Site-Specific N-Glycosylation Analysis of Human Immunoglobulin E

IgE glycosylation and impact on structure and function: A systematic review

The impact of human IgE glycosylation on structure, function and disease mechanisms is not fully elucidated, and heterogeneity in different studies renders drawing conclusions challenging. Previous reviews discussed IgE glycosylation focusing on specific topics such as health versus disease, FcεR binding or impact on function. We present the first systematic review of human IgE glycosylation conducted utilizing the PRISMA guidelines. We sought to define the current consensus concerning the roles of glycosylation on structure, biology and disease. Despite diverse analytical methodologies, source, expression systems and the sparsity of data on IgE antibodies from non-allergic individuals, collectively evidence suggests differential glycosylation profiles, particularly in allergic diseases compared with healthy states, and indicates functional impact, and contributions to IgE-mediated hypersensitivities and atopic diseases. Beyond allergic diseases, dysregulated terminal glycan structures, including sialic acid, may regulate IgE metabolism. Glycan sites such as N394 may contribute to stabilizing IgE structure, with alterations in these glycans likely influencing both structure and IgE-FcεR interactions. This systematic review therefore highlights critical IgE glycosylation attributes in health and disease that may be exploitable for therapeutic intervention, and the need for novel analytics to explore pertinent research avenues.

Production of therapeutic glycoproteins in glycoengineered plant: old farm for new crops

Plant-based expression systems have emerged as promising avenues for the production of recombinant N-linked glycoproteins. This review offers insights into the evolution and progress of plant glycoengineering. It delves into the distinctive features of plant-derived N-glycans, the diverse range of plant hosts employed for glycoprotein synthesis, and the advancements in glycoengineering strategies aimed at generating glycoproteins with N-glycan structures akin to those produced in mammalian cell lines. Furthermore, alternative strategies for augmenting glycoengineering efforts and the current spectrum of applications for plant-produced N-glycan recombinant proteins are examined, underscoring their potential significance in biopharmaceutical manufacturing.

The B Cell Response and Formation of Allergenic and Anti-Allergenic Antibodies in Food Allergy

Food allergies are a growing public health concern worldwide, especially in children and young adults. Allergen-specific IgE plays a central role in the pathogenesis of food allergies, but their titers poorly correlate with allergy development. Host immune systems yield allergen-specific immunoglobulin (Ig)A, IgE and IgG subclasses with low or high affinities and differential Fc N-glycosylation patterns that can affect the allergic reaction to food in multiple ways. High-affinity IgE is required to induce strong mast cell activation eventually leading to allergic anaphylaxis, while low-affinity IgE can even inhibit the development of clinically relevant allergic symptoms. IgA and IgG antibodies can inhibit IgE-mediated mast cell activation through various mechanisms, thereby protecting IgE-positive individuals from allergy development. The production of IgE and IgG with differential allergenic potential seems to be affected by the signaling strength of individual B cell receptors, and by cytokines from T cells. This review provides an overview of the diversity of the B cell response and the diverse roles of antibodies in food allergy.

On the complexity of IgE: The role of structural flexibility and glycosylation for binding its receptors

It is well established that immunoglobulin E (IgE) plays a crucial role in atopy by binding to two types of Fcε receptors (FcεRI and FcεRII, also known as CD23). The cross-linking of FcεRI-bound IgE on effector cells, such as basophils and mast cells, initiates the allergic response. Conversely, the binding of IgE to CD23 modulates IgE serum levels and antigen presentation. In addition to binding to FcεRs, IgE can also interact with other receptors, such as certain galectins and, in mice, some FcγRs. The binding strength of IgE to its receptors is affected by its valency and glycosylation. While FcεRI shows reduced binding to IgE immune complexes (IgE-ICs), the binding to CD23 is enhanced. There is no evidence that galectins bind IgE-ICs. On the other hand, IgE glycosylation plays a crucial role in the binding to FcεRI and galectins, whereas the binding to CD23 seems to be independent of glycosylation. In this review, we will focus on receptors that bind to IgE and examine how the glycosylation and complexation of IgE impact their binding.

High-throughput immunoaffinity enrichment and N-glycan analysis of human plasma haptoglobin

Haptoglobin (Hp) is a positive acute phase protein, synthesized in the liver, with four N-glycosylation sites carrying mainly complex type N-glycans. Its glycosylation is altered in different types of diseases but still has not been extensively studied mainly due to analytical challenges, especially the lack of a fast, efficient, and robust high-throughput Hp isolation procedure. Here, we describe the development of a high-throughput method for Hp enrichment from human plasma, based on monolithic chromatographic support in immunoaffinity mode and downstream Hp N-glycome analysis by hydrophilic interaction ultrahigh-performance liquid chromatography with fluorescent detection (HILIC-UHPLC-FLR). Chromatographic monolithic supports in a 96-well format enable fast, efficient, and robust Hp enrichment directly from diluted plasma samples. The N-glycome analysis demonstrated that a degree of Hp deglycosylation differs depending on the conditions used for N-glycan release and on the specific glycosylation site, with Asn 241 being the most resistant to deglycosylation under tested conditions. HILIC-UHPLC-FLR analysis enables robust quantification of 28 individual chromatographic peaks, in which N-glycan compositions were determined by UHPLC coupled to electrospray ionization quadrupole time of flight mass spectrometry. The developed analytical approach enables fast evaluation of total Hp N-glycosylation and is applicable in large-scale studies.

Generation and Characterization of Native and Sialic Acid-Deficient IgE

Efficient characterization of IgE antibodies and their glycan structures is required for understanding their function in allergy and in the emerging AllergoOncology field for antibody immunotherapy. We report the generation, glyco-profiling and functional analysis of native and sialic acid-deficient glyco-engineered human IgE. The antibodies produced from human embryonic kidney cells were purified via a human IgE class-specific affinity matrix and structural integrity was confirmed by SDS-PAGE and size-exclusion chromatography (SEC). Purified IgEs specific for the tumor-associated antigens Chondroitin Sulfate Proteoglycan 4 (CSPG4-IgE) and Human Epidermal Growth Factor Receptor 2 (HER2-IgE) were devoid of by-products such as free light chains. Using neuraminidase-A, we generated sialic acid-deficient CSPG4-IgE as example glyco-engineered antibody. Comparative glycan analyses of native and glyco-engineered IgEs by Hydrophilic interaction liquid chromatography (HILIC)-high performance liquid chromatography (HPLC) indicated loss of sialic acid terminal residues and differential glycan profiles. Native and glyco-engineered CSPG4-IgEs recognized Fc receptors on the surface of human FcεRI-expressing rat basophilic leukemia RBL-SX38 cells, and of CD23/FcεRII-expressing human RPMI-8866 B-lymphocytes and bound to CSPG4-expressing A2058 human melanoma cells, confirming Fab-mediated recognition. When cross-linked on the cell surface, both IgEs triggered RBL-SX38 degranulation. We demonstrate efficient generation and functional competence of recombinant native and sialic acid-deficient IgEs.

Identification and characterization of circulating immune complexes in IgA nephropathy

The underlying pathology of immunoglobulin A (IgA) nephropathy (IgAN), the most common glomerulonephritis worldwide, is driven by the deposition of immune complexes containing galactose-deficient IgA1 [Tn(+)IgA1] in the glomerular mesangium. Here, we report that novel anti-Tn circulating immune complexes (anti-Tn CICs) contain predominantly IgM, representing large macromolecular complexes of ~1.2 megadaltons to several megadalton sizes together with Tn(+)IgA1 and some IgG. These complexes are significantly elevated in sera of patients with IgAN, which contains higher levels of complement C3, compared to healthy individuals. Anti-Tn CICs are bioactive and induce specific proliferation of human renal mesangial cells. We found that these anti-Tn CICs can be dissociated with small glycomimetic compounds, which mimic the Tn antigen of Tn(+)IgA1, releasing IgA1 from anti-Tn CICs. This glycomimetic compound can also significantly inhibit the proliferative activity of anti-Tn CICs of patients with IgAN. These findings could enhance both the diagnosis of IgAN and its treatment, as specific drug treatments are now unavailable.

