Recombinant parainfluenza virus 5 expressing clade 2.3.4.4b H5 hemagglutinin protein confers broad protection against H5Ny influenza viruses.

The global circulation of clade 2.3.4.4b H5Ny highly pathogenic avian influenza viruses (HPAIVs) in poultry and wild birds, increasing mammal infections, continues to pose a public health threat and may even form a pandemic. An efficacious vaccine against H5Ny HPAIVs is crucial for emergency use and pandemic preparedness. In this study, we developed a parainfluenza virus 5 (PIV5)-based vaccine candidate expressing hemagglutinin (HA) protein of clade 2.3.4.4b H5 HPAIV, termed rPIV5-H5, and evaluated its safety and efficacy in mice and ferrets. Our results demonstrated that intranasal immunization with a single dose of rPIV5-H5 could stimulate H5-specific antibody responses, moreover, a prime-boost regimen using rPIV5-H5 stimulated robust humoral, cellular, and mucosal immune responses in mice. Challenge study showed that rPIV5-H5 prime-boost regimen provided sterile immunity against lethal clade 2.3.4.4b H5N1 virus infection in mice and ferrets. Notably, rPIV5-H5 prime-boost regimen provided protection in mice against challenge with lethal doses of heterologous clades 2.2, 2.3.2, and 2.3.4 H5N1, and clade 2.3.4.4h H5N6 viruses. These results revealed that rPIV5-H5 can elicit protective immunity against a diverse clade of highly pathogenic H5Ny virus infection in mammals, highlighting the potential of rPIV5-H5 as a pan-H5 influenza vaccine candidate for emergency use.IMPORTANCEClade 2.3.4.4b H5Ny highly pathogenic avian influenza viruses (HPAIVs) have been widely circulating in wild birds and domestic poultry all over the world, leading to infections in mammals, including humans. Here, we developed a recombinant PIV5-vectored vaccine candidate expressing the HA protein of clade 2.3.4.4b H5 virus. Intranasal immunization with rPIV5-H5 in mice induced airway mucosal IgA responses, high levels of antibodies, and robust T-cell responses. Importantly, rPIV5-H5 conferred complete protection in mice and ferrets against clade 2.3.4.4b H5N1 virus challenge, the protective immunity was extended against heterologous H5Ny viruses. Taken together, our data demonstrate that rPIV5-H5 is a promising vaccine candidate against diverse H5Ny influenza viruses in mammals.

Decaffeination and Neuraminidase Inhibitory Activity of Arabica Green Coffee (Coffea arabica) Beans: Chlorogenic Acid as a Potential Bioactive Compound.

Coffee has been studied for its health benefits, including prevention of several chronic diseases, such as type 2 diabetes mellitus, cancer, Parkinson's, and liver diseases. Chlorogenic acid (CGA), an important component in coffee beans, was shown to possess antiviral activity against viruses. However, the presence of caffeine in coffee beans may also cause insomnia and stomach irritation, and increase heart rate and respiration rate. These unwanted effects may be reduced by decaffeination of green bean Arabica coffee (GBAC) by treatment with dichloromethane, followed by solid-phase extraction using methanol. In this study, the caffeine and chlorogenic acid (CGA) level in the coffee bean from three different areas in West Java, before and after decaffeination, was determined and validated using HPLC. The results showed that the levels of caffeine were reduced significantly, with an order as follows: Tasikmalaya (2.28% to 0.097% (97 ppm), Pangalengan (1.57% to 0.049% (495 ppm), and Garut (1.45% to 0.00002% (0.2 ppm). The CGA levels in the GBAC were also reduced as follows: Tasikmalaya (0.54% to 0.001% (118 ppm), Pangalengan (0.97% to 0.0047% (388 ppm)), and Garut (0.81% to 0.029% (282 ppm). The decaffeinated samples were then subjected to the H5N1 neuraminidase (NA) binding assay to determine its bioactivity as an anti-influenza agent. The results show that samples from Tasikmalaya, Pangalengan, and Garut possess NA inhibitory activity with IC50 of 69.70, 75.23, and 55.74 µg/mL, respectively. The low level of caffeine with a higher level of CGA correlates with their higher levels of NA inhibitory, as shown in the Garut samples. Therefore, the level of caffeine and CGA influenced the level of NA inhibitory activity. This is supported by the validation of CGA-NA binding interaction via molecular docking and pharmacophore modeling; hence, CGA could potentially serve as a bioactive compound for neuraminidase activity in GBAC.

