Differences in sialic acid-galactose linkages in the chicken egg amnion and allantois influence human influenza virus receptor specificity and variant selection

Molecular evolution of influenza A/H3N2 viruses in the province of Québec (Canada) during the 1997-2000 period

In this study, we compared antigenic (hemagglutination inhibition (HI) assay) and molecular (sequencing of the hemagglutinin (HA1) gene) characterization of influenza isolates collected in the Province of Québec (Canada) during the last three flu seasons (1997-2000). Twenty-three isolates were tested by a standard HI assay and 37 by sequencing of the HA1 gene for their homology to the A/H3N2 vaccine strains A/Wuhan/359/95 (1997-1998) and A/Sydney/5/97 (1998-1999 and 1999-2000). By HI, two isolates were antigenically similar to A/Wuhan/359/95 (both from 1997 to 1998), 16 were similar to A/Sydney/5/97 (1997-2000) and no conclusions could be inferred for the other five isolates due to identical HI titers for the two vaccine strains (n=4) or insufficient viral titer (n=1). Sequence analysis revealed that four isolates from 1997 to 1998 were related to A/Wuhan/359/95 whereas the others (n=4) from 1997 to 1998, as well as all isolates from 1998 to 1999 (n=18) and 1999 to 2000 (n=11) were closer to A/Sydney/5/97. The mean number of amino acid differences for the 33 A/Sydney/5/97-like isolates compared with the homologous vaccine strain was 6.3 (1.9%), 9.2 (2.8%) and 13.6 (4.1%) for those collected in 1997-1998, 1998-1999, and 1999-2000, respectively. Phylogenetic analysis confirmed that a progressive drift occurred among our A/H3N2 influenza isolates over the last three flu seasons. Furthermore, it revealed that isolates collected during the last two flu seasons were in fact more related to A/Panama/2007/99 (2000-2001 vaccine strain) than to A/Sydney/5/97. Our studies suggest that molecular analysis of the HA1 gene should complement the HI assay for a more accurate analysis of influenza A virus drift.

Apoptosis by influenza viruses correlates with efficiency of viral mRNA synthesis

A mutant influenza virus, A/NWS-Mvi, grows well in the presence of exogenous sialidase activity sufficient to remove all cell surface sialic acids. Related wild-type viruses grow very poorly under these conditions, although mutant and wild-type viruses bind to desialylated cells with similar efficiency and show similar reduction of binding to sialidase-treated cells compared to native cells. Here we examine entry, transcription, translation, and RNA replication and find that, although the viruses appear to utilize the same entry pathway, the mutant NWS-Mvi transcribes and replicates RNA to higher levels than the wild-type strains. The kinetics of replication in multi-cycle infection show that this enhancement of RNA synthesis facilitates growth where entry is restricted. The hemagglutinin (HA) protein of NWS-Mvi lyses red blood cells 0.1 pH unit higher than wild-type viruses. This higher fusion pH may allow more efficient release of nucleocapsids from endosomes and contribute to the enhanced RNA synthesis. The efficient RNA synthesis assists virus survival at low inocula or under stringent growth conditions, such as the presence of antiviral agents. NWS-Mvi induces apoptosis in infected cells more readily than wild-type viruses, apparently as a consequence of enhanced production of viral mRNA. Since growth of NWS-Mvi is more efficient, apoptosis may play a positive role in viral replication by removing cells that have already been infected from those capable of making more virus.

Anti-influenza drugs and neuraminidase inhibitors

Each year, influenza viruses are responsible for considerable illness, complications and mortality. An effective treatment will have a major impact on the severe personal and economic burden that this disease incurs. There are several points in the influenza life cycle that may be potentially inhibited. One critical point is the release of newly synthesized virions from the host cell surface. Viral neuraminidase (NA) cleaves the virus from host cell sialic acid residues allowing infection of other host cells. Rationally designed NA inhibitors that block the viral life cycle are now in the clinic and these molecules are effective and safe for the treatment of influenza. Compared with other anti-influenza agents the NA inhibitors are well tolerated, effective against all influenza types and there has been little evidence of the emergence of viral resistance. NA inhibitors provide an important new therapeutic weapon for the management of influenza infection.

