IL-36γ is a pivotal inflammatory player in periodontitis-associated bone loss

Oral microbiome, periodontal disease and systemic bone-related diseases in the era of homeostatic medicine

BACKGROUND

Homeostasis is a state of self-regulation and dynamic equilibrium, maintaining the good physiological functions of each system in living organisms. In the oral cavity, the interaction between the host and the oral microbiome forms oral microbial homeostasis. Physiological bone remodeling and renewal can occur under the maintenance of oral microbial homeostasis. The imbalance of bone homeostasis is a key mechanism leading to the occurrence of systemic bone-related diseases. Considering the importance of oral microbial homeostasis in the maintenance of bone homeostasis, it still lacks a complete understanding of the relationship between oral microbiome, periodontal disease and systemic bone-related diseases.

AIM OF REVIEW

This review focuses on the homeostatic changes, pathogenic routes and potential mechanisms in the oral microbiome in periodontal disease and systemic bone-related diseases such as rheumatoid arthritis, osteoarthritis, osteoporosis and osteomyelitis. Additionally, this review discusses oral microbiome-based diagnostic approaches and explores probiotics, mesenchymal stem cells, and oral microbiome transplantation as promising treatment strategies.

KEY SCIENTIFIC CONCEPTS OF REVIEW

This review highlights the association between oral microbial homeostasis imbalance and systemic bone-related diseases, and highlights the possibility of remodeling oral microbial homeostasis for the prevention and treatment of systemic bone-related diseases.

Floxed Il1rl2 Locus with mCherry Reporter Element Reveals Distinct Expression Patterns of the IL-36 Receptor in Barrier Tissues

IL-36 cytokines are emerging as beneficial in immunity against pathogens and cancers but can also be detrimental when dysregulated in autoimmune and autoinflammatory conditions. Interest in targeting IL-36 activity for therapeutic purposes is rapidly growing, yet many unknowns about the functions of these cytokines remain. Thus, the availability of robust research tools is essential for both fundamental basic science and pre-clinical studies to fully access outcomes of any manipulation of the system. For this purpose, a floxed Il1rl2, the gene encoding the IL-36 receptor, mouse strain was developed to facilitate the generation of conditional knockout mice. The targeted locus was engineered to contain an inverted mCherry reporter sequence that upon Cre-mediated recombination will be flipped and expressed under the control of the endogenous Il1rl2 promoter. This feature can be used to confirm knockout in individual cells but also as a reporter to determine which cells express the IL-36 receptor IL-1RL2. The locus was confirmed to function as intended and further used to demonstrate the expression of IL-1RL2 in barrier tissues. Il1rl2 expression was detected in leukocytes in all barrier tissues. Interestingly, strong expression was observed in epithelial cells at locations in direct contact with the environment such as the skin, oral mucosa, the esophagus, and the upper airways, but almost absent from epithelial cells at more inward facing sites, including lung alveoli, the small intestine, and the colon. These findings suggest specialized functions of IL-1RL2 in outward facing epithelial tissues and cells. The generated mouse model should prove valuable in defining such functions and may also facilitate basic and translational research.

Investigating the diagnostic potential of IL-1β, IL-10, and IL-36γ in gingival crevicular fluid in patients with different periodontal conditions

BACKGROUND

The study aimed to analyze cytokine levels, including interleukin (IL)-1β, IL-10, and IL-36γ, to investigate the link between pro- and anti-inflammatory responses in periodontal conditions and assess their potential as diagnostic biomarkers for distinguishing between different types of periodontal conditions.

METHODS

80 systemically healthy non-smokers (25 periodontally healthy, 25 with gingivitis, 30 with periodontitis) were included. Clinical periodontal parameters were recorded, and gingival crevicular fluid (GCF) samples were obtained. Receiver operating characteristic (ROC) curve analysis was applied to determine the diagnostic value of cytokines.

RESULTS

IL-36γ had the highest sensitivity for diagnosing periodontitis, although its specificity for identifying those without periodontitis was relatively low. The combination of IL-1β and IL-36γ was the most effective in differentiating periodontitis from periodontal health. IL-10 was found to be an acceptable discriminator for distinguishing gingivitis from healthy conditions. However, its sensitivity and specificity for identifying gingivitis were lower. The combination of the three cytokines showed the highest ability to distinguish between periodontitis and gingivitis.

