PB2 residue 473 contributes to the mammalian virulence of H7N9 avian influenza virus by modulating viral polymerase activity via ANP32A

Since the first human infection reported in 2013, H7N9 avian influenza virus (AIV) has been regarded as a serious threat to human health. In this study, we sought to identify the virulence determinant of the H7N9 virus in mammalian hosts. By comparing the virulence of the SH/4664 H7N9 virus, a non-virulent H9N2 virus, and various H7N9-H9N2 hybrid viruses in infected mice, we first pinpointed PB2 as the primary viral factor accounting for the difference between H7N9 and H9N2 in mammalian virulence. We further analyzed the in vivo effects of individually mutating H7N9 PB2 residues different from the closely related H9N2 virus and consequently found residue 473, alongside the well-known residue 627, to be critical for the virulence of the H7N9 virus in mice and the activity of its reconstituted viral polymerase in mammalian cells. The importance of PB2-473 was further strengthened by studying reverse H7N9 substitutions in the H9N2 background. Finally, we surprisingly found that species-specific usage of ANP32A, a family member of host factors connecting with the PB2-627 polymorphism, mediates the contribution of PB2 473 residue to the mammalian adaption of AIV polymerase, as the attenuating effect of PB2 M473T on the viral polymerase activity and viral growth of the H7N9 virus could be efficiently complemented by co-expression of chicken ANP32A but not mouse ANP32A and ANP32B. Together, our studies uncovered the PB2 473 residue as a novel viral host range determinant of AIVs via species-specific co-opting of the ANP32 host factor to support viral polymerase activity.IMPORTANCEThe H7N9 avian influenza virus has been considered to have the potential to cause the next pandemic since the first case of human infection reported in 2013. In this study, we identified PB2 residue 473 as a new determinant of mouse virulence and mammalian adaptation of the viral polymerase of the H7N9 virus and its non-pathogenic H9N2 counterparts. We further demonstrated that the variation in PB2-473 is functionally linked to differential co-opting of the host ANP32A protein in supporting viral polymerase activity, which is analogous to the well-known PB2-627 polymorphism, albeit the two PB2 positions are spatially distant. By providing new mechanistic insight into the PB2-mediated host range determination of influenza A viruses, our study implicated the potential existence of multiple PB2-ANP32 interfaces that could be targets for developing new antivirals against the H7N9 virus as well as other mammalian-adapted influenza viruses.

New Viral Diseases and New Possible Remedies by Means of the Pharmacology of the Renin-Angiotensin System

All strains of SARS-CoV-2, as well as previously described SARS-CoV and MERS-CoV, bind to ACE2, the cell membrane receptor of β-coronaviruses. Monocarboxypeptidase ACE2 activity stops upon viral entry into cells, leading to inadequate tissue production of angiotensin 1-7 (Ang1-7). Acute lung injury due to the human respiratory syncytial virus (hRSV) or avian influenza A H7N9 and H5N1 viruses is also characterized by significant downregulation of lung ACE2 and increased systemic levels of angiotensin II (Ang II). Restoration of Ang1-7 anti-inflammatory, antifibrotic, vasodilating, and natriuretic properties was attempted at least in some COVID-19 patients through i.v. infusion of recombinant human ACE2 or intranasal administration of the modified ACE2 protein, with inconsistent clinical results. Conversely, use of ACE inhibitors (ACEis), which increase ACE2 cell expression, seemed to improve the prognosis of hypertensive patients with COVID-19. To restore Ang1-7 tissue levels in all these viral diseases and avoid the untoward effects frequently seen with ACE2 systemic administration, a different strategy may be hypothesized. Experimentally, when metallopeptidase inhibitors block ACE2, neprilysin (NEP), highly expressed in higher and lower airways, starts cleaving angiotensin I (Ang I) into Ang1-7. We suggest a discerning use of ACEis in normohypertensive patients with β-coronavirus disease as well as in atypical pneumonia caused by avian influenza viruses or hRSV to block the main ACE-dependent effects: Ang II synthesis and Ang1-7 degradation into angiotensin 1-5. At the same time, i.v.-infused Ang I, which is not hypertensive provided ACE is inhibited, may become the primary substrate for local Ang1-7 synthesis via ubiquitous NEP; i.e., NEP could replace inadequate ACE2 function if Ang I was freely available. Moreover, inhibitors of chymase, a serine endopeptidase responsible for 80% of Ang II-forming activity in tissues and vessel walls, could protect patients with atypical pneumonia from Ang II-mediated microvascular damage without reducing arterial blood pressure.