Plasticity of the Influenza Virus H5 HA Protein.

Since the emergence of highly pathogenic avian influenza viruses of the H5 subtype, the major viral antigen, hemagglutinin (HA), has undergone constant evolution, resulting in numerous genetic and antigenic (sub)clades. To explore the consequences of amino acid changes at sites that may affect the antigenicity of H5 viruses, we simultaneously mutated 17 amino acid positions of an H5 HA by using a synthetic gene library that, theoretically, encodes all combinations of the 20 amino acids at the 17 positions. All 251 mutant viruses sequenced possessed ≥13 amino acid substitutions in HA, demonstrating that the targeted sites can accommodate a substantial number of mutations. Selection with ferret sera raised against H5 viruses of different clades resulted in the isolation of 39 genotypes. Further analysis of seven variants demonstrated that they were antigenically different from the parental virus and replicated efficiently in mammalian cells. Our data demonstrate the substantial plasticity of the influenza virus H5 HA protein, which may lead to novel antigenic variants.IMPORTANCE The HA protein of influenza A viruses is the major viral antigen. In this study, we simultaneously introduced mutations at 17 amino acid positions of an H5 HA expected to affect antigenicity. Viruses with ≥13 amino acid changes in HA were viable, and some had altered antigenic properties. H5 HA can therefore accommodate many mutations in regions that affect antigenicity. The substantial plasticity of H5 HA may facilitate the emergence of novel antigenic variants.

Identification of a universal antigen epitope of influenza A virus using peptide microarray.

BACKGROUND: Hemagglutinin is a major surface protein in influenza A virus (IAV), and HA2 is relative conserved among different IAVs. It will be meaningful to identify broad-spectrum epitopes based on the HA2 protein. RESULTS: Overlapping peptides of the HA2 protein of the H5N1 IAV A/Mallard/Huadong/S/2005 were synthesized and loaded on modified silica gel film to form a microarray, and antisera against different subtypes of IAVs were used to screen universal epitopes. The selected epitope was further confirmed by western blotting using anti-peptide immune serum and viruses rescued with amino acid substitution. The results showed that 485-FYHKCDNECME-495 of the H5 14th peptide in HA2 had broad-spectrum binding activity with antisera against H1, H3, H4, H5, H6, H7, H8, H9, and H10 subtype IAV. Substitution of amino acids (K or D) in rescued viruses resulted in decreased serum binding, indicating that they were critical residues for serum binding activity. In Immune Epitope Database, some epitopes containing 14-4 peptide were confirmed as MHC-II-restricted CD4 T cell epitope and had effects on releasing IL-2 or IFN. CONCLUSION: The identified epitope should be a novel universal target for detection and vaccine design and its ability to generate immune protection needs further exploration.

The N-terminal residual arginine19 of influenza A virus NS1 protein is required for its nuclear localization and RNA binding.

RNA binding ability and cellular distribution are important for nonstructural protein 1 (NS1) of influenza A virus to act as a viral regulatory factor to control virus life cycle. In this study, we identified that the N-terminal residues 19-21 of NS1 are a highly conserved motif depending on all the available NS1 full length sequence of H5N1 influenza A virus from NCBI database. Site-directed mutation analysis demonstrated that the R19 residue of NS1 is critical for its RNA binding and nuclear localization. Furthermore, the residue R19 of NS1 was identified to be critical for regulating M1 mRNA splicing and NS1 nuclear export. Biological analysis of the rescued viruses indicated that the R19A mutation of NS1 did not interfere the replication of H5N1 virus during infection and attenuated the virulence of H5N1 virus in mice.

MARCH8 Inhibits Ebola Virus Glycoprotein, Human Immunodeficiency Virus Type 1 Envelope Glycoprotein, and Avian Influenza Virus H5N1 Hemagglutinin Maturation.

