Early apoptosis of porcine alveolar macrophages limits avian influenza virus replication and pro-inflammatory dysregulation

PB1-F2 of low pathogenicity H7N7 restricts apoptosis in avian cells

Avian influenza viruses (AIV) pose a continuous challenge to global health and economy. While countermeasures exist to control outbreaks in poultry, the persistent circulation of AIV in wild aquatic and shorebirds presents a significant challenge to effective disease prevention efforts. PB1-F2 is a non-structural protein expressed from a second open reading frame (+1) of the polymerase basic 1 (PB1) segment. The sequence and length of the PB1-F2 protein can vary depending on the host of origin. While avian isolates typically carry full-length PB1-F2, isolates from mammals, often express truncated forms. The selective advantage of the full-length PB1-F2 in avian isolates is not fully understood. Most research on the role of PB1-F2 in influenza virus replication has been conducted in mammalian systems, where PB1-F2 interfered with the host immune response and induced apoptosis. Here, we used Low Pathogenicity (LP) AIV H7N7 expressing full-length PB1-F2 as well as a knockout mutant. We found that the full-length PB1-F2 of LPAIV prolonged survival of infected cells by limiting apoptotic cell death. Furthermore, PB1-F2 knockout LPAIV significantly decreased MHC-I expression on fibroblasts, delayed tissue healing and increased phagocytic uptake of infected cells, whereas LPAIV expressing PB1-F2 has limited effects. These findings indicate that full-length PB1-F2 enables AIV to cause prolonged infections without severely harming the avian host. Our observations may explain maintenance of AIV in the natural bird reservoir in absence of severe clinical signs.

Porcine γδ T cells express cytotoxic cell-associated markers and display killing activity but are not selectively cytotoxic against PRRSV- or swIAV-infected macrophages

Background

Gamma-delta (γδ) T cells are a major immune cell subset in pigs. Approximately 50% of circulating T cells are γδ T cells in young pigs and up to 30% in adult sows. Despite this abundance, the functions of porcine γδ T cells are mostly unidentified. In humans and mice, activated γδ T cells exhibit broad innate cytotoxic activity against a wide variety of stressed, infected, and cancerous cells through death receptor/ligand-dependent and perforin/granzyme-dependent pathways. However, so far, it is unknown whether porcine γδ T cells have the ability to perform cytotoxic functions.

Methods

In this study, we conducted a comprehensive phenotypic characterization of porcine γδ T cells isolated from blood, lung, and nasal mucosa. To further analyze the cytolytic potential of γδ T cells, in vitro cytotoxicity assays were performed using purified γδ T cells as effector cells and virus-exposed or mock-treated primary porcine alveolar macrophages as target cells.

Results

Our results show that only CD2+ γδ T cells express cytotoxic markers (CD16, NKp46, perforin) with higher perforin and NKp46 expression in γδ T cells isolated from lung and nasal mucosa. Moreover, we found that γδ T cells can exhibit cytotoxic functions in a cell-cell contact and degranulation-dependent manner. However, porcine γδ T cells did not seem to specifically target Porcine Reproductive and Respiratory Syndrome Virus or swine Influenza A Virus-infected macrophages, which may be due to viral escape mechanisms.

Conclusion

Porcine γδ T cells express cytotoxic markers and can exhibit cytotoxic activity in vitro. The specific mechanisms by which porcine γδ T cells recognize target cells are not fully understood but may involve the detection of cellular stress signals.

Sialic Acid Receptor Specificity in Mammary Gland of Dairy Cattle Infected with Highly Pathogenic Avian Influenza A(H5N1) Virus

In March 2024, the US Department of Agriculture's Animal and Plant Health Inspection Service reported detection of highly pathogenic avian influenza (HPAI) A(H5N1) virus in dairy cattle in the United States for the first time. One factor that determines susceptibility to HPAI H5N1 infection is the presence of specific virus receptors on host cells; however, little is known about the distribution of the sialic acid (SA) receptors in dairy cattle, particularly in mammary glands. We compared the distribution of SA receptors in the respiratory tract and mammary gland of dairy cattle naturally infected with HPAI H5N1. The respiratory and mammary glands of HPAI H5N1-infected dairy cattle are rich in SA, particularly avian influenza virus-specific SA α2,3-gal. Mammary gland tissues co-stained with sialic acids and influenza A virus nucleoprotein showed predominant co-localization with the virus and SA α2,3-gal. HPAI H5N1 exhibited epitheliotropism within the mammary gland, and we observed rare immunolabeling within macrophages.

