Structural basis for receptor specificity of influenza B virus hemagglutinin

The roles of hemagglutinin Phe-95 in receptor binding and pathogenicity of influenza B virus

Diverged ~4000 years ago, influenza B virus has several important differences from influenza A virus, including lower receptor-binding affinity and highly restricted host range. Based on our prior structural studies, we hypothesized that a single-residue difference in the receptor-binding site of hemagglutinin (HA), Phe-95 in influenza B virus versus Tyr-98 in influenza A/H1-H15, is possibly a key determinant for the low receptor-binding affinity. Here we demonstrate that the mutation Phe95→Tyr in influenza B virus HA restores all three hydrogen bonds made by Tyr-98 in influenza A/H1-15 HA and has the potential to enhance receptor binding. However, the full realization of this potential is influenced by the local environment into which the mutation is introduced. The binding and replication of the recombinant viruses correlate well with the receptor-binding capabilities of HA. These results are discussed in relation to the roles of Phe-95 in receptor binding and pathogenicity of influenza B virus.

Comparison of the Infectivity and Transmission of Contemporary Canine and Equine H3N8 Influenza Viruses in Dogs

Phylogenetic analyses indicate that canine influenza viruses (CIVs) (H3N8) evolved from contemporary equine influenza virus (EIV). Despite the genetic relatedness of EIV and CIV, recent evidence suggests that CIV is unable to infect, replicate, and spread among susceptible horses. To determine whether equine H3N8 viruses have equally lost the ability to infect, cause disease, and spread among dogs, we evaluated the infectivity and transmissibility of a recent Florida sublineage EIV isolate in dogs. Clinical signs, nasal virus shedding, and serological responses were monitored in dogs for 21 days after inoculation. Real-time reverse transcription-PCR and hemagglutination inhibition assays showed that both the viruses have maintained the ability to infect and replicate in dogs and result in seroconversion. Transmission of EIV from infected to sentinel dogs, however, was restricted. Furthermore, both CIV and EIV exhibited similar sialic acid- α 2,3-gal receptor-binding preferences upon solid-phase binding assays. The results of the in vivo experiments reported here suggesting that dogs are susceptible to EIV and previous reports by members of our laboratory showing limited CIV infection in horses have been mirrored in CIV and EIV infections studies in primary canine and equine respiratory epithelial cells.

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.

Colorimetric viral detection based on sialic acid stabilized gold nanoparticles

Sialic acid reduced and stabilized gold nanoparticles (d=20.1±1.8 nm) were synthesized by a simple one-pot, green method without chemically modifying sialic acid for colorimetric detection of influenza virus. The gold nanoparticles showed target-specific aggregation with viral particles via hemagglutinin-sialic acid binding. A linear correlation was observed between the change in optical density and dilution of chemically inactivated influenza B/Victoria and influenza B/Yamagata. Virus dilution (hemagglutination assay titer, 512) of 0.156 vol% was readily detected. The upper limit of the linearity can be extended with the use of more sialic acid-gold nanoparticles.

Human monoclonal antibodies broadly neutralizing against influenza B virus

Influenza virus has the ability to evade host immune surveillance through rapid viral genetic drift and reassortment; therefore, it remains a continuous public health threat. The development of vaccines producing broadly reactive antibodies, as well as therapeutic strategies using human neutralizing monoclonal antibodies (HuMAbs) with global reactivity, has been gathering great interest recently. Here, three hybridoma clones producing HuMAbs against influenza B virus, designated 5A7, 3A2 and 10C4, were prepared using peripheral lymphocytes from vaccinated volunteers, and were investigated for broad cross-reactive neutralizing activity. Of these HuMAbs, 3A2 and 10C4, which recognize the readily mutable 190-helix region near the receptor binding site in the hemagglutinin (HA) protein, react only with the Yamagata lineage of influenza B virus. By contrast, HuMAb 5A7 broadly neutralizes influenza B strains that were isolated from 1985 to 2006, belonging to both Yamagata and Victoria lineages. Epitope mapping revealed that 5A7 recognizes 316G, 318C and 321W near the C terminal of HA1, a highly conserved region in influenza B virus. Indeed, no mutations in the amino acid residues of the epitope region were induced, even after the virus was passaged ten times in the presence of HuMAb 5A7. Moreover, 5A7 showed significant therapeutic efficacy in mice, even when it was administered 72 hours post-infection. These results indicate that 5A7 is a promising candidate for developing therapeutics, and provide insight for the development of a universal vaccine against influenza B virus.

