To observe the damage effect of irradiation on intestine of C57 mice and investigate the protective effect and role of human α-defensin 5（HD5）in the injury.

Intestinal α-defensin knockout mice（MMP7^-/- mice）were constructed based on C57 mice, and HD5 gene was transferred into MMP7^-/- mice to construct and breed a HD5 transgenic mice（HD5tg mice）. The experiment was divided into wild-type C57 mice group（WT group）, MMP7^-/- mice group and HD5tg mice group. After the mice were irradiated with 5 Gy X-ray（1.26 Gy/min）, the survival status of mice within 30 d was observed and statistically analyzed, and the ileal tissues were collected at 0, 2 and 10 d. The pathological injury of the ileum was observed with HE staining, the cell apoptosis in the mice intestinal mucosa was evaluated by TUNEL, and the proliferation and differentiation of ileum intestinal stem cells were analyzed with immunofluorescence assay and PAS staining. The protein expression of TLR4, MyD88 and NF-κB was detected with Western blotting, and the transcription levels of IL-1, IL-6 and IL-10 in intestinal mucosa were measured with RT-qPCR.

MMP7^-/- mice and HD5tg mice were successfully constructed in this study. At 30 d after irradiation, the survival rates of WT, MMP7^-/- and HD5tg groups were 50%（10/20）, 35%（7/20）and 55%（11/20）, respectively. The survival rates of WT and HD5tg groups were significantly higher than that of MMP7^-/- group（P<0.05）, but there was no statistic difference between WT group and HD5tg group. When compared with MMP7^-/- group, HD5 transfer significantly improved the pathological damage of villi and crypt structure in the small intestine of mice after irradiation, inhibited the apoptosis of intestinal mucosal cells, promoted the proliferation and differentiation of intestinal stem cells, and decreased transcription levels of inflammatory factors via inhibiting TLR4-MyD88-NF-κB signaling pathway（P<0.05）.

HD5 promotes the renewal of intestinal mucosal cells and tissue repair and inhibit inflammatory injury by improving intestinal epithelial barrier dysfunction induced by irradiation, and thus, enhance the anti-irradiation ability of C57 mice.