The anti-inflammatory and antibacterial mechanisms of bone marrow mesenchymal stem cells (MSCs) ameliorating lung injury in chronic obstructive pulmonary disease (COPD) mice induced by cigarette smoke and Haemophilus Parainfluenza (HPi) were studied.The experiment was divided into four groups in vivo:control group,COPD group,COPD+HPi group,and COPD+HPi+MSCs group.The indexes of emphysematous changes,inflammatory reaction and lung injury score,and antibacterial effects were evaluated in all groups.As compared with control group,emphysematous changes were significantly aggravated in COPD group,COPD+HPi group and COPD+HPi+MSCs group (P＜0.01),the expression of necrosis factor-kappaB (NF-κB) signal pathway and proinflammatory cytokines in bronchoalveolar lavage fluid (BALF) were increased (P＜0.01),and the phagocytic activity of alveolar macrophages was downregulated (P＜0.01).As compared with COPD group,lung injury score,inflammatory cells and proinflammatory cytokines were significantly increased in the BALF of COPD+HPi group and COPD+HPi+MSCs group (P＜0.01).As compared with COPD+HPi group,the expression of tumor necrosis factor-α stimulated protein/gene 6 (TSG-6) was increased,the NF-κB signal pathway was depressed,proinflammatory cytokine was significantly reduced,the anti-inflammatory cytokine IL-10 was increased,and lung injury score was significantly reduced in COPD+HPi+MSCs group.Meanwhile,the phagocytic activity of alveolar macrophages was significantly enhanced and bacterial counts in the lung were decreased.The results indicated cigarette smoke caused emphysematous changes in mice and the phagocytic activity of alveolar macrophages was decreased.The lung injury of acute exacerbation of COPD mice induced by cigarette smoke and HPi was alleviated through MSCs transplantation,which may be attributed to the fact that MSCs could promote macrophages into anti-inflammatory phenotype through secreting TSG-6,inhibit NF-κB signaling pathway,and reduce inflammatory response through reducing proinflammatory cytokines and promoting the expression of the anti-inflammatory cytokine.Simultaneously,MSCs could enhance phagocytic activity of macrophages and bacterial clearance.Meanwhile,we detected anti-inflammatory and antibacterial activity of macrophages regulated by MSCs in vitro.As compared with RAW264.7+HPi+CSE group,the expression of NF-κB p65,IL-1β,IL-6 and TNF-α was significantly reduced,and the phagocytic activity of macrophages was significantly increased in RAW264.7+HPi+CSE+MSCs group (P＜0.01).The result indicated the macrophages co-cultured with MSCs may inhibit NF-κB signaling pathway and promote phagocytosis by paracrine mechanism.