mda-5, but not RIG-I, is a common target for paramyxovirus V proteins

IFIH1 (MDA5) is required for innate immune detection of intron-containing RNA expressed from the HIV-1 provirus

Intron-containing RNA expressed from the HIV-1 provirus activates type 1 interferon in primary human blood cells, including CD4+ T cells, macrophages, and dendritic cells. To identify the innate immune receptor required for detection of intron-containing RNA expressed from the HIV-1 provirus, a loss-of-function screen was performed with short hairpin RNA-expressing lentivectors targeting twenty-one candidate genes in human monocyte-derived dendritic cells. Among the candidate genes tested, only knockdown of XPO1 (CRM1), IFIH1 (MDA5), or MAVS prevented activation of the interferon-stimulated gene ISG15. The importance of IFIH1 protein was demonstrated by rescue of the knockdown with nontargetable IFIH1 coding sequence. Inhibition of HIV-1-induced ISG15 by the IFIH1-specific Nipah virus V protein, and by IFIH1-transdominant 2-CARD domain-deletion or phosphomimetic point mutations, indicates that IFIH1 (MDA5) filament formation, dephosphorylation, and association with MAVS are all required for innate immune activation in response to HIV-1 transduction. Since both IFIH1 (MDA5) and DDX58 (RIG-I) signal via MAVS, the specificity of HIV-1 RNA detection by IFIH1 was demonstrated by the fact that DDX58 knockdown had no effect on activation. RNA-Seq showed that IFIH1 knockdown in dendritic cells globally disrupted the induction of IFN-stimulated genes by HIV-1. Finally, specific enrichment of unspliced HIV-1 RNA by IFIH1 (MDA5), over two orders of magnitude, was revealed by formaldehyde cross-linking immunoprecipitation (f-CLIP). These results demonstrate that IFIH1 is the innate immune receptor for intron-containing RNA from the HIV-1 provirus and that IFIH1 potentially contributes to chronic inflammation in people living with HIV-1, even in the presence of effective antiretroviral therapy.

A Comparative Assessment of the Pathogenic Potential of Newly Discovered Henipaviruses

Recent advances in high-throughput sequencing technologies have led to the discovery of a plethora of previously unknown viruses in animal samples. Some of these newly detected viruses are closely related to human pathogens. A prime example are the henipaviruses. Both Nipah (NiV) and Hendra virus (HeV) cause severe disease in humans. Henipaviruses are of zoonotic origin, and animal hosts, including intermediate hosts, play a critical role in viral transmission to humans. The natural reservoir hosts of NiV and HeV seem to be restricted to a few fruit bat species of the Pteropus genus in distinct geographic areas. However, the recent discovery of novel henipa- and henipa-like viruses suggests that these viruses are far more widespread than was originally thought. To date, these new viruses have been found in a wide range of animal hosts, including bats, shrews, and rodents in Asia, Africa, Europe, and South America. Since these viruses are closely related to human pathogens, it is important to learn whether they pose a threat to human health. In this article, we summarize what is known about the newly discovered henipaviruses, highlight differences to NiV and HeV, and discuss their pathogenic potential.

The W195 Residue of the Newcastle Disease Virus V Protein Is Critical for Multiple Aspects of Viral Self-Regulation through Interactions between V and Nucleoproteins

The transcription and replication of the Newcastle disease virus (NDV) strictly rely on the viral ribonucleoprotein (RNP) complex, which is composed of viral NP, P, L and RNA. However, it is not known whether other viral non-RNP proteins participate in this process for viral self-regulation. In this study, we used a minigenome (MG) system to identify the regulatory role of the viral non-RNP proteins V, M, W, F and HN. Among them, V significantly reduced MG-encoded reporter activity compared with the other proteins and inhibited the synthesis of viral mRNA and cRNA. Further, V interacted with NP. A mutation in residue W195 of V diminished V-NP interaction and inhibited inclusion body (IB) formation in NP-P-L-cotransfected cells. Furthermore, a reverse-genetics system for the highly virulent strain F48E9 was established. The mutant rF48E9-VW195R increased viral replication and apparently enhanced IB formation. In vivo experiments demonstrated that rF48E9-VW195R decreased virulence and retarded time of death. Overall, the results indicate that the V-NP interaction of the W195 mutant V decreased, which regulated viral RNA synthesis, IB formation, viral replication and pathogenicity. This study provides insight into the self-regulation of non-RNP proteins in paramyxoviruses.

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.

Molecular biology of canine parainfluenza virus V protein and its potential applications in tumor immunotherapy

Canine parainfluenza virus (CPIV) is a zoonotic virus that is widely distributed and is the main pathogen causing canine infectious respiratory disease (CIRD), also known as "kennel cough," in dogs. The CPIV-V protein is the only nonstructural protein of the virus and plays an important role in multiple stages of the virus life cycle by inhibiting apoptosis, altering the host cell cycle and interfering with the interferon response. In addition, studies have shown that the V protein has potential applications in the field of immunotherapy in oncolytic virus therapy or self-amplifying RNA vaccines. In this review, the biosynthesis, structural characteristics and functions of the CPIV-V protein are reviewed with an emphasis on how it facilitates viral immune escape and its potential applications in the field of immunotherapy. Therefore, this review provides a scientific basis for research into the CPIV-V protein and its potential applications.

IFIH1 (MDA5) is required for innate immune detection of intron-containing RNA expressed from the HIV-1 provirus

Antiretroviral therapy (ART) suppresses HIV-1 viremia and prevents progression to AIDS. Nonetheless, chronic inflammation is a common problem for people living with HIV-1 on ART. One possible cause of inflammation is ongoing transcription from HIV-1 proviruses, whether or not the sequences are competent for replication. Previous work has shown that intron-containing RNA expressed from the HIV-1 provirus in primary human blood cells, including CD4+ T cells, macrophages, and dendritic cells, activates type 1 interferon. This activation required HIV-1 rev and was blocked by the XPO1 (CRM1)-inhibitor leptomycin. To identify the innate immune receptor required for detection of intron-containing RNA expressed from the HIV-1 provirus, a loss-of-function screen was performed with shRNA-expressing lentivectors targeting twenty-one candidate genes in human monocyte derived dendritic cells. Among the candidate genes tested, only knockdown of XPO1 (CRM1), IFIH1 (MDA5), or MAVS prevented activation of the IFN-stimulated gene ISG15. The importance of IFIH1 protein was demonstrated by rescue of the knockdown with non-targetable IFIH1 coding sequence. Inhibition of HIV-1-induced ISG15 by the IFIH1-specific Nipah virus V protein, and by IFIH1-transdominant inhibitory CARD-deletion or phosphomimetic point mutations, indicates that IFIH1 filament formation, dephosphorylation, and association with MAVS, are all required for innate immune activation in response to HIV-1 transduction. Since both IFIH1 and DDX58 (RIG-I) signal via MAVS, the specificity of HIV-1 RNA detection by IFIH1 was demonstrated by the fact that DDX58 knockdown had no effect on activation. RNA-Seq showed that IFIH1-knockdown in dendritic cells globally disrupted the induction of IFN-stimulated genes. Finally, specific enrichment of unspliced HIV-1 RNA by IFIH1 was revealed by formaldehyde crosslinking immunoprecipitation (f-CLIP). These results demonstrate that IFIH1 is required for innate immune activation by intron-containing RNA from the HIV-1 provirus, and potentially contributes to chronic inflammation in people living with HIV-1.