Glycosylation generates an efficacious and immunogenic vaccine against H7N9 influenza virus

Zoonotic avian influenza viruses pose severe health threats to humans. Of several viral subtypes reported, the low pathogenic avian influenza H7N9 virus has since February 2013 caused more than 1,500 cases of human infection with an almost 40% case-fatality rate. Vaccination of poultry appears to reduce human infections. However, the emergence of highly pathogenic strains has increased concerns about H7N9 pandemics. To develop an efficacious H7N9 human vaccine, we designed vaccine viruses by changing the patterns of N-linked glycosylation (NLG) on the viral hemagglutinin (HA) protein based on evolutionary patterns of H7 HA NLG changes. Notably, a virus in which 2 NLG modifications were added to HA showed higher growth rates in cell culture and elicited more cross-reactive antibodies than did other vaccine viruses with no change in the viral antigenicity. Developed into an inactivated vaccine formulation, the vaccine virus with 2 HA NLG additions exhibited much better protective efficacy against lethal viral challenge in mice than did a vaccine candidate with wild-type (WT) HA by reducing viral replication in the lungs. In a ferret model, the 2 NLG-added vaccine viruses also induced hemagglutination-inhibiting antibodies and significantly suppressed viral replication in the upper and lower respiratory tracts compared with the WT HA vaccines. In a mode of action study, the HA NLG modification appeared to increase HA protein contents incorporated into viral particles, which would be successfully translated to improve vaccine efficacy. These results suggest the strong potential of HA NLG modifications in designing avian influenza vaccines.

This study shows that changing the pattern of N-glycosylation of the pathogenic avian influenza H7N9 virus hemagglutinin protein increases the amount of hemagglutinin incorporated into the viral membrane; the candidate vaccine virus induces neutralizing antibodies and protects animal models from lethal viral challenge.

Virus subtype-specific suppression of MAVS aggregation and activation by PB1-F2 protein of influenza A (H7N9) virus

Human infection with avian influenza A (H5N1) and (H7N9) viruses causes severe respiratory diseases. PB1-F2 protein is a critical virulence factor that suppresses early type I interferon response, but the mechanism of its action in relation to high pathogenicity is not well understood. Here we show that PB1-F2 protein of H7N9 virus is a particularly potent suppressor of antiviral signaling through formation of protein aggregates on mitochondria and inhibition of TRIM31-MAVS interaction, leading to prevention of K63-polyubiquitination and aggregation of MAVS. Unaggregated MAVS accumulated on fragmented mitochondria is prone to degradation by both proteasomal and lysosomal pathways. These properties are proprietary to PB1-F2 of H7N9 virus but not shared by its counterpart in WSN virus. A recombinant virus deficient of PB1-F2 of H7N9 induces more interferon β in infected cells. Our findings reveal a subtype-specific mechanism for destabilization of MAVS and suppression of interferon response by PB1-F2 of H7N9 virus.

Exactly why avian influenza A (H5N1) and (H7N9) viruses cause severe diseases in humans remains unclear. PB1-F2 protein encoded by influenza A virus is one virulence factor that might make a difference. In this study we show that PB1-F2 protein of H7N9 virus is particularly strong in the suppression of host antiviral defense. This was achieved by inhibiting a key protein in cell signaling named MAVS. PB1-F2 directs MAVS for degradation and prevents MAVS from forming protein aggregates required for full activation. A recombinant virus in which PB1-F2 of H7N9 has been deleted can activate host antiviral response robustly. Our findings reveal a novel mechanism by which PB1-F2 protein of H7N9 virus prevents MAVS aggregation and promotes MAVS degradation, leading to the suppression of host antiviral defense.

