Restriction of amino acid change in influenza A virus H3HA: comparison of amino acid changes observed in nature and in vitro

Expression of recombinant Florida clade 2 hemagglutinin in baculovirus expression system: A step for subunit vaccine development against H3N8 equine influenza virus

Background

Equine influenza (EI) is a transmissible viral respiratory sickness of the Equidae family. Two viruses, H7N7 and H3N8 caused EI; however, H7N7 has not been detected for decades. H3N8 has circulated and bifurcated into Eurasian and American lineages. The latter subsequently diversified into Kentucky, South America, and Florida sub-lineages. Florida clade 1 (FC1) and Florida clade 2 (FC2) strains are the only circulating EI viruses (EIVs) in the meantime. Immunization is considered the major means for the prevention and control of EI infection. Using disparate technologies and platforms, several vaccines have been developed and commercialized. According to the recommendations of the World Organization for Animal Health (WOAH), all commercial vaccines shall comprise representatives of both FC1 and FC2 strains. Unfortunately, most of the commercially available vaccines were not updated to incorporate a representative of FC2 strains.

Aim

The purpose of this research was to develop a new EI vaccine candidate that incorporates the hemagglutinin (HA) antigen from the currently circulating FC2.

Methods

In this study, we report the expression of the full-length recombinant HA gene of FC2 in the baculovirus expression system.

Results

The HA recombinant protein has been proven to maintain its biological characteristics by hemadsorption (HAD) and hemagglutination tests. Moreover, using a reference-specific serum, the specificity of the HA has been confirmed through the implementation of immunoperoxidase and western immunoblotting assays.

Conclusion

In conclusion, we report the expression of specific biologically active recombinant HA of FC2, which would act as a foundation for the generation of an updated EI subunit or virus vector vaccine candidates.

Generation and properties of one strain of H3N2 influenza virus with enhanced replication

H3N2 canine influenza virus (CIV) has been circulating in many countries since 2008. The epidemic spread of CIV could be a concern for public health because of the close contact between humans and companion animals. In this study, we used Madin-Darby canine kidney (MDCK) cells as a coinfection model of H3N2 CIV and the pandemic (2009) H1N1 influenza virus to investigate the possibility of genetic mutation or recombination. One of the resultant progeny viruses, designated as CP15, was identified with a significantly increased replication ability. For this viral strain all segments exhibit a homology close to 100 % with its parental strain A/Canine/Jiangsu/06/2010 (JS/10), except for two site mutations K156E and R201 K which occur in the receptor-binding sites of hemagglutinin (HA) and antigen binding sites of neuraminidase (NA), respectively. Virus growth in MDCK cells showed that CP15 had a higher virus titer (more than 10 times) than JS/10. Consistent with this, CP15 exhibited extensive tissue tropism and higher viral RNA loads in the spleen, kidney and lung of mice challenged with this virus compared to JS/10. However, body weight loss and lung injure score due to CP15 infection were greatly reduced. Importantly, anti-CP15 serum antibodies could confer a high neutralization activity against JS/10. These findings indicated that the CP15 strain of high replication ability represents a promising candidate to develop an efficient CIV vaccine.

Novel reassortant swine H3N2 influenza A viruses in Germany

Analysis of 228 H3N2 swine influenza A virus isolates collected between 2003 and 2015 in Germany revealed important changes in molecular epidemiology. The data indicate that a novel reassortant, Rietberg/2014-like swine H3N2, emerged in February 2014 in Northern Germany. It is comprised of a hemagglutinin gene of seasonal H3N2 (A/Denmark/129/2005-like), a neuraminidase gene of Emmelsbuell/2009-like swine H1N2 and the internal gene cassette of pandemic H1N1 viruses. Together with Danish swine H3N2 strains of 2013-2015 with identical genome layout, the Rietberg/2014-like viruses represent a second swine H3N2 lineage which cocirculates with a variant of the Gent/1984-like swine H3N2 lineage. This variant, named Gent1984/Diepholz-like swine H3N2, has a Gent/1984-like HA and a Diepholz/2008-like NA; the origin of the internal gene cassette likely derived from avian-like swine H1N1. The first isolate of the Gent1984/Diepholz reassortant emerged in Northern Germany in September 2011 whereas the last German Gent/1984-like isolate was collected in October 2011.