Viral evasion of the interferon response at a glance

Re-emerging and new viral pathogens have caused significant morbidity and mortality around the world, as evidenced by the recent monkeypox, Ebola and Zika virus outbreaks and the ongoing COVID-19 pandemic. Successful viral infection relies on tactical viral strategies to derail or antagonize host innate immune defenses, in particular the production of type I interferons (IFNs) by infected cells. Viruses can thwart intracellular sensing systems that elicit IFN gene expression (that is, RIG-I-like receptors and the cGAS-STING axis) or obstruct signaling elicited by IFNs. In this Cell Science at a Glance article and the accompanying poster, we review the current knowledge about the major mechanisms employed by viruses to inhibit the activity of intracellular pattern-recognition receptors and their downstream signaling cascades leading to IFN-based antiviral host defenses. Advancing our understanding of viral immune evasion might spur unprecedented opportunities to develop new antiviral compounds or vaccines to prevent viral infectious diseases.

Porcine Respirovirus 1 Suppresses Host Type I Interferon Production and the JAK-STAT Signaling Pathway

Porcine respirovirus 1 (PRV1), first reported in Hong Kong, is currently widely spread in several countries. Our knowledge of the clinical significance and the pathogenicity of this virus is still limited. In this study, we studied the interactions between PRV1 and host innate immune responses. PRV1 exhibited strong inhibitory effects on the production of interferon (IFN), ISG15, and RIG-I induced by SeV infection. Our data generated in vitro suggest that multiple viral proteins can suppress host type I interferon production and signaling, including N, M, and P/C/V/W. The P gene products disrupt both IRF3 and NF-κB dependent type I IFN production and block type I IFN signaling pathway by sequestering STAT1 in the cytoplasm. The V protein disrupts both MDA5 signaling and RIG-I signaling through interaction with TRIM25 and RIG-I, V protein blocks RIG-I polyubiquitination, which is required for RIG-I activation. V protein also binds to MDA5, which may contribute to its inhibitory effect on MDA5 signaling. These findings indicate that PRV1 antagonizes host innate immune responses using various mechanisms, which provides important insights into the pathogenicity of PRV1.

Immune correlates of protection for SARS-CoV-2, Ebola and Nipah virus infection

Correlates of protection (CoP) are biological parameters that predict a certain level of protection against an infectious disease. Well-established correlates of protection facilitate the development and licensing of vaccines by assessing protective efficacy without the need to expose clinical trial participants to the infectious agent against which the vaccine aims to protect. Despite the fact that viruses have many features in common, correlates of protection can vary considerably amongst the same virus family and even amongst a same virus depending on the infection phase that is under consideration. Moreover, the complex interplay between the various immune cell populations that interact during infection and the high degree of genetic variation of certain pathogens, renders the identification of immune correlates of protection difficult. Some emerging and re-emerging viruses of high consequence for public health such as SARS-CoV-2, Nipah virus (NiV) and Ebola virus (EBOV) are especially challenging with regards to the identification of CoP since these pathogens have been shown to dysregulate the immune response during infection. Whereas, virus neutralising antibodies and polyfunctional T-cell responses have been shown to correlate with certain levels of protection against SARS-CoV-2, EBOV and NiV, other effector mechanisms of immunity play important roles in shaping the immune response against these pathogens, which in turn might serve as alternative correlates of protection. This review describes the different components of the adaptive and innate immune system that are activated during SARS-CoV-2, EBOV and NiV infections and that may contribute to protection and virus clearance. Overall, we highlight the immune signatures that are associated with protection against these pathogens in humans and could be used as CoP.

Avian Influenza NS1 Proteins Inhibit Human, but Not Duck, RIG-I Ubiquitination and Interferon Signaling

The nonstructural protein 1 (NS1) of influenza A viruses is an important virulence factor that controls host cell immune responses. In human cells, NS1 proteins inhibit the induction of type I interferon by several mechanisms, including potentially, by preventing the activation of the retinoic acid-inducible gene I (RIG-I) receptor by the ubiquitin ligase tripartite motif-containing protein 25 (TRIM25). It is unclear whether the inhibition of human TRIM25 is a universal function of all influenza A NS1 proteins or is strain dependent. It is also unclear if NS1 proteins similarly target the TRIM25 of mallard ducks, a natural reservoir host of avian influenza viruses with a long coevolutionary history and unique disease dynamics. To answer these questions, we compared the ability of five different NS1 proteins to interact with human and duck TRIM25 using coimmunoprecipitation and microscopy and assessed the consequence of this on RIG-I ubiquitination and signaling in both species. We show that NS1 proteins from low-pathogenic and highly pathogenic avian influenza viruses potently inhibit RIG-I ubiquitination and reduce interferon promoter activity and interferon-beta protein secretion in transfected human cells, while the NS1 of the mouse-adapted PR8 strain does not. However, all the NS1 proteins, when cloned into recombinant viruses, suppress interferon in infected alveolar cells. In contrast, avian NS1 proteins do not suppress duck RIG-I ubiquitination and interferon promoter activity, despite interacting with duck TRIM25. IMPORTANCE Influenza A viruses are a major cause of human and animal disease. Periodically, avian influenza viruses from wild waterfowl, such as ducks, pass through intermediate agricultural hosts and emerge into the human population as zoonotic diseases with high mortality rates and epidemic potential. Because of their coevolution with influenza A viruses, ducks are uniquely resistant to influenza disease compared to other birds, animals, and humans. Here, we investigate a mechanism of influenza A virus interference in an important antiviral signaling pathway that is orthologous in humans and ducks. We show that NS1 proteins from four avian influenza strains can block the coactivation and signaling of the human RIG-I antiviral receptor, while none block the coactivation and signaling of duck RIG-I. Understanding host-pathogen dynamics in the natural reservoir will contribute to our understanding of viral disease mechanisms, viral evolution, and the pressures that drive it, which benefits global surveillance and outbreak prevention.

Host-pathogen protein-nucleic acid interactions: A comprehensive review

Recognition of pathogen-derived nucleic acids by host cells is an effective host strategy to detect pathogenic invasion and trigger immune responses. In the context of pathogen-specific pharmacology, there is a growing interest in mapping the interactions between pathogen-derived nucleic acids and host proteins. Insight into the principles of the structural and immunological mechanisms underlying such interactions and their roles in host defense is necessary to guide therapeutic intervention. Here, we discuss the newest advances in studies of molecular interactions involving pathogen nucleic acids and host factors, including their drug design, molecular structure and specific patterns. We observed that two groups of nucleic acid recognizing molecules, Toll-like receptors (TLRs) and the cytoplasmic retinoic acid-inducible gene (RIG)-I-like receptors (RLRs) form the backbone of host responses to pathogen nucleic acids, with additional support provided by absent in melanoma 2 (AIM2) and DNA-dependent activator of Interferons (IFNs)-regulatory factors (DAI) like cytosolic activity. We review the structural, immunological, and other biological aspects of these representative groups of molecules, especially in terms of their target specificity and affinity and challenges in leveraging host-pathogen protein-nucleic acid interactions (HP-PNI) in drug discovery.

JMM Profile: Avian paramyxovirus type-1 and Newcastle disease: a highly infectious vaccine-preventable viral disease of poultry with low zoonotic potential

Newcastle disease (ND) is a highly contagious disease of poultry caused by virulent avian paramyxovirus-1 (APMV-1) (previously termed avian avulavirus-1 and avian orthoavulavirus-1). APMV-1 is endemic in poultry in many developing countries, whilst outbreaks still occur in developed countries, affecting both commercial and backyard flocks. ND outbreaks can have substantial economic consequences due to high mortality rates and the imposition of trade restrictions. APMV-1 nucleic acid can be detected from swabs or tissues of suspected cases by PCR. Evidence of infection or vaccination may be demonstrated by the presence of specific antibodies against HN in serum samples. No anti-viral treatments exist, but vaccines are available, although there are currently concerns over their efficacy.

