Receptor-binding properties of swine influenza viruses isolated and propagated in MDCK cells

Receptor specificity of subtype H1 influenza A viruses isolated from swine and humans in the United States

The evolution of classical swine influenza viruses receptor specificity preceding the emergence of the 2009 H1N1 pandemic virus was analyzed in glycan microarrays. Classical swine influenza viruses from the α, β, and γ antigenic clusters isolated between 1945 and 2009 revealed a binding profile very similar to that of 2009 pandemic H1N1 viruses, with selectivity for α2-6-linked sialosides and very limited binding to α2-3 sialosides. Despite considerable genetic divergence, the 'human-like' H1N1 viruses circulating in swine retained strong binding preference for α2-6 sialylated glycans. Interspecies transmission of H1N1 influenza viruses from swine to humans or from humans to swine has not driven selection of viruses with distinct novel receptor binding specificities. Classical swine and human seasonal H1N1 influenza viruses have conserved specificity for similar α2-6-sialoside receptors in spite of long term circulation in separate hosts, suggesting that humans and swine impose analogous selection pressures on the evolution of receptor binding function.

N-glycans from porcine trachea and lung: predominant NeuAcα2-6Gal could be a selective pressure for influenza variants in favor of human-type receptor

It is known that pigs acted as "mixing vessels" for genesis of a new reassortant influenza strain responsible for pandemic H1N1 2009. However, the host factors driving the evolution of a reassorted virus in pigs to 'jump species' resulting in a human outbreak remain unclear. N-glycans derived from the porcine respiratory tract were enzymatically released, fluorescent labeled with 2-aminopyridine, separated according to charge, size and hydrophobicity, and structurally identified by a two-dimensional (size and hydrophobicity) HPLC mapping technique and MALDI-TOF mass spectrometry before and after exo-glycosidase digestion. We found a 3-, 5-, and 13-fold increases in NeuAcα2-6, a preferable human influenza receptor, over NeuAcα2-3, an avian influenza receptor, from upper and lower parts of the porcine trachea towards the porcine lung, a major target organ for swine virus replication. The large proportion of NeuAcα2-6 may exert selective pressure for selection of influenza variants with altered receptor preference for this human-type α2-6 receptor, a crucial first step for generating a human pandemic.

Glycan analysis and influenza A virus infection of primary swine respiratory epithelial cells: the importance of NeuAc{alpha}2-6 glycans

To better understand influenza virus infection of pigs, we examined primary swine respiratory epithelial cells (SRECs, the primary target cells of influenza viruses in vivo), as a model system. Glycomic profiling of SRECs by mass spectrometry revealed a diverse range of glycans terminating in sialic acid or GalαGal. In terms of sialylation, α2-6 linkage was more abundant than α2-3, and NeuAc was more abundant than NeuGc. Virus binding and infection experiments were conducted to determine functionally important glycans for influenza virus infection, with a focus on recently emerged swine viruses. Infection of SRECs with swine and human viruses resulted in different infectivity levels. Glycan microarray analysis with a high infectivity "triple reassortant" virus ((A/Swine/MN/593/99 (H3N2)) that spread widely throughout the North American swine population and a lower infectivity human virus isolated from a single pig (A/Swine/ONT/00130/97 (H3N2)) showed that both viruses bound exclusively to glycans containing NeuAcα2-6, with strong binding to sialylated polylactosamine and sialylated N-glycans. Treatment with mannosamine precursors of sialic acid (to alter NeuAc/NeuGc abundances) and linkage-specific sialidases prior to infection indicated that the influenza viruses tested preferentially utilize NeuAcα2-6-sialylated glycans to infect SRECs. Our data indicate that NeuAcα2-6-terminated polylactosamine and sialylated N-glycans are important determinants for influenza viruses to infect SRECs. As NeuAcα2-6 polylactosamine glycans play major roles in human virus infection, the importance of these receptor components in virus infection of swine cells has implications for transmission of viruses between humans and pigs and for pigs as possible adaptation hosts of novel human 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.

