The pathogenic role of IgA1 O-linked glycosylation in the pathogenesis of IgA nephropathy

Numerous abnormalities of the IgA immune system have been reported in IgAN but the most consistent finding remains aberrant IgA1 O-linked glycosylation of the IgA1 hinge region. The defect comprises reduced galactosylation of O-linked N-acetylgalactosamine residues with or without changes in the terminal sialylation of the O-linked sugars. Aberrant O-galactosylation has been found in serum IgA1, in IgA1 isolated from tonsillar lymphocytes, and in IgA1 eluted from mesangial deposits. There is evidence that changes in IgA1 O-galactosylation lead to IgA immune complex formation and mesangial IgA deposition. Mesangial cells exposed to these IgA immune complexes proliferate and adopt a pro-inflammatory phenotype; they secrete cytokines, chemokines, growth factors and extracellular matrix components promoting glomerular inflammation and glomerulosclerosis. Recent evidence suggests that the control of IgA1 O-glycosylation is linked to class switching from IgD to IgA1 synthesis and that the pattern of IgA1 O-glycosylation may be programmed at the time of initial antigen encounter. IgA1 glycosylation varies between systemic and mucosal sites and the association of aberrant IgA1 galactosylation with low affinity, polymeric IgA1 antibodies against mucosal antigens suggests undergalactosylated IgA1 may in fact be a mucosal glycoform of IgA1. Although suited to the mucosal compartment, when these IgA1 glycoforms enter the systemic circulation in appreciable quantities they deposit in the mesangium and trigger glomerular inflammation. This review will discuss the evidence for the role of IgA1 O-glycosylation in the pathogenesis of IgAN and propose an explanation for the presence of aberrantly O-glycosylated IgA1 in the circulation of patients with IgAN.     

Abnormalities of IgA1 production in IgA nephropathy

SUMMARY: IgA nephropathy (IgAN) is characterized by the mesangial deposition of polymeric IgA1 (plgA1). the original view that this plgA1 is derived from the mucosal immune system can no longer be sustained. Studies of duodenal mucosa and marrow indicate increased production of plgA1 in the marrow and decreased production in the mucosa. These changes are consistent with immunization studies showing exaggerated and prolonged plgA responses to systemic immunization, and reduced mucosal responses to mucosal neoantigens. However, the IgA1 and IgG systemic responses to mucosal antigen are increased in IgAN, a finding consistent with impairment in oral tolerance, the process by which systemic immune responses, to mucosal antigen challenge are normally suppressed. Both IgA1 production and the induction of oral tolerance are under T-cell control. T-cell populations involved in these processes include γδ T cells, Tr cells and T-helper (Th)3 cells; cytokines with a key role in the control of IgA production include interleukin (IL)-10 and transforming growth factor (TGF)-β. There is evidence of abnormal γδ T-cell V region usage in both mucosa and marrow in IgAN. Increased expression of relevant cytokines has also been reported in circulating T cells in IgAN. the increased O-glycosylation of circulating IgA1 in IgAN may also be further evidence of a shift in the production of mucosal-type plgA1 from the mucosa to marrow. These findings suggest that the specific lymphocyte homing mechanisms that normally maintain oral tolerance and control the site of IgA production require further study in IgAN.     

MicroRNAs in IgA nephropathy

IgA nephropathy (IgAN) is the most common primary glomerulonephritis worldwide. It is considered that the pathogenesis of IgAN involves the ‘multiple hit theory’ and the immune-inflammatory mechanism; however, these theories have certain limitations. The gold standard for diagnosing IgAN is still renal biopsy. Although renal biopsy is accurate, it is traumatic and is associated with some risks and limitations, so there is a need for non-invasive diagnostic methods. According to recent studies, microRNAs (miRNAs) play important roles in the occurrence and development of IgAN; thus, they provide the possibility of the noninvasive diagnosis of IgAN and also have some value in predicting prognosis. This review summarizes the current research status of miRNAs in the occurrence, development, diagnosis, and prognosis of IgAN. We also highlight some interesting and challenging points that require further study.     

IgA serology in recurrent and non-recurrent IgA nephropathy after renal transplantation

