It is not just AIV: from avian to swine-origin influenza virus

Implications of protein conformations to modifying novel inhibitor Oseltamivir for 2009 H1N1 influenza A virus by simulation and docking studies

Recently three FDA approved existing drugs, namely-Oseltamivir, Peramivir and Zanamivir, used against Neuraminidase (NA) for the inhibitory effect on the process of viral progeny release to inhibit infection. All NA subtypes has been divided into two groups (Group 1 and Group 2) based on phylogenetic study. Oseltamivir and Zanamivir drugs are designed for Group 2 NA but are also used against 2009 H1N1 NA that lies in Group 1. There is no specific drug available for H1N1 and, consequently, there is an urgent requirement for the same. The structure-based drug design and fragment-based drug design methods are used for building more effective and economic drug molecules. In this work, the fragment-based drug development followed by fragment evolution on the basis of protein conformations after every 10 ns of 100 ns simulation. There are two analogs of Oseltamivir acid drug discovered in this study. Only analog 1, along with Oseltamivir acid, were then docked with the native protein. The analog 1 (benzoic acid inhibitor 11) exhibited higher binding affinity value of - 10.70 kcal/mol in comparison to its predecessor. The concept of conformations and protein-ligand interactions can be useful in designing new drugs for H1N1 with high specific binding.

Hemagglutinin-specific CD4+ T-cell responses following 2009-pH1N1 inactivated split-vaccine inoculation in humans

Influenza A virus remains a major threat to public health, and the inactivated split-virus vaccine is the most prevalent vaccine used worldwide. However, our knowledge about cellular immune responses to the inactivated influenza virus vaccine and its correlation with humoral responses are yet limited, which has restricted our understanding of the vaccine's protective mechanisms. Herein, in two clinical trials, T-cell responses specific for both previously identified human leucocyte antigen (HLA)-I-restricted epitopes from influenza virus and hemagglutinin (HA) protein were longitudinally investigated before, during, and after a two-dose vaccination with the inactivated 2009 pandemic H1N1 (2009-pH1N1) vaccine. A robust antibody response in all of the donors after vaccination was observed. Though no CD8⁺ T-cell responses to known epitopes were detected, HA-specific T-cell responses were primed following vaccination, and the responses were found to be mainly CD4⁺ T-cell dependent. However, HA-specific T-cells circulating in peripheral blood dropped to baseline levels 6weeks after vaccination, but humoral immune responses maintained a high level for 4months post-vaccination. Significant correlations between the magnitude of the HA-specific T-cell responses and hemagglutination inhibition antibody titers were demonstrated, indicating a priming role of HA-specific T-cells for humoral immune responses. In conclusion, our study indicates that HA-specific CD4⁺ T-cell responses can be primed by the inactivated 2009-pH1N1 vaccine, which may coordinate with the elicitation of antibody protection. These findings would benefit a better understanding of the immune protective mechanisms of the widely used inactivated 2009-pH1N1 vaccine.

Altered viral replication and cell responses by inserting microRNA recognition element into PB1 in pandemic influenza A virus (H1N1) 2009

Objective. MicroRNAs (miRNAs) are endogenous noncoding RNAs that spatiotemporally modulate mRNAs in a posttranscriptional manner. Engineering mutant viruses by inserting cell-specific miRNA recognition element (MRE) into viral genome may alter viral infectivity and host responses in vital tissues and organs infected with pandemic influenza A virus (H1N1) 2009 (H1N1pdm). Methods. In this study, we employed reverse genetics approach to generate a recombinant H1N1pdm with a cell-specific miRNA target sequence inserted into its PB1 genomic segment to investigate whether miRNAs are able to suppress H1N1pdm replication. We inserted an MRE of microRNA-let-7b (miR-let-7b) into the open reading frame of PB1 to test the feasibility of creating a cell-restricted H1N1pdm virus since let-7b is abundant in human bronchial epithelial cells. Results. miR-let-7b is rich in human bronchial epithelial cells (HBE). Incorporation of the miR-let-7b-MRE confers upon the recombinant H1N1pdm virus susceptibility to miR-let-7b targeting, suggesting that the H1N1pdm and influenza A viruses can be engineered to exert the desired replication restrictive effect and decrease infectivity in vital tissues and organs. Conclusions. This approach provides an additional layer of biosafety and thus has great potential for the application in the rational development of safer and more effective influenza viral vaccines.

