NOD1 and NOD2 mediate sensing of periodontal pathogens

NOD2 contributes to Parvimonas micra-induced bone resorption in diabetic rats with experimental periodontitis

BACKGROUND

Type 2 diabetes mellitus (T2DM) may affect the oral microbial community, exacerbating periodontal inflammation; however, its pathogenic mechanisms remain unclear. As nucleotide-binding oligomerization domain 2 (NOD2) plays a crucial role in the activation during periodontitis (PD), it is hypothesized that changes in the oral microbial community due to diabetes enhance periodontal inflammation through the activation of NOD2.

METHODS

We collected subgingival plaque from 180 subjects who were categorized into two groups based on the presence or absence of T2DM. The composition of oral microbiota was detected by 16S rRNA high-throughput sequencing. In animal models of PD with or without T2DM, we assessed alveolar bone resorption by micro-computerized tomography and used immunohistochemistry to detect NOD2 expression in alveolar bone. Primary osteoblasts were cultured in osteogenic induction medium with high or normal glucose and treated with inactivated bacteria. After 24 h of inactivated bacteria intervention, the osteogenic differentiation ability was detected by alkaline phosphatase (ALP) staining, and the expressions of NOD2 and interleukin-12 (IL-6) were detected by western blot.

RESULTS

The relative abundance of Parvimonas and Filifactor in the T2DM group was increased compared to the group without T2DM. In animal models, alveolar bone mass was decreased in PD, particularly in T2DM with PD (DMPD) group, compared to controls. Immunohistochemistry revealed NOD2 in osteoblasts from the alveolar bone in both the PD group and DMPD group, especially in the DMPD group. In vitro, intervention with inactivated Parvimonas significantly reduced ALP secretion of primary osteoblasts in high glucose medium, accompanied by increased expression of NOD2 and IL-6.

CONCLUSIONS

The results suggest that T2DM leading to PD may be associated with the activation of NOD2 by Parvimonas.

Identification of disease-associate variants of aggressive periodontitis using genome-wide association studies

Aggressive periodontitis (AgP), Stage III or IV and Grade C according to the new periodontitis classification, is characterized by the rapid destruction of periodontal tissues in the systemically healthy population and often causes premature tooth loss. The presence of familial aggregation suggests the involvement of genetic factors in the pathogenesis. However, the genes associated with the onset and progression of the disease and details of its pathogenesis have not yet been fully identified. In recent years, the genome-wide approach (GWAS), a comprehensive genome analysis method using bioinformatics, has been used to search for disease-related genes, and the results have been applied in genomic medicine for various diseases, such as cancer. In this review, we discuss GWAS in the context of AgP. First, we introduce the relationship between single-nucleotide polymorphisms (SNPs) and susceptibility to diseases and how GWAS is useful for searching disease-related SNPs. Furthermore, we summarize the recent findings of disease-related genes using GWAS on AgP inside and outside Japan and a possible mechanism of the pathogenesis of AgP based on available literature and our research findings. These findings will lead to advancements in the prevention, prognosis, and treatment of AgP.

Prevotella genus and its related NOD-like receptor signaling pathway in young males with stage III periodontitis

Background

As periodontitis progresses, the oral microbiota community changes dynamically. In this study, we evaluated the dominant bacteria and their roles in the potential pathway in young males with stage III periodontitis.

Methods

16S rRNA sequencing was performed to evaluate variations in the composition of oral bacteria between males with stage I and III periodontitis and identify the dominant bacteria of each group. Function prediction was obtained based on 16S rRNA sequencing data. The inhibitor of the predominant pathway for stage III periodontitis was used to investigate the role of the dominant bacteria in periodontitis in vivo and in vitro.

Results

Chao1 index, Observed Species and Phylogenetic Diversity (PD) whole tree values were significantly higher in the stage III periodontitis group. β-diversity suggested that samples could be divided according to the stages of periodontitis. The dominant bacteria in stage III periodontitis were Prevotella, Prevotella_7, and Dialister, whereas that in stage I periodontitis was Cardiobacterium. KEGG analysis predicted that variations in the oral microbiome may be related to the NOD-like receptor signaling pathway. The inhibitor of this pathway, NOD-IN-1, decreased P. intermedia -induced Tnf-α mRNA expression and increased P. intermedia -induced Il-6 mRNA expression, consistent with the ELISA results. Immunohistochemistry confirmed the down-regulation of TNF-α and IL-6 expressions by NOD-IN-1 in P. intermedia-induced periodontitis.

Conclusion

The composition of the oral bacteria in young males varied according to the stage of periodontitis. The species richness of oral microtia was greater in young males with stage III periodontitis than those with stage I periodontitis. Prevotella was the dominant bacteria in young males with stage III periodontitis, and inhibition of the NOD-like receptor signaling pathway can decrease the periodontal inflammation induced by P. intermedia.

