Molecular basis of IgA nephropathy

IgA nephropathy (IgAN) is the most common glomerulonephritis worldwide and remains an important cause of end-stage renal failure. However, the basic molecular mechanism(s) underlying abnormal IgA synthesis, selective mesangial deposition with ensuing mesangial cell proliferation and extracellular matrix expansion remains poorly understood. Notably, the severity of tubulointerstitial lesions better predicts the renal progression than the degree of glomerular lesions. The task of elucidating the molecular basis of IgAN is made especially challenging by the fact that both environmental and genetic components likely contribute to the development and progression of IgAN. This review will summarize the earlier works on the structure of the IgA molecule, mechanisms of mesangial IgA deposition and pathophysiologic effects of IgA on mesangial cells following mesangial deposition. Recently, a series of important advances in the area of communication between the glomerular mesangium and renal tubular cells have emerged. These novel findings regarding the molecular pathogenesis of IgAN will be helpful in designing future directions for therapy.     

IL-6 and its role in IgA nephropathy development

IL-6 is considered one of the well characterized cytokines exhibiting homeostatic, pro- and anti-inflammatory activities, depending on the receptor variant and the induced intracellular cis- or trans-signaling responses. IL-6-activated pathways are involved in the regulation of cell proliferation, survival, differentiation, and cell metabolism changes.Deviations in IL-6 levels or abnormal response to IL-6 signaling are associated with several autoimmune diseases including IgA nephropathy (IgAN), one of most frequent primary glomerulonephritis worldwide. IgAN is associated with increased plasma concentration of IL-6 and increased plasma concentration of aberrantly galactosylated IgA1 immunoglobulin (Gd-IgA1). Gd-IgA1 is specifically recognized by autoantibodies, leading to the formation of circulating immune complexes (CIC) with nephritogenic potential, since CIC deposited in the glomerular mesangium induce mesangial cells proliferation and glomerular injury. Infection of the upper respiratory or digestive tract enhances IL-6 production and in IgAN patients is often followed by the macroscopic hematuria.This review recapitulates general aspects of IL-6 signaling and summarizes experimental evidences about IL-6 involvement in the etiopathogenesis of IgA nephropathy through the production of Gd-IgA1 and regulation of mesangial cell proliferation.     

Uncoupling of Glomerular IgA Deposition and Disease Progression in Alymphoplasia Mice with IgA Nephropathy

Previous clinical and experimental studies have indicated that cells responsible for IgA nephropathy (IgAN), at least in part, are localized in bone marrow (BM). Indeed, we have demonstrated that murine IgAN can be experimentally reconstituted by bone marrow transplantation (BMT) from IgAN prone mice in not only normal mice, but also in alymphoplasia mice (aly/aly) independent of IgA⁺ cells homing to mucosa or secondary lymphoid tissues. The objective of the present study was to further assess whether secondary lymph nodes (LN) contribute to the progression of this disease. BM cells from the several lines of IgAN prone mice were transplanted into aly/aly and wild-type mice (B6). Although the transplanted aly/aly showed the same degree of mesangial IgA and IgG deposition and the same serum elevation levels of IgA and IgA-IgG immune-complexes (IC) as B6, even in extent, the progression of glomerular injury was observed only in B6. This uncoupling in aly/aly was associated with a lack of CD4⁺ T cells and macrophage infiltration, although phlogogenic capacity to nephritogenic IC of renal resident cells was identical between both recipients. It is suggested that secondary LN may be required for the full progression of IgAN after nephritogenic IgA and IgA/IgG IC deposition.     

LG3 fragment of endorepellin is a possible biomarker of severity in IgA nephropathy

IgA nephropathy (IgAN), the most common primary glomerulonephritis, is characterized by deposition of IgA in the glomerular mesangium. The diagnosis of IgAN still requires a kidney biopsy that cannot easily be repeated in the same patient during follow‐up. Therefore, identification of noninvasive urinary biomarkers would be very useful for monitoring patients with IgAN. We first used bidimensional electrophoresis (2DE) coupled to MALDI‐TOF‐TOF and Western blot to identify some urinary biomarkers associated with IgAN. Urine of IgAN patients showed an increase of albumin fragments, α‐1‐antitrypsin and α‐1‐β‐glycoprotein, along with a decrease of a single spot that was identified as the laminin G‐like 3 (LG3) fragment of endorepellin. The urinary proteomes of 43 IgAN patients were compared to those of 30 healthy individuals by ELISA. Quantification of LG3 confirmed a significant decrease in the urine of IgAN patients compared to healthy controls, except in ten patients in whom LG3 was increased. These ten patients had a more severe disease with lower glomerular filtration rate values. We found a significant inverse correlation between LG3 levels and glomerular filtration rate in the 43 patients with IgAN, which was not observed in 65 patients with other glomerular diseases including membranous nephropathy (23), lupus nephropathy (13), focal segmental glomerulosclerosis (15), diabetic nephropathy (14), and six patients with nonglomerular diseases. Therefore, we suggest that the LG3 fragment of endorepellin could be associated with IgAN severity and might be related to pathogenesis of IgAN.     

