Human metapneumovirus glycoprotein G disrupts mitochondrial signaling in airway epithelial cells

Targeting PRMT7-mediated monomethylation of MAVS enhances antiviral innate immune responses and inhibits RNA virus replication

RIG-I-like receptors (RLRs)-mitochondrial antiviral signaling protein (MAVS) are crucial for type I interferon (IFN) signaling pathway and innate immune responses triggered by RNA viruses. However, the regulatory molecular mechanisms underlying RNA virus-activated type I IFN signaling pathway remain incompletely understood. Here, we found that protein arginine methyltransferase 7 (PRMT7) serves as a negative regulator of the type I IFN signaling pathway by interacting with MAVS and catalyzing monomethylation of arginine 232 (R232me1) in MAVS. RNA virus infection leads to the downregulation and dissociation of PRMT7 from MAVS as well as the decrease of R232me1 methylation, enhancing MAVS/RIG-I interaction, MAVS aggregation, type I IFN signaling activation, and antiviral immune responses. Knock-in mice with MAVS R232 substituted with lysine (MavsR232K-KI) are more resistant to Vesicular Stomatitis Virus infection due to enhanced antiviral immune responses. PiPRMT7-MAVS, a short peptide inhibitor designed to interrupt the interaction between PRMT7 and MAVS, inhibits R232me1 methylation, thereby enhancing MAVS/RIG-I interaction, promoting MAVS aggregation, activating type I IFN signaling, and bolstering antiviral immune responses to suppress RNA virus replication. Moreover, the clinical relevance of PRMT7 is highlighted that it is significantly downregulated in RNA virus-infected clinical samples, such as blood samples from severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and Ebola virus, as well as H1N1-infected bronchial epithelial cells. Our findings uncovered that PRMT7-mediated arginine methylation plays critical roles in regulating MAVS-mediated antiviral innate immune responses, and targeting arginine methylation might represent a therapeutic avenue for treating RNA viral infection.

Micro-CT Features of Lung Consolidation, Collagen Deposition and Inflammation in Experimental RSV Infection Are Aggravated in the Absence of Nrf2

Severe respiratory syncytial virus (RSV) infections in early life have been linked to the development of chronic airway disease. RSV triggers the production of reactive oxygen species (ROS), which contributes to inflammation and enhanced clinical disease. NF-E2-related factor 2 (Nrf2) is an important redox-responsive protein that helps to protect cells and whole organisms from oxidative stress and injury. The role of Nrf2 in the context of viral-mediated chronic lung injury is not known. Herein, we show that RSV experimental infection of adult Nrf2-deficient BALB/c mice (Nrf2-/-; Nrf2 KO) is characterized by enhanced disease, increased inflammatory cell recruitment to the bronchoalveolar compartment and a more robust upregulation of innate and inflammatory genes and proteins, compared to wild-type Nrf2+/+ competent mice (WT). These events that occur at very early time points lead to increased peak RSV replication in Nrf2 KO compared to WT mice (day 5). To evaluate longitudinal changes in the lung architecture, mice were scanned weekly via high-resolution micro-computed tomography (micro-CT) imaging up to 28 days after initial viral inoculation. Based on micro-CT qualitative 2D imaging and quantitative reconstructed histogram-based analysis of lung volume and density, we found that RSV-infected Nrf2 KO mice developed significantly greater and prolonged fibrosis compared to WT mice. The results of this study underscore the critical role of Nrf2-mediated protection from oxidative injury, not only in the acute pathogenesis of RSV infection but also in the long-term consequences of chronic airway injury.