Re-emergence of H3N2 strains carrying potential neutralizing mutations at the N-linked glycosylation site at the hemagglutinin head, post the 2009 H1N1 pandemic

Background

Seasonally prevalent H1N1 and H3N2 influenza A viruses have evolved by antigenic drift; this evolution has resulted in the acquisition of asparagine (N)-linked glycosylation sites (NGSs) in the globular head of hemagglutinin (HA), thereby affecting the antigenic and receptor-binding properties, as well as virulence. An epidemiological survey indicated that although the traditional seasonal H1N1 strain had disappeared, H3N2 became predominant again in the seasons (2010–11 and 2011–12) immediately following the H1N1 pandemic of 2009. Interestingly, although the 2009 pandemic H1N1 strain (H1N1pdm09) lacks additional NGSs, clinically isolated H3N2 strains obtained during these seasons gained N (Asn) residues at positions 45 and 144 of HA that forms additional NGSs.

Methods

To investigate whether these NGSs are associated with re-emergence of H3N2 within the subtype, we tested the effect of amino acid substitutions on neutralizing activity by using the antisera raised against H3N2 strains with or without additional NGSs. Furthermore, because the N residue at position 144 of HA was identified as the site of mismatch between the vaccine and epidemic strains of 2011–2012, we generated mutant viruses by reverse genetics and tested the functional importance of this particular NGS for antibody-mediated neutralization by intranasal inoculation of mice.

Results

The results indicated that amino acid substitution at residue 144 significantly affected neutralization activity, acting as an escape mutation.

Conclusions

Our data suggest that the newly acquired NGSs in the HA globular head may play an important role in the re-emergence of endemic seasonal H3N2 strain by aiding the escape from humoral immunity.

Electron-correlated fragment-molecular-orbital calculations for biomolecular and nano systems

Recent developments in the fragment molecular orbital (FMO) method for theoretical formulation, implementation, and application to nano and biomolecular systems are reviewed. The FMO method has enabled ab initio quantum-mechanical calculations for large molecular systems such as protein-ligand complexes at a reasonable computational cost in a parallelized way. There have been a wealth of application outcomes from the FMO method in the fields of biochemistry, medicinal chemistry and nanotechnology, in which the electron correlation effects play vital roles. With the aid of the advances in high-performance computing, the FMO method promises larger, faster, and more accurate simulations of biomolecular and related systems, including the descriptions of dynamical behaviors in solvent environments. The current status and future prospects of the FMO scheme are addressed in these contexts.

Extreme genetic fragility of the HIV-1 capsid

Genetic robustness, or fragility, is defined as the ability, or lack thereof, of a biological entity to maintain function in the face of mutations. Viruses that replicate via RNA intermediates exhibit high mutation rates, and robustness should be particularly advantageous to them. The capsid (CA) domain of the HIV-1 Gag protein is under strong pressure to conserve functional roles in viral assembly, maturation, uncoating, and nuclear import. However, CA is also under strong immunological pressure to diversify. Therefore, it would be particularly advantageous for CA to evolve genetic robustness. To measure the genetic robustness of HIV-1 CA, we generated a library of single amino acid substitution mutants, encompassing almost half the residues in CA. Strikingly, we found HIV-1 CA to be the most genetically fragile protein that has been analyzed using such an approach, with 70% of mutations yielding replication-defective viruses. Although CA participates in several steps in HIV-1 replication, analysis of conditionally (temperature sensitive) and constitutively non-viable mutants revealed that the biological basis for its genetic fragility was primarily the need to coordinate the accurate and efficient assembly of mature virions. All mutations that exist in naturally occurring HIV-1 subtype B populations at a frequency >3%, and were also present in the mutant library, had fitness levels that were >40% of WT. However, a substantial fraction of mutations with high fitness did not occur in natural populations, suggesting another form of selection pressure limiting variation in vivo. Additionally, known protective CTL epitopes occurred preferentially in domains of the HIV-1 CA that were even more genetically fragile than HIV-1 CA as a whole. The extreme genetic fragility of HIV-1 CA may be one reason why cell-mediated immune responses to Gag correlate with better prognosis in HIV-1 infection, and suggests that CA is a good target for therapy and vaccination strategies.

