Hinge-Region O-Glycosylation of Human Immunoglobulin G3 (IgG3)

The importance of IgG glycosylation-What did we learn after analyzing over 100,000 individuals

All four subclasses of immunoglobulin G (IgG) antibodies have glycan structures attached to the protein part of the IgG molecules. Glycans linked to the Fc portion of IgG are found in all IgG antibodies, while about one-fifth of IgG antibodies in plasma also have glycans attached to the Fab portion of IgG. The IgG3 subclass is characterized by more complex glycosylation compared to other IgG subclasses. In this review, we discuss the significant influence that glycans exert on the structural and functional properties of IgG. We provide a comprehensive overview of how the composition of these glycans can affect IgG's effector functions by modulating its interactions with Fcγ receptors and other molecules such as the C1q component of complement, which in turn influence various immune responses triggered by IgG, including antibody-dependent cell-mediated cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC). In addition, the importance of glycans for the efficacy of therapeutics like monoclonal antibodies and intravenous immunoglobulin (IVIg) therapy is discussed. Moreover, we offer insights into IgG glycosylation characteristics and roles derived from general population, disease-specific, and interventional studies. These studies indicate that IgG glycans are important biomarkers and functional effectors in health and disease.

Functional comparison of Fc-engineering strategies to improve anti-HIV-1 antibody effector functions

Substantial reduction of the intact proviral reservoir is essential towards HIV-1 cure. In vivo administration of broadly neutralizing antibodies (bNAbs) targeting the HIV-1 envelope glycoprotein (Env) trimer can decrease the viral reservoir, through Fc-mediated killing of infected cells. In this study, we compared three commonly used antibody engineering strategies to enhance Fc-mediated effector functions: (i) glyco-engineering, (ii) protein engineering, and (iii) subclass/hinge modifications in a panel of anti-HIV-1 antibodies. We found that antibody-dependent cellular phagocytosis (ADCP) was improved by elongating the hinge domain and switching to an IgG3 constant domain. In addition, potent NK cell activation and ADCC activity was observed for afucosylated antibodies and antibodies bearing the GASDALIE mutations. The combination of these engineering strategies further increased NK cell activation and induced antibody dependent cytotoxicity (ADCC) of infected cells at low antibody concentrations. The bNAb N6 was most effective at killing HIV-1 infected cells, likely due to its high affinity and optimal angle of approach. Overall, the findings of this study are applicable to other antibody formats, and can aid the development of effective immunotherapies and antibody-based treatments for HIV-1 cure strategies.

A First Insight into the Developability of an Immunoglobulin G3: A Combined Computational and Experimental Approach

Immunoglobulin G 3 (IgG3) monoclonal antibodies (mAbs) are high-value scaffolds for developing novel therapies. Despite their wide-ranging therapeutic potential, IgG3 physicochemical properties and developability characteristics remain largely under-characterized. Protein-protein interactions elevate solution viscosity in high-concentration formulations, impacting physicochemical stability, manufacturability, and the injectability of mAbs. Therefore, in this manuscript, the key molecular descriptors and biophysical properties of a model anti-IL-8 IgG1 and its IgG3 ortholog are characterized. A computational and experimental framework was applied to measure molecular descriptors impacting their downstream developability. Findings from this approach underpin a detailed understanding of the molecular characteristics of IgG3 mAbs as potential therapeutic entities. This work is the first report examining the manufacturability of IgG3 for high-concentration mAb formulations. While poorer conformational and colloidal stability and elevated solution viscosity were observed for IgG3, future efforts controlling surface potential through sequence-engineering of solvent-accessible patches can be used to improve biophysical parameters that dictate mAb developability.

The genetics and epidemiology of N- and O-immunoglobulin A glycomics

BACKGROUND

Immunoglobulin (Ig) glycosylation modulates the immune response and plays a critical role in ageing and diseases. Studies have mainly focused on IgG glycosylation, and little is known about the genetics and epidemiology of IgA glycosylation.

METHODS

We generated, using a novel liquid chromatography-mass spectrometry method, the first large-scale IgA glycomics dataset in serum from 2423 twins, encompassing 71 N- and O-glycan species.

RESULTS

We showed that, despite the lack of a direct genetic template, glycosylation is highly heritable, and that glycopeptide structures are sex-specific, and undergo substantial changes with ageing. We observe extensive correlations between the IgA and IgG glycomes, and, exploiting the twin design, show that they are predominantly influenced by shared genetic factors. A genome-wide association study identified eight loci associated with both the IgA and IgG glycomes (ST6GAL1, ELL2, B4GALT1, ABCF2, TMEM121, SLC38A10, SMARCB1, and MGAT3) and two novel loci specifically modulating IgA O-glycosylation (C1GALT1 and ST3GAL1). Validation of our findings in an independent cohort of 320 individuals from Qatar showed that the underlying genetic architecture is conserved across ancestries.

CONCLUSIONS

Our study delineates the genetic landscape of IgA glycosylation and provides novel potential functional links with the aetiology of complex immune diseases, including genetic factors involved in IgA nephropathy risk.

Immunoglobulin G glycosylation and its alterations in aging-related diseases

Immunoglobulin G (IgG) is an important serum glycoprotein and a major component of antibodies. Glycans on IgG affect the binding of IgG to the Fc receptor or complement C1q, which in turn affects the biological activity and biological function of IgG. Altered glycosylation patterns on IgG emerge as important biomarkers in the aging process and age-related diseases. Key aging-related alterations observed in IgG glycosylation include reductions in galactosylation and sialylation, alongside increases in agalactosylation, and bisecting GlcNAc. Understanding the role of IgG glycosylation in aging-related diseases offers insights into disease mechanisms and provides opportunities for the development of diagnostic and therapeutic strategies. This review summarizes five aspects of IgG: an overview of IgG, IgG glycosylation, IgG glycosylation with inflammation mediation, IgG glycan changes with normal aging, as well as the relevance of IgG glycan changes to aging-related diseases. This review provides a reference for further investigation of the regulatory mechanisms of IgG glycosylation in aging-related diseases, as well as for evaluating the potential of IgG glycosylation changes as markers of aging and aging-related diseases.

Engineering Agonistic Bispecifics to Investigate the Influence of Distance on Surface-Mediated Complement Activation

The development of agonists capable of activating the human complement system by binding to the C1 complex presents a novel approach for targeted cell killing. Bispecific nanobodies and Abs can successfully use C1 for this purpose; however, efficacy varies significantly between epitopes, Ab type, and bispecific design. To address this variability, we investigated monomeric agonists of C1 in the form of bispecific nanobodies, which lack Fc domains that lead to oligomerization in Abs. These therefore offer an ideal opportunity to explore the geometric parameters crucial for C1 activation. In this study, we explored the impact of linker length as a metric for Ag and epitope location. DNA nanotechnology and protein engineering allowed us to design linkers with controlled lengths and flexibilities, revealing a critical range of end-to-end distances for optimal complement activation. We discovered that differences in complement activation were not caused by differential C1 activation or subsequent cleavage of C4, but instead impacted C4b deposition and downstream membrane lysis. Considering the importance of Ab class and subclass, this study provides insights into the structural requirements of C1 binding and activation, highlighting linker and hinge engineering as a potential strategy to enhance potency over specific cellular targets. Additionally, using DNA nanotechnology to modify geometric parameters demonstrated the potential for synthetic biology in complement activation. Overall, this research offers valuable insights into the design and optimization of agonists for targeted cell killing through complement activation.

