Influenza A virus proteins NS1 and hemagglutinin along with M2 are involved in stimulation of autophagy in infected cells

Autophagy, EVs, and Infections: A Perfect Question for a Perfect Time

Autophagy, a highly conserved process, serves to maintain cellular homeostasis in response to an extensive variety of internal and external stimuli. The classic, or canonical, pathway of autophagy involves the coordinated degradation and recycling of intracellular components and pathogenic material. Proper regulation of autophagy is critical to maintain cellular health, as alterations in the autophagy pathway have been linked to the progression of a variety of physiological and pathological conditions in humans, namely in aging and in viral infection. In addition to its canonical role as a degradative pathway, a more unconventional and non-degradative role for autophagy has emerged as an area of increasing interest. This process, known as secretory autophagy, is gaining widespread attention as many viruses are believed to use this pathway as a means to release and spread viral particles. Moreover, secretory autophagy has been found to intersect with other intracellular pathways, such as the biogenesis and secretion of extracellular vesicles (EVs). Here, we provide a review of the current landscape surrounding both degradative autophagy and secretory autophagy in relation to both aging and viral infection. We discuss their key features, while describing their interplay with numerous different viruses (i.e. hepatitis B and C viruses, Epstein-Barr virus, SV40, herpesviruses, HIV, chikungunya virus, dengue virus, Zika virus, Ebola virus, HTLV, Rift Valley fever virus, poliovirus, and influenza A virus), and compare secretory autophagy to other pathways of extracellular vesicle release. Lastly, we highlight the need for, and emphasize the importance of, more thorough methods to study the underlying mechanisms of these pathways to better advance our understanding of disease progression.

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.

Autophagy: The multi-purpose bridge in viral infections and host cells

Autophagy signaling pathway is involved in cellular homeostasis, developmental processes, cellular stress responses, and immune pathways. The aim of this review is to summarize the relationship between autophagy and viruses. It is not possible to be fully comprehensive, or to provide a complete "overview of all viruses". In this review, we will focus on the interaction of autophagy and viruses and survey how human viruses exploit multiple steps in the autophagy pathway to help viral propagation and escape immune response. We discuss the role that macroautophagy plays in cells infected with hepatitis C virus, hepatitis B virus, rotavirus gastroenteritis, immune cells infected with human immunodeficiency virus, and viral respiratory tract infections both influenza virus and coronavirus.

Autophagy diminishes the early interferon-β response to influenza A virus resulting in differential expression of interferon-stimulated genes

Influenza A virus (IAV) infection perturbs metabolic pathways such as autophagy, a stress-induced catabolic pathway that crosstalks with cellular inflammatory responses. However, the impact of autophagy perturbation on IAV gene expression or host cell responses remains disputed. Discrepant results may be a reflection of in vivo studies using cell-specific autophagy-related (Atg) gene-deficient mouse strains, which do not delineate modification of developmental programmes from more proximal effects on inflammatory response. In vitro experiments can be confounded by gene expression divergence in wild-type cultivated cell lines, as compared to those experiencing long-term absence of autophagy. With the goal to investigate cellular processes within cells that are competent or incompetent for autophagy, we generated a novel experimental cell line in which autophagy can be restored by ATG5 protein stabilization in an otherwise Atg5-deficient background. We confirmed that IAV induced autophagosome formation and p62 accumulation in infected cells and demonstrated that perturbation of autophagy did not impact viral infection or replication in ATG5-stablized cells. Notably, the induction of interferon-stimulated genes (ISGs) by IAV was diminished when cells were autophagy competent. We further demonstrated that, in the absence of ATG5, IAV-induced interferon-β (IFN-β) expression was increased as compared to levels in autophagy-competent lines, a mechanism that was independent of IAV non-structural protein 1. In sum, we report that induction of autophagy by IAV infection reduces ISG expression in infected cells by limiting IFN-β expression, which may benefit viral replication and spread.

Influenza A virus-induced autophagy contributes to enhancement of virus infectivity by SOD1 downregulation in alveolar epithelial cells

Infection with influenza A virus (IAV) A/WSN/1933 (H1N1) causes oxidative stress and severe lung injury. We have demonstrated that the generation of reactive oxygen species (ROS) during IAV infection is tightly regulated by superoxide dismutase 1 (SOD1) and correlated with viral replication in alveolar epithelial cells. However, the molecular mechanism underlying SOD1 reduction during IAV infection is uncertain. Here we demonstrate that the autophagy pathway is activated by IAV infection and involved in enhanced ROS generation in the early phase of infection. We observed that IAV infection induced autophagic vacuolation, leading to autophagic degradation of cellular proteins, including the protease sensitive antioxidant SOD1. Silencing of the microtubule-associated protein 1A/1B-light chain 3 (LC3) gene in A549 cells supported the critical role of autophagy in the ROS increase. The decrease in viral titer and viral polymerase activity caused by LC3 silencing or the autophagy inhibitor clearly evidenced the involvement of autophagy in the control of ROS generation and viral infectivity. Therefore, we concluded that early stage IAV infection induces autophagic degradation of antioxidant enzyme SOD1, thereby contributing to increased ROS generation and viral infectivity in alveolar epithelial cells.