N-glycosylation of mannose receptor (CD206) regulates glycan binding by C-type lectin domains

The macrophage mannose receptor (MR, CD206) is a transmembrane endocytic lectin receptor, expressed in selected immune and endothelial cells, and is involved in immunity and maintaining homeostasis. Eight of the ten extracellular domains of the MR are C-type lectin domains (CTLDs) which mediate the binding of mannose, fucose, and GlcNAc in a calcium-dependent manner. Previous studies indicated that self-glycosylation of MR regulates its glycan binding. To further explore this structure-function relationship, we studied herein a recombinant version of mouse MR CTLD4-7 fused to human Fc-portion of IgG (MR-Fc). The construct was expressed in different glycosylation-mutant cell lines to study the influence of differential glycosylation on receptor glycan-binding properties. We conducted site-specific N- and O-glycosylation analysis and glycosylation site characterization using mass spectrometry by which several novel O-glycosylation sites were identified in mouse MR and confirmed in human full-length MR. This information guided experiments evaluating the receptor functionality by glycan microarray analysis in combination with glycan-modifying enzymes. Treatment of active MR-Fc with combinations of exoglycosidases, including neuraminidase and galactosidases, resulted in the loss of trans-binding (binding of MR CTLDs to non-MR glycans), due to unmasking of terminal, nonreducing GlcNAc in N-glycans of the MR CTLDs. Regalactosylation of N-glycans rescues mannose binding by MR-Fc. Our results indicate that glycans within the MR CTLDs act as a regulatory switch by masking and unmasking self-ligands, including terminal, nonreducing GlcNAc in N-glycans, which could control MR activity in a tissue- and cell-specific manner or which potentially affect bacterial pathogenesis in an immunomodulatory fashion.

Site-specific N-glycosylation analysis of human thyroid thyroglobulin by mass spectrometry-based Glyco-analytical strategies

Human thyroglobulin (Tg), which has many glycosylation sites, is an essential protein produced by the human thyroid glands. Although human Tg N-glycans play critical roles in the cellular events of the Thyroid gland, the site-specific distribution of glycan structures has not been studied in detail. This study aimed to profile human Tg N-glycosylation sites and their glycan contents by using high-throughput glyco-analytical strategies, including glycopeptide and glycan levels. The sulfated complex and hybrid type N-glycan species were determined by the analysis of the human Tg samples with HPLC-HILIC-FLD-MS/MS. It was found that all fucosylated N-glycans carried fucose residue on their N-glycan core structure. The human Tg was digested with multiple enzymes by applying both in-gel and in-solution protocols to enhance site-specific glycosylation analysis. In total, 17 out of 20 N-glycosylation sites were characterized. It was noticed that 6 N-glycosylation sites contain only high-mannose type glycans, while other regions include complex and hybrid type glycans. In addition, sulfated glycoform structures were detected at the glycopeptide level in glycosylation sites containing complex and hybrid type glycans. It is expected that the results obtained from this study will contribute to functional studies to be conducted on human Tg protein. BIOLOGICAL SIGNIFICANCE: N-glycans of human thyroglobulin modulate thyroid hormone synthesis both in vivo and in vitro. Therefore, a comprehensive analysis of the N-glycosylation sites of human thyroglobulin is essential to improve our understanding of the function of its N-glycans. The present research significantly expanded the knowledge regarding N-glycosylation profiles of human thyroid thyroglobulin protein. For instance, as highlighted here, sulfated N-glycan structures were characterized using comprehensive glyco-analytical strategies. N-glycan patterns for the sites Asn110, Asn1869, and Asn2122 were described for the first time in this current work. In addition, N-glycan structures containing core-fucosylation and bisecting types were confirmed for all determined glycosylation sites.

Defects and countermeasures in laboratory diagnosis of rare IgE multiple myeloma

BACKGROUND

IgE multiple myeloma (MM) is a rare subtype of MM that is easily misdiagnosed. We report a rare case of IgE-MM and investigate the application of the SLiM-CRAB criteria to screen for high-risk smoldering MM (SMM) patients, so as to summarize the causes and methods used to prevent missed diagnosis or misdiagnosis in IgE-MM.

METHODS

The serum monoclonal protein (M-protein) classification and IgE quantification was performed and sent to several individual institutions. The results were collected and the causes of IgE detection defects were analyzed.

RESULTS

Upon admission to our hospital, the patient's serum free kappa light chain was 1,069.9 mg/L, free lambda light chain was 9.2 mg/L, and free kappa/lambda ratio was 115.9, which met the SLiM criteria, but without CRAB features. Immunofixation electrophoresis (IF) showed "M-like protein aggregation bands" in all lanes. After pretreatment with 1% β-mercaptoethanol to depolymerize the aggregation of monoclonal protein, the "M-like protein aggregation bands disappeared. The other five institutions did not provide the correct typing results. The quantification of serum IgE was as high as 2.06×10⁷ IU/mL, whereas 7 other testing institutions reported IgE levels ranging from 1.0 to 1100 IU/mL.

CONCLUSION

High-risk biomarkers in SLiM criteria can achieve good therapeutic effects in rare IgE-MM patients. Serum immunofixation performed without antisera against IgE, insufficient identification of the lytic bands produced by high macromolecule aggregation in IF, and the absence of a prozone effect avoidance procedure during IgE quantitative detection are the primary causes of missed diagnosis or misdiagnosis in patients with IgE-MM.

Sialylation as an Important Regulator of Antibody Function

Antibodies play a critical role in linking the adaptive immune response to the innate immune system. In humans, antibodies are categorized into five classes, IgG, IgM, IgA, IgE, and IgD, based on constant region sequence, structure, and tropism. In serum, IgG is the most abundant antibody, comprising 75% of antibodies in circulation, followed by IgA at 15%, IgM at 10%, and IgD and IgE are the least abundant. All human antibody classes are post-translationally modified by sugars. The resulting glycans take on many divergent structures and can be attached in an N-linked or O-linked manner, and are distinct by antibody class, and by position on each antibody. Many of these glycan structures on antibodies are capped by sialic acid. It is well established that the composition of the N-linked glycans on IgG exert a profound influence on its effector functions. However, recent studies have described the influence of glycans, particularly sialic acid for other antibody classes. Here, we discuss the role of glycosylation, with a focus on terminal sialylation, in the biology and function across all antibody classes. Sialylation has been shown to influence not only IgG, but IgE, IgM, and IgA biology, making it an important and unappreciated regulator of antibody function.

A Recombinant Ig Fragment (IgCw-γεκ) Comprising the Cγ1-Cε2-4 and Cκ Domains Is an Alternative Reagent to Human IgE

Human IgE is useful for immunological assays, such as sensitization of FcεRI-positive cells and IgE measurement. In this study, we report the development of a recombinant Ig fragment, designated IgCw-γεκ, as an alternative reagent to human IgE. IgCw-γεκ (∼130 kDa) comprises two hybrid constant H chain regions (Cγ₁-Cε_2-4, each ∼53 kDa) and two constant κ L chains (C_κ, each ∼12 kDa) and lacks a V domain. The presence of Cγ₁ instead of Cε₁ within the H chain increased the production yield and facilitated assembly of the H and L chains. IgCw-γεκ was produced in cultured human embryonic kidney 293F cells, with a yield of ∼27 mg/l. IgCw-γεκ bound to human FcεRIαRs expressed on the surface of rat basophilic leukemia-2H3 cells. A β-hexosaminidase release assay revealed that the biological activity of IgCw-γεκ was comparable with that of IgE. The IgE concentration measured using IgCw-γεκ as a standard was similar to that measured using IgE as a standard. These results suggest that the IgCw-γεκ molecule retains the basic characteristics of IgE, but does not cross-react with Ags, making it an alternative to the IgE isotype references used in a variety of immunological assays.

