Differential B lymphocyte galactosyltransferase activity in the MRL mouse model of rheumatoid arthritis

Divergent Golgi trafficking limits B cell-mediated IgG sialylation

The degree of α2,6-linked sialylation on IgG glycans is associated with a variety of inflammatory conditions and is thought to drive IgG anti-inflammatory activity. Previous findings revealed that ablation of β-galactoside α2,6-sialyltransferase 1 (ST6Gal1) in B cells failed to alter IgG sialylation in vivo, yet resulted in the loss of B cell surface α2,6 sialylation, suggesting divergent pathways for IgG and cell surface glycoprotein glycosylation and trafficking. Employing both B cell hybridomas and ex vivo murine B cells, we discovered that IgG was poorly sialylated by ST6Gal1 and highly core fucosylated by α1,6-fucosyltransferase 8 (Fut8) in cell culture. In contrast, cell surface glycoproteins on IgG-producing cells showed the opposite pattern by flow cytometry, with high α2,6 sialylation and low α1,6 fucosylation. Paired studies further revealed that ex vivo B cell-produced IgG carried significantly less sialylation compared with IgG isolated from the plasma of matched animals, providing evidence that IgG sialylation increases after release in vivo. Finally, confocal analyses demonstrated that IgG poorly localized to subcellular compartments rich in sialylation and ST6Gal1, and strongly to regions rich in fucosylation and Fut8. These findings support a model in which IgG subcellular trafficking diverges from the canonical secretory pathway by promoting Fut8-mediated core fucosylation and limiting exposure to and modification by ST6Gal1, providing a mechanism for why B cell-expressed ST6Gal1 is dispensable for IgG sialylation in vivo.

Immunoglobulin G Glycosylation Changes in Aging and Other Inflammatory Conditions

Among the multiple roles played by protein glycosylation, the fine regulation of biological interactions is one of the most important. The asparagine 297 (Asn₂₉₇) of IgG heavy chains is decorated by a diantennary glycan bearing a number of galactose and sialic acid residues on the branches ranging from 0 to 2. In addition, the structure can present core-linked fucose and/or a bisecting GlcNAc. In many inflammatory and autoimmune conditions, as well as in metabolic, cardiovascular, infectious, and neoplastic diseases, the IgG Asn₂₉₇-linked glycan becomes less sialylated and less galactosylated, leading to increased expression of glycans terminating with GlcNAc. These conditions alter also the presence of core-fucose and bisecting GlcNAc. Importantly, similar glycomic alterations are observed in aging. The common condition, shared by the above-mentioned pathological conditions and aging, is a low-grade, chronic, asymptomatic inflammatory state which, in the case of aging, is known as inflammaging. Glycomic alterations associated with inflammatory diseases often precede disease onset and follow remission. The aberrantly glycosylated IgG glycans associated with inflammation and aging can sustain inflammation through different mechanisms, fueling a vicious loop. These include complement activation, Fcγ receptor binding, binding to lectin receptors on antigen-presenting cells, and autoantibody reactivity. The complex molecular bases of the glycomic changes associated with inflammation and aging are still poorly understood.

The history of IgG glycosylation and where we are now

IgG glycosylation is currently at the forefront of both immunology and glycobiology, likely due in part to the widespread and growing use of antibodies as drugs. For over four decades, it has been recognized that the conserved N-linked glycan on asparagine 297 found within the second Ig domain of the heavy chain (CH2) that helps to comprise Fc region of IgG plays a special role in IgG structure and function. Changes in galactosylation, fucosylation and sialylation are now well-established factors, which drive differential IgG function, ranging from inhibitory/anti-inflammatory to activating complement and promoting antibody-dependent cellular cytotoxicity. Thus, if we are to truly understand how to design and deploy antibody-based drugs with maximal efficacy and evaluate proper vaccine responses from a protective and functional perspective, a deep understanding of IgG glycosylation is essential. This article is intended to provide a comprehensive review of the IgG glycosylation field and the impact glycans have on IgG function, beginning with the earliest findings over 40 years ago, in order to provide a robust foundation for moving forward.

