Crystal structure of a novel asymmetrically engineered Fc variant with improved affinity for FcI3Rs

Enhancing the effector function by optimizing the interaction between Fc and FcI3 receptor (FcI3R) is a promising approach to enhance the potency of anticancer monoclonal antibodies (mAbs). To date, a variety of Fc engineering approaches to modulate the interaction have been reported, such as afucosylation in the heavy chain Fc region or symmetrically introducing amino acid substitutions into the region, and there is still room to improve FcI3R binding and thermal stability of the CH2 domain with these approaches. Recently, we have reported that asymmetric Fc engineering, which introduces different substitutions into each Fc region of heavy chain, can further improve the FcI3R binding while maintaining the thermal stability of the CH2 domain by fine-tuning the asymmetric interface between the Fc domain and FcI3R. However, the structural mechanism by which the asymmetrically engineered Fc improved FcI3R binding remained unclear. In order to elucidate the mechanism, we solved the crystal structure of a novel asymmetrically engineered Fc, asym-mAb23, in complex with FcI3RIIIa. Asym-mAb23 has enhanced binding affinity for both FcI3RIIIa and FcI3RIIa at the highest level of previously reported Fc variants. The structural analysis reveals the features of the asymmetrically engineered Fc in comparison with symmetric Fc and how each asymmetrically introduced substitution contributes to the improved interaction between asym-mAb23 and FcI3RIIIa. This crystal structure could be utilized to enable us to design a more potent asymmetric Fc.