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

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.

Degalactosylated and/or Denatured IgA, but Not Native IgA in Any Form, Bind to Mannose-Binding Lectin

Mannose-binding lectin (MBL) is reported to bind to agalactosyl IgG, but not to normally galactosylated (native) IgG. It was recently reported that serum polymeric IgA in its native form reacts with MBL, whereas a more recent report has claimed that native IgD and IgE, and possibly IgM, do not. This led us to investigate whether IgA is truly reactive with MBL. To accomplish this, we collected purified human Igs, of various classes, subclasses, and allotypes, and tested their ability to bind to MBL using an ELISA method. Among these preparations, only one (monoclonal IgA2m(2):Kur) exhibited significant MBL binding. In particular, polymeric or monomeric forms of our normal serum IgA preparation lacked any ability to bind to MBL whatsoever. However, all the Ig preparations which had not bound to MBL became able to do so when they were degalactosylated with a galactosidase treatment, and the binding was further enhanced by acidic denaturation of the Igs. Among the degalactosylated and/or acid-denatured IgA, the IgA2 subclass exhibited a higher level of MBL binding than did IgA1. Our results suggest that MBL does not bind to native Igs (viewed in principle as "self" components), and that only Igs with abnormal glycosylation (degalactosylated forms) and/or denaturation would be MBL reactive.

Self-aggregated deglycosylated IgA1 with or without IgG were associated with the development of IgA nephropathy

IgA nephropathy (IgAN) is the most common primary glomerulonephritis, with various pathological phenotypes. Our previous study suggested that aberrant glycosylation of serum IgA1 was associated with different pathological phenotypes of IgAN, and substantial evidence indicated that deglycosylated IgA1 had an increased tendency to form macromolecules. The aim of the current study was to investigate the composition of IgA1-containing macromolecules in different pathological phenotypes of IgAN. Sera from 10 patients with mild mesangial proliferative IgAN (mIgAN), 10 with focal proliferative sclerosing IgAN (psIgAN) and 10 healthy blood donors were collected. The sera were applied and IgA1 binding proteins (IgA1-BP) were eluted from the columns immobilized with desialylated IgA1 (DesIgA1/Sepharose) or desialylated/degalactosylated IgA1 (DesDeGalIgA1/Sepharose), respectively. The amounts of IgA1 and IgG and the glycoform of IgA1 in the IgA1-BP were detected by enzyme-linked immunosorbent assays (ELISAs) and were compared between patients with different pathological phenotypes and normal controls. The amount of IgA1 in IgA1-BP eluted from both columns was significantly higher in patients with both pathological phenotypes of IgAN than in normal controls. In IgA1-BP eluted from DesDeGalIgA1/Sepharose, the desialylation of IgA1 was much more pronounced in patients with both pathological phenotypes of IgAN than in normal controls, while the degalactosylation of IgA1 was much more pronounced only in patients with psIgAN than in normal controls. Furthermore, the amount of IgG in IgA1-BP eluted from DesDeGalIgA1/Sepharose was significantly higher in patients with psIgAN than in normal controls. In patients with psIgAN, the amount of IgG eluted from DesDeGalIgA1/Sepharose was much greater than from DesIgA1/Sepharose. In conclusion, self-aggregated deglycosylated IgA1 with or without IgG were associated with the development of IgAN.

IgA-containing immune complexes in the urine of IgA nephropathy patients

BACKGROUND

Sera of IgA nephropathy (IgAN) patients contain high levels of circulating immune complexes composed of IgA1 molecules with aberrantly glycosylated hinge-region O-linked oligosaccharides and IgG or IgA1 antibodies with anti-glycan or anti-hinge-region peptide specificities. Due to damaged sieving properties of the glomerular capillary wall in IgAN, these immune complexes may appear in the urine.

METHODS

We collected urine samples from 29 patients with biopsy-proven IgAN (Group I), 27 proteinuric patients with non-IgA nephropathies (Group II) and 28 healthy volunteers (Group III). The levels of urinary IgA and IgG and IgA-IgG-containing immune complexes were measured by ELISA and standardized for urinary creatinine concentrations.

RESULTS

The urinary IgA and IgG levels were significantly higher in Groups I and II than in Group III. Although the excretion of IgA as a fraction of total urinary protein was not significantly greater in IgAN patients than in patients with other renal diseases, the excretion of aberrantly glycosylated IgA1 was observed by western blot in 68% of the IgAN patients but in none of the healthy controls. The urinary levels of IgA-IgG immune complexes were significantly higher in Group I than in Groups II (P < 0.01) and III (P < 0.05). There was no significant difference in the levels between Groups II and III. These immune complexes had a molecular mass between 650-850 kDa, as shown by size-exclusion chromatography.

CONCLUSION

The amounts of urinary IgA-IgG-containing immune complexes were significantly higher in patients with IgAN than in patients with non-IgA nephropathies or healthy controls.

Binding capacity of in vitro deglycosylated IgA1 to human mesangial cells

IgA nephropathy (IgAN) is the most common glomerular disease and it is characterized by deposition of IgA1 molecules in mesangium. Recent studies had demonstrated that serum and mesangial IgA1 in IgAN were deglycosylated and IgA1 could bind to human mesangial cells (HMC) through a novel receptor. The aim of the current study is to investigate and compare the binding capacities of different in vitro deglycosylated IgA1 on human mesangial cells. Serum IgA1 was purified by jacalin affinity chromatography and then was desialylated (DesIgA1) and/or degalactosylated (Des/DeGalIgA1) with neuraminidase and/or beta-galactosidase. The efficacy of deglycosylations was assessed by Peanut agglutinin (PNA) and Vicia villosa (VV) lectin. The sizes of normal IgA1 and deglycosylated IgA1 were determined by Sephacryl S-300 chromatography and binding capacities to primary HMC were evaluated by radioligand binding assays. Normal IgA1 and deglycosylated IgA1 could bind to HMC in a dose-dependent, saturable manner. The maximal binding capacities and binding sites/cell of DesIgA1 and Des/DeGalIgA were significantly higher than that of normal IgA1. However, more aggregated IgA1 was found in DesIgA1 and Des/DeGalIgA1. Scatchard analysis revealed a similar Kd of normal IgA1 and deglycosylated IgA1. The current study suggested that the binding capacities of DesIgA1 and Des/DeGalIgA1 to HMC were significantly higher than that of normal IgA1, which at least in part was due to more macromolecular IgA1 in deglycoslated IgA1. However, there were no significant differences in the affinities of normal IgA1, DesIgA1 and Des/DeGalIgA1 with HMC. Deglycosylated IgA1 might play an important role in pathogenesis of IgAN.

The glycans deficiencies of macromolecular IgA1 is a contributory factor of variable pathological phenotypes of IgA nephropathy

Recent evidence has suggested that IgA1-containing macromolecules and the glycosylation of IgA1 in sera from patients with IgAN might involve the pathogenesis of IgAN. However, whether the different histological phenotypes can be attributed or not to the aberrant glycosylation of macromolecular IgA1 has not yet been elucidated. The aim of the current study is to investigate the glycosylation of IgA1 molecules in serum IgA1-containing macromolecules and their association with pathological phenotypes of IgAN. Sera was collected from 40 patients with IgAN and 20 donors. Twenty patients had mild mesangial proliferative IgAN, the remaining 20 had focal proliferative sclerosing IgAN. Polyethylene glycol 6000 was used to precipitate the macromolecules from sera of patients and controls. Biotinylated lectins were used in an enzyme-linked immunosorbent assay (ELISA) to examine different glycans on IgA1 molecules. The alpha2,6 sialic acid was detected by elderberry bark lectin (SNA) and the exposure of terminal galactose (Gal) and N-acetylgalactosamine (GalNAc) were detected by Arachis hypogaea (PNA) and Vilsa villosa lectin (VVL), respectively. The IgA1 glycans levels corrected by IgA1 concentrations were compared between patients and controls. Reduced terminal alpha2,6 sialic acid of IgA1 (79.89 +/- 25.17 versus 62.12 +/- 24.50, P = 0.034) was demonstrated only in precipitates from sera of patients with focal proliferative sclerosing IgAN, compared with those from controls. Reduced galactosylation of IgA1 molecules in precipitates was demonstrated in patients with both mild mesangial proliferative IgAN and focal proliferative sclerosing IgAN compared with normal controls (24.52 +/- 18.71 versus 76.84 +/- 32.59 P = 0.000 and 33.48 +/- 25.36 versus 76.84 +/- 32.59 P = 0.000). However, no significant difference was found in IgA1 glycosylation in the supernatant between patients and normal controls (P > 0.05). The glycosylation deficiency of IgA1 existed only in serum IgA1-containing macromolecules of patients with IgAN, and was associated with the renal pathological phenotypes. This suggests that aberrant glycosylation of IgA1 in serum macromolecules might be a contributory factor in the pathogenesis of IgAN.

