Objective: To investigate the protective effect of dexmedetomidine (DEX) on bleomycin (BLM) -induced pulmonary fibrosis in mice and its effect on toll-like receptor 4/myeloid differentiation factor88 (TLR4/MyD88) signaling pathway. Methods: Experimental mouse pulmonary fibrosis was induced by injection of BLM. The mice were grouped into: saline (Sham) group

BLM group

BLM+DEX group

BLM+PFD group

and BLM+DEX+LPS group. The pathological changes of lung tissue were analyzed by hematoxylin and eosin (HE) and Masson staining; the expression of fibronectin (Fn)

α-smooth muscle actin (α-SMA) and type Ⅰ collagen (collagen Ⅰ) was measured by immunohistochemistry; the apoptosis in lung tissue was measured by TdT mediated dUTP nick end labeling (TUNEL) ; the total cell number and protein content in bronchoalveolar lavage fluid (BALF) were measured; and the TLR4/MyD88 signaling pathway and apoptosis-related protein levels in lung tissue were detected by Western blot. Results: Compared with the Sham group

the alveolitis score [(2.56±0.24) points vs. (0.15±0.02) points] and Ashcroft score [(5.68±0.52) points vs. (0.09±0.01) points] in lung tissue after BLM stimulation increased significantly

the expression of Fn [(63.63±5.48) % vs. (25.12±2.16) %]

α-SMA [(58.63 ± 5.03) % vs. (17.56±1.25) %] and collagenⅠ [(55.32 ± 5.16) % vs. (12.03±1.20) %] increased (P<0.05)

and the degrees of lung inflammation and fibrosis increased. Meantime

the rate of TUNEL positive cells increased

the level of bcl-2 associated X protein (Bax) protein in the cells increased

and the level of B-cell lymphoma-2 (Bcl-2) protein decreased (P<0.05) ; the total cell number [(3.54±0.06) ×105/mL vs. (0.98±0.07) ×105/mL] and protein content [(2.85±0.20) mg/mL vs. (0.29±0.03) mg/mL] increased in BALF (P<0.05) ; the protein levels of TLR4 and MyD88 in lung tissue were increased (P<0.05). DEX treatment attenuated BLM-induced pulmonary fibrosis and inflammation in mice

decreased the rate of TUNEL-positive cells

and reversed the expression of Fn

α-SMA

collagen I

Bax and Bcl-2 (P<0.05) ; meanwhile

DEX also decreased the total cell number and protein content in BLM-induced mouse BALF (P<0.05) ; in addition

DEX reduced TLR4 and MyD88 protein levels in BLM-induced mouse lung tissue (P<0.05). Pirfenidone (PFD) and DEX had similar effects; lipase (LPS) was able to partially reverse the protective effect of DEX on BLM-induced pulmonary fibrosis in mice (P<0.05). Conclusion: DEX may improve BLM-induced pulmonary fibrosis in mice by inhibiting TLR4/MyD88 signaling pathway.