Case Report: A rare case of concurrence of IgG4-related tubulointerstitial nephritis and renal Amyloid A amyloidosis

Immunoglobulin G4-related disease (IgG4-RD) is a systemic immune-mediated fibroinflammatory condition affecting multiple organs. IgG4-related tubulointerstitial nephritis (IgG4-TIN) is the predominant pattern of kidney involvement. Amyloid A (AA) amyloidosis is a systemic amyloidosis that develops secondary to chronic inflammation or infection, most frequently affecting the kidneys. The association between IgG4-RD and AA amyloidosis is rarely reported. Herein, we report a case of a 56-year-old Chinese man presenting with a one-year history of dizziness and fatigue. The clinical evaluation and laboratory findings showed multiple enlarged lymph nodes, elevated serum creatinine, and increased levels of IgG4 and C-reactive protein. A kidney biopsy revealed IgG4-TIN. Furthermore, patchy congophilic amyloid deposits in the interstitium and arteriolar walls were positive for AA protein by immunohistochemical staining. Subsequent cervical lymph node biopsy showed IgG4-related lymphadenopathy. With prednisone and cyclophosphamide treatment, the patient achieved complete remission of renal function and a noticeable decrease in IgG4 and C-reactive protein levels. This is the first reported case to our knowledge of IgG4-related lymphadenopathy, IgG4-TIN, concurrent with renal AA amyloidosis. Clinicians should be aware that AA amyloidosis may occur in patients with IgG4-TIN, warranting further investigation into the underlying mechanisms linking AA amyloidosis to IgG4-RD.