Engineering antibody Fc domains for improved therapeutic function
Therapeutic antibodies have achieved exceptional clinical success in the treatment of cancer and other human diseases. Now, new approaches are required to enhance the potency of antibodies to further increase the number of patients responding to therapy. By engineering the antibody Fc domain through mutation of the amino acid sequence, binding affinity to activating or inhibitory Fc receptors on effector cells can be increased to modulate the cellular immune response. However, attaining selectivity for closely related Fc receptors has proved challenging and the technique has not been applied to access the function of antibody isotypes other than IgG. Here we present new methods for enhancing antibody potency using both hybrid IgA/G and aglycosylated Fc domains. In the first instance, a chimeric antibody Fc domain has been created by combining residues from IgA with those from IgG. The new variant, MutD, introduces binding to FcαRI while retaining affinity for certain members of the FcγR family. ADCC assays show MutD, when part of a full length trastuzumab antibody against Her2 antigen, can kill Her2-overexpressing tumor cell lines as effectively as IgA antibodies. Moreover, MutD shows improved assembly compared to IgA and thus provides access to potent FcαRI function while overcoming the expression and purification barriers that have limited the use of IgA as a therapeutic. Alternatively, aglycosylated antibodies may be engineered for exceptional effector function. Glycans anchored to residue N297 of the antibody IgG Fc domain are typically critical in mediating binding toward the FcγRs. Yet, using a full length bacterial IgG display system, we have isolated aglycosylated Fc1004 with mutations that confer a 160-fold increase in the affinity toward the low affinity FcγRIIa-R131 allele as well as high selectivity against binding to the remarkably homologous inhibitory receptor, FcγRIIb. Incorporation of this engineered Fc into trastuzumab resulted in a 75% increase in tumor cell phagocytosis by macrophages compared to that of the parental glycosylated trastuzumab with medium Her2-expressing cancer cells. In vivo testing of Fc1004 using NOD/SCID mouse model, reconstituted by adoptive transfer of leukocytes from FcγRIIa-R131 homozygous donors, showed a promising reduction in tumor burden in SkBr-3 Her2+ xenografts.