Glycobiology of Aging

Glycosylation is one of the most frequent post-translational modification of proteins. Many membrane and secreted proteins are decorated by sugar chains mainly linked to asparagine (N-linked) or to serine or threonine (O-linked). The biosynthesis of the sugar chains is mainly controlled by the activity of their biosynthetic enzymes: the glycosyltransferases. Glycosylation plays multiple roles, including the fine regulation of the biological activity of glycoproteins. Inflammaging is a chronic low grade inflammatory status associated with aging, probably caused by the continuous exposure of the immune system to inflammatory stimuli of endogenous and exogenous origin. The aging-associated glycosylation changes often resemble those observed in inflammatory conditions. One of the most reproducible markers of calendar and biological aging is the presence of N-glycans lacking terminal galactose residues linked to Asn₂₉₇ of IgG heavy chains (IgG-G0). Although the mechanism(s) generating IgG-G0 remain unclear, their presence in a variety of inflammatory conditions suggests a link with inflammaging. In addition, these aberrantly glycosylated IgG can exert a pro-inflammatory effect through different mechanisms, triggering a self-fueling inflammatory loop. A strong association with aging has been documented also for the plasmatic forms of glycosyltrasferases B4GALT1 and ST6GAL1, although their role in the extracellular glycosylation of antibodies does not appear likely. Siglecs, are a group of sialic acid binding mammalian lectins which mainly act as inhibitory receptors on the surface of immune cells. In general activity of Siglecs appears to be associated with long life, probably because of their ability to restrain aging-associated inflammation.

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.

Glycan analysis of monoclonal antibodies secreted in deposition disorders indicates that subsets of plasma cells differentially process IgG glycans

OBJECTIVE

To compare the glycosylation of polyclonal serum IgG heavy chains in a patient with rheumatoid arthritis (RA) with that of monoclonal serum IgG heavy chains in the same patient during an episode of heavy-chain deposition disease (HCDD), to establish whether glycosylation processing is specific for subsets of B cells.

METHODS

Serum IgG was purified using a HiTrap protein G column. Immunoglobulins were run on sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels, and IgG glycans were isolated from gel bands and fluorescently labeled. Glycans were analyzed by normal-phase high-performance liquid chromatography and by liquid chromatography-electrospray ionization-mass spectrometry.

RESULTS

The glycosylation of serum immunoglobulins from a patient with seronegative RA and HCDD was analyzed. The predominant immunoglobulin was a truncated glycosylated gamma3 heavy chain, and a small amount of polyclonal IgG was also present. The glycan profile showed that the monoclonal gamma3 heavy chain contained fully galactosylated biantennary glycans with significantly less fucose but more sialic acid than in IgG3 from healthy controls. In contrast, the polyclonal IgG showed an RA-like profile, with a predominance of fucosylated biantennary glycans and low levels of galactosylation. The glycan profile of serum IgG obtained from the same patient during disease remission resembled a typical RA profile.

CONCLUSION

These data indicate that different types of B cells process a particular set of IgG glycoforms.

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.

Changes in the galactose content of IgG during humoral immune responses

The Fc region of IgG bears two oligosaccharides of variable composition. The serum level of one variant which lacks terminal galactose and sialic acid (agalactosyl IgG) is raised in a number of autoimmune diseases and animal models thereof. Here it is shown that such changes in IgG glycosylation occur during non-pathological humoral immune responses. It was found that if specific pathogen free (SPF) CBA/Ca mice are transferred from a sterile to a conventional environment, their levels of total serum IgG rise whereas the degree of IgG galactosylation falls. Next, mice were immunised with bovine serum albumin (BSA) in incomplete Freund's adjuvant. As anti-BSA titres rose the antibodies became less galactosylated and later, as the titres fell, the antibodies became more galactosylated. By contrast, there was little or no variation in the relative galactosylation of total IgG. It is considered that the galactosylation of IgG antibodies varies during an immune response.

A family of human beta4-galactosyltransferases. Cloning and expression of two novel UDP-galactose:beta-n-acetylglucosamine beta1, 4-galactosyltransferases, beta4Gal-T2 and beta4Gal-T3