Influenza virus is classified into types A, B and C. Influenza A virus is further divided into many subtypes, all of which exist in animals, indicating pandemic potential. By contrast, influenza B virus circulates almost exclusively in humans and, as there is no evidence for reassortment with influenza A virus, there is no indication of pandemic potential. Hence, there is far less accumulated research information regarding influenza B virus than influenza A virus. Influenza B virus, which is classified into two phylogenetic lineages, does, however, cause annual epidemics in humans and is therefore as essential to control as influenza A virus. Recently, the development of a universal vaccine and therapeutic strategies using human monoclonal antibodies (HuMAbs) has been gathering great interest. The present study reports a HuMAb neutralizing a wide range of influenza B viruses of both lineages. This HuMAb recognizes the conserved region of hemagglutinin. Moreover, therapeutic efficacy of this HuMAb was also confirmed by in vivo animal experiments. Thus, this study provides insight for the development of broad-spectrum therapeutics and a universal prophylactic vaccine against influenza B virus.

Isolation of a novel swine influenza virus from Oklahoma in 2011 which is distantly related to human influenza C viruses

Of the Orthomyxoviridae family of viruses, only influenza A viruses are thought to exist as multiple subtypes and has non-human maintenance hosts. In April 2011, nasal swabs were collected for virus isolation from pigs exhibiting influenza-like illness. Subsequent electron microscopic, biochemical, and genetic studies identified an orthomyxovirus with seven RNA segments exhibiting approximately 50% overall amino acid identity to human influenza C virus. Based on its genetic organizational similarities to influenza C viruses this virus has been provisionally designated C/Oklahoma/1334/2011 (C/OK). Phylogenetic analysis of the predicted viral proteins found that the divergence between C/OK and human influenza C viruses was similar to that observed between influenza A and B viruses. No cross reactivity was observed between C/OK and human influenza C viruses using hemagglutination inhibition (HI) assays. Additionally, screening of pig and human serum samples found that 9.5% and 1.3%, respectively, of individuals had measurable HI antibody titers to C/OK virus. C/OK virus was able to infect both ferrets and pigs and transmit to naive animals by direct contact. Cell culture studies showed that C/OK virus displayed a broader cellular tropism than a human influenza C virus. The observed difference in cellular tropism was further supported by structural analysis showing that hemagglutinin esterase (HE) proteins between two viruses have conserved enzymatic but divergent receptor-binding sites. These results suggest that C/OK virus represents a new subtype of influenza C viruses that currently circulates in pigs that has not been recognized previously. The presence of multiple subtypes of co-circulating influenza C viruses raises the possibility of reassortment and antigenic shift as mechanisms of influenza C virus evolution.

Mapping the phosphoproteome of influenza A and B viruses by mass spectrometry

Protein phosphorylation is a common post-translational modification in eukaryotic cells and has a wide range of functional effects. Here, we used mass spectrometry to search for phosphorylated residues in all the proteins of influenza A and B viruses--to the best of our knowledge, the first time such a comprehensive approach has been applied to a virus. We identified 36 novel phosphorylation sites, as well as confirming 3 previously-identified sites. N-terminal processing and ubiquitination of viral proteins was also detected. Phosphorylation was detected in the polymerase proteins (PB2, PB1 and PA), glycoproteins (HA and NA), nucleoprotein (NP), matrix protein (M1), ion channel (M2), non-structural protein (NS1) and nuclear export protein (NEP). Many of the phosphorylation sites detected were conserved between influenza virus genera, indicating the fundamental importance of phosphorylation for all influenza viruses. Their structural context indicates roles for phosphorylation in regulating viral entry and exit (HA and NA); nuclear localisation (PB2, M1, NP, NS1 and, through NP and NEP, of the viral RNA genome); and protein multimerisation (NS1 dimers, M2 tetramers and NP oligomers). Using reverse genetics we show that for NP of influenza A viruses phosphorylation sites in the N-terminal NLS are important for viral growth, whereas mutating sites in the C-terminus has little or no effect. Mutating phosphorylation sites in the oligomerisation domains of NP inhibits viral growth and in some cases transcription and replication of the viral RNA genome. However, constitutive phosphorylation of these sites is not optimal. Taken together, the conservation, structural context and functional significance of phosphorylation sites implies a key role for phosphorylation in influenza biology. By identifying phosphorylation sites throughout the proteomes of influenza A and B viruses we provide a framework for further study of phosphorylation events in the viral life cycle and suggest a range of potential antiviral targets.

