Influenza A virus protein PB1-F2 translocates into mitochondria via Tom40 channels and impairs innate immunity

Effects of the PA-X and PB1-F2 Proteins on the Virulence of the 2009 Pandemic H1N1 Influenza A Virus in Mice

There have been several previous reports showing that PA-X and PB1-F2 proteins can regulate innate immune responses and may play roles in the adaptation of influenza viruses to new hosts. In this research, we investigated, for the first time, the combined effects of PA-X and PB1-F2 proteins on viral virulence in mice. Based on the 2009 pH1N1 A/Guangdong/1057/2010 virus backbone, four viruses encoding different combinations of full-length or truncated PA-X and PB1-F2 proteins were rescued by a reverse genetic engineering system. We analyzed viral replication, host-shutoff activity, in vitro viral pathogenicity and in vivo host immune response. We found that simultaneously expressing the full-length PA-X and PB1-F2 proteins enhanced viral replication in vitro through increasing the accumulation of the RNP complex protein and enhanced viral pathogenicity in mice during the early stage of infection. Furthermore, PA-X and PB1-F2 simultaneously regulated the host innate response, and different forms of PB1-F2 proteins may have impacts on the host shutoff activity induced by the PA-X protein. Our results provide a better understanding of the mechanisms of PA-X and PB1-F2 proteins during viral replication, pathogenicity and host immune response.

Porcine deltacoronavirus nucleocapsid protein antagonizes IFN-β production by impairing dsRNA and PACT binding to RIG-I

Porcine deltacoronavirus (PDCoV) is an emerging swine enteropathogenic coronavirus that causes watery diarrhea, vomiting and mortality in newborn piglets. Previous studies have suggested that PDCoV infection antagonizes RIG-I-like receptor (RLR)-mediated IFN-β production to evade host innate immune defense, and PDCoV-encoded nonstructural protein nsp5 and accessory protein NS6 are associated with this process. However, whether the structural protein(s) of PDCoV also antagonize IFN-β production remains unclear. In this study, we found that PDCoV nucleocapsid (N) protein, the most abundant viral structural protein, suppressed Sendai virus (SEV)-induced IFN-β production and transcription factor IRF3 activation, but did not block IFN-β production induced by overexpressing RIG-I/MDA5. Furthermore, study revealed that PDCoV N protein interacted with RIG-I and MDA5 in an in vitro overexpression system and evident interactions between N protein and RIG-I could be detected in the context of PDCoV infection, which interfered with the binding of dsRNA and protein activator of protein kinase R (PACT) to RIG-I. Together, our results demonstrate that PDCoV N protein is an IFN antagonist and utilizes diverse strategies to attenuate RIG-I recognition and activation.

PA-X antagonises MAVS-dependent accumulation of early type I interferon messenger RNAs during influenza A virus infection

The sensing of viral nucleic acids by the innate immune system activates a potent antiviral response in the infected cell, a key component of which is the expression of genes encoding type I interferons (IFNs). Many viruses counteract this response by blocking the activation of host nucleic acid sensors. The evolutionarily conserved influenza A virus (IAV) protein PA-X has been implicated in suppressing the host response to infection, including the expression of type I IFNs. Here, we characterise this further using a PA-X-deficient virus of the mouse-adapted PR8 strain to study activation of the innate immune response in a mouse model of the early response to viral infection. We show that levels of Ifna4 and Ifnb1 mRNAs in the lungs of infected mice were elevated in the absence of PA-X and that this was completely dependent on MAVS. This therefore suggests a role for PA-X in preventing the accumulation of early type I IFN mRNAs in the lung during IAV infection.

Human metapneumovirus activates NOD-like receptor protein 3 inflammasome via its small hydrophobic protein which plays a detrimental role during infection in mice

NOD-like receptor protein 3 (NLRP3) inflammasome activation triggers caspase-1 activation-induced maturation of interleukin (IL)-1β and IL-18 and therefore is important for the development of the host defense against various RNA viral diseases. However, the implication of this protein complex in human metapneumovirus (HMPV) disease has not been fully studied. Herein, we report that NLRP3 inflammasome plays a detrimental role during HMPV infection because NLRP3 inflammasome inhibition protected mice from mortality and reduced weight loss and inflammation without impacting viral replication. We also demonstrate that NLRP3 inflammasome exerts its deleterious effect via IL-1β production since we observed reduced mortality, weight loss and inflammation in IL-1β-deficient (IL-1β^-/-) mice, as compared to wild-type animals during HMPV infection. Moreover, the effect on these evaluated parameters was not different in IL-1β^-/- and wild-type mice treated with an NLRP3 inflammasome inhibitor. The production of IL-1β was also abrogated in bone marrow derived macrophages deficient for NLRP3. Finally, we show that small hydrophobic protein-deleted recombinant HMPV (HMPV ΔSH) failed to activate caspase-1, which is responsible for IL-1β cleavage and maturation. Furthermore, HMPV ΔSH-infected mice had less weight loss, showed no mortality and reduced inflammation, as compared to wild-type HMPV-infected mice. Thus, NLRP3 inflammasome activation seems to be triggered by HMPV SH protein in HMPV disease. In summary, once activated by the HMPV SH protein, NLRP3 inflammasome promotes the maturation of IL-1β, which exacerbates HMPV-induced inflammation. Therefore, the blockade of IL-1β production by using NLRP3 inflammasome inhibitors might be a novel potential strategy for the therapy and prevention of HMPV infection.

Human metapneumovirus (HMPV), a negative-stranded, enveloped RNA virus, is recognized as one of the leading causes of acute respiratory disease in children since its discovery in 2001. Nevertheless, there is currently no licensed vaccine for the prevention of HMPV infection and treatment modalities are limited to the use of ribavirin, a weak antiviral agent or immunoglobulins. NOD-like receptor protein 3 (NLRP3) inflammasome has been shown to be involved in the pathogenesis of several RNA viral diseases but its role during HMPV infection has not been fully studied. Here, we report for the first time that NLRP3 inflammasome is activated by the small hydrophobic protein of HMPV, leading to the release of IL-1β, which has the potential to exacerbate inflammation. However, NLRP3 inflammasome has no direct influence on viral replication. Thus, IL-1β-mediated inflammatory process plays an important role during HMPV infection and, therefore, anti-IL-1β strategies such as the use of NLRP3 inhibitors may be a novel potential approach for the prevention and therapy of HMPV disease.

The absent in melanoma 2 (AIM2) inflammasome in microbial infection

Inflammasomes play a very important role in the host defense against multiple pathogenic microbes, including bacteria and viruses. Inflammasomes are multiprotein complex platforms that mediate the processing of the two most important inflammatory cytokines, pro-IL-1β and pro-IL-18, to their active forms. The inflammasome is formed by the apoptosis-associated speck-like protein containing a CARD (ASC), procaspase-1 and a sensor protein, either a NOD-like receptor (NLR) or an absent in melanoma 2 (AIM2)-like receptor. The sensor molecule determines inflammasome specificity by detecting specific and conserved microbial products or cell stress signals. Compared with the other inflammasomes, there is much more unknown about the activation or regulation mechanisms of the AIM2 inflammasome. In this review, we will discuss these mechanisms and the specific roles of the AIM2 inflammasome in response to diverse pathogens.