Beyond Titer: Expanding the Scope of Clinical Autoantibody Testing

Background

Autoantibodies that bind self-antigens are a hallmark of autoimmune diseases, but can also be present in healthy individuals. Clinical assays that detect and titer antigen-specific autoantibodies are an important component of the diagnosis and monitoring of autoimmune diseases. Autoantibodies may contribute to disease pathogenesis via effector functions that are dictated by both the antigen-binding site and constant domain.

Content

In this review, we discuss features of antibodies, in addition to antigen-binding specificity, which determine effector function. These features include class, subclass, allotype, and glycosylation. We discuss emerging data indicating that analysis of these antibody features may be informative for diagnosis and monitoring of autoimmune diseases. We also consider methodologies to interrogate these features and consider how they could be implemented in the clinical laboratory.

Summary

Future autoantibody assays may incorporate assessment of additional antibody features that contribute to autoimmune disease pathogenesis and provide added clinical value.

Extending the capabilities of intact-mass analyses to monoclonal immunoglobulins of the E-isotype (IgE)

Mass spectrometry (MS) has become an indispensable tool in structural characterization and quality control of monoclonal antibodies (mAbs). Intact-mass analysis is a particularly attractive option that provides a powerful and cost-effective means to not only confirm the structural integrity of the protein, but also probe its interactions with therapeutic targets. To a certain extent, this success can be attributed to relatively modest glycosylation levels exhibited by IgG molecules, which limits their structural heterogeneity and enables straightforward mass measurements at the intact molecule level. The recent surge of interest in expanding the repertoire of mAbs to include other classes of immunoglobulins places a premium on efforts to adapt the IgG-tailored experimental strategies to other classes of antibodies, but their dramatically higher levels of glycosylation may create insurmountable obstacles. The monoclonal murine IgE antibody explored in this work provides a challenging model system, as its glycosylation level exceeds that of conventional IgG mAbs by a factor of nine. The commercial sample, which included various IgE fragments, yields a poorly resolved ionic signal in intact-mass measurements, from which little useful information can be extracted. However, coupling MS measurements with the limited charge reduction of select polycationic species in the gas phase gives rise to well-defined charge ladders, from which both ionic masses and charges can be readily determined. The measurements reveal significant variation of the extent of glycosylation within intact IgE molecules, as well as the presence of low-molecular weight impurities in the commercial IgE sample. Furthermore, incubation of the monoclonal IgE with its antigen (ovalbumin) gives rise to the formation of complexes with varying stoichiometries, which can also be uniquely identified using a combination of native MS, limited charge reduction in the gas phase and data fitting procedures. This work demonstrates that following appropriate modifications, intact-mass analysis measurements can be successfully applied to mAbs beyond the IgG isotype, providing a wealth of information not only on the mass distribution of the intact IgE molecules, but also their large-scale conformational integrity, the integrity of their covalent structure, and their interactions with antigens.

Glycoengineering of Therapeutic Antibodies with Small Molecule Inhibitors

Monoclonal antibodies (mAbs) are one of the cornerstones of modern medicine, across an increasing range of therapeutic areas. All therapeutic mAbs are glycoproteins, i.e., their polypeptide chain is decorated with glycans, oligosaccharides of extraordinary structural diversity. The presence, absence, and composition of these glycans can have a profound effect on the pharmacodynamic and pharmacokinetic profile of individual mAbs. Approaches for the glycoengineering of therapeutic mAbs—the manipulation and optimisation of mAb glycan structures—are therefore of great interest from a technological, therapeutic, and regulatory perspective. In this review, we provide a brief introduction to the effects of glycosylation on the biological and pharmacological functions of the five classes of immunoglobulins (IgG, IgE, IgA, IgM and IgD) that form the backbone of all current clinical and experimental mAbs, including an overview of common mAb expression systems. We review selected examples for the use of small molecule inhibitors of glycan biosynthesis for mAb glycoengineering, we discuss the potential advantages and challenges of this approach, and we outline potential future applications. The main aim of the review is to showcase the expanding chemical toolbox that is becoming available for mAb glycoengineering to the biology and biotechnology community.

Brilliant glycans and glycosylation: Seq and ye shall find

Proteoglycosylation is the addition of monosaccharides or glycans to the protein peptide chain. This is a common post-translational modification of proteins with a variety of biological functions. At present, more than half of all biopharmaceuticals in clinic are modified by glycosylation. Most glycoproteins are potential drug targets and biomarkers for disease diagnosis. Therefore, in-depth study of glycan structure of glycoproteins will ultimately improve the sensitivity and specificity of glycoproteins for clinical disease detection. With the deepening of research, the function and application value of glycans and glycosylation has gradually emerged. This review systematically introduces the latest research progress of glycans and glycosylation. It encompasses six cancers, four viruses, and their latest discoveries in Alzheimer's disease, allergic diseases, congenital diseases, gastrointestinal diseases, inflammation, and aging.

Tumor cells express pauci- and oligomannosidic N-glycans in glycoproteins recognized by the mannose receptor (CD206)

The macrophage mannose receptor (CD206, MR) is an endocytic lectin receptor which plays an important role in homeostasis and innate immunity, however, the endogenous glycan and glycoprotein ligands recognized by its C-type lectin domains (CTLD) have not been well studied. Here we used the murine MR CTLD4-7 coupled to the Fc-portion of human IgG (MR-Fc) to investigate the MR glycan and glycoprotein recognition. We probed 16 different cancer and control tissues using the MR-Fc, and observed cell- and tissue-specific binding with varying intensity. All cancer tissues and several control tissues exhibited MR-Fc ligands, intracellular and/or surface-located. We further confirmed the presence of ligands on the surface of cancer cells by flow cytometry. To characterize the fine specificity of the MR for glycans, we screened a panel of glycan microarrays. Remarkably, the results indicate that the CTLD4-7 of the MR is highly selective for specific types of pauci- and oligomannose N-glycans among hundreds of glycans tested. As lung cancer tissue and the lung cancer cell line A549 showed intense MR-Fc binding, we further investigated the MR glycoprotein ligands in those cells by immunoprecipitation and glycoproteomic analysis. All enriched glycoproteins, of which 42 were identified, contained pauci- or oligomannose N-glycans, confirming the microarray results. Our study demonstrates that the MR CTLD4-7 is highly selective for pauci- and oligomannosidic N-glycans, structures that are often elevated in tumor cells, and suggest a potential role for the MR in tumor biology.