Escherichia coli and Pichia pastoris: microbial cell-factory platform for -full-length IgG production

Owing to the unmet demand, the pharmaceutical industry is investigating an alternative host to mammalian cells to produce antibodies for a variety of therapeutic and research applications. Regardless of some disadvantages, Escherichia coli and Pichia pastoris are the preferred microbial hosts for antibody production. Despite the fact that the production of full-length antibodies has been successfully demonstrated in E. coli, which has mostly been used to produce antibody fragments, such as: antigen-binding fragments (Fab), single-chain fragment variable (scFv), and nanobodies. In contrast, Pichia, a eukaryotic microbial host, is mostly used to produce glycosylated full-length antibodies, though hypermannosylated glycan is a major challenge. Advanced strategies, such as the introduction of human-like glycosylation in endotoxin-edited E. coli and cell-free system-based glycosylation, are making progress in creating human-like glycosylation profiles of antibodies in these microbes. This review begins by explaining the structural and functional requirements of antibodies and continues by describing and analyzing the potential of E. coli and P. pastoris as hosts for providing a favorable environment to create a fully functional antibody. In addition, authors compare these microbes on certain features and predict their future in antibody production. Briefly, this review analyzes, compares, and highlights E. coli and P. pastoris as potential hosts for antibody production.

Are Changes in Serum IgG Glycosylation Related to the Severe Course of SARS-CoV-2 Infection and Recovery Process? In Search of New Diagnostic and Prognostic Biomarkers

Introduction

Immunoglobulin G (IgG) glycosylation affects its effector functions and is essential in many steps of the inflammatory cascade. Therefore, it may be an important parameter for assessing the body's immune response during the course of COVID-19 (Coronavirus disease 2019).

Methods

The N- and O-glycosylation of serum IgG in severe COVID-19 patients (n=87), convalescents (n=50), and healthy subjects (n=65) were examined using a modified lectin-ELISA method with specific biotinylated lectins. The obtained data were analyzed using STATISTICA 13.3PL software.

Results

We showed significantly higher expression of Lewisx oligosaccharide structures in severe COVID-19 patients than in the other two groups. Moreover, significantly lower expression of Lewisy sugar structures in IgG glycans was observed in the convalescents when compared with COVID-19 patients and healthy subjects. The lowest expression of highly branched N-glycans in cases of severe COVID-19 indicates that the development of the disease is associated with the presence of typical IgG biantennary N-glycans. The lack of significant differences in the expression of Tn antigen in IgG between studied groups and the significantly lower expression of T antigen in convalescents compared to the patients with severe COVID-19 and healthy subjects indicates a decrease in the content of the T antigen in IgG O-glycans in subjects recovered from COVID-19. Substantially higher reactivities of IgG O-glycans with Jacalin observed in COVID-19 patients and convalescents in comparison to the control group were most probably caused by increased expression of core 3 O-glycans in IgG.

Conclusion

Severe COVID-19 is accompanied by the expression in serum IgG of sialylated biantennary and highly branched N-glycans, decorated by fucose of Lewisx and Lewisy structures. The higher reactivity of IgG O-glycans with Jacalin in severe COVID-19 patients and convalescents indicates that the disease development and the recovery process are most probably accompanied by increased expression of the core 3 O-glycans.

N-linked Fc glycosylation is not required for IgG-B-cell receptor function in a GC-derived B-cell line

IgG secreted by B cells carry asparagine N(297)-linked glycans in the fragment crystallizable (Fc) region. Changes in Fc glycosylation are related to health or disease and are functionally relevant, as IgG without Fc glycans cannot bind to Fcɣ receptors or complement factors. However, it is currently unknown whether ɣ-heavy chain (ɣHC) glycans also influence the function of membrane-bound IgG-B-cell receptors (BCR) and thus the outcome of the B-cell immune response. Here, we show in a germinal center (GC)-derived human B-cell line that ɣHC glycans do not affect membrane expression of IgG-BCRs. Furthermore, antigen binding or other BCR-facilitated mechanisms appear unaffected, including BCR downmodulation or BCR-mediated signaling. As expected, secreted IgG lacking Fc glycosylation is unable to carry out effector functions. Together, these observations indicate that IgG-Fc glycosylation serves as a mechanism to control the effector functions of antibodies, but does not regulate the activation of IgG-switched B cells, as its absence had no apparent impact on BCR function.

Impact of structural modifications of IgG antibodies on effector functions

Immunoglobulin G (IgG) antibodies are a critical component of the adaptive immune system, binding to and neutralizing pathogens and other foreign substances. Recent advances in molecular antibody biology and structural protein engineering enabled the modification of IgG antibodies to enhance their therapeutic potential. This review summarizes recent progress in both natural and engineered structural modifications of IgG antibodies, including allotypic variation, glycosylation, Fc engineering, and Fc gamma receptor binding optimization. We discuss the functional consequences of these modifications to highlight their potential for therapeutical applications.

The Influence of Human IgG Subclass and Allotype on Complement Activation

Complement activation via the classical pathway is initiated when oligomeric Igs on target surfaces are recognized by C1 of the complement cascade. The strength of this interaction and activation of the complement system are influenced by structural variation of the Ab, including Ab isotype, subclass, and glycosylation profile. Polymorphic variants of IgG have also been described to influence Fc-dependent effector functions. Therefore, we assessed complement binding, deposition, and complement-dependent cytotoxicity (CDC) of 27 known IgG allotypes with anti-trinitrophenyl specificity. Differences between allotypes within subclasses were minor for IgG1, IgG3, and IgG4 allotypes, and more substantial for IgG2. Allelic variant IGHG2*06, containing a unique serine at position 378 in the CH3 domain, showed less efficient complement activation and CDC compared with other IgG2 polymorphisms. We also observed variable cell lysis between IgG1 and IgG3, with IgG3 being superior in lysis of human RBCs and Ramos cells, and IgG1 being superior in lysis of Raji and Wien133 cells, demonstrating that a long-standing conundrum in the literature depends on cellular context. Furthermore, we compared IgG1 and IgG3 under different circumstances, showing that Ag density and Ab hinge length, but not complement regulators, define the context dependency of Ab-mediated CDC activity. Our results point toward a variation in the capacity of IgG subclasses to activate complement due to single amino acid changes and hinge length differences of allotypes to activate complement, which might give new insights on susceptibility to infectious, alloimmune, or autoimmune diseases and aid the design of Ab-based therapeutics.

Modulating antibody effector functions by Fc glycoengineering

Antibody based drugs, including IgG monoclonal antibodies, are an expanding class of therapeutics widely employed to treat cancer, autoimmune and infectious diseases. IgG antibodies have a conserved N-glycosylation site at Asn297 that bears complex type N-glycans which, along with other less conserved N- and O-glycosylation sites, fine-tune effector functions, complement activation, and half-life of antibodies. Fucosylation, galactosylation, sialylation, bisection and mannosylation all generate glycoforms that interact in a specific manner with different cellular antibody receptors and are linked to a distinct functional profile. Antibodies, including those employed in clinical settings, are generated with a mixture of glycoforms attached to them, which has an impact on their efficacy, stability and effector functions. It is therefore of great interest to produce antibodies containing only tailored glycoforms with specific effects associated with them. To this end, several antibody engineering strategies have been developed, including the usage of engineered mammalian cell lines, in vitro and in vivo glycoengineering.

Complement is activated by elevated IgG3 hexameric platforms and deposits C4b onto distinct antibody domains

IgG3 is unique among the IgG subclasses due to its extended hinge, allotypic diversity and enhanced effector functions, including highly efficient pathogen neutralisation and complement activation. It is also underrepresented as an immunotherapeutic candidate, partly due to a lack of structural information. Here, we use cryoEM to solve structures of antigen-bound IgG3 alone and in complex with complement components. These structures reveal a propensity for IgG3-Fab clustering, which is possible due to the IgG3-specific flexible upper hinge region and may maximise pathogen neutralisation by forming high-density antibody arrays. IgG3 forms elevated hexameric Fc platforms that extend above the protein corona to maximise binding to receptors and the complement C1 complex, which here adopts a unique protease conformation that may precede C1 activation. Mass spectrometry reveals that C1 deposits C4b directly onto specific IgG3 residues proximal to the Fab domains. Structural analysis shows this to be caused by the height of the C1-IgG3 complex. Together, these data provide structural insights into the role of the unique IgG3 extended hinge, which will aid the development and design of upcoming immunotherapeutics based on IgG3.

