Receptor binding specificity of recent human H3N2 influenza viruses

The receptor preference of influenza viruses

OBJECTIVES

The cell surface receptor used by an influenza virus to infect that cell is an N-acetyl neuraminic acid (NANA) residue terminally linked by an alpha2,3 or alpha2,6 bond to a carbohydrate moiety of a glycoprotein or glycolipid. Our aim was to determine a quick and technically simple method to determine cell receptor usage by whole influenza A virus particles.

METHODS

We employed surface plasmon resonance to detect the binding of viruses to fetuin, a naturally occurring glycoprotein that has both alpha2,3- and alpha2,6-linked NANA, and free 3'-sialyllactose or 6'-sialyllactose to compete virus binding. All virus stocks were produced in embryonated chicken's eggs.

RESULTS

The influenza viruses tested bound preferentially to NANAalpha2,3Gal or to NANAalpha2,6Gal, or showed no preference. Two PR8 viruses had different binding preferences. Binding preferences of viruses correlated well with their known biological properties.

CONCLUSIONS

Our data suggest that it is not easy to predict receptor usage by influenza viruses. However, direct experimental determination as described here can inform experiments concerned with viral pathogenesis, biology and structure. In principle, the methodology can be used for any virus that binds to a terminal NANA residue.

Neuraminidase receptor binding variants of human influenza A(H3N2) viruses resulting from substitution of aspartic acid 151 in the catalytic site: a role in virus attachment?

Changes in the receptor binding characteristics of human H3N2 viruses have been evident from changes in the agglutination of different red blood cells (RBCs) and the reduced growth capacity of recently isolated viruses, particularly in embryonated eggs. An additional peculiarity of viruses circulating in 2005 to 2009 has been the poor inhibition of hemagglutination by postinfection ferret antisera for many viruses isolated in MDCK cells, including homologous reference viruses. This was shown not to be due to an antigenic change in hemagglutinin (HA) but was shown to be the result of a mutation in aspartic acid 151 of neuraminidase (NA) to glycine, asparagine, or alanine, which caused an oseltamivir-sensitive agglutination of RBCs. The D151G substitution was shown to cause a change in the specificity of NA such that it acquired the capacity to bind receptors, which were refractory to enzymatic cleavage, without altering its ability to remove receptors for HA. Thus, the inhibition of NA-dependent agglutination by the inclusion of oseltamivir carboxylate in the assay was effective in restoring the anti-HA specificity of the hemagglutination inhibition (HI) assay for monitoring antigenic changes in HA. Since the NA-dependent binding activity did not affect virus neutralization, and virus populations in clinical specimens possessed, at most, low levels of the "151 mutant," the biological significance of this feature of NA in, for example, immune evasion is unclear. It is apparent, however, that an important role of aspartic acid 151 in the activity of NA may be to restrict the specificity of the NA interaction and its receptor-destroying activity to complement that of HA receptor binding.

Structure and Receptor binding properties of a pandemic H1N1 virus hemagglutinin

The 3D-structure of the major surface viral antigen from the recent H1N1 pandemic influenza virus (A/Darwin/2001/2009) was determined to 2.8 Å resolution. The structure was used to analyze changes in the HA that have emerged during the first 11 months of the pandemic and have raised public health concerns. Receptor binding properties of this protein reveals a strict preference for human-type receptors.

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.

Cytosolic delivery mediated via electrostatic surface binding of protein, virus, or siRNA cargos to pH-responsive core-shell gel particles

We recently described a strategy for intracellular delivery of macromolecules, utilizing pH-responsive "core-shell" structured gel particles. These cross-linked hydrogel particles disrupt endosomes with low toxicity by virtue of physical sequestration of an endosome-disrupting "proton sponge" core inside a nontoxic hydrophilic shell. Here we tested the efficacy of this system for cytosolic delivery of a broad range of macromolecular cargos, and demonstrate the delivery of proteins, whole viral particles, or siRNA oligonucleotides into the cytosol of dendritic cells and epithelial cells via core-shell particles. We assessed the functional impact of particle delivery for vaccine applications and found that cytosolic delivery of protein antigens in dendritic cells via the core-shell particles promotes priming of CD8(+) T-cells at 100-fold lower doses than soluble protein. Functional gene knockdown following delivery of siRNA using the particles was demonstrated in epithelial cells. Based on these findings, these materials may be of interest for a broad range of biomedical applications.

