A Case of IgA Nephropathy With Membranoproliferative Glomerulonephritis‐Like Features

ABSTRACT A 73‐year‐old man was referred due to the onset of nephrotic‐range proteinuria. He had been diagnosed with rheumatoid arthritis 18 years prior and had achieved remission with treatment, including methotrexate and janus kinase (JAK) inhibitor. Although routine follow‐ups had not revealed any urinary abnormalities, subsequent tests detected proteinuria and hematuria in the absence of infection or other symptoms. As the urinary abnormalities persisted, with a serum albumin decrease and proteinuria measuring 5.7 g/day, indicating nephrotic syndrome, the patient was referred to our hospital for further evaluation, and a renal biopsy was performed. Light microscopy revealed mesangial cell proliferation, endocapillary proliferation and double‐contoured basement membranes. Immunofluorescence microscopy showed IgA‐dominant deposits in both mesangial areas and glomerular capillary walls. Transmission electron microscopy demonstrated electron‐dense deposits in the mesangium and subendothelial regions, leading to the diagnosis of membranoproliferative glomerulonephritis (MPGN)‐type IgA nephropathy. Immunostaining with the Gd‐IgA1 (galactose‐deficient IgA1)‐specific antibody (KM55) was positive, consistent with the diagnosis. Following the initiation of steroid therapy, proteinuria rapidly decreased, achieving complete remission within 5 months. IgA nephropathy with MPGN‐like features often presents as nephrotic syndrome, differing from the typical pathological and clinical presentation of IgA nephropathy, making differentiation from secondary MPGN and other diseases sometimes challenging. This case suggests that KM55 staining may offer additional information in differentiating atypical IgA nephropathy with non‐classical pathological features.