Neutralizing epitopes specific for influenza B virus Yamagata group strains are in the 'loop'

Substitutions near the HA receptor binding site explain the origin and major antigenic change of the B/Victoria and B/Yamagata lineages

Influenza B virus primarily infects humans, causing seasonal epidemics globally. Two antigenic variants-Victoria-like and Yamagata-like-were detected in the 1980s, of which the molecular basis of emergence is still incompletely understood. Here, the antigenic properties of a unique collection of historical virus isolates, sampled from 1962 to 2000 and passaged exclusively in mammalian cells to preserve antigenic properties, were determined with the hemagglutination inhibition assay and an antigenic map was built to quantify and visualize the divergence of the lineages. The antigenic map revealed only three distinct antigenic clusters-Early, Victoria, and Yamagata-with relatively little antigenic diversity in each cluster until 2000. Viruses with Victoria-like antigenic properties emerged around 1972 and diversified subsequently into two genetic lineages. Viruses with Yamagata-like antigenic properties evolved from one lineage and became clearly antigenically distinct from the Victoria-like viruses around 1988. Recombinant mutant viruses were tested to show that insertions and deletions (indels), as observed frequently in influenza B virus hemagglutinin, had little effect on antigenic properties. In contrast, amino-acid substitutions at positions 148, 149, 150, and 203, adjacent to the hemagglutinin receptor binding site, determined the main antigenic differences between the Early, Victoria-like, and Yamagata-like viruses. Surprisingly, substitutions at two of the four positions reverted in recent viruses of the Victoria lineage, resulting in antigenic properties similar to viruses circulating ∼50 y earlier. These data shed light on the antigenic diversification of influenza viruses and suggest there may be limits to the antigenic evolution of influenza B virus.

Three Types of Broadly Reacting Antibodies against Influenza B Viruses Induced by Vaccination with Seasonal Influenza Viruses

We analyzed the antibody (Ab) repertoire against influenza B viruses induced by vaccination with seasonal influenza viruses in one individual who had never been vaccinated until 2009. The vaccine used in this study comprised B/Massachusetts/2/2012 (Yamagata lineage), A/Texas/50/2012 (H3N2), and A/California/7/2009 (H1N1). One month after the subject received two vaccinations, blood (200 ml) was obtained and peripheral mononuclear cells were prepared, and a large Ab library was constructed using phage display technology. The library was screened with HA-enriched fraction of B/Massachusetts/2/2012 and B/Brisbane/60/2008 (Victoria lineage) virus, and a total of 26 Abs that potentially bound to hemagglutinin (HA) molecules were isolated. Their binding activities to six influenza B viruses, three of Yamagata lineage and three of Victoria lineage, and two influenza A viruses, H1N1 and H3N2, were examined. The Abs showed cross-reactivity at three different levels. The first type bound to all Yamagata lineage viruses. The second type bound to both Yamagata and Victoria lineage viruses. The third type bound to both influenza A and B viruses. These results indicate that common epitopes exist on HA molecules of influenza virus at various levels, and humans have capability to produce Abs that bind to such common epitopes.

Determination of influenza B identity and potency in quadrivalent inactivated influenza vaccines using lineage-specific monoclonal antibodies

Co-circulation of two antigenically and genetically distinct lineages of influenza B virus, represented by prototype viruses B/Victoria/2/1987 and B/Yamagata/16/1988, has led to the development of quadrivalent influenza vaccines that contain two influenza B antigens. The inclusion of two influenza B antigens presents challenges for the production and regulation of inactivated quadrivalent vaccines, including the potential for cross-reactivity of the reagents used in identity and potency assays because of the relative close relatedness of the hemagglutinin (HA) from the two virus lineages. Monoclonal antibodies (mAbs) specific for the two lineages of influenza B HA were generated and characterized and used to set-up simple identity tests that distinguish the influenza B antigens in inactivated trivalent and quadrivalent vaccines. The lineage-specific mAbs bound well to the HA of influenza B strains included in influenza vaccines over a period of more than 10 years, suggesting that identity tests using such lineage-specific mAbs would not necessarily have to be updated with every influenza B vaccine strain change. These lineage-specific mAbs were also used in an antibody capture ELISA format to quantify HA in vaccine samples, including monovalent, trivalent, and quadrivalent vaccine samples from various manufacturers. The results demonstrated correlation with HA values determined by the traditional single radial immunodiffusion (SRID) assay. Further, the antibody-capture ELISA was able to distinguish heat-stressed vaccine from unstressed vaccine, and was similar to the SRID in quantifying the resultant loss of potency. These mAb reagents should be useful for further development of antibody-based alternative influenza B identity and potency assays.