Pathogenic Role of a Proliferation-Inducing Ligand (APRIL) in Murine IgA Nephropathy

A proliferation-inducing ligand (APRIL) is a member of the tumor necrosis factor (TNF) superfamily. Despite advances in clinical and genetic studies, the details of the pathological roles of APRIL in IgA nephropathy (IgAN) remain to be fully defined. The present study aimed to further assess the pathological role of APRIL using a mouse model of IgAN. Mice with IgAN designated “grouped ddY” (gddY) were intraperitoneally administered an anti-APRIL monoclonal antibody (anti-APRIL Ab) or control IgG (Control Ab) twice each week for 2 weeks starting during the early stage of IgAN (6–7 weeks of age). Urinary albumin, serum IgA, and glomerular IgA deposition were evaluated. We further assessed the inflammatory responses during treatment by measuring the levels of the chemokine fractalkine (FKN) and its receptor CX3CR1 as well as the level of peripheral blood monocytosis. Anti-APRIL Ab treatment significantly decreased albuminuria and tissue damage combined with decreases in serum IgA levels and deposition of glomerular IgA. In contrast, the abundance of IgA⁺/B220⁺ or CD138⁺/B220⁺ B cells in the spleen and bone marrow, respectively, was unchanged. Treating gddY mice with anti-April Ab reduced the overexpression of FKN/CX3CR1 in the kidney and the increase in the population of circulating Gr1⁻/CD115⁺ monocytes. The size of the population of Gr1⁻/CD115⁺ monocytes correlated with renal FKN and urinary albumin levels. Moreover, mice treated with anti-APRIL Ab exhibited reduced progression of IgAN, serum IgA levels, and glomerular IgA deposition as well as an attenuated inflammatory process mediated by FKN-associated activation of monocytes. To the best of our knowledge, this is the first study to implicate the APRIL signal transduction pathway in the pathogenesis of nephrogenic IgA production. Moreover, our findings identify APRIL as a potential target of therapy.     

Difference in IgA1 O-glycosylation between IgA deposition donors and IgA nephropathy recipients

IgA nephropathy (IgAN) is the most common form of primary glomerulonephritis, and disease recurrence often occurs after transplantation. On the other hands, Asymptomatic IgA deposition (IgAD) is occasionally observed in donated kidney. It is recognized that IgAD does not progress to IgAN, but the mechanism has not demonstrated yet. In IgAN, aberrant IgA1 O-glycan structure in the hinge region (HR) of serum IgA is suggested as one of the most convincing key mediators. However, little is known about IgA1 O-glycan structure in IgAD patients. Herein, we investigated the prevalence of IgAD in living renal transplant donors in our cohort. IgAD was observed in 21(13.0%) among 161 renal transplant donors and have statistically significant blood relationship with IgAN recipients (28.6% in relatives vs. 9.8% in non-relatives, respectively; p?=?0.0073). Next, we evaluated the IgA1 O-glycan structure of serum IgA from IgAN recipients (n?=?26), IgAD donors (n?=?17), and non-IgAD helthy donors (n?=?27) using matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI–TOF MS). The numbers of GalNAc and Gal and the Gal/GalNAc ratio in the HR of the IgAN recipients had significantly lower comparing to the IgAD and non-IgAD healthy donors. The decreased Gal/GalNAc ratio in IgAN recipients means the increased ratio of galactose-deficient IgA1. To the best of our knowledge, this is the first report to compare the O-glycan structures in IgAN recipients and IgAD donors using MALDI–TOF MS. We concluded that IgAD was more common in IgAN related donors. Overall, decreased GalNAc and Gal contents in HR could play a material pathogenic role in IgAN.     

