Host defense against oral microbiota by bone-damaging T cells

Taxonomic and Gene Category Analyses of Subgingival Plaques from a Group of Japanese Individuals with and without Periodontitis

Periodontitis is an inflammation of tooth-supporting tissues, which is caused by bacteria in the subgingival plaque (biofilm) and the host immune response. Traditionally, subgingival pathogens have been investigated using methods such as culturing, DNA probes, or PCR. The development of next-generation sequencing made it possible to investigate the whole microbiome in the subgingival plaque. Previous studies have implicated dysbiosis of the subgingival microbiome in the etiology of periodontitis. However, details are still lacking. In this study, we conducted a metagenomic analysis of subgingival plaque samples from a group of Japanese individuals with and without periodontitis. In the taxonomic composition analysis, genus Bacteroides and Mycobacterium demonstrated significantly different compositions between healthy sites and sites with periodontal pockets. The results from the relative abundance of functional gene categories, carbohydrate metabolism, glycan biosynthesis and metabolism, amino acid metabolism, replication and repair showed significant differences between healthy sites and sites with periodontal pockets. These results provide important insights into the shift in the taxonomic and functional gene category abundance caused by dysbiosis, which occurs during the progression of periodontal disease.

Interleukin-17 induces pyroptosis in osteoblasts through the NLRP3 inflammasome pathway in vitro

OBJECTIVE

Interleukin-17 (lL-17), a pro-inflammatory cytokine produced by Th17 cells, is also considered to play an important role in bone metabolism, but the exact mechanism of bone destruction remains unclear. In this study, we explored whether IL-17 could induce osteoblasts pyroptosis in vitro.

METHODS

The murine primary osteoblasts were isolated from the calvarial bones of mice. The proliferation of osteoblasts was evaluated by cell counting kit-8 (CCK-8) assay. The mRNA levels of NOD-like receptor family pyrin domain containing 3 (NLRP3), apoptosis associated speck like protein containing a card (ASC), caspase-1, gasdermin-D (GSDMD), IL-1β and receptor activator of nuclear factor-kappa B ligand (RANKL) were measured by real-time quantitative PCR. Pyroptosis after IL-17 treatment was evaluated by lactate dehydrogenase (LDH) Release Assay Kit and the morphological characteristics of osteoblasts were observed via Scanning Electron Microscopy (SEM). Pyroptosis associated proteins, cleaved IL-1β and RANKL were evaluated through western blot. The release of IL-1β and RANKL was measured by ELISA. In addition, calcium nodule was tested by alizarin red staining.

RESULTS

High concentration IL-17 (100 ng/mL) could affect the proliferation of osteoblasts, promote the gene expression of NLRP3, caspase-1, GSDMD, IL-1β and RANKL. In contrast to control group, osteoblasts treated with IL-17 had the appearance of numerous pores, swelling and rupture. Also, the release of LDH, IL-1β and RANKL increased in the presence of IL-17. However, inhibition of NLRP3 prevented activation of the NLRP3 inflammasome, thereby restoring osteoblasts morphology and function.

CONCLUSION

IL-17 induced osteoblasts pyroptosis, and the pyroptosis of osteoblasts may prompt the release of IL-1β and RANKL,which may further contribute to disruption of bone metabolism. Besides, the NLRP3 inflammasome pathway was involved in the pyroptosis of osteoblasts.

Reduced differentiation of intestinal epithelial cells in wasting marmoset syndrome

Wasting marmoset syndrome (WMS) is a serious disease in captive common marmoset (Callithrix jacchus) colonies. Because of the high mortality rates, elucidation of the underlying mechanisms is essential. In this study, we compared the histopathology, the number of each epithelial cell in the jejunum and colon, and the expression patterns of some molecular markers between healthy and WMS-affected marmosets. Atrophy of villi in the jejunum and mononuclear cell infiltration in the lamina propria were observed in the intestinal tract of WMS-affected marmosets. Although the numbers of transient amplifying cells and tuft cells were increased, the number of goblet cells was obviously decreased in the jejunum and colon of WMS-affected marmosets compared to healthy marmosets. In addition, the number of enterocytes in the jejunum was decreased in WMS animals. There was no apparent difference in the numbers of stem cells, enteroendocrine cells, or Paneth cells. The expression of β-catenin and Tcf7l2 was increased in WMS, and the co-existence of β-catenin and Tcf7l2/Cyclin D1 was observed around the crypts in WMS-affected marmosets. These findings suggest that cell proliferation continues, but cell differentiation is halted in the intestinal tract due to the enhanced β-catenin/Tcf7l2/Cyclin D1signaling pathway in WMS, which results in malfunction of the villus and mucosa.

RvE1 Impacts the Gingival Inflammatory Infiltrate by Inhibiting the T Cell Response in Experimental Periodontitis

Periodontitis is a chronic inflammatory disease associated with the formation of dysbiotic plaque biofilms and characterized by the progressive destruction of the alveolar bone. The transition from health to disease is characterized by a shift in periodontal immune cell composition, from mostly innate (neutrophils) to adaptive (T lymphocytes) immune responses. Resolvin E1 (RvE1) is a specialized pro-resolution mediator (SPMs), produced in response to inflammation, to enhance its resolution. Previous studies have indicated the therapeutic potential of RvE1 in periodontal disease; however, the impact of RvE1 in the microbial-elicited osteoclastogenic immune response remains uncharacterized in vivo. In the present study, we studied the impact of RvE1 on the gingival inflammatory infiltrate formation during periodontitis in a mouse model. First, we characterized the temporal-dependent changes of the main immune cells infiltrating the gingiva by flow cytometry. Then, we evaluated the impact of early or delayed RvE1 administration on the gingival immune infiltration and cervical lymph nodes composition. We observed a consistent inhibitory outcome on T cells -particularly effector T cells- and a protective effect on regulatory T cells (Tregs). Our data further demonstrated the wide range of actions of RvE1, its preventive role in the establishment of the adaptive immune response during inflammation, and bone protective capacity.

Connection between Periodontitis-Induced Low-Grade Endotoxemia and Systemic Diseases: Neutrophils as Protagonists and Targets

Periodontitis is considered a promoter of many systemic diseases, but the signaling pathways of this interconnection remain elusive. Recently, it became evident that certain microbial challenges promote a heightened response of myeloid cell populations to subsequent infections either with the same or other pathogens. This phenomenon involves changes in the cell epigenetic and transcription, and is referred to as ''trained immunity''. It acts via modulation of hematopoietic stem and progenitor cells (HSPCs). A main modulation driver is the sustained, persistent low-level transmission of lipopolysaccharide from the periodontal pocket into the peripheral blood. Subsequently, the neutrophil phenotype changes and neutrophils become hyper-responsive and prone to boosted formation of neutrophil extracellular traps (NET). Cytotoxic neutrophil proteases and histones are responsible for ulcer formations on the pocket epithelium, which foster bacteremia and endoxemia. The latter promote systemic low-grade inflammation (SLGI), a precondition for many systemic diseases and some of them, e.g., atherosclerosis, diabetes etc., can be triggered by SLGI alone. Either reverting the polarized neutrophils back to the homeostatic state or attenuation of neutrophil hyper-responsiveness in periodontitis might be an approach to diminish or even to prevent systemic diseases.