Membrane-associated RING-CH-type 8 (MARCH8) strongly blocks human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein (Env) incorporation into virions by downregulating its cell surface expression, but the mechanism is still unclear. We now report that MARCH8 also blocks the Ebola virus (EBOV) glycoprotein (GP) incorporation via surface downregulation. To understand how these viral fusion proteins are downregulated, we investigated the effects of MARCH8 on EBOV GP maturation and externalization via the conventional secretion pathway. MARCH8 interacted with EBOV GP and furin when detected by immunoprecipitation and retained the GP/furin complex in the Golgi when their location was tracked by a bimolecular fluorescence complementation (BiFC) assay. MARCH8 did not reduce the GP expression or affect the GP modification by high-mannose N-glycans in the endoplasmic reticulum (ER), but it inhibited the formation of complex N-glycans on the GP in the Golgi. Additionally, the GP O-glycosylation and furin-mediated proteolytic cleavage were also inhibited. Moreover, we identified a novel furin cleavage site on EBOV GP and found that only those fully glycosylated GPs were processed by furin and incorporated into virions. Furthermore, the GP shedding and secretion were all blocked by MARCH8. MARCH8 also blocked the furin-mediated cleavage of HIV-1 Env (gp160) and the highly pathogenic avian influenza virus H5N1 hemagglutinin (HA). We conclude that MARCH8 has a very broad antiviral activity by prohibiting different viral fusion proteins from glycosylation and proteolytic cleavage in the Golgi, which inhibits their transport from the Golgi to the plasma membrane and incorporation into virions.IMPORTANCE Enveloped viruses express three classes of fusion proteins that are required for their entry into host cells via mediating virus and cell membrane fusion. Class I fusion proteins are produced from influenza viruses, retroviruses, Ebola viruses, and coronaviruses. They are first synthesized as a type I transmembrane polypeptide precursor that is subsequently glycosylated and oligomerized. Most of these precursors are cleaved en route to the plasma membrane by a cellular protease furin in the late secretory pathway, generating the trimeric N-terminal receptor-binding and C-terminal fusion subunits. Here, we show that a cellular protein, MARCH8, specifically inhibits the furin-mediated cleavage of EBOV GP, HIV-1 Env, and H5N1 HA. Further analyses uncovered that MARCH8 blocked the EBOV GP glycosylation in the Golgi and inhibited its transport from the Golgi to the plasma membrane. Thus, MARCH8 has a very broad antiviral activity by specifically inactivating different viral fusion proteins.

The adaptability of H9N2 avian influenza A virus to humans: A comparative docking simulation study.

Influenza A virus, the H9N2 subtype, is an avian influenza virus that has long been circulating in the worldwide poultry industry and is occasionally found to be transmissible to humans. Evidence from genomic analysis suggests that H9N2 provides the genes for the H5N1 and H7N9 subtypes, which have been found to infect mammals and pose a threat to human health. However, due to the lack of a structural model of the interaction between H9N2 and host cells, the mechanism of the extensive adaptability and strong transformation capacity of H9N2 is not fully understood. In this paper, we collected 40 representative H9N2 virus samples reported recently, mainly in China and neighboring countries, and investigated the interactions between H9N2 hemagglutinin and the mammalian receptor, the polysaccharide α-2,6-linked lactoseries tetrasaccharide c, at the atomic level using docking simulation tools. We categorized the mutations of studied H9N2 hemagglutinin according to their effects on ligand-binding interactions and the phylogenetic analysis. The calculations indicated that all the studied H9N2 viruses can establish a tight binding with LSTc although the mutations caused a variety of perturbations to the local conformation of the binding pocket. Our calculations suggested that a marginal equilibrium is established between the conservative ligand-receptor interaction and the conformational dynamics of the binding pocket, and it might be this equilibrium that allows the virus to accommodate mutations to adapt to a variety of environments. Our results provided a way to understand the adaptive mechanisms of H9N2 viruses, which may help predict its propensity to spread in mammals.

Structure of avian influenza hemagglutinin in complex with a small molecule entry inhibitor.

