Mesangial IgA Research Articles

Genetic polymorphisms in the promoter and 5′ UTR region of the Fc α receptor (CD89) are not associated with a risk of IgA nephropathy

The molecular mechanisms of immunoglobulin A glomerulonephritis (IgAN), the most prevalent form of primary glomerulonephritis, remain poorly understood. Recently, the essential role of soluble Fc alpha receptor (FcalphaR) in the formation of the pathogenic immune complex has been revealed. We screened genomic DNA samples from patients with IgAN and those with other glomerular diseases for polymorphisms in the promoter and the 5'-untranslated region region of the FcalphaR gene by direct nucleotide sequencing. We found three common polymorphisms in this region, T-114C, T-27C, and T+56C from the putative transcription initiation site. Each genotype was determined in 151 patients with IgAN and 163 patients with other glomerular diseases shown to have no mesangial IgA deposition by renal biopsy. The haplotype analysis revealed tight linkage disequilibrium among them. An association study for the genotype, allele, and haplotype frequencies of the polymorphisms between the patients with histologically proven IgAN and those with other glomerular diseases showed no significant difference in the genotype, allele, and haplotype distributions between the two groups. The present study indicates that the analyzed polymorphisms of the FcalphaR gene do not appear to be primarily involved in the susceptibility to IgAN.

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

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. 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. 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. 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.

Association of mesangial IgM with IgM deposits in the macula densa: an indication of non-specific macromolecule transport rather than immune reactant?

We observed IgM deposits in the macula densa of the distal convoluted tubule in some renal biopsies with mesangial IgM deposits and did a systematic study to investigate the frequency of this phenomenon. We compared the findings with those in IgA disease. A total of 30 renal biopsies with either isolated predominantly mesangial IgM, or mesangial IgA (+/-IgM) deposition, were retrieved from the files and reviewed independently by both authors. Eight showed strong macula densa IgM deposits and another three showed weak deposits in the macula densa on immunoperoxidase staining. A total of 14 biopsies also showed mesangial IgA deposition but IgA was not seen in the macula densa. These results confirm the association of IgM deposits in the macula densa with mesangial IgM, and suggest that mesangial IgM deposits may be a reflection of non-specific macromolecule transport rather than an immune reactant.

Primary IgA nephropathy in children: association of clinical and pathological findings with prognosis.

Primary IgA nephropathy is a disease characterized by recurrent macroscopic or microscopic hematuria and diffuse mesangial IgA deposition. Although IgA nephropathy had previously been suggested to have a benign prognosis, long term follow-up of the patients revealed that it might lead to chronic renal failure. In this study, the association of the initial clinical and laboratory findings with the renal histological changes was evaluated in 14 cases with primary IgA nephropathy who were at follow-up with a mean duration of 43.07 +/- 16.88 months. Finally the correlation between the clinicopathological findings and prognosis was investigated. In 92.8% of the patients, macroscopic hematuria was the presenting complaint. Proteinuria was detected in 42.9% of the cases mild proteinuria in 14.3% and moderate in 28.6%. Renal biopsy specimens, evaluated according to Churg-Sobin's classification, showed grade 1 changes in 35.7% cases, grade 2 in 35.7%, grade 3 in 14.3% and grade 4 in 14.3%. Both the patients with grade 4 histology had moderate proteinuria, and developed chronic renal failure requiring hemodialysis. Prognosis was found to be associated with the degree of proteinuria and the severity of the histopathological findings.

The molecular pathogenesis and experimental therapy of IgA nephropathy: recent advances and future directions.

In 1968 Berger and Hinglais published the first description of IgA nephropathy (IgAN). In the ensuing 30 years, extensive clinical, epidemiologic, and immunologic characterizations of primary (idiopathic) glomerulonephritis with IgA as the predominant or co-dominant immunoglobulin deposited in the mesangia of all glomeruli, have established the features of IgAN as a distinct glomerular disease entity. Despite these efforts, the basic molecular mechanism(s) which mediate abnormal mesangial IgA deposition with ensuing extracellular matrix expansion and mesangial cell proliferation remains poorly understood, definitive diagnosis still depends on histologic examination of renal biopsy specimens, and widely accepted standards for effective therapy remain to be defined. This review will begin with a summary of the earlier 'descriptive' histopathologic and clinical epidemiologic work which firmly established the distinct immunohistologic features of IgAN, the most common glomerulonephritis among patients undergoing renal biopsy and a major cause of renal failure worldwide. In recent years, a series of important advances in the areas of molecular pathogenesis and experimental therapy have emerged, reflected in a "molecular" paradigm shift in the techniques and approaches applied to the study of IgAN. Representative studies will be critically evaluated to highlight both the strengths and potential weaknesses of each of these approaches. Throughout, the author will offer a personal perspective on promising areas of new investigation and potential approaches to the identification of disease/susceptibility genes involved in the development and progression of IgAN, the application of these discoveries through the development of clinically useful molecular diagnostic tests, and the rational design of novel therapeutic strategies.

