Activation of tubular epithelial cells by mesangial-derived TNF-alpha: glomerulotubular communication in IgA nephropathy

BACKGROUND: IgA nephropathy (IgAN), characterized by mesangial IgA deposition, runs a variable clinical course with tubulointerstitial damage and renal failure in no less than 30% of patients. Histologically, IgA is rarely detected in renal tubules. The direct toxicity by IgA on renal tubules remains uncertain. We hypothesize that mediators released from human mesangial cells (HMC) triggered by IgA deposition may lead to activation of proximal tubular epithelial cells (PTEC). METHODS: The binding of IgA to PTEC or HMC was assessed by flow cytometry. IgA-HMC medium was prepared by collecting the spent medium in which growth arrested HMC were incubated with IgA isolated from patients with IgAN, healthy control subjects, or other nephritic control patients. PTEC was cultured with the IgA-HMC medium in the presence or absence of neutralizing antibodies to TNF-alpha, IL-1beta, TGF-beta, or PDGF. Gene expression and protein synthesis of TNF-alpha, MIF, or ICAM-1 by PTEC were determined by RT-PCR and ELISA, respectively. RESULTS: The binding of IgA isolated from patients with IgAN to PTEC was increased when compared to binding of IgA from healthy control subjects (P < 0.005). However, the binding to PTEC was less than one tenth that of HMC in IgAN. The binding to PTEC was not mediated through known IgA receptors, as shown by competitive binding assays and gene expression of the receptors. Despite the in vitro binding, PTEC cultured with isolated IgA exhibited no increased cell proliferation or enhanced synthesis of TNF-alpha, MIF, or sICAM-1. However, when PTEC were cultured with IgA-HMC medium prepared from IgAN patients, there was enhanced proliferation of PTEC (P < 0.001) and increased synthesis of TNF-alpha, MIF, and sICAM-1 when compared with PTEC cultured with IgA-HMC medium from control subjects (P < 0.001). The synthesis of MIF and sICAM-1 by PTEC cultured with IgA-HMC medium was reduced by neutralizing antibodies to TNF-alpha (P < 0.001) but not by neutralizing antibodies to IL-1beta, TGF-beta, or PDGF. CONCLUSION: Our finding implicates that TNF-alpha released from the mesangium after IgA deposition activates renal tubular cells. The glomerulotubular communication could play an important role in the pathogenesis of tubulointerstitial damage in IgAN.     

Novel mechanisms of tubulointerstitial injury in IgA nephropathy: a new therapeutic paradigm in the prevention of progressive renal failure

IgA nephropathy (IgAN) runs a highly variable clinical course with frequent involvement of tubulointerstitial damage. Notably, renal progression correlates more closely with the severity of tubulointerstitial lesions than with the degree of glomerular lesions In IgAN. Mesangial IgA deposition induces local release of cytokines, complement, and angiotensin II leading to glomerular inflammation. It remains unclear how mesangial IgA deposition leads to tubulointerstitial injury in IgAN. Moreover, IgA deposits are rarely detected in renal interstitium in IgAN. We hypothesize that mediators released from mesangial cells triggered by IgA deposition leads to activation of proximal tubular epithelial cells. Our preliminary findings implicate a glomerulotubular cross talk with mediators released from the mesangium contributing to the pathogenesis of tubulointerstitial damage in IgAN. We have also found the expression of angiotensin II subtype-1 receptor or angiotensin II subtype-2 receptor in proximal tubular epithelial cells differs from that of mesangial cells. One potential therapeutic approach is to counterbalance the growth-stimulatory effects of angiotensin II through subtype-1 receptor in tubular epithelial cells by subtype-2 receptor-mediated apoptosis and growth inhibition. These novel findings may provide clinicians new therapeutic approach for selective blockade of the RAS in IgAN.     