Activation of regulatory T cells triggers specific changes in glycosylation associated with Siglec-1-dependent inflammatory responses

Background: Siglec-1 is a macrophage lectin-like receptor that mediates sialic acid-dependent cellular interactions. Its upregulation on macrophages in autoimmune disease was shown previously to promote inflammation through suppressing the expansion of regulatory T cells (Tregs). Here we investigate the molecular basis for Siglec-1 binding to Tregs using in vitro-induced cells as a model system. Methods: Glycosylation changes that affect Siglec‑1 binding were studied by comparing activated and resting Tregs using RNA-Seq, glycomics, proteomics and binding of selected antibodies and lectins. A proximity labelling and proteomics strategy was used to identify Siglec-1 counter-receptors expressed on activated Tregs. Results: Siglec-1 binding was strongly upregulated on activated Tregs, but lost under resting conditions. Glycomics revealed changes in N-glycans and glycolipids following Treg activation and we observed changes in expression of multiple 'glycogenes' that could lead to the observed increase in Siglec-1 binding. Proximity labelling of intact, living cells identified 49 glycoproteins expressed by activated Tregs that may function as Siglec-1 counter-receptors. These represent ~5% of the total membrane protein pool and were mainly related to T cell activation and proliferation. We demonstrate that several of these counter-receptors were upregulated following activation of Tregs and provide initial evidence that their altered glycosylation may also be important for Siglec-1 binding. Conclusions: We provide the first comprehensive analysis of glycan changes that occur in activated Tregs, leading to recognition by the macrophage lectin, Siglec-1 and suppression of Treg expansion. We furthermore provide insights into glycoprotein counter-receptors for Siglec-1 expressed by activated Tregs that are likely to be important for suppressing Treg expansion.

Afucosylated IgG Targets FcγRIV for Enhanced Tumor Therapy in Mice

Simple Summary

Cancer treatments are increasingly based on therapeutic antibodies to clear tumors. While in vivo mouse models are useful to predict effectiveness of human antibodies it is not completely clear how useful these models are to test antibodies engineered with enhanced effector functions designed for humans. One of the changes considered for many new antibody-based drugs is the removal of fucose (resulting in afucosylated IgG) which enhances IgG-Fc receptor (FcγR) mediated effector functions in humans through FcγRIIIa. Here we show that afucosylated human IgG1 also have enhanced effector functions against peritoneal metastasis of melanoma cells in mice through the evolutionary related mouse FcγRIV. This shows that afucosylated human IgG is functionally recognized across species and shows that mouse tumor models can be used to assess the therapeutic potential of afucosylated IgG1.

Abstract

Promising strategies for maximizing IgG effector functions rely on the introduction of natural and non-immunogenic modifications. The Fc domain of IgG antibodies contains an N-linked oligosaccharide at position 297. Human IgG antibodies lacking the core fucose in this glycan have enhanced binding to human (FcγR) IIIa/b, resulting in enhanced antibody dependent cell cytotoxicity and phagocytosis through these receptors. However, it is not yet clear if glycan-enhancing modifications of human IgG translate into more effective treatment in mouse models. We generated humanized hIgG1-TA99 antibodies with and without core-fucose. C57Bl/6 mice that were injected intraperitoneally with B16F10-gp75 mouse melanoma developed significantly less metastasis outgrowth after treatment with afucosylated hIgG1-TA99 compared to mice treated with wildtype hhIgG1-TA99. Afucosylated human IgG1 showed stronger interaction with the murine FcγRIV, the mouse orthologue of human FcγRIIIa, indicating that this glycan change is functionally conserved between the species. In agreement with this, no significant differences were observed in tumor outgrowth in FcγRIV^-/- mice treated with human hIgG1-TA99 with or without the core fucose. These results confirm the potential of using afucosylated therapeutic IgG to increase their efficacy. Moreover, we show that afucosylated human IgG1 antibodies act across species, supporting that mouse models can be suitable to test afucosylated antibodies.

Glycan-specific IgG anti-IgE autoantibodies are protective against allergic anaphylaxis in a murine model

BACKGROUND

IgE causes anaphylaxis in type I hypersensitivity diseases by activating degranulation of effector cells such as mast cells and basophils. The mechanisms that control IgE activity and prevent anaphylaxis under normal conditions are still enigmatic.

OBJECTIVE

We aimed to unravel how anti-IgE autoantibodies are induced and we aimed to understand their role in regulating serum IgE level and allergic anaphylaxis.

METHODS

We immunized mice with different forms of IgE and tested anti-IgE autoantibody responses and their specificities. We then analyzed the effect of those antibodies on serum kinetics and their in vitro and in vivo impact on anaphylaxis. Finally, we investigated anti-IgE autoantibodies in human sera.

RESULTS

Immunization of mice with IgE-immune complexes induced glycan-specific anti-IgE autoantibodies. The anti-IgE autoantibodies prevented effector cell sensitization, reduced total IgE serum levels, protected mice from passive and active IgE sensitization, and resulted in cross-protection against different allergens. Furthermore, glycan-specific anti-IgE autoantibodies were present in sera from subjects with allergy and subjects without allergy.

CONCLUSION

In conclusion, this study provided the first evidence that in the murine model, the serum level and anaphylactic activity of IgE may be downregulated by glycan-specific IgG anti-IgE autoantibodies.

The SPPL3-Defined Glycosphingolipid Repertoire Orchestrates HLA Class I-Mediated Immune Responses

HLA class I (HLA-I) glycoproteins drive immune responses by presenting antigens to cognate CD8+ T cells. This process is often hijacked by tumors and pathogens for immune evasion. Because options for restoring HLA-I antigen presentation are limited, we aimed to identify druggable HLA-I pathway targets. Using iterative genome-wide screens, we uncovered that the cell surface glycosphingolipid (GSL) repertoire determines effective HLA-I antigen presentation. We show that absence of the protease SPPL3 augmented B3GNT5 enzyme activity, resulting in upregulation of surface neolacto-series GSLs. These GSLs sterically impeded antibody and receptor interactions with HLA-I and diminished CD8+ T cell activation. Furthermore, a disturbed SPPL3-B3GNT5 pathway in glioma correlated with decreased patient survival. We show that the immunomodulatory effect could be reversed through GSL synthesis inhibition using clinically approved drugs. Overall, our study identifies a GSL signature that inhibits immune recognition and represents a potential therapeutic target in cancer, infection, and autoimmunity.

Mass Spectrometry-Based Methods for Immunoglobulin G N-Glycosylation Analysis

Mass spectrometry and its hyphenated techniques enabled by the improvements in liquid chromatography, capillary electrophoresis, novel ionization, and fragmentation modes are truly a cornerstone of robust and reliable protein glycosylation analysis. Boost in immunoglobulin G (IgG) glycan and glycopeptide profiling demands for both applied biomedical and research applications has brought many new advances in the field in terms of technical innovations, sample preparation, improved throughput, and confidence in glycan structural characterization. This chapter summarizes mass spectrometry basics, focusing on IgG and monoclonal antibody N-glycosylation analysis on several complexity levels. Different approaches, including antibody enrichment, glycan release, labeling, and glycopeptide preparation and purification, are covered and illustrated with recent breakthroughs and examples from the literature omitting excessive theoretical frameworks. Finally, selected highly popular methodologies in IgG glycoanalytics such as liquid chromatography-mass spectrometry and matrix-assisted laser desorption ionization are discussed more thoroughly yet in simple terms making this text a practical starting point either for the beginner in the field or an experienced clinician trying to make sense out of the IgG glycomic or glycoproteomic dataset.

Glycosylation of Plant-Produced Immunoglobulins

Many economically important protein-based therapeutics like monoclonal antibodies are glycosylated. Due to the recognized importance of this type of posttranslational modification, glycoengineering of expression systems to obtain highly active and homogenous therapeutics is an emerging field. Although most of the monoclonal antibodies on the market are still produced in mammalian expression platforms, plants are emerging as an alternative cost-effective and scalable production platform that allows precise engineering of glycosylation to produce targeted human glycoforms at large homogeneity. Apart from producing more effective antibodies, pure glycoforms are required in efforts to link biological functions to specific glycan structures. Much is already known about the role of IgG1 glycosylation and this antibody class is the dominant recombinant format that has been expressed in plants. By contrast, little attention has been paid to the glycoengineering of recombinant IgG subtypes and the other four classes of human immunoglobulins (IgA, IgD, IgE, and IgM). Except for IgD, all these antibody classes have been expressed in plants and the glycosylation has been analyzed in a site-specific manner. Here, we summarize the current data on glycosylation of plant-produced monoclonal antibodies and discuss the findings in the light of known functions for these glycans.

