Importance of a CDR H3 basal residue in VH/VL interaction of human antibodies

Although the cooperativity of the V-H and V-L domains of an antibody in antigen binding has been extensively studied, the interaction between the V-H and V-L domains had not received sufficient attention. To systematically investigate the relationship between the amino acid sequence and V-H/V-L interaction strength, we here used a set of anti-bovine serum albumin antibodies having a single human framework for V-H (V3-23/DP-47 and JH4b) and Vk (O12/O2/DPK9 and Jk1), but with different V-H/V-L interaction strengths. By phage display of a V-H mini-library and analysis of the interaction of amino acids with immobilized V-L fragments, the residue at H95 (Kabat numbering) at the beginning of seven CDR H3 residues was found to play a key role in determining the V-H/V-L interaction. On saturation mutagenesis of H95, Gly showed the strongest interaction, while Asp, Asn, and Glu showed lesser interaction in that order. The generality of the rule was confirmed by the test with urine-derived human L chain instead of a particular V-L. The results demonstrate that H95 plays a central role in deciding the V-H/V-L interaction of human Fvs that have most commonly found frameworks.