Periodontal ligament cells under mechanical force regulate local immune homeostasis by modulating Th17/Treg cell differentiation

Objectives Improper orthodontic force often causes root resorption or destructive bone resorption. There is evidence that T helper 17 (Th17) cells and regulatory T (Treg) cells may be actively involved in bone remodeling during tooth movement. In a combination of in vitro and in vivo studies, we investigated the effect of human periodontal ligament cells (hPDLCs) on Th17/Treg cells under different orthodontic forces and corticotomy. Material and methods hPDLCs were cultured in vitro and subjected to different mechanical forces. The expression of interleukin (IL)-6 and transforming growth factor (TGF)-beta in the supernatant and the mRNA levels of hypoxia inducible factor (HIF)-1 alpha, Notch1, and TGF-beta in hPDLCs were investigated. Supernatants were collected and co-cultured with activated CD4(+)T cells, and the differentiation of Th17/Treg cells was analyzed by flow cytometry. We also established an animal model of tooth movement with or without corticotomy. The tooth movement distance, alveolar bone height, and root resorption were analyzed using micro-computed tomography. Expression of interleukin (IL)-17A, forkhead Box P3 (Foxp3), and IL-6 were analyzed using immunohistochemistry, while osteoclasts were evaluated by tartrate-resistant acid phosphatase (TRAP) staining. The mRNA levels of IL-17A, IL-6, Foxp3, IL-10, HIF-1 alpha, notch1, and C-X-C motif chemokine ligand 12 (CXCL12) in alveolar bone and gingiva were investigated. Results Heavy force repressed cell viability and increased the mortality rate of hPDLCs; it also improved the expression of IL-6, declined the expression of TGF-beta, and promoted the mRNA expression level of HIF-1 alpha. The expression of TGF-beta and Notch1 mRNA decreased and then increased. The supernatant of hPDLCs under heavy force promotes the polarization of Th17 cells. The heavy force caused root resorption and decreased alveolar bone height and increased the positive area of IL-17A immunohistochemical staining and the expression of IL-17A, IL-6, HIF-1 alpha, and Notch1 mRNA. Corticotomy accelerated tooth movement, increased the proportion of Foxp3-positive cells, and up-regulated the expression of Foxp3, IL-10, and CXCL12 mRNA. Conclusions During orthodontic tooth movement, the heavy force causes root resorption and inflammatory bone destruction, which could be associated with increased expression of Th17 cells and IL-6. Corticotomy can accelerate tooth movement without causing root resorption and periodontal bone loss, which may be related to the increased expression of Treg cells.