Determination of cold-adapted influenza virus (Orthomyxoviridae: Alphainfluenzavirus) polymerase activity by the minigenome method with a fluorescent protein.

INTRODUCTION: Polymerase proteins PB1 and PB2 determine the cold-adapted phenotype of the influenza virus A/Krasnodar/101/35/59 (H2N2), as was shown earlier. OBJECTIVE: The development of the reporter construct to determine the activity of viral polymerase at 33 and 37 °C using the minigenome method. MATERIALS AND METHODS: Co-transfection of Cos-1 cells with pHW2000 plasmids expressing viral polymerase proteins PB1, PB2, PA, NP (minigenome) and reporter construct. RESULTS: Based on segment 8, two reporter constructs were created that contain a direct or inverted NS1-GFP-NS2 sequence for the expression of NS2 and NS1 proteins translationally fused with green fluorescent protein (GFP), which allowed the evaluation the transcriptional and/or replicative activity of viral polymerase. CONCLUSION: Polymerase of virus A/Krasnodar/101/35/59 (H2N2) has higher replicative and transcriptional activity at 33 °C than at 37 °C. Its transcriptional activity is more temperature-dependent than its replicative activity. The replicative and transcriptional activity of polymerase A/Puerto Rico/8/34 virus (H1N1, Mount Sinai variant) have no significant differences and do not depend on temperature.

[Latex Agglutination as an Alternative to the Hemagglutination Reaction of Influenza Viruses].

As an alternative to the classical method of erythrocyte hemagglutination, a latex agglutination assay based on the interaction of influenza viruses with the sialoglycoprotein fetuin immobilized on the surface of polystyrene microspheres has been developed. Twelve influenza A virus strains of different subtypes and two influenza B viruses of different lines were tested. Simultaneous titration of viruses using the classical hemagglutination test and the proposed latex agglutination assay showed similar sensitivity and a high degree of correlation (R = 0.94). The obtained microspheres can be used for titration of viruses that recognize and bind sialylated glycans as receptors. In particular, latex aggregation was also induced by the Newcastle disease virus.

[Mathematical model for assessing the level of cross-immunity between strains of influenza virus subtype H3N2].

INTRODUCTION: The WHO regularly updates influenza vaccine recommendations to maximize their match with circulating strains. Nevertheless, the effectiveness of the influenza A vaccine, specifically its H3N2 component, has been low for several seasons. The aim of the study is to develop a mathematical model of cross-immunity based on the array of published WHO hemagglutination inhibition assay (HAI) data. MATERIALS AND METHODS: In this study, a mathematical model was proposed, based on finding, using regression analysis, the dependence of HAI titers on substitutions in antigenic sites of sequences. The computer program we developed can process data (GISAID, NCBI, etc.) and create real-time databases according to the set tasks. RESULTS: Based on our research, an additional antigenic site F was identified. The difference in 1.6 times the adjusted R2, on subsets of viruses grown in cell culture and grown in chicken embryos, demonstrates the validity of our decision to divide the original data array by passage histories. We have introduced the concept of a degree of homology between two arbitrary strains, which takes the value of a function depending on the Hamming distance, and it has been shown that the regression results significantly depend on the choice of function. The provided analysis showed that the most significant antigenic sites are A, B, and E. The obtained results on predicted HAI titers showed a good enough result, comparable to similar work by our colleagues. CONCLUSION: The proposed method could serve as a useful tool for future forecasts, with further study to confirm its sustainability.

[Monoclonal antibodies to hemagglutinin of influenza A/H7N3 virus (Orthomyxoviridae: Alphainfluenzavirus: Influenza A virus)].

