Galactose-deficient IgA1 in sera of IgA nephropathy patients is present in complexes with IgG

Naturally occurring structural isomers in serum IgA1 o-glycosylation

IgA is the most abundantly produced antibody and plays an important role in the mucosal immune system. Human IgA is represented by two isotypes, IgA1 and IgA2. The major structural difference between these two subclasses is the presence of nine potential sites of O-glycosylation in the hinge region between the first and second constant region domains of the heavy chain. Thr(225), Thr(228), Ser(230), Ser(232) and Thr(236) have been identified as the predominant sites of O-glycan attachment. The range and distribution of O-glycan chains at each site within the context of adjacent sites in this clustered region create a complex heterogeneity of surface epitopes that is incompletely defined. We previously described the analysis of IgA1 O-glycan heterogeneity by use of high resolution LC-MS and electron capture dissociation tandem MS to unambiguously localize all amino acid attachment sites in IgA1 (Ale) myeloma protein. Here, we report the identification and elucidation of IgA1 O-glycopeptide structural isomers that occur based on amino acid position of the attached glycans (positional isomers) and the structure of the O-glycan chains at individual sites (glycan isomers). These isomers are present in a model IgA1 (Mce1) myeloma protein and occur naturally in normal human serum IgA1. Variable O-glycan chains attached to Ser(230), Thr(233) or Thr(236) produce the predominant positional isomers, including O-glycans composed of a single GalNAc residue. These findings represent the first definitive identification of structural isomeric IgA1 O-glycoforms, define the single-site heterogeneity for all O-glycan sites in a single sample, and have implications for defining epitopes based on clustered O-glycan variability.

Primary IgA nephropathy: new insights into pathogenesis

Since its first description more than 40 years ago, IgA nephropathy has become the most common pattern of primary glomerulonephritis identified in all areas of the world where renal biopsy is routinely performed. This review discusses advances in our understanding of the pathogenesis of IgA nephropathy, principally focusing on work published in the past 5 years. It has been recognized for some time that one of the most consistent features of IgA nephropathy is an alteration in the complement of serum IgA1 O-glycoforms, with an overrepresentation of poorly galactosylated IgA1 O-glycoforms both in the serum and mesangial deposits of patients with IgA nephropathy. New data suggest that poorly galactosylated IgA1 O-glycoforms might act either as autoantigens driving the formation of glycan-specific antibodies, or antigens for cross-reactive antimicrobial antibodies. Formation of these circulating and mesangial IgA-containing immune complexes appears pivotal to the pathogenesis of IgA nephropathy and there is strong in vitro data to support their role in activation of mesangial cells, induction of podocyte injury, and activation of proximal tubular epithelial cells. Genetic factors are likely to influence many facets of pathogenesis both in primary and familial IgA nephropathy, however, to date work in this area has failed to identify consistent candidate genes.

Development of IgA nephropathy-like glomerulonephritis associated with Wiskott-Aldrich syndrome protein deficiency

Wiskott-Aldrich syndrome (WAS) is a rare X-linked disorder caused by mutations in the WAS gene. Glomerulonephritis is a frequent complication, however, histopathological data from affected patients is scarce because the thrombocytopenia that affects most patients is a contraindication to renal biopsies. We found that WASp-deficient mice develop proliferative glomerulonephritis reminiscent of human IgA nephropathy (IgAN). We examined whether increased aberrant IgA production is associated with the development of glomerulonephritis in WASp-deficient mice. Serum IgA and IgA production by splenic B cells was increased in WASp-deficient mice compared to wild-type (WT) mice. A lectin-binding study revealed a reduced ratio of sialylated and galactosylated IgA in the sera from old WASp-deficient mice. Circulating IgA-containing immune complexes showed significantly higher titers in WASp-deficient mice compared to WT mice. These results indicate that the increased IgA production and aberrant glycosylation of IgA may be critically involved in the pathogenesis of glomerulonephritis in WAS.

IgA1 immune complexes from pediatric patients with IgA nephropathy activate cultured human mesangial cells

BACKGROUND

Circulating immune complexes (CIC) containing galactose (Gal)-deficient IgA1 from adults with IgA nephropathy (IgAN) induce proliferation of cultured mesangial cells, but activities of CIC from pediatric patients with the disease have not been studied.

METHODS

CIC of different sizes were isolated from sera of pediatric and adult IgAN patients and their effects on cultured human mesangial cells (MC) were assessed by measuring cellular proliferation, expression of IL-6 and IL-8 and laminin and phosphotyrosine signaling.

RESULTS

Large CIC from pediatric IgAN patients (>800 kDa) containing Gal-deficient IgA1 stimulated cellular proliferation, whereas in some patients, smaller CIC were inhibitory. Addition of stimulatory and inhibitory CIC to MC differentially altered phosphorylation patterns of three major tyrosine-phosphorylated proteins of molecular mass 37, 60 and 115 kDa. The stimulatory CIC transiently increased tyrosine-phosphorylation of the 37-kDa protein and decreased phosphorylation of the other two proteins, whereas the inhibitory CIC increased phosphorylation of all three proteins. Furthermore, we investigated the influence of IgA1-containing CIC from sera of children with IgAN with clinically active disease (i.e., abnormal urinalysis and/or serum creatinine concentration) or inactive disease (i.e., normal urinalysis and serum creatinine concentration) on the expression of IL-6 and IL-8 genes by mesangial cells. Real-time reverse transcription-polymerase chain reaction results showed that the CIC from a patient with active disease stimulated MC to express the two cytokine genes at higher levels than did the CIC from a patient with inactive disease. Moreover, stimulatory CIC increased production of the extracellular matrix protein laminin.

CONCLUSION

These data indicate that sera of pediatric IgAN patients contain biologically active CIC with Gal-deficient IgA1.

The pathogenesis of IgA nephropathy

PURPOSE OF REVIEW

This review will analyze contemporary information concerning the possible pathogenetic mechanisms involved in IgA nephropathy, emphasizing studies in humans rather than experimental animals.

RECENT FINDINGS

Deposition of IgA in the glomeruli, the hallmark of IgA nephropathy, may be a quite common phenomenon. Aberrant O-linked galactosylation of IgA subclass (IgA1) appears to play a central role and 'auto-immunity' to a conformational epitope related to glycans at the hinge region of IgA1 is apparently required. Both a circulating immune complex and an in-situ immune complex mechanism have been advanced. Mediator systems, such as complement activation and engagement of innate immune system, also play prominent roles in determining the clinical onset and severity of disease. Genetic influences are evident but the fine details of genetic predisposition and its impact on outcomes still need to be further elucidated.

SUMMARY

Progress in understanding the details of the pathogenesis of IgA nephropathy will lead to a better means of diagnosis (including noninvasive tests for diagnosis), more accurate individualized prognosis and personalized treatment regimens for this globally distributed and very common primary glomerular disease.