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

OBJECTIVES

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Bacterial subversion of NLR-mediated immune responses

Members of the mammalian Nod-like receptor (NLR) protein family are important intracellular sensors for bacteria. Bacteria have evolved under the pressure of detection by host immune sensing systems, leading to adaptive subversion strategies to dampen immune responses for their benefits. These include modification of microbe-associated molecular patterns (MAMPs), interception of innate immune pathways by secreted effector proteins and sophisticated instruction of anti-inflammatory adaptive immune responses. Here, we summarise our current understanding of subversion strategies used by bacterial pathogens to manipulate NLR-mediated responses, focusing on the well-studied members NOD1/2, and the inflammasome forming NLRs NLRC4, and NLRP3. We discuss how bacterial pathogens and their products activate these NLRs to promote inflammation and disease and the range of mechanisms used by bacterial pathogens to evade detection by NLRs and to block or dampen NLR activation to ultimately interfere with the generation of host immunity. Moreover, we discuss how bacteria utilise NLRs to facilitate immunotolerance and persistence in the host and outline how various mechanisms used to attenuate innate immune responses towards bacterial pathogens can also aid the host by reducing immunopathologies. Finally, we describe the therapeutic potential of harnessing immune subversion strategies used by bacteria to treat chronic inflammatory conditions.

Pathogenic Mechanisms of Fusobacterium nucleatum on Oral Epithelial Cells

Periodontitis is an oral chronic inflammatory disease and may cause tooth loss in adults. Oral epithelial cells provide a barrier for bacteria and participate in the immune response. Fusobacterium nucleatum (F. nucleatum) is one of the common inhabitants of the oral cavity and has been identified as a potential etiologic bacterial agent of oral diseases, such as periodontitis and oral carcinomas. F. nucleatum has been shown to be of importance in the development of diverse human cancers. In the dental biofilm, it exhibits a structural role as a bridging organism, connecting primary colonizers to the largely anaerobic secondary colonizers. It expresses adhesins and is able to induce host cell responses, including the upregulation of defensins and the release of chemokines and interleukins. Like other microorganisms, its detection is achieved through germline-encoded pattern-recognition receptors (PRRs) and pathogen-associated molecular patterns (PAMPs). By identification of the pathogenic mechanisms of F. nucleatum it will be possible to develop effective methods for the diagnosis, prevention, and treatment of diseases in which a F. nucleatum infection is involved. This review summarizes the recent progress in research targeting F. nucleatum and its impact on oral epithelial cells.

Periodontitis and diabetes mellitus co-morbidity: A molecular dialogue

BACKGROUND

Type 2 diabetes mellitus (T2DM) and periodontitis are two biologically linked diseases that often coexist in complex interaction. While periodontitis may lead to insulin receptor desensitization, diabetes may increase the expression of inflammatory cytokines, such as Tumor Necrosis Factor-α (TNF-α) and Interleukin 6 (IL-6), in the gingival crevicular fluid and activate osteoclasts via Receptor activator of nuclear factor kappa-Β ligand (RANK-L) production, leading to bone resorption. However, the association between the two diseases processes, where one may exacerbate the progression of the other, is unclear. In addition, both diseases have similar mechanistic themes, such as chronic inflammation and oxidative stress. This review aimed to investigate the pathophysiological and molecular mechanisms underlying T2DM and periodontitis.

HIGHLIGHT

Uncontrolled diabetes is often associated with severe periodontitis, measured by clinical attachment loss. Alteration in the oral microbiome composition, which may activate the host inflammatory response and lead to irreversible oxidative stress, is a common finding in both diseases. An understanding of the molecular crosstalk between the two disease processes is crucial for developing therapeutic targets that inhibit bone resorption and halt the progression of periodontitis in patients with diabetes.

CONCLUSION

The Oral microbiome composition in T2DM and periodontitis shifts toward dysbiosis, favoring bacterial pathogens, such as Fusobacteria and Porphyromonas species. Both conditions are marked by pro-inflammatory immune activity via the activation of Interleukin 17 (IL-17), Interleukin 1 (IL-1), TNF-α, and Nuclear Factor Kappa Beta (NF-κB). Common molecular crosstalk signaling appears to involve advanced glycation end products (AGEs) and oxidative stress. Thus, future drug targets are multifactorial, ranging from modulatory of host inflammatory response to preventing the accumulation of AGEs and oxidative free radicals.

Is the oral microbiome a source to enhance mucosal immunity against infectious diseases?

