Loss of periodontal ligament fibroblasts by RIPK3-MLKL-mediated necroptosis in the progress of chronic periodontitis

Periodontal homeostasis is maintained by the dynamic equilibrium between cell death, differentiation and proliferation of resident cells in the periodontal microenvironment. Loss of resident periodontal ligament fibroblasts (PDLFs) has been a major challenge in the periodontal treatment. This study aimed to investigate the exact role of necroptotic cell death in periodontal diseases. Elevated levels of receptor-interacting protein serine-threonine kinases -1 (RIPK1), phosphorylated RIPK3, mixed lineage kinase domain-like protein (MLKL), phosphorylated MLKL and FLIP(L) were observed in gingival tissues collected from patients with untreated chronic periodontitis; whereas no difference in caspase 8 was observed between the periodontitis and healthy control group. In contrast to the high incidence of necroptotic cell death in monocytes during live P. gingivalis infection with a low multiplicity of infection (MOI), necroptosis was only observed in PDLFs with a high MOI. Priming PDLFs with frozen thawed monocytes enhanced proinflammatory responses to P. gingivalis infection; moreover, frozen thawed monocytes stimulation triggered RIPK1, RIPK3 and MLKL-mediated-necroptotic cell death in PDLFs. These results indicated that RIPK3 and MLKL-mediated-necroptotic cell death participated in the pathogenesis of periodontitis, and DAMPs released from monocytes after P. gingivalis stimulation by necroptosis triggered not only inflammatory responses, but also necroptosis of PDLFs.