Immunomodulatory properties and therapeutic application of mesenchymal stem cells

Mesenchymal stem cells (MSCs) are multi-potent progenitor cells that are isolated from the bone marrow and several adult organs and tissues. These cells possess remarkable immunosuppressive properties and can inhibit the proliferation and function of the major immune cell populations, including T cells, B cells and natural killer (NK) cells; modulate the activities of dendritic cells (DCs); and induce regulatory T cells both in vivo and in vitro. These unique properties make MSCs ideal candidates for clinical application as immunosuppressants. The immunomodulatory effect of MSCs is mediated by a non-specific anti-proliferative action of these cells, which is dependent on cell-cell contact or secreted soluble factors such as indoleamine 2,3-dioxygenase (IDO), prostaglandin E(2) (PGE(2) ), nitric oxide (NO), histocompatibility leucocyte antigen-G (HLA-G), transforming growth factor (TGF)-β, interferon (IFN)-γ and interleukin (IL)-1β. Considerable progress has been obtained in preclinical studies on MSCs, including those on their ability to activate allogeneic cells. This review examines the current understanding of the immunomodulatory properties of MSCs and its therapeutic implication for immune-mediated diseases and transplant rejection.

Aberrantly glycosylated IgA1 as a factor in the pathogenesis of IgA nephropathy

Predominant or codominant immunoglobulin (Ig) A deposition in the glomerular mesangium characterizes IgA nephropathy (IgAN). Accumulated glomerular IgA is limited to the IgA1 subclass and usually galactose-deficient. This underglycosylated IgA may play an important role in the pathogenesis of IgAN. Recently, antibodies against galactose-deficient IgA1 were found to be well associated with the development of IgAN. Several therapeutic strategies based on corticosteroids or other immunosuppressive agents have been shown to at least partially suppress the progression of IgAN. On the other hand, several case reports of kidney transplantation or acquired IgA deficiency uncovered a remarkable ability of human kidney to remove mesangial IgA deposition, resulting in the long-term stabilization of kidney function. Continuous exposure to circulating immune complexes containing aberrantly glycosylated IgA1 and sequential immune response seems to be essential in the disease progression of IgAN. Removal of mesangial IgA deposition may be a challenging, but fundamental approach in the treatment of IgAN.

Genetic studies of IgA nephropathy: past, present, and future

Immunoglobulin A nephropathy (IgAN) is the most common form of primary glomerulonephritis worldwide and an important cause of kidney disease in young adults. Highly variable clinical presentation and outcome of IgAN suggest that this diagnosis may encompass multiple subsets of disease that are not distinguishable by currently available clinical tools. Marked differences in disease prevalence between individuals of European, Asian, and African ancestry suggest the existence of susceptibility genes that are present at variable frequencies in these populations. Familial forms of IgAN have also been reported throughout the world but are probably underrecognized because associated urinary abnormalities are often intermittent in affected family members. Of the many pathogenic mechanisms reported, defects in IgA1 glycosylation that lead to formation of immune complexes have been consistently demonstrated. Recent data indicates that these IgA1 glycosylation defects are inherited and constitute a heritable risk factor for IgAN. Because of the complex genetic architecture of IgAN, the efforts to map disease susceptibility genes have been difficult, and no causative mutations have yet been identified. Linkage-based approaches have been hindered by disease heterogeneity and lack of a reliable noninvasive diagnostic test for screening family members at risk of IgAN. Many candidate-gene association studies have been published, but most suffer from small sample size and methodological problems, and none of the results have been convincingly validated. New genomic approaches, including genome-wide association studies currently under way, offer promising tools for elucidating the genetic basis of IgAN.

Clustered O-glycans of IgA1: defining macro- and microheterogeneity by use of electron capture/transfer dissociation

IgA nephropathy (IgAN) is the most common primary glomerulonephritis in the world. Aberrantly glycosylated IgA1, with galactose (Gal)-deficient hinge region (HR) O-glycans, plays a pivotal role in the pathogenesis of the disease. It is not known whether the glycosylation defect occurs randomly or preferentially at specific sites. We have described the utility of activated ion-electron capture dissociation (AI-ECD) mass spectrometric analysis of IgA1 O-glycosylation. However, locating and characterizing the entire range of O-glycan attachment sites are analytically challenging due to the clustered serine and threonine residues in the HR of IgA1 heavy chain. To address this problem, we analyzed all glycoforms of the HR glycopeptides of a Gal-deficient IgA1 myeloma protein, mimicking the aberrant IgA1 in patients with IgAN, by use of a combination of IgA-specific proteases + trypsin and AI-ECD Fourier transform ion cyclotron resonance (FT-ICR) tandem mass spectrometry (MS/MS). The IgA-specific proteases provided a variety of IgA1 HR fragments that allowed unambiguous localization of all O-glycosylation sites in the six most abundant glycoforms, including the sites deficient in Gal. Additionally, this protocol was adapted for on-line liquid chromatography (LC)-AI-ECD MS/MS and LC-electron transfer dissociation MS/MS analysis. Our results thus represent a new clinically relevant approach that requires ECD/electron transfer dissociation-type fragmentation to define the molecular events leading to pathogenesis of a chronic kidney disease. Furthermore, this work offers generally applicable principles for the analysis of clustered sites of O-glycosylation.

Recognition of galactose-deficient O-glycans in the hinge region of IgA1 by N-acetylgalactosamine-specific snail lectins: a comparative binding study

Aberrancies in IgA1 glycosylation have been linked to the pathogenesis of IgA nephropathy (IgAN), a kidney disease characterized by deposits of IgA1-containing immune complexes in the glomerular mesangium. IgA1 from IgAN patients is characterized by the presence of galactose (Gal)-deficient O-glycans in the hinge region that can act as epitopes for anti-glycan IgG or IgA1 antibodies. The resulting circulating immune complexes are trapped in the glomerular mesangium of the kidney where they trigger localized inflammatory responses by activating mesangial cells. Certain lectins recognize the terminal N-acetylgalactosamine (GalNAc)-containing O-glycans on Gal-deficient IgA1 and can be potentially used as diagnostic tools. To improve our understanding of GalNAc recognition by these lectins, we have conducted binding studies to assess the interaction of Helix aspersa agglutinin (HAA) and Helix pomatia agglutinin (HPA) with Gal-deficient IgA1. Surface plasmon resonance spectroscopy revealed that both HAA and HPA bind to a Gal-deficient synthetic hinge region glycopeptide (HR-GalNAc) as well as various aberrantly glycosylated IgA1 myeloma proteins. Despite having six binding sites, both HAA and HPA bind IgA1 in a functionally bivalent manner, with the apparent affinity for IgA1 related to the number of exposed GalNAc groups in the IgA1 hinge. Finally, HAA and HPA were shown to discriminate very effectively between the IgA1 secreted by cell lines derived from peripheral blood cells of patients with IgAN and that from cells of healthy controls. These studies provide insight into lectin recognition of the Gal-deficient IgA1 hinge region and lay the groundwork for the development of reliable diagnostic tools for IgAN.