Circulating pattern and genomic characteristics of influenza B viruses in Taiwan from 2003 to 2014

BACKGROUND/PURPOSE

Influenza B viruses are antigenically classified into Yamagata and Victoria lineages according to their hemagglutinin (HA) proteins. These two lineages are known to either appear sequentially or cocirculate in Taiwan.

METHODS

Taiwanese influenza B viral HA and neuraminidase (NA) sequences between 2003 and 2014 were determined and analyzed. A time-scaled phylogenetic tree was constructed to decipher the evolutionary trends of these sequences, and the reassortment between the two lineages. Positively selected amino acids were predicted, demonstrating the adaptive mutations of the circulating pattern.

RESULTS

The HA phylogenetic tree revealed that the Victoria lineage evolved into a ladder-like pattern, whereas the Yamagata lineage exhibited complex topology with several independently evolved clades on which viruses from different influenza seasons interlaced. For several seasons, HA sequences were found to be dominated by strains of the same lineage as the corresponding vaccine strain. Inspecting these sequences revealed that frequent mutations occurred in neutralizing epitopes and glycosylation sites. Amino acid positions 212 and 214 of N-glycosylation sites, which are known to be critical determinants of receptor-binding specificity, were found to be subject to positive selection. No drug-resistant sites were noticed in the NA sequences. In addition, we identified several cases of NA reassortment with an overall incidence rate of 6% for the investigated Taiwan strains.

CONCLUSION

We highlighted the interplay between mutations in the glycosylation sites and epitope during HA evolution. These are crucial molecular signatures to be monitored for influenza B epidemics in the future.

Genetic and antigenic typing of seasonal influenza virus breakthrough cases from a 2008-2009 vaccine efficacy trial

Estimations of the effectiveness of vaccines against seasonal influenza virus are guided by comparisons of the antigenicities between influenza virus isolates from clinical breakthrough cases with strains included in a vaccine. This study examined whether the prediction of antigenicity using a sequence analysis of the hemagglutinin (HA) gene-encoded HA1 domain is a simpler alternative to using the conventional hemagglutination inhibition (HI) assay, which requires influenza virus culturing. Specimens were taken from breakthrough cases that occurred in a trivalent influenza virus vaccine efficacy trial involving >43,000 participants during the 2008-2009 season. A total of 498 influenza viruses were successfully subtyped as A(H3N2) (380 viruses), A(H1N1) (29 viruses), B(Yamagata) (23 viruses), and B(Victoria) (66 viruses) from 603 PCR- or culture-confirmed specimens. Unlike the B strains, most A(H3N2) (377 viruses) and all A(H1N1) viruses were classified as homologous to the respective vaccine strains based on their HA1 domain nucleic acid sequence. HI titers relative to the respective vaccine strains and PCR subtyping were determined for 48% (182/380) of A(H3N2) and 86% (25/29) of A(H1N1) viruses. Eighty-four percent of the A(H3N2) and A(H1N1) viruses classified as homologous by sequence were matched to the respective vaccine strains by HI testing. However, these homologous A(H3N2) and A(H1N1) viruses displayed a wide range of relative HI titers. Therefore, although PCR is a sensitive diagnostic method for confirming influenza virus cases, HA1 sequence analysis appeared to be of limited value in accurately predicting antigenicity; hence, it may be inappropriate to classify clinical specimens as homologous or heterologous to the vaccine strain for estimating vaccine efficacy in a prospective clinical trial.