Amino acid 138 in the HA of a H3N2 subtype influenza A virus increases affinity for the lower respiratory tract and alveolar macrophages in pigs

Influenza A virus (FLUAV) infects a wide range of hosts and human-to-swine spillover events are frequently reported. However, only a few of these human viruses have become established in pigs and the host barriers and molecular mechanisms driving adaptation to the swine host remain poorly understood. We previously found that infection of pigs with a 2:6 reassortant virus (hVIC/11) containing the hemagglutinin (HA) and neuraminidase (NA) gene segments from the human strain A/Victoria/361/2011 (H3N2) and internal gene segments of an endemic swine strain (sOH/04) resulted in a fixed amino acid substitution in the HA (A138S, mature H3 HA numbering). In silico analysis revealed that S138 became predominant among swine H3N2 virus sequences deposited in public databases, while 138A predominates in human isolates. To understand the role of the HA A138S substitution in the adaptation of a human-origin FLUAV HA to swine, we infected pigs with the hVIC/11A138S mutant and analyzed pathogenesis and transmission compared to hVIC/11 and sOH/04. Our results showed that the hVIC/11A138S virus had an intermediary pathogenesis between hVIC/11 and sOH/04. The hVIC/11A138S infected the upper respiratory tract, right caudal, and both cranial lobes while hVIC/11 was only detected in nose and trachea samples. Viruses induced a distinct expression pattern of various pro-inflammatory cytokines such as IL-8, TNF-α, and IFN-β. Flow cytometric analysis of lung samples revealed a significant reduction of porcine alveolar macrophages (PAMs) in hVIC/11A138S-infected pigs compared to hVIC/11 while a MHCIIlowCD163neg population was increased. The hVIC/11A138S showed a higher affinity for PAMs than hVIC/11, noted as an increase of infected PAMs in bronchoalveolar lavage fluid (BALF), and showed no differences in the percentage of HA-positive PAMs compared to sOH/04. This increased infection of PAMs led to an increase of granulocyte-monocyte colony-stimulating factor (GM-CSF) stimulation but a reduced expression of peroxisome proliferator-activated receptor gamma (PPARγ) in the sOH/04-infected group. Analysis using the PAM cell line 3D4/21 revealed that the A138S substitution improved replication and apoptosis induction in this cell type compared to hVIC/11 but at lower levels than sOH/04. Overall, our study indicates that adaptation of human viruses to the swine host involves an increased affinity for the lower respiratory tract and alveolar macrophages.

The role of PB1-F2 in adaptation of high pathogenicity avian influenza virus H7N7 in chickens

Avian influenza viruses (AIV) of the H7N7 subtype are enzootic in the wild bird reservoir in Europe, cause infections in poultry, and have sporadically infected humans. The non-structural protein PB1-F2 is encoded in a second open frame in the polymerase segment PB1 and its sequence varies with the host of origin. While mammalian isolates predominantly carry truncated forms, avian isolates typically express full-length PB1-F2. PB1-F2 is a virulence factor of influenza viruses in mammals. It modulates the host immune response, causing immunopathology and increases pro-inflammatory responses. The role of full-length PB1-F2 in IAV pathogenesis as well as its impact on virus adaptation and virulence in poultry remains enigmatic. Here, we characterised recombinant high pathogenicity AIV (HPAIV) H7N7 expressing or lacking PB1-F2 in vitro and in vivo in chickens. In vitro, full-length PB1-F2 modulated viability of infected chicken fibroblasts by limiting apoptosis. In chickens, PB1-F2 promoted gastrointestinal tropism, as demonstrated by enhanced viral replication in the gut and increased cloacal shedding. PB1-F2's effects on cellular immunity however were marginal. Overall, chickens infected with full-length PB1-F2 virus survived for shorter periods, indicating that PB1-F2 is also a virulence factor in bird-adapted viruses.