Aberrantly glycosylated IgA1 induces mesangial cells to produce platelet-activating factor that mediates nephrin loss in cultured podocytes

The reaction of mesangial cells with aberrantly glycosylated IgA1 has been implicated in the etiology of IgA nephropathy (IgAN). Tumor necrosis factor, which is assumed to mediate the interaction between mesangial cells and podocytes, also induces the expression of platelet-activating factor (PAF). In this study, we determined whether PAF affects the expression of nephrin (an adhesion molecule critical to glomerular permselectivity) and cytoskeletal F-actin organization in podocytes. We treated human mesangial cells with atypically glycosylated IgA1 either prepared in vitro or derived from the sera of patients with IgAN. We then prepared conditioned media from these cells and added them to cultured human podocytes in the presence of PAF receptor antagonists. Podocytes transfected to overexpress acetylhydrolase, the main catabolic enzyme of PAF, served as controls. Downregulation of nephrin expression and F-actin reorganization occurred when podocytes were cultured with mesangial cell-conditioned medium. Preincubation of podocytes with a PAF receptor antagonist prevented the loss and redistribution of nephrin. In control podocytes overexpressing acetylhydrolase, nephrin loss was abrogated. Our results suggest that atypically glycosylated IgA-induced PAF from mesangial cells is a mediator of podocyte changes, which, when more directly tested elsewhere, were found to be associated with proteinuria. Hence, it is possible that these in vitro findings may be relevant to the proteinuria of IgAN.

Toll-like receptor 4 expression is increased in circulating mononuclear cells of patients with immunoglobulin A nephropathy

We investigated Toll-like receptors (TLR-3, -4 and -7) expression in circulating mononuclear cells of patients with immunoglobulin A nephropathy (IgAN), a disease with debated relationships with mucosal immunity. TLR-4 expression (detected by fluorescence activated cell sorter) and mRNA transcriptional levels (Taqman) were significantly higher in patients with IgAN than in healthy controls (P = 0.00200 and P = 0.0200). TLR-3 and TLR-7 were not modified significantly. In IgAN patients proteinuria was correlated significantly with TLR-4 expression (P = 0.0312). In a group of nephrotic syndromes, TLR-3, -4 and -7 expression was similar to healthy controls. A significant difference in TLR-4 expression and mRNA levels was found between very active IgAN patients (proteinuria > 1 g/1.73 m(2)/day in association with severe microscopic haematuria) and inactive patients (proteinuria < 0.5 g/1.73 m(2)/day, with absent or minimal haematuria). No correlation with levels of aberrantly glycosylated IgA1, age, renal biopsy features or therapy was found. This study shows for the first time an up-regulation of TLR-4 in circulating mononuclear cells of patients with IgAN, particularly in association with proteinuria and heavy microscopic haematuria.

Clinical impact of altered immunoglobulin levels in Henoch-Schönlein purpura

BACKGROUND

The aim of the present study was the identification of immunological features, present at the time of diagnosis, that would predict the severity of Henoch-Schönlein purpura and its outcome.

METHODS

A cohort study was carried out in a tertiary pediatric hospital of 69 children with Henoch-Schönlein purpura, in whom serum complement components C3, C4 and IgA, IgM, IgG were repeatedly determined.

RESULTS

During the acute phase of the disease in 54/69 patients (78.3%) immunological imbalances were observed. In 24/54 cases (44.4%) certain complications involving the kidneys and the gastrointestinal tract were noted as opposed to in 3/15 children (20%) without immunologic abnormalities. In 50/69 children (72.5%), elevated serum IgA was detected and 16 of them (32%) developed renal involvement while only 1/19 children (5.3%) with normal IgA concentration had renal involvement. Considering separately the group of 9/69 children (13%) with increased IgM and those with normal IgM levels (53/69; 76.8%), irrespective of IgA and IgG concentration, we found a comparable percentage of children who had both renal and intestinal involvement without, however, developing severe complications, which were exclusively seen in patients with increased IgA (5/7 children) and reduced IgM levels. Serum C3 fraction was elevated in 26 children (37.7%) and in 73% of cases it was associated with increased serum IgA values.

CONCLUSION

Renal involvement was seen in 32% of children with increased IgA values. Most importantly, elevated IgA concentration along with reduced IgM levels was associated with higher prevalence of severe complications.

Aberrantly glycosylated IgA1 in IgA nephropathy patients is recognized by IgG antibodies with restricted heterogeneity

IgA nephropathy (IgAN) is characterized by circulating immune complexes composed of galactose-deficient IgA1 and a glycan-specific IgG antibody. These immune complexes deposit in the glomerular mesangium and induce the mesangioproliferative glomerulonephritis characteristic of IgAN. To define the precise specificities and molecular properties of the IgG antibodies, we generated EBV-immortalized IgG-secreting lymphocytes from patients with IgAN and found that the secreted IgG formed complexes with galactose-deficient IgA1 in a glycan-dependent manner. We cloned and sequenced the heavy- and light-chain antigen-binding domains of IgG specific for galactose-deficient IgA1 and identified an A to S substitution in the complementarity-determining region 3 of the variable region of the gene encoding the IgG heavy chain in IgAN patients. Furthermore, site-directed mutagenesis that reverted the residue to alanine reduced the binding of recombinant IgG to galactose-deficient IgA1. Finally, we developed a dot-blot assay for the glycan-specific IgG antibody that differentiated patients with IgAN from healthy and disease controls with 88% specificity and 95% sensitivity and found that elevated levels of this antibody in the sera of patients with IgAN correlated with proteinuria. Collectively, these findings indicate that glycan-specific antibodies are associated with the development of IgAN and may represent a disease-specific marker and potential therapeutic target.

O-linked oligosaccharides of the IgA1 hinge region: roles of its aberrant structure in the occurrence and/or progression of IgA nephropathy

Primary IgA nephropathy (IgAN) has been regarded as an immune complex-mediated glomerulonephritis characterized immunohistologically by the predominant deposition of IgA in the glomerular mesangial area with a variety of histopathologic injuries (Clarkson et al. in Ann Rev Med 38:157-168, 1987). In 1992, the characteristic structure of O-linked oligosaccharides (O-glycans) in the IgA1 hinge and its possible aberrancy were simultaneously and independently proposed by Mesteckey et al. (Cont Nephrol 104:172-182, 1993), and our group (Cont Nephrol 104:217, 1993) at the International Congress of Nephrology (IgA Nephropathy 25th year) held in Nancy, France. Since then, the aberrancy has been confirmed by several research groups and is suspected to play a role in the occurrence and/or the progression of IgAN. At the end of the 1980s, I took an interest in the existence of O-glycans in the hinge region of IgA1 and have pursued the structure of the carbohydrate chains. Since an excellent review on the structure and the role of the carbohydrate in IgA molecules was recently published by Narita et al. (Clin Ex Nephrol 12:332-338, 2008), this review focuses on the process by which I developed the idea of aberrant O-glycosylation in IgA1 molecules in IgAN patients and summarizes our recent observations concerning IgA1 molecules.