Liquid PRF Reduces the Inflammatory Response and Osteoclastogenesis in Murine Macrophages

Macrophage activation and osteoclastogenesis are hallmarks of inflammatory osteolysis and may be targeted by the local application of liquid platelet-rich fibrin (PRF). Liquid PRF is produced by a hard spin of blood in the absence of clot activators and anticoagulants, thereby generating an upper platelet-poor plasma (PPP) layer, a cell-rich buffy coat layer (BC; termed concentrated-PRF or C-PRF), and the remaining red clot (RC) layer. Heating PPP has been shown to generate an albumin gel (Alb-gel) that when mixed back with C-PRF generates Alb-PRF having extended working properties when implanted in vivo. Evidence has demonstrated that traditional solid PRF holds a potent anti-inflammatory capacity and reduces osteoclastogenesis. Whether liquid PRF is capable of also suppressing an inflammatory response and the formation of osteoclasts remains open. In the present study, RAW 264.7 and primary macrophages were exposed to lipopolysaccharides (LPS), lactoferrin, and agonists of Toll-like receptors (TLR3 and TLR7) in the presence or absence of lysates prepared by freeze-thawing of liquid PPP, BC, Alb-gel, and RC. For osteoclastogenesis, primary macrophages were exposed to receptor activator of nuclear factor kappa B ligand (RANKL), macrophage colony-stimulating factor (M-CSF), and human transforming growth factor-β1 (TGF-β1) in the presence or absence of PPP, BC, Alb-gel, RC lysates and hemoglobin. We show here that it is mainly the lysates prepared from PPP and BC that consistently reduced the agonist-induced expression of interleukin 6 (IL6) and cyclooxygenase-2 (COX2) in macrophages, as determined by RT-PCR and immunoassay. With respect to osteoclastogenesis, lysates from PPP and BC but also from RC, similar to hemoglobin, reduced the expression of osteoclast marker genes tartrate-resistant acid phosphatase (TRAP) and cathepsin K, as well as TRAP histochemical staining. These findings suggest that liquid PRF holds a potent in vitro heat-sensitive anti-inflammatory activity in macrophages that goes along with an inhibition of osteoclastogenesis.

Relationship between NAFLD and Periodontal Disease from the View of Clinical and Basic Research, and Immunological Response

Periodontal disease is an inflammatory disease caused by pathogenic oral microorganisms that leads to the destruction of alveolar bone and connective tissues around the teeth. Although many studies have shown that periodontal disease is a risk factor for systemic diseases, such as type 2 diabetes and cardiovascular diseases, the relationship between nonalcoholic fatty liver disease (NAFLD) and periodontal disease has not yet been clarified. Thus, the purpose of this review was to reveal the relationship between NAFLD and periodontal disease based on epidemiological studies, basic research, and immunology. Many cross-sectional and prospective epidemiological studies have indicated that periodontal disease is a risk factor for NAFLD. An in vivo animal model revealed that infection with periodontopathic bacteria accelerates the progression of NAFLD accompanied by enhanced steatosis. Moreover, the detection of periodontopathic bacteria in the liver may demonstrate that the bacteria have a direct impact on NAFLD. Furthermore, Porphyromonas gingivalis lipopolysaccharide induces inflammation and accumulation of intracellular lipids in hepatocytes. Th17 may be a key molecule for explaining the relationship between periodontal disease and NAFLD. In this review, we attempted to establish that oral health is essential for systemic health, especially in patients with NAFLD.

Near-infrared 940-nm diode laser photobiomodulation of inflamed periodontal ligament stem cells

Photobiomodulation (PBM) is an acceptable method of stimulating stem cells through its non-invasive absorption by the cell photoreceptors and the induction of cellular response. The current research was aimed at evaluating the effect of near-infrared PBM on proliferation and osteogenic differentiation in inflamed periodontal ligament stem cells (I-PDLSCs). I-PDLSCs were isolated and characterized. Third passage cells were irradiated with 940-nm laser at an output power of 100 mW in a continuous wave. A fluence of 4 J/cm² in three sessions at 48-h intervals was applied and compared with non-irradiated controls. Cell viability and proliferation were evaluated by MTT assay. Alkaline phosphatase activity, quantitative Alizarin red staining test, and q-RT-PCR were used to evaluate the osteogenic properties of the I-PDLSCs in four groups of (a) osteogenic differentiation medium + laser (ODM + L), (b) osteogenic differentiation medium without laser (ODM), (c) non-osteogenic differentiation medium + laser (L), and (d) non-osteogenic differentiation medium (control). There was a non-significant increase in the viability of cells at 48- and 72-h post last laser irradiation. Alizarin red staining revealed no significant stimulatory effect of PBM at 14 and 21 days. However, alkaline phosphatase activity was significantly higher in the L + ODM group. Expression of osteogenic-related genes had a statistically significant increase at 21-day post irradiation. The irradiation used in the present study showed no significant increase in the proliferation of I-PDLSCs by PBM. However, expression levels of osteogenic-related genes and alkaline phosphatase activity were significantly increased in irradiated groups.

Effects of Erythromycin on Osteoclasts and Bone Resorption via DEL-1 Induction in Mice

Macrolides are used to treat various infectious diseases, including periodontitis. Furthermore, macrolides are known to have immunomodulatory effects; however, the underlying mechanism of their action remains unclear. DEL-1 has emerged as an important factor in homeostatic immunity and osteoclastogenesis. Specifically, DEL-1 is downregulated in periodontitis tissues. Therefore, in the present study, we investigated whether the osteoclastogenesis inhibitory effects of erythromycin (ERM) are mediated through upregulation of DEL-1 expression. We used a ligature-induced periodontitis model in C57BL/6Ncrl wild-type or DEL-1-deficient mice and in vitro cell-based mechanistic studies to investigate how ERM inhibits alveolar bone resorption. As a result of measuring alveolar bone resorption and gene expression in the tooth ligation model, ERM treatment reduced bone loss by increasing DEL-1 expression and decreasing the expression of osteoclast-related factors in wild-type mice. In DEL-1-deficient mice, ERM failed to suppress bone loss and gene expression of osteoclast-related factors. In addition, ERM treatment downregulated osteoclast differentiation and calcium resorption in in vitro experiments with mouse bone marrow-derived macrophages. In conclusion, ERM promotes the induction of DEL-1 in periodontal tissue, which may regulate osteoclastogenesis and decrease inflammatory bone resorption. These findings suggest that ERM may exert immunomodulatory effects in a DEL-1-dependent manner.

Maintaining homeostatic control of periodontal bone tissue

Alveolar bone is a unique osseous tissue due to the proximity of dental plaque biofilms. Periodontal health and homeostasis are mediated by a balanced host immune response to these polymicrobial biofilms. Dysbiotic shifts within dental plaque biofilms can drive a proinflammatory immune response state in the periodontal epithelial and gingival connective tissues, which leads to paracrine signaling to subjacent bone cells. Sustained chronic periodontal inflammation disrupts "coupled" osteoclast-osteoblast actions, which ultimately result in alveolar bone destruction. This chapter will provide an overview of alveolar bone physiology and will highlight why the oral microbiota is a critical regulator of alveolar bone remodeling. The ecology of dental plaque biofilms will be discussed in the context that periodontitis is a polymicrobial disruption of host homeostasis. The pathogenesis of periodontal bone loss will be explained from both a historical and current perspective, providing the opportunity to revisit the role of fibrosis in alveolar bone destruction. Periodontal immune cell interactions with bone cells will be reviewed based on our current understanding of osteoimmunological mechanisms influencing alveolar bone remodeling. Lastly, probiotic and prebiotic interventions in the oral microbiota will be evaluated as potential noninvasive therapies to support alveolar bone homeostasis and prevent periodontal bone loss.

Exosomes from TNF-α-treated human gingiva-derived MSCs enhance M2 macrophage polarization and inhibit periodontal bone loss

Mesenchymal stem cell (MSC)-derived exosome plays a central role in the cell-free therapeutics involving MSCs and the contents can be customized under disease-associated microenvironments. However, optimal MSC-preconditioning to enhance its therapeutic potential is largely unknown. Here, we show that preconditioning of gingival tissue-derived MSCs (GMSCs) with tumor necrosis factor-alpha (TNF-α) is ideal for the treatment of periodontitis. TNF-α stimulation not only increased the amount of exosome secreted from GMSCs, but also enhanced the exosomal expression of CD73, thereby inducing anti-inflammatory M2 macrophage polarization. The effect of GMSC-derived exosomes on inflammatory bone loss were examined by ligature-induced periodontitis model in mice. Local injection of GMSC-derived exosomes significantly reduced periodontal bone resorption and the number of tartrate-resistant acid phosphatase (TRAP)-positive osteoclasts, and these effects were further enhanced by preconditioning of GMSCs with TNF-α. Thus, GMSC-derived exosomes also exhibited anti-osteoclastogenic activity. Receptor activator of NF-κB ligand (RANKL) expression was regulated by Wnt5a in periodontal ligament cells (PDLCs), and exosomal miR-1260b was found to target Wnt5a-mediated RANKL pathway and inhibit its osteoclastogenic activity. These results indicate that significant ability of the TNF-α-preconditioned GMSC-derived exosomes to regulate inflammation and osteoclastogenesis paves the way for establishment of a therapeutic approach for periodontitis.