Influenza viral matrix 1 protein aggravates viral pathogenicity by inducing TLR4-mediated reactive oxygen species production and apoptotic cell death

Influenza virus is one of the most challenging viruses threating human health. Since infection with influenza virus triggers inflammatory responses and induces cell death, the molecular and cellular mechanisms by which the virus-infected cells undergo apoptotic and necrotic cell death have been widely studied. However, most of the studies have focused on the molecular events occurring in the cytosol and there is limited information on the physiological correlation between virus-induced cell death and the viral pathogenesis in vivo. In this study, we demonstrate that the influenza virus matrix 1 (M1) protein is released from virus-infected cells and triggers apoptotic cell death of lung epithelial and pulmonary immune cells, through the activation of Toll-like receptor 4 (TLR4) signaling. Treatment with M1 protein led to robust cellular inflammatory responses, such as the production of proinflammatory cytokines and cellular reactive oxygen species (ROS), and induction of cell death. When M1 protein was administered in vivo, it induced the activation of inflammatory responses and cell death in the lungs. Furthermore, the administration of M1 aggravated lung pathology and mortality of the virus-infected mice in a TLR4-dependent manner. These results demonstrate that M1 is an important pathogenic factor contributing to influenza virus pathogenicity by enhancing cell death in the lungs, thereby expanding our understanding of the molecular mechanism of influenza virus-induced cell death through the interaction with an innate immune receptor.

Alveolar Macrophages Participate in the Promotion of Influenza Virus Infection by Aflatoxin B1 at an Early Stage

Aflatoxin B1 (AFB1), one of the most common environmental mycotoxin contaminations in food and feed, poses significant threats to human and animal health. Our previous study indicated that even non-toxic AFB1 concentrations could promote influenza virus replication and induce influenza virus-infected alveolar macrophages polarizing from M1 (immunostimulatory phenotype) to M2 (immunosuppressive phenotype) over time. However, whether AFB1 promotes influenza replication via modulating the polarization of alveolar macrophages is unknown. Here, we specifically depleted alveolar macrophages using clodronate-containing liposomes in swine influenza virus (SIV)-infected mice to explore the mechanism the promotion of SIV replication by AFB1. The results show that the depletion of alveolar macrophages significantly alleviated the AFB1-induced weight loss, inflammatory responses, and lung and immune organ damage of the SIV-infected mice after 14 days and greatly diminished the AFB1-promoted SIV replication. In contrast, the depletion of alveolar macrophages did not alleviate the AFB1-induced weight loss, and lung and immune organ damage of the SIV-infected mice after 28 days and slightly diminished the AFB1-promoted SIV replication. Collectively, the data indicate that alveolar macrophages play a crucial role the promotion of SIV infection by AFB1 in the early rather than late stage, and AFB1 can promote SIV replication by inducing alveolar macrophages to polarize towards M1 macrophages. This research provides novel targets for reducing the risk of AFB1-promoted influenza virus infection.

Comparative transcriptomics reveals small RNA composition and differential microRNA responses underlying interferon-mediated antiviral regulation in porcine alveolar macrophages

Previous studies have shown that interferon-mediated antiviral activity is subtype-dependent. Using a whole transcriptome procedure, we aimed to characterize the small RNA transcriptome (sRNA-Seq) and specifically the differential microRNA (miRNA) responses in porcine alveolar macrophages (PAMs) upon antiviral activation during viral infection and interferon (IFN) stimulation. Data showed that near 90% of the qualified reads of sRNA were miRNAs, and about 10% of the other sRNAs included rRNA, snoRNA, snRNA, and tRNA in order of enrichment. As the majority of sRNA (>98%) were commonly detected in all PAM samples under different treatments, about 2% sRNA were differentially expressed between the different antiviral treatments. Focusing on miRNA, 386 miRNA were profiled, including 331 known and 55 novel miRNA sequences, of which most were ascribed to miRNA families conserved among vertebrates, particularly mammalian species. Of the miRNA profiles comparably generated across the different treatments, in general, significantly differentially expressed miRNA (SEM) demonstrated that: (1) the wild-type and vaccine strains of a porcine arterivirus (a.k.a., PRRSV) induced nearly reversed patterns of up- or down-regulated SEMs; (2) similar SEM patterns were found among the treatments by the vaccine strain and antiviral IFN-α1/-ω5 subtypes; and (3) the weak antiviral IFN-ω1, however, remarked a suppressive SEM pattern as to SEMs upregulated in the antiviral treatments by the vaccine and IFN-α1/-ω5 subtypes. Further articulation identified SEMs commonly or uniquely expressed in different treatments, and experimentally validated that some SEMs including miR-10b and particularly miR-9-1 acted significantly in regulation of differential antiviral reactions stimulated by different IFN subtypes. Therefore, this study provides a general picture of porcine sRNA composition and pinpoints key SEMs underlying antiviral regulation in PAMs correlated to a typical respiratory RNA virus in pigs.