Mucosal tissues act as a barrier throughout the oral, nasopharyngeal, lung, and intestinal systems, offering first-line protection against potential pathogens. Conventionally, vaccines are applied parenterally to induce serotype-dependent humoral response but fail to drive adequate mucosal immune protection for viral infections such as influenza, HIV, and coronaviruses. Oral mucosa, however, provides a vast immune repertoire against specific microbial pathogens and yet is shaped by an ever-present microbiome community that has co-evolved with the host over thousands of years. Adjuvants targeting mucosal T-cells abundant in oral tissues can promote soluble-IgA (sIgA)-specific protection to confer increased vaccine efficacy. Th17 cells, for example, are at the center of cell-mediated immunity and evidence demonstrates that protection against heterologous pathogen serotypes is achieved with components from the oral microbiome. At the point of entry where pathogens are first encountered, typically the oral or nasal cavity, the mucosal surfaces are layered with bacterial cohabitants that continually shape the host immune profile. Constituents of the oral microbiome including their lipids, outer membrane vesicles, and specific proteins, have been found to modulate the Th17 response in the oral mucosa, playing important roles in vaccine and adjuvant designs. Currently, there are no approved adjuvants for the induction of Th17 protection, and it is critical that this research is included in the preparedness for the current and future pandemics. Here, we discuss the potential of oral commensals, and molecules derived thereof, to induce Th17 activity and provide safer and more predictable options in adjuvant engineering to prevent emerging infectious diseases.

Effect of Reactive Oxygen Species on the Endoplasmic Reticulum and Mitochondria during Intracellular Pathogen Infection of Mammalian Cells

Oxidative stress, particularly reactive oxygen species (ROS), are important for innate immunity against pathogens. ROS directly attack pathogens, regulate and amplify immune signals, induce autophagy and activate inflammation. In addition, production of ROS by pathogens affects the endoplasmic reticulum (ER) and mitochondria, leading to cell death. However, it is unclear how ROS regulate host defense mechanisms. This review outlines the role of ROS during intracellular pathogen infection, mechanisms of ROS production and regulation of host defense mechanisms by ROS. Finally, the interaction between microbial pathogen-induced ROS and the ER and mitochondria is described.

NOD1-Targeted Immunonutrition Approaches: On the Way from Disease to Health

Immunonutrition appears as a field with great potential in modern medicine. Since the immune system can trigger serious pathophysiological disorders, it is essential to study and implement a type of nutrition aimed at improving immune system functioning and reinforcing it individually for each patient. In this sense, the nucleotide-binding oligomerization domain-1 (NOD1), one of the members of the pattern recognition receptors (PRRs) family of innate immunity, has been related to numerous pathologies, such as cancer, diabetes, or cardiovascular diseases. NOD1, which is activated by bacterial-derived peptidoglycans, is known to be present in immune cells and to contribute to inflammation and other important pathways, such as fibrosis, upon recognition of its ligands. Since immunonutrition is a significant developing research area with much to discover, we propose NOD1 as a possible target to consider in this field. It is relevant to understand the cellular and molecular mechanisms that modulate the immune system and involve the activation of NOD1 in the context of immunonutrition and associated pathological conditions. Surgical or pharmacological treatments could clearly benefit from the synergy with specific and personalized nutrition that even considers the health status of each subject.

Receptor-interacting protein kinase 2 contributes to host innate immune responses against Fusobacterium nucleatum in macrophages and decidual stromal cells

PROBLEM

Chorioamnionitis is caused by a bacterial infection that ascends from the vagina and can cause adverse pregnancy outcomes (APOs). Fusobacterium nucleatum (F. nucleatum) is a periodontal pathogen associated with the occurrence of APOs. In this study, we evaluated whether receptor-interacting protein kinase 2 (Ripk2), an adaptor protein of the cytosolic receptors nucleotide-binding oligomerization domain (NOD)1 and NOD2, in macrophages and human decidual stromal cells (hDSCs) contributes to immune responses against F. nucleatum.

METHOD OF STUDY

Bone marrow-derived macrophages (BMDMs) isolated from wild-type (WT) and Ripk2-deficient mice and hDSCs were cultured with F. nucleatum (MOI 1, 10, 100). BMDMs and hDSCs were assessed using enzyme-linked immunosorbent assay, Western blot analysis, real-time PCR, and nitrite assay.

RESULTS

Fusobacterium nucleatum-induced production of IL-6, but not of TNF-α and IL-10, was lower in Ripk2-deficient BMDMs than in WT cells. Western blotting revealed a decrease in F. nucleatum-induced p65 phosphorylation in Ripk2-deficient macrophages, whereas mitogen-activated protein kinases activation was comparable between WT and Ripk2-deficient cells. The production of nitric oxide (NO) in response to F. nucleatum and the gene and protein expression of inducible NO synthase was impaired in Ripk2-deficient BMDMs. In hDSCs, F. nucleatum upregulated the gene and protein expression of NOD1, NOD2, and Ripk2 in a time-dependent manner. F. nucleatum also increased the production of IL-6, CXCL8, and CCL2, whereas this production was decreased by the Ripk2 inhibitors SB203580 and PP2.

CONCLUSIONS

In conclusion, Ripk2 signaling appears to contribute to the F. nucleatum-induced immune response and can be a preventive and therapeutic target against APOs.