Tubular expression of angiotensin II receptors and their regulation in IgA nephropathy

Enhanced renal expression for the renin-angiotensin system (RAS) is detected in IgA nephropathy (IgAN). Previous data showed an altered glomerular expression of angiotensin II type 1 receptor (AT1R), suggesting a regulatory response to high intrarenal angiotensin II (Ang II) concentration in IgAN. In this study, the expression and regulation of Ang II receptors were examined in human proximal tubular epithelial cells (PTEC) in IgAN. Tubular expression of AT1R and Ang II type 2 receptor (AT2R) was increased in IgAN. In vitro culture experiment showed that the upregulation of Ang II receptors was not due to the direct effect of IgA but the indirect effect after IgA deposition on human mesangial cell. When PTEC were cultured with conditioned culture medium from human mesangial cells activated with IgA, Ang II production was upregulated, leading to inflammation and apoptosis via the AT1R and AT2R, respectively. Sequential expression of Ang II receptors determined the injury of PTEC induced by mediators in the conditioned medium. The initial interaction between Ang II and AT1R activated both protein kinase C and mitogen-activated protein kinase pathways, leading to inflammatory responses. This early AT1R-dependent event was followed by upregulation of AT2R expression and continued Ang II release. The interaction between Ang II and AT2R subsequently led to expression of cleaved poly[ADP-ribose] polymerase through downregulation of the mitogen-activated protein kinase pathway. The data suggest that appropriate control of Ang II receptor activities in PTEC may ameliorate tubulointerstitial injury in IgAN.     

Engagement of transferrin receptor by polymeric IgA1: evidence for a positive feedback loop involving increased receptor expression and mesangial cell proliferation in IgA nephropathy

IgA nephropathy (IgAN), the most common primary glomerulonephritis in the world, is characterized by IgA immune complex-mediated mesangial cell proliferation. The transferrin receptor (TfR) was identified previously as an IgA1 receptor, and it was found that, in biopsies of patients with IgAN, TfR is overexpressed and co-localizes with IgA1 mesangial deposits. Here, it is shown that purified polymeric IgA1 (pIgA1) is a major inducer of TfR expression (three- to four-fold increase) in quiescent human mesangial cells (HMC). IgA-induced but not cytokine-induced HMC proliferation is dependent on TfR engagement as it is inhibited by both TfR1 and TfR2 ectodomains as well as by the anti-TfR mAb A24. It is dependent on the continued presence of IgA1 rather than on soluble factors released during IgA1-mediated activation. In addition, pIgA1-induced IL-6 and TGF-beta production from HMC was specifically inhibited by mAb A24, confirming that pIgA1 triggers a TfR-dependent HMC activation. Finally, upregulation of TfR expression induced by sera from patients with IgAN but not from healthy individuals was dependent on IgA. It is proposed that deposited pIgA1 or IgA1 immune complexes could initiate a process of auto-amplification involving hyperexpression of TfR, allowing increased IgA1 mesangial deposition. Altogether, these data unveil a functional cooperation between pIgA1 and TfR for IgA1 deposition and HMC proliferation and activation, features that are commonly implicated in the chronicity of mesangial injuries observed in IgAN and that could explain the recurrence of IgA1 deposits in the mesangium after renal transplantation.     

Mesangial expression of angiotensin II receptor in IgA nephropathy and its regulation by polymeric IgA1

BACKGROUND: Enhanced gene expression for the renin-angiotensin system (RAS) is detected in glomerular mesangial cells in IgA nephropathy (IgAN). Preliminary studies showed a reduced glomerular gene expression of angiotensin II subtype 1 receptor (AT1R), suggesting a regulatory response to high intrarenal angiotensin II (Ang II) concentration in IgAN. METHODS: We examined the effect of polymeric IgA1 (pIgA1) from patients with IgAN on the expression of Ang II receptors in cultured human mesangial cells (HMC). RESULTS: Polymeric IgA1 from patients with IgAN down-regulated the expression of AT1R in HMC in a dose-dependent manner. When similar experiments were conducted with addition of an angiotensin-converting enzyme inhibitor (captopril) or an AT1R antagonist (losartan), there was a significant increase in the expression of AT1R. Blockade of Ang II with captopril or losartan alone resulted in a stepwise increase of AT1R in cultured HMC. Down-regulation of Ang II subtype 2 receptor (AT2R) was not observed in HMC cultured with pIgA1 from patients with IgAN. The acute suppressive effect of pIgA1 from IgAN on the expression of AT1R was confirmed in HMC incubated with IgA isolated from 15 IgAN patients, 15 healthy subjects, and other glomerulonephritides control subjects. Reduced glomerular expression of AT1R (but not AT2R) was also demonstrated in renal biopsies from patients with IgAN. CONCLUSION: Our findings demonstrate an altered AT1R expression in HMC in response to raised intrarenal Ang II in IgAN. Our in vitro studies also support that an imbalance of AT1R and AT2R activity in HMC following exposure to pIgA plays a significant pathogenetic role in the inflammatory injury in IgAN.     

