Osteoimmunology in Periodontitis; a paradigm for Th17/IL-17 inflammatory bone loss

Abstract

Periodontitis is a prevalent human disease of inflammation-induced bone destruction. Through studies in patient lesions of rare and common forms of periodontitis and animal model experimentation, Th17/IL-17 related immune pathways have emerged as mediators of disease pathology. In this focused review, we examine mechanisms of induction, amplification and pathogenicity of Th17 cells in periodontitis.

Introduction

The term osteoimmunology was coined more than two decades ago [1] to describe the interdisciplinary research field that deals with the cross-regulation between bone cells and the immune system. Indeed, the osseous and immune systems are intimately interconnected. During development, physical proximity brings together bone with the immune system, as the bone marrow provides the intimate microenvironment where hematopoiesis primarily occurs giving rise to blood and immune cell types [2]. With increased appreciation of the primary role of the stromal/osseous microenvironment in regulating hematopoiesis under both physiologic and pathologic conditions [3,4], it has become evident that there is constant communication between the osseous and immune system during development. Additionally, one of the key cell types of the osseous system, the osteoclast is of myeloid origin [5]. However, the first appreciation regarding the direct communication of the immune and osseous systems came from the setting of inflammation in the context of periodontitis, when “osteoclast activating factor” was discovered as an immune cytokine stimulating osteoclastogenesis [6]. This factor was later renamed IL-1β. In fact, the context of inflammatory-bone diseases became the setting where most of the biological mechanisms related to immune triggering of bone destruction/erosion have been discovered. Through this work it became evident that shared immune mediators and cell types have been implicated in the pathology of various inflammatory bone diseases such as rheumatoid arthritis (RA), osteoarthritis (OA) and periodontitis. As such, the myeloid derived cytokines IL-1β, Tumor necrosis factor (TNF) ɑ and IL-6 have emerged as common mediators and druggable targets for a variety of inflammatory bone diseases [7–12]. Beyond myeloid cells, T cells and associated factors have been well established as mediators of inflammatory bone loss [13]. In particular, the T cell subset Th17 and its associated cytokines (IL-23/IL-17) have emerged as pathogenic drivers for various forms of inflammatory bone loss diseases with successful therapeutic targeting of these pathways, particularly related to specific forms of inflammatory arthritis (psoriatic arthritis (PsA) and subtype of RA patients) [14].

In this focused review, we will present basic concepts related to the oral inflammatory bone disease periodontitis and discuss unique and shared mechanisms to the induction and pathogenic role of Th17-related immune responses in periodontitis as compared with other inflammatory bone diseases.

1. Periodontitis is a prototypical disease of inflammation-induced bone loss

Periodontitis is one of the most prevalent human inflammatory diseases which affects oral mucosa (gingiva) and structural tissues which support the dentition, including connective tissue, tooth-associated cementum and alveolar bone. In its severe forms, periodontitis affects approximately 8% of the general population in the United States [15] and has been epidemiologically associated with the co-occurrence of various comorbidities including diabetes, cardiovascular disease, and RA [16–19].

Disease is characterized by the accumulation of a “dysbiotic” microbiome on the root surface of the tooth and mucosal inflammation which becomes pathogenic leading to tissue destruction with loss of tooth supporting structures including connective tissue, tooth cementum and bone and ultimately loss of teeth in severe cases. In health, teeth “sit” inside the alveolar (jaw) bone, which provides support. The alveolar bone is connected to the tooth root surface (a hard tissue structure named cementum) through connective tissue fibers (namely, periodontal ligament). The tooth is typically supported by bone through most of its root, and this support ends 1–2 mm below the clinical crown (the part of a tooth above gum) of the tooth. This 1–2 mm is the area where the oral mucosa connects with the tooth through a very few layers of epithelium (namely Junctional epithelium (JE)) which is connected to the tooth via hemidesmosomes [20,21]. (Fig.1) With inflammation, JE becomes quickly ulcerated and disconnected from the tooth, allowing for microbial translocation. When inflammation is limited to the gingival mucosal tissues, the disease is termed gingivitis and is typically reversible with removal of the dysbiotic microbiome. However, in susceptible individuals’ inflammation becomes more destructive leading to further periodontal tissue destruction: destruction of the connection between tooth and bone, destruction of bone and migration of the epithelium with formation of a periodontal pocket. (Fig. 1)

Figure 1.

