Immune recruitment or suppression by glycan engineering of endogenous and therapeutic antibodies

The IgG-specific endoglycosidases EndoS and EndoS2 are distinguished by conformation and antibody recognition

The IgG-specific endoglycosidases EndoS and EndoS2 from Streptococcus pyogenes can remove conserved N-linked glycans present on the Fc region of host antibodies to inhibit Fc-mediated effector functions. These enzymes are therefore being investigated as therapeutics for suppressing unwanted immune activation, and have additional application as tools for antibody glycan remodeling. EndoS and EndoS2 differ in Fc glycan substrate specificity due to structural differences within their catalytic glycosyl hydrolase domains. However, a chimeric EndoS enzyme with a substituted glycosyl hydrolase from EndoS2 loses catalytic activity, despite high structural homology between the two enzymes, indicating either mechanistic divergence of EndoS and EndoS2, or improperly-formed domain interfaces in the chimeric enzyme. Here, we present the crystal structure of the EndoS2-IgG1 Fc complex determined to 3.0 Å resolution. Comparison of complexed and unliganded EndoS2 reveals relative reorientation of the glycosyl hydrolase, leucine-rich repeat and hybrid immunoglobulin domains. The conformation of the complexed EndoS2 enzyme is also different when compared to the earlier EndoS-IgG1 Fc complex, and results in distinct contact surfaces between the two enzymes and their Fc substrate. These findings indicate mechanistic divergence of EndoS2 and EndoS. It will be important to consider these differences in the design of IgG-specific enzymes, developed to enable customizable antibody glycosylation.

Introduction of Carbonyl Groups into Antibodies

Antibodies and their derivatives (scFv, Fabs, etc.) represent a unique class of biomolecules that combine selectivity with the ability to target drug delivery. Currently, one of the most promising endeavors in this field is the development of molecular diagnostic tools and antibody-based therapeutic agents, including antibody-drug conjugates (ADCs). To meet this challenge, it is imperative to advance methods for modifying antibodies. A particularly promising strategy involves the introduction of carbonyl groups into the antibody that are amenable to further modification by biorthogonal reactions, namely aliphatic, aromatic, and α-oxo aldehydes, as well as aliphatic and aryl-alkyl ketones. In this review, we summarize the preparation methods and applications of site-specific antibody conjugates that are synthesized using this approach.

IgG glycans in health and disease: Prediction, intervention, prognosis, and therapy

Immunoglobulin (IgG) glycosylation is a complex enzymatically controlled process, essential for the structure and function of IgG. IgG glycome is relatively stable in the state of homeostasis, yet its alterations have been associated with aging, pollution and toxic exposure, as well as various diseases, including autoimmune and inflammatory diseases, cardiometabolic diseases, infectious diseases and cancer. IgG is also an effector molecule directly involved in the inflammation processes included in the pathogenesis of many diseases. Numerous recently published studies support the idea that IgG N-glycosylation fine-tunes the immune response and plays a significant role in chronic inflammation. This makes it a promising novel biomarker of biological age, and a prognostic, diagnostic and treatment evaluation tool. Here we provide an overview of the current state of knowledge regarding the IgG glycosylation in health and disease, and its potential applications in pro-active prevention and monitoring of various health interventions.

The role of N-glycosylation in B-cell biology and IgG activity. The aspects of autoimmunity and anti-inflammatory therapy

The immune system is strictly regulated by glycosylation through the addition of highly diverse and dynamically changing sugar structures (glycans) to the majority of immune cell receptors. Although knowledge in the field of glycoimmunology is still limited, numerous studies point to the key role of glycosylation in maintaining homeostasis, but also in reflecting its disruption. Changes in oligosaccharide patterns can lead to impairment of both innate and acquired immune responses, with important implications in the pathogenesis of diseases, including autoimmunity. B cells appear to be unique within the immune system, since they exhibit both innate and adaptive immune activity. B cell surface is rich in glycosylated proteins and lectins which recognise glycosylated ligands on other cells. Glycans are important in the development, selection, and maturation of B cells. Changes in sialylation and fucosylation of cell surface proteins affect B cell signal transduction through BCRs, CD22 inhibitory coreceptor and Siglec-G. Plasmocytes, as the final stage of B cell differentiation, produce and secrete immunoglobulins (Igs), of which IgGs are the most abundant N-glycosylated proteins in human serum with the conserved N-glycosylation site at Asn297. N-oligosaccharide composition of the IgG Fc region affects its secretion, structure, half-life and effector functions (ADCC, CDC). IgG N-glycosylation undergoes little change during homeostasis, and may gradually be modified with age and during ongoing inflammatory processes. Hyperactivated B lymphocytes secrete autoreactive antibodies responsible for the development of autoimmunity. The altered profile of IgG N-glycans contributes to disease progression and remission and is sensitive to the application of therapeutic substances and immunosuppressive agents. In this review, we focus on the role of N-glycans in B-cell biology and IgG activity, the rearrangement of IgG oligosaccharides in aging, autoimmunity and immunosuppressive therapy.

