A Potent Germline-like Human Monoclonal Antibody Targets a pH-Sensitive Epitope on H7N9 Influenza Hemagglutinin

The H7N9 influenza virus causes high-mortality disease in humans but no effective therapeutics are available. Here we report a human monoclonal antibody, m826, that binds to H7 hemagglutinin (HA) and protects against H7N9 infection. m826 binds to H7N9 HA with subnanomolar affinity at acidic pH and 10-fold lower affinity at neutral pH. The high-resolution (1.9 Å) crystal structure of m826 complexed with H7N9 HA indicates that m826 binds an epitope that may be fully exposed upon pH-induced conformational changes in HA. m826 fully protects mice against lethal challenge with H7N9 virus through mechanisms likely involving antibody-dependent cell-mediated cytotoxicity. Interestingly, immunogenetic analysis indicates that m826 is a germline antibody, and m826-like sequences can be identified in H7N9-infected patients, healthy adults, and newborn babies. These m826 properties offer a template for H7N9 vaccine immunogens, a promising candidate therapeutic, and a tool for exploring mechanisms of virus infection inhibition by antibodies.