Tropism and innate host responses of the 2009 pandemic H1N1 influenza virus in ex vivo and in vitro cultures of human conjunctiva and respiratory tract

The novel pandemic influenza H1N1 (H1N1pdm) virus of swine origin causes mild disease but occasionally leads to acute respiratory distress syndrome and death. It is important to understand the pathogenesis of this new disease in humans. We compared the virus tropism and host-responses elicited by pandemic H1N1pdm and seasonal H1N1 influenza viruses in ex vivo cultures of human conjunctiva, nasopharynx, bronchus, and lung, as well as in vitro cultures of human nasopharyngeal, bronchial, and alveolar epithelial cells. We found comparable replication and host-responses in seasonal and pandemic H1N1 viruses. However, pandemic H1N1pdm virus differs from seasonal H1N1 influenza virus in its ability to replicate in human conjunctiva, suggesting subtle differences in its receptor-binding profile and highlighting the potential role of the conjunctiva as an additional route of infection with H1N1pdm. A greater viral replication competence in bronchial epithelium at 33 degrees C may also contribute to the slight increase in virulence of the pandemic influenza virus. In contrast with highly pathogenic influenza H5N1 virus, pandemic H1N1pdm does not differ from seasonal influenza virus in its intrinsic capacity for cytokine dysregulation. Collectively, these results suggest that pandemic H1N1pdm virus differs in modest but subtle ways from seasonal H1N1 virus in its intrinsic virulence for humans, which is in accord with the epidemiology of the pandemic to date. These findings are therefore relevant for understanding transmission and therapy.

Context-specific target definition in influenza a virus hemagglutinin-glycan receptor interactions

Protein-glycan interactions are important regulators of a variety of biological processes, ranging from immune recognition to anticoagulation. An important area of active research is directed toward understanding the role of host cell surface glycans as recognition sites for pathogen protein receptors. Recognition of cell surface glycans is a widely employed strategy for a variety of pathogens, including bacteria, parasites, and viruses. We present here a representative example of such an interaction: the binding of influenza A hemagglutinin (HA) to specific sialylated glycans on the cell surface of human upper airway epithelial cells, which initiates the infection cycle. We detail a generalizable strategy to understand the nature of protein-glycan interactions both structurally and biochemically, using HA as a model system. This strategy combines a top-down approach using available structural information to define important contacts between glycans and HA, with a bottom-up approach using data-mining and informatics approaches to identify the common motifs that distinguish glycan binders from nonbinders. By probing protein-glycan interactions simultaneously through top-down and bottom-up approaches, we can scientifically validate a series of observations. This in turn provides additional confidence and surmounts known challenges in the study of protein-glycan interactions, such as accounting for multivalency, and thus truly defines concepts such as specificity, affinity, and avidity. With the advent of new technologies for glycomics-including glycan arrays, data-mining solutions, and robust algorithms to model protein-glycan interactions-we anticipate that such combination approaches will become tractable for a wide variety of protein-glycan interactions.

Tissue and host tropism of influenza viruses: importance of quantitative analysis

It is generally accepted that human influenza viruses preferentially bind to cell-surface glycoproteins/glycolipids containing sialic acids in alpha2,6-linkage; while avian and equine influenza viruses preferentially bind to those containing sialic acids in alpha2,3-linkage. Even though this generalized view is accurate for H3 subtype isolates, it may not be accurate and absolute for all subtypes of influenza A viruses and, therefore, needs to be reevaluated carefully and realistically. Some of the studies published in major scientific journals on the subject of tissue tropism of influenza viruses are inconsistent and caused confusion in the scientific community. One of the reasons for the inconsistency is that most studies were quantitative descriptions of sialic acid receptor distributions based on lectin or influenza virus immunohistochemistry results with limited numbers of stained cells. In addition, recent studies indicate that alpha2,3- and alpha2,6-linked sialic acids are not the sole receptors determining tissue and host tropism of influenza viruses. In fact, determinants for tissue and host tropism of human, avian and animal influenza viruses are more complex than what has been generally accepted. Other factors, such as glycan topology, concentration of invading viruses, local density of receptors, lipid raft microdomains, coreceptors or sialic acid-independent receptors, may also be important. To more efficiently control the global spread of pandemic influenza such as the current circulating influenza A H1N1, it is crucial to clarify the determinants for tissue and host tropism of influenza viruses through quantitative analysis of experimental results. In this review, I will comment on some conflicting issues related to tissue and host tropism of influenza viruses, discuss the importance of quantitative analysis of lectin and influenza virus immunohistochemistry results and point out directions for future studies in this area, which should lead to a better understanding of tissue and host tropism of influenza viruses.