Ubiquitination of the Cytoplasmic Domain of Influenza A Virus M2 Protein Is Crucial for Production of Infectious Virus Particles

Virus replication is mediated by interactions between the virus and host. Here, we demonstrate that influenza A virus membrane protein 2 (M2) can be ubiquitinated. The lysine residue at position 78, which is located in the cytoplasmic domain of M2, is essential for M2 ubiquitination. An M2-K78R (Lys78→Arg78) mutant, which produces ubiquitination-deficient M2, showed a severe defect in the production of infectious virus particles. M2-K78R mutant progeny contained more hemagglutinin (HA) proteins, less viral RNAs, and less internal viral proteins, including M1 and NP, than the wild-type virus. Furthermore, most of the M2-K78R mutant viral particles lacked viral ribonucleoproteins upon examination by electron microscopy and exhibited slightly lower densities. We also found that mutant M2 colocalized with the M1 protein to a lesser extent than for the wild-type virus. These findings may account for the reduced incorporation of viral ribonucleoprotein into virions. By blocking the second round of virus infection, we showed that the M2 ubiquitination-defective mutant exhibited normal levels of virus replication during the first round of infection, thereby proving that M2 ubiquitination is involved in the virus production step. Finally, we found that the M2-K78R mutant virus induced autophagy and apoptosis earlier than did the wild-type virus. Collectively, these results suggest that M2 ubiquitination plays an important role in infectious virus production by coordinating the efficient packaging of the viral genome into virus particles and the timing of virus-induced cell death.IMPORTANCE Annual epidemics and recurring pandemics of influenza viruses represent very high global health and economic burdens. The influenza virus M2 protein has been extensively studied for its important roles in virus replication, particularly in virus entry and release. Rimantadine, one of the most commonly used antiviral drugs, binds to the channel lumen near the N terminus of M2 proteins. However, viruses that are resistant to rimantadine have emerged. M2 undergoes several posttranslational modifications, such as phosphorylation and palmitoylation. Here, we reveal that ubiquitination mediates the functional role of M2. A ubiquitination-deficient M2 mutant predominately produced virus particles either lacking viral ribonucleoproteins or containing smaller amounts of internal viral components, resulting in lower infectivity. Our findings offer insights into the mechanism of influenza virus morphogenesis, particularly the functional role of M1-M2 interactions in viral particle assembly, and can be applied to the development of new influenza therapies.

Autophagy in Negative-Strand RNA Virus Infection

Autophagy is a homoeostatic process by which cytoplasmic material is targeted for degradation by the cell. Viruses have learned to manipulate the autophagic pathway to ensure their own replication and survival. Although much progress has been achieved in dissecting the interplay between viruses and cellular autophagic machinery, it is not well understood how the cellular autophagic pathway is utilized by viruses and manipulated to their own advantage. In this review, we briefly introduce autophagy, viral xenophagy and the interaction among autophagy, virus and immune response, then focus on the interplay between NS-RNA viruses and autophagy during virus infection. We have selected some exemplary NS-RNA viruses and will describe how these NS-RNA viruses regulate autophagy and the role of autophagy in NS-RNA viral replication and in immune responses to virus infection. We also review recent advances in understanding how NS-RNA viral proteins perturb autophagy and how autophagy-related proteins contribute to NS-RNA virus replication, pathogenesis and antiviral immunity.

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.

Differential immune response of influenza A virus-infected dendritic cells and association with autophagy

Aim: We sought to study the responses of dendritic cells (DCs) after direct stimulation by different influenza A viruses. Materials & methods: Using bone marrow-derived DCs (BMDCs) as a model, we measured the expression of surface markers, cytokine production and the priming effect on CD4+ naive T cells. Results & conclusion: We found that all of the tested viruses induced BMDC maturation. Cytokine expression assays also demonstrated that activated BMDCs secrete higher levels of cytokines. Similar to the maturation degree, well-stimulated BMDCs induced higher levels of naive CD4+ T-cell activation. Furthermore, we found that the PR8 and WSN influenza A viruse-induced BMDC functional activation was at least partially influenced by autophagy.

