Objective:To explore the function of IL-9 and PU.1 on genesis and development of pulmonary fibrosis,and the effect of active vitamin D3[1,25(OH)2VD3] on the expression levels of this two factors during the pathogenesis of fibrosis.Methods: 90 male SD rats were randomly divided into model group,treatment group,control group (n=30).Bleomycin(5 mg/kg) was injected into the trachea of rats to establish the model of pulmonary fibrosis in the model group and treatment group,while the control group was injected with isopyknic sterile saline.The treatment group,the model group and the control group were injected intraperitoneally with active vitamin D3,solvent of vitamin D3 (propylene glycol) and sterile saline on the 2nd day after surgery respectively.All injections were carried out once every other day.10 rats were euthanized at 14th,21st and 28th day in each group in turn.After obtaining lung tissues from experimental rats,the pathological change of lung was compared by hematoxylin-eosin staining.The difference of collagen fiber and hydroxyproline content were compared by the Masson staining and basic-hydrolysis method respectively.The mRNA and protein expression of IL-9 and PU.1 in lung tissue were detected by Real-time PCR and immunohistochemical technology respectively.The expression of IL-9 in serum was detected by ELISA.Results: Fibrosis appeared in lungs of experimental rats treated with bleomycin after 14 days,and more and more aggravated with time.At three time points,the hydroxyproline content in model group and treatment group were significantly higher than that of control group,and the treatment group was significantly lower than the model group.At three time points,the expression of IL-9 and PU.1 in model group and treatment group were risen gradually,and obviously higher than that in control group.On the 14th and 21st day,the expression of two factors in treatment group was significantly lower than model group;on the 28th day,there was no statistically significant difference between treatment group and model group(P>0.05).In model group and treatment group,the expression of two factors on 21st day was significantly higher than that on 14th day;there was no statistically significant difference between the 28th day and the 21st day.Conclusion: IL-9 and PU.1 may play a profibrotic role at early stage of pulmonary fibrosis induced by bleomycin.The active vitamin D3 may lower the expression level of PU.1,and then reduce the secretion of IL-9,thus may play an inhibiting effect on genesis and development of pulmonary fibrosis in rats.