STING inhibition alleviates bone resorption in apical periodontitis

The goal of this study was to investigate the potential effects of an immunotherapeutic drug targeting STING to suppress the overreactive innate immune response and relieve the bone defect in apical periodontitis.

We established an apical periodontitis mouse model in Sting^-/- and WT mice in vivo. The progression of apical periodontitis was analysed by micro-CT analysis and H&E staining. The expression level and localization of STING in F4/80⁺ cells were identified by IHC and immunofluorescence staining. RANKL in periapical tissues was tested by IHC staining. TRAP staining was used to detect osteoclasts. To clarify the effect of STING inhibitor C-176 as an immunotherapeutic drug, mice with apical periodontitis were treated with C-176 and the bone loss was identified by H&E, TRAP, RANKL staining and micro-CT. Bone marrow-derived macrophages (BMMs) were isolated from Sting^-/- and WT mice and induced to osteoclasts in a lipopolysaccharide (LPS)-induced inflammatory environment in vitro. Moreover, WT BMMs were treated with C-176 to determine the effect on osteoclast differentiation by TRAP staining. The expression levels of osteoclast-related genes were tested using qRT-PCR.

Compared to WT mice, the bone resorption and inflammatory cell infiltration were reduced in exposed Sting^-/- mice. In the exposed WT group, STING was activated mainly in F4/80⁺ macrophages. Histological staining revealed the less osteoclasts and lower expression of osteoclast-related factor RANKL in Sting^-/- mice. The treatment of the STING inhibitor C-176 in an apical periodontitis mice model alleviated inflammation progression and bone loss, similar to the effect observed in Sting^-/- mice. Expression of RANKL and osteoclast number in periapical tissues were also decreased after C-176 administration. In vitro, TRAP staining showed fewer positive cells and qRT-PCR reflected decreased expression of osteoclastic marker, Src and Acp5 were detected during osteoclastic differentiation in Sting^-/- and C-176 treated BMMs.

STING was activated and was proven to be a positive factor in bone loss and osteoclastogenesis in apical periodontitis. The STING inhibitor C-176 administration could alleviate the bone loss via modulating local immune response, which provided immunotherapy to the treatment of apical periodontitis.