Hemagglutinin and neuraminidase genes of influenza B viruses circulating in Riyadh, Saudi Arabia during 2010-2011: evolution and sequence analysis

Influenza viruses are known as continuing threats to human public health every year worldwide. Evolutionary dynamics of influenza B viruses in humans are in a unique progression having two lineages; B/Yam and B/Vic-like viruses, which are circulating simultaneously worldwide. There is a considerable lack of data on influenza B viruses circulating in Saudi Arabia. During the winter-spring season of 2010-2011, 80 nasopharyngeal aspirates were collected from hospitalized patients with flu-like symptoms in Riyadh. Screening of samples by one-step RT-PCR identified three (3.8%) influenza B viruses. Sequencing of hemagglutinin (HA) and neuraminidase (NA) genes was performed to analyze influenza B viruses circulating in Riyadh as compared to the globally circulating strains. Several common and six unique amino acid substitutions were observed for both HA and NA genes of influenza B Saudi strains. Three unique substitutions (T182A, D196N, and K254R) were identified in HA gene of the B/Yam-like Riyadh strains. In NA gene, a unique common substitution (D53G) was found in all Riyadh strains, while two unique substitutions (L38P, G233R) were recognized only in B/Vic-like Riyadh strains. Riyadh strains were also found to contain N-glycosylation site in HA gene of both B/Vic and B/Yam lineages at positions 197-199 (NET) and 196-198 (NNK/DNK), respectively. The significance of these mutations on the antigenicity of both lineages is discussed herein. The unique changes observed in HA and NA genes of influenza B Riyadh strains support strongly the need for continuous surveillance and monitoring of new evolving strains that might pose threat to the Saudi community.

Structural basis for the divergent evolution of influenza B virus hemagglutinin

Influenza A and B viruses are responsible for the severe morbidity and mortality worldwide in annual influenza epidemics. Currently circulating influenza B virus belongs to the B/Victoria or B/Yamagata lineage that was diverged from each other about 30-40 years ago. However, a mechanistic understanding of their divergent evolution is still lacking. Here we report the crystal structures of influenza B/Yamanashi/166/1998 hemagglutinin (HA) belonging to B/Yamagata lineage and its complex with the avian-like receptor analogue. Comparison of these structures with those of undiverged and diverged influenza B virus HAs, in conjunction with sequence analysis, reveals the molecular basis for the divergent evolution of influenza B virus HAs. Furthermore, HAs of diverged influenza B virus strains display much stronger molecular interactions with terminal sialic acid of bound receptors, which may allow for a different tissue tropism for current influenza B viruses, for which further investigation is required.

Surveillance and molecular characterization of human influenza B viruses during 2006-2010 revealed co-circulation of Yamagata-like and Victoria-like strains in eastern India

Acute respiratory illness (ARI) is one of the major health problems in tropical countries of Asia, like India where approximately 0.5 million children in the age group of < 5 years die annually. Previously we have reported the genetic characterization of influenza A (Inf-A) strains circulating in Kolkata, eastern India. This study was initiated to characterize the genetic diversity of the circulating influenza B (Inf-B) viruses. Of 3035 nasal/throat swabs, 494 (16.3%) samples were identified as influenza A/B positive by real time RT-PCR, of which 244 samples were confirmed having Inf-B infection. Comparison of nucleotide (nt) and amino acid (aa) sequences of HA and NA gene of Inf-B viruses revealed co-circulation of B/Yamagata and B/Victoria lineages. Of the 32 randomly selected Inf-B strains from Kolkata, seventeen strains possessed reassorted NA gene. There was a single Histidine to Asparagine substitution in the 131st position which is a part of 120 loop on HA1 region along with a deletion at position 178 in the Kolkata strains belonging to the Yamagata lineage. Amino acid substitution was observed at position 198 on NA gene in the strains B/Kol/542/2006, B/Kol/1373/2008, B/Kol/1880/2008, B/Kol/2044/2008 and in all the representative strains isolated during 2009 with respect to the circulating vaccine strains. This substitution is responsible for reduced sensitivity of neuraminidase inhibitors. The results highlight the importance of monitoring Inf-B viruses for development of antiviral resistance among circulating strains.

