Complement 3 mediates periodontal destruction in patients with type 2 diabetes by regulating macrophage polarization in periodontal tissues

OBJECTIVES: Diabetes aggravates the risk and severity of periodontitis, but the specific mechanism remains confused. Complement 3 (C3) is closely related to complications of type 2 diabetes (T2DM). In the present study, we concentrated on whether C3 mediates the development of periodontitis in T2DM. MATERIALS AND METHODS: Levels of C3 in blood and gingival crevicular fluid (GCF) of patients were measured first. A C3‐knockout diabetic mouse model was established, real‐time PCR, Western blotting and histological investigation were performed to evaluate the progress of periodontitis. Microcomputed tomography (micro‐CT) and TRAP staining were performed to detect alveolar bone resorption. Immunofluorescence was performed to detect polarization of macrophages. RESULTS: Our data showed that C3 levels were elevated in the blood and GCF of T2DM patients compared with non‐diabetic individuals. Increased C3 was closely related to the upregulation of inflammatory cytokines including interleukin (IL)‐1, IL‐6 and tumour necrosis factor‐alpha (TNF‐α), as well as the decline of the bone volume density (BMD) and bone volume over total volume (BV/TV) of the alveolar bones in diabetic mice. The deletion of C3 inhibited inflammatory cytokines and rescued the decreased BMD and BV/TV of the alveolar bones. C3‐mediated polarization of macrophages was responsible for the damage. CONCLUSION: T2DM‐related upregulation of C3 contributes to the development of periodontitis by promoting macrophages M1 polarization and inhibiting M2 polarization, triggering a pro‐inflammatory effect on periodontal tissues.