Retrospective screening of serum IgG glycosylation biomarker for primary Sjögren's syndrome using lectin microarray

Background

Primary Sjögren's syndrome (PSS) is a systemic autoimmune disease resulting in significant loss of systemic gland secretory function. IgG glycosylation abnormalities had been found to play important roles in autoimmune diseases. Here, we aim to explore the specific changes of IgG glycosylation in PSS patient serum that could serve as potential biomarkers for disease diagnosis and differential diagnosis.

Method

From 2012 to 2018, patients diagnosed with PSS or primary biliary cholangitis (PBC) admitted consecutively to the department of Rheumatology at Peking Union Medical College Hospital were retrospectively included in this study. Glycan profiles of serum IgG from 40 PSS patients, 50 PBC patients, and 38 healthy controls were detected with lectin microarray containing 56 lectins. Lectins with significantly different signal intensity among groups were selected and validated by lectin blot assay.

Results

Lectin microarray analysis revealed that binding levels of Amaranthus Caudatus Lectin (ACL, prefers glycan Galβ3GalNAc, P = 0.011), Morniga M Lectin (MNA-M, prefers glycan mannose. P = 0.013), and Lens Culinaris Agglutinin (LCA, prefers glycan fucose) were significantly increased, while Salvia sclarea Agglutinin (SSA, prefers glycan sialylation, P = 0.001) was significantly decreased in PSS patients compared to PBC group. Compared to healthy controls, MNA-M (P = 0.001) and LCA (P = 0.028) were also significantly increased, while Phaseolus Vulgaris Erythroagglutinin and Phaseolus Vulgaris Leucoagglutinin (PHA-E and PHA-L, prefer glycan galactose, P = 0.004 and 0.006) were significantly decreased in PSS patients. The results of LCA and MNA-M were further confirmed using lectin blot assay.

Conclusion

Changes in serum IgG glycosylation in PSS increased binding levels of LCA and MNA-M lectins using microarray techniques compared to PBC patients and healthy controls, which could provide potential diagnostic value. Increased core fucose and mannose alteration of IgG may play important roles in PSS disease.

Altered glycosylation profiles of serum IgG in Takayasu arteritis

BACKGROUND

Takayasu arteritis (TAK) is an autoimmune inflammatory disorder with an undefined etiology. This study aimed to characterize the glycosylation profiles of serum immunoglobulin G (IgG) in patients with TAK.

METHODS

Lectin microarrays containing 56 types of lectins were used to detect the glycan levels of serum IgG in 164 patients with TAK, 128 patients with atherosclerosis used as disease controls (DCs), and 100 healthy controls (HCs). Differentially altered glycosylation patterns between TAK and control groups as well as between TAK subgroups were identified and further validated by lectin blot. The classification performance of the TAK-specific glycosylation change was measured by receiver-operating characteristic (ROC) curve analysis.

RESULTS

Lectin microarray analysis revealed significantly increased N-Acetylgalactosamine (GalNAc) levels in the TAK group compared to the DC and HC groups (all p < 0.01). For TAK subgroups, significantly decreased mannosylation was observed in patients with active TAK compared to patients with inactive disease (p < 0.01). These differences were validated by lectin blot. In addition, GalNAc levels exhibited a considerable potential for discriminating patients with TAK from patients with atherosclerosis, with an area under the curve of 0.749 (p < 0.001), a sensitivity of 71.7%, and a specificity of 73.8%.

CONCLUSIONS

Serum IgG in patients with TAK displayed disease-specific glycosylation alterations. Aberrant GalNAc glycosylation showed substantial value as a diagnostic biomarker. The potential proinflammatory properties of the abnormal glycans may provide new insights into the role of humoral immunity in the pathogenesis of TAK.

The relationship between chronic immune response and neurodegenerative damage in long COVID-19

In the past two years, the world has faced the pandemic caused by the severe acute respiratory syndrome 2 coronavirus (SARS-CoV-2), which by August of 2022 has infected around 619 million people and caused the death of 6.55 million individuals globally. Although SARS-CoV-2 mainly affects the respiratory tract level, there are several reports, indicating that other organs such as the heart, kidney, pancreas, and brain can also be damaged. A characteristic observed in blood serum samples of patients suffering COVID-19 disease in moderate and severe stages, is a significant increase in proinflammatory cytokines such as interferon-α (IFN-α), interleukin-1β (IL-1β), interleukin-2 (IL-2), interleukin-6 (IL-6) and interleukin-18 (IL-18), as well as the presence of autoantibodies against interferon-α (IFN-α), interferon-λ (IFN-λ), C-C motif chemokine ligand 26 (CCL26), CXC motif chemokine ligand 12 (CXCL12), family with sequence similarity 19 (chemokine (C-C motif)-like) member A4 (FAM19A4), and C-C motif chemokine ligand 1 (CCL1). Interestingly, it has been described that the chronic cytokinemia is related to alterations of blood-brain barrier (BBB) permeability and induction of neurotoxicity. Furthermore, the generation of autoantibodies affects processes such as neurogenesis, neuronal repair, chemotaxis and the optimal microglia function. These observations support the notion that COVID-19 patients who survived the disease present neurological sequelae and neuropsychiatric disorders. The goal of this review is to explore the relationship between inflammatory and humoral immune markers and the major neurological damage manifested in post-COVID-19 patients.

Exploration of quantitative site-specific serum O-glycoproteomics with isobaric labeling for the discovery of putative O-glycoprotein biomarkers

PURPOSE

Exploration study of site-specific isobaric-TMT-labeling quantitative serum O-glycoproteomics for the discovery of putative O-glycoprotein cancer biomarkers.

EXPERIMENTAL DESIGN

Sera of 10 breast cancer patients was used as the exploration cohort. More abundant N-glycosylation was first removed with PNGase F. After tryptic digestion of de-N-glycosylated serum proteome, the TMT-labeled O-glycopeptides mixture was prepared and analyzed with RPLC-MS/MS. Site-specific qualitative and quantitative database search of O-glycopeptides was carried out with pGlyco 3.0. The same raw datasets were also searched with intact N-glycopeptide search engine GPSeeker to exclude possible interference of N-glycosylation. The final IDs were checked manually with GlcNAc-containing glycosite-determining fragment ions for confirmation.

RESULTS

With the control of spectrum-level FDR ≤ 1% and manual validation, 299 O-glycopeptides corresponding to 83 O-glycosites and 66 O-glycoproteins were identified, and 13 O-glycopeptides were found differentially expressed. Most interestingly, differential O-glycosylation was observed for IgG1 and IgG3, which is an interesting putative biomarker panel.

CONCLUSION AND CLINICAL RELEVANCE

Isobaric-labeling site-specific quantitative O-glycoproteomics is currently a state-of-the-art instrumental platform for discovery of putative seral cancer biomarkers. Differential seral O-glycosylation was observed in the IgG1 and IgG3.

