Effective Phagocytosis of Low Her2 Tumor Cell Lines with Engineered, Aglycosylated IgG Displaying High FcγRIIa Affinity and Selectivity

Glycans anchored to residue N297 of the antibody IgG Fc domain are critical in mediating binding toward Fc gamma Rs to direct both adaptive and innate immune responses. However, using a full length bacterial IgG display system, we have isolated aglycosylated Fc domains with mutations that confer up to a 160-fold increase in the affinity toward the low affinity Fc gamma RIIa-R131 allele as well as high selectivity against binding to the remarkably homologous human inhibitory receptor, Fc gamma RIIb. The mutant Pc domain (AglycoT-Fc1004) contained a total of 5 amino acid substitutions that conferred an activating to inhibitory ratio of 25 (A/I ratio; FcyRIIa-R131:Fc gamma RIIb). Incorporation of this engineered Fc into trastuzumab, an anti-Her2 antibody, resulted in a 75% increase in tumor cell phagocytosis by macrophages compared to that of the parental glycosylated trastuzumab with both medium and low Her2-expressing cancer cells. A mathematical model has been developed to help explain how receptor affinity and the A/I ratio relate to improved antibody dependent cell-mediated phagocytosis. Our model provides guidelines for the future engineering of Fc domains with enhanced effector function.