The HIV-1 capsid protein (CA) is absolutely essential for viral replication and there is, therefore, intense evolutionary pressure for HIV-1 CA to conserve its functions. However, HIV-1 CA is also a key target of the host immune response, which should provide evolutionary pressure to diversify CA sequence. Genetic robustness, or fragility, is defined as the ability, or lack thereof, of a biological entity to preserve function in the face of sequence changes. Thus, it should be advantageous to HIV-1 CA to evolve genetic robustness. Here, we present the results of extensive, random mutagenesis of single amino acids in CA that reveal an extreme genetic fragility. Although CA participates in several steps in HIV-1 replication, the biological basis for its genetic fragility was primarily the need to participate in the efficient and proper assembly of mature virion particles. The extreme genetic fragility of HIV-1 CA may be one reason why immune responses to it correlate with better prognosis in HIV-1 infection, and suggests that CA is a good target for therapy and vaccination strategies.

Detection of positive selection eliminating effects of structural constraints in hemagglutinin of H3N2 human influenza A virus

In the evolutionary studies of proteins, the average effect of natural selection operating on amino acid mutations may be examined by comparing the numbers of synonymous (dS) and nonsynonymous (dN) substitutions that have accumulated during the same time period. In this method, destabilizing mutations occurring across protein molecules may interfere with detection of natural selection, particularly positive selection, operating on other mutations. Here an attempt to detect positive selection eliminating effects of structural constraints is demonstrated using hemagglutinin (HA) of H3N2 human influenza A virus as an example. Compatible and incompatible amino acids were inferred at each site from the computational analysis of three-dimensional structure using the thermodynamic stability as an indicator, and natural selection was examined by comparing dS and dN among compatible amino acids. In the analysis of 2701 nucleotide sequences for the entire coding region of HA, the new method identified twice as many positively selected amino acid sites as the ordinary method (16 and 4 sites in the former method without and with correction for multiple testing, respectively, and 8 and 2 sites in the latter method). Positively selected sites were involved in epitopes, receptor-binding pocket, epistasis, and stabilization, which appeared to be biologically reasonable. Nevertheless, there still appeared to be several problems, which may largely render this method conservative. It may be effective to analyze many densely sampled sequences in this method.

Reactivity of human convalescent sera with influenza virus hemagglutinin protein mutants at antigenic site A

How the antibodies of individual convalescent human sera bind to each amino acid residue at the antigenic sites of hemagglutinin (HA) of influenza viruses, and how the antigenic drift strains of influenza viruses are selected by human sera, is not well understood. In our previous study, it was found by a binding assay with a chimeric HA between A/Kamata/14/91 (Ka/91) and A/Aichi/2/68 that convalescent human sera, following Ka/91 like (H3N2) virus infection, bind to antigenic site A of Ka/91 HA. Here using chimeric HAs possessing single amino acid substitutions at site A, it was determined how those human sera recognize each amino acid residue at antigenic site A. It was found that the capacity of human sera to recognize amino acid substitutions at site A differs from one person to another and that some amino acid substitutions result in all convalescent human sera losing their binding capacity. Among these amino acid substitutions, certain ones might be selected by chance, thus creating successive antigenic drift. Phylogenetic analysis of the drift strains of Ka/91 showed amino acid substitutions at positions 133, 135 and 145 were on the main stream of the phylogenetic tree. Indeed, all of the investigated convalescent sera failed to recognize one of them.