Potential targeted therapy based on deep insight into the relationship between the pulmonary microbiota and immune regulation in lung fibrosis

Pulmonary fibrosis is an irreversible disease, and its mechanism is unclear. The lung is a vital organ connecting the respiratory tract and the outside world. The changes in lung microbiota affect the progress of lung fibrosis. The latest research showed that lung microbiota differs in healthy people, including idiopathic pulmonary fibrosis (IPF) and acute exacerbation-idiopathic pulmonary fibrosis (AE-IPF). How to regulate the lung microbiota and whether the potential regulatory mechanism can become a necessary targeted treatment of IPF are unclear. Some studies showed that immune response and lung microbiota balance and maintain lung homeostasis. However, unbalanced lung homeostasis stimulates the immune response. The subsequent biological effects are closely related to lung fibrosis. Core fucosylation (CF), a significant protein functional modification, affects the lung microbiota. CF regulates immune protein modifications by regulating key inflammatory factors and signaling pathways generated after immune response. The treatment of immune regulation, such as antibiotic treatment, vitamin D supplementation, and exosome micro-RNAs, has achieved an initial effect in clearing the inflammatory storm induced by an immune response. Based on the above, the highlight of this review is clarifying the relationship between pulmonary microbiota and immune regulation and identifying the correlation between the two, the impact on pulmonary fibrosis, and potential therapeutic targets.

Engineering nucleotide sugar synthesis pathways for independent and simultaneous modulation of N-glycan galactosylation and fucosylation in CHO cells

Glycosylation of recombinant therapeutics like monoclonal antibodies (mAbs) is a critical quality attribute. N-glycans in mAbs are known to affect various effector functions, and thereby therapeutic use of such glycoproteins can depend on a particular glycoform profile to achieve desired efficacy. However, there are currently limited options for modulating the glycoform profile, which depend mainly on over-expression or knock-out of glycosyltransferase enzymes that can introduce or eliminate specific glycans but do not allow predictable glycoform modulation over a range of values. In this study, we demonstrate the ability to predictably modulate the glycoform profile of recombinant IgG. Using CRISPR/Cas9, we have engineered nucleotide sugar synthesis pathways in CHO cells expressing recombinant IgG for combinatorial modulation of galactosylation and fucosylation. Knocking out the enzymes UDP-galactose 4'-epimerase (Gale) and GDP-L-fucose synthase (Fx) resulted in ablation of de novo synthesis of UDP-Gal and GDP-Fuc. With Gale knock-out, the array of N-glycans on recombinantly expressed IgG is narrowed to agalactosylated glycans, mainly A2F glycan (89%). In the Gale and Fx double knock-out cell line, agalactosylated and afucosylated A2 glycan is predominant (88%). In the double knock-out cell line, galactosylation and fucosylation was entirely dependent on the salvage pathway, which allowed for modulation of UDP-Gal and GDP-Fuc synthesis and intracellular nucleotide sugar availability by controlling the availability of extracellular galactose and fucose. We demonstrate that the glycoform profile of recombinant IgG can be modulated from containing predominantly agalactosylated and afucosylated glycans to up to 42% and 96% galactosylation and fucosylation, respectively, by extracellular feeding of sugars in a dose-dependent manner. By simply varying the availability of extracellular galactose and/or fucose, galactosylation and fucosylation levels can be simultaneously and independently modulated. In addition to achieving the production of tailored glycoforms, this engineered CHO host platform can cater to the rapid synthesis of variably glycoengineered proteins for evaluation of biological activity.

Characterization of core fucosylation via sequential enzymatic treatments of intact glycopeptides and mass spectrometry analysis

Core fucosylation of N-linked glycoproteins has been linked to the functions of glycoproteins in physiological and pathological processes. However, quantitative characterization of core fucosylation remains challenging due to the complexity and heterogeneity of N-linked glycosylation. Here we report a mass spectrometry-based method that employs sequential treatment of intact glycopeptides with enzymes (STAGE) to analyze site-specific core fucosylation of glycoproteins. The STAGE method utilizes Endo F3 followed by PNGase F treatment to generate mass signatures for glycosites that are formerly modified by core fucosylated N-linked glycans. We benchmark the STAGE method and use it to characterize site specific core fucosylation of glycoproteins from human hepatocellular carcinoma and pancreatic ductal adenocarcinoma, resulting in the identification of 1130 and 782 core fucosylated glycosites, respectively. These results indicate that our STAGE method enables quantitative characterization of core fucosylation events from complex protein mixtures, which may benefit our understanding of core fucosylation functions in various diseases.

Core Fucosylation Regulates the Function of Pre-BCR, BCR and IgG in Humoral Immunity

Most of the membrane molecules involved in immune response are glycosylated. N-glycans linked to asparagine (Asn) of immune molecules contribute to the protein conformation, surface expression, stability, and antigenicity. Core fucosylation catalyzed by core fucosyltransferase (FUT8) is the most common post-translational modification. Core fucosylation is essential for evoking a proper immune response, which this review aims to communicate. First, FUT8 deficiency suppressed the interaction between μHC and λ5 during pre-BCR assembly is given. Second, we described the effects of core fucosylation in B cell signal transduction via BCR. Third, we investigated the role of core fucosylation in the interaction between helper T (T_H) cells and B cells. Finally, we showed the role of FUT8 on the biological function of IgG. In this review, we discussed recent insights into the sites where core fucosylation is critical for humoral immune responses.