CONCLUSION

The levels of IL-1β, IL-10, and IL-36γ in GCF may provide insights into periodontal health and disease status. Further studies are needed to validate these results and explore the potential of these cytokines in periodontal disease management.

Interleukin-38: A crucial player in periodontitis

BACKGROUND

The objective of this study was to compare the levels of gingival crevicular fluid (GCF), salivary, and serum matrix metalloproteinase-9, interleukin (IL)-17, IL-36γ, and IL-38 in individuals with healthy periodontium, gingivitis, and periodontitis and to evaluate their correlations with clinical periodontal parameters.

MATERIALS AND METHODS

Ninety systemically healthy and nonsmoking volunteers divided into a healthy (H) group (n = 30), a gingivitis (G) group (n = 30), and a periodontitis (P) group (n = 30) were included in this study. Clinical periodontal parameters of volunteers were recorded, and GCF, unstimulated saliva, and serum samples were collected. Data analysis was done with enzyme-linked immunosorbent assays. The Kruskal-Wallis test and Bonferroni correction were used for multiple comparisons and post hoc statistical analyses.

RESULTS

The group H had significantly lower clinical parameters than the group P (p < 0.001). GCF and salivary IL-36γ and IL-38 levels were significantly higher in the group P than in the H and G groups (p < 0.05). Positive correlations between biochemical findings and clinical periodontal parameters were observed.

CONCLUSIONS

IL-36γ and IL-38 levels in GCF, saliva, and serum correlate with clinical periodontal parameters and may play a role in determining the activity of periodontitis.

IL-36 Regulates Neutrophil Chemotaxis and Bone Loss at the Oral Barrier

Tissue-specific mechanisms regulate neutrophil immunity at the oral barrier, which plays a key role in periodontitis. Although it has been proposed that fibroblasts emit a powerful neutrophil chemotactic signal, how this chemotactic signal is driven has not been clear. The objective of this study was to investigate the site-specific regulatory mechanisms by which fibroblasts drive powerful neutrophil chemotactic signals within the oral barrier, with particular emphasis on the role of the IL-36 family. The present study found that IL-36γ, agonist of IL-36R, could promote neutrophil chemotaxis via fibroblast. Single-cell RNA sequencing data disclosed that IL36G is primarily expressed in human and mouse gingival epithelial cells and mouse neutrophils. Notably, there was a substantial increase in IL-36γ levels during periodontitis. In vitro experiments demonstrated that IL-36γ specifically activates gingival fibroblasts, leading to chemotaxis of neutrophils. In vivo experiments revealed that IL-36Ra inhibited the infiltration of neutrophils and bone resorption, while IL-36γ promoted their progression in the ligature-induced periodontitis mouse model. In summary, these data elucidate the function of the site-enriched IL-36γ in regulating neutrophil immunity and bone resorption at the oral barrier. These findings provide new insights into the tissue-specific pathophysiology of periodontitis and offer a promising avenue for prevention and treatment through targeted intervention of the IL-36 family.

Mechanism and application of Lactobacillus in type 2 diabetes-associated periodontitis

Type 2 diabetes mellitus (T2DM) accelerates the progression of periodontitis through diverse pathways. Abnormal immune responses, excessive activation of inflammation, increased levels of advanced glycation end products, and oxidative stress have defined roles in the pathophysiological process of T2DM-associated periodontitis. Furthermore, in the periodontium of diabetic individuals, there are high levels of advanced glycation end-products and glucose. Meanwhile, progress in microbiomics has revealed that dysbacteriosis caused by T2DM also contributes to the progression of periodontitis. Lactobacillus, owing to its fine-tuning function in the local microbiota, has sparked tremendous interest in this field. Accumulating research on Lactobacillus has detailed its beneficial role in both diabetes and oral diseases. In this study, we summarize the newly discovered mechanisms underlying Lactobacillus-mediated improvement of T2DM-associated periodontitis and propose the application of Lactobacillus in the clinic.

Illuminating the oral microbiome: cellular microbiology

Epithelial cells line mucosal surfaces such as in the gingival crevice and provide a barrier to the ingress of colonizing microorganisms. However, epithelial cells are more than a passive barrier to microbial intrusion, and rather constitute an interactive interface with colonizing organisms which senses the composition of the microbiome and communicates this information to the underlying cells of the innate immune system. Microorganisms, for their part, have devised means to manipulate host cell signal transduction pathways to favor their colonization and survival. Study of this field, which has become known as cellular microbiology, has revealed much about epithelial cell physiology, bacterial colonization and pathogenic strategies, and innate host responses.