Biochemical Variations in Cytolytic Activity of Ortho- and Paramyxoviruses in Human Lung Tumor Cell Culture

Human lung cancer cells (Calu-3 line) were studied for the development of apoptosis, necrosis, and autophagy in response to infection with ortho- and paramyxoviruses. Biochemical pathways underlying various mechanisms of cell death differed for different viruses. When infected with murine Sendai paramyxovirus, Calu-3 cells demonstrated typical necrotic features such as cell swelling (but not shrinkage), lack of chromatin DNA laddering, of caspase 3 and 8 activation, and of apoptotic cleavage of poly(ADP-ribose) polymerase (PARP) protein; an activation of antiapoptotic protein kinase Akt was also revealed. In contrast, infection with avian influenza virus A/FPV/Rostock/34 (H7N1 subtype) or Newcastle disease virus (NDV, avian paramyxovirus) caused the development of typical apoptotic markers such as cell shrinkage, ladder-type chromosomal DNA fragmentation, caspase 3 and 8 activation, and proteolytic cleavage of PARP in the absence of Akt activation. Notably, no upregulation of p53 protein phosphorylation was observed in all infected cells, which indicates that p53 is not involved in the virus-induced death of Calu-3 cells. Cell death caused by the influenza virus was accompanied by overstimulation of autophagy, whereas no stimulation of autophagy was observed in the NDV-infected cells. Infection with Sendai virus caused moderate stimulation of autophagy, which suggests that the mechanism of the virus-induced cell death and the balance between autophagy and cell death in infected cancer cells depend on the virus type and might significantly differ even for closely related viruses. Therefore, an optimal strategy for oncolytic virus-mediated destruction of tumor cells in cancer patients requires selection of the most appropriate oncolytic virus based on the mechanism of its cytolytic action in a particular type of tumor.

Autophagy induction regulates influenza virus replication in a time-dependent manner

PURPOSE

Autophagy plays a key role in host defence responses against microbial infections by promoting degradation of pathogens and participating in acquired immunity. The interaction between autophagy and viruses is complex, and this pathway is hijacked by several viruses. Influenza virus (IV) interferes with autophagy through its replication and increases the accumulation of autophagosomes by blocking lysosome fusion. Thus, autophagy could be an effective area for antiviral research.

METHODOLOGY

In this study, we evaluated the effect of autophagy on IV replication. Two cell lines were transfected with Beclin-1 expression plasmid before (prophylactic approach) and after (therapeutic approach) IV inoculation.Results/Key findings. Beclin-1 overexpression in the cells infected by virus induced autophagy to 26 %. The log10haemagglutinin titre and TCID50 (tissue culture infective dose giving 50 % infection) of replicating virus were measured at 24 and 48 h post-infection. In the prophylactic approach, the virus titre was enhanced significantly at 24 h post-infection (P≤0.01), but it was not significantly different from the control at 48 h post-infection. In contrast, the therapeutic approach of autophagy induction inhibited the virus replication at 24 and 48 h post-infection. Additionally, we showed that inhibition of autophagy using 3-methyladenine reduced viral replication.

CONCLUSION

This study revealed that the virus (H1N1) titre was controlled in a time-dependent manner following autophagy induction in host cells. Manipulation of autophagy during the IV life cycle can be targeted both for antiviral aims and for increasing viral yield for virus production.

Edible bird's nest modulate intracellular molecular pathways of influenza A virus infected cells

BACKGROUND

Edible Bird's Nest (EBN) as a popular traditional Chinese medicine is believed to have health enhancing and antiviral activities against influenza A virus (IAV); however, the molecular mechanism behind therapeutic effects of EBN is not well characterized.

METHODS

In this study, EBNs that underwent different enzymatic preparation were tested against IAV infected cells. 50% cytotoxic concentration (CC50) and 50% inhibitory concentration (IC50) of the EBNs against IAV strain A/Puerto Rico/8/1934(H1N1) were determined by HA and MTT assays. Subsequently, the sialic acid content of the used EBNs were analyzed by fluorometric HPLC. Western Blotting and immunofluorescent staining were used to investigate the effects of EBNs on early endosomal trafficking and autophagy process of influenza virus.

RESULTS

This study showed that post inoculations of EBNs after enzymatic preparations have the highest efficacy to inhibit IAV. While CC50 of the tested EBNs ranged from 27.5-32 mg/ml, the IC50 of these compounds ranged between 2.5-4.9 mg/ml. EBNs could inhibit IAV as efficient as commercial antiviral agents, such as amantadine and oseltamivir with different mechanisms of action against IAV. The antiviral activity of these EBNs correlated with the content of N-acetyl neuraminic acid. EBNs could affect early endosomal trafficking of the virus by reducing Rab5 and RhoA GTPase proteins and also reoriented actin cytoskeleton of IAV infected cells. In addition, for the first time this study showed that EBNs can inhibit intracellular autophagy process of IAV life cycle as evidenced by reduction of LC3-II and increasing of lysosomal degradation.

CONCLUSIONS

The results procured in this study support the potential of EBNs as supplementary medication or alternative to antiviral agents to inhibit influenza infections. Evidently, EBNs can be a promising antiviral agent; however, these natural compounds should be screened for their metabolites prior to usage as therapeutic approach.

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.