Structural Organization and Dynamic Characteristics of the Binding Site for Conformational Rearrangement Inhibitors in Hemagglutinins from H3N2 and H7N9 Influenza Viruses

Computer models of hemagglutinins from the H3N2 and H7N9 influenza viruses were developed to study structural organization and dynamic characteristics of the binding site for the conformational rearrangement inhibitors. The metadynamics was used to map the binding site free energy and to define the volume of its most energetically favorable states. It was demonstrated by simulation of the umifenovir (Arbidol) interaction with hemagglutinin that ligand binding requires an increase in the binding site volume and deformation of its most energetically favorable state. We also identified amino acid residues directly involved in the ligand binding that determine the binding efficiency, as well as the dynamic behavior of the binding site. The revealed features of the binding site structural organization of the influenza virus hemagglutinin should be taken into account when searching for new antiviral drugs capable to modulate its functional properties.

Generation of neutralizing and non-neutralizing monoclonal antibodies against H7N9 influenza virus

The H7N9 viruses have been circulating for six years. The insertion of a polybasic cleavage site in the haemagglutinin (HA) protein of H7N9 has resulted in the emergence of a highly pathogenic (HP) avian influenza virus. Currently, there are limited studies on neutralizing monoclonal antibodies(mAbs) against HP H7N9 AIVs. In this study, mice were immunized with inactivated H7N9 vaccine of A/ZJU01/PR8/2013 to produce murine mAbs. Finally, two murine mAbs against the HA of low pathogenic (LP) virus were produced and characterized. Characterization included determining mAbs binding breadth and affinity, in vitro neutralization capacity, and potential in vivo protection. Two of these mAbs, 1H10 and 2D1, have been identified to have therapeutic and prophylactic efficacy against the HP strain in mouse passive transfer-viral challenge experiments. The mAb 1H10 was most efficacious, even if the treatment-time was as late as 72 h post-infection, or the therapeutic dose was as low as 1 mg/kg; and it was confirmed to have haemagglutination inhibition and neutralizing activity on both LP-and HP-H7N9 strains. Further study indicated that the protection provided by 2D1 was mediated by antibody-dependent cellular cytotoxicity. The mAbs described here provide promising results and merit further development into potential antiviral therapeutics for H7N9 infection.

Structural and Molecular Characterization of the Hemagglutinin from the Fifth-Epidemic-Wave A(H7N9) Influenza Viruses

The avian influenza A(H7N9) virus continues to cause human infections in China and is a major ongoing public health concern. Five epidemic waves of A(H7N9) infection have occurred since 2013, and the recent fifth epidemic wave saw the emergence of two distinct lineages with elevated numbers of human infection cases and broader geographic distribution of viral diseases compared to the first four epidemic waves. Moreover, highly pathogenic avian influenza (HPAI) A(H7N9) viruses were also isolated during the fifth epidemic wave. Here, we present a detailed structural and biochemical analysis of the surface hemagglutinin (HA) antigen from viruses isolated during this recent epidemic wave. Results highlight that, compared to the 2013 virus HAs, the fifth-wave virus HAs remained a weak binder to human glycan receptor analogs. We also studied three mutations, V177K-K184T-G219S, that were recently reported to switch a 2013 A(H7N9) HA to human-type receptor specificity. Our results indicate that these mutations could also switch the H7 HA receptor preference to a predominantly human binding specificity for both fifth-wave H7 HAs analyzed in this study.IMPORTANCE The A(H7N9) viruses circulating in China are of great public health concern. Here, we report a molecular and structural study of the major surface proteins from several recent A(H7N9) influenza viruses. Our results improve the understanding of these evolving viruses and provide important information on their receptor preference that is central to ongoing pandemic risk assessment.

Development of an Influenza A Master Virus for Generating High-Growth Reassortants for A/Anhui/1/2013(H7N9) Vaccine Production in Qualified MDCK Cells