Surface glycoproteins of influenza A H3N2 virus modulate virus replication in the respiratory tract of ferrets

The hemagglutinin (HA) genes of the influenza A H3N2 subtype viruses isolated from 1968 to 2010 have evolved substantially but their neuraminidase (NA) genes have been relatively less divergent. The H3N2 viruses isolated since 1995 were found to replicate in the lower respiratory tract of ferrets less efficiently than the earlier isolates. To evaluate whether the HA or/and NA or the internal protein gene segments of the H3N2 virus affected viral replication in the respiratory tract of ferrets, recombinant A/California/07/2004 (CA04) (H3N2) virus and its reassortants that contained the same CA04 internal protein gene segments and the HA and/or NA of A/Udorn/309/1972 (UD72) or A/Wuhan/359/1995 (WH95) H3N2 viruses were generated and evaluated for their replication in the respiratory tract of ferrets. All the reassortant viruses replicated efficiently in the upper respiratory tract of ferrets, but their replication in the lower respiratory tract of ferrets varied. In contrast to the UD72-HA reassortant virus that replicated efficiently in the lungs of ferrets, the virus with the WH95-HA or the CA04-HA either replicated modestly or did not replicate in the lungs of ferrets. The reassortants with the WH95-HA and UD72-NA or CA04-NA had the tendency to lose a N-linked glycosylation site at residue 246 in the HA, resulting in viral lung titer of 100-fold higher than the virus with the HA and NA from WH95. The UD72-NA had the highest neuraminidase activity and increased viral replication by up to 100-fold in tissue culture cells during early infection. Thus, our data indicate that both the HA and NA glycoproteins play important roles in viral replication of the H3N2 influenza virus in ferrets.

Molecular and antigenic characterization of reassortant H3N2 viruses from turkeys with a unique constellation of pandemic H1N1 internal genes

Triple reassortant (TR) H3N2 influenza viruses cause varying degrees of loss in egg production in breeder turkeys. In this study we characterized TR H3N2 viruses isolated from three breeder turkey farms diagnosed with a drop in egg production. The eight gene segments of the virus isolated from the first case submission (FAV-003) were all of TR H3N2 lineage. However, viruses from the two subsequent case submissions (FAV-009 and FAV-010) were unique reassortants with PB2, PA, nucleoprotein (NP) and matrix (M) gene segments from 2009 pandemic H1N1 and the remaining gene segments from TR H3N2. Phylogenetic analysis of the HA and NA genes placed the 3 virus isolates in 2 separate clades within cluster IV of TR H3N2 viruses. Birds from the latter two affected farms had been vaccinated with a H3N4 oil emulsion vaccine prior to the outbreak. The HAl subunit of the H3N4 vaccine strain had only a predicted amino acid identity of 79% with the isolate from FAV-003 and 80% for the isolates from FAV-009 and FAV-0010. By comparison, the predicted amino acid sequence identity between a prototype TR H3N2 cluster IV virus A/Sw/ON/33853/2005 and the three turkey isolates from this study was 95% while the identity between FAV-003 and FAV-009/10 isolates was 91%. When the previously identified antigenic sites A, B, C, D and E of HA1 were examined, isolates from FAV-003 and FAV-009/10 had a total of 19 and 16 amino acid substitutions respectively when compared with the H3N4 vaccine strain. These changes corresponded with the failure of the sera collected from turkeys that received this vaccine to neutralize any of the above three isolates in vitro.

