An increased polymeric IgA level is not a prognostic marker for progressive IgA nephropathy
Paul J. M. van der Boog,
Cees van Kooten,
Anneke van Seggelen,
Marko Mallat,
Ngaisah Klar-Mohamad,
Johan W. de Fijter and
Mohamed R. Daha
Department of Nephrology, Leiden University Medical Center, Leiden, the Netherlands
Correspondence and offprint requests to: P.J.M. van der Boog, MD, Department of Nephrology, C3-P, Leiden University Medical Center, PO Box 9600, 2300 RC Leiden, the Netherlands. Email: p.j.m.van_der_boog{at}lumc.nl
 |
Abstract
|
---|
Background. Elution of IgA from renal biopsies of patients with primary IgA nephropathy (IgAN) has suggested that mesangial IgA deposits are mainly multimeric in nature. This macromolecular IgA consists of dimeric and polymeric IgA and may be derived from the circulation. In children with IgAN, circulating macromolecular IgA levels correlate with bouts of macroscopic haematuria, but in adults a correlation with disease activity is less clear. Therefore, we have designed a novel method to assess the levels of polymeric IgA (pIgA) in sera from patients and controls.
Methods. A novel precipitation assay using recombinant CD89 was developed to measure pIgA. Polymeric IgA levels were measured in serum samples obtained from healthy volunteers (n = 21) and patients with IgAN (n = 51). Subsequently, serum pIgA levels were correlated with clinical parameters of disease.
Results. Serum pIgA levels were significantly increased in patients with IgAN. However, pIgA concentrations relative to total IgA were significantly lower in sera of patients with IgAN. No correlation was found between serum pIgA levels and clinical parameters of IgAN, such as decline of glomerular filtration rate, haematuria or proteinuria.
Conclusions. Although absolute levels of serum pIgA are increased in patients with IgAN as compared with controls, levels of pIgA relative to total serum IgA are lower. No significant correlation was found between serum concentrations of pIgA and clinical parameters of disease. These data support the notion that it is not the size alone, but the physicochemical composition of the macromolecular IgA that is the key factor leading to mesangial deposition.
Keywords: CD89; IgA nephropathy; immunoglobulin A; macromolecular IgA; polymeric IgA
 |
Introduction
|
---|
Primary immunoglobulin-A nephropathy (IgAN) is the most common form of glomerulonephritis worldwide [1]. The hallmark of the disease is deposition of IgA in the glomerular mesangium, but the mechanism leading to renal deposition is still unknown. The recurrence of IgA deposition in renal allografts [2], a reduced mucosal IgA response to primary mucosal immunization [3] and an enhanced systemic macromolecular IgA response to systemic immunization [4] support a primary defect in circulating IgA, rather than in the kidney. About half of the patients with IgAN have elevated serum IgA levels, but increased production alone is not sufficient to develop IgAN. Therefore, abnormal physicochemical properties of circulating IgA, such as size, charge [5] and glycosylation [6,7], might play a role. Recently, it has been shown that mesangial cells have a better ability to bind undergalactosylated IgA-containing complexes of patients with IgAN as compared with circulating complexes of healthy controls [8].
Mesangial IgA consists mainly of the IgA1 subclass and at least in part of macromolecular IgA [9]. Most studies indirectly demonstrated the nature of this macromolecular IgA using the ability of the J-chain and secretory component to bind to the deposited IgA. In two elution studies with a limited number of renal biopsies, macromolecular IgA constituted a substantial fraction of the mesangial IgA [10,11].
Circulating macromolecular IgA may be dimeric [dIgA; molecular weight (MW) 335 kDa] or polymeric (pIgA; MW >335 kDa). Dimeric IgA contains two molecules of monomeric IgA (mIgA) covalently bound by a J-chain. Polymeric IgA are IgA-containing complexes, which contain IgA and other proteins, such as food antigens [12], fibronectin [13], rheumatoid factor [14] and/or Fc
RI/CD89 [15,16]. Elevated levels of IgA1 and IgA1-containing circulating complexes are found in sera of most patients with IgAN [17,18]. In children with IgAN, macromolecular IgA levels are found to correlate with bouts of macroscopic haematuria [19,20], but in adults the elevation of serum macromolecular IgA and its correlation with disease activity is less clear [3].