Antibody class-switching as a strategy to improve HIV-1 neutralization

Broadly neutralizing antibodies (bNAbs), when administered through passive immunization, are protective against HIV-1 infection. Current HIV-1 vaccine strategies are aimed at guiding the immune system to make bNAbs by mimicking their development during infection. Somatic hypermutation of the variable region is known to be crucial for the development of bNAbs. More recently, however, studies have shown how class-switch recombination (CSR) resulting in the generation of different antibody isotypes may serve as an additional mechanism through which antibodies can gain neutralization breadth and potency. In this review, we discuss the importance of different antibody isotypes for HIV-1 neutralization breadth and potency and how this information can be leveraged to improve passive and active immunization against HIV-1.

Direct Detection of Glycated Human Serum Albumin and Hyperglycosylated IgG3 in Serum, by MALDI-ToF Mass Spectrometry, as a Predictor of COVID-19 Severity

The prefusion spike protein of SARS-CoV-2 binds advanced glycation end product (AGE)-glycated human serum albumin (HSA) and a higher mass (hyperglycosylated/glycated) immunoglobulin (Ig) G3, as determined by matrix assisted laser desorption mass spectrometry (MALDI-ToF). We set out to investigate if the total blood plasma of patients who had recovered from acute respiratory distress syndrome (ARDS) as a result of COVID-19, contained more glycated HSA and higher mass (glycosylated/glycated) IgG3 than those with only clinically mild or asymptomatic infections. A direct serum dilution, and disulphide bond reduction, method was developed and applied to plasma samples from SARS-CoV-2 seronegative (n = 30) and seropositive (n = 31) healthcare workers (HCWs) and 38 convalescent plasma samples from patients who had been admitted with acute respiratory distress (ARDS) associated with COVID-19. Patients recovering from COVID-19 ARDS had significantly higher mass AGE-glycated HSA and higher mass IgG3 levels. This would indicate that increased levels and/or ratios of hyper-glycosylation (probably terminal sialic acid) IgG3 and AGE glycated HSA may be predisposition markers for the development of COVID-19 ARDS as a result of SARS-CoV2 infection. Furthermore, rapid direct analysis of serum/plasma samples by MALDI-ToF for such humoral immune correlates of COVID-19 presents a feasible screening technology for the most at risk; regardless of age or known health conditions.

Enhanced neutralization potency of an identical HIV neutralizing antibody expressed as different isotypes is achieved through genetically distinct mechanisms

Antibodies with the same variable region can exist as multiple isotypes with varying neutralization potencies, though the mechanism for this is not fully defined. We previously isolated an HIV-directed IgA1 monoclonal antibody (mAb), CAP88-CH06, and showed that IgA1 and IgG3 isotypes of this antibody demonstrated enhanced neutralization compared to IgG1. To explore the mechanism behind this, hinge region and constant heavy chain (CH1) chimeras were constructed between the IgA1, IgG3 and IgG1 mAbs and assessed for neutralization activity, antibody-dependent cellular phagocytosis (ADCP) and antibody-dependent cellular cytotoxicity (ADCC). Hinge chimeras revealed that the increased neutralization potency and phagocytosis of the IgG3 isotype was attributed to its longer hinge region. In contrast, for IgA1, CH1 chimeras showed that this region was responsible both for enhanced neutralization potency and decreased ADCP, though ADCC was not affected. Overall, these data show that the enhanced neutralization potency of CAP88-CH06 IgG3 and IgA1, compared to IgG1, is achieved through distinct mechanisms. Understanding the influence of the hinge and CH1 regions on Fab domain function may provide insights into the engineering of therapeutic antibodies with increased neutralization potency.

Changes in Serum IgG Glycosylation Patterns for Abdominal Aortic Aneurysm Patients

Background: B cells and autoantibodies play an important role in the pathogenesis of abdominal aortic aneurysm (AAA). IgG glycosylations are highly valued as potential disease biomarkers and therapeutic targets. Methods: Lectin microarray was applied to analyze the expression profile of serum IgG glycosylation in 75 patients with AAA, 68 autoimmune disease controls, and 100 healthy controls. Lectin blots were performed to validate the differences. The clinical relevance of lectins binding from the microarray results was explored in AAA patients. Results: Significantly lower binding level of SBA (preferred GalNAc) was observed for the AAA group compared with DCs (p < 0.001) and HCs (p = 0.049). A significantly lower binding level of ConA (preferred mannose) was observed in patients with aneurysm diameter >5 cm. Significantly higher binding of CSA (preferred GalNAc) was present for dyslipidemia patients, whereas a lower binding level of AAL (preferred fucose) was observed for hypertensive patients. Patients with diabetes had lower binding levels of IRA (preferred GalNAc) and HPA (preferred GalNAc) compared with those not with DM. PTL-L (R = 0.36, p = 0.0015, preferred GalNAc) was positively associated with aneurysm diameters, whereas DSL (R = 0.28, p = 0.014, preferred (GlcNAc)2-4) was positively associated with patients’ age. Symptomatic patients had a lower binding level of ConA (p = 0.032), and patients with coronary heart disease had higher binding levels of STL (p = 0.0029, preferred GlcNAc). Patients with ILT bound less with black bean crude (p = 0.04, preferred GalNAc). Conclusions: AAA was associated with a decreased IgG binding level of SBA (recognizing glycan GalNAc). Symptomatic patients with aneurysm <5 cm had a higher binding level of ConA (preferred mannose). Coronary heart disease and elder age were associated with increased IgG bisecting GlcNAc. IgG O-glycosylation (GalNAc) may play an important role in AAA pathogenesis and progression.

O-Glycosylation Changes in Serum Immunoglobulin G Are Associated with Inflammation Development in Advanced Endometriosis

Endometriosis is a gynecological disease, the pathogenesis of which seems to be directly related to inflammatory processes with an immune basis. Our study aimed to analyze the O-glycosylation of native serum IgG and IgG isolated from sera of women with advanced endometriosis, without endometriosis but with benign gynecological diseases, and from a control group of healthy women, in the context of its utility for differentiation of advanced endometriosis from the other two groups of women studied. For the analysis of serum IgG O-glycosylation and the expression of multi-antennary N-glycans, lectin-ELISA with lectins specific to O-glycans (MPL, VVL, and Jacalin) and highly branched N-glycans (PHA-L) was used. The relative reactivities of isolated serum IgG O-linked glycans with specific lectins as well as the MPL/VVL O-glycosylation ratio were significantly higher in patients with advanced endometriosis and those with other gynecological diseases when compared to the control group of healthy women. We also showed significantly higher expression of PHA-L-reactive multi-antennary N-glycans in isolated IgG in the advanced endometriosis and the non-endometriosis groups in comparison to the control group. Additionally, significantly higher expression of Jacalin-reactive O-glycans in isolated IgG was observed in the non-endometriosis than in the advanced endometriosis group. The results of the ROC curve and cluster analysis additionally confirmed that the lectin-based analysis of isolated serum IgG O-glycosylation and the expression of highly branched N-glycans may help distinguish women with advanced endometriosis from healthy women. Moreover, the analysis of the expression of Jacalin-reactive i-IgG O-glycans may be helpful in differentiation between women with advanced endometriosis and patients with other gynecological diseases with an inflammatory background. In the case of non-endometriosis patients, the observed differences were most probably caused by increased expression of core 3 type O-glycans.

