Monoclonal antibodies (mAbs), particularly IgGs, are among the most successful therapeutic proteins used in oncology, autoimmune, and infectious disease indications. One important attribute affecting their clinical performance is N-linked glycosylation at Asn-297 in the CH2 domain of the Fc region. The heterogeneity of glycan structures at this site can influence mAbs’ pharmacokinetics (PK), pharmacodynamics (PD), immunogenicity, and effector functions such as antibody-dependent cellular cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC).
IgG-Fc glycans are usually biantennary complex oligosaccharides with a conserved GlcNAc₂Man₃ core. Variation comes from the presence or absence of terminal sugars such as fucose, galactose, sialic acid, or bisecting GlcNAc. This glycosylation impacts not only structural stability and receptor interactions but also in vivo clearance and immunogenicity, particularly when antibodies are expressed in non-human cell lines. Therefore, understanding glycosylation is needed in antibody development pipelines.
Fc-associated glycans can affect the stability and conformation of the antibody’s CH2 domain. Glycans stabilize this domain through non-covalent interactions, reducing molecular flexibility and promoting a more "open" conformation conducive to Fcγ receptor binding. Fcγ receptors (FcγRs) are proteins found on the surface of immune cells (like macrophages, natural killer (NK) cells, dendritic cells, etc.) that bind to the Fc region of IgG antibodies. Deglycosylated IgGs adopt a "closed" conformation and are prone to thermal instability and structural degradation.
Related: CHOK1-FUT8 Afucosylated Antibody Expression
Glycoform variability is influenced by the host expression system. CHO cells generally produce glycan profiles similar to endogenous human IgG, whereas murine cell lines (NS0, SP2/0) may incorporate non-human glycans that can be immunogenic in humans. Cell line selection and control of glycosylation profiles are key quality attributes in therapeutic antibody development. Regulatory guidance emphasizes the need for consistent glycoform profiles, particularly for biosimilars.
In addition to recombinant glycoforms, endogenous alterations in B-cell glycosylation can contribute to immunogenic potential. B cells express glycosylated surface receptors and lectins that modulate their activation and tolerance. For example, changes in sialylation and fucosylation of the B-cell receptor (BCR) and co-inhibitory receptors like CD22 and Siglec-G affect downstream signaling thresholds, potentially predisposing hyperactive B cells to autoantibody production.
Related: Exploring Afucosylated Antibodies: Mechanism of Action and Therapeutic Impact
The neonatal Fc receptor (FcRn) governs the recycling and extended half-life of IgG molecules. This receptor binds in a pH-dependent manner to the Fc region at the CH2–CH3 interface, a site distinct from the glycosylation site. Consequently, glycosylation does not significantly alter FcRn binding or IgG half-life.
However, glycan-mediated clearance pathways can reduce antibody half-life. Specifically, glycoproteins with terminal mannose or galactose residues are targeted by mannose receptors or asialoglycoprotein receptors (ASGPR), leading to hepatic uptake and accelerated clearance. Studies in mice demonstrate that antibodies enriched in high-mannose glycans (e.g., Man8, Man9) exhibit faster serum clearance compared to complex-type fucosylated antibodies. Conversely, desialylation exposes galactose residues, reducing half-life due to ASGPR-mediated clearance.
These findings highlight the importance of minimizing high-mannose and desialylated species in therapeutic mAbs to preserve favorable PK properties.
Core fucose is a key regulator of ADCC, with its absence markedly increasing affinity for FcγRIIIa on NK cells, enhancing ADCC activity up to 100-fold. Multiple studies have confirmed this relationship across IgG subclasses, and clinically approved afucosylated antibodies such as obinutuzumab and mogamulizumab exploit this to improve therapeutic efficacy in B-cell malignancies.
Insights from autoimmune disease models reinforce the importance of fucosylation in modulating ADCC. For example, IgGs from patients with autoimmune thyroiditis and rheumatoid arthritis often exhibit reduced core fucosylation, which correlates with increased cytotoxicity and disease severity. Furthermore, therapeutic antibodies such as rituximab and pertuzumab show significantly enhanced ADCC when afucosylated, underscoring the therapeutic relevance of modulating fucose content. In vitro models have demonstrated that removal of both fucose and sialic acid from pertuzumab leads to a 20-fold increase in ADCC, suggesting that tandem modulation of glycan components can yield additive or synergistic effects on effector function.
Galactosylation does not significantly impact ADCC but enhances CDC by increasing C1q binding affinity. Engineering higher G2 content in rituximab has been shown to improve CDC-mediated tumor cell lysis. While effects may be subclass-specific, galactosylation is a rational strategy to boost CDC in complement-reliant mechanisms.
