Fc γ receptor compositional heterogeneity: Considerations for immunotherapy development

Abstract

The antibody-binding crystallizable fragment (Fc) γ receptors (FcγRs) are expressed by leukocytes and activate or suppress a cellular response once engaged with an antibody-coated target. Therapeutic mAbs that require FcγR binding for therapeutic efficacy are now frontline treatments for multiple diseases. However, substantially fewer development efforts are focused on the FcγRs, despite accounting for half of the antibody-receptor complex. The recent success of engineered cell-based immunotherapies now provides a mechanism to introduce modified FcγRs into the clinic. FcγRs are highly heterogeneous because of multiple functionally distinct alleles for many genes, the presence of membrane-tethered and soluble forms, and a high degree of post-translational modification, notably asparagine-linked glycans. One significant factor limiting FcγR improvement is the fundamental lack of knowledge regarding endogenous receptor forms present in the human body. This review describes the composition of FcγRs isolated from primary human leukocytes, summarizes recent efforts to engineer FcγRs, and concludes with a description of potential FcγR features to enrich for enhanced function. Further understanding FcγR biology could accelerate the development of new clinical therapies targeting immune-related disease.

Keywords: Fc receptor; antibody; glycobiology; glycoprotein; immunotherapy.

Figure 1

NK cells naturally target multiple antigens by binding the conserved crystallizable fragment (Fc) of IgG, unlike chimeric antigen receptor (CAR)–T cells or CAR–NK cells that are programmed to recognize a single antigen.A, NK cell Fc γ receptor IIIa/CD16a associates with either CD3ζ or the Fc ε receptor γ chain to activate an NK cell after ligation to an antibody-bound target (70, 71). The CAR-T example shown represents the tisagenlecleucel (Kymriah) therapy that targets CD19 on B-cell lymphoma. This CAR–NK construct likewise recently showed success (3, 72). Both CARs contain custom transmembrane and activation domains. B, Fc γ receptors bind to the crystallizable fragment of IgG. IgG, immunoglobulin G; NK, natural killer.

Figure 2

Simplified mammalian N-glycan processing scheme showing the three main N-glycan types and heterogeneity found on secreted proteins. Multiple arrows indicate multiple processing steps. Note that remodeling reactions occur without a template or proof-reading mechanisms. N-glycan, glycans attached to Asn residues.

Figure 3

The predominant Fc γ receptor glycoforms identified in the tissue or serum from healthy donors. The N-glycans are scaled roughly to the appropriate size. The NK cell N-glycans increase the mass of the extracellular domain by 40%. The CD16 N45 glycan contributes to protein stability, the CD16a N162 glycan composition impacts antibody-binding affinity, and the CD32a N64 glycan inhibits antibody binding through cis-interactions on the neutrophil surface. N-glycan, glycans attached to Asn residues; NK, natural killer.

Figure 4

Extensive carbohydrate modifications provide FcγRs with distinct functions. Some enhance antibody-binding affinity, some repress activity, and some likely bind cell matrix proteins. FcγRs, Fc γ receptors.