Asymmetric evolution in viral overlapping genes is a source of selective protein adaptation

Overlapping genes represent an intriguing puzzle, as they encode two proteins whose ability to evolve is constrained by each other. Overlapping genes can undergo “symmetric evolution” (similar selection pressures on the two proteins) or “asymmetric evolution” (significantly different selection pressures on the two proteins). By sequence analysis of 75 pairs of homologous viral overlapping genes, I evaluated their accordance with one or the other model. Analysis of nucleotide and amino acid sequences revealed that half of overlaps undergo asymmetric evolution, as the protein from one frame shows a number of substitutions significantly higher than that of the protein from the other frame. Interestingly, the most variable protein (often known to interact with the host proteins) appeared to be encoded by the de novo frame in all cases examined. These findings suggest that overlapping genes, besides to increase the coding ability of viruses, are also a source of selective protein adaptation.

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A dataset of 80 pairs of homologous overlapping genes from viruses is examined.

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Its analysis reveals that half of overlapping genes undergo asymmetric evolution.

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The most variable gene product is that encoded by the de novo overlapping gene.

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Overlapping genes evolving asymmetrically are a source of selective protein adaptation.

The Differential Expression of Mitochondrial Function-Associated Proteins and Antioxidant Enzymes during Bovine Herpesvirus 1 Infection: A Potential Mechanism for Virus Infection-Induced Oxidative Mitochondrial Dysfunction

Reactive oxidative species (ROS) are important inflammatory mediators. Electrons escaping from the mitochondrial electron transport chain (ETC) during oxidative phosphorylation (OXPHOS) in the mitochondrial respiratory chain (RC) complexes contribute to ROS production. The cellular antioxidant enzymes are important for maintaining ROS release at the physiological levels. It has been reported that BoHV-1 infection induces overproduction of ROS and oxidative mitochondrial dysfunction in cell cultures. In this study, we found that chemical interruption of RC complexes by TTFA (an inhibitor of RC complex II), NaN₃ (an inhibitor of RC complex IV), and oligomycin A (an inhibitor of ATP synthase) consistently decreased virus productive infection, suggesting that the integral processes of RC complexes are important for the virus replication. The virus infection significantly increased the expression of subunit SDHB (succinate dehydrogenase) and MTCO1 (cytochrome c oxidase subunit I), critical components of RC complexes II and IV, respectively. The expression of antioxidant enzymes including superoxide dismutase 1 (SOD1), SOD2, catalase (CAT), and glutathione peroxidase 4 (GPX4) was differentially affected following the virus infection. The protein TFAM (transcription factor A, mitochondrial) stimulated by either nuclear respiratory factor 1 (NRF1) or NRF2 is a key regulator of mitochondrial biogenesis. Interestingly, the virus infection at the late stage (at 16 h after infection) stimulated TFAM expression but decreased the levels of both NRF1 and NRF2, indicating that virus infection activated TFAM signaling independent of either NRF1 or NRF2. Overall, this study provided evidence that BoHV-1 infection altered the expression of molecules associated with RC complexes, antioxidant enzymes, and mitochondrial biogenesis-related signaling NRF1/NRF2/TFAM, which correlated with the previous report that virus infection induces ROS overproduction and mitochondrial dysfunction.

Cellular Proteostasis During Influenza A Virus Infection-Friend or Foe?

In order to efficiently replicate, viruses require precise interactions with host components and often hijack the host cellular machinery for their own benefit. Several mechanisms involved in protein synthesis and processing are strongly affected and manipulated by viral infections. A better understanding of the interplay between viruses and their host-cell machinery will likely contribute to the development of novel antiviral strategies. Here, we discuss the current knowledge on the interactions between influenza A virus (IAV), the causative agent for most of the annual respiratory epidemics in humans, and the host cellular proteostasis machinery during infection. We focus on the manipulative capacity of this virus to usurp the cellular protein processing mechanisms and further review the protein quality control mechanisms in the cytosol and in the endoplasmic reticulum that are affected by this virus.

The cellular localization of avian influenza virus PB1-F2 protein alters the magnitude of IFN2 promoter and NFκB-dependent promoter antagonism in chicken cells

The accessory protein, PB1-F2, of influenza A virus (IAV) functions in a chicken host to prolong infectious virus shedding and thus the transmission window. Here we show that this delay in virus clearance by PB1-F2 in chickens is accompanied by reduced transcript levels of type 1 interferon (IFN)-induced genes and NFκB-activated pro-inflammation cytokines. In vitro, two avian influenza isolate-derived PB1-F2 proteins, H9N2 UDL01 and H5N1 5092, exhibited the same antagonism of the IFN and pro-inflammation induction pathways seen in vivo, but to different extents. The two PB1-F2 proteins had different cellular localization in chicken cells, with H5N1 5092 being predominantly mitochondrial-associated and H9N2 UDL being cytoplasmic but not mitochondrial-localized. We hypothesized that PB1-F2 localization might influence the functionality of the protein during infection and that the protein sequence could alter cellular localization. We demonstrated that the sequence of the C-terminus of PB1-F2 determined cytoplasmic localization in chicken cells and this was linked with protein instability. Mitochondrial localization of PB1-F2 resulted in reduced antagonism of an NFκB-dependent promoter. In parallel, mitochondrial localization of PB1-F2 increased the potency of chicken IFN 2 induction antagonism. We suggest that mitochondrial localization of PB1-F2 restricts interaction with cytoplasmic-located IKKβ, reducing NFκB-responsive promoter antagonism, but enhances antagonism of the IFN2 promoter through interaction with the mitochondrial adaptor MAVS. Our study highlights the differential mechanisms by which IAV PB1-F2 protein can dampen the avian host innate signalling response.

Influenza M2 protein regulates MAVS-mediated signaling pathway through interacting with MAVS and increasing ROS production

Influenza A virus can evade host innate immune response that is involved in several viral proteins with complicated mechanisms. To date, how influenza A M2 protein modulates the host innate immunity remains unclear. Herein, we showed that M2 protein colocalized and interacted with MAVS (mitochondrial antiviral signaling protein) on mitochondria, and positively regulated MAVS-mediated innate immunity. Further studies revealed that M2 induced reactive oxygen species (ROS) production that was required for activation of macroautophagy/autophagy and enhancement of MAVS signaling pathway. Importantly, the proton channel activity of M2 protein was demonstrated to be essential for ROS production and antagonizing the autophagy pathway to control MAVS aggregation, thereby enhancing MAVS signal activity. In conclusion, our studies provided novel insights into mechanisms of M2 protein in modulating host antiviral immunity and uncovered a new mechanism into biology and pathogenicity of influenza A virus. Abbreviations: AKT/PKB: AKT serine/threonine kinase; Apo: apocynin; ATG5: autophagy related 5; BAPTA-AM: 1,2-Bis(2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid tetrakis; BECN1: beclin 1; CARD: caspase recruitment domain; CCCP: carbonyl cyanide m-chlorophenylhydrazone; CQ: chloroquine; DCF: dichlorodihyd-rofluorescein; DPI: diphenyleneiodonium; DDX58: DExD/H-box helicase 58; eGFP: enhanced green fluorescent protein; EGTA: ethylene glycol-bis(2-aminoethylether)-N,N,N',N'-tetraacetic acid; ER: endoplasmic reticulum; hpi: hours post infection; IAV: influenza A virus; IFN: interferon; IP: immunoprecipitation; IRF3: interferon regulatory factor 3; ISRE: IFN-stimulated response elements; LIR: LC3-interacting region; MAP1LC3B/LC3B: microtubule associated protein 1 light chain 3 beta; MAVS: mitochondrial antiviral signaling protein; MMP: mitochondrial membrane potential; MOI, multiplicity of infection; mRFP: monomeric red fluorescent protein; MTOR: mechanistic target of rapamycin kinase; NC: negative control; NFKB/NF-κB: nuclear factor kappa B; PI3K: class I phosphoinositide 3-kinase; RLR: RIG-I-like-receptor; ROS: reactive oxygen species; SEV: sendai virus; TM: transmembrane; TMRM: tetramethylrhodamine methylester; VSV: vesicular stomatitis virus.

