Antigenically intact hemagglutinin in circulating avian and swine influenza viruses and potential for H3N2 pandemic

Abstract

The 2009 swine-origin H1N1 influenza, though antigenically novel to the population at the time, was antigenically similar to the 1918 H1N1 pandemic influenza, and consequently was considered to be "archived" in the swine species before reemerging in humans. Given that the H3N2 is another subtype that currently circulates in the human population and is high on WHO pandemic preparedness list, we assessed the likelihood of reemergence of H3N2 from a non-human host. Using HA sequence features relevant to immune recognition, receptor binding and transmission we have identified several recent H3 strains in avian and swine that present hallmarks of a reemerging virus. IgG polyclonal raised in rabbit with recent seasonal vaccine H3 fail to recognize these swine H3 strains suggesting that existing vaccines may not be effective in protecting against these strains. Vaccine strategies can mitigate risks associated with a potential H3N2 pandemic in humans.

Figure 1. Trends in antigenic evolution over time.

Antigenic identity of HA from human, avian and swine species relative to pandemics that of note: (a) 1918–19 H1N1, (b) 1957–58 H2N2, (c) 1968–69 H3N2 plotted against time of isolation (x-axis). To generate the plots, 2, 927 human, 166 avian, 950 swine HAs of H1 subtype; 117 human and 163 avian HAs of H2 subtype; 3,632 human, 756 avian and 347 swine HAs of H3 subtype were used. The two black arrows in Fig. 1a correspond to A/New Jersey/76 (H1N1) and A/Wisconsin/4754/1994 (H1N1), both of which caused human infections following pig-human interspecies transmission and have high AI values similar to the 2009 pandemic H1N1 strains. Dotted trendlines are added to graphically display the antigenic drift in avian vs. human H2 (Fig. 1b). The slope of the avian H2 trendline is 0.0845, whereas the slope of the human H2 trendline is −1.866. The dotted horizontal line indicates cutoff AI values (70% (H1); 70% (H2); 70% & 49% (H3)).The data points were jittered slightly on y-axis to avoid large overlaps (AI ~ AI + ε, where −1< ε <1).

Figure 2. Binding of anti-A/Brisbane/10/2007(H3N2) pAb to H3 strains measured by ELISA.

Tested were 2 swine H3N2 HAs (A/swine/Chonburi/05CB2/2005, A/swine/Nakhon pathom/NIAH586-2/2005), pandemic H3N2 HA (A/Aichi/2/1968), 1 seasonal H3N2 HA (A/Wisconsin/67/2005). The seasonal vaccine H3N2 HA (A/Brisbane/10/2007) and a representative H7N7 HA (A/Netherlands/219/2003) were used as positive and negative controls, respectively.

Figure 3. Genetic, antigenic and glycosylation-pattern relatedness of 1968 pandemic H3N2 HA to seasonal, swine and avian H3 HA.

(a) Sequence alignment of the expanded globular head region (residues 50–328) of the HAs listed in Table 4. Antigenic sites A, B, C, D, E of H3 HA are highlighted in green, magenta, cyan, grey and yellow, respectively. In each sequence, the Asn residue associated with the N-linked glycosylation sites (Asn-X-Ser/Asn-X/Thr) is marked in red. (b) Surface rendered three-dimensional structural models of trimeric HA1 globular head of representative pandemic (middle), seasonal (right) and swine (left) HAs. The view of trimer is along axis perpendicular to 3-fold symmetry axis to give a complete picture of the antigenic and glycosylation sites. The homology models of HA1 chain were generated using SWISS-MODEL automated modeling server (http://swissmodel.expasy.org/) and the trimannosyl N-linked glycosylation at the sites were added in silico using GlyProt server (http://www.glycosciences.de/modeling/glyprot/php/main.php). The trimeric HA1 was generated by superimposing three copies of homology-modeled glycosylated monomer with corresponding monomers in the trimer crystal structure (PDB ID: 1HGE). The antigenic sites A–E are marked on the structure. With 1968 pandemic HA as reference (antigenic sites shown in red), the structural similarity of the antigenic sites in seasonal and swine HAs to the reference HA is shown in different shades of red (duller shade representing low similarity to brighter shade representing high similarity).

Figure 4. Glycan microarray analysis of representative H3 HAs.

Dose dependent binding of A/swine/Nakhon pathom/NIAH586-2/2005 (a), and A/swine/Chonburi/05CB2/2005 (b) HAs to representative avian and human glycan receptors on the glycan array platform is shown. Both these HAs show high affinity binding to both human receptors (6′SLN-LN) and avian receptors (3′SLN-LN and 3′SLN-LN-LN).