Adaptation of influenza A viruses to cells expressing low levels of sialic acid leads to loss of neuraminidase activity

Influenza A viruses possess two virion surface proteins, hemagglutinin (HA) and neuraminidase (NA). The HA binds to sialyloligosaccharide viral receptors, while the NA removes sialic acids from the host cell and viral sialyloligosaccarides. Alterations of the HA occur during adaptation of influenza viruses to new host species, as in the 1957 and 1968 influenza pandemics. To gain a better understanding of the contributions of the HA and possibly the NA to this process, we generated cell lines expressing reduced levels of the influenza virus receptor determinant, sialic acid, by selecting Madin-Darby canine kidney cells resistant to a lectin specific for sialic acid linked to galactose by alpha(2-3) or alpha(2-6) linkages. One of these cell lines had less than 1/10 as much N-acetylneuraminic acid as its parent cell line. When serially passaged in this cell line, human H3N2 viruses lost sialidase activity due to a large internal deletion in the NA gene, without alteration of the HA gene. These findings indicate that NA mutations can contribute to the adaptation of influenza A virus to new host environments and hence may play a role in the transmission of virus across species.

Infection of human airway epithelia with H1N1, H2N2, and H3N2 influenza A virus strains

Three subtypes of influenza A virus cause human disease: H1N1, H2N2, and H3N2. Although all result in respiratory illness, little is known about how these subtypes infect differentiated airway epithelia. Therefore, we assayed A/PR/8/34 (H1N1), A/Japan/305/57 (H2N2), and X31 (H3N2) influenza virus strains for binding and infection on fully differentiated primary cultures of airway epithelia isolated from human bronchus, grown on semiporous filters at an air-liquid interface. In this model system, viral infectivity was highest when virus was applied to the apical versus the basolateral surface; Japan was most infectious, followed by PR8. The X31 strain showed very low levels of infectivity. Confocal microscopy and fluorescence-resonance energy transfer studies indicated that Japan virus could enter and fuse with cellular membranes, while infection with X31 virions was greatly inhibited. Japan virus could also productively infect human trachea explant tissues. These data show that influenza viruses with SAalpha2,3Gal binding specificity, like Japan, productively infect differentiated human airway epithelia from the apical surface. These data are important to consider in the development of pseudotyped recombinant viral vectors for gene transfer to human airway epithelia for gene therapy.

Receptor binding and membrane fusion in virus entry: the influenza hemagglutinin

Hemagglutinin (HA) is the receptor-binding and membrane fusion glycoprotein of influenza virus and the target for infectivity-neutralizing antibodies. The structures of three conformations of the ectodomain of the 1968 Hong Kong influenza virus HA have been determined by X-ray crystallography: the single-chain precursor, HA0; the metastable neutral-pH conformation found on virus, and the fusion pH-induced conformation. These structures provide a framework for designing and interpreting the results of experiments on the activity of HA in receptor binding, the generation of emerging and reemerging epidemics, and membrane fusion during viral entry. Structures of HA in complex with sialic acid receptor analogs, together with binding experiments, provide details of these low-affinity interactions in terms of the sialic acid substituents recognized and the HA residues involved in recognition. Neutralizing antibody-binding sites surround the receptor-binding pocket on the membrane-distal surface of HA, and the structures of the complexes between neutralizing monoclonal Fabs and HA indicate possible neutralization mechanisms. Cleavage of the biosynthetic precursor HA0 at a prominent loop in its structure primes HA for subsequent activation of membrane fusion at endosomal pH (Figure 1). Priming involves insertion of the fusion peptide into a charged pocket in the precursor; activation requires its extrusion towards the fusion target membrane, as the N terminus of a newly formed trimeric coiled coil, and repositioning of the C-terminal membrane anchor near the fusion peptide at the same end of a rod-shaped molecule. Comparison of this new HA conformation, which has been formed for membrane fusion, with the structures determined for other virus fusion glycoproteins suggests that these molecules are all in the fusion-activated conformation and that the juxtaposition of the membrane anchor and fusion peptide, a recurring feature, is involved in the fusion mechanism. Extension of these comparisons to the soluble N-ethyl-maleimide-sensitive factor attachment protein receptor (SNARE) protein complex of vesicle fusion allows a similar conclusion.

