Lipopolysaccharide inhibits osteogenic differentiation of periodontal ligament stem cells partially through toll-like receptor 4-mediated ephrinB2 downregulation

Kaempferol combats the osteogenic differentiation damage of periodontal ligament stem cells in periodontitis via regulating EphrinB2-mediated PI3K/Akt and P38 pathways

BACKGROUND

The osteogenic differentiation of periodontal ligament stem cells (PDLSCs) plays a fundamental role in endogenous bone regeneration during periodontitis treatment, yet achieving consistent differentiation under inflammatory conditions remains clinically challenging. Kaempferol, a phytochemical flavonol, has demonstrated osteoprotective efficacy in osteoporosis and bone repair models. However, whether kaempferol exerts pro-osteogenic effects on PDLSCs within the pathologically complex microenvironment of periodontitis, and through what molecular mechanisms, remains unexplored.

PURPOSE

This study aimed to systematically characterize the therapeutic efficacy of kaempferol in restoring osteogenic differentiation of human PDLSCs under inflammatory stress, and promoting bone regeneration in a mice periodontitis model, and elucidate novel molecular targets and downstream mechanisms mediating these regenerative actions.

METHODS

An in vitro inflammatory microenvironment was established using lipopolysaccharide (LPS)-stimulated human PDLSCs to mimic periodontitis-induced osteogenic impairment. Osteogenic recovery was assessed through alkaline phosphatase (ALP), alizarin red S staining, quantitative real-time polymerase chain reaction (qRT-PCR), and Western blot analysis of osteogenesis-related markers (ALP, RUNX2, OSX, OPN). The bioinformatics, network pharmacology and siRNA transfection were performed to identify EphrinB2 as kaempferol's putative cellular target. Downstream PI3K/Akt and p38 MAPK pathway activation was evaluated through phosphoprotein analysis. In vivo validation employed micro-CT quantification of alveolar bone loss and immunohistochemical profiling of pathways key proteins in a mice periodontitis model.

RESULTS

Kaempferol dose-dependently rescued LPS-impaired osteogenic differentiation in human PDLSCs, especially at 10 μM, where kaempferol significantly reversed suppressed ALP activity, mineralized nodule formation, and transcriptional and protein expression of osteogenic markers (ALP, RUNX2, OSX, OPN). Mechanistically, kaempferol upregulated the key target EphrinB2 under inflammatory stress, thereby reactivating the downstream PI3K/Akt and p38 pathways. In periodontitis mice, kaempferol administration (10 mg/kg) significantly promoted the periodontal expression of OPN and EphrinB2, restored the phosphorylation of PI3K, AKT, and P38, attenuating alveolar bone loss by 63.8 % (BV/TV: 72.4 % ± 2.07 vs. 44.2 % ± 3.19 in CON).

CONCLUSION

Kaempferol could rescue PDLSCs' osteogenic differentiation and mitigates bone loss in periodontitis microenvironments by targeting EphrinB2 to activate PI3K/Akt and P38 pathways. This work underscores kaempferol's potential as a natural therapeutic for reversing pathological bone resorption and promoting periodontal regeneration.

Osteogenic mechanism of deciduous teeth periodontal ligament stem cells in inflammatory environment

This study aimed to illustrate the biological behavior and changes in cell function during the progression of apical periodontitis in deciduous teeth and to explore the underlying molecular mechanism. Deciduous teeth periodontal ligament stem cells (DePDLSCs) were derived and their identity was confirmed. The viability, inflammation, and osteogenic ability of cells were tested by exposing them to various concentrations of lipopolysaccharide (LPS) (0-100 μg/mL) using the cell counting kit-8 (CCK-8) assay, reverse transcription polymerase chain reaction (real-time PCR), alkaline phosphatase (ALP) staining, and ALP activity assay. In addition, osteogenic-induced cells with and without 10 μg/mL LPS were harvested for high-throughput sequencing. Based on sequencing data, proinflammatory factors and ALP expression were measured after interference with the PI3K-AKT signaling pathway activator, 740Y-P. LPS biphasically affected the proliferation and osteogenesis of DePDLSCs. Low concentrations of LPS showed stimulatory effects, whereas inhibitory effects were observed at high concentrations. Sequencing analysis showed that the PI3K-AKT signaling pathway was significantly downregulated when DePDLSCs were treated with 10 μg/mL LPS. The LPS-induced inflammation and osteogenesis inhibition of DePDLSCs were partially rescued by 740Y-P treatment. In conclusion, LPS affected DePDLSCs proliferation and osteogenesis in a biphasic manner. Moderate activation of PI3K-AKT signaling pathway was beneficial for osteogenic differentiation and anti-inflammatory effect in DePDLSCs. This research may provide etiological probes for apical periodontitis and its treatment.