Monoclonal antibodies in man that neutralized H3N2 influenza viruses were classified into three groups with distinct strain specificity: 1968-1973, 1977-1993 and 1997-2003

We tried to reveal the strain specificity of neutralizing mAbs against H3N2 influenza viruses in individuals. A large number of B lymphocytes of a pediatrician were collected by apheresis and two Ab libraries were constructed at 2004 and 2007 by using the phage-display technology. The libraries were screened against 12 different H3 strains of flu isolated between 1968 and 2004. Large numbers of clones that bound to the Ags were isolated and mAbs that specifically bound to H3 strain viruses were selected. Their binding activity to the 12 strains and neutralizing activity were studied by ELISA and focus reduction test, respectively. Furthermore, the binding activity to hemagglutinin (HA) was examined by Western blot. The majority of clones showing the neutralizing activity turned out to be anti-HA mAbs and could be divided into three major groups showing distinct strain specificity: 1968-1973, 1977-1993 and 1997-2003.

Cocirculation of antigenic variants and the vaccine-type virus during the 2004-2005 influenza B virus epidemics in Japan

In the 2004-2005 season, there was a large epidemic of the influenza B virus Yamagata group in Kobe, Japan. In hemagglutination inhibition tests, most of the clinical isolates from Kobe showed antigenicities similar to those of previous isolates (the vaccine-type virus). Only a few antigenic variants were isolated around the peak of the epidemic; however, Kobe residents developed antibodies against the variants during the season. The antigenic variants showed a one-point mutation of a nucleotide in the HA1 gene (C440A or G421A), which resulted in the substitution of one amino acid in the 150 loop of the HA molecule (T147N or G141R). The 150 loop is one of four epitopes of the hemagglutinin molecule of the influenza B virus. We established a system to detect one-point differences in the nucleotides of the 150 loop by means of high-resolution melting curve analysis with LCGreen. With this system, the isolates were determined to be the vaccine-type virus, antigenic variants, or a mixture of both. Some isolates were shown to be mixtures although they had been recognized as the vaccine-type virus with the hemagglutination inhibition tests. Thus, the antigenic variants appeared in the early period of the epidemic and were cocirculating with the vaccine-type virus during the epidemic.

Diversifying selective pressure on influenza B virus hemagglutinin

Influenza B virus hemagglutinin (HA) is a major surface glycoprotein with frequent amino acid substitutions. However, the roles of antibody selection in the amino acid substitutions of HA were still poorly understood. In order to gain insights into this important issue, an analysis was conducted on a total of 271 HA1 sequences of influenza B virus strains isolated during 1940-2007. In this analysis, phylogenetic analysis by maximum likelihood (PAML) package was used to detect the existence of positive selection and to identify positively selected sites on HA1. Strikingly, all the positively selected sites were located in the four major epitopes (120-loop, 150-loop, 160-loop, and 190-helix) of HA identified in previous studies, thus supporting a predominant role of antibody selection in HA evolution. Of particular significance is the involvement of the 120-loop in positive selection, which may become increasingly important in future field isolates. Despite the absence of different subtypes, influenza B virus HA continued to evolve into new sublineages, within which the four major epitopes were targeted selectively in positive selection. Thus, any newly emerging strains need to be placed in the context of their evolutionary history in order to understand and predict their epidemic potential.

