H5N1 receptor specificity as a factor in pandemic risk

Abstract

The high pathogenicity of H5N1 viruses in sporadic infections of humans has raised concerns for its potential to acquire the ability to transmit between humans and emerge as a highly pathogenic pandemic virus. Because avian and human influenza viruses differ in their specificity for recognition of their host cell receptors, receptor specificity represents one barrier for efficient transmission of avian viruses in human hosts. Over the last century, each influenza virus pandemic has coincided with the emergence of virus with an immunologically distinct hemagglutinin exhibiting a 'human-type' receptor specificity, distinct from that of viruses with the same hemagglutinin circulating in zoonotic species. Recent studies suggest that it is possible for H5N1 to acquire human type receptor specificity, but this has not occurred in nature. This review covers what is known about the molecular basis for the switch between avian and human-type receptor specificity for influenza viruses that have successfully adapted to man, the potential for H5N1 to evolve to human-type receptor specificity and its relevance to pandemic risk.

Keywords: H5N1; Hemagglutinin; Pandemic risk; Receptor specificity; Sialic acid.

Figure 1

Influenza virus hemagglutinin receptor specificity assays in plate format. (A). Virus or recombinant hemagglutinin (HA) is adsorbed to the plate. Biotinylated polymers displaying multivalent α2–3Sia or α2–6Sia are then added, and after washing, bound sialoside is detected with labeled streptavidin (Gambaryan and Matrosovich, 1992). (B). Sialoside is bound to the plate, followed by detection of bound virus (Chandrasekaran et al., 2008; Totani et al., 2003; Yamada et al., 2006).

Figure 2

Examples of common sialic acid containing glycans found in glycoproteins and glycolipids on cells of mammalian and avian hosts of influenza. Illustrated are linear fragments that occur as variations at the tips of glycoprotein or glycolipid glycans, 1–8 (L), intact O-linked Core 1–4 glycans linked to threonine or serine, 9–21 (O1-O4), and N-linked glycans linked to asparagine, 22–27 (N). In most cases, sialic acids (purple diamonds) can be linked in either α2–3 or α2–-6 linkage to the next sugar (e.g galactose).

Figure 3

Structural basis for human-type receptor specificity in H1, H2 and H3 hemagglutinins. (A) Structure of the hemaggluninin receptor-binding domain (RBD) from Cal/04/09 H1N1 (PDBID: 3UBE). Structural features that frame the RBD including the 130 and 200 loops and the 190 helix are labeled in red. Side chains are shown in green for amino acids involved in binding sialic acid (Y95, W153, H183 and Y195) or are known to influence receptor binding in human influenza viruses (D190, D225, Q226 and G228,). (B) Tabular representation for amino acids involved conferring human type receptor specificity in red, and corresponding sequences in avian isolates. (C) Structure H2 hemagglutinin (A/Sin/1/52 H2N2, PDBID: 2WR) co-crystallized with the α2–6 sialoside, LSTc. Side chains of amino acids that confer specificity for α2–6 sialosides at positions S226 and L228 are shown in green. (D) Structure representation of H2 A/Sin/1/52 H2N2 co-crystallized with the α2–3 sialoside LSTb (PDBID: 2WRB), showing only the terminal NeuAc-Gal sequence, and the ‘avianized’ mutations S226G and L228Q. (E) Structure H1 derived from Cal/04/09 H1N1 co crystallized with LSTc (PDBID: 3UBE). Side chains of amino acids that confer specificity for α2–6 sialosides at positions D190 and D225 are shown in green. (F) Structure of H1 derived from Cal/04/09 H1N1 co crystallized with the α2–3 sialoside LSTb (PDBID: 3UBQ), showing only the terminal NeuAc-Gal sequence, and the ‘avianized’ mutations D190E and D225G. Structural representations are made using the PyMOL Molecular Graphics System, Version 1.5.0.4 Schrödinger, LLC http://www.pymol.org. with ‘avianized’ mutations modeled in Swiss Model (Arnold et al., 2006).

Figure 4

H5 structures of WT and transmissible H5 viruses. (A) Structure representation of H5 derived from H5N1 with complex N-glycans attached at its N-glycosylation sites (PDBID: 2IBX). The N-linked glycan at position 158 is shown in yellow, and was modeled by Glyprot (Bohne-Lang and von der Lieth, 2005). Side chains of amino acids at position 158, 160, 196, 224, 226 and 228 known to influence receptor specificity and transmissibility are highlighted in green. (B) Transmissible H5 HA from Chen et al. (Chen et al., 2012) with mutations at position 196, 226 and 228 shown in green. The H5 used in this study was from Egret/EG/06, which does not contains a NxT/S sequon at position 158. (C) Transmissible H5 HA from Imai et al. (Imai et al., 2012) with side chains of mutations at position 158, 224 and 226 shown in green. (D) Transmissible H5 HA from Herfst et al. (Herfst et al., 2012) with side chains of mutations at positions 160, 226 and 228 shown in green. In panels A-D a glycan is modeled in appropriate to the specificity of that hemagglutinin for illustration only. Panel has superimposed the NeuAcα2–3Gal structure from LSTb co-crystallized with H1N1 Cal/04/09 (PDBID: 3UBQ). Panels B-D have the superimposed the α2–6 sialoside from LSTc co-crystalized with H2 A/Sin/1/52 H2N2 (PDBID: 2WR7). Structural representations are made using the PyMOL Molecular Graphics System, Version 1.5.0.4 Schrödinger, LLC http://www.pymol.org.

Figure 5

Comparison of ELISA-type and glycan array assays for assessing receptor specificity of H5 mutations. Shown is the influence of E190G and Q196R mutations on receptor specificity of H5 hemagglutinin assessed in two different assays. (A) Receptor specificity of mutations made on the A/Vietnam/1203/04 background the ELISA-type assay. (B) Receptor specificity of mutations made on the A/Vietnam/1203/04 (right) and clade 2.2 A/Egret/Egypt/06 (right) strains using a glycan array with a variety of 2–3Sia and 2–6Sia. Data are excerpted and reproduced with permission with full details and list of glycans in the glycan array available in the original article (Chen et al., 2012).