Tissue-specific expression of renin-angiotensin system components in IgA nephropathy

BACKGROUND: The renin-angiotensin II system (RAS) has been implicated in the development of glomerulonephritis. The aims of this study were to determine (1) the expression of RAS components, angiotensin (Ang II)-forming enzymes [angiotensin-I-converting enzyme (ACE) and chymase], and Ang II receptors, and (2) the correlation between RAS expression and severity of tissue injury in IgA nephropathy (IgAN). METHODS: The expression levels of ACE, chymase, and Ang II type 1 and type 2 receptor (AT1R and AT2R) mRNAs were determined by in situ hybridization in renal specimens from 18 patients with IgAN, 5 patients with non-IgA mesangial proliferative glomerulonephritis (non-IgAN) and 10 patients with nonmesangial proliferative glomerulonephritis (minimal change nephrotic syndrome, n = 5, and membranous nephropathy, n = 5). Normal portions of surgically resected kidney served as control. RESULTS: In normal kidney, a few mesangial cells and glomerular and tubular epithelial cells weakly expressed ACE, chymase and AT1R mRNAs. In IgAN and non-IgAN samples, ACE, chymase, AT1R and AT2R mRNAs were expressed in resident glomerular cells, including mesangial cells, glomerular epithelial cells and cells of Bowman's capsule. The glomerular expressions in IgAN were stronger than in minimal change nephrotic syndrome and membranous nephropathy. In IgAN, the expressions in glomeruli correlated with the degree of mesangial hypercellularity, whereas the expression levels were weaker at the area of mesangial expansion. IgAN with severe tubulointerstitial injury showed expression of ACE, chymase, AT1R and AT2R mRNAs in atrophic tubules and infiltrating cells and such expression correlated with the degree of tubulointerstitial damage. CONCLUSION: Our results suggest that renal cells can produce RAS components and that locally synthesized Ang II may be involved in tissue injury in IgAN through Ang II receptors in the kidney.     

Polymeric IgA increases the synthesis of macrophage migration inhibitory factor by human mesangial cells in IgA nephropathy.

BACKGROUND: It has been suggested that polymeric IgA (pIgA) or IgA immune complexes play a significant pathogenic role in IgA nephropathy (IgAN). Macrophage migration inhibitory factor (MIF) shares many activities with other pro-inflammatory cytokines. In human glomerulonephritis, including IgAN, glomerular expression of MIF is found to correlate with progressive renal injury. We hypothesized that deposition of pIgA within the kidney may lead to enhanced synthesis of MIF by mesangial cells. METHODS: In this study we examined the effect of pIgA and monomeric IgA (mIgA) from randomly selected patients with IgAN in clinical quiescence on the gene expression and protein synthesis of MIF in cultured human mesangial cells (HMC). RESULTS: Both pIgA and mIgA from IgAN patients or matched healthy controls increased MIF gene expression and protein synthesis in a dose-dependent fashion. The magnitude of MIF protein induction by pIgA (100 microg/ml) was similar to that of tumour necrosis factor-alpha (TNF-alpha) at 10 pg/ml. In all subjects, the induction of MIF was higher for pIgA when compared with mIgA (P < 0.01). Furthermore, the up-regulation of MIF synthesis by either pIgA or mIgA was significantly higher in IgAN patients than in healthy controls (P < 0.05). Similarly, pIgA and mIgA were able to induce TNF-alpha gene expression and protein synthesis in mesangial cells. Incubation of mesangial cells with neutralizing antibody to TNF-alpha reduced the MIF synthesis induced by pIgA. CONCLUSION: We demonstrate that pIgA is capable of inducing MIF and TNF-alpha production in HMC, which may play a major pathogenic role in IgAN. Induction of MIF can be partially blocked by neutralizing antibody to TNF-alpha, suggesting the possibility that up-regulation of MIF synthesis in HMC is mediated via an amplifying proinflammatory loop involving TNF-alpha.     

