Humanization of a mouse monoclonal antibody by CDR-grafting: the importance of framework residues on loop conformation

A mouse monoclonal antibody (mAb 425) with therapeutic potentialwas ‘humanized’ in two ways. Firstly the mouse variableregions from mAb 425 were spliced onto human constant regionsto create a chimeric 425 antibody. Secondly, the mouse complementarity–determiningregions (CDRs) from mAb 425 were grafted into human variableregions, which were then joined to human constant regions, tocreate a reshaped human 425 antibody. Using a molecular modelof the mouse mAb 425 variable regions, framework residues (FRs)that might be critical for antigen-binding were identified.To test the importance of these residues, nine versions of thereshaped human 425 heavy chain variable (VH) regions and twoversions of the reshaped human 425 light chain variable (VJregions were designed and constructed. The recombinant DNAscoding for the chimeric and reshaped human light and heavy chainswere coexpressed transiently in COS cells. In antigen-bindingassays and competition-binding assays, the reshaped human antibodieswere compared with mouse 425 antibody and to chimeric 425 antibody.The different versions of 425–reshaped human antibodyshowed a wide range of avidities for antigen, indicating thatsubstitutions at certain positions in the human FRs significantlyinfluenced binding to antigen. Why certain individual FR residuesinfluence antigen-binding is discussed. One version of reshapedhuman 425 antibody bound to antigen with an avidity approachingthat of the mouse 425 antibody.