Autophagy is involved in regulating the immune response of dendritic cells to influenza A (H1N1) pdm09 infection

Autophagy can mediate antiviral immunity. However, it remains unknown whether autophagy regulates the immune response of dendritic cells (DCs) to influenza A (H1N1) pdm09 infection. In this study, we found that infection with the H1N1 virus induced DC autophagy in an endocytosis-dependent manner. Compared with autophagy-deficient Beclin-1(+/-) mice, we found that bone-marrow-derived DCs from wild-type mice (WT BMDCs) presented a more mature phenotype on H1N1 infection. Wild-type BMDCs secreted higher levels of interleukin-6 (IL-6), tumour necrosis factor- α (TNF-α), interferon-β (IFN-β), IL-12p70 and IFN-γ than did Beclin-1(+/-) BMDCs. In contrast to Beclin-1(+/-) BMDCs, H1N1-infected WT BMDCs exhibited increased activation of extracellular signal-regulated kinase, Jun N-terminal kinase, p38, and nuclear factor-κB as well as IFN regulatory factor 7 nuclear translocation. Blockade of autophagosomal and lysosomal fusion by bafilomycin A1 decreased the co-localization of H1N1 viruses, autophagosomes and lysosomes as well as the secretion of IL-6, TNF-α and IFN-β in H1N1-infected BMDCs. In contrast to Beclin-1(+/-) BMDCs, H1N1-infected WT BMDCs were more efficient in inducing allogeneic CD4(+) T-cell proliferation and driving T helper type 1, 2 and 17 cell differentiation while inhibiting CD4(+) Foxp3(+) regulatory T-cell differentiation. Moreover, WT BMDCs were more efficient at cross-presenting the ovalbumin antigen to CD8(+) T cells. We consistently found that Beclin-1(+/-) BMDCs were inferior in their inhibition of H1N1 virus replication and their induction of H1N1-specific CD4(+) and CD8(+) T-cell responses, which produced lower levels of IL-6, TNF-α and IFN-β in vivo. Our data indicate that autophagy is important in the regulation of the DC immune response to H1N1 infection, thereby extending our understanding of host immune responses to the virus.

Autophagy is involved in regulating influenza A virus RNA and protein synthesis associated with both modulation of Hsp90 induction and mTOR/p70S6K signaling pathway

Influenza A virus (IAV) infection triggers autophagosome formation, but inhibits the fusion of autophagosomes with lysosomes. However, the role of autophagy in IAV replication is still largely unclarified. In this study, we aim to reveal the role of autophagy in IAV replication and the molecular mechanisms underlying the regulation. By using autophagy-deficient (Atg7(-/-)) MEFs, we demonstrated that autophagy deficiency significantly reduced the levels of viral proteins, mRNA and genomic RNAs (vRNAs) without affecting viral entry. We further found that autophagy deficiency lead to a transient increase in phosphorylation of mTOR and its downstream targets including 4E-BP1 and S6 at a very early stage of IAV infection, and markedly suppressed p70S6K phosphorylation at the late stage of IAV infection. Furthermore, autophagy deficiency resulted in impairment of Hsp90 induction in response to IAV infection. These results indicate that IAV regulates autophagy to benefit the accumulation of viral elements (synthesis of viral proteins and genomic RNA) during IAV replication. This regulation is associated with modulation of Hsp90 induction and mTOR/p70S6K signaling pathway. Our results provide important evidence for the role of autophagy in IAV replication and the mechanisms underlying the regulation.

Hemagglutinin of Influenza A Virus Antagonizes Type I Interferon (IFN) Responses by Inducing Degradation of Type I IFN Receptor 1

Influenza A virus (IAV) employs diverse strategies to circumvent type I interferon (IFN) responses, particularly by inhibiting the synthesis of type I IFNs. However, it is poorly understood if and how IAV regulates the type I IFN receptor (IFNAR)-mediated signaling mode. In this study, we demonstrate that IAV induces the degradation of IFNAR subunit 1 (IFNAR1) to attenuate the type I IFN-induced antiviral signaling pathway. Following infection, the level of IFNAR1 protein, but not mRNA, decreased. Indeed, IFNAR1 was phosphorylated and ubiquitinated by IAV infection, which resulted in IFNAR1 elimination. The transiently overexpressed IFNAR1 displayed antiviral activity by inhibiting virus replication. Importantly, the hemagglutinin (HA) protein of IAV was proved to trigger the ubiquitination of IFNAR1, diminishing the levels of IFNAR1. Further, influenza A viral HA1 subunit, but not HA2 subunit, downregulated IFNAR1. However, viral HA-mediated degradation of IFNAR1 was not caused by the endoplasmic reticulum (ER) stress response. IAV HA robustly reduced cellular sensitivity to type I IFNs, suppressing the activation of STAT1/STAT2 and induction of IFN-stimulated antiviral proteins. Taken together, our findings suggest that IAV HA causes IFNAR1 degradation, which in turn helps the virus escape the powerful innate immune system. Thus, the research elucidated an influenza viral mechanism for eluding the IFNAR signaling pathway, which could provide new insights into the interplay between influenza virus and host innate immunity.