Replication, pathogenesis and transmission of pandemic (H1N1) 2009 virus in non-immune pigs

The declaration of the human influenza A pandemic (H1N1) 2009 (H1N1/09) raised important questions, including origin and host range [1], [2]. Two of the three pandemics in the last century resulted in the spread of virus to pigs (H1N1, 1918; H3N2, 1968) with subsequent independent establishment and evolution within swine worldwide [3]. A key public and veterinary health consideration in the context of the evolving pandemic is whether the H1N1/09 virus could become established in pig populations [4]. We performed an infection and transmission study in pigs with A/California/07/09. In combination, clinical, pathological, modified influenza A matrix gene real time RT-PCR and viral genomic analyses have shown that infection results in the induction of clinical signs, viral pathogenesis restricted to the respiratory tract, infection dynamics consistent with endemic strains of influenza A in pigs, virus transmissibility between pigs and virus-host adaptation events. Our results demonstrate that extant H1N1/09 is fully capable of becoming established in global pig populations. We also show the roles of viral receptor specificity in both transmission and tissue tropism. Remarkably, following direct inoculation of pigs with virus quasispecies differing by amino acid substitutions in the haemagglutinin receptor-binding site, only virus with aspartic acid at position 225 (225D) was detected in nasal secretions of contact infected pigs. In contrast, in lower respiratory tract samples from directly inoculated pigs, with clearly demonstrable pulmonary pathology, there was apparent selection of a virus variant with glycine (225G). These findings provide potential clues to the existence and biological significance of viral receptor-binding variants with 225D and 225G during the 1918 pandemic [5].

Generation of live attenuated novel influenza virus A/California/7/09 (H1N1) vaccines with high yield in embryonated chicken eggs

Several live attenuated influenza virus A/California/7/09 (H1N1) (CA09) candidate vaccine variants that possess the hemagglutinin (HA) and neuraminidase (NA) gene segments from the CA09 virus and six internal protein gene segments from the cold-adapted influenza virus A/Ann Arbor/6/60 (H2N2) virus were generated by reverse genetics. The reassortant viruses replicated relatively poorly in embryonated chicken eggs. To improve virus growth in eggs, reassortants expressing the HA and NA of CA09 were passaged in MDCK cells and variants exhibiting large-plaque morphology were isolated. These variants replicated at levels approximately 10-fold higher than the rate of replication of the parental strains in embryonated chicken eggs. Sequence analysis indicated that single amino acid changes at positions 119, 153, 154, and 186 were responsible for the improved growth properties in MDCK cells and eggs. In addition, the introduction of a mutation at residue 155 that was previously shown to enhance the replication of a 1976 swine influenza virus also significantly improved the replication of the CA09 virus in eggs. Each variant was further evaluated for receptor binding preference, antigenicity, attenuation phenotype, and immunogenicity. Mutations at residues 153, 154, and 155 drastically reduced viral antigenicity, which made these mutants unsuitable as vaccine candidates. However, changes at residues 119 and 186 did not affect virus antigenicity or immunogenicity, justifying their inclusion in live attenuated vaccine candidates to protect against the currently circulating 2009 swine origin H1N1 viruses.

Combining molecular dynamics with bayesian analysis to predict and evaluate ligand-binding mutations in influenza hemagglutinin

Influenza virus attaches to and infects target cells via binding of cell-surface glycans by the viral hemagglutinin. This binding specificity is considered a major reason why avian influenza is typically poorly transmitted between humans, while swine influenza is better transmitted due to glycan similarity between the human and swine upper respiratory tract. Predicting mutations that control glycan binding is thus important to continued surveillance against new pandemic influenza strains. We have designed a molecular-dynamics approach for scoring potential mutants with predictive power for both receptor-binding-domain and allosteric mutations similar to those identified from clinical isolates of avian influenza. We have performed thousands of simulations of 17 different hemagglutinin mutants totaling >1 ms in length and employ a bayesian model to rank mutations that disrupt the stability of the hemagglutinin-ligand complex. Based on our simulations, we predict a significantly increased k(off) for seven of these mutants. This means of using molecular dynamics analysis to make experimentally verifiable predictions offers a potentially general method to identify ligand-binding mutants, particularly allosteric ones. Our analysis of ligand dissociation provides a means to evaluate mutants prior to experimental mutagenesis and testing and constitutes an important step toward understanding the determinants of ligand binding by H5N1 influenza.

Alterations in receptor-binding properties of swine influenza viruses of the H1 subtype after isolation in embryonated chicken eggs