Location is everything: protein translocations as a viral infection strategy

Protein movement between different subcellular compartments is an essential aspect of biological processes, including transcriptional and metabolic regulation, and immune and stress responses. As obligate intracellular parasites, viruses are master manipulators of cellular composition and organization. Accumulating evidences have highlighted the importance of infection-induced protein translocations between organelles. Both directional and temporal, these translocation events facilitate localization-dependent protein interactions and changes in protein functions that contribute to either host defense or virus replication. The discovery and characterization of protein movement is technically challenging, given the necessity for sensitive detection and subcellular resolution. Here, we discuss infection-induced translocations of host and viral proteins, and the value of integrating quantitative proteomics with advanced microscopy for understanding the biology of human virus infections.

Fibrillar structures induced by a plant reovirus target mitochondria to activate typical apoptotic response and promote viral infection in insect vectors

Numerous plant viruses that cause significant agricultural problems are persistently transmitted by insect vectors. We wanted to see if apoptosis was involved in viral infection process in the vector. We found that a plant reovirus (rice gall dwarf virus, RGDV) induced typical apoptotic response during viral replication in the leafhopper vector and cultured vector cells, as demonstrated by mitochondrial degeneration and membrane potential decrease. Fibrillar structures formed by nonstructural protein Pns11 of RGDV targeted the outer membrane of mitochondria, likely by interaction with an apoptosis-related mitochondrial protein in virus-infected leafhopper cells or nonvector insect cells. Such association of virus-induced fibrillar structures with mitochondria clearly led to mitochondrial degeneration and membrane potential decrease, suggesting that RGDV Pns11 was the inducer of apoptotic response in insect vectors. A caspase inhibitor treatment and knockdown of caspase gene expression using RNA interference each reduced apoptosis and viral accumulation, while the knockdown of gene expression for the inhibitor of apoptosis protein improved apoptosis and viral accumulation. Thus, RGDV exploited caspase-dependent apoptotic response to promote viral infection in insect vectors. For the first time, we directly confirmed that a nonstructural protein encoded by a persistent plant virus can induce the typical apoptotic response to benefit viral transmission by insect vectors.

Camouflage and interception: how pathogens evade detection by intracellular nucleic acid sensors

Intracellular DNA and RNA sensors play a vital part in the innate immune response to viruses and other intracellular pathogens, causing the secretion of type I interferons, cytokines and chemokines from infected cells. Pathogen RNA can be detected by retinoic-acid inducible gene I-like receptors in the cytosol, whereas cytosolic DNA is recognized by DNA sensors such as cyclic GMP-AMP synthase (cGAS). The resulting local immune response, which is initiated within hours of infection, is able to eliminate many pathogens before they are able to establish an infection in the host. For this reason, all viruses, and some intracellular bacteria and protozoa, need to evade detection by nucleic acid sensors. Immune evasion strategies include the sequestration and modification of nucleic acids, and the inhibition or degradation of host factors involved in innate immune signalling. Large DNA viruses, such as herpesviruses, often use multiple viral proteins to inhibit signalling cascades at several different points; for instance herpes simplex virus 1 targets both DNA sensors cGAS and interferon-γ-inducible protein 16, as well as the adaptor protein STING (stimulator of interferon genes) and other signalling factors in the pathway. Viruses with a small genome encode only a few immunomodulatory proteins, but these are often multifunctional, such as the NS1 protein from influenza A virus, which inhibits RNA sensing in multiple ways. Intracellular bacteria and protozoa can also be detected by nucleic acid sensors. However, as the type I interferon response is not always beneficial for the host under these circumstances, some bacteria subvert, rather than evade, these signalling cascades for their own gain.

Effects of PB1-F2 on the pathogenicity of H1N1 swine influenza virus in mice and pigs

Although several studies have exploited the effects of PB1-F2 in swine influenza viruses, its contribution to the pathogenicity of swine influenza viruses remains unclear. Herein, we investigated the effects of PB1-F2 on the pathogenicity of influenza virus using a virulent H1N1 A/swine/Kansas/77778/2007 (KS07) virus, which expresses a full-length PB1-F2, in mice and pigs. Using reverse genetics, we generated the wild-type KS07 (KS07_WT), a PB1-F2 knockout mutant (KS07_K/O) and its N66S variant (KS07_N66S). KS07_K/O showed similar pathogenicity in mice to the KS07_WT, whereas KS07_N66S displayed enhanced virulence when compared to the other two viruses. KS07_WT exhibited more efficient replication in lungs and nasal shedding in infected pigs than the other two viruses. Pigs infected with the KS07_WT had higher pulmonary levels of granulocyte-macrophage colony-stimulating factor, IFN-γ, IL-6 and IL-8 at 3 and 5 days post-infection, as well as lower levels of IL-2, IL-4 and IL-12 at 1 day post-infection compared to those infected with the KS07_K/O. These results indicate that PB1-F2 modulates KS07 H1N1 virus replication, pathogenicity and innate immune responses in pigs and the single substitution at position 66 (N/S) in the PB1-F2 plays a critical role in virulence in mice. Taken together, our results provide new insights into the effects of PB1-F2 on the virulence of influenza virus in swine and support PB1-F2 as a virulence factor of influenza A virus in a strain- and host-dependent manner.

Interplay between Cellular Metabolism and Cytokine Responses during Viral Infection

Metabolism and immune responses are two fundamental biological processes that serve to protect hosts from viral infection. As obligate intracellular pathogens, viruses have evolved diverse strategies to activate metabolism, while inactivating immune responses to achieve maximal reproduction or persistence within their hosts. The two-way virus-host interaction with metabolism and immune responses choreograph cytokine production via reprogramming metabolism of infected cells/hosts. In return, cytokines can affect the metabolism of virus-infected and bystander cells to impede viral replication processes. This review aims to summarize our current understanding of the cross-talk between metabolic reprogramming and cytokine responses, and to highlight future potential research topics. Although the focus is placed on viral pathogens, relevant findings from other microbes are integrated to provide an overall picture, particularly when corresponding information on viral infection is lacking.

Influenza virus replication raises the temperature of cells

Influenza virus invades the cell by binding sialic acid on the cell membrane through haemagglutinin (HA), and then genome replication and transcription are carried out in the nucleus to produce progeny virus. Multiplication of influenza virus requires metabolites, such as nucleotides and amino acids, as well as cellular machinery to synthesize its genome and proteins, thereby producing viral particles. Influenza virus infection forces the start of several metabolic systems in the cell, which consume or generate large amounts of energy. Thus, the viral multiplication processes involved in both genome replication and transcription are considered to require large numbers of nucleotides. The high-level consumption of nucleotides generates large amounts of energy, some of which is converted into heat, and this heat may increase the temperature of cells. To address this question, we prepared a tool based on rhodamine B fluorescence, which we used to measure the temperatures of influenza virus-infected and uninfected cells. The results indicated that influenza virus multiplication increased the temperature of cells by approximately 4 °C - 5 °C, ATP levels in the cells decreased at 3 h after infection, and mitochondrial membrane potential decreased with multiplication level. Thus, the increase in cellular temperature during influenza virus infection appears to be due to the massive consumption of ATP over a short period.