Diagnostic Significance of Selected Serum Inflammatory Markers in Women with Advanced Endometriosis

Endometriosis is a gynecological disease, the pathogenesis of which seems to be directly associated with inflammatory processes. Serum concentrations of IL-1β, IL-6, hs-CRP, IgG, YKL 40 and PRL, in comparison to the well-known CA 125 levels, were studied with the aim of identifying an additional noninvasive inflammatory marker or set of markers characteristic for endometriosis. The study group included 43 women with endometriosis (E), 35 women with benign gynecological disorders but without endometriosis (NE, non-endometriosis) as a comparative group, and a control group consisting of 18 healthy subjects (C). The serum concentrations of IL-1β, IL-6, hs-CRP, YKL-40, PRL and CA 125 were significantly higher in the E group (median values: 0.41 pg/mL, 2.42 pg/mL, 2.33 mg/L, 79.30 ng/mL, 21.88 ng/mL and 68.00 U/mL, respectively) than in the control group (median values: 0.21 pg/mL, 0.98 pg/mL, 0.52 mg/L, 49.77 ng/mL, 12.08 ng/mL and 12.20 U/mL respectively), with the significance of p = 0.011, p < 0.001, p = 0.028, p = 0.005, p < 0.001 and p < 0.001, respectively. The IgG concentrations were significantly lower in the endometriosis group (median value: 1061.21 mg/dL) as compared to healthy women (median value: 1210.50 mg/dL; p = 0.025). Significant differences in concentrations of IL-6 (p = 0.040), hs-CRP (p = 0.007) and CA 125 (p < 0.001) were observed in stage III vs. stage IV of endometriosis. Significantly higher concentrations of IL-6 (p = 0.010), hs-CRP (p = 0.037) and PRL (p < 0.001) were observed in the NE group vs. the control group. Only CA 125 concentrations were significantly higher in endometriosis patients as compared to the non-endometriosis group (p < 0.001). The proposed panel of inflammatory markers, especially IL-6, PRL and CA 125, may become a useful tool to identify women with advanced endometriosis who could qualify for treatment.

Modeling of the Biological Activity of Monoclonal Antibodies Based on the Glycosylation Profile

The influence of the glycosylation profile of IgG on biological activity is known, but it is not clear which glycoforms have the highest impact on the main mechanism of action. The aim of this study was to design a mathematical model for predicting the antibody-dependent cellular cytotoxicity (ADCC) activity and the Fc gamma IIIa receptors' (FcɣRIIIa) relative binding of rituximab drug products based on their glycosylation profile. An additional goal was to identify the glycoforms that have the greatest impact on these mechanisms of action. For these purposes, the glycosylation profile was examined by hydrophilic interaction ultra-performance liquid chromatography (HILIC-UPLC), ADCC was assessed using a Promega kit, and FcɣRIIIa's binding affinity was assessed by surface plasmon resonance (SPR) analysis of a group of >50 rituximab drug products. Based on the results, mathematical models for the ADCC and FcɣRIIIa binding affinity prediction were designed using JMP 13.2.0. The quality of the model and the influence of sample size and heterogeneity on the reliability were verified. The results allow for the evaluation of rituximab drug products' activity based on their glycosylation profile and show that with a sufficiently large and differentiated dataset, it is possible to generate models for different monoclonal antibodies.

Importance and Monitoring of Therapeutic Immunoglobulin G Glycosylation

The complex diantennary-type oligosaccharides at Asn297 residues of the IgG heavy chains have a profound impact on the safety and efficacy of therapeutic IgG monoclonal antibodies (mAbs). Fc glycosylation of a mAb is an established critical quality attribute (CQA), and its oligosaccharide profile is required to be thoroughly characterized by state-of-the-art analytical methods. The Fc oligosaccharides are highly heterogeneous, and the differentially glycosylated species (glycoforms) of IgG express unique biological activities. Glycoengineering is a promising approach for the production of selected mAb glycoforms with improved effector functions, and non- and low-fucosylated mAbs exhibiting enhanced antibody-dependent cellular cytotoxicity activity have been approved or are under clinical evaluation for treatment of cancers, autoimmune/chronic inflammatory diseases, and infection. Recently, the chemoenzymatic glycoengineering method that allows for the transfer of structurally defined oligosaccharides to Asn-linked GlcNAc residues with glycosynthase has been developed for remodeling of IgG-Fc oligosaccharides with high efficiency and flexibility. Additionally, various glycoengineering methods have been developed that utilize the Fc oligosaccharides of IgG as reaction handles to conjugate cytotoxic agents by "click chemistry", providing new routes to the design of antibody-drug conjugates (ADCs) with tightly controlled drug-antibody ratios (DARs) and homogeneity. This review focuses on current understanding of the biological relevance of individual IgG glycoforms and advances in the development of next-generation antibody therapeutics with improved efficacy and safety through glycoengineering.

The Essential Functions and Detection of Bisecting GlcNAc in Cell Biology

The N-glycans of mammalian glycoproteins vary greatly in structure, and the biological importance of these variations is mostly unknown. It is widely acknowledged that the bisecting N-acetylglucosamine (GlcNAc) structure, a β1,4-linked GlcNAc attached to the core β-mannose residue, represents a special type of N-glycosylated modification, and it has been reported to be involved in various biological processes, such as cell adhesion, fertilization and fetal development, neuritogenesis, and tumor development. In particular, the occurrence of N-glycans with a bisecting GlcNAc modification on proteins has been proven, with many implications for immune biology. Due to the essential functions of bisecting GlcNAc structures, analytical approaches to this modification are highly required. The traditional approach that has been used for bisecting GlcNAc determinations is based on the lectin recognition of Phaseolus vulgaris erythroagglutinin (PHA-E); however, poor binding specificity hinders the application of this method. With the development of mass spectrometry (MS) with high resolution and improved sensitivity and accuracy, MS-based glycomic analysis has provided precise characterization and quantification for glycosylation modification. In this review, we first provide an overview of the bisecting GlcNAc structure and its biological importance in neurological systems, immune tolerance, immunoglobulin G (IgG), and tumor metastasis and development and then summarize approaches to its determination by MS for performing precise functional studies. This review is valuable for those readers who are interested in the importance of bisecting GlcNAc in cell biology.