BACKGROUND: This study investigated whether abnormal circulation of macromolecularIgA and IgA with altered glycosylation or electrical chargeplays a role in the recurrence of IgA nephropathy (IgAN) aftertransplantation. STUDY DESIGN: A total of 92 renal transplant patients were enrolled; 52 IgANpatients and 40 with other non-IgAN. The IgAN group included10 patients showing IgA mesangial deposits in the grafted kidneys(recurrent group) and 10 who did not (immunohistochemicallyproven non-recurrent group). In addition another 22 IgAN transplantpatients were clinically free of recurrent disease. METHODS: The analyses included macromolecular IgA (IgAIC) detected bythe conglutinin assay (K), heavy IgA precipitated in 2.5% polyethyleneglycol (PEG), IgA-fibronectin aggregates (IgA/F Aggr), mixedIgA/IgGIC, IgA binding to mesangial matrix components (fibronectin,laminin, type IV collagen) or polycations (poly-L-lysine) andIgA with altered glycosylation (Jacalin-binding assay). RESULTS: After transplantation, IgAN patients displayed significantlyhigher mean levels for each variable measured than non-IgAN(ANOVA, P <0.05). By stepwise regression analysis, the bindingof IgA to fibronectin had the highest coefficient. By comparingdata in recurrent and clinically non-recurrent IgAN, we observedthat two groups could be distinguished by the results of thetwo assays for macromolecular IgA (conglutinin IgAIC and IgA-fibronectinaggregates) and IgA with increased affinity for type IV collagen(P <0.05). When the selected group of immunohistochemicallyproven non-recurrent IgAN was compared to the recurrent one,a statistically significant difference was found only for thebinding of IgA to type IV collagen (P<0.05). Data from thistest were significantly related with proteinuria (P<0.05)and microscopic haematuria (P <0.04). CONCLUSION: Even though the IgA serology of renal transplant IgAN patientsshows peculiar features and recurrent and non-recurrent IgANdiffer in many aspects, the prevalence of positive data in thetwo groups had no predictive value. This suggests that the recurrenceof IgAN is modulated by factors affecting the interaction betweencirculating abnormal IgA and mesangial cells and/or matrix.     

Immunological studies of IgA nephropathy in blacks reveal elevations of serum IgA2 as well as IgA1

Although IgA nephropathy (IgAN) is recognized worldwide as themost common primary glomerulonephritis, the prevalence of thisdisease among American blacks is strikingly low despite thefrequency of other renal disorders. We have previously describedthe clinical features of 27 black patients enrolled in a multicentreIgAN database; in this paper we report several immunologicalparameters of the disease in this population. Quantificationof serum immunoglobulins revealed significantly higher concentrationsof total IgA, IgAl and IgA2 (P=0.0001, 0.002 and 0.005 respectively)in the patients, but no significant increases in IgG or IgM.Examination of immunoglobulin synthesis by peripheral bloodlymphocytes indicated relatively few differences in the secretionof immunoglobulins by patients compared to healthy Americanblacks. The spontaneous production of total IgA, IgA1, and IgA2in patients was depressed compared to the control subjects (P=0.02,0.04, 0.03,), yet the ratio of IgA1:IgA2 was normal. Stimulationwith poke-weed mitogen enhanced secretion of immunoglobulinin both subject groups. However, a significantly greater IgA1:IgA2ratio was noted in the patients (P=0.002). Circulating immunecomplexes containing C3 and IgA as well as C3 and IgM were elevatedin the patients (P=0.0006, 0.0003 and 0.02, respectively). Theseimmunological aberrancies did not correlate with clinical manifestationsof disease. These data suggest the immune abnormalities of blackIgAN patients are similar to, but not identical with, thoseof white patients.     

Animal models of IgA nephropathy: Formulating therapeutic strategies

Summary: This review seeks to highlight those aspects of ongoing research with animal models that may facilitate development of effective and perhaps specific therapy of IgAN. Oral immunization is typically accompanied by a predominantly Th2 response; defects in the evolution of this typical response, instigated by cyclophosphamide and/or oestradiol, elicit IgAN in mice. The Th2 cytokines that predominate in orally immunized mice, and in patients with IgAN, promote abnormally glycosylated IgA, similar to the IgA in patients. Immune complexes prepared with IgA antibody bearing altered glycosyl residues show enhanced rates of glomerular uptake and altered rates of clearance from the circulation. When immune responses to infectious pathogens are localized to sites remote from a new infection with a serologically related organism, ‘misdirected’ responses permit pathogen replication and antigen production concomitant with strong host antibody responses. The resultant immune complexes elicit IgAN in mice. Similar ‘misdirected’ immune responses may arise in J chain-deficient subjects. Improved pharmacologic control of B cell trafficking and/or function could rectify these defects. Correction of deficiency in clearance of IgA immune complexes could benefit patients with secondary IgAN. Finally, manipulation of glomerular haemodynamics and/or production or response to intraglomerular cytokines and growth factors might ameliorate disease or forestall progression to glomerulosclerosis. Animal models have served as a conceptual springboard for a wide variety of clinical investigations, and in turn clinical information has guided the design and goal of experimental investigation. Hopefully, this synergy can continue, ultimately resulting in effective and specific therapy.     

Suppressive effects of Perilla frutescens on IgA nephropathy in HIGA mice.