Peripheral B lymphocyte beta1,3-galactosyltransferase and chaperone expression in immunoglobulin A nephropathy

PURPOSE

Aberrant O-glycosylation of serum IgA(1) is presumed to be one of the main pathogenesis of immunoglobulin A nephropathy (IgAN). beta1,3-galactosyltransferase (beta1,3GT), whose activity requires coexistence of a specific chaperone, is the main enzyme which participate in the glycosylation process. The current study was carried out to elucidate the expression level of beta1,3GT (C1GALT1) and its chaperone (Cosmc) in IgAN, and their relationships with clinical features as well as IgA glycosylation level.

DESIGN, SETTING AND SUBJECTS

Forty-one patients with IgAN, 21 patients with non-IgAN glomerulonephritis and 26 normal controls were included in the present study. Peripheral B lymphocytes were isolated, and then expression level of C1GALT1 and Cosmc were quantitatively measured by real-time reverse transcriptase polymerase chain reaction (RT-PCR). Serum IgA level and glycosylation level were determined by enzyme-linked immunosorbent assay (ELISA) and VV lectin-binding method. Correlation analysis was performed between C1GALT1/Cosmc expression levels and clinical manifestations (severe proteinuria, renal dysfunction, gross haematuria).

RESULTS

B-lymphocyte Cosmc gene expression level was significantly lower in IgAN patients than that of normal control and non-IgAN patients (P<0.05), whilst no apparent disparity was observed in C1GALT1 expression level. Cosmc expression showed a negative correlation with IgA O-glycosylation level indicated by VV lectin-binding assay. Statistical analysis also indicated that the level of Cosmc expression was negatively correlated with severe proteinuria (P<0.05) instead of gross haematuria (P>0.05).

CONCLUSION

These data suggested that the aberrant IgA O-glycosylation in IgAN was resulted from a downregulation of beta1,3GT chaperone (Cosmc) expression in B lymphocyte, which is closely associated with clinical characteristics of the disease. This downregulation might be one of the fundamental pathogenic abnormalities in IgAN.

Aberrantly glycosylated serum IgA1 are closely associated with pathologic phenotypes of IgA nephropathy

BACKGROUND

IgA nephropathy (IgAN) is the most common glomerulonephritis with various histologic and clinical phenotypes. The mechanisms underlying the pathogenesis of IgAN remained unclear. But now altered O-glycosylation of serum IgA1 observed in these patients was considered to be a key contributory factor. The aim of the current study is to investigate whether aberrantly glycosylated IgA1 was associated with pathologic phenotypes of IgAN.

METHODS

Sera from 107 patients with IgAN recently diagnosed were collected. Fifty patients were with mild mesangial proliferative IgAN, the others were with focal proliferative and sclerosing IgAN. Sera from 22 normal blood donors were used as normal controls. Biotinylated lectins were used in enzyme-linked immunosorbent assay (ELISA) to examine different glycans on IgA1 molecules. The alpha2,6 sialic acid was detected by elderberry bark lectin (SNA), the exposure of terminal galactose (Gal) and N-acetylgalactosamine (GalNAc) were detected by arachis hypogaea [peanut agglutinin (PNA)] and vilsa villosa lectin (VVL), respectively. The serum IgA1 glycans levels corrected by serum IgA1 concentrations were compared between patients and controls.

RESULTS

Reduced terminal alpha2,6 sialic acid (1.16 +/- 0.21 vs. 0.98 +/- 0.31) (P= 0.008) and galactosylation (0.30 +/- 0.29 vs. 0.16 +/- 0.19) (P= 0.029) increased exposure of (GalNAc) (0.00 vs. 0.03) (P= 0.024) were demonstrated in serum IgA1 from patients with IgAN as compared with those in controls. More important, the exposures of 2,6 sialic acid and Gal were significantly decreased, especially in patients with focal proliferative and sclerosing IgAN compared with that in patients with mild mesangial proliferative IgAN (0.91 +/- 0.34 vs. 1.05 +/- 0.25) (P= 0.014) (0.108 +/- 0.137 vs. 0.221 +/- 0.219) (P= 0.018). However, no significant difference was found between patients with mild mesangial proliferative IgAN and normal controls (P > 0.05). The exposure of GalNAc of serum IgA1 from patients with focal proliferative and sclerosing IgAN was significantly higher than that of controls (P= 0.017), but had no statistical difference with that of patients with mild mesangial proliferative IgAN.

CONCLUSION

The desialylation and degalactosylation of IgA1 in sera of patients with IgAN were closely associated with pathologic phenotypes.

IgA1-containing immune complexes in IgA nephropathy differentially affect proliferation of mesangial cells

BACKGROUND

Sera of patients with IgA nephropathy (IgAN) contain circulating immune complexes (CIC) composed of galactose-deficient IgA1 complexed with antiglycan antibodies. The role of these CIC in the pathogenesis of IgAN is not known.

METHODS

We studied how proliferation of cultured mesangial cells (MC) is affected by CIC prepared from sera of IgAN patients and healthy control subjects using size-exclusion chromatography. CIC-containing fractions were added to serum-starved MC in culture, and cell proliferation was measured using (3)H-thymidine incorporation. The results were confirmed by staining MC using an antibody against proliferating cell nuclear antigen.

RESULTS

The incubation of starved MC with serum fractions with M(r) 800 to 900 kD, rich with galactose-deficient IgA1, stimulated proliferation, while fractions with smaller complexes were inhibitory. Furthermore, CIC-containing larger molecular mass fractions isolated from serum of an IgAN patient collected during an episode of macroscopic hematuria stimulated MC proliferation more than CIC obtained during a subsequent quiescent phase. To examine the role of IgA, we removed IgA1 from serum before fractionation. The resultant IgA1-depleted fractions were devoid of stimulatory IgA-CIC. Sera of IgAN patients were also fractionated after addition of desialylated galactose-deficient polymeric IgA1 to form additional immune complexes. Supplementation with a small quantity of this IgA1 increased cellular proliferation in assays using serum fractions of M(r)>/=800 to 900 kD; uncomplexed IgA1 did not affect MC proliferation significantly. In contrast, supplementation with a larger quantity of this IgA1 inhibited cellular proliferation in assays using serum fractions of M(r) 700 to 800 kD.

CONCLUSION

Overall, these findings suggest that CIC containing aberrantly glycosylated IgA1 affect proliferation of MC in vitro and, thus, likely play a role in the pathogenesis of IgAN.

Role of macromolecular IgA in IgA nephropathy

Primary IgA nephropathy (IgAN) is the most common form of primary glomerulonephritis, leading to progressive renal failure in almost one third of the patients. The disease is characterized by mesangial deposits of IgA. The pathogenesis of IgAN remains incompletely understood. The basic abnormality of this disorder lies within the IgA immune system rather than in the kidney. Elevated levels of IgA and IgA-containing complexes are found in sera of most patients with IgAN, but increased levels alone are not sufficient to develop IgAN. Therefore abnormal physicochemical properties of circulating IgA, such as size, charge, and glycosylation may play a role. This is supported by the presence of altered glycosylation of serum and mesangial IgA in patients with IgAN. Although the precise origin and nature of the mesangial IgA deposits are still uncertain, they contain at least in part macromolecular IgA, which may be derived from circulating IgA-containing complexes. Recently, novel insights have been obtained in the molecular composition of circulating high-molecular-weight IgA, which might include complexes with underglycosylated IgA1 and IgA-CD89 complexes. In this review various aspects of macromolecular IgA in relation to IgAN will be discussed.