Activity of alpha2,6-sialyltransferase and its gene expression in peripheral B lymphocytes in patients with IgA nephropathy

It is known that aberrant sialylation of IgA1 is involved in the pathogenesis of IgA nephropathy (IgAN). We hypothesize that aberrant sialylation of serum IgA1 may result from changes in the activity of alpha2,6-sialyltransferase (alpha2,6-ST) or expression of its coding gene ST6GALNAC2 in peripheral B lymphocytes. Sixty patients with IgAN and 20 healthy controls were enrolled. Peripheral B lymphocytes were isolated by CD-19-positive magnetic beads. The expression level of ST6GALNAC2 was quantitatively analysed by real-time reverse-transcriptase polymerase chain reaction (PCR). Serum IgA1 and sialylation levels were detected by enzyme-linked immunosorbent assay (ELISA) and specific lectin-binding ELISA. Activity of alpha2,6-ST was measured by specific lectin-binding ELISA. Expression of ST6GALNAC2 in B peripheral lymphocytes was significantly lower in patients with IgAN than that in normal controls (3.7 +/- 2.2 versus 6.3 +/- 2.3, P = 0.016); alpha2,6-ST activity in B lymphocytes was correlated positively with the level of alpha2,6-sialic acid in serum IgA1 in patients (n = 42) and controls (n = 12) (r = 0.37, P = 0.007). However, alpha2,6-ST activity did not differ between patients with IgAN and controls (1.19 +/- 1.43 versus 1.06 +/- 1.17, P > 0.05). These data suggested that reduced sialylation of serum IgA1 may result from decreased expression of ST6GALNAC2. The factors affecting activity of alpha2,6-ST in the sialylation of IgA1 need to be further investigated.

T cell cytokine polarity as a determinant of immunoglobulin A (IgA) glycosylation and the severity of experimental IgA nephropathy

Immunoglobulin A (IgA) glycosylation, recognized as an important pathogenic factor in IgA nephropathy (IgAN), is apparently controlled by the polarity of T helper (Th) cytokine responses. To examine the role of cytokine polarity in IgAN, inbred mice were immunized by intraperitoneal priming with inactivated Sendai virus (SeV) emulsified in either complete Freund's adjuvant (CFA) or incomplete Freund's adjuvant (IFA), which promote Th1- or Th2-immune response, respectively, and then boosted identically twice orally with aqueous suspensions of inactivated virus. Next, some mice were challenged intranasally with infectious SeV. Mice primed with CFA or IFA had equal reductions in nasal viral titre relative to non-immune controls, and equally increased serum levels of SeV-specific IgA antibody. Mice primed with CFA showed higher SeV-specific IgG than those with IFA. Splenocytes from mice primed with IFA produced copious amounts of interleukin (IL)-4 and IL-5, but little interferon-gamma and IL-2; those primed with CFA had reciprocal cytokine recall responses. Total serum IgA and especially SeV-specific IgA from mice primed with IFA showed a selective defect in sialylation and galactosylation. Although the frequency and intensity of glomerular deposits and haematuria did not differ, glomerulonephritis in mice primed with IFA and challenged with infectious virus was more severe than in those given CFA, as judged by serum creatinine level. We conclude that the polarity of T cell cytokines controls the pattern of IgA glycosylation and exerts direct or indirect effects on functional glomerular responses to immune complex deposition.

IgA nephropathy and Henoch-Schönlein purpura nephritis

PURPOSE OF REVIEW

IgA nephropathy and Henoch-Schönlein purpura nephritis are common glomerular disorders in pediatrics that can potentially progress to end-stage renal disease in some patients. This review summarizes our current understanding of the pathogenesis of these closely related conditions and discusses the rationale for development of diagnostic tests and prognostic markers. The review also presents the best data for long-term outcome, clinical markers of prognosis, and the results of randomized controlled trials.

RECENT FINDINGS

Our understanding of the defective galactosylation of O-linked glycans in the hinge region of human IgA1 and its role in the pathogenesis of IgA nephropathy and Henoch-Schönlein purpura nephritis has evolved over the past decade. This review discusses studies that suggest that demonstration of galactose-deficient IgA1 in the serum may become an important diagnostic tool for these conditions. Proteomic techniques for development of biomarkers for diagnosis and prognosis show promise. Although data from randomized controlled trials have failed to support the use of immunosuppressive agents in pediatric IgA nephropathy and Henoch-Schönlein purpura nephritis, recent data indicate that angiotensin converting enzyme inhibitor therapy is indicated for reduction of proteinuria.

SUMMARY

Childhood IgA nephropathy and Henoch-Schönlein purpura nephritis have the potential for serious morbidity, either during childhood or later in adulthood. In the future clinical tests will be used for noninvasive diagnosis and as markers for judging response to treatment, particularly in those individuals at highest risk for eventual progression to end-stage renal disease.

IgA1-secreting cell lines from patients with IgA nephropathy produce aberrantly glycosylated IgA1

Aberrant glycosylation of IgA1 plays an essential role in the pathogenesis of IgA nephropathy. This abnormality is manifested by a deficiency of galactose in the hinge-region O-linked glycans of IgA1. Biosynthesis of these glycans occurs in a stepwise fashion beginning with the addition of N-acetylgalactosamine by the enzyme N-acetylgalactosaminyltransferase 2 and continuing with the addition of either galactose by beta1,3-galactosyltransferase or a terminal sialic acid by a N-acetylgalactosamine-specific alpha2,6-sialyltransferase. To identify the molecular basis for the aberrant IgA glycosylation, we established EBV-immortalized IgA1-producing cells from peripheral blood cells of patients with IgA nephropathy. The secreted IgA1 was mostly polymeric and had galactose-deficient O-linked glycans, characterized by a terminal or sialylated N-acetylgalactosamine. As controls, we showed that EBV-immortalized cells from patients with lupus nephritis and healthy individuals did not produce IgA with the defective galactosylation pattern. Analysis of the biosynthetic pathways in cloned EBV-immortalized cells from patients with IgA nephropathy indicated a decrease in beta1,3-galactosyltransferase activity and an increase in N-acetylgalactosamine-specific alpha2,6-sialyltransferase activity. Also, expression of beta1,3-galactosyltransferase was significantly lower, and that of N-acetylgalactosamine-specific alpha2,6-sialyltransferase was significantly higher than the expression of these genes in the control cells. Thus, our data suggest that premature sialylation likely contributes to the aberrant IgA1 glycosylation in IgA nephropathy and may represent a new therapeutic target.

IgA glycosylation and IgA immune complexes in the pathogenesis of IgA nephropathy

Circulating immune complexes containing aberrantly glycosylated IgA1 play a pivotal role in the pathogenesis of IgA nephropathy (IgAN). A portion of IgA1 secreted by IgA1-producing cells in patients with IgAN is galactose-deficient and consequently recognized by anti-glycan IgG or IgA1 antibodies. Some of the resultant immune complexes in the circulation escape normal clearance mechanisms, deposit in the renal mesangium, and induce glomerular injury. Recent studies of the origin of these aberrant molecules, their glycosylation profiles, and mechanisms of biosynthesis have provided new insight into the autoimmune nature of the pathogenesis of this common renal disease. An imbalance in the activities of the pertinent glycosyltransferases in the IgA1-producing cells favors production of molecules with galactose-deficient O-linked glycans at specific sites in the hinge region of the alpha heavy chains. By using sophisticated analytic methods, it may be possible to define biomarkers for diagnostic purposes and identify new therapeutic targets for a future disease-specific therapy.