Posttranslational modifications optimize the ability of SARS-CoV-2 spike for effective interaction with host cell receptors

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spike glycoprotein is the prime target for vaccines, diagnostics, and therapeutic antibodies against the virus. While anchored in the viral envelope, for effective virulence, the spike needs to maintain structural flexibility to recognize the host cell surface receptors and bind to them, a property that can heavily depend upon the dynamics of the unresolved domains, most prominently the stalk. Construction of the complete, membrane-bound spike model and the description of its dynamics are critical steps in understanding the inner working of this key element of the viral infection by SARS-CoV-2. Combining homology modeling, protein-protein docking, and molecular dynamics (MD) simulations, we have developed a full spike structure in a native membrane. Multimicrosecond MD simulations of this model, the longest known single trajectory of the full spike, reveal conformational dynamics employed by the protein to explore the surface of the host cell. In agreement with cryogenic electron microscopy (cryo-EM), three flexible hinges in the stalk allow for global conformational heterogeneity of spike in the fully glycosylated system mediated by glycan-glycan and glycan-lipid interactions. The dynamical range of the spike is considerably reduced in its nonglycosylated form, confining the area explored by the spike on the host cell surface. Furthermore, palmitoylation of the membrane domain amplifies the local curvature that may prime the fusion. We show that the identified hinge regions are highly conserved in SARS coronaviruses, highlighting their functional importance in enhancing viral infection, and thereby, provide points for discovery of alternative therapeutics against the virus.

Determination of IgG1 and IgG3 SARS-CoV-2 Spike Protein and Nucleocapsid Binding-Who Is Binding Who and Why?

The involvement of immunoglobulin (Ig) G3 in the humoral immune response to SARS-CoV-2 infection has been implicated in the pathogenesis of acute respiratory distress syndrome (ARDS) in COVID-19. The exact molecular mechanism is unknown, but it is thought to involve this IgG subtype’s differential ability to fix, complement and stimulate cytokine release. We examined the binding of convalescent patient antibodies to immobilized nucleocapsids and spike proteins by matrix-assisted laser desorption/ionization–time of flight (MALDI-ToF) mass spectrometry. IgG3 was a major immunoglobulin found in all samples. Differential analysis of the spectral signatures found for the nucleocapsid versus the spike protein demonstrated that the predominant humoral immune response to the nucleocapsid was IgG3, whilst for the spike protein it was IgG1. However, the spike protein displayed a strong affinity for IgG3 itself, as it would bind from control plasma samples, as well as from those previously infected with SARS-CoV-2, similar to the way protein G binds IgG1. Furthermore, detailed spectral analysis indicated that a mass shift consistent with hyper-glycosylation or glycation was a characteristic of the IgG3 captured by the spike protein.

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.

IgG N-glycans

Glycosylation, one of the most common post-translational modifications in mammalian cells, impacts many biological processes such as cell adhesion, proliferation and differentiation. As the most abundant glycoprotein in human serum, immunoglobulin G (IgG) plays a vital role in immune response and protection. There is a growing body of evidence suggests that IgG structure and function are modulated by attached glycans, especially N-glycans, and aberrant glycosylation is associated with disease states. In this chapter, we review IgG glycan repertoire and function, strategies for profiling IgG N-glycome and recent studies. Mass spectrometry (MS) based techniques are the most powerful tools for profiling IgG glycome. IgG glycans can be divided into high-mannose, biantennary complex and hybrid types, modified with mannosylation, core-fucosylation, galactosylation, bisecting GlcNAcylation, or sialylation. Glycosylation of IgG affects antibody half-life and their affinity and avidity for antigens, regulates crystallizable fragment (Fc) structure and Fcγ receptor signaling, as well as antibody effector function. Because of their critical roles, IgG N-glycans appear to be promising biomarkers for various disease states. Specific IgG glycosylation can convert a pro-inflammatory response to an anti-inflammatory activity. Accordingly, IgG glycoengineering provides a powerful approach to potentially develop effective drugs and treat disease. Based on the understanding of the functional role of IgG glycans, the development of vaccines with enhanced capacity and long-term protection are possible in the near future.

Poly-IgA Complexes and Disease Severity in IgA Nephropathy

BACKGROUND AND OBJECTIVES

Poly-IgA immune complex formation and glomerular deposition play a key role in IgA nephropathy. Our study sought to develop a new methodology for one-step serologic detection of poly-IgA levels.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

A novel ELISA method using recombinant CD89 as a "capturing" probe was established for detecting poly-IgA immune complex in plasma. We applied semiquantitative measurements of these poly-IgA indices in patients recruited at Peking University First Hospital who had IgA nephropathy or other kidney disease types, as compared with healthy controls. The longitudinal trend of the poly-IgA index and the association with pathologic parameters and treatment responses were evaluated. Finally, we analyzed the molecular composition of poly-IgA complexes in patients by mass spectrometry.

RESULTS

Recombinant CD89-mounted ELISA plates specifically captured plasma poly-IgA. The levels of poly-IgA immune complex (26.7 [interquartile range (IQR) 17.1-42.6] U/ml) in IgA nephropathy were significantly higher than those in healthy controls (15.5 [IQR 10.7-20.0] U/ml; P<0.001) or in controls with non-IgA nephropathy disease (14.8 [IQR 10.5-21.9] U/ml; P<0.001). Higher levels of poly-IgA immune complex were associated with lower eGFR and worse kidney outcome. Accuracy parameters and concordant statistics showed good discrimination between IgA nephropathy and healthy controls based on poly-IgA index levels (area under the curve [AUC], 0.78; 95% confidence interval [95% CI], 0.72 to 0.83; P<0.001), significantly outperforming galactose-deficient IgA1 levels (AUC, 0.70; P=0.05). Corticosteroid and immunosuppressant treatments lowered poly-IgA indices. After a recombinant CD89-directed workflow in conjunction with mass spectrometry, we also analyzed the molecular composition of IgA immune complex in patients with IgA nephropathy.

CONCLUSIONS

Higher level of recombinant CD89-bound poly-IgA immune complex was associated with the severity of the disease and with treatment response to steroids and immunosuppressants.

Nicotiana spp. for the Expression and Purification of Functional IgG3 Antibodies Directed Against the Staphylococcus aureus Alpha Toxin

Immunoglobulin subclass IgG1 is bound and neutralized effectively by Staphylococcus aureus protein A, allowing the bacterium to evade the host’s adaptive immune response. In contrast, the IgG3 subclass is not bound by protein A and can be used to treat S. aureus infections, including drug-resistant strains such as methicillin-resistant Staphylococcus aureus (MRSA). However, the yields of recombinant IgG3 are generally low because this subclass is prone to degradation, and recovery is hindered by the inability to use protein A as an affinity ligand for antibody purification. Here, we investigated plants (Nicotiana spp.) as an alternative to microbes and mammalian cell cultures for the production of an IgG3 antibody specific for the S. aureus alpha toxin. We targeted recombinant IgG3 to different subcellular compartments and tested different chromatography conditions to improve recovery and purification. Finally, we tested the antigen-binding capacity of the purified antibodies. The highest IgG3 levels in planta (&gt;130 mg kg−1 wet biomass) were achieved by targeting the endoplasmic reticulum or apoplast. Although the purity of IgG3 exceeded 95% following protein G chromatography, product recovery requires further improvement. Importantly, the binding affinity of the purified antibodies was in the nanomolar range and thus comparable to previous studies using murine hybridoma cells as the production system.

Solution structures of human myeloma IgG3 antibody reveal extended Fab and Fc regions relative to the other IgG subclasses

Human immunoglobulin G subclass 3 (IgG3) possesses a uniquely long hinge region that separates its Fab antigen-binding and Fc receptor-binding regions. Owing to this hinge length, the molecular structure of full-length IgG3 remains elusive, and the role of the two conserved Fc glycosylation sites are unknown. To address these issues, we subjected glycosylated and deglycosylated human myeloma IgG3 to multidisciplinary solution structure studies. Using analytical ultracentrifugation, the elongated structure of IgG3 was determined from the reduced sedimentation coefficients s020,w of 5.82 to 6.29 S for both glycosylated and deglycosylated IgG3. X-ray and neutron scattering showed that the Guinier RG values were 6.95 nm for glycosylated IgG3 and were unchanged after deglycosylation, again indicating an elongated structure. The distance distribution function P(r) showed a maximum length of 25 to 28 nm and three distinct maxima. The molecular structure of IgG3 was determined using atomistic modeling based on molecular dynamics simulations of the IgG3 hinge and Monte Carlo simulations to identify physically realistic arrangements of the Fab and Fc regions. This resulted in libraries containing 135,135 and 73,905 glycosylated and deglycosylated IgG3 structures, respectively. Comparisons with the X-ray and neutron scattering curves gave 100 best-fit models for each form of IgG3 that accounted for the experimental scattering curves. These models revealed the first molecular structures for full-length IgG3. The structures exhibited relatively restricted Fab and Fc conformations joined by an extended semirigid hinge, which explains the potent effector functions of IgG3 relative to the other subclasses IgG1, IgG2, and IgG4.