Impact of Fc N-glycan sialylation on IgG structure

Human IgG antibodies containing terminal alpha 2,6-linked sialic acid on their Fc N-glycans have been shown to reduce antibody-dependent cell-mediated cytotoxicity and possess anti-inflammatory properties. Although terminal sialylation on complex N-glycans can happen via either an alpha 2,3-linkage or an alpha 2,6-linkage, sialic acids on human serum IgG Fc are almost exclusively alpha 2,6-linked. Recombinant IgGs expressed in Chinese hamster ovary (CHO) cells, however, have sialic acids through alpha 2,3-linkages because of the lack of the alpha 2,6-sialyltransferase gene. The impact of different sialylation linkages to the structure of IgG has not been determined. In this work, we investigated the impact of different types of sialylation to the conformational stability of IgG through hydrogen/deuterium exchange (HDX) and limited proteolysis experiments. When human-derived and CHO-expressed IgG1 were analyzed by HDX, sialic acid-containing glycans were found to destabilize the CH2 domain in CHO-expressed IgG, but not human-derived IgG. When structural isomers of sialylated glycans were chromatographically resolved and identified in the limited proteolysis experiment, we found that only alpha 2,3-linked sialic acid on the 6-arm (the major sialylated glycans in CHO-expressed IgG1) destabilizes the CH2 domain, presumably because of the steric effect that decreases the glycan-CH2 domain interaction. The alpha 2,6-linked sialic acid on the 3-arm (the major sialylated glycan in human-derived IgG), and the alpha 2,3-linked sialic acid on the 3-arm, do not have this destabilizing effect.

A Novel Broad-Spectrum Elastase-Like Serine Protease From the Predatory Bacterium Bdellovibrio bacteriovorus Facilitates Elucidation of Site-Specific IgA Glycosylation Pattern

The increased interest in predatory bacteria due to their ability to kill antibiotic resistant bacteria has also highlighted their inherent plethora of hydrolytic enzymes, and their potential as natural sources of novel therapeutic agents and biotechnological tools. Here, we have identified and characterized a novel protease from the predatory bacterium Bdellovibrio bacteriovorus: BspE (Bdellovibrio elastase-like serine protease). Mapping preferential sites of proteolytic activity showed a single proteolytic cleavage site of native plasma IgA (pIgA) in the Fc-tail; as well as in the secretory component (SC) of secretory IgA (SIgA). Proteolysis of other native immunoglobulins and plasma proteins was either absent (IgG1 and 2, IgM, albumin and orosomucoid) or unspecific with multiple cleavage sites (IgG3 and 4, IgE, IgD). BspE displayed a broad activity against most amino acid bonds in shorter peptides and denatured proteins, with a slight preference for hydrolysis C-terminal of Y, V, F, S, L, R, P, E, and K. BspE autoproteolysis results in numerous cleavage products sustaining activity for more than 6 h. The enzymatic activity remained stable at pH 5.0-9.0 but was drastically reduced in the presence of MnCl₂ and completely inhibited by ZnCl₂. The hydrolysis of pIgA was subsequently utilized for the specific glycan characterization of the released pIgA Fc-tail (Asn⁴⁵⁹). Besides contributing to the basic knowledge of Bdellovibrio biology and proteases, we propose that BspE could be used as a potential tool to investigate the importance, and biological function of the pIgA Fc-tail. IMPORTANCE Antibodies are well-established as key components of the immune system, and the importance of antibody glycosylation is steadily gaining recognition. Modifications of antibodies by glycosylation creates a vast repertoire of antibody glycovariants with distinctive and diverse functions in the immune system. Most of the available information regarding antibody glycosylation is based on studies with IgG, which have contributed greatly to the advance of therapeutic antibody treatments. However, much is still unknown regarding the importance of glycosylation and the Fc-structure for the remaining antibody classes. Such research has proven to be technically challenging and demonstrates a need for novel tools to facilitate such investigations. Here we have identified and characterized a novel protease from B. bacteriovorus, facilitating the study of plasma IgA by cleaving the Fc-tail, including the Asn⁴⁵⁹ N-glycan. This further highlights the potential of B. bacteriovorus as a source to identify potential novel biotechnological tools.

Pharmacologic Considerations in the Disposition of Antibodies and Antibody-Drug Conjugates in Preclinical Models and in Patients

The rapid advancement in the development of therapeutic proteins, including monoclonal antibodies (mAbs) and antibody-drug conjugates (ADCs), has created a novel mechanism to selectively deliver highly potent cytotoxic agents in the treatment of cancer. These agents provide numerous benefits compared to traditional small molecule drugs, though their clinical use still requires optimization. The pharmacology of mAbs/ADCs is complex and because ADCs are comprised of multiple components, individual agent characteristics and patient variables can affect their disposition. To further improve the clinical use and rational development of these agents, it is imperative to comprehend the complex mechanisms employed by antibody-based agents in traversing numerous biological barriers and how agent/patient factors affect tumor delivery, toxicities, efficacy, and ultimately, biodistribution. This review provides an updated summary of factors known to affect the disposition of mAbs/ADCs in development and in clinical use, as well as how these factors should be considered in the selection and design of preclinical studies of ADC agents in development.