Preliminary investigations on the role of Drp-1 dependent mitochondrial fission in attenuating RLR downstream signaling during nervous necrosis virus infection

Member of the dynamin family of large GTPases, dynamin-related protein 1 (Drp1) dependent mitochondrial fission is an intricate process regulating both cellular and organ dynamics. Present study shows that NNV perturbs mitochondrial dynamics by promoting Drp-1 dependent mitochondrial fission, which attenuates MAVS mediated downstream signaling. NNV infected SISS cells revealed induction in Drp1 expression and subsequent translocation into mitochondria. The level of MAVS expression was up-regulated over a period of 24 hpi and declined with the progression of NNV infection at 48 and 72 hpi confirmed by western blot and mRNA transcript analysis. Drp-1 displayed its association with fragmented mitochondria and the transcript abundance was significant post infection along with Mff. Expression levels of IRF-3 IFN-1 and Mx followed a similar pattern with abundant expression at 48 hpi and diminished expression during the further period. Importantly, silencing of Drp1 caused significant elevation in the RLR downstream molecules and reduction in viral RNA expression. These results suggest that NNV-induced mitochondrial fission serve to attenuate host RLR signaling. This provides an illustration of host-pathogen interaction in which the virus evades innate immunity by enhancing mitochondrial fission and perturbs MAVS, and the downstream molecules.

Operation of mitochondrial machinery in viral infection-induced immune responses

Mitochondria have been recognized as ancient bacteria that contain evolutionary endosymbionts. Metabolic pathways and inflammatory signals interact within mitochondria in response to different stresses, such as viral infections. In this commentary, we address several interesting questions, including (1) how do mitochondrial machineries participate in immune responses; (2) how do mitochondria mediate antiviral immunity; (3) what mechanisms involved in mitochondrial machinery, including the downregulation of mitochondrial DNA (mtDNA), disturbances of mitochondrial dynamics, and the induction of mitophagy and regulation of apoptosis, have been adopted by viruses to evade antiviral immunity; (4) what mechanisms involve the regulation of mitochondrial machineries in antiviral therapeutics; and (5) what are the potential challenges and perspectives in developing mitochondria-targeting antiviral treatments? This commentary provides a comprehensive review of the roles and mechanisms of mitochondrial machineries in immunity, viral infections and related antiviral therapeutics.

Swine Influenza Virus Induces RIPK1/DRP1-Mediated Interleukin-1 Beta Production

Nucleotide-binding domain and leucine-rich repeat-containing protein 3 (NLRP3) inflammasome plays a pivotal role in modulating lung inflammation in response to the influenza A virus infection. We previously showed that the swine influenza virus (SIV) infection induced NLRP3 inflammasome-mediated IL-1β production in primary porcine alveolar macrophages (PAMs), and we were interested in examining the upstream signaling events that are involved in this process. Here, we report that the SIV-infection led to dynamin-related protein 1 (DRP1) phosphorylation at serine 579 and mitochondrial fission in PAMs. IL-1β production was dependent on the reactive oxygen species (ROS) production, and DRP1 phosphorylation resulted in the upregulation of the NLRP3 inflammasome. Furthermore, the requirement of the kinase activity of receptor-interacting protein kinase 1 (RIPK1) for the IL-1β production and RIPK1-DRP1 association suggested that RIPK1 is an upstream kinase for DRP1 phosphorylation. Our results reveal a critical role of the RIPK1/DRP1 signaling axis, whose activation leads to mitochondrial fission and ROS release, in modulating porcine NLRP3 inflammasome-mediated IL-1β production in SIV-infected PAMs.

Programmed Cell Death in the Pathogenesis of Influenza

Influenza is a respiratory disease induced by infection by the influenza virus, which is a member of Orthomyxoviridae family. This infectious disease has serious impacts on public health systems and results in considerable mortality and economic costs throughout the world. Based on several experimental studies, massive host immune reaction is associated with the disease severity of influenza. Programmed cell death is typically induced during virus infection as a consequence of host immune reaction to limit virus spread by eliminating niches for virus propagation without causing inflammation. However, in some viral infectious diseases, such as influenza, in the process of immune reaction, aberrant induction of programmed cell death disturbs the maintenance of organ function. Current reports show that there are different types of programmed cell death that vary in terms of molecular mechanisms and/or associations with inflammation. In addition, these novel types of programmed cell death are associated with pathogenesis rather than suppressing virus propagation in the disease course. Here, we review our current understanding of mechanisms of programmed cell death in the pathogenesis of influenza.

Immunomodulatory Nonstructural Proteins of Influenza A Viruses

Influenza epidemics and pandemics still represent a severe public health threat and cause significant morbidity and mortality worldwide. As intracellular parasites, influenza viruses are strongly dependent on the host cell machinery. To ensure efficient production of progeny viruses, viral proteins extensively interfere with cellular signalling pathways to inhibit antiviral responses or to activate virus-supportive functions. Here, we review various functions of the influenza virus nonstructural proteins NS1, PB1-F2, and PA-X in infected cells and how post-transcriptional modifications of these proteins affect the viral life cycle. Furthermore, we discuss newly discovered interactions between these proteins and the antiviral interferon response.

Composition-dependent membrane disruption by the proapoptotic protein PB1F2 from HK97 influenza A virus

PB1F2 is a proapoptotic protein encoded by an alternative reading frame in the influenza A virus. Its accumulation accelerates mitochondrial fragmentation by decreasing the mitochondrial membrane potential following translocation into the mitochondrial inner membrane space, but the mechanistic underpinnings remain unclear. Herein, the PB1F2 from HK97 was expressed and purified in soluble form. The interaction between PB1F2 and the mitochondrial membrane were investigated using three membrane mimics, liposomes, bicelles, and nanodiscs. We show that the interactions between PB1F2 and membrane mimics depend on lipid type and are time- and dose-dependent. The primary membrane target of PB1F2 is phosphatidylcholine, the lipid that forms the major component of mitochondrial inner membranes. PB1F2 disrupts the integrity of lipid membranes by forming micelle-like PB1F2-lipid assemblies.

Ectromelia Virus Affects Mitochondrial Network Morphology, Distribution, and Physiology in Murine Fibroblasts and Macrophage Cell Line

Mitochondria are multifunctional organelles that participate in numerous processes in response to viral infection, but they are also a target for viruses. The aim of this study was to define subcellular events leading to alterations in mitochondrial morphology and function during infection with ectromelia virus (ECTV). We used two different cell lines and a combination of immunofluorescence techniques, confocal and electron microscopy, and flow cytometry to address subcellular changes following infection. Early in infection of L929 fibroblasts and RAW 264.7 macrophages, mitochondria gathered around viral factories. Later, the mitochondrial network became fragmented, forming punctate mitochondria that co-localized with the progeny virions. ECTV-co-localized mitochondria associated with the cytoskeleton components. Mitochondrial membrane potential, mitochondrial fission–fusion, mitochondrial mass, and generation of reactive oxygen species (ROS) were severely altered later in ECTV infection leading to damage of mitochondria. These results suggest an important role of mitochondria in supplying energy for virus replication and morphogenesis. Presumably, mitochondria participate in transport of viral particles inside and outside of the cell and/or they are a source of membranes for viral envelope formation. We speculate that the observed changes in the mitochondrial network organization and physiology in ECTV-infected cells provide suitable conditions for viral replication and morphogenesis.

