#410 Novel renal pathology and treatment of TIN with monoclonal plasma cells invasion in a patient with Sjögren's syndrome and IgA monoclonal gammopathy
Authors: Hesaka Atsushi , Hirata Yutaro , Kubota Takuya , Sakai Shinsuke , Doi Yohei , Namba-Hamano Tomoko , Takahashi Atsushi , Mizui Masayuki , Kaimori Jun-Ya , Isaka Yoshitaka 
Published: May 2024

Abstract

Background and Aims
Patients with Sjögren's syndrome (SS) may occasionally present with monoclonal gammopathy of undetermined significance (MGUS), of the rare IgA type. Additionally, they may present with tubulointerstitial nephritis (TIN) infiltrated by polyclonal plasmacytes and lymphocytes.
Previous reports indicate the development of IgA-type MGUS and TIN with monoclonal plasma cells in some SS patients, which are considered distinct from monoclonal gammopathy of renal significance (MGRS) because there is no evidence that monoclonal gammopathy affects nephropathy. Although treatment with prednisolone (PSL) has shown beneficial effects, the influence of monoclonal plasma cells on nephropathy remains unclear and a standardized treatment approach is lacking.
Here we present a new renal pathological insight through a case of such a patient as described above, who underwent renal biopsy before and after PSL treatment. The aim is to propose a treatment strategy based on the findings of the second renal biopsy.

Method and Results (Case Presentation)
A 75-year-old Japanese man with primary complaint of dry mouth was diagnosed with SS by lip biopsy and anti-Ro/SSA and La/SSB antibody tests. Monoclonal proteins were found in both blood and urine. After immunoelectrophoresis, bone marrow puncture, and 18F-FDG PET/CT, we identified it as IgA kappa-type MGUS. At the same time, he developed renal dysfunction and was admitted to our hospital (week 1, Fig. 1) for a renal biopsy. The biopsy revealed severe and diffuse TIN infiltrated by lymphocytes with abundant interstitial plasma cell invasion (Fig. 2a). Minimal changes were observed in the glomeruli. Periodic acid–Schiff staining revealed Dutcher bodies in the nucleus of plasma cells. Immunohistochemical staining for immunoglobulins and light chains showed predominantly positive results for IgA and kappa (Fig. 2b). Monoclonality was confirmed by in situ hybridization. The infiltrates were positive for CD 45, CD 79a, and CD 138, partially positive for CD 19 and CD 20, and negative for CD 5, CD 10, and CD 23. Congo-red staining showed no evidence of amyloid. Immunofluorescence and electron microscopy revealed no detectable immunological deposits in the glomeruli and tubules. Mutations in the myeloid differentiation primary response gene 88 were not detected in the kidney. We diagnosed the condition as acute TIN by lymphocytes with IgA kappa-positive monoclonal plasma cells, distinct from MGRS. Considering the lack of evidence from previous reports on the impact of monoclonal plasma cells on nephropathy, we decided to treat the patient primarily for TIN with SS, initiating PSL at 30 mg/day (0.5 mg/kg/day). Renal function improved rapidly with the treatment, allowing for a gradual reduction in the dose of PSL; however, a low dose of PSL (10 mg/day) was required to prevent relapse. As a complication of PSL, the patient eventually developed a lumbar compression fracture, prompting us to evaluate the effect of PSL on renal function by a renal re-biopsy. The re-biopsy revealed significant recovery of TIN by lymphocytes (Fig. 2c). Other findings, including abundant interstitial plasma cells invasion were comparable to the initial biopsy. We diagnosed the condition as recovered TIN by lymphocytes with IgA kappa-positive monoclonal plasma cells. We concluded that TIN by lymphocytes, rather than monoclonal plasma cell invasion, primarily affected the nephropathy, and PSL mainly improved the former. The Nudix hydrolase 15 codon139 in blood was identified as Cys/Cys, indicating a high risk of severe adverse reactions to azathioprine in Asians. Therefore, we decided to concurrently administer mycophenolate mofetil, proven effective in the treatment of TIN with SS.

Conclusion
In the management patients with SS and IgA monoclonal gammopathy, the focus should be on TIN by lymphocytes rather than monoclonal plasma cell invasion. This case, considering similar cases, suggests that it should be defined as a new disease category. We demonstrate new renal pathology and treatment insights based on renal re-biopsy in a patient treated with PSL.