Expression of osteoclastogenic and anti-osteoclastogenic cytokines differs in mouse gingiva injected with lipopolysaccharide, peptidoglycan, or both

OBJECTIVE

Bacterial substances in subgingival biofilm evoke alveolar bone resorption. We previously reported that gingival injection of bacterial lipopolysaccharide (LPS) and peptidoglycan (PGN) induced alveolar bone resorption in mice. However, the mechanism by which LPS and PGN induce osteoclast formation has not been investigated. The aim of this study is to clarify the role of osteoclastogenic and anti-osteoclastogenic cytokines in the alveolar bone resorption induced by LPS and PGN.

MATERIALS

LPS from Escherichia coli, PGN from Staphylococcus aureus, or both were injected into the gingiva of mice every 48 h for a total of 13 times. Alveolar bone resorption was assessed histochemically by tartrate-resistant acid phosphatase staining. Expression of the receptor activator of nuclear factor-κB ligand (RANKL), tumor necrosis factor (TNF)-α, interleukin (IL)-17, and IL-10 were analyzed by immunostaining. To analyze the role of these cytokines, RANKL-pretreated mouse bone marrow macrophages were stimulated with LPS, PGN, or LPS + PGN with or without anti-TNF-α antibody, IL-17, or IL-10.

RESULTS

Alveolar bone resorption was induced by both LPS and PGN and exacerbated by LPS + PGN. LPS induced higher RANKL expression than PGN. Expression of TNF-α and IL-10 was correlated with bone resorption. PGN injections induced the strongest expression of IL-17, followed by LPS + PGN and LPS. In an in vitro osteoclastogenesis assay, anti-TNF-α antibody and IL-10 inhibited osteoclast formation, but IL-17 promoted it.

CONCLUSION

LPS, PGN, or LPS + PGN injections induce distinctive expression of TNF-α, IL-10, and IL-17, suggesting that the composition of these bacterial ligands in dental plaque is critical for alveolar bone resorption.

NOD2/c-Jun NH2-Terminal Kinase Triggers Mycoplasma ovipneumoniae-Induced Macrophage Autophagy

Mycoplasma ovipneumoniae belongs to Mycoplasma, a genus containing the smallest self-replicating microorganisms, and causes infectious pleuropneumonia in goats and sheep. Nucleotide-binding oligomerization domain-containing protein (NOD2), an intracellular pattern recognition receptor, interacts with muramyl dipeptide (MDP) to recognize bacterial peptidoglycans and is involved in autophagy induction. However, there have been no reports about NOD recognition of mycoplasmas or M. ovipneumoniae-induced autophagy. In this study, we sought to determine the role of NOD2 in M. ovipneumoniae-induced autophagy using Western blotting, immunofluorescence, real-time PCR (RT-PCR), and color-changing unit (CCU) analysis. M. ovipneumoniae infection markedly increased NOD2 but did not increase NOD1 expression in RAW 264.7 cells. Treating RAW 264.7 cells with MDP significantly increased colocalization of M. ovipneumoniae and LC3, whereas treatment with NOD inhibitor, NOD-IN-1, decreased colocalization of M. ovipneumoniae and LC3. Furthermore, suppressing NOD2 expression with small interfering RNA (siRNA)-NOD2 failed to trigger M. ovipneumoniae-induced autophagy by detecting autophagy markers Atg5, beclin1, and LC3-II. In addition, M. ovipneumoniae infection significantly increased the phosphorylated c-Jun NH₂-terminal kinase (p-JNK)/JNK, p-Bcl-2/Bcl-2, beclin1, Atg5, and LC3-II ratios in RAW 264.7 cells. Treatment with JNK inhibitor, SP600126, or siRNA-NOD2 did not increase this reaction. These findings suggested that M. ovipneumoniae infection activated NOD2, and both NOD2 and JNK pathway activation promoted M. ovipneumoniae-induced autophagy. This study provides new insight into the NOD2 reorganization mechanism and the pathogenesis of M. ovipneumoniae infection.IMPORTANCE M. ovipneumoniae, which lacks a cell wall, causes infectious pleuropneumonia in goats and sheep. In the present study, we focused on the interaction between NOD and M. ovipneumoniae, as well as its association with autophagy. We showed for the first time that NOD2 was activated by M. ovipneumoniae even when peptidoglycans were not present. We also observed that both NOD2 and JNK pathway activation promoted M. ovipneumoniae-induced autophagy.