Anatomy of tooth and periodontal tissue.

Tooth supporting structures include the gingiva (oral mucosa), the tooth root structure cementum, the alveolar bone and the periodontal ligament which connects the cementum to bone. Oral mucosa has three distinct epithelial regions, Junctional epithelium (JE): the area where the oral mucosa connects to the tooth through a very thin epithelium layer, the Oral gingival epithelium (OGE): Epithelial layer which is exposed to the outside of the tooth and the Oral sulcular (crevicular) epithelium (OSE): In periodontitis, the dysbiotic microbiome on the root surface triggers mucosal inflammation in the gingiva and subsequent loss of connective tissue attachment and bone loss.

2. Dysbiotic microbiome as a disease trigger in periodontitis

The surface of the tooth root (subgingival area) is colonized by a complex microbial biofilm, even in the setting of health. In fact, the subgingival microbiome is one of the most complex microbial communities in the human body [22,23]. However, in the setting of periodontitis, the subgingival microbiome undergoes significant changes, which have been well characterized by multiple research groups [17,24,25]. This disease-associated dysbiotic microbiome is characterized by a significant increase in total microbial biomass and an increase in microbial diversity with over-representation of particular periodontitis-associated species, including Porphyromonas gingivalis, Tannerella forsythia, Treponema denticola: a triad of bacteria known as the “red complex”, and Filifactor alocis [24,26–28]. This dysbiotic microbiome is thought to have increased virulence potentially associated with its ability to trigger excessive inflammatory responses [29], but also associated with the adaptation into an anaerobic and inflammatory environment. In turn, local inflammation is thought to provide nutrients to the microbial community and further perpetuate microbial dysbiosis, creating a vicious self-reinforcing cycle [17,24].

Importantly, microbial dysbiosis is not sufficient to precipitate periodontitis. Therefore, bacterial plaque due to lack of oral and dental care does not necessarily lead to periodontitis, but is thought to trigger disease in individuals with host susceptibility or in cooperation with additional risk factors, such as additional disease comorbidities and/or environmental factors such as smoking [30]. However, because microbial triggering is a key event for the initiation and perpetuation of periodontal disease pathogenesis, current standard of care treatments relies on the removal of microbial biofilm [31].

Current treatment for periodontitis is largely nonspecific, broadly aimed at removing the dysbiotic microbiome and when necessary intervening surgically. Removal of microbial biofilm is typically accomplished with mechanical instrumentation and in some cases with use of adjunct antibiotic treatment, with further surgical interventions in severe cases [32]. However, there are no targeted treatments approved to date for inhibiting specific mechanisms involved in inflammatory bone loss of periodontitis. Furthermore, whether distinct biological mechanisms should be targeted in the treatment of particular subgroups of periodontitis patients also remains unclear. Currently, we lack compelling evidence to subclassify periodontitis patients based on diverse mechanisms implicated. Further evidence is needed to identify valuable targets and potentially identify patient groups where select modulation may be applicable [33]. Further understanding of disease’s triggers, susceptibility and pathogenesis may inform therapeutic intervention but also progress the field of immune-osseous interaction in inflammatory disease.

3. A primer on Th17 biology

In 2006, different labs separately reported on a newly identified helper T cell subset, the IL-17-producing CD4+ helper T cells termed Th17 cells [34–36]. The transcription factor RAR-related orphan receptor gamma (RORγt) was first defined as the signature transcriptional regulator for Th17, yet additional transcription factors have been shown to be involved in the differentiation, commitment and pathogenicity of Th17 including the nuclear receptor RORɑ, the basic leucine zipper transcriptional factor ATF-like (BATF) and interferon-regulatory factor 4 (IRF4), aryl hydrocarbon receptor (Ahr) [37]. Differentiation of Th17 cell was originally shown to depend on cooperative signaling from the cytokines transforming growth factor beta (TGF-β) and IL-6, and there after additional cytokine molecules have been demonstrated to trigger Th17 differentiation in various contexts including the interleukins, IL-1β, IL-21 and IL-23 [38].

