Abstract: BACKGROUND: Studies have found that human dental pulp stem cells can treat periapical periodontitis bone defects under hypoxia preconditioning, but whether dental pulp stem cell exosomes under hypoxia preconditioning can carry the biological information of blasts and affect polarization of macrophages is unclear.  
OBJECTIVE: To study the effect of hypoxia-treated dental pulp stem cells exosomes on macrophage polarization.
METHODS:  Passage 3 dental pulp stem cells were cultured in a normal and hypoxic incubator. The medium was replaced once every 48 hours. Cell culture supernatants were collected at the confluence of 80%-90%. Exosomes were extracted by differential centrifugation. Human monocyte cell line THP-1 was induced to differentiate into macrophages, and co-incubated with PBS, normal culture, and hypoxic cultured dental pulp stem cells for 48 hours. Levels of inflammatory factors interleukin 10 and tumor necrosis factor alpha in supernatant were detected using ELISA. RT-qPCR was utilized to detect macrophage CD163, interleukin 1 receptor antagonist, transforming growth factor β1, chemokine CCL2, and tumor necrosis factor α mRNA expression. Flow cytometry was utilized to detect the expression of CD11b+CD163+ macrophages. Western blot assay was used to detect the activation of NF-κB and STAT3 signaling pathways.  
RESULTS AND CONCLUSION: (1) Compared with the PBS group, the level of interleukin-10 in the supernatant of macrophages in the normal cultured dental pulp stem cell-derived exosomes group significantly increased (P < 0.05), and the level of tumor necrosis factor alpha significantly reduced (P < 0.05). The expression of CD163, transforming growth factor β1, and chemokine CCL2 mRNA in macrophages increased significantly (P < 0.05); the expression of tumor necrosis factor alpha mRNA decreased significantly (P < 0.05), and the positive rate of CD11b+CD163+ macrophages increased significantly (P < 0.05); the phosphorylation level of p65 significantly reduced (P < 0.05); the expression of IκBα significantly increased (P < 0.05), and the phosphorylation level of STAT3 also significantly increased (P < 0.001). (2) Compared with the normal cultured dental pulp stem cell-derived exosomes group, the level of interleukin-10 in the supernatant of macrophages significantly increased in the hypoxic cultured dental pulp stem cell-derived exosomes group (P < 0.001), and the level of tumor necrosis factor alpha significantly reduced (P < 0.01); CD163, transforming growth factor β1, chemokine CCL2 and interleukin 1 receptor antagonist mRNA expression significantly increased (P < 0.01); tumor necrosis factor alpha mRNA expression also significantly decreased (P < 0.01); the positive rate of CD11b+CD163+ macrophages significantly increased (P < 0.01); the phosphorylation level of p65 significantly reduced (P < 0.01); the expression of IκBα significantly increased (P < 0.01), and the phosphorylation level of STAT3 significantly increased (P < 0.01). (3) It is concluded that normal culture and hypoxic culture of exosomes derived from dental pulp stem cells can promote the M2 polarization of macrophages, and the M2 polarization of macrophages is more obvious under hypoxic culture, and its mechanism may be to activate the STAT3 signaling pathway and inhibit the NF-κB signaling pathway.

Key words: stem cells, dental pulp stem cells, hypoxia, exosomes, macrophages, polarization, STAT3, NF-κB, signaling pathway