Single-domain antibodies directed against hemagglutinin and neuraminidase protect against influenza B viruses

Influenza B viruses are antigenically diverse and contribute significantly to the annual influenza burden. Here we report influenza B virus neutralizing single-domain antibodies that target highly conserved regions of the hemagglutinin and neuraminidase. Structural studies by single particle electron cryo-microscopy (cryo-EM) revealed that one of these single-domain antibodies prevents the conformational transition of the viral hemagglutinin to the post-fusion state by targeting a quaternary epitope spanning two protomers in the hemagglutinin-stem region. A second single-domain antibody broadly inhibits influenza B neuraminidase activity, including an oseltamivir-resistant neuraminidase, and its complex with neuraminidase elucidated by single particle cryo-EM established that it binds to residues in the neuraminidase catalytic site. Head-to-tail fusions of these single-domain antibodies led to bispecific binders that further improved the neutralization breadth and potency against influenza B viruses. These single-domain antibodies, fused to a human IgG1-Fc domain, fully protected female mice against an otherwise lethal influenza B virus challenge. Our findings underscore the potential of engineered single-domain antibodies to help control influenza B virus infections.