Gene polymorphism in IgA nephropathy

SUMMARY: IgA nephropathy (IgAN) is the most common primary glomerulonephritis in the world. Since its original description in 1968, a large body of clinical, epidemiological and immunological studies of its pathogenesis has emerged. However, the basic molecular mechanisms of abnormal mesangial IgA deposition have remained obscure. In recent years, much clinical and experimental evidence has indicated the presence of genetic factors in the development and progression of IgAN. The search for susceptibility loci has centred on the major histocompatibility complex (MHC) using disease association in family and population studies. In addition, genes outside the realm of the MHC have been reported. The evidence for genetic factors in the onset and progression of IgAN is reviewed in this paper. The major loci influencing disease susceptibility include HLA‐DR4, ‐DQw7 and ‐DQβ, C4 null and T‐cell receptor Cα genes, whereas those affecting the natural course of IgAN include HLA‐DQβ, angiotensin‐converting enzyme D/D, T‐cell receptor Cβ and endothelial cell nitric oxide synthase 4a alleles.

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

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. 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. 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. 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.

Association of gene polymorphism of polymeric immunoglobulin receptor and IgA nephropathy.

In order to explore the possibility that genetic predisposition to dysfunction of mucosal immunity and the IgA processing pathway plays a role in the pathogenesis of mesangial IgA1 deposition in IgAN, we examined the possible association of the gene polymorphism of pIgR in the patients with and without IgAN. Genomic DNA of peripheral blood cells was isolated from 372 individuals including 172 histologically confirmed IgAN patients. Segments of the pIgR gene were PCR amplified and restriction fragment length polymorphism was determined as A1 and A2 with and without Pvu II site, respectively. The pIgR genotype distribution was significantly different between the patients with IgAN and those without IgAN. Allele frequency of A2 was higher in IgAN than in other renal diseases (A1 and A2; 0.516 and 0.484 in IgAN, 0.641 and 0.359 in others, chi2 = 9.84, P = 0.0017, Odds ratio = 1.71). Moreover, the subjects with A2A2 genotype were associated with a relatively low level of serum IgA only in the patients with IgAN but not in other renal diseases. The difference of allele frequencies was more remarkable in the patients with a serum IgA level of less than 300 mg/dl (A1 and A2; 0.439 and 0.561 in IgAN, 0.702 and 0.298 in others, chi2 = 12.44, P = 0.0004, Odds ratio = 3.01). This is the first demonstration of the pIgR gene polymorphisms in IgAN which are associated with its clinical phenotype. Gene polymorphisms of pIgR may be candidate genetic markers of susceptibility to IgAN.

Live donor renal allograft in end-stage renal failure patients from immunoglobulin A nephropathy.

The purpose of this study was to attempt to resolve two important issues, i.e. to determine (1) whether the course of recurrent immunoglobulin A nephropathy (IgAN) is benign, and (2) whether it is advisable to use a related donor. We evaluated the long-term outcome, in terms of recurrence and graft survival, after live related or unrelated donor renal transplantation, and assessed the validity of the use of related donors in 90 grafts in 89 IgAN patients. Ten-year graft survival for IgAN patients was 66%, compared with 84% for 107 reference recipients who had other kinds of glomerulonephritis (GN), and with 69% in 90 other recipients who had non-GN renal failure (P=0.27). In 43 grafts, 54 event graft biopsies were performed, documenting the presence of mesangial IgA deposits in 19 of those grafts. In eight grafts, lesions were accompanied by chronic rejection (CR). Ten-year cumulative recurrence was 44%. Ten grafts were lost: by CR (n=3) or acute rejection (n=1) in 24 recurrence-free recipients, by CR (n=2) or recurrence (n=2) in 19 recurrent patients, and by patient death (n=2) in 46 patients devoid of graft biopsy. We found no difference in 10-year graft survival between the recurrent and recurrence-free patients (63% vs. 74%, P=0.98), or the proportion of related donors (68% vs. 83%, P=0.25). The presence or matching of HLA B12, B35, or DR4 did not affect the recurrence. Recurrence increased to 44% with longer follow-up, but this did not limit the graft outcome. Recurrence was not affected by the kind of live donor. We conclude that live related or unrelated kidneys should be offered to IgAN patients.

