Structural basis for the broad antigenicity of the computationally optimized influenza hemagglutinin X6

Influenza causes significant morbidity and mortality. As an alternative approach to current seasonal vaccines, the computationally optimized broadly reactive antigen (COBRA) platform has been previously applied to hemagglutinin (HA). This approach integrates wild-type HA sequences into a single immunogen to expand the breadth of accessible antibody epitopes. Adding to previous studies of H1, H3, and H5 COBRA HAs, we define the structural features of another H1 subtype COBRA, X6, that incorporates HA sequences from before and after the 2009 H1N1 influenza pandemic. We determined structures of this antigen alone and in complex with COBRA-specific as well as broadly reactive and functional antibodies, analyzing its antigenicity. We found that X6 possesses features representing both historic and recent H1 HA strains, enabling binding to both head- and stem-reactive antibodies. Overall, these data confirm the integrity of broadly reactive antibody epitopes of X6 and contribute to design efforts for a next-generation vaccine. [Display omitted] •X6 captures glycosylation and antigenic features of pre-2009 and post-2009 H1 HAs•Vaccine-induced COBRA-reactive mAbs can have broad H1 reactivity and neutralization•X6 possesses intact head and stem epitopes characteristic of several H1 viruses Current influenza vaccines confer narrow, relatively strain-specific protection. Through analyses of a next-generation vaccine, X6, Nagashima et al. describe the characteristics responsible for enhanced antibody-dependent breadth. Glycans and antibody-binding sites of the major surface protein, hemagglutinin (HA), were conserved from circulating strains from before and after the 2009 H1N1 pandemic.