Characterization of glycan binding specificities of influenza B viruses with correlation with hemagglutinin genotypes and clinical features

The carbohydrate binding specificities are different among avian and human influenza A viruses and may affect the tissue tropism and transmission of these viruses. The glycan binding biology for influenza B, however, has not been systematically characterized. Glycan binding specificities of influenza B viral isolates were analyzed and correlated to hemagglutinin (HA) genotypes and clinical manifestations. A newly developed solution glycan array was applied to characterize the receptor binding specificities of influenza B virus clinical isolates from 2001 to 2007 in Taiwan. Thirty oligosaccharides which include α-2,3 and α-2,6 linkage glycans were subjected to analysis. The glycan binding patterns of 53 influenza B isolates could be categorized into three groups and were well correlated to their HA genotypes. The Yamagata-like strains predominantly bound to α-2,6-linkage glycan (24:29, 83%) while Victoria-like strains preferentially bound to both α-2,3- and α-2,6-linkage glycans (13:24, 54%). A third group of viruses bound to sulfated glycans and these all belonged to Victoria-like strains. Based on the HA sequences, Asn-163, Glu-198, Ala-202, and Lys-203 were conserved among Victoria-like strains which may influence their carbohydrate recognition. The viruses bound to dual type glycans were more likely to be associated with the development of bronchopneumonia and gastrointestinal illness than those bound only to α-2,6 sialyl glycans (P < 0.05). Glycan binding analyses provide additional information to monitor the antigenic shift, tissue tropism, and transmission capability of influenza B viruses, and will contribute to virus surveillance and vaccine strain selection.

Receptor binding surveillance of influenza clinical isolates

Aims: The outbreak of highly pathogenic avian influenza H5N1 in poultry and the influenza A pandemic have wreaked havoc on public health. Viruses alter their carbohydrate binding preferences, thereby causing pandemics all over the world. In this study, we tried to investigate the carbohydrate binding specificity of influenza clinical isolates. Materials & methods: Biotin-conjugated polyacrylamide-based glycan epitopes were immobilized on UltraBind™ membranes and used to survey the glycan-binding preference of influenza clinical isolates, including seasonal influenza A, A(H1N1)pdm09 and influenza B viruses. In addition, the DNA sequences of influenza B virus hemagglutinin were analyzed. Results: Human influenza A, especially the A(H1N1)pdm09 viruses, accepted α2,6 and α2,3 sialylated glycans, sulfated glycans and α2,8 sialosides. Although all influenza B clinical isolates bound strongly to NeuAc, 6´-sialyl lactose and sialyl biantennary N-glycan, some viruses also recognized sulfated and α2,3 sialylated glycans. According to the nucleotide sequences of viral hemagglutinin, influenza B viruses that exhibited weak interaction with sulfated and α2,3 sialylated glycans showed fewer charged amino acids. Conclusion: The substrate specificities of influenza clinical isolates were surveyed. Influenza A exhibited more complicated glycan-binding patterns than influenza B viruses. Our findings provided a systematic investigation of receptor-binding specificities for influenza clinical isolates, as well as useful information for exploring viral tropism.

Receptor binding by influenza virus: using computational techniques to extend structural data

Influenza attaches to host cells via hemagglutinin binding of cell-surface glycans. These relatively low-affinity interactions involving flexible ligands are critical in determining tissue and host specificity, but their dynamic nature complicates structural characterization of hemagglutinin-receptor complexes. Molecular simulation can assist in analyzing glycan and protein flexibility in crystallized complexes, assessing how binding might change under mutation or altered glycosylation patterns, and evaluating how soluble ligands may relate to physiological presentation on the plasma membrane. Molecular dynamics simulation also has the potential to help integrate structural and dynamic data sources. Here we review recent progress from analysis of molecular dynamics simulation and outline challenges for the future.

