Structural changes in the oligosaccharide moiety of human IgG with aging

N-glycomic biomarkers of biological aging and longevity: a link with inflammaging

Glycosylation is a frequent co/post-translational modification of proteins which modulates a variety of biological functions. The analysis of N-glycome, i.e. the sugar chains N-linked to asparagine, identified new candidate biomarkers of aging such as N-glycans devoid of galactose residues on their branches, in a variety of human and experimental model systems, such as healthy old people, centenarians and their offspring and caloric restricted mice. These agalactosylated biantennary structures mainly decorate Asn297 of Fc portion of IgG (IgG-G0), and are present also in patients affected by progeroid syndromes and a variety of autoimmune/inflammatory diseases. IgG-G0 exert a pro-inflammatory effect through different mechanisms, including the lectin pathway of complement, binding to Fcγ receptors and formation of autoantibody aggregates. The age-related accumulation of IgG-G0 can contribute to inflammaging, the low-grade pro-inflammatory status that characterizes elderly, by creating a vicious loop in which inflammation is responsible for the production of aberrantly glycosylated IgG which, in turn, would activate the immune system, exacerbating inflammation. Moreover, recent data suggest that the N-glycomic shift observed in aging could be related not only to inflammation but also to alteration of important metabolic pathways. Thus, altered N-glycans are both powerful markers of aging and possible contributors to its pathogenesis.

Comparison of the Fc glycosylation of fetal and maternal immunoglobulin G

Human immunoglobulin G (IgG) molecules are composed of two Fab portions and one Fc portion. The glycans attached to the Fc portions of IgG are known to modulate its biological activity as they influence interaction with both complement and various cellular Fc receptors. IgG glycosylation changes significantly with pregnancy, showing a vast increase in galactosylation and sialylation and a concomitant decrease in the incidence of bisecting GlcNAc. Maternal IgGs are actively transported to the fetus by the neonatal Fc receptor (FcRn) expressed in syncytiotrophoblasts in the placenta, providing the fetus and newborn with immunological protection. Two earlier reports described significant differences in total glycosylation between fetal and maternal IgG, suggesting a possible glycosylation-selective transport via the placenta. These results might suggest an alternative maternal transport pathway, since FcRn binding to IgG does not depend on Fc-glycosylation. These early studies were performed by releasing N-glycans from total IgG. Here, we chose for an alternative approach analyzing IgG Fc glycosylation at the glycopeptide level in an Fc-specific manner, providing glycosylation profiles for IgG1 and IgG4 as well as combined Fc glycosylation profiles of IgG2 and 3. The analysis of ten pairs of fetal and maternal IgG samples revealed largely comparable Fc glycosylation for all the analyzed subclasses. Average levels of galactosylation, sialylation, bisecting GlcNAc and fucosylation were very similar for the fetal and maternal IgGs. Our data suggest that the placental IgG transport is not Fc glycosylation selective.

The role of sialic acid as a modulator of the anti-inflammatory activity of IgG

Immunoglobulin G (IgG) molecules can have two completely opposing activities. They can be very potent pro-inflammatory mediators on the one hand, directing the effector functions of the innate immune system towards infected cells, tumor cells or healthy tissues in the case of autoimmune diseases. On the other hand, a mixture of IgG molecules purified from the blood of ten thousands of healthy donors is used as an anti-inflammatory treatment for many autoimmune diseases since several decades. It has become evident only recently that certain residues in the sugar moiety attached to the IgG constant fragment can dramatically alter the pro- and anti-inflammatory activities of IgG. This review will focus on sialic acid residues as a modulator of the anti-inflammatory activity and provide an overview of situations where serum IgG glycosylation and sialylation is altered and which molecular and cellular pathways may be involved in this immunomodulatory pathway.

Hypogalactosylation of serum N-glycans fails to predict clinical response to methotrexate and TNF inhibition in rheumatoid arthritis

Introduction

Rheumatoid arthritis (RA) is associated with hypogalactosylation of immunoglobulin G (IgG). We examined whether a proxy measure for galactosylation of IgG N-glycans could predict response to therapy or was differentially affected by methotrexate (MTX) or TNF blockade.

Methods

Using a previously defined normal phase high-performance liquid chromatography approach, we ascertained the galactosylation status of whole serum N-glycans in two well-defined RA clinical cohorts: the Autoimmune Biomarkers Collaborative Network (n = 98) and Nested I (n = 64). The ratio of agalactosylated to monogalactosylated N-glycans in serum (sG0/G1) was determined before and during therapy with MTX or TNF inhibition and correlated with anticitrullinated peptide antibody (ACPA) status and clinical response as assessed by 28-joint Disease Activity Score utilizing C-reactive peptide and European League Against Rheumatism response criteria.

