After the catheter was inserted into the trachea of the mice, 200 μL of cold PBS (10010049, Gibco, MA, USA) was injected into the lung using a syringe. Then, BALF samples were collected into the syringe. After the BALF samples were centrifuged for 10 min (300 × g), the supernatants were stored at −80°C for ELISA experiment. The precipitate, which was the cells in BALF, was further re-suspended in 200 μL of PBS and used for hematoxylin-eosin staining. Cells were counted by automatic cell counting and vitality analysis system.