Human metapneumovirus M2-2 protein inhibits RIG-I signaling by preventing TRIM25-mediated RIG-I ubiquitination

Retinoic acid-inducible gene I (RIG-I) is a receptor that senses viral RNA and interacts with mitochondrial antiviral signaling (MAVS) protein, leading to the production of type I interferons and inflammatory cytokines to establish an antiviral state. This signaling axis is initiated by the K63-linked RIG-I ubiquitination, mediated by E3 ubiquitin ligases such as TRIM25. However, many viruses, including several members of the family Paramyxoviridae and human respiratory syncytial virus (HRSV), a member of the family Pneumoviridae, escape the immune system by targeting RIG-I/TRIM25 signaling. In this study, we screened human metapneumovirus (HMPV) open reading frames (ORFs) for their ability to block RIG-I signaling reconstituted in HEK293T cells by transfection with TRIM25 and RIG-I CARD (an N-terminal CARD domain that is constitutively active in RIG-I signaling). HMPV M2-2 was the most potent inhibitor of RIG-I/TRIM25-mediated interferon (IFN)-β activation. M2-2 silencing induced the activation of transcription factors (IRF and NF-kB) downstream of RIG-I signaling in A549 cells. Moreover, M2-2 inhibited RIG-I ubiquitination and CARD-dependent interactions with MAVS. Immunoprecipitation revealed that M2-2 forms a stable complex with RIG-I CARD/TRIM25 via direct interaction with the SPRY domain of TRIM25. Similarly, HRSV NS1 also formed a stable complex with RIG-I CARD/TRIM25 and inhibited RIG-I ubiquitination. Notably, the inhibitory actions of HMPV M2-2 and HRSV NS1 are similar to those of V proteins of several members of the Paramyxoviridae family. In this study, we have identified a novel mechanism of immune escape by HMPV, similar to that of Pneumoviridae and Paramyxoviridae family members.

Role of human metapneumovirus glycoprotein G in modulation of immune responses

Human metapneumovirus (hMPV) is an important pathogen responsible for acute respiratory tract infections in children, the elderly, and immunocompromised patients, with no effective treatment or vaccine currently available. Knowledge of virus- and host-specific mechanisms contributing to the pathogenesis of hMPV infection is still limited. Studies have shown that hMPV surface glycoprotein G is an important virulence factor, by inhibiting innate immune signaling in airway epithelial cells and immune cells. In this study, we investigated the role of G protein in modulating innate and adaptive immune responses in mice infected with a recombinant virus with deletion of G protein (rhMPV-ΔG). Results show that rhMPV-ΔG was strongly attenuated, as it did not induce significant clinical disease, airway obstruction and airway hyperresponsiveness (AHR), compared to infection with a control strain (rhMPV-WT). By analysis of cells in bronchoalveolar fluid and lung tissue, as well as cytokine production, we found that G protein mediates aspects of both innate and adaptive immune responses, including neutrophils, dendritic cells, natural killer cells and B cells. Lung T cells recruited in response to rhMPV-ΔG had a significantly higher activated phenotype compared to those present after rhMPV-WT infection. Despite highly attenuation characterized by low levels of replication in the lung, rhMPV-ΔG was able to induce neutralizing antibodies and to protect mice from a secondary hMPV challenge. However, challenged mice that had received rhMPV-ΔG as primary infection showed some signs of lung disease at the earliest time points, which were less evident in mice that had received the rhMPV-WT strain as primary infection. These results demonstrate some of the mechanisms by which G protein could contribute to airway disease and modulate immune response to hMPV infection.

MAVS: A Two-Sided CARD Mediating Antiviral Innate Immune Signaling and Regulating Immune Homeostasis

Mitochondrial antiviral signaling protein (MAVS) functions as a "switch" in the immune signal transduction against most RNA viruses. Upon viral infection, MAVS forms prion-like aggregates by receiving the cytosolic RNA sensor retinoic acid-inducible gene I-activated signaling and further activates/switches on the type I interferon signaling. While under resting state, MAVS is prevented from spontaneously aggregating to switch off the signal transduction and maintain immune homeostasis. Due to the dual role in antiviral signal transduction and immune homeostasis, MAVS has emerged as the central regulation target by both viruses and hosts. Recently, researchers show increasing interest in viral evasion strategies and immune homeostasis regulations targeting MAVS, especially focusing on the post-translational modifications of MAVS, such as ubiquitination and phosphorylation. This review summarizes the regulations of MAVS in antiviral innate immune signaling transduction and immune homeostasis maintenance.

