The interleukin-6 signal regulates orthodontic tooth movement and pain.

Orthodontic tooth movement (OTM) is accomplished by controlling the mechanical loading onto the bone around the roots of target teeth. The precise orthodontic force induces osteoclastic bone resorption on the compression side and osteoblastic bone formation on the tension side of the alveolar bone. Orthodontic intervention causes inflammation in the periodontal ligament (PDL), which manifests as acute pain. Because inflammation is deeply connected to bone remodeling, it has been indicated that the inflammation after orthodontic intervention affects both the movement of teeth and generation of pain. However, the precise mechanisms underlying the immune regulation of OTM and the related pain are not well elucidated. Here, we found from the search of a public database that the interleukin (IL)-6 family of cytokines are highly expressed in the PDL by mechanical loading. The IL-6 signal was activated in the PDL after orthodontic intervention. The signal promoted OTM by inducing osteoclastic bone resorption. IL-6 was found to increase the number of osteoclasts by suppressing apoptosis and increasing their responsiveness to macrophage colony-stimulating factor (M-CSF) and receptor activator of NF-κB ligand (RANKL). Furthermore, IL-6 signal was shown to elicit orthodontic pain by inducing neuroinflammation in the trigeminal ganglion (TG). Taken together, it was demonstrated that the IL-6 signal regulates tooth movement and pain during orthodontic treatment. It was also indicated that local blockade of the IL-6 signal is a promising therapeutic option in orthodontic treatment, targeting both tooth movement and pain.