PGC-1α inhibits NLRP3 signaling through transcriptional activation of POP1 to alleviate inflammation and strengthen osteogenic differentiation of lipopolysaccharide-induced human periodontal stem cells

Periodontitis is a chronic infectious disease that affects the oral health of adults. Periodontal stem cells (PDLSCs) have good self-renewal and multipotential differentiation abilities to maintain the integrity of periodontal support structure and repair defects. This study aimed to elucidate the role of peroxisome proliferator activated receptor-γ co-activator 1-α (PGC-1α) in lipopolysaccharide (LPS)-induced PDLSCs and the underlying mechanisms related to predicated that pyrin domain (PYD)-only protein 1 (POP1). Notably downregulated PGC-1α and POP1 expression was observed in LPS-induced PDLSCs. PGC-1α or POP1 overexpression significantly reduced the inflammation and enhanced the osteogenic differentiation of LPS-treated PDLSCs. Particularly, PGC-1 bound to POP1 promoter region and upregulated POP1 expression. Moreover, POP1 knockdown ameliorated the impacts of PGC-1α overexpression on the inflammation and osteogenic differentiation in LPS-induced PDLSCs. Besides, PGC-1α inactivated NLRP3 signaling in LPS-treated PDLSCs, which was reversed by POP1 knockdown. Taken together, PGC-1α inhibits NLRP3 signaling through transcriptional activation of POP1, thereby alleviating inflammation and strengthening osteogenic differentiation of LPS-induced PDLSCs.

Toll-like receptor 4 deficiency affects the balance of osteoclastogenesis and osteoblastogenesis in periodontitis

Toll-like receptor 4 (TLR4) acts as a double-edged sword in the occurrence and development of periodontitis. While the activation of TLR4 in macrophages aids in clearing local pathogens, it can also disrupt innate immune responses, upsetting microecological balance and accelerating the destruction of periodontal bone tissues. To date, the effects of TLR4 on osteogenesis and osteoclastogenesis in periodontitis have not been comprehensively studied. In this study, we investigated the development of periodontitis in the Tlr4-/- mice by ligating their second molars with silk threads. Compared to wild-type (WT) mice, Tlr4-/- mice demonstrated increased resistance to periodontitis-associated bone destruction, as evidenced by decreased bone resorption and enhanced bone regeneration. Mechanistically, the deletion of Tlr4 not only inhibited osteoclast formation by reducing the expression of NFATc1, CTSK and TRAP, but also enhanced osteogenic abilities through increased expression of OCN, OPN and RUNX2. In conclusion, TLR4 tips the balance of osteoclastogenesis and osteogenesis, thereby promoting periodontal bone destruction in periodontitis.

Mesenchymal Stromal Cells Derived from Dental Tissues: Immunomodulatory Properties and Clinical Potential

Mesenchymal stem/stromal cells (MSCs) are multipotent cells located in different areas of the human body. The oral cavity is considered a potential source of MSCs because they have been identified in several dental tissues (D-MSCs). Clinical trials in which cells from these sources were used have shown that they are effective and safe as treatments for tissue regeneration. Importantly, immunoregulatory capacity has been observed in all of these populations; however, this function may vary among the different types of MSCs. Since this property is of clinical interest for cell therapy protocols, it is relevant to analyze the differences in immunoregulatory capacity, as well as the mechanisms used by each type of MSC. Interestingly, D-MSCs are the most suitable source for regenerating mineralized tissues in the oral region. Furthermore, the clinical potential of D-MSCs is supported due to their adequate capacity for proliferation, migration, and differentiation. There is also evidence for their potential application in protocols against autoimmune diseases and other inflammatory conditions due to their immunosuppressive capacity. Therefore, in this review, the immunoregulatory mechanisms identified at the preclinical level in combination with the different types of MSCs found in dental tissues are described, in addition to a description of the clinical trials in which MSCs from these sources have been applied.