BACKGROUND: Perilla frutescens (perilla) is a herbal medicine used in Japanese traditional Kampo medicine. The present study was conducted to evaluate the anti-nephritic effects of perilla in HIGA mice that spontaneously develop high levels of serum immunoglobulin A (IgA) along with mesangial IgA deposition. METHODS: A perilla decoction and its major active constituent, rosmarinic acid (RsA), were orally administrated to 10-week-old HIGA mice for 16 weeks. At study completion, we measured proteinuria and serum IgA levels and generated histological scores from kidney specimens. In addition, we measured concentrations of IgA in culture media of intestinal Peyer's patch cells and spleen cells obtained from the HIGA mice. RESULTS: Perilla suppressed proteinuria, proliferation of glomerular cells, serum levels of IgA, glomerular IgA and IgG depositions in HIGA mice. Cultured Peyer's patch cells and spleen cells from perilla-treated mice produced significantly less IgA than controls. Rosmarinic acid, by itself, suppressed serum IgA levels and glomerular IgA deposition in HIGA mice. Cultured spleen cells from RsA-treated mice produced less IgA than controls. CONCLUSIONS: The perilla decoction may suppress IgA nephropathy, in part, through modulation of the intestinal mucosal immune system. These effects were caused by RsA acting synergistically with other constituents.     

Low-antigen-content diet in the treatment of patients with IgA nephropathy

Since dietary macromolecular antigens can be involved in thepathogenesis of IgA nephropathy (IgAN), the effect of a low-antigen-contentdiet was evaluated in 21 patients (10 women, 11 men, mean age27.7±10 years) with immunohistochemical findings of activeIgAN. The diet was followed for a 14–24-week period (mean18.8±6); in all cases the effects of the treatment wereevaluated by clinical and serological parameters, and in 11patients also by repeat renal biopsy. After dietetic therapy a significant reduction of urinary proteinswas recorded (P< 0.001); in particular, heavy proteinuria(>1 g/day), present in 12 cases during the 6 months precedingthe treatment, was markedly reduced or disappeared in 11. Atposttreatment control biopsy mesangial and parietal depositsof immunoglobulins, complement C₅, fraction and fibrinogen weresignificantly reduced. The improvement of the objective parameterssuch as heavy proteinuria, a strong predictor of a poor prognosis,and of immunohistochemical alterations indicate that a low-antigendiet can positively affect patients with IgAN. These resultscould be ascribed to a reduction of nephritogenic food antigeninput and to a putative functional restoration of the mononuclearphagocytic system.     

Lack of association between NPHS2 gene polymorphisms and sporadic IgA nephropathy

AIM: IgA nephropathy (IgAN) is the most common primary form of glomerulonephritis worldwide. In the present study, the genetic structure of the NPHS2 gene was studied to verify if podocin plays a role in the pathogenesis of IgAN. METHODS: Clinical characteristics and DNA samples were collected from 26 Chinese children with sporadic IgAN. A direct sequencing was performed after polymerase chain reaction amplification to all the eight exons of the NPHS2 gene. RESULTS: Three synonymous variants as known polymorphisms (954T-->C homozygous, 1038A-->G heterozygous and homozygous) were found in 3, 4 and 1 patients, respectively. There was no significant difference in the genotypic and allelic frequencies of 954T > C and 1038A > G polymorphisms between the patients and normal controls. CONCLUSION: No significant difference in the genotypic and allelic frequencies of the identified 954T > C and 1038A > G polymorphisms between the patients and normal controls was found.     

Henoch-Schönlein syndrome and IgA nephropathy: A case report suggesting a common pathogenesis

An 8-year-old Caucasian male presented with two episodes of gross hematuria but was otherwise asymptomatic. Serum IgA levels were markedly elevated and a renal biopsy showed mesangial proliferative glomerulonephritis with immunofluorescent and electron microscopy findings consistent with IgA nephropathy (IgAN). Two years later he developed abdominal pain, rectal bleeding, gross hematuria and a classic purpuric rash of Henoch-Schönlein syndrome (HSS). Serum IgA levels continued to be elevated and 3 years later in follow-up he is clinically well. These observations support the concept that HSS and IgAN are variants of the same process. The reverse situation has been reported in a 15-year-old female who developed HSS at 4 years of age and IgAN at age 15 years.     