Related: CHOK1-FUT8 Afucosylated Antibody Expression
Sialylation exhibits context-dependent effects. In CHO-derived antibodies, terminal N-acetylneuraminic acid (NANA) in α-2,3 linkage is common, while murine lines can express NGNA, linked to immunogenicity. Functionally, sialylation may reduce ADCC and CDC by sterically hindering FcγR and C1q binding. However, sialylated Fc structures are also associated with anti-inflammatory properties, notably in intravenous immunoglobulin (IVIG) therapy, by enhancing binding to SIGN-R1/DCSIGN and upregulating FcγRIIb expression on immune cells.
Sialylation’s immunomodulatory role is under investigation for autoimmune disease treatment but remains poorly characterized in clinical mAbs due to its variable influence on effector functions.
Inclusion of bisecting N-acetylglucosamine (GlcNAc) enhances FcγRIIIa binding and ADCC. Recombinant antibodies from standard CHO lines lack this residue due to insufficient GnT-III activity. Overexpression of GnT-III or use of specialized cell lines can introduce bisecting GlcNAc and increase therapeutic potency. However, data are mixed on whether this modification acts independently or synergistically with afucosylation.
High-mannose glycans (Man5–Man9) increase ADCC, likely due to lack of fucosylation. However, these glycoforms reduce CDC efficacy and promote rapid clearance. Their content is typically limited to <1% in human serum IgG, but can be elevated in certain recombinant systems. Optimization requires balancing effector enhancement with pharmacokinetic liabilities.
Host cell modification is the most direct route for altering glycan profiles. For example, FUT8 knockout in CHO cells yields afucosylated antibodies with improved ADCC. Overexpression of GnT-III introduces bisecting GlcNAc, further boosting FcγRIIIa interaction. Environmental and media adjustments can also modulate glycosylation; supplementation with galactose, uridine, and manganese enhances galactosylation and CDC activity.
Use of glycosylation inhibitors (e.g., kifunensine) during culture can trap antibodies in high-mannose states for mechanistic studies or ADC payload loading. Gene editing tools (CRISPR/Cas9, siRNA) allow precise engineering of glycosylation pathways, enabling scalable and reproducible glycoform control.
For post-expression modification, chemoenzymatic glycoengineering enables site-specific remodeling of Fc glycans. Endoglycosidases (ENGases) like EndoS and EndoA cleave native glycans and allow reattachment of defined glycan structures using glycosynthase mutants and activated glycan oxazolines. This approach has produced homogeneous glycoforms such as G0, G2, and fully sialylated variants with controlled functional profiles.
N-glycan engineering also supports site-specific ADC generation. Fc glycans can be selectively oxidized to introduce aldehyde groups at vicinal diols (e.g., in fucose or galactose), enabling chemoselective conjugation of cytotoxic drugs via hydrazone or oxime linkages. Such strategies minimize heterogeneity and preserve antigen-binding affinity.
Modifying specific glycoforms in the Fc region not only enables site-specific payload attachment but may also alter antibody biodistribution and half-life. For example, galactose exposure following sialic acid removal can trigger hepatic uptake via ASGPRs, potentially reducing circulation time of glycoengineered ADCs. Thus, consideration of receptor-mediated clearance mechanisms linked to glycan motifs is critical when designing stable and effective ADCs.
Innovations in glycoengineering may allow researchers to fine-tune therapeutic properties, reduce immunogenic risk, and enable advanced formats such as ADCs.
Among the various glycoengineering approaches, afucosylation remains a key strategy to enhance ADCC through increased FcγRIIIa binding affinity. Biointron offers a CHOK1-FUT8 knockout expression platform for the production of afucosylated monoclonal antibodies. This system eliminates core fucosylation via targeted disruption of the FUT8 gene, enabling the generation of IgG molecules with significantly improved ADCC potency while maintaining high expression yields and consistent glycosylation profiles.
References:
Boune, S., Hu, P., Epstein, A. L., & Khawli, L. A. (2020). Principles of N-Linked Glycosylation Variations of IgG-Based Therapeutics: Pharmacokinetic and Functional Considerations. Antibodies, 9(2), 22. https://doi.org/10.3390/antib9020022
Trzos, S., & Pocheć, E. (2023). The role of N-glycosylation in B-cell biology and IgG activity. The aspects of autoimmunity and anti-inflammatory therapy. Frontiers in Immunology, 14, 1188838. https://doi.org/10.3389/fimmu.2023.1188838
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