IMPORTANCE

Influenza A virus (IAV) infection causes significant morbidity and mortality worldwide and remains a major health concern. When triggered by influenza viral infection, host cells produce type I interferon (IFN) to block viral replication. Although IAV was shown to have diverse strategies to evade this powerful, IFN-mediated antiviral response, it is not well-defined if IAV manipulates the IFN receptor-mediated signaling pathway. Here, we uncovered that influenza viral hemagglutinin (HA) protein causes the degradation of type I IFN receptor subunit 1 (IFNAR1). HA promoted phosphorylation and polyubiquitination of IFNAR1, which facilitated the degradation of this receptor. The HA-mediated elimination of IFNAR1 notably decreased the cells' sensitivities to type I IFNs, as demonstrated by the diminished expression of IFN-induced antiviral genes. This discovery could help us understand how IAV regulates the host innate immune response to create an environment optimized for viral survival in host cells.

Autophagy postpones apoptotic cell death in PRRSV infection through Bad-Beclin1 interaction

Autophagy and apoptosis play significant roles in PRRSV infection and replication. However, the interaction between these 2 processes in PRRSV replication is still far from been completely understood. In our studies, the exposure of MARC-145 cells to PRRSV confirmed the activation of autophagy and subsequent induction of apoptosis. The inhibition of autophagy by 3-methyladenine (3-MA) caused a significant increase in PRRSV-induced apoptosis, showing a potential connection between both mechanisms. Moreover, we observed an increase in Bad expression (a pro-apoptotic protein) and Beclin1 (an autophagy regulator) in virus-infected cells up to 36h. Co-immunoprecipitation assays showed the formation of Bad and Beclin1 complex in PRRSV infected cells. Accordingly, Bad co-localized with Beclin1 in MARC-145 infected cells. Knockdown of Beclin1 significantly decreased PRRSV replication and PRRSV-induced autophagy, while Bad silencing resulted in increased autophagy and enhanced viral replication. Furthermore, PRRSV infection phosphorylated Bad (Ser112) to promote cellular survival. These results demonstrate that autophagy can favor PRRSV replication by postponing apoptosis through the formation of a Bad-Beclin1 complex.

Influenza lung injury: mechanisms and therapeutic opportunities

In this Perspectives, we discuss some recent developments in the pathogenesis of acute lung injury following influenza infection, with an emphasis on promising therapeutic leads. Damage to the alveolar-capillary barrier has been quantified in mice, and agents have been identified that can help to preserve barrier integrity, such as vasculotide, angiopoietin-like 4 neutralization, and sphingosine 1-phosphate mimics. Results from studies using mesenchymal stem cells have been disappointing, despite promising data in other types of lung injury. The roles of fatty acid binding protein 5, prostaglandin E2, and the interplay between IFN-γ and STAT1 in epithelial signaling during infection have been addressed in vitro. Finally, we discuss the role of autophagy in inflammatory cytokine production and the viral life cycle and the opportunities this presents for intervention.

Protection against Influenza A Virus Challenge with M2e-Displaying Filamentous Escherichia coli Phages

Human influenza viruses are responsible for annual epidemics and occasional pandemics that cause severe illness and mortality in all age groups worldwide. Matrix protein 2 (M2) of influenza A virus is a tetrameric type III membrane protein that functions as a proton-selective channel. The extracellular domain of M2 (M2e) is conserved in human and avian influenza A viruses and is being pursued as a component for a universal influenza A vaccine. To develop a M2e vaccine that is economical and easy to purify, we genetically fused M2e amino acids 2-16 to the N-terminus of pVIII, the major coat protein of filamentous bacteriophage f88. We show that the resulting recombinant f88-M2e2-16 phages are replication competent and display the introduced part of M2e on the phage surface. Immunization of mice with purified f88-M2e2-16 phages in the presence of incomplete Freund's adjuvant, induced robust M2e-specific serum IgG and protected BALB/c mice against challenge with human and avian influenza A viruses. Thus, replication competent filamentous bacteriophages can be used as efficient and economical carriers to display conserved B cell epitopes of influenza A.