Generating human artery and vein cells from pluripotent stem cells highlights the arterial tropism of Nipah and Hendra viruses

Stem cell research endeavors to generate specific subtypes of classically defined "cell types." Here, we generate >90% pure human artery or vein endothelial cells from pluripotent stem cells within 3-4 days. We specified artery cells by inhibiting vein-specifying signals and vice versa. These cells modeled viral infection of human vasculature by Nipah and Hendra viruses, which are extraordinarily deadly (∼57%-59% fatality rate) and require biosafety-level-4 containment. Generating pure populations of artery and vein cells highlighted that Nipah and Hendra viruses preferentially infected arteries; arteries expressed higher levels of their viral-entry receptor. Virally infected artery cells fused into syncytia containing up to 23 nuclei, which rapidly died. Despite infecting arteries and occupying ∼6%-17% of their transcriptome, Nipah and Hendra largely eluded innate immune detection, minimally eliciting interferon signaling. We thus efficiently generate artery and vein cells, introduce stem-cell-based toolkits for biosafety-level-4 virology, and explore the arterial tropism and cellular effects of Nipah and Hendra viruses.

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.

Henipavirus Immune Evasion and Pathogenesis Mechanisms: Lessons Learnt from Natural Infection and Animal Models

Nipah henipavirus (NiV) and Hendra henipavirus (HeV) are zoonotic emerging paramyxoviruses causing severe disease outbreaks in humans and livestock, mostly in Australia, India, Malaysia, Singapore and Bangladesh. Both are bat-borne viruses and in humans, their mortality rates can reach 60% in the case of HeV and 92% for NiV, thus being two of the deadliest viruses known for humans. Several factors, including a large cellular tropism and a wide zoonotic potential, con-tribute to their high pathogenicity. This review provides an overview of HeV and NiV pathogenicity mechanisms and provides a summary of their interactions with the immune systems of their different host species, including their natural hosts bats, spillover-hosts pigs, horses, and humans, as well as in experimental animal models. A better understanding of the interactions between henipaviruses and their hosts could facilitate the development of new therapeutic strategies and vaccine measures against these re-emerging viruses.

Amplification of immunity by engineering chicken MDA5 combined with the C terminal domain (CTD) of RIG-I

Innate immune system is triggered by pattern recognition receptors (PRRs) recognition. Retinoic acid-inducible gene 1 (RIG-I) is a major sensor that recognizes RNA ligands. However, chickens have no homologue of RIG-I; instead, they rely on melanoma differentiation-associated protein 5 (MDA5) to recognize RNA ligands, which renders chickens susceptible to infection by influenza A viruses (IAVs). Here, we engineered the cMDA5 viral RNA sensing domain (C-terminal domain, CTD) such that it functions similarly to human RIG-I (hRIG-I) by mutating histidine 925 into phenylalanine, a key residue for hRIG-I RNA binding loop function, or by swapping the CTD of cMDA5 with that of hRIG-I or duck RIG-I (dRIG-I). The engineered cMDA5 gene was expressed in cMDA5 knockout DF-1 cells, and interferon-beta (IFN-β) activity and expression of interferon-related genes were measured after transfection of cells with RNA ligands of hRIG-I or human MDA5 (hMDA5). We found that both mutant cMDA5 and engineered cMDA5 triggered significantly stronger interferon-mediated immune responses than wild-type cMDA5. Moreover, engineered cMDA5 reduced the IAV titer by 100-fold compared with that in control cells. Collectively, engineered cMDA5/RIG-I CTD significantly enhanced interferon-mediated immune responses, making them invaluable strategies for production of IAV-resistant chickens. KEY POINTS: • Mutant chicken MDA5 with critical residue of RIG-I (phenylalanine) enhanced immunity. • Engineered chicken MDA5 with CTD of RIG-I increased IFN-mediated immune responses. • Engineered chicken MDA5 reduced influenza A virus titers by up to 100-fold.

Nipah Virus V Protein Binding Alters MDA5 Helicase Folding Dynamics

Nipah virus (NiV) is an emerging and deadly zoonotic paramyxovirus that is responsible for periodic epidemics of acute respiratory illness and encephalitis in humans. Previous studies have shown that the NiV V protein antagonizes host antiviral immunity, but the molecular mechanism is incompletely understood. To address this gap, we biochemically characterized NiV V binding to the host pattern recognition receptor MDA5. We find that the C-terminal domain of NiV V (VCTD) is sufficient to bind the MDA5SF2 domain when recombinantly co-expressed in bacteria. Analysis by hydrogen-deuterium exchange mass spectrometry (HDX-MS) studies revealed that NiV VCTD is conformationally dynamic, and binding to MDA5 reduces the dynamics of VCTD. Our results also suggest that the β-sheet region in between the MDA5 Hel1, Hel2, and Hel2i domains exhibits rapid HDX. Upon VCTD binding, these β-sheet and adjacent residues show significant protection. Collectively, our findings suggest that NiV V binding disrupts the helicase fold and dynamics of MDA5 to antagonize host antiviral immunity.

Evolution of RNA sensing receptors in birds

Birds are important hosts for many RNA viruses, including influenza A virus, Newcastle disease virus, West Nile virus and coronaviruses. Innate defense against RNA viruses in birds involves detection of viral RNA by pattern recognition receptors. Several receptors of different classes are involved, such as endosomal toll-like receptors and cytoplasmic retinoic acid-inducible gene I-like receptors, and their downstream adaptor proteins. The function of these receptors and their antagonism by viruses is well established in mammals; however, this has received less attention in birds. These receptors have been characterized in a few bird species, and the completion of avian genomes will permit study of their evolution. For each receptor, functional work has established ligand specificity and activation by viral infection. Engagement of adaptors, regulation by modulators and the supramolecular organization of proteins required for activation are incompletely understood in both mammals and birds. These receptors bind conserved nucleic acid agonists such as single- or double-stranded RNA and generally show purifying selection, particularly the ligand binding regions. However, in birds, these receptors and adaptors differ between species, and between individuals, suggesting that they are under selection for diversification over time. Avian receptors and signalling pathways, like their mammalian counterparts, are targets for antagonism by a variety of viruses, intent on escape from innate immune responses.

Chicken ISG12(2) attenuates Newcastle disease virus and enhances the efficiency of Newcastle disease vaccine via activating immune pathways

Low virulence and strong immunogenicity are quite important for Newcastle disease virus (NDV) producing Newcastle disease (ND) living-attenuated vaccine. However, immunogenicity of NDV positively correlates to its virulence. Usually, the velogenic NDV induces stronger immune responses of poultry than the lentogenic strain, but virulent NDV poses a risk for chicken. In this study, we identified the chicken interferon (IFN)-stimulated gene 12-2 (ISG12(2)) not only attenuated NDV, but also increased immunogenicity of ND vaccine strain. Firstly, we found that NDV infection or IFNs stimulation induced expression of chicken ISG12(2) that reinforced expression of IFNs. Overexpression or knockdown proved that chicken ISG12(2) inhibited NDV replication. Then, recombinant NDV LaSota strains (rLaSota/Fmut/ISG12(2) and rLaSota/ISG12(2)), expressing ISG12(2), were rescued. Pathogenicity tests showed that ISG12(2) expression attenuated NDV. RNA-seq or RT-qPCR demonstrated that, comparing to rLaSota/Fmut and rLaSota, rLaSota/Fmut/ISG12(2) and rLaSota/ISG12(2) induced hosts to produce cytokines enriching in innate and adaptive immune pathways in vitro and in vivo. Finally, we showed that rLaSota/ISG12(2) vaccination improved immune condition of chicken to quickly response NDV infection and then enhance protection. These results suggest that chicken ISG12(2) is a potential novel molecular adjuvant to regulate immune responses, which decrease virulence and increase immunogenicity of NDV. The chicken ISG12(2) may contribute to development of high efficient poultry vaccine. This article is protected by copyright. All rights reserved.