B-cell O-galactosyltransferase activity, and expression of O-glycosylation genes in bone marrow in IgA nephropathy

In IgA nephropathy (IgAN), pathogenic IgA1 is likely derived from bone marrow (BM) cells and exhibits reduced O-galactosylation. Defective O-galactosylation may arise from the compromised expression or function of the enzyme beta-galactosyltransferase and/or its molecular chaperone (Cosmc). We measured B-cell O-galactosylation activity and the relative gene expression of beta-galactosyltransferase and Cosmc in peripheral blood and BM taken from patients with IgAN and controls. O-galactosylation activity was measured in peripheral and BM B cells by the incorporation of radiolabeled galactose into an asialo-mucin acceptor. Gene expression of beta-galactosyltransferase and Cosmc was measured by real-time PCR and related to that of the enzyme GalNAc-T2 (UDP-N-acetyl-alpha-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase-2), which synthesizes the core O-glycan. Neither the B-cell O-galactosylation activity nor the gene expression of the enzyme or chaperone was different between patients and controls. However, the relationships between the O-glycosylation of serum IgA1, galactosylation activity, and beta-galactosyltransferase gene expression showed different patterns in IgAN and controls. In IgAN, O-galactosylation activity correlated with beta-galactosyltransferase gene expression, but not with IgA1 O-glycosylation, suggesting that factors other than the availability of beta-galactosyltransferase or Cosmc are responsible for altered IgA1 O-glycosylation.

Aberrant IgA1 glycosylation is inherited in familial and sporadic IgA nephropathy

IgA nephropathy (IgAN) is a complex trait determined by genetic and environmental factors. Most IgAN patients exhibit a characteristic undergalactosylation of the O-glycans of the IgA1 hinge region, which promotes formation and glomerular deposition of immune complexes. It is not known whether this aberrant glycosylation is the result of an acquired or inherited defect, or whether the presence of aberrant IgA1 glycoforms alone can produce IgAN. A newly validated lectin enzyme-linked immunosorbent assay (ELISA) was used to determine the serum level of galactose-deficient IgA1 (Gd-IgA1) in a cohort of 89 IgAN patients and 266 of their relatives. High Gd-IgA1 levels (> or =95th percentile for controls) were observed in all 5 available patients with familial IgAN, in 21 of 45 (47%) of their at-risk relatives (assuming autosomal dominant inheritance), and in only 1 of 19 (5%) of unrelated individuals who married into the family. This provides evidence that abnormal IgA1 glycosylation is an inherited rather than acquired trait. Similarly, Gd-IgA1 levels were high in 65 of 84 (78%) patients with sporadic IgAN and in 50 of 202 (25%) blood relatives. Heritability of Gd-IgA1 was estimated at 0.54 (P = 0.0001), and segregation analysis suggested the presence of a major dominant gene on a polygenic background. Because most relatives with abnormal IgA1 glycoforms were asymptomatic, additional cofactors must be required for IgAN to develop. The fact that abnormal IgA1 glycosylation clusters in most but not all families suggests that measuring Gd-IgA1 may help distinguish patients with different pathogenic mechanisms of disease.

Serological and genetic factors in early recurrence of IgA nephropathy after renal transplantation

BACKGROUND

The relative role of IgA anomalies and genetic factors in IgA nephropathy (IgAN) recurrence after transplantation has never been investigated in a single cohort.

METHODS

Sixty-one transplanted patients who had IgAN as an original disease (30 with biopsy-proved early recurrence, median 2.9 yr post-transplant), and 120 controls, were investigated for aberrantly glycosylated IgA1, IgA binding to mesangial matrix, macromolecular IgA (IgA/fibronectin and uteroglobulin/IgA/fibronectin complexes), and polymorphisms of cytokines [tumor necrosis factor alpha (TNFalpha), interleukin 10 (IL-10), IL-6, interferon gamma and transforming growth factor beta 1] and renin-angiotensin system (angiotensinogen converting enzyme, angiotensin II receptor 1, and angiotensinogen) genes.

RESULTS

At multivariate logistic regression analysis, recurrence showed a border-line association with aberrantly glycosylated IgA1 [odds ratio (OR) 8.172, p = 0.077], and was significantly less frequent in carriers of -308 AG/AA TNF-alpha"high producer" genotype (OR 0.125, p = 0.036) and -1082, -819, -592 ACC/ATA IL-10 "low producer" (OR 0.038, p = 0.009) genotypes.

CONCLUSION

High levels of aberrantly glycosylated IgA1 do not appear to play a strong crucial role in recurrence of IgAN. Polymorphisms of TNFalpha and IL-10 known to condition Th1 prevalence were associated with protection from early recurrence of IgAN.

The pathogenic role of IgA1 O-linked glycosylation in the pathogenesis of IgA nephropathy

Numerous abnormalities of the IgA immune system have been reported in IgAN but the most consistent finding remains aberrant IgA1 O-linked glycosylation of the IgA1 hinge region. The defect comprises reduced galactosylation of O-linked N-acetylgalactosamine residues with or without changes in the terminal sialylation of the O-linked sugars. Aberrant O-galactosylation has been found in serum IgA1, in IgA1 isolated from tonsillar lymphocytes, and in IgA1 eluted from mesangial deposits. There is evidence that changes in IgA1 O-galactosylation lead to IgA immune complex formation and mesangial IgA deposition. Mesangial cells exposed to these IgA immune complexes proliferate and adopt a pro-inflammatory phenotype; they secrete cytokines, chemokines, growth factors and extracellular matrix components promoting glomerular inflammation and glomerulosclerosis. Recent evidence suggests that the control of IgA1 O-glycosylation is linked to class switching from IgD to IgA1 synthesis and that the pattern of IgA1 O-glycosylation may be programmed at the time of initial antigen encounter. IgA1 glycosylation varies between systemic and mucosal sites and the association of aberrant IgA1 galactosylation with low affinity, polymeric IgA1 antibodies against mucosal antigens suggests undergalactosylated IgA1 may in fact be a mucosal glycoform of IgA1. Although suited to the mucosal compartment, when these IgA1 glycoforms enter the systemic circulation in appreciable quantities they deposit in the mesangium and trigger glomerular inflammation. This review will discuss the evidence for the role of IgA1 O-glycosylation in the pathogenesis of IgAN and propose an explanation for the presence of aberrantly O-glycosylated IgA1 in the circulation of patients with IgAN.

Role of aberrant glycosylation of IgA1 molecules in the pathogenesis of IgA nephropathy

Studies of the properties of immune complexes (IC) in the circulation, urine, and mesangium of IgA nephropathy (IgAN) patients have provided data relevant to the pathogenesis of this disease. IC contain predominantly polymeric IgA1 molecules which are deficient in galactose (Gal) residues on O-linked glycan chains in the hinge region (HR) of their heavy (H) chains. As a result of this aberrancy, a novel antigenic determinant(s) involving N-acetylgalactosamine (GalNAc) and perhaps sialic acid (SA) of O-linked glycans is generated and recognized by naturally occurring GalNAc-specific antibodies. Thus, IC in IgAN consist of Gal-deficient IgA1 molecules as an antigen, and GalNAc-specific IgG and/or IgA1 as an antibody. IgG antibodies to Gal-deficient IgA1 are probably induced by cross-reactive microbial antigens; they are present at variable levels not only in humans with or without IgAN but also in many phylogenetically diverse vertebrate species. Incubation of human mesangial cells with IC from sera of IgAN patients indicated that stimulation of cellular proliferative activity was restricted to the large (>800 kDa) complexes. These findings suggest that experimental approaches that prevent the formation of large Gal-deficient IgA1-IgG IC may be applied ultimately in an immunologically mediated therapy.