Survival of effector CD8+ T cells during influenza infection is dependent on autophagy

The activation and expansion of effector CD8(+) T cells are essential for controlling viral infections and tumor surveillance. During an immune response, T cells encounter extrinsic and intrinsic factors, including oxidative stress, nutrient availability, and inflammation, that can modulate their capacity to activate, proliferate, and survive. The dependency of T cells on autophagy for in vitro and in vivo activation, expansion, and memory remains unclear. Moreover, the specific signals and mechanisms that activate autophagy in T effector cells and their survival are not known. In this study, we generated a novel inducible autophagy knockout mouse to study T cell effector responses during the course of a virus infection. In response to influenza infection, Atg5(-/-) CD8(+) T cells had a decreased capacity to reach the peak effector response and were unable to maintain cell viability during the effector phase. As a consequence of Atg5 deletion and the impairment in effector-to-memory cell survival, mice fail to mount a memory response following a secondary challenge. We found that Atg5(-/-) effector CD8(+) T cells upregulated p53, a transcriptional state that was concomitant with widespread hypoxia in lymphoid tissues of infected mice. The onset of p53 activation was concurrent with higher levels of reactive oxygen species (ROS) that resulted in ROS-dependent apoptotic cell death, a fate that could be rescued by treating with the ROS scavenger N-acetylcysteine. Collectively, these results demonstrate that effector CD8(+) T cells require autophagy to suppress cell death and maintain survival in response to a viral infection.

Antiviral effect of methylated flavonol isorhamnetin against influenza

Influenza is an infectious respiratory disease with frequent seasonal epidemics that causes a high rate of mortality and morbidity in humans, poultry, and animals. Influenza is a serious economic concern due to the costly countermeasures it necessitates. In this study, we compared the antiviral activities of several flavonols and other flavonoids with similar, but distinct, hydroxyl or methyl substitution patterns at the 3, 3′, and 4′ positions of the 15-carbon flavonoid skeleton, and found that the strongest antiviral effect was induced by isorhamnetin. Similar to quercetin and kaempferol, isorhamnetin possesses a hydroxyl group on the C ring, but it has a 3′-methyl group on the B ring that is absent in quercetin and kaempferol. Co-treatment and pre-treatment with isorhamnetin produced a strong antiviral effect against the influenza virus A/PR/08/34(H1N1). However, isorhamnetin showed the most potent antiviral potency when administered after viral exposure (post-treatment method) in vitro. Isorhamnetin treatment reduced virus-induced ROS generation and blocked cytoplasmic lysosome acidification and the lipidation of microtubule associated protein1 light chain 3-B (LC3B). Oral administration of isorhamnetin in mice infected with the influenza A virus significantly decreased lung virus titer by 2 folds, increased the survival rate which ranged from 70–80%, and decreased body weight loss by 25%. In addition, isorhamnetin decreased the virus titer in ovo using embryonated chicken eggs. The structure-activity relationship (SAR) of isorhamnetin could explain its strong anti-influenza virus potency; the methyl group located on the B ring of isorhamnetin may contribute to its strong antiviral potency against influenza virus in comparison with other flavonoids.

Involvement of fish signal transducer and activator of transcription 3 (STAT3) in nodavirus infection induced cell death

The Janus kinase (JAK)-signal transducer and activator of transcription (STAT) signaling pathway is an important signaling pathway activated by interferons in response to virus infection. Fish STAT3 has been demonstrated to be involved in Singapore grouper iridovirus (SGIV) infection and virus induced paraptosis, but its effects on the replication of other fish viruses still remained uncertain. Here, the roles of grouper STAT3 (Ec-STAT3) in red spotted grouper nervous necrosis virus (RGNNV) infection were investigated. The present data showed that the distribution of phosphorylated Ec-STAT3 was altered in RGNNV infected fish cells, and the promoter activity of STAT3 was significantly increased during virus infection, suggesting that STAT3 activation was involved in RGNNV infection. Using STAT3 specific inhibitor, we found that inhibition of Ec-STAT3 in vitro did not affect the transcription and protein synthesis of RGNNV coat protein (CP), however, the severity of RGNNV induced vacuolation and autophagy was significantly increased. Meanwhile, at the late stage of virus infection, RGNNV induced necrotic cell death was significantly decreased after inhibition of Ec-STAT3. Further studies indicated that Ec-STAT3 inhibition significantly increased the transcript level of autophagy related genes, including UNC-51-like kinase 2 (ULK2) and microtubule-associated protein 1 light chain 3-II (LC3-II) induced by RGNNV infection. Moreover, the expression of several pro-inflammatory factors, including TNFα, IL-1β and IL-8 were mediated by Ec-STAT3 during RGNNV infection. Together, our results not only firstly revealed that STAT3 exerted novel roles in response to fish virus infection, but also provided new insights into understanding the roles of STAT3 in different forms of programmed cell death.

RASSF4 promotes EV71 replication to accelerate the inhibition of the phosphorylation of AKT

Enterovirus 71 (EV71) is a neurotropic virus that causes hand, foot and mouth disease (HFMD), occasionally leading to death. As a member of the RAS association domain family (RASSFs), RASSF4 plays important roles in cell death, tumor development and signal transduction. However, little is known about the relationship between RASSF4 and EV71. Our study reveals for the first time that RASSF4 promotes EV71 replication and then accelerates AKT phosphorylation inhibition in EV71-infected 293T cells, suggesting that RASSF4 may be a potential new target for designing therapeutic measures to prevent and control EV71 infection.