Myocardial injury and bacterial pneumonia contribute to the pathogenesis of fatal influenza B virus infection

BACKGROUND

Influenza B virus infection causes rates of hospitalization and influenza-associated pneumonia similar to seasonal influenza A virus infection and accounts for a substantial percentage of all influenza-related hospitalizations and deaths among those aged <18 years; however, the pathogenesis of fatal influenza B virus infection is poorly described.

METHODS

Tissue samples obtained at autopsy from 45 case patients with fatal influenza B virus infection were evaluated by light microscopy and immunohistochemical assays for influenza B virus, various bacterial pathogens, and complement components C4d and C9, to identify the cellular tropism of influenza B virus, characterize concomitant bacterial pneumonia, and describe the spectrum of cardiopulmonary injury.

RESULTS

Viral antigens were localized to ciliated respiratory epithelium and cells of submucosal glands and ducts. Concomitant bacterial pneumonia, caused predominantly by Staphylococcus aureus, was identified in 38% of case patients and occurred with significantly greater frequency in those aged >18 years. Pathologic evidence of myocardial injury was identified in 69% of case patients for whom cardiac tissue samples were available for examination, predominantly in case patients aged <18 years.

CONCLUSIONS

Our findings suggest that bacterial pneumonia and cardiac injury contribute to fatal outcomes after infection with influenza B virus and that the frequency of these manifestations may be age related.

Viruses and sialic acids: rules of engagement

Viral infections are initiated by specific attachment of a virus particle to receptors at the surface of the host cell. For many viruses, these receptors are glycans that are linked to either a protein or a lipid. Glycans terminating in sialic acid and its derivatives serve as receptors for a large number of viruses, including several human pathogens. In combination with glycan array analyses, structural analyses of complexes of viruses with sialylated oligosaccharides have provided insights into the parameters that underlie each interaction. Here, we compare the currently available structural data on viral attachment proteins in complex with sialic acid and its variants. The objective is to define common parameters of recognition and to provide a platform for understanding the determinants of specificity. This information could be of use for the prediction of the location of sialic acid binding sites in viruses for which structural information is still lacking. An improved understanding of the principles that govern the recognition of sialic acid and sialylated oligosaccharides would also advance efforts to develop efficient antiviral agents.

Influenza virus entry

As all the enveloped viruses, the entry of influenza viruses includes a number of steps in host cell infection. This chapter summarizes the current knowledge of the entry pathway and the role of the fusion protein of influenza virus, hemagglutinin, in this process. Hemagglutinin (HA) is a trimeric glycoprotein that is present in multiple copies in the membrane envelope of influenza virus. HA contains a fusion peptide, a receptor binding site, a metastable structural motif, and the transmembrane domain. The first step of influenza virus entry is the recognition of the host cell receptor molecule, terminal α-sialic acid, by HA. This multivalent attachment by multiple copies of trimetric HA triggers endocytosis of influenza virus that is contained in the endosome. The endosome-trapped virus traffics via a unidirectional pathway to near the nucleus. At this location, the interior pH of the endosome becomes acidic that induces a dramatic conformational change in HA to insert the fusion peptide into the host membrane, induce juxtaposition of the two membranes, and form a fusion pore that allows the release of the genome segments of influenza virus. HA plays a key role in the entire entry pathway. Inhibitors of virus entry are potentially effective antiviral drugs of influenza viruses.

Phenotypic and genotypic characterization of influenza virus mutants selected with the sialidase fusion protein DAS181

BACKGROUND

influenza viruses (IFVs) frequently achieve resistance to antiviral drugs, necessitating the development of compounds with novel mechanisms of action. DAS181 (Fludase), a sialidase fusion protein, may have a reduced potential for generating drug resistance due to its novel host-targeting mechanism of action.

METHODS

IFV strains B/Maryland/1/59 and A/Victoria/3/75 (H3N2) were subjected to >30 passages under increasing selective pressure with DAS181. The DAS181-selected IFV isolates were characterized in vitro and in mice.