IgA in serum exists predominantly in the monomeric form. A minor portion of serum IgA also exists as dIgA and only a small portion is found as pIgA. In the present study, an assay was developed to assess serum levels of pIgA in controls and patients with IgAN. In patients there was a significant increase in pIgA levels as compared with controls. However, no direct relationship between pIgA levels and disease parameters of IgAN was found.
 |
Subjects and methods
|
---|
Human subjects
In the present study we included 21 healthy volunteers, three patients with IgA deficiency, three patients with IgA myeloma and 51 patients with biopsy-confirmed primary IgAN. The latter were defined by mesangial deposits of IgA as the dominant isotype. None of the patients had clinical or laboratory evidence of HenochSchoenlein purpura, systemic lupus erythematosus, liver disease or received immunosuppressive therapy. Patients were included in the study between October 1998 and February 1999. Venous blood and urine samples were collected and clinical characteristics of the patient group were obtained (Table 1). From all patients, clinical and laboratory data, such as the estimated glomerular filtration rate (GFR) using the CockcroftGault formula and urine sediments, were obtained retrospectively (observation time: 5.9±0.5 years) and prospectively (follow-up time: 3.7±0.2 years). Informed consent was obtained from all subjects.
Radioimmunoassay for polymeric IgA
In order to quantify serum levels of pIgA, a radioimmunoassay (RIA) specific for pIgA was developed. To set up this assay, we used the observation that the human myeloid IgA-receptor Fc
RI/CD89 has a higher affinity for high molecular weight IgA as compared with mIgA [21]. Therefore, recombinant CD89 or CD89-Fc fusion proteins [22] were labelled with 125I by the chloramine T method. The binding of IgA to recombinant CD89 or CD89-Fc was determined in a fluid-phase RIA. In order to prevent non-specific precipitation, 25 µl heat-inactivated fetal calf serum was added to 25 µl serum samples and subsequently mixed with 50 µl phosphate-buffered saline containing 20 000 c.p.m. of 125I-CD89 or 125I-CD89-Fc. The mixtures were incubated overnight at 0°C and the next day 1 ml 7% polyethylene glycol (PEG) 6000 (Schuchardt, Hohenbrunn, Germany) was added, mixed and kept on ice for 1 h. Subsequently, the samples were centrifuged at 3000 r.p.m. (MSE Mistral 3000, 1500 g) for 20 min at 4°C and the supernatant was removed. Radioactivity in the pellet was measured in a gamma counter. All samples were assessed in duplicate. Concentrations in samples were calculated relative to a standard that was included in each experiment. As standard, purified pIgA of a patient with an IgA myeloma was used (IgA concentration of 1 mg/ml). Initially, we purified pIgA from myeloma serum following standard methods, including anion-exchange chromatography and gel filtration as described before [23]. In this preparation of pIgA we assessed the protein concentration using the Pierce BCA Protein Assay (Pierce, Munich, Germany) with purified mIgA as a standard. With this pIgA we found that it was specifically precipitated in the 125I-CD89-Fc assay. For the standard used in the present study we isolated the pIgA-containing fraction from myeloma serum by HR200 gel-filtration and quantified the IgA content in this standard by enzyme-linked immunosorbent assay (ELISA) using the pIgA purified myeloma IgA.
When different pools of the myeloma IgA (pIgA, dIgA and mIgA) were assessed for reactivity with radioactively labelled CD89-Fc fusion protein (125I-CD89-Fc) in the fluid-phase PEG-precipitation assay, a dose-dependent relationship between the pIgA concentration and precipitation of 125I-CD89-Fc was shown (Figure 1). No significant precipitation was found with dIgA and mIgA. Similar results were obtained with radioactively labelled CD89 (data not shown). The detection limit of pIgA in this assay was 5 µg pIgA/ml.