Gnarled-trunk evolutionary model of influenza A virus hemagglutinin

Human influenza A viruses undergo antigenic changes with gradual accumulation of amino acid substitutions on the hemagglutinin (HA) molecule. A strong antigenic mismatch between vaccine and epidemic strains often requires the replacement of influenza vaccines worldwide. To establish a practical model enabling us to predict the future direction of the influenza virus evolution, relative distances of amino acid sequences among past epidemic strains were analyzed by multidimensional scaling (MDS). We found that human influenza viruses have evolved along a gnarled evolutionary pathway with an approximately constant curvature in the MDS-constructed 3D space. The gnarled pathway indicated that evolution on the trunk favored multiple substitutions at the same amino acid positions on HA. The constant curvature was reasonably explained by assuming that the rate of amino acid substitutions varied from one position to another according to a gamma distribution. Furthermore, we utilized the estimated parameters of the gamma distribution to predict the amino acid substitutions on HA in subsequent years. Retrospective prediction tests for 12 years from 1997 to 2009 showed that 70% of actual amino acid substitutions were correctly predicted, and that 45% of predicted amino acid substitutions have been actually observed. Although it remains unsolved how to predict the exact timing of antigenic changes, the present results suggest that our model may have the potential to recognize emerging epidemic strains.

Reassortant between human-Like H3N2 and avian H5 subtype influenza A viruses in pigs: a potential public health risk

BACKGROUND

Human-like H3N2 influenza viruses have repeatedly been transmitted to domestic pigs in different regions of the world, but it is still uncertain whether any of these variants could become established in pig populations. The fact that different subtypes of influenza viruses have been detected in pigs makes them an ideal candidate for the genesis of a possible reassortant virus with both human and avian origins. However, the determination of whether pigs can act as a "mixing vessel" for a possible future pandemic virus is still pending an answer. This prompted us to gather the epidemiological information and investigate the genetic evolution of swine influenza viruses in Jilin, China.

METHODS

Nasopharyngeal swabs were collected from pigs with respiratory illness in Jilin province, China from July 2007 to October 2008. All samples were screened for influenza A viruses. Three H3N2 swine influenza virus isolates were analyzed genetically and phylogenetically.

RESULTS

Influenza surveillance of pigs in Jilin province, China revealed that H3N2 influenza viruses were regularly detected from domestic pigs during 2007 to 2008. Phylogenetic analysis revealed that two distinguishable groups of H3N2 influenza viruses were present in pigs: the wholly contemporary human-like H3N2 viruses (represented by the Moscow/10/99-like sublineage) and double-reassortant viruses containing genes from contemporary human H3N2 viruses and avian H5 viruses, both co-circulating in pig populations.

CONCLUSIONS

The present study reports for the first time the coexistence of wholly human-like H3N2 viruses and double-reassortant viruses that have emerged in pigs in Jilin, China. It provides updated information on the role of pigs in interspecies transmission and genetic reassortment of influenza viruses.

Epidemiologic study of human influenza virus infection in South Korea from 1999 to 2007: origin and evolution of A/Fujian/411/2002-like strains

Influenza epidemics arise through the accumulation of viral genetic changes, culminating in a novel antigenic type that is able to escape host immunity. Following an outbreak of the A/Fujian/411/2002-like strains in Asia, including China, Japan, and South Korea, in 2002, Australia and New Zealand experienced substantial outbreaks of the same strains in 2003, and subsequently worldwide outbreaks occurred in the 2003-2004 season. The emergence of A/Fujian/411/2002-like strains coincided with a higher level of influenza-like illness in South Korea than what is seen at the peak of a normal season, and there was at least a year's difference between South Korea and the United States. Genetic evolution of human influenza A/H3N2 viruses was monitored by sequence analysis of hemagglutinin (HA) genes collected in Asia, including 269 (164 new) HA genes isolated in South Korea from 1999 to 2007. The Fujian-like influenza strains were disseminated with rapid sequence variation across the antigenic sites of the HA1 domain, which sharply distinguished between the A/Moscow/10/1999-like and A/Fujian/411/2002-like strains. This fast variation, equivalent to approximately 10 amino acid changes within a year, occurred in Asia and would be the main cause of the disappearance of the reassortants, although the reassortant and nonreassortant Fujian-like strains circulated simultaneously in Asia.

