Broadly neutralizing anti-hepatitis B virus antibody reveals a complementarity determining region H3 lid-opening mechanism.

The humanized monoclonal antibody HzKR127 recognizes the preS1 domain of the human hepatitis B virus surface proteins with a broadly neutralizing activity in vivo. We present the crystal structures of HzKR127 Fab and its complex with a major epitope peptide. In the complex structure, the bound peptide forms a type IV beta-turn followed by 3(10) helical turn, the looped-out conformation of which provides a structural basis for broad neutralization. Upon peptide binding, the antibody undergoes a dramatic complementarity determining region H3 lid opening. To understand the structural implication of the virus neutralization, we carried out comprehensive alanine-scanning mutagenesis of all complementarity determining region residues in HzKR127 Fab. The functional mapping of the antigen-combining site demonstrates the specific roles of major binding determinants in antigen binding, contributing to the rational design for maximal humanization and affinity maturation of the antibody.