INTRODUCTION: Variants of influenza virus A/H7 have the same high pandemic potential as A/H5. However, the information about the antigenic structure of H7 hemagglutinin (НА) is considerably inferior in quantitative terms to similar data for H5 НА.The aims of the study were development and characterization of the monoclonal antibodies (MAbs) panel for HA subtype H7 of the influenza A virus. MATERIAL AND METHODS: Viruses were accumulated in 10-day-old chicken embryos. Purification and concentration of the virus, determination of protein concentration, preparation of MAbs and ascitic fluids, hemagglutination and hemagglutination inhibition (HI) tests, assessment of antibodies' activity in indirect enzyme-linked immunosorbent assay (ELISA), as well as determination of MAbs isotypes and neutralization reaction (NR) were carried out by standard methods. RESULTS: The obtained MAbs to А/mallard/Netherlands/12/2000 (H7N3) strain were studied in HI test with a set of strains of different years of isolation belonging to different evolutionary groups. MAbs had a reduced reactivity compared to the immunogen-virus for all the studied strains. Cross-interaction of MAbs 9E11 and 9G12 in HI test with influenza A/H15 virus has been observed. DISCUSSION: Influenza A agent with H7 HA variant could serve as a potential cause of a future pandemic. Development of the MAbs panel for subtype H7 HA is an urgent task for both veterinary medicine and public health. CONCLUSION: The obtained MAbs can be used not only for epitope mapping of the H7 HA molecule (currently insufficiently studied) and as reagents for diagnostic assays, but also for determining common («universal¼) epitopes in HA of different strains of this subtype.

[The Effect of I155T, K156Q, K156E and N186K Mutations in Hemagglutinin on the Virulence and Reproduction of Influenza A/H5N1 Viruses].

The continued circulation of influenza A virus subtype H5 may cause the emergence of new potential pandemic virus variants, which can be transmitted from person to person. The occurrence of such variants is mainly related to mutations in hemagglutinin (HA). Previously we discovered mutations in H5N1 influenza virus hemagglutinin, which contributes to virus immune evasion. The purpose of this work was to study the role of these mutations in changing other, non-antigenic properties of the virus and the possibility of their maintenance in the viral population. Mutations were introduced into the HA gene of a recombinant H5N1 influenza A virus (VNH5N1-PR8/CDC-RG) using site-specific mutagenesis. The "variant" viruses were investigated and compared with respect to replication kinetics in chicken embryos, thermostability, reproductive activity at different temperatures (33, 37 and 40°C), and virulence for mice. Amino acid substitutions I155T, K156Q, K156E+V138A, N186K led to a decrease in thermal stability, replication activity of the mutant viruses in chicken embryos, and virulence for mice, although these effects differed between the variants. The K156Q and N186K mutations reduced viral reproduction at elevated temperature (40°C). The analysis of the frequency of these mutations in natural isolates of H5N1 influenza viruses indicated that the K156E/Q and N186K mutations have little chance to gain a foothold during evolution, in contrast to the I155T mutation, which is the most responsible for antigenic drift. The A138V and N186K mutations seem to be adaptive in mammalian viruses.

Variability of nonpathogenic influenza virus H5N3 under immune pressure.

Mutations arising in influenza viruses that have undergone immune pressure may promote a successful spread of mutants in nature. In order to evaluate the variability of nonpathogenic influenza virus A/duck/Moscow/4182-C/2010(H5N3) and to determine the common epitopes between it and highly pathogenic H5N1 avian influenza viruses (HPAIV), a set of escape mutants was selected due to action of MABs specific against A/chicken/Pennsylvania/8125/83(H5N2), A/Vietnam/1203/04(H5N1) and A/duck/Novosibirsk/56/05(H5N1) viruses. The complete genomes of escape mutants were sequenced and amino acid point mutations were determined in HA, NA, PA, PB1, PB2, M1, M2, and NP proteins. Comprehensive analysis of the acquired mutations was performed using the Influenza Research Database (https://www.fludb.org) and revealed that all mutations were located inside short linear epitopes, in positions characterized by polymorphisms. Most of the mutations found were characterized as substitutions by predominant or alternative amino acids existing in nature. Antigenic changes depended only on substitutions at positions 126, 129, 131, 145 and 156 of HA (H3 numbering). The positions 126, 145 and 156 were common for HA/H5 of different phylogenetic lineages of H5N1 HPAIV (arisen from A/goose/Guangdong/1/96) and low pathogenic American and Eurasian viruses. Additionally, mutation S145P increased the temperature of HA heat inactivation, compared to wild-type, as was proved by reverse genetics. Moreover, nonpathogenic A/duck/Moscow/4182-C/2010(H5N3) and H5N1 HPAI viruses have the same structure of short linear epitopes in HA (145-157) and internal proteins (PB2: 186-200, 406-411; PB1: 135-143, 538-546; PA: 515-523; NP: 61-68; M1: 76-84; M2: 45-53). These facts may indicate that H5 wild duck nonpathogenic virus could be used as vaccine against H5N1 HPAIV. Keywords: avian influenza virus; H5 hemagglutinin; escape mutants; genetic analysis; phenotypic properties; site-specific mutagenesis.