Induced opening of influenza virus neuraminidase N2 150-loop suggests an important role in inhibitor binding

The recently discovered 150-cavity (formed by loop residues 147–152, N2 numbering) adjacent to the enzymatic active site of group 1 influenza A neuraminidase (NA) has introduced a novel target for the design of next-generation NA inhibitors. However, only group 1 NAs, with the exception of the 2009 pandemic H1N1 NA, possess a 150-cavity, and no 150-cavity has been observed in group 2 NAs. The role of the 150-cavity played in enzymatic activity and inhibitor binding is not well understood. Here, we demonstrate for the first time that oseltamivir carboxylate can induce opening of the rigid closed N2 150-loop and provide a novel mechanism for 150-loop movement using molecular dynamics simulations. Our results provide the structural and biophysical basis of the open form of 150-loop and illustrates that the inherent flexibility and the ligand induced flexibility of the 150-loop should be taken into consideration for future drug design.

Influenza neuraminidase operates via a nucleophilic mechanism and can be targeted by covalent inhibitors

Development of novel influenza neuraminidase inhibitors is critical for preparedness against influenza outbreaks. Knowledge of the neuraminidase enzymatic mechanism and transition-state analogue, 2-deoxy-2,3-didehydro-N-acetylneuraminic acid, contributed to the development of the first generation anti-neuraminidase drugs, zanamivir and oseltamivir. However, lack of evidence regarding influenza neuraminidase key catalytic residues has limited strategies for novel neuraminidase inhibitor design. Here, we confirm that influenza neuraminidase conserved Tyr406 is the key catalytic residue that may function as a nucleophile; thus, mechanism-based covalent inhibition of influenza neuraminidase was conceived. Crystallographic studies reveal that 2α,3ax-difluoro-N-acetylneuraminic acid forms a covalent bond with influenza neuraminidase Tyr406 and the compound was found to possess potent anti-influenza activity against both influenza A and B viruses. Our results address many unanswered questions about the influenza neuraminidase catalytic mechanism and demonstrate that covalent inhibition of influenza neuraminidase is a promising and novel strategy for the development of next-generation influenza drugs.

Structure and receptor binding specificity of hemagglutinin H13 from avian influenza A virus H13N6

Interspecies transmission (host switching/jumping) of influenza viruses is a key scientific question that must be addressed. In addition to the vigorous research on highly pathogenic avian influenza viruses (HPAIVs), studies of the mechanism of interspecies transmission of low-pathogenic avian influenza viruses (LPAIVs) could also provide insights into host tropism and virulence evolution. Influenza A viruses harboring hemagglutinin (HA) H13 (e.g., H13N6) are LPAIVs. In this study, soluble H13 HA glycoprotein was purified, and its receptor binding activity was characterized. The results revealed that H13 exclusively binds the avian α2-3-linked sialic acid receptor; no binding to the mammalian α2-6-linked sialic acid receptor was detected. Furthermore, the molecular basis of the H13 receptor binding specificity was revealed by comparative analysis of the crystal structures of both receptor-bound H13 and H5 HAs, which might be contributed by the hydrophobic residue V186. Work with an H13N186 mutant confirmed the importance of V186 in the receptor binding specificity of H13 HA, which shows that the mutant protein reduced the binding of an avian receptor analog but increased the binding of a human receptor analog. Detailed structural analysis also demonstrated that the conserved binding sites of the recently well-studied broadly neutralizing human monoclonal antibodies targeting the HA2 domain are found in H13. Our results expand our understanding of virulence evolution, receptor binding preference, and species tropism of the LPAIVs and HPAIVs.

Molecular basis of the receptor binding specificity switch of the hemagglutinins from both the 1918 and 2009 pandemic influenza A viruses by a D225G substitution

Influenza A virus uses sialic acids as cell entry receptors, and there are two main receptor forms, α2,6 linkage or α2,3 linkage to galactose, that determine virus host ranges (mammalian or avian). The receptor binding hemagglutinins (HAs) of both 1918 and 2009 pandemic H1N1 (18H1 and 09H1, respectively) influenza A viruses preferentially bind to the human α2,6 linkage receptor. A single D225G mutation in both H1s switches receptor binding specificity from α2,6 linkage binding to dual receptor binding. However, the molecular basis for this specificity switch is not fully understood. Here, we show via H1-ligand complex structures that the D225G substitution results in a loss of a salt bridge between amino acids D225 and K222, enabling the key residue Q226 to interact with the avian receptor, thereby obtaining dual receptor binding. This is further confirmed by a D225E mutant that retains human receptor binding specificity with the salt bridge intact.