Results

RA patients from both cohorts exhibited elevation of sG0/G1 at baseline. Improvement in clinical scores correlated with a reduction in sG0/G1 (Spearman's ρ = 0.31 to 0.37; P < 0.05 for each cohort). However, pretreatment sG0/G1 was not predictive of clinical response. Changes in sG0/G1 were similar in the MTX and TNF inhibitor groups. Corrected for disease activity, ACPA positivity correlated with higher sG0/G1.

Conclusions

Baseline serum N-glycan hypogalactosylation, an index previously correlated with hypogalactosylation of IgG N-glycans, did not distinguish patients with rheumatoid arthritis who were likely to experience a favorable clinical response to MTX or TNF blockade. Clinical improvement was associated with partial glycan normalization. ACPA-positive patients demonstrated enhanced N-glycan aberrancy compared with ACPA-negative patients.

Changes in antigen-specific IgG1 Fc N-glycosylation upon influenza and tetanus vaccination

Antibody effector functions have been shown to be influenced by the structure of the Fc N-glycans. Here we studied the changes in plasma or serum IgG Fc N-glycosylation upon vaccination of 10 Caucasian adults and 10 African children. Serum/plasma IgG was purified by affinity chromatography prior to and at two time points after vaccination. Fc N-glycosylation profiles of individual IgG subclasses were determined for both total IgG and affinity-purified anti-vaccine IgG using a recently developed fast nanoliquid chromatography-electrospray ionization MS (LC-ESI-MS) method. While vaccination had no effect on the glycosylation of total IgG, anti-vaccine IgG showed increased levels of galactosylation and sialylation upon active immunization. Interestingly, the number of sialic acids per galactose increased during the vaccination time course, suggesting a distinct regulation of galactosylation and sialylation. In addition we observed a decrease in the level of IgG1 bisecting N-acetylglucosamine whereas no significant changes were observed for the level of fucosylation. Our data indicate that dependent on the vaccination time point the infectious agent will encounter IgGs with different glycosylation profiles, which are expected to influence the antibody effector functions relevant in immunity.

Fc-glycosylation of IgG1 is modulated by B-cell stimuli

We have recently shown that IgG1 directed against antigens thought to be involved in the pathogenesis of rheumatoid arthritis harbor different glycan moieties on their Fc-tail, as compared with total sera IgG1. Given the crucial roles of Fc-linked N-glycans for the structure and biological activity of IgG, Fc-glycosylation of antibodies is receiving considerable interest. However, so far little is known about the signals and factors that could influence the composition of these carbohydrate structures on secreted IgG produced by B lymphocytes. Here we show that both "environmental" factors, such as all-trans retinoic acid (a natural metabolite of vitamin A), as well as factors stimulating the innate immune system (i.e. CpG oligodeoxynucleotide, a ligand for toll-like receptor 9) or coming from the adaptive immune system (i.e. interleukin-21, a T-cell derived cytokine) can modulate IgG1 Fc-glycosylation. These factors affect Fc-glycan profiles in different ways. CpG oligodeoxynucleotide and interleukin-21 increase Fc-linked galactosylation and reduce bisecting N-acetylglucosamine levels, whereas all-trans retinoic acid significantly decreases galactosylation and sialylation levels. Moreover, these effects appeared to be stable and specific for secreted IgG1 as no parallel changes of the corresponding glycans in the cellular glycan pool were observed. Interestingly, several other cytokines and molecules known to affect B-cell biology and antibody production did not have an impact on IgG1 Fc-coupled glycan profiles. Together, these data indicate that different stimuli received by B cells during their activation and differentiation can modulate the Fc-linked glycosylation of secreted IgG1 without affecting the general cellular glycosylation machinery. Our study, therefore, furthers our understanding of the regulation of IgG1 glycosylation at the cellular level.

Decreased levels of bisecting GlcNAc glycoforms of IgG are associated with human longevity

Background

Markers for longevity that reflect the health condition and predict healthy aging are extremely scarce. Such markers are, however, valuable in aging research. It has been shown previously that the N-glycosylation pattern of human immunoglobulin G (IgG) is age-dependent. Here we investigate whether N-linked glycans reflect early features of human longevity.

