IgG glycosylation in autoimmune-prone strains of mice

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.

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.

Profiling of IgG N-glycome during mouse aging: Fucosylated diantennary glycans containing one Neu5Gc-linked LacNAc are associated with age

N-glycosylation of immunoglobulin G (IgG) has been reported to change in human aging and in some age-related diseases. To further understand the molecular processes that determine these alterations, a detailed examination of individual IgG N-glycans with aging remains required. Mouse is the most commonly used model animal in studies of aging and age-related diseases, and mice have the advantage of relatively controllable genetic and environment variations compared to human. In this study, we systemically investigated the changes in serum IgG N-glycome in C57BL/6 mice during aging at 12 time points (6-80 weeks) via ultraperformance liquid chromatography with fluorescence detection. The study demonstrated several important findings. First, four chromatographic IgG N-glycan peaks were identified for the first time, including a high-mannose glycan, a monoantennary glycan, and two afucosylated glycans. Second, most of the IgG glycan levels changed significantly and presented pronounced gender-related differences from 6 to 12 weeks. Interestingly, all the IgG glycan levels tended to be similar between male and female mice at 12 weeks. Third, the level of fucosylated diantennary glycans containing one N-glycolylneuraminic acid (Neu5Gc)-linked N-acetyllactosamine (LacNAc) decreased gradually and showed a significant negative correlation with age from 24 to 80 weeks (r = -0.716, p < 0.0001), which was not sex-specific. SIGNIFICANCE: More comprehensive profile of murine IgG N-glycans by ultraperformance liquid chromatography with fluorescence detection was shown in this study with four newly identified chromatographic murine IgG N-glycan peaks. The majority of IgG N-glycans showed substantial stage-specific changes and sex-related differences during mouse aging, indicating a strict regulatory mechanism of glycan synthesis. The level of fucosylated diantennary glycans containing one Neu5Gc-linked LacNAc was significantly negatively correlated with age from 24 to 80 weeks, suggesting its great potential as an aging biomarker. The detailed characteristics of IgG N-glycosylation with aging in C57BL/6 mice demonstrated in the present study could provide essential reference data for studying the function and mechanism of IgG glycosylation in age-related researches based on C57BL/6 mouse models.

Profiling and genetic control of the murine immunoglobulin G glycome

Immunoglobulin G (IgG) glycosylation is essential for function of the immune system, but the genetic and environmental factors that underlie its inter-individual variability are not well defined. The Collaborative Cross (CC) genetic resource harnesses over 90% of the common genetic variation of the mouse. By analyzing the IgG glycome composition of 95 CC strains, we made several important observations: (i) glycome variation between mouse strains was higher than between individual humans, despite all mice having the same environmental influences; (ii) five genetic loci were found to be associated with murine IgG glycosylation; (iii) variants outside traditional glycosylation site motifs affected glycome variation; (iv) bisecting N-acetylglucosamine (GlcNAc) was produced by several strains although most previous studies have reported the absence of glycans containing the bisecting GlcNAc on murine IgGs; and (v) common laboratory mouse strains are not optimal animal models for studying effects of glycosylation on IgG function.

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.

A longitudinal study of the relationship between galactosylation degree of IgG and rheumatoid factor titer and avidity during long-term immunization of rabbits with BSA

Although IgG with reduced content of galactose has been implicated as important in the autoimmune rheumatoid factor (RF) response in rheumatoid arthritis (RA), relatively little is known about the temporal relationship between RF and the degree of galactosylation of IgG in vivo. We established an experimental model for studying the dynamic association between changes in the relative extent of galactosylation of IgG antigen(s) and the main parameters of RF activity, such as the titer, specificity and functional affinity/avidity. Rabbits hyperimmunized with BSA were used for examining the influence of long-term antigenic stimulation on the galactosylation status of IgG and rheumatoid factor production. The results showed that the galactosylation profile of IgG varied during the humoral anti-BSA response in rabbits and that the accompanying RF response fluctuated in titer and binding avidity for differently galactosylated IgG. The immune complexes (IC) were found to be composed of differently galactosylated IgG differing in capacity to inhibit the agglutination activity of RF. Moreover, the ability of circulating RF to react avidly with rather small IC was associated with a lower content of galactose in complexed IgG. The results suggest that a certain dynamic relationship exists between the oligosaccharide moiety of IgG and the titer and avidity of RF during the normal anti-BSA response of rabbits.