Physiologically, Th17 cells are encountered at barrier surfaces such as the gastrointestinal tract and skin and mediate critical immuno-protective functions associated with barrier defense [39]. Th17 cells and their signature cytokine IL-17A have been clearly demonstrated to induce epithelial barrier defenses, to promote barrier integrity and to play a central role in the recruitment of neutrophils during injury and infection [39]. Through these critical functions, Th17 cells have been shown to promote antibacterial and antifungal barrier defenses. In fact, humans with genetic defects in molecules involved in Th17 development and IL-17 signaling present a dominant phenotype of oral and mucocutaneous fungal infections (Candidiasis) and cutaneous and lung bacterial infections [40]. Yet, beyond physiologic immuno-protective roles, Th17 cells have been implicated in chronic inflammatory and autoimmune diseases. Experimental works in animal models have demonstrated a role for Th17 and related cytokines in experimental autoimmune encephalomyelitis (EAE), collagen-induced arthritis (CIA), models of inflammatory bowel disease (IBD), ankylosing spondylitis (AS), psoriasis, experimental uveitis and periodontitis [41]. In humans, inhibition of the cytokine IL-17 has been very successful in the treatment of psoriasis and PsA, while inhibition of the upstream mediator IL-23 has been implemented for the treatment of both psoriasis as well as Crohn’s disease, demonstrating a critical role of these pathways in human autoimmunity and inflammation [41]. (Table1)

4. Th17 cells are pathogenic drivers of inflammatory bone loss in periodontitis

Th17 cells and associated pathways have been associated with immune-osseous interactions implicated in the pathogenesis of diseases associated with inflammatory bone loss such as RA, AS, OA and periodontitis [2].

Upregulation of Th17 and associated responses have been documented in the majority of arthritis mouse models, with the exception of the TNF overproducing models [42,43]. Furthermore, antibody inhibition of IL-17A or IL-17 receptor A (IL-17RA) has resulted in decreased clinical scores and bone loss in a variety of arthritis animal models including the CIA mouse model, the formalin-fixed Borrelia burgdorferi immunize mouse model and rat adjuvant-induced model [44–46]. Additionally, antigen-specific (collagen type 2-specific) Th17 have been found in opposition to osteoclasts in the subchondral area in inflamed joints of the CIA model [47]; and administration of IL-17 into a normal mouse joint induced cartilage degradation [48]. Th17 cells and IL-17A have been documented to be enriched in RA patient joint synovium. The experiment of human bone explant cultures has demonstrated that blockade of the bone-derived endogenous IL-17 by specific inhibitors resulted in a protective effect: inhibition of bone destruction. Those reports strongly suggested Th17/IL-17 as therapeutic targets in RA [49,50]. Yet, inhibition of IL-17A in patients with RA has shown modest results of efficacy, particularly in specific subsets of patients non-responsive to TNF inhibitors [49]. Additionally, targeting of IL-17 related pathways has shown efficacy particularly for the treatment of PsA and promise for spondyloarthritis (SpA), suggesting a role in particular disease subsets of inflammatory bone loss [2,49].

Similar to other inflammatory bone diseases, the Th17/IL-17 axis has been implicated in the pathogenesis of periodontitis. Work from many laboratories has clearly demonstrated the upregulation of cytokines related to the Th17 response, including IL-17A, IL-17F, IL-23(p19), IL-21 in tissue lesions of patients with both chronic and aggressive forms of periodontal disease [51]. In fact, levels of the cytokines IL-17A, IL-23 and IL-21 within periodontal mucosal tissues have been shown to correlate with the severity of bone destruction [52–55]. Furthermore, the expansion of Th17 cells has been documented both in common [55–57] and rare genetic forms of periodontitis [58].

In experimental models of periodontal disease, Th17 cells accumulate after disease induction [55,59,60] and play a pathogenic role as drivers of inflammation and bone destruction in these models. Indeed, inhibition of either the differentiation of Th17 cells through genetic models (Signal transducer and activator of transcription 3: Stat3, CD4^CreStat3f^l/fl mice and Lck^CreRorc^fl/f mice) or the signature cytokines IL-17A and IL-17F significantly suppressed gingival inflammation and bone destruction in various models of periodontitis [55,59].

In humans, whether Th17 cells and related cytokines are pathogenic drivers of periodontal bone loss has not been conclusively established. However, patients with a deficiency in Th17 differentiation due to a loss of function mutation in the STAT3 gene, have been shown to have very low Th17 cells in the circulation and in gingival tissues with reduced gingival inflammation and periodontal bone loss compared to the general populations [55]. Furthermore, inhibition of p40 (antibody: ustekinumab), the common chain of IL-23 and IL-12 has led to significant reduction of periodontal inflammation in a genetic form of periodontitis, Leukocyte adhesion deficiency (LAD) [61] and has led to the initiation of a clinical trial for the treatment of LAD-associated immune pathologies with ustekinumab [62]. (Table 1)

Table 1.