The Effect of I155T, K156Q, K156E and N186K Mutations in Hemagglutinin on the Virulence and Reproduction of Influenza A/H5N1 Viruses.

The continued circulation of influenza A virus subtype H5 may cause the emergence of new potential pandemic virus variants, which can be transmitted from person to person. The occurrence of such variants is mainly related to mutations in hemagglutinin (HA). Previously we discovered mutations in H5N1 influenza virus hemagglutinin, which contributes to virus immune evasion. The purpose of this work was to study the role of these mutations in changing other, non-antigenic properties of the virus and the possibility of their maintenance in the viral population. Mutations were introduced into the HA gene of a recombinant H5N1 influenza A virus (VNH5N1-PR8/CDC-RG) using site-specific mutagenesis. The "variant" viruses were investigated and compared with respect to replication kinetics in chicken embryos, thermostability, reproductive activity at different temperatures (33, 37 and 40°C), and virulence for mice. Amino acid substitutions I155T, K156Q, K156E+V138A, N186K led to a decrease in thermal stability, replication activity of the mutant viruses in chicken embryos, and virulence for mice, although these effects differed between the variants. The K156Q and N186K mutations reduced viral reproduction at elevated temperature (40°C). The analysis of the frequency of these mutations in natural isolates of H5N1 influenza viruses indicated that the K156E/Q and N186K mutations have little chance to gain a foothold during evolution, in contrast to the I155T mutation, which is the most responsible for antigenic drift. The A138V and N186K mutations seem to be adaptive in mammalian viruses.

[Change of phenotypic properties of escape mutants and readaptants of influenza virus A (H1N1)pdm09 under the influence of selected mutations in the molecule of hemagglutinin.]

INTRODUCTION: After the emergence and spread of pandemic H1N1 viruses in 2009, antigenic epitopes recognized by neutralizing antibodies against the hemagglutinin of influenza A/Moscow/01/09(H1N1)pdm09 viruses were studied. PURPOSE: The purpose of the study was to obtain readapted variants of the virus from a low-virulent escapemutant that has an increased affinity of the avian and the human types cellular receptors compared to the wild type and the comparative study of their antigenic and receptor specificity. MATERIAL AND METHODS: Viruses were accumulated in 10-day-old chicken embryos. The MAB panel against HA of influenza virus strain A/IIV-Moscow/01/09(H1N1)sw1 was used in the form of ascites fluids from mice. Immunization of mice, HI testing, elution of viruses from chicken erythrocytes, PCR and sequencing of readapted variants were performed by standard methods. RESULTS: The amino acid substitution A198E acquired in the process of readaptation leads to changes in the antigenic specificity. A correlation was found between a decrease in virulence of a low-virulent escape mutant associated with the substitution D190N in the hemagglutinin molecule and an increase in the hemagglutinating titer to inhibitors in normal mouse serum. Viruses with low affinity of cellular receptor analogs and carrying amino acid substitutions have an increased ability to elute from chicken erythrocytes. DISCUSSION: The results discuss the effect of mutations in the HA molecule of the influenza A(H1N1) pdm09 virus to the change in antigen specificity; virulence for mice, adsorption-elution at cellular receptors. CONCLUSION: A comparative study of the antigenic specificity and receptor-binding activity of the escape mutants was conducted for the hemagglutinin of the influenza virus A/Moscow/01/2009 (H1N1)swl, and the readapted variants obtained for one of the escape mutants with reduced virulence for mouse. Monitoring the pleiotropic effect of mutations in the hemagglutinin H1 molecule is necessary to predict variants of the virus with pandemic potential.

Solution Structure, Self-Assembly, and Membrane Interactions of the Matrix Protein from Newcastle Disease Virus at Neutral and Acidic pH.