Cross-allele cytotoxic T lymphocyte responses against 2009 pandemic H1N1 influenza A virus among HLA-A24 and HLA-A3 supertype-positive individuals

Lack of a universal vaccine against all serotypes of influenza A viruses and recent progress on T cell-related vaccines against influenza A virus illuminate the important role of human leukocyte antigen (HLA)-restricted cytotoxic T lymphocytes (CTLs) in anti-influenza virus immunity. However, the diverse HLA alleles among humans complicate virus-specific cellular immunity research, and elucidation of cross-HLA allele T cell responses to influenza virus specificity requires further detailed work. An ideal CTL epitope-based vaccine would cover a broad spectrum of epitope antigens presented by most, if not all, of the HLAs. Here, we evaluated the 2009 pandemic influenza A (H1N1) virus-specific T cell responses among the HLA-A24(+) population using a rationally designed peptide pool during the 2009 pandemic. Unexpectedly, cross-HLA allele T cell responses against the influenza A virus peptides were detected among both HLA-A11(+) and HLA-A24(+) donors. Furthermore, we found cross-responses in the entire HLA-A3 supertype population (including HLA-A11, -A31, -A33, and -A30). The cross-allele antigenic peptides within the peptide pool were identified and characterized, and the crystal structures of the major histocompatibility complex (MHC)-peptide complexes were determined. The subsequent HLA-A24-defined cross-allele peptides recognized by the HLA-A11(+) population were shown to mildly bind to the HLA-A*1101 molecule. Together with the structural models, these results partially explain the cross-allele responses. Our findings elucidate the promiscuity of the cross-allele T cell responses against influenza A viruses and are beneficial for the development of a T cell epitope-based vaccine applied in a broader population.

Structural and functional characterization of neuraminidase-like molecule N10 derived from bat influenza A virus

The recent discovery of the unique genome of influenza virus H17N10 in bats raises considerable doubt about the origin and evolution of influenza A viruses. It also identifies a neuraminidase (NA)-like protein, N10, that is highly divergent from the nine other well-established serotypes of influenza A NA (N1-N9). The structural elucidation and functional characterization of influenza NAs have illustrated the complexity of NA structures, thus raising a key question as to whether N10 has a special structure and function. Here the crystal structure of N10, derived from influenza virus A/little yellow-shouldered bat/Guatemala/153/2009 (H17N10), was solved at a resolution of 2.20 Å. Overall, the structure of N10 was found to be similar to that of the other known influenza NA structures. In vitro enzymatic assays demonstrated that N10 lacks canonical NA activity. A detailed structural analysis revealed dramatic alterations of the conserved active site residues that are unfavorable for the binding and cleavage of terminally linked sialic acid receptors. Furthermore, an unusual 150-loop (residues 147-152) was observed to participate in the intermolecular polar interactions between adjacent N10 molecules of the N10 tetramer. Our study of influenza N10 provides insight into the structure and function of the sialidase superfamily and sheds light on the molecular mechanism of bat influenza virus infection.

Insights into avian influenza virus pathogenicity: the hemagglutinin precursor HA0 of subtype H16 has an alpha-helix structure in its cleavage site with inefficient HA1/HA2 cleavage

With a new serotype (H17) of hemagglutinin (HA) recently being discovered, there are now 17 serotypes (H1 to H17) of influenza A viruses in total. It is believed that HA is initially expressed as a precursor of HA0 and then cleaved into HA1 and HA2, forming a disulfide bond-linked complex, for its full function. Structural data show that a loop structure exists in the cleavage site between HA1 and HA2, and this flexible loop is crucial for the efficient cleavage of HA0. Here, the crystal structures of H16 (a low-pathogenicity avian influenza virus) in their HA0 form (H16HA0) have been solved at 1.7-Å and 2.0-Å resolutions. To our surprise, an α-helix element in the cleavage site which inserts into the negatively charged cavity with the key residue R329 hidden behind the helix was observed. In vitro trypsin cleavage experiments demonstrated inefficient cleavage of H16HA0 under both neutral and low-pH conditions. The results provide new insights into influenza A virus pathogenicity; both the relatively stable α-helix structure in the flexible cleavage loop and inaccessibility of the cleavage site likely contribute to the low pathogenicity of avian influenza A virus. Furthermore, compared to all of the HAs whose structures have been solved, H16 is a good reference for assigning the HA subtypes into two groups on the basis of the three-dimensional structure, which is consistent with the phylogenetic grouping. We conclude that in light of the current H16HA0 structure, the natural α-helix element might provide a new opportunity for influenza virus inhibitor design.

