Structural evidence for recognition of a single epitope by two distinct antibodies

The structure of a complex between the hemagglutinin of influenza virus and the Fab of a neutralizing antibody was determined by X-ray crystallography at 2.8 Angstrom resolution. This antibody and another which has only 56% s equence identity bind to the same epitope with very similar affinities and in the same orientation. One third of the interactions is conserved in the two complexes; a significant proportion of the interactions that differ are established by residues of the H3 complementarity-determining regions (CDR ) which adopt distinct conformations in the two antibodies. This demonstrat es that there is a definite flexibility in the selection of antibodies that bind to a given epitope, despite the high affinity of their complexes. Thi s flexibility allows the humoral immune response to be redundant, a feature that may be useful in achieving longer lasting protection against evolving viral pathogens. (C) 2000 Wiley-Liss, Inc.