View larger version (14K):
[in this window]
[in a new window]
|
Fig. 1. Dose-dependent reactivity of mIgA, dIgA and pIgA with 125I-CD89-Fc in a fluid-phase PEG-precipitation assay. The pools of mIgA, dIgA and pIgA were purified from sera of a patient with an IgA myeloma.
|
|
In order to find out whether the precipitation assay is specific for IgA, sera from nine controlsthree patients with IgA deficiency and five umbilical cord serawere assessed for reactivity with CD89-Fc and relationship to the standard of pIgA. Control sera clearly showed positivity in the assay, while there was very little binding to CD89-Fc in umbilical cord serum or sera of patients with IgA deficiency (Figure 2).

View larger version (11K):
[in this window]
[in a new window]
|
Fig. 2. Detection of pIgA with the PEG-precipitation assay showing pIgA serum levels in nine healthy controls (NHS), three patients with IgA-deficiency (IgA-def) and five umbilical cord sera (UCS). Data are means±SD.
|
|
In order to investigate the specificity of the precipitation test for pIgA in sera from normals and patients, sera were fractionated by gel filtration. Fractions were assessed for IgA content by ELISA and for binding to CD89-Fc in the precipitation assay. The results of a representative example of serum from a healthy subject are depicted in Figure 3A. Significant positivity of the CD89-Fc precipitation assay was seen exclusively in the pIgA fractions.

View larger version (23K):
[in this window]
[in a new window]
|
Fig. 3. Specificity of the precipitation assay for pIgA. (A) Normal human serum was fractionated on a HR200 Superdex column and fractions were assessed for IgA concentrations by ELISA and for binding to CD89-Fc. (B) Peak pIgA fractions of nine fractionated sera were assessed for IgA by ELISA and for reactivity with 125I-CD89-Fc in the precipitation assay. There was a linear relationship between concentrations of IgA and precipitation in the 125I-CD89-Fc assay.
|
|
From sera of six healthy subjects and three patients with IgAN, the peaks of pIgA were further analysed. Polymeric IgA concentrations were measured by ELISA and by the precipitation assay using CD89-Fc. As shown in Figure 3B, there was a linear relationship between pIgA measured by ELISA and binding to CD89-Fc (R2 = 0.87). Therefore, we concluded that CD89-Fc is a suitable and representative ligand for the detection of pIgA in whole serum.
Detection of IgA by ELISA
For quantification of IgA in samples, ELISA wells were coated with polyclonal rabbit-
-huIgA antibody (DAKO, Glostrup, Denmark) (concentration 1 µg/ml), followed by serial 2-fold dilutions of samples for 1 h at 37°C. Bound IgA was subsequently detected by affinity-purified biotin-labelled goat F(ab')2 fragments directed against IgA heavy chains (concentration 0.5 µg/ml; Tago, Burlingame, CA, USA). IgA concentrations in samples were calculated relative to a known standard of IgA, which was included in each plate. The detection limit of IgA was 2 ng/ml.
Gel filtration
Serum samples (2 ml) from a number of controls or patients with IgAN were separated by size on 26/60 HR200 Superdex columns and fractions were assessed for IgA by ELISA and binding to radioactively labelled CD89 or CD89-Fc by the above described precipitation assay.
Western blot analysis
To assess the size and composition of IgA in fractions of columns, size-fractionated serum samples were subjected to 6% SDSpolyacrylamide gels under non-reducing conditions and blotted onto polyvinylidine difluoride membranes (Millipore, Bedford, MA, USA), as described before [15]. IgA was detectable with the monoclonal anti-IgA antibody 4E8 (IgG1) at a concentration of 2 µg/ml [24]. The presence of the J-chain in Western blots was determined with a rabbit polyclonal antiserum (kindly provided by Per Brandtzaeg, University of Oslo, Norway), as described previously [15].
Urine analysis
Proteinuria (g/24 h) was analysed in 24 h urine samples. To quantify erythrocyturia, 10 ml urine from freshly collected samples were centrifuged for 5 min at 2000 r.p.m. (Beckman Allegra 6R Centrifuge, 800 g) and analysed by microscopy. Erythrocyturia was recorded as the number of erythrocytes per high-power field.