Contribution of mutation I142M in fusion protein and Q44R in matrix protein of Newcastle disease virus to virulence in ducks

Although verogenic Newcastle disease viruses (NDVs) generally cause subclinical infection in waterfowls such as ducks, NDVs with high virulence in waterfowl have been sporadically reported. We previously reported that the NDV d5a20b strain, which is obtained by serial passaging of the velogenic 9a5b strain in domestic ducks, showed increased virulence in ducks (Hidaka et al., 2021). The d5a20b strain had 11 amino acid substitutions in its P/V, M, F, HN, and L proteins as compared to 9a5b. In the present study, we generated a series of recombinant (r) NDVs with these amino acid substitutions to identify the molecular basis of virulence of NDV in ducks, and evaluated their influences on virulence and in vitro viral properties. Each of the single amino acid substitutions in either the F protein I142M or the M protein Q44R contributed to the enhancement of intracerebral and intranasal pathogenicity in domestic ducks. The cell-cell fusion activity of the virus with F I142M was five times higher than that of the parental r9a5b. The virus with M Q44R rapidly replicated in duck embryo fibroblasts. Additionally, the rM+F+HN strain, which has the same amino acid sequences as d5a20b in M, F, and HN proteins, showed the highest level of virulence and replication efficiency among the generated recombinant viruses, nearly comparable to rd5a20b. These results suggest that multiple factors are involved in the high growth ability of NDV in duck cells, leading to increased virulence in vivo.

Nipah virus W protein harnesses nuclear 14-3-3 to inhibit NF-κB-induced proinflammatory response

Nipah virus (NiV) is a highly pathogenic emerging bat-borne Henipavirus that has caused numerous outbreaks with public health concerns. It is able to inhibit the host innate immune response. Since the NF-κB pathway plays a crucial role in the innate antiviral response as a major transcriptional regulator of inflammation, we postulated its implication in the still poorly understood NiV immunopathogenesis. We report here that NiV inhibits the canonical NF-κB pathway via its nonstructural W protein. Translocation of the W protein into the nucleus causes nuclear accumulation of the cellular scaffold protein 14-3-3 in both African green monkey and human cells infected by NiV. Excess of 14-3-3 in the nucleus was associated with a reduction of NF-κB p65 subunit phosphorylation and of its nuclear accumulation. Importantly, W-S449A substitution impairs the binding of the W protein to 14-3-3 and the subsequent suppression of NF-κB signaling, thus restoring the production of proinflammatory cytokines. Our data suggest that the W protein increases the steady-state level of 14-3-3 in the nucleus and consequently enhances 14-3-3-mediated negative feedback on the NF-κB pathway. These findings provide a mechanistic model of W-mediated disruption of the host inflammatory response, which could contribute to the high severity of NiV infection.

Cytosolic and nuclear recognition of virus and viral evasion

The innate immune system is the first line of host defense, which responds rapidly to viral infection. Innate recognition of viruses is mediated by a set of pattern recognition receptors (PRRs) that sense viral genomic nucleic acids and/or replication intermediates. PRRs are mainly localized either to the endosomes, the plasma membrane or the cytoplasm. Recent evidence suggested that several proteins located in the nucleus could also act as viral sensors. In turn, these important elements are becoming the target for most viruses to evade host immune surveillance. In this review, we focus on the recent progress in the study of viral recognition and evasion.

The Expression of Hemagglutinin by a Recombinant Newcastle Disease Virus Causes Structural Changes and Alters Innate Immune Sensing

Recombinant Newcastle disease viruses (rNDV) have been used as bivalent vectors for vaccination against multiple economically important avian pathogens. NDV-vectored vaccines expressing the immunogenic H5 hemagglutinin (rNDV-H5) are considered attractive candidates to protect poultry from both highly pathogenic avian influenza (HPAI) and Newcastle disease (ND). However, the impact of the insertion of a recombinant protein, such as H5, on the biological characteristics of the parental NDV strain has been little investigated to date. The present study compared a rNDV-H5 vaccine and its parental NDV LaSota strain in terms of their structural and functional characteristics, as well as their recognition by the innate immune sensors. Structural analysis of the rNDV-H5 demonstrated a decreased number of fusion (F) and a higher number of hemagglutinin-neuraminidase (HN) glycoproteins compared to NDV LaSota. These structural differences were accompanied by increased hemagglutinating and neuraminidase activities of rNDV-H5. During in vitro rNDV-H5 infection, increased mRNA expression of TLR3, TLR7, MDA5, and LGP2 was observed, suggesting that the recombinant virus is recognized differently by sensors of innate immunity when compared with the parental NDV LaSota. Given the growing interest in using NDV as a vector against human and animal diseases, these data highlight the importance of thoroughly understanding the recombinant vaccines’ structural organization, functional characteristics, and elicited immune responses.

The chicken-derived velogenic Newcastle disease virus can acquire high pathogenicity in domestic ducks via serial passaging

Velogenic Newcastle disease virus (NDV) strains, which show high mortality in chickens, generally do not cause severe disease in waterfowl such as ducks. To elucidate the difference in the pathogenic mechanisms of NDV between chickens and ducks, a chicken-derived velogenic strain (9a5b) was passaged in domestic ducks five times in their air sacs, followed by 20 times in their brains. Eventually, 9a5b acquired higher intracerebral and intranasal pathogenicity in ducks. The intracerebral pathogenicity index (ICPI) value increased from 1.10 to 1.88. All one-week-old ducks intranasally inoculated with the passaged virus (d5a20b) died by 5 days post-inoculation, whereas 70% of the ducks inoculated with parental 9a5b survived for 8 days. The d5a20b strain replicated in broader systemic tissues in ducks compared with the 9a5b strain. The velogenic profile of 9a5b in chickens was maintained after passaging in ducks. The d5a20b suppressed IFN-β gene expression in duck embryo fibroblasts and replicated more rapidly than 9a5b. A total of 11 amino acid substitutions were found in the P, V, M, F, HN, and L proteins of d5a20b. These results suggest that chicken-derived velogenic NDVs have the potential to become virulent in both chickens and ducks during circulation in domesticated waterfowl populations. RESEARCH HIGHLIGHTSChicken-derived NDV acquired high pathogenicity in ducks with serial passaging.The passaged NDV showed intracerebral and intranasal pathogenicity in ducks.The passaged NDV efficiently replicated in systemic tissues in ducks.Of 11 amino acid substitutions some or all are likely involved in pathogenicity.

