The Emerging Importance of IgG Fab Glycosylation in Immunity

Abstract

Human IgG is the most abundant glycoprotein in serum and is crucial for protective immunity. In addition to conserved IgG Fc glycans, ∼15–25% of serum IgG contains glycans within the variable domains. These so-called “Fab glycans” are primarily highly processed complex-type biantennary N-glycans linked to N-glycosylation sites that emerge during somatic hypermutation. Specific patterns of Fab glycosylation are concurrent with physiological and pathological conditions, such as pregnancy and rheumatoid arthritis. With respect to function, Fab glycosylation can significantly affect stability, half-life, and binding characteristics of Abs and BCRs. Moreover, Fab glycans are associated with the anti-inflammatory activity of IVIgs. Consequently, IgG Fab glycosylation appears to be an important, yet poorly understood, process that modulates immunity.

Structural Analysis of Variable Domain Glycosylation of Anti-Citrullinated Protein Antibodies in Rheumatoid Arthritis Reveals the Presence of Highly Sialylated Glycans

“…It has been estimated that up to 10 -25% of healthy donor IgG molecules carry Fab-linked N-glycans (8). In line with this, our current data indicate that plasma and SF IgG (depleted of ACPA) of RA patients exhibit a median level of Fab-glycosylation of around 17%, albeit with variation between donors.…”

“…Next to Fc-glycosylation, we have recently reported that ACPA-IgG are extensively glycosylated in their variable domain; a feature that may modulate the function of ACPA-IgG and that could be involved in the pathophysiology of RA (8). So far, however, a detailed structural analysis of ACPA Fablinked glycans was lacking (13).…”

“…In addition to Fc-linked N-glycans, ϳ15-25% of IgG molecules in human serum contain N-linked glycans present in the Fab-region (8). Fab-glycans can also modulate cellular function and have been implicated in the emergence of lymphoma's such as follicular lymphoma, diffuse large B-cell lymphoma and Burkitt's lymphoma B-cells, presumably through the provision of aberrant Fab-glycosylated B-cell receptor cross-linking via the glycan to lectins (9 -12).…”

“…This suggests that ACPA-producing B-cells with an N-glycosylation site in the BCR variable domain might have a selective advantage compared with other ACPA-producing B-cells. Finally, Fab glycosylation could also confer biological effector function to ACPA-IgG such as binding to certain lectins expressed on immune cells (8).…”

Data-independent oxonium ion profiling of multi-glycosylated biotherapeutics

“…The importance of this type of glycosylation is underscored by its ability to modulate IgG antigen binding and affect the anti-inflammatory properties of IVIG. 34 Another challenging type of glycosylation present in a variety of immunoglobulins, O-glycosylation, 35 is especially hard to predict because a consensus amino acid sequence motif has not been identified. Therefore, new methods that can better characterize the presence of these challenging glycosylation varieties are warranted.…”