Statistical analysis
The MannWhitney U-test and the KruskalWallis test were used for statistical analysis.
 |
Results
|
---|
Quantification of macromolecular IgA by gel filtration
In humans, serum IgA is present as mIgA (MW 160 kDa), J-chain-containing dIgA (MW 335 kDa) and pIgA (MW >335 kDa). To quantify the amount of serum pIgA in humans, sera obtained from nine healthy volunteers were fractionated by gel filtration and analysed for IgA concentration by ELISA. In Figure 4A, a representative example is shown, which shows the presence of a high monomeric and a lower dimeric IgA peak. Next, the fractions were analysed for the presence of monomeric and dimeric IgA by Western blot analysis. Under non-reducing conditions, the fractions containing mIgA exhibited two bands when stained with a heavy chain-specific monoclonal antibody against IgA. This is in agreement with our earlier findings [15,16]. Fractions filtering between 116 and 128 ml exhibited an extra band at 335 kDa (Figure 4B). When stained with anti-J-chain, the upper bands of the polymeric and dimeric fractions showed the presence of the J-chain (data not shown), compatible with earlier results [15]. Although the exact composition of pIgA is still unknown, the present data show that these complexes contain both mIgA and dIgA. By Western blot analysis, it is not possible to distinguish between the pIgA and dIgA in the fractions. In order to quantify serum levels of pIgA, a RIA specific for pIgA was developed as described in the Subjects and methods.


View larger version (42K):
[in this window]
[in a new window]
|
Fig. 4. Analysis of the size of IgA in normal serum. (A) Normal human serum was fractionated on a HR200 Superdex column. Fractions were assessed for IgA concentrations by ELISA. (B) Selected fractions (116140) were subsequently subjected to Western blot analysis under non-reducing conditions and analysed for IgA with specific anti-IgA antibodies.
|
|
Polymeric IgA in patients with IgAN and healthy controls
Serum samples from 51 patients with IgAN and 21 controls were assessed for total IgA by ELISA and for pIgA by the precipitation assay with 125I-CD89-Fc. No significant correlation between age and serum levels of total IgA or pIgA was found (data not shown). In the patient group, IgA serum levels were significantly higher as compared with controls (median: 3.9 vs 1.9 mg/ml; P<0.001). Serum levels of pIgA, measured by CD89-Fc precipitation, were also significantly higher in the patient group compared with the control group (median: 64 vs 50 µg/ml; P = 0.002). However, when the pIgA concentration relative to the total serum IgA concentration was calculated for each serum sample, the pIgA/IgA ratio was significantly lower in the patient group as compared with healthy controls (median: 1.9% vs 3.2 %; P = 0.04) (Figure 5).

View larger version (14K):
[in this window]
[in a new window]
|
Fig. 5. Levels of serum IgA (A) and serum pIgA (B) and pIgA/total IgA ratios (C) in 51 patients with IgAN and 21 healthy controls.
|
|
When pIgA levels were related to total IgA levels in all serum samples, no significant correlation was found (data not shown).
Correlation between serum pIgA and clinical parameters in IgAN
In the majority of patients with IgAN, the disease has an indolent course, manifested by persistent or intermittent microscopic haematuria. Proteinuria and impaired renal function are risk factors for progressive disease. Polymeric IgA is thought to constitute a substantial fraction of the mesangial IgA and, in this study, we have found that absolute pIgA levels are higher in patients with IgAN. Therefore, we correlated absolute pIgA levels with clinical parameters of disease. Patients with microscopic haematuria were divided into four groups, based on different patterns of microscopic haematuria during the whole follow-up period up to 9.6 years (group I: no or minimal haematuria; group II: only previous haematuria; group III: intermittent haematuria; and group IV: persistent haematuria). No association was found between pIgA levels and the different patterns of haematuria (Figure 6A). The presence and degree of proteinuria was evaluated using 24 h urine samples at the time of inclusion. No association with pIgA levels was found when patients were divided in a group with
1 g/24 h and a group with >1 g/24 h (Figure 6B). Further, the decline in the GFR per year, calculated in both the preceding period (5.9±0.5 years) (data not shown) and the period following (3.7±0.2 years) the time of inclusion, was not associated with pIgA levels (Figure 6C). In addition, no correlations were found between serum IgA or relative pIgA levels and any of the above-mentioned clinical parameters of the disease (data not shown).