New insights into inflammatory osteoclast precursors as therapeutic targets for rheumatoid arthritis and periodontitis

Rheumatoid arthritis (RA) and periodontitis are chronic inflammatory diseases leading to increased bone resorption. Preventing this inflammatory bone resorption is a major health challenge. Both diseases share immunopathogenic similarities and a common inflammatory environment. The autoimmune response or periodontal infection stimulates certain immune actors, leading in both cases to chronic inflammation that perpetuates bone resorption. Moreover, RA and periodontitis have a strong epidemiological association that could be explained by periodontal microbial dysbiosis. This dysbiosis is believed to be involved in the initiation of RA via three mechanisms. (i) The dissemination of periodontal pathogens triggers systemic inflammation. (ii) Periodontal pathogens can induce the generation of citrullinated neoepitopes, leading to the generation of anti-citrullinated peptide autoantibodies. (iii) Intracellular danger-associated molecular patterns accelerate local and systemic inflammation. Therefore, periodontal dysbiosis could promote or sustain bone resorption in distant inflamed joints. Interestingly, in inflammatory conditions, the existence of osteoclasts distinct from "classical osteoclasts" has recently been reported. They have proinflammatory origins and functions. Several populations of osteoclast precursors have been described in RA, such as classical monocytes, a dendritic cell subtype, and arthritis-associated osteoclastogenic macrophages. The aim of this review is to synthesize knowledge on osteoclasts and their precursors in inflammatory conditions, especially in RA and periodontitis. Special attention will be given to recent data related to RA that could be of potential value in periodontitis due to the immunopathogenic similarities between the two diseases. Improving our understanding of these pathogenic mechanisms should lead to the identification of new therapeutic targets involved in the pathological inflammatory bone resorption associated with these diseases.

Molecular Basis beyond Interrelated Bone Resorption/Regeneration in Periodontal Diseases: A Concise Review

Periodontitis is the sixth most common chronic inflammatory disease, destroying the tissues supporting the teeth. There are three distinct stages in periodontitis: infection, inflammation, and tissue destruction, where each stage has its own characteristics and hence its line of treatment. Illuminating the underlying mechanisms of alveolar bone loss is vital in the treatment of periodontitis to allow for subsequent reconstruction of the periodontium. Bone cells, including osteoclasts, osteoblasts, and bone marrow stromal cells, classically were thought to control bone destruction in periodontitis. Lately, osteocytes were found to assist in inflammation-related bone remodeling besides being able to initiate physiological bone remodeling. Furthermore, mesenchymal stem cells (MSCs) either transplanted or homed exhibit highly immunosuppressive properties, such as preventing monocytes/hematopoietic precursor differentiation and downregulating excessive release of inflammatory cytokines. In the early stages of bone regeneration, an acute inflammatory response is critical for the recruitment of MSCs, controlling their migration, and their differentiation. Later during bone remodeling, the interaction and balance between proinflammatory and anti-inflammatory cytokines could regulate MSC properties, resulting in either bone formation or bone resorption. This narrative review elaborates on the important interactions between inflammatory stimuli during periodontal diseases, bone cells, MSCs, and subsequent bone regeneration or bone resorption. Understanding these concepts will open up new possibilities for promoting bone regeneration and hindering bone loss caused by periodontal diseases.

Treponema denticola Induces Interleukin-36γ Expression in Human Oral Gingival Keratinocytes via the Parallel Activation of NF-κB and Mitogen-Activated Protein Kinase Pathways

The oral epithelial barrier acts as both a physical barrier to the abundant oral microbiome and a sentry for the immune system that, in health, constrains the accumulation of the polymicrobial plaque biofilm. The immune homeostasis during gingivitis that is largely protective becomes dysregulated, unproductive, and destructive to gingival tissue as periodontal disease progresses to periodontitis. The progression to periodontitis is associated with the dysbiosis of the oral microbiome, with increasing prevalences and abundances of periodontal pathogens such as Treponema denticola. Despite the association of T. denticola with a chronic inflammatory disease, relatively little is known about gingival epithelial cell responses to T. denticola infection. Here, we characterized the transcriptome of gingival keratinocytes following T. denticola challenge and identified interleukin-36γ (IL-36γ) as the most differentially expressed cytokine. IL-36γ expression is regulated by p65 NF-κB and the activation of both the Jun N-terminal protein kinase (JNK) and p38 mitogen-activated protein kinase (MAPK) pathways downstream of Toll-like receptor 2 (TLR2). Finally, we demonstrate for the first time that mitogen- and stress-activated kinase 1 (MSK1) contributes to IL-36γ expression and may link the activation of MAPK and NF-κB signaling. These findings suggest that the interactions of T. denticola with the gingival epithelium lead to elevated IL-36γ expression, which may be a critical inducer and amplifier of gingival inflammation and subsequent alveolar bone loss.