ISG15-dependent activation of the sensor MDA5 is antagonized by the SARS-CoV-2 papain-like protease to evade host innate immunity

Activation of the RIG-I-like receptors, retinoic-acid inducible gene I (RIG-I) and melanoma differentiation-associated protein 5 (MDA5), establishes an antiviral state by upregulating interferon (IFN)-stimulated genes (ISGs). Among these is ISG15, the mechanistic roles of which in innate immunity still remain enigmatic. In the present study, we report that ISG15 conjugation is essential for antiviral IFN responses mediated by the viral RNA sensor MDA5. ISGylation of the caspase activation and recruitment domains of MDA5 promotes its oligomerization and thereby triggers activation of innate immunity against a range of viruses, including coronaviruses, flaviviruses and picornaviruses. The ISG15-dependent activation of MDA5 is antagonized through direct de-ISGylation mediated by the papain-like protease of SARS-CoV-2, a recently emerged coronavirus that has caused the COVID-19 pandemic. Our work demonstrates a crucial role for ISG15 in the MDA5-mediated antiviral response, and also identifies a key immune evasion mechanism of SARS-CoV-2, which may be targeted for the development of new antivirals and vaccines to combat COVID-19.

Peste des petits ruminants virus non-structural C protein inhibits the induction of interferon-β by potentially interacting with MAVS and RIG-I

Peste des petits ruminants virus (PPRV) causes an acute and highly contagious disease in domestic and wild small ruminants throughout the world, mainly by invoking immunosuppression in its natural hosts. It has been suggested that the non-structural C protein of PPRV helps in evading host responses but the molecular mechanisms by which it antagonizes the host responses have not been fully characterized. Here, we report the antagonistic effect of PPRV C protein on the expression of interferon-β (IFN-β) through both MAVS and RIG-I mediated pathways in vitro. Dual luciferase reporter assay and direct expression of IFN-β mRNA analysis indicated that PPRV C significantly down regulates IFN-β via its potential interaction with MAVS and RIG-I signaling molecules. Results further indicated that PPRV C protein significantly suppresses endogenous and exogenous IFN-β-induced anti-viral effects in PPRV, EMCV and SVS infections in vitro. Moreover, PPRV C protein not only down regulates IFN-β but also the downstream cytokines of interferon stimulated genes 56 (ISG56), ISG15, C-X-C motif chemokine (CXCL10) and RIG-I mediated activation of IFN promoter elements of ISRE and NF-κB. Further, this study deciphers that PPRV C protein could significantly inhibit the phosphorylation of STAT1 and interferes with the signal transmission in JAK-STAT signaling pathway. Collectively, this study indicates that PPRV C protein is important for innate immune evasion and disease progression.

Investigating the Interaction between Negative Strand RNA Viruses and Their Hosts for Enhanced Vaccine Development and Production

The current pandemic has highlighted the ever-increasing risk of human to human spread of zoonotic pathogens. A number of medically-relevant zoonotic pathogens are negative-strand RNA viruses (NSVs). NSVs are derived from different virus families. Examples like Ebola are known for causing severe symptoms and high mortality rates. Some, like influenza, are known for their ease of person-to-person transmission and lack of pre-existing immunity, enabling rapid spread across many countries around the globe. Containment of outbreaks of NSVs can be difficult owing to their unpredictability and the absence of effective control measures, such as vaccines and antiviral therapeutics. In addition, there remains a lack of essential knowledge of the host–pathogen response that are induced by NSVs, particularly of the immune responses that provide protection. Vaccines are the most effective method for preventing infectious diseases. In fact, in the event of a pandemic, appropriate vaccine design and speed of vaccine supply is the most critical factor in protecting the population, as vaccination is the only sustainable defense. Vaccines need to be safe, efficient, and cost-effective, which is influenced by our understanding of the host–pathogen interface. Additionally, some of the major challenges of vaccines are the establishment of a long-lasting immunity offering cross protection to emerging strains. Although many NSVs are controlled through immunisations, for some, vaccine design has failed or efficacy has proven unreliable. The key behind designing a successful vaccine is understanding the host–pathogen interaction and the host immune response towards NSVs. In this paper, we review the recent research in vaccine design against NSVs and explore the immune responses induced by these viruses. The generation of a robust and integrated approach to development capability and vaccine manufacture can collaboratively support the management of outbreaking NSV disease health risks.

Modulation of Early Host Innate Immune Response by an Avipox Vaccine Virus' Lateral Body Protein

The avian pathogen fowlpox virus (FWPV) has been successfully used as a vaccine vector in poultry and humans, but relatively little is known about its ability to modulate host antiviral immune responses in these hosts, which are replication-permissive and nonpermissive, respectively. FWPV is highly resistant to avian type I interferon (IFN) and able to completely block the host IFN-response. Microarray screening of host IFN-regulated gene expression in cells infected with 59 different, nonessential FWPV gene knockout mutants revealed that FPV184 confers immunomodulatory capacity. We report that the FPV184-knockout virus (FWPVΔ184) induces the cellular IFN response as early as 2 h postinfection. The wild-type, uninduced phenotype can be rescued by transient expression of FPV184 in FWPVΔ184-infected cells. Ectopic expression of FPV184 inhibited polyI:C activation of the chicken IFN-β promoter and IFN-α activation of the chicken Mx1 promoter. Confocal and correlative super-resolution light and electron microscopy demonstrated that FPV184 has a functional nuclear localisation signal domain and is packaged in the lateral bodies of the virions. Taken together, these results provide a paradigm for a late poxvirus structural protein packaged in the lateral bodies, capable of suppressing IFN induction early during the next round of infection.

Regulatory effects of chicken TRIM25 on the replication of ALV-A and the MDA5-mediated type I interferon response

This study focuses on the immunoregulatory effects of chicken TRIM25 on the replication of subgroup A of avian leukosis virus (ALV-A) and the MDA5-mediated type I interferon response. The ALV-A-SDAU09C1 strain was inoculated into DF1 cells and 1-day-old SPF chickens, and the expression of TRIM25 was detected at different time points after inoculation. A recombinant overexpression plasmid containing the chicken TRIM25 gene (TRIM25-GFP) was constructed and transfected into DF1 cells to analyse the effects of the overexpression of chicken TRIM25 on the replication of ALV-A and the expression of MDA5, MAVS and IFN-β. A small interfering RNA targeting chicken TRIM25 (TRIM25-siRNA) was prepared and transfected into DF1 cells to assess the effects of the knockdown of chicken TRIM25 on the replication of ALV-A and the expression of MDA5, MAVS and IFN-β. The results showed that chicken TRIM25 was significantly upregulated at all time points both in ALV-A-infected cells and in ALV-A-infected chickens. Overexpression of chicken TRIM25 in DF1 cells dramatically decreased the antigenic titres of ALV-A in the cell supernatant and upregulated the relative expression of MDA5, MAVS and IFN-β induced by ALV-A or by poly(I:C); in contrast, knockdown of chicken TRIM25 significantly increased the antigenic titres of ALV-A and downregulated the relative expression of MDA5, MAVS and IFN-β. It can be concluded that chicken TRIM25 can inhibit the replication of ALV-A and upregulate the MDA5 receptor-mediated type I interferon response in chickens. This study can help improve the understanding of the antiviral activities of chicken TRIM25 and enrich the knowledge of antiviral responses in chickens.

Synthetic rewiring and boosting type I interferon responses for visualization and counteracting viral infections

Mammalian first line of defense against viruses is accomplished by the interferon (IFN) system. Viruses have evolved numerous mechanisms to reduce the IFN action allowing them to invade the host and/or to establish latency. We generated an IFN responsive intracellular hub by integrating the synthetic transactivator tTA into the chromosomal Mx2 locus for IFN-based activation of tTA dependent expression modules. The additional implementation of a synthetic amplifier module with positive feedback even allowed for monitoring and reacting to infections of viruses that can antagonize the IFN system. Low and transient IFN amounts are sufficient to trigger these amplifier cells. This gives rise to higher and sustained—but optionally de-activatable—expression even when the initial stimulus has faded out. Amplification of the IFN response induced by IFN suppressing viruses is sufficient to protect cells from infection. Together, this interfaced sensor/actuator system provides a toolbox for robust sensing and counteracting viral infections.