AFF4 regulates osteogenic potential of human periodontal ligament stem cells via mTOR-ULK1-autophagy axis

Scaffold protein AF4/FMR2 family member 4 (AFF4) has been found to play a role in osteogenic commitment of stem cells. However, function of AFF4 in human periodontal ligament stem cells (hPDLSCs) has not been studied yet. This present study aims to investigate the biological effect of AFF4 on osteogenic differentiation of hPDLSCs and potential mechanistic pathway. First, AFF4 expression profile was evaluated in conditions of periodontitis and osteogenic differentiation of hPDLSCs by immunohistochemical staining, western blot and qRT-PCR. Next, si-RNA mediated knockdown and lentiviral transduction mediated overexpression of AFF4 were adopted to explore impact of AFF4 on osteogenic capacity of hPDLSCs. Then, possible mechanistic pathway was identified. At last, pharmacological agonist of autophagy, rapamycin, was utilized to affirm the role of autophagy in AFF4-regulated osteogenesis of hPDLSCs. First, AFF4 expressions were significantly lower in inflamed periodontal tissues and lipopolysaccharides-treated hPDLSCs than controls, and were up-regulated during osteogenic differentiation of hPDLSCs. Next, osteogenic potential of hPDLSCs was impaired by AFF4 knockdown and potentiated by AFF4 overexpression. Moreover, AFF4 was found to positively regulate autophagic activity in hPDLSCs. At last, rapamycin treatment was shown to be able to partly restore AFF4 knockdown-suppressed osteogenic differentiation. Our study demonstrates that AFF4 regulates osteogenic potential of hPDLSCs via targeting autophagic activity. The involvement of AFF4 in periodontal homeostasis was identified for the first time.

Toll-like receptor and C-type lectin receptor agonists attenuate osteogenic differentiation in human dental pulp stem cells

BACKGROUND

This study aimed to investigate the effects of various toll-like receptor (TLR) and C-type lectin receptor (CLR) ligands on osteogenic differentiation in human dental pulp stem cells (hDPSCs).

METHODS

hDPSCs were cultured and treated with various concentrations (0.01, 0.1, 1.0, and 10 µg/mL) of TLR or CLR agonists (PG-LPS, E.coli LPS, poly(I:C), Pam3CSK4, Furfurman, and Zymosan). Cell viability was determined by MTT assay. The effects of TLR and CLR agonists on osteogenic differentiation of hDPSCs were measured by alkaline phosphatase (ALP) activity, Alizarin Red S staining, and Von Kossa staining. In addition, the mRNA expression of osteogenesis-related genes (ALP, COL1A1, RUNX2, OSX, OCN and DMP1) was examined by RT-qPCR. A non-parametric analysis was employed for the statistical analyses. The statistically significant difference was considered when p < 0.05.

RESULTS

Treatment with TLR and CLR agonists was associated with an increase in hDPSCs' colony-forming unit ability. Compared with the control group, TLR and CLR agonists significantly inhibited the osteogenic differentiation of hDPSCs by decreasing the ALP activity, mineralised nodule formation, and mRNA expression levels of osteogenesis-related genes (ALP, COL1A1, RUNX2, OSX, OCN and DMP1). The inhibition of TRIF but not Akt signalling rescued the effects of TLR and CLR agonist attenuating hDPSCs' mineralisation.

CONCLUSIONS

The activation of TLRs or CLRs exhibited an inhibitory effect on osteogenic differentiation of hDPSCs via the TRIF-dependent signalling pathway.

LIPUS as a potential strategy for periodontitis treatment: A review of the mechanisms

Periodontitis is a chronic inflammatory condition triggered by oral bacteria. A sustained inflammatory state in periodontitis could eventually destroy the alveolar bone. The key objective of periodontal therapy is to terminate the inflammatory process and reconstruct the periodontal tissues. The traditional Guided tissue regeneration (GTR) procedure has unstable results due to multiple factors such as the inflammatory environment, the immune response caused by the implant, and the operator's technique. Low-intensity pulsed ultrasound (LIPUS), as acoustic energy, transmits the mechanical signals to the target tissue to provide non-invasive physical stimulation. LIPUS has positive effects in promoting bone regeneration, soft-tissue regeneration, inflammation inhibition, and neuromodulation. LIPUS can maintain and regenerate alveolar bone during an inflammatory state by suppressing the expression of inflammatory factors. LIPUS also affects the cellular behavior of periodontal ligament cells (PDLCs), thereby protecting the regenerative potential of bone tissue in an inflammatory state. However, the underlying mechanisms of the LIPUS therapy are still yet to be summarized. The goal of this review is to outline the potential cellular and molecular mechanisms of periodontitis-related LIPUS therapy, as well as to explain how LIPUS manages to transmit mechanical stimulation into the signaling pathway to achieve inflammatory control and periodontal bone regeneration.