HA plays a critical role in influenza infection and, thus HA is a potential target for antivirals. Recently, our laboratories have described a novel fusion inhibitor, termed CBS1117, with EC50 ∼3 µM against group 1 HA. In this work, we characterize the binding properties of CBS1117 to avian H5 HA by x-ray crystallography, NMR, and mutagenesis. The x-ray structure of the complex shows that the compound binds near the HA fusion peptide, a region that plays a critical role in HA-mediated fusion. NMR studies demonstrate binding of CBS1117 to H5 HA in solution and show extensive hydrophobic contacts between the compound and HA surface. Mutagenesis studies further support the location of the compound binding site proximal to the HA fusion peptide and identify additional amino acids that are important to compound binding. Together, this work gives new insights into the CBS1117 mechanism of action and can be exploited to further optimize this compound and better understand the group specific activity of small-molecule inhibitors of HA-mediated entry.

Structures of the MHC-I molecule BF2*1501 disclose the preferred presentation of an H5N1 virus-derived epitope.

Lethal infections by strains of the highly-pathogenic avian influenza virus (HPAIV) H5N1 pose serious threats to both the poultry industry and public health worldwide. A lack of confirmed HPAIV epitopes recognized by cytotoxic T lymphocytes (CTLs) has hindered the utilization of CD8+ T-cell-mediated immunity and has precluded the development of effectively diversified epitope-based vaccination approaches. In particular, an HPAIV H5N1 CTL-recognized epitope based on the peptide MHC-I-ß2m (pMHC-I) complex has not yet been designed. Here, screening a collection of selected peptides of several HPAIV strains against a specific pathogen-free pMHC-I (pBF2*1501), we identified a highly-conserved HPAIV H5N1 CTL epitope, named HPAIV-PA123-130 We determined the structure of the BF2*1501-PA123-130 complex at 2.1 Å resolution to elucidate the molecular mechanisms of a preferential presentation of the highly-conserved PA123-130 epitope in the chicken B15 lineage. Conformational characteristics of the PA123-130 epitope with a protruding Tyr-7 residue indicated that this epitope has great potential to be recognized by specific TCRs. Moreover, significantly increased numbers of CD8+ T cells specific for the HPAIV-PA123-130 epitope in peptide-immunized chickens indicated that a repertoire of CD8+ T cells can specifically respond to this epitope. We anticipate that the identification and structural characterization of the PA123-130 epitope reported here could enable further studies of CTL immunity against HPAIV H5N1. Such studies may aid in the development of vaccine development strategies using well-conserved internal viral antigens in chickens.

Optimization of 4-Aminopiperidines as Inhibitors of Influenza A Viral Entry That Are Synergistic with Oseltamivir.

Vaccination is the most prevalent prophylactic means for controlling seasonal influenza infections. However, an effective vaccine usually takes at least 6 months to develop for the circulating strains. Therefore, new therapeutic options are needed for the acute treatment of influenza infections to control this virus and prevent epidemics/pandemics from developing. We have discovered fast-acting, orally bioavailable acylated 4-aminopiperidines with an effective mechanism of action targeting viral hemagglutinin (HA). Our data show that these compounds are potent entry inhibitors of influenza A viruses. We present docking studies that suggest an HA binding site for these inhibitors on H5N1. Compound 16 displayed a significant decrease of viral titer when evaluated in the infectious assays with influenza virus H1N1 (A/Puerto Rico/8/1934) or H5N1 (A/Vietnam/1203/2004) strains and the oseltamivir-resistant strain with the most common H274Y mutation. In addition, compound 16 showed significant synergistic activity with oseltamivir in vitro.

Iridoid, phenylethanoid and flavonoid glycosides from Forsythia suspensa.

As part of our continuing efforts to explore bioactive compounds from natural resources, a new iridoid glycoside, adoxosidic acid-6'-oleuroperic ester (1), together with one known phenylethanoid glycoside (2) and two known flavonoid glycosides (3-4) were isolated from the fruit of Forsythia suspensa. The structure of the new compound (1) was elucidated through 1D and 2D NMR spectroscopic data and HR-ESIMS. Interestingly, compound 1 was a monoterpene ester of one iridoid glycoside. Compounds 2-4 were identified as calceolarioside A (2), kaempferol-3-O-rutinoside (3), kampferol-3-O-robinobioside (4) on the basis of NMR spectroscopic data analyses and comparison with the data reported in the literature. The antiviral activity aganisist influenza A (H5N1) virus of compound 1 was studied as well.

A conserved histidine in Group-1 influenza subtype hemagglutinin proteins is essential for membrane fusion activity.

