Concurrent isolated IgG2-positive membranous nephropathy and malignant B-cell lymphoma

A recent systematic review showed that hematological malignancy is often complicated by membranous nephropathy (MN). Histologically, the deposition of IgG subclasses other than IgG4 may imply secondary MN, such as malignancy-associated MN (M-MN). We describe a very rare case of concurrent isolated IgG2-positive MN and B-cell lymphoma. An 83-year-old woman was hospitalized at our institute for facial and lower extremity edema persisting for 2 months. Laboratory tests showed urinary protein level of 10.8 g/day, serum albumin level of 1.6 g/dl, and serum creatinine level of 2.34 mg/dl. Soon after diagnosis of nephrotic syndrome, treatment with corticosteroid was initiated, but it proved to be ineffective. Renal biopsy showed isolated IgG2-positive MN with highly infiltrated CD20-positive lymphoid cells in the kidney. Computed tomography revealed systemic lymphadenopathy, and aberrant B-cells with immunoglobulin light chain restriction were detected in peripheral blood and bone marrow, which led to the diagnosis of mature B-cell lymphoma. Although rituximab (375 mg/m²/week) was administered, the patient suddenly died from gastrointestinal bleeding on day 40 of hospitalization. It is, thus, necessary to consider hematological malignancy when a diagnosis of MN is made. Further studies are expected to elucidate the pathogenesis and to help establish the adequate treatment for this rare situation.