High Mobility Group Box 1 Expression in Oral Inflammation and Regeneration

High mobility group box 1 (HMGB1) is a non-histone DNA-binding protein of about 30 kDa. It is released from a variety of cells into the extracellular milieu in response to inflammatory stimuli and acts on specific cell-surface receptors, such as receptors for advanced glycation end-products (RAGE), Toll-like receptor (TLR)2, TLR4, with or without forming a complex with other molecules. HMGB1 mediates various mechanisms such as inflammation, cell migration, proliferation, and differentiation. On the other hand, HMGB1 enhances chemotaxis acting through the C-X-C motif chemokine ligand (CXCL)12/C-X-C chemokine receptor (CXCR)4 axis and is involved in regeneration. In the oral cavity, high levels of HMGB1 have been detected in the gingival tissue from periodontitis and peri-implantitis patients, and it has been shown that secreted HMGB1 induces pro-inflammatory cytokine expression, such as interleukin (IL)-1β, IL-6, and tumor necrosis factor (TNF)-α, which prolong inflammation. In contrast, wound healing after tooth extraction or titanium dental implant osseointegration requires an initial acute inflammation, which is regulated by secreted HMGB1. This indicates that secreted HMGB1 regulates angiogenesis and bone remodeling by osteoclast and osteoblast activation and promotes bone healing in oral tissue repair. Therefore, HMGB1 can prolong inflammation in the periodontal tissue and, conversely, can regenerate or repair damaged tissues in the oral cavity. In this review, we highlight the role of HMGB1 in the oral cavity by comparing its function and regulation with its function in other diseases. We also discuss the necessity for further studies in this field to provide more specific scientific evidence for dentistry.

Aggressive periodontitis and NOD2 variants

Aggressive periodontitis (AgP) occurs at an early age and causes rapid periodontal tissue destruction. Nucleotide-binding oligomerization domain-containing protein 2 (NOD2) encodes a protein with two caspase recruitment domains and eleven leucine-rich repeats. This protein is expressed mainly in peripheral blood leukocytes and is involved in immune response. NOD2 variants have been associated with increased susceptibility to Crohn's disease, and recently, NOD2 was reported as a causative gene in AgP. The present study aimed to identify potential NOD2 variants in an AgP cohort (a total of 101 patiens: 37 patients with positive family histories and 64 sporadic patients). In the familial group, six patients from two families had a reported heterozygous missense variant (c.C931T, p.R311W). Four patients in the sporadic group had a heterozygous missense variant (c.C1411T, p.R471C), with no reported association to the disease. Overall, two NOD2 variants, were identified in 10% of our AgP cohort. These variants were different from the major variants reported in Crohn's disease. More cases need to be investigated to elucidate the role of NOD2 variants in AgP pathology.

Pilot GWAS of caries in African-Americans shows genetic heterogeneity

BACKGROUND

Dental caries is the most common chronic disease in the US and disproportionately affects racial/ethnic minorities. Caries is heritable, and though genetic heterogeneity exists between ancestries for a substantial portion of loci associated with complex disease, a genome-wide association study (GWAS) of caries specifically in African Americans has not been performed previously.

METHODS

We performed exploratory GWAS of dental caries in 109 African American adults (age > 18) and 96 children (age 3-12) from the Center for Oral Health Research in Appalachia (COHRA1 cohort). Caries phenotypes (DMFS, DMFT, dft, and dfs indices) assessed by dental exams were tested for association with 5 million genotyped or imputed single nucleotide polymorphisms (SNPs), separately in the two age groups. The GWAS was performed using linear regression with adjustment for age, sex, and two principal components of ancestry. A maximum of 1 million adaptive permutations were run to determine empirical significance.

RESULTS

No loci met the threshold for genome-wide significance, though some of the strongest signals were near genes previously implicated in caries such as antimicrobial peptide DEFB1 (rs2515501; p = 4.54 × 10- 6) and TUFT1 (rs11805632; p = 5.15 × 10- 6). Effect estimates of lead SNPs at suggestive loci were compared between African Americans and Caucasians (adults N = 918; children N = 983). Significant (p < 5 × 10- 8) genetic heterogeneity for caries risk was found between racial groups for 50% of the suggestive loci in children, and 12-18% of the suggestive loci in adults.

CONCLUSIONS

The genetic heterogeneity results suggest that there may be differences in the contributions of genetic variants to caries across racial groups, and highlight the critical need for the inclusion of minorities in subsequent and larger genetic studies of caries in order to meet the goals of precision medicine and to reduce oral health disparities.

The impact of Aggregatibacter actinomycetemcomitans biofilm-derived effectors following antimicrobial photodynamic therapy on cytokine production in human gingival fibroblasts

BACKGROUND

Antimicrobial photodynamic therapy (aPDT) is an effective adjunctive therapeutic modality for the treatment of local infections, including periodontitis and peri-implantitis. After receiving aPDT, microbial cells in the biofilm structure may produce and/ or release soluble biofilm-derived effectors (BDEs), which may affect the biology of the host cells in the community context of their surrounding microenvironment. Given the fact that no study has investigated the role of BDEs following aPDT in the pathogenesis of infectious diseases, the aim of the current study was to determine the effect of BDEs of Aggregatibacter actinomycetemcomitans following exposure to sub-lethal doses of indocyanine green (ICG)-aPDT on human gingival fibroblasts (HGFs) in terms of cytokines produced.