Influenza A viruses enter host cells through the endocytic pathway, where acidification triggers conformational changes of the viral hemagglutinin (HA) to drive membrane fusion. During this process, the HA fusion peptide is extruded from its buried position in the neutral pH structure and targeted to the endosomal membrane. Conserved ionizable residues near the fusion peptide may play a role in initiating these structural rearrangements. We targeted highly conserved histidine residues in this region, at HA1 position 17 of Group-2 HA subtypes and HA2 position 111 of Group-1 HA subtypes, to determine their role in fusion activity. WT and mutant HA proteins representing several subtypes were expressed and characterized, revealing that His 111 is essential for HA functional activity of Group-1 subtypes, supporting continued efforts to target this region of the HA structure for vaccination strategies and the design of antiviral compounds.

Molecular dynamics simulation studies on influenza A virus H5N1 complexed with sialic acid and fluorinated sialic acid.

Abbreviations HA Hemagglutinin MD Molecular Dynamics MM-PBSA Molecular Mechanics Poisson-Boltzmann Surface Area NA Neuraminidase NAMD Nanoscale Molecular Dynamic Simulation PMEMD Particle Mesh Ewald Molecular Dynamics RMSD Root-Mean-Square Deviation RMSF Root-Mean-Square Fluctuation SIA sialic acid VMD Visual Molecular Dynamics Communicated by Ramaswamy H. Sarma.

Impact of tetramerization on the ligand recognition of N1 influenza neuraminidase via MMGBSA approach.

Influenza virus neuraminidase (NA) is a homotetrameric surface protein that, in contrast to other non-influenza NAs, requires a quaternary assembly to exhibit enzymatic activity, suggesting that the oligomeric state significantly impacts the active site of influenza NA. Nevertheless, most structure-based drug design studies have been reported by employing the monomeric state in the closed or open-loop due to the computational cost of employing the tetrameric NA. In this work, we present MD simulations coupled to the MMGBSA approach of avian N1 type NA in its monomeric and tetrameric closed and open-loop state both with and without the inhibitor oseltamivir and its natural substrate, sialic acid. Structural and energetic analyses revealed that the tetrameric state impacts flexibility as well as the map of interactions participating in stabilizing the protein-ligand complexes with respect to the monomeric state. It was observed that the tetrameric state exerts dissimilar effects in binding affinity, characteristic of positive and negative cooperativity for oseltamivir and sialic acid, respectively. Based on our results, to perform a confident structure-based drug design, as well as to evaluate the impact of key mutations through MD simulations, it is important to consider the tetrameric state closed-loop state.

Pandemic Avian Influenza and Intra/Interhaemagglutinin Subtype Electrostatic Variation among Viruses Isolated from Avian, Mammalian, and Human Hosts.

Host jump can result in deadly pandemic events when avian influenza A viruses broaden their host specificity and become able to infect mammals, including humans. Haemagglutinin-the major capsid protein in influenza A viruses-is subjected to high rate mutations, of which several occur at its "head": the receptor-binding domain that mediates specific binding to host cell receptors. Such surface-changing mutations may lead to antigenically novel influenza A viruses hence in pandemics by host jump and in vaccine escape by antigenic drift. Changes in haemagglutinin surface electrostatics have been recently associated with antigenic drift and with clades evolution and spreading in H5N1 and H9N2 viruses. We performed a comparative analysis of haemagglutinin surface electrostatics to investigate clustering and eventual fingerprints among representative pandemic (H5 and H7) and nonpandemic (H4 and H6) avian influenza viral subtypes. We observed preferential sorting of viruses isolated from mammalian/human hosts among these electrostatic clusters of a subtype; however, sorting was not "100% specific" to the different clusters. Therefore, electrostatic fingerprints can help in understanding, but they cannot explain alone the host jumping mechanism.

Antigenic characterization of highly pathogenic avian influenza A(H5N1) viruses with chicken and ferret antisera reveals clade-dependent variation in hemagglutination inhibition profiles.