The pH of activation of the hemagglutinin protein regulates H5N1 influenza virus pathogenicity and transmissibility in ducks

While the molecular mechanism of membrane fusion by the influenza virus hemagglutinin (HA) protein has been studied extensively in vitro, the role of acid-dependent HA protein activation in virus replication, pathogenesis, and transmission in vivo has not been characterized. To investigate the biological significance of the pH of activation of the HA protein, we compared the properties of four recombinant viruses with altered HA protein acid stability to those of wild-type influenza virus A/chicken/Vietnam/C58/04 (H5N1) in vitro and in mallards. Membrane fusion by wild-type virus was activated at pH 5.9. Wild-type virus had a calculated environmental persistence of 62 days and caused extensive morbidity, mortality, shedding, and transmission in mallards. An N114K mutation that increased the pH of HA activation by 0.5 unit resulted in decreased replication, genetic stability, and environmental stability. Changes of +0.4 and -0.5 unit in the pH of activation by Y23H and K58I mutations, respectively, reduced weight loss, mortality, shedding, and transmission in mallards. An H24Q mutation that decreased the pH of activation by 0.3 unit resulted in weight loss, mortality, clinical symptoms, and shedding similar to those of the wild type. However, the HA-H24(1)Q virus was shed more extensively into drinking water and persisted longer in the environment. The pH of activation of the H5 HA protein plays a key role in the propagation of H5N1 influenza viruses in ducks and may be a novel molecular factor in the ecology of influenza viruses. The data also demonstrate that H5N1 neuraminidase activity increases the pH of activation of the HA protein in vitro.

Sialic acids as regulators of molecular and cellular interactions

The wide occurrence of sialic acids (Sia) in various chemical forms linked as monomers or polymers in an outstanding position in a multitude of complex carbohydrates of animals and microorganisms renders them as most versatile function modulators in cell biology and pathology. A survey is presented of recent advances in the study of the influences that Sias have as bulky hydrophilic and electronegatively charged monosaccharides on animal cells and on their interaction with microorganisms. Some highlights are: sialylation leads to increased anti-inflammatory activity of IgG antibodies, facilitates the escape of microorganisms from the host's immune system, and in polymeric form is involved in the regulation of embryogenesis and neuronal growth and function. The role of siglecs in immunoregulation, the dynamics of lymphocyte binding to selectins and the interactions of toxins, viruses, and other microorganisms with the host's Sia are now better understood. N-Glycolylneuraminic acid from food is antigenic in man and seems to have pathogenic potential. Sia O-acetylation mediated by various eukaryotic and prokaryotic O-acetyltransferases modulates the affinity of these monosaccharides to mammalian and microbial receptors and hinders apoptosis. The functionally versatile O-acetylated ganglioside GD3 is an onco-fetal antigen.

Comparative efficacy of inactivated and live attenuated influenza vaccines

BACKGROUND

The efficacy of influenza vaccines may vary from year to year, depending on a variety of factors, and may differ for inactivated and live attenuated vaccines.

METHODS

We carried out a randomized, double-blind, placebo-controlled trial of licensed inactivated and live attenuated influenza vaccines in healthy adults during the 2007-2008 influenza season and estimated the absolute and relative efficacies of the two vaccines.

RESULTS

A total of 1952 subjects were enrolled and received study vaccines in the fall of 2007. Influenza activity occurred from January through April 2008, with the circulation of influenza types A (H3N2) (about 90%) and B (about 9%). Absolute efficacy against both types of influenza, as measured by isolating the virus in culture, identifying it on real-time polymerase-chain-reaction assay, or both, was 68% (95% confidence interval [CI], 46 to 81) for the inactivated vaccine and 36% (95% CI, 0 to 59) for the live attenuated vaccine. In terms of relative efficacy, there was a 50% (95% CI, 20 to 69) reduction in laboratory-confirmed influenza among subjects who received inactivated vaccine as compared with those given live attenuated vaccine. The absolute efficacy against the influenza A virus was 72% (95% CI, 49 to 84) for the inactivated vaccine and 29% (95% CI, -14 to 55) for the live attenuated vaccine, with a relative efficacy of 60% (95% CI, 33 to 77) for the inactivated vaccine.

