Abstract： Objective To establish the ischemia reperfusion injury model in rat after lung transplantation(LT)and explore the expression of high mobility group box 1 protein(HMGB1)after intravenous injection with bone marrow mesenchymal stem cells(MSCs).Methods Forty healthy 8-10 weeks male SD rats were randomly divided into four groups including the sham-operated group,ischemia-reperfusion(IR),Saline-IR and MSC-IR group.The sham-operated rats were only conducted thoracotomy without lung transplantation and the rest groups were respectively conducted with the left LT,left LT followed by 1 ml saline and left LT followed by 1 ml MSCs(1.0×107/ml).Four groups of rats were killed at 24 h after reperfusion.The blood and left lung tissue were collected.Oxygenation index(OI)and the ratio of wet/dry in four groups were detected and histological sections stained with hematoxylin and eosin(HE)were made.HMGB1 levels in serum were detected with ELISA.Real-time PCR and Western blot were performed to detect the expression of HMGB1 in mRNA and protein levels.Results The OI in four groups were respectively 383 ±15,174 ±24,170 ±30 and 217 ±21.OI in IR and Saline-IR group decreased compared with the sham-operated group,all P＜0.01.The OI increased after injection with MSCs compared with IR group,P＜0.01.The histological images showed the marked inflammatory infiltrates and interalveolar septal thickening in IR group.Treatment with MSCs reduced inflammatory injury.The ratio of wet/dry in IR group and Saline-IR group increased compared with the sham-operated group((5.38 ±0.19),(5.24 ±0.15)vs(4.16 ±0.12),all P＜0.05).Ratio in MSC-IR group decreased compared with the IR group(4.47 ± 0.14)vs(5.38 ±0.19),P＜0.05.ELISA results showed that HMGB1 level increased significantly in IR group(287 ±37)ng/ml,Saline-IR group(260 ±24)ng/ml and MSC-IR group(101 ±14)ng/ml when compared with the sham-operated group(41 ±5)ng/ml.The serum HMGB1 level in IR group was positively correlated with the OI(r=0.759,P＜0.05)and wet/dry ratio(r=0.725,P＜0.05).RT-PCR showed that HMGB1 mRNA level in sham-operated group was the lowest and increased significantly in IR group,while decreased significantly in MSC-IR group compared with IR group and Saline-IR group(P＜0.01).The results of HMGB1 expression at protein level by Western blot were consistent with the mRNA level.Conclusion Lung transplantation can induce the expression of HMGB 1 but HMGB1 level of lung tissue decreased significantly after the treatment with MSCs,which indicated that MSCs might play an important role in protecting transplanted lung via HMGB 1.