Swine influenza virus triggers ferroptosis in A549 cells to enhance virus replication

Background

Recently, Influenza A virus (IAV) has been shown to activate several programmed cell death pathways that play essential roles in host defense. Indeed, cell death caused by viral infection may be mediated by a mixed pattern of cell death instead of a certain single mode. Ferroptosis is a novel form of regulated cell death (RCD) that is mainly mediated by iron-dependent lipid peroxidation. Based on the proteomic data, we wondered whether IAV causes ferroptosis in host cells.

Method

In this study, a quantitative proteomics approach based on an iTRAQ combined with LC–MS/MS was used to profile proteins expressed in A549 cells infected with H1N1 swine influenza virus (SIV). Meanwhile, we measured the intracellular iron content, reactive oxygen species (ROS) release and lipid peroxidation in response to SIV infection. Finally, a drug experiment was conducted to investigate the effects of ferroptosis on modulating SIV survival.

Results

The bioinformatics analysis revealed several proteins closely relevant to iron homeostasis and transport, and the ferroptosis signaling pathway are highly enriched in response to SIV infection. In our experiment, aberrant expression of iron-binding proteins disrupted labile iron uptake and storage after SIV infection. Meanwhile, SIV infection inhibited system the Xc⁻/GPX4 axis resulting in GSH depletion and the accumulation of lipid peroxidation products. Notably, cell death caused by SIV as a result of iron-dependent lipid peroxidation can be partially rescued by ferroptosis inhibitor. Additionally, blockade of the ferroptotic pathway by ferrostatin-1 (Fer-1) treatment decreased viral titers and inflammatory response.

Conclusions

This study revealed a new mode of cell death induced by IAV infection, and our findings might improve the understanding of the underlying mechanism involved in the interaction of virus and host cells.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12985-022-01825-y.

Innate immune responses at the asymptomatic stage of influenza A viral infections of Streptococcus pneumoniae colonized and non-colonized mice

Seasonal Influenza A virus (IAV) infections can promote dissemination of upper respiratory tract commensals such as Streptococcus pneumoniae to the lower respiratory tract resulting in severe life-threatening pneumonia. Here, we aimed to compare innate immune responses in the lungs of healthy colonized and non-colonized mice after IAV challenge at the initial asymptomatic stage of infection. Responses during a severe bacterial pneumonia were profiled for comparison. Cytokine and innate immune cell imprints of the lungs were analyzed. Irrespective of the colonization status, mild H1N1 IAV infection was characterized by a bi-phasic disease progression resulting in full recovery of the animals. Already at the asymptomatic stage of viral infection, the pro-inflammatory cytokine response was as high as in pneumococcal pneumonia. Flow cytometry analyses revealed an early influx of inflammatory monocytes into the lungs. Neutrophil influx was mostly limited to bacterial infections. The majority of cells, except monocytes, displayed an activated phenotype characterized by elevated CCR2 and MHCII expression. In conclusion, we show that IAV challenge of colonized healthy mice does not automatically result in severe co-infection. However, a general local inflammatory response was noted at the asymptomatic stage of infection irrespective of the infection type.

Comparison of cellular immune responses to avian influenza virus in two genetically distinct, highly inbred chicken lines