Sialic acid recognition is a key determinant of influenza A virus tropism in murine trachea epithelial cell cultures

Influenza A virus interacts with specific types of sialic acid during attachment and entry into susceptible cells. The precise amino acids in the hemagglutinin protein that control sialic acid binding specificity and affinity vary among antigenic subtypes. For H3 subtypes, amino acids 226 and 228 are critical for differentiating between alpha2,3- and alpha2,6-linked forms of sialic acid (SA). We demonstrate that position 190 of the HA from A/Udorn/307/72 (H3N2) plays an important role in the recognition of alpha2,3-SA, as changing the residue from a glutamic acid to an aspartic acid led to alteration of red blood cell hemagglutination and a complete loss of replication in differentiated, murine trachea epithelial cell cultures which express only alpha2,3-SA. This amino acid change had a minimal effect on virus replication in MDCK cells, suggesting subtle changes in receptor recognition by the H3 hemagglutinin can lead to significant alterations in cell and species tropism.

Genetic Analyses of an H3N8 Influenza Virus Isolate, Causative Strain of the Outbreak of Equine Influenza at the Kanazawa Racecourse in Japan in 2007

In August 2007, an outbreak of equine influenza occurred among vaccinated racehorses with Japanese commercial equine influenza vaccine at Kanazawa Racecourse in Ishikawa prefecture in Japan. Apparent symptoms were pyrexia (38.2-41.0 degrees C) and nasal discharge with or without coughing, although approximately half of the infected horses were subclinical. All horses had been shot with a vaccine that contained two inactivated H3N8 influenza virus strains [A/equine/La Plata/93 (La Plata/93) of American lineage and A/equine/Avesta/93 (Avesta/93) of European lineage] and an H7N7 strain (A/equine/Newmarket/1/77). Influenza virus, A/equine/Kanazawa/1/2007 (H3N8) (Kanazawa/07), was isolated from one of the nasal swab samples of diseased horses. Phylogenetic analysis indicated that Kanazawa/07 was classified into the American sublineage Florida. In addition, four amino acid substitutions were found in the antigenic sites B and E in the HA1 subunit protein of Kanazawa/07 in comparison with that of La Plata/93. Hemagglutination-inhibition (HI) test using 16 serum samples from recovering horses revealed that 1.4- to 8-fold difference in titers between Kanazawa/07 and either of the vaccine strains. The present findings suggest that Japanese commercial inactivated vaccine contributed to reducing the morbidity rate and manifestation of the clinical signs of horses infected with Kanazawa/07 that may be antigenically different from the vaccine strains.

Effects of single-point amino acid substitutions on the structure and function neuraminidase proteins in influenza A virus

In order to clarify the effect of amino acid substitutions on the structure and function of the neuraminidase (NA) protein of influenza A virus, we introduced single-point amino acid substitutions into the NA protein of the A/Tokyo/3/67 (H2N2) strain using PCR-based random mutation. The rate of tolerant random one amino acid substitutions in the NA protein was 47%. Rates of tolerant substitutions for the stalk and for the surface and inner portion of the head region of the NA protein were 79, 54, and 19%, respectively. Deleterious changes, such as those causing the NA protein to stop at the Golgi/endoplasmic reticulum, were scattered throughout the protein. On the other hand, the ratio of mutations with which the NA protein lost neuraminidase activity, but was transported to the cell surface, decreased in proportion to the distance from the structural center of enzyme active site. In order to investigate the effect of accumulated amino acid substitutions on the structural character of the N2NA protein during evolution, the same amino acid substitutions were introduced by site-directed mutagenesis at 23 homologous positions on N2 proteins of A/Tokyo/3/67, A/Bangkok/15/85 (H3N2), and A/Mie/1/2004 (H3N2). The results showed a shift, or discordance, in tolerance at some of the positions. An increase in discordance was correlated with the interval in years between virus strains, and the discordance rate was estimated to be 0.6-0.7% per year.

