TLR3 activation modulates immunomodulatory properties of human periodontal ligament cells

Abstract

Background Toll‐like receptors (TLR) are a group of receptors that play roles in the innate immune system. Human periodontal ligament cells (hPDL cells) express several TLRs, including TLR3, a nucleotide sensing receptor that recognizes double‐stranded RNA from viral infection. However, its role in hPDL cells is unclear. The aim of this study was to investigate the responses of hPDL cells in terms of immunomodulation after TLR3 engagement. Methods HPDL cells were treated with various doses of poly I:C, a TLR3 activator. The expression of interferon‐gamma (IFNγ), indoleamine 2,3 dioxygenase (IDO), and human leukocyte antigen G (HLA‐G) was determined. Chemical inhibitors and small interfering RNA (siRNA) were used to confirm the role of TLR3. Coculture with human peripheral blood mononuclear cells (PBMCs) with poly I:C‐activated hPDL cells was performed. Results Endosomal TLR3 in hPDL cells was observed by immunocytochemistry. Addition of poly I:C significantly enhanced the expression and secretion of IFNγ, IDO, and HLA‐G. Knockdown of TLR3 using siRNA decreased the poly I:C‐induced expression of these three molecules. Bafilomycin‐A, an inhibitor of auto‐phagosome and lysosome fusion, inhibited poly I:C‐induced IDO and HLA‐G expression, whereas cycloheximide and a TLR3‐neutralizing antibody had no effect. In co‐culture experiments, poly I:C‐activated hPDL cells inhibited PBMCs proliferation and increased mRNA expression of forkhead box P3 (FOXP3), a transcription factor which is a marker of regulatory T cells. Conclusion Our findings indicated that TLR3 engagement of hPDL cells induced immunosuppressive properties of these cells. Because immunosuppressive properties play an important role in tissue healing and regeneration, activation of TLR3 may help to attenuate tissue destruction by limiting the inflammatory process and perhaps initiate the healing and regeneration process of the periodontium.

Shear Stress Enhances the Paracrine-Mediated Immunoregulatory Function of Human Periodontal Ligament Stem Cells via the ERK Signalling Pathway

“…For investigation of T cell proliferation, the activated CD4 + T cells were co-cultured with the conditioned medium for 3 days [7]. The CD4 + T cells were incubated with 7-hydroxy-10-oxidophenoxazin-10-ium-3-one, sodium (resazurin) (cat.…”

“…From an immunomodulatory standpoint, hPDLSCs possess striking features allowing them to escape immune recognition, inhibit activated immune cell function and regulate inflammatory cytokines during tissue regeneration [1,[7][8][9][10][11][12][13]. These properties were initially recognised in MSCs that possess the ability to regulate the proliferation and differentiation of immune cells during an inflammatory response in order to promote tissue healing [14][15][16].…”

“…The depletion of tryptophan through the IDO-kynurenine pathway modulates immune response by inhibiting T cell proliferation and promoting Treg differentiation [24][25][26][27]. The role of IDO as an immune suppressor was shown to be activated in the presence of proinflammatory cytokines, for instance, IL-1β [28], IL-12 [29], Toll-like receptors (TLRs) [7,30], cyclooxygenase 2 (COX2)/prostaglandin E2 (PGE2) [31,32], interferon gamma (IFN-γ) [1,30,33] as well as TGF-β1 [34], leading to suppression of inflammation. Recently, our group showed that TLR3 induced the expression IFN-γ-independent IDO, resulting in inhibition of peripheral blood mononuclear cell (PBMC) proliferation and upregulation of a Treg-specific gene marker, forkhead box P3 or FOXP3 [7].…”

“…The role of IDO as an immune suppressor was shown to be activated in the presence of proinflammatory cytokines, for instance, IL-1β [28], IL-12 [29], Toll-like receptors (TLRs) [7,30], cyclooxygenase 2 (COX2)/prostaglandin E2 (PGE2) [31,32], interferon gamma (IFN-γ) [1,30,33] as well as TGF-β1 [34], leading to suppression of inflammation. Recently, our group showed that TLR3 induced the expression IFN-γ-independent IDO, resulting in inhibition of peripheral blood mononuclear cell (PBMC) proliferation and upregulation of a Treg-specific gene marker, forkhead box P3 or FOXP3 [7]. Additionally, mechanical stimuli have also been applied in vitro to regulate immunomodulatory properties of both PDL cells [35,36] and bone marrow-derived MSCs (BM-MSCs) [37] PDL cells are considered as mechanosensory cells that can perceive and respond to mechanical stress driven by mastication, speech or orthodontic movement [38].…”

The Th17/Treg cell balance: crosstalk among the immune system, bone and microbes in periodontitis

“…To inhibit the immune‐inflammatory response of Th17s, proinflammatory cytokines and TLRs stimulate DCs, gingival fibroblasts, periodontal ligament stem cells etc. to express an immunosuppressive enzyme called indoleamine 2,3‐dioxygenase (IDO) 94‐96 . IDO can directly inhibit the maturation of DCs mediated by bacterial antigens or inhibit the proliferation of antigen‐specific CD4 + T cells, which is beneficial for achieving immune tolerance or reducing inflammation 97 .…”