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• Indirubin alleviated BLM-induced pulmonary fibrosis. • Indirubin inhibited fibroblast to myofibroblast differentiation by repression TGF-β/Smad signaling. • Indirubin had no significant toxicity to mice and cells. Idiopathic pulmonary fibrosis (IPF) is a chronic fibrosing interstitial lung disease with a poor prognosis. Indirubin, a compound obtained from indigo-bearing plants or mollusks of the family Muricidae, has various bioactivities, including anti-tumor activity and anti-inflammation effect. However, whether indirubin could mediate its therapeutic effects on bleomycin (BLM)-induced pulmonary fibrosis has not been addressed. The impacts of indirubin on bleomycin (BLM)-induced pulmonary fibrosis were evaluated by pathological staining, western blot, RT-PCR and immunofluorescent staining. The effects of indirubin on fibroblast differentiation and related signaling were next investigated to demonstrate the underlying mechanisms. The results indicated that indirubin-treated mice exhibited a definitively improved survival rate than that of the BLM-induced mice in a dose-depend manner. Additionally, administration of indirubin significantly alleviated inflammatory cells infiltration in BLM mice. Importantly, indirubin provided protection for mice against BLM-induced pulmonary fibrosis as manifested by the attenuating expression of fibrotic hallmarks, including fibronectin, collagen I and α-smooth muscle actin (α-SMA). Subsequently, we provided in vitro evidence revealing that indirubin suppressed fibroblast to myofibroblast differentiation by repressed TGF-β/Smad signaling in a dose-dependent manner. Notably, our data showed that indirubin seemed to be safe in mice and fibroblasts. Overall, indirubin could protect the mice against BLM-induced pulmonary fibrosis by alleviated fibroblast differentiation and may be therapeutically beneficial for IPF patients. [ABSTRACT FROM AUTHOR]