Highly pathogenic avian influenza (HPAI) A(H5N1) viruses pose a significant economic burden to the poultry industry worldwide and have pandemic potential. Poultry vaccination against HPAI A(H5N1) viruses has been an important component of HPAI control measures and has been performed in Vietnam since 2005. To systematically assess antigenic matching of current vaccines to circulating field variants, we produced a panel of chicken and ferret antisera raised against historical and contemporary Vietnamese reference viruses representing clade variants that were detected between 2001 and 2014. The antisera were used for hemagglutination inhibition (HI) assays to generate data sets for analysis by antigenic cartography, allowing for a direct comparison of results from chicken or ferret antisera. HI antigenic maps, developed with antisera from both hosts, revealed varying patterns of antigenic relationships and clustering of viruses that were dependent on the clade of viruses analyzed. Antigenic relationships between existing poultry vaccines and circulating field viruses were also aligned with in vivo protection profiles determined by previously reported vaccine challenge studies. Our results establish the feasibility and utility of HPAI A(H5N1) antigenic characterization using chicken antisera and support further experimental and modeling studies to investigate quantitative relationships between genetic variation, antigenic drift and correlates of poultry vaccine protection in vivo.

Characterisation of the antigenic epitopes in the subunit 2 haemagglutinin of avian influenza virus H5N1.

Monitoring avian influenza (AI) infection and detecting silent infection in vaccinated chickens has been challenging due to the lack of effective serological diagnostic assays to differentiate between vaccinated and infected animals. Very few studies have identified suitable proteins in AI virus that can be used in successfully differentiating infected from vaccinated animals (DIVA). An HA2 peptide: HA2 position 197-201 (HA position 488-516) described by Khurana et al. (J Virol 85(23):12455-12463, 2011), was shown to have DIVA ability by differentiating H5N1-infected human sera in ELISA. In order to explore the capacity of the HA2 protein, as a DIVA reagent in chickens, four overlapping recombinant HA2 proteins, were expressed in E. coli and tested for reaction with H5N1 sera obtained from infected and vaccinated chickens. Recombinant protein HA2_B2 (380-461) was able to generate a detectable reaction with both H5N1 infected and vaccinated chicken sera but recombinant protein HA2_B4 (483-565) reacted strongly only with sera obtained from chickens infected with live virus, confirming its suitability as a DIVA antigen. Further analysis of the HA2 using several overlapping peptides suggested that positions 380-461 and 483-565 were antigenic in mouse and chicken. This study, for the first time, identified novel antigenic epitopes on the H5N1 HA2 subunit. Two epitopes, found in the HA2 ectodomain, have never been described for AIV infection in any animal species. Also one HA2 epitope was found to have high potential as a DIVA antigen.

Amino acid substitutions affecting aspartic acid 605 and valine 606 decrease the interaction strength between the influenza virus RNA polymerase PB2 '627' domain and the viral nucleoprotein.

The influenza virus RNA genome is transcribed and replicated in the context of the viral ribonucleoprotein (vRNP) complex by the viral RNA polymerase. The nucleoprotein (NP) is the structural component of the vRNP providing a scaffold for the viral RNA. In the vRNP as well as during transcription and replication the viral polymerase interacts with NP but it is unclear which parts of the polymerase and NP mediate these interactions. Previously the C-terminal '627' domain (amino acids 538-693) of PB2 was shown to interact with NP. Here we report that a fragment encompassing amino acids 146-185 of NP is sufficient to mediate this interaction. Using NMR chemical shift perturbation assays we show that amino acid region 601 to 607 of the PB2 '627' domain interacts with this fragment of NP. Substitutions of these PB2 amino acids resulted in diminished RNP activity and surface plasmon resonance assays showed that amino acids D605 was essential for the interaction with NP and V606 may also play a partial role in the interaction. Collectively these results reveal a possible interaction surface between NP and the PB2 subunit of the RNA polymerase complex.

pH Optimum of Hemagglutinin-Mediated Membrane Fusion Determines Sensitivity of Influenza A Viruses to the Interferon-Induced Antiviral State and IFITMs.