TH17cells in different inflammatory/autoimmune diseases.

5. Disease-associated microbiota trigger Th17 accumulation in periodontitis

A unique aspect of periodontitis compared to other inflammatory bone diseases, such as RA and OA, is that it is initiated by a dysbiotic microbiome [26,59]. While in settings of autoimmune inflammatory bone loss Th17 are thought to be specific to autoantigen triggers, in periodontitis models Th17 cells expand in oral mucosal gingival tissues, in response to the accumulation of a dysbiotic microbiome [55,59]. Indeed, broad spectrum antibiotic treatment inhibited expansion of Th17 cells in experimental periodontitis and associated bone loss [55,59–61,63,64].

Dependence of Th17 on microbes for their induction at barrier sites was first demonstrated in the gastrointestinal tract and skin, with germ-free mice having significantly lower frequency of Th17 cells in the lower intestine [65] and skin [66]. In the gingival mucosa, Th17 physiologically accumulates with age, even in the absence of live commensal microbiota with germ free (GF) mice having comparable proportions and numbers of Th17 cells to specific pathogen-free (SPF) counterparts [67]. In the healthy gingiva, local damage from mastication becomes a tissue-specific trigger for the induction of homeostatic Th17 cells [67]. However, in disease, further amplification of Th17 depends on the accumulation of dysbiotic microbiomes. While specific microbiota capable of triggering Th17 immunity have not been defined to date, experimental inhibition of anaerobes using the narrow spectrum antibiotic Metronidazole inhibited Th17 induction without corresponding reduction of total microbial load. The result indicates that the specific class of microbiota are implicated in periodontitis and Th17 induction [55]. Supporting the role of anaerobes in triggering pathogenic inflammatory responses, Metronidazole is the antibiotic of choice of treatment of aggressive forms of periodontitis [68]. Induction of Th17 cells in the gingiva is shown to depend on cognate antigen recognition in health [67] and T cell receptor (TCR) engagement is also evident during disease expansion of Th17 in periodontitis [55], yet specific antigen recognition and dependence is not detailed to date.

In the gastrointestinal (GI) tract and skin, particular microbiota have been shown to trigger Th17 induction. Segmented filamentous bacteria (SFB) in the gut, and Staphylococcus epidermidis in the skin were the first commensal microbes shown to specifically induce Th17 cell differentiation in their respective barriers in the setting of health [66,69]. In the GI tract, SFB-specific induction of Th17 cells in health and during inflammation depended on the cooperative action of the acute phase protein serum amyloid A (SAA) and cytokine IL-23 both in health and disease states [70–73]. While in the skin, S. epidermidis induction of Th17 depended on the cytokine IL-1β. Antigen specificity of Th17 to be commensal specific has also been shown in the gut [74]. Commensal-induction of Th17 has also been demonstrated in the setting of disease. SFB has been shown to trigger Th17 in the gut not only during heath but also in the setting of inflammation. Actinobacterium Eggerthella lenta, a human microbe, has been associated with induction of Th17 in mice and humans in the gut in the setting of IBD [75]. Additionally, Actinobacterium Bifidobacterium adolescentis has been shown to trigger induction of Th17 cells in an antigen-specific manner and to contribute to severity of arthritis in mice [75].

While amplification of Th17 during disease has been clearly shown to be microbe-dependent in experimental models of periodontitis, to date specific constituents of the microbiome that may be implicated in disease-associated Th17 induction have not been identified in mice or humans. Furthermore, it remains unclear whether Th17 in periodontitis are antigen-specific towards particular periodontitis related pathobionts. Induction of Th17 cells in the gingiva is shown to depend on cognate antigen recognition in health [67] and T cell receptor (TCR) engagement is also evident during disease expansion of Th17 in periodontitis [55], yet specific antigen recognition and dependence is not detailed to date.

