N-linked glycans are present in 15–25% of human IgG antibodies’ variable domain (heavy chain variable domain (VH) or light chain variable domain (VL)) regions
[26][27]. These N-glycosylation sites encoded by the V-region genes (so-called Fab N-glycans) are a result of somatic hypermutation
[26][28][29], because very few germline alleles carry N-glycosylation consensus sequences (NXS/T)
[30]. In recent years, more and more evidences indicate that Fab N-glycans can influence antibody binding affinity. Several mechanisms on how N-glycan in antibody V-regions impacts epitope binding have been proposed, including the bulk size of N-glycan to fill out the space between the antigen epitope and the antibody paratope
[31], charge–charge interaction between N-glycan sialic acids and the antigen
[17][28], and through steric hinderance effects that affect the binding
[32]. The IgG4 subclass has the highest prevalence of V-region glycosylation (44% versus 11%–15% in other subclasses)
[28]. IgE has a two-fold higher propensity for Fab glycans than IgA or IgG1, suggesting that elevated Fab glycosylation might be a hallmark of Th2-like responses
[33]. A large portion of autoantibodies in rheumatoid arthritis and certain B-cell lymphomas were found to contain Fab N-glycans
[34][35][36], which are also present in human anti-idiotype autoantibodies to adalimumab and infliximab
[28]. Removing N-glycans from the complementarity-determining regions (CDRs) of antibodies can lead to a significant decrease in the antibody binding affinity
[28][37][38]. Removing N-glycan located within the antigen-binding sites of a human IgG alloantibody decreases its neutralization towards factor VIII (FVIII) procoagulant activity without losing its binding affinity, suggesting that its Fab glycan blocks the interaction between FVIII and the chaperone partner through steric hinderance
[32]. Fab glycans in the framework or constant regions play additional roles in increasing antibody stability
[29] and in vivo half-life
[39].
The structure of N-glycans within the V region are different from those rigid under-sialylated biantennary Fc-glycans attached to Asn297 in the Fc region, because they are typically surface-exposed α2,6-linked sialylated complex biantennary glycans
[37][40][41]. The negatively charged sialic acid on these V-region glycans have been found to contribute to the increased binding affinity
[28][38][40]. This data indicate that the introduction of N-linked glycans to variable domains is an additional layer for immune repertoire diversification
[17].
Engineering N-glycans into antibody-binding sites has been utilized for therapeutic rational design (
Figure 2A). Engineering an N-linked consensus site into an ibalizumab light chain recognizes human immunodeficiency virus (HIV)’s envelope glycoprotein gp120 with a loss of an N-glycan in the V5 loop, which is otherwise resistant to the HIV-1-neutralizing activity
[31]. Similarly, introducing Fab glycans into adalimumab enhances the TNFα binding of two antibody glycovariants by two-fold
[28]. Introducing Fab N-glycans can be a way to decrease antigen-binding poly-reactivity and self-reactivity
[42][43]. The introduction of an N-linked glycan into an antibody-variable domain also has been employed for improving antibody solubility
[44][45]. Although engineering in Fab N-glycosylation can increase manufacturing challenges, the high degree of conformational dynamics from glycans can enhance the chemical diversity of antibody paratopes and thus the functionalities.