IL-17 response mediates acute lung injury induced by the 2009 pandemic influenza A (H1N1) virus

The 2009 flu pandemic involved the emergence of a new strain of a swine-origin H1N1 influenza virus (S-OIV H1N1) that infected almost every country in the world. Most infections resulted in respiratory illness and some severe cases resulted in acute lung injury. In this report, we are the first to describe a mouse model of S-OIV virus infection with acute lung injury and immune responses that reflect human clinical disease. The clinical efficacy of the antiviral oseltamivir (Tamiflu) administered in the early stages of S-OIV H1N1 infection was confirmed in the mouse model. Moreover, elevated levels of IL-17, Th-17 mediators and IL-17-responsive cytokines were found in serum samples of S-OIV-infected patients in Beijing. IL-17 deficiency or treatment with monoclonal antibodies against IL-17-ameliorated acute lung injury induced by the S-OIV H1N1 virus in mice. These results suggest that IL-17 plays an important role in S-OIV-induced acute lung injury and that monoclonal antibodies against IL-17 could be useful as a potential therapeutic remedy for future S-OIV H1N1 pandemics.

Structural and functional analysis of laninamivir and its octanoate prodrug reveals group specific mechanisms for influenza NA inhibition

The 2009 H1N1 influenza pandemic (pH1N1) led to record sales of neuraminidase (NA) inhibitors, which has contributed significantly to the recent increase in oseltamivir-resistant viruses. Therefore, development and careful evaluation of novel NA inhibitors is of great interest. Recently, a highly potent NA inhibitor, laninamivir, has been approved for use in Japan. Laninamivir is effective using a single inhaled dose via its octanoate prodrug (CS-8958) and has been demonstrated to be effective against oseltamivir-resistant NA in vitro. However, effectiveness of laninamivir octanoate prodrug against oseltamivir-resistant influenza infection in adults has not been demonstrated. NA is classified into 2 groups based upon phylogenetic analysis and it is becoming clear that each group has some distinct structural features. Recently, we found that pH1N1 N1 NA (p09N1) is an atypical group 1 NA with some group 2-like features in its active site (lack of a 150-cavity). Furthermore, it has been reported that certain oseltamivir-resistant substitutions in the NA active site are group 1 specific. In order to comprehensively evaluate the effectiveness of laninamivir, we utilized recombinant N5 (typical group 1), p09N1 (atypical group 1) and N2 from the 1957 pandemic H2N2 (p57N2) (typical group 2) to carry out in vitro inhibition assays. We found that laninamivir and its octanoate prodrug display group specific preferences to different influenza NAs and provide the structural basis of their specific action based upon their novel complex crystal structures. Our results indicate that laninamivir and zanamivir are more effective against group 1 NA with a 150-cavity than group 2 NA with no 150-cavity. Furthermore, we have found that the laninamivir octanoate prodrug has a unique binding mode in p09N1 that is different from that of group 2 p57N2, but with some similarities to NA-oseltamivir binding, which provides additional insight into group specific differences of oseltamivir binding and resistance.

The influenza neuraminidase (NA) enzyme is the most successful drug target against the seasonal and pandemic flu. The 2009 H1N1 flu pandemic led to record sales of the NA inhibitors oseltamivir (Tamiflu) and zanamivir (Relenza). Recently, a new drug, laninamivir (Inavir), has been approved for use in Japan can also be administered effectively using a single dose via its octanoate prodrug (CS-8958), however its effectiveness against oseltamivir-resistant influenza infection has not been demonstrated in clinical studies. In this study we comprehensively evaluate the effectiveness of laninamivir and its prodrug using NA from different groups with different active site features. We expressed and purified a group 2 NA from the 1957 pandemic H2N2, an atypical group 1 NA from the 2009 H1N1 pandemic and a group 1 NA from avian H12N5. NA inhibition was assayed and NAs were further crystallized with each inhibitor to determine the structural basis of their action. We found that laninamivir inhibition is highly potent for each NA, however binding and inhibition of laninamivir and its prodrug showed group specific preferences. Our results provide the structural and functional basis of NA inhibition using classical and novel inhibitors, with NAs from multiple serotypes with different properties.