Pathogenesis of Berger's disease: recent advances on the involvement of immunoglobulin A and their receptors

Immunoglobulin A (IgA) nephropathy or Berger's disease is the most common form of primary glomerulonephritis in the world and one of the first cause of end-stage renal failure. IgA nephropathy is characterized by the accumulation in mesangial areas of immune complexes containing polymeric IgA1. While epidemiology and clinical studies of IgA nephropathy are well established, the mechanism(s) underlying disease development is poorly understood. The pathogenesis of this disease involves the deposition of polymeric and undergalactosylated IgA1 in the mesangium. Quantitative and structural changes of IgA1 play a key role in the development of the disease due to functional abnormalities of two IgA receptors: The FcalphaR (CD89) expressed by blood myeloid cells and the transferrin receptor (CD71) on mesangial cells. Abnormal IgA induce the release of soluble CD89 which is responsible for the formation of circulating IgA complexes. These complexes may be trapped by CD71 that is overexpressed on mesangial cells in IgA nephropathy patients allowing pathogenic IgA complex formation.     

天然免疫分子在糖基化异常IgA致肾小球足细胞损伤中的免疫调节作用

IgA肾病(IgA nephropathy,IgAN)位居各类肾小球疾病之首,是一组以IgA为主的免疫球蛋白在肾小球系膜区沉积为特征的免疫介导性肾小球疾病,也是引起患者终末期肾衰竭最常见的病因之一。足细胞是继系膜细胞与IgA肾病关系的新近关注热点,其系一类位于基底膜最外层的上皮细胞,并是构成肾小球滤过屏障的核心成份。目前认为,足细胞损伤及其生物学行为在IgA肾病等疾病起始进展乃至终末期肾衰中起关键作用。近年伴随着对上皮细胞尤其细胞转分化(EMT)现象在足细胞损伤机制中重要意义的认识,人们注意到糖基化异常IgA在足细胞EMT发生中的诱发作用,以及足细胞EMT过程中的病生理调控机制与IgA肾病等肾小球疾病发生发展的关系。为此,该文进一步基于足细胞的生物学特性以及免疫调节新的视角,探讨天然免疫分子在糖基化异常IgA致足细胞损伤中的调控作用,拟为进一步阐释IgA肾病发病机制及其相关研究乃至临床治疗提供新的思路。     

Suppression of adiponectin by aberrantly glycosylated IgA1 in glomerular mesangial cells in vitro and in vivo

The pathogenesis of IgA nephropathy (IgAN) may be associated with the mesangial deposition of aberrantly glycosylated IgA1. To identify mediators affected by aberrantly glycosylated IgA1 in cultured human mesangial cells (HMCs), we generated enzymatically modified desialylated and degalactosylated (deSial/deGal) IgA1. The state of deglycosylated IgA1 was confirmed by lectin binding to Helix aspersa (HAA) and Sambucus nigra (SNA). In the cytokine array analysis, 52 proteins were upregulated and 34 were downregulated in HMCs after stimulation with deSial/deGal IgA1. Among them, the secretion of adiponectin was suppressed in HMCs after stimulation with deSial/deGal IgA1. HMCs expressed mRNAs for adiponectin and its type 1 receptor, but not the type 2 receptor. Moreover, we revealed a downregulation of adiponectin expression in the glomeruli of renal biopsy specimens from patients with IgAN compared to those with lupus nephritis. We also demonstrated that aberrantly glycosylated IgA1 was deposited in the mesangium of patients with IgAN by dual staining of HAA and IgA. Moreover, the urinary HAA/SNA ratio of lectin binding was significantly higher in IgAN compared to other kidney diseases. Since adiponectin has anti-inflammatory effects, including the inhibition of adhesion molecules and cytokines, these data suggest that the local suppression of this adipokine by aberrantly glycosylated IgA1 could be involved in the regulation of glomerular inflammation and sclerosis in IgAN.     

Lung injury after deposition of IgA immune complexes. Requirements for CD18 and L-arginine.