ELAC2, an Enzyme for tRNA Maturation, Plays a Role in the Cleavage of a Mature tRNA to Produce a tRNA-Derived RNA Fragment During Respiratory Syncytial Virus Infection

Respiratory syncytial virus (RSV) is the most common cause of lower respiratory tract infection in young children. However, effective treatment against RSV is unavailable. tRNA-derived RNA fragments (tRFs) are a recently discovered family of non-coding RNAs. We made an early observation that RSV infection causes significant induction of tRFs, which are mainly derived from the 5’-end of mature tRNAs (tRF5). However, their functions and biogenesis mechanism are not fully understood. Herein, we identified an enzyme responsible for the induction of a functional tRF5 derived from tRNA-Gln-CTG (tRF5-GlnCTG). We found that tRF5-GlnCTG promotes RSV replication and its induction, assessed by Northern blot and a new qRT-PCR-based method, is regulated by ribonuclease ELAC2. ELAC2-mediated tRF5 induction has never been reported. We also found that ELAC2 is associated with RSV N and NS1 proteins. Given the fact that tRF5-GlnCTG plays a role in RSV replication, the identification of ELAC2 being responsible for tRF5-GlnCTG induction could provide new insights into therapeutic strategy development against RSV infection.

Regulation of MAVS Expression and Signaling Function in the Antiviral Innate Immune Response

Viral infection is controlled by host innate immune cells that express specialized receptors for viral components. Engagement of these pattern recognition receptors triggers a series of signaling pathways that culminate in the production of antiviral mediators such as type I interferons. Mitochondrial antiviral-signaling protein (MAVS) acts as a central hub for signal transduction initiated by RIG-I-like receptors, which predominantly recognize viral RNA. MAVS expression and function are regulated by both post-transcriptional and post-translational mechanisms, of which ubiquitination and phosphorylation play the most important roles in modulating MAVS function. Increasing evidence indicates that viruses can escape the host antiviral response by interfering at multiple points in the MAVS signaling pathways, thereby maintaining viral survival and replication. This review summarizes recent studies on the mechanisms by which MAVS expression and signaling are normally regulated and on the various strategies employed by viruses to antagonize MAVS activity, which may provide new insights into the design of novel antiviral agents.

Strain-Dependent Impact of G and SH Deletions Provide New Insights for Live-Attenuated HMPV Vaccine Development

Human metapneumovirus (HMPV) is a major pediatric respiratory pathogen with currently no specific treatment or licensed vaccine. Different strategies to prevent this infection have been evaluated, including live-attenuated vaccines (LAV) based on SH and/or G protein deletions. This approach showed promising outcomes but has not been evaluated further using different viral strains. In that regard, we previously showed that different HMPV strains harbor distinct in vitro fusogenic and in vivo pathogenic phenotypes, possibly influencing the selection of vaccine strains. In this study, we investigated the putative contribution of the low conserved SH or G accessory proteins in such strain-dependent phenotypes and generated recombinant wild type (WT) and SH- or G-deleted viruses derived from two different patient-derived HMPV strains, A1/C-85473 and B2/CAN98-75. The ΔSH and ΔG deletions led to different strain-specific phenotypes in both LLC-MK2 cell and reconstituted human airway epithelium models. More interestingly, the ΔG-85473 and especially ΔSH-C-85473 recombinant viruses conferred significant protection against HMPV challenge and induced immunogenicity against a heterologous strain. In conclusion, our results show that the viral genetic backbone should be considered in the design of live-attenuated HMPV vaccines, and that a SH-deleted virus based on the A1/C-85473 HMPV strain could be a promising LAV candidate as it is both attenuated and protective in mice while being efficiently produced in a cell-based system.

