Does Glycosylation as a modifier of Original Antigenic Sin explain the case age distribution and unusual toxicity in pandemic novel H1N1 influenza?

Abstract

Background: A pandemic novel H1N1 swine-origin influenza virus has emerged. Most recently the World Health Organization has announced that in a country-dependent fashion, up to 15% of cases may require hospitalization, often including respiratory support. It is now clear that healthy children and young adults are disproportionately affected, most unusually among those with severe respiratory disease without underlying conditions. One possible explanation for this case age distribution is the doctrine of Original Antigenic Sin, i.e., novel H1N1 may be antigenically similar to H1N1 viruses that circulated at an earlier time. Persons whose first exposure to influenza viruses was to such similar viruses would be relatively immune. However, this principle is not sufficient to explain the graded susceptibility between ages 20 and 60, the reduced susceptibility in children below age 10, and the unusual toxicity observed.

Methods: We collected case data from 11 countries, about 60% of all cases reported through mid-July 2009. We compared sequence data for the hemagglutinin of novel H1N1 with sequences of H1N1 viruses from 1918 to the present. We searched for sequence differences that imply loss of antigenicity either directly through amino acid substitution or by the appearance of sites for potential glycosylation proximal to sites known to be antigenic in humans. We also considered T-cell epitopes.

Results: In our composite, over 75% of confirmed cases of novel H1N1 occurred in persons < or = 30 years old, with peak incidence in the age range 10-19 years. Less than 3% of cases occurred in persons over 65, with a gradation in incidence between ages 20 and 60 years.The sequence data indicates that novel H1N1 is most similar to H1N1 viruses that circulated before 1943. Novel H1N1 lacks glycosylation sites on the globular head of hemagglutinin (HA1) near antigenic regions, a pattern shared with the 1918 pandemic strain and H1N1 viruses that circulated until the early 1940s. Later H1N1 viruses progressively added new glycosylation sites likely to shield antigenic epitopes, while T-cell epitopes were relatively unchanged.

Conclusions: In this evolutionary context, Original Antigenic Sin exposure should produce an immune response increasingly mismatched to novel H1N1 in progressively younger persons. We suggest that it is this mismatch that produces both the gradation in susceptibility and the unusual toxicity. Several murine studies suggest specific cell types as a likely basis of the unusual toxicity. These studies also point to widely available pharmaceutical agents as plausible candidates for mitigating the toxic effects. The principle of Original Antigenic Sin modified by glycosylation appears to explain both the case age distribution and the unusual toxicity pattern of the novel H1N1 pandemic. In addition, it suggests pharmaceutical agents for immediate investigation for mitigation potential, and provides strategic guidance for the distribution of pandemic mitigation resources of all types.

Figure 1

The age distribution of frequency of cases of novel H1N1 in ten countries on five continents confirmed through the last date available in July, 2009.

Figure 2

Phylogenetic analysis of the HA1 of H1N1 influenza viruses. Phylogenetic tree constructed using the neighbor-joining method and bootstrap analysis (n = 500) to determine the best-fitting tree for the HA1 region of the HA gene. Selected, relevant strains of the H1N1 subtype are listed by year of isolation. Small case "a" and "b" designate when two distinct strains co-circulated in the same year. Bootstrap values ≥ 70% are shown at branchpoints.

Figure 3

Glycosylation of H1N1 viruses, 1918-2009. A) HA glycosylation status of selected, relevant H1N1 viruses from 1918 to 2009 is shown. Beige coloration shows potential glycosylation sites in the fusion domain at positions 21, 33, 289, and 154 (HA2). Blue coloration represents glycosylation status at positions 63, 81, 94, and 271 in the vestigial esterase domain. Green coloration indicates glycosylation status at positions 129 (or 131), 156, and 163 on the globular head near the receptor binding site. The site labeled 129 had the attachment asparagine at position 131 through 1984, but at position 129 from 1986 to the present; these overlapping sites are mutually exclusive so are represented together. B) Structural representations of the H1N1 HA trimer based on the crystal structure of the 1918 pandemic strain [27]. Sites for potential glycosylation have been superimposed on the structure in light blue and labeled to correspond to the chart in Figure 3A.