Acute lung injury produced by deposition of IgA immune complexes is complement-dependent, neutrophil-independent, oxygen radical-mediated, and may be a result of the formation of the hydroxyl radical (HO) generated directly or indirectly from activated lung macrophages. The current studies were designed to evaluate further the pathophysiologic events that occur after intrapulmonary deposition of IgA immune complexes. Pretreatment of rats with the human recombinant soluble complement receptor-1 resulted in marked attenuation of IgA immune complex-induced lung injury. Intravenous administration of antibody to CD18, but not antibody to CD11b, was highly protective against lung injury. Treatment of animals with either anti-endothelial leukocyte-adhesion molecule-1 or anti-TNF-alpha, both of which were highly protective against IgG immune complex-induced lung injury, had no protective effects in the model of IgA immune complex-induced lung injury. Immunohistochemical analysis revealed up-regulation of the endothelial leukocyte adhesion molecule-1 in the pulmonary vasculature after deposition of IgA immune complexes. This up-regulation was TNF-alpha-dependent. The arginine analog, NG-monomethyl-L-arginine, was highly protective against IgA immune complex-induced lung injury. This protective effect was reversed by the co-presence of L-arginine (but not D-arginine). Protective interventions against IgA immune complex-induced lung injury were inversely correlated with the numbers of macrophages that could be retrieved by lung lavage. These data suggest fundamental differences in the pathogenesis of lung injury after intrapulmonary deposition of IgA immune complexes, as compared with injury caused by deposition of IgG immune complexes. In the latter, neutrophils, intrapulmonary generation of TNF-alpha, and up-regulation of pulmonary vascular endothelial leukocyte-adhesion molecule-1 are required for the full development of lung injury, whereas no such requirements appear in the case of IgA immune complex-induced lung injury. Full expression of IgA immune complex-induced lung injury also appears to require L-arginine, suggesting a possible role for nitric oxide or its derivatives in events ultimately leading to injury.     

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.     

Insulin-like growth factor binding protein-1 levels are increased in patients with IgA nephropathy

The mechanisms underlying the pathogenesis of immunoglobulin A (IgA) nephropathy (IgAN) are not well understood. In this study, we examined gene expression profiles in kidneys obtained from mice with high serum IgA levels (HIGA mice), which exhibit features of human IgAN. Female inbred HIGA, established from the ddY line, were used in these experiments. Serum IgA levels, renal IgA deposition, mesangial proliferation, and glomerulosclerosis were increased in 32-week-old HIGA mice in comparison to ddY animals. By microarray analysis, five genes were observed to be increased by more than 2.5-fold in 32-week-old HIGA in comparison to 16-week-old HIGA; these same five genes were decreased more than 2.5-fold in 32-week-old ddY in comparison to 16-week-old ddY mice. Of these five genes, insulin-like growth factor (IGF) binding protein (IGFBP)-1 exhibited differential expression between these mouse lines, as confirmed by quantitative RT-PCR. In addition, serum IGFBP-1 levels were significantly higher in patients with IgAN than in healthy controls. In patients with IgAN, these levels correlated with measures of renal function, such as estimated glomerular filtration rate (eGFR), but not with sex, age, serum IgA, C3 levels, or IGF-1 levels. Pathologically, serum IGFBP-1 levels were significantly associated with the severity of renal injury, as assessed by mesangial cell proliferation and interstitial fibrosis. These results suggest that increased IGFBP-1 levels are associated with the severity of renal pathology in patients with IgAN.     

Imbalance of T cell immunoregulatory subsets in primary IgA nephropathy

The peripheral blood distribution of T cell subsets was evaluated in a group of patients with primary IgA nephropathy (IgAN). Results showed that the frequency of helper (CD4+) and suppressor (CD8+) T lymphocytes in IgAN overlapped that seen in healthy blood donors. In addition, the helper T cell subset (CD4+ CDW29+ and CD4+ CD45R+ cells, respectively) proportion was normal, while with particular reference to suppressor T cell subpopulations, a significant decrease of CD8+ CD11+ lymphocytes (the true suppressor cells) was observed in IgAN. These data were further confirmed by the demonstration that monocyte chemotactic responsiveness triggered by lymphocyte-derived chemotactic factor (LDCF), a lymphokine released by CD8+ CD11- cells, was higher in IgAN than in controls. These data suggest that the low frequency of CD8+ CD11+ cells may be responsible for the impaired T cell immunoregulatory activity in patients with IgAN.     

A Panel of Serum Biomarkers Differentiates IgA Nephropathy from Other Renal Diseases

Background and Objectives

There is increasing evidence that galactose-deficient IgA1 (Gd-IgA1) and Gd-IgA1-containing immune complexes are important for the pathogenesis of IgA nephropathy (IgAN). In the present study, we assessed a novel noninvasive multi-biomarker approach in the diagnostic test for IgAN.

Materials and Methods

We compared serum levels of IgA, IgG, Gd-IgA1, Gd-IgA1-specific IgG and Gd-IgA1-specific IgA in 135 IgAN patients, 79 patients with non-IgAN chronic kidney disease (CKD) controls and 106 healthy controls. Serum was collected at the time of kidney biopsy from all IgAN and CKD patients.

