An 8-month-old boy with infantile nephrotic syndrome caused by semaphorin 3B-associated membranous nephropathy.

We present a case of nephrotic syndrome caused by semaphorin 3B-associated membranous nephropathy. The patient was an 8-month-old male infant who presented with severe proteinuria and hypertension. He was treated with prednisolone (PSL) for nephrotic syndrome; however, remission was not achieved within 4 weeks. He was diagnosed with steroid-resistant nephrotic syndrome and underwent kidney biopsy. Pathological examination revealed membranous nephropathy with IgG deposits on both the glomerular basement membrane (GBM) and the tubular basement membrane (TBM). He was treated with cyclosporine (CsA) in addition to PSL and achieved complete remission. However, frequent relapses occurred after the discontinuation or tapering of immunosuppressants. Two years after treatment initiation, a second biopsy was performed and showed worsening of the disease, which required treatment with several immunosuppressants to achieve complete remission. After that, we performed additional immunostaining for semaphorin 3B, which confirmed the diagnosis of semaphorin 3B-associated membranous nephropathy. Although extremely rare in infantile cases, semaphorin 3B-associated membranous nephropathy should be considered in the differential diagnosis, as strong treatment with immunosuppressants might be needed. In addition, mycophenolate mofetil showed an effective clinical response in this case, indicating that it can be considered for future treatment strategies.