MATERIALS AND METHODS

In this study, we evaluated the effect of biofilm-conditioned medium (BCM) resulting from the treatment of A. actinomycetemcomitans biofilm with a sub-lethal dose of aPDT on cytokines production, including IL-6, IL-8, CXCL10, TGF-β, and bFGF of HGFs using enzyme-linked immunoassays (ELISA). The sensitivity of cytokines to BDEs was determined by micro-titer plates.

RESULTS

The maximal sub-lethal dose of ICG-PDT was 20.15 μM/mL ICG at a fluence of 31.2 J/cm². The BCM of ICG-PDT-treated viable A. actinomycetemcomitans significantly reduced IL-6, IL-8, and CXCL10 levels compared to the BCM of untreated viable A. actinomycetemcomitans (78-, 93-, and 61.6-fold reduction, respectively; all P < 0.01). TGF-β and bFGF were strongly induced by BCM of ICG-PDT treated viable A. actinomycetemcomitans (by 57.7 and 36.1 folds, respectively; both P < 0.05). The BCM of untreated viable A. actinomycetemcomitans degraded most of the CxCL10, TGF-β and bFGF (58.8, 61.5, and 71.6%, respectively) in 24 h, while it degraded 9.3% of IL-6 and 15.1% of IL-8 after 24 h.

CONCLUSION

The results of the current study revealed that a sub-lethal dose of ICG-aPDT through the effect of BCM on HGFs could not only significantly reduce the production of pro-inflammatory cytokines but also promoted their role in periodontal regeneration due to increasing the bFGF level. Altogether, ICG-aPDT, with it's antimicrobial effects reduces inflammation and induces of tissue regeneration resulting from BCM, can be considered an efficient adjunctive therapeutic method for the treatment of local infections.

Role of NOD1/NOD2 receptors in Fusobacterium nucleatum mediated NETosis

Polymorphonuclear neutrophils (PMNs) are indispensable in fighting infectious microbes by adopting various antimicrobial strategies including phagocytosis and neutrophil extracellular traps (NETs). Although the role and importance of PMNs in periodontal disease are well established, the specific molecular mechanisms involved in NET formation are yet to be characterized. In the present study, we sought to determine the role of periodontal pathogen on NET formation by utilizing Fusobacterium nucleatum. Our data demonstrates that F. nucleatum activates neutrophils and induces robust NETosis in a time-dependent manner via the upregulation of the Nucleotide oligomerization domain 1 (NOD1) and NOD2 receptors. Furthermore, CRISPR/Cas9 knockout of HL-60 cells and the use of ligands/inhibitors confirmed the involvement of NOD1 and NOD2 receptors in F. nucleatum-mediated NET formation. When treated with NOD1 and NOD2 inhibitors, we observed a significant downregulation of peptidylarginine deiminase 4 (PAD4) activity. In addition, neutrophils showed a significant increase and decrease of myeloperoxidase (MPO) and neutrophil elastase (NE) when treated with NOD1/NOD2 ligands and inhibitors, respectively. Taken together, CRISPR/Cas9 knockout of NOD1/NOD2 HL-60 cells and inhibitors of NOD signaling confirmed the role of NLRs in F. nucleatum-mediated NETosis. Our data demonstrates an important pathway linking NOD1 and NOD2 to NETosis by F. nucleatum, a prominent microbe in periodontal biofilms. This is the first study to elucidate the role of NOD-like receptors in NETosis and their downstream signaling network.

Porphyromonas gulae Activates Unprimed and Gamma Interferon-Primed Macrophages via the Pattern Recognition Receptors Toll-Like Receptor 2 (TLR2), TLR4, and NOD2

Porphyromonas gulae is an anaerobic, Gram-negative coccobacillus that has been associated with periodontal disease in companion animals. The aims of this study were to analyze the ligation of pattern recognition receptors by P. gulae and the subsequent activation of macrophages. Exposure of HEK cells transfected with Toll-like receptors (TLRs) or NOD-like receptors to P. gulae resulted in the ligation of TLR2, TLR4, and NOD2. The effects of this engagement of receptors were investigated by measuring the synthesis of nitric oxide (NO), CD86 expression, and inflammatory cytokine production by wild-type, TLR2^-/-, and TLR4^-/- macrophages. The addition of P. gulae to unprimed and gamma interferon (IFN-γ)-primed (M1 phenotype) macrophages significantly increased the surface expression of CD86, but only M1 macrophages produced nitric oxide. P. gulae-induced expression of CD86 on unprimed macrophages was dependent on both TLR2 and TLR4, but CD86 expression and NO production in M1 macrophages were only TLR2 dependent. P. gulae induced an increase in secretion of interleukin-1α (IL-1α), IL-1β, IL-6, IL-12p70, IL-13, tumor necrosis factor alpha (TNF-α), granulocyte colony-stimulating factor (G-CSF), monocyte chemoattractant protein 1 (MCP-1), and macrophage inflammatory protein 1α (MIP-1α) by M1 macrophages compared to that by unprimed controls. Among these cytokines, secretion of IL-6 and TNF-α by M1 macrophages was dependent on either TLR2 or TLR4. Our data indicate that TLR2 and TLR4 are important for P. gulae activation of unprimed macrophages and that activation and effector functions induced in M1 macrophages by P. gulae are mainly dependent on TLR2. In conclusion, P. gulae induces a strong TLR2-dependent inflammatory M1 macrophage response which may be important in establishing the chronic inflammation associated with periodontal disease in companion animals.