Inducible Lung Epithelial Resistance Requires Multisource Reactive Oxygen Species Generation To Protect against Viral Infections

Viral pneumonias cause profound worldwide morbidity, necessitating novel strategies to prevent and treat these potentially lethal infections. Stimulation of intrinsic lung defenses via inhalation of synergistically acting Toll-like receptor (TLR) agonists protects mice broadly against pneumonia, including otherwise-lethal viral infections, providing a potential opportunity to mitigate infectious threats. As intact lung epithelial TLR signaling is required for the inducible resistance and as these cells are the principal targets of many respiratory viruses, the capacity of lung epithelial cells to be therapeutically manipulated to function as autonomous antiviral effectors was investigated. Our work revealed that mouse and human lung epithelial cells could be stimulated to generate robust antiviral responses that both reduce viral burden and enhance survival of isolated cells and intact animals. The antiviral protection required concurrent induction of epithelial reactive oxygen species (ROS) from both mitochondrial and dual oxidase sources, although neither type I interferon enrichment nor type I interferon signaling was required for the inducible protection. Taken together, these findings establish the sufficiency of lung epithelial cells to generate therapeutically inducible antiviral responses, reveal novel antiviral roles for ROS, provide mechanistic insights into inducible resistance, and may provide an opportunity to protect patients from viral pneumonia during periods of peak vulnerability.IMPORTANCE Viruses are the most commonly identified causes of pneumonia and inflict unacceptable morbidity, despite currently available therapies. While lung epithelial cells are principal targets of respiratory viruses, they have also been recently shown to contribute importantly to therapeutically inducible antimicrobial responses. This work finds that lung cells can be stimulated to protect themselves against viral challenges, even in the absence of leukocytes, both reducing viral burden and improving survival. Further, it was found that the protection occurs via unexpected induction of reactive oxygen species (ROS) from spatially segregated sources without reliance on type I interferon signaling. Coordinated multisource ROS generation has not previously been described against viruses, nor has ROS generation been reported for epithelial cells against any pathogen. Thus, these findings extend the potential clinical applications for the strategy of inducible resistance to protect vulnerable people against viral infections and also provide new insights into the capacity of lung cells to protect against infections via novel ROS-dependent mechanisms.

Virulent PB1-F2 residues: effects on fitness of H1N1 influenza A virus in mice and changes during evolution of human influenza A viruses

Specific residues of influenza A virus (IAV) PB1-F2 proteins may enhance inflammation or cytotoxicity. In a series of studies, we evaluated the function of these virulence-associated residues in the context of different IAV subtypes in mice. Here, we demonstrate that, as with the previously assessed pandemic 1968 (H3N2) IAV, PB1-F2 inflammatory residues increase the virulence of H1N1 IAV, suggesting that this effect might be a universal feature. Combining both inflammatory and cytotoxic residues in PB1-F2 enhanced virulence further, compared to either motif alone. Residues from these virulent motifs have been present in natural isolates from human seasonal IAV of all subtypes, but there has been a trend toward a gradual reduction in the number of virulent residues over time. However, human IAV of swine and avian origin tend to have more virulent residues than do the human-adapted seasonal strains, raising the possibility that donation of PB1 segments from these zoonotic viruses may increase the severity of some seasonal human strains. Our data suggest the value of surveillance of virulent residues in both human and animal IAV to predict the severity of influenza season.

Genome-wide identification of interferon-sensitive mutations enables influenza vaccine design

In conventional attenuated viral vaccines, immunogenicity is often suboptimal. Here we present a systematic approach for vaccine development that eliminates interferon (IFN)-modulating functions genome-wide while maintaining virus replication fitness. We applied a quantitative high-throughput genomics system to influenza A virus that simultaneously measured the replication fitness and IFN sensitivity of mutations across the entire genome. By incorporating eight IFN-sensitive mutations, we generated a hyper-interferon-sensitive (HIS) virus as a vaccine candidate. HIS virus is highly attenuated in IFN-competent hosts but able to induce transient IFN responses, elicits robust humoral and cellular immune responses, and provides protection against homologous and heterologous viral challenges. Our approach, which attenuates the virus and promotes immune responses concurrently, is broadly applicable for vaccine development against other pathogens.

Influenza Virus: A Master Tactician in Innate Immune Evasion and Novel Therapeutic Interventions

Influenza is a contagion that has plagued mankind for many decades, and continues to pose concerns every year, with millions of infections globally. The frequent mutations and recombination of the influenza A virus (IAV) cast a looming threat that antigenically novel strains/subtypes will rise with unpredictable pathogenicity and fear of it evolving into a pandemic strain. There have been four major influenza pandemics, since the beginning of twentieth century, with the great 1918 pandemic being the most severe, killing more than 50 million people worldwide. The mechanisms of IAV infection, host immune responses, and how viruses evade from such defensive responses at the molecular and structural levels have been greatly investigated in the past 30 years. While this has advanced our understanding of virus–host interactions and human immunology, and has led to the development of several antiviral drugs, they have minimal impact on the clinical outcomes of infection. The heavy use of these drugs has also imposed selective pressure on IAV to evolve and develop resistance. Vaccination remains the cornerstone of public health efforts to protect against influenza; however, rapid mass-production of sufficient vaccines is unlikely to occur immediately after the beginning of a pandemic. This, therefore, requires novel therapeutic strategies against this continually emerging infectious virus with higher specificity and cross-reactivity against multiple strains/subtypes of IAVs. This review discusses essential virulence factors of IAVs that determine sustainable human-to-human transmission, the mechanisms of viral hijacking of host cells and subversion of host innate immune responses, and novel therapeutic interventions that demonstrate promising antiviral properties against IAV. This hopefully will promote discussions and investigations on novel avenues of prevention and treatment strategies of influenza, that are effective and cross-protective against multiple strains/subtypes of IAV, in preparation for the advent of future IAVs and pandemics.

NS1 Protein of 2009 Pandemic Influenza A Virus Inhibits Porcine NLRP3 Inflammasome-Mediated Interleukin-1 Beta Production by Suppressing ASC Ubiquitination