Comparison of epitope structures of H3HAs through protein modeling of influenza A virus hemagglutinin: mechanism for selection of antigenic variants in the presence of a monoclonal antibody

Starting with nine plaques of influenza A/Kamata/14/91(H3N2) virus, we selected mutants in the presence of monoclonal antibody 203 (mAb203). In total, amino acid substitutions were found at nine positions (77, 80, 131, 135, 141, 142, 143, 144 and 146), which localized in the antigenic site A of the hemagglutinin (HA). The escape mutants differed in the extent to which they had lost binding to mAb203. HA protein with substitutions of some amino acid residues created by site-directed mutagenesis in the escape mutants retained the ability to bind to mAb203. Changes in the amino acid character affecting charge or hydrophobicity accounted for the binding capacity to the antibody of the HA with most of the substitutions in the escape mutants and binding-positive mutants. However, the effect of some amino acid substitutions remained unexplained. A three-dimensional model of the 1991 HA was constructed and used to analyze substituted amino acids in these mutants for the accessible surface hydrophobic and hydrophilic characters. One amino acid substitution in an escape mutant and another amino acid substitution in a binding-positive mutant seemed to be explained by the changes noted on this model.

Genetic and antigenic analysis of epidemic influenza viruses isolated during 2006-2007 season in Taiwan

Influenza viruses are some of the most active pathogens in Taiwan. The monitoring influenza activity has been coordinated by the Centers for Diseases Control, Taiwan, and the surveillance is based on integrated clinical and virological surveillance components. Data from sentinel physician networks and other sources, mainly hospitals were collected. During 2006-07 season, a total of 1724 cases of laboratory-confirmed influenza were reported by collaborating laboratories and sentinels, which was five fold higher than during the corresponding part of the 2005-06 season. Of the Taiwan isolates analyzed using post-infection ferret antisera, 1.5% were H1N1 (A/Hi), 21.5% H3N2 (A/H3), and 77.0% influenza B viruses. This reflects the predominance of influenza B viruses during 2006-07 season. In addition, continued antigenic drift was seen with the A/I-B viruses compared with the previous season's reference strains. However, an increasing number of recent A/H3 isolates characterized in our report were amantadine sensitive. Preparation for an influenza pandemic is presently a high priority in Taiwan. Laboratory-based surveillance systems must be timely in order to be effective. The data presented here highlights the need to characterize the circulating strains both antigenically and genetically during regular surveillance. Any contribution of individual genes or gene combinations to usual or unusual epidemic characteristics might thus be identified ensuring that virus strains can be selected for vaccine formulation that will most closely match the circulating viruses.

Genetic and epidemiological analysis of influenza virus epidemics in Taiwan during 2003 to 2006

The genetic characterization of Taiwanese influenza A and B viruses on the basis of analyses of pairwise amino acid variations, genetic clustering, and phylogenetics was performed. A total of 548, 2,123, and 1,336 sequences of the HA1 genes of influenza A virus subtypes H1 and H3 and influenza B virus, respectively, collected during 2003 to 2006 from an island-wide surveillance network were determined. Influenza A virus H3 showed activity during all periods, although it was dominant only in the winters of 2002-2003 and 2003-2004. Instead, influenza B virus and influenza A virus H1 were dominant in the winters of 2004-2005 and 2005-2006, respectively. Additionally, two influenza A virus H3 peaks were found in the summers of 2004 and 2005. From clustering analysis, similar characteristics of high sequence diversity and short life spans for the influenza A virus H1 and H3 clusters were observed, despite their distinct seasonal patterns. In contrast, clusters with longer life spans and fewer but larger clusters were found among the influenza B viruses. We also noticed that more amino acid changes at antigenic sites, especially at sites B and D in the H3 viruses, were found in 2003 and 2004 than in the following 2 years. The only epidemic of the H1 viruses, which occurred in the winter of 2005-2006, was caused by two genetically distinct lineages, and neither of them showed apparent antigenic changes compared with the antigens of the vaccine strain. For the influenza B viruses, the multiple dominant lineages of Yamagata-like strains with large genetic variations observed reflected the evolutionary pressure caused by the Yamagata-like vaccine strain. On the other hand, only one dominant lineage of Victoria-like strains circulated from 2004 to 2006.