Determination of aberrant O-glycosylation in the IgA1 hinge region by electron capture dissociation fourier transform-ion cyclotron resonance mass spectrometry

In a number of human diseases of chronic inflammatory or autoimmune character, immunoglobulin molecules display aberrant glycosylation patterns of N- or O-linked glycans. In IgA nephropathy, 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 Gal deficiency in IgA1 proteins occurs randomly or preferentially at specific sites. To develop experimental approaches to address this question, the synthetic IgA1 hinge region and hinge region from a naturally Gal-deficient IgA1 myeloma protein have been analyzed by 9.4 tesla Fourier transform-ion cyclotron resonance mass spectrometry. Fourier transform-ion cyclotron resonance mass spectrometry offers two complementary fragmentation techniques for analysis of protein glycosylation by tandem mass spectrometry. Infrared multiphoton dissociation of isolated myeloma IgA1 hinge region peptides confirms the amino acid sequence of the de-glycosylated peptide and positively identifies a series of fragments differing in O-glycosylation. To localize sites of O-glycan attachment, synthetic IgA1 HR glycopeptides and HR from a naturally Gal-deficient polymeric IgA1 myeloma protein were analyzed by electron capture dissociation and activated ion-electron capture dissociation. Multiple sites of O-glycan attachment (including sites of Gal deficiency) in myeloma IgA1 HR glycoforms were identified (in all but one case uniquely). These results represent the first direct identification of multiple sites of O-glycan attachment in IgA1 hinge region by mass spectrometry, thereby enabling future characterization at the molecular level of aberrant glycosylation of IgA1 in diseases such as IgA nephropathy.

Staphylococcus aureus cell envelope antigen is a new candidate for the induction of IgA nephropathy

BACKGROUND

IgA nephropathy is the most common form of glomerulonephritis worldwide. We previously reported a novel form of glomerulonephritis with glomerular IgA deposits following methicillin-resistant Staphylococcus aureus (S. aureus) infection. We investigated the role of S. aureus related antigens in the immunopathogenesis of IgA nephropathy by producing several monoclonal antibodies against S. aureus surface antigens and determining the epitopes of deposited antigens in patients with IgA nephropathy.

METHODS

Cell membrane proteins were isolated from cultured S. aureus. Mouse monoclonal antibodies against these proteins were generated, and their target epitopes were determined by antibody affinity chromatography and amino acid sequence analysis, and by monoclonal antibody screening of Escherichia coli clones transfected with plasmids from the Lambda S. aureus Genomic Library. Renal biopsy specimens from 116 patients with IgA nephropathy and 122 patients with other forms of renal disease were examined for glomerular antigen depositions by immunofluorescence microscopy.

RESULTS

. The major antigen recognized by monoclonal antibodies against S. aureus cell membrane was identified as the S. aureus cell envelope antigen designated 'probable adhesin' (ACCESSION AP003131-77, Protein ID; BAB41819.1). In 68.1% (79/116) of renal biopsy specimens from patients with IgA nephropathy, S. aureus cell envelope antigen was localized in the glomeruli, and the data confirmed that S. aureus cell envelope antigen was co-localized with IgA antibody in the glomeruli. No deposition of this antigen was detected in the glomeruli of patients with non-immune complex deposit forms of glomerulonephritis.

CONCLUSION

S. aureus cell envelope antigen is a new candidate for the induction of IgA nephropathy.

IgA nephropathy and tonsils--an approach from the structure of IgA1 produced by tonsillar lymphocytes

Human immunoglobulin A1 (IgA1), which is the predominant subtype to be deposited in glomeruli in IgA nephropathy (IgAN), has a unique mucine-like structure in its hinge region. Namely, it contains O-glycans and proline-rich peptides We previously observed underglycosylation of the hinge region in serum and deposited IgA1 in IgAN. On the other hand, clinical development and exacerbation of IgAN are frequently preceded by episodes of upper respiratory tract infection, and palatine tonsils represent the predominant immunocompetent tissue of the upper respiratory tract. Therefore, we hypothesized that tonsils were one of the origins of glomerular IgA1 in IgAN, and investigated the O-glycan structure of IgA1 produced by tonsillar lymphocytes (tonsillar IgA1). A significant increase in asialo-agalacto type O-glycans was found in the tonsillar IgA1 hinge in IgAN. These results suggest that the tonsils produce underglycosylated IgA1 molecules, which enter the bloodstream and are then deposited in the glomeruli.

Pediatric IgA nephropathies: clinical aspects and therapeutic approaches

The pediatric IgA nephropathies are IgA nephrothapy (Berger's Disease) and Henoch-Schönlein purpura nephritis. Both conditions are reviewed in detail with respect to epidemiology, clinical features, outcome, prognostic markers, and therapeutic approaches. For both conditions variable disease severity and outcome along with the lack of conclusive evidence for efficacy of treatment based on randomized clinical trials makes it difficult to make strong recommendations regarding therapy.

Inhibition of B cell death causes the development of an IgA nephropathy in (New Zealand white x C57BL/6)F(1)-bcl-2 transgenic mice

Little is known about the pathogenic mechanisms of IgA nephropathy, despite being the most prevalent form of glomerulonephritis in humans. We report in this study that in (New Zealand White (NZW) x C57BL/6)F(1) mice predisposed to autoimmune diseases, the expression of a human bcl-2 (hbcl-2) transgene in B cells promotes a CD4-dependent lupus-like syndrome characterized by IgG and IgA hypergammaglobulinemia, autoantibody production, and the development of a fatal glomerulonephritis. Histopathological analysis of glomerular lesions reveals that the glomerulonephritis observed in these animals resembles that of human IgA nephropathy. The overexpression of Bcl-2 in B cells selectively enhances systemic IgA immune responses to T-dependent Ags. Significantly, serum IgA purified from (NZW x C57BL/6)F(1)-hbcl-2 transgenic mice, but not from nontransgenic littermates, shows reduced levels of galactosylation and sialylation and an increased ability to deposit in the glomeruli, as observed in human patients with IgA nephropathy. Our results indicate that defects in the regulation of B lymphocyte survival associated with aberrant IgA glycosylation may be critically involved in the pathogenesis of IgA nephropathy, and that (NZW x C57BL/6)F(1)-hbcl-2 Tg mice provide a new experimental model for this form of glomerulonephritis.

Detection of gender difference and epitope specificity of IgG antibody activity against IgA1 hinge portion in IgA nephropathy patients by using synthetic hinge peptide and glycopeptide probes

BACKGROUND AND AIMS

There are many reports on the presence of an incompletely glycosylated O-linked oligosaccharide(s) on the IgA1 hinge region in some immunoglobulin (IgA) nephropathy patients. Furthermore, the production of an antibody against the naked hinge peptide portion was reported in an IgA nephropathy patient. In this report, characterization of the IgG antibody against the hinge portion was carried out by using synthetic hinge glycopeptide probes.

METHODS AND RESULTS

The following synthetic hinge peptide and glycopeptides were prepared: 19mer peptide, V-P-S-T-P-P-T-P-S-P-S-T-P-P-T-P-S-P-S (designated HP), the peptide having a single alpha-linked GalNAc residue at positions 4, 7, 9, 11 and 15 (4 GN - 15 GN, respectively) and the same peptide having five GalNAc residues at all five positions (GN5). The mean value of the antibody activity against these probes was compared with each other. The highest activity against the naked hinge peptide (HP) and lowest activity against the fully glycosylated hinge peptide (GN5) were obvious. As attachment of GalNAc to position 4 or 11 on the peptide brought about a significant reduction of the activity against the naked hinge peptide, the P-S-T-P sequence included in both positions was thought to be the most probable site recognized by these antibodies. As an additional unexpected observation, a gender difference in this antibody activity against all the probes was found. The antibody activity in a female was significantly higher compared with that in a male.