The Utilization of Autoantibodies in Approaches to Precision Health

Precision health (PH) applied to autoimmune disease will need paradigm shifts in the use and application of autoantibodies and other biomarkers. For example, autoantibodies combined with other multi-analyte “omic” profiles will form the basis of disease prediction allowing for earlier intervention linked to disease prevention strategies, as well as earlier, effective and personalized interventions for established disease. As medical intervention moves to disease prediction and a model of “intent to PREVENT,” diagnostics will include an early symptom/risk-based, as opposed to a disease-based approach. Newer diagnostic platforms that utilize emerging megatrends such as deep learning and artificial intelligence and close the gaps in autoantibody diagnostics will benefit from paradigm shifts thereby facilitating the PH agenda.

N-glycan Remodeling Using Mannosidase Inhibitors to Increase High-mannose Glycans on Acid α-Glucosidase in Transgenic Rice Cell Cultures

Glycoengineering of plant expression systems is a prerequisite for the production of biopharmaceuticals that are compatible with animal-derived glycoproteins. Large amounts of high-mannose glycans such as Man₇GlcNAc₂, Man₈GlcNAc₂, and Man₉GlcNAc₂ (Man7/8/9), which can be favorably modified by chemical conjugation of mannose-6-phosphate, are desirable for lysosomal enzyme targeting. This study proposed a rice cell-based glycoengineering strategy using two different mannosidase inhibitors, kifunensine (KIF) and swainsonine (SWA), to increase Man7/8/9 glycoforms of recombinant human acid α-glucosidase (rhGAA), which is a therapeutic enzyme for Pompe disease. Response surface methodology was used to investigate the effects of the mannosidase inhibitors and to evaluate the synergistic effect of glycoengineering on rhGAA. Both inhibitors suppressed formation of plant-specific complex and paucimannose type N-glycans. SWA increased hybrid type glycans while KIF significantly increased Man7/8/9. Interestingly, the combination of KIF and SWA more effectively enhanced synthesis of Man7/8/9, especially Man9, than KIF alone. These changes show that SWA in combination with KIF more efficiently inhibited ER α-mannosidase II, resulting in a synergistic effect on synthesis of Man7/8/9. In conclusion, combined KIF and SWA treatment in rice cell culture media can be an effective method for the production of rhGAA displaying dominantly Man7/8/9 glycoforms without genetic manipulation of glycosylation.

IgG Glyco-Engineering to Improve IVIg Potency

Intravenous immunoglobulins (IVIg) are used in the treatment of different autoimmune and inflammatory diseases, such as immune thrombocytopenia and hemolytic anemia. One of the modes of action of IVIg is preventing phagocytosis of autoantibody-opsonized blood cells by saturation of the Fc-gamma receptors of macrophages in spleen and liver. IgG contains a fixed glycan, which is in most cases biantennary, at the asparagine residue at position 297 in the Fc tail. This glycan consists of a core structure of N-acetyl glucosamine (GlcNAc) and mannose groups, variably extended with core fucose, bisecting GlcNAc as well as terminal galactose and sialic acid. This structural glycan influences the binding affinity of IgG to Fc-gamma receptors. By glyco-engineering, we generated monoclonal IgG antibodies with different Fc-tail glycans and tested both their opsonizing and blocking capacity in a phagocytosis assay of IgG-opsonized erythrocytes with human monocyte-derived macrophages. In contrast to a lack of effect in opsono-phagocytosis, these IgG glycovariants differentially inhibited the uptake of opsonized erythrocytes. The level of bisecting GlcNAc and galactosylation had unexpectedly larger impact than core fucosylation, and suggest that targeted modifications different from the core fucose may well improve the immunomodulating efficacy of IVIg treatment.

Efficient Assembly of Quantum Dots with Homogenous Glycans Derived from Natural N-Linked Glycoproteins

Coating inorganic nanoparticles with polyethylene glycol (PEG)-appended ligands, as means to preserve their physical characteristics and promote steric interactions with biological systems, including enhanced aqueous solubility and reduced immunogenicity, has been explored by several groups. Conversely, macromolecules present in the human serum and on the surface of cells are densely coated with hydrophilic glycans that act to reduce nonspecific interactions, while facilitating specific binding and interactions. In particular, N-linked glycans are abundant on the surface of most serum proteins and are composed of a branched architecture that is typically characterized by a significant level of molecular heterogeneity. Here we provide two distinct methodologies, covalent bioconjugation and self-assembly, to functionalize two types of Quantum Dots with a homogeneous, complex-type N-linked glycan terminated with a sialic acid moiety. A detailed physical and functional characterization of these glycan-coated nanoparticles has been performed. Our findings support the potential use of such fluorescent platforms to sense glycan-involved biological processes, such as lectin recognition and sialidase-mediated hydrolysis.

Signature of Antibody Domain Exchange by Native Mass Spectrometry and Collision-Induced Unfolding

The development of domain-exchanged antibodies offers a route to high-affinity targeting to clustered multivalent epitopes, such as those associated with viral infections and many cancers. One strategy to generate these antibodies is to introduce mutations into target antibodies to drive domain exchange using the only known naturally occurring domain-exchanged anti-HIV (anti-human immunodeficiency virus) IgG1 antibody, 2G12 , as a template. Here, we show that domain exchange can be sensitively monitored by ion-mobility mass spectrometry and gas-phase collision-induced unfolding. Using native 2G12 and a mutated form that disrupts domain exchange such that it has a canonical IgG1 architecture ( 2G12 I19R ), we show that the two forms can be readily distinguished by their unfolding profiles. Importantly, the same signature of domain exchange is observed for both intact antibody and isolated Fab fragments. The development of a mass spectrometric method to detect antibody domain exchange will enable rapid screening and selection of candidate antibodies engineered to exhibit this and other unusual quaternary antibody architectures.

