Heavy Chain/Light Chain Antibody Immunofluorescence to Identify Monoclonal Plasma Cells in a Case of Plasma Cell-Rich Acute Interstitial Nephritis.

Heavy/light chain (HLC) antibodies can be used to quantify intact HLC pairs. In immunofluorescence studies, they allow differentiation of monoclonal versus polyclonal immunoglobulin deposits in kidney diseases that occur in the setting of monoclonal gammopathy. Here, we present a case of a patient with acute kidney injury with first kidney biopsy suggestive of acute interstitial nephritis with a polymorphous infiltrate of plasma cells. Routine immunofluorescence did not show a monotypic plasma cell infiltrate. Serum protein electrophoresis and immunofixation revealed monoclonal immunoglobulin A (IgA) lambda. She improved with steroid therapy, but kidney function worsened after steroids were stopped. She underwent a second kidney biopsy, which showed plasma cell-rich interstitial infiltrate with a population of IgA lambda-restricted plasma cells on routine immunofluorescence. In light of this finding, Hevylite HLC antibody was used to reassess the first biopsy, which confirmed the presence of a population of plasma cells with IgA lambda restriction. Because of the presence of monotypic plasma cells, anti-CD38 monoclonal antibody (daratumumab) was initiated.