A synopsis of recent developments defining how N-glycosylation impacts immunoglobulin G structure and function

Abstract

Therapeutic monoclonal antibodies (mAbs) are the fastest growing group of drugs with 11 new antibodies or antibody-drug conjugates approved by the Food and Drug Administration in 2018. Many mAbs require effector function for efficacy, including antibody-dependent cell-mediated cytotoxicity triggered following contact of an immunoglobulin G (IgG)-coated particle with activating crystallizable fragment (Fc) γ receptors (FcγRs) expressed by leukocytes. Interactions between IgG1 and the FcγRs require post-translational modification of the Fc with an asparagine-linked carbohydrate (N-glycan). Though the structure of IgG1 Fc and the role of Fc N-glycan composition on disease were known for decades, the underlying mechanism of how the N-glycan affected FcγR binding was not defined until recently. This review will describe the current understanding of how N-glycosylation impacts the structure and function of the IgG1 Fc and describe new techniques that are poised to provide the next critical breakthroughs.

Keywords: Fc gamma receptor; N-glycosylation; antibody; immunoglobulin G.

Figure 1

The IgG1 Fc N-glycan is essential for binding to the Fc γ receptors. (A) IgG1 Fc is a heterotetramer consisting of two Fabs and a single Fc that binds receptors. (B) The Fc N-glycan forms an interface with Cγ2 residues through noncovalent intramolecular interactions. The volume of interacting residues is shown with a gray background, and individual N-glycan and protein residues are highlighted. Blue squares represent N-acetylglucosamine residues; green circles, mannose; and yellow circles, galactose.

Figure 2

The different Fc N-glycan compositions provide Fc with variable affinity for CD16a. (A) N-glycan compositions as discussed in the text; note the abbreviated identifier to the left of the cartoon figures. (B) IgG1 Fc with each different N-glycan binds with different affinity to CD16a. These K_D values were adapted directly from (Subedi and Barb 2016) but are consistent with other reports (Yamaguchi et al. 2006; Thomann et al. 2015; Dekkers et al. 2017). GlcNAc, N-acetylglucosamine; Neu5Ac, N-acetylneuraminic acid.

Figure 3

IgG1 Fc Cγ2 domain structure. Residues that are within 5 Å of CD16a are highlighted in red. These contacts are draw for residues that form contacts in either the Fc a or b chain. Strands are labeled with letters.