Low pathogenicity avian influenza causes mild disease involving the respiratory, gastrointestinal, and reproductive systems of wild and domestic birds. Avian influenza research often emphasizes the effect of the virus genetics on disease, but the influence of host genetics on resistance to infection is not well understood. The genetic determinants of enhanced resistance to influenza can be explored by using genetically distinct, highly inbred chicken lines that differ in susceptibility to influenza. In this study, we compared the mucosal cellular immune responses between the relatively resistant Fayoumi M43 chicken line and the relatively susceptible Leghorn GB2 chicken line after challenging with low pathogenicity avian influenza virus (LPAIV) H6N2. The birds were inoculated at 21 days of age with 10⁷ 50 % egg infective dose (EID₅₀) LPAIV H6N2 via nasal and tracheal routes in two separate experiments. Clinical signs were recorded, tracheal swabs were collected to measure viral titer, and tracheas and lungs were harvested for flow cytometric analysis of macrophage, B cell, and T cell populations at 4 days post-infection (dpi) (Experiments 1 and 2) and 6 dpi (Experiment 2). Blood and tears were also collected at 7 and 14 dpi (Experiment 1) to measure antibody levels. Compared to both the non-challenged Fayoumis and the relatively susceptible Leghorn chickens, relatively resistant Fayoumi chickens challenged with LPAIV demonstrated enhanced MHC class I expression on antigen-presenting cells and increased macrophage, B cell, and T cell frequencies in the trachea, which were associated with reduced tracheal viral titers at 4 dpi. In contrast, MHC class I expression and immune cell frequencies in the trachea were not different between challenged Leghorns and non-challenged Leghorns. Furthermore, Leghorns shed higher virus titers in their trachea compared to Fayoumis. Challenged Fayoumis and Leghorns both produced AIV-specific IgY detected in the serum and tears, but AIV-specific IgA was not detected in the tears. In this study, we provide new insight into immune mechanisms of enhanced resistance to avian influenza in chickens, which may lead to improved vaccination strategies and breeding programs.

Swine MicroRNAs ssc-miR-221-3p and ssc-miR-222 Restrict the Cross-Species Infection of Avian Influenza Virus

Avian influenza virus (AIV) can cross species barriers to infect humans and other mammals. However, these species-cross transmissions are most often dead-end infections due to host restriction. Current research about host restriction focuses mainly on the barriers of cell membrane, nuclear envelope, and host proteins; whether microRNAs (miRNAs) play a role in host restriction is largely unknown. In this study, we used porcine alveolar macrophage (PAM) cells as a model to elucidate the role of miRNAs in host range restriction. During AIV infection, 40 dysregulation expressed miRNAs were selected in PAM cells. Among them, two Sus scrofa (ssc; swine) miRNAs, ssc-miR-221-3p and ssc-miR-222, could inhibit the infection and replication of AIV in PAM cells by directly targeting viral genome and inducing cell apoptosis via inhibiting the expression of anti-apoptotic protein HMBOX1. Avian but not swine influenza virus caused upregulated expressions of ssc-miR-221-3p and ssc-miR-222 in PAM cells. We further found that NF-κB P65 was more effectively phosphorylated upon AIV infection and that P65 functioned as a transcription activator to regulate the AIV-induced expression of miR-221-3p/222 Importantly, we found that ssc-miR-221-3p and ssc-miR-222 could also be specifically upregulated upon AIV infection in newborn pig tracheal epithelial (NPTr) cells and also exerted anti-AIV function. In summary, our study indicated that miRNAs act as a host barrier during cross-species infection of influenza A virus.IMPORTANCE The host range of an influenza A virus is determined by species-specific interactions between virus and host cell factors. Host miRNAs can regulate influenza A virus replication; however, the role of miRNAs in host species specificity is unclear. Here, we show that the induced expression of ssc-miR-221-3p and ssc-miR-222 in swine cells is modulated by NF-κB P65 phosphorylation in response to AIV infection but not swine influenza virus infection. ssc-miR-221-3p and ssc-miR-222 exerted antiviral function via targeting viral RNAs and causing apoptosis by inhibiting the expression of HMBOX1 in host cells. These findings uncover miRNAs as a host range restriction factor that limits cross-species infection of influenza A virus.

Assessment of replication of bovine herpesvirus type 4 in human glioblastoma and breast cancer cells as a potential oncolytic virus

Oncolytic viruses have been extensively used in cancer treatment due to their tropism, selective replication only in tumor cells, and possible synergic interaction with other therapeutics. Different researchers have demonstrated that bovine herpesvirus 4 (BoHV-4), a member of the gammaherpesviridae family, has oncolytic potential in some human-origin cancer cell lines like glioma through the selective replication strategy. Using four apoptosis detection methods, namely MTT, LDH, TUNEL, and Annexin V assays, we evaluated the apoptotic effect of BoHV-4 Movar33/63 reference strain along with a recombinant BoHV-4 expressing EGFP in U87 MG cells (human glioblastoma cell line), MDA MB-231 (human breast cancer cell line), and MCF10a (non-tumorigenic human mammary epithelial cell line). Our findings indicate that this virus can replicate and induce apoptosis in these cell lines and hinder in vitro proliferation in a dose-dependent manner. In conclusion, BoHV-4 has in vitro potential as a novel oncolytic virus in human cancer therapy. However, its replication potential in the MCF10a cells as a non-tumorigenic human mammary epithelial cell line is a concern in using this virus in cancer therapy, at least against human mammary tumors. Further studies must therefore be conducted to examine the specific apoptotic pathways induced by this virus to move on to further experiments.