The direct and indirect effects of glycans on immune function

The biological impact of glycans is as diverse and complex as the impact of proteins on biology. Familiar roles include those as a protein folding checkpoint in the endoplasmic reticulum and as a modulator of the serum half-life of secreted glycoproteins, but it has become clear over the last several decades that glycans are key signaling moieties, participate in cell-cell interactions and modulate the function of individual proteins, to name but a few examples. In the immune system, the majority of microbial "patterns" are glycans or glycoconjugates, while virtually all cell surface receptors are glycoproteins, and antibody glycosylation critically influences antibody function. In order to provide a simple contextual framework to understand the myriad roles, glycans play in immunity, we propose that glycan effects are considered direct or indirect, depending on their direct participation or their indirect effects on other components in a given biological process or pathway. Here, we present the published evidence that supports this framework, which ultimately leads to the conclusion that we should learn to embrace the complexity inherent to the glycome and its potential as a largely uncharted but target rich area of new therapeutic investigation.

Glycosylation of random IgG distinguishes seropositive and seronegative rheumatoid arthritis

The N-glycosylation of human immunoglobulins, especially IgGs, plays a critical role in determining affinity of IgGs towards their effector (pro- and anti-inflammatory) receptors. However, it is still not clear whether altered glycosylation is involved in only antibody-dependent disorders like seropositive rheumatoid arthritis (RA) or also in pathologies with similar clinical manifestations, but no specific autoantibodies like seronegative RA. The clarification of that uncertainty was the aim of the current study. Another study aim was the detection of specific glycan forms responsible for altered exposure of native glycoepitopes. We studied sera from seropositive RA (n = 15) and seronegative RA (n = 12) patients for exposure of glycans in native IgG molecules, followed by determination of specific glycans by capillary electrophoresis with laser-induced fluorescent detection (CE-LIF). Aged-matched groups of normal healthy donors (NHD) and samples of intravenous immunoglobulin IgG preparations (IVIG) served as controls. There was significantly stronger binding of Lens culinaris agglutinin (LCA) and Aleuria aurantia lectin (AAL) lectins towards IgG from seropositive RA compared to seronegative RA or NHD. CE-LIF analysis revealed statistically significant increases in bisecting glycans FA2BG2 (p = .006) and FABG2S1 (p = .005) seropositive RA, accompanied by decrease of bisecting monogalactosylated glycan FA2(6)G1 (p = .074) and non-bisecting monosialylated glycan FA2(3)G1S1 (p = .055). The results suggest that seropositive RA is distinct from seronegative RA in terms of IgG glycan moieties, attributable to specific immunoglobulin molecules present in seropositive disease. These glycans were determined to be bisecting GlcNAc-bearing forms FA2BG2 and FABG2S1, and their appearance increased the availability of LCA and AAL lectin-binding sites in native IgG glycoepitopes.

Conformational Plasticity of the Immunoglobulin Fc Domain in Solution

Fragment crystallizable (Fc) region of immunoglobulin G (IgG) antibody binds to specific Fc receptors (FcγRs) to control antibody effector functions. Currently, engineered specific Fc-FcγR interactions are validated with a static conformation derived from the crystal structure. However, computational evidence suggests that the conformational variability of Fcs plays an important role in receptor recognition. Here we elucidate Fc flexibility of IgG1, IgG2, and IgG1 Fc with mutations (M255Y/S257T/T259E) in solution by small-angle X-ray scattering (SAXS). Measured SAXS profiles and experimental parameters show variations in flexibility between Fc isotypes. We develop and apply a modeling tool for an accurate conformational sampling of Fcs followed by SAXS fitting. Revealed conformational variability of the CH2 domain as low as 10 Å in displacement, illustrates the power of the atomistic modeling combined with SAXS. This inexpensive SAXS-based approach offers to improve the engineering of antibodies for tailoring Fc receptor interactions through altering and measuring Fc flexibility.

Characterization of novel endo-β-N-acetylglucosaminidases from Sphingobacterium species, Beauveria bassiana and Cordyceps militaris that specifically hydrolyze fucose-containing oligosaccharides and human IgG

Endo-β-N-acetylglucosaminidase (ENGase) catalyzes hydrolysis of N-linked oligosaccharides. Although many ENGases have been characterized from various organisms, so far no fucose-containing oligosaccharides-specific ENGase has been identified in any organism. Here, we screened soil samples, using dansyl chloride (Dns)-labeled sialylglycan (Dns-SG) as a substrate, and discovered a strain that exhibits ENGase activity in the culture supernatant; this strain, named here as strain HMA12, was identified as a Sphingobacterium species by 16S ribosomal RNA gene analysis. By draft genome sequencing, five candidate ENGase encoding genes were identified in the genome of this strain. Among them, a recombinant protein purified from Escherichia coli expressing the candidate gene ORF1188 exhibited fucose-containing oligosaccharides-specific ENGase activity. The ENGase exhibited optimum activities at very acidic pHs (between pH 2.3–2.5). A BLAST search using the sequence of ORF1188 identified two fungal homologs, one in Beauveria bassiana and the other in Cordyceps militaris. Recombinant ORF1188, Beauveria and Cordyceps ENGases released the fucose-containing oligosaccharides residues from rituximab (immunoglobulin G) but not the high-mannose-containing oligosaccharides residues from RNase B, a result that not only confirmed the substrate specificity of these novel ENGases but also suggested that natural glycoproteins could be their substrates.

