Myeloperoxidase Antineutrophil Cytoplasmic Antibody (MPO-ANCA)-Associated Usual Interstitial Pneumonia: Pathogenic Mechanisms, Diagnostic Challenges, and Clinical Implications

Usual interstitial pneumonia (UIP) has traditionally been regarded as synonymous with idiopathic pulmonary fibrosis (IPF) in the absence of an identifiable cause. However, emerging evidence demonstrates that a subset of patients with a UIP pattern exhibit myeloperoxidase antineutrophil cytoplasmic antibodies (MPO-ANCAs), suggesting a distinct and evolving clinical phenotype at the intersection of fibrotic lung disease and systemic autoimmunity. This structured narrative review aims to synthesize current evidence regarding the clinical, diagnostic, and therapeutic implications of MPO-ANCA-associated UIP. Preferred Reporting Items for Systematic Reviews and Meta-Analyses-like methodology was employed to identify and select relevant studies from PubMed (MEDLINE) and Google Scholar published between 2020 and 2026. After screening and eligibility assessment, 18 studies were included in the qualitative synthesis. Data were extracted on clinical presentation, radiological features, serological findings, therapeutic strategies, and outcomes. The available evidence indicates that pulmonary fibrosis may precede or coexist with ANCA-associated vasculitis, particularly microscopic polyangiitis, supporting the concept of UIP as a potential early or lung-dominant manifestation of systemic disease. This phenotype is characterized by a dual trajectory, combining progressive fibrosis with the risk of severe vasculitic complications such as diffuse alveolar hemorrhage. Radiological findings alone are insufficient for differentiation, highlighting the importance of MPO-ANCA testing for early phenotyping. Therapeutic management remains challenging due to the lack of standardized guidelines, requiring individualized and multidisciplinary approaches that balance antifibrotic and immunosuppressive strategies. MPO-ANCA-associated UIP should be recognized as a distinct and dynamic clinical entity with significant diagnostic and prognostic implications. Early identification and structured follow-up are essential to optimize outcomes and guide timely therapeutic interventions.