Methodology/Principal Findings

The Leiden Longevity Study (LLS) consists of nonagenarian sibling pairs, their offspring, and partners of the offspring serving as control. IgG subclass specific glycosylation patterns were obtained from 1967 participants in the LLS by MALDI-TOF-MS analysis of tryptic IgG Fc glycopeptides. Several regression strategies were applied to evaluate the association of IgG glycosylation with age, sex, and longevity. The degree of galactosylation of IgG decreased with increasing age. For the galactosylated glycoforms the incidence of bisecting GlcNAc increased as a function of age. Sex-related differences were observed at ages below 60 years. Compared to males, younger females had higher galactosylation, which decreased stronger with increasing age, resulting in similar galactosylation for both sexes from 60 onwards. In younger participants (<60 years of age), but not in the older age group (>60 years), decreased levels of non-galactosylated glycoforms containing a bisecting GlcNAc reflected early features of longevity.

Conclusions/Significance

We here describe IgG glycoforms associated with calendar age at all ages and the propensity for longevity before middle age. As modulation of IgG effector functions has been described for various IgG glycosylation features, a modulatory effect may be expected for the longevity marker described in this study.

Aberrant IgG galactosylation precedes disease onset, correlates with disease activity, and is prevalent in autoantibodies in rheumatoid arthritis

OBJECTIVE

To examine the association between aberrant IgG galactosylation and disease parameters in rheumatoid arthritis (RA).

METHODS

Analysis of N-glycan in serum samples from multiple cohorts was performed. The IgG N-glycan content and the timing of N-glycan aberrancy relative to disease onset were compared in healthy subjects and in patients with RA. Correlations between aberrant galactosylation and disease activity were assessed in the RA cohorts. The impact of disease activity, sex, age, anti-cyclic citrullinated peptide (anti-CCP) antibody titer, disease duration, and C-reactive protein level on aberrant galactosylation was determined using multivariate analysis. The N-glycan content was also compared between epitope affinity-purified autoantibodies and the remaining IgG repertoire in RA patients.

RESULTS

Our results confirm the aberrant galactosylation of IgG in RA patients as compared with healthy controls (mean +/- SD 1.36 +/- 0.43 versus 1.01 +/- 0.23; P < 0.0001). We observed a significant correlation between levels of aberrant IgG galactosylation and disease activity (Spearman's rho = 0.37, P < 0.0001). This correlation was higher in women (Spearman's rho = 0.60, P < 0.0001) than in men (Spearman's rho = 0.16, P = 0.10). Further, aberrant IgG galactosylation substantially predated the onset of arthritis and the diagnosis of RA (3.5 years) and resided selectively in the anticitrullinated antigen fraction.

CONCLUSION

Our findings identify aberrant IgG galactosylation as a dysregulated component of the humoral immune response in RA that begins prior to disease onset, associates with disease activity in a sex-specific manner, and resides preferentially in autoantibodies.

Immunoglobulin G glycopeptide profiling by matrix-assisted laser desorption ionization Fourier transform ion cyclotron resonance mass spectrometry

Immunoglobulin G (IgG) fragment crystallizable (Fc) glycosylation is essential for Fc-receptor-mediated activities. Changes in IgG Fc glycosylation have been found to be associated with various diseases. Here we describe a high-throughput IgG glycosylation profiling method. Sample preparation is performed in 96-well plate format: IgGs are purified from 2 microL of human plasma using immobilized protein A. IgGs are cleaved with trypsin, and the resulting glycopeptides are purified by reversed-phase or hydrophilic interaction solid-phase extraction. Glycopeptides are analyzed by intermediate pressure matrix-assisted laser desorption ionization Fourier transform ion cyclotron resonance mass spectrometry (MALDI-FTICR-MS). Notably, both dihydroxybenzoic acid (DHB) and alpha-cyano-4-hydroxycinnamic acid (CHCA) matrixes allowed the registration of sialylated as well as nonsialylated glycopeptides. Data were automatically processed, and IgG isotype-specific Fc glycosylation profiles were obtained. The entire method showed an interday variation below 10% for the six major glycoforms of both IgG1 and IgG2. The method was found suitable for isotype-specific high-throughput IgG glycosylation profiling from human plasma. As an example we successfully applied the method to profile the IgG glycosylation of 62 human samples.