However, cytokine dependence for gingival Th17 induction in health and disease has been explored. Th17 cells depend on the cytokine IL-6 for their induction in health and disease [55,59]. IL-6 KO mice had almost complete abrogation of gingival Th17 in periodontitis, although both IL-6 and IL-23 are necessary in disease for Th17 induction [55]. Of interest in the tongue and in the setting of candida infection, Th17 cells are shown to depend on IL-1 indicating tissue and disease specificity for the induction of IL-17 immunity even within the oral mucosal niches [76]. However, while the cellular source of IL-6 in mice has been identified to be the oral epithelium, the cellular source of IL-23 and mechanisms related to IL-23 induction in disease both in mice and humans are not fully elucidated. IL-23 has been linked to pathogenicity of Th17 in other settings [77] consistent with its role in other settings; it appears to be a switch between health and disease associated with Th17 in experimental periodontitis. Indeed, IL-23 is currently explored as a disease target for a Mendelian form of periodontitis in LAD-1.

IL-23 has been previously associated with pathogenicity in inflammatory bone diseases including periodontitis, RA and SpA. Increased levels of IL-23 have been documented in lesions from common and rare forms of periodontitis [51]. In RA, increased levels of IL-23 have been documented in serum and synovial fluid, suggesting IL-23 as a disease biomarker [78]. Furthermore, polymorphisms of the IL-23 receptor (IL-23R) are a risk factor for AS and PsA, which indicates that IL-23 is also involved in the pathogenesis of SpA [79]. IL-23R is expressed by pathogenic Th17 in experimental arthritis [80]. Additionally, IL-23 synergizes with other cytokines such as IL-17 and TNFα to mediate inflammatory bone loss. Finally, IL-23 inhibition has been shown to attenuate paw swelling and joint destruction in CIA rats [81], but has also been clinically effective in reducing clinical manifestations of PsA and SpA. (Table 1)

6. Th17-mediated bone destruction in periodontitis

To date, most of the existing evidence points to a pathogenic role for Th17 cells in experimental models of periodontitis, primarily through its signature cytokine IL-17. Indeed, inhibition of cytokine IL-17A or IL-17A/F has led to significant protection from inflammatory bone loss in the ligature model of periodontitis [55,59]. IL-17 inhibition has also led to protection from natural age-dependent periodontal bone loss [67] and in the context of diabetes-associated periodontitis [82], Developmental endothelial locus-1 (DEL-1) deficiency associated periodontitis [83] and Leukocyte adhesion deficiency type1 (LAD-1) associated periodontitis in mice [58].

One mechanism by which the IL-17 cytokine appears to mediate inflammatory bone destruction is through excessive recruitment of neutrophils and neutrophil mediated immunopathology. Indeed, in the ligature experimental periodontitis (LIP) model and DEL-1 KO mice with spontaneous periodontitis, inhibition of IL-17 led to inhibition of neutrophil granulopoiesis factors and signature chemokines: granulocyte-colony stimulating factor (G-CSF), chemokine (C-X-C motif) ligand (CXCL) 1, CXCL2), leading to reduced neutrophil recruitment to tissues [63,84]. Furthermore, while abrogation of neutrophil recruitment [85] [58] is linked to periodontitis, reduction in neutrophil numbers (using monoclonal antibodies) has led to protection from periodontal bone loss, demonstrating a role for neutrophil-mediated destruction in periodontal models [63]. Neutrophil accumulation is a well-documented feature not only in experimental periodontitis but importantly in human disease. Furthermore, neutrophil activation and neutrophil-mediated pathways including production of reactive oxygen species (ROS), neutrophil proteases and neutrophil extracellular traps (NETs) have been suspected as inflammatory triggers in periodontitis [56,86–88] Indeed, neutrophil activation and NETosis has been implicated in a variety of inflammatory pathologies, including inflammatory bone loss in RA [56,89]. Recently, our lab has documented that neutrophil activation through Fibrin-CD11b binding mediates inflammatory bone loss, partially through activation of NETosis [90].

However, it is clear that IL-17 has pleiotropic roles, beyond neutrophil recruitment, even in the context of periodontitis. A clear example of this is IL-17 mediating inflammatory bone loss in models of defective neutrophil transmigration into tissues. In Lymphocyte Function-associated Antigen 1 (LFA-1) KO mice, neutrophils are unable to transmigrate into tissues. However, IL-17A is a pathogenic driver of inflammatory bone loss in the absence of tissue neutrophils. In this context, inhibition of IL-17 is associated with overall reduction in inflammatory molecules but also reduced expression of the classic osteoclastogenic factor: receptor activator of nuclear factor kappa-Β ligand (RANKL). Yet, the cellular targets for IL-17 signaling in this context are not clear. While it has been clearly shown that osteoblast and periodontal ligament cell expression of RANKL is critical for periodontal bone loss [59], it is still not clear whether IL-17 directly or indirectly signals to these cell subsets to mediate bone resorption. (Fig.2)

Figure 2.