The DEL-1/β3 integrin axis promotes regulatory T cell responses during inflammation resolution

FOXP3+CD4+ regulatory T cells (Tregs) are critical for immune homeostasis and respond to local tissue cues, which control their stability and function. We explored here whether developmental endothelial locus-1 (DEL-1), which, like Tregs, increases during resolution of inflammation, promotes Treg responses. DEL-1 enhanced Treg numbers and function at barrier sites (oral and lung mucosa). The underlying mechanism was dissected using mice lacking DEL-1 or expressing a point mutant thereof, or mice with T cell-specific deletion of the transcription factor RUNX1, identified by RNA sequencing analysis of the DEL-1-induced Treg transcriptome. Specifically, through interaction with αvβ3 integrin, DEL-1 promoted induction of RUNX1-dependent FOXP3 expression and conferred stability of FOXP3 expression upon Treg restimulation in the absence of exogenous TGF-β1. Consistently, DEL-1 enhanced the demethylation of the Treg-specific demethylated region (TSDR) in the mouse Foxp3 gene and the suppressive function of sorted induced Tregs. Similarly, DEL-1 increased RUNX1 and FOXP3 expression in human conventional T cells, promoting their conversion into induced Tregs with increased TSDR demethylation, enhanced stability, and suppressive activity. We thus uncovered a DEL-1/αvβ3/RUNX1 axis that promotes Treg responses at barrier sites and offers therapeutic options for modulating inflammatory/autoimmune disorders.

Ovariectomy induces bone loss via microbial-dependent trafficking of intestinal TNF+ T cells and Th17 cells

Estrogen deficiency causes a gut microbiome-dependent expansion of BM Th17 cells and TNF-α-producing T cells. The resulting increased BM levels of IL-17a (IL-17) and TNF stimulate RANKL expression and activity, causing bone loss. However, the origin of BM Th17 cells and TNF+ T cells is unknown. Here, we show that ovariectomy (ovx) expanded intestinal Th17 cells and TNF+ T cells, increased their S1P receptor 1-mediated (S1PR1-mediated) egress from the intestine, and enhanced their subsequent influx into the BM through CXCR3- and CCL20-mediated mechanisms. Demonstrating the functional relevance of T cell trafficking, blockade of Th17 cell and TNF+ T cell egress from the gut or their influx into the BM prevented ovx-induced bone loss. Therefore, intestinal T cells are a proximal target of sex steroid deficiency relevant for bone loss. Blockade of intestinal T cell migration may represent a therapeutic strategy for the treatment of postmenopausal bone loss.

Act Locally, Act Globally-Microbiota, Barriers, and Cytokines in Atherosclerosis

Atherosclerosis is a lipid-driven chronic inflammatory disease that is characterized by the formation and progressive growth of atherosclerotic plaques in the wall of arteries. Atherosclerosis is a major predisposing factor for stroke and heart attack. Various immune-mediated mechanisms are implicated in the disease initiation and progression. Cytokines are key mediators of the crosstalk between innate and adaptive immune cells as well as non-hematopoietic cells in the aortic wall and are emerging players in the regulation of atherosclerosis. Progression of atherosclerosis is always associated with increased local and systemic levels of pro-inflammatory cytokines. The role of cytokines within atherosclerotic plaque has been extensively investigated; however, the cell-specific role of cytokine signaling, particularly the role of cytokines in the regulation of barrier tissues tightly associated with microbiota in the context of cardiovascular diseases has only recently come to light. Here, we summarize the knowledge about the function of cytokines at mucosal barriers and the interplay between cytokines, barriers, and microbiota and discuss their known and potential implications for atherosclerosis development.

Local and systemic mechanisms linking periodontal disease and inflammatory comorbidities

Periodontitis, a major inflammatory disease of the oral mucosa, is epidemiologically associated with other chronic inflammation-driven disorders, including cardio-metabolic, neurodegenerative and autoimmune diseases and cancer. Emerging evidence from interventional studies indicates that local treatment of periodontitis ameliorates surrogate markers of comorbid conditions. The potential causal link between periodontitis and its comorbidities is further strengthened by recent experimental animal studies establishing biologically plausible and clinically consistent mechanisms whereby periodontitis could initiate or aggravate a comorbid condition. This multi-faceted ‘mechanistic causality’ aspect of the link between periodontitis and comorbidities is the focus of this Review. Understanding how certain extra-oral pathologies are affected by disseminated periodontal pathogens and periodontitis-associated systemic inflammation, including adaptation of bone marrow haematopoietic progenitors, may provide new therapeutic options to reduce the risk of periodontitis-associated comorbidities.

Periodontitis has been causally linked to the development of other chronic inflammatory diseases outside the oral mucosa. In this Review, George Hajishengallis and Triantafyllos Chavakis consider the molecular basis of these links.

γδT Cells Are Essential for Orthodontic Tooth Movement

Sustained mechanical forces applied to tissue are known to shape local immunity. In the oral mucosa, mechanical stress, either naturally induced by masticatory forces or externally via mechanical loading during orthodontic tooth movement (OTM), is translated, in part, by T cells to alveolar bone resorption. Nevertheless, despite being considered critical for OTM, depletion of CD4⁺ and CD8⁺ T cells is reported to have no impact on tooth movement, thus questioning the function of αβT cells in OTM-associated bone resorption. To further address the role of T cells in OTM, we first characterized the leukocytes residing in the periodontal ligament (PDL), the tissue of interest during OTM, and compared it to the neighboring gingiva. Unlike the gingiva, monocytes and neutrophils represent the major leukocytes of the PDL. These myeloid cells were also the main leukocytes in the PDL of germ-free mice, although at lower levels than SPF mice. T lymphocytes were more enriched in the gingiva than the PDL, yet in both tissues, the relative fraction of the γδT cells was higher than the αβ T cells. We thus sought to examine the role of γδT cells in OTM. γδT cells residing in the PDL were mainly Vγ6⁺ and produced interleukin (IL)-17A but not interferon-γ. Using Tcrd-GDL mice allowing conditional ablation of γδT cells in vivo, we demonstrate that OTM was greatly diminished in the absence of γδT cells. Further analysis revealed that ablation of γδT cells decreased early IL-17A expression, monocyte and neutrophil recruitment, and the expression of the osteoclastogenic molecule receptor activator of nuclear factor-κβ ligand. This, eventually, resulted in reduced numbers of osteoclasts in the pressure site during OTM. Collectively, our data suggest that γδT cells are essential in OTM for translating orthodontic mechanical forces to bone resorption, required for relocating the tooth in the alveolar bone.

Local destruction from distant action

Discussion on the identification of an osteoclast precursor population, which emerges in the bone marrow after systemic infection with a periodontal pathogen.

Analysis of Experimental Ligature-Induced Periodontitis Model in Mice

Periodontitis is one of the most prevalent chronic inflammatory diseases in humans. However, the disease has been hard to study, majorly because it has been difficult to establish a reproducible animal model. Nonetheless, the ligature-induced periodontitis model in rodent has shown some promise. Here we describe a simplified systematic method to analyze periodontal pathogenesis using quantitative polymerase chain reaction, immunohistochemistry, and bone phenotype in ligature-induced periodontitis murine model. We provide detailed experimental methods and also provide notes that will help to carry out the procedure successfully.

RANKL and osteoimmunology in periodontitis

Periodontitis, one of the most common infectious diseases in humans, is characterized by inflammation of the periodontal tissue and subsequent destruction of the alveolar bone, which ultimately leads to tooth loss. Recently, it was revealed that the osteoclastic bone damage that occurs during periodontitis is dependent on the receptor activator of NF-kB ligand (RANKL) produced by osteoblastic cells and periodontal ligament cells. Immune cells provide essential cues for the RANKL induction that takes place during periodontal inflammation. The knowledge accumulated and experimental tools established in the field of "osteoimmunology" have made crucial contributions to a better understanding of periodontitis pathogenesis and, reciprocally, the investigation of periodontitis has provided important insights into the field. This review discusses the molecular mechanisms underlying periodontal bone loss by focusing on the osteoimmune interactions and RANKL.

