H7N9 avian influenza virus causes severe infections and might have the potential to trigger a major pandemic. Molecular determinants of human humoral immune response to N9 neuraminidase (NA) proteins, which exhibit unusual features compared with seasonal influenza virus NA proteins, are ill-defined. We isolated 35 human monoclonal antibodies (mAbs) from two H7N9 survivors and two vaccinees. These mAbs react to NA in a subtype-specific manner and recognize diverse antigenic sites on the surface of N9 NA, including epitopes overlapping with, or distinct from, the enzyme active site. Despite recognizing multiple antigenic sites, the mAbs use a common mechanism of action by blocking egress of nascent virions from infected cells, thereby providing an antiviral prophylactic and therapeutic protection in vivo in mice. Studies of breadth, potency, and diversity of antigenic recognition from four subjects suggest that vaccination with inactivated adjuvanted vaccine induce NA-reactive responses comparable to that of H7N9 natural infection.
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