RESULTS

despite extensive passaging, DAS181-selected viruses exhibited a very low level of resistance to DAS181, which ranged between 3- and 18-fold increase in EC(50). DAS181-selected viruses displayed an attenuated phenotype in vitro, as exhibited by slower growth, smaller plaque size and increased particle to pfu ratios relative to wild-type virus. Further, the DAS181 resistance phenotype was unstable and was substantially reversed over time upon DAS181 withdrawal. In mice, the DAS181-selected viruses exhibited no greater virulence than their wild-type counterparts. Genotypic and phenotypic analysis of DAS181-selected viruses revealed mutations in the haemagglutinin (HA) and neuraminidase (NA) molecules and also changes in HA and NA function.

CONCLUSIONS

results indicate that resistance to DAS181 is minimal and unstable. The DAS181-selected IFV isolates exhibit reduced fitness in vitro, likely due to altered HA and NA functions.

Genetic sequence analysis of influenza viruses and illness severity in ill children previously vaccinated with live attenuated or inactivated influenza vaccine

In a large comparative study in 2004-2005, children aged 6-59 months vaccinated with live attenuated influenza vaccine (LAIV) experienced 55% fewer cases of culture-confirmed influenza illness compared with trivalent inactivated influenza vaccine (TIV) recipients. To better understand the characteristics of the breakthrough influenza illnesses, we analyzed the HA1 genetic sequence for all available samples and examined disease severity by strain and treatment group. All 48 A/H1N1 viruses were well-matched to the vaccine, whereas all 276 A/H3N2 viruses and 349 (96%) influenza B viruses were mismatched to the vaccine. The incidence of fever or lower respiratory illness did not differ by strain; however, LAIV recipients had less febrile disease and fewer lower respiratory illnesses than TIV recipients. Viruses of each influenza B lineage caused more illnesses than A/H1N1 viruses; strategies to enhance protection against multiple influenza B lineages should be pursued.

Shift in oligosaccharide specificities of hemagglutinin and neuraminidase of influenza B viruses resistant to neuraminidase inhibitors

Influenza virus neuraminidase inhibitors (NAIs), currently used as anti-influenza drugs, can lead to the appearance of drug-resistant variants. Resistance to NAIs appears due to mutations in the active site of the neuraminidase (NA) molecule that decrease the NA enzymatic activity and sometimes in the hemagglutinin (HA) that decrease its affinity for cell receptors and, therefore, reduce the requirement for NA activity in releasing mature virions from infected cells. Using a set of sialo-oligosaccharides, we evaluated changes in the receptor-binding specificity of the HA and substrate specificity of the NA of influenza B viruses that had acquired resistance to NAIs. The oligosaccharide specificity of two pairs of field influenza B viruses, namely: i) B/Memphis/20/96 and its NAI-resistant variant, B/Memphis/20-152K/96, containing mutation R152K in the NA and 5 amino acid substitutions in the HA1, and ii) B/Hong Kong/45/2005 and its NAI-resistant variant B/Hong Kong/36/2005, containing a single R371K mutation in the NA, was evaluated. Wild type viruses bound strictly to a "human type" receptor, alpha2-6-sialo-oligosaccharide 6;SLN, but desialylated it is approximately 8 times less efficiently than the alpha2-3 sialosaccharides. Both drug-resistant viruses demonstrated the ability to bind to "avian type" receptors, alpha2-3 sialo-oligosaccharides (such as 3;SLN), whereas their affinity for 6;SLN was noticeably reduced in comparison with corresponding wild type viruses. Thus, the development of the NAI resistance in the studied influenza B viruses was accompanied by a readjustment of HA-NA oligosaccharide specificities.

Structural Biology and the Design of Effective Vaccines for HIV-1 and Other Viruses

Structural biology provides a wealth of information about the three-dimensional organization and chemical makeup of proteins. An understanding of atomic-level structure offers enormous potential to design rationally proteins that stimulate specific immune responses. Yet current vaccine development efforts makes little use of structural information. At the Vaccine Research Center, a major goal is to apply structural techniques to vaccine design for challenging pathogens, that include human immunodeficiency virus type 1 (HIV-1) and other enveloped viruses such as influenza, Ebola, and respiratory syncytial viruses. Our three-part strategy involves 1.) the definition of the functional viral spike at the atomic level 2.) achieving a structural understanding of how neutralizing antibodies recognize the spike, and 3.) rational development of proteins that can elicit a specific antibody response. Overall, our strategy aims to incorporate information about viral spike-antibody interactions, to assimilate immunogenic feedback, and to leverage recent advances in immunofocusing and computational biology.