Pandemic threat posed by avian influenza A viruses

Influenza pandemics, defined as global outbreaks of the disease due to viruses with new antigenic subtypes, have exacted high death tolls from human populations. The last two pandemics were caused by hybrid viruses, or reassortants, that harbored a combination of avian and human viral genes. Avian influenza viruses are therefore key contributors to the emergence of human influenza pandemics. In 1997, an H5N1 influenza virus was directly transmitted from birds in live poultry markets in Hong Kong to humans. Eighteen people were infected in this outbreak, six of whom died. This avian virus exhibited high virulence in both avian and mammalian species, causing systemic infection in both chickens and mice. Subsequently, another avian virus with the H9N2 subtype was directly transmitted from birds to humans in Hong Kong. Interestingly, the genes encoding the internal proteins of the H9N2 virus are genetically highly related to those of the H5N1 virus, suggesting a unique property of these gene products. The identification of avian viruses in humans underscores the potential of these and similar strains to produce devastating influenza outbreaks in major population centers. Although highly pathogenic avian influenza viruses had been identified before the 1997 outbreak in Hong Kong, their devastating effects had been confined to poultry. With the Hong Kong outbreak, it became clear that the virulence potential of these viruses extended to humans.

Host mediated variation and receptor binding specificity of influenza viruses

There are at least two possible mechanisms for the host range variation of influenza A viruses. First mechanism is the pressure of the antibody, this results has already reported by us, in 1989(12). In this case, Ser 205 which is located in antigenic site D in the hemagglutinin trimer. When Ser 205 is substituted to Tyr, receptor binding specificity become 2-6 predominant rather than 2-3 predominant. Ser205 is located far away from the receptor binding pocket in the subunit itself, however, very close to the pocket in the next subunit. When the Ser 205 is substituted to bulky amino acid Tyr, the three dimensional structure of the receptor binding pocket in next subunit may be influenced to become 2-6 binding property rather than 2-3 binding. Second mechanism is the selective pressure for the appearance of host cell variant with altered receptor binding specificities. In this case, amino acid 226 located in receptor binding pocket is clitical for the recognition of 2-6 and 2-3 linkages. Only single amino acid alteration of Leu 226 to Gln made remarkable change of receptor binding specificity 2-6 to 2-3. In this study, we also identified amino acid 155 Thr, 158 Glu, and 228 Ser of the hemagglutinin which may be clitical for the recognition of the molecular species of sialic acid, such as Neu5Ac and Neu5Gc. Very recently, we reported the amino acid residues contributing to the substrate specificity of the influenza A virus sialidase. If these amino acid alteration occurres in the host animal bodies in nature, the virus aquires the new receptor binding specificity which allows the infection to the different animal species, such as bird to human population. This process may be one of the important mechanism for the host mediated variation, and the influenza virus transmission between different hosts, and also emerging of new subtypes of influenza viruses in human population.

Change in receptor-binding specificity of recent human influenza A viruses (H3N2): a single amino acid change in hemagglutinin altered its recognition of sialyloligosaccharides

Human H3N2 influenza A viruses were known to preferentially bind to sialic acid (SA) in alpha2,6Gal linkage on red blood cells (RBC). However, H3N2 viruses isolated in MDCK cells after 1992 did not agglutinate chicken RBC (CRBC). Experiments with point-mutated hemagglutinin (HA) of A/Aichi/51/92, one of these viruses, revealed that an amino acid change from Glu to Asp at position 190 (E190D) was responsible for the loss of ability to bind to CRBC. A/Aichi/51/92 did not agglutinate CRBC treated with alpha2, 3-sialidase, suggesting that SAalpha2,3Gal on CRBC might not inhibit the binding of the virus to SAalpha2,6Gal on CRBC. However, the virus agglutinated derivatized CRBC resialylated with SAalpha2, 6Galbeta1,4GlcNAc. These findings suggested that the E190D change might have rendered the HA able to distinguish sialyloligosaccharides on the derivatized CRBC containing the SAalpha2,6Galbeta1,4GlcNAc sequence from those on the native CRBC.