In 2013, the first case of human infection with an avian influenza A virus (H7N9) was reported in China, and the human infection with this virus has continued as of 2016. At the request of the WHO, we have successfully developed candidate reassortant vaccine virus using A/Anhui/1/2013 and the high egg-growth master virus A/PR/8/1934. Recent plans regarding influenza vaccine production include using cell-cultured systems in Japan and several other countries. However, egg-based vaccine viruses are not always suitable for cell-cultured vaccine production due to potential issues with growth, protein yield and antigenic stability. Therefore, in this study, we have developed a high-growth master virus (hg-PR8) adapted to qualified NIID-MDCK cells that are competent for vaccine production. The virus hg-PR8 was obtained after 20 serial passages of A/Puerto Rico/8/1934 (PR8) in NIID-MDCK cells. The viral titer of hg-PR8 was 10^8.6 plaque-forming units per milliliter (PFU/mL). Seven amino acid substitutions were identified in the amino acid sequences of PB2, PB1, PA, NA, M and NS of hg-PR8 compared to the sequence of the original PR8 (org-PR8) strain. The growth capacities of the reassortant viruses, which possess heterologous internal genes from hg-PR8 or org-PR8, indicated that the amino acid changes in PB2 and NS2 similarly affected growth capacity in NIID-MDCK cells. To assess the suitability of hg-PR8 as a master virus, we generated 6:2 reassortant viruses possessing the HA and NA segments from A/Anhui/1/2013 (H7N9) and the remaining segments from hg-PR8. The virus titers of the reassortant strains were 10⁷−10⁸ PFU/mL. The antigenicity of the viruses was stable during ten passages of the viruses in NIID-MDCK cells. In comparison with the egg-based reassortant vaccine viruses with identical HA and NA segments, the hg-PR8-based viruses showed 1.5- to 2-fold higher protein yields in NIID-MDCK cells.

RIG-I ligand enhances the immunogenicity of recombinant H7HA protein

Avian H7N9 influenza virus infection with fatal outcomes continues to pose a pandemic threat and highly immunogenic vaccines are urgently needed. In this report we show that baculovirus-derived recombinant H7 hemagglutinin protein, when delivered with RIG-I ligand, induced enhanced antibody and T cell responses and conferred protection against lethal challenge with a homologous H7N9 virus. These findings indicate the potential utility of RIG-I ligands as vaccine adjuvants to increase the immunogenicity of recombinant H7 hemagglutinin.

Preliminary Proteomic Analysis of A549 Cells Infected with Avian Influenza Virus H7N9 and Influenza A Virus H1N1

A newly emerged H7N9 influenza virus poses high risk to human beings. However, the pathogenic mechanism of the virus remains unclear. The temporal response of primary human alveolar adenocarcinoma epithelial cells (A549) infected with H7N9 influenza virus and H1N1 influenza A virus (H1N1, pdm09) were evaluated using the proteomics approaches (2D-DIGE combined with MALDI-TOF-MS/MS) at 24, 48 and 72 hours post of the infection (hpi). There were 11, 12 and 33 proteins with significant different expressions (P<0.05) at 24, 48 and 72hpi, especially F-actin-capping protein subunit alpha-1 (CAPZA1), Ornithine aminotransferase (OAT), Poly(rC)-binding protein 1 (PCBP1), Eukaryotic translation initiation factor 5A-1 (EIF5A) and Platelet-activating factor acetylhydrolaseⅠb subunit beta (PAFAH1B2) were validated by western-blot analysis. The functional analysis revealed that the differential proteins in A549 cells involved in regulating cytopathic effect. Among them, the down-regulation of CAPZA1, OAT, PCBP1, EIF5A are related to the death of cells infected by H7N9 influenza virus. This is the first time show that the down-regulation of PAFAH1B2 is related to the later clinical symptoms of patients infected by H7N9 influenza virus. These findings may improve our understanding of pathogenic mechanism of H7N9 influenza virus in proteomics.

The Novel H7N9 Influenza A Virus NS1 Induces p53-Mediated Apoptosis of A549 Cells

BACKGROUND

H7N9, emerged as an avian influenza virus outbreak in Eastern China in early 2013, and represented another major threat to global health. Roles of its NS1 protein, an essential viral factor, in regulating apoptosis remain unknown.

METHODS

Apoptotic effect and features of H7N9/NS1 in the human A549 alveolar basal epithelial cell line were examined by caspase 3/7 activity assay and western blotting of apoptotic associated proteins. Effects of H7N9/NS1on mitochondrial membrane potential were investigated by flow cytometry.