CONCLUSIONS

In the 2007-2008 season, the inactivated vaccine was efficacious in preventing laboratory-confirmed symptomatic influenza A (predominately H3N2) in healthy adults. The live attenuated vaccine also prevented influenza illnesses but was less efficacious. (ClinicalTrials.gov number, NCT00538512.)

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.

New development of glycan arrays

The development of glycan arrays has enabled the high-sensitivity and high-throughput analysis of carbohydrate-protein interactions and contributed to significant advances in glycomics. A number of new array platforms that allow for qualitative and quantitative analysis of mono- and multivalent interactions on surfaces have been developed recently. Glycan arrays are not only a powerful tool for basic research, but also a promising technique for medical diagnosis, and detection of pathogens and cancers. These studies also have led to the design of efficient carbohydrate-based antimicrobial or anticancer vaccines.

XPS and SPR analysis of glycoarray surface density

Despite the fact that the carbohydrate microarray has seen increasing use within the field of glycobiology, the surface chemistry of the glycoarray remains largely unexplored. Motivated by the need to develop surface analytical techniques to characterize carbohydrate-modified surfaces, we developed a quantitative X-ray photoelectron spectroscopy (XPS) and surface plasmon resonance imaging (SPR imaging) method to study glycan biosensors. We performed a comparative analysis on the relative coverage of mixed self-assembled monolayers (SAMs) on gold, consisting of a thiol-functionalized trimannoside (Manalpha1,2Manalpha1,2Manalpha-OEG-SH) at varying concentrations (0-100%) mixed separately with two thiol-containing polyethylene glycol oligomers. XPS C1s core level analysis was used to identify the O-C-O functionality unique to the carbohydrate acetal moiety and to separate and quantify the relative coverage of sugar in carbohydrate/OEG mixed SAMs. XPS spectra of the mixed SAMs demonstrated a proportional increase in the acetal signature of the glycan with increasing sugar concentration. To relate surface glycan density with biological function, we carried out a kinetic analysis of Concanavalin A (ConA) binding to SAMs of varying densities of carbohydrate using SPR imaging. We observed protein binding that was highly dependent on both glycan density and the nature of the OEG-thiol used in the mixed self-assembly. These results illustrate the utility of surface analytical techniques such as XPS and SPR in carbohydrate biosensor characterization and optimization.

Deletions of neuraminidase and resistance to oseltamivir may be a consequence of restricted receptor specificity in recent H3N2 influenza viruses

Background

Influenza viruses attach to cells via sialic acid receptors. The viral neuraminidase (NA) is needed to remove sialic acids so that newly budded virions can disperse. Known mechanisms of resistance to NA inhibitors include mutations in the inhibitor binding site, or mutations in the hemagglutinin that reduce avidity for sialic acid and therefore reduce the requirement for NA activity.

Results

Influenza H3N2 isolates A/Oklahoma/323/03 (Fujian-like), A/Oklahoma/1992/05 (California-like), and A/Oklahoma/309/06 (Wisconsin-like) lost NA activity on passage in MDCK cells due to internal deletions in the NA-coding RNA segment. The viruses grow efficiently in MDCK cells despite diminished NA activity. The full length NA enzyme activity is sensitive to oseltamivir but replication of A/Oklahoma/323/03 and A/Oklahoma/309/06 in MDCK cells was resistant to this inhibitor, indicating that NA is not essential for replication. There was no change in HA activity or sequence after the NA activity was lost but the three viruses show distinct, quite restricted patterns of receptor specificity by Glycan Array analysis. Extensive predicted secondary structure in RNA segment 6 that codes for NA suggests the deletions are generated by polymerase skipping over base-paired stem regions. In general the NA deletions were not carried into subsequent passages, and we were unable to plaque-purify virus with a deleted NA RNA segment.

Conclusion

H3N2 viruses from 2003 to the present have reduced requirement for NA when passaged in MDCK cells and are resistant to NA inhibitors, possibly by a novel mechanism of narrow receptor specificity such that virus particles do not self-aggregate. These viruses delete internal regions of the NA RNA during passage and are resistant to oseltamivir. However, deletions are independently generated at each passage, suggesting that virus with a full length NA RNA segment initiates the first round of infection.