Baricitinib alleviates lipopolysaccharide‑induced human periodontal ligament stem cell injury and promotes osteogenic differentiation by inhibiting JAK/STAT signaling

Periodontitis is the chronic inflammation of the periodontal tissue. The present study aimed to investigate the role of baricitinib, a Janus kinase (JAK)1/2 inhibitor, in periodontitis by using a lipopolysaccharide (LPS)-induced human periodontal ligament stem cell (PDLSC) model. The viability of PDLSCs stimulated by LPS was assessed in the presence of baricitinib by Cell Counting Kit-8 assay. The induction of oxidative stress was evaluated by detecting the intracellular reactive oxygen species (ROS) levels, superoxide dismutase (SOD) activity and glutathione (GSH) content. ELISA and reverse transcription-quantitative PCR were used to determine the levels of inflammatory factors TNF-α, IL-1β and IL-6. Alkaline phosphatase (ALP) activity and alizarin red staining were used to assess the osteogenic differentiation of PDLSCs. The expression levels of osteogenic differentiation- and JAK/signal transducer and activator of transcription (STAT) signaling-associated proteins were estimated with western blotting. RO8191, an agonist of the JAK/STAT pathway, was used to treat PDLSCs to investigate the regulatory mechanism of baricitinib. The results indicated that baricitinib elevated the LPS-induced decrease in cell viability. LPS-triggered oxidative stress and inflammation were inhibited by baricitinib, as demonstrated by the decreased levels of ROS, TNF-α, IL-1β, IL-6 and increased levels of SOD and GSH. In addition, baricitinib caused a marked elevation in ALP activity and mineralization ability of PDLSCs, as determined by the upregulated osteocalcin and Runt-related transcription factor 2 expression. Moreover, the expression levels of phosphorylated (p)-JAK1, p-JAK2 and p-STAT3 were downregulated by baricitinib in a dose-dependent manner. Furthermore, addition of RO8191 restored the effect of baricitinib on the induction of oxidative stress, inflammation and osteogenic differentiation of PDLSCs exposed to LPS. Collectively, these findings suggested that baricitinib alleviated oxidative stress and inflammation and promoted osteogenic differentiation of LPS-induced PDLSCs by inhibiting JAK/STAT signaling.

Osteogenic differentiation of periodontal membrane stem cells in inflammatory environments

Periodontitis is a common disease that is difficult to treat, and if not controlled in time, it causes severe conditions, such as alveolar bone resorption and tooth loosening and loss. Periodontal ligament stem cells constitute a promising cell source for regenerative treatment of periodontitis due to their high osteogenic differentiation capacity. PDLSC osteogenesis plays a central role in periodontal regeneration through successive cytokine-mediated signaling pathways and various biochemical and physicochemical factors. However, this process is inhibited in the inflammatory periodontitis environment due to high concentrations of lipopolysaccharide. Here, we review the mechanisms that influence the osteogenic differentiation of periodontal stem cells in this inflammatory microenvironment.

Activation of the pattern recognition receptor NOD1 in periodontitis impairs the osteogenic capacity of human periodontal ligament stem cells via p38/MAPK signalling

OBJECTIVES

Nucleotide oligomerization domain receptor 1 (NOD1) mediates host recognition of pathogenic bacteria in periodontium. However, the specific role of NOD1 in regulating osteogenesis is unclear. Therefore, this study focused on the activation status of NOD1 in periodontitis and its effect on the osteogenic capacity of human periodontal ligament stem cells (hPDLSCs) as well as the underlying mechanism.

METHODS

Histological staining and Western blot were utilized to assess NOD1 expression in the periodontium of people with or without periodontitis. HPDLSCs were cultured under NOD1 agonist or antagonist treatment. Q-PCR and Western blot were employed to assess the expression of osteogenic marker genes and proteins. Alizarin red staining and alkaline phosphatase staining were used to determine the osteogenic capability of hPDLSCs. The activation of downstream signalling was determined and specific inhibitors were utilized to confirm the signalling pathway in NOD1-regulated osteogenesis.

RESULTS

NOD1 expression is significantly elevated in periodontitis. With NOD1 activated by particular agonist tri-DAP, the osteogenic potential of hPDLSCs was impaired. NOD1 antagonist co-incubation partially restored the decreased osteogenesis in hPDLSCs. P38/MAPK was phosphorylated in tri-DAP-induced NOD1 activation. The inhibitor of p38 rescued the suppression of osteogenesis induced by tri-DAP in hPDLSCs.

