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Notes: Background: Incidental IgA deposition in glomerular mesangium exists in 10–20% of autopsy kidneys1,2 or renal allograft donors.3 In the present study, we examined the clinicopathological features of incidental mesangial IgA deposition in renal biopsy from patients with minimal change nephrotic syndrome (MCNS) to understand the significance of mesangial IgA deposition in MCNS and pathogenesis of IgA nephropathy.Patients and Methods: From January 1994 to September 2000, 63 patients were diagnosed with MCNS by renal biopsy at Kidney Center, Tokyo Women’s Medical University. Mesangial IgA and C3 deposition was examined by immunofluorescence staining using frozen sections. The frequency of IgA and C3 deposition in MCNS and clinicopathological features of IgA-positive patients with MCNS were investigated.Results: The mesangial IgA deposition was present in 15 out of 63 patients (23.8%). Among these 15 patients, codeposition of C3 was present in 10 patients (66.7%) (〈link href="#f1"〉Fig. 1). The serum IgA concentration was significantly higher in the IgA-positive patients than in the IgA-negative patients (309 ± 75 mg/dL versus 245 ± 106 mg/dL, P = 0.043) (〈link href="#f2"〉Fig. 2). The urinary red blood cell count was higher in IgA-positive patients than in IgA-negative patients, although not significantly different (11.7 ± 12.7 counts/HPF versus 5.3 ± 4.0 counts/HPF, P = 0.067) (〈link href="#f3"〉Fig. 3). Other clinical parameters (age, sex, amount of proteinuria, serum creatinine and creatinine clearance) were not significantly different. Histologically, no significant differences were observed between IgA-positive and IgA-negative patients in following parameters: grade of mesangial cell proliferation and mesangial matrix increase, extents of tubular atrophy and interstitial fibrosis and grade of vascular sclerosis. After steroid treatment, all 15 patients with mesangial IgA deposition had become complete remission, although three patients once relapsed proteinuria. The haematuria also disappeared after steroid treatment in these patients.〈figure xml:id="f1"〉1〈mediaResource alt="image" href="urn:x-wiley:13205358:NEP14:NEP_14_f1"/〉The frequency of mesangial IgA and C3 deposition in MCNS patients (n = 63). The mesangial IgA deposition was present in 15 out of 63 patients (23.8%). Among these 15 patients, codeposition of C3 was present in 10 patients (66.7%).〈figure xml:id="f2"〉2〈mediaResource alt="image" href="urn:x-wiley:13205358:NEP14:NEP_14_f2"/〉The serum IgA concentration of the MCNS patients with and without mesangial IgA deposition. The serum IgA concentration was significantly higher in IgA-positive patients (n = 15) than in IgA-negative patients (n = 48) (309 ± 75 mg/dL vs 245 ± 106 mg/dL, P = 0.043).〈figure xml:id="f3"〉3〈mediaResource alt="image" href="urn:x-wiley:13205358:NEP14:NEP_14_f3"/〉The urinary red blood cell counts of the MCNS patients with and without mesangial IgA deposition. The urinary red blood cell count was higher in IgA-positive patients (n = 15) than in IgA-negative patients (n = 48), although not significantly different (11.7 ± 12.7 counts/HPF vs 5.3 ± 4.0 counts/HPF, P = 0.067).Conclusion: The incidental mesangial IgA deposition was frequently observed in MCNS patients (15/60 patients, 23.8%). The phenomenon of mesangial IgA deposition in MCNS patients was related to higher serum IgA concentration and might cause slight haematuria. However, no influence of mesangial IgA deposition was found on the renal function and the clinical outcome of MCNS after treatment.