Polymeric IgA1 from patients with IgA nephropathy upregulates transforming growth factor-beta synthesis and signal transduction in human mesangial cells via the renin-angiotensin system

The effects of polymeric IgA1 (pIgA1) and monomeric IgA1 (mIgA1) from patients with IgA nephropathy (IgAN) on the renin-angiotensin system (RAS) and TGF-beta synthesis were examined in cultured human mesangial cells (HMC). Both pIgA1 and mIgA1 induced renin gene expression in HMC, in a dose-dependent manner. Similar findings were observed for TGF-beta gene and protein expression. The values measured in HMC incubated with pIgA1 were significantly higher than those in HMC incubated with equivalent amounts of mIgA1. When similar experiments were performed with the addition of either captopril or losartan, there was a significant increase in the renin gene expression by HMC, whereas the synthesis of TGF-beta was markedly reduced. The TGF-beta signal transduction pathways in HMC were studied by measuring the receptor-regulated Smad proteins (Smad 2 and 3) and common-partner Smad proteins (Smad 4). pIgA1 from patients with IgAN upregulated Smad activity in HMC, and the activity observed in HMC that had been preincubated with pIgA1 was readily suppressed with optimal concentrations of captopril or losartan. The effects of pIgA1 on the RAS were further examined in HMC incubated with IgA isolated from 30 patients with IgAN, 30 healthy subjects, and disease control subjects with other diseases. pIgA1 induction of angiotensin II or TGF-beta synthesis in HMC was significantly greater with preparations from patients with IgAN, compared with healthy or disease control subjects. The findings support a pathogenetic role of pIgA1 in IgAN through upregulation of the RAS and TGF-beta, leading to chronic renal failure with renal fibrosis.     

Future directions in the treatment of IgA nephropathy

IgA nephropathy (IgAN) is the most common primary glomerulonephritis yet its etiology remains uncertain. Recent data suggest a structural aberration of the IgA molecule in IgAN that may exert pathophysiologic effects on target cells, reduce clearance of IgA-immune complexes (IC), or favor mesangial IC trapping. Mesangial reactivity to immune complexes triggers off the release of cytokines and the alteration of prostaglandin and thromboxane A(2) production promoting mesangial cell proliferation. Angiotensin II-induced mesangial cells contraction with efferent arteriolar vasodilatation initiates glomerular injury and eventually lead to glomerulosclerosis following increased local production of transforming growth factor-beta (TGF-beta) and platelet-derived growth factor (PDGF). This paper highlights the potential therapeutic strategies in the future. These strategies include: (i) decreasing the synthesis of IgA-IC; (ii) limiting the mesangial uptake of IgA-IC; (iii) antagonizing the effect of PDGF and TGF-beta to reduce mesangial proliferation and glomerulosclerosis; and (iv) reducing the noxious glomerular injury due to infiltrating neutrophils. The effective treatment of IgAN requires a better clarification of the pathogenesis of the nephropathy. Future therapeutic attempts to slow down the renal deterioration should target at prevention of mesangial IgA deposition and the amelioration of inflammatory injury induced by infiltrating neutrophils and the released cytokines.     

Interleukin 4 co-stimulates the PDGF-BB- and bFGF-mediated proliferation of mesangial cells and myofibroblasts