ISG15-dependent Activation of the RNA Sensor MDA5 and its Antagonism by the SARS-CoV-2 papain-like protease

Activation of the RIG-I-like receptors, RIG-I and MDA5, establishes an antiviral state by upregulating interferon (IFN)-stimulated genes (ISGs). Among these is ISG15 whose mechanistic roles in innate immunity still remain enigmatic. Here we report that ISGylation is essential for antiviral IFN responses mediated by the viral RNA sensor MDA5. ISG15 conjugation to the caspase activation and recruitment domains of MDA5 promotes the formation of higher-order assemblies of MDA5 and thereby triggers activation of innate immunity against a range of viruses including coronaviruses, flaviviruses and picornaviruses. The ISG15-dependent activation of MDA5 is antagonized through direct de-ISGylation mediated by the papain-like protease (PLpro) of SARS-CoV-2, a recently emerged coronavirus that causes the COVID-19 pandemic. Our work demonstrates a crucial role for ISG15 in the MDA5-mediated antiviral response, and also identifies a novel immune evasion mechanism of SARS-CoV-2, which may be targeted for the development of new antivirals and vaccines to combat COVID-19.

Innate immune responses to duck Tembusu virus infection

The disease caused by duck Tembusu virus (DTMUV) is characterized by severe egg-drop in laying ducks. Currently, the disease has spread to most duck-raising areas in China, leading to great economic losses in the duck industry. In the recent years, DTMUV has raised some concerns, because of its expanding host range and increasing pathogenicity, as well as the potential threat to public health. Innate immunity is crucial for defending against invading pathogens in the early stages of infection. Recently, studies on the interaction between DTMUV and host innate immune response have made great progress. In the review, we provide an overview of DTMUV and summarize current advances in our understanding of the interaction between DTMUV and innate immunity, including the host innate immune responses to DTMUV infection through pattern recognition receptors (PRRs), signaling transducer molecules, interferon-stimulated genes (ISGs), and the immune evasion strategies employed by DTMUV. The aim of the review is to gain an in-depth understanding of DTMUV pathogenesis to facilitate future studies.

Human Type I Interferon Antiviral Effects in Respiratory and Reemerging Viral Infections

Type I interferons (IFN-I) are a group of related proteins that help regulate the activity of the immune system and play a key role in host defense against viral infections. Upon infection, the IFN-I are rapidly secreted and induce a wide range of effects that not only act upon innate immune cells but also modulate the adaptive immune system. While IFN-I and many IFN stimulated genes are well-known for their protective antiviral role, recent studies have associated them with potential pathogenic functions. In this review, we summarize the current knowledge regarding the complex effects of human IFN-I responses in respiratory as well as reemerging flavivirus infections of public health significance and the molecular mechanisms by which viral proteins antagonize the establishment of an antiviral host defense. Antiviral effects and immune modulation of IFN-stimulated genes is discussed in resisting and controlling pathogens. Understanding the mechanisms of these processes will be crucial in determining how viral replication can be effectively controlled and in developing safe and effective vaccines and novel therapeutic strategies.

Seek and hide: the manipulating interplay of measles virus with the innate immune system

The innate immune system is the first line of defense against infections with pathogens. It provides direct antiviral mechanisms to suppress the viral life cycle at multiple steps. Innate immune cells are specialized to recognize pathogen infections and activate and modulate adaptive immune responses through antigen presentation, co-stimulation and release of cytokines and chemokines. Measles virus, which causes long-lasting immunosuppression and immune-amnesia, primarily infects and replicates in innate and adaptive immune cells, such as dendritic cells, macrophages, T cells and B cells. To achieve efficient replication, measles virus has evolved multiple mechanisms to manipulate innate immune responses by both stimulation and blocking of specific signals necessary for antiviral immunity. This review will highlight our current knowledge in this and address open questions.

Impact of Respiratory Syncytial Virus Infection on Host Functions: Implications for Antiviral Strategies

Respiratory syncytial virus (RSV) is one of the leading causes of viral respiratory tract infection in infants, the elderly, and the immunocompromised worldwide, causing more deaths each year than influenza. Years of research into RSV since its discovery over 60 yr ago have elucidated detailed mechanisms of the host-pathogen interface. RSV infection elicits widespread transcriptomic and proteomic changes, which both mediate the host innate and adaptive immune responses to infection, and reflect RSV's ability to circumvent the host stress responses, including stress granule formation, endoplasmic reticulum stress, oxidative stress, and programmed cell death. The combination of these events can severely impact on human lungs, resulting in airway remodeling and pathophysiology. The RSV membrane envelope glycoproteins (fusion F and attachment G), matrix (M) and nonstructural (NS) 1 and 2 proteins play key roles in modulating host cell functions to promote the infectious cycle. This review presents a comprehensive overview of how RSV impacts the host response to infection and how detailed knowledge of the mechanisms thereof can inform the development of new approaches to develop RSV vaccines and therapeutics.

Norovirus Replication in Human Intestinal Epithelial Cells Is Restricted by the Interferon-Induced JAK/STAT Signaling Pathway and RNA Polymerase II-Mediated Transcriptional Responses

Human noroviruses (HuNoV) are a leading cause of viral gastroenteritis worldwide and a significant cause of morbidity and mortality in all age groups. The recent finding that HuNoV can be propagated in B cells and mucosa-derived intestinal epithelial organoids (IEOs) has transformed our ability to dissect the life cycle of noroviruses. Using transcriptome sequencing (RNA-Seq) of HuNoV-infected intestinal epithelial cells (IECs), we have found that replication of HuNoV in IECs results in interferon (IFN)-induced transcriptional responses and that HuNoV replication in IECs is sensitive to IFN. This contrasts with previous studies that suggested that the innate immune response may play no role in the restriction of HuNoV replication in immortalized cells. We demonstrated that inhibition of Janus kinase 1 (JAK1)/JAK2 enhanced HuNoV replication in IECs. Surprisingly, targeted inhibition of cellular RNA polymerase II-mediated transcription was not detrimental to HuNoV replication but instead enhanced replication to a greater degree than blocking of JAK signaling directly. Furthermore, we demonstrated for the first time that IECs generated from genetically modified intestinal organoids, engineered to be deficient in the interferon response, were more permissive to HuNoV infection. Taking the results together, our work revealed that IFN-induced transcriptional responses restrict HuNoV replication in IECs and demonstrated that inhibition of these responses mediated by modifications of the culture conditions can greatly enhance the robustness of the norovirus culture system.IMPORTANCE Noroviruses are a major cause of gastroenteritis worldwide, and yet the challenges associated with their growth in culture have greatly hampered the development of therapeutic approaches and have limited our understanding of the cellular pathways that control infection. Here, we show that human intestinal epithelial cells, which represent the first point of entry of human noroviruses into the host, limit virus replication by induction of innate responses. Furthermore, we show that modulating the ability of intestinal epithelial cells to induce transcriptional responses to HuNoV infection can significantly enhance human norovirus replication in culture. Collectively, our findings provide new insights into the biological pathways that control norovirus infection but also identify mechanisms that enhance the robustness of norovirus culture.

