Abstract
Many broadly reactive human monoclonal antibodies against the hemagglutinin (HA) stem of influenza A virus have been developed for therapeutic applications. These antibodies typically inhibit viral entry steps, especially the HA conformational change that is required for membrane fusion. To better understand the mechanisms by which such antibodies inhibit viral replication, we established broadly reactive human anti-HA stem antibodies and determined the properties of these antibodies by examining their reactivity with 18 subtypes of HA, evaluating their in vivo protective efficacy, identifying their epitopes, and characterizing their inhibitory mechanisms. Among the eight human monoclonal antibodies we generated, which recognized at least 3 subtypes of the soluble HA antigens tested, clone S9-1-10/5-1 reacted with 18 subtypes of HA and protected mice from lethal infection with H1N1pdm09, H3N2, H5N1, and H7N9 viruses. This antibody recognized the HA2 helix A in the HA stem, and inhibited virus particle release from infected cells but did not block viral entry completely. These results show that broadly reactive human anti-HA stem antibodies can exhibit protective efficacy by inhibiting virus particle release. These findings expand our knowledge of the mechanisms by which broadly reactive stem-targeting antibodies inhibit viral replication and provide valuable information for universal vaccine development.