ABCA1 and autoimmune pulmonary alveolar proteinosis: These results suggest a mechanism explaining the pathogenesis of autoimmune PAP (Fig. 4): reduced GM-CSF-PU.1-PPARγ axis signaling in alveolar macrophages → reduced ABCG1/ABCA1 expression → reduced cholesterol clearance → secondarily reduced surfactant uptake and clearance (not impaired surfactant phospholipid catabolism) → surfactant accumulation in pulmonary alveoli → clinical manifestations.