Involvement of NLRP10 in IL-1α induction of oral epithelial cells by periodontal pathogens

This study investigated the pathogenesis of periodontitis and the role of nucleotide-binding oligomerization domain-like receptor protein 10 (NLRP10). The human oral epithelial cell line HOK-16B was infected with two periodontal pathogens, Tannerella forsythia and Fusobacterium nucleatum, at various MOIs. RT-PCR and immunoblotting demonstrated that infection increased mRNA and protein expression of NLRP10, respectively. The siRNA-mediated NLRP10 knockdown significantly reduced IL-1α expression and secretion. Both bacteria induced phosphorylation of ERK, JNK and p38 MAP kinases in HOK-16B cells. NLRP10 knockdown impaired ERK phosphorylation only. ERK inhibition significantly decreased the expression of T. forsythia- and F. nucleatum-induced IL-1α. Our data suggest that NLRP10 is involved in activating the ERK signalling pathway in HOK-16B cells infected with T. forsythia and F. nucleatum. This pathway likely augments the pro-inflammatory cytokine IL-1α levels, which may play a critical role in periodontitis.

Association of NOD2 Mutations with Aggressive Periodontitis

Aggressive periodontitis (AgP) is characterized by rapid alveolar bone destruction and tooth loss early in life, and its etiology remains unclear. To explore the genetic risk factors of AgP, we performed genome-wide single-nucleotide polymorphism genotyping for identity-by-descent mapping and identified 32 distinct candidate loci, followed by whole exome sequencing with 2 pedigrees of AgP consisting of 3 cases and 1 control in 1 family and 2 sibling cases in the other. After variant filtering procedures and validation by targeted Sanger sequencing, we identified 2 missense mutations at 16q12 in NOD2 (p.Ala110Thr and p.Arg311Trp), which encodes nucleotide-binding oligomerization domain protein 2. We further examined 94 genetically unrelated AgP patients by targeted sequencing of NOD2 and found that 2 patients among them also carried the p.Arg311Trp variant. Furthermore, we found 3 additional missense mutations in this gene (p.His370Tyr, p.Arg459Cys, and p.Ala868Thr). These mutations either had not been previously observed or are extremely rare (frequency <0.001) in Asian populations. NOD2 plays a crucial role in innate immunity as an intracellular receptor initiating nuclear factor κB-dependent and mitogen-activated protein kinase-dependent gene transcription. These results demonstrated NOD2 as a novel gene involved in AgP.

PKC, ERK/p38 MAP kinases and NF-κB targeted signalling play a role in the expression and release of IL-1β and CXCL8 in Porphyromonas gingivalis-infected THP1 cells

Porphyromonas gingivalis is a keystone pathogen in periodontitis and is gaining importance in cardiovascular pathogenesis. Protease-activated receptors (PARs), toll-like receptors (TLRs) and nucleotide-binding oligomerization domain (NOD) on monocytes recognize the structural components on P. gingivalis, inducing inflammatory intermediates. Here, we elucidate the modulation of PARs, TLRs, NODs, and the role of MAPK and NF-κB in IL-1β and CXCL8 release. THP1 cells were stimulated with P. gingivalis wild-type W50 and its isogenic gingipain mutants: Rgp mutant E8 and Kgp mutant K1A. We observed modulation of PARs, TLRs, NOD, IL-1β and CXCL8 expression by P. gingivalis. Gingipains hydrolyse IL-1β and CXCL8, which is more evident for IL-1β accumulation at 24 h. Inhibition of PKC (protein kinase C), p38 and ERK (extracellular signal-regulated kinases) partially reduced P. gingivalis-induced IL-1β at 6 h, whereas PKC and ERK reduced CXCL8 at both 6 and 24 h. Following NF-κB inhibition, P. gingivalis-induced IL-1β and CXCL8 were completely suppressed to basal levels. Overall, TLRs, PARs and NOD possibly act in synergy with PKC, MAPK ERK/p38 and NF-κB in P. gingivalis-induced IL-1β and CXCL8 release from THP1 cells. These pro-inflammatory cytokines could affect leucocytes in circulation and exacerbate other vascular inflammatory conditions such as atherosclerosis.