Protective role of anti-synthetic hinge peptide antibody for glomerular deposition of hypoglycosylated IgA1

BACKGROUND

The KM mouse lacks endogenous genes for immunoglobulins and carries the entire human IgH locus and the IgLk transgene. Therefore, human IgA1 does not provoke a hetero-immune response. We had observed mesangial IgA deposits in KM mice given desialo-degalacto (DeS/DeGal) IgA1.

METHODS

In this study, the mice were immunized with synthetic IgA1 hinge (glyco-)peptide before administration of DeS/DeGal IgA1, and the effects of the pre-immunization were evaluated. Mice were divided into sHP, 5GalNAc-sHP and non-immunization groups. In two pre-immunization groups, KLH-conjugated sHP or KLH-5GalNAc-sHP, which has five GalNAc residues, was subcutaneously given three times every 2 weeks. Two weeks after the final pre-immunization, DeS/DeGal IgA1 was administered daily for 5 weeks. Serial serum levels of anti-sHP and anti-IgA1 antibodies were evaluated by ELISA. On the day of the last administration of IgA1, renal biopsy was performed.

RESULTS

Mesangial IgA deposits were observed in all non-immunized mice. In pre-immunized mice, IgA deposition was not detected in 6 of 13 sHP mice and 1 of 4 5GalNAc-sHP mice. The intensities of IgA deposits were significantly different between sHP groups and non-immunized (P = 0.003) groups. There was a significant inverse correlation between the intensities of IgA deposits and the anti-sHP antibody titers (P = 0.016).

CONCLUSIONS

These results suggest that the anti-IgA1 hinge peptide antibody plays a role in the inhibition of glomerular IgA deposition.

Microbial IgA protease removes IgA immune complexes from mouse glomeruli in vivo: potential therapy for IgA nephropathy

The hallmark of IgA nephropathy (IgAN), the most common form of glomerulonephritis, is the presence of mesangial deposits containing IgA, specifically the IgA1 subclass, as the most prominent component. The deposited IgA is considered to be part of an immune complex. The family of enzymes known as bacterial IgA proteases exhibits substrate specificity that is essentially limited to the hinge region of IgA1. Here we demonstrate the ability of systemically administered IgA protease to remove glomerular IgA immune complexes, both the antigen and antibody components, in a passive mouse model of IgAN. Thus, IgA protease may have potential as a therapeutic agent for human IgAN.

Serum levels of galactose-deficient IgA in children with IgA nephropathy and Henoch-Schönlein purpura

IgA nephropathy and Henoch-Schönlein purpura nephritis (HSPN) are related diseases characterized by deposits of IgA1-containing immune complexes in the renal mesangium. Adult patients with IgA nephropathy have aberrantly glycosylated IgA1 (galactose-deficient O-linked glycans) in the circulation and renal deposits. However, IgA1 glycosylation has not been studied in pediatric patients with IgA nephropathy. Using our quantitative lectin enzyme-linked immunosorbent assay (ELISA) test, we measured serum levels of galactose-deficient IgA1 of children with IgA nephropathy and HSPN and controls. Children with IgA nephropathy and HSPN had serum levels higher than those of healthy children or renal-disease controls with C1q nephropathy. Furthermore, lectin ELISA identified patients with HSPN whose clinical course mimicked that of IgA nephropathy. In summary, pediatric patients with IgA nephropathy and HSPN have an aberrancy in the glycosylation in IgA1 O-linked glycans that is similar to that in adults with IgA nephropathy.

Aberrant sialylation of serum IgA1 was associated with prognosis of patients with IgA nephropathy

Aberrant glycosylation of serum IgA1 was considered as an initial event and involvement in the pathogenesis of IgAN. We previously demonstrated that aberrant glycosylation of serum IgA1 was associated with pathologic phenotype of IgAN. The present study is to investigate if abnormal sialylation of IgA1 affects renal survival of IgAN. 127 patients with biopsy-proven IgAN were enrolled and followed up to 8 years. Seventy-nine healthy and 75 patients with non-IgAN renal diseases were selected as controls. Alpha 2, 6 sialic acid (SA) of serum IgA1 was measured by sandwich-ELISA. Renal survival rate was estimated by Kaplan-Meier method. Alpha 2, 6 SA level in patients with IgAN was lower than that in healthy controls (0.92+/-0.14 vs. 0.98+/-0.12, P=0.001) and non-IgAN glomerulonephritis (0.92+/-0.14 vs. 1.00+/-0.18, n=53, P=0.001). Patients with IgAN in Low SA Group were no significant differences compared with patients in Normal SA Group in age, gender, hypertension, serum creatinine, and excretion of proteinuria. Renal cumulative survival rate was 53.3% in patients in Low SA Group and 83.5% in Normal SA Group (P=0.0008). The lower the alpha 2, 6 SA level of serum IgA1 in patients with IgAN was, the worse their renal survival rate was. Although patients in Low SA Group had worse renal function evaluated by eGFR, there was no significant difference in various CKD stages in non-IgAN renal function controls (n=42, P=0.352). Alpha 2, 6 SA level of serum IgA1 was associated with the prognosis of patients with IgAN and could serve as a predictor of poor prognosis in IgAN.

Immunopathogenesis of IgAN

The defining hallmark of IgA nephropathy (IgAN) is deposition of polymeric IgA1 in the glomerular mesangium accompanied by a mesangial proliferative glomerulonephritis. The mechanisms involved in mesangial polymeric IgA1 deposition and the initiation of inflammatory glomerular injury remain unclear. This lack of a complete understanding of the pathogenesis of IgAN has meant that there is still no treatment known to modify mesangial deposition of IgA. Increasing evidence, however, supports the importance of IgA-containing immune complex formation as a pivotal factor driving mesangial IgA deposition and triggering of glomerular injury. A number of potentially important changes to the IgA1 molecule have been identified in IgAN, which may contribute to immune complex formation. These changes suggest that the polymeric IgA1 that deposits in IgA nephropathy is derived from mucosally primed plasma cells. The presence of this IgA in the circulation reflects displacement of mucosal B lineage cells to systemic sites and may be the result of mishoming of lymphocytes trafficking along the mucosa-bone marrow axis.

Analysis of O-glycan heterogeneity in IgA1 myeloma proteins by Fourier transform ion cyclotron resonance mass spectrometry: implications for IgA nephropathy

IgA nephropathy (IgAN) is the most common form of primary glomerulonephritis. In IgAN, IgA1 molecules with incompletely galactosylated O-linked glycans in the hinge region (HR) are present in mesangial immunodeposits and in circulating immune complexes. It is not known whether the galactose deficiency in IgA1 proteins occurs randomly or preferentially at specific sites. We have previously demonstrated the first direct localization of multiple O-glycosylation sites on a single IgA1 myeloma protein by use of activated ion-electron capture dissociation (AI-ECD) Fourier transform ion cyclotron resonance (FT-ICR) tandem mass spectrometry. Here, we report the analysis of IgA1 O-glycan heterogeneity by use of FT-ICR MS and liquid chromatography FT-ICR MS to obtain unbiased accurate mass profiles of IgA1 HR glycopeptides from three different IgA1 myeloma proteins. Additionally, we report the first AI-ECD fragmentation on an individual IgA1 O-glycopeptide from an IgA1 HR preparation that is reproducible for each IgA1 myeloma protein. These results suggest that future analysis of IgA1 HR from IgAN patients and normal healthy controls should be feasible.

