Improving Antibody Therapeutics by Manipulating the Fc Domain: Immunological and Structural Considerations

Abstract

Interactions between the crystallizable fragment (Fc) domain of antibodies and a plethora of cellular Fc receptors (FcRs) or soluble proteins form a critical link between humoral and innate immunity. In particular, the immunoglobulin G Fc domain is critical for the clearance of target cells by processes that include (a) cytotoxicity, phagocytosis, or complement lysis; (b) modulation of inflammation; (c) antigen presentation; (d) antibody-mediated receptor clustering; and (e) cytokine release. More than 30 Fc-engineered antibodies aimed primarily at tailoring these effects for optimal therapeutic outcomes are in clinical evaluation or have already been approved. Nonetheless, our understanding of how FcR engagement impacts various immune cell phenotypes is still largely incomplete. Recent insights into FcR biology coupled with advances in Fc:FcR structural analysis, Fc engineering, and mouse models that recapitulate human biology are helping to fill in existing knowledge gaps. These advances will provide a blueprint on how to fine-tune the Fc domain to achieve optimal therapeutic efficacy.

Keywords: Fc engineering; Fc receptors; cytotoxicity; myeloid cells; signaling; structure.

Figure 1

Ig:FcR engagement triggers a spectrum of effector functions, bridging innate and adaptive immune responses. Ig binding on the antigen initiates the complement cascade (❶), activating NK cells to release cytokines and cytotoxic proteins and leading to ADCC (❷). Neutrophils engulf and degrade ICs while releasing inflammatory mediators (❸). Ig-coated pathogens are phagocytosed by macrophages (❹) and dendritic cells (❺). This facilitates the presentation of antigen by MHC class molecules, leading to T cell stimulation. FcRs on B cells can cause receptor clustering (e.g., 4–1BB on T cells) (❻), leading to receptor activation. Finally, inhibitory FcR signaling (❼) can counteract B cell receptor activation, thus promoting tolerance. Abbreviations: ADCC, antibody-dependent cell-mediated cytotoxicity; ADCP, antibody-dependent cell-mediated phagocytosis; FcR, Fc receptor; IC, immune complex; Ig, immunoglobulin, MHC, major histocompatibility; NK, natural killer.

Figure 2

Structure of the IgG1 Fc (PDB ID: 1HZH) and the Fc binding receptors. The two chains of the Fc are colored light and dark gray, and the N297 attached glycan is pink. The binding sites of the FcγRs (green), C1q (purple), and FcRn (blue) on the Fc are highlighted. Structures of the extracellular domains of the FcγRs (FcγRI PDB ID: 3RJD, FcγRIIa PDB ID: 3R5Y, FcγRIIb/c PDB ID: 2FCB, FcγRIIIa/b PDB ID: 5VU0), FcRn (PDB ID: 5WHK, α-chain in blue, β2m in red), and C1q head (PDB ID: 1PK6) are also depicted. The cellular expression and signaling molecules directly downstream of the Fc receptors are indicated. FcγRIIIa can signal through CD3ζ/ZAP-70 in NK cells, as well as the FcRγ common chain. There is evidence of inhibitory ITAMi signaling for FcγRIIa and FcγRIIIa. Abbreviations: DC, dendritic cell; Fc, crystallizable fragment; FcγR, Fc gamma receptor; FcRn, neonatal Fc receptor; GPI, glycosylphosphatidylinositol; IgG1, immunoglobulin G1; ITAM, immunoreceptor tyrosine-based activation motif; ITAMi, inhibitory ITAM signaling; ITIM, immunoreceptor tyrosine-based inhibition motif; NK, natural killer; PDB ID, Protein Data Bank identifier; SHIP-1, SH2 domain-containing inositol polyphosphate 5-phosphatase 1; Syk, spleen tyrosine kinase; ZAP-70: zeta-chain-associated protein kinase 70.