BACKGROUND: Although many mediators involved in the pathogenesis of fibrosis are known, its precise mechanism is still unknown. In vitro experiments may contribute to the recognition of cellular changes which also take place during fibrosis. METHODS: Renal tubular epithelial cells (EPC), mesangial cells (MC) and glomerular endothelial cells (GEDC) as well as endothelial cells (EDC) and myofibroblasts (MF) from cattle were isolated to measure the proliferation and protein synthesis in the presence of individual and combined cytokines/growth factors in cell cultures. RESULTS: Cytokines stimulating or permitting the proliferation of myofibroblast-like cells (MFLC) (MC and MF), caused damage of endothelial cells (EDC, GEDC), whereas EPC were stable. The proliferation of MFLC was strongly stimulated by PDGF-BB and bFGF and elevated more than twofold in the presence of interleukin 4 (IL-4), but IL-4 alone had no effect. Furthermore, the proliferation of transdifferentiated endothelial cells (TEC), obtained by incubation of EDC with TNFalpha and bFGF, was stimulated with both PDGF-BB/IL-4 and bFGF/IL-4 in the same way and proved to be stable with respect to TNFalpha. CONCLUSION: Interleukin 4 co-stimulates the PDGF-BB- and bFGF-mediated proliferation of MC, MF, and TEC. TNFalpha does not inhibit the proliferation of extracellular matrix-synthesizing cells, but has an inhibitory or even toxic effect on EDC and GEDC. It may be concluded that cytokines released in inflamed renal tissue influence tubulointerstitial cells in different ways, resulting in progressive tissue damage and fibrosis in which the EDC would be the most sensitive cells. Thus, we speculate that microvascular injury in these areas leads to ischemia and malnutrition of tubular EPC and may be responsible for ongoing tubular damage and resulting renal interstitial fibrosis.     

Intrarenal synthesis of IL-6 in IgA nephropathy

Summary: Recent in vitro studies have shown the synthesis of interleukin-6 (IL-6) in glomerular mesangial and epithelial cells, and suggested the involvement of IL-6 in mesangial proliferative glomerulonephritis. However, the expression site of IL-6 mRNA in renal tissue of IgA nephropathy (IgAN), the most common chronic mesangial proliferative glomerulonephritis, remains obscure. to localize IL-6 mRNA in renal biopsy specimens of IgAN, we used nonradioactive in situ hybridization (ISH) developed in our laboratory, sensitive in detecting individual cells positive for a specific mRNA. In some sections, periodic acid-Schiff staining was performed after ISH in order to identify the topographical relation between IL-6 mRNA positive cells and glomerular basement membrane and mesangial area. In situ hybridization for IL-6 mRNA and immunohistochemistry for CD3 and CD68, markers for lymphocytes and monocytes, respectively, were also performed on serial sections to examine the contribution of infiltrated mononuclear cells to cells positive for IL-6 mRNA in glomeruli. Glomerular resident cells, including glomerular mesangial and epithelial cells and cells of Bowman's capsule, as well as tubular epithelial cells and infiltrated mononuclear cells expressed IL-6 mRNA. We also compared the localization of IL-6 mRNA and protein and showed different distribution between the gene product and protein. the expression of IL-6 mRNA correlated with the degree of mesangial cell proliferation and tubulointerstitial changes. Our results indicate that IL-6 is synthesized in renal tissues of IgAN and suggest that the increased IL-6 expression may be important in the pathogenesis of IgAN.     

Additive effect of PPAR-�� agonist and ARB in treatment of experimental IgA nephropathy

Our recent in vitro study demonstrated peroxisome proliferator-activated receptor-γ (PPAR−γ) agonist potentiated the anti-inflammatory effect of angiotensin receptor blocker (ARB) in tubular epithelial cell under milieu mimicking IgA nephropathy (IgAN). Here we studied the therapeutic effect of combining a PPAR-γ agonist, rosiglitazone (Ros), with an ARB, losartan (Los), in experimental IgAN induced in Lewis rats by oral and intravenous immunization with bovine gamma-globulin (BGG). The rats were randomly divided into six groups: control, IgAN, IgAN with unilateral nephrectomy (IgAN/1K), and IgAN/1K receiving Ros, Los, or Ros + Los. Medication was given 1 week after nephrectomy until killing. Rats developing IgAN had hematuria, mesangial hypercellularity with IgA deposition, glomerular damage, and tubulointerstitial infiltration of CD25+ leukocytes accompanied by increased renal expression of TGF-β, AngII receptor subtype-1 (ATR1) and ICAM-1. The renal histopathology, albuminuria, and renal expression of TGF-β, ATR1 and ICAM-1 worsened with unilateral nephrectomy. Ros or Los reduced the renal expression of PCNA, TGF-β, ATR1, and ICAM-1 in IgAN rats with nephrectomy. Despite no difference between rats treated with monotherapy, combined therapy offered additive effect with decreased renal expression of TGF-β, ATR1 and ICAM-1 and attenuation of renal injury. Our animal study suggests combined PPAR-γ agonist and ARB holds promise for future therapy for IgAN.     