IgA myeloma presenting as Henoch-Schönlein purpura with nephritis

IgA nephropathy (IgAN) and Henoch-Schönlein purpura (HSP) are both characterized by IgA-mediated tissue injury, including mesangial proliferative glomerulonephritis. Abnormalities of IgA1 glycosylation are described in IgA nephropathy and HSP nephritis. IgA-antineutrophil cytoplasmic antibodies (ANCA) have been inconsistently described in the serum of patients with HSP. In IgA myeloma, the paraprotein-mediated renal lesion is typically cast nephropathy; IgAN or HSP have only rarely been reported in myeloma even when an IgA paraprotein is circulating in large concentrations. We report the case of a 50-year-old man with IgA myeloma who presented with HSP including nephritis and rapidly progressive renal failure. His IgA1 had altered O-glycosylation in the pattern seen in IgAN and also contained an IgA-ANCA. This case adds further weight to the evidence that IgA1 O-glycosylation abnormalities predispose to mesangial IgA deposition and also that IgA-ANCA may have a pathogenic role in the development of HSP.

Interleukin-6-deficient mice refractory to IgA dysregulation but not anorexia induction by vomitoxin (Deoxynivalenol) ingestion

Dietary exposure to the trichothecene vomitoxin (VT) causes feed refusal and elevates IgA production in the mouse. Based on the observations that IL-6 can cause anorexia and promote IgA production and that gene expression of this cytokine is increased in vivo and ex vivo on VT exposure, we hypothesized that IL-6 is an essential cytokine in VT-induced feed refusal and IgA dysregulation. To test this hypothesis, the effects of dietary VT on feed intake, weight gain, serum IgA levels and kidney mesangial IgA deposition in an IL-6-“knockout” mouse (B6129-IL6 tmi Kopf ) were compared to those in both a corresponding “wild-type” (B6129F2) and a previously characterized “sentinel” strain (B6C3F1) that possess the intact gene for this cytokine. IL-6 deficiency did not alter the capacity of VT to cause feed refusal or impair weight gain. VT-fed B6129F2 and B6C3F1 mice had significantly higher serum IgA concentrations than did their corresponding controls fed clean diet, whereas significant differences were not observed between IL-6 KO mice fed VT or control diets. Kidneys taken from VT-fed wild-type and sentinel mice had significantly increased mesangial IgA deposition as compared to controls. While slight increases in mesangial IgA were observed in VT-fed IL-6 KO mice, mean fluorescence intensities were significantly less than that found in the corresponding wild-type and sentinel strains. IL-6 KO mice appeared to be less prone to the development of microscopic haematuria following VT exposure than were the corresponding wild-type and sentinel strains. In total, the results suggested that IL-6-deficient mice were refractory to VT-induced dysregulation of IgA production and development of IgA nephropathy, whereas chronic VT-mediated nutritional effects related to feed intake and weight gain were unaffected.

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Absence of CD89, polymeric immunoglobulin receptor, and asialoglycoprotein receptor on human mesangial cells.