Human Metapneumovirus: Mechanisms and Molecular Targets Used by the Virus to Avoid the Immune System

Human metapneumovirus (hMPV) is a respiratory virus, first reported the year 2001. Since then, it has been described as one of the main etiological agents that causes acute lower respiratory tract infections (ALRTIs), which is characterized by symptoms such as bronchiolitis, wheezing and coughing. Susceptible population to hMPV-infection includes newborn, children, elderly and immunocompromised individuals. This viral agent is a negative-sense, single-stranded RNA enveloped virus, that belongs to the Pneumoviridae family and Metapneumovirus genus. Early reports—previous to 2001—state several cases of respiratory illness without clear identification of the responsible pathogen, which could be related to hMPV. Despite the similarities of hMPV with several other viruses, such as the human respiratory syncytial virus or influenza virus, mechanisms used by hMPV to avoid the host immune system are still unclear. In fact, evidence indicates that hMPV induces a poor innate immune response, thereby affecting the adaptive immunity. Among these mechanisms, is the promotion of an anergic state in T cells, instead of an effective polarization or activation, which could be induced by low levels of cytokine secretion. Further, the evidences support the notion that hMPV interferes with several pattern recognition receptors (PRRs) and cell signaling pathways triggered by interferon-associated genes. However, these mechanisms reported in hMPV are not like the ones reported for hRSV, as the latter has two non-structural proteins that are able to inhibit these pathways. Several reports suggest that viral glycoproteins, such as G and SH, could play immune-modulator roles during infection. In this work, we discuss the state of the art regarding the mechanisms that underlie the poor immunity elicited by hMPV. Importantly, these mechanisms will be compared with those elicited by other common respiratory viruses.

Human Metapneumovirus Small Hydrophobic Protein Inhibits Interferon Induction in Plasmacytoid Dendritic Cells

Human metapneumovirus (hMPV), a leading cause of respiratory tract infections in infants, encodes a small hydrophobic (SH) protein of unknown function. Here we show that infection of plasmacytoid dendritic cells (pDCs) with a recombinant virus lacking SH expression (rhMPV-ΔSH) enhanced the secretion of type I interferons (IFNs), which required TLR7 and MyD88 expression. HMPV SH protein inhibited TLR7/MyD88/TRAF6 signaling leading to IFN gene transcription, identifying a novel mechanism by which paramyxovirus SH proteins modulate innate immune responses.

Human metapneumovirus - what we know now

Human metapneumovirus (HMPV) is a leading cause of acute respiratory infection, particularly in children, immunocompromised patients, and the elderly. HMPV, which is closely related to avian metapneumovirus subtype C, has circulated for at least 65 years, and nearly every child will be infected with HMPV by the age of 5. However, immunity is incomplete, and re-infections occur throughout adult life. Symptoms are similar to those of other respiratory viral infections, ranging from mild (cough, rhinorrhea, and fever) to more severe (bronchiolitis and pneumonia). The preferred method for diagnosis is reverse transcription-polymerase chain reaction as HMPV is difficult to culture. Although there have been many advances made in the past 16 years since its discovery, there are still no US Food and Drug Administration-approved antivirals or vaccines available to treat HMPV. Both small animal and non-human primate models have been established for the study of HMPV. This review will focus on the epidemiology, transmission, and clinical manifestations in humans as well as the animal models of HMPV pathogenesis and host immune response.