H7N9 influenza A virus activation of necroptosis in human monocytes links innate and adaptive immune responses

We previously demonstrated that avian influenza A H7N9 virus preferentially infected CD14⁺ monocyte in human peripheral blood mononuclear cells (PBMCs), which led to apoptosis. To better understand H7N9 pathogenesis in relation to monocyte cell death, we showed here that extensive phosphorylation of mixed lineage kinase domain-like (MLKL) protein occurred concurrently with the activation of caspases-8, -9 and -3 in H7N9-infected monocytes at 6 h post infection (hpi), indicating that apoptosis and necroptosis pathways were simultaneously activated. The apoptotic morphology was readily observed in H7N9-infected monocytes with transmission electron microscopy (TEM), while the pan-caspase inhibitor, IDN6556 (IDN), accelerated cell death through necroptosis as evidenced by the increased level of pMLKL accompanied with cell swelling and plasma membrane rupture. Most importantly, H7N9-induced cell death could only be stopped by the combined treatment of IDN and necrosulfonamide (NSA), a pMLKL membrane translocation inhibitor, but not by individual inhibition of caspase or RIPK3. Our data further showed that activation of apoptosis and necroptosis pathways in monocytes differentially contributed to the immune response of monocytes upon H7N9 infection. Specifically, caspase inhibition significantly enhanced, while RIPK3 inhibition reduced the early expression of type I interferons and cytokine/chemokines in H7N9-infected monocytes. Moreover, culture supernatants from IDN-treated H7N9-infected monocyte promoted the expression of co-stimulatory molecule CD80, CD83 and CD86 on freshly isolated monocytes and monocyte-derived dendritic cells (MDCs) and enhanced the capacity of MDCs to induce CD3⁺ T-cell proliferation in vitro. In contrast, these immune stimulatory effects were abrogated by using culture supernatants from H7N9-infected monocyte with RIPK3 inhibition. In conclusion, our findings indicated that H7N9 infection activated both apoptosis and necroptosis in monocytes. An intact RIPK3 activity is required for upregulation of innate immune responses, while caspase activation suppresses the immune response.

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.

Dissecting host cell death programs in the pathogenesis of influenza

Influenza A virus (IAV) is a pulmonary pathogen, responsible for significant yearly morbidity and mortality. Due to the absence of highly effective antiviral therapies and vaccine, as well as the constant threat of an emerging pandemic strain, there is considerable need to better understand the host-pathogen interactions and the factors that dictate a protective versus detrimental immune response to IAV. Even though evidence of IAV-induced cell death in human pulmonary epithelial and immune cells has been observed for almost a century, very little is known about the consequences of cell death on viral pathogenesis. Recent study indicates that both the type of cell death program and its kinetics have major implications on host defense and survival. In this review, we discuss advances in our understanding of cell death programs during influenza virus infection, in hopes of fostering new areas of investigation for targeted clinical intervention.

The induction and consequences of Influenza A virus-induced cell death

Infection with Influenza A virus (IAV) causes significant cell death within the upper and lower respiratory tract and lung parenchyma. In severe infections, high levels of cell death can exacerbate inflammation and comprise the integrity of the epithelial cell barrier leading to respiratory failure. IAV infection of airway and alveolar epithelial cells promotes immune cell infiltration into the lung and therefore, immune cell types such as macrophages, monocytes and neutrophils are readily exposed to IAV and infection-induced death. Although the induction of cell death through apoptosis and necrosis following IAV infection is a well-known phenomenon, the molecular determinants responsible for inducing cell death is not fully understood. Here, we review the current understanding of IAV-induced cell death and critically evaluate the consequences of cell death in aiding either the restoration of lung homoeostasis or the progression of IAV-induced lung pathologies.