RESULTS

The expression of H7N9/NS1 in A549 cells activated caspase 3/7 and increased the protein levels of cleaved caspase 7 and cleaved poly (ADP-ribose) polymerase (PARP). H7N9/NS1-expressing A549 cells displayed a decrease in mitochondrial membrane potential. In addition, H7N9/NS1 increased the protein levels of total p53, p53 phosphorylated at Ser46 and Ser37, activated caspase 9, and the Bax/Bcl-2 ratio.

CONCLUSION

Our results suggest that H7N9/NS1 protein causes the accumulation of p53 by increasing phosphorylation levels of p53 and the induction of mitochondrial dysfunction, which may contribute to H7N9/NS1-induced apoptosis in A549 cells.

[Establishment of a lentivirus-mediated rapid eukaryotic expression system of novel avian influenza H7N9 virus hemagglutinin gene]

OBJECTIVE

To establish a rapid eukaryotic expression system of hemagglutinin (HA) gene of novel avian influenza H7N9 using lentiviral vector, express the recombinant protein and study its functions in human embryonic kidney HEK293T cells.

METHODS

The full-length HA gene was amplified from H7N9 genomic RNA by reverse transcription PCR (RT-PCR) and linked with pMD18-T vector to generate pMD18-T-HA plasmid. Blunt-end HA gene with Kozak sequence was amplified from pMD18-T-HA vector, and then pLenti-HA-V5 expression vector was constructed by Topo cloning for transient expression in HEK293T cells. Expression of HA-V5 recombinant protein was confirmed by immunofluorescence assay (IFA) and Western blotting. Hemagglutination test was performed to evaluate the biological activity of the recombinant protein.

RESULTS

The full-length HA gene (1 683 bp) was obtained and eukaryotic expression plasmid was constructed successfully. A recombinant protein with relative molecular mass (Mr) 70 000 was expressed and the antigenicity and binding specificity to positive serum were demonstrated by IFA and Western blotting. The hemagglutination activity was proved by hemagglutination test. IFA and Western blotting showed that the Mr 70 000 recombinant protein had an immuoreactivity to positive serum. The hemagglutination activity was confirmed by hemagglutination test.

CONCLUSION

The rapid eukaryotic expression system of HA gene was successfully constructed, which laid a solid foundation for further research on subunit vaccine development, neutralizing epitope mapping and packaging pseudovirus.

Structural differences between the avian and human H7N9 hemagglutinin proteins are attributable to modifications in salt bridge formation: a computational study with implications in viral evolution

Influenza A hemagglutinin (HA) is a homotrimeric glycoprotein composed of a fibrous globular stem supporting a globular head containing three sialic acid binding sites responsible for infection. The H7N9 strain has consistently infected an avian host, however, the novel 2013 strain is now capable of infecting a human host which would imply that the HA in both strains structurally differ. A better understanding of the structural differences between the avian and human H7N9 strains may shed light into viral evolution and transmissibility. In this study, we elucidated the structural differences between the avian and human H7N9 strains. Throughout the study, we generated HA homology models, verified the quality of each model, superimposed HA homology models to determine structural differences, and, likewise, elucidated the probable cause for these structural differences. We detected two different types of structural differences between the novel H7N9 human and representative avian strains, wherein, one type (Pattern-1) showed three non-overlapping regions while the other type (Pattern-2) showed only one non-overlapping region. In addition, we found that superimposed HA homology models exhibiting Pattern-1 contain three non-overlapping regions designated as: Region-1 (S157₁-A160₁); Region-3 (R262₁-S265₁); and Region-4 (S270₁-D281₁), whereas, superimposed HA homology models showing Pattern-2 only contain one non-overlapping region designated as Region-2 (S137₁-S145₁). We attributed the two patterns we observed to either the presence of salt bridges involving the E114₁ residue or absence of the R141₁:D77₁ salt bridge. Interestingly, comparison between the human H7N7 and H7N9 HA homology models showed high structural similarity. We propose that the putative absence of the R141₁:D77₁ salt bridge coupled with the putative presence of the E114₁:R262₁ and E114₁:K264₁ salt bridges found in the 2013 H7N9 HA homology model is associated to human-type receptor binding. This highlights the possible significance of HA salt bridge formation modifications in viral infectivity, immune escape, transmissibility and evolution.