Abstract: BACKGROUND: Radiotherapy for head and neck tumors can easily cause xerostomia, seriously affecting the quality of life of patients. In recent years, engineered stem cells and their paracrine factors have shown therapeutic potential in the repair of salivary gland injury. However, there is currently no experimental study on the application of amniotic mesenchymal stem cell-derived exosome in radiation-induced salivary gland injury.
OBJECTIVE: To preliminarily explore the repair effect of exosome derived from human amniotic mesenchymal stem cells on radiation-induced submandibular gland injury.
METHODS: Human amniotic mesenchymal stem cell exosomes were extracted and identified by ultrafiltration and ultracentrifugation. SD rats were randomly divided into a control group, a radiation injury group, and a radiation injury+exosome group. An in vitro model of radiation-induced submandibular gland injury was constructed using the submandibular gland tissue of SD rats irradiated with 18 Gy of radiation. One day after radiation modeling, exosome derived from human amniotic mesenchymal stem cells was injected into the submandibular gland in situ. Samples are taken at 1, 3, 7, and 14 days to detect the resting salivary flow rate. The structure of the submandibular gland tissue was observed by hematoxylin-eosin staining. The expression of glycogen particles in the submandibular gland tissue was observed by Periodic Acid-Schiff staining. Fibrosis in the submandibular gland tissue was observed by Masson staining. The secretion of salivary amylase was detected by enzyme-linked immunosorbent assay. The expression of aquaporin and tight junction proteins in submandibular gland tissue was observed by immunofluorescence staining. Real-time fluorescence quantitative PCR was used to detect the relative expression levels of aquaporins and salivary amylase mRNA in submandibular gland tissue. TUNEL assay was used to detect the apoptosis rate of submandibular gland tissues in each group.
RESULTS AND CONCLUSION: After radiomodeling, compared with the radiation injury group, (1) hematoxylin-eosin staining observed that the submandibular gland tissue structure in the radiation injury+exosome group was restored, the nucleoli increased, the number of acinus increased, and the acinar atrophy improved. (2) Glycogen staining observed that the number and density of positive zymogen granules in the acinar cytoplasm of the radiation injury+exosome group gradually increased. (3) Masson staining results observed that the number and density of positive collagen fibers in the interstitium and around the ducts in the radiation injury+exosome group gradually decreased, the degree of fibrosis decreased, and the collagen deposition decreased. (4) The salivary flow rate in the radiation injury+exosome group increased (P < 0.05). The fluorescence intensity of aquaporin-5 was enhanced (P < 0.05) and the gene expression was significantly enhanced (P < 0.01). The fluorescence distribution of tight junction protein 4 was weakened and the fluorescence intensity decreased (P < 0.05, 
P < 0.01). The content of salivary amylase increased (P < 0.05) and gene expression were significantly increased (P < 0.01). The number of positive apoptotic cells decreased (P < 0.05, P < 0.01). It is indicated that local injection of exosome derived from human amniotic mesenchymal stem cells could improve the pathological morphology of submandibular gland tissue, promote saliva flow rate and amylase expression, and may play a functional repair role in radioactive submandibular gland injury by inhibiting acinar apoptosis.

Key words: human amniotic mesenchymal stem cell exosome, rat, submandibular gland, radiation injury repair, xerostomia, engineered stem cells