Cross-Neutralization of 1918 and 2009 Influenza Viruses: Role of Glycans in Viral Evolution and Vaccine Design

The 1918 and 2009 pandemic influenza viruses are both inhibited by antibodies directed to an exposed region of the viral spike, but this region becomes shielded by glycans in influenza seasonal strains. Remembrance of Flus Past For those who hate needles, this past winter was not a good one. Not only did we have to get the usual seasonal flu shot, doctors recommended that we also get a second shot against another type of flu, a pandemic virus called 2009 H1N1. As the U.S. Centers for Disease Control warned, “A seasonal vaccine will not protect you against 2009 H1N1.” Also odd was the fact that the 2009 H1N1 pandemic flu seemed to spare older people; those age 65 and older were not considered at high risk as they are for seasonal flu. Wei et al. have now worked out why the 2009 H1N1 pandemic flu has these properties, showing how the virus is different from seasonal flu virus but similar to the pandemic flu that swept the globe in 1918. The authors injected mice with seasonal flu viruses as well as with pandemic viruses from 1918 and 2009. The resulting antibodies raised in the mice could inhibit both pandemic viruses in culture and protected mice from infection with either 2009 or 1918 pandemic flu. Antibodies raised to the seasonal flu virus did not have this protective effect, although they protected against seasonal flu perfectly well. In investigating why, Wei et al. found that the key inhibitory antibodies raised to the pandemic flu strains bound to the exposed top of the spike protein, a molecule that projects from the virus and helps it to infect host cells. This immunogenic part of the spike is very similar in the 1918 and 2009 pandemic viruses. Even more interesting is how the seasonal flu escapes from these antibodies. Its spike protein has two sites, not present in the pandemic flu spike protein, to which sugar groups are added, shielding the seasonal flu spike protein from inhibition by the antibodies that act against the pandemic strains. Pandemic flu viruses evolve into seasonal flu varieties, and the authors suggest that one of the key evolutionary steps is the acquisition of the sites for sugar groups on the spike protein. These changes allow the virus to infect people with preexisting immunity to pandemic flu. The results of Wei et al. may also explain the relative resistance of older people to the present flu pandemic: Persistent immunity to the 1918 flu or its close relatives from childhood may inhibit the unprotected spike protein of the current 2009 pandemic flu virus and, thus, its ability to infect host cells. New strains of H1N1 influenza virus have emerged episodically over the last century to cause human pandemics, notably in 1918 and recently in 2009. Pandemic viruses typically evolve into seasonal forms that develop resistance to antibody neutralization, and cross-protection between strains separated by more than 3 years is uncommon. Here, we define the structural basis for cross-neutralization between two temporally distant pandemic influenza viruses—from 1918 and 2009. Vaccination of mice with the 1918 strain protected against subsequent lethal infection by 2009 virus. Both were resistant to antibodies directed against a seasonal influenza, A/New Caledonia/20/1999 (1999 NC), which was insensitive to antisera to the pandemic strains. Pandemic strain–neutralizing antibodies were directed against a subregion of the hemagglutinin (HA) receptor binding domain that is highly conserved between the 1918 and the 2009 viruses. In seasonal strains, this region undergoes amino acid diversification but is shielded from antibody neutralization by two highly conserved glycosylation sites absent in the pandemic strains. Pandemic HA trimers modified by glycosylation at these positions were resistant to neutralizing antibodies to wild-type HA. Yet, antisera generated against the glycosylated HA mutant neutralized it, suggesting that the focus of the immune response can be selectively changed with this modification. Collectively, these findings define critical determinants of H1N1 viral evolution and have implications for vaccine design. Immunization directed to conserved receptor binding domain subregions of pandemic viruses could potentially protect against similar future pandemic viruses, and vaccination with glycosylated 2009 pandemic virus may limit its further spread and transformation into a seasonal influenza.