Regulated glycosylation patterns of IgG during alloimmune responses against human platelet antigens

Various biological activities of immunoglobulin G (IgG) including antibody-dependent cellular cytotoxicity (ADCC) are modulated by the structural features of the N-glycans in the Fc part. In this study, we describe a population of IgG1 alloantibodies which are formed during pregnancy against human platelet antigens (HPA) of the fetus, causing fetoneonatal alloimmune thrombocytopenia. By analyzing the Fc-glycosylation of the pathogenic, affinity-purified IgG1 alloantibodies at the glycopeptide level using mass spectrometry, we found markedly decreased levels of core-fucosylation as well as increased levels of galactosylation and sialylation as compared to glycosylation patterns of total serum IgG1 of the same patients. Because IgG1 Fc-core-fucosylation is known to influence ADCC activity, modulation of core-fucosylation may have a profound effect on disease severity and prognosis. Studies in large patient cohorts will have to be performed to establish such correlations. Moreover, experiments in animal models as well as in vitro immunological tests will be needed to unravel the mechanisms regulating IgG Fc glycosylation.

Immunoglobulin 1 (IgG1) Fc-glycosylation profiling of anti-citrullinated peptide antibodies from human serum

In several autoimmune disorders, including rheumatoid arthritis (RA), autoantibodies are thought to be the driving force of pathogenicity. Glycosylation of the Fc-part of human Igs is known to modulate biological activity. Hitherto, glycosylation of human IgG-Fc has been analyzed predominantly at the level of total serum IgG, revealing reduced galactosylation in RA. Given the pathogenic relevance of autoantibodies in RA, we wished, in the present study, to address the question whether distinct Fc-glycosylation features are observable at the level of antigen-specific IgG subpopulations. For this purpose, we have developed a method for the microscale purification and Fc-glycosylation analysis of anti-citrullinated peptide antibodies (ACPA). ACPA represent a group of autoantibodies that occur with unique specificity in RA patients. Their presence is associated with increased inflammatory disease activity and rapid joint destruction. Results indicate that ACPA of the IgG1 subclass vary considerably from total serum IgG1 with respect to Fc-galactosylation, with galactosylation being higher on ACPA than on serum IgG1 for some patients, while other patients show higher galactosylation on serum IgG1 than on ACPA. Using this method, studies can be performed on the biological and clinical relevance of ACPA glycosylation within RA patient cohorts.

Pneumococcal vaccination in older adults induces antibodies with low opsonic capacity and reduced antibody potency

The primary mode of prevention of adult disease from Streptococcus pneumoniae is vaccination with anti-capsular polysaccharide vaccine; however, its effects are less in the targeted older population than in young persons. Few studies have examined the mechanism behind this limited effectiveness. We have measured antibody concentrations and opsonization titers for multiple serotypes amongst both old adults and young, healthy controls. To avoid specificity problems associated with pneumococcal antibody ELISA, we absorbed the serum samples with c-polysaccharide and capsular polysaccharide of 22F type. Antibody concentrations were found to be similar for six out of the seven tested serotypes, while opsonization titers were significantly higher in six out of seven serotypes in the younger population. Antibody potency, as measured by the ratio of opsonization titer to antibody concentration, was found to be significantly higher for the younger subjects for all serotypes. We conclude that, while all ages of adults make similar concentrations of antibodies in response to pneumococcal vaccine, the effectiveness of those antibodies is significantly reduced in the older adult population.

Comparison of fluorescent labels for oligosaccharides and introduction of a new postlabeling purification method

Labeling of oligosaccharides with fluorescent dyes is the prerequisite for their sensitive analysis by high-performance liquid chromatography (HPLC). In this work, we present a fast new postlabeling cleanup procedure that requires no device other than the reaction vial itself. The procedure can be applied to essentially all labeling reagents. We also compare the performance of 15 different labels for N-glycan analysis in various analytical procedures. We took special care to prevent obscuring influences from incomplete derivatization and signal quenching by impurities. Procainamide emerged as more sensitive than anthranilic acid for normal-phase HPLC, but its chromatographic performance was not convincing. 2-aminopyridine was the label with the lowest retention on reversed-phase and graphitic carbon columns and, thus, appears to be most suitable for glycan fractionation by multidimensional HPLC. Most glycan derivatives performed better than native sugars in matrix-assisted laser desorption/ionization-mass spectrometry (MALDI-MS) and electrospray ionization-MS (ESI-MS), but the gain was small and hardly sufficient to compensate for sample loss during preparation.