Sweet Rules: Linking Glycosylation to Antibody Function

Antibodies produced upon infections with pathogenic microorganisms are essential for clearing primary infections and for providing the host with long-lasting immunity. Moreover, antibodies have become the most widely used platform for developing novel therapies against cancer and autoimmunity, requiring an in-depth understanding of how antibodies mediate their activity in vivo and which factors modulate pro- or anti-inflammatory antibody activities. Since the discovery that select residues present in the sugar domain attached to the immunoglobulin G (IgG) fragment crystallizable (Fc) region can modulate both, pro- and anti-inflammatory effector functions, a wealth of studies has focused on understanding how IgG glycosylation is regulated and how this knowledge can be used to optimize therapeutic antibody activity. With the introduction of glycoengineered afucosylated antibodies in cancer therapy and the initiation of clinical testing of highly sialylated anti-inflammatory antibodies the proof-of-concept that understanding antibody glycosylation can lead to clinical innovation has been provided. The focus of this review is to summarize recent insights into how antibody glycosylation is regulated in vivo and how select sugar residues impact IgG function.

Monitoring of immunoglobulin N- and O-glycosylation in health and disease

Protein N- and O-glycosylation are well known co- and post-translational modifications of immunoglobulins. Antibody glycosylation on the Fab and Fc portion is known to influence antigen binding and effector functions, respectively. To study associations between antibody glycosylation profiles and (patho) physiological states as well as antibody functionality, advanced technologies and methods are required. In-depth structural characterization of antibody glycosylation usually relies on the separation and tandem mass spectrometric (MS) analysis of released glycans. Protein- and site-specific information, on the other hand, may be obtained by the MS analysis of glycopeptides. With the development of high-resolution mass spectrometers, antibody glycosylation analysis at the intact or middle-up level has gained more interest, providing an integrated view of different post-translational modifications (including glycosylation). Alongside the in-depth methods, there is also great interest in robust, high-throughput techniques for routine glycosylation profiling in biopharma and clinical laboratories. With an emphasis on IgG Fc glycosylation, several highly robust separation-based techniques are employed for this purpose. In this review, we describe recent advances in MS methods, separation techniques and orthogonal approaches for the characterization of immunoglobulin glycosylation in different settings. We put emphasis on the current status and expected developments of antibody glycosylation analysis in biomedical, biopharmaceutical and clinical research.

IgE Antibodies against Cancer: Efficacy and Safety

Immunoglobulin E (IgE) antibodies are well known for their role in allergic diseases and for contributions to antiparasitic immune responses. Properties of this antibody class that mediate powerful effector functions may be redirected for the treatment of solid tumours. This has led to the rise of a new class of therapeutic antibodies to complement the armamentarium of approved tumour targeting antibodies, which to date are all IgG class. The perceived risk of type I hypersensitivity reactions following administration of IgE has necessitated particular consideration in the development of these therapeutic agents. Here, we bring together the properties of IgE antibodies pivotal to the hypothesis for superior antitumour activity compared to IgG, observations of in vitro and in vivo efficacy and mechanisms of action, and a focus on the safety considerations for this novel class of therapeutic agent. These include in vitro studies of potential hypersensitivity, selection of and observations from appropriate in vivo animal models and possible implications of the high degree of glycosylation of IgE. We also discuss the use of ex vivo predictive and monitoring clinical tools, as well as the risk mitigation steps employed in, and the preliminary outcomes from, the first-in-human clinical trial of a candidate anticancer IgE therapeutic.

Cosmc controls B cell homing

The molecular mechanisms regulating lymphocyte homing into lymph nodes are only partly understood. Here, we report that B cell-specific deletion of the X-linked gene, Cosmc, and the consequent decrease of protein O-glycosylation, induces developmental blocks of mouse B cells. After transfer into wild-type recipient, Cosmc-null B cells fail to home to lymph nodes as well as non-lymphoid organs. Enzymatic desialylation of wild-type B cells blocks their migration into lymph nodes, indicating a requirement of sialylated O-glycans for proper trafficking. Mechanistically, Cosmc-deficient B cells have normal rolling and firm arrest on high endothelium venules (HEV), thereby attributing their inefficient trafficking to alterations in the subsequent transendothelial migration step. Finally, Cosmc-null B cells have defective chemokine signaling responses. Our results thus demonstrate that Cosmc and its effects on O-glycosylation are important for controlling B cell homing.

SARS-CoV-2 Spike Protein Interacts with Multiple Innate Immune Receptors

The spike (S) glycoprotein in the envelope of SARS-CoV-2 is densely glycosylated but the functions of its glycosylation are unknown. Here we demonstrate that S is recognized in a glycan-dependent manner by multiple innate immune receptors including the mannose receptor MR/CD206, DC-SIGN/CD209, L-SIGN/CD209L, and MGL/CLEC10A/CD301. Single-cell RNA sequencing analyses indicate that such receptors are highly expressed in innate immune cells in tissues susceptible to SARS-CoV-2 infection. Binding of the above receptors to S is characterized by affinities in the picomolar range and consistent with S glycosylation analysis demonstrating a variety of N- and O-glycans as receptor ligands. These results indicate multiple routes for SARS-CoV-2 to interact with human cells and suggest alternative strategies for therapeutic intervention.

Sialylation of immunoglobulin E is a determinant of allergic pathogenicity

Approximately one-third of the world's population suffers from allergies¹. Exposure to allergens crosslinks immunoglobulin E (IgE) antibodies that are bound to mast cells and basophils, triggering the release of inflammatory mediators, including histamine². Although IgE is absolutely required for allergies, it is not understood why total and allergen-specific IgE concentrations do not reproducibly correlate with allergic disease^3-5. It is well-established that glycosylation of IgG dictates its effector function and has disease-specific patterns. However, whether IgE glycans differ in disease states or affect biological activity is completely unknown⁶. Here we perform an unbiased examination of glycosylation patterns of total IgE from individuals with a peanut allergy and from non-atopic individuals without allergies. Our analysis reveals an increase in sialic acid content on total IgE from individuals with a peanut allergy compared with non-atopic individuals. Removal of sialic acid from IgE attenuates effector-cell degranulation and anaphylaxis in several functional models of allergic disease. Therapeutic interventions-including removing sialic acid from cell-bound IgE with a neuraminidase enzyme targeted towards the IgE receptor FcεRI, and administering asialylated IgE-markedly reduce anaphylaxis. Together, these results establish IgE glycosylation, and specifically sialylation, as an important regulator of allergic disease.

Simultaneous Immunoglobulin A and G Glycopeptide Profiling for High-Throughput Applications

Immunoglobulin (Ig) glycosylation is recognized for its influence on Ig turnover and effector functions. However, the large-scale profiling of Ig glycosylation in a biomedical setting is challenged by the existence of different Ig isotypes and subclasses, their varying serum concentrations, and the presence of multiple glycosylation sites per Ig. Here, a high-throughput nanoliquid chromatography (LC)- mass spectrometry (MS)-based method for simultaneous analysis of IgG and IgA glycopeptides was developed and applied on a serum sample set from 185 healthy donors. Sample preparation from minute amounts of serum was performed in 96-well plate format. Prior to trypsin digestion, IgG and IgA were enriched simultaneously, followed by a one-step denaturation, reduction, and alkylation. The obtained nanoLC-MS data were subjected to semiautomated, targeted feature integration and quality control. The combined and simplified protocol displayed high overall method repeatability, as assessed using pooled plasma and serum standards. Taking all samples together, 143 individual N- and O-glycopeptides were reliably quantified. These glycopeptides were attributable to 11 different peptide backbones, derived from IgG1, IgG2/3, IgG4, IgA1, IgA2, and the joining chain from dimeric IgA. Using this method, novel associations were found between IgA N- and O-glycosylation and age. Furthermore, previously reported associations of IgG Fc glycosylation with age in healthy individuals were confirmed. In conclusion, the new method paves the way for high-throughput multiprotein plasma glycoproteomics.