Newcastle disease virus (NDV) is an enveloped paramyxovirus. The matrix protein of the virus (M-NDV) has an innate propensity to produce virus-like particles budding from the plasma membrane of the expressing cell without recruiting other viral proteins. The virus predominantly infects the host cell via fusion with the host plasma membrane or, alternatively, can use receptor-mediated endocytic pathways. The question arises as to what are the mechanisms supporting such diversity, especially concerning the assembling and membrane binding properties of the virus protein scaffold under both neutral and acidic pH conditions. Here, we suggest a novel method of M-NDV isolation in physiological ionic strength and employ a combination of small-angle X-ray scattering, atomic force microscopy with complementary structural techniques, and membrane interaction measurements to characterize the solution behavior/structure of the protein as well as its binding to lipid membranes at pH 4.0 and pH 7.0. We demonstrate that the minimal structural unit of the protein in solution is a dimer that spontaneously assembles in a neutral milieu into hollow helical oligomers by repeating the protein tetramers. Acidic pH conditions decrease the protein oligomerization state to the individual dimers, tetramers, and octamers without changing the density of the protein layer and lipid membrane affinity, thus indicating that the endocytic pathway is a possible facilitator of NDV entry into a host cell through enhanced scaffold disintegration.IMPORTANCE The matrix protein of the Newcastle disease virus (NDV) is one of the most abundant viral proteins that regulates the formation of progeny virions. NDV is an avian pathogen that impacts the economics of bird husbandry due to its resulting morbidity and high mortality rates. Moreover, it belongs to the Avulavirus subfamily of the Paramyxoviridae family of Mononegavirales that include dangerous representatives such as respiratory syncytial virus, human parainfluenza virus, and measles virus. Here, we investigate the solution structure and membrane binding properties of this protein at both acidic and neutral pH to distinguish between possible virus entry pathways and propose a mechanism of assembly of the viral matrix scaffold. This work is fundamental for understanding the mechanisms of viral entry as well as to inform subsequent proposals for the possible use of the virus as an adequate template for future drug or vaccine delivery.

Signal Immune Reactions of Macrophages Differentiated from THP-1 Monocytes to Infection with Pandemic H1N1PDM09 Virus and H5N2 and H9N2 Avian Influenza A Virus.

In culture of THP-1 cells differentiated into macrophages with PMA (THP-PMA macrophages) infected with influenza viruses of subtypes H1, H5 and H9, we measured the expression of TLR7 and RIG1 receptor genes, sensors of viral RNA and ribonucleoprotein, and the levels of production of inflammatory cytokines IL-1ß, TNFα, IL-10, and IFNα. The sensitivity and inflammatory response of THP-PMA macrophages to pandemic influenza A virus H1N1pdm09 and avian influenza H5N2 and H9N2 viruses correlate with the intracellular level of their viral RNA and activation of the RIG1 gene. Abortive infection is accompanied by intensive macrophage secretion of TNFα, IL-1ß, and toxic factors inducing cell death. Activity of endosomal TLR7 receptor gene changed insignificantly in 24 h after infection and significantly decreased in 48 and 72 h under the action of H5N2 and H9N2, which correlated with manifestation of the cytopathogenic effect of these viruses. H5N2 and H9N2 avian viruses in THP-PMA macrophages are strong activators of the expression of the gene of the cytoplasmic RIG1 receptor 24 and 48 h after infection, and the pandemic virus H1N1pdm09 is a weak stimulator of RIG1 gene. Avian influenza H5N2 and H9N2 viruses are released by rapid induction of the inflammatory response in macrophages. At the late stages of infection, we observed a minor increase in IL-10 secretion in macrophages and, probably, the polarization of a part of the population in type M2. The studied influenza A viruses are weak inductors of IFN in THP-PMA macrophages. In the culture medium of THP-PMA macrophages infected with H9N2 and H5N2 viruses, MTT test revealed high levels of toxic factors causing the death of Caco-2 cells. In contrast to avian viruses, pandemic virus H1N1pdm09 did not induce production of toxic factors.

[Three Mutations in the Stalk Region of Hemagglutinin Affect the pH of Fusion and Pathogenicity of H5N1 Influenza Virus].