View larger version (16K):
[in this window]
[in a new window]
|
Fig. 6. Levels of serum pIgA in patients with IgAN with clinical parameters, such as (A) the course of microscopic haematuria during the whole follow-up period (group I: no or minimal haematuria; group II: only previous haematuria; group III: intermittent haematuria; and group IV: persistent haematuria), (B) the degree of proteinuria (less or equals to or more than 1 g/24 h) and (C) the decline of Cockcroft clearance per year, calculated from the start of serum collection.
|
|
 |
Discussion
|
---|
Previous studies have suggested that IgA found in kidneys from patients with IgAN contains predominantly macromolecular IgA [9]. These IgA immune deposits are likely derived from the circulation. In order to find out whether there is a relationship between circulating pIgA (MW>335 kDa) and clinical parameters of the disease, we first developed a new method that detects predominantly pIgA. In this assay, sera from patients and controls were incubated first with a radioactively labelled chimeric construct of CD89 (CD89-Fc) and, subsequently, pIgACD89-Fc complexes were precipitated with PEG. The assay was proven to be specific for pIgA and it was found that circulating pIgA was significantly increased in patients with IgAN. However, no correlation was found with clinical parameters of disease.
IgA is present in the circulation mainly as mIgA. When size-fractionated serum samples were analysed on Western blot under non-reducing conditions, two main bands of mIgA were seen, as observed previously [15,16]. At present, the molecular difference of these two proteins is not known. We excluded that the upper band consists of IgACD89 complexes [15,16]. Differences in MW might be caused by association with other proteins or, alternatively, could be the result of differences in glycosylation. Dimeric IgA (two mIgA proteins covalently bound by a J-chain) and pIgA make up only
15% of the systemic pool. Recently, we showed that high molecular mass complexes of CD89-IgA can be distinguished from J-chain-containing dIgA and we hypothesized that CD89 might contribute to polymerization of serum IgA [15]. The present data are in agreement with our earlier findings that under non-reducing conditions pIgA contains both monomeric and/or dimeric IgA components.
Mesangial IgA has been found to be mainly macromolecular [9], but the precise composition is still unknown. In the majority of studies, the detection of macromolecular IgA in the renal mesangium was based on the presence of a J-chain and binding to a secretory component, which are both qualitative measures. In two studies, IgA from a limited number of renal biopsies was eluted and subjected to high-pressure liquid chromatography [10] or density gradient ultracentrifugation [11]. The eluted mesangial IgA consisted mainly of macromolecular IgA. Two other studies showed the presence of highly undergalactosylated IgA1 in eluates of renal tissue containing mesangial IgA [25,26]. Altered O-glycosylation of serum IgA might favour complex formation and it has been found that undergalactosylated IgA-containing immune complexes bind more efficiently to mesangial cells than circulating immune complexes from healthy controls [8]. Although IgACD89 complexes have been suggested to play a role in the pathogenesis of mesangial IgA deposition, until now CD89 could not be demonstrated in renal biopsies [8,27]. It would be interesting to look for a correlation between serum levels of pIgA and (immuno-)histological parameters. However, in our patient population, the time between renal biopsy and serum collection varied from 2 to 18 years, which makes it difficult to analyse these data in a reliable fashion. Since no correlation between pIgA levels and decline in renal function was found in the present study, we think it is less likely to find a clear correlation between pIgA levels and (immuno-)histological data.