CDG and immune response: From bedside to bench and back

Glycosylation is an essential biological process that adds structural and functional diversity to cells and molecules, participating in physiological processes such as immunity. The immune response is driven and modulated by protein-attached glycans that mediate cell-cell interactions, pathogen recognition and cell activation. Therefore, abnormal glycosylation can be associated with deranged immune responses. Within human diseases presenting immunological defects are congenital disorders of glycosylation (CDG), a family of around 130 rare and complex genetic diseases. In this review, we have identified 23 CDG with immunological involvement, characterized by an increased propensity to-often life-threatening-infection. Inflammatory and autoimmune complications were found in 7 CDG types. CDG natural history(ies) and the mechanisms behind the immunological anomalies are still poorly understood. However, in some cases, alterations in pathogen recognition and intracellular signaling (eg, TGF-β1, NFAT, and NF-κB) have been suggested. Targeted therapies to restore immune defects are only available for PGM3-CDG and SLC35C1-CDG. Fostering research on glycoimmunology may elucidate the involved pathophysiological mechanisms and open new therapeutic avenues, thus improving CDG patients' quality of life.

In Planta Glycan Engineering and Functional Activities of IgE Antibodies

Human immunoglobulin E (IgE) is the most extensively glycosylated antibody isotype so glycans attached to the seven N-glycosites (NGS) in its Fab and Fc domains may modulate its functions. However, targeted modification of glycans in multiply glycosylated proteins remains a challenge. Here, we applied an in vivo approach that allows the manipulation of IgE N-glycans, using a trastuzumab equivalent IgE (HER2-IgE) as a model. Taking advantage of plant inherent features, i.e., synthesis of largely homogeneous complex N-glycans and susceptibility to glycan engineering, we generated targeted glycoforms of HER2-IgE largely resembling those found in serum IgE. Plant-derived HER2-IgE exhibited N-glycans terminating with GlcNAc, galactose or sialic acid, lacking, or carrying core fucose and xylose. We were able to not only modulate the five NGSs naturally decorated with complex N-glycans, but to also induce targeted glycosylation at the usually unoccupied NGS6, thus increasing the overall glycosylation content of HER2-IgE. Recombinant human cell-derived HER2-IgE exhibited large N-glycan heterogeneity. All HER2-IgE variants demonstrated glycosylation-independent binding to the target antigen and the high affinity receptor FcεRI, and subsequent similar capacity to trigger mast cell degranulation. In contrast, binding to the low affinity receptor CD23 (FcεRII) was modulated by the glycan profile, with increased binding to IgE variants with glycans terminating with GlcNAc residues. Here we offer an efficient in planta approach to generate defined glycoforms on multiply glycosylated IgE, allowing the precise exploration of glycosylation-dependent activities.

IgE Glycosylation in Health and Disease

IgE are absolutely required for initiation of allergy reactions, which affect over 20% of the world's population. IgE are the least prevalent immunoglobulins in circulation with 12-h and 2-day half-lives in mouse and human serum, respectively, but an extended tissue half-life of 3-weeks bound to the surface of mast cells by the high affinity IgE receptor, FcεRI (Gould and Sutton 2008). Although the importance of glycosylation to IgG biology is well established, less is known regarding the contribution of IgE glycosylation to allergic inflammation. IgE has seven and nine N-linked glycosylation sites distributed across human and murine constant chains, respectively. Here we discuss studies that have analyzed IgE glycosylation and its function, and how IgE glycosylation contributions to health and disease.

IgE Antibodies: From Structure to Function and Clinical Translation

Immunoglobulin E (IgE) antibodies are well known for their role in mediating allergic reactions, and their powerful effector functions activated through binding to Fc receptors FcεRI and FcεRII/CD23. Structural studies of IgE-Fc alone, and when bound to these receptors, surprisingly revealed not only an acutely bent Fc conformation, but also subtle allosteric communication between the two distant receptor-binding sites. The ability of IgE-Fc to undergo more extreme conformational changes emerged from structures of complexes with anti-IgE antibodies, including omalizumab, in clinical use for allergic disease; flexibility is clearly critical for IgE function, but may also be exploited by allosteric interference to inhibit IgE activity for therapeutic benefit. In contrast, the power of IgE may be harnessed to target cancer. Efforts to improve the effector functions of therapeutic antibodies for cancer have almost exclusively focussed on IgG1 and IgG4 subclasses, but IgE offers an extremely high affinity for FcεRI receptors on immune effector cells known to infiltrate solid tumours. Furthermore, while tumour-resident inhibitory Fc receptors can modulate the effector functions of IgG antibodies, no inhibitory IgE Fc receptors are known to exist. The development of tumour antigen-specific IgE antibodies may therefore provide an improved immune functional profile and enhanced anti-cancer efficacy. We describe proof-of-concept studies of IgE immunotherapies against solid tumours, including a range of in vitro and in vivo evaluations of efficacy and mechanisms of action, as well as ex vivo and in vivo safety studies. The first anti-cancer IgE antibody, MOv18, the clinical translation of which we discuss herein, has now reached clinical testing, offering great potential to direct this novel therapeutic modality against many other tumour-specific antigens. This review highlights how our understanding of IgE structure and function underpins these exciting clinical developments.

An oligosaccharyltransferase from Leishmania major increases the N-glycan occupancy on recombinant glycoproteins produced in Nicotiana benthamiana

N-glycosylation is critical for recombinant glycoprotein production as it influences the heterogeneity of products and affects their biological function. In most eukaryotes, the oligosaccharyltransferase is the central-protein complex facilitating the N-glycosylation of proteins in the lumen of the endoplasmic reticulum (ER). Not all potential N-glycosylation sites are recognized in vivo and the site occupancy can vary in different expression systems, resulting in underglycosylation of recombinant glycoproteins. To overcome this limitation in plants, we expressed LmSTT3D, a single-subunit oligosaccharyltransferase from the protozoan Leishmania major transiently in Nicotiana benthamiana, a well-established production platform for recombinant proteins. A fluorescent protein-tagged LmSTT3D variant was predominately found in the ER and co-located with plant oligosaccharyltransferase subunits. Co-expression of LmSTT3D with immunoglobulins and other recombinant human glycoproteins resulted in a substantially increased N-glycosylation site occupancy on all N-glycosylation sites except those that were already more than 90% occupied. Our results show that the heterologous expression of LmSTT3D is a versatile tool to increase N-glycosylation efficiency in plants.