Glycosylation profiling of immunoglobulin G (IgG) subclasses from human serum

All four subclasses of human serum IgG contain a single N-glycosylation site in the constant region of their heavy chain, which is occupied by biantennary, largely core-fucosylated and partially truncated oligosaccharides, that may carry a bisecting N-acetylglucosamine and sialic acid residues. IgG glycosylation has been shown to be altered under various physiological and pathological circumstances. IgG N-glycan profiles vary with age, and galactosylation for example is enhanced during pregnancy. Several diseases including rheumatoid arthritis are associated with a reduction in galactosylation of the IgG N-glycans. Here, we describe a robust method for the isolation of IgG subclasses using protein A (binds IgG1, IgG2, and IgG4) and protein G (binds additionally IgG3) at the 96-well plate level, which is suitable for automation. Isolated IgGs were digested with trypsin, and obtained glycopeptides were analyzed by nano-LC-MS. Glycopeptides were characterized by CID as well as electron transfer dissociation (ETD). The method provided glycosylation profiles for IgG1, IgG2, IgG3, and IgG4 and revealed distinct differences in N-glycosylation between the four IgG subclasses. The changes in galactosylation associated with rheumatoid arthritis could readily be monitored. This method is suitable for the subclass-specific analysis of IgG glycosylation from clinical samples.

Endoglycosidase treatment abrogates IgG arthritogenicity: Importance of IgG glycosylation in arthritis

The glycosylation status of IgG has been implicated in the pathology of rheumatoid arthritis. Earlier, we reported the identification of a novel secreted endo-beta-N-acetylglucosaminidase (EndoS), secreted by Streptococcus pyogenes that specifically hydrolyzes the beta-1,4-di-N-acetylchitobiose core of the asparagine-linked glycan of human IgG. Here, we analyzed the arthritogenicity of EndoS-treated collagen type II (CII)-specific mouse mAb in vivo. Endoglycosidase treatment of the antibodies inhibited the induction of arthritis in (BALB/c x B10.Q) F1 mice and induced a milder arthritis in B10.RIII mice as compared with the severe arthritis induced by non-treated antibodies. Furthermore, EndoS treatment did not affect the binding of IgG to CII and their ability to activate complement, but it resulted in reduced IgG binding to FcgammaR and disturbed the formation of stable immune complexes. Hence, the asparagine-linked glycan on IgG plays a crucial role in the development of arthritis.

Structural alterations in outer arms of IgG oligosaccharides in patients with Werner syndrome

Werner syndrome (WS) is a heredofamilial disorder characterized by clinicopathological premature aging. In healthy individuals, structural alteration of serum IgG oligosaccharides is known to be an aging phenotype. In the present study, we determined and compared oligosaccharide structures of serum IgG among WS patients, healthy age-sex-matched individuals, and healthy elderly individuals from both sexes in order to reveal whether WS patients exhibit an aging phenotype in terms of IgG oligosaccharide structure. Sialylation and galactosylation levels of IgG oligosaccharides from WS patients were similar to those from healthy elderly individuals in which sialylation and galactosylation levels were significantly lower than those from the healthy age-sex-matched individuals. In contrast, the bisecting N-acetylglucosaminylation level of IgG oligosaccharides from WS patients was comparable to that from the healthy age-sex-matched controls and significantly lower than that of the healthy elderly controls. There was no significant sexual difference in these modifications of IgG oligosaccharides. These results suggest that WS patients exhibit an aging phenotype for structural alterations such as sialylation and galactosylation in the outer arms of IgG oligosaccharides.

Glycosylation of an immunoglobulin produced from a murine hybridoma cell line: The effect of culture mode and the anti-apoptotic gene,bcl-2

The impact of bcl-2 over-expression on the glycosylation pattern of an antibody produced by a bcl-2 transfected hybridoma cell line (TB/C3.bcl-2) was investigated in suspension batch, continuous and high cell density culture (Flat hollow fibre, Tecnomouse system). In all culture modes bcl-2 over-expression resulted in higher cell viability. Analysis of the glycans from the IgG of batch cultures showed that >95% of the structures were neutral core fucosylated asialo biantennary oligosaccharides with variable terminal galactosylation (G0f, G1f and G2f) consistent with previous analysis of glycans from the conserved site at Asn-297 of the IgG protein. The galactosylation index (GI) was determined as an indicator of the glycan profile (=(G2 + 0.5* G1)/(G0 + G1 + G2)). GI values in control cultures were comparable to bcl-2 cultures during exponential growth (0.53) but declined toward the end of the culture when there was a loss in cell viability. Low dilution rates in chemostat culture were associated with reduced galactosylation of the IgG glycans in both cell lines. However, at the higher dilution rates the GI for IgG was consistently higher in the TB/C3.bcl-2 cultures. In the hollow fibre bioreactor the galactosylation of the IgG glycans was considerably lower than in suspension batch or continuous cultures with GI values averaging 0.38. Similar low galactosylation values have been found previously for high density cell cultures and these are consistent with the low values obtained when the dissolved oxygen level is maintained at a low value (10%) in controlled suspension cultures of hybridomas.