The Role of the Immune Microenvironment in Bone Regeneration

Bone is an active tissue, being constantly renewed in healthy individuals with participation of the immune system to a large extent. Any imbalance between the processes of bone formation and bone resorption is linked to various inflammatory bone diseases. The immune system plays an important role in tissue formation and bone resorption. Recently, many studies have demonstrated complex interactions between the immune and skeletal systems. Both of immune cells and cytokines contribute to the regulation of bone homeostasis, and bone cells, including osteoblasts, osteoclasts, osteocytes, also influence the cellular functions of immune cells. These crosstalk mechanisms between the bone and immune system finally emerged, forming a new field of research called osteoimmunology. Therefore, the immune microenvironment is crucial in determining the speed and outcome of bone healing, repair, and regeneration. In this review, we summarise the role of the immune microenvironment in bone regeneration from the aspects of immune cells and immune cytokines. The elucidation of immune mechanisms involved in the process of bone regeneration would provide new therapeutic targets for improving the curative effects of bone injury treatment.

RANKL as the master regulator of osteoclast differentiation

RANKL, the essential cue for osteoclast differentiation, is the membrane-bound factor expressed by osteoclastogenesis-supporting cells such as osteoblasts and osteocytes. In vivo evidence indicates that RANKL functions as the indispensable and irreplaceable in the program of osteoclast differentiation. The reason why RANKL plays a critical role in osteoclastogenesis is discussed from the viewpoint of the distinct signaling pathways mediated by co-stimulatory receptors and the key transcription factor NFATc1.

The Role of the Oral Microbiota Related to Periodontal Diseases in Anxiety, Mood and Trauma- and Stress-Related Disorders

The prevalence of anxiety, mood and trauma- and stress-related disorders are on the rise; however, efforts to develop new and effective treatment strategies have had limited success. To identify novel therapeutic targets, a comprehensive understanding of the disease etiology is needed, especially in the context of the holobiont, i.e., the superorganism consisting of a human and its microbiotas. Much emphasis has been placed on the role of the gut microbiota in the development, exacerbation, and persistence of psychiatric disorders; however, data for the oral microbiota are limited. The oral cavity houses the second most diverse microbial community in the body, with over 700 bacterial species that colonize the soft and hard tissues. Periodontal diseases encompass a group of infectious and inflammatory diseases that affect the periodontium. Among them, periodontitis is defined as a chronic, multi-bacterial infection that elicits low-grade systemic inflammation via the release of pro-inflammatory cytokines, as well as local invasion and long-distance translocation of periodontal pathogens. Periodontitis can also induce or exacerbate other chronic systemic inflammatory diseases such as atherosclerosis and diabetes and can lead to adverse pregnancy outcomes. Recently, periodontal pathogens have been implicated in the etiology and pathophysiology of neuropsychiatric disorders (such as depression and schizophrenia), especially as dysregulation of the immune system also plays an integral role in the etiology and pathophysiology of these disorders. This review will discuss the role of the oral microbiota associated with periodontal diseases in anxiety, mood and trauma- and stress-related disorders. Epidemiological data of periodontal diseases in individuals with these disorders will be presented, followed by a discussion of the microbiological and immunological links between the oral microbiota and the central nervous system. Pre-clinical and clinical findings on the oral microbiota related to periodontal diseases in anxiety, mood and trauma- and stress-related phenotypes will be reviewed, followed by a discussion on the bi-directionality of the oral-brain axis. Lastly, we will focus on the oral microbiota associated with periodontal diseases as a target for future therapeutic interventions to alleviate symptoms of these debilitating psychiatric disorders.

Gastrin-Releasing Peptide (GRP) Stimulates Osteoclastogenesis in Periodontitis

Periodontitis is a chronic inflammatory disease with alveolar bone resorption and subsequent tooth loss as its ultimate outcomes. Gastrin-releasing peptide (GRP) is a neuropeptide with growth-stimulatory and tumorigenic properties, and neuropeptides have previously been suggested to play a role in the complex cascade of chemical activity associated with periodontal inflammation. In this study, GRP treatment enhanced the differentiation of bone marrow-derived macrophages (BMMs) into osteoclasts, and gastrin-releasing peptide receptor (GRPR) antagonists suppressed the pro-osteoclastogenic effect of GRP. Grpr-siRNA knockdown resulted in a significantly lower number of osteoclasts formed as compared with the control. Interestingly, gene expression analysis indicated downregulation of Grp and Grpr expressions in BMMs during osteoclastogenesis. Moreover, ligature-induced periodontitis model in mice and gingival samples from patients with periodontitis displayed increased immunostaining of GRP in the oral epithelium. Subsequently, stimulation of mouse primary epithelial cells (ECs) and HaCaT cells, human epidermal keratinocytes, with lipopolysaccharides (LPS) of Porphyromonas gingivalis or live P. gingivalis upregulated Grp and Grpr expressions. Finally, coculture of P. gingivalis-stimulated ECs and BMMs using Transwell system revealed that the differentiation of BMMs was induced when subjected to paracrine activation by LPS- as well as live-P. gingivalis stimulated ECs. Taken together, our results demonstrate that the pro-osteoclastogenic properties of BMMs may be modulated by GRP produced by ECs in the periodontal microenvironment.

NETs Are Double-Edged Swords with the Potential to Aggravate or Resolve Periodontal Inflammation

Periodontitis is a general term for diseases characterised by inflammatory destruction of tooth-supporting tissues, gradual destruction of the marginal periodontal ligament and resorption of alveolar bone. Early-onset periodontitis is due to disturbed neutrophil extracellular trap (NET) formation and clearance. Indeed, mutations that inactivate the cysteine proteases cathepsin C result in the massive periodontal damage seen in patients with deficient NET formation. In contrast, exaggerated NET formation due to polymorphonuclear neutrophil (PMN) hyper-responsiveness drives the pathology of late-onset periodontitis by damaging and ulcerating the gingival epithelium and retarding epithelial healing. Despite the gingival regeneration, periodontitis progression ends with almost complete loss of the periodontal ligament and subsequent tooth loss. Thus, NETs help to maintain periodontal health, and their dysregulation, either insufficiency or surplus, causes heavy periodontal pathology and edentulism.

Stepwise cell fate decision pathways during osteoclastogenesis at single-cell resolution

Osteoclasts are the exclusive bone-resorbing cells, playing a central role in bone metabolism, as well as the bone damage that occurs under pathological conditions^1,2. In postnatal life, haematopoietic stem-cell-derived precursors give rise to osteoclasts in response to stimulation with macrophage colony-stimulating factor and receptor activator of nuclear factor-κB ligand, both of which are produced by osteoclastogenesis-supporting cells such as osteoblasts and osteocytes^1-3. However, the precise mechanisms underlying cell fate specification during osteoclast differentiation remain unclear. Here, we report the transcriptional profiling of 7,228 murine cells undergoing in vitro osteoclastogenesis, describing the stepwise events that take place during the osteoclast fate decision process. Based on our single-cell transcriptomic dataset, we find that osteoclast precursor cells transiently express CD11c, and deletion of receptor activator of nuclear factor-κB specifically in CD11c-expressing cells inhibited osteoclast formation in vivo and in vitro. Furthermore, we identify Cbp/p300-interacting transactivator with Glu/Asp-rich carboxy-terminal domain 2 (Cited2) as the molecular switch triggering terminal differentiation of osteoclasts, and deletion of Cited2 in osteoclast precursors in vivo resulted in a failure to commit to osteoclast fate. Together, the results of this study provide a detailed molecular road map of the osteoclast differentiation process, refining and expanding our understanding of the molecular mechanisms underlying osteoclastogenesis.

Regulatory T cell phenotype and anti-osteoclastogenic function in experimental periodontitis

The alveolar bone resorption is a distinctive feature of periodontitis progression and determinant for tooth loss. Regulatory T lymphocytes (Tregs) display immuno-suppressive mechanisms and tissue repairing functions, which are critical to support periodontal health. Tregs may become unstable and dysfunctional under inflammatory conditions, which can even accelerate tissue destruction. In this study, experimental periodontitis was associated with the progressive and increased presence of Th17 and Treg-related mediators in the gingiva (IL-6, IL-17A, IL-17F, RANKL, IL-10, TGF-β and GITR; P < 0.05), and the proliferation of both Treg and Th17 cells in cervical lymph nodes. Tregs from cervical lymph nodes had reduced Foxp3 expression (> 25% MFI loss) and increased IL-17A expression (> 15%), compared with Tregs from spleen and healthy controls. Tregs gene expression analysis showed a differential signature between health and disease, with increased expression of Th17-associated factors in periodontitis-derived Tregs. The ex vivo suppression capacity of Tregs on osteoclastic differentiation was significantly lower in Tregs obtained from periodontally diseased animals compared to controls (P < 0.05), as identified by the increased number of TRAP⁺ osteoclasts (P < 0.01) in the Tregs/pre-osteoclast co-cultures. Taken together, these results demonstrate that Tregs become phenotypically unstable and lose anti-osteoclastogenic properties during experimental periodontitis; thus, further promoting the Th17-driven bone loss.

