Biomechanical force-primed periodontal ligament stem cells exhibit a tolerance effect against bacterial inflammation

As mechanosensory cells, periodontal ligament stem cells (PDLSCs) react to mechanical force through proliferative, immunomodulatory, and regenerative actions that promote bone deposition. This study aimed to investigate how biomechanical compressive forces modulate PDLSCs' self-tolerance to proinflammatory responses.

PDLSCs were cultured and characterized using flow cytometry. Intermittent compressive force (ICF) was applied to the cells using a computerized-controlled apparatus, with a force of 1.5 g/cm² at 0.23 Hz for 24 hours. TLR4 activation was induced using lipopolysaccharides (LPS) from P. gingivalis, in the presence or absence of ICF. Pathway inhibitors targeting TGF-β receptor type 1, Rho kinase, and NF-kB were applied to investigate the signaling pathways. Pro-inflammatory cytokine levels were measured using qPCR and ELISA. Western blotting was performed to assess the protein expression of TLR4 and TGF-β1. Immunofluorescence staining was used to localize TLR4. Osteogenic differentiation and IDO enzymatic activity were assessed.

Our results showed that PDLSCs primed with ICF developed tolerance to self-inflammatory responses when exposed to LPS, as indicated by reduced levels of inflammatory factors such as interleukin (IL)-1β, IL-6, tumor necrosis factor (TNF)-α, and interferon (IFN)-γ. This tolerance, specific to ICF-primed PDLSCs, was partially mediated by the NF-kB p65 signaling pathway. Additionally, ICF enhanced PDLSC immunosuppressive properties and restored osteogenic differentiation, which had been delayed by LPS/TLR4 activation. Notably, TLR4 responded directly to ICF stimulation.

This study demonstrated that priming inflamed PDLSCs with biomechanical compressive force induces a tolerance effect against infection-induced inflammation, promoting bone regeneration.

This study underscores the critical role of tolerance mechanisms in maintaining PDLSCs homeostasis, highlights the intricate interplay between biomechanical forces and immune modulation, and provides new insights into manipulating stem cells and developing therapeutic strategies to enhance bone and tissue regeneration in immune-related disorders, particularly periodontal disease.