Gingival Th17/IL-17 axis in health and periodontal disease.

In health IL-6, primarily derived by oral epithelial cells in response to mechanical forces of mastication, induces Th17. In periodontitis, homeostatic IL-6 and IL-23 induced by the dysbiotic microbiota leads to pathogenic expansion of Th17 which drives immune pathology through their cytokine IL17A via neutrophil dependent and independent mechanisms. Ultimately osteoclast activation mediates alveolar bone loss.

7. Potential features of pathogenicity of periodontitis - associated Th17 cells

While the pathogenic role for IL-17 has been well documented in animal models of periodontitis, it is unclear whether Th17 cells have additional, cell intrinsic pathogenic features that can contribute to periodontal immunopathology. In other inflammatory settings, Th17 cells have been documented to acquire a pathogenic gene signature with additional factors (beyond IL-17 secretion) implicated in their pathogenicity. Early studies defined unique transcriptional signatures of proinflammatory Th17 cells in models of EAE [77] and determined the ability of “pathogenic” Th17 cells to drive disease through, not only IL-17 secretion but also secretion of additional mediators such as Interferon gamma (IFNɣ) and Granulocyte-macrophage colony-stimulating factor (GM-CSF) [91]. Additionally, while “pathogenic” signatures may have unique features depending on tissue and disease, a shared feature has been their dependence on IL-23 and their expression of STAT3. In fact, IL23 dependance for pathogenic Th17 has justified targeting IL23 versus IL17 in certain diseases. [92].

In the gingiva, specific “pathogenic” transcriptional signatures have not been documented that would distinguish health vs disease specific Th17 populations. Furthermore, Th17 subpopulations capable of inducing disease upon transfer have also not been defined. However, Th17 in periodontitis do share features previously associated with pathogenicity in other disease settings. In human disease, Th17 cells have been shown to be majority tissue resident, effector memory T cells (Trem) [93] which secrete predominantly IL-17A, with smaller proportions of cells secreting GM-CSF, IFNɣ and IL-22 [55]. Transcriptional profiles of Th17 in human periodontitis also reveal expression of IL17F, CCR6 and STAT3 [56]. In experimental models, while health-associated Th17 cells depend on IL-6, expansion of Th17 in LIP necessitates IL-23, similar to IL-23 dependence of Th17 in most pathologies [55]. Furthermore, depletion of Stat3 in CD4 cells resulted in protection from periodontitis, indicating a role for Stat3 in disease-associated Th17 cells [55]. However, periodontitis-associated Th17 cells are not shown to co-secrete IFNγ, a pathogenic Th17 cytokine in other settings [55,59,60].

Another, potential pathogenic feature of periodontitis-associated Th17 is the fact that they were converted from Foxp3+ cells in the setting of inflammation. In RA models CD25^lowFoxp3⁺CD4⁺ T cells have been shown to lose Foxp3 expression (called exFoxp3 cells) and undergo transdifferentiation into Th17 cells. Fate mapping analysis demonstrated that IL-17-expressing exFoxp3 Th17 cells accumulated in inflamed joints and were more potent osteoclastogenic T cells than were naȉve CD4+ T cells-derived Th17 cells. Notably, exFoxp3 Th17 cells were characterized by the expression of SRY-Box transcription factor 4 (Sox4), chemokine (C-C motif) receptor 6 (CCR6), chemokine ligand 20 (CCL20), IL-23R and receptor activator of NF-κB ligand RANKL [80]. In periodontitis, exFoxp3 Th17 cells express more Th17 signature cytokines (Il17a and Il17f), transcription factor Rort and RANKL [59] than Th17 in health. Increased IL-17A/F production can contribute to pathogenicity. RANKL expression on T cells also contributes to inflammatory bone loss, albeit not as the major source of RANKL-mediated osteoclastogenesis [59]. In arthritis models the major source of RANKL contributing to osteoclastogenesis are synovial fibroblasts, not T cells [94,95]. Although Th17 cells have been shown to secrete RANKL, RANKL on Th17 cells alone is not sufficient for the induction of osteoclast differentiation [96].

