Objective To observe the expression of YKL-40 protein in the pulmonary tissues of rats with bleomycin-induced pulmonary fibrosis(PF). MethodsThe PF model group was induced with intratracheal instillation of bleomycin solution (7.5 mg/kg) and the control group was treated with normal saline. On day 7, 14, 21 after bleomycin challenge, rats were sacrificed and the pulmonary tissues were harvested. H-E staining, Masson staining and Szapiel score were employed to determine alveolitis and pulmonary fibrosis. Expressions of YKL-40 in lung tissues were detected by real-time RT-PCR, Western blotting analysis and immunohistochemistry method. ResultsBleomycin instillation induced alveolitis in the lung of rats, with inflammation score being significantly higher on day 7 (2.8±0.45, P<0.01) and on day 21 (1.8±0.84, P<0.05) compared with that of control group (0.42±0.25). Pulmonary fibrosis degrees in model group was significantly higher on day 14 (1.7±0.73, P<0.05) and on day 21 (2.9±0.56, P<0.01) compared with that of control group (0.2±0.45). YKL-40 mRNA (YKL-40/β-actin) expression was significantly increased on day 7 (3.71±0.25) after bleomycin treatment compared with the control group (P<0.05). Western blotting analysis showed that YKL-40 protein expression was significantly increased on day 14(0.56±0.24,P<0.05) and on day 21(1.15±0.19, P<0.01) after bleomycin treatment compared with the control group (0.23±0.07). The results of immunohistochemistry showed that bleomycin up-regulated YKL-40 expression in a time-dependent manner, and YKL-40 expression was mainly located in the smooth muscle cells, alveolar macrophages, alveolar epithelium and fibroblasts. ConclusionYKL-40 expression may contribute to the pulmonary fibrosis, and may participate in the pathogenesis of pulmonary fibrosis.