[Accumulation of amino acid substitutions promotes irreversible structural changes in the hemagglutinin of human influenza AH3 virus during evolution]

During protein evolution the amino acid substitutions accumulate with time. However, the effect of accumulation of the amino acid substitutions to structural changes has not been estimated well. We will propose that the discordance of amino acid substitution on the HA protein of influenza A virus is useful for the assessment of structural changes during evolution. Discordance value can be obtained from the experimental data of tolerance or intolerance by introducing site directed mutagenesis at the homologous positions of two HA proteins holding the same amino acid residues. The value of discordance correlated to the number of amino acid differences among proteins. In the H3HA discordance rate was calculated to be 0.45% per one amino acid change. Furthermore, discordance of amino acid substitutions suggests that tolerable amino acid substitutions in different order have a probability of promoting irreversible divergence of the HA protein to different subtypes.

Accumulation of amino acid substitutions promotes irreversible structural changes in the hemagglutinin of human influenza AH3 virus during evolution

In order to clarify the effect of an accumulation of amino acid substitutions on the hemadsorption character of the influenza AH3 virus hemagglutinin (HA) protein, we introduced single-point amino acid changes into the HA1 domain of the HA proteins of influenza viruses isolated in 1968 (A/Aichi/2/68) and 1997 (A/Sydney/5/97) by using PCR-based random mutation or site-directed mutagenesis. These substitutions were classified as positive or negative according to their effects on the hemadsorption activity. The rate of positive substitutions was about 50% for both strains. Of 44 amino acid changes that were identical in the two strains with regard to both the substituted amino acids and their positions in the HA1 domain, 22% of the changes that were positive in A/Aichi/2/68 were negative in A/Sydney/5/97 and 27% of the changes that were negative in A/Aichi/2/68 were positive in A/Sydney/5/97. A similar discordance rate was also seen for the antigenic sites. These results suggest that the accumulation of amino acid substitutions in the HA protein during evolution promoted irreversible structural changes and therefore that antigenic changes in the H3HA protein may not be limited.

Effect of the addition of oligosaccharides on the biological activities and antigenicity of influenza A/H3N2 virus hemagglutinin

Influenza A/H3N2 viruses have developed an increased number of glycosylation sites on the globular head of the hemagglutinin (HA) protein since their appearance in 1968. Here, the effect of addition of oligosaccharide chains to the HA of A/H3N2 viruses on its biological activities was investigated. We constructed seven mutant HAs of A/Aichi/2/68 virus with one to six glycosylation sites on the globular head, as found in natural isolates, by site-directed mutagenesis and analyzed their intracellular transport, receptor binding, and cell fusion activities. The glycosylation sites of mutant HAs correspond to representative A/H3N2 isolates (A/Victoria/3/75, A/Memphis/6/86, or A/Sydney/5/97). The results showed that all the mutant HAs were transported to the cell surface as efficiently as wild-type HA. Although mutant HAs containing three to six glycosylation sites decreased receptor binding activity, their cell fusion activity was not affected. The reactivity of mutant HAs having four to six glycosylation sites with human sera collected in 1976 was much lower than that of wild-type HA. Thus, the addition of new oligosaccharides to the globular head of the HA of A/H3N2 viruses may have provided the virus with an ability to evade antibody pressures by changing antigenicity without an unacceptable defect in biological activity.