C41-09 Pulmonary-Limited PR3-ANCA Vasculitis Presenting With Interstitial Lung Disease

Introduction Antineutrophil cytoplasmic antibody-associated vasculitis represents a spectrum of small-vessel inflammatory disorders that most commonly involve the kidneys, upper respiratory tract, and lungs. The two principal autoantigens are proteinase-3 and myeloperoxidase, which generally correspond to granulomatosis with polyangiitis and microscopic polyangiitis, respectively. Interstitial lung disease is increasingly recognized as a pulmonary manifestation of these disorders, particularly in myeloperoxidase-positive disease. Pulmonary-limited proteinase-3-positive vasculitis is rare and may closely resemble idiopathic pulmonary fibrosis, making early diagnosis challenging. Description A 73-year-old man with a history of antiphospholipid antibody syndrome, prior pulmonary embolism receiving long-term anticoagulation, and chronic thromboembolic pulmonary hypertension presented with several months of progressive exertional shortness of breath and fatigue. High-resolution computed tomography of the chest demonstrated basilar-predominant subpleural reticulation with mild traction bronchiectasis, suggestive of a usual interstitial pneumonia pattern. Laboratory evaluation revealed a strongly positive proteinase-3 antineutrophil cytoplasmic antibody, while myeloperoxidase antibody was negative. He had no renal, sinonasal, or dermatologic involvement. Rheumatology consultation supported a diagnosis of proteinase-3-associated vasculitis confined to the lungs, and treatment with rituximab was planned. Several weeks later, the patient was readmitted with worsening hypoxemia and diffuse ground-glass opacities on repeat imaging. Infectious testing, including serum procalcitonin, was negative. High-dose corticosteroid therapy led to marked improvement in oxygenation and clinical status. He was transitioned to a tapering oral regimen and discharged for outpatient immunosuppressive therapy initiation. Discussion Proteinase-3-associated vasculitis is classically linked to granulomatous inflammation of the upper airways and kidneys, whereas interstitial lung disease is more frequently seen in myeloperoxidase-positive vasculitis. The pathophysiologic mechanism of fibrosis in antineutrophil cytoplasmic antibody-associated vasculitis remains unclear but may involve neutrophil-mediated capillaritis followed by fibroproliferation. The absence of extrapulmonary findings and a fibrotic imaging pattern in this patient support the existence of a pulmonary-limited phenotype. Recognition of this presentation is critical, as immunosuppressive therapy can stabilize or reverse inflammatory activity and prevent irreversible scarring. Conclusion Pulmonary-limited proteinase-3 antineutrophil cytoplasmic antibody vasculitis is an uncommon and underrecognized cause of interstitial lung disease. Clinicians should maintain a high index of suspicion in patients presenting with fibrotic lung disease and positive autoantibody testing. Early initiation of corticosteroids and B-cell-targeted therapy may prevent progressive loss of lung function and improve clinical outcomes.