Isolation and characterization of H4N6 avian influenza viruses from pigs with pneumonia in Canada

In October 1999, H4N6 influenza A viruses were isolated from pigs with pneumonia on a commercial swine farm in Canada. Phylogenetic analyses of the sequences of all eight viral RNA segments demonstrated that these are wholly avian influenza viruses of the North American lineage. To our knowledge, this is the first report of interspecies transmission of an avian H4 influenza virus to domestic pigs under natural conditions.

Apical secretion and sialylation of soluble dipeptidyl peptidase IV are two related events

The role of glycans in the apical targeting of proteins in epithelial cells remains a debated question. We have expressed the mouse soluble dipeptidyl peptidase IV (DPP IV ectodomain) in kidney (MDCK) and in intestinal (Caco-2) epithelial cell lines, as a model to study the role of glycosylation in apical targeting. The mouse DPP IV ectodomain was secreted mainly into the apical medium by MDCK cells. Exposure of MDCK cells to GalNac-alpha-O-benzyl, a drug previously described as an inhibitor of mucin O-glycosylation, produced a protein with a lower molecular weight. In addition this treatment resulted in a decreased apical secretion and an increased basolateral secretion of mouse DPP IV ectodomain. When expressed in Caco-2 cells, the mouse DPP IV ectodomain was secreted mainly into the basolateral medium. However, BGN was still able to decrease the amount of apically secreted protein and to increase its basolateral secretion. Neuraminidase digestion showed that the most striking effect of BGN was a blockade of DPP IV sialylation in both MDCK and Caco-2 cells. These results indicate that a specific glycosylation step, namely, sialylation, plays a key role in the control of the apical targeting of a secreted DPP IV both in MDCK and Caco-2 cells.

Characterization of low virulent strains of highly pathogenic A/Hong Kong/156/97 (H5N1) virus in mice after passage in embryonated hens' eggs

Avian influenza A H5N1 viruses were isolated from humans for the first time in Hong Kong in 1997. The virulence of A/Hong Kong/156/97 (HK156) strain in mice was found to change significantly depending on the passage history of the virus. Madin-Darby canine kidney (MDCK) cell-grown parental virus and three of its clones derived from mouse brain showed high pathogenicity in mice after intranasal or intracerebral infection. In contrast, the egg-derived parental virus HK156-E3 and its cloned viruses were markedly less pathogenic in mice. It appeared that differences in pathogenicity among viruses derived from MDCK cells and eggs were due to their ability or inability to disseminate from the lungs to the brain. Sequence analysis of the entire protein coding regions of all eight RNA genome segments revealed a total of six conserved amino acid differences in the HA1 domain (residue 211) of the HA protein, as well as the PB1 (residues 456 and 712), PA (residue 631), NP (residue 127), and NS1 (residue 101) proteins that correlated with observed changes in virulence and neurovirulence of HK156 virus in mice. Thus it was evident that the passaging of HK156 in embryonated eggs led to the adaptation and selection of variants demonstrating markedly decreased pathogenicity and neurovirulence in mice that appeared to be attributable to specific amino acid changes in the HA and internal proteins.

Selection of receptor-binding variants of human influenza A and B viruses in baby hamster kidney cells

Cultivation of human influenza viruses in the allantoic cavity of embryonated chicken eggs leads to a selection of receptor-binding variants with amino acid substitutions on the globular head of the hemagglutinin (HA) molecule. Such selection can be avoided by growing the human viruses in Madin Darby canine kidney (MDCK) cells. In the present study, we tested whether baby hamster kidney (BHK) cells select receptor-binding mutants of human influenza viruses. After isolating H1N1, H3N2, and type B influenza viruses from clinical samples in MDCK cells, we passaged them in either BHK cells or chicken eggs. The BHK-grown viruses differed from their MDCK-grown counterparts by virtue of mutations in the HA: 225D --> G (H1N1 virus), 128T --> A and 226I --> V (H3N2), and 187N --> D (type B) (H3 numbering). Variants with different substitutions were selected by passaging of the same MDCK-grown parents in eggs: 141L --> H, 208R --> H, and 225D --> G (H1N1), 194L --> I (H3N2), and 137G --> R (B). Compared with their MDCK-grown counterparts, both BHK- and egg-grown viruses possessed a higher affinity for the cellular membranes of BHK cells and of the chorioallantoic cells of chicken embryos and for a 3'-sialylgalactose-containing synthetic sialylglycopolymer. By contrast, changes in the affinity of mutants for a 6'-sialyl-(N-acetyllactosamine)-containing sialylglycopolymer varied from negative to positive. Fluorescence-activated cell-sorting analysis with linkage-specific lectins showed that the density of the 6'-sialyl-(N-acetyllactosamine)-containing receptors is substantially lower on the surface of BHK cells than on MDCK cells, providing an explanation for the growth restriction of human viruses in the former cells. Our data demonstrate that cultures of BHK cells, like eggs, can select receptor-binding variants of human influenza viruses.