Highly Efficient Enrichment of O-GalNAc Glycopeptides by Using Immobilized Metal Ion Affinity Chromatography

Proteomics analysis of O-GalNAc glycosylation is important for the screening of biomarkers and the assessment of therapeutic responses. However, its analysis still faces challenges due to the poor performance of currently available enrichment methods. In this study, an enrichment method was established on the basis of Ti-IMAC(IV) materials, which could enrich the intact O-GalNAc glycopeptides via both the hydrophilic interaction and affinity interaction. This method enabled nearly 200 intact O-GalNAc glycopeptides identified from only 0.1 μL of human serum. This was nearly 2-fold different from that of the HILIC method. An in-depth analysis of the O-GalNAc glycosylation was performed, and 2093 intact glycopeptides were identified from 7.2 μL of human serum samples. This is the largest O-GalNAc glycosylation database of human serum from a trace amount of sample. Furthermore, 52 significantly changed intact O-GalNAc glycopeptides were determined by the quantitative analysis of hepatocellular carcinoma (HCC) and control serum samples, indicating the potential applications of this enrichment method in biomarker discovery.

Ig Glycosylation in Ulcerative Colitis: It's Time for New Biomarkers

Background: Ulcerative colitis (UC) is a chronic relapsing disease, which needs a continue monitoring, especially during biological therapies. An increasing number of patients is treated with anti-Tumor Necrosis factor (TNF) drugs, and current research is focalized to identify biomarkers able to monitor the disease and to predict therapeutic outcome.

Methods: We enrolled consecutive UC patients treated with anti-TNF, naïve to biologic drugs. Therapeutic outcome was evaluated after 54 weeks of treatment in terms of clinical remission (Partial Mayo Score -PMS- <2) and mucosal healing (Mayo Endoscopic Score <2). On serum samples collected at baseline and after 54 weeks of treatment, a Lectin-based ELISA assay was performed, and specific glycosylation patterns were evaluated by biotin-labelled lectins. We have also collected 21 healthy controls (NHS) samples, age and sex-matched.

Results: Out of 44 UC patients enrolled, 22 achieved clinical remission and mucosal healing after 54 weeks. At baseline, when Protein A was used as coating, UC patients non-responders showed a reduced reactivity to Jacalin (JAC) in comparison with NHS (p = 0.04). After one year of treatment, a decrease in JAC binding was seen only in responders, in comparison with baseline (p = 0.04). When JAC binding was tested selecting IgG by means of Fab anti-IgG Fab, UC patients displayed an increased reactivity after anti-TNF therapy (p < 0,0001 vs controls). At baseline, PMS inversely correlates with JAC binding when Fab anti-IgG Fab was used in solid phase (r² = 0,2211; p = 0,0033). Patients with higher PMS at baseline (PMS ≥5) presented lower binding capacity for JAC in comparison with NHS and with lower PMS patients (p = 0,0135 and p = 0,0089, respectively).

Conclusion: Ig glycosylation was correlated with clinical and endoscopic activity in patients with UC. JAC protein A-selected Ig showed a possible role in predicting therapeutic effectiveness. If these data would be confirmed, Ig glycosylation could be used as biomarker in UC.

Sialylated immunoglobulin G: a promising diagnostic and therapeutic strategy for autoimmune diseases

Human immunoglobulin G (IgG), especially autoantibodies, has major implications for the diagnosis and management of a wide range of autoimmune diseases. However, some healthy individuals also have autoantibodies, while a portion of patients with autoimmune diseases test negative for serologic autoantibodies. Recent advances in glycomics have shown that IgG Fc N-glycosylations are more reliable diagnostic and monitoring biomarkers than total IgG autoantibodies in a wide variety of autoimmune diseases. Furthermore, these N-glycosylations of IgG Fc, particularly sialylation, have been reported to exert significant anti-inflammatory effects by upregulating inhibitory FcγRIIb on effector macrophages and reducing the affinity of IgG for either complement protein or activating Fc gamma receptors. Therefore, sialylated IgG is a potential therapeutic strategy for attenuating pathogenic autoimmunity. IgG sialylation-based therapies for autoimmune diseases generated through genetic, metabolic or chemoenzymatic modifications have made some advances in both preclinical studies and clinical trials.

Altered immunoglobulin G glycosylation in patients with isolated hyperprolactinaemia

Prolactin is a peptide hormone produced in the anterior pituitary, which increase in several physiological and pathological situations. It is unclear if hyperprolactinaemia may affect glycosylation of immunoglobulin G (IgG). Twenty-five patients with hyperprolactinemia and 22 healthy control subjects were included in the study. The groups had similar age and gender distribution. A panel of hormonal and haematological analyses, creatinine, glucose, liver enzymes and immunoglobulins were measured by routine clinical methods. IgG was purified from serum by Protein G Sepharose. Sialic acid was released from IgG by use of neuraminidase followed by quantification on high performance anion-exchange chromatography with pulsed amperometric detection. Tryptic glycopeptides of IgG was analysed by matrix-assisted laser desorption/ionization-time of flight mass spectrometry. Hormone and immunoglobulin levels were similar in the two groups, except for IgA and prolactin. Significantly higher IgG1 and IgG2/3 galactosylation was found in the patient group with hyperprolactinaemia compared to controls. (A significant correlation between prolactin and IgG2/3 galactosylation (Rs 0.61, p<0.001) was found for samples with prolactin values below 2000 mIU/L. The relative amount of sialylated and bisecting glycans on IgG did not differ between patients and controls. The four macroprolactinaemic patients showed decreased relative amount of bisecting IgG2/3 glycans. Hyperprolactinaemia was found to be associated with increased galactosylation of IgG1and IgG2/3. This may have impact on IgG interactions with Fc-receptors, complement and lectins, and consequently lead to an altered immune response.

Coming together at the hinges: Therapeutic prospects of IgG3

The human IgG3 subclass is conspicuously absent among the formats for approved monoclonal antibody therapies and Fc fusion protein biologics. Concern about the potential for rapid degradation, reduced plasma half-life, and increased immunogenicity due to marked variation in allotypes has apparently outweighed the potential advantages of IgG3, which include high affinity for activating Fcγ receptors, effective complement fixation, and a long hinge that appears better suited for low abundance targets. This review aims to highlight distinguishing features of IgG3 and to explore its functional role in the immune response. We present studies of natural immunity and recombinant antibody therapies that elucidate key contributions of IgG3 and discuss historical roadblocks that no longer remain clearly relevant. Collectively, this body of evidence motivates thoughtful reconsideration of the clinical advancement of this distinctive antibody subclass for treatment of human diseases. Abbreviations: ADCC - Antibody-Dependent Cell-mediated CytotoxicityADE - Antibody-dependent enhancementAID - Activation-Induced Cytidine DeaminaseCH - Constant HeavyCHF - Complement factor HCSR - Class Switch RecombinationEM - Electron MicroscopyFab - Fragment, antigen bindingFc - Fragment, crystallizableFcRn - Neonatal Fc ReceptorFcγR - Fc gamma ReceptorHIV - Human Immunodeficiency VirusIg - ImmunoglobulinIgH - Immunoglobulin Heavy chain geneNHP - Non-Human Primate.