GMP-17-positive T-lymphocytes in renal tubules predict progression in early stages of IgA nephropathy

Treatment of patients with IgA nephropathy (IgAN) depends on a reliable assessment of disease progression based on measurements of glomerular filtration rate (GFR), proteinuria, hypertension, and tubulointerstitial changes. We sought to determine whether progression could be predicted from analysis of glomerular and tubulointerstitial inflammation in biopsies taken at an early stage of IgAN. We retrospectively analyzed biopsies from 50 patients, relating the subsequent clinical course to infiltration with B- and T-lymphocytes, granule membrane protein of 17 kDa (GMP-17) positive cytotoxic T cells, macrophages, fibroblasts, and tubulointerstitial expression of human leukocyte antigen-D related (HLA-DR). At biopsy, 19 patients had decreased GFR while 13 of 31 patients with normal GFR and progressive IgAN differed significantly from 18 non-progressors in the level of proteinuria and in the severity of scores for mesangial proliferation, tubular atrophy, interstitial fibrosis, and interstitial infiltrates. On multivariate regression analysis these differences disappeared; however, associations with GMP-17-positive cytotoxic T-lymphocytes in intact renal tubules and of B-lymphocytes in the interstitium remained significant. Our study may have identified a marker of disease progression in early stages of IgAN.     

Association of angiotensin-converting enzyme gene polymorphism and renal pathology in Japanese children with IgA nephropathy.

Polymorphism of the gene that codes for angiotensin I-converting enzyme (ACE) is associated with increased severity of immunoglobulin A (IgA) nephropathy in adult patients. We evaluated the relationship between the polymorphism of ACE genotypes and the pathological and clinical findings in Japanese children with IgA nephropathy. Patients with moderate/diffuse mesangial proliferation, glomerular sclerosis and tubulointerstitial damage showed a significant increase of the D/D type compared to those who had mild/focal mesangial proliferation, without glomerular sclerosis or tubulointerstitial damage (p < 0.05). Proteinuria at the first renal biopsy was significantly higher in the former group compared with the latter group except glomerular sclerosis (p < 0.01). IgA nephropathy patients with tubulointerstitial damage also showed an increased serum creatinine level compared to patients without the damage (p < 0.03). We conclude that ACE gene polymorphism may be correlated with the prognosis of IgA nephropathy in Japanese children.     

Pathogenic role of the IgA molecule in IgA nephropathy

SUMMARY: Deposits of IgA together with complement in different body tissues support the hypothesis that IgA can trigger inflammatory mechanisms. IgA nephropathy (IgAN) is characterized by predominant mesangial IgA₁ deposits of a polymeric nature. So far, the mechanism of polymeric IgA₁ deposition in the kidney mesangium is poorly understood in IgAN. the exact pathophysiological sequel preceding renal fibrosis following the mesangial deposition of IgA immune complexes remains speculative. Recent in vitro studies revealed that binding of IgA to mesangial cells led to increased expression of growth factors, cytokines, and integrins. the release of these proinflammatory factors is likely to enhance inflammatory injury. In addition, the local renin-angiotensin system present in renal tissues also contributes to renal fibrosis through the activation of transforming growth factor-β. the question of whether polymeric IgA isolated from patients with IgAN exerted any upregulatory effect on the synthesis of macrophage migration inhibitory factor (MIF) and components of the renin-angiotensin system in human mesangial cells was explored. the in vitro studies revealed that polymeric IgA from IgAN patients upregulated the gene expression of renin and MIF in human mesangial cells in a dose-dependent manner. These findings further support the notion that glomerular deposition of IgA is not only a pathological epiphenomenon of IgAN, but that polymeric IgA exerts a pathophysiologic effect on the mesangial cells leading to renal fibrosis.     

