Structural and genetic basis for development of broadly neutralizing influenza antibodies

The events leading to the generation of broadly neutralizing antibodies to influenza viruses, which may hold the key to developing a universal flu vaccine, are elucidated. Vaccine-friendly anti-influenza antibodies The study of broadly neutralizing antibodies to influenza virus may pave the way for the generation of a universal vaccine. Here, Daniel Lingwood et al . define the minimal requirements for high-affinity binding of such broadly neutralizing antibodies. They show that binding does not involve light chains, and that most of the crucial heavy-chain contacts are germline encoded. Membrane-bound antibodies are shown to function despite their initially very low affinity. Influenza viruses take a yearly toll on human life despite efforts to contain them with seasonal vaccines. These viruses evade human immunity through the evolution of variants that resist neutralization. The identification of antibodies that recognize invariant structures on the influenza haemagglutinin (HA) protein have invigorated efforts to develop universal influenza vaccines. Specifically, antibodies to the highly conserved stem region of HA neutralize diverse viral subtypes. These antibodies largely derive from a specific antibody gene, heavy-chain variable region IGHV1-69 , after limited affinity maturation from their germline ancestors 1 , 2 , but how HA stimulates naive B cells to mature and induce protective immunity is unknown. To address this question, we analysed the structural and genetic basis for their engagement and maturation into broadly neutralizing antibodies. Here we show that the germline-encoded precursors of these antibodies act as functional B-cell antigen receptors (BCRs) that initiate subsequent affinity maturation. Neither the germline precursor of a prototypic antibody, CR6261 (ref. 3 ), nor those of two other natural human IGHV1-69 antibodies, bound HA as soluble immunoglobulin-G (IgG). However, all three IGHV1-69 precursors engaged HA when the antibody was expressed as cell surface IgM. HA triggered BCR-associated tyrosine kinase signalling by germline transmembrane IgM. Recognition and virus neutralization was dependent solely on the heavy chain, and affinity maturation of CR6261 required only seven amino acids in the complementarity-determining region (CDR) H1 and framework region 3 (FR3) to restore full activity. These findings provide insight into the initial events that lead to the generation of broadly neutralizing antibodies to influenza, informi