A review of T helper 17 cell-related cytokines in serum and saliva in periodontitis

Periodontitis is a chronic inflammatory disease with a complex underlying immunopathology. Cytokines, as molecular mediators of inflammation, play a role in all stages of disease progression. T helper 17 (Th17) cells are thought to play a role in periodontitis. Th17 cell development and maintenance requires a pro-inflammatory cytokine milieu, with many of the cytokines implicated in the pathogenesis of periodontitis. Serum and saliva are easily accessible biofluids which can represent the systemic and local environment to promote the development of Th17 cells. Here we review human clinical studies that investigate IL-1β, IL-4, IL-6, IL-10, IL-17A, IL-17F, IL-21, IL-22, IL-23, IL-25, IL-31, IL-33, IFN-γ, sCD40L and TNF-α in serum and saliva in periodontitis. We highlight their putative role in the pathogenesis of periodontitis and place them within a wider context of animal and other clinical studies.

Current understanding of periodontal disease pathogenesis and targets for host-modulation therapy

Recent advances indicate that periodontitis is driven by reciprocally reinforced interactions between a dysbiotic microbiome and dysregulated inflammation. Inflammation is not only a consequence of dysbiosis but, via mediating tissue dysfunction and damage, fuels further growth of selectively dysbiotic communities of bacteria (inflammophiles), thereby generating a self-sustained feed-forward loop that perpetuates the disease. These considerations provide a strong rationale for developing adjunctive host-modulation therapies for the treatment of periodontitis. Such host-modulation approaches aim to inhibit harmful inflammation and promote its resolution or to interfere directly with downstream effectors of connective tissue and bone destruction. This paper reviews diverse strategies targeted to modulate the host periodontal response and discusses their mechanisms of action, perceived safety, and potential for clinical application.

The Role of the Microbiome in Driving RA-Related Autoimmunity

Once referred to as "normal commensal flora" the human microbiome plays an integral role between health and disease. The host mucosal surface replete with a multitude of immune cells is a vast arena constantly sensing and responding to antigen presentation and microbial by-products. It is this key role that may allow the microbiome to prime or protect the host from autoimmune disease. Rheumatoid arthritis (RA) is a chronic, disabling inflammatory condition characterized by a complex multifactorial etiology. The presence of certain genetic markers has been proven to increase susceptibility to RA however it does not guarantee disease development. Given low concordance rates demonstrated in monozygotic twin studies there is a clear implication for the involvement of external players in RA pathogenesis. Since the historical description of rheumatoid factor, numerous additional autoantibodies have been described in the sera of RA patients. The presence of anti-cyclic citrullinated protein antibody is now a standard test, and is associated with a more severe disease course. Interestingly these antibodies are detectable in patient's sera long before the clinical signs of RA occur. The production of autoantibodies is driven by the lack of tolerance of the immune system, and how tolerance is broken is a crucial question for understanding RA development. Here we review current literature on the role of the microbiome in RA development including periodontal, gut and lung mucosa, with particular focus on proposed mechanisms of host microbiome interactions. We discuss the use of Mendelian randomization to assign causality to the microbiome and present considerations for future studies.

Sclerostin expression in trabecular bone is downregulated by osteoclasts

Bone tissues have trabecular bone with a high bone turnover and cortical bone with a low turnover. The mechanisms by which the turnover rate of these bone tissues is determined remain unclear. Osteocytes secrete sclerostin, a Wnt/β-catenin signaling antagonist, and inhibit bone formation. We found that sclerostin expression in cortical bone is more marked than in trabecular bone in Sost reporter mice. Leukemia inhibitory factor (LIF) secreted from osteoclasts reportedly suppressed sclerostin expression and promoted bone formation. Here, we report that osteoclasts downregulate sclerostin expression in trabecular bone and promote bone turnover. Treatment of C57BL/6 mice with an anti-RANKL antibody eliminated the number of osteoclasts and LIF-positive cells in trabecular bone. The number of sclerostin-positive cells was increased in trabecular bone, while the number of β-catenin-positive cells and bone formation were decreased in trabecular bone. Besides, Tnfsf11 heterozygous (Rankl^+/−) mice exhibited a decreased number of LIF-positive cells and increased number of sclerostin-positive cells in trabecular bone. Rankl^+/− mice exhibited a decreased number of β-catenin-positive cells and reduced bone formation in trabecular bone. Furthermore, in cultured osteoclasts, RANKL stimulation increased Lif mRNA expression, suggesting that RANKL signal increased LIF expression. In conclusion, osteoclasts downregulate sclerostin expression and promote trabecular bone turnover.

Dendritic cell derived exosomes loaded with immunoregulatory cargo reprogram local immune responses and inhibit degenerative bone disease in vivo

Chronic bone degenerative diseases represent a major threat to the health and well-being of the population, particularly those with advanced age. This study isolated exosomes (EXO), natural nano-particles, from dendritic cells, the “directors” of the immune response, to examine the immunobiology of DC EXO in mice, and their ability to reprogram immune cells responsible for experimental alveolar bone loss in vivo. Distinct DC EXO subtypes including immune-regulatory (regDC EXO), loaded with TGFB1 and IL10 after purification, along with immune stimulatory (stimDC EXO) and immune “null” immature (iDCs EXO) unmodified after purification, were delivered via I.V. route or locally into the soft tissues overlying the alveolar bone. Locally administrated regDC EXO showed high affinity for inflamed sites, and were taken up by both DCs and T cells in situ. RegDC EXO-encapsulated immunoregulatory cargo (TGFB1 and IL10) was protected from proteolytic degradation. Moreover, maturation of recipient DCs and induction of Th17 effectors was suppressed by regDC EXO, while T-regulatory cell recruitment was promoted, resulting in inhibition of bone resorptive cytokines and reduction in osteoclastic bone loss. This work is the first demonstration of DC exosome-based therapy for a degenerative alveolar bone disease and provides the basis for a novel treatment strategy.

The Bacterial Connection between the Oral Cavity and the Gut Diseases

More than 100 trillion symbiotic microorganisms constitutively colonize throughout the human body, including the oral cavity, the skin, and the gastrointestinal tract. The oral cavity harbors one of the most diverse and abundant microbial communities within the human body, second to the community that resides in the gastrointestinal tract, and is composed of >770 bacterial species. Advances in sequencing technologies help define the precise microbial landscape in our bodies. Environmental and functional differences render the composition of resident microbiota largely distinct between the mouth and the gut and lead to the development of unique microbial ecosystems in the 2 mucosal sites. However, it is apparent that there may be a microbial connection between these 2 mucosal sites in the context of disease pathogenesis. Accumulating evidence indicates that resident oral bacteria can translocate to the gastrointestinal tract through hematogenous and enteral routes. The dissemination of oral microbes to the gut may exacerbate various gastrointestinal diseases, including irritable bowel syndrome, inflammatory bowel disease, and colorectal cancer. However, the precise role that oral microbes play in the extraoral organs, including the gut, remains elusive. Here, we review the recent findings on the dissemination of oral bacteria to the gastrointestinal tract and their possible contribution to the pathogenesis of gastrointestinal diseases. Although little is known about the mechanisms of ectopic colonization of the gut by oral bacteria, we also discuss the potential factors that allow the oral bacteria to colonize the gut.