HIF-1α drives the transcription of NOG to inhibit osteogenic differentiation of periodontal ligament stem cells in response to hypoxia

Osteogenic differentiation of periodontal ligament stem cells (PDLSCs) is limited in hypoxia, and HIF-1α is key to the response to hypoxia. However, its mechanisms remain largely unknown. This study discovered an osteogenesis-related gene sensitive to hypoxia in PDLSCs, and investigated the molecular mechanisms between HIF-1α and the gene. NOG, a gene that negatively regulates osteogenesis, was discovered by RNA-seq. Under normoxic conditions, HIF-1α overexpression led to enhanced expression of NOG/Noggin and inhibited the expression of osteogenesis-related genes, while inhibition of HIF-1α reversed this effect. The expression of HIF-1α, NOG/Noggin and the osteogenesis-related genes were detected by qRT-PCR or Western blot. Mechanistically, we verified that HIF-1α binds to the hypoxia response element (-1505 to -1502) in the promotor of NOG to enhance secretion of Noggin by chromatin immunoprecipitation and a dual-luciferase reporter assay. IHC staining findings in an animal model verified that Noggin-associated osteogenic differentiation was inhibited in hypoxia. NOG displayed a concordant relationship with HIF-1α, and secreted more with increasing of HIF-1α. Hypoxia stabilized HIF-1α, which bound to the HRE (-1505 to -1502) of the NOG promotor to enhance NOG transcription resulted in inhibiting osteogenic differentiation of PDLSCs. This study offers a promising therapy for periodontitis.

The mechanism on Prevotella melaninogenica promoting the inflammatory progression of oral lichen planus

Oral lichen planus (OLP) is a common chronic inflammatory disease occurring in the oral mucosa. Bacteria is a key driver of mucosal immune response and can induce changes in gene expression and function of epithelial keratinocytes. IL-36γ can induce the expression of antimicrobial peptides, cytokines and chemokines, and is widely involved in many chronic inflammatory diseases. Our aim is to explore the role of IL-36γ in pathological process of OLP when Prevotella melaninogenica (P. melaninogenica) invades oral mucosa. The expression of IL-36γ in OLP lesions and mice was detected by immunohistochemistry. Recombinant human IL-36Gamma (rhIL-36γ) was used to treat oral keratinocytes and the expression levels of inflammatory cytokines were detected by qRT-PCR and ELISA. The expression of IL-36γ and TRPV1 was detected by western blotting following co-culturing P. melaninogenica with oral keratinocytes. The mRNA expression of IL-36γ was detected by qRT-PCR. From our results, IL-36γ was upregulated in OLP lesions. Exogenous rhIL-36γ promoted the expression of pro-inflammatory cytokines and antibacterial peptides in oral keratinocytes. The expression of IL-36γ was significantly increased following the stimulation of P. melaninogenica in oral keratinocytes and mice. TRPV1 activation was induced by P. melaninogenica and its activation enhanced the expression of IL-36γ. IL-36Ra could reduce the inflammation in OLP in vitro. In summary, overexpression of IL-36γ in OLP lesions could promote its pathogenesis by inducing inflammation. P. melaninogenica invasion of oral keratinocytes could induce the expression of IL-36γ by the activation of TRPV1, thereby regulating the interaction between bacteria and oral epithelial cells.

Down-regulated IL36RN expression based on peripheral blood mononuclear cells and plasma of periodontitis patients and its clinical significance

BACKGROUND

The role of IL-36 receptor antagonist (IL36RN), a mutated gene expression of IL-36 in periodontitis patients with peripheral blood mononuclear cells (PBMC) and plasma remains to be undetermined.