The replication and pathogenicity of influenza A viruses (IAVs) critically depend on their ability to tolerate the antiviral interferon (IFN) response. To determine a potential role for the IAV hemagglutinin (HA) in viral sensitivity to IFN, we studied the restriction of IAV infection in IFN-ß-treated human epithelial cells by using 2:6 recombinant IAVs that shared six gene segments of A/Puerto Rico/8/1934 virus (PR8) and contained HAs and neuraminidases of representative avian, human, and zoonotic H5N1 and H7N9 viruses. In A549 and Calu-3 cells, viruses displaying a higher pH optimum of HA-mediated membrane fusion, H5N1-PR8 and H7N9-PR8, were less sensitive to the IFN-induced antiviral state than their counterparts with HAs from duck and human viruses, which fused at a lower pH. The association between a high pH optimum of fusion and reduced IFN sensitivity was confirmed by using HA point mutants of A/Hong Kong/1/1968-PR8 that differed solely by their fusion properties. Furthermore, similar effects of the viral fusion pH on IFN sensitivity were observed in experiments with (i) primary human type II alveolar epithelial cells and differentiated cultures of human airway epithelial cells, (ii) nonrecombinant zoonotic and pandemic IAVs, and (iii) preparations of IFN-α and IFN-λ1. A higher pH of membrane fusion and reduced sensitivity to IFN correlated with lower restriction of the viruses in MDCK cells stably expressing the IFN-inducible transmembrane proteins IFITM2 and IFITM3, which are known to inhibit viral fusion. Our results reveal that the pH optimum of HA-driven membrane fusion of IAVs is a determinant of their sensitivity to IFN and IFITM proteins.IMPORTANCE The IFN system constitutes an important innate defense against viral infection. Substantial information is available on how IAVs avoid detection by sensors of the IFN system and disable IFN signaling pathways. Much less is known about the ability of IAVs to tolerate the antiviral activity of IFN-induced cellular proteins. The IFN-induced proteins of the IFITM family block IAV entry into target cells and can restrict viral spread and pathogenicity. Here we show for the first time that the sensitivity of IAVs to the IFN-induced antiviral state and IFITM2 and IFITM3 proteins depends on the pH value at which the viral HA undergoes a conformational transition and mediates membrane fusion. Our data imply that the high pH optimum of membrane fusion typical of zoonotic IAVs of gallinaceous poultry, such as H5N1 and H7N9, may contribute to their enhanced virulence in humans.

Genetic Predisposition To Acquire a Polybasic Cleavage Site for Highly Pathogenic Avian Influenza Virus Hemagglutinin.

Highly pathogenic avian influenza viruses with H5 and H7 hemagglutinin (HA) subtypes evolve from low-pathogenic precursors through the acquisition of multiple basic amino acid residues at the HA cleavage site. Although this mechanism has been observed to occur naturally only in these HA subtypes, little is known about the genetic basis for the acquisition of the polybasic HA cleavage site. Here we show that consecutive adenine residues and a stem-loop structure, which are frequently found in the viral RNA region encoding amino acids around the cleavage site of low-pathogenic H5 and H7 viruses isolated from waterfowl reservoirs, are important for nucleotide insertions into this RNA region. A reporter assay to detect nontemplated nucleotide insertions and deep-sequencing analysis of viral RNAs revealed that an increased number of adenine residues and enlarged stem-loop structure in the RNA region accelerated the multiple adenine and/or guanine insertions required to create codons for basic amino acids. Interestingly, nucleotide insertions associated with the HA cleavage site motif were not observed principally in the viral RNA of other subtypes tested (H1, H2, H3, and H4). Our findings suggest that the RNA editing-like activity is the key mechanism for nucleotide insertions, providing a clue as to why the acquisition of the polybasic HA cleavage site is restricted to the particular HA subtypes.IMPORTANCE Influenza A viruses are divided into subtypes based on the antigenicity of the viral surface glycoproteins hemagglutinin (HA) and neuraminidase. Of the 16 HA subtypes (H1 to -16) maintained in waterfowl reservoirs of influenza A viruses, H5 and H7 viruses often become highly pathogenic through the acquisition of multiple basic amino acid residues at the HA cleavage site. Although this mechanism has been known since the 1980s, the genetic basis for nucleotide insertions has remained unclear. This study shows the potential role of the viral RNA secondary structure for nucleotide insertions and demonstrates a key mechanism explaining why the acquisition of the polybasic HA cleavage site is restricted to particular HA subtypes in nature. Our findings will contribute to better understanding of the ecology of influenza A viruses and will also be useful for the development of genetically modified vaccines against H5 and H7 influenza A viruses with increased stability.