Anti-Inflammatory Activity of Immunoglobulin G Resulting from Fc Sialylation

Immunoglobulin G (IgG) mediates pro- and anti-inflammatory activities through the engagement of its Fc fragment (Fc) with distinct Fcg receptors (FcgRs). One class of Fc-FcgR interactions generates pro-inflammatory effects of immune complexes and cytotoxic antibodies. In contrast, therapeutic intravenous gamma globulin and its Fc fragments are anti-inflammatory. We show here that these distinct properties of the IgG Fc result from differential sialylation of the Fc core polysaccharide. IgG acquires anti-inflammatory properties upon Fc sialylation, which is reduced upon the induction of an antigen-specific immune response. This differential sialylation may provide a switch from innate anti-inflammatory activity in the steady state to generating adaptive pro-inflammatory effects upon antigenic challenge.

A critical role for sialylation in cryoglobulin activity of murine IgG3 monoclonal antibodies

Cryoprecipitating IgG3 autoantibodies have been shown to play a significant role in the development of murine lupus-like autoimmune syndrome. However, the structural basis of IgG3 cryoprecipitation still remains to be defined. In view of the implication of positively charged amino acid residues present in variable regions in IgG3 cryoglobulin activity, we explored the role of terminal sialic acids in oligosaccharide side chains for the cryogenic activity of IgG3 mAb. Comparative oligosaccharide structural analysis of different cryogenic and non-cryogenic IgG3 mAb showed an inverse correlation between the extent of sialylation and cryogenic activity. The inhibitory role of sialylation was further confirmed by the demonstration of enrichment of less and more sialylated IgG3 in cryoprecipitated and noncryoprecipitated fractions, respectively, separated from four different cryogenic IgG3 mAb. Significantly, the sialic acid contents of the latter fraction became comparable to those of non-cryogenic IgG3 mAb. Finally, we observed that highly sialylated non-cryogenic IgG3 mAb was more potent in the inhibition of cryoprecipitation of cryogenic IgG3 mAb. Our results thus suggest that the content of negatively charged sialic acids in oligosaccharide side chains is one of the critical factors to determine IgG3 cryoglobulin activity, along with amino acid sequences of the IgG3 variable regions.

Polylactosamine synthesis and branch formation of N-glycans in β1,4-galactosyltransferase-1-deficient mice

Analysis of glycans from erythrocyte membrane glycoproteins from beta1,4-galactosyltransferase-1 (beta4GalT-1)-deficient mice revealed moderately decreased galactosylation but comparable polylactosamine content compared to control beta4GalT-1(+/-) mice. The increased expression of more branched N-glycans was observed in beta4GalT-1(-/-) mice, and its extent was more remarkable in elder beta4GalT-1(-/-) mice (28 weeks old) than in younger beta4GalT-1(-/-) mice (6-9 weeks old). In relation to this issue, the less galactosylation of biantennary glycans was observed in the elder group, suggesting that beta4GalTs actually compete with N-acetylglucosaminyltransferases IV and V in erythroid cells. In contrast, approximately 80% of core 2 O-glycans were not beta1,4-galactosylated regardless of age of the knockout mice. These results suggest that beta4GalT-1 expressed in erythroid cells may regulate a constant branch formation of N-glycans and plays a predominant role in beta1,4-galactosylation of core 2 O-glycan.

Hypogalactosylation of serum IgG in patients with coeliac disease

Coeliac disease (CD) is described as an autoimmune enteropathy associated with the presence of IgG and IgA antigliadin and antitransglutaminase autoantibodies. While of diagnostic significance, the role of these autoantibodies in the immunopathogenesis of CD is elucidated. An inappropriate T cell immune response to gluten is also involved in the pathogenesis of CD, as evidenced by autoantibody switching. The N-glycans released from serum IgG of CD patients and three groups of healthy controls, of differing age ranges, were analysed by NH2-high performance liquid chromatography (HPLC). The fucosylated biantennary N- glycans were the most abundant neutral oligosaccharides; in particular, the agalacto form (G0F) showed a mean value of 42% (s.d. +/- 7.4), 30% (s.d. +/- 5.9), 26% (s.d. +/- 4.2) and 35% (s.d. +/- 6.8) for CD patients, healthy children, healthy adults under 40 and healthy adults over 40 years old, respectively. The ratio of asialo agalacto fucosylated biantenna to asialo monogalacto fucosylated biantenna (G0F)/(G1F) for CD patients showed a significant increase compared to healthy children (P < 0.0002), healthy adults under 40 (P < 0.0002) and healthy adults over 40 years old (P < 0.01). Hypogalactosylation was more pronounced for CD patients than for the patients with other autoimmune diseases such as rheumatoid arthritis or psoriatic arthritis.