Detection of Nuclear Protein Profile Changes by Human Metapneumovirus M2-2 Protein Using Quantitative Differential Proteomics

Human metapneumovirus (hMPV) is a leading cause of lower respiratory infection in pediatric populations globally. This study examined proteomic profile changes in A549 cells infected with hMPV and two attenuated mutants with deleted PDZ domain-binding motif(s) in the M2-2 protein. These motifs are involved in the interruption of antiviral signaling, namely the interaction between the TNF receptor associated factor (TRAF) and mitochondrial antiviral-signaling (MAVS) proteins. The aim of this study was to provide insight into the overall and novel impact of M2-2 motifs on cellular responses via an unbiased comparison. Tandem mass tagging, stable isotope labeling, and high-resolution mass spectrometry were used for quantitative proteomic analysis. Using quantitative proteomics and Venn analysis, 1248 common proteins were detected in all infected samples of both technical sets. Hierarchical clustering of the differentiated proteome displayed distinct proteomic signatures that were controlled by the motif(s). Bioinformatics and experimental analysis confirmed the differentiated proteomes, revealed novel cellular biological events, and implicated key pathways controlled by hMPV M2-2 PDZ domain-binding motif(s). This provides further insight for evaluating M2-2 mutants as potent vaccine candidates.

Modulation of Host Immunity by the Human Metapneumovirus

Globally, as a leading agent of acute respiratory tract infections in children <5 years of age and the elderly, the human metapneumovirus (HMPV) has gained considerable attention. As inferred from studies comparing vaccinated and experimentally infected mice, the acquired immune response elicited by this pathogen fails to efficiently clear the virus from the airways, which leads to an exaggerated inflammatory response and lung damage. Furthermore, after disease resolution, there is a poor development of T and B cell immunological memory, which is believed to promote reinfections and viral spread in the community. In this article, we discuss the molecular mechanisms that shape the interactions of HMPV with host tissues that lead to pulmonary pathology and to the development of adaptive immunity that fails to protect against natural infections by this virus.

Functional motifs responsible for human metapneumovirus M2-2-mediated innate immune evasion

Human metapneumovirus (hMPV) is a major cause of lower respiratory infection in young children. Repeated infections occur throughout life, but its immune evasion mechanisms are largely unknown. We recently found that hMPV M2-2 protein elicits immune evasion by targeting mitochondrial antiviral-signaling protein (MAVS), an antiviral signaling molecule. However, the molecular mechanisms underlying such inhibition are not known. Our mutagenesis studies revealed that PDZ-binding motifs, 29-DEMI-32 and 39-KEALSDGI-46, located in an immune inhibitory region of M2-2, are responsible for M2-2-mediated immune evasion. We also found both motifs prevent TRAF5 and TRAF6, the MAVS downstream adaptors, to be recruited to MAVS, while the motif 39-KEALSDGI-46 also blocks TRAF3 migrating to MAVS. In parallel, these TRAFs are important in activating transcription factors NF-kB and/or IRF-3 by hMPV. Our findings collectively demonstrate that M2-2 uses its PDZ motifs to launch the hMPV immune evasion through blocking the interaction of MAVS and its downstream TRAFs.

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This manuscript describes a molecular mechanism underlying the immune evasion of hMPV.

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Results create the design basis for safer and more effective hMPV vaccines/therapeutic molecules.

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We demonstrate the contribution of TRAFs in antiviral responses to hMPV infection.

Recent advances on viral manipulation of NF-κB signaling pathway

NF-κB transcription factors regulate the expression of hundreds of genes primarily involved in immune responses. Signaling events leading to NF-κB activation constitute a major antiviral immune pathway. To replicate and persist within their hosts, viruses have evolved diverse strategies to evade and exploit cellular NF-κB immune signaling cascades for their benefit. We summarize recent studies concerning viral manipulation of the NF-κB signaling pathway downstream of pattern recognition receptors. Signal transduction mediated by pattern recognition receptors is a research frontier for both infectious disease and innate immunology.

Premature infants have impaired airway antiviral IFNγ responses to human metapneumovirus compared to respiratory syncytial virus

BACKGROUND

It is unknown why human metapneumovirus (HMPV) and respiratory syncytial virus (RSV) cause severe respiratory infection in children, particularly in premature infants. Our aim was to investigate if there are defective airway antiviral responses to these viruses in young children with history of prematurity.