Earlier studies showed no correlation between serum macromolecular IgA and disease activity [3], but in these studies total macromolecular IgA was measured and no distinction was made between dimeric and polymeric IgA. In the present study we have used an assay specific for pIgA. In the patient group, serum pIgA levels were significantly increased, but there was no correlation between pIgA serum levels and clinical parameters of disease, such as course of haematuria, proteinuria and change in GFR. These results are in agreement with the observation that patients with IgA myeloma may have extremely high pIgA levels without mesangial IgA deposition [9]. Therefore, pIgA may be different in a qualitative way or only a small part of circulating pIgA is of pathogenic relevance and selectively trapped in the renal mesangium.
Although the pIgA concentration assessed in the present assay was higher in patients with IgAN as compared with the control group, the ratio of pIgA to total IgA was significantly lower in patients (1.9% vs 3.2%). Less binding of patient pIgA to CD89 could provide an explanation, but earlier results have not shown a reduced binding of pIgA from patients with IgAN to CD89 as compared with pIgA from healthy controls [28]. Macromolecular IgA contains at least in part IgACD89 complexes and, due to occupation of the CD89-binding site of the IgA-protein, these complexes might not be detected by the present precipitation assay. However, recently we have shown that IgACD89 immune complex levels are not different in patients with IgAN and healthy controls [16].
In conclusion, absolute levels of pIgA are higher in patients with IgAN as compared with healthy controls. However, there is no correlation with clinical parameters of the disease.
Conflict of interest statement. None declared.
 |
References
|
---|
- Schena FP. A retrospective analysis of the natural history of primary IgA nephropathy worldwide. Am J Med 1990; 89: 209215[ISI][Medline]
- van der Boog PJ, de Fijter JW, Bruijn JA, van Es LA. Recurrence of IgA nephropathy after renal transplantation. Ann Med Interne (Paris) 1999; 150: 137142[ISI][Medline]
- de Fijter JW, Eijgenraam JW, Braam CA et al. Deficient IgA1 immune response to nasal cholera toxin subunit B in primary IgA nephropathy. Kidney Int 1996; 50: 952961[ISI][Medline]
- Layward L, Allen AC, Harper SJ, Hattersley JM, Feehally J. Increased and prolonged production of specific polymeric IgA after systemic immunization with tetanus toxoid in IgA nephropathy. Clin Exp Immunol 1992; 88: 394398[ISI][Medline]
- Leung JC, Tang SC, Lam MF, Chan TM, Lai KN. Charge-dependent binding of polymeric IgA1 to human mesangial cells in IgA nephropathy. Kidney Int 2001; 59: 277285[CrossRef][ISI][Medline]
- Allen AC, Bailey EM, Barratt J, Buck KS, Feehally J. Analysis of IgA1 O-glycans in IgA nephropathy by fluorophore-assisted carbohydrate electrophoresis. J Am Soc Nephrol 1999; 10: 17631771[Abstract/Free Full Text]
- Hiki Y, Tanaka A, Kokubo T et al. Analyses of IgA1 hinge glycopeptides in IgA nephropathy by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. J Am Soc Nephrol 1998; 9: 577582[Abstract]
- Novak J, Vu HL, Novak L et al. Interactions of human mesangial cells with IgA and IgA-containing immune complexes. Kidney Int 2002; 62: 465475[CrossRef][ISI][Medline]
- Harper SJ, Feehally J. The pathogenic role of immunoglobulin A polymers in immunoglobulin A nephropathy. Nephron 1993; 65: 337345[ISI][Medline]
- Monteiro RC, Halbwachs-Mecarelli L, Roque-Barreira MC et al. Charge and size of mesangial IgA in IgA nephropathy. Kidney Int 1985; 28: 666671[ISI][Medline]