Level of galactosylation determines cryoglobulin activity of murine IgG3 monoclonal rheumatoid factor

Autoantibodies of the cryoprecipitating IgG3 isotype have been shown to play a significant role in the development of murine lupus-like autoimmune syndrome. At present, the structural basis of IgG3 cryoprecipitation and its role in autoantibody pathogenicity remain to be defined. Using molecular variants of an IgG3 monoclonal rheumatoid factor, 6-19, derived from an autoimmune MRL-Fas(lpr) mouse, we have investigated the implication of charged residues in the heavy-chain variable (VH) region, potential CH3-linked oligosaccharides, and galactosylation of CH2-linked oligosaccharides in its cryoglobulin activity. The cryoglobulin activity of the IgG3 6-19 mutant bearing more negatively charged residues at VH 6 and 23 was found to be reduced but still highly significant, whereas that of the mutant lacking a potential CH3 glycosylation site remained unchanged. In marked contrast, IgG3 6-19 variants obtained from 6-19 heavy-chain transgenic mice displayed barely detectable cryoglobulin activity associated with an increased level of galactosylation in the CH2 oligosaccharide side chains. Thus, our data strongly suggest that the cryoglobulin activity of IgG3 6-19 autoantibody is critically determined by levels of galactosylation in the CH2 oligosaccharide side chains, whereas VH residues play a secondary role in 6-19 IgG3 cryoglobulin activity.

Structural studies on IgG oligosaccharides of patients with primary Sjögren's syndrome

Sjögren's syndrome (SS) is an autoimmune disease, and some patients have been found to have SS complicated with rheumatoid arthritis (RA), in which IgG is known to carry abnormal N-linked oligosaccharides. In order to investigate the relationship between SS and RA, the structures of N-linked oligosaccharides of IgG from 12 primary SS patients without RA, 9 RA patients, and 8 healthy individuals were analyzed using reversed-phase high-performance liquid chromatography, in combination with sequential exoglycosidase treatment and matrix-assisted laser desorption ionization time-of-flight mass spectrometry. All of the IgG samples obtained from primary SS patients, RA patients, and healthy individuals contained the same series of biantennary complex-type oligosaccharides, but the ratio of each oligosaccharide differed among these 3 groups. The incidence of galactose-lacking N-linked oligosaccharides obtained from the IgG of RA patients was significantly higher than that from healthy individuals, but that from the serum IgG of primary SS patients varied among individuals. The patients with primary SS were classified into two groups based on the galactosylation levels of IgG oligosaccharides; one group exhibits galactosylation levels as low as those of RA patients and another exhibits levels similar to those of healthy individuals. Measurement of levels of rheumatoid factor (RF) revealed that primary SS patients with a high incidence of RF belonged to the low galactosylation group, as did RA patients. These results suggest that appearance of IgG carrying abnormal N-linked oligosaccharides in primary SS may be related to future complication with RA.

Abnormal IgG galactosylation in MRL-lpr/lpr mice: pathogenic role in the development of arthritis

MRL-lpr/lpr (MRL/lpr) mice spontaneously develop arthritis by an increase in the incidence of agalactosylated oligosaccharides in serum IgG, similar to rheumatoid arthritis patients. However, whether this association has a pathogenic significance is still unknown. In this study, we analyzed the oligosaccharide structure of serum IgG in various MRL mice with or without arthritis, to clarify the relationship between the oligosaccharide abnormality and the development of arthritis. The level of agalactosylation in serum IgG was comparable in both arthritis-free MRL/lpr and MRL-+/+ (MRL/+) mice at 6 weeks of age. In contrast, the incidence of IgG lacking galactose markedly increased in MRL/lpr mice at 6 months of age (the age at which arthritis occurred), compared with that from age-matched MRL/+ mice without arthritis. However, the proportion of agalactosylated IgG increased similarly in anti-CD4 monoclonal antibody-treated MRL/lpr mice at 6 months of age, despite the absence of the development of arthritis, because of depletion of CD4+ T cells. These results suggest that the abnormality in IgG galactosylation of MRL/lpr mice developed in an age-dependent manner, but it did so independently of CD4+ T cell-dependent B-cell activation and is not a consequence of the development of arthritis.