METHODS

Nasal airway secretions were collected from 140 children ≤ 3 y old without detectable virus (n = 80) or with PCR-confirmed HMPV or RSV infection (n = 60). Nasal protein levels of IFNγ, CCL5/RANTES, IL-10, IL-4, and IL-17 were determined using a multiplex magnetic bead immunoassay.

RESULTS

Full-term children with HMPV and RSV infection had increased levels of nasal airway IFNγ, CCL5, and IL-10 along with an elevation in Th1 (IFNγ)/Th2 (IL-4) ratios, which is expected during antiviral responses. In contrast, HMPV-infected premature children (< 32 wk gestation) did not exhibit increased Th1/Th2 ratios or elevated nasal airway secretion of IFNγ, CCL5, and IL-10 relative to uninfected controls.

CONCLUSION

Our study is the first to demonstrate that premature infants have defective IFNγ, CCL5/RANTES, and IL-10 airway responses during HMPV infection and provides novel insights about the potential reason why HMPV causes severe respiratory disease in children with history of prematurity.

RIG-I-like receptor regulation in virus infection and immunity

Mammalian cells have the intrinsic capacity to detect viral pathogens and to initiate an antiviral response that is characterized by the induction of interferons (IFNs) and proinflammatory cytokines. A delicate regulation of the signaling pathways that lead to cytokine production is needed to ensure effective clearance of the virus, while preventing tissue damage caused by excessive cytokine release. Here, we focus on the mechanisms that modulate the signal transduction triggered by RIG-I-like receptors (RLRs) and their adaptor protein MAVS, key components of the host machinery for sensing foreign RNA. Specifically, we summarize recent advances in understanding how RLR signaling is regulated by posttranslational and posttranscriptional mechanisms, microRNAs (miRNAs) and autophagy. We further discuss how viruses target these regulatory mechanisms for immune evasion.

Effect of in vitro syncytium formation on the severity of human metapneumovirus disease in a murine model

Human metapneumovirus (HMPV) is an important cause of acute respiratory tract infections (ARTI) in children, elderly individuals and immunocompromised patients. In vitro, different HMPV strains can induce variable cytopathic effects ranging from large multinucleated syncytia to focal cell rounding. In this study, we investigated the impact of different in vitro phenotypes of two HMPV strains on viral replication and disease severity in a BALB/c mouse model. We first generated two recombinant GFP-expressing HMPV viruses: C-85473, a syncytium-inducing strain (rC-85473) belonging to the A1 subtype and CAN98-75, a focal cell rounding-inducing strain (rCAN98-75) of the B2 subtype. We subsequently exchanged the F genes of both strains to create the chimeric viruses rC-85473_F and rCAN98-75_F. We demonstrated that the F protein was the sole protein responsible for the syncytium phenotype and that viruses carrying a syncytium-inducing F protein replicated to significantly higher titers in vitro. In vivo, however, the virulence and replicative capacity of the different HMPV strains did not appear to be solely dependent on the F gene but also on the viral background, with the strains containing the C-85473 background inducing more weight loss as well as increased lung viral titers, pro-inflammatory cytokines and inflammation than strains containing the CAN98-75 background. In conclusion, the F protein is the main determinant of syncytium formation and replication kinetics in vitro, although it is not the only factor implicated in HMPV disease severity in mice.

Recent vaccine development for human metapneumovirus

Human metapneumovirus (hMPV) and respiratory syncytial virus, its close family member, are two major causes of lower respiratory tract infection in the paediatric population. hMPV is also a common cause of worldwide morbidity and mortality in immunocompromised patients and older adults. Repeated infections occur often, demonstrating a heavy medical burden. However, there is currently no hMPV-specific prevention treatment. This review focuses on the current literature on hMPV vaccine development. We believe that a better understanding of the role(s) of viral proteins in host responses might lead to efficient prophylactic vaccine development.