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.

Emergence of oseltamivir-resistant influenza A/H3N2 virus with altered hemagglutination pattern in a hematopoietic stem cell transplant recipient

BACKGROUND

Persistent influenza virus replication during antiviral therapy in patients undergoing hematopoietic stem cell transplantation (HSCT) could promote the emergence of antiviral drug resistance.

OBJECTIVES

To follow the viral genotypic and drug susceptibility changes in a patient who developed progressive influenza A/H3N2 pneumonia despite oseltamivir therapy after haploidentical HSCT.

STUDY DESIGN

Direct genotypic analysis of the neuraminidase (NA) and hemagglutinin (HA) genes in successive bronchoalveolar lavage specimens was employed in combination with hemagglutination and NA enzymatic activity assays of the corresponding viral isolates.

RESULTS

The emergence of NA oseltamivir-resistance mutation R292K was detected by 12 days of oseltamivir treatment with 44,286-fold increase in oseltamivir IC50. Resurgence of wild type viral population was identified by 7 days after cessation of oseltamivir. Sequential HA mutations R228S and A138S were identified and associated with a shift in the HA receptor binding pattern reflected by loss of the ability to agglutinate chicken erythrocytes.

CONCLUSIONS

These rapid evolutionary changes warrant close virologic monitoring of immunocompromised patients treated for influenza infection, and raise concern about the efficacy of mono-drug therapy for influenza-associated disease in HSCT recipients.

Changes in H5N1 influenza virus hemagglutinin receptor binding domain affect systemic spread

The HA of influenza virus is a receptor-binding and fusion protein that is required to initiate infection. The HA receptor-binding domain determines the species of sialyl receptors recognized by influenza viruses. Here, we demonstrate that changes in the HA receptor-binding domain alter the ability of the H5N1 virus to spread systemically in mice. The A/Vietnam/1203/04 (VN1203) and A/Hong Kong/213/03 (HK213) viruses are consistently lethal to domestic chickens but differ in their pathogenicity to mammals. Insertion of the VN1203 HA and neuraminidase (NA) genes into recombinant HK213 virus expanded its tissue tropism and increased its lethality in mice; conversely, insertion of HK213 HA and NA genes into recombinant VN1203 virus decreased its systemic spread and lethality. The VN1203 and HK213 HAs differ by 10 aa, and HK213 HA has shown greater binding affinity for synthetic alpha2,6-linked sialyl receptor. Introduction of an S227N change and removal of N-linked glycosylation at residue 158 increased the alpha2,6-binding affinity of VN1203 HA. Recombinant VN1203 virus carrying the S227N change alone or with the residue-158 glycosylation site removed showed reduced lethality and systemic spread in mice but not in domestic chickens. Wild-type VN1203 virus exhibited the greatest efficiency in systemic spread after intramuscular inoculation and in infection of mouse bone marrow-derived dendritic cells and conventional pulmonary dendritic cells. These results show that VN1203 HA glycoprotein confers pathogenicity by facilitating systemic spread in mice; they also suggest that a minor change in receptor binding domain may modulate the virulence of H5N1 viruses.

Recent avian H5N1 viruses exhibit increased propensity for acquiring human receptor specificity

Adaptation of avian influenza viruses for replication and transmission in the human host is believed to require mutations in the hemagglutinin glycoprotein (HA) which enable binding to human alpha2-6 sialosides and concomitant reduction in affinity for avian alpha2-3 linked sialosides. Here, we show by glycan microarray analyses that the two mutations responsible for such specificity changes in 1957 H2N2 and 1968 H3N2 pandemic viruses, when inserted into recombinant HAs or intact viruses of some recent avian H5N1 isolates (clade 2.2), impart such attributes. This propensity to adapt to human receptors is primarily dependent on arginine at position 193 within the receptor-binding site, as well as loss of a vicinal glycosylation site. Widespread occurrence of these susceptible H5N1 clade 2.2 influenza strains has already occurred in Europe, the Middle East, and Africa. Thus, these avian strains should be considered high-risk, because of their significantly lower threshold for acquiring human receptor specificity and, therefore, warrant increased surveillance and further study.