Noncognate binding to histones of IgG from patients with idiopathic systemic lupus erythematosus

The mechanism of attachment of circulating immune complexes (CIC) to glomerular basement membranes (GBM) has not yet been elucidated. Since it has been proposed that histone may be the ligand between GBM and DNA/anti-DNA CIC, we explored by ELISA and Western blots the nature of the interaction of IgG with histone on solid phase. Cognate binding of IgG anti-histone antibody was characteristically dependent on in its F(ab')(2) fragment and was inhibited by free histone. On the other hand, heat-aggregated IgG, a model for CIC, and IgG from most patients with idiopathic SLE had a characteristic noncognate binding behavior to histone: it was dependent on Fcgamma rather than on the F(ab')(2) fragment and was not effectively inhibited by free histones. Also, binding to histone of in vitro generated DNA/anti-DNA immune complexes was not dependent on DNA as a histone ligand, but on Fcgamma. Finally, there was good agreement between the binding of this IgG to histone and to C1q. We concluded that: (1) altered IgG and/or CIC bind to solid-phase-attached histone primarily through their Fcgamma and (2) CIC may mimic IgG antihistone antibodies in solid-phase immunoassays.

Binding and Uptake of Agalactosyl IgG by Mannose Receptor on Macrophages and Dendritic Cells

Increased levels of agalactosyl IgG (G0 IgG) are found in several autoimmune diseases, including rheumatoid arthritis, in which they are correlated with severity of the disease. To investigate whether structural alteration of IgG may lead to aberrant processing and presentation of IgG peptides as autoantigens, we have studied uptake of G0 IgG by human dendritic cells and macrophages cultured from PBMC. We found that enzymatic removal of terminal galactose residues, which exposes N-acetylglucosamine residues, increases uptake of soluble IgG mediated by mannose receptor on macrophages and dendritic cells. Efficient uptake appears to require recycling of the receptor, can be blocked by saccharides or Abs reactive with mannose receptor, and is dependent upon the state of maturation of the dendritic cells. No differences between IgG isotypes in ability to be internalized by APC were identified, suggesting that uptake would not be limited to a particular subset of Abs. These results suggest a novel pathway by which Abs or Ag-Ab complexes can be taken into dendritic cells and macrophages, and potentially generate epitopes recognized by T cells. These findings may have particular relevance for autoimmune disorders characterized by high levels of G0 IgG.

The effect on immunoglobulin glycosylation of altering in vivo production of immunoglobulin G

The effect on murine immunoglobulin G (IgG) glycosylation of altering IgG production in vivo was assessed in interleukin (IL)-6 transgenic and CD4 knockout mice. C57BL/6 mice carrying the IL-6 transgene showed increased levels of circulating IgG. This was associated with decreased levels of galactose on the IgG oligosaccharides. No decrease in beta4-galactosyltransferase mRNA or in enzyme activity was seen in IL-6 transgenic mice. MRL-lpr/lpr mice normally have elevated levels of circulating IgG, again accompanied by decreased levels of IgG galactose. Disruption of the CD4 gene in MRL-lpr/lpr mice led to a substantial decrease in the concentration of circulating IgG, but IgG galactose levels remained low. Thus, an enforced decrease in IgG levels in the lymphoproliferative MRL-lpr/lpr mice did not alter the percentage of agalactosyl IgG in these mice, suggesting that agalactosyl IgG production is not simply caused by excessive IgG synthesis leading to an insufficient transit time in the trans-Golgi, but rather to a molecular defect in the interaction between galactosyltransferase and the immunoglobulin heavy chain.

Galactosylation of serum IgG and autoantibodies in murine models of autoimmune haemolytic anaemia

A number of systemic autoimmune diseases are associated with increased levels of the agalactosyl (G0) IgG isoforms that lack a terminal galactose from the CH2 domain oligosaccharide. The current aim was to determine whether the galactosylation of serum IgG is also reduced in a classic antibody-mediated, organ-specific autoimmune condition, and whether the pathogenic autoantibodies are preferentially G0. In two murine forms of autoimmune haemolytic anaemia (AIHA), sera and autoantibodies eluted from erythrocytes were obtained, and the levels of G0 measured using a lectin-binding assay. Serum IgG galactosylation was unaffected following the induction of AIHA in CBA/Igb mice by immunization with rat erythrocytes, but in all animals with the disease the IgG autoantibodies generated were more G0 than the sera. The anti-rat erythrocyte antibodies were similar to the autoantibodies in being preferentially G0, and when CBA/Igb mice were immunized with canine erythrocytes as a control foreign antigen, there was again a bias towards the production of G0 IgG antibodies. In NZB mice with chronic, spontaneous AIHA, the concentration and galactosylation of both serum IgG and autoantibodies were lower than in the induced model, and the ratio of G0 IgG in the serum and erythrocyte eluates varied markedly between different individuals. Our interpretation of these results is that changes in serum IgG or autoantibody galactosylation are not consistent in different models of AIHA, and that production of low galactosyl antibodies can be a feature of a normal immune response.

The role of glycosylation in autoimmune disease

Oligosaccharide structures play a key role in the antigenicity of a number of clinically important antigens such as blood group determinants. Interest in glycobiology has increased dramatically amongst immunologists during the last few years due to the fact that oligosaccharides also play a central role in adhesion and homing events during inflammatory processes (1), comprise powerful xenotransplantation antigens (2), and may provide targets for tumor immunotherapy (3). Additionally, alterations in glycosylation are now known to occur in a number of autoimmune diseases. This review will first discuss some general aspects of protein glycosylation and then explore some of the autoimmune diseases in which the role of glycosylation has been examined.

