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Molecular Structures of Native HA Trimers on 2009 H1N1 Pandemic Influenza Virus Complexed with Neutralizing Antibodies.
Antigenic variation of influenza virus hemagglutinin (HA) remains the principal challenge in developing more effective vaccines. Here, we report determination of the 3D structures of HA trimers on intact 2009 H1N1 pandemic virus particles in the absence and presence of neutralizing antibodies. We studied HA trimer structures bound to H17-L10, an antibody that recognizes a quaternary epitope near the head region of the trimer, and to C179, a broadly neutralizing antibody that is reactive to the conserved stem region of HA and neutralizes viruses expressing a broad range of HA subtypes (H1, H2, H5, and H9). We deduce the locations of the molecular surfaces of HA involved in interaction with each of these antibodies. Despite the dense packing of HA trimers on the viral surface, and the location of the stem region close to the viral membrane, we show that ∼75% of HA trimers have C179 bound to the stem domain. Thus, the majority of HA trimers on intact virions are available to bind anti-stem antibodies that target conserved HA epitopes, suggesting that universal influenza vaccines that elicit such antibodies could be effective.