Th1 polarization in murine IgA nephropathy directed by bone marrow-derived cells

IgA nephropathy is the most common form of progressive glomerulonephritis although the pathophysiology of this nephropathy is unclear. The ddY mouse is a spontaneous animal model with variable incidence and extent of glomerular injury mimicking human IgA nephropathy. Here, we transplanted bone marrow cells from 20-week-old ddY mice with beginning or quiescent IgA nephropathy into irradiated similar ddY mice, C57Bl/6 (Th1 prone) mice, or BALB/c (Th2 prone) mice. Serum IgA/IgG complex and Th1/Th2 polarization of spleen cells was determined by enzyme-linked immunosorbent assay and confirmed by fluorescent cytometric analysis. The ddY mice with commencing IgA nephropathy demonstrated strong polarization toward Th1, while those with quiescent disease were Th2 polarized. Serum levels of IgA/IgG2a immune complex significantly correlated with the severity of the glomerular lesions. Bone marrow taken from mice with commencing IgA nephropathy conferred IgA nephropathy with Th1 polarization in recipient-quiescent mice, while transplantation from the quiescent mice ablated glomerular injury and mesangial IgA/IgG deposition in those commencing IgA disease. However, adoptive transfer of CD4(+) T cells from those whose disease began failed to induce any IgA deposition or renal injury. Our study suggests that bone marrow cells, presuming IgA producing cells, may initiate this disease. Th1 cells may be involved in the pathophysiology of the disease after glomerular IgA deposition.

The genetics of IgA nephropathy

IgA nephropathy is the most common form of primary glomerulonephritis. Variations in clinical manifestations indicate that a diagnosis of IgA nephropathy encompasses multiple disease subsets that cannot be distinguished on the basis of renal pathology or clinical variables alone. Familial forms of the disease have been reported throughout the world, but are probably under-recognized because associated urinary abnormalities are often intermittent in affected family members. IgA nephropathy has complex determination, with different genes probably causing disease in different patient subgroups. Of the many pathogenic mechanisms reported, defects in IgA1 glycosylation that lead to formation of immune complexes have been consistently implicated. Here, we present the evidence for genetic contributions to the disease, review clinical patterns of familial disease, and summarize some of the most promising genetic studies conducted to date. Linkage-based approaches to the study of familial forms of the disease have identified significant or suggestive loci on chromosomes 6q22-23, 2q36, 4q26-31, 17q12-22 and 3p24-23, but no causal gene has yet been identified. Many interesting, but poorly replicated, genetic association studies have also been reported. We discuss recent developments in analytic tools that should enable genetic studies of sporadic forms of disease by the genome-wide association approach.

Patients with IgA nephropathy have increased serum galactose-deficient IgA1 levels

Immunoglobulin A (IgA) nephropathy is the most prevalent form of glomerulonephritis worldwide. A renal biopsy is required for an accurate diagnosis, as no convenient biomarker is currently available. We developed a serological test based upon the observation that this nephropathy is characterized by undergalactosylated IgA1 in the circulation and in mesangial immune deposits. In the absence of galactose, the terminal saccharide of O-linked chains in the hinge region of IgA1 is terminal or sialylated N-acetylgalactosamine. A lectin from Helix aspersa, recognizing N-acetylgalactosamine, was used to develop an enzyme-linked immunosorbent assay that measures galactose-deficient IgA1 in serum. The median serum lectin-binding IgA1 level was significantly higher for 153 Caucasian adult patients with IgA nephropathy without progression to end-stage renal disease as compared with that for 150 healthy Caucasian adult controls. As the lectin-binding IgA1 levels for the controls were not normally distributed, the 90th percentile was used for determination of significant elevation. Using a value of 1076 U/ml as the upper limit of normal, 117 of the 153 patients with IgA nephropathy had an elevated serum lectin-binding IgA1 level. The sensitivity as a diagnostic test was 76.5%, with specificity 94%; the positive predictive value was 88.6% and the negative predictive value was 78.9%. We conclude that this lectin-binding assay may have potential as a noninvasive diagnostic test for IgA nephropathy.

Hypercomplementemia in adult patients with IgA nephropathy

IgA nephropathy (IgAN) is the most common form of chronic glomerulonephritis. Although glomerular deposition of complement components is well known, the evidence of serological complement activation in IgAN is inconclusive. We hypothesized that serum levels of complement components and regulatory proteins in patients with IgAN are correlated with its pathogenesis. In the present study we measured complement components in 50 patients with IgAN and 50 healthy volunteers. C5, C1 inhibitor, factor B, C4 binding protein, factor H, and factor I were measured with the use of single radial immunodiffusion. Mannose-binding lectin (MBL) and properdin (P) were measured by enzyme-linked immunosorbent assay (ELISA). The correlations among complements in the sera of patients with clinical gradings for IgAN (i.e., the good prognosis group, relatively good prognosis group, relatively poor prognosis group, and poor prognosis group) were evaluated. CH50, C4, factor B, P, factor I, and factor H were significantly higher in IgAN patients than in healthy controls. There were significant correlations between C5 and C4 binding protein, between C3 and C5, or between C4 and factor B in patients with IgAN. In the poor prognosis group, C4 binding protein was significantly higher than in the other groups of IgAN patients. hypercomplementemia occurs in IgAN and is associated with an increase in complement regulatory protein (CRP). C4 binding protein analyses can be used to predict disease prognosis.

Differential binding characteristics of native monomeric and polymeric immunoglobulin A1 (IgA1) on human mesangial cells and the influence of in vitro deglycosylation of IgA1 molecules

Recent studies had demonstrated that serum and mesangial immunoglobulin A1 (IgA1) in patients with IgA nephropathy (IgAN) were polymeric and deglycosylated. The current study was to investigate the binding characteristics of monomeric and polymeric normal human IgA1 on mesangial cells and the influence of in vitro deglycosylation of IgA1 molecules. The normal human IgA1 was desialylated and degalactosylated with specific enzymes, respectively. The monomeric IgA1 (mIgA1) and polymeric IgA1 (pIgA1) were separated by Sephacryl S-300 chromatography. The binding capacities of the mIgA1 and pIgA1 to primary human mesangial cells (HMC) were evaluated by classical radioligand assay. Both the native mIgA1 and pIgA1 could bind to HMC in a dose-dependent and saturable manner. The maximal binding capacity of the native pIgA1 were significantly higher than that of the native mIgA1 (P < 0.05). However, the affinity of the native mIgA1 was almost 100 times higher than that of the native pIgA1. After deglycosylation, binding of the two deglycosylated mIgA1 to HMC could not be detected. However, the maximal binding capacities of the two deglycosylated pIgA1 to HMC were increased significantly compared with that of native pIgA1. The affinity of the two deglycosylated pIgA1 was similar to that of native pIgA1 (P > 0.05). The current study suggests differential binding characteristics of native monomeric and polymeric IgA1 on mesangial cells. Glycosylation of IgA1 molecules could significantly affect the binding of IgA1 on HMC.