Periodontal manifestations of inflammatory bowel disease: emerging epidemiologic and biologic evidence

Inflammatory bowel disease and periodontitis are both described as a disproportionate mucosal inflammatory response to a microbial environment in susceptible patients. Moreover, these two conditions share major environmental and lifestyle-related risk factors. Despite this intriguing pathogenic parallel, large-scale studies and basic research have only recently considered periodontal outcomes as relevant data. There are mounting and consistent arguments, from recent epidemiologic studies and animal models, that these two conditions might be related. This article is a comprehensive and critical up-to-date review of the current evidence and future prospects in understanding the biologic and epidemiologic relationships between periodontal status and inflammatory bowel disease.

Role of NOD2 and RIP2 in host-microbe interactions with Gram-negative bacteria: insights from the periodontal disease model

NOD2 is a member of the NLR family of proteins that participate in the activation of the innate immune response. RIP2 is a downstream kinase activated by both NOD1 and NOD2. There is scarcity of information regarding the relevance of NOD2 in periodontitis, a chronic inflammatory condition characterized by inflammatory bone resorption. We used NOD2-KO and RIP2-KO mice in a model of microbial-induced periodontitis. Heat-killed Aggregatibacter actinomycetemcomitans was injected in the gingival tissues three times/wk for 4 wk. Bone resorption was assessed by μCT analysis; osteoclasts were identified by immunohistochemical staining for TRAP and inflammation was assessed using a severity score system in H/E-stained sections. In vitro studies using primary macrophages assessed the response macrophages using qPCR-based array and multi-ligand ELISA. Bone resorption and osteoclastogenesis were significantly reduced in NOD2-KO mice. Severity of inflammation was not affected. qPCR-focused arrays and multi-ligand ELISA showed that expression of pro-inflammatory mediators was reduced in NOD2- and RIP2-deficient cells. RANKL-induced osteoclastogenesis was impaired in NOD2- and RIP2-deficient macrophages. We conclude that NOD2 is important for osteoclast differentiation and inflammatory bone resorption in vivo and also for the macrophage response to Gram-negative bacteria.

Transcriptome sequencing of gingival biopsies from chronic periodontitis patients reveals novel gene expression and splicing patterns

BACKGROUND

Periodontitis is the most common chronic inflammatory disease caused by complex interaction between the microbial biofilm and host immune responses. In the present study, high-throughput RNA sequencing was utilized to systemically and precisely identify gene expression profiles and alternative splicing.

METHODS

The pooled RNAs of 10 gingival tissues from both healthy and periodontitis patients were analyzed by deep sequencing followed by computational annotation and quantification of mRNA structures.

RESULTS

The differential expression analysis designated 400 up-regulated genes in periodontitis tissues especially in the pathways of defense/immunity protein, receptor, protease, and signaling molecules. The top 10 most up-regulated genes were CSF3, MAFA, CR2, GLDC, SAA1, LBP, MME, MMP3, MME-AS1, and SAA4. The 62 down-regulated genes in periodontitis were mainly cytoskeletal and structural proteins. The top 10 most down-regulated genes were SERPINA12, MT4, H19, KRT2, DSC1, PSORS1C2, KRT27, LCE3C, AQ5, and LCE6A. The differential alternative splicing analysis revealed unique transcription variants in periodontitis tissues. The EDB exon was predominantly included in FN1, while exon 2 was mostly skipped in BCL2A1.

CONCLUSIONS

These findings using RNA sequencing provide novel insights into the pathogenesis mechanism of periodontitis in terms of gene expression and alternative splicing.

Withaferin A inhibits inflammatory responses induced by Fusobacterium nucleatum and Aggregatibacter actinomycetemcomitans in macrophages

Periodontitis is a progressive chronic inflammatory disease and a major cause of tooth loss in humans. As a withanolides, withaferin A (WA) is known to exhibit strong anti‑inflammatory activity. The present study examined whether WA inhibited inflammatory responses in macrophages in response to two representative periodontal pathogens, Fusobacterium nucleatum and Aggregatibacter actinomycetemcomitans. Murine bone marrow‑derived macrophages (BMDMs) were used in this study and cytokine production in culture supernatants was measured by enzyme‑linked immunosorbent assays. Western blot analysis was performed to determine the activation of nuclear factor‑κB and mitogen‑activated protein kinases (MAPKs) and the expression of inducible nitric oxide synthase (iNOS), toll‑like receptor (TLR) 2 and TLR4. The production of nitric oxide (NO) was determined by the Griess reaction. WA treatment was shown to decrease interleukin (IL)‑6 and tumor necrosis factor (TNF)‑α production in BMDMs in response to F. nucleatum and A. actinomycetemcomitans in a dