The Rare Occurrence of Tissue PLA2R-Positive Membranous Nephropathy in a Patient With Sjögren's Syndrome and Renal Tubular Acidosis

<b>Aims/Background</b> Membranous nephropathy (MN) is categorized into primary and secondary forms based on its etiology, with autoimmune diseases significantly contributing to the secondary form. The phospholipase A2 receptor (PLA2R) antibody has been widely used as a specific marker for primary MN. However, explaining PLA2R positivity in MN patients who also exhibit typical features of Sjögren's syndrome (SS) is challenging. <b>Case Presentation</b> A 51-year-old male patient presented with acute systemic edema, hypertension, macroalbuminuria, and hypoproteinemia, confirming a diagnosis of nephrotic syndrome. Concurrently, SS, characterized by dry mouth and dry eye symptoms, positive specific antibodies, and positive labial gland biopsy results, was also confirmed. Renal pathology indicated MN (stage I-II) with moderate secondary chronic renal tubulointerstitial lesions. The cluster of differentiation 3 (CD3)+ and CD20+ immunostaining of renal tissue was consistent with renal interstitial damage associated with SS, and the clinical manifestations aligned with the typical presentation of renal tubular acidosis (RTA). PLA2R was negative in the serum but positive in the renal tissue. <b>Conclusion</b> SS is one of the autoimmune diseases associated with secondary membranous nephropathy, which has a low clinical incidence. In this particular case, the diagnosis of SS was confirmed, and the histopathological examination revealed typical tubulointerstitial lesions clinically accompanied by RTA. However, the specific antibody PLA2R, which is typically associated with primary MN, was positive in renal tissue. Additionally, IgG immunodeposition was not observed. Further investigation is warranted to ascertain whether the MN is secondary to SS.