Previously, an attenuated variant Ku/at was obtained from the highly pathogenic avian influenza virus A/chicken/Kurgan/3/2005 (H5N1) by a reverse selection method aimed at increasing the virus resistance to a proteolytic cleavage and acidic pH values. In the Ku/at, 10 mutations in proteins PB2, PB1, HA, NA, and NS1 occurred. In comparison with the parental strain, the pH of the conformational transition of the viral glycoprotein hemagglutinin (HA) and virulence for mice and chickens have decreased in an attenuated variant. The purpose of this work is to clarify the role of three mutations in the stalk region of HA: Asp54Asn in HA1 and Val48Ile and Lys131Thr in HA2 (H3 HA numbering). To attain these ends, analogous substitutions were introduced into HA with a deleted polybasic cleavage site (important for pathogenicity) of the recombinant A/Vietnam/1203/04-PR8/CDC-RG (H5N1) virus, and so we created the VN3x-PR variant. Viruses VN3x-PR and Ku/at with the same three mutations, but different proteolytic cleavage sites in HA, as well as the corresponding initial viruses, were tested for pathogenicity in mice and in the erythrocyte hemolysis test. Compared with the parental strains, the virulence of their mutant variants in the case of intranasal infection of BALB/c mice decreased by 4-5 orders of magnitude, and the pH of the conformational transition of HA decreased from 5.70-5.80 to 5.25-5.30, which is typical for low pathogenic natural isolates. Thus, as a result of the study, the attenuating role of these three mutations in HA has been proved, a correlation was established between the pH value of the HA conformational transition and the virulence of H5N1 influenza viruses, and it was shown that the polybasic cleavage site of the H5 HA does not always determine high pathogenicity of the virus.

Predicting the Evolutionary Variability of the Influenza A Virus.

The influenza A virus remains one of the most common and dangerous human health concerns due to its rapid evolutionary dynamics. Since the evolutionary changes of influenza A viruses can be traced in real time, the last decade has seen a surge in research on influenza A viruses due to an increase in experimental data (selection of escape mutants followed by examination of their phenotypic characteristics and generation of viruses with desired mutations using reverse genetics). Moreover, the advances in our understanding are also attributable to the development of new computational methods based on a phylogenetic analysis of influenza virus strains and mathematical (integro-differential equations, statistical methods, probability-theory-based methods) and simulation modeling. Continuously evolving highly pathogenic influenza A viruses are a serious health concern which necessitates a coupling of theoretical and experimental approaches to predict the evolutionary trends of the influenza A virus, with a focus on the H5 subtype.

[Changes in the phenotypic properties of highly pathogenic influenza A virus of H5N1 subtype induced by N186I and N186T point mutations in hemagglutinin].

The change in the phenotypic properties resulting from amino acid substitutions in the hemagglutinin (HA) molecule is an important link in the evolutionary process of influenza viruses. It is believed to be one of the mechanisms of the emergence of highly pathogenic strains of influenza A viruses, including subtype H5N1. Using the site-directed mutagenesis, we introduced mutations in the HA gene of the H5N1 subtype of influenza A virus. The obtained virus variants were analyzed and compared using the following parameters: optimal pH of conformational transition (according to the results of the hemolysis test), specificity of receptor binding (using a set of synthetic analogues of cell surface sialooligosaccharides), thermoresistance (heat-dependent reduction of hemagglutinin activity), virulence in mice, and the kinetics of replication in chicken embryos, and reproductive activity at different temperatures (RCT-based). N186I and N186T mutations in the HA protein increased the virulence of the original virus in mice. These mutations accelerated virus replication in the early stages of infection in chicken embryos and increased the level of replication at late stages. In addition, compared to the original virus, the mutant variants replicated more efficiently at lower temperatures. The obtained data clearly prove the effect of amino acid substitutions at the 186 position of HA on phenotypic properties of the H5N1 subtype of influenza A.

[Effect of amino acid substitutions in small subunit of avian H5N2 influenza virus hemagglutinin on selection of the mutants resistant to neutralizing monoclonal antibodies].

Changes associated with the resistance to physical and chemical factors in the hemagglutinin (HA) of influenza A viruses may play an important role in the selection of different influenza variants during circulation in nature. Here, we studied the escape mutants of influenza virus A/mallard/Pennsylvania/10218/84 (H5N2) that were selected by the monoclonal antibody. The escape mutant m4F11(4) carried a single amino acid substitution in large subunit (HA1) of the HA, S145P1, and two ones, m4G10(10) and m4G10(6), had additional amino acid changes in the small subunit (HA2), namely: L124F2 and L124F2 + N79D2, respectively. As it has been found the substitutions appeared in the HA2 of m4G(10) and m4G(6) viruses compensated negative effect of the S145P1 mutation and provided a significant increase in the viral replication ability at the early stage of infection in embryonated chicken eggs as well as in HA thermostability in comparison with m4F11(4) mutant. Phenotypic properties that provide advantages in the process of virus replication can play a role of the positive selection factor in viral population.

