NLRP3 inflammasome mediates M1 macrophage polarization and IL-1β production in inflammatory root resorption

Aims To explore the involvement of NOD-like receptor protein 3 (NLRP3) inflammasome and M1 macrophage in root resorption (RR). Methods A rat RR model was established by excessive orthodontic force. After different force-loading time, the expression levels of NLRP3, caspase-1, and interleukin-1 beta (IL-1 beta) and distribution of M1 macrophages were analysed by immunohistochemistry and immunofluorescence staining in vivo. Then, the mechanism of NLRP3 activation was further verified by macrophage and human periodontal ligament cell (hPDLC) co-culture system in vitro. The production levels of NLRP3, caspase-1, pro-caspase-1, and IL-1 beta in M1 macrophages in the co-culture system were detected by Western blot, and the polarization of CD68+IL-1 beta+ M1 macrophages was detected by immunofluorescence staining. Results In the rat RR model, NLRP3, caspase-1, IL-1 beta, and M1 macrophages were expressed in periodontal ligament, mainly concentrated around RR areas. Force-pre-treated hPDLCs promoted M1 macrophage polarization and the production of NLRP3, caspase-1, and IL-1 beta in M1 macrophages in co-culture system. When MCC950, an inhibitor of NLRP3 inflammasome, was added, NLRP3 activation and M1 macrophage polarization were inhibited. Conclusions In periodontal tissues, hPDLCs stimulated by force promoted M1 macrophage polarization and increased IL-1 beta production by activating NLRP3 inflammasome in M1 macrophages, thus initiating the occurrence of RR.