CONCLUSION

Because the frequency of incidence of IgA nephropathy is known to be slightly higher in males, this gender difference might indicate a protective meaning to remove aberrantly glycosylated molecules from the patient's serum.

IgA nephropathy: an update

PURPOSE OF REVIEW

The elution of nephrectomy specimens from patients with IgA nephropathy yields IgA1 with galactose-deficient glycans in the hinge region. In this review, we summarize recent advances in our understanding of the role of the aberrant immunoglobulin in the pathogenesis of this form of glomerulonephritis. In the absence of a disease-specific therapy, we discuss current therapeutic approaches.

RECENT FINDINGS

Galactose-deficient IgA1 forms macromolecular complexes that bind to mesangial cells and stimulate them to proliferate, synthesize various cytokines and chemokines, and secrete extracellular matrix proteins. Whereas progress has been made in understanding the glycosylation pathways of IgA1 O-linked glycans and binding galactose-deficient IgA1-complexes to mesangial cells, there is still no IgA nephropathy-specific therapy. The current approach to suppress the effects of angiotensin II, by angiotensin-converting enzyme inhibitors, angiotensin II receptor type 1 blockers, or both, as a cornerstone of the therapy of IgA nephropathy has been strengthened by recent studies. Treatment with glucocorticoids, cyclophosphamide, or both, may be appropriate for a subset of IgA nephropathy patients.

SUMMARY

A better understanding of the mechanisms underlying the synthesis of galactose-deficient IgA1, the formation of circulating immune complexes, and interactions with mesangial cells will provide further insights into the pathogenetic mechanisms that culminate in the glomerular and interstitial damage of IgA nephropathy, and could identify novel therapeutic targets in the prevention and management of this renal disease.

The role of fish oil/omega-3 fatty acids in the treatment of IgA nephropathy

Beneficial effects of omega-3 polyunsaturated fatty acids (n-3 PUFA) have been reported in recent epidemiologic studies and randomized clinical trials in a variety of cardiovascular and autoimmune diseases. Fish and marine oils are the most abundant and convenient sources of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), the two major n-3 fatty acids that serve as substrates for cyclooxygenase and lipoxygenase pathways leading to less potent inflammatory mediators than those produced through the n-6 PUFA substrate, arachidonic acid. N-3 PUFA can also suppress inflammatory and/or immunologic responses through eicosanoid-independent mechanisms. Although the pathophysiology of IgA nephropathy is incompletely understood, it is likely that n-3 PUFA prevents renal disease progression by interfering with a number of effector pathways triggered by mesangial immune-complex deposition. In addition, potential targets of n-3 PUFA relevant to renal disease progression could be similar to those involved in preventing the development and progression of cardiovascular disease by lowering blood pressure, reducing serum lipid levels, decreasing vascular resistance, or preventing thrombosis. In IgA nephropathy, efficacy of n-3 PUFA contained in fish oil supplements has been tested with varying results. The largest randomized clinical trial performed by our collaborative group provided strong evidence that treatment for 2 years with a daily dose of 1.8 g of EPA and 1.2 g of DHA slowed the progression of renal disease in high-risk patients. These benefits persisted after 6.4 years of follow up. With safety, composition, and dosing convenience in mind, we can recommend two products that are available as pharmaceutical-grade fish-oil concentrates, Omacor (Pronova Biocare, Oslo, Norway) and Coromega (European Reference Botanical Laboratories, Carlsbad, CA).

Aberrant glycosylation in IgA nephropathy (IgAN)

Immunoglobulin A nephropathy (IgAN) patients exhibit circulating IgA1 with reduced galactose (Gal) and/or sialic acid (Neu5Ac) and increased exposure of N-acetylgalactosamine (GalNAc). These IgA glycoforms fix complement and in mesangial cells regulate integrin expression, enhance nitric oxide synthase (NOS) activity, decrease endothelial growth factor synthesis, meanwhile depressing proliferation and increasing apoptosis. Drugs can be targeted to the effects enhanced by aberrantly glycosylated IgA1 on mesangial cells. Recent data suggest that aberrant IgA1 glycosylation may modulate clinical expression and progression of IgAN.

Pathogenesis of IgA nephropathy

Immunoglobulin A (IgA) nephropathy is an immune-complex-mediated glomerulonephritis characterized by the presence of immunoglobulin A deposits in mesangial and paramesangial regions. The patients with IgA nephropathy present with varying clinical symptoms (eg, microhematuria with preserved renal function or progressive deterioration of renal functions resulting in end-stage renal disease). The factors involved in the pathogenetic mechanisms of IgA nephropathy include (1). environmental factors, (2). genetic factors, (3). abnormality of the IgA1 molecule, and (4). various inflammatory mediators. The gene polymorphism studies for human leukocyte antigen (HLA), renin-angiotensin-aldosterone system, and selectin gene clusters, suggest a certain degree of genetic predisposition in patients for IgA nephropathy. Also, the genome-wide screening in familial IgA nephropathy showed linkage of IgA nephropathy to the 6q22-23 chromosome. Besides genetic influence in its pathogenesis, aberrant galactosylation in serum IgA and IgA1 eluted from kidneys with IgA nephropathy has been observed, and conceivably such abnormalities induce the expression of various cytokines, interleukin (IL)-6, platelet-derived growth factor (PDGF), tumor necrosis factor (TNF)-alpha, and transforming growth factor (TGF)-beta1 in the renal cells, which contributes to further glomerular injury. Despite an enormous amount of information available in the literature, further studies are needed to delineate the precise pathogenetic mechanisms involved in primary IgA nephropathy and also to facilitate the development of newer therapeutic interventions.

IgA1 molecules produced by tonsillar lymphocytes are under-O-glycosylated in IgA nephropathy

BACKGROUND

Human serum immunoglobulin A1 (IgA1) has a unique mucine-like structure in its hinge region that contains O-glycans and proline-rich peptides. We previously reported the under-O-glycosylation of the hinge in serum IgA1 and deposited IgA1 in glomeruli (glomerular IgA1) in IgA nephropathy. The clinical development and exacerbation of IgA nephropathy frequently are preceded by episodes of upper respiratory tract infections. Therefore, tonsils, which represent the predominant immunocompetent tissue of the upper respiratory tract, may be related to the pathogenesis of IgA nephropathy. In this study, we investigated the O-glycan structure of IgA1 produced by tonsillar lymphocytes (tonsillar IgA1), suspecting that tonsillar IgA1 is one of the origins of glomerular IgA1 in patients with IgA nephropathy.

METHODS

Extracted tonsils were obtained from 7 patients with IgA nephropathy and 5 patients with chronic tonsillitis as controls. Tonsillar lymphocytes separated from extracted tonsils were cultured for 7 days, and IgA1 in the culture medium was purified. The varieties of O-glycans in tonsillar IgA1 were determined from the molecular weights measured by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry.

RESULTS

A significant increase in the percentage of asialo-agalacto type O-glycans was found in tonsillar IgA1 in 4 of 7 patients with IgA nephropathy (57.1%) compared with controls. Between the IgA nephropathy and control groups, the difference was statistically significant (P = 0.047).

CONCLUSION

This study provides precise information about the structure of O-glycans in tonsillar IgA1 in patients with IgA nephropathy. Our results suggest that tonsils produced the underglycosylated IgA1 molecules in patients with IgA nephropathy.