Lectin and Liquid Chromatography-Based Methods for Immunoglobulin (G) Glycosylation Analysis

Immunoglobulin (Ig) glycosylation has been shown to dramatically affect its structure and effector functions. Ig glycosylation changes have been associated with different diseases and show a promising biomarker potential for diagnosis and prognosis of disease advancement. On the other hand, therapeutic biomolecules based on structural and functional features of Igs demand stringent quality control during the production process to ensure their safety and efficacy. Liquid chromatography (LC) and lectin-based methods are routinely used in Ig glycosylation analysis complementary to other analytical methods, e.g., mass spectrometry and capillary electrophoresis. This chapter covers analytical approaches based on LC and lectins used in low- and high-throughput N- and O-glycosylation analysis of Igs, with the focus on immunoglobulin G (IgG) applications. General principles and practical examples of the most often used LC methods for Ig purification are described, together with typical workflows for N- and O-glycan analysis on the level of free glycans, glycopeptides, subunits, or intact Igs. Lectin chromatography is a historical approach for the analysis of lectin-carbohydrate interactions and glycoprotein purification but is still being used as a valuable tool in Igs purification and glycan analysis. On the other hand, lectin microarrays have found their application in the rapid screening of glycan profiles on intact proteins.

Glycosylation and Aging

Human lifespan has increased significantly in the last 200 years, emphasizing our need to age healthily. Insights into molecular mechanisms of aging might allow us to slow down its rate or even revert it. Similar to aging, glycosylation is regulated by an intricate interplay of genetic and environmental factors. The dynamics of glycopattern variation during aging has been mostly explored for plasma/serum and immunoglobulin G (IgG) N-glycome, as we describe thoroughly in this chapter. In addition, we discuss the potential functional role of agalactosylated IgG glycans in aging, through modulation of inflammation level, as proposed by the concept of inflammaging. We also comment on the potential to use the plasma/serum and IgG N-glycome as a biomarker of healthy aging and on the interventions that modulate the IgG glycopattern. Finally, we discuss the current knowledge about animal models for human plasma/serum and IgG glycosylation and mention other, less explored, instances of glycopattern changes during organismal aging and cellular senescence.

On enzymatic remodeling of IgG glycosylation; unique tools with broad applications

The importance of IgG glycosylation has been known for many years not only by scientists in glycobiology but also by human pathogens that have evolved specific enzymes to modify these glycans with fundamental impact on IgG function. The rise of IgG as a major therapeutic scaffold for many cancer and immunological indications combined with the availability of unique enzymes acting specifically on IgG Fc-glycans have spurred a range of applications to study this important post-translational modification on IgG. This review article introduces why the IgG glycans are of distinguished interest, gives a background on the unique enzymatic tools available to study the IgG glycans and finally presents an overview of applications utilizing these enzymes for various modifications of the IgG glycans. The applications covered include site-specific glycan transglycosylation and conjugation, analytical workflows for monoclonal antibodies and serum diagnostics. Additionally, the review looks ahead and discusses the importance of O-glycosylation for IgG3, Fc-fusion proteins and other new formats of biopharmaceuticals.

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.

A novel glycosidase plate-based assay for the quantification of galactosylation and sialylation on human IgG

Changes in human IgG galactosylation and sialylation have been associated with several inflammatory diseases which are a major burden on the health care system. A large body of work on well-established glycomic and glycopeptidomic assays has repeatedly demonstrated inflammation-induced changes in IgG glycosylation. However, these assays are usually based on specialized analytical instrumentation which could be considered a technical barrier for uptake by some laboratories. Hence there is a growing demand for simple biochemical assays for analyzing these glycosylation changes. We have addressed this need by introducing a novel glycosidase plate-based assay for the absolute quantification of galactosylation and sialylation on IgG. IgG glycoproteins are treated with specific exoglycosidases to release the galactose and/or sialic acid residues. The released galactose monosaccharides are subsequently used in an enzymatic redox reaction that produces a fluorescence signal that is quantitative for the amount of galactosylation and, in-turn, sialylation on IgG. The glycosidase plate-based assay has the potential to be a simple, initial screening assay or an alternative assay to the usage of high-end analytical platforms such as HILIC-FLD-MSn when considering the analysis of galactosylation and sialylation on IgG. We have demonstrated this by comparing our assay to an industrial established HILIC-FLD-MSn glycomic analysis of 15 patient samples and obtained a Pearson’s r correlation coefficient of 0.8208 between the two methods.

Interlaboratory Study for Characterizing Monoclonal Antibodies by Top-Down and Middle-Down Mass Spectrometry

The Consortium for Top-Down Proteomics (www.topdownproteomics.org) launched the present study to assess the current state of top-down mass spectrometry (TD MS) and middle-down mass spectrometry (MD MS) for characterizing monoclonal antibody (mAb) primary structures, including their modifications. To meet the needs of the rapidly growing therapeutic antibody market, it is important to develop analytical strategies to characterize the heterogeneity of a therapeutic product's primary structure accurately and reproducibly. The major objective of the present study is to determine whether current TD/MD MS technologies and protocols can add value to the more commonly employed bottom-up (BU) approaches with regard to confirming protein integrity, sequencing variable domains, avoiding artifacts, and revealing modifications and their locations. We also aim to gather information on the common TD/MD MS methods and practices in the field. A panel of three mAbs was selected and centrally provided to 20 laboratories worldwide for the analysis: Sigma mAb standard (SiLuLite), NIST mAb standard, and the therapeutic mAb Herceptin (trastuzumab). Various MS instrument platforms and ion dissociation techniques were employed. The present study confirms that TD/MD MS tools are available in laboratories worldwide and provide complementary information to the BU approach that can be crucial for comprehensive mAb characterization. The current limitations, as well as possible solutions to overcome them, are also outlined. A primary limitation revealed by the results of the present study is that the expert knowledge in both experiment and data analysis is indispensable to practice TD/MD MS.

Expression Profiling and Glycan Engineering of IgG Subclass 1-4 in Nicotiana benthamiana

IgG, the main serum immunoglobulin isotype, exists in four subclasses which selectively appear with distinctive glycosylation profiles. However, very little is known about the biological consequences mainly due to the difficulties in the generation of distinct IgG subtypes with targeted glycosylation. Here, we show a comprehensive expression and glycan modulation profiling of IgG variants in planta that are identical in their antigen binding domain but differ in their subclass appearance. While IgG1, 2, and 4 exhibit similar expression levels and purification yields, IgG3 is generated only at low levels due to the in planta degradation of the heavy chain. All IgG subtypes are produced with four distinct N-glycosylation profiles, differing in sugar residues previously shown to impact IgG activities, i.e., galactosylation, sialylation and core fucosylation. Affinity purified IgG variants are shown to be fully assembled to heterodimers but display different biochemical/physical features. All subtypes are equally well amenable to targeted glycosylation, except sialylated IgG4 which frequently accumulates substantial fractions of unusual oligo-mannosidic structures. IgG variants show significant differences in aggregate formation and endotoxin contamination which are eliminated by additional polishing steps (size exclusion chromatography, endotoxin removal treatments). Collectively we demonstrate the generation of 16 IgG variants at high purity and large glycan homogeneity which constitute an excellent toolbox to further study the biological impact of the two main Fc features, subclass and glycosylation.