Crystallizable Fragment Glycoengineering for Therapeutic Antibodies Development

Monoclonal antibody (mAb)-based therapeutics are the fastest growing class of human pharmaceuticals. They are typically IgG1 molecules with N-glycans attached to the N297 residue on crystallizable fragment (Fc). Different Fc glycoforms impact their effector function, pharmacokinetics, stability, aggregation, safety, and immunogenicity. Fc glycoforms affect mAbs effector functions including antibody-dependent cell-mediated cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC) by modulating the Fc-FcγRs and Fc-C1q interactions. While the terminal galactose enhances CDC activity, the fucose significantly decreases ADCC. Defucosylated immunoglobulin Gs (IgGs) are thus highly pursued as next-generation therapeutic mAbs with potent ADCC at reduced doses. A plethora of cell glycoengineering and chemoenzymatic glycoengineering strategies is emerging to produce IgGs with homogenous glycoforms especially without core fucose. The chemoenzymatic glycosylation remodeling also offers useful avenues for site-specific conjugations of small molecule drugs onto mAbs. Herein, we review the current progress of IgG-Fc glycoengineering. We begin with the discussion of the structures of IgG N-glycans and biosynthesis followed by reviewing the impact of IgG glycoforms on antibody effector functions and the current Fc glycoengineering strategies with emphasis on Fc defucosylation. Furthermore, we briefly discuss two novel therapeutic mAbs formats: aglycosylated mAbs and Fc glycan specific antibody-drug conjugates (ADCs). The advances in the understanding of Fc glycobiology and development of novel glycoengineering technologies have facilitated the generation of therapeutic mAbs with homogenous glycoforms and improved therapeutic efficacy.

Microheterogeneity of Recombinant Antibodies: Analytics and Functional Impact

Antibodies are typical examples of biopharmaceuticals which are composed of numerous, almost infinite numbers of potential molecular entities called variants or isoforms, which constitute the microheterogeneity of these molecules. These variants are generated during biosynthesis by so-called posttranslational modification, during purification or upon storage. The variants differ in biological properties such as pharmacodynamic properties, for example, Antibody Dependent Cellular Cytotoxicity, complement activation, and pharmacokinetic properties, for example, serum half-life and safety. Recent progress in analytical technologies such as various modes of liquid chromatography and mass spectrometry has helped to elucidate the structure of a lot of these variants and their biological properties. In this review the most important modifications (glycosylation, terminal modifications, amino acid side chain modifications, glycation, disulfide bond variants and aggregation) are reviewed and an attempt is made to give an overview on the biological properties, for which the reports are often contradictory. Even though there is a deep understanding of cellular and molecular mechanism of antibody modification and their consequences, the clinical proof of the effects observed in vitro and in vivo is still not fully rendered. For some modifications such as core-fucosylation of the N-glycan and aggregation the effects are clear and should be monitored, but with others such as C-terminal lysine clipping the reports are contradictory. As a consequence it seems too early to tell if any modification can be safely ignored.

Decoding the Human Immunoglobulin G-Glycan Repertoire Reveals a Spectrum of Fc-Receptor- and Complement-Mediated-Effector Activities

Glycosylation of the immunoglobulin G (IgG)-Fc tail is required for binding to Fc-gamma receptors (FcγRs) and complement-component C1q. A variety of IgG1-glycoforms is detected in human sera. Several groups have found global or antigen-specific skewing of IgG glycosylation, for example in autoimmune diseases, viral infections, and alloimmune reactions. The IgG glycoprofiles seem to correlate with disease outcome. Additionally, IgG-glycan composition contributes significantly to Ig-based therapies, as for example IVIg in autoimmune diseases and therapeutic antibodies for cancer treatment. The effect of the different glycan modifications, especially of fucosylation, has been studied before. However, the contribution of the 20 individual IgG glycoforms, in which the combined effect of all 4 modifications, to the IgG function has never been investigated. Here, we combined six glyco-engineering methods to generate all 20 major human IgG1-glycoforms and screened their functional capacity for FcγR and complement activity. Bisection had no effect on FcγR or C1q-binding, and sialylation had no- or little effect on FcγR binding. We confirmed that hypo-fucosylation of IgG1 increased binding to FcγRIIIa and FcγRIIIb by ~17-fold, but in addition we showed that this effect could be further increased to ~40-fold for FcγRIIIa upon simultaneous hypo-fucosylation and hyper-galactosylation, resulting in enhanced NK cell-mediated antibody-dependent cellular cytotoxicity. Moreover, elevated galactosylation and sialylation significantly increased (independent of fucosylation) C1q-binding, downstream complement deposition, and cytotoxicity. In conclusion, fucosylation and galactosylation are primary mediators of functional changes in IgG for FcγR- and complement-mediated effector functions, respectively, with galactose having an auxiliary role for FcγRIII-mediated functions. This knowledge could be used not only for glycan profiling of clinically important (antigen-specific) IgG but also to optimize therapeutic antibody applications.