MATERIALS AND METHODS

Our study discovered the IL36RN expression through GEO public databases and further validated by PBMC and plasma of periodontitis patients and healthy participants. A total of 194 participants of public datasets, consisting of 97 cases of periodontitis and 97 cases of healthy control were retrospectively evaluated and explored the gene enrichment pathways and clinical significance of IL36RN expression accompanied by three different cytokines. Furthermore, the clinical significance of IL36RN was evaluated in mild-to-severe patients of periodontitis by the receiver operating curve (ROC) using the area under the curve (AUC).

RESULTS

IL36RN expressions were notably down-regulated in PBMC and plasma of periodontitis patients. Further, a positive correlation of IL36RN expression was significantly observed between PBMC and plasma of periodontitis patients while IL36RN expression was negatively correlated to serum-based three different cytokines of periodontitis patients. Meanwhile, the ROC-AUCs achieved a significantly higher range from 0.80 to 0.87 with PBMC of mild-to-severe and moderate-to-severe periodontitis patients whereas similar patients with plasma obtained a significant AUC range from 0.73 to 0.83.

CONCLUSION

IL36RN can distinctively be detectable in periodontitis patients with PBMC and plasma, which can act as a down-regulated mutated gene that might play an effective role in causing periodontitis. IL36RN may involve by other inflammatory cytokines in the pathogenesis of periodontitis.

Akkermansia muciniphila and Its Pili-Like Protein Amuc_1100 Modulate Macrophage Polarization in Experimental Periodontitis

Periodontitis is a chronic inflammatory disease triggered by dysbiosis of the oral microbiome. Porphyromonas gingivalis is strongly implicated in periodontal inflammation, gingival tissue destruction, and alveolar bone loss through sustained exacerbation of the host response. Recently, the use of other bacterial species, such as Akkermansia muciniphila, has been suggested to counteract inflammation elicited by P. gingivalis In this study, the effects of A. muciniphila and its pili-like protein Amuc_1100 on macrophage polarization during P. gingivalis infection were evaluated in a murine model of experimental periodontitis. Mice were gavaged with P. gingivalis alone or in combination with A. muciniphila or Amuc_1100 for 6 weeks. Morphometric analysis demonstrated that the addition of A. muciniphila or Amuc_1100 significantly reduced P. gingivalis-induced alveolar bone loss. This decreased bone loss was associated with a proresolutive phenotype (M2) of macrophages isolated from submandibular lymph nodes as observed by flow cytometry. Furthermore, the expression of interleukin 10 (IL-10) at the RNA and protein levels was significantly increased in the gingival tissues of the mice and in macrophages exposed to A. muciniphila or Amuc_1100, confirming their anti-inflammatory properties. This study demonstrates the putative therapeutic interest of the administration of A. muciniphila or Amuc_1100 in the management of periodontitis through their anti-inflammatory properties.

IL-1 Superfamily Members and Periodontal Diseases

Periodontitis is a complex, multifactorial chronic disease involving continuous interactions among bacteria, host immune/inflammatory responses, and modifying genetic and environmental factors. More than any other cytokine family, the interleukin (IL)-1 family includes key signaling molecules that trigger and perpetuate periodontal inflammation. Over the years, the IL-1 family expanded to include 11 members of cytokines, some with agonist activity (IL-1α, IL-1β, IL-18, IL-33, IL-36α, IL-36β, and IL-36γ), receptor antagonists (IL-1Ra, IL-36Ra), and 2 anti-inflammatory cytokines (IL-37, IL-38). The IL-1 receptor antagonist (IL-1Ra) has emerged as a pivotal player in the defense against periodontitis. IL-33 primarily induces the production of Th2-associated cytokines but acts as an "alarmin" via stimulation of mast cells. The IL-36 subclass of cytokines may be important in regulating mucosal inflammation and homeostasis. IL-37 suppresses innate and acquired immune responses. IL-38 is the most recent member of the IL-1 superfamily and has anti-inflammatory properties similar to those of IL-37 but through different receptors. However, limited evidence exists regarding the role of IL-37 and IL-38 in periodontitis. Despite the development of IL-1 blocking agents, therapeutic blockade of select IL-1 family members for periodontitis has only been partially investigated in preclinical and clinical research, while the development of IL-37 and IL-38 as novel anti-inflammatory drugs has not been considered adequately. Here, we review the key properties of the IL-1 family members and provide insights into targeting or promoting select cytokines as new therapeutic agents.