CONCLUSIONS

Our study revealed the expression status of NOD1 in periodontitis. Its activation greatly decreased the osteogenic capacity of hPDLSCs which was mediated by the phosphorylation of p38 downstream signalling.

TLR-4 targeting contributes to the recovery of osteoimmunology in periodontitis

OBJECTIVE

The aim of this study was to determine the potential role of TLR-4 in the osteoimmunological imbalance of periodontitis.

BACKGROUND

Although current evidence supports that TLR-4 plays an important role in the inflammatory response of periodontal tissues triggered by microorganisms, little information is available regarding the function of TLR-4 in the osteoimmune regulation of homeostasis in periodontitis.

METHODS

Human gingival epithelial cells (HGEC) were isolated from the gingival tissues of 3 healthy volunteers and the expression of osteoclastogenic cytokines was evaluated by ELISA and real time RT-PCR. In addition, 30 C57BL/6 mice were used and randomly divided into three groups: control group, periodontitis group (CP) and periodontitis+TAK-242 (a specific inhibitor of TLR-4) group (TAK-242) and the expression of osteoclastogenic cytokines and the osteoclast density in the periodontal tissue were evaluated by immunohistochemical staining and tartrate resistant acid phosphatase staining. Moreover, micro-computed tomography (Micro-CT) was used to assess bone resorption.

RESULTS

The in vitro results showed that TAK-242 blocked the overproduction of IL-1, IL-6, TNF-α and RANKL in HGEC treated with LPS. The in vivo results revealed that TAK-242 also effectively decreased these osteoclastogenic cytokines in periodontal tissue of mice with periodontitis. More importantly, Micro-CT analysis showed a significant reduction of the alveolar bone loss in the TAK-242 group compared with the CP group. Furthermore, the TRAP staining showed a significant lower density of osteoclasts in the alveolar bone area of the TAK-242 group.

CONCLUSION

TLR-4 inhibition decreased the differentiation of osteoclast through the inhibition of the overproduction of osteoclastogenic cytokines and the prevention of the alveolar bone absorption in mouse periodontitis models. Therefore, the use of TAK-242 might contribute to the recovery of the osteoimmunological homeostasis and might provide a potential strategy to treat periodontal diseases.

The virulence factor GroEL directs the osteogenic and adipogenic differentiation of human periodontal ligament stem cells through the involvement of JNK/MAPK and NF-κB signaling

OBJECTIVE

GroEL, a bacterial metabolite, is an important stimulator of inflammation. The aim of this study is to confirm the effect of the virulence factor GroEL on differentiation potential of periodontal ligament (PDL) stem cells (PDLSCs) and the potential mechanisms.

METHODS

PDLSCs were obtained from extracted human premolars. GroEL was administered to osteogenic- and adipogenic-induced hPDLSCs. Alkaline phosphatase (ALP) staining, Alizarin Red staining and Oil Red staining were performed. Gene and protein expression were separately measured by qPCR and Western blotting. The expression and localization of activated signaling factors were confirmed by immunofluorescence staining. The inhibitors of myeloid differentiation factor 88 (MyD88, an adaptor protein of TLRs), JNK/MAPK and NF-κB signaling were used to verify their specific effects.

RESULTS

First, we found that GroEL inhibited the osteogenic differentiation and enhanced the adipogenic differentiation of hPDLSCs. Next, we found that GroEL increased the expression of TLR2 and TLR4 and GroEL activated JNK/MAPK and NF-κB signaling, which can be blocked by inhibition of MyD88. Finally, we found that inhibition of MyD88 restored GroEL-induced osteogenic and adipogenic differentiation and blocking JNK/MAPK or NF-κB signaling partly restored GroEL effects.

CONCLUSION

In the current study, we revealed a potential interaction between bacteria and host cells by showing that GroEL directs the osteogenic and adipogenic differentiation of hPDLSCs by the involvement of JNK/MAPK and NF-κB signaling. This study provides evidence that bacterial products can influence the differentiation of stem cells and reveals potential effect of GroEL on the context of tissue regeneration.