One-pot enzymatic glycan remodeling of a therapeutic monoclonal antibody by endoglycosidase S (Endo-S) from Streptococcus pyogenes

A facile, one-pot enzymatic glycan remodeling of antibody rituximab to produce homogeneous high-mannose and hybrid type antibody glycoforms is described. This method was based on the unique substrate specificity of the endoglycosidase S (Endo-S) from Streptococcus pyogenes. While Endo-S efficiently hydrolyzes the bi-antennary complex type IgG Fc N-glycans, we found that Endo-S did not hydrolyze the "ground state" high-mannose or hybrid glycoforms, and only slowly hydrolyzed the highly activated high-mannose or hybrid N-glycan oxazolines. Moreover, we found that wild-type Endo-S could efficiently use high-mannose or hybrid glycan oxazolines for transglycosylation without product hydrolysis. The combination of the remarkable difference in substrate specificity of Endo-S allows the deglycosylation of heterogeneous rituximab and the transglycosylation with glycan oxazoline to take place in one-pot without the need of isolating the deglycosylated intermediate or changing the enzyme to afford the high-mannose type, hybrid type, and some selectively modified truncated form of antibody glycoforms.

Enhanced Effector Functions Due to Antibody Defucosylation Depend on the Effector Cell Fcγ Receptor Profile

Abs of the IgG isotype are glycosylated in their Fc domain at a conserved asparagine at position 297. Removal of the core fucose of this glycan greatly increases the affinity for FcγRIII, resulting in enhanced FcγRIII-mediated effector functions. Normal plasma IgG contains ∼94% fucosylated Abs, but alloantibodies against, for example, Rhesus D (RhD) and platelet Ags frequently have reduced fucosylation that enhances their pathogenicity. The increased FcγRIII-mediated effector functions have been put to use in various afucosylated therapeutic Abs in anticancer treatment. To test the functional consequences of Ab fucosylation, we produced V-gene-matched recombinant anti-RhD IgG Abs of the four different subclasses (IgG1-4) with and without core fucose (i.e., 20% fucose remaining). Binding to all human FcγR types and their functional isoforms was assessed with surface plasmon resonance. All hypofucosylated anti-RhD IgGs of all IgG subclasses indeed showed enhanced binding affinity for isolated FcγRIII isoforms, without affecting binding affinity to other FcγRs. In contrast, when testing hypofucosylated anti-RhD Abs with FcγRIIIa-expressing NK cells, a 12- and 7-fold increased erythrocyte lysis was observed with the IgG1 and IgG3, respectively, but no increase with IgG2 and IgG4 anti-RhD Abs. Notably, none of the hypofucosylated IgGs enhanced effector function of macrophages, which, in contrast to NK cells, express a complex set of FcγRs, including FcγRIIIa. Our data suggest that the beneficial effects of afucosylated biologicals for clinical use can be particularly anticipated when there is a substantial involvement of FcγRIIIa-expressing cells, such as NK cells.

Crystal Structure of a Homogeneous IgG-Fc Glycoform with the N-Glycan Designed to Maximize the Antibody Dependent Cellular Cytotoxicity

N-glycosylation on IgG modulates Fc conformation and effector functions. An IgG-Fc contains a human sialo-complex type (hSCT) glycan of biantennary structure with two α2,6-sialylations and without core-fucosylation is an optimized glycoform developed to enhance the antibody dependent cellular cytotoxicity (ADCC). hSCT modification not only enhances the binding affinity to Fc receptors in the presence of antigen but also in some cases provides gain-of-function effector activity. We used enzymatic glyco-engineering to prepare an IgG-Fc with homogeneous hSCT attached to each C_H2 domain and solved its crystal structure. A compact form and an open form were observed in an asymmetric unit in the crystal. In the compact structure, the double glycan latches from the two hSCT chains stabilize the C_H2 domains in a closed conformation. In the open structure, the terminal sialic acid (N-acetylneuraminic acid or NeuNAc) residue interacts through water-mediated hydrogen bonds with the D249-L251 helix, to modulate the pivot region of the C_H2-C_H3 interface. The double glycan latches and the sialic acid modulation may be mutually exclusive. This is the first crystal structure of glyco-engineered Fc with enhanced effector activities. This work provides insights into the relationship between the structural stability and effector functions affected by hSCT modification and the development of better antibodies for therapeutic applications.

Glycosynthase Mutants of Endoglycosidase S2 Show Potent Transglycosylation Activity and Remarkably Relaxed Substrate Specificity for Antibody Glycosylation Remodeling

Glycosylation can exert a profound impact on the structures and biological functions of antibodies. Glycosylation remodeling using the endoglycosidase-catalyzed deglycosylation and transglycosylation approach is emerging as a promising platform to produce homogeneous glycoforms of antibodies, but the broad application of this method will require the availability of highly efficient glycosynthase mutants. We describe in this paper a systematic site-directed mutagenesis of an endoglycosidase from Streptococcus pyogenes of serotype M49 (Endo-S2) and the evaluation of the resulting mutants for their hydrolysis and transglycosylation activities. We found that mutations at the Asp-184 residue gave mutants that demonstrated significantly different properties, some possessed potent transglycosylation activity with diminished hydrolysis activity but others did not, which would be otherwise difficult to predict without the comparative study. In contrast to the previously reported Endo-S mutants that are limited to action on complex type N-glycans, the Endo-S2 glycosynthases described here, including D184M and D184Q, were found to have remarkably relaxed substrate specificity and were capable of transferring three major types (complex, high-mannose, and hybrid type) of N-glycans for antibody glycosylation remodeling. In addition, the Endo-S2 glycosynthase mutants were found to be much more active in general than the Endo-S mutants for transglycosylation. The usefulness of these Endo-S2 glycosynthase mutants was exemplified by an efficient glycosylation remodeling of two therapeutic monoclonal antibodies, rituximab and trastuzumab (Herceptin).