The role of autophagy in microbial infection and immunity

The autophagy pathway represents an evolutionarily conserved cell recycling process that is activated in response to nutrient deprivation and other stress signals. Over the years, it has been linked to an array of cellular functions. Equally, a wide range of cell-intrinsic, as well as extracellular, factors have been implicated in the induction of the autophagy pathway. Microbial infections represent one such factor that can not only activate autophagy through specific mechanisms but also manipulate the response to the invading microbe’s advantage. Moreover, in many cases, particularly among viruses, the pathway has been shown to be intricately involved in the replication cycle of the pathogen. Conversely, autophagy also plays a role in combating the infection process, both through direct destruction of the pathogen and as one of the key mediating factors in the host defense mechanisms of innate and adaptive immunity. Further, the pathway also plays a role in controlling the pathogenesis of infectious diseases by regulating inflammation. In this review, we discuss various interactions between pathogens and the cellular autophagic response and summarize the immunological functions of the autophagy pathway.

Baicalin inhibits autophagy induced by influenza A virus H3N2

Baicalin, a natural product isolated from Scutellariaradix, has been reported to have significant in vivo and in vitro anti-influenza virus activity, but the underlying mechanism remains poorly understood. In this study, we found that baicalin inhibited autophagy induced by influenza virus A3/Beijing/30/95 (H3N2) in both A549 and Ana-1 cells. The results showed that H3N2 induced autophagy by suppressing mTOR signaling pathway, which however could be significantly inhibited by baicalin. Baicalin could suppress the expression of Atg5-Atg12 complex and LC3-II, and attenuate autophagy induced by starvation. Thus, the inhibition of autophagy induced by virus may account for the antiviral activities of baicalin against H3N2. Autophagy may be a potential marker in developing novel anti-influenza drugs.

Compounds with anti-influenza activity: present and future of strategies for the optimal treatment and management of influenza. Part I: Influenza life-cycle and currently available drugs

Influenza is a contagious respiratory acute viral disease characterized by a short incubation period, high fever and respiratory and systemic symptoms. The burden of influenza is very heavy. Indeed, the World Health Organization (WHO) estimates that annual epidemics affect 5-15% of the world's population, causing up to 4-5 million severe cases and from 250,000 to 500,000 deaths. In order to design anti-influenza molecules and compounds, it is important to understand the complex replication cycle of the influenza virus. Replication is achieved through various stages. First, the virus must engage the sialic acid receptors present on the free surface of the cells of the respiratory tract. The virus can then enter the cells by different routes (clathrin-mediated endocytosis or CME, caveolae-dependent endocytosis or CDE, clathrin-caveolae-independent endocytosis, or macropinocytosis). CME is the most usual pathway; the virus is internalized into an endosomal compartment, from which it must emerge in order to release its nucleic acid into the cytosol. The ribonucleoprotein must then reach the nucleus in order to begin the process of translation of its genes and to transcribe and replicate its nucleic acid. Subsequently, the RNA segments, surrounded by the nucleoproteins, must migrate to the cell membrane in order to enable viral assembly. Finally, the virus must be freed to invade other cells of the respiratory tract. All this is achieved through a synchronized action of molecules that perform multiple enzymatic and catalytic reactions, currently known only in part, and for which many inhibitory or competitive molecules have been studied. Some of these studies have led to the development of drugs that have been approved, such as Amantadine, Rimantadine, Oseltamivir, Zanamivir, Peramivir, Laninamivir, Ribavirin and Arbidol. This review focuses on the influenza life-cycle and on the currently available drugs, while potential antiviral compounds for the prevention and treatment of influenza are considered in the subsequent review.

ER stress, autophagy, and RNA viruses

Endoplasmic reticulum (ER) stress is a general term for representing the pathway by which various stimuli affect ER functions. ER stress induces the evolutionarily conserved signaling pathways, called the unfolded protein response (UPR), which compromises the stimulus and then determines whether the cell survives or dies. In recent years, ongoing research has suggested that these pathways may be linked to the autophagic response, which plays a key role in the cell's response to various stressors. Autophagy performs a self-digestion function, and its activation protects cells against certain pathogens. However, the link between the UPR and autophagy may be more complicated. These two systems may act dependently, or the induction of one system may interfere with the other. Experimental studies have found that different viruses modulate these mechanisms to allow them to escape the host immune response or, worse, to exploit the host's defense to their advantage; thus, this topic is a critical area in antiviral research. In this review, we summarize the current knowledge about how RNA viruses, including influenza virus, poliovirus, coxsackievirus, enterovirus 71, Japanese encephalitis virus, hepatitis C virus, and dengue virus, regulate these processes. We also discuss recent discoveries and how these will produce novel strategies for antiviral treatment.