Intracellular sensing of viral genomes and viral evasion

During viral infection, virus-derived cytosolic nucleic acids are recognized by host intracellular specific sensors. The efficacy of this recognition system is crucial for triggering innate host defenses, which then stimulate more specific adaptive immune responses against the virus. Recent studies show that signal transduction pathways activated by sensing proteins are positively or negatively regulated by many modulators to maintain host immune homeostasis. However, viruses have evolved several strategies to counteract/evade host immune reactions. These systems involve viral proteins that interact with host sensor proteins and prevent them from detecting the viral genome or from initiating immune signaling. In this review, we discuss key regulators of cytosolic sensor proteins and viral proteins based on experimental evidence.

Antagonism of STAT1 by Nipah virus P gene products modulates disease course but not lethal outcome in the ferret model

Nipah virus (NiV) is a pathogenic paramyxovirus and zoononis with very high human fatality rates. Previous protein over-expression studies have shown that various mutations to the common N-terminal STAT1-binding motif of the NiV P, V, and W proteins affected the STAT1-binding ability of these proteins thus interfering with he JAK/STAT pathway and reducing their ability to inhibit type-I IFN signaling, but due to differing techniques it was unclear which amino acids were most important in this interaction or what impact this had on pathogenesis in vivo. We compared all previously described mutations in parallel and found the amino acid mutation Y116E demonstrated the greatest reduction in binding to STAT1 and the greatest reduction in interferon antagonism. A similar reduction in binding and activity was seen for a deletion of twenty amino acids constituting the described STAT1-binding domain. To investigate the contribution of this STAT1-binding motif in NiV-mediated disease, we produced rNiVs with complete deletion of the STAT1-binding motif or the Y116E mutation for ferret challenge studies (rNiVM-STAT1blind). Despite the reduced IFN inhibitory function, ferrets challenged with these rNiVM-STAT1blind mutants had a lethal, albeit altered, NiV-mediated disease course. These data, together with our previously published data, suggest that the major role of NiV P, V, and W in NiV-mediated disease in the ferret model are likely to be in the inhibition of viral recognition/innate immune signaling induction with a minor role for inhibition of IFN signaling.

Constitutively Active MDA5 Proteins Are Inhibited by Paramyxovirus V Proteins

Excessive interferon (IFN) production and signaling can lead to immunological and developmental defects giving rise to autoimmune diseases referred to collectively as "type I interferonopathies." A subset of these diseases is caused by monogenic mutations affecting proteins involved in nucleic acid sensing, homeostasis, and metabolism. Interferonopathic mutations in the cytosolic antiviral sensor MDA5 render it constitutively hyperactive, resulting in chronic IFN production and IFN-stimulated gene expression. Few therapeutic options are available for patients with interferonopathic diseases, but a large number of IFN evasion and antagonism strategies have evolved in viral pathogens that can counteract IFN production and signaling to enhance virus replication. To test the hypothesis that these natural IFN suppressors could be used to subdue the activity of interferonopathic signaling proteins, hyperactive MDA5 variants were assessed for susceptibility to a family of viral MDA5 inhibitors. In this study, Paramyxovirus V proteins were tested for their ability to counteract constitutively active MDA5 proteins. Results indicate that the V proteins are able to bind to and disrupt the signaling activity of these MDA5 proteins, irrespective of their specific mutations, reducing IFN production and IFN-stimulated gene expression to effectively suppress the hyperactive antiviral response.

Newcastle Disease Virus V Protein Degrades Mitochondrial Antiviral Signaling Protein To Inhibit Host Type I Interferon Production via E3 Ubiquitin Ligase RNF5

Paramyxovirus establishes an intimate and complex interaction with the host cell to counteract the antiviral responses elicited by the cell. Of the various pattern recognition receptors in the host, the cytosolic RNA helicases interact with viral RNA to activate the mitochondrial antiviral signaling protein (MAVS) and subsequent cellular interferon (IFN) response. On the other hand, viruses explore multiple strategies to resist host immunity. In this study, we found that Newcastle disease virus (NDV) infection induced MAVS degradation. Further analysis showed that NDV V protein degraded MAVS through the ubiquitin-proteasome pathway to inhibit IFN-β production. Moreover, NDV V protein led to proteasomal degradation of MAVS through Lys362 and Lys461 ubiquitin to prevent IFN production. Further studies showed that NDV V protein recruited E3 ubiquitin ligase RNF5 to polyubiquitinate and degrade MAVS. Compared with levels for wild-type NDV infection, V-deficient NDV induced attenuated MAVS degradation and enhanced IFN-β production at the late stage of infection. Several other paramyxovirus V proteins showed activities of degrading MAVS and blocking IFN production similar to those of NDV V protein. The present study revealed a novel role of NDV V protein in targeting MAVS to inhibit cellular IFN production, which reinforces the fact that the virus orchestrates the cellular antiviral response to its own benefit.IMPORTANCE Host anti-RNA virus innate immunity relies mainly on the recognition by retinoic acid-inducible gene I and melanoma differentiation-associated protein 5 and subsequently initiates downstream signaling through interaction with MAVS. On the other hand, viruses have developed various strategies to counteract MAVS-mediated signaling. The mechanism for paramyxoviruses regulating MAVS to benefit their infection remains unknown. In this article, we demonstrate that the V proteins of NDV and several other paramyxoviruses target MAVS for ubiquitin-mediated degradation through E3 ubiquitin ligase RING-finger protein 5 (RNF5). MAVS degradation leads to the inhibition of the downstream IFN-β pathway and therefore benefits virus proliferation. Our study reveals a novel mechanism of NDV evading host innate immunity and provides insight into the therapeutic strategies for the control of paramyxovirus infection.

Human Hemoglobin Subunit Beta Functions as a Pleiotropic Regulator of RIG-I/MDA5-Mediated Antiviral Innate Immune Responses

Hemoglobin is an important oxygen-carrying protein and plays crucial roles in establishing host resistance against pathogens and in regulating innate immune responses. The hemoglobin subunit beta (HB) is an essential component of hemoglobin, and we have previously demonstrated that the antiviral role of the porcine HB (pHB) is mediated by promoting type I interferon pathways. Thus, considering the high homology between human HB (hHB) and pHB, we hypothesized that hHB also plays an important role in the antiviral innate immunity. In this study, we characterized hHB as a regulatory factor for the replication of RNA viruses by differentially regulating the RIG-I- and MDA5-mediated antiviral signaling pathways. Furthermore, we showed that hHB directly inhibited MDA5-mediated signaling by reducing the MDA5-double-stranded RNA (dsRNA) interaction. Additionally, hHB required hHB-induced reactive oxygen species (ROS) to promote RIG-I-mediated signaling through enhancement of K63-linked RIG-I ubiquitination. Taken together, our findings suggest that hHB is a pleiotropic regulator of RIG-I/MDA5-mediated antiviral responses and further highlight the importance of the intercellular microenvironment, including the redox state, in regulating antiviral innate immune responses.IMPORTANCE Hemoglobin, the most important oxygen-carrying protein, is involved in the regulation of innate immune responses. We have previously reported that the porcine hemoglobin subunit beta (HB) exerts antiviral activity through regulation of type I interferon production. However, the antiviral activities and the underlying mechanisms of HBs originating from other animals have been poorly understood. Here, we identified human HB (hHB) as a pleiotropic regulator of the replication of RNA viruses through regulation of RIG-I/MDA5-mediated signaling pathways. hHB enhances RIG-I-mediated antiviral responses by promoting RIG-I ubiquitination depending on the hHB-induced reactive oxygen species (ROS), while it blocks MDA5-mediated antiviral signaling by suppressing the MDA5-dsRNA interaction. Our results contribute to an understanding of the crucial roles of hHB in the regulation of the RIG-I/MDA5-mediated signaling pathways. We also provide novel insight into the correlation of the intercellular redox state with the regulation of antiviral innate immunity.

