Influenza virus presents a significant and persistent threat to public health worldwide and current vaccines provide immunity to viral isolates similar to the vaccine strain. High affinity antibodies against a conserved epitope could provide immunity to the diverse influenza subtypes and protection against future pandemic viruses. Co-crystal structures were determined at 2.2 and 2.7 Å resolutions for broadly neutralizing human antibody CR6261 Fab in complexes with the major surface antigen (hemagglutinin, HA) from viruses responsible for the 1918 H1N1 influenza pandemic and a recent lethal case of H5N1 avian influenza. In contrast to all other structurally characterized influenza antibodies, CR6261 recognizes a highly conserved helical region in the membrane-proximal stem of HA1/HA2. The antibody neutralizes the virus by blocking conformational rearrangements associated with membrane fusion. The CR6261 epitope identified here should accelerate the design and implementation of improved vaccines that can elicit CR6261-like antibodies, as well as antibodybased therapies for the treatment of influenza.Over the past century, three human influenza A pandemics (1918 H1N1 Spanish, 1957 H2N2 Asian, and 1968 have killed ∼50-100 million people worldwide. Each pandemic virus was derived, at least in part, from an avian influenza virus by direct interspecies transmission or exchange of genetic material between avian and human viruses (1-4). In each case, a novel hemagglutinin (HA) envelope glycoprotein was acquired that was antigenically distinct from the HAs of the human viruses in circulation at that time. HA is the primary target of neutralizing antibodies and rapidly and continuously accumulates mutations to escape recognition by the immune system. In pandemic years, HAs are shuffled from the vast reservoir of 16 HA subtypes in avian viruses into a circulating human virus to evade prevailing immunity in the human population. Thus, while many factors likely contribute to virulence and transmissibility, immune evasion is critical for the rapid spread of pandemic and epidemic viruses.Several small molecules are in use for treatment of influenza. Most notable are neuraminidase (NA) inhibitors, oseltamivir (Tamiflu) and zanamivir (Relenza), that prevent release of nascent virions, and amantadine (5) that interferes with the M2 channel proton conducting activity. However, excessive use leads to resistant viruses (6-8) that often show surprisingly little attenuation from the escape mutations, thereby contributing to rapid spread worldwide (6).

Identification of broadly neutralizing antibodies against influenza A viruses has raised hopes for the development of monoclonal antibody-based immunotherapy and ‘universal’ vaccines for influenza. However, a significant part of the annual flu burden is caused by two cocirculating, antigenically distinct lineages of influenza B viruses. Here we report human monoclonal antibodies, CR8033, CR8071 and CR9114, which protect mice against lethal challenge from both lineages. Antibodies CR8033 and CR8071 recognize distinct conserved epitopes in the head region of the influenza B hemagglutinin (HA), whereas CR9114 binds a conserved epitope in the HA stem and protects against lethal challenge with influenza A and B viruses. These antibodies may inform on development of monoclonal antibody-based treatments and a universal flu vaccine for all influenza A and B viruses.

Current flu vaccines provide only limited coverage against seasonal strains of influenza viruses. The identification of VH1-69 antibodies that broadly neutralize almost all influenza A group 1 viruses constituted a breakthrough in the influenza field. Here we report the isolation and characterization of a human monoclonal antibody CR8020 with broad neutralizing activity against most group 2 viruses, including H3N2 and H7N7, which cause severe human infection. The crystal structure of Fab CR8020 with the 1968 pandemic H3 hemagglutinin (HA) reveals a highly conserved epitope in the HA stalk distinct from the epitope recognized by the VH1-69 group 1 antibodies. Thus, a cocktail of two antibodies may be sufficient to neutralize most influenza A subtypes and, hence, enable development of a universal flu vaccine and broad spectrum antibody therapies.