IgA nephropathy (IgAN) is characterized by raised serum IgA and predominant mesangial IgA deposits of polymeric nature. The expression of IgA receptor molecules in white blood cells and glomerular mesangial cells has recently attracted much attention in relation to the uptake of IgA by these cells. This study investigates the expression of IgA Fc receptor (Fc alphaR1 or CD89), asialoglycoprotein receptor (ASGPR), and polymeric Ig receptor (pIgR) in cultured glomerular mesangial cells. Using a sensitive nested reverse transcription-PCR, mRNA encoding for Fc alphaR1, pIgR, or the H2 chain of ASGPR was not demonstrated on human mesangial cells. U937, HepG2, and HT29 cell lines, used as positive controls, strongly expressed the Fc alphaR1, ASGPR, and pIgR mRNA, respectively, under similar experimental conditions. Flow cytometry also demonstrated the presence of surface proteins for Fc alphaR1, ASGPR, and pIgR on the respective control cell lines but not on human mesangial cells. Expression of Fc alphaR1 mRNA on cultured U937 cells was upregulated by tumor necrosis factor-alpha. However, tumor necrosis factor-alpha, interleukin-1beta, or transforming growth factor-beta failed to induce the expression of Fc alphaR1 on human mesangial cells. Human serum IgA or secretory IgA bound to human mesangial cells, HepG2, or the U937 cell line in a dose-dependent manner. The binding of purified IgA to human mesangial cells was not blocked by preincubation with human IgG, IgM, orosomucoid, asialo-orosomucoid, anti-CD89 antibody (My43), or anti-secretory component antibody. The present study concluded that there was an absence of Fc alphaR1, ASGPR, or pIgR on human mesangial cells. These findings suggest that the predominant binding of human IgA to human mesangial cells is mediated by other mechanisms.

Binding sites for carrier-immobilized carbohydrates in the kidney: implication for the pathogenesis of Henoch-Schönlein purpura and/or IgA nephropathy.

Henoch-Schönlein purpura is a common vasculitis of childhood affecting the skin, joints, gastrointestinal tract, and kidney. The mesangial deposition of IgA1 is the most critical factor for the prognosis of patients with this disease. The aberrant glycosylation of the IgA1 subclass with the absence of terminally located galactose and presence of only alpha-N-acetylgalactosamine in O-linked oligosaccharides in the hinge region of IgA1 represents a prominent difference from the normal IgA1. These alterations prompt the supposition that the sugar part may guide IgA deposition by recognition of endogenous lectins on the mesangium. Owing to the limited knowledge about the expression of carbohydrate-binding sites in the human kidney we initiated the study of this aspect with a class of tools which are suitable to map the lectinome of cells. Employing biotinylated neoglycoconjugates, glycosaminoglycans, and sulphated polysaccharides we monitored the presence of accessible carbohydrate-binding sites in control kidneys represented by tumour-free areas of kidneys with Grawitz tumour and in biopsies from patients with Henoch-Schönlein purpura-associated IgA nephropathy. Using frozen sections, no expression of any tested carbohydrate-binding site(s) was observed in the endothelial and the mesangial cells in glomeruli of the control kidneys as well as in the biopsies from Henoch-Schönlein purpura IgA nephropathic kidneys, in contrast to the tubules. The N-acetylgalactosamine-binding sites were expressed only in the inner layer of Bowman's capsule of 20% of glomeruli of the control kidney from one patient with Grawitz tumour and one biopsy from a patient with Henoch-Schönlein purpura-associated IgA nephropathy. However, the macrophages in the glomeruli of patients with IgA nephropathy and interstitial macrophages from both studied groups, i.e. without and with IgA nephropathy, harbour capacity to recognize carrier-immobilized alpha-N-acetylgalactosamine. Access to this binding site for the neoligand conjugate can be blocked by the monoclonal antibody MEM-18 recognizing CD14 antigen. The possibility for a participation of macrophage deposition of IgA1 in mesangium via a lectin mechanism involving this binding capacity warrants further studies.

Ultrastructural Localization of Dominantly Increased Fibronectin in the Markedly Thickened Glomerular Basement Membrane in a Selectively Mated Murine High IgA Strain (HIGA Mice)

To clarify which matrix component(s) contributes to glomerular sclerosis with mesangial IgA deposits in a murine high serum IgA strain (HIGA) derived from ddY mice, morphological and immunopathological analyses of glomeruli were performed in comparison with original ddY and BALB/c mice as controls. Significantly increased thickness of the glomerular basement membrane (GBM), especially the lamina densa, was observed in HIGA mice. Immunofluorescent staining showed marked increases in levels of fibronectin and laminin in both the mesangium and capillary wall in aged HIGA mice. Analysis of the distribution of immunogold-labeled antibody in GBM revealed a significant increase (p < 0.0001) and specific orientation of fibronectin in the endothelial side, which suggested that mesangial fibronectin produced at high levels due to IgA deposition extended to the endothelial side of GBM and contributed to the thickening of GBM with further development to glomerulosclerosis in the HIGA mice.