Initiation of O-glycan synthesis in IgA1 hinge region is determined by a single enzyme, UDP-N-acetyl-alpha-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase 2

The hinge region of human immunoglobulin A1 (*IgA1) possesses multiple O-glycans, of which synthesis is initiated by the addition of GalNAc to serine or threonine through the activity of UDP-N-acetyl-alpha-D-galactosamine:polypeptide N-acetylgalactosaminyltransferases (pp-GalNAc-Ts). We found that six pp-GalNAc-Ts, pp-GalNAc-T1, -T2, -T3, -T4, -T6, and -T9, were expressed in B cells, IgA-bearing B cells, and NCI-H929 IgA myeloma cells. pp-GalNAc-T activities of these six enzymes for a synthetic IgA hinge peptide, which has nine possible O-glycosylation sites, were examined using a reversed phase-high performance liquid chromatography, a matrix-assisted laser desorption ionization time of flight mass spectrometry, and peptide sequencing analysis. pp-GalNAc-T2 showed the strongest activity transferring GalNAc to a maximum of eight positions. Other pp-GalNAc-Ts exhibited different substrate specificities from pp-GalNAc-T2; however, their activities were extremely weak. It was reported that the IgA1 hinge region possesses a maximum of five O-glycans, and their amino acid positions have been determined. We found that pp-GalNAc-T2 selectively transferred GalNAc residues to the same five positions. These results strongly suggested that pp-GalNAc-T2 is an essential enzyme for initiation of O-linked glycosylation of the IgA1 hinge region.

A unique molecular chaperone Cosmc required for activity of the mammalian core 1 beta 3-galactosyltransferase

Human core 1 beta3-galactosyltransferase (C1beta3Gal-T) generates the core 1 O-glycan Galbeta1-3GalNAcalpha1-SerThr (T antigen), which is a precursor for many extended O-glycans in animal glycoproteins. We report here that C1beta3Gal-T activity requires expression of a molecular chaperone designated Cosmc (core 1 beta3-Gal-T-specific molecular chaperone). The human Cosmc gene is X-linked (Xq23), and its cDNA predicts a 318-aa transmembrane protein ( approximately 36.4 kDa) with type II membrane topology. The human lymphoblastoid T cell line Jurkat, which lacks C1beta3Gal-T activity and expresses the Tn antigen GalNAcalpha1-SerThr, contains a normal gene and mRNA encoding C1beta3Gal-T, but contains a mutated Cosmc with a deletion introducing a premature stop codon. Expression of Cosmc cDNA in Jurkat cells restored C1beta3Gal-T activity and T antigen expression. Without Cosmc, the C1beta3Gal-T is targeted to proteasomes. Expression of active C1beta3Gal-T in Hi-5 insect cells requires coexpression of Cosmc. Overexpression of active C1beta3Gal-T in mammalian cell lines also requires coexpression of Cosmc, indicating that endogenous Cosmc may be limiting. A small portion of C1beta3Gal-T copurifies with Cosmc from cell extracts, demonstrating physical association of the proteins. These results indicate that Cosmc acts as a specific molecular chaperone in assisting the foldingstability of C1beta3Gal-T. The identification of Cosmc, a uniquely specific molecular chaperone required for a glycosyltransferase expression in mammalian cells, may shed light on the molecular basis of acquired human diseases involving altered O-glycosylation, such as IgA nephropathy, Tn syndrome, Henoch-Schönlein purpura, and malignant transformation, all of which are associated with a deficiency of C1beta3Gal-T activity.

In human IgA nephropathy uteroglobin does not play the role inferred from transgenic mice

BACKGROUND

Uteroglobin (UG)-knockout and UG-antisense transgenic mice develop clinical and pathological features of immunoglobulin A (IgA) nephropathy with heavy proteinuria. These models suggested that UG, an anti-inflammatory protein with high affinity for fibronectin (Fn), prevents the formation of IgA-Fn complexes and mesangial deposits in mice. We aim to elucidate whether similar mechanisms underlie the development and severity of human IgA nephropathy.

METHODS

Specific enzyme-linked immunosorbent assays were devised to detect serum levels of UG binding to Fn or incorporated into IgA-Fn complexes and IgA binding to Fn or collagen IV. Sera from 75 patients with IgA nephropathy with normal renal function and various degrees of proteinuria (0.2 to 5 g/d of protein) stable over the previous 3 months without therapy were investigated and compared with healthy controls.

RESULTS

Levels of UG binding to Fn were similar in patients with IgA nephropathy and healthy controls. UG incorporated into circulating IgA-Fn complexes, as well as levels of IgA-Fn complexes and IgA binding Fn and collagen IV, were significantly greater in patients than healthy controls. Greater amounts of UG incorporated into IgA-Fn complexes reduced the risk for proteinuria with protein greater than 1 g/d (odds ratio = 0.67; P < 0.001). Logistic regression analysis assigned a predictive value for proteinuria persistently greater than 1 g/d of protein to lower amounts of UG incorporated into IgA-Fn complexes (R = -0.267; P = 0.008) and increased binding of IgA to collagen IV (R = 0.214; P = 0.0003).

CONCLUSION

This first report of human IgA nephropathy after the publication of the mouse model shows that UG is not reduced in circulation and is even increased in IgA-Fn complexes. Because aberrant IgA1 glycosylation is the event initiating IgA nephropathy in humans, we speculate that the enhanced incorporation of UG into IgA-Fn complexes might represent feedback to reduce the formation of macromolecular aggregates.

Quantitative analysis of IgA1 binding protein prepared from human serum by hypoglycosylated IgA1/Sepharose affinity chromatography

The binding protein to a hypoglycosylated IgA1/Sepharose (IgA1-BP) could be prepared from human sera. IgG was a major component in the IgA1-BP. A Protein A column was used to remove the IgG; however, about half of the IgA1-BP was passed from the column [Biochem. Biophys. Res. Commun., 264 (1999) 424]. Quantitative analysis of the passed fraction (PAP) by laser nepherometry indicated that it was composed of a fairly large amount of IgA, IgM and complement C3 besides IgG. The relative content of IgG:IgA:IgM:C3:C4 was 25:10:41:22:2 in the PAP fraction. Meanwhile, the Protein A bound-fraction was essentially composed of IgG (78%) and IgM (19%). The total amount of IgA1-BP was not different between the sera from IgA nephropathy patients and other nephropathy patients. With respect to the IgA content in the IgA1-BP from IgA nephropathy patients, it was significantly higher than that from other nephropathy patients. It was found that the IgA1-BP from some IgA nephropathy patients contained a few micrograms of aberrant IgA per ml of serum. Thus, the obtained results suggested the preferential deposition of the self-aggregated IgA composed of hypoglycosylated IgA1 and co-deposition of IgG, IgM and C3 in the glomeruli in an IgA nephropathy patient.

Interactions of human mesangial cells with IgA and IgA-containing immune complexes

BACKGROUND

IgA nephropathy (IgAN) is characterized by IgA1-containing immune complexes in mesangial deposits and in the circulation. The circulating immune complexes (CIC) are composed of galactose- (Gal) deficient IgA1 and IgG or IgA1 antibodies specific for the Gal-deficient IgA1; interactions of these CIC with mesangial cells (MC) were studied.

METHODS

Binding, internalization, and catabolic degradation of myeloma IgA1 protein as a standard control and the isolated CIC were studied using human MC, hepatoma cell line HepG2 expressing the asialoglycoprotein receptor (ASGP-R), and monocyte-like cell line U937 expressing the Fc(alpha)-R (CD89). Biochemical and molecular approaches were used to assess expression of CD89 and ASGP-R by MC.

RESULTS

At 4 degrees C, radiolabeled IgA1 bound to MC and HepG2 cells in a dose-dependent and saturable manner. The binding was inhibited by IgA-containing CIC or excess IgA1 or its Fc fragment but not by the Fab fragment of IgA1. At 37 degrees C, the cell-bound IgA1 was internalized and catabolized. In addition to IgA1, HepG2 cells also bound (in a Ca2+-dependent manner), internalized, and catabolized asialoorosomucoid (ASOR), other asialo-(AS)-glycoproteins, and secretory component (SC). The binding by MC appeared to be restricted to IgA1 or AS-IgA1 and was not Ca2+-dependent. Furthermore, MC and HepG2 cells internalized and catabolized IgA1-containing CIC. Using RT-PCR with ASGP-R- or CD89-specific primers, mRNAs of the two respective genes were not detected in MC.