Over-expression of suppressor of cytokine signaling 3 inhibits the proliferation of human mesangial cells stimulated by aggregated IgA1 from IgA nephropathy patients

Objective To investigate the effect of suppressor of cytokine signaling 3 (SOCS3) on the proliferation of human mesangial cells stimulated by aggregated IgA1 (aIgA1) from patients with IgA nephropathy(IgAN), and explore its possible mechanism. Methods Serum monomeric IgA1 was isolated with jacalin affinity and Sephacryl S-200 HR chromatography from IgAN patients, and then heated to aggregated form (aIgA1). Human glomerular mesangial cells(HMC) were transfected with Adv-SOCS3-IRES2-EGFP for 48 hours, and incubated with aIgA1 for 12-48 h. The cells were divided into blank control group, IgA1 group, IgA1+Adv-EGFP group and IgA1+Adv-SOCS3-IRES2-EGFP group. The mesangial cell proliferation was observed through MTT, and the levels of SOCS3, TLR4, TGF-β1 protein and mRNA were detected through Western blotting and real-time PCR. Results HMC proliferation was promoted significantly after IgA1 stimulated at 24 h. Compared with control group, the protein and mRNA expression of SOCS3, TLR4, TGF-β1 were significantly increased in IgA1 group (P＜0.05). Compared with IgA1 group and IgA1+Adv-EGFP group, MTT absorbency was obviously reduced after incubation with aIgA1 for 24 h and 48 h in IgA+Adv-SOCS3-IRES2-EGFP group, and the protein and mRNA expression of TLR4 and TGF-β1 were significantly decreased in IgA1+Adv-SOCS3-EGFP group (P＜0.05). Conclusion Over-expression of SOCS3 may inhibit the proliferation of HMC stimulated by aIgA1, partly through down-regulating the expression of TLR4 and TGF-β1.     

Glycosylation and size of IgA1 are essential for interaction with mesangial transferrin receptor in IgA nephropathy

Transferrin receptor (TfR) has been identified as a candidate IgA1 receptor expressed on human mesangial cells (HMC). TfR binds IgA1 but not IgA2, co-localizes with mesangial IgA1 deposits, and is overexpressed in patients with IgA nephropathy (IgAN). Here, structural requirements of IgA1 for its interaction with mesangial TfR were analyzed. Polymeric but not monomeric IgA1 interacted with TfR on cultured HMC and mediates internalization. IgA1 binding was significantly inhibited (>50%) by soluble forms of both TfR1 and TfR2, confirming that TfR serves as mesangial IgA1 receptor. Hypogalactosylated serum IgA1 from patients with IgAN bound TfR more efficiently than IgA1 from healthy individuals. Serum IgA immune complexes from patients with IgAN containing aberrantly glycosylated IgA1 bound more avidly to TfR than those from normal individuals. This binding was significantly inhibited by soluble TfR, highlighting the role of TfR in mesangial IgA1 deposition. For addressing the potential role of glycosylation sites in IgA1-TfR interaction, a variety of recombinant dimeric IgA1 molecules were used in binding studies on TfR with Daudi cells that express only TfR as IgA receptor. Deletion of either N- or O-linked glycosylation sites abrogated IgA1 binding to TfR, suggesting that sugars are essential for IgA1 binding. However, sialidase and beta-galactosidase treatment of IgA1 significantly enhanced IgA1/TfR interaction. These results indicate that aberrant glycosylation of IgA1 as well as immune complex formation constitute essential factors favoring mesangial TfR-IgA1 interaction as initial steps in IgAN pathogenesis.     

Chronic tubulointerstitial disease in IgA nephropathy

Summary: Considerable clinical and experimental evidence points to the functional importance of tubulointerstitial disease (TID) in IgA nephropathy (IgAN). Current thinking is that the proteinuria/haematuria in IgAN causes tubular injury and that this is followed by tubular cell neoantigen expression (human leucocyte antigens [HLA] ICAM-1 and VCAM-1), with T cell (γδ cell) activation, macrophage recruitment, the release of cytokines (notably transforming growth factor-β [TGF-β] and platelet derived growth factor [PDGF]), and the growth of interstitial fibroblasts. Agents that have been effective in preventing experimental TID in animals include anti-TGF-β, decorin, IL-1 receptor antagonists and angiotensin converting enzyme (ACE) inhibitors. Thus the tubulointerstitial component of IgAN might well provide an important new window of opportunity for therapeutic intervention in this disease. Tubulointerstitial disease must be monitored more closely as a prognostic factor in future clinicopathological studies of IgAN.