Results

Each serum marker was significantly elevated in IgAN patients compared to CKD (P<0.001) and healthy controls (P<0.001). While 41% of IgAN patients had elevated serum Gd-IgA1 levels, 91% of these patients exhibited Gd-IgA1-specific IgG levels above the 90th percentile for healthy controls (sensitivity 89%, specificity 92%). Although up to 25% of CKD controls, particularly those with immune-mediated glomerular diseases including lupus nephritis, also had elevated serum levels of Gd-IgA1-specific IgG, most IgAN patients had elevated levels of Gd-IgA1-specific antibody of both isotypes. Serum levels of Gd-IgA1-specific IgG were associated with renal histological grading. Furthermore, there was a trend toward higher serum levels of Gd-IgA1-specific IgG in IgAN patients with at least moderate proteinuria (≥1.0 g/g), compared to patients with less proteinuria.

Conclusions

Serum levels of Gd-IgA1-specific antibodies are elevated in most IgAN patients, and their assessment, together with serum levels of Gd-IgA1, improves the specificity of the assays. Our observations suggest that a panel of serum biomarkers may be helpful in differentiating IgAN from other glomerular diseases.     

ST6Gal1 is up‐regulated and associated with aberrant IgA1 glycosylation in IgA nephropathy: An integrated analysis of the transcriptome

Galactose‐deficient IgA1 (Gd‐IgA1) plays a crucial role in the development of Immunoglobulin A nephropathy (IgAN), however, the underlying pathogenic mechanisms driving Gd‐IgA1 production in B cells are not well understood. In this study, RNA‐seq analysis identified 337 down‐regulated and 405 up‐regulated genes in B cells from 17 patients with IgAN and 6 healthy controls. Among them, ST6Gal1, which was associated with IgAN in a previous genome‐wide association study (GWAS), was up‐regulated in IgAN and significantly positive correlated with elevated Gd‐IgA1. In addition, we identified increased plasma ST6Gal1 levels in 100 patients with IgAN, which were associated with higher levels of proteinuria, plasma IgA, Gd‐IgA1 levels, greater degrees of systemic complement activation including C3a, Bb, C4d, MAC and a lower proportion classified as C2 grade (crescent proportion ≥25%). Interesting, in vitro, recombinant ST6Gal1 (rST6Gal1) exposure reduced the production of Gd‐IgA1 in cultured peripheral blood mononuclear cells from IgAN patients. rST6Gal1 stimuli also increased expression of C1GALT1, which were well‐known proportional to the decrease in galactose deficiency of IgA1. In conclusions, we identified increased plasma ST6Gal1 levels and the association of ST6Gal1 with disease severity of IgAN. Additionally, rST6Gal1 administration in vitro increased expression of C1GALT1 and reduced the production of Gd‐IgA1.     

Cytokines Alter IgA1 O-Glycosylation by Dysregulating C1GalT1 and ST6GalNAc-II Enzymes

IgA nephropathy (IgAN), the most common primary glomerulonephritis, is characterized by renal immunodeposits containing IgA1 with galactose-deficient O-glycans (Gd-IgA1). These immunodeposits originate from circulating immune complexes consisting of anti-glycan antibodies bound to Gd-IgA1. As clinical disease onset and activity of IgAN often coincide with mucosal infections and dysregulation of cytokines, we hypothesized that cytokines may affect IgA1 O-glycosylation. We used IgA1-secreting cells derived from the circulation of IgAN patients and healthy controls and assessed whether IgA1 O-glycosylation is altered by cytokines. Of the eight cytokines tested, only IL-6 and, to a lesser degree, IL-4 significantly increased galactose deficiency of IgA1; changes in IgA1 O-glycosylation were robust for the cells from IgAN patients. These cytokines reduced galactosylation of the O-glycan substrate directly via decreased expression of the galactosyltransferase C1GalT1 and, indirectly, via increased expression of the sialyltransferase ST6GalNAc-II, which prevents galactosylation by C1GalT1. These findings were confirmed by siRNA knockdown of the corresponding genes and by in vitro enzyme reactions. In summary, IL-6 and IL-4 accentuated galactose deficiency of IgA1 via coordinated modulation of key glycosyltransferases. These data provide a mechanism explaining increased immune-complex formation and disease exacerbation during mucosal infections in IgAN patients.     