8. Th17 pathways to osteoclastogenesis

Similar to other inflammatory bone loss diseases, osteoclast activation is the final step towards induction of bone destruction in periodontitis. Indeed, upregulation of the classical osteoclastogenic factor RANKL and accumulation of osteoclasts on the bone surface are features for both human and experimental periodontitis [97,98]. RANKL and its decoy receptor Osteoprotegerin (OPG) play very important roles in osteoclast differentiation and activation [99]. RANKL, which can be membrane bound or secreted, binds to RANK expressed on osteoclasts to activate them into bone resorbing phagocytes. Alternatively, the decoy receptor OPG which is expressed on a variety of cell types, inhibits and regulates their functions [100,101]. Upregulation of RANKL and accumulation of osteoclasts on the alveolar bone surface is indeed a classical finding in all types of periodontitis models [102,103]. Furthermore, an increased RANKL/OPG ratio has been reported in gingival tissues from periodontitis patients compared to healthy volunteers in multiple clinical studies [104]. In animal models, RANKL inhibition through osteoprotegerin has been shown to inhibit bone loss in experimental periodontitis [97]. Furthermore, mice deficient in OPG have been shown to develop severe alveolar bone loss with increased presence of osteoclasts [97,105].

While RANKL is expressed by multiple cell types, it appears that RANKL from osteoblasts and periodontal ligament cells most significantly contribute to osteoclastogenesis and bone loss, with T cell expressed RANKL contributing modestly to osteoclastogenesis, in the ligature periodontitis model [59].

It is not well understood whether Th17 and the IL-17 cytokines contribute directly or indirectly to RANKL upregulation and osteoclastogenesis in periodontitis. Nuclear localization of NF-kB signaling in osteoblasts has been shown to be a key for osteoclastogenesis in the oral cavity [106], however whether IL-17 (alone and/or in coordination with other proinflammatory cytokines) contributes to this process is not well determined in periodontitis. Consistent with a role for IL17 in directly mediating RANKL expression, in RA experimental models, IL-17 has been shown to upregulate RANKL in synovial fibroblasts to mediate inflammatory bone loss [95]. (Fig.2)

9. Shared mechanisms in Periodontitis and RA

Th17 and IL-17 related pathways emerge as shared culprits involved in inflammatory bone loss for both periodontitis and RA. Importantly, a strong epidemiological connection between periodontitis and RA has been well defined over the years. Indeed, patients with RA present with a higher incidence and severity of periodontal disease compared to the general population [19]. Significant periodontitis has also been documented in cohorts of new onset RA patients (NORA) [107], instigating the idea that periodontitis may even be an initiating factor for the development of RA [19]. Periodontitis- associated microbiota have been implicated as triggers of early events in RA [108], but also systemic inflammation caused by periodontitis, has been shown to induce epigenetic rewiring of the bone marrow niche, leading to sustained systemic inflammation and increased susceptibility to RA [109]. Whether periodontitis associated microbial and/or host factors contribute to the emergence of Th17-related immunopathology in both diseases, or if shared genetic susceptibilities underlie the emergence of Th17 immunopathology in a subset of RA and periodontitis patients is yet to be defined. Moreover, given the evident co-occurrence of the two disease entities, and the shared clinical feature of inflammation-induced bone destruction further understanding of mechanisms underlying shared susceptibility and pathogenesis will be very valuable.

Conclusions

Periodontitis shares features with distal inflammatory and autoimmune conditions such as inflammation-driven pathology of psoriasis in the skin, colitis in the GI tract and inflammatory bone loss in arthritis. A common feature of all these conditions both in humans and experimental models is the amplification of Th17/IL-17 mediated immune responses. While distinct local triggers appear to mediate Th17 induction in the various disease entities, some mechanisms of triggering and pathogenicity appear to be shared throughout the disease process. Comparing and contrasting features of Th17 immunity in various contexts will not only help advance fundamental understanding of relevant immune mechanisms but may aid in targeting common mechanisms in various disease settings.

Highlights.

• Periodontitis is a very prevalent disease of inflammation-induced bone destruction

• Th17/IL-17 -related pathways are upregulated in tissues from periodontitis patients

• Th17 cells are pathogenic drivers of inflammatory bone loss in experimental periodontitis

• A dysbiotic microbiome triggers Th17 amplification in periodontitis

• Th17- mediated bone loss in periodontitis largely depends on IL-17