Toll-Like Receptors and Dental Mesenchymal Stromal Cells

Dental mesenchymal stromal cells (MSCs) are a promising tool for clinical application in and beyond dentistry. These cells possess multilineage differentiation potential and immunomodulatory properties. Due to their localization in the oral cavity, these cells could sometimes be exposed to different bacteria and viruses. Dental MSCs express various Toll-like receptors (TLRs), and therefore, they can recognize different microorganisms. The engagement of TLRs in dental MSCs by various ligands might change their properties and function. The differentiation capacity of dental MSCs might be either inhibited or enhanced by TLRs ligands depending on their nature and concentrations. Activation of TLR signaling in dental MSCs induces the production of proinflammatory mediators. Additionally, TLR ligands alter the immunomodulatory ability of dental MSCs, but this aspect is still poorly explored. Understanding the role of TLR signaling in dental MSCs physiology is essential to assess their role in oral homeostasis, inflammatory diseases, and tissue regeneration.

The virulence factor GroEL promotes gelatinase secretion from cells in the osteoblast lineage: Implication for direct crosstalk between bacteria and adult cells

OBJECTIVE

The aim of this study was to demonstrate the influence of the virulence factor GroEL on osteoblast behavior by characterizing the changes of secreted gelatinases.

DESIGN

ELISA was performed to detect GroEL from samples from patients with or without apical periodontitis. An apical periodontitis model was established in rats and the expression of MMP-2, MMP-9 and NF-κB was evaluated by immunofluorescence staining. The primary osteoblasts and osteoblast-like MC3T3 cells were stimulated with recombinant GroEL, and gelatin zymography was used to determine the activity and expression of MMP-2 and MMP-9. Western blot was used to screen signaling pathways, and immunofluorescence staining was performed to confirm the activated signaling.

RESULTS

First, we found expression of GroEL to be higher in oral saliva, gingival crevicular fluid and periradicular granulation tissue of patients with apical periodontitis than it was in healthy control patients. We next found that recombinant GroEL could increase the activity of the gelatinases, MMP-2 and MMP-9, which were secreted by both primary osteoblasts and MC3T3 cells. In a rat apical periodontitis model, strong expression of gelatinases was confirmed. Then, we found that GroEL-enhanced gelatinase activity was mediated through activation of NF-κB signaling. Acetylated NF-κB accumulated in the cell nucleus and bound to the promoter of MMP-2 and MMP-9 genes, thus initiating their high expression.

CONCLUSION

This study reveals a direct interaction between oral bacteria and adult cells by demonstrating that gelatinase secretion is induced by GroEL, which partially explains bone resorption through gelatinase activation.

Chronic Exposure of Gingival Fibroblasts to TLR2 or TLR4 Agonist Inhibits Osteoclastogenesis but Does Not Affect Osteogenesis

Chronic exposure to periodontopathogenic bacteria such as Porphyromonas gingivalis and the products of these bacteria that interact with the cells of the tooth surrounding tissues can ultimately result in periodontitis. This is a disease that is characterized by inflammation-related alveolar bone degradation by the bone-resorbing cells, the osteoclasts. Interactions of bacterial products with Toll-like receptors (TLRs), in particular TLR2 and TLR4, play a significant role in this chronic inflammatory reaction, which possibly affects osteoclastic activity and osteogenic capacity. Little is known about how chronic exposure to specific TLR activators affects these two antagonistic activities. Here, we studied the effect of TLR activation on gingival fibroblasts (GF), cells that are anatomically close to infiltrating bacterial products in the mouth. These were co-cultured with naive osteoclast precursor cells (i.e., monocytes), as part of the peripheral blood mononuclear cells (PBMCs). Activation of GF co-cultures (GF + PBMCs) with TLR2 or TLR4 agonists resulted in a weak reduction of the osteoclastogenic potential of these cultures, predominantly due to TLR2. Interestingly, chronic exposure, especially to TLR2 agonist, resulted in increased release of TNF-α at early time points. This effect, was reversed at later time points, thus suggesting an adaptation to chronic exposure. Monocyte cultures primed with M-CSF + RANKL, led to the formation of bone-resorbing osteoclasts, irrespective of being activated with TLR agonists. Late activation of these co-cultures with TLR2 and with TLR4 agonists led to a slight decrease in bone resorption. Activation of GF with TLR2 and TLR4 agonists did not affect the osteogenic capacity of the GF cells. In conclusion, chronic exposure leads to diverse reactions; inhibitory with naive osteoclast precursors, not effecting already formed (pre-)osteoclasts. We suggest that early encounter of naive monocytes with TLR agonists may result in differentiation toward the macrophage lineage, desirable for clearing bacterial products. Once (pre-)osteoclasts are formed, these cells may be relatively insensitive for direct TLR stimulation. Possibly, TLR activation of periodontal cells indirectly stimulates osteoclasts, by secreting osteoclastogenesis stimulating inflammatory cytokines.