Uteroglobin interacts with the heparin-binding site of fibronectin and prevents fibronectin-IgA complex formation found in IgA-nephropathy

Immunoglobulin A (IgA)-nephropathy (IgAN) is the most common primary renal glomerular disease in the world that has no effective treatment. High levels of circulating IgA-fibronectin (Fn) complexes, characteristically found in IgAN patients, are suggested to cause abnormal deposition of IgA and Fn in the renal glomeruli of these patients causing renal failure. We previously reported that binding of Fn to uteroglobin (UG), a multifunctional anti-inflammatory protein, inhibits Fn-IgA heteromerization. However, the specific site of Fn-UG interaction until now remained unidentified. We report here that UG interacts with the heparin-binding site of Fn and propose that small molecules competing for interaction with this site may reduce the level of circulating Fn-IgA complexes in IgAN.     

DNA Methylation in Cosmc Promoter Region and Aberrantly Glycosylated IgA1 Associated with Pediatric IgA Nephropathy

IgA nephropathy (IgAN) is one of the most common glomerular diseases leading to end-stage renal failure. Elevation of aberrantly glycosylated IgA1 is a key feature of it. The expression of the specific molecular chaperone of core1ß1, 3galactosyl transferase (Cosmc) is known to be reduced in IgAN. We aimed to investigate whether the methylation of CpG islands of Cosmc gene promoter region could act as a possible mechanism responsible for down-regulation of Cosmc and related higher secretion of aberrantly glycosylated IgA1in lymphocytes from children with IgA nephropathy. Three groups were included: IgAN children (n = 26), other renal diseases (n = 11) and healthy children (n = 13). B-lymphocytes were isolated and cultured, treated or not with IL-4 or 5-Aza-2’-deoxycytidine (AZA). The levels of DNA methylation of Cosmc promotor region were not significantly different between the lymphocytes of the three children populations (P = 0.113), but there were significant differences between IgAN lymphocytes and lymphocytes of the other two children populations after IL-4 (P<0.0001) or AZA (P<0.0001). Cosmc mRNA expression was low in IgAN lymphocytes compared to the other two groups (P<0.0001). The level of aberrantly glycosylated IgA1 was markedly higher in IgAN group compared to the other groups (P<0.0001). After treatment with IL-4, the levels of Cosmc DNA methylation and aberrantly glycosylated IgA1 in IgAN lymphocytes were remarkably higher than the other two groups (P<0.0001) with more markedly decreased Cosmc mRNA content (P<0.0001). After treatment with AZA, the levels in IgAN lymphocytes were decreased, but was still remarkably higher than the other two groups (P<0.0001), while Cosmc mRNA content in IgAN lymphocytes were more markedly increased than the other two groups (P<0.0001). The alteration of DNA methylation by IL-4 or AZA specifically correlates in IgAN lymphocytes with alterations in Cosmc mRNA expression and with the level of aberrantly glycosylated IgA1 (r = −0.948, r = 0. 707). Our results suggested that hypermethylation of Cosmc promoter region could be a key mechanism for the reduction of Cosmc mRNA expression in IgAN lymphocytes with associated increase in aberrantly glycosylated IgA1.     

Disruption of Smad4 Expression in T Cells Leads to IgA Nephropathy-Like Manifestations

The link between glomerular IgA nephropathy (IgAN) and T helper 2 (Th2) response has been implicated, however, the mechanisms are poorly defined because of the lack of an appropriate model. Here we report a novel murine model characterized by lineage-restricted deletion of the gene encoding MAD homologue 4 (Smad4) in T cells (Smad4^{co/co;Lck-cre}). Loss of Smad4 expression in T cells results in overproduction of Th2 cytokines and high serum IgA levels. We found that Smad4^{co/co;Lck-cre} mice exhibited massive glomerular IgA deposition, increased albumin creatinine ratio, aberrant glycosylated IgA, IgA complexed with IgG1 and IgG2a, and polymeric IgA, all known features of IgAN in humans. Furthermore, we examined the β1, 4-galactosyltransferases (β4GalT) enzyme which is involved in the synthesis of glycosylated murine IgA, and we found reduced β4GalT2 and β4GalT4 mRNA levels in B cells. These findings indicate that Smad4^{co/co;Lck-cre} mice could be a useful model for studying the mechanisms between IgAN and Th2 response, and further, disruption of Smad4-dependent signaling in T cells may play an important role in the pathogenesis of human IgAN and contributing to a Th2 T cell phenotype.