- Tomino Y, Sakai H, Miura M, Endoh M, Nomoto Y. Detection of polymeric IgA in glomeruli from patients with IgA nephropathy. Clin Exp Immunol 1982; 49: 419425[ISI][Medline]
- Coppo R, Amore A, Roccatello D. Dietary antigens and primary immunoglobulin A nephropathy. J Am Soc Nephrol 1992; 2: S173S180[Abstract]
- Jennette JC, Wieslander J, Tuttle R, Falk RJ. Serum IgA-fibronectin aggregates in patients with IgA nephropathy and HenochSchonlein purpura: diagnostic value and pathogenic implications. The Glomerular Disease Collaborative Network. Am J Kidney Dis 1991; 18: 466471[ISI][Medline]
- Sinico RA, Fornasieri A, Maldifassi P, Colasanti G, d'Amico G. The clinical significance of IgA rheumatoid factor in idiopathic IgA mesangial nephropathy (Berger's disease). Clin Nephrol 1988; 30: 182186[ISI][Medline]
- van der Boog PJ, van Zandbergen G, de Fijter JW et al. Fc alpha RI/CD89 circulates in human serum covalently linked to IgA in a polymeric state. J Immunol 2002; 168: 12521258[Abstract/Free Full Text]
- van der Boog PJ, de Fijter JW, van Kooten C et al. Complexes of IgA with FcalphaRI/CD89 are not specific for primary IgA nephropathy. Kidney Int 2003; 63: 514521[CrossRef][ISI][Medline]
- Schena FP, Pastore A, Ludovico N et al. Increased serum levels of IgA1IgG immune complexes and anti-F(ab')2 antibodies in patients with primary IgA nephropathy. Clin Exp Immunol 1989; 77: 1520[ISI][Medline]
- Tomana M, Novak J, Julian BA et al. Circulating immune complexes in IgA nephropathy consist of IgA1 with galactose-deficient hinge region and antiglycan antibodies. J Clin Invest 1999; 104: 7381[Abstract/Free Full Text]
- Davin JC, Foidart JB, Mahieu PR. Relation between biological IgA abnormalities and mesangial IgA deposits in isolated hematuria in childhood. Clin Nephrol 1987; 28: 7380[ISI][Medline]
- Feehally J, Beattie TJ, Brenchley PE et al. Sequential study of the IgA system in relapsing IgA nephropathy. Kidney Int 1986; 30: 924931[ISI][Medline]
- Reterink TJ, van Zandbergen G, van Egmond M et al. Size-dependent effect of IgA on the IgA Fc receptor (CD89). Eur J Immunol 1997; 27: 22192224[ISI][Medline]
- van Zandbergen G, Westerhuis R, Mohamad NK et al. Crosslinking of the human Fc receptor for IgA (FcalphaRI/CD89) triggers FcR gamma-chain-dependent shedding of soluble CD89. J Immunol 1999; 163: 58065812[Abstract/Free Full Text]
- Hiemstra PS, Gorter A, Stuurman ME, van Es LA, Daha MR. Activation of the alternative pathway of complement by human serum IgA. Eur J Immunol 1987; 17: 321326[ISI][Medline]
- de Fijter JW, van den Wall Bake AW, Braam CA, van Es LA, Daha MR. Immunoglobulin A subclass measurement in serum and saliva: sensitivity of detection of dimeric IgA2 in ELISA depends on the antibody used. J Immunol Methods 1995; 187: 221232[CrossRef][ISI][Medline]
- Allen AC, Bailey EM, Brenchley PE et al. Mesangial IgA1 in IgA nephropathy exhibits aberrant O-glycosylation: observations in three patients. Kidney Int 2001; 60: 969973[CrossRef][ISI][Medline]
- Hiki Y, Odani H, Takahashi M et al. Mass spectrometry proves under-O-glycosylation of glomerular IgA1 in IgA nephropathy. Kidney Int 2001; 59: 10771085[CrossRef][ISI][Medline]
- Launay P, Grossetete B, Arcos-Fajardo M et al. Fcalpha receptor (CD89) mediates the development of immunoglobulin A (IgA) nephropathy (Berger's disease). Evidence for pathogenic soluble receptorIgA complexes in patients and CD89 transgenic mice. J Exp Med 2000; 191: 19992009[Abstract/Free Full Text]
- van Zandbergen G, van Kooten C, Mohamad NK et al. Reduced binding of immunoglobulin A (IgA) from patients with primary IgA nephropathy to the myeloid IgA Fc-receptor, CD89. Nephrol Dial Transplant 1998; 13: 30583064[Abstract]
Received for publication: 16. 2.04
Accepted in revised form: 9. 6.04