Oncolytic Immunotherapy: Dying the Right Way is a Key to Eliciting Potent Antitumor Immunity

Oncolytic viruses (OVs) are novel immunotherapeutic agents whose anticancer effects come from both oncolysis and elicited antitumor immunity. OVs induce mostly immunogenic cancer cell death (ICD), including immunogenic apoptosis, necrosis/necroptosis, pyroptosis, and autophagic cell death, leading to exposure of calreticulin and heat-shock proteins to the cell surface, and/or released ATP, high-mobility group box 1, uric acid, and other damage-associated molecular patterns as well as pathogen-associated molecular patterns as danger signals, along with tumor-associated antigens, to activate dendritic cells and elicit adaptive antitumor immunity. Dying the right way may greatly potentiate adaptive antitumor immunity. The mode of cancer cell death may be modulated by individual OVs and cancer cells as they often encode and express genes that inhibit/promote apoptosis, necroptosis, or autophagic cell death. We can genetically engineer OVs with death-pathway-modulating genes and thus skew the infected cancer cells toward certain death pathways for the enhanced immunogenicity. Strategies combining with some standard therapeutic regimens may also change the immunological consequence of cancer cell death. In this review, we discuss recent advances in our understanding of danger signals, modes of cancer cell death induced by OVs, the induced danger signals and functions in eliciting subsequent antitumor immunity. We also discuss potential combination strategies to target cells into specific modes of ICD and enhance cancer immunogenicity, including blockade of immune checkpoints, in order to break immune tolerance, improve antitumor immunity, and thus the overall therapeutic efficacy.

The regulation of autophagy by influenza A virus

Influenza A virus is a dreadful pathogen of animals and humans, causing widespread infection and severe morbidity and mortality. It is essential to characterize the influenza A virus-host interaction and develop efficient counter measures against the viral infection. Autophagy is known as a catabolic process for the recycling of the cytoplasmic macromolecules. Recently, it has been shown that autophagy is a critical mechanism underlying the interaction between influenza A virus and its host. Autophagy can be induced by the infection with influenza A virus, which is considered as a necessary process for the viral proliferation, including the accumulation of viral elements during the replication of influenza A virus. On the other hand, influenza A virus can inhibit the autophagic formation via interaction with the autophagy-related genes (Atg) and signaling pathways. In addition, autophagy is involved in the influenza virus-regulated cell deaths, leading to significant changes in host apoptosis. Interestingly, the high pathogenic strains of influenza A virus, such as H5N1, stimulate autophagic cell death and appear to interplay with the autophagy in distinct ways as compared with low pathogenic strains. This review discusses the regulation of autophagy, an influenza A virus driven process.

Commentary on the regulation of viral proteins in autophagy process

The ability to subvert intracellular antiviral defenses is necessary for virus to survive as its replication occurs only in the host cells. Viruses have to modulate cellular processes and antiviral mechanisms to their own advantage during the entire virus life cycle. Autophagy plays important roles in cell regulation. Its function is not only to catabolize aggregate proteins and damaged organelles for recycling but also to serve as innate immunity to remove intracellular pathogenic elements such as viruses. Nevertheless, some viruses have evolved to negatively regulate autophagy by inhibiting its formation. Even more, some viruses have employed autophagy to benefit their replication. To date, there are more and more growing evidences uncovering the functions of many viral proteins to regulate autophagy through different cellular pathways. In this review, we will discuss the relationship between viruses and autophagy and summarize the current knowledge on the functions of viral proteins contributing to affect autophagy process.

Autophagy mediates avian influenza H5N1 pseudotyped particle-induced lung inflammation through NF-κB and p38 MAPK signaling pathways

Since avian influenza virus H5N1-induced hypercytokemia plays a key role in acute lung injury, understanding its molecular mechanism is highly desirable for discovering therapeutic targets against H5N1 infection. In the present study, we investigated the role of autophagy in H5N1-induced lung inflammation by using H5N1 pseudotyped viral particles (H5N1pps). The results showed that H5N1pps significantly induced autophagy both in A549 human lung epithelial cells and in mouse lung tissues, which was primarily due to hemagglutinin (HA) of H5N1 virus. Blocking autophagy with 3-methyladenine (an autophagy inhibitor) or siRNA knockdown of autophagy-related genes (beclin1 and atg5) dramatically attenuated H5N1pp-induced proinflammatory cytokines and chemokines, such as IL-1β, TNF-α, IL-6, CCL2, and CCL5, both in vitro and in vivo. Autophagy-mediated inflammatory responses involved the activation of NF-κB and p38 MAPK signaling pathways, which required the presence of clathrin but did not rely on p62 or autophagosome-lysosome fusion. On the other hand, the activation of NF-κB also promoted H5N1pp-induced autophagosome formation. These data indicated a positive feedback loop between autophagy and NF-κB signaling cascade, which could exacerbate H5N1pp-induced lung inflammation. Our data demonstrated an essential role of autophagy in H5N1pp-triggered inflammatory responses, and targeting the autophagic pathway could be a promising strategy to treat H5N1 virus-caused lung inflammation.