BLAST analysis of expressed sequence tags (ESTs) using the coding sequence of the human UDP-galactose:beta-N-acetylglucosamine beta1, 4-galactosyltransferase, designated beta4Gal-T1, revealed a large number of ESTs with identical as well as similar sequences. ESTs with sequences similar to that of beta4Gal-T1 could be grouped into at least two non-identical sequence sets. Analysis of the predicted amino acid sequence of the novel ESTs with beta4Gal-T1 revealed conservation of short sequence motifs as well as cysteine residues previously shown to be important for the function of beta4Gal-T1. The likelihood that the identified ESTs represented novel galactosyltransferase genes was tested by cloning and sequencing of the full coding region of two distinct genes, followed by expression. Expression of soluble secreted constructs in the baculovirus system showed that these genes represented genuine UDP-galactose:beta-N-acetylglucosamine beta1, 4-galactosyltransferases, thus designated beta4Gal-T2 and beta4Gal-T3. Genomic cloning of the genes revealed that they have identical genomic organizations compared with beta4Gal-T1. The two novel genes were located on 1p32-33 and 1q23. The results demonstrate the existence of a family of homologous galactosyltransferases with related functions. The existence of multiple beta4-galactosyltransferases with the same or overlapping functions may be relevant for interpretation of biological functions previously assigned to beta4Gal-T1.

Agalactosyl IgG and beta-1,4-galactosyltransferase gene expression in rheumatoid arthritis patients and in the arthritis-prone MRL lpr/lpr mouse

Reduced galactosylation of immunoglobulin G (IgG) is well documented in rheumatoid arthritis (RA), but the reason for this defect is still unknown. There is some evidence supporting a defect in the biosynthetic pathway, and a reduction in the level of beta-1,4-galactosyltransferase (beta-1,4-GalTase) enzyme activity. Since glycosyltransferases are, in general, regulated at the level of transcription, we have measured the level of beta-1,4-GalTase gene expression in B cells from patients with RA and normal control individuals. We found no significant difference in mRNA levels for the transferase in these two groups (P > 0.7). MRL/Mp-lpr/lpr (MRL-lpr) mice develop a spontaneous arthritis with increased levels of agalactosyl IgG (G0). In spite of a significant reduction in the level of beta-1,4-GalTase mRNA in total spleen lymphocytes from MRL-lpr mice compared with the congenic MRL/Mp-(+/+) (MRL-(+/+) mice and with CBA/Ca mice, we found comparable levels of the beta-1,4-GalTase mRNA in purified B cells from both spleen and lymph nodes of the three strains. Amongst the lymphoid compartments examined, the spleen and peripheral blood were found to be the major contributors of G0 in MRL-lpr mice. These data indicate that in neither human RA, nor in an animal model of this disease, is reduced IgG galactosylation caused by impaired expression of the beta-1,4-GalTase gene in B lymphocytes. Furthermore, splenic B cells, which have normal levels of beta-1,4-GalTase mRNA, appear to be a major source of G0 in MRL-lpr mice.

Evaluation of beta 1,4-galactosyltransferase in rheumatoid arthritis and its role in the glycosylation network associated with this disease

Evidence indicating an important link between glycosylation changes and autoimmune rheumatic disease is presented. Attention is especially focused on the interrelationship between reduced galactosylation of the oligosaccharides of IgG, auto-sensitization which is thought to be of central importance in the pathogenesis of rheumatoid arthritis (RA), and the enzyme beta 1,4-galactosyltransferase (GTase) that catalyses the addition of galactose to the oligosaccharide chains on this molecule. Data are presented to indicate that GTase undergoes a variety of normal and disease associated changes. These variations are believed to contribute to the pathological processes in rheumatoid disease, and a hypothesis is suggested, whereby disease is associated with the dysregulation of an integrated glycosylation network, comprising IgG galactosylation, lymphocytic GTase and anti-GTase antibodies, that is a component of the normal immune system.

Beta 1,4-galactosyltransferase variations in rheumatoid arthritis

Evidence indicating an important link between glycosylation changes and autoimmune rheumatic disease is presented. Attention is especially focused on the interrelationship between reduced galactosylation of the oligosaccharides of IgG, auto-sensitization to which is thought to be of central importance in the pathogenesis of rheumatoid arthritis (RA), and the enzyme beta 1,4-Galactosyltransferase (GTase) that catalyses the addition of galactose to the oligosaccharide chains on this molecule. Data are presented to indicate that GTase undergoes a variety of normal and disease associated changes. These variations are believed to contribute to the pathological processes in rheumatoid disease, and a hypothesis is suggested, whereby disease is associated with the dysregulation of an integrated glycosylation network, comprising IgG galactosylation, lymphocytic GTase and anti-GTase antibodies, that is a component of the normal immune system.