Monoclonal Interstitial Nephritis with IgA-Kappa Plasma Cells Deposition in Sjögren Syndrome: A Case Report

Introduction: In Sjögren syndrome, tubulointerstitial involvement is a frequent finding, characterized by lymphoplasmacytic infiltration, tubular atrophy, and interstitial fibrosis. Nevertheless, descriptions of monoclonal interstitial infiltrates remain scarce. These observations suggest that beyond the classical spectrum of glomerular or tubular injury, a subset of patients with Sjögren syndrome may present atypical renal manifestations potentially associated with underlying monoclonal gammopathy of renal significance. This highlights the importance of accurately characterizing infiltrates and carefully considering monoclonal gammopathy of renal significance (MGRS) as a potential contributor to disease heterogeneity and long-term kidney outcomes. Case Presentation: An 83-year-old woman with Sjögren syndrome presented with a rapidly progressive decline in kidney function, without proteinuria. Immunologic and viral workup was unremarkable. A kidney biopsy revealed marked interstitial plasma cell infiltration, as demonstrated by CD138+ staining and IgA-kappa restriction by immunohistochemistry, with no glomerular involvement. Bone marrow evaluation showed <10% plasma cells, excluding multiple myeloma. Serum electrophoresis confirmed an IgA-kappa monoclonal spike and abnormal free light chain ratio. Chemotherapy with bortezomib, dexamethasone, and lenalidomide resulted in an improvement of kidney function and normalization of IgA levels. Conclusion: The interstitium is frequently overlooked in MGRS classification; however, interstitial monoclonal plasma cell infiltration has recently been described in a subgroup of patients with Sjögren syndrome. It may represent a novel entity within MGRS, potentially enhancing prognosis through plasma cell-directed therapies.