Effects of egg-adaptation on the receptor-binding properties of human influenza A and B viruses

Propagation of human influenza viruses in embryonated chicken eggs (CE) results in the selection of variants with amino acid substitutions near the receptor-binding site of the hemagglutinin (HA) molecule. To evaluate the mechanisms by which these substitutions enable human virus growth in CE, we studied the binding of 10 human influenza A (H1N1, H3N2) and B strains, isolated and propagated solely in MDCK cells, and of their egg-adapted counterparts to preparations of cellular membranes, gangliosides, sialylglycoproteins, and sialyloligosaccharides. All egg-adapted variants differed from nonadapted strains by increased binding to the plasma membranes of chorio-allantoic (CAM) cells of CE and by the ability to bind to CAM gangliosides. In addition, there was no decrease in affinity for inhibitors within allantoic fluid. These findings indicate that growth of human influenza viruses in CE is restricted because of their inefficient binding to receptors on CAM cells and that gangliosides can play an important role in virus binding and/or penetration. The effects of the egg-adaptation substitutions on the receptor-binding properties of the viruses include (i) enhancement of virus binding to the terminal Sia(alpha2-3)Gal determinant (substitutions in HA positions 190, 225 of H1N1 strains and in position 186 of H3N2 strains); (ii) a decrease of steric interference with more distant parts of the Sia(alpha2-3Gal)-containing receptors (a loss of glycosylation sites in positions 163 of H1 HA and 187 of type B HA); and (iii) enhanced ionic interactions with the negatively charged molecules due to charged substitutions at the tip of the HA [187, 189, 190 (H1), and 145, 156 (H3)]. Concomitantly with enhanced binding to Sia(alpha2-3)Gal-terminated receptors, all egg-adapted variants decreased their affinity for equine macroglobulin, a glycoprotein bearing terminal 6'-sialyl(N-acetyllactosamine)-moieties.

Origin and evolution of the 1918 "Spanish" influenza virus hemagglutinin gene

The "Spanish" influenza pandemic killed over 20 million people in 1918 and 1919, making it the worst infectious pandemic in history. Here, we report the complete sequence of the hemagglutinin (HA) gene of the 1918 virus. Influenza RNA for the analysis was isolated from a formalin-fixed, paraffin-embedded lung tissue sample prepared during the autopsy of a victim of the influenza pandemic in 1918. Influenza RNA was also isolated from lung tissue samples from two additional victims of the lethal 1918 influenza: one formalin-fixed, paraffin-embedded sample and one frozen sample obtained by in situ biopsy of the lung of a victim buried in permafrost since 1918. The complete coding sequence of the A/South Carolina/1/18 HA gene was obtained. The HA1 domain sequence was confirmed by using the two additional isolates (A/New York/1/18 and A/Brevig Mission/1/18). The sequences show little variation. Phylogenetic analyses suggest that the 1918 virus HA gene, although more closely related to avian strains than any other mammalian sequence, is mammalian and may have been adapting in humans before 1918.

Mink lung epithelial cells: unique cell line that supports influenza A and B virus replication

We have demonstrated for the first time that a mink lung epithelial cell line (Mv1Lu) supports the replication of influenza A and B viruses, including the recently isolated H5N1 avian and human Hong Kong strains, to titers comparable to those in MDCK cells. These results suggest that Mv1Lu cells might serve as an alternative system for the isolation and cultivation of influenza A and B viruses and may be useful for vaccine development.