Chitosan hydrogel incorporated with dental pulp stem cell-derived exosomes alleviates periodontitis in mice via a macrophage-dependent mechanism

Periodontitis is caused by host immune-inflammatory response to bacterial insult. A high proportion of pro-inflammatory macrophages to anti-inflammatory macrophages leads to the pathogenesis of periodontitis. As stem cell-derived exosomes can modulate macrophage phenotype, dental pulp stem cell-derived exosomes (DPSC-Exo) can effectively treat periodontitis. In this study, we demonstrated that DPSC-Exo-incorporated chitosan hydrogel (DPSC-Exo/CS) can accelerate the healing of alveolar bone and the periodontal epithelium in mice with periodontitis. Gene Ontology (GO) term enrichment analysis showed that treatment with DPSC-Exo/CS ameliorated periodontal lesion by suppressing periodontal inflammation and modulating the immune response. Specifically, DPSC-Exo/CS facilitated macrophages to convert from a pro-inflammatory phenotype to an anti-inflammatory phenotype in the periodontium of mice with periodontitis, the mechanism of which could be associated with miR-1246 in DPSC-Exo. These results not only shed light on the therapeutic mechanism of DPSC-Exo/CS but also provide the basis for developing an effective therapeutic approach for periodontitis.

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DPSC-Exo/CS accelerates the healing of periodontal tissues in mice with periodontitis.

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DPSC-Exo/CS ameliorates periodontitis by modulating the immune response.

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The immunomodulatory effects of DPSC-Exo/CS are associated with miR-1246 in DPSC-Exo.

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DPSC-Exo/CS is a promising therapy for periodontitis.

Inhibition of CCL2 by bindarit alleviates diabetes-associated periodontitis by suppressing inflammatory monocyte infiltration and altering macrophage properties

Diabetes-associated periodontitis (DP) aggravates diabetic complications and increases mortality from diabetes. DP is caused by diabetes-enhanced host immune-inflammatory responses to bacterial insult. In this study, we found that persistently elevated CCL2 levels in combination with proinflammatory monocyte infiltration of periodontal tissues were closely related to DP. Moreover, inhibition of CCL2 by oral administration of bindarit reduced alveolar bone loss and increased periodontal epithelial thickness by suppressing periodontal inflammation. Furthermore, bindarit suppressed the infiltration of proinflammatory monocytes and altered the inflammatory properties of macrophages in the diabetic periodontium. This finding provides a basis for the development of an effective therapeutic approach for treating DP.

Effect of VEGFC on lymph flow and inflammation-induced alveolar bone loss

The lymphatic system plays a crucial role in the maintenance of tissue fluid homeostasis and the immunological response to inflammation. The effects of lymphatic drainage dysfunction on periodontitis have not been well studied. Here we show that lymphatic vessel endothelial receptor 1 (LYVE1)+ /podoplanin (PDPN)+ lymphatic vessels (LVs) are increased in the periodontal tissues, with accumulation close to the alveolar bone surface, in two murine periodontitis models: rheumatoid arthritis (RA)-associated periodontitis and ligature-induced periodontitis. Further, PDPN+ /alpha-smooth muscle actin (αSMA)- lymphatic capillaries are increased, whereas PDPN+ /αSMA+ collecting LVs are decreased significantly in the inflamed periodontal tissues. Both mouse models of periodontitis have delayed lymph flow in periodontal tissues, increased TRAP-positive osteoclasts, and significant alveolar bone loss. Importantly, the local administration of adeno-associated virus for vascular endothelial growth factor C, the major growth factor that promotes lymphangiogenesis, increases the area and number of PDPN+ /αSMA+ collecting LVs, promotes local lymphatic drainage, and reduces alveolar bone loss in both models of periodontitis. Lastly, LYVE1+ /αSMA- lymphatic capillaries are increased, whereas LYVE1+ /αSMA+ collecting LVs are decreased significantly in gingival tissues of patients with chronic periodontitis compared with those of clinically healthy controls. Thus, our findings reveal an important role of local lymphatic drainage in periodontal inflammation-mediated alveolar bone loss. © 2020 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

The presence of neutrophils causes RANKL expression in periodontal tissue, giving rise to osteoclast formation

BACKGROUNDS AND OBJECTIVE

Increased neutrophil infiltration and osteoclast formation are key characteristics of periodontitis. The effect of these neutrophils on osteoclast formation in periodontitis remains unclear. Therefore, we investigated the effects of neutrophils on osteoclast formation in a neutrophil-deficient mouse model of periodontitis.

METHODS

Anti-Ly6G antibody (Ab) was used for neutrophil depletion in two mouse models: periodontitis and air pouch. In the periodontitis experiments, mice were divided into PBS-administered control (C), control Ab-administered periodontitis (P), and anti-Ly6G Ab-administered periodontitis (P + Ly6G) groups. Periodontitis was induced by ligature of mandibular first molars. In the air pouch experiments, mice were divided into PBS-administered (C), LPS and control Ab-administered (LPS), and LPS and anti-Ly6G Ab-administered (LPS + Ly6G) groups. Neutrophil migration into air pouches was induced by LPS injection. Flow cytometry was used to examine CD11b⁺ Ly6G⁺ neutrophils in the blood of periodontitis mice and CD11b⁺ Ly6G⁺ RANKL⁺ neutrophils in exudates of air pouch mice. In periodontal tissue, Ly6G⁺ neutrophil and RANKL⁺ cell numbers in periodontal ligament and alveolar bone areas were estimated using immunohistochemistry, osteoclast numbers were measured using TRAP assay, and alveolar bone loss was determined by H&E staining.

RESULTS

In blood, CD11b⁺ Ly6G⁺ neutrophils were found in greater percentage in the P group than in the C group on days 3 and 7. However, the percentage of neutrophils was lower in the P + Ly6G group than in the C and P groups. In periodontal tissue, the numbers of Ly6G⁺ neutrophils and RANKL⁺ cells were lower in the P + Ly6G group than in the P group on day 3. Ly6G⁺ neutrophil numbers decreased more in the P + Ly6G group than in the P group on day 7, but RANKL⁺ cell numbers did not decrease in the P + Ly6G group. In exudates, the number of CD11b⁺ Ly6G⁺ RANKL⁺ neutrophils was greater in the LPS group than in the C and LPS + Ly6G groups. On days 3 and 7, the numbers of osteoclasts and alveolar bone loss were greater in periodontal tissue in the P and P + Ly6G groups than in the C group. Interestingly, there were fewer osteoclasts in the P + Ly6G group than in the P group on day 3.

CONCLUSION

Neutrophil deficiency caused a reduction in numbers of both RANKL⁺ cells and osteoclasts in periodontitis-induced tissues only on day 3. Furthermore, in the LPS-injected air pouch model, neutrophil deficiency reduced the influx of RANKL⁺ neutrophils. These findings suggest that the presence of neutrophils induces RANKL expression and could induce osteoclast formation in the early stages of periodontitis.

A Simplified and Effective Method for Generation of Experimental Murine Periodontitis Model

Periodontitis, a common disease that can lead to bone destruction, periodontal attachment loss, and tooth loss, is the major cause for oral tissue engineering. Experimental periodontitis is a suitable disease-model for studying bone regeneration and the potential therapeutic role of biomaterials on periodontal tissue engineering, as this in vivo model could be employed to mimic the natural host response under bacteria-caused oral pathological environments. Although large animals with ligature-induced periodontitis have mostly been used for experiments, a mouse model is a better choice for several reasons. Inserting ligature threads through the interproximal space between the teeth is the key step in establishing a periodontitis model, and it is easy to achieve in large animals, but difficult in mice due to the limited operating space. In this work, we provide a new and proven approach for periodontitis induction in mice using C+ nickel-titanium root canal files and stainless-steel ligature wires. The validity of this method was assessed by evaluating alveolar bone loss via micro-CT and detecting periodontal inflammation by histological staining and qPCR after the treatments. Progressive alveolar bone loss was observed from day 3 after the ligature-placement. Infiltration and accumulation of F4/80+ macrophage was also detected. In accordance with the histological results, there was upregulation of the expression levels of the inflammatory genes Il1β, Tnf-α, and Il6 in gingival tissues isolated from the ligation sites. Our results suggest that this novel method could resolve the difficulty of ligature-placement in mice and consequently contribute to further use of mouse models for studying the pathological mechanisms of periodontitis and developing potential periodontal tissue regeneration strategies. C+ files, which are made of nickel-titanium, are tough, elastic, and sufficiently thin to pass through the interproximal space between the teeth after pre-bending to form an appropriate angle, thus providing an access for ligature wire insertion. As a common tool in the dental clinic, it is familiar to researchers of oral biology, and can provide the feasibility for wide application of our method.

