Enhanced antibody-mediated cellular cytotoxicity of germline-like anti-HER2 antibodies through a point mutation in complementarity-determining regions

Human epidermal growth factor receptor 2 (HER2) is highly overexpressed on the surface of breast cancer cells, presenting an attractive target for therapeutic antibodies. Antibody-dependent cellular cytotoxicity (ADCC) is a central mechanism underlying the antitumor activities of anti-HER2 monoclonal antibodies. Here, we engineered Ab5, a germline-like anti-HER2 monoclonal antibody derived from a naïve human antibody library. Our aim was to enhance the ADCC properties of Ab5 by affinity maturation. Through an in silico aided mutagenesis analysis, we identified an Ab5 mutant termed Ab5m, exhibiting enhanced affinity compared with the parental Ab5. Computational modeling predicted that a crucial interacting residue, aspartic acid 31 on the complementarity-determining region 1 of the heavy chain involved in the important charged interactions with HER2 domain II. Substitution of aspartic acid with glutamic acid decreased the interaction energies, resulting in a remarkable affinity enhancement of Ab5m-scFv (KD = 0.2 nM) compared with parental Ab5-scFv (KD = 1.5 nM). This affinity maturation translated into the obvious improvement in ADCC and notable enhancement of tumor ablation in vivo, either alone or in combination with anti-B7-H3 antibodies. These findings suggest that the potential of affinity optimization as a strategy to enhance the ADCC properties of human germline antibodies.