CONCLUSIONS

The data showed that the ability of MC to bind IgA1 and IgA1-containing CIC in vitro was mediated by an IgA receptor that was different from CD89 or ASGP-R and had a higher affinity for IgA-CIC than for uncomplexed IgA.

[Berger's disease in childhood]

Berger's disease or IgA nephropathy (NIgA) is the most common form of glomerulonephritis in the world. In children macroscopic haematuria is the first sign in about 80% of the patients. Renal failure appears in 20% of cases after twenty years of follow-up. The most important prognosis indicators are a nephrotic syndrome at the onset, a proteinuria > 1 g/24 hours, diffuse tubulo-interstitial lesions and extracapillary proliferation with crescents in more than 50% of the glomeruli. The pathogenic mechanisms are just emerging and involve a disrupted process of the systemic tolerance to mucosal antigen with abnormal mucosal gamma delta T cell repertoire, abnormally glycosylated IgA1 molecules and a down-regulation of Fc alpha receptors on blood cells. After IgA deposition, the mechanisms of mesangial cell damage and activation involve vascular factors as endothelin/nitric oxide system, cytokines and growth factors such as interleukine-6, platelet derived growth factor and transforming growth factor beta. There is no curative treatment but steroids are useful in diffuse proliferative extracapillary forms, when histological activity score is high with a short delay between diagnosis and treatment, or for moderately severe NIgA with normal renal function.

Polymorphism of the cytokine genes and IgA nephropathy

BACKGROUND

IgA nephropathy (IgAN) is a form of chronic glomerulonephritis of unknown etiology and pathogenesis. Cytokine gene polymorphisms regulate cytokine production and play a role in immune and inflammatory responses; these immunological responses thus are possibly involved in the etiology and pathogenesis of IgAN.

METHODS

We studied by polymerase chain reaction (PCR) polymorphisms of important cytokine genes of inflammation interleukin-1 (IL-1beta), tumor necrosis factor-alpha (TNF-alpha), interleukin-6 (IL-6), and interleukin-1 receptor antagonist (IL-1Ra) in 167 patients with IgAN and 400 healthy blood donor controls. IgAN patients had been followed up for 6 to 17 (median 11) years from renal biopsy.

RESULTS

Carriage of the IL-1beta allele 2 (IL1beta2) or IL-1Ra allele 2 (IL1RN*2) was associated with an increased risk of IgAN. These alleles were highly linked and the odds ratio (OR) of IgAN for carriage of both alleles was 1.8 (95% confidence interval 1.2 to 2.6; P = 0.002). Carriage of the TNF-alpha allele 2 (TNF2) was associated with a decreased risk of IgAN (OR 0.5, range 0.3 to 0.7; P = 0.001). The risk of IgAN was found to be highest in those carrying IL1beta2 and IL1RN*2 but not TNF2 as compared to those who did not carry both of these IL-1 cluster genes and were carriers of TNF2 (OR 5.0 (2.4-10.3); P < 0.001). None of the polymorphisms studied was associated with poor prognosis.

CONCLUSION

Carriage of IL1beta2 and IL1RN*2 together with non-carriage of TNF2 is associated with increased susceptibility, but not with a prognosis of IgAN.

Expression of Fc alpha/mu receptor by human mesangial cells: a candidate receptor for immune complex deposition in IgA nephropathy

IgA nephropathy is characterized by the deposition of IgA immune complexes in the glomerular mesangium, but the mechanisms responsible for this are not well understood. Human mesangial cells (HMCs) can bind IgA but do not express known IgA receptors. We show here that primary HMCs express mRNA for a novel receptor, the Fc alpha/mu receptor (Fcalpha/muR), and that receptor expression is upregulated by IL-1. We also detected mRNA for a novel receptor variant in HMCs that may encode a soluble form of the receptor. Fcalpha/muR was expressed in a heterologous system which showed that the receptor was approximately 58 kDa in weight and was only minimally N-glycosylated. As predicted from the characteristics of the murine homologue, the expressed human Fcalpha/muR was able to bind IgA and IgM, but not IgG. These results suggest that Fcalpha/muR may be the receptor responsible for mesangial IgA deposition in IgA nephropathy.

Purification, characterization, and subunit structure of rat core 1 Beta1,3-galactosyltransferase

The O-linked oligosaccharides (O-glycans) in mammalian glycoproteins are classified according to their core structures. Among the most common is the core 1 disaccharide structure consisting of Galbeta1-->3GalNAcalpha1-->Ser/Thr, which is also the precursor for many extended O-glycan structures. The key enzyme for biosynthesis of core 1 O-glycan from the precursor GalNAc-alpha-Ser/Thr is UDP-Gal:GalNAc-alpha-Ser/Thr beta3-galactosyltransferase (core1 beta3-Gal-T). Core 1 beta3-Gal-T activity, which requires Mn2+, was solubilized from rat liver membranes and purified 71,034-fold to apparent homogeneity (>90% purity) in 5.7% yield by ion exchange chromatography on SP-Sepharose, affinity chromatography on immobilized asialo-bovine submaxillary mucin, and gel filtration chromatography on Superose 12. The purified enzyme is free of contaminating glycosyltransferases. Two peaks of core 1 beta3-Gal-T activity were identified in the final step on Superose 12. One peak of activity contained protein bands on non-reducing SDS-PAGE of approximately 84- and approximately 86-kDa disulfide-linked dimers, whereas the second peak of activity contained monomers of approximately 43 kDa. Reducing SDS-PAGE of these proteins gave approximately 42- and approximately 43-kDa monomers. Both the 84/86-kDa dimers and the 42/43-kDa monomers have the same novel N-terminal sequence. The purified enzyme, which is remarkably stable, has an apparent Km for UDP-Gal of 630 microm and an apparent Vmax of 206 micromol/mg/h protein using GalNAcalpha1-O-phenyl as the acceptor. The reaction product was generated using asialo-bovine submaxillary mucin as an acceptor; treatment with O-glycosidase generated the expected disaccharide Galbeta1-->3GalNAc. These studies demonstrate that activity of the core 1 beta1,3-Gal-T from rat liver is contained within a single, novel, disulfide-bonded, dimeric enzyme.

Association of the uteroglobin gene polymorphism with IgA nephropathy

Immunoglobulin A (IgA) nephropathy results from the abnormal deposition of IgA in the renal mesangium. Genetic factors may be involved in the development and progression of IgA nephropathy. Uteroglobin (UG) is a steroid-inducible, cytokine-like, multifunctional protein with anti-inflammatory and immunomodulatory properties. The knockout or antisense mouse of the UG gene develops renal disease similar to IgA nephropathy. We analyzed the UG gene as a candidate for a predisposing factor in 61 Japanese patients with IgA nephropathy (23 children, 38 adults) and detected only the G38A mutation. The gene frequency of the G38A mutation in patients was 0.43, not significantly different from the frequency of 0.36 in healthy controls. However, the frequency of patients homozygous for G38A was twice that of controls, and a significant increase was seen in child patients. We measured serum UG levels in patients and healthy adults. A significant decrease in serum UG levels in homozygotes of G38A compared with homozygotes of G38 was detected only in adult women patients and controls. There is no information on where serum UG is produced or how UG may work in association with IgA nephropathy. However, it is possible that the effect of G38A may be apparent under such stimulation as sex steroids or infections, and homozygotes of the G38A mutation cannot produce sufficient UG in response to stimulation and may be predisposed to IgA nephropathy, especially in childhood.