The secreted protein DEL-1 activates a β3 integrin-FAK-ERK1/2-RUNX2 pathway and promotes osteogenic differentiation and bone regeneration

The integrin-binding secreted protein developmental endothelial locus-1 (DEL-1) is involved in the regulation of both the initiation and resolution of inflammation in different diseases, including periodontitis, an oral disorder characterized by inflammatory bone loss. Here, using a mouse model of bone regeneration and in vitro cell-based mechanistic studies, we investigated whether and how DEL-1 can promote alveolar bone regeneration during resolution of experimental periodontitis. Compared with WT mice, mice lacking DEL-1 or expressing a DEL-1 variant with an Asp-to-Glu substitution in the RGD motif ("RGE point mutant"), which does not interact with RGD-dependent integrins, exhibited defective bone regeneration. Local administration of DEL-1 or of its N-terminal segment containing the integrin-binding RGD motif, but not of the RGE point mutant, reversed the defective bone regeneration in the DEL-1-deficient mice. Moreover, DEL-1 (but not the RGE point mutant) promoted osteogenic differentiation of MC3T3-E1 osteoprogenitor cells or of primary calvarial osteoblastic cells in a β3 integrin-dependent manner. The ability of DEL-1 to promote in vitro osteogenesis, indicated by induction of osteogenic genes such as the master transcription factor Runt-related transcription factor-2 (Runx2) and by mineralized nodule formation, depended on its capacity to induce the phosphorylation of focal adhesion kinase (FAK) and of extracellular signal-regulated kinase 1/2 (ERK1/2). We conclude that DEL-1 can activate a β3 integrin-FAK-ERK1/2-RUNX2 pathway in osteoprogenitors and promote new bone formation in mice. These findings suggest that DEL-1 may be therapeutically exploited to restore bone lost due to periodontitis and perhaps other osteolytic conditions.

Toll-Like Receptor Signaling and Immune Regulatory Lymphocytes in Periodontal Disease

Periodontitis is known to be initiated by periodontal microbiota derived from biofilm formation. The microbial dysbiotic changes in the biofilm trigger the host immune and inflammatory responses that can be both beneficial for the protection of the host from infection, and detrimental to the host, causing tissue destruction. During this process, recognition of Pathogen-Associated Molecular Patterns (PAMPs) by the host Pattern Recognition Receptors (PRRs) such as Toll-like receptors (TLRs) play an essential role in the host–microbe interaction and the subsequent innate as well as adaptive responses. If persistent, the adverse interaction triggered by the host immune response to the microorganisms associated with periodontal biofilms is a direct cause of periodontal inflammation and bone loss. A large number of T and B lymphocytes are infiltrated in the diseased gingival tissues, which can secrete inflammatory mediators and activate the osteolytic pathways, promoting periodontal inflammation and bone resorption. On the other hand, there is evidence showing that immune regulatory T and B cells are present in the diseased tissue and can be induced for the enhancement of their anti-inflammatory effects. Changes and distribution of the T/B lymphocytes phenotype seem to be a key determinant of the periodontal disease outcome, as the functional activities of these cells not only shape up the overall immune response pattern, but may directly regulate the osteoimmunological balance. Therefore, interventional strategies targeting TLR signaling and immune regulatory T/B cells may be a promising approach to rebalance the immune response and alleviate bone loss in periodontal disease. In this review, we will examine the etiological role of TLR signaling and immune cell osteoclastogenic activity in the pathogenesis of periodontitis. More importantly, the protective effects of immune regulatory lymphocytes, particularly the activation and functional role of IL-10 expressing regulatory B cells, will be discussed.

Systemic Th17 response in the presence of periodontal inflammation

BACKGROUND

The relationship between periodontitis and the pathogenesis of other inflammatory diseases, such as diabetes, rheumatoid arthritis and obesity has been an important topic of study in recent decades. The Th17 pathway plays a significant role in how local inflammation can influence systemic inflammation in the absence of systemic pathology.

OBJECTIVE

To determine Th17 biased-cells in systemically healthy patients in the presence of generalized chronic periodontitis.

METHODOLOGY

A total of 28 patients were recruited without systemic inflammatory pathology, which was determined by clinical history, the Health Assessment Questionnaire (HAQ) and rheumatoid factor detection. Of these patients, 13 were diagnosed as healthy/gingivitis (H/G) and 15 as generalized chronic periodontitis (GCP). Th17 (CD4+CD161+) cells and Th17IL23R+ (CD4+CD161+IL-23R+) cells were quantified by flow cytometry, based on the total cells and on the lymphocyte region, termed the "enriched population" (50,000 events for each).

RESULTS

The percentages of Th17 cells of the H/G and periodontitis groups were similar on total cells and enriched population (19 vs 21.8; p=4.134 and 19.6 vs 21.8; p=0.55). However, Th17IL23R+ cells differ significantly between periodontally healthy patients and generalized chronic periodontitis patients in both total cell (0.22% vs 0.65%; p=0.0004) and enriched populations (0.2% vs 0.75%; p=0.0266).

CONCLUSIONS

GCP patients (otherwise systemically healthy) were characterized by increased Th17-proinflammatory cell phenotype positive for the IL-23 receptor in peripheral blood. The proportion of Th17 cells that are negative for the IL-23 receptor in the peripheral blood of systemically healthy patients seemed to be unaffected by the presence or absence of chronic periodontitis.

Periodontitis: a host mediated disruption of microbial homeostasis

PURPOSE OF REVIEW

In a prolific scientific career, Dr. Robert J. Genco dedicated himself to enriching our understanding of the pathogenesis of periodontitis. During a period of time in the 1970s and 1980s, when periodontitis was considered a classic infectious disease, Bob had the foresight to investigate and characterize the immune/inflammatory response in periodontitis, particularly Juvenile Periodontitis. His leadership in this area brought to the fore our appreciation of host-microbiome interactions that many years later (2008) culminated in the realization that periodontitis is a fundamental inflammatory disease. In this review, the question of how the host regulates the inflammatory response will be addressed in the context of how more recently-discovered pathways of resolution of inflammation play a role in disease pathogenesis.

RECENT FINDINGS

The host inflammatory response to commensal organisms creates excess inflammation in susceptible individuals and likely drives the dysbiosis of the oral microbiome observed in people with Periodontitis. In periodontal health, the oral microbiome is in balance with the host response. It is the loss of this symbiotic relationship with excess inflammation and microbiome dysbiosis that characterizes progressive disease. In recent years, the role of mediators of resolution of inflammation in the loss of balance and their potential use as therapeutics to restore homeostasis has extended our knowledge of how the host drives immune responses to affect oral dysbiosis.

SUMMARY

Dr. Genco provided the foundation for our ever-emerging understanding host-microbial interactions. The discovery of inflammation resolution pathways has furthered our knowledge in periodontal homeostasis. More studies are needed to understand how the host regulates the microbiome to fulfill the ultimate goal of more efficient therapeutics for periodontitis and related inflammatory diseases.

Indigenous Microbiota Protects against Inflammation-Induced Osteonecrosis

Medication-related osteonecrosis of the jaw (MRONJ) is a rare intraoral lesion that occurs in patients undergoing long-term and/or high-dose therapy with nitrogen-containing bisphosphonates, a RANKL inhibitor, antiangiogenic agents, or mTOR inhibitors. The presence of pathogenic bacteria is highly associated with advanced stages of MRONJ lesions; however, the exact role of indigenous microbes in MRONJ development is unknown. Here, we report that the normal oral flora in mice protects against inflammation-induced osteonecrosis. In mice that developed osteonecrosis following tooth extraction, there was increased bacterial infiltration when compared with healed controls. Antibiotic-mediated oral dysbiosis led to a local inhibition of bone resorption in the presence of ligature-induced periodontitis (LIP). There was no significant difference in empty lacunae, necrotic bone formation, osteoclast number, and surface area in antibiotic-treated as compared with conventionally colonized mice following extraction of healthy teeth after zoledronic acid infusions. However, extraction of LIP teeth led to increased empty lacunae, necrotic bone, and osteoclast surface area in antibiotic- and zoledronic acid-treated mice as compared with conventionally colonized mice. Our findings suggest that the presence of the indigenous microbiota protects against LIP-induced osteonecrosis.