Abnormal IgG galactosylation and arthritis in MRL-Fas(lpr) or MRL-FasL(gld) mice are under the control of the MRL genetic background

MRL mice bearing the lpr (Fas) or gld (Fas ligand) mutation, MRL-Fas(lpr) or MRL-FasL(gld), respectively, develop arthritis similar to rheumatoid arthritis, but C3H and C57BL/6 mice bearing such mutations do not. In MRL-Fas(lpr) mice, agalactosylated oligosaccharides in serum IgG increase significantly in comparison to MRL-+/+ mice without arthritis. In this study, an increased level of agalactosylation in IgG, as compared to MRL-+/+, was found in both MRL-Fas(lpr) and MRL-FasL(gld) mice. In contrast, the incidence of IgG without galactose was comparable among C3H-Fas(lpr), C3H-FasL(gld), and C3H-+/+ mice as well as between C57BL/6-Fas(lpr) and C57BL/6-+/+ mice. These results suggest that the increase in agalactosylated IgG and the development of arthritis in MRL-Fas(lpr) and MRL-FasL(gld) mice are controlled by the MRL genetic background.

Rapid and simple preparation of N-linked oligosaccharides by cellulose-column chromatography

As a means of preparing N-linked oligosaccharides from hydrazinolysates of glycoproteins in a rapid and simple manner, a method has been developed using cellulose-column chromatography. Hydrazinolysates of human IgG, containing a series of biantennary complex type oligosaccharides, were applied to a cellulose column equilibrated with (4:1:1, v/v) 1-butanol-ethanol-water. The N-linked oligosaccharides were eluted with (1:1, v/v) ethanol-water, and analyzed by HPLC in combination with sequential glycosidase digestion. The oligosaccharides, with or without sialic acid, were quantitatively recovered in the fraction eluted with (1:1, v/v) ethanol-water without UV-detectable contamination by impurities derived from protein or the cellulose. Other types of N-linked oligosaccharides of alpha1-acid glycoprotein (tetraantennary complex-type), ovalbumin (hybrid-type), and ribonuclease B (high mannose-type) were also quantitatively prepared from the hydrazinolysates by elution of the cellulose column with (1:1, v/v) ethanol-water and these had as high a quality as those prepared by conventional paper chromatography.

Role of galactosylation in the renal pathogenicity of murine immunoglobulin G3 monoclonal cryoglobulins

Cryoglobulin activity associated with murine immunoglobulin G3 (IgG3) has been shown to play a significant role in the development of murine lupuslike glomerulonephritis. A fraction, but not all, IgG3 monoclonal antibodies are capable of inducing a severe acute lupuslike glomerulonephritis as a result of direct localization of IgG3 cryoglobulins, suggesting the importance of qualitative features of cryoglobulins in their nephritogenic activities. Here a remarkable difference is shown in the renal pathogenicity of 2 murine IgG3 monoclonal cryoglobulins, identical in the amino acid sequences of their heavy and light chains but different in galactosylation patterns of oligosaccharide side chains because of their synthesis in different myeloma cells. The antibody lacking the capacity to induce severe glomerulonephritis displayed an increased proportion of galactosylated heavy chains. Changes in conformation, as revealed by gel filtration analysis, reduced cryoglobulin activity, and accelerated clearance could account for the lack of the renal pathogenicity of the more galactosylated variant. This observation provides a direct demonstration for the role of IgG galactosylation in the pathogenic potential of cryoglobulins.

High galactosylation of oligosaccharides in umbilical cord blood IgG, and its relationship to placental function

N-linked oligosaccharides on human serum IgGs have been reported to modulate IgG function. We studied umbilical cord blood to determine whether neonatal IgGs have characteristic structures related to developmental and pathological status. Oligosaccharide patterns of serum IgG from 45 umbilical cord blood samples were characterized by HPLC, and compared with those of serum IgG from 11 normal adults. Oligosaccharyl amines from purified IgG were released by recombinant N-glycanase, labeled with fluorescence reagent FMOC (9-fluorenylmethyl chloroformate), and analyzed quantitatively by high-pressure liquid chromatography (HPLC). Increased galactosylation was observed in cord blood. The ratio of galactosylated to non-galactosylated oligosaccharides on IgG was 7.90+/-3.92 (mean+/-S.D.) in cord blood, significantly higher than the ratio in adults (1.60+/-0.62, P<0.0001). There were weak but not significant correlations between the ratio and birth weight, gestation period, mother's age, and no correlation with serum IgG concentration. The ratio was lower for premature or intra-uterine growth retarded neonates. Our results, in conjunction with previous reports that galactosylated IgG stimulates Fc-mediated phagocytosis of monocytes, suggest that increased galactosylation of IgG enhances neonatal immunity.