The Fap1 fimbrial adhesin is a glycoprotein: antibodies specific for the glycan moiety block the adhesion of Streptococcus parasanguis in an in vitro tooth model

Streptococcus parasanguis is a primary colonizer of the tooth surface and plays a pivotal role in the formation of dental plaque. The fimbriae of S. parasanguis are important in mediating adhesion to saliva-coated hydroxylapatite (SHA), an in vitro tooth adhesion model. The Fap1 adhesin has been identified as the major fimbrial subunit, and recent studies suggest that Fap1 is a glycoprotein. Monosaccharide analysis of Fap1 purified from the culture supernatant of S. parasanguis indicated the presence of rhamnose, glucose, galactose, N-acetylglucosamine and N-acetylgalactosamine. A glycopeptide moiety was isolated from a pronase digest of Fap1 and purified by immunoaffinity chromatography. The monosaccharide composition of the purified glycopeptide was similar to that of the intact molecule. The functionality of the glycan moiety was determined using monoclonal antibodies (MAbs) specific for the intact Fap1 glycoprotein. These antibodies were grouped into two categories based on their ability to block adhesion of S. parasanguis to SHA and their corresponding specificity for either protein or glycan epitopes of the Fap1 protein. 'Non-blocking' MAb epitopes were mapped to unique protein sequences in the N-terminus of the Fap1 protein using non-glycosylated recombinant Fap1 proteins (rFap1 and drFap1) expressed in Escherichia coli. In contrast, the 'blocking' antibodies did not bind to the recombinant Fap1 proteins, and were effectively competed by the binding to the purified glycopeptide. These data suggest that the 'blocking' antibodies are specific for the glycan moiety and that the adhesion of S. parasanguis is mediated by sugar residues associated with Fap1.

T cell cytokines determine the severity of experimental IgA nephropathy by regulating IgA glycosylation

Hyperfunction of Th2 cells and aberrant glycosylation of IgA have been proposed independently as factors in the pathogenesis of IgA nephropathy (IgAN), the most common form of glomerulonephritis. To investigate the relationship between Th2 cytokines and IgA glycosylation in the genesis of IgAN, we induced IgAN in C3HeB and BALB/c mice by oral immunization and intranasal challenge with Sendai virus. Although both strains of mice developed microhaematuria and glomerular IgA immune deposits to similar degrees, only BALB/c mice developed significant renal insufficiency. More profound reductions of terminal galactosylation and sialylation occurred in Sendai virus-specific IgA from BALB/c versus C3HeB mice, and splenocytes from immunized BALB/c mice produced more Th2 and less Th1 cytokines compared to C3HeB mice when stimulated with antigen in vitro. Furthermore, the decreased glycosylation of IgA elicited by Th2 cytokines in vitro was blunted by the addition of IFN-gamma. We conclude that increased production of Th2 cytokines can lead to abnormalities in IgA glycosylation, which in turn promote heightened phlogistic responses to IgA immune complexes lodging in the glomerulus. We suggest that a relative or absolute increase in Th2 cytokine production in response to mucosal infection is a significant pathogenic factor in human IgAN.

Schönlein-henoch purpura in children and adults: diagnosis, pathophysiology and management

Schönlein-Henoch syndrome is a form of systemic small-vessel vasculitis, characterised by vascular and/or mesangial immunoglobulin A1 deposits. The main clinical manifestations are vascular purpura, predominating on the lower limbs, and articular, gastrointestinal and renal symptoms. Pulmonary, cardiac, genital and neurological symptoms have also be observed. The syndrome usually affects children, whereas it is rare in adults. The frequency of renal involvement varies between individual studies (from 20 to 100%). Renal manifestations are usually mild and transient, although chronic nephropathies may occur. Overall, an estimated 2% of children with Schönlein-Henoch purpura progress to renal failure and up to 20% of children with nephritis treated in specialised centres require haemodialysis. The renal prognosis appears to be worse in adults. Aetiological investigations are required, as a triggering factor is found in approximately half the patients (e.g. viral, bacterial and parasitic infections, drugs, toxins, systemic diseases and cancer). Dapsone has beneficial effects on cutaneous, gastrointestinal and articular manifestations in adults, especially those with chronic forms. Corticosteroids may be useful for refractory abdominal pain. Methylprednisolone pulse therapy, immunosuppressive drugs (e.g. cyclophosphamide and azathioprine), plasma exchange and polyclonal immunoglobulin therapy are beneficial in very rare life-threatening forms of the disease and in rare instances where renal function is compromised.

Progress in molecular and genetic studies of IgA nephropathy

Several new findings emerged recently from biochemical, genetic, and molecular studies of patients with IgA nephropathy. It appears that immunoglobulin A1-secreting cells of IgA nephropathy patients produce increased amounts of aberrantly glycosylated IgA1 in which the O-linked glycans in the hinge region are deficient in the content of galactose. The galactose-deficient IgA1 in the circulation is recognized by naturally occurring antibodies with anti-glycan specificity, and immune complexes are formed. These circulating immune complexes escape hepatic degradation and eventually are deposited in the kidney mesangium. Resident mesangial cells bind the IgA-containing immune complexes with the involvement of a novel IgA receptor and become activated. A familial form of IgA nephropathy has been linked to chromosome 6q22-23. Recent progress in molecular analyses of IgA nephropathy thus defines this disease as an autoimmune process with a novel IgA mesangial receptor and certain genetically determined traits.

Glycosylation of circulating IgA in patients with IgA nephropathy modulates proliferation and apoptosis of mesangial cells

Abnormalities in circulating IgA1 have been demonstrated in patients with IgA nephropathy (IgAN). This study addresses the question of the functional significance of this alteration in creating mesangial injury. Biologic effects of selected IgA glycoforms isolated from serum of IgAN patients and controls and in vitro deglycosylated normal IgA were tested on cultured human mesangial cells (MC). IgA glycoforms, ranging from 250 to 500 kD molecular weight, were isolated by lectin affinity chromatography followed by HPLC. IgA and IgG content was measured by enzyme-linked immunosorbent assay. HPLC fractions were incubated with MC to evaluate proliferation and apoptosis rates and nitric oxide synthesis. Moreover, MC were conditioned with in vitro desialylated and degalactosylated normal IgA. Patients with IgAN displayed increased levels of IgA glycoforms exposing sialic acid in alpha2,6 linkage with N-acetylgalactosamine (Neu5Acalpha2,6GalNAc) (P < 0.02) and GalNAc (P < 0.05), indicating truncation of O-linked glycans of IgA1. Moreover, IgA glycoforms with increased exposure of mannose were observed (P < 0.03), suggesting a defective N-linked glycosylation. No modification in IgG glycosylation was detected. When incubated with MC, the IgA glycoforms isolated from patients with increased exposure of GalNAc, Neu5Acalpha2,6GalNAc, or mannose, significantly depressed the proliferation and increased the apoptotic rate and nitric oxide synthesis activity of cultured MC, in comparison with fractions isolated from controls. Similarly, in vitro desialylated and degalactosylated IgAs significantly depressed the proliferation and enhanced the apoptosis rates of MC. In conclusion, a significant modulation of several human MC functions exerted by serum IgA with increased exposure of GalNAc, Neu5Acalpha2,6GalNAc, and mannose residues isolated from IgAN patients is reported for the first time.

Mesangial IgA1 in IgA nephropathy exhibits aberrant O-glycosylation: observations in three patients

BACKGROUND

In IgA nephropathy (IgAN), circulating IgA1 molecules display an abnormal pattern of O-glycosylation. This abnormality may potentially contribute to mesangial IgA1 deposition, but this is unproven because the O-glycosylation of mesangial IgA1 has not been analyzed.

METHODS

IgA1 was eluted from glomeruli isolated from the kidneys of three IgAN patients obtained after nephrectomy or at postmortem. Serum from these patients, other patients with IgAN, and controls was subjected to the same treatment as the glomerular eluates. The O-glycosylation of eluted and serum IgA1 was measured by lectin binding using an enzyme-linked immunosorbent assay-based system.

RESULTS

In all three cases, the lectin binding of IgA1 eluted from the glomeruli of IgAN patients was markedly higher than that of the serum IgA1 of the same individual, and also all but one of a series of serum IgA1 samples from other patients and controls.

