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BACKGROUND:Bone marrow mesenchymal stem cels have a therapeutic effect on acute lung injury, but the mechanism is unclear. If the mechanism is understood, the majority of patients with acute lung injury can obtain a benefit. OBJECTIVE:To explore the possible mechanism underlying bone marrow mesenchymal stem cels in the treatment of acute lung injury with sepsis in rats. METHODS: (1) Thirty-six adult Wistar rats were randomly divided into three groups, sham operation group (sham group), sepsis group and bone marrow mesenchymal stem cels group (cel treatment group). In the sepsis and cel treatment groups, animal models of sepsis with acute lung injury were established by cecal ligation and puncture, while in the sham group, the cecum was not ligated and punctured. Then, 1 mL normal saline was injected via the femoral vein in the sepsis and sham groups, and 1 mL bone marrow mesenchymal stem cel suspension (1×109/L) was injected into the cel treatment group. After 6 hours, interleukin 10 and macrophage inflammatory protein-2 levels in serum were measured in the three groups. Lung tissues were taken for pathological observation using hematoxylin-eosin staining. (2) Rat alveolar macrophages were obtained by bronchoalveolar lavage, seeded into 24-wel culture plates, and divided into three groups: control group (group A), sepsis model group (group B) and intervention group of bone marrow mesenchymal stem cels (group C). Normal saline, septic plasma, and co-intervention of septic plasma and mesenchymal stem cels were used in the groups A, B, C, respectively. Then, cels in the three groups were cultured in a 5% CO2 incubator at 37℃ for 1 hour. After that, alveolar macrophages were taken to detect whether nuclear factor-κB (P65) protein entered into the nucleus using laser scanning confocal microscopy. RESULTS AND CONCLUSION: (1) The results of animal experiments showed that compared with the sham group, the macrophage inflammatory protein-2 levels in the sepsis group and cel treatment group were significantly increased (P < 0.05), but the macrophage inflammatory protein-2 level in the cel treatment group was significantly lower than that in the sepsis group (P < 0.05); there were no significant differences in serum interleukin 10 levels among the three groups (P > 0.05); inflammatory cel infiltration, interstitial pulmonary edema and pulmonary hemorrhage existed in the sepsis and cel treatment groups, but these symptoms were significantly reduced in the cel treatment group compared with the sepsis group. (2) Results from cel experiments showed that compared with the group A, in group B and group C, the number of nuclear factor-κB (P65) proteins into the nucleus was significantly higher (P < 0.05), but it was lower in the group C than the group B (P < 0.05). These findings indicate that bone marrow mesenchymal stem cels in acute lung injury with sepsis can regulate nuclear factor-κB (P65) protein of alveolar macrophages into the nucleus, reduce expression of macrophage inflammatory protein-2, and thereby play a protective role in the lungvia reducing neutrophil infiltration. Temporarily, this study cannot explain whether bone marrow mesenchymal stem cels have an effect on interleukin 10.