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.

Selective replication of oncolytic virus M1 results in a bystander killing effect that is potentiated by Smac mimetics

Oncolytic virotherapy is a treatment modality that uses native or genetically modified viruses that selectively replicate in and kill tumor cells. Viruses represent a type of pathogen-associated molecular pattern and thereby induce the up-regulation of dozens of cytokines via activating the host innate immune system. Second mitochondria-derived activator of caspases (Smac) mimetic compounds (SMCs), which antagonize the function of inhibitor of apoptosis proteins (IAPs) and induce apoptosis, sensitize tumor cells to multiple cytokines. Therefore, we sought to determine whether SMCs sensitize tumor cells to cytokines induced by the oncolytic M1 virus, thus enhancing a bystander killing effect. Here, we report that SMCs potentiate the oncolytic effect of M1 in vitro, in vivo, and ex vivo. This strengthened oncolytic efficacy resulted from the enhanced bystander killing effect caused by the M1 virus via cytokine induction. Through a microarray analysis and subsequent validation using recombinant cytokines, we identified IL-8, IL-1A, and TRAIL as the key cytokines in the bystander killing effect. Furthermore, SMCs increased the replication of M1, and the accumulation of virus protein induced irreversible endoplasmic reticulum stress- and c-Jun N-terminal kinase-mediated apoptosis. Nevertheless, the combined treatment with M1 and SMCs had little effect on normal and human primary cells. Because SMCs selectively and significantly enhance the bystander killing effect and the replication of oncolytic virus M1 specifically in cancer cells, this combined treatment may represent a promising therapeutic strategy.

Influenza Virus Overcomes Cellular Blocks To Productively Replicate, Impacting Macrophage Function

Whether influenza virus replication in macrophages is productive or abortive has been a topic of debate. Utilizing a panel of 28 distinct human, avian, and swine influenza viruses, we found that only a small subset can overcome cellular blocks to productively replicate in murine and primary human macrophages. Murine macrophages have two cellular blocks. The first block is during viral entry, where virions with relatively acid-stable hemagglutinin (HA) proteins are rendered incapable of pH-induced triggering for membrane fusion, resulting in lysosomal degradation. The second block is downstream of viral replication but upstream of late protein synthesis. In contrast, primary human macrophages only have one cellular block that occurs after late protein synthesis. To determine the impact of abortive replication at different stages of the viral life cycle or productive replication on macrophage function, we assessed cytotoxicity, nitric oxide or reactive oxygen species production, and phagocytosis. Intriguingly, productive viral replication decreased phagocytosis of IgG-opsonized bioparticles and Fc receptor CD16 and CD32 surface levels, a function, to our knowledge, never before reported for an RNA virus. These data suggest that replication in macrophages affects cellular function and plays an important role in pathogenesis during infection in vivo IMPORTANCE: Macrophages are a critical first line of defense against respiratory pathogens. Thus, understanding how viruses evade or exploit macrophage function will provide greater insight into viral pathogenicity and antiviral responses. We previously showed that only a subset of highly pathogenic avian (HPAI) H5N1 influenza virus strains could productively replicate in murine macrophages through a hemagglutinin (HA)-mediated mechanism. These studies expand upon this work and demonstrate that productive replication is not specific to unique HPAI H5N1 viruses; an H1N1 strain (A/WSN/33) can also replicate in macrophages. Importantly, we identify two cellular blocks limiting replication that can be overcome by an avian-like pH of activation for nuclear entry and a yet-to-be-identified mechanism(s) to overcome a postnuclear entry block. Overcoming these blocks reduces the cell's ability to phagocytose IgG-opsonized bioparticles by decreasing Fc receptor surface levels, a mechanism previously thought to occur during bacterial and DNA viral infections.

Role of Autophagy and Apoptosis in the Postinfluenza Bacterial Pneumonia

The risk of influenza A virus (IAV) is more likely caused by secondary bacterial infections. During the past decades, a great amount of studies have been conducted on increased morbidity from secondary bacterial infections following influenza and provide an increasing number of explanations for the mechanisms underlying the infections. In this paper, we first review the recent research progress that IAV infection increased susceptibility to bacterial infection. We then propose an assumption that autophagy and apoptosis manipulation are beneficial to antagonize post-IAV bacterial infection and discuss the clinical significance.