Role of dietary antioxidants in human metapneumovirus infection

Human metapneumovirus (hMPV) is a major cause of respiratory tract infections in children, elderly and immunocompromised hosts, for which no vaccine or treatment are currently available. Oxidative stress and inflammatory responses represent important pathogenic mechanism(s) of hMPV infection. Here, we explored the potential protective role of dietary antioxidants in hMPV infection. Treatment of airway epithelial cells with resveratrol and quercetin during hMPV infection significantly reduced cellular oxidative damage, inflammatory mediator secretion and viral replication, without affecting viral gene transcription and protein synthesis, indicating that inhibition of viral replication occurred at the level of viral assembly and/or release. Modulation of proinflammatory mediator expression occurred through the inhibition of transcription factor nuclear factor (NF)-κB and interferon regulatory factor (IRF)-3 binding to their cognate site of endogenous gene promoters. Our results indicate the use of dietary antioxidants as an effective treatment approach for modulating hMPV induced lung oxidative damage and inflammation.

Host-Viral Interactions: Role of Pattern Recognition Receptors (PRRs) in Human Pneumovirus Infections

Acute respiratory tract infection (RTI) is a leading cause of morbidity and mortality worldwide and the majority of RTIs are caused by viruses, among which respiratory syncytial virus (RSV) and the closely related human metapneumovirus (hMPV) figure prominently. Host innate immune response has been implicated in recognition, protection and immune pathological mechanisms. Host-viral interactions are generally initiated via host recognition of pathogen-associated molecular patterns (PAMPs) of the virus. This recognition occurs through host pattern recognition receptors (PRRs) which are expressed on innate immune cells such as epithelial cells, dendritic cells, macrophages and neutrophils. Multiple PRR families, including Toll-like receptors (TLRs), RIG-I-like receptors (RLRs) and NOD-like receptors (NLRs), contribute significantly to viral detection, leading to induction of cytokines, chemokines and type I interferons (IFNs), which subsequently facilitate the eradication of the virus. This review focuses on the current literature on RSV and hMPV infection and the role of PRRs in establishing/mediating the infection in both in vitro and in vivo models. A better understanding of the complex interplay between these two viruses and host PRRs might lead to efficient prophylactic and therapeutic treatments, as well as the development of adequate vaccines.

Regulation of RIG-I-like receptor signaling by host and viral proteins

Vertebrate innate immunity is characterized by an effective immune surveillance apparatus, evolved to sense foreign structures, such as proteins or nucleic acids of invading microbes. RIG-I-like receptors (RLRs) are key sensors of viral RNA species in the host cell cytoplasm. Activation of RLRs in response to viral RNA triggers an antiviral defense program through the production of hundreds of antiviral effector proteins including cytokines, chemokines, and host restriction factors that directly interfere with distinct steps in the virus life cycle. To avoid premature or abnormal antiviral and proinflammatory responses, which could have harmful consequences for the host, the signaling activities of RLRs and their common adaptor molecule, MAVS, are delicately controlled by cell-intrinsic regulatory mechanisms. Furthermore, viruses have evolved multiple strategies to modulate RLR-MAVS signal transduction to escape from immune surveillance. Here, we summarize recent progress in our understanding of the regulation of RLR signaling through host factors and viral antagonistic proteins.