Identification and characterization of CMP-NeuAc:GalNAc-IgA1 alpha2,6-sialyltransferase in IgA1-producing cells

Glycosylation defects occur in several human diseases. In IgA nephropathy, IgA1 contains O-glycans that are galactose-deficient and consist mostly of core 1 alpha2,6 sialylated N-acetylgalactosamine, a configuration suspected to prevent beta1,3 galactosylation. We confirmed the same aberrancy in IgA1 secreted by the human DAKIKI B cell line. Biochemical assays indicated CMP-NeuAc:GalNAc-IgA1 alpha2,6-sialyltransferase activity in this cell line. However, a candidate enzyme, ST6-GalNAcI, was not transcribed in DAKIKI cells, B cells isolated from blood, or Epstein-Barr virus (EBV)-immortalized IgA1-producing cells from the blood of IgAN patients and healthy controls. Instead, ST6-GalNAcII transcription was detected at a high level. Expression of the ST6-GalNAcII gene and activity of the CMP-NeuAc:GalNAc-IgA1 alpha2,6-sialyltransferase were higher in IgA1-producing cell lines from IgAN patients than in such cells from healthy controls. These data are the first evidence that human cells that lack ST6-GalNAcI can sialylate core 1 GalNAc-Ser/Thr.

Reactivities of N-acetylgalactosamine-specific lectins with human IgA1 proteins

Lectins are proteins with specificity of binding to certain monosaccharides or oligosaccharides. They can detect abnormal glycosylation patterns on immunoglobulins in patients with various chronic inflammatory diseases, including rheumatoid arthritis and IgA nephropathy (IgAN). However, lectins exhibit binding heterogeneity, depending on their source and methods of isolation. To characterize potential differences in recognition of terminal N-acetylgalactosamine (GalNAc) on IgA1, we evaluated the binding characteristics of several commercial preparations of GalNAc-specific lectins using a panel of IgA1 and, as controls, IgA2 and IgG myeloma proteins. These lectins originated from snails Helix aspersa (HAA) and Helix pomatia (HPA), and the plant Vicia villosa (VV). Only HAA and HPA bound exclusively to IgA1, with its O-linked glycans composed of GalNAc, galactose, and sialic acid. In contrast, VV reacted with sugars of both IgA subclasses and IgG, indicating that it also recognized N-linked glycans without GalNAc. Furthermore, HAA and HPA from several manufacturers differed in their ability to bind various IgA1 myeloma proteins and other GalNAc-containing glycoproteins in ELISA and Western blot. For serum samples from IgAN patients, HAA was the optimal lectin to study IgA1 glycosylation in ELISA and Western blot assays, including identification of the sites of attachment of the aberrant glycans. The galactose-deficient glycans were site-specific, localized mostly at Thr228 and/or Ser230. Because of the heterogeneity of GalNAc-specific lectins, they should be carefully characterized with appropriate substrates before undertaking any study.

Variants of C1GALT1 gene are associated with the genetic susceptibility to IgA nephropathy

IgA nephropathy (IgAN) is a polygenic disorder and the precise role of genetic factors remains elusive. Increasing evidences have implicated the aberrant galactosylation of IgA1 molecules in the pathogenesis of IgAN. The galactosyltransferase, core 1 beta3-Gal-T, and its chaperone, Cosmc, play important roles in beta1,3 glycosylation of IgA1 molecule. A case-control association study was performed to investigate the association between single-nucleotide polymorphisms (SNPs) of C1GALT1 and C1GALT1C1 genes and the susceptibility to IgAN. A total of 1164 subjects were enrolled, including 670 IgAN patients and 494 geographically matched healthy controls. Five SNPs, -734C/T, -465A/G, -330G/T, -292C/-, and 1365G/A in C1GALT1 were selected as tagging SNPs. The D allele and DD genotype of -292C/- in IgAN patients were significantly lower than in the controls (P<0.01). The frequency of haplotype YATIG (Y=C or T) was significantly lower in patients than in controls (0.0719 vs 0.1168, P=2.775 x 10(-4), odds ratio (OR)=0.70). The haplotype YAGDA (0.1236 vs 0.0791, P=3.815 x 10(-3), OR=1.77) and YATDG (0.0840 vs 0.0298, P=1.258 x 10(-5), OR=3.03) were significantly higher in patients than in controls. The present study suggested that the polymorphisms of C1GALT1 gene were associated with the genetic susceptibility to IgAN in Chinese population.

Structural analyses of O-glycan sugar chains on IgA1 hinge region using SELDI-TOFMS with various lectins

The aim of the study was to develop a simple and precise method for identifying glycosylation of the IgA hinge region using surface-enhanced laser desorption/ionization (SELDI)-TOFMS with a lectin-coupled ProteinChip array. Serum IgA was isolated using an anti-IgA antibody column. Following reduction, alkylation, and trypsin digestion, the IgA fragments were applied on the ProteinChip coupled with jacalin, peanut agglutinin (PNA), or Vilsa villosa lectin (VVL). The SELDI-TOFMS peaks corresponding to the fragments containing IgA1 hinge glycopeptides trapped by each lectin were compared. The jacalin-, PNA-, and VVL-immobilized ProteinChips detected 13, 4, and 2 peaks, respectively. One major peak was confirmed as a glycopeptide by MS/MS analysis. These results suggest that a lectin-immobilized ProteinChip assay can be used to simplify the procedures for the analyses of the O-glycans in IgA1 hinge. This method potentially makes it possible to identify a disease-specific glycoform by selecting the appropriate ligand-coupled ProteinChip array.

A pathogenic role for secretory IgA in IgA nephropathy

IgA nephropathy (IgAN) is characterized by deposits of IgA in the renal mesangium. It is thought that deposits of IgA mainly involve high molecular weight (HMW) IgA1. However, there is limited information on the exact composition of HMW IgA in these deposits. In this study, we investigated the presence of secretory IgA (SIgA) in human serum and in the glomerular deposits of a patient with IgAN. Furthermore, we analyzed the interaction of SIgA with mesangial cells. With enzyme-linked immunosorbent assay, SIgA concentrations in the serum of IgAN patients and healthy controls were measured. Both patients and controls had circulating SIgA that was restricted to the HMW fractions. Patients tended to have higher levels of SIgA, but this difference was not significant. However, in patients with IgAN, high serum SIgA concentrations were associated with hematuria. Binding of size-fractionated purified serum IgA and SIgA to mesangial cells was investigated with flow cytometry. These studies showed stronger binding of SIgA to primary mesangial cells compared to binding of serum IgA. Importantly, after isolation and elution of glomeruli from a nephrectomized transplanted kidney from a patient with recurrent IgAN, we demonstrated a 120-fold accumulation of SIgA compared to IgA1 in the eluate. In conclusion, we have demonstrated that SIgA strongly binds to human mesangial cells, and is present in significant amounts in serum. Furthermore, we showed that SIgA is accumulated in the glomeruli of an IgAN patient. These data suggest an important role for SIgA in the pathogenesis of IgAN.