Tubulointerstitial nephritis with predominant monotypic plasma cell infiltration in three cases of Sjögren's syndrome: case reports and literature review.

Tubulointerstitial nephritis (TIN) is a common kidney manifestation of Sjögren's syndrome, typically characterized by interstitial infiltration of T and B cells, with plasma cells being more prominent than in other etiologies. While most plasma cell infiltration in TIN is polytypic and the importance of the immunoglobulin isotypes expressed by the infiltrating plasma cells in the pathogenesis is unknown, recent reports have identified rare cases with monotypic IgA-positive plasma cell infiltration accompanied by monoclonal gammopathy of undetermined significance (MGUS). Here, we present three cases of Sjögren syndrome-associated TIN characterized by monotypic plasma cell infiltration. In all cases, the isotype of the infiltrating plasma cells was consistent with that of the predominant serum immunoglobulin. One patient exhibited IgA-type MGUS, one had IgM-type MGUS, and one showed elevated serum IgG levels along with IgG-positive plasma cell infiltration, but without detectable paraproteinemia. Notably, serum immunoglobulin levels decreased in parallel with improvement in kidney function. These findings suggest a potential link between the systemic immunoglobulin profile and local immunopathology of the kidneys in Sjögren's syndrome. Further case accumulation is needed to clarify the clinical significance and pathophysiology of monotypic plasma cell infiltration in TIN associated with Sjögren's syndrome.