Expression and physical association of Fc alpha receptor and Fc receptor gamma chain in human mesangial cells.

Most intensive investigations on the pathogenesis of IgA nephropathy have focused on the process before IgA deposition and the characteristics of IgA/IgA immune complex (IgA IC), but it still remains uncertain whether mesangial IgA ICs may cause glomerular injuries directly or are only secondary events of another pathological process. To assess the role of IgA ICs in IgA nephropathy, we investigated the characteristics of Fc alpha receptor (Fc alphaR) and FcR gamma chain which is a signalling subunit of FcR in human mesangial cells (MCs). Gene expression of Fc alphaR and FcR gamma chain of human cultured MCs was examined by RT-PCR and subsequent Southern blot analyses. Sequence analyses after subcloning were also performed for further confirmation. Expression of Fc alphaR and FcR gamma chain at the protein level and their physical association in MCs were determined by immunoprecipitation after stimulation of the cells with heat-aggregated IgA. Two distinct cDNA products were amplified from each cultured MC line. The sequence of the major product of approximately 900 bp was completely identical to that of Fc alphaR previously described. The smaller product had a 288 bp deletion which corresponded to exon 2 encoding the extracellular domain 2 of Fc alphaR. Gene expression of FcR gamma chain was also confirmed. Furthermore, we proved the physical association of Fc alphaR with the FcR gamma chain by co-immunoprecipitation under stimulation with a high dose of the heat-aggregated IgA. These findings suggested that polymeric IgA and/or IgA IC can directly activate MCs via Fc alphaR associated with the gamma chain. Our data also indicated that phenotypic variations of Fc alphaR occur on MC, such as splicing forms, the chain association and/or the alpha chain expression itself, which may contribute to the pathogenesis of IgA nephropathy.

Effects of Vomitoxin Ingestion on Murine Models for Systemic Lupus Erythematosus

Dietary exposure to the trichothecene vomitoxin (VT) results in reduced body weight gain, elevated serum IgA, terminal differentiation of Peyer's patch B cells to IgA secreting plasma cells haematuria, and increased kidney mesangial IgA accumulation in B6C3F 1 mice and other inbred strains. These effects closely mimic a human autoimmune-like kidney disease known as IgA nephropathy. Using NZBW/F 1, MRL/lpr, and BXSB mouse strains as models of systemic lupus erythematosus, we assessed whether consumption of diet containing 5 ppm or 10 ppm VT will similarly affect mice genetically prone to autoimmunity. Reduced weight gains were seen in NZBW/F 1 and MRL/lpr mice fed both doses of VT within 2–3 weeks. In contrast, VT had little effect on weight gain by BXSB mice. Serum Ig levels in all three strains generally did not differ from control mice. Haematuria was significantly increased when all three strains were fed VT. In NZBW/F 1 Peyer's patch cultures stimulated with lipopolysaccharide (LPS), prior VT exposure significantly increased the IgG and IgM secretion but had no effect on IgA. In MRL/lpr Peyer's patch cultures stimulated with LPS, VT exposure increased IgA secretion but not IgM or IgG. BXSB Peyer's patch cultures prepared from VT treatment groups produced significantly more IgA than controls when cultured with LPS or Concanavalin A. Whereas mesangial deposition of IgA and IgG was significantly lower in the treatment groups of NZBW/F 1 and MRL/lpr mice compared with control, BXSB mice had significantly higher IgA, IgG, and complement (C 3) deposition when fed VT. The results suggest that although dietary VT differentially affected mice with aberrant immune systems, these strains did not appear to be any more sensitive to the mycotoxin than were more immunologically robust inbred strains.

Influence of alpha-tocopherol over the time course of experimental IgA nephropathy.