Changes in H3 influenza A virus receptor specificity during replication in humans

Influenza A viruses of the H3 subtype caused the 1968 Hong Kong pandemic, the hemagglutinin (HA) gene being introduced into humans following a reassortment event with an avian virus. Receptor specificity and serum inhibitor sensitivity of the HA of influenza A viruses are linked to the host species. Human H3 viruses preferentially recognize N-acetyl sialic acid linked to galactose by alpha2,6 linkages (Neu5Acalpha2,6Gal) and are sensitive to serum inhibitors, whereas avian and equine viruses preferentially recognize Neu5Acalpha2,3Gal linkages and are resistant to serum inhibitors. We have examined the receptor specificity and serum inhibitor sensitivity of H3 human influenza A viruses from the time they were introduced into the human population to gain insight into the mechanism of viral molecular evolution and host tropism. All of the viruses were sensitive to neutralization and hemagglutination inhibition by horse serum. Early H3 viruses were resistant to pig and rabbit serum inhibitors. Viruses isolated after 1977 were uniformly sensitive to inhibition by pig and rabbit sera. The recognition of Neu5Acalpha2,3Gal or Neu5Acalpha2,6Gal linkages was not correlated with the serum sensitivity. These data showed that the receptor specificity of HA, measured as inhibitor sensitivity, has changed during replication in humans since its introduction from an avian virus.

Effects of host-dependent glycosylation of hemagglutinin on receptor-binding properties on H1N1 human influenza A virus grown in MDCK cells and in embryonated eggs

There is growing evidence that the receptor-binding characteristics of influenza viruses are affected by the host-dependent glycosylation of viral hemagglutinin (HA). To better understand these effects, we propagated two variants of the human influenza virus USSR/90/77 (which differed by the mutation Asn131 reversible Asp131 in the glycosylation sequon of their HA) in either embryonated chicken eggs or MDCK cell. Those variants were then compared for their ability to bind soluble receptor analogs and to attach to receptors represented on a solid phase. The carbohydrate chain at position 131 of the HA (CHO 131) interfered with virus binding to soluble Sia2-6Gal-containing macromolecular receptors, but had little or no effect on its binding to Sia2-3Gal-containing macromolecules. This specificity could be explained by the different orientation of the asialic parts of the 2-3-linked sialosides versus 2-6-linked sialosides with respect to the receptor-binding site (Eisen et al., 1997, Virology 232, 19-31). In the case of virus attachment to solid-phase immobilized receptors, MDCK-grown viruses bound substantially more weakly than their egg-grown counterparts to receptors of avian origin, whereas binding to mammalian cell membranes was only marginally affected by differences in host-specific glycosylation of the virus. Our data indicated that the effects of the carbohydrate side chain of HA on virus receptor-binding activity are dependent on both the cells in which the virus was grown and the nature of the cellular receptors or intercellular inhibitors to which the virus binds.

Molecular mechanisms of serum resistance of human influenza H3N2 virus and their involvement in virus adaptation in a new host

H3N2 human influenza viruses that are resistant to horse, pig, or rabbit serum possess unique amino acid mutations in their hemagglutinin (HA) protein. To determine the molecular mechanisms of this resistance, we characterized the receptor-binding properties of these mutants by measuring their affinity for total serum protein inhibitors and for soluble receptor analogs. Pig serum-resistant variants displayed a markedly decreased affinity for total pig serum sialylglycoproteins (which contain predominantly 2-6 linkage between sialic acid and galactose residues) and for the sialyloligosaccharide 6'-sialyl(N-acetyllactosamine). These properties correlated with the substitution 186S-->I in HA1. The major inhibitory activity in rabbit serum was found to be a beta inhibitor with characteristics of mannose-binding lectins. Rabbit serum-resistant variants exhibited decreased sensitivity to this inhibitor due to the loss of a glycosylation sequon at positions 246 to 248 of the HA. In addition to a somewhat reduced affinity for 6'-sialyl(N-acetyllactosamine)-containing receptors, horse serum-resistant variants lost the ability to bind the viral neuraminidase-resistant 4-O-acetylated sialic acid moieties of equine alpha2-macroglobulin because of the mutation 145N-->K/D in their HA1. These results indicate that influenza viruses become resistant to serum inhibitors because their affinity for these inhibitors is reduced. To determine whether natural inhibitors play a role in viral evolution during interspecies transmission, we compared the receptor-binding properties of H3N8 avian and equine viruses, including two strains isolated during the 1989 to 1990 equine influenza outbreak, which was caused by an avian virus in China. Avian strains bound 4-O-acetylated sialic acid residues of equine alpha2-macroglobulin, whereas equine strains did not. The earliest avian-like isolate from a horse influenza outbreak bound to this sialic acid with an affinity similar to that of avian viruses; a later isolate, however, displayed binding properties more similar to those of classical equine strains. These data suggest that the neuraminidase-resistant sialylglycoconjugates present in horses exert selective pressure on the receptor-binding properties of avian virus HA after its introduction into this host.