The inflammasome represents a molecular platform for innate immune regulation and controls proinflammatory cytokine production. The NLRP3 inflammasome is comprised of NLRP3, ASC, and procaspase-1. When the NLRP3 inflammasome is activated, it causes ASC speck formation and caspase-1 activation, resulting in the maturation of interleukin-1β (IL-1β). The NLRP3 inflammasome is regulated at multiple levels, with one level being posttranslational modification. Interestingly, ubiquitination of ASC has been reported to be indispensable for the activation of the NLRP3 inflammasome. Influenza A virus (IAV) infection induces NLRP3 inflammasome-dependent IL-1β secretion, which contributes to the host antiviral defense. However, IAVs have evolved multiple antagonizing mechanisms, one of which is executed by viral NS1 protein to suppress the NLRP3 inflammasome. In this study, we compared IL-1β production in porcine alveolar macrophages in response to IAV infection and found that the 2009 pandemic H1N1 induced less IL-1β than swine influenza viruses (SIVs). Further study revealed that the NS1 C terminus of pandemic H1N1 but not that of SIV was able to significantly inhibit NLRP3 inflammasome-mediated IL-1β production. This inhibitory function was attributed to impaired ASC speck formation and suppression of ASC ubiquitination. Moreover, we identified two target lysine residues, K110 and K140, which are essential for both porcine ASC ubiquitination and NLRP3 inflammasome-mediated IL-1β production. These results revealed a novel mechanism by which the NS1 protein of the 2009 pandemic H1N1 suppresses NLRP3 inflammasome activation.IMPORTANCE Influenza A virus (IAV) infection activates the NLRP3 inflammasome, resulting in the production of IL-1β, which contributes to the host innate immune response. ASC, an adaptor protein of NLRP3, forms specks that are critical for inflammasome activation. Here, we report that the NS1 C terminus of the 2009 pandemic H1N1 has functions to suppress porcine IL-1β production by inhibiting ASC speck formation and ASC ubiquitination. Furthermore, the ubiquitination sites on porcine ASC were identified. The information gained here may contribute to an in-depth understanding of porcine inflammasome activation and regulation in response to different IAVs, helping to further enhance our knowledge of innate immune responses to influenza virus infection in pigs.

CSFV induced mitochondrial fission and mitophagy to inhibit apoptosis

Classical swine fever virus (CSFV), which causes typical clinical characteristics in piglets, including hemorrhagic syndrome and immunosuppression, is linked to hepatitis C and dengue virus. Oxidative stress and a reduced mitochondrial transmembrane potential are disturbed in CSFV-infected cells. The balance of mitochondrial dynamics is essential for cellular homeostasis. In this study, we offer the first evidence that CSFV induces mitochondrial fission and mitophagy to inhibit host cell apoptosis for persistent infection. The formation of mitophagosomes and decline in mitochondrial mass relevant to mitophagy were detected in CSFV-infected cells. CSFV infection increased the expression and mitochondrial translocation of Pink and Parkin. Upon activation of the PINK1 and Parkin pathways, Mitofusin 2 (MFN2), a mitochondrial fusion mediator, was ubiquitinated and degraded in CSFV-infected cells. Mitophagosomes and mitophagolysosomes induced by CSFV were, respectively, observed by the colocalization of LC3-associated mitochondria with Parkin or lysosomes. In addition, a sensitive dual fluorescence reporter (mito-mRFP-EGFP) was utilized to analyze the delivery of mitophagosomes to lysosomes. Mitochondrial fission caused by CSFV infection was further determined by mitochondrial fragmentation and Drp1 translocation into mitochondria using a confocal microscope. The preservation of mitochondrial proteins, upregulated apoptotic signals and decline of viral replication resulting from the silencing of Drp1 and Parkin in CSFV-infected cells suggested that CSFV induced mitochondrial fission and mitophagy to enhance cell survival and viral persistence. Our data for mitochondrial fission and selective mitophagy in CSFV-infected cells reveal a unique view of the pathogenesis of CSFV infection and provide new avenues for the development of antiviral strategies.

Critical Role of HAX-1 in Promoting Avian Influenza Virus Replication in Lung Epithelial Cells

The PB1-F2 protein of influenza A virus has been considered a virulence factor, but its function in inducing apoptosis may be of disadvantage to viral replication. Host mechanisms to regulate PB1-F2-induced apoptosis remain unknown. We generated a PB1-F2-deficient avian influenza virus (AIV) H9N2 and found that the mutant virus replicated less efficiently in human lung epithelial cells. The PB1-F2-deficient virus produced less apoptotic cells, indicating that PB1-F2 of the H9N2 virus promotes apoptosis, occurring at the early stage of infection, in the lung epithelial cells. To understand how host cells regulate PB1-F2-induced apoptosis, we explored to identify cellular proteins interacting with PB1-F2 and found that HCLS1-associated protein X-1 (HAX-1), located mainly in the mitochondria as an apoptotic inhibitor, interacted with PB1-F2. Increased procaspase-9 activations, induced by PB1-F2, could be suppressed by HAX-1. In HAX-1 knockdown A549 cells, the replication of AIV H9N2 was suppressed in parallel to the activation of caspase-3 activation, which increased at the early stage of infection. We hypothesize that HAX-1 promotes AIV replication by interacting with PB1-F2, resulting in the suppression of apoptosis, prolonged cell survival, and enhancement of viral replication. Our data suggest that HAX-1 may be a promoting factor for AIV H9N2 replication through desensitizing PB1-F2 from its apoptotic induction in human lung epithelial cells.

Human cytomegalovirus-encoded US9 targets MAVS and STING signaling to evade type I interferon immune responses

Human cytomegalovirus (HCMV) has evolved sophisticated immune evasion mechanisms that target both the innate and adaptive immune responses. However, how HCMV encoded proteins are involved in this immune escape is not clear. Here, we show that HCMV glycoprotein US9 inhibits the IFN-β response by targeting the mitochondrial antiviral-signaling protein (MAVS) and stimulator of interferon genes (STING)-mediated signaling pathways. US9 accumulation in mitochondria attenuates the mitochondrial membrane potential, leading to promotion of MAVS leakage from the mitochondria. Furthermore, US9 disrupts STING oligomerization and STING–TBK1 association through competitive interaction. Intriguingly, US9 blocks interferon regulatory factor 3 (IRF3) nuclear translocation and its cytoplasmic domain is essential for inhibiting IRF3 activation. Mutant HCMV lacking US7-16 is impaired in antagonism of MAVS/STING-mediated IFN-β expression, an effect that is reversible by the introduction of US9. Our findings indicate that HCMV US9 is an antagonist of IFN signaling to persistently evade host innate antiviral responses.

MAVS and STING signaling are central to interferon-inducing antiviral immunity. Here, the authors show how the human cytomegalovirus protein US9 can evade this immunity by antagonizing these pathways.

Evolution and Virulence of Influenza A Virus Protein PB1-F2

PB1-F2 is an accessory protein of most human, avian, swine, equine, and canine influenza A viruses (IAVs). Although it is dispensable for virus replication and growth, it plays significant roles in pathogenesis by interfering with the host innate immune response, inducing death in immune and epithelial cells, altering inflammatory responses, and promoting secondary bacterial pneumonia. The effects of PB1-F2 differ between virus strains and host species. This can at least partially be explained by the presence of multiple PB1-F2 sequence variants, including premature stop codons that lead to the expression of truncated PB1-F2 proteins of different lengths and specific virulence-associated residues that enhance susceptibility to bacterial superinfection. Although there has been a tendency for human seasonal IAV to gradually reduce the number of virulence-associated residues, zoonotic IAVs contain a reservoir of PB1-F2 proteins with full length, virulence-associated sequences. Here, we review the molecular mechanisms by which PB1-F2 may affect influenza virulence, and factors associated with the evolution and selection of this protein.

The essential role of mitochondrial dynamics in antiviral immunity

Viruses alter cellular physiology and function to establish cellular environment conducive for viral proliferation. Viral immune evasion is an essential aspect of viral persistence and proliferation. The multifaceted mitochondria play a central role in many cellular events such as metabolism, bioenergetics, cell death, and innate immune signaling. Recent findings accentuate that viruses regulate mitochondrial function and dynamics to facilitate viral proliferation. In this review, we will discuss how viruses exploit mitochondrial dynamics to modulate mitochondria-mediated antiviral innate immune response during infection. This review will provide new insight to understanding the virus-mediated alteration of mitochondrial dynamics and functions to perturb host antiviral immune signaling.

