The extent of pyroptosis varies in different stages of apical periodontitis

Apical periodontitis (AP) is an inflammation affecting the periapical region of tooth root. Microbial pathogens activate inflammasomes and promote the production of pro-inflammatory cytokines. Caspase-1-mediated pyroptosis is a possible mechanism involved in the initiation and progression of AP. The purpose of this study was to evaluate the role of caspase-1 and pyroptosis on AP at different stages. Human periapical inflammatory tissue was collected to study chronic AP stage. Human periodontal ligament fibroblasts (hPDLFs) were stimulated with lipopolysaccharide in vitro for 24 h to simulate early AP stage. Experimental AP rat model was established to study acute AP stage from 0 d to 28 d. The results showed that NLRP3, cleaved caspase-1 and Interleukin (IL)-1β were enhanced in all AP stages. Caspase-1 activation was detectable in most cells. However, the level of pyroptosis was in accordance with the degree of AP inflammation. Early and chronic AP showed a comparable hemostasis state, with pyroptosis remaining in a reduced level. On the contrary, extensive pyroptosis accelerated inflammation and induced cell death in acute AP. VX765, a caspase-1 inhibitor, was used in an experimental AP rat model. The results showed that VX765 suppressed bone loss, suggesting a role of pyroptosis on bone resorption in acute AP. VX765 also inhibited the expressions of IL-1β, Monocyte chemoattractant protein-1 (MCP-1), IL-6 and IL-8 in vitro, thus decreased inflammatory responses during AP. In conclusion, caspase-1 and pyroptosis contributed to AP inflammation and lesion and pyroptosis extent was in line with AP progression.