Microscopic polyangiitis atypically presenting with tubulointerstitial nephritis

A 65-year-old woman was admitted with a 4-week history of non-specific constitutional symptoms. Microscopic hematuria, proteinuria and mild renal insufficiency together with the presence of serum antineutrophil cytoplasmic antibodies prompted to perform a renal biopsy. The specimen showed tubulointerstitial nephritis without glomerular change. However, she developed purpura on lower limbs and hemoptysis, along with diffuse pulmonary infiltrates on chest radiograph, 2 weeks after admission. Skin and lung biopsies demonstrated leukocytoclastic vasculitis and diffuse alveolar hemorrhage, respectively. Microscopic polyangiitis was diagnosed based on clinical and pathological criteria. Clinical improvement occurred after intensive immunosuppressive therapy was given. This case illustrates an unusual renal presentation of tubulointerstitial nephritis in microscopic polyangiitis. The possible pathogenetic mechanism will be discussed.