Influenza A Virus H7 nanobody recognizes a conserved immunodominant epitope on hemagglutinin head and confers heterosubtypic protection

Influenza remains a persistent global health challenge, largely due to the virus’ continuous antigenic drift and shift, which impede the development of a universal vaccine. To address this, the identification of broadly neutralizing antibodies and their epitopes is crucial. Nanobodies, with their unique characteristics and binding capacity, offer a promising avenue to identify such epitopes. Here, we isolated and purified a hemagglutinin (HA)-specific nanobody that recognizes an H7 subtype of influenza A virus. Notably, the nanobody, named E10, exhibited broad-spectrum binding, cross-group neutralization and in vivo protection across various influenza A subtypes. Through phage display and in vitro characterization, we demonstrated that E10 specifically targets an epitope on HA head. This epitope is part of the conserved lateral patch of HA head and proved to be highly immunodominant upon H7 infection. Importantly, immunization with a peptide including the E10 epitope elicited cross-reactive antibodies and mediated partial protection from lethal viral challenge. Our data highlight the potential of E10 and its associated epitope as a candidate for future influenza prevention strategies.