N- and O-glycosylation Analysis of Human C1-inhibitor Reveals Extensive Mucin-type O-Glycosylation

Human C1-inhibitor (C1-Inh) is a serine protease inhibitor and the major regulator of the contact activation pathway as well as the classical and lectin complement pathways. It is known to be a highly glycosylated plasma glycoprotein. However, both the structural features and biological role of C1-Inh glycosylation are largely unknown. Here, we performed for the first time an in-depth site-specific N- and O-glycosylation analysis of C1-Inh combining various mass spectrometric approaches, including C18-porous graphitized carbon (PGC)-LC-ESI-QTOF-MS/MS applying stepping-energy collision-induced dissociation (CID) and electron-transfer dissociation (ETD). Various proteases were applied, partly in combination with PNGase F and exoglycosidase treatment, in order to analyze the (glyco)peptides. The analysis revealed an extensively O-glycosylated N-terminal region. Five novel and five known O-glycosylation sites were identified, carrying mainly core1-type O-glycans. In addition, we detected a heavily O-glycosylated portion spanning from Thr82-Ser121 with up to 16 O-glycans attached. Likewise, all known six N-glycosylation sites were covered and confirmed by this site-specific glycosylation analysis. The glycoforms were in accordance with results on released N-glycans by MALDI-TOF/TOF-MS/MS. The comprehensive characterization of C1-Inh glycosylation described in this study will form the basis for further functional studies on the role of these glycan modifications.

Recent advances in mass spectrometry-based glycoproteomics

Protein glycosylation plays fundamental roles in many biological processes as one of the most common, and the most complex, posttranslational modification. Alterations in glycosylation profile are now known to be associated with many diseases. As a result, the discovery and detailed characterization of glycoprotein disease biomarkers is a primary interest of biomedical research. Advances in mass spectrometry (MS)-based glycoproteomics and glycomics are increasingly enabling qualitative and quantitative approaches for site-specific structural analysis of protein glycosylation. While the complexity presented by glycan heterogeneity and the wide dynamic range of clinically relevant samples like plasma, serum, cerebrospinal fluid, and tissue make comprehensive analyses of the glycoproteome a challenging task, the ongoing efforts into the development of glycoprotein enrichment, enzymatic digestion, and separation strategies combined with novel quantitative MS methodologies have greatly improved analytical sensitivity, specificity, and throughput. This review summarizes current MS-based glycoproteomics approaches and highlights recent advances in its application to cancer biomarker and neurodegenerative disease research.

Human disease glycomics: technology advances enabling protein glycosylation analysis - part 2

INTRODUCTION

The changes in glycan structures have been attributed to disease states for several decades. The surface glycosylation pattern is a signature of physiological state of a cell. In this review we provide a link between observed substructural glycan changes and a range of diseases. Areas covered: We highlight biologically relevant glycan substructure expression in cancer, inflammation, neuronal diseases and diabetes. Furthermore, the alterations in antibody glycosylation in a disease context are described. Expert commentary: Advances in technologies, as described in Part 1 of this review have now enabled the characterization of specific glycan structural markers of a range of disease states. The requirement of including glycomics in cross-disciplinary omics studies, such as genomics, proteomics, epigenomics, transcriptomics and metabolomics towards a systems glycobiology approach to understanding disease mechanisms and management are highlighted.

Trapping IgE in a closed conformation by mimicking CD23 binding prevents and disrupts FcεRI interaction

Anti-IgE therapeutics interfere with the ability of IgE to bind to its receptors on effector cells. Here we report the crystal structure of an anti-IgE single-domain antibody in complex with an IgE Fc fragment, revealing how the antibody inhibits interactions between IgE and the two receptors FcεRI and CD23. The epitope overlaps only slightly with the FcεRI-binding site but significantly with the CD23-binding site. Solution scattering studies of the IgE Fc reveal that antibody binding induces a half-bent conformation in between the well-known bent and extended IgE Fc conformations. The antibody acts as functional homolog of CD23 and induces a closed conformation of IgE Fc incompatible with FcεRI binding. Notably the antibody displaces IgE from both CD23 and FcεRI, and abrogates allergen-mediated basophil activation and facilitated allergen binding. The inhibitory mechanism might facilitate strategies for the future development of anti-IgE therapeutics for treatment of allergic diseases.

IgE is linked to allergic diseases and there is a great interest in developing anti-IgE therapeutics. Here the authors characterize the binding of human IgE Fc to a single domain antibody (sdab) and show that the sdab induces a closed conformation, which prevents and disrupts IgE binding to its receptor FcεRI and abrogates allergen mediated activation.

Structure and Dynamics of Immunoglobulin G Glycoproteins

Immunoglobulin G (IgG) is a major serum glycoprotein that exerts the role of antibody in the immune system. This multifunctional glycoprotein couples antigen recognition with a variety of effector functions promoted via interactions with various IgG-binding proteins. Given its versatile functionality, IgG has recently been used for therapeutic interventions. Evidence indicates that the carbohydrate moieties of IgG glycoproteins critically affect their antibody functions, particularly the effector functions mediated by the interactions with Fcγ receptors (FcγRs). N-glycans at specific positions of FcγR also contribute both positively and negatively to the interactions with IgG. The integration of multilateral biophysical approaches, including X-ray crystallography, nuclear magnetic resonance spectroscopy, and molecular dynamics simulations, has provided structural insights into the mechanisms underlying the glycofunctions of this interacting system. The N-glycans of IgG and FcγR mediate their interactions by either strengthening or weakening the affinity on the basis of their glycoforms. Moreover, the N-glycosylation of IgG-Fc is a prerequisite to maintain the integrity of the quaternary structure of the sites interacting with the effector molecules and can also control functionally relevant local conformations. The biopharmaceutical significance of these glycan functions is discussed from a structural point of view.

Investigation of O-glycosylation heterogeneity of recombinant coagulation factor IX using LC–MS/MS

Aim: Recombinant coagulation factor IX (rFIX) has extraordinarily multiple post-translational modifications including N-glycosylation and O-glycosylation which have a drastic effect on biological functions and in vivo recovery. Unlike N-glycosylation extensively characterized, there are a few studies on O-glycosylation due to its intrinsic complexity. In-depth O-glycosylation analysis is necessary to better understand and assess pharmacological activity of rFIX. Results: We determined unusual O-glycosylations including O-fucosylation and O-glucosylation which were located at Serine 53 and 61, respectively in EGF domain. Other O-glycosylations bearing core 1 glycan moiety were found on activation peptide. Conclusion: This is the first comprehensive study to characterize O-glycosylation of rFIX using MS-based glycomic and glycoproteomic approaches. Site-specific profiling will be a powerful platform to determine bioequivalence of biosimilars.

Thermal sensitivity and flexibility of the Cε3 domains in immunoglobulin E

Immunoglobulin E (IgE) is the antibody that plays a central role in the mechanisms of allergic diseases such as asthma. Interactions with its receptors, FcεRI on mast cells and CD23 on B cells, are mediated by the Fc region, a dimer of the Cε2, Cε3 and Cε4 domains. A sub-fragment lacking the Cε2 domains, Fcε3–4, also binds to both receptors, although receptor binding almost exclusively involves the Cε3 domains. This domain also contains the N-linked glycosylation site conserved in other isotypes. We report here the crystal structures of IgE-Fc and Fcε3–4 at the highest resolutions yet determined, 1.75 Å and 2.0 Å respectively, revealing unprecedented detail regarding the carbohydrate and its interactions with protein domains. Analysis of the crystallographic B-factors of these, together with all earlier IgE-Fc and Fcε3–4 structures, shows that the Cε3 domains exhibit the greatest intrinsic flexibility and quaternary structural variation within IgE-Fc. Intriguingly, both well-ordered carbohydrate and disordered polypeptide can be seen within the same Cε3 domain. A simplified method for comparing the quaternary structures of the Cε3 domains in free and receptor-bound IgE-Fc structures is presented, which clearly delineates the FcεRI and CD23 bound states. Importantly, differential scanning fluorimetric analysis of IgE-Fc and Fcε3–4 identifies Cε3 as the domain most susceptible to thermally-induced unfolding, and responsible for the characteristically low melting temperature of IgE.

