Toll-Like Receptor 9-Mediated Inflammation Triggers Alveolar Bone Loss in Experimental Murine Periodontitis

Abstract

Chronic periodontitis is a local inflammatory disease induced by a dysbiotic microbiota and leading to destruction of the toothsupporting structures. Microbial nucleic acids are abundantly present in the periodontium, derived through release after phagocytic uptake of microbes and/or from biofilm-associated extracellular DNA. Binding of microbial DNA to its cognate receptors, such as Toll-like receptor 9 (TLR9), can trigger inflammation. In this study, we utilized TLR9 knockout (TLR9 ؊/؊ ) mice and wild-type (WT) controls in a murine model of Porphyromonas gingivalis-induced periodontitis and report the first in vivo evidence that TLR9 signaling mediates the induction of periodontal bone loss. P. gingivalis-infected WT mice exhibited significantly increased bone loss compared to that in sham-infected WT mice or P. gingivalis-infected TLR9 ؊/؊ mice, which were resistant to bone loss. Consistent with this, the expression levels of interleukin 6 (IL-6), tumor necrosis factor (TNF), and receptoractivator of nuclear factor kappa B ligand (RANKL) were significantly elevated in the gingival tissues of the infected WT mice but not in infected TLR9؊/؊ mice compared to their levels in controls. Ex vivo studies using splenocytes and bone marrow-derived macrophages revealed significantly diminished cytokine production in TLR9؊/؊ cells relative to the cytokine production in WT cells in response to P. gingivalis, thereby implicating TLR9 in inflammatory responses to this organism. Intriguingly, compared to the cytokine production in WT cells, TLR9؊/؊ cells exhibited significantly decreased proinflammatory cytokine production upon challenge with lipopolysaccharide (LPS) (TLR4 agonist) or Pam3Cys (TLR2 agonist), suggesting possible cross talk between TLR9, TLR4, and TLR2. Collectively, our results provide the first proof-of-concept evidence implicating TLR9-triggered inflammation in periodontal disease pathogenesis, thereby identifying a new potential therapeutic target to control periodontal inflammation.

Interplay of Toll-Like Receptor 9, Myeloid Cells, and Deubiquitinase A20 in Periodontal Inflammation

“…Our previous study identified that in vivo TLR9 signaling facilitates inflammatory and osteoclastogenic cytokine activation and subsequent alveolar bone loss in a chronic periodontitis model using Porphyromonas gingivalis oral gavage (18). To better understand the mechanism by which TLR9 contributes to periodontitis pathogenesis, we utilized the mechanically induced ligature model for periodontal disease in which silk sutures were placed in between all molar teeth.…”

“…Specifically for TLR9, in vivo evidence revealed that TLR9-deficient (TLR9 Ϫ/Ϫ ) mice are resistant to periodontitis using an oral gavage model of periodontitis. This provided the first proof-of-concept evidence that nucleic acid sensors are involved in periodontitis (18). Furthermore, TLR9 has been shown to modulate TLR2-and TLR4-triggered inflammation, suggesting cross talk between these sensors, possibly through downstream signaling pathways, in the course of periodontal inflammation (18).…”

“…This provided the first proof-of-concept evidence that nucleic acid sensors are involved in periodontitis (18). Furthermore, TLR9 has been shown to modulate TLR2-and TLR4-triggered inflammation, suggesting cross talk between these sensors, possibly through downstream signaling pathways, in the course of periodontal inflammation (18). Hence, understanding the mechanisms by which TLR9 contributes to periodontal inflammation can provide important insights on how to control aberrant periodontal inflammation and identify therapeutic targets and disease biomarkers critical both for local and systemic outcomes.…”

“…In this regard, disease outcomes are often guided by the polymicrobial nature of the disease, leading to the engagement of multiple innate sensors, the activation of downstream inflammatory signaling cascades, and the inability of the host to resolve inflammation, which in turn provides nourishment to feed the dysbiosis. A number of PRRs have been implicated in periodontal disease pathogenesis, including plasma membrane-associated TLR2, TLR4, complement receptors, and more recently, intracellular innate sensors, such as TLR9 and nucleotide-binding oligomerization domain-containing (NOD) receptors (17)(18)(19)(20)(21)(22). The mechanisms by which TLR2 and complement receptors cause tissue damage in periodontitis are well documented, with P. gingivalis, a keystone periodontal pathogen, instigating cross talk between these receptors in neutrophils to prevent the antimicrobial response while allowing the proinflammatory response to ensue (17).…”

Local Induction of B Cell Interleukin-10 Competency Alleviates Inflammation and Bone Loss in Ligature-Induced Experimental Periodontitis in Mice

“…Hassan et al (38) suggested that the level of CD40L/CD40 signaling was the determinant of a tilt to a pathogenic role or a protective role. It has also been shown that TLR9-mediated inflammation triggers alveolar bone loss in experimental murine periodontitis (39). Thus, the anti-inflammatory effect of B10 cells may be neutralized by the inflammation upregulated due to TLR9 activation in response to high-dose CpG.…”