Host Cellular Receptors for the Peste des Petits Ruminant Virus

Peste des Petits Ruminant (PPR) is an important transboundary, OIE-listed contagious viral disease of primarily sheep and goats caused by the PPR virus (PPRV), which belongs to the genus Morbillivirus of the family Paramyxoviridae. The mortality rate is 90–100%, and the morbidity rate may reach up to 100%. PPR is considered economically important as it decreases the production and productivity of livestock. In many endemic poor countries, it has remained an obstacle to the development of sustainable agriculture. Hence, proper control measures have become a necessity to prevent its rapid spread across the world. For this, detailed information on the pathogenesis of the virus and the virus host interaction through cellular receptors needs to be understood clearly. Presently, two cellular receptors; signaling lymphocyte activation molecule (SLAM) and Nectin-4 are known for PPRV. However, extensive information on virus interactions with these receptors and their impact on host immune response is still required. Hence, a thorough understanding of PPRV receptors and the mechanism involved in the induction of immunosuppression is crucial for controlling PPR. In this review, we discuss PPRV cellular receptors, viral host interaction with cellular receptors, and immunosuppression induced by the virus with reference to other Morbilliviruses.

Suicide gene‑armed measles vaccine virus for the treatment of AML

Virotherapy comprises a novel therapeutic approach to selectively eliminate cancer cells. Preclinical, as well as clinical data have demonstrated the efficacy of tumor‑selective (oncolytic) viruses in hematological malignancies. In this study, we infected AML cell lines and primary AML cells from patients with measles vaccine virus either expressing GFP or armed with super cytosine deaminase, which converts the prodrug, 5‑fluorocytosine, into the chemotherapeutic compound, 5‑fluorouracil. Target cell density of the measles entry receptor, CD46, infection rates of targeted leukemic cells, tumor cell viability, and apoptotic rates were determined. We found that measles vaccine virus infected the leukemic blasts and profoundly diminished the number and viability of leukemic cells via the induction of apoptosis. The conversion of 5‑fluorocytosine to 5‑fluorouracil exerted a potent additive tumoricidal effect. This was also observed in cases when leukemic cells displayed only moderate susceptibility to the oncolytic virus and hence direct oncolysis. Taken together, in this study, we provide a first characterization of the combinatorial use of measles vaccine virus and 5‑fluorouracil for treatment of AML. Our approach to site‑specifically produce the active drug and combine this agent with the direct lytic effect of virotherapy may overcome present limitations and constitutes a feasible method with which to introduce 5‑fluorouracil in the treatment of AML.

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

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

Histone Deacetylase Inhibitors Enhance Cell Killing and Block Interferon-Beta Synthesis Elicited by Infection with an Oncolytic Parainfluenza Virus

Previous results have shown that infection with the cytoplasmic-replicating parainfluenza virus 5 mutant P/V-CPI- sensitizes cells to DNA damaging agents, resulting in the enhanced killing of airway cancer cells. Here, we have tested the hypothesis that histone deacetylase (HDAC) inhibitors can also act with P/V-CPI- infection to enhance cancer cell killing. Using human small cell lung cancer and laryngeal cancer cell lines, 10 HDAC inhibitors were tested for their effect on viability of P/V-CPI- infected cells. HDAC inhibitors such as scriptaid enhanced caspase-3/7, -8 and -9 activity induced by P/V-CPI- and overall cell toxicity. Scriptaid-mediated enhanced killing was eliminated in lung cancer cells that were engineered to express a protein which sequesters double stranded RNA. Scriptaid also enhanced cancer cell killing by two other negative strand RNA viruses - the La Crosse virus and vesicular stomatitis virus. Scriptaid treatment enhanced the spread of the P/V-CPI- virus through a population of cancer cells, and suppressed interferon-beta induction through blocking phosphorylation and nuclear translocation of Interferon Regulatory Factor 3 (IRF-3). Taken together, these data support a role for combinations of a cytoplasmic-replicating RNA virus such as the P/V-CPI- mutant along with chemotherapeutic agents.

Goose MAVS functions in RIG-I-mediated IFN-β signaling activation

Mitochondrial antiviral-signaling protein (MAVS) is an essential adaptor protein in retinoic acid-inducible gene I (RIG-I)-mediated antiviral innate immunity in mammals. In this study, the goose MAVS gene (goMAVS) was identified. The 2019 bp-long goMAVS exhibits 96.2% amino acid similarity compared to the predicted goMAVS. Quantitative real-time polymerase chain reactions showed that goMAVS mRNA was widely expressed in different tissues. The overexpression of goMAVS in goose embryo fibroblast cells up-regulated the mRNA levels of goose interferon-stimulated genes. We concluded that MAVS mediates the activation of type I interferon (IFN) pathway in a species-specific manner. We further demonstrated that a CARD-like domain, transmembrane domain and two previously unidentified domains of goMAVS were essential for the activation of type I IFN pathway. GoMAVS inhibited Newcastle disease virus replication by activating type I IFN pathways, especially at the early stages of infection. Finally, the interaction between goMAVS and goose RIG-I was confirmed. The CARD domain of goMAVS plays a vital role in the interaction. Together, we identified goMAVS as a goRIG-I interactive protein and concluded that goMAVS is involved in the activation of type I IFN pathways in goose cells.

The interferon antagonistic activities of the V proteins of NDV correlated with their virulence

Protein V of Newcastle disease virus (NDV) serves as interferon (IFN) antagonist, and NDV stains with different pathogenicity show different abilities in inhibition IFN expression. To further reveal the relationship between viral virulence and their IFN-antagonistic activity derived from protein V, six NDV strains with three different pathotypes were used in this study and their V gene were cloned into eukaryotic expression vector. The V gene derived from different NDV strains were expressed in same level in cells after transfection according to the results from Western blotting. And these proteins showed different interferon-antagonistic activities based on interferon expression using Luciferase Reporter Assay and ELISA. The expression of IFN and viral virulence index, mean death time, have a good linear relationship indicating a good correlation between viral virulence and IFN antagonism of their V Protein.

Truncated chicken MDA5 enhances the immune response to inactivated NDV vaccine

Melanoma Differentiation-Associated protein 5 (MDA5) is a cytoplasmic sensor for viral invasion and plays an important role in regulation of the immune response against Newcastle disease virus (NDV) in chickens. MDA5 was used as an adjuvant to enhance the humoral immune response against influenza virus. In the current study, truncated chicken MDA5 [1-483 aa, chMDA5(483aa)] expressed by recombinant adenovirus was administered to specific-pathogen-free (SPF) chickens to improve the immune response induced by inactivated NDV vaccine. A total of 156 SPF chickens were divided into six groups, and after two rounds of immunization, the humoral immune response, cell-mediated immune (CMI) response and the protective efficacy of the vaccines against NDV challenge were evaluated. The results showed that co-administration of chMDA5(483aa) expressed by adenovirus increased the NDV-specific antibody response by 1.7 times and chickens received chMDA5(483aa) also gained a higher level of CMI response. Consistently, the protective efficacy of the inactivated NDV vaccine against virulent NDV (vNDV) challenge was improved by co-administrate with chMDA5(483aa), as indicated by the reduced morbidity and pathological lesions, lower levels of viral load in organs and reduced virus shedding. Our study demonstrated that chMDA5(433aa) expressed by adenovirus could enhance the immune efficacy of inactivated NDV vaccine in chickens and could be a potential adjuvant candidate in developing chicken NDV vaccines.