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The Cε3 domains of IgE are most susceptible to thermally induced unfolding determined by differential scanning fluorimetry.

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The Cε3 domains are responsible for the characteristically low melting temperature of IgE.

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The Cε3 domains exhibit the greatest intrinsic flexibility.

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Quaternary structural diversity of Cε3 domains is compared across all known structures using a simplified single parameter.

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Human IgE-Fc and Fcε3–4 domain structures are determined at the highest resolutions yet reported (1.75 Å and 2.0 Å).

Exploring Site-Specific N-Glycosylation of HEK293 and Plant-Produced Human IgA Isotypes

The full potential of recombinant Immunoglobulin A as therapeutic antibody is not fully explored, owing to the fact that structure-function relationships of these extensively glycosylated proteins are not well understood. Here monomeric IgA1, IgA2m(1), and IgA2m(2) variants of the anti-HER2 antibody (IgG1) trastuzumab were expressed in glyco-engineered Nicotiana benthamiana plants and in human HEK293-6E cells. All three IgA isotypes were purified and subjected to biophysical and biochemical characterization. While no differences in assembly, antigen binding, and glycosylation occupancy were observed, both systems vary tremendously in terms of glycan structures and heterogeneity of glycosylation. Mass-spectrometric analysis of site-specific glycosylation revealed that plant-produced IgAs carry mainly complex-type biantennary N-glycans. HEK293-6E-produced IgAs, on the contrary, showed very heterogeneous N-glycans with high levels of sialylation, core-fucose, and the presence of branched structures. The site-specific analysis revealed major differences between the individual N-glycosylation sites of each IgA subtype. Moreover, the proline-rich hinge region from HEK293-6E cell-derived IgA1 was occupied with mucin-type O-glycans, whereas IgA1 from N. benthamiana displayed numerous plant-specific modifications. Interestingly, a shift in unfolding of the CH2 domain of plant-produced IgA toward lower temperatures can be observed with differential scanning calorimetry, suggesting that distinct glycoforms affect the thermal stability of IgAs.

Identification of Fc Gamma Receptor Glycoforms That Produce Differential Binding Kinetics for Rituximab

Fc gamma receptors (FcγR) bind the Fc region of antibodies and therefore play a prominent role in antibody-dependent cell-based immune responses such as ADCC, CDC and ADCP. The immune effector cell activity is directly linked to a productive molecular engagement of FcγRs where both the protein and glycan moiety of antibody and receptor can affect the interaction and in the present study we focus on the role of the FcγR glycans in this interaction. We provide a complete description of the glycan composition of Chinese hamster ovary (CHO) expressed human Fcγ receptors RI (CD64), RIIa_{Arg131/His131} (CD32a), RIIb (CD32b) and RIIIa_{Phe158/Val158} (CD16a) and analyze the role of the glycans in the binding mechanism with IgG. The interactions of the monoclonal antibody rituximab with each FcγR were characterized and we discuss the CHO-FcγRIIIa_{Phe158/Val158} and CHO-FcγRI interactions and compare them to the equivalent interactions with human (HEK293) and murine (NS0) produced receptors. Our results reveal clear differences in the binding profiles of rituximab, which we attribute in each case to the differences in host cell-dependent FcγR glycosylation. The glycan profiles of CHO expressed FcγRI and FcγRIIIa_{Phe158/Val158} were compared with the glycan profiles of the receptors expressed in NS0 and HEK293 cells and we show that the glycan type and abundance differs significantly between the receptors and that these glycan differences lead to the observed differences in the respective FcγR binding patterns with rituximab. Oligomannose structures are prevalent on FcγRI from each source and likely contribute to the high affinity rituximab interaction through a stabilization effect. On FcγRI and FcγRIIIa large and sialylated glycans have a negative impact on rituximab binding, likely through destabilization of the interaction. In conclusion, the data show that the IgG1-FcγR binding kinetics differ depending on the glycosylation of the FcγR and further support a stabilizing role of FcγR glycans in the antibody binding interaction.

AllergoOncology - the impact of allergy in oncology: EAACI position paper

Th2 immunity and allergic immune surveillance play critical roles in host responses to pathogens, parasites and allergens. Numerous studies have reported significant links between Th2 responses and cancer, including insights into the functions of IgE antibodies and associated effector cells in both antitumour immune surveillance and therapy. The interdisciplinary field of AllergoOncology was given Task Force status by the European Academy of Allergy and Clinical Immunology in 2014. Affiliated expert groups focus on the interface between allergic responses and cancer, applied to immune surveillance, immunomodulation and the functions of IgE-mediated immune responses against cancer, to derive novel insights into more effective treatments. Coincident with rapid expansion in clinical application of cancer immunotherapies, here we review the current state-of-the-art and future translational opportunities, as well as challenges in this relatively new field. Recent developments include improved understanding of Th2 antibodies, intratumoral innate allergy effector cells and mediators, IgE-mediated tumour antigen cross-presentation by dendritic cells, as well as immunotherapeutic strategies such as vaccines and recombinant antibodies, and finally, the management of allergy in daily clinical oncology. Shedding light on the crosstalk between allergic response and cancer is paving the way for new avenues of treatment.

Recombinant plant-derived human IgE glycoproteomics

The increasing biotechnological interest in human IgE antibodies demands advanced systems which allow their proper expression. However, this is still a challenge due to the complexity of the molecule, particularly regarding the diverse N-glycosylation pattern. Here, we present the expression of recombinant IgE in wild type and glycan-engineered Nicotiana benthamiana plants and in-depth N-glycosylation analyses. Mass spectrometric profiling revealed that plant IgE has a site occupancy rate that ranges from non-occupied at glycosite 6 (GS6) to 100% occupancy at GS1 and 2. Similarly to human cell-derived IgE, plant versions carry complex N-glycans at GS1-5 and oligomannosidic structures at GS7. Computational modelling suggests that spatial position (or orientation) of glycans can impair processing or site occupancy on adjacent glycosites. IgE expressed in glycoengineered and wild type plants carry, respectively, GnGn and plant-typical GnGnXF structures at large homogeneity. This contrasts with the glycan diversity of HEK cell-derived IgE, carrying at least 20 different glycoforms. Importantly, IgE glycoengineering allows the control of its glycosylation, a so far unmet need when using well-established expression systems. This enables the elucidation of possible carbohydrate-dependent IgE functions.

SIGNIFICANCE

Targeted glycosylation of recombinant proteins may provide an advantage in therapeutic applications. Despite increasing biotechnological interest in IgE antibodies, knowledge and impact of glycosylation on this antibody class are scarce. With the ability to glyco-engineer recombinant IgE, we provide an important step towards the generation of IgE with other targeted N-glycans. This will facilitate detailed structure-function studies and may lead to the production of IgE with optimized activities.

IgE binds asymmetrically to its B cell receptor CD23

The antibody IgE plays a central role in allergic disease mechanisms. Its effector functions are controlled through interactions between the Fc region and two principal cell surface receptors FcεRI and CD23. The interaction with FcεRI is primarily responsible for allergic sensitization and the inflammatory response, while IgE binding to CD23 is involved in the regulation of IgE synthesis and allergen transcytosis. Here we present the crystal structure of a CD23/IgE-Fc complex and conduct isothermal titration calorimetric binding studies. Two lectin-like "head" domains of CD23 bind to IgE-Fc with affinities that differ by more than an order of magnitude, but the crystal structure reveals only one head bound to one of the two identical heavy-chains in the asymmetrically bent IgE-Fc. These results highlight the subtle interplay between receptor binding sites in IgE-Fc and their affinities, the understanding of which may be exploited for therapeutic intervention in allergic disease.