The p68 autoantigen characteristic of rheumatoid arthritis is reactive with carbohydrate epitope specific autoantibodies

OBJECTIVE

The autoantigen p68 is a target of autoantibodies as well as autoreactive T cells with a high specificity in rheumatoid arthritis (RA). The binding characteristics of the autoantibodies to their antigen were now analysed biochemically and cytologically.

METHODS

Deglycosylation techniques as well as lectin and sugar competition experiments were performed to p68 to discover if the antibodies detected a glycoepitope, Its antigenicity was investigated applying anti-p68 antibodies derived from RA patients in comparison with polyclonal rabbit anti-p68 antibodies.

RESULTS

p68 specific antibodies from RA patients did not to bind to p68 that had been deglycosylated by alkaline beta-elimination, O-glycosidase or periodate treatment. In contrast, binding of p68 specific antibodies raised in rabbit was unaffected by either deglycosylation protocol. Furthermore, lectins specific for the carbohydrate N-acetylglucosamine competed with p68 specific antibodies from RA patients for antigen bindings. N-acetylglucosamine by itself also competed with patient derived anti-p68 antibodies for p68 binding. Again, rabbit and anti-p68 antibodies did not elicit these competitive effects. Applying cytoimmunofluorescence, p68 was present in the cytoplasm or endoplasmic reticulum and also in low abundance on the cell surface. Under heatshock conditions, p68 was detectable in the nucleus.

CONCLUSIONS

Autoimmunity to p68 during RA is carried by anti-carbohydrate autoantibodies. The carbohydrate modification of p68 appears to be N-acetylglucosamine, which may reflect the regulation of intracellular localisation of the antigen. It is hypothesised that a shift in glycosylation pattern accompanied by an unphysiological localisation of the antigen could trigger antigenicity of p68 during the pathogenesis of IRA.

Bias in murine IgG isotype immobilisation. Implications for IgG glycoform analysis ELISA procedures

This study investigates immobilisation of murine IgG in various ELISA procedures. Monoclonal murine IgG isotypes and polyclonal IgG from sera were studied. Similar binding curves to plastic were found for all four individual murine IgG isotypes. Single isotypes displayed different affinities for both protein A and protein G, in particular IgG1 was poorly and IgG3 strongly bound to both of these proteins. When mixtures of the isotypes were bound to either plastic, protein A or protein G, competition was observed in which IgG3 was dominant. Paradoxically, studies on the binding rates of single isotypes direct to plastic revealed that IgG3 had the slowest binding rate. Heating of bound IgGs resulted in significant but isotypically non-selective losses from the plates. The data demonstrate that despite obtaining equivalent individual IgG isotype binding curves, mixtures of IgG isotypes behave very differently, with competition for binding occurring even on plastic. The IgG isotype levels of murine sera were measured for individual mice, and the capture efficiency of each IgG isotype by protein A determined at different serum dilutions. Comparisons were made between the observed capture levels of IgG isotypes and their known serum levels. At all dilutions tested, greater than expected binding of IgG3, IgG2b and IgG2a was observed. At a serum dilution of 1/100 the binding of these three isotypes was increased 16-, 2.9- and 0.4-fold, respectively. These increases were balanced by a decrease in IgG1 binding which was the most prevalent serum IgG isotype. The results described above suggest that capture techniques are biased and unlikely to provide a coating of IgG isotypes that accurately reflects that of the serum. This bias is derived from the specificity of the individual isotypes for either protein A or protein G, and the errors further compounded by direct competition between isotypes whatever the capture surface. Induced coalescence of IgG3 may explain the latter observations.

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.

Reduced galactosyltransferase mRNA levels are associated with the agalactosyl IgG found in arthritis-prone MRL-lpr/lpr strain mice

MRL-lpr/lpr strain mice have defectively glycosylated IgG. This may be related to the rheumatoid arthritis (RA)-like disease that occurs in these mice, because a similar glycosylation defect is seen in human subjects with RA. Whilst it is known that this defect is associated with reduced activity of the beta-1,4-galactosyltransferase (beta-1,4-GalTase) enzyme, the cause of this reduced activity is at present unknown. We have therefore examined the molecular genetics of beta-1,4-GalTase in MRL-lpr/lpr mice. Using 10 different restriction endonucleases we found no evidence for a polymorphic variant of the gene in glycosylation-defective mice. However, the level of mRNA for beta-1,4-GalTase was lowest in the MRL-lpr/lpr mice, the strain with the most poorly galactosylated IgG of the four strains examined. Thus, the reduced level of IgG oligosaccharide galactosylation found in MRL-lpr/lpr strain mice appears to be related to either an altered transcriptional level of, or altered mRNA stability for, beta-1,4-GalTase in lymphocytes from these mice.