Alterations of the receptor-binding properties of swine influenza A viruses (SIVs) during their isolation in embryonated chicken eggs have not been well studied. In this study, the receptor-binding properties of classical H1 SIVs isolated solely in eggs or Madin-Darby canine kidney (MDCK) cells were examined. Sequencing analysis revealed substitutions of D190V/N or D225G in the haemagglutinin (HA) proteins in egg isolates, whereas MDCK isolates retained HA genes identical to those of the original viruses present in the clinical samples. Egg isolates with substitution of either D190V/N or D225G had increased haemagglutinating activity for mouse and sheep erythrocytes, but reduced activity for rabbit erythrocytes. Additionally, egg isolates with D225G had increased haemagglutination activity for chicken erythrocytes. A direct binding assay using a sialyl glycopolymer that possessed either a 5-N-acetylneuraminic acid (Neu5Ac) alpha2,6galactose (Gal) or a Neu5Acalpha2,3Gal linkage revealed that the egg isolates used in this study showed higher binding activity to the Neu5Acalpha2,3Gal receptor than MDCK isolates. Increased binding activity of the egg isolates to the Neu5Acalpha2,3Gal receptor was also confirmed by haemagglutination assay with resialylated chicken erythrocytes by Galbeta1,3/4GlcNAcalpha2,3-sialyltransferase. These observations were reinforced by flow-cytometric and N-glycan analyses of the erythrocytes. The alpha2,3-linked sialic acids were expressed predominantly on the surface of mouse and sheep erythrocytes. Chicken erythrocytes expressed Neu5Acalpha2,3Gal more abundantly than Neu5Acalpha2,6Gal, and rabbit erythrocytes expressed both 5-N-glycolylneuraminic acid (Neu5Gc) alpha2,6Gal and Neu5Acalpha2,6Gal. Our results demonstrate clearly that classical H1 SIVs undergo alterations in receptor-binding activity associated with an amino acid substitution in the HA protein during isolation and propagation in embryonated chicken eggs.

Different evolutionary trajectories of European avian-like and classical swine H1N1 influenza A viruses

In 1979, a lineage of avian-like H1N1 influenza A viruses emerged in European swine populations independently from the classical swine H1N1 virus lineage that had circulated in pigs since the Spanish influenza pandemic of 1918. To determine whether these two distinct lineages of swine-adapted A/H1N1 viruses evolved from avian-like A/H1N1 ancestors in similar ways, as might be expected given their common host species and origin, we compared patterns of nucleotide and amino acid change in whole genome sequences of both groups. An analysis of nucleotide compositional bias across all eight genomic segments for the two swine lineages showed a clear lineage-specific bias, although a segment-specific effect was also apparent. As such, there appears to be only a relatively weak host-specific selection pressure. Strikingly, despite each lineage evolving in the same species of host for decades, amino acid analysis revealed little evidence of either parallel or convergent changes. These findings suggest that although adaptation due to evolutionary lineages can be distinguished, there are functional and structural constraints on all gene segments and that the evolutionary trajectory of each lineage of swine A/H1N1 virus has a strong historical contingency. Thus, in the context of emergence of an influenza A virus strain via a host switch event, it is difficult to predict what specific polygenic changes are needed for mammalian adaptation.

Analysis of a point mutation in H5N1 avian influenza virus hemagglutinin in relation to virus entry into live mammalian cells

Binding to and infection of human cells is essential for avian influenza virus transmission. Since virus binding is not always predictive for efficient infection of the cells, here we wished to investigate how hemagglutinin (HA) mutations of avian influenza virus H5N1 influence virus post-binding events in a single cycle of replication. One mutation observed in H5 HA of avian and natural human isolates from mainland China, Hong Kong, Vietnam and Thailand was identified and analyzed. The effects of the mutation on receptor binding, fusion and virus entry into cultured cells were investigated using hemadsorption, polykaryon formation and pseudotyped virus that express luciferase in the cytoplasm of transduced cell. Our results revealed that replacing aspartic acid at residue 94 with asparagine enhanced virus fusion activity and increased the binding of HA to sialic acid alpha2,6 galactose, while it decreased pseudotyped virus entry into cells expressing the avian type receptor, sialic acid alpha2,3 galactose. Our result may have implications for the understanding of the role of HA mutations in virus entry into live cells that exclusively display one type of receptor.

Genetic and phenotypic characterization of a low-pathogenicity avian influenza H11N9 virus

An H11N9 low-pathogenicity avian influenza virus, A/duck/WA/663/97, was isolated from a sick Mandarin duck kept in an outdoor bird exhibit. Genetic and phenotypic characterization of the virus suggested that it originated from free-flying birds, a concept supported by genetic similarity with waterfowl isolates from the same geographic area and time period. This duck-origin virus had genetic features typical of H11 and N9 viruses, including no neuraminidase stalk deletion, no differences in putative glycosylation sites in either surface protein, and no addition of basic amino acid residues at the hemagglutinin cleavage site compared to published sequences. It replicated in both avian and mammalian cells in vitro, and experimentally challenged chickens developed mild acute upper respiratory lesions but no clinical signs of disease. It elicited immune responses in chickens, resulting in seroconversion in all infected birds, although antibody titers remained low over the experimental period.