CONCLUSIONS

The higher lectin binding of glomerular compared with serum IgA1 suggests that O-glycosylated IgA1 molecules abnormally and selectively deposit in the kidney. These results provide the first evidence that mesangial IgA1 is abnormally O-glycosylated, and support a direct role for abnormal IgA1 O-glycosylation in the mechanism of mesangial IgA deposition in IgAN.

Mass spectrometry proves under-O-glycosylation of glomerular IgA1 in IgA nephropathy

BACKGROUND

The IgA1 molecule, which is predominantly deposited in glomeruli in IgA nephropathy (IgAN), is a unique serum glycoprotein because it has O-glycan side chains in its hinge region. Our study was conducted to investigate the O-glycan structure in the glomerular IgA1 in IgAN.

METHODS

The IgA1 was separated from 290 renal biopsy specimens of 278 IgAN patients and from four serum IgA1 samples (IgAN, 2; control, 2). The variety of O-glycan glycoform was determined by estimating the precise molecular weights of the IgA1 hinge glycopeptides using matrix-assisted laser desorption ionization time of flight mass spectrometry.

RESULTS

The peak distribution of IgA1 hinge glycopeptides clearly shifted to lesser molecular weights in both glomerular and serum IgA1 in IgAN compared with the serum IgA1 of controls. In the five major peaks of IgA1 hinge glycopeptides in each sample, the numbers of carbohydrates composing O-glycans (GalNAc, Gal, and NANA) in the deposited and serum IgA1 in IgAN patients were significantly fewer than those in the serum IgA1 in the control groups.

CONCLUSION

The O-glycan side chains in the hinge of the glomerular IgA1 were highly underglycosylated in IgAN. These results indicate that the decreased sialylation and galactosylation of the IgA1 hinge glycopeptides play a crucial role in its glomerular deposition in IgAN.

What is the difference between IgA nephropathy and Henoch-Schönlein purpura nephritis?

Henoch-Schönlein purpura nephritis (HSPN) and IgA nephropathy (IgAN) are considered to be related diseases since both can be encountered consecutively in the same patient, they have been described in twins, and bear identical pathological and biological abnormalities. Apart from the presence of extrarenal clinical signs found only in HSPN, other differences are noticed between the two diseases. The peak age ranges between 15 and 30 years for a diagnosis of IgAN, whereas HSPN is mainly seen in childhood. Nephritic and/or nephrotic syndromes are more often seen at presentation in HSPN. In contrast to IgAN, HSPN has been described in association with hypersensitivity. Endocapillary and extracapillary inflammations as well as fibrin deposits in the glomerulus are more frequent in HSPN. No major biological differences have been found between the two illnesses, except for a larger size of circulating IgA-containing complexes (IgA-CC) and a greater incidence of increased plasma IgE levels in HSPN. As tissue infiltration by leukocytes is a major feature of HSPN vasculitis, a possible role of a more potent activation of the latter cells by IgA-CC and/or circulating chemokines in HSPN should be considered. Further studies are required to elucidate this possible mechanism as well as the role of hypersensitivity in HSPN.

The interaction of selective plant lectins with neuraminidase-treated and untreated IgA1 from the sera of IgA nephropathy patients

A study on the binding interaction of lectins from Artocarpus heterophyllus (jacalin), Glycine max and Sambucus nigra with standardised quantity of IgA from the IgA nephropathy patients and normal controls was performed. The Glycine max lectin demonstrated higher affinity towards the serum IgA of IgAN patients as compared to normal controls. However, the affinity binding was lower in cases ofjacalin and the Sambucus nigra lectin. When serum samples were treated with neuraminidase, the differential jacalin affinity binding between IgA1 of patients and normal controls was abrogated. Our data are in support of the view that the O-linked oligosaccharide moieties of the patients IgA1 were generally lacking in galactose and sialic acid residues.

Analysis of the microheterogeneity of the IgA1 hinge glycopeptide having multiple O-linked oligosaccharides by capillary electrophoresis

It was found that the self-aggregation of IgA1 was closely connected with the glycoform of a mucin-type sugar chain on its hinge portion. In this report, normal human serum IgA1 was separated into two subfractions by a jacalin column. The elution condition, 25 mM galactose, used here was similar to that reported for the glycoprotein with a single mucin-type sugar chain per molecule. The IgA1 eluted under this condition was substantially the monomeric form. In contrast, the remaining IgA1 eluted from the column with 0. 8 M galactose was substantially the aggregated form. An analytical method for the microheterogeneity of the IgA1 hinge glycopeptide (HGP33) was developed to determine the difference between these IgA1 fractions by capillary electrophoresis (CE). Native HGP33 from both IgA1 fractions was separated into peaks 1-11, depending on their glycoforms. Because the sialic acid-rich component migrated slowly on CE, the 25 mM fraction was abundant in the sialic acid-rich components (peaks 7-11), but the 0.8 M fraction was abundant in the sialic acid-poor components (peaks 1-4). Comparison of the number of sugar chains per hinge peptide indicated that the 25 mM fraction was relatively well glycosylated. Thus, application of CE analysis to the HGP33 indicated that the monomeric IgA1 was composed of a relatively complete molecule with respect to the glycoform rather than the aggregated IgA1.

Charge-dependent binding of polymeric IgA1 to human mesangial cells in IgA nephropathy

BACKGROUND

IgA nephropathy (IgAN) is characterized by raised serum IgA1 and predominant mesangial IgA1 deposits of polymeric nature. The mechanism of polymeric IgA1 (pIgA1) deposition in the kidney mesangium is poorly understood in IgAN. It has been suggested that increased sialic acid content and anionic charge of the pIgA1 molecules may be operational in the IgA1 deposition in human mesangial cells (HMCs). The present study examined the binding of pIgA1 with different surface charges to HMCs. The binding characteristics of IgA1 to HMCs in the presence of polycation (poly-L-lysine) or polyanion (heparin) were also investigated.

METHODS

IgA1 was purified in sera from patients with IgAN and from healthy controls by jacalin affinity chromatography. IgA1 was further separated into pIgA1 and monomeric IgA1 (mIgA1) by fast protein liquid chromatography (FPLC). pIgA1 or mIgA1 with different net charges on their surface were resolved by ion exchange chromatography (IEC) with a Mono Q column. The binding characteristics of pIgA1 and mIgA1 to HMCs in the presence or absence of polycation or polyanion were examined by flow cytometry.

RESULTS

In patients with IgAN, the absolute amount of mIgA1 and pIgA1 is significantly higher than that of healthy controls (P < 0. 001). There was significant increase in binding of pIgA1 from patients with IgAN to HMC and cell lysate. pIgA1 that interacted strongly with the ion exchanger also bound more to HMCs when compared with IgA1 interacted weakly with the ion exchanger (P < 0. 001). The anionic charged pIgA1 from patients was significantly higher than that of healthy controls (P < 0.001). Preincubation with poly-L-lysine increased the binding of pIgA1 to HMCs. The binding of pIgA1 to HMCs was decreased by preincubation with heparin.

CONCLUSIONS

The binding of IgA to HMCs is charge dependent. Polymeric IgA with the highest net negative charge binds more to HMCs. Preincubation with polyanion decreased the binding of polymeric IgA to HMCs. These results suggest an important role for anionic charge in IgA1 deposition onto the kidney mesangial cells.

Henoch-Schönlein purpura

Although Henoch-Schönlein purpura (HSP) can occur at any age from infancy to adulthood, it is overwhelmingly a disease of childhood. Indeed, HSP is the most common vasculitis syndrome affecting children. The clinical features of HSP have been well documented, and the diagnosis is generally not difficult. However, there are substantial gaps in our understanding of the etiology, pathogenesis, and treatment of HSP. This article briefly reviews the clinical aspects of HSP and new information concerning therapy. The major focus of this review is recent information concerning abnormalities of immunoglobulin A1 glycosylation and the role of aberrantly glycosylated immunoglobulin A1 in the pathogenesis of HSP.