Development and Function of γδT Cells in the Oral Mucosa

To successfully withstand a wide variety of microbial and mechanical challenges, the immune system of the oral mucosa is composed of tissue-resident and specially recruited leukocytes. These leukocytes facilitate the establishment and maintenance of local homeostasis but are also capable to cause oral pathologies when are unrestrained. γδT cells represent an important tissue-resident innate T-cell population in various mucosal and nonmucosal barrier tissues, in which they are ideally located to assist in immunosurveillance, tissue repair, and homeostasis. Whereas most works studying γδT cells were focused on tissues such as the skin and intestine, these cells in the oral mucosa were only recently thoroughly studied. The findings obtained by those studies appear to be both complementary and contradicting, likely reflecting differences in the experimental settings and the type of transgenic mouse modalities employed by each study. Nevertheless, oral γδT cells were shown to consist of developmentally distinct tissue-resident Vγ6 cells and circulating Vγ1 and Vγ4 subsets that are independently maintained in the oral mucosa. In the gingiva, a particularly challenging barrier tissue due to its proximity to the dental plaque, γδT cells are strategically positioned close to the plaque and represent the major source of IL-17. While this suggests that γδT cells might be involved in controlling the dental biofilm, conflicting data were reported in this regard. In vivo studies have shown that γδT cells either play a protective role during age-associated bone loss or, alternatively, have no impact in this process. Also, recent reports suggested opposing data concerning the impact of γδT cells in experimental periodontitis based on the ligature model. This review summarizes and discusses the most up-to-date literature on oral γδT cells, providing a balanced perspective regarding our current understanding on the development of oral γδT cells and their role under physiologic conditions and certain oral pathologies.

RANKL biology: bone metabolism, the immune system, and beyond

Receptor activator of NF-κB (RANK) ligand (RANKL) induces the differentiation of monocyte/macrophage-lineage cells into the bone-resorbing cells called osteoclasts. Because abnormalities in RANKL, its signaling receptor RANK, or decoy receptor osteoprotegerin (OPG) lead to bone diseases such as osteopetrosis, the RANKL/RANK/OPG system is essential for bone resorption. RANKL was first discovered as a T cell-derived activator of dendritic cells (DCs) and has many functions in the immune system, including organogenesis, cellular development. The essentiality of RANKL in the bone and the immune systems lies at the root of the field of "osteoimmunology." Furthermore, this cytokine functions beyond the domains of bone metabolism and the immune system, e.g., mammary gland and hair follicle formation, body temperature regulation, muscle metabolism, and tumor development. In this review, we will summarize the current understanding of the functions of the RANKL/RANK/OPG system in biological processes.

Alveolar Bone Protection by Targeting the SH3BP2-SYK Axis in Osteoclasts

Periodontitis is a bacterially induced chronic inflammatory condition of the oral cavity where tooth-supporting tissues including alveolar bone are destructed. Previously, we have shown that the adaptor protein SH3-domain binding protein 2 (SH3BP2) plays a critical role in inflammatory response and osteoclastogenesis of myeloid lineage cells through spleen tyrosine kinase (SYK). In this study, we show that SH3BP2 is a novel regulator for alveolar bone resorption in periodontitis. Micro-CT analysis of SH3BP2-deficient (Sh3bp2 ^-/- ) mice challenged with ligature-induced periodontitis revealed that Sh3bp2 ^-/- mice develop decreased alveolar bone loss (male 14.9% ± 10.2%; female 19.0% ± 6.0%) compared with wild-type control mice (male 25.3% ± 5.8%; female 30.8% ± 5.8%). Lack of SH3BP2 did not change the inflammatory cytokine expression and osteoclast induction. Conditional knockout of SH3BP2 and SYK in myeloid lineage cells with LysM-Cre mice recapitulated the reduced bone loss without affecting both inflammatory cytokine expression and osteoclast induction, suggesting that the SH3BP2-SYK axis plays a key role in regulating alveolar bone loss by mechanisms that regulate the bone-resorbing function of osteoclasts rather than differentiation. Administration of a new SYK inhibitor GS-9973 before or after periodontitis induction reduced bone resorption without affecting inflammatory reaction in gingival tissues. In vitro, GS-9973 treatment of bone marrow-derived M-CSF-dependent macrophages suppressed tartrate-resistant acid phosphatase (TRAP)-positive osteoclast formation with decreased mineral resorption capacity even when GS-9973 was added after RANKL stimulation. Thus, the data suggest that SH3BP2-SYK is a novel signaling axis for regulating alveolar bone loss in periodontitis and that SYK can be a potential therapeutic target to suppress alveolar bone resorption in periodontal diseases. © 2019 American Society for Bone and Mineral Research. © 2019 American Society for Bone and Mineral Research.

The Interrelationship Between Diabetes, IL-17 and Bone Loss

PURPOSE OF REVIEW

Diabetes has a detrimental effect on bone, increasing the risk of fracture and formation of osteolytic lesions such as those seen in periodontitis. Several diabetic complications are caused by diabetes-enhanced inflammation. This review examines mechanisms by which IL-17 contributes to diabetes-enhanced periodontitis and other effects of IL-17 on bone.

RECENT FINDINGS

IL-17 upregulates anti-bacterial defenses, yet its expression is also linked to a destructive host response in the periodontium. Periodontal disease is caused by bacteria that stimulate an inflammatory response. Diabetes-enhanced IL-17 increases gingival inflammation, which alters the composition of the oral microbiota to increase its pathogenicity. In addition, IL-17 can induce osteoclastogenesis by upregulation of TNF and RANKL in a number of cell types, and IL-17 has differential effects on osteoblasts and their progenitors. Increased IL-17 production caused by diabetes alters the pathogenicity of the oral microbiota and can promote periodontal bone resorption.

PTH induces bone loss via microbial-dependent expansion of intestinal TNF+ T cells and Th17 cells

Bone loss is a frequent but not universal complication of hyperparathyroidism. Using antibiotic-treated or germ-free mice, we show that parathyroid hormone (PTH) only caused bone loss in mice whose microbiota was enriched by the Th17 cell-inducing taxa segmented filamentous bacteria (SFB). SFB⁺ microbiota enabled PTH to expand intestinal TNF⁺ T and Th17 cells and increase their S1P-receptor-1 mediated egress from the intestine and recruitment to the bone marrow (BM) that causes bone loss. CXCR3-mediated TNF⁺ T cell homing to the BM upregulated the Th17 chemoattractant CCL20, which recruited Th17 cells to the BM. This study reveals mechanisms for microbiota-mediated gut-bone crosstalk in mice models of hyperparathyroidism that may help predict its clinical course. Targeting the gut microbiota or T cell migration may represent therapeutic strategies for hyperparathyroidism.

Regulation of host-microbe interactions at oral mucosal barriers by type 17 immunity

The oral mucosa is a primary barrier site and a portal for entry of microbes, food, and airborne particles into the gastrointestinal tract. Nonetheless, mucosal immunity at this barrier remains understudied compared with other anatomical barrier sites. Here, we review basic aspects of oral mucosal histology, the oral microbiome, and common and clinically significant diseases that present at oral mucosal barriers. We particularly focus on the role of interleukin-17 (IL-17)/T helper 17 (TH17) responses in protective immunity and inflammation in the oral mucosa. IL-17/TH17 responses are highly relevant to maintaining barrier integrity and preventing pathogenic infections by the oral commensal fungus Candida albicans On the other hand, aberrant IL-17/TH17 responses are implicated in driving the pathogenesis of periodontitis and consequent bone and tooth loss. We discuss distinct IL-17-secreting T cell subsets, emphasizing their regulation and function in oropharyngeal candidiasis and periodontitis.

Osteoimmunology - Bidirectional dialogue and inevitable union of the fields of bone and immunity

Bone is a critically important part of the skeletal system that is essential for body support and locomotion. The immune system protects against pathogens and is active in host defense. These two seemingly distinct systems in fact interact with each other, share molecules and create a collaborative regulatory system called the "osteoimmune system". The most representative osteoimmune molecule is receptor activator of NF-κB ligand (RANKL), which plays multiple roles in the osteoimmune system under both physiological and pathological conditions such as rheumatoid arthritis and cancer metastasis to bone. Based on accumulating evidence for such mutual dependence, it is concluded that the relationship between bone and the immune system did not develop by accident but as a necessary consequence of evolution. Here I describe the history of and recent advances in osteoimmunology, providing a perspective in the contexts of both science and medicine.