Molecular comparisons of full length metapneumovirus (MPV) genomes, including newly determined French AMPV-C and -D isolates, further supports possible subclassification within the MPV Genus

Four avian metapneumovirus (AMPV) subgroups (A-D) have been reported previously based on genetic and antigenic differences. However, until now full length sequences of the only known isolates of European subgroup C and subgroup D viruses (duck and turkey origin, respectively) have been unavailable. These full length sequences were determined and compared with other full length AMPV and human metapneumoviruses (HMPV) sequences reported previously, using phylogenetics, comparisons of nucleic and amino acid sequences and study of codon usage bias. Results confirmed that subgroup C viruses were more closely related to HMPV than they were to the other AMPV subgroups in the study. This was consistent with previous findings using partial genome sequences. Closer relationships between AMPV-A, B and D were also evident throughout the majority of results. Three metapneumovirus "clusters" HMPV, AMPV-C and AMPV-A, B and D were further supported by codon bias and phylogenetics. The data presented here together with those of previous studies describing antigenic relationships also between AMPV-A, B and D and between AMPV-C and HMPV may call for a subclassification of metapneumoviruses similar to that used for avian paramyxoviruses, grouping AMPV-A, B and D as type I metapneumoviruses and AMPV-C and HMPV as type II.

Human metapneumovirus infection induces significant changes in small noncoding RNA expression in airway epithelial cells

Small noncoding RNAs (sncRNAs), such as microRNAs (miRNA), virus-derived sncRNAs, and more recently identified tRNA-derived RNA fragments, are critical to posttranscriptional control of genes. Upon viral infection, host cells alter their sncRNA expression as a defense mechanism, while viruses can circumvent host defenses and promote their own propagation by affecting host cellular sncRNA expression or by expressing viral sncRNAs. Therefore, characterizing sncRNA profiles in response to viral infection is an important tool for understanding host–virus interaction, and for antiviral strategy development. Human metapneumovirus (hMPV), a recently identified pathogen, is a major cause of lower respiratory tract infections in infants and children. To investigate whether sncRNAs play a role in hMPV infection, we analyzed the changes in sncRNA profiles of airway epithelial cells in response to hMPV infection using ultrahigh-throughput sequencing. Of the cloned sncRNAs, miRNA was dominant in A549 cells, with the percentage of miRNA increasing in a time-dependent manner after the infection. In addition, several hMPV-derived sncRNAs and corresponding ribonucleases for their biogenesis were identified. hMPV M2-2 protein was revealed to be a key viral protein regulating miRNA expression. In summary, this study revealed several novel aspects of hMPV-mediated sncRNA expression, providing a new perspective on hMPV–host interactions.

Human metapneumovirus M2-2 protein inhibits innate immune response in monocyte-derived dendritic cells

Human metapneumovirus (hMPV) is a leading cause of lower respiratory infection in young children, the elderly and immunocompromised patients. Repeated hMPV infections occur throughout life. However, immune evasion mechanisms of hMPV infection are largely unknown. Recently, our group has demonstrated that hMPV M2-2 protein, an important virulence factor, contributes to immune evasion in airway epithelial cells by targeting the mitochondrial antiviral-signaling protein (MAVS). Whether M2-2 regulates the innate immunity in human dendritic cells (DC), an important family of immune cells controlling antigen presenting, is currently unknown. We found that human DC infected with a virus lacking M2-2 protein expression (rhMPV-ΔM2-2) produced higher levels of cytokines, chemokines and IFNs, compared to cells infected with wild-type virus (rhMPV-WT), suggesting that M2-2 protein inhibits innate immunity in human DC. In parallel, we found that myeloid differentiation primary response gene 88 (MyD88), an essential adaptor for Toll-like receptors (TLRs), plays a critical role in inducing immune response of human DC, as downregulation of MyD88 by siRNA blocked the induction of immune regulatory molecules by hMPV. Since M2-2 is a cytoplasmic protein, we investigated whether M2-2 interferes with MyD88-mediated antiviral signaling. We found that indeed M2-2 protein associated with MyD88 and inhibited MyD88-dependent gene transcription. In this study, we also identified the domains of M2-2 responsible for its immune inhibitory function in human DC. In summary, our results demonstrate that M2-2 contributes to hMPV immune evasion by inhibiting MyD88-dependent cellular responses in human DC.