Heterogeneous selective pressure acting on influenza B Victoria- and Yamagata-like hemagglutinins

As a consequence of immune pressure, influenza virus hemagglutinin presents some of its amino acids under positive selection. Several authors have reported the existence of influenza A hemagglutinin codons under positive selective pressure (PSP). In this framework, the present work objectives were to demonstrate the presence of PSP and evaluate its effects on Victoria- and Yamagata-like influenza B viruses. Methodology adopted consisted in estimating the acceptance rate of nonsynonymous substitutions (omega = dN/dS) that describe the strength of selective pressure and identifying codons that may be positively selected, applying a set of continuous-time Markov chain codon-substitution models. Two groups of HA1 sequences (140 from Yamagata and 60 from Victoria lineage) were used. All the model maximum-likelihood estimates were obtained using codeml software application (PAML 3.15). The hypothesis of no existence of sites under PSP was rejected for both lineages (p < 0.001), using likelihood ratio tests. These results demonstrate the presence of positive selection acting on hemagglutinin of both Yamagata- and Victoria-like influenza B viruses. Several different sites were identified to be under PSP on Yamagata and Victoria hemagglutinins. Sites found with a posterior probability > 0.95 were codons 197 and 199 in both lineages, codon 75 in the Yamagata lineage, and codon 129 in the Victoria lineage. The detected amino acids are located at or near antigenic sites in influenza A virus H3 hemagglutinin.

Crystal structure of unliganded influenza B virus hemagglutinin

Here we report the crystal structure of hemagglutinin (HA) from influenza B/Hong Kong/8/73 (B/HK) virus determined to 2.8 A. At a sequence identity of approximately 25% to influenza A virus HAs, B/HK HA shares a similar overall structure and domain organization. More than two dozen amino acid substitutions on influenza B virus HAs have been identified to cause antigenicity alteration in site-specific mutants, monoclonal antibody escape mutants, or field isolates. Mapping these substitutions on the structure of B/HK HA reveals four major epitopes, the 120 loop, the 150 loop, the 160 loop, and the 190 helix, that are located close in space to form a large, continuous antigenic site. Moreover, a systematic comparison of known HA structures across the entire influenza virus family reveals evolutionarily conserved ionizable residues at all regions along the chain and subunit interfaces. These ionizable residues are likely the structural basis for the pH dependence and sensitivity to ionic strength of influenza HA and hemagglutinin-esterase fusion proteins.

Detection of antigenic variants of the influenza B virus by melting curve analysis with LCGreen

Automated, high-throughput detection methods for single-nucleotide polymorphisms have been applied to the routine genotyping of genetic polymorphisms influencing drug metabolism. Melting curve analysis with LCGreen was introduced recently as one such technique which can be performed rapidly and easily. This technique was used to detect antigenic variants of the influenza B virus. The antigenic variants and vaccine-type strains of the influenza B virus are isolated from clinical specimens of one epidemic season, and they usually differ in one nucleotide in the HA1 gene, corresponding to one amino-acid substitution. By means of melting curve analysis with LCGreen, an antigenic variant clone and a vaccine-type clone were clearly distinguished. In addition, the proportions of the antigenic variants in the mixture-type isolates were estimated. The clinical isolates were detected as the vaccine-type strains, antigenic variants, or a mixture of both. It became clear that humans were infected with a mixture of the vaccine-type strains and the antigenic variants for a certain period after which the viral antigenicities vary. This technique will contribute to the analysis of antigenic shifts in influenza B virus.

Genetic analysis and evaluation of the reassortment of influenza B viruses isolated in Taiwan during the 2004-2005 and 2006-2007 epidemics

Influenza B viruses were predominant in Taiwan during the 2004-2005 epidemic and both Victoria and Yamagata lineage viruses co-circulated. A reassortant influenza B virus that contained a Victoria lineage hemagglutinin (HA) gene and Yamagata lineage neuraminidase (NA) gene appeared first in 2002 and became predominant during the 2004-2005 epidemic. During the 2006-2007 epidemic, an influenza B outbreak occurred in Taiwan and only Victoria lineage viruses circulated. We characterized the viruses isolated in the 2006-2007 epidemic and found that the HA genes of influenza B viruses from that epidemic were highly similar to those from the 2004-2005 epidemic. We also analyzed the NA genes of isolates from the 2006-2007 epidemic and found that they all belonged to the Yamagata lineage and formed a new genetic subclade. Comparison of isolates from the 2004-2005 and 2006-2007 epidemics revealed four substitutions, N220K, E320D, K343R and E404K in NA genes. Although the HA sequences from the 2006-2007 epidemic were similar to those from the 2004-2005 epidemic, the NA sequences differed, suggesting distinct patterns of evolution of the HA and NA genes from 2004-2007 in Taiwan. This study emphasizes that the evolution of the NA genes may contribute to reemergence of influenza B viruses.