Adjustment of receptor-binding and neuraminidase substrate specificities in avian-human reassortant influenza viruses

Balanced action of hemagglutinin (HA) and neuraminidase (NA) is an important condition of influenza virus efficient replication, but a role of HA and NA specificities at oligosaccharide level in maintaining such a balance remains poorly studied. Avian virus HA binds exclusively and NA digests efficiently alpha2-3-sialylated carbohydrate chains, while human virus HA interacts with alpha2-6 chains and low-active NA cleaves both alpha2-3- and alpha2-6-sialosides. Reassortment between viruses leading to appearance of avian virus HA and human virus NA on the virion surface often resulted in decreasing the replicative potential of the formed variants because of disturbance of a functional balance between "alien" HA and NA. A restoration of the reassortant productivity happened due to the appearance of amino acid substitutions in HA and, sometimes, NA. Here, a role of NA and HA oligosaccharide specificities in a restoration of HA-NA functional balance in high-yield passage variants was studied. Postreassortment changes in HA receptor-binding and NA substrate specificities for three reassortant/passage variant virus pairs towards 3'SiaLac, 3'SiaLacNAc, SiaLe(c), SiaLe(a), SiaLe(x), 6'SiaLac, and 6'SiaLacNAc were determined. Selection of the high-yield variants of the human-avian reassortants led either to twofold decrease in the affinity of HA for most alpha2-3-sialosides and the appearance of affinity for alpha2-6-sialosides (H3N2 reassortant), or to decreasing the HA affinity for SiaLe(c) and SiaLe(a) (H3N1 reassortant), or to enhancing the ability of NA to discriminate between alpha2-3/2-6 substrates (H4N1 reassortant). Thus, all postreassortment changes in oligosaccharide specificities of "alien" HA and NA were directed towards their adjustment to each other, but by different manner.

Amino acid 226 in the hemagglutinin of H4N6 influenza virus determines binding affinity for alpha2,6-linked sialic acid and infectivity levels in primary swine and human respiratory epithelial cells

Avian lineage H4N6 influenza viruses previously isolated from pigs differ at hemagglutinin amino acids 226 and 228 from H4 subtype viruses isolated from birds. Using a parental H4N6 swine isolate and hemagglutinin mutant viruses (at residues 226 and/or 228), we determined that viruses which contain L226 had a higher affinity for sialic acid alpha2,6 galactose (SAalpha2,6Gal) and a higher infectivity level for primary swine and human respiratory epithelial cells, whereas viruses which contain Q226 had lower SAalpha2,6Gal affinity and lower infectivity levels for both types of cells. Using specific neuraminidases, we found that irrespective of their relative binding preferences, all of the influenza viruses examined utilized SAalpha2,6Gal to infect swine and human cells.

Genetic compatibility and virulence of reassortants derived from contemporary avian H5N1 and human H3N2 influenza A viruses

The segmented structure of the influenza virus genome plays a pivotal role in its adaptation to new hosts and the emergence of pandemics. Despite concerns about the pandemic threat posed by highly pathogenic avian influenza H5N1 viruses, little is known about the biological properties of H5N1 viruses that may emerge following reassortment with contemporary human influenza viruses. In this study, we used reverse genetics to generate the 63 possible virus reassortants derived from H5N1 and H3N2 viruses, containing the H5N1 surface protein genes, and analyzed their viability, replication efficiency, and mouse virulence. Specific constellations of avian–human viral genes proved deleterious for viral replication in cell culture, possibly due to disruption of molecular interaction networks. In particular, striking phenotypes were noted with heterologous polymerase subunits, as well as NP and M, or NS. However, nearly one-half of the reassortants replicated with high efficiency in vitro, revealing a high degree of compatibility between avian and human virus genes. Thirteen reassortants displayed virulent phenotypes in mice and may pose the greatest threat for mammalian hosts. Interestingly, one of the most pathogenic reassortants contained avian PB1, resembling the 1957 and 1968 pandemic viruses. Our results reveal the broad spectrum of phenotypes associated with H5N1/H3N2 reassortment and a possible role for the avian PB1 in the emergence of pandemic influenza. These observations have important implications for risk assessment of H5N1 reassortant viruses detected in surveillance programs.