Na(+)/K (+)-ATPase β1 subunit interacts with M2 proteins of influenza A and B viruses and affects the virus replication

Interplay between the host and influenza virus has a pivotal role for the outcome of infection. The matrix proteins M2/BM2 from influenza (A and B) viruses are small type III integral membrane proteins with a single transmembrane domain, a short amino-terminal ectodomain and a long carboxy-terminal cytoplasmic domain. They function as proton channels, mainly forming a membrane-spanning pore through the transmembrane domain tetramer, and are essential for virus assembly and release of the viral genetic materials in the endosomal fusion process. However, little is known about the host factors which interact with M2/BM2 proteins and the functions of the long cytoplasmic domain are currently unknown. Starting with yeast two-hybrid screening and applying a series of experiments we identified that the β1 subunit of the host Na(+)/K(+)-ATPase β1 subunit (ATP1B1) interacts with the cytoplasmic domain of both the M2 and BM2 proteins. A stable ATP1B1 knockdown MDCK cell line was established and we showed that the ATP1B1 knockdown suppressed influenza virus A/WSN/33 replication, implying that the interaction is crucial for influenza virus replication in the host cell. We propose that influenza virus M2/BM2 cytoplasmic domain has an important role in the virus-host interplay and facilitates virus replication.

The 2009 pandemic H1N1 neuraminidase N1 lacks the 150-cavity in its active site

Influenza A virus neuraminidase can be classified into groups 1 and 2 on the basis of its primary structure. The main structural feature of group 1 neuraminidase is an extra cavity in the active site, the 150-cavity. Here we present the crystal structure of neuraminidase from the 2009 pandemic H1N1 influenza strain. In contrast to other characterized N1 neuraminidases, which are all members of group 1, 2009 H1N1 neuraminidase does not have a 150-cavity.

In silico characterization of the functional and structural modules of the hemagglutinin protein from the swine-origin influenza virus A (H1N1)-2009

The 2009 swine-origin influenza virus (S-OIV, H1N1 subtype) has developed into a new pandemic influenza as announced by the World Health Organization. In order to uncover clues about the determinants for virulence and pathogenicity of the virus, we characterized the functional modules of the surface glycoprotein hemagglutinin (HA), the most important protein in molecular epidemiology and pathogenesis of influenza viruses. We analyzed receptor binding sites, basic patch, neutralization antibody epitopes and T cell epitopes in the HA protein of the current S-OIV according to the corresponding functional and structural modules previously characterized in other H1 HA molecules or HA molecules of other subtypes. We compared their differences and similarities systematically. Based on the amino acids defined as the functional and structural modules, the HA protein of 2009 S-OIV should specifically bind to the human 2,6-receptor. The D225G/E mutation in HA, which is found in some isolates, may confer dual binding specificity to the 2,3- and 2,6-receptor based on previously reported work. This HA variant contains two basic patches, one of which results in increased basicity, suggesting enhanced membrane fusion function. The 2009 S-OIV HA also has an extra glycosylation site at position 276. Four of the five antibody neutralization epitopes identified in A/RP/8/34(H1N1) were exposed, but the other was hidden by a glycosylation site. The previously identified cytotoxic T cell epitopes in various HA molecules were summarized and their corresponding sequences in 2009 S-OIV HA were defined. These results are critical for understanding the pathogenicity of the virus and host immune response against the virus.

Crystal structure of the swine-origin A (H1N1)-2009 influenza A virus hemagglutinin (HA) reveals similar antigenicity to that of the 1918 pandemic virus

Influenza virus is the causative agent of the seasonal and occasional pandemic flu. The current H1N1 influenza pandemic, announced by the WHO in June 2009, is highly contagious and responsible for global economic losses and fatalities. Although the H1N1 gene segments have three origins in terms of host species, the virus has been named swine-origin influenza virus (S-OIV) due to a predominant swine origin. 2009 S-OIV has been shown to highly resemble the 1918 pandemic virus in many aspects. Hemagglutinin is responsible for the host range and receptor binding of the virus and is therefore a primary indicator for the potential of infection. Primary sequence analysis of the 2009 S-OIV hemagglutinin (HA) reveals its closest relationship to that of the 1918 pandemic influenza virus, however, analysis at the structural level is necessary to critically assess the functional significance. In this report, we report the crystal structure of soluble hemagglutinin H1 (09H1) at 2.9 Å, illustrating that the 09H1 is very similar to the 1918 pandemic HA (18H1) in overall structure and the structural modules, including the five defined antiboby (Ab)-binding epitopes. Our results provide an explanation as to why sera from the survivors of the 1918 pandemics can neutralize the 2009 S-OIV, and people born around the 1918 are resistant to the current pandemic, yet younger generations are more susceptible to the 2009 pandemic.