Progressive lung fibrosis in aged mice induced by repetitive bleomycin: Exacerbation by long-term PM₂.₅ inhalation

Idiopathic pulmonary fibrosis (IPF) is a progressive and fatal interstitial lung disease marked by spatially and temporally heterogeneous fibrotic remodeling. Although epidemiological studies suggest that prolonged exposure to fine particulate matter (PM2.5) accelerates IPF progression, the molecular mechanisms underlying this effect remain poorly defined. The objective of this study was to establish an experimental model that better reflects the chronicity and pathological complexity of IPF. To this end, we developed a repetitive low-dose bleomycin (BLM) model in aged mice and subjected them to long-term PM2.5 inhalation. This approach induced sustained fibrosis with histological features resembling usual interstitial pneumonia (UIP), including alveolar epithelial hyperplasia, bronchiolar metaplasia, cystic remodeling, and fibroblastic foci. Transcriptomic profiling revealed extensive gene expression reprogramming, with a greater overlap with human IPF signatures than observed in the single-dose model. Notably, 27.5 % of differentially expressed genes were shared with human IPF lungs, particularly those involved in extracellular matrix remodeling, epithelial repair, and immune modulation. PM2.5 exposure further exacerbated fibrotic remodeling, reduced survival, and increased fibrotic marker expression. Additional transcriptomic analysis identified PM2.5-specific gene signatures enriched in mitotic dysregulation, chromatin remodeling, and epithelial stress responses, with key hub genes such as BNIP3, SERPINE1, and BIRC5. In conclusion, our repetitive BLM model in aged mice effectively recapitulates the histopathological and molecular landscape of IPF and provides a physiologically relevant platform for studying chronic fibrogenesis. This model also enables mechanistic investigation of environmental exacerbators such as PM2.5, offering new insights into gene-environment interactions that drive pollutant-induced fibrotic progression.