Generation of the influenza B viruses with improved growth phenotype by substitution of specific amino acids of hemagglutinin

Variability in growth characteristics of influenza B viruses remains a serious limitation in the manufacture of inactivated influenza vaccines. Currently, serial passage in eggs is the strategy used in most instances for selection of high growth virus variants. In previous studies we found that adaptation of the strain B/Victoria/504/2000 to high growth in eggs was associated with changes only in hemagglutinin (HA). The high growth phenotype was associated with acquisition of either two (R162M and D196Y) or three (G141E, R162M and D196Y) amino acid (AA) substitutions, predicted to be near the receptor-binding domain of HA. In the present study we analyzed, using reverse genetics, the contribution to virus growth of each of these AA substitutions and determined their effect on antigenic properties. We found that G141E and R162M were most favorable for virus growth; however, only R162M could improve virus growth without antigenic alteration. Substitution D196Y had least effect on virus growth but substantially altered antigenic properties. Additional virus variants with AA substitutions at positions 126, 129, 137 and 141 were generated and characterized. The AA changes advantageous for growth of B/Victoria/504/2000 were also tested in the context of the HA of the B/Beijing/184/93, a virus with stable low-growth phenotype. All of the tested AA substitutions improved the replicative capabilities of the corresponding viruses, but only N126D and K129E had no effect on antigenicity. The results of our studies demonstrate that introduction of specific AA substitutions into viral HA can improve viral replicative efficiency while preserving the original antigenic properties.

Characterization of the epidemic influenza B viruses isolated during 2004-2005 season in Taiwan

To characterize the antigenic and genetic relationships of influenza B viruses isolated during the 2004-2005 season, a total of 11,707 clinical respiratory samples were tested of which 1572 (13.5%) were positive for influenza (463 type A and 1109 type B influenza). Of the type B viruses, 348 isolates collected in different parts of Taiwan were further analyzed. Viruses belonging to both influenza B lineages, B/Yamagata/16/88 (B/Yam) and B/Victoris/2/87 (B/Vic) were detected, although an increasing number of B/Vic lineage isolates was obtained as the season progressed. Recent B/Vic-lineage isolates were found to have additional amino acid substitutions compared to isolates from previous seasons, indicating that viruses of this lineage continue to evolve significantly and may have the capacity to become the dominant influenza B viruses worldwide. Results presented in this report demonstrate that antigenically and genetically distinct viruses within both B/Vic and B/Yam lineages co-circulate and that reassortment among these two lineages occurs frequently contributing to the genetic diversity of the circulating strains.

Discovery of the neutralizing epitope common to influenza B virus victoria group isolates in Japan

Monoclonal antibody 9B2 possesses hemagglutination inhibition activity against all the 2002/2003 influenza B virus Victoria group isolates in Kobe, Japan, as well as representative strains isolated between 1987 and 1997. The 9B2 epitope localizes three-dimensionally in the vicinity of antigenic site A of the hemagglutinin molecule, and amino acid substitutions in this region affected the binding of 9B2.

Variation of the conserved neutralizing epitope in influenza B virus victoria group isolates in Japan

For almost 20 years, the neutralizing-epitope site specific for influenza B virus Victoria group isolates was conserved at the "tip" of the hemagglutinin molecule; however, it was not detected in half of the isolates from the 2002-2003 epidemic in Japan. Amino acid substitutions (D164E or N165K) were observed at the "tip", and the epitope was altered. The viral antigenicities were affected, and human antibodies did not substantially inhibit the hemagglutination in the hemagglutination inhibition tests. It is suspected that such variants will be important in future epidemics.