[Effect of mutations changing the antigenic specificity on the receptor-binding activity of the influenza virus hemagglutinin of H1 and H5 subtypes].

The influenza virus hemagglutinin (HA) is an envelope virus glycoprotein responsible for the attachment of the virus particles to cells via binding terminal sialic acid residues of cell surface oligosaccharides. In our previous works on influenza A virus escape mutants, that is, mutants resistant to the neutralization effect of monoclonal antibodies, we encountered amino acid changes in the vicinity of receptor-binding pocket of the HA. In this work the degree of the affinity to both alpha-2, -3, and alpha-2, -6, -sialoglycoconjugates was assessed for escape mutants of influenza H1 and H5 viruses. The data demonstrate that the decrease of the positive electrostatic charge of the HA molecule surface resulting from amino acid changes conferring resistance to monoclonal antibodies may lead to a lowering of the affinity to sialic acid-containing analogs of cell receptors. The results are discussed in the context of the evolution of HA in natural circulation of H1 and H5 influenza viruses.

Yields of virus reassortants containing the HA gene of pandemic influenza 2009 virus.

KEYWORDS: influenza virus; reassortment; virus yield; gene constellation.

[High-yield reassortant virus containing hemagglutinin and neuraminidase genes of pandemic influenza A/Moscowl/01/2009 (H1N1) virus].

The crossing of influenza A/Moscow/01/2009 (H1N1) virus and reassortant strain X31 (H3N2) containing the genes of internal and non-structural proteins of A/Puerto Rico/8/34 (H1N1) strain gave rise to reassortant virus ReM8. The reassortant contained hemagglutinin (HA) and neuraminidase (NA) genes of pandemic 2009 influenza virus and 6 genes of high-yield A/Puerto Rico/8/34 (H1N1) strain. The reassortant ReM8 produced higher yields in the embryonated chicken eggs than the parent pandemic virus, as suggested by infectivity and HA activity titration as well as by ELISA and the measurement of HA protein content by scanning electrophoresis in polyacrylamide gel slabs. High immunogenicity of ReM8 reassortant was demonstrated by immune protection studies in mice. The reassortant virus ReM8 is suitable as a candidate strain for the production of inactivated and subunit influenza vaccines.

Readaptation of a low-virulence influenza H9 escape mutant in mice: the role of changes in hemagglutinin as revealed by site-specific mutagenesis.

In our earlier studies, we showed that an escape mutant of mouse-adapted H9N2 influenza virus carrying a T198N amino acid change in heamagglutinin (HA) has a lowered virulence for mice. The readaptation of this mutant to mice was associated with N198S or N198D reverse mutations. In this study, single-gene reassortants having HA gene of the wild-type virus, its low-virulence escape mutant, or a readapted variant were generated by site-specific mutagenesis and assayed for virulence. The results showed that antibody-selected mutations in the HA of H9 influenza virus can decrease mortality and virus accumulation in mouse lungs, though not in nasal turbinates, and the effect may be compensated by reverse mutations in the course of passaging.

[Amino acid substitutions in the hemagglutinin of H5 influenza virus changing the antigenic specificity and virus virulence].

In our earlier studies, we mapped the hemagglutinin antigenic epitopes of H5 influenza virus by selecting mutants resistant to the neutralizing effect of the antibody (escape mutants). Several escape mutants were shown to have a lowered virulence for mice. The readaptation of low-virulent escape mutants to mice resulted in the restoration of virulence. In the present communication. We present data on the assay of virulence of single-gene reassortants containing HA genes of the wild-type virus, low-virulent escape mutant, or re-adapted variant, and the other genes of a mouse-adapted H9N2 Influenza virus. The results demonstrate that the amino acid change S145F (H3 numbering) in the hemagglutinin ensuring the resistance to a monoclonal antibody can be deleterious to virulence, and that the damaging effect on virulence may be compensated for by additional amino acid changes in position 186 in the hemagglutinin arising in the course of virus passaging in mice. The data indicate that the compensational mutations restoring the pathogenic potential of antigenic variants may be regarded as an additional factor in the evolution of influenza virus hemagglutinin.