Abstract
In the oral mechanical environment, periodontal ligament cells (PDL cells) contribute to maintaining periodontal tissue homeostasis. Recent studies showed that exosomes, which are small vesicles secreted by various types of cells, play a pivotal role in cell-to-cell communication in biological processes. We examined the secretion of exosomes from PDL cells stimulated with cyclic stretch and their role in the inflammatory response of macrophages using the human macrophage cell line THP-1 and human primary monocytes/macrophages. We prepared supernatants from human PDL cells (PDL-sup) stimulated with cyclic stretch. The treatment of macrophages with PDL-sup, but not PDL-sup from unstimulated PDL cells, inhibited the production of IL-1β in LPS/nigericin-stimulated macrophages. The pretreatment of PDL cells with GW4869, an inhibitor of exosome secretion, or siRNA for Rab27B, which controls exosome secretion, abrogated the inhibitory effects of PDL-sup. A transmission electron microscopy analysis demonstrated the existence of exosomes with diameters ranging between 30 and 100 nm in PDL-sup, suggesting that exosomes in PDL-sup contribute to this inhibition. An immunofluorescence microscopy analysis revealed that exosomes labeled with PKH67, a fluorescent dye, were incorporated by macrophages as early as 2 h after the addition of exosomes. Purified exosomes inhibited IL-1β production in LPS/nigericin-stimulated macrophages and the nuclear translocation of NF-κB as well as NF-κB p65 DNA-binding activity in LPS-stimulated macrophages, suggesting that exosomes suppress IL-1β production by inhibiting the NF-κB signaling pathway. Our results indicate that PDL cells in mechanical environments contribute to the maintenance of periodontal immune/inflammatory homeostasis by releasing exosomes.
Highlights
Periodontal tissue is defined as tissue that supports teeth, and includes the root cementum, periodontal ligament (PDL), alveolar bone, and gingiva
We investigated whether cyclic stretch acted on human PDL cells to promote regulation of the inflammatory response of macrophages using the THP-1 human monocyte-like cell line
We examined whether culture supernatants of PDL cells exposed to cyclic stretch inhibited the induction of pyroptosis by nigericin in macrophages primed with LPS by detecting dead cells stained by propidium iodide (PI)
Summary
Periodontal tissue is defined as tissue that supports teeth, and includes the root cementum, periodontal ligament (PDL), alveolar bone, and gingiva. Macrophages have a central role in both the inflammatory process and repair of tissue damage Among these processes, activation of inflammasomes occurs as part of the inflammatory response. Pyrin domain-containing 3 (NLRP3) is the best-characterized member of the NLR family, and it is involved in responses to various stimuli that include pathogen-associated molecules such as nigericin [6] and damage-associated molecular patterns such as extracellular adenosine triphosphate (ATP) [7]. Since periodontal lesions are often exposed to bacterial infection and/or injury, mechanisms that tightly regulate the NLRP3 inflammasomes of macrophages are required to maintain periodontal homeostasis. It currently remains unclear whether PDL cells are regulatory cells for immune/inflammatory homeostasis
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