BACKGROUND: Our previous findings indicate that inflammation-activated bone marrow mesenchymal stem cell conditioned medium (MSC-CM) contribute to repairing the structure and function of the small intestine after radiation-induced acute intestinal injury. However, it is unclear whether the repair effect can be achieved by regulating small intestinal stem cells. OBJECTIVE: To investigate the effects of inflammation-activated bone marrow MSC-CM on the small intestinal epithelial stem cells after acute radiation-induced intestinal injury and to further discuss the repairing mechanism. METHODS: Bone marrow mesenchymal stem cells of Sprague-Dawley rats were separated, cultured and identified. Then, the bone marrow mesenchymal stem cells were co-cultured with normal or radiation-induced IEC-6 cell lines in the Transwell system for 24 hours. Inflammation-activated bone marrow mesenchymal stem cells were cultured alone for 48 hours. Non-activated MSC-CM (MSC-CMNOR) and MSC-CM under radiation-induced inflammatory condition (MSC-CMIR) were collected. Adult Sprague-Dawley rats (provided by the Experimental Center of Sun Yat-Sen University North Campus) were randomly divided into four groups with 20 rats in each group: control group, radiation group, radiation+MSC-CMNOR group and radiation+MSC-CMIR group. The rats in the latter three groups were exposed to one-off 14 Gy whole abdominal radiation to make a rat model of acute radiation-induced small intestinal injury. Three-day continuous administration beginning within 4 hours after successful modeling was given via the tail vein and intraperitoneal implantation of Alzet micro-osmotic pumps: EMEM-F12 (200 μL/d) for the radiation group, MSC-CMNOR for radiation+MSC-CMNOR group and MSC-CMIR for radiation+MSC-CMIR group. There was 2 mL of concentrated conditioned medium in the pump which was released at a constant rate of 10 μL/h into the abdominal cavity after implantation. Intestinal samples were collected at 1, 3, 5, 7 days after radiation for immunochemistry staining, western blot and qRT-PCR detection. RESULTS AND CONCLUSION: (1) On the 3rd day after radiation, Lgr5 positive cells, which were actively proliferating on the base of crypts, became significantly reduced compared with the normal control group, and there was nearly no existing Lgr5 positive cells. However, after infusion of MSC-CMIR, Lgr5 positive intestinal stem cells were significantly increased compared with the radiation group, while in the radiation+MSCNOR group, there was no significant increase in Lgr5 positive intestinal stem cells. (2) On the 3rd day after radiation injury, Bmi1 positive intestinal stem cells were almost invisible. After infusion of MSC-CMIR, Bmi1 positive intestinal stem cells increased significantly, and it was observed not only in the +4 cell position but also in the common position used to be Lgr5 stem cells, indicating that Bmi1 stem cells could differentiate into Lgr5 positive cells to act its repairing effect. (3) Western blot and qRT-PCR further confirmed that the radiation+MSC-CMIR group was significantly higher on the Lgr5 expression level than the radiation group and the radiation+MSC-CMNOR group, and it returned to the normal level on the 7th day after the continuous high expression level. The repair effect of radiation+MSC-CMNOR group was weaker, and only on the 7th day, the expression level of Lgr5 was statistically different from the radiation group. To conclude, inflammation-activated bone marrow MSC-CM exert a protective effect on the small intestinal epithelial stem cells after acute radiation-induced intestinal injury