Proteomics analysis of differential expression of cellular proteins in response to avian H9N2 virus infection in human cells

We present the first proteomic analysis on the cellular responses to avian influenza virus (H9N2) infection in a human cell line in different time courses in order to search for target proteins for viral pathogenesis/adaptation studies. By using 2-DE coupled with MALDI-TOF MS and nano-ESI-MS/MS, we identified a set of differentially expressed cellular proteins, including cytoplasmic actin, cytokeratin, prohibitin, enoyl-CoA hydratase, peptide-prolyl cis-trans isomerase A (PPIase A), chloride intracellular channel protein 1, pyruvate dehydrogenase E1 component subunit beta, adenine phosphoribosyltransferase, guanine nucleotide-binding protein subunit beta, nucleoside diphosphate kinase A, elongation factor 1-beta and splicing factor, arginine/serine rich 1. The most significant changes in different time courses were found in cytoplasmic actin and cytokeratin, both of which constituted the major components of cytoskeleton network in the cells. The obtained data suggested a possible role of the cytoskeleton during avian influenza virus infection of mammalian cells, which might help for better understanding of the dynamics of avian influenza virus and host interaction in mammalian cell setting.

6-sulfo sialyl Lewis X is the common receptor determinant recognized by H5, H6, H7 and H9 influenza viruses of terrestrial poultry

Background

Influenza A viruses of domestic birds originate from the natural reservoir in aquatic birds as a result of interspecies transmission and adaptation to new host species. We previously noticed that influenza viruses isolated from distinct orders of aquatic and terrestrial birds may differ in their fine receptor-binding specificity by recognizing the structure of the inner parts of Neu5Acα2-3Gal-terminated sialyloligosaccharide receptors. To further characterize these differences, we studied receptor-binding properties of a large panel of influenza A viruses from wild aquatic birds, poultry, pigs and horses.

Results

Using a competitive solid-phase binding assay, we determined viral binding to polymeric conjugates of sialyloligosaccharides differing by the type of Neu5Acα-Gal linkage and by the structure of the more distant parts of the oligosaccharide chain. Influenza viruses isolated from terrestrial poultry differed from duck viruses by an enhanced binding to sulfated and/or fucosylated Neu5Acα2-3Gal-containing sialyloligosaccharides. Most of the poultry viruses tested shared a high binding affinity for the 6-sulfo sialyl Lewis X (Su-SLe^x). Efficient binding of poultry viruses to Su-SLe^x was often accompanied by their ability to bind to Neu5Acα2-6Gal-terminated (human-type) receptors. Such a dual receptor-binding specificity was demonstrated for the North American and Eurasian H7 viruses, H9N2 Eurasian poultry viruses, and H1, H3 and H9 avian-like virus isolates from pigs.

Conclusion

Influenza viruses of terrestrial poultry differ from ancestral duck viruses by enhanced binding to sulfated and/or fucosylated Neu5Acα2-3Gal-terminated receptors and, occasionally, by the ability to bind to Neu5Acα2-6Gal-terminated (human-type) receptors. These findings suggest that the adaptation to receptors in poultry can enhance the potential of an avian virus for avian-to-human transmission and pandemic spread.

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.

Effective replication of human influenza viruses in mice lacking a major alpha2,6 sialyltransferase

The hemagglutinins of influenza viruses isolated from humans typically prefer binding to sialic acid in an alpha2,6 linkage. Presumably, the virus uses the presence of these receptors on the respiratory tract to gain entrance into the host cell. The ST6Gal I sialyltransferase knock-out mouse lacks the main enzyme necessary for the attachment of alpha2,6 sialic acid to N-linked glycoproteins on the cell surface. Yet even in the absence of detectable alpha2,6 sialic acid in the mouse respiratory tract, human influenza viruses can still infect these mice and grow to similar titers in the lung and trachea as compared to wild-type animals. This work demonstrates that the presence of a major alpha2,6 sialic acid on N-linked glycoproteins is not essential for human influenza virus infection in mice.

Influenza A virus infection of primary differentiated airway epithelial cell cultures derived from Syrian golden hamsters

The ability of several different influenza A virus strains to infect and replicate in primary, differentiated airway epithelial cell cultures from Syrian golden hamsters was investigated. All virus strains tested replicated equivalently in the cultures and displayed a preference for infecting nonciliated cells. This tropism correlated with the expression of both alpha2,3- and alpha2,6-linked sialic acid on the nonciliated cells. In contrast, the ciliated cells did not have detectable alpha2,6-linked sialic acid and expressed only low amounts of alpha2,3-linked sialic acid. In contrast to clinical isolates, laboratory strains of influenza A virus infected a limited number of ciliated cells at late times post-infection. The presence of alpha2,3- and alpha2,6-linked sialic acid residues on the same cell type suggests that Syrian golden hamsters and differentiated airway epithelial cell cultures derived from hamsters may provide a system for studying the reassortment of influenza A virus strains which utilize different forms of sialic acid as a primary virus receptor.