Checks and Balances between Autophagy and Inflammasomes during Infection

Autophagy and inflammasome complex assembly are physiological processes that control homeostasis, inflammation, and immunity. Autophagy is a ubiquitous pathway that degrades cytosolic macromolecules or organelles, as well as intracellular pathogens. Inflammasomes are multi-protein complexes that assemble in the cytosol of cells upon detection of pathogen- or danger-associated molecular patterns. A critical outcome of inflammasome assembly is the activation of the cysteine protease caspase-1, which activates the pro-inflammatory cytokine precursors pro-IL-1β and pro-IL-18. Studies on chronic inflammatory diseases, heart diseases, Alzheimer's disease, and multiple sclerosis revealed that autophagy and inflammasomes intersect and regulate each other. In the context of infectious diseases, however, less is known about the interplay between autophagy and inflammasome assembly, although it is becoming evident that pathogens have evolved multiple strategies to inhibit and/or subvert these pathways and to take advantage of their intricate crosstalk. An improved appreciation of these pathways and their subversion by diverse pathogens is expected to help in the design of anti-infective therapeutic interventions.

MCPIP1 attenuates the innate immune response to influenza A virus by suppressing RIG-I expression in lung epithelial cells

The pattern recognition receptor retinoic acid-inducible gene I (RIG-I) reportedly plays a key role in sensing influenza A virus (IAV) infection and activating type I interferon (IFN) response. MCP-1-induced protein 1 (MCPIP1) can directly degrade cytokine mRNAs, such as IL-6, IL-12, IL-1β, and IL-2, by functioning as an RNase. Here, we initially observed that MCPIP1 exhibited virus supportive functions later in the course of IAV infection in A549 cells, and negatively regulated IAV-induced RIG-I-dependent innate antiviral response. Exogenous overexpression of MCPIP1 suppressed the expression of RIG-I, whereas shRNA-mediated inhibition of endogenous MCPIP1 enhanced RIG-I expression. The results of experiments with actinomycin D and luciferase assay demonstrated that MCPIP1 reduced RIG-I expression through destabilizing its mRNA. Various mutants of functional domains of MCPIP1 further confirmed that the inhibitory effect of MCPIP1 on RIG-I expression required RNase activity but not deubiquitinase activity. Finally, the overexpression of several IAV proteins, which have the ability to inhibit the host IFN response at different levels, induced MCPIP1 expression, especially non-structural protein 1 (NS1). Conclusively, these data demonstrate the MCPIP1 contributes to attenuate IAV-induced host antiviral response by suppressing RIG-I expression.

Exploring and Exploiting Proteome Organization during Viral Infection

Viral replication in eukaryotes is a process inherently organized in both space and time. Viral components target subcellular organelles to access host machineries required for replication and spread. Diverse viruses are known to alter organelle shape, composition, function, and dynamics as part of their replication cycles. Here, we highlight recent advances in microscopy and proteomic methods that have helped and will continue to help define mechanisms used by viruses to exploit host proteome organization.

RLR-mediated antiviral innate immunity requires oxidative phosphorylation activity

Mitochondria act as a platform for antiviral innate immunity, and the immune system depends on activation of the retinoic acid-inducible gene I (RIG-I)-like receptors (RLR) signaling pathway via an adaptor molecule, mitochondrial antiviral signaling. We report that RLR-mediated antiviral innate immunity requires oxidative phosphorylation (OXPHOS) activity, a prominent physiologic function of mitochondria. Cells lacking mitochondrial DNA or mutant cells with respiratory defects exhibited severely impaired virus-induced induction of interferons and proinflammatory cytokines. Recovery of the OXPHOS activity in these mutants, however, re-established RLR-mediated signal transduction. Using in vivo approaches, we found that mice with OXPHOS defects were highly susceptible to viral infection and exhibited significant lung inflammation. Studies to elucidate the molecular mechanism of OXPHOS-coupled immune activity revealed that optic atrophy 1, a mediator of mitochondrial fusion, contributes to regulate the antiviral immune response. Our findings provide evidence for functional coordination between RLR-mediated antiviral innate immunity and the mitochondrial energy-generating system in mammals.

Inflammasomes on the Crossroads of Innate Immune Recognition and Metabolic Control

Inflammasomes are protein complexes formed upon encounter of microbial or damage-associated stimuli. The main output of inflammasome assembly is activation of caspase-1, a protease involved in both pro-inflammatory and host-protective responses. Defined bacterial or viral ligands have been identified for the inflammasome-forming receptors AIM2, NLRP1, and NLRC4. The signals activating other inflammasomes, NLRP3, NLRP6, and pyrin, are less well understood. Recent studies implicated several low-molecular-weight compounds traditionally linked to metabolism, not immunity, in modulation of inflammasome signaling. Furthermore, genetic, pharmacological, or pathogen-mediated interference with energy metabolism also affects inflammasome activation. Here we review the findings on how microbial- and host-derived metabolites regulate activation of the NLRP3 and NLRP6 inflammasomes. We discuss the different models of how glycolysis and mitochondrial metabolism control the NLRP3 inflammasome. Finally, we summarize the findings on metabolic control of pyrin and point to open questions to be addressed to broaden our understanding of metabolism-inflammasome interactions.

Influenza virus protein PB1-F2 interacts with CALCOCO2 (NDP52) to modulate innate immune response

PB1-F2 is a viral protein encoded by influenza A viruses (IAVs). PB1-F2 is implicated in virulence by triggering immune cell apoptosis and enhancing inflammation. To obtain an insight into the molecular mechanisms of PB1-F2-mediated virulence, we used the yeast two-hybrid approach to find new PB1-F2 cellular interactors. This allowed us to identify calcium-binding and coiled-coil domain 2 (CALCOCO2, also known as NDP52) as a binding partner of PB1-F2. Binding of PB1-F2 to CALCOCO2 was confirmed by pull-down. Surface plasmon resonance binding experiments enabled us to estimate the dissociation constant (Kd) of the two partners to be around 20 nM. Using bioinformatics tools, we designed a CALCOCO2 interaction map based on previous knowledge and showed a strong connection between this protein and the type I interferon production pathways and the I-κB kinase/NF-κB signalling pathway. NF-κB reporter assays in which CALCOCO2, MAVS and PB1-F2 were co-expressed showed a cooperation of these three proteins to increase the inflammatory response. By contrast, PB1-F2 inhibits the TBK1-dependent activation of an ISRE reporter plasmid. We also demonstrated that the signal transducer TRAF6 is implicated in the enhancement of NF-κB activity mediated by PB1-F2/CALCOCO2 binding. Altogether, this report provides evidence of an interaction link between PB1-F2 and human proteins, and allows a better understanding of the involvement of PB1-F2 in the pathologic process mediated by IAV.

Innate recognition of influenza virus and vaccine development

Understanding the mechanisms by which influenza viruses are recognized by the innate immune system to elicit a protective adaptive immune response is essential for the development of effective vaccines. We have demonstrated that synthetic double-stranded RNA poly(I:C) is an effective adjuvant for intranasal influenza vaccine. Furthermore, we found that influenza virus activated the NLR　family, pyrin domain-containing 3 (NLRP3) inflammasome via its M2 protein. Inflammasome activation in the lung coupled with priming signals from the commensal microbiota in the gut are essential for the generation of influenza virus-specific adaptive immune responses. These results provide a useful basis for developing effective vaccines against influenza viruses.