The present study investigated the pathogenesis and the time course of kidney injury in experimental IgA nephropathy. In order to determine an appropriate period in the course of experimental IgA nephropathy to study renal injury and repair, we examined proteinuria and IgA deposition in the renal mesangium after 4, 8, and 16 weeks of mucosal challenge by bovine gamma globulins (BGG) provided in the drinking water. The hallmark of IgA deposition in the mesangium was present after 4 weeks and 8 weeks of BGG inoculation, but by 16 weeks, the mesangial IgA deposition had resolved. In addition, we confirmed our previous report on the beneficial effects of alpha-tocopherol in reducing proteinuria in IgA nephropathy at 8 weeks, and extended this observation to investigate the effects of dietary supplementation of alpha-tocopherol at both 4 weeks and 16 weeks. Proteinuria resolved spontaneously at 16 weeks. There is oxidative stress, as suggested by the elevation in plasma and renal malondialdehyde content, and increased fibrogenic cytokine message, as suggested by elevated transforming growth factor beta1 mRNA. These increases were clearly blunted by alpha-tocopherol at both 4 weeks and 8 weeks. Treatment with alpha-tocopherol was associated with a significant reduction in the severity of proteinuria. Thus, our data suggest that the period between 4 and 8 weeks of BGG vaccination could be relevant in designing an appropriate model to study the molecular biology of the pathogenesis of renal injury and the effects of treatment. The 16-week model may be useful in exploring gene expression involved with spontaneous resolution.

Increased sialylation of polymeric immunoglobulin A1: Mechanism of selective glomerular deposition in immunoglobulin A nephropathy?

Immunoglobulin A nephropathy (IgAN) is characterized by raised serum IgA and predominant mesangial IgA deposits of polymeric nature. The abnormal glycosylation of the carbohydrate moieties in the hinge region of the IgA molecule has recently attracted much attention. In this study we investigated the galactosylation and sialylation of monomeric and polymeric IgA1 isolated from patients with IgAN. Total IgA1 in serum samples from patients with IgAN or from healthy controls was isolated with a jacalin-agarose column as jacalin-bound protein (JBP). Monomeric and polymeric IgA1 were distinctly separated by fast protein liquid chromatography. Lectin binding assays were designed to examine the sialylation and the expression of terminal galactose and N-acetyl galactosamine of the O-linked carbohydrate in the hinge region of the IgA molecule. Reduced terminal galactosylation was demonstrated in serum IgA and monomeric IgA1 isolated from patients with IgAN as compared with results in healthy control subjects. However, a reduction in terminal galactosylation was not found in polymeric IgA1 isolated from patients with IgAN. Instead, increased sialylation of IgA1 (α2–3 linked to galactose) was demonstrated in polymeric IgA1. This abnormality of IgA1 could bear considerable implication on the pathogenesis of IgAN, because the masking effect of sialic acid may hinder the clearance of polymeric IgA1 by the asialoglycoprotein receptor (ASGP-R) of the liver cells. An increase in the sialylated content would also render the polymeric IgA from patients with IgAN more anionic. These immunochemical properties may contribute to the selective glomerular deposition of polymeric IgA1 in IgAN.

Reduced binding of immunoglobulin A (IgA) from patients with primary IgA nephropathy to the myeloid IgA Fc-receptor, CD89.

Primary IgA nephropathy (IgAN) is associated with elevated levels of circulating IgA and is characterized by deposition of primarily IgA1 in the renal mesangium. It has not yet been clarified which mechanisms govern the deposition of IgA1 in the mesangium. One of the factors which may play a role in trapping of IgA in the mesangial area is the interaction of IgA with specific IgA receptors (Fc alphaR, CD89) on the mesangial cells. In the present study IgA derived from patients with IgAN and controls was investigated for its interaction with human CD89, expressed on the surface of the murine B cell line IIA1.6. IgA binding to CD89 expressing cells was specific, concentration dependent and binding of dIgA and pIgA occurred in a more efficient fashion than that of mIgA. IgA binding to CD89 directly from serum of patients compared to controls showed no significant difference. However these experiments are affected by differences in IgA concentration and combinations of different sizes of IgA. Using purified fractions of mIgA, dIgA, and pIgA isolated from serum, a significantly reduced binding of mIgA to CD89 from patients compared to controls was observed. Finally, the binding of aIgA2 to CD89 was less inhibited using mIgA from patients with IgAN compared to controls. The reduced binding of mIgA to CD89 seems to contradict a direct role for CD89 in deposition of IgA. However reduced binding of mIgA to CD89 may affect IgA clearance, leading to higher serum IgA. Furthermore, since it has been demonstrated that mIgA can interfere with binding of di- and pIgA, CD89 could still contribute to pIgA deposition in the mesangial area.