Plasma and antibody glycomic biomarkers of time to HIV rebound and viral setpoint

OBJECTIVE

HIV cure research urgently needs to identify pre-analytic treatment interruption (ATI) biomarkers of time-to-viral-rebound and viral setpoint to mitigate the risk of ATI and accelerate development of a cure. We previously reported that galactosylated IgG glycans, G2, negatively correlate with cell-associated HIV DNA and RNA during antiretroviral therapy (ART). We hypothesized that this and other plasma glycomic traits can predict time-to-viral-rebound and viral setpoint upon ART cessation.

DESIGN

We profiled the circulating glycomes (plasma and bulk IgG) of two geographically distinct cohorts: Philadelphia Cohort - 24 HIV-infected, ART-suppressed individuals who had participated in an open-ended ATI study without concurrent immunomodulatory agents. Johannesburg Cohort - 23 HIV-infected, ART-suppressed individuals who had participated in a 2-week ATI.

METHODS

Capillary electrophoresis and lectin microarray were used for glycomic analyses. Cox proportional-hazards model and log-rank test were used for statistical analyses.

RESULTS

Higher pre-ATI levels of the IgG glycan, G2, were significantly associated with a longer time-to-viral-rebound (hazard ratio = 0.12, P = 0.05). In addition to G2, we identified several predictive glycomic traits in plasma, for example, levels of FA2BG1, a non-sialylated, core-fucosylated glycan, associated with a longer time-to-viral-rebound (hazard ratio = 0.023, P = 0.05), whereas FA2G2S1, a sialylated glycan, associated with a shorter time-to-viral-rebound (hazard ratio = 24.1, P = 0.028). Additionally, pre-ATI plasma glycomic signatures associated with a lower viral setpoint, for example, T-antigen (Galβ1-3GalNAc) (r = 0.75, P = 0.0007), or a higher viral setpoint, for example, polylactosamine (r = -0.58, P = 0.01). These results were initially validated in the Johannesburg Cohort.

CONCLUSION

We describe first-in-class, non-invasive, plasma and IgG glycomic biomarkers that inform time-to-viral-rebound and viral setpoint in two geographically distinct cohorts.

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.

Considerations for the Design of Antibody-Based Therapeutics

Antibody-based proteins have become an important class of biologic therapeutics, due in large part to the stability, specificity, and adaptability of the antibody framework. Indeed, antibodies not only have the inherent ability to bind both antigens and endogenous immune receptors but also have proven extremely amenable to protein engineering. Thus, several derivatives of the monoclonal antibody format, including bispecific antibodies, antibody-drug conjugates, and antibody fragments, have demonstrated efficacy for treating human disease, particularly in the fields of immunology and oncology. Reviewed here are considerations for the design of antibody-based therapeutics, including immunological context, therapeutic mechanisms, and engineering strategies. First, characteristics of antibodies are introduced, with emphasis on structural domains, functionally important receptors, isotypic and allotypic differences, and modifications such as glycosylation. Then, aspects of therapeutic antibody design are discussed, including identification of antigen-specific variable regions, choice of expression system, use of multispecific formats, and design of antibody derivatives based on fragmentation, oligomerization, or conjugation to other functional moieties. Finally, strategies to enhance antibody function through protein engineering are reviewed while highlighting the impact of fundamental biophysical properties on protein developability.

Igh locus structure and evolution in Platyrrhines: new insights from a genomic perspective

Non-human primates have been used as animal models because of their phylogenetic closeness to humans. However, the genetic differences between humans and non-human primates must be considered to select the appropriate animal models. Recently, New World monkeys (Platyrrhines) have generated a higher interest in biomedical research, especially in assessing vaccine safety and immunogenicity. Given the continued and renewed interest in Platyrrhines as biomedical models, it is a necessary to have a better and more complete understanding of their immune system and its implications for research. Immunoglobulins (Ig) are the main proteins that mediate humoral immunity. These proteins have evolved as part of an adaptive immune response system derived from ancient vertebrates. There are at least four Ig classes in Prosimians, whereas five have been reported in Catarrhines. Information on the structure and evolution of the loci containing immunoglobulin heavy chain constant genes (Igh) in Platyrrhines, however, is limited. Here, Igh loci were characterized in 10 Platyrrhines using the available whole genome sequences. Human and Macaca Igh loci were also assessed to compare them with their Platyrrhines counterparts. Differences in Igh locus structure were observed between Platyrrhines and Catarrhines. Noteworthy changes occur in the γ gene, which encodes a key Ig involved in organism defense that would favor protection after vaccination. The remarkable differences between the immunoglobulin proteins of Platyrrhines and Catarrhines warrant a cautionary message to biomedical researchers.

Current outlook on radionuclide delivery systems: from design consideration to translation into clinics

Radiopharmaceuticals have proven to be effective agents, since they can be successfully applied for both diagnostics and therapy. Effective application of relevant radionuclides in pre-clinical and clinical studies depends on the choice of a sufficient delivery platform. Herein, we provide a comprehensive review on the most relevant aspects in radionuclide delivery using the most employed carrier systems, including, (i) monoclonal antibodies and their fragments, (ii) organic and (iii) inorganic nanoparticles, and (iv) microspheres. This review offers an extensive analysis of radionuclide delivery systems, the approaches of their modification and radiolabeling strategies with the further prospects of their implementation in multimodal imaging and disease curing. Finally, the comparative outlook on the carriers and radionuclide choice, as well as on the targeting efficiency of the developed systems is discussed.

Recombinant human monoclonal HLA antibodies of different IgG subclasses recognising the same epitope: Excellent tools to study differential effects of donor-specific antibodies

In the field of transplantation, the humoural immune response against mismatched HLA antigens of the donor is associated with inferior graft survival, but not in every patient. Donor‐specific HLA antibodies (DSA) of different immunoglobulin G (IgG) subclasses may have differential effects on the transplanted organ. Recombinant technology allows for the generation of IgG subclasses of a human monoclonal antibody (mAb), while retaining its epitope specificity. In order to enable studies on the biological function of IgG subclass HLA antibodies, we used recombinant technology to generate recombinant human HLA mAbs from established heterohybridomas. We generated all four IgG subclasses of a human HLA class I and class II mAb and showed that the different subclasses had a comparable affinity, normal human Fc glycosylation, and retained HLA epitope specificity. For both mAbs, the IgG1 and IgG3 isotypes were capable of binding complement component 3d (C3d) and efficient in complement‐dependent cell lysis against their specific targets, while the IgG2 and IgG4 subclasses were not able to induce cytotoxicity. Considering the fact that the antibody‐binding site and properties remained unaffected, these IgG subclass HLA mAbs are excellent tools to study the function of individual IgG subclass HLA class I and class II‐specific antibodies in a controlled fashion.

Site-specific N- and O-glycosylation analysis of atacicept

The Fc-fusion protein atacicept is currently under clinical investigation for its biotherapeutic application in autoimmune diseases owing to its ability to bind the two cytokines B-Lymphocyte Stimulator (BLyS) and A PRoliferation-Inducing Ligand (APRIL). Like typical recombinant IgG-based therapeutics, atacicept is a glycoprotein whose glycosylation-related heterogeneity arises from the glycosylation-site localization, site-specific occupation and structural diversity of the attached glycans. Here, we present a first comprehensive site-specific N- and O-glycosylation characterization of atacicept using mass spectrometry-based workflows. First, N- and O-glycosylation sites and their corresponding glycoforms were identified. Second, a relative quantitation of the N-glycosylation site microheterogeneity was achieved by glycopeptide analysis, which was further supported by analysis of the released N-glycans. We confirmed the presence of one N-glycosylation site, carrying 47 glycoforms covering 34 different compositions, next to two hinge region O-glycosylation sites with core 1-type glycans. The relative O-glycan distribution was analyzed based on the de-N-glycosylated intact protein species. Overall, N- and O-glycosylation were consistent between two individual production batches.