Studies of the binding properties of influenza hemagglutinin receptor-site mutants

Site-specific mutations have been made in the influenza hemagglutinin (HA) receptor binding site to assess the contribution of individual amino acid residues to receptor recognition. Screening of mutant HAs, expressed using recombinant vaccinia virus-infected cells, for their abilities to bind human erythrocytes indicated that substitutions involving conserved residues Y98F, H183F, and L194A severely restricted binding and that the substitution W153A prevented cell surface expression of HA. Mutation of residues E190 and S228 that are in positions to form hydrogen bonds with the 9-OH of sialic acid appeared to increase erythrocyte binding slightly, as did the substitution G225R. Substitutions of other residues that are directly or indirectly involved in receptor binding, S136T, S136A, Y195F, G225D, and L226P, had intermediate effects on binding between these two extremes. Estimates of changes in receptor binding specificity based on inhibition of binding to erythrocytes by nonimmune horse sera indicated that mutants G225R and L226P, unlike wild-type HA, were not inhibited; Y195F and G225D mutants were, like wild type, inhibited; and erythrocyte binding by mutants S136A, S136T, E190A, and S228G was only partially inhibited. Viruses containing mutant HAs Y98F, S136T, G225D, and S228G that cover the range of erythrocyte binding properties observed were also constructed by transfection. All four transfectant viruses replicated in MDCK cells and embryonated hens' eggs as efficiently as wild-type X-31 virus, although the Y98F mutant virus was unable to agglutinate erythrocytes. Mutant MDCK cells that have reduced levels of cell surface sialic acids were susceptible to infection by S136T, G225D, and S228G transfectant viruses and by wild type but not by the Y98F transfectant virus.

Specification of receptor-binding phenotypes of influenza virus isolates from different hosts using synthetic sialylglycopolymers: non-egg-adapted human H1 and H3 influenza A and influenza B viruses share a common high binding affinity for 6'-sialyl(N-acetyllactosamine)

Synthetic sialylglycoconjugates bearing 3'-sialyllactose, 6'-sialyllactose, or 6'-sialyl(N-acetyllactosamine) moieties attached to the polyacrylic acid carrier (P-3-SL, P-6-SL, and P-6-SLN, respectively) were prepared and tested for their ability to bind to influenza virus isolates from different hosts in a competitive solid phase assay. The virus panel included egg-grown avian and porcine strains, as well as human viruses isolated and propagated solely in mammalian (MDCK) cells and their egg-adapted variants. A clear correlation was observed between the pattern of virus binding of two glycopolymers, P-3-SL and P-6-SLN, and the host species from which the virus was derived. Avian isolates displayed a high binding affinity for P-3-SL and a two to three orders of magnitude lower affinity for P-6-SLN. By contrast, all non-egg-adapted human A and B viruses bound P-6-SLN strongly but did not bind P-3-SL. Unlike the "authentic" human strains, their egg-adapted counterparts acquired an ability to bind P-3-SL, indicative of a shift in the receptor-binding phenotype toward the recognition of Neu5Ac2-3Gal-terminated sugar sequences. Among the porcine viruses and human isolates with porcine hemagglutinin, few displayed an avian-like binding phenotype, while others differed from both avian and human strains by a reduced ability to discriminate between P-3-SL and P-6-SLN. Our data show that sialylglycopolymers may become a useful tool in studies on molecular mechanisms of interspecies transfer, tissue specificity, and other structure-function relationships of the influenza virus hemagglutinin.