Changes of the receptor-binding properties of influenza B virus B/Victoria/504/2000 during adaptation in chicken eggs

Selection of high-growth virus variants of strain B/Victoria/504/2000 by serial passage in eggs resulted in three amino acid substitutions, G141E, R162M, and D196Y, in the vicinity of the receptor-binding pocket of viral hemagglutinin. Virus variants containing the identified amino acid substitutions, individually or in various combinations, were constructed using reverse genetics and analyzed for their receptor-binding properties using glycan microarray platform. Three different patterns of virus binding were revealed. A low-growth virus variant, corresponding to the original egg-derived virus B/Victoria/504/2000 prior to acquisition of amino acid changes G141E, R162M, and D196Y, had a clear preference for the oligosaccharide chains terminated with alpha2-6-linked sialic acid with very weak binding of the glycans terminated with alpha2-3-linked sialic acid. Amino acid substitutions R162M and D196Y had similar effects, resulting in viruses that bound with high efficiency almost all terminally sialylated glycans represented on the array regardless of the type of glycosidic linkage. In contrast, substitution of G141E alone, or in combinations with the other two amino acid substitutions, significantly restricted virus glycan-binding capabilities. All virus variants possessing this substitution lost the ability to bind glycans with alpha2-6 glycosidic linkage as well as most of the glycans with alpha2-3 glycosidic linkage. Linear penta- and heptasaccharide chains represented at the non-reducing end by alpha2-3 sialylated Type-II motif (LacNAc) were the only structures bound with high affinity by the virus variants with G141E substitution. In all cases when the effects on virus binding of individual amino acid substitutions differed, the effect of R162M was subordinate to the effect of either G141E or D196Y.

Isolation and molecular characterization of equine H3N8 influenza viruses from pigs in China

During 2004-2006 swine influenza virus surveillance, two strains of H3N8 influenza viruses were isolated from pigs in central China. Sequence and phylogenetic analyses of eight gene segments revealed that the two swine isolates were of equine origin and most closely related to European equine H3N8 influenza viruses from the early 1990s. Comparison of hemagglutinin (HA) amino acid sequences showed several important substitutions. One substitution caused the loss of a potential glycosylation site, and two substitutions, located at the cleavage site and adjacent to the receptor-binding pocket, respectively, had been reported previously in canine H3 HAs. This expansion of host range of equine H3N8 influenza viruses with mutations in the HA protein might raise the possibility of transmission of these viruses to humans.

Microbial recognition of human cell surface glycoconjugates

Infection by pathogens is generally initiated by the specific recognition of host epithelia surfaces and subsequent adhesion is essential for invasion. In their infection strategy, microorganisms often use sugar-binding proteins, that is lectins and adhesins, to recognize and bind to host glycoconjugates where sialylated and fucosylated oligosaccharides are the major targets. The lectin/glycoconjugate interactions are characterized by their high specificity and most of the time by multivalency to generate higher affinity of binding. Recent crystal structures of viral, bacterial, and parasite receptors in complex with human histo-blood group epitopes or sialylated derivatives reveal new folds and novel sugar-binding modes. They illustrate the tight specificity between tissue glycosylation and lectins.

Evolving complexities of influenza virus and its receptors

Sialic acids (Sias) are regarded as receptors for influenza viruses and are usually bound to galactose (Gal) in an alpha2-3 or alpha2-6 configuration. The detection of these Sia configurations in tissues has commonly been through the use of plant lectins that are able to identify which cells contain Siaalpha2-3- and Siaalpha2-6-linked glycans, although other techniques for receptor distribution have been used. Initial experiments indicated that avian versus human influenza virus binding was determined by either Siaalpha2-6 or Siaalpha2-3 expression. In this review, we suggest that the distribution and detection of these terminal Siaalpha2-3- and Siaalpha2-6-linked receptors within the respiratory tract might not be as clear cut as has been reported. We will also review how other viral and receptor components might act as determinants for successful viral replication and transmission. Understanding these additional components is important in comprehending the infection and the transmission of both existing human influenza viruses and newly emerging avian influenza viruses.

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.