(A) Schematic representation of the animal experiment involving PDGFRα^CreERT (control) and PDGFRα^CreERT WT1^OE (cWT1^OE) mice. Mice were treated repeatedly with bleomycin via intratracheal administration and with tamoxifen via intraperitoneal injection. (B) Quantification of Wt1 gene transcripts by RT-PCR in total lung RNA isolated from control and cWT1^OE mice (n = 6/group). **P < 0.01, by 2-tailed Student’s t test. (C) Representative confocal images of lung sections from control and cWT1^OE mice co-immunostained for WT1 (red), vimentin (green), and DAPI (blue). Scale bars: 20 μm. (D) Representative images of Masson’s trichrome–stained lung sections from control and cWT1^OE mice. Original magnification, ×4 and ×20, respectively. Scale bars: 1,500 μm and 200 μm, respectively. (E) The percentage of fibrotic area was quantified in control and cWT1^OE mice using BZ-X image analysis (n = 6/group). ****P < 0.0001, by 2-tailed Student’s t test. (F) Hydroxyproline levels were measured in the right lungs of control and cWT1^OE mice (n = 6/group). ***P < 0.001, by 2-tailed Student’s t test. (G) Lung resistance was assessed using FlexiVent in control and cWT1^OE mice treated with bleomycin (n = 6/group). *P < 0.01, by 2-tailed Student’s t test. (H) Proliferation of fibroblasts was quantified using a BrdU incorporation assay in lung cultures from control and cWT1^OE mice treated with bleomycin (n = 3/group). *P < 0.05, by 2-tailed Student’s t test. (I) Fibroblasts from the lung cultures of control and cWT1^OE mice treated with bleomycin were treated with anti-Fas antibody for 24 hours, followed by TUNEL staining (red). Representative confocal images are shown; nuclei are stained with DAPI (blue). Original magnification, ×20. Scale bars: 100 μm. (J) Percentage of TUNEL⁺ fibroblasts in total DAPI⁺ fibroblasts (n = 3/group). *P < 0.05 and **P < 0.01, by 1-way ANOVA. Data are representative of 2 independent experiments with similar findings.