Antigenic characterisation of influenza B virus with a new microneutralisation assay: comparison to haemagglutination and sequence analysis

Although the haemagglutination inhibition assay is considered the "gold standard" for antigenic characterisation of influenza viruses, some limitations of this technique are well known. A new microneutralisation assay, as a tool for antigenic characterisation of influenza B viruses, has been standardised and its performance evaluated in comparison with the haemagglutination inhibition test in the light of molecular characterisation of the haemagglutinin. Twelve B viruses belonging to the two lineages and the four sub-lineages discriminated by phylogenetic analysis of HA were tested. The microneutralisation assay clearly distinguishes viruses belonging to different lineages and, in addition, discriminates strains belonging to different sub-lineages that are poorly or not discriminated using the haemagglutination inhibition test. This new microneutralisation assay could provide a useful tool for antigenic characterisation of circulating influenza viruses and contribute, together with the haemagglutination inhibition test and sequence analysis of the haemagglutinin and neuraminidase, in the choice of the strain for use in vaccine composition.

Genetic diversity of influenza B virus: the frequent reassortment and cocirculation of the genetically distinct reassortant viruses in a community

To characterize the genetic diversity of influenza B viruses isolated during one influenza season, the antigenic and genetic relationships among 20 strains of influenza B virus isolated in February and March 2001 at one pediatric clinic in Yamagata City, Japan, were investigated. The HA gene and seven other gene segments were phylogenetically divided into three distinct sublineages (Harbin/7/94-, Tokyo/6/98-, and Shiga/T30/98-related lineage) of the Yamagata/16/88-like lineage. The NS genes of the viruses belonging to the Harbin/7/94-related lineage have additional three nucleotides at positions 439-447, and were phylogenetically distinguishable from those of the currently circulating Yamagata/16/88- and Victoria/2/87-like lineages, but were closely related to that of the Yamagata/16/88-like lineage isolated before 1994. Moreover, four strains of influenza B virus isolated in the same community between 2002 and 2003 were further examined. Phylogenetic analysis revealed that a virus of Victoria/2/87-like lineage isolated in 2003 had acquired the NA, NS, M, and PA gene segments from a Shiga/T30/98-like virus, and two strains of Harbin/7/94-related lineage had acquired the various gene segments from Shiga/T30/98-like virus through a reassortment event. These results indicate that genetically distinct multiple viruses can combine to cause an influenza B epidemic in a community and that the frequent reassortment among these viruses plays a role in generating the genetic diversity of influenza B viruses.

Mutational pattern of influenza B viruses adapted to high growth replication in embryonated eggs

Improved replication of influenza viruses in embryonated chicken eggs (CE) permits increased vaccine production and availability. We investigated the growth properties of influenza B viruses in relation to specific mutations occurring after serial passage in CE. In serial passage experiments yielding high growth variants of B/Victoria/504/2000, mutations predicted to alter amino acid (AA) composition occurred only near the receptor-binding pocket of the hemagglutinins (HA) and in no other genes. Two B/Victoria/504/2000 high growth variants had the same AA substitutions in HA (R162M and D196Y), but the higher yield variant had a third substitution (G141E), which also altered antigenic characteristics. In a serial passage experiment yielding a high growth variant of B/Hong Kong/330/2001, mutations predicted to alter AA composition occurred only in PB2 and NP in domains predicted to relate to RNP formation and function. Our results indicate that adaptation of influenza B viruses to high-yield replication by serial passage in CE requires few mutations either in internal or external genes. Specific modifications of genes or a combination of genes could be used to optimize or create influenza B viruses for specific growth substrates.

Influenza B virus victoria group with a new glycosylation site was epidemic in Japan in the 2002-2003 season

In the 2002-2003 season, influenza B virus Victoria strains were epidemic after a 6-year absence in Kobe City, Japan. They reacted poorly to the immune ferret sera prepared for use against the previous strain. An amino acid substitution in the HA1 region caused them to acquire an N-linked glycosylation site.