The influenza pandemics of 1957 and 1968 were caused by hybrid viruses consisting of a mixture of human and avian influenza genes. The introduction of avian genes resulted in a sudden change of the virus surface antigens, allowing its worldwide spread due to lack of immunity in the population. The highly pathogenic avian influenza H5N1 virus has continued its spread in domestic and wild birds in Asia, Europe, and Africa. Although H5N1 infection in humans is rare and person-to-person transmission is very inefficient, the steady accumulation of human cases has raised concern over the possible reassortment between H5N1 and human seasonal influenza resulting in a virus with new surface antigens and pandemic potential. In this study, we used recombinant DNA technology to generate a systematic collection of hybrid viruses (with genes from human and avian viruses) bearing H5N1 surface antigens and analyzed their properties in cell culture and in mice. The H5N1 hybrid viruses revealed a broad range of viability and multiplication capacity in cell cultures. In addition, several H5N1 hybrid viruses were highly virulent in mice. Results from this systematic analysis provide important insight to support risk assessment of reassortant H5N1 avian influenza viruses.

Human-like receptor specificity does not affect the neuraminidase-inhibitor susceptibility of H5N1 influenza viruses

If highly pathogenic H5N1 influenza viruses acquire affinity for human rather than avian respiratory epithelium, will their susceptibility to neuraminidase (NA) inhibitors (the likely first line of defense against an influenza pandemic) change as well? Adequate pandemic preparedness requires that this question be answered. We generated and tested 31 recombinants of A/Vietnam/1203/04 (H5N1) influenza virus carrying single, double, or triple mutations located within or near the receptor binding site in the hemagglutinin (HA) glycoprotein that alter H5 HA binding affinity or specificity. To gain insight into how combinations of HA and NA mutations can affect the sensitivity of H5N1 virus to NA inhibitors, we also rescued viruses carrying the HA changes together with the H274Y NA substitution, which was reported to confer resistance to the NA inhibitor oseltamivir. Twenty viruses were genetically stable. The triple N158S/Q226L/N248D HA mutation (which eliminates a glycosylation site at position 158) caused a switch from avian to human receptor specificity. In cultures of differentiated human airway epithelial (NHBE) cells, which provide an ex vivo model that recapitulates the receptors in the human respiratory tract, none of the HA-mutant recombinants showed reduced susceptibility to antiviral drugs (oseltamivir or zanamivir). This finding was consistent with the results of NA enzyme inhibition assay, which appears to predict influenza virus susceptibility in vivo. Therefore, acquisition of human-like receptor specificity does not affect susceptibility to NA inhibitors. Sequence analysis of the NA gene alone, rather than analysis of both the NA and HA genes, and phenotypic assays in NHBE cells are likely to adequately identify drug-resistant H5N1 variants isolated from humans during an outbreak.

If the avian influenza H5N1 viruses adapt to human hosts, the first step is likely to be a switch in the preference of their viral hemagglutinin (HA) glycoprotein to bind to human rather than avian cell receptors. Such a switch may also alter virus susceptibility to neuraminidase (NA) inhibitors, which are anti-influenza drugs that are likely to be the first line of defense against a pandemic. We generated recombinant A/Vietnam/1203/04-like (H5N1) viruses carrying HA mutations previously shown to alter receptor specificity or affinity. We also discovered a previously unknown route (three simultaneous HA amino acid substitutions) by which highly pathogenic H5N1 viruses can adapt to human receptors. We then used a novel cell-culture–based system (differentiated human airway epithelial NHBE cells) to evaluate the recombinant viruses' resistance to NA inhibitors. None of the HA-mutant recombinants showed reduced drug susceptibility. Our results indicate that the tested HA mutations are unlikely to cause resistance to NA inhibitors in vivo. The NHBE system meets the need for an appropriate cell-culture–based system for phenotypic characterization of drug resistance.

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.

Glycan arrays: biological and medical applications

Carbohydrates and their conjugates are involved in various biological events, including viral and bacterial infection, the immune response, differentiation and development, and the progression of tumor cell metastasis. Glycan arrays are a new technology that has enabled the high-sensitivity and rapid analysis carbohydrate–protein interaction and contribute to significant advances in glycomics. Glycan arrays use a minute amount of materials and can be used for high-throughput profiling and quantitative analysis and provide information for the development of carbohydrate-based vaccines and new drug discovery.

