Pirfenidone regulates fibrocyte pool size in the lungs of bleomycin (BLM)-treated mice. Lung digests stained for CD45 and collagen-I (Col-I) were examined with fluorescence-activated cell sorter analysis. (A) CD45+ Col-I + fibrocytes in lung digests were examined using the logical gates depicted on day 14 of BLM treatment. (B) CD45+ Col-I + fibrocytes in lung digests from BLM-treated mice administered pirfenidone were examined on day 14. Pirfenidone administration decreased CD45+ Col-I + fibrocytes from 26.0% to 10.4%. Results of the analyses of cells treated with isotype-matched control immunoglobulin G for anti-CD45 and anti-Col-I antibodies on day 14 of BLM treatment are shown in (C) and (D), respectively. (E) The absolute number of fibrocytes in the lungs of BLM-treated mice was increased from 1.4 × 104 cells/mL (saline treated) to 3.5 × 105 cells/mL by BLM treatment. Pirfenidone treatment attenuated this increase to 1.7 × 104 cells/mL. Data shown represent the means ± standard error from 2 independent experiments with 6 mice per group. PFD, pirfenidone.