ability of HPIV1 to inhibit IFN signaling. The F170S mutation in HPIV1 C did not prevent perinuclear co-localization with Stat1, but apparently weakened this interaction such that, upon IFN stimulation, Stat1 was translocated to the nucleus to induce an antiviral response.

The nucleocapsid protein of measles virus blocks host interferon response

Measles virus (MV) belongs to the genus Morbillivirus of the family Paramyxoviridae. A number of paramyxoviruses inhibit host interferon (IFN) signaling pathways in host immune systems by various mechanisms. Inhibition mechanisms have been described for many paramyxoviruses. Although there are inconsistencies among previous reports concerning MV, it appears that P/V/C proteins interfere with the pathways. In this study, we confirmed the effects of MV P gene products of a wild MV strain on IFN pathways and examined that of other viral proteins on it. Interestingly, we found that N protein acts as an IFN-α/β and γ-antagonist as strong as P gene products. We further investigated the mechanisms of MV-N inhibition, and revealed that MV-N blocks the nuclear import of activated STAT without preventing STAT and Jak activation or STAT degradation, and that the nuclear translocation of MV-N is important for the inhibition. The inhibitory effect of the N protein was observed as a common feature of other morbilliviruses. The results presented in this report suggest that N protein of MV as well as P/V/C proteins is involved in the inhibition of host IFN signaling pathways.

Nipah and hendra virus interactions with the innate immune system

Nipah virus and Hendra virus are related, highly pathogenic paramyxoviruses with unusually broad host ranges. Henipaviruses encode several proteins that block innate immune responses, and these are likely to serve as virulence factors. Specfically, four virus-encoded proteins, the phosphoprotein (P), the V protein, the W protein, and the C protein have each been demonstrated to counteract aspects of the interferon (IFN)-α/β response, a key component of the innate immune response to virus infection. The available data indicate that V and W can inhibit the production of IFNα/β in response to various stimuli, while the P, V, and W proteins also block the ability of IFNs to signal and induce an antiviral state in cells. The C protein also inhibits the antiviral effects of IFNα/β by a poorly characterized mechanism. Reverse genetics systems, which allow the generation of recombinant viruses bearing specific mutations, have demonstrated the importance of the viral IFN-antagonists for replication. With these systems in hand, the field is now poised to define how specific viral IFN-antagonist functions influence viral pathogenesis.