Quantitative biochemical rationale for differences in transmissibility of 1918 pandemic influenza A viruses

The human adaptation of influenza A viruses is critically governed by the binding specificity of the viral surface hemagglutinin (HA) to long (chain length) alpha2-6 sialylated glycan (alpha2-6) receptors on the human upper respiratory tissues. A recent study demonstrated that whereas the 1918 H1N1 pandemic virus, A/South Carolina/1/1918 (SC18), with alpha2-6 binding preference transmitted efficiently, a single amino acid mutation on HA resulted in a mixed alpha2-3 sialylated glycan (alpha2-3)/alpha2-6 binding virus (NY18) that transmitted inefficiently. To define the biochemical basis for the observed differences in virus transmission, in this study, we have developed an approach to quantify the multivalent HA-glycan interactions. Analysis of the molecular HA-glycan contacts showed subtle changes resulting from the single amino acid variations between SC18 and NY18. The effect of these changes on glycan binding is amplified by multivalency, resulting in quantitative differences in their long alpha2-6 glycan binding affinities. Furthermore, these differences are also reflected in the markedly distinct binding pattern of SC18 and NY18 HA to the physiological glycans present in human upper respiratory tissues. Thus, the dramatic lower binding affinity of NY18 to long alpha2-6 glycans, as against a mixed alpha2-3/6 binding, correlates with its inefficient transmission. In summary, this study establishes a quantitative biochemical correlate for influenza A virus transmission.

Glycan topology determines human adaptation of avian H5N1 virus hemagglutinin

A switch in specificity of avian influenza A viruses' hemagglutinin (HA) from avian-like (alpha2-3 sialylated glycans) to human-like (alpha2-6 sialylated glycans) receptors is believed to be associated with their adaptation to infect humans. We show that a characteristic structural topology--and not the alpha2-6 linkage itself--enables specific binding of HA to alpha2-6 sialylated glycans and that recognition of this topology may be critical for adaptation of HA to bind glycans in the upper respiratory tract of humans. An integrated biochemical, analytical and data mining approach demonstrates that HAs from the human-adapted H1N1 and H3N2 viruses, but not H5N1 (bird flu) viruses, specifically bind to long alpha2-6 sialylated glycans with this topology. This could explain why H5N1 viruses have not yet gained a foothold in the human population. Our findings will enable the development of additional strategies for effective surveillance and potential therapeutic interventions for H5N1 and possibly other influenza A viruses.

Stabilizing the glycosylation pattern of influenza B hemagglutinin following adaptation to growth in eggs

The currently circulating influenza B viruses from both antigenic lineages contain an N-linked glycosylation site in the hemagglutinin (HA) protein at positions of 196 or 197. However, egg adaptation caused the loss of the glycosylation site that could impact virus antigenicity and vaccine efficacy. The effect of the 196/197 glycosylation site on influenza B virus growth and antigenicity was systemically evaluated in this study by the molecular approach. Paired recombinant 6:2 reassortant influenza B vaccine strains, with or without the 196/197 glycosylation site, were generated by reverse genetics and the glycosylation site was retained in MDCK cells. In contrast, all the viruses that contained the introduced glycosylation site were unable to grow in eggs and rapidly lost the glycosylation site once adapted to grow in eggs. We showed that glycosylation affected virus binding to the alpha-2,3-linked sialic acid receptor and affected virus antigenicity as tested by postinfected ferret sera. We have further identified that the Arginine residue at amino acid position 141 (141R) can stabilize the 196/197 glycosylation site without affecting virus antigenicity. Thus, the 141R could be introduced into vaccine strains to retain the 196/197 glycosylation site for influenza B vaccines.

Human parainfluenza viruses hPIV1 and hPIV3 bind oligosaccharides with alpha2-3-linked sialic acids that are distinct from those bound by H5 avian influenza virus hemagglutinin

We investigated the binding of human parainfluenza virus types 1 and 3 (hPIV1 and hPIV3, respectively) to the glycan array of the Consortium for Functional Glycomics and binding and their release from erythrocytes under conditions where neuraminidase is inactive or active. hPIV1 and hPIV3 bind modifications of Neu5Acalpha2-3Galbeta1-4GlcNAc, including the sialyl-Lewis(x) motif and structures containing 6-sulfogalactose. hPIV1 and hPIV3 thus bind typical N-linked glycans, in contrast to avian influenza virus H5 hemagglutinin (J. Stevens, O. Blixt, T. M. Tumpey, J. K. Taubenberger, J. C. Paulson, and I. A. Wilson, Science 312:404-410, 2006), which binds less-common motifs. While the receptor is not the sole determinant of tropism, hPIV or H5 influenza virus infection of specific cells that express receptors may contribute to their different pathologies.