Virological Surveillance of Influenza A Subtypes Isolated in 2014 from Clinical Outbreaks in Canadian Swine

Influenza A viruses (IAVs) are respiratory pathogens associated with an acute respiratory disease that occurs year-round in swine production. It is currently one of the most important pathogens in swine populations, with the potential to infect other host species including humans. Ongoing research indicates that the three major subtypes of IAV—H1N1, H1N2, and H3N2—continue to expand in their genetic and antigenic diversity. In this study, we conducted a comprehensive genomic analysis of 16 IAVs isolated from different clinical outbreaks in Alberta, Manitoba, Ontario, and Saskatchewan in 2014. We also examined the genetic basis for probable antigenic differences among sequenced viruses. On the basis of phylogenetic analysis, all 13 Canadian H3N2 viruses belonged to cluster IV, eight H3N2 viruses were part of the IV-C cluster, and one virus belonged to the IV-B and one to the IV-D cluster. Based on standards used in this study, three H3N2 viruses could not be clearly classified into any currently established group within cluster IV (A to F). Three H1N2 viruses were part of the H1α cluster.

Regulation and functions of NLRP3 inflammasome during influenza virus infection

The NLRP3 inflammasome constitutes a major antiviral host defense mechanism during influenza virus infection. Inflammasome assembly in virus-infected cells facilitates autocatalytic processing of pro-caspase-1 and subsequent cleavage and secretion of proinflammatory cytokines IL-1β and IL-18. The NLRP3 inflammasome is critical for induction of both innate and adaptive immune responses during influenza virus infection. Inflammasome-dependent antiviral responses also regulate immunopathology and tissue repair in the infected lungs. The regulation of NLRP3 inflammasome assembly is an area of active research and recent studies have unraveled multiple cellular and viral factors involved in inflammasome assembly. Emerging studies have also identified the cross talk between inflammasome activation and programmed cell death pathways in influenza virus-infected cells. Here, we review the current literature regarding regulation and functions of NLRP3 inflammasome during influenza virus infection.

Hochuekkito, a Japanese Herbal Medicine, Restores Metabolic Homeostasis between Mitochondrial and Glycolytic Pathways Impaired by Influenza A Virus Infection

BACKGROUND

Hochuekkito (HKT), a traditional Japanese herbal medicine (Kampo), has been used to treat symptoms of several diseases. In a recent clinical study, HKT was shown to be protective against the influenza virus infection. However, the underlying mechanism of the prophylactic effect is not clear. Mitochondrial and glycolytic pathways play important roles in cellular energy metabolism to maintain biological functions. These metabolic pathways are affected by the influenza virus infection. In this study, we examined the relationship between the preventive effects of HKT against the influenza virus infection and cellular energy metabolism in mitochondria and glycolysis using Madin-Darby canine kidney cells and influenza A/PR/8/34 (H1N1) virus (IAV).

METHODS

Mitochondrial and glycolytic metabolic pathways were evaluated on the basis of the oxygen consumption rate (OCR) and extracellular acidification rate (ECAR), respectively, using the XF24 Extracellular Analyzer.

RESULTS

The OCR/ECAR ratio in IAV-infected cells was lower than that in control cells. Cells that were treated with HKT before IAV infection showed a metabolic pattern similar to that in the control cells (increase in both OCR and ECAR).

CONCLUSIONS

Our results suggest that HKT not only activates both mitochondrial and glycolytic energy metabolism in IAV-infected cells but also helps maintain metabolic homeostasis similar to that in noninfected cells.

Host and Viral Modulation of RIG-I-Mediated Antiviral Immunity

Innate immunity is the first line of defense against invading pathogens. Rapid and efficient detection of pathogen-associated molecular patterns via pattern-recognition receptors is essential for the host to mount defensive and protective responses. Retinoic acid-inducible gene-I (RIG-I) is critical in triggering antiviral and inflammatory responses for the control of viral replication in response to cytoplasmic virus-specific RNA structures. Upon viral RNA recognition, RIG-I recruits the mitochondrial adaptor protein mitochondrial antiviral signaling protein, which leads to a signaling cascade that coordinates the induction of type I interferons (IFNs), as well as a large variety of antiviral interferon-stimulated genes. The RIG-I activation is tightly regulated via various posttranslational modifications for the prevention of aberrant innate immune signaling. By contrast, viruses have evolved mechanisms of evasion, such as sequestrating viral structures from RIG-I detections and targeting receptor or signaling molecules for degradation. These virus–host interactions have broadened our understanding of viral pathogenesis and provided insights into the function of the RIG-I pathway. In this review, we summarize the recent advances regarding RIG-I pathogen recognition and signaling transduction, cell-intrinsic control of RIG-I activation, and the viral antagonism of RIG-I signaling.

PB1-F2 Peptide Derived from Avian Influenza A Virus H7N9 Induces Inflammation via Activation of the NLRP3 Inflammasome

The emergence of avian H7N9 influenza A virus in humans with associated high mortality has highlighted the threat of a potential pandemic. Fatal H7N9 infections are characterized by hyperinflammation and increased cellular infiltrates in the lung. Currently there are limited therapies to address the pathologies associated with H7N9 infection and the virulence factors that contribute to these pathologies. We have found that PB1-F2 derived from H7N9 activates the NLRP3 inflammasome and induces lung inflammation and cellular recruitment that is NLRP3-dependent. We have also shown that H7N9 and A/Puerto Rico/H1N1 (PR8)PB1-F2 peptide treatment induces significant mitochondrial reactive oxygen production, which contributes to NLRP3 activation. Importantly, treatment of cells or mice with the specific NLRP3 inhibitor MCC950 significantly reduces IL-1β maturation, lung cellular recruitment, and cytokine production. Together, these results suggest that PB1-F2 from H7N9 avian influenza A virus may be a major contributory factor to disease pathophysiology and excessive inflammation characteristic of clinical infections and that targeting the NLRP3 inflammasome may be an effective means to reduce the inflammatory burden associated with H7N9 infections.

Dengue Virus Perturbs Mitochondrial Morphodynamics to Dampen Innate Immune Responses

With no antiviral drugs or widely available vaccines, Dengue virus (DENV) constitutes a public health concern. DENV replicates at ER-derived cytoplasmic structures that include substructures called convoluted membranes (CMs); however, the purpose of these membrane alterations remains unclear. We determine that DENV nonstructural protein (NS)4B, a promising drug target with unknown function, associates with mitochondrial proteins and alters mitochondria morphology to promote infection. During infection, NS4B induces elongation of mitochondria, which physically contact CMs. This restructuring compromises the integrity of mitochondria-associated membranes, sites of ER-mitochondria interface critical for innate immune signaling. The spatio-temporal parameters of CM biogenesis and mitochondria elongation are linked to loss of activation of the fission factor Dynamin-Related Protein-1. Mitochondria elongation promotes DENV replication and alleviates RIG-I-dependent activation of interferon responses. As Zika virus infection induces similar mitochondria elongation, this perturbation may protect DENV and related viruses from innate immunity and create a favorable replicative environment.

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DENV NS4B induces mitochondria elongation during viral infection

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Elongated mitochondria and virus-induced convoluted membranes are physically linked

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NS4B inhibits activation of the mitochondrial fission factor DRP1

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Mitochondria elongation alleviates DENV-induced RIG-I-dependent innate immunity