In idiopathic pulmonary fibrosis (IPF), the usual interstitial pneumonia (UIP) pattern on HRCT/histology is characterized by basal, subpleural honeycombing with fibroblastic foci. Which molecular pathway is considered central to the pathogenesis of irreversible fibrosis in IPF?
- A TGF-β1-mediated epithelial-to-mesenchymal transition (EMT) and myofibroblast activation: repetitive alveolar epithelial injury leads to aberrant TGF-β1 secretion by AEC2 cells, activating SMAD2/3-driven transcription of collagen I/III, alpha-SMA, and fibronectin in fibroblasts, establishing a self-reinforcing fibrotic loop ✓
- B Th2 cytokine IL-4/IL-13 drives M2 macrophage polarization, which directly secretes collagen and constitutes the bulk of fibrotic tissue
- C Chronic neutrophilic inflammation causes elastase-mediated matrix destruction and subsequent collagen overrepair in a manner identical to emphysema
- D Viral (EBV, CMV) persistence in type II pneumocytes drives IFN-γ production, causing macrophage-driven collagen secretion
Explanation
IPF pathogenesis centers on recurrent micro-injury to alveolar epithelial type 2 (AEC2) cells (from inhaled agents, ER stress, telomere shortening due to TERT/TERC mutations) leading to aberrant repair. TGF-β1 secreted by injured AEC2 cells and alveolar macrophages is the master fibrogenic cytokine: it activates SMAD2/3 signaling in fibroblasts and fibrocytes, driving their differentiation into myofibroblasts (alpha-SMA+) and transcription of collagen I, III, fibronectin, and CTGF. Myofibroblasts form the 'fibroblastic foci' pathognomonic of UIP. Pirfenidone (TGF-β inhibitor) and nintedanib (PDGFR/VEGFR/FGFR TKI) slow disease progression by targeting this pathway.
Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.
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