IgA肾病患者肾组织中增殖细胞核抗原表达

用免疫组化方法研究了２０例原发性ＩｇＡ肾病患者肾组织中增殖细胞核抗原（ＰＣＮＡ）表达。结果显示ＩｇＡ肾病患者肾组织中ＰＣＮＡ表达增加，肾小球内ＰＣＮＡ阳性细胞数、肾小管和间质中ＰＣＮＡ阳性细胞百分数均与肾组织学损害程度呈正相关；临床－病理研究显示肾小管中ＰＣＮＡ阳性细胞百分数分别与２４小时尿蛋白量、血清肌酐浓度（Ｓｃｒ）呈正相关；肾间质中ＰＣＮＡ阳性细胞百分数亦与２４小时尿蛋白量呈正相关。肾脏细胞增殖程度作为一项判断肾小球肾炎组织学损害程度和预后的指标值得进一步研究。     

Pathogenesis of idiopathic IgA nephropathy

Despite a prodigious amount of work on the physiology of IgA production in man, and many studies on the immunopathology of IgA nephropathy, ranging from the immunogenetics to the immune response to chemical characteristics of the IgA, we are hardly any nearer to defining the pathogenesis of this disease. One of the main changes in our understanding has been to recognise that the bone marrow, now known to produce normally one-third of the body's IgA, overproduces this immunoglobulin in IgA nephropathy. This alters the previous notion that IgA nephropathy was due simply to IgA production in the mucosa, although a mucosal component is not excluded. Certain characteristics of the IgA in the diseased kidney and the circulation have been defined: it is of subclass IgA1 and has a higher proportion of light chains and negative charge than in normal subjects. The specificities of the IgA, either in the kidney or in complexes, have not helped to clarify the pathogenesis. They have been found for a wide range of endogenous and exogenous antigens, suggesting that the antibody activity represents polyclonal B cell activation. These findings have not helped to confirm the prevailing theory that IgA nephropathy is an immune complex disease. Other theories put forward are that IgA nephropathy is an autoimmune disease, glomerular components or IgA itself being among the candidate antigens, or that there is primary dysregulation of the IgA immune system. At this stage of development in our understanding of this common nephropathy, it is important to guard against the assumption that idiopathic IgA nephropathy is one disease and is the result of a single pathogenetic mechanism.     

A case report suggesting a common pathogenesis for IgA nephropathy and Henoch-Schönlein purpura

A 7-year-old boy who had been followed for asymptomatic haematuria and elevated serum IgA levels developed Henoch-Schönlein purpura (HSP) after a streptococcal infection of the tonsils. Findings on renal biopsy were compatible with mild IgA nephropathy (IgAN); tonsillectomy was also performed as he had chronic tonsillitis. This case suggests that there is a common pathogenesis for IgAN and HSP, at least in some patients.     

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

BACKGROUND: Sera of IgA nephropathy (IgAN) patients contain high levels of circulating immune complexes composed of IgA1 molecules with aberrantly glycosylated hinge-region O-linked oligosaccharides and IgG or IgA1 antibodies with anti-glycan or anti-hinge-region peptide specificities. Due to damaged sieving properties of the glomerular capillary wall in IgAN, these immune complexes may appear in the urine. METHODS: We collected urine samples from 29 patients with biopsy-proven IgAN (Group I), 27 proteinuric patients with non-IgA nephropathies (Group II) and 28 healthy volunteers (Group III). The levels of urinary IgA and IgG and IgA-IgG-containing immune complexes were measured by ELISA and standardized for urinary creatinine concentrations. RESULTS: The urinary IgA and IgG levels were significantly higher in Groups I and II than in Group III. Although the excretion of IgA as a fraction of total urinary protein was not significantly greater in IgAN patients than in patients with other renal diseases, the excretion of aberrantly glycosylated IgA1 was observed by western blot in 68% of the IgAN patients but in none of the healthy controls. The urinary levels of IgA-IgG immune complexes were significantly higher in Group I than in Groups II (P < 0.01) and III (P < 0.05). There was no significant difference in the levels between Groups II and III. These immune complexes had a molecular mass between 650-850 kDa, as shown by size-exclusion chromatography. CONCLUSION: The amounts of urinary IgA-IgG-containing immune complexes were significantly higher in patients with IgAN than in patients with non-IgA nephropathies or healthy controls.     

Specific binding of circulating IgA antibodies in patients with IgA nephropathy

Detection of circulating IgA antibodies which are specific in patients with IgA nephropathy is described. Freeze and thawed extracts of pharyngeal cells obtained from patients with IgA nephropathy, other glomerular diseases, and healthy adults were cultured with fibroblasts such as Vero or Hel cells at 37 degrees C for 2 weeks. Serum samples were obtained from these patients and healthy adults. The cultured fibroblasts were fixed on slide glasses, and then incubated with the serum samples from the same or other patients with IgA nephropathy. The cells were stained with FITC-labeled heavy-chain specific anti-human IgA antiserum and then examined with a fluorescent microscope. It was demonstrated that the IgA antibodies in sera obtained from patients with IgA nephropathy or HSP nephritis were bound with the nuclear regions of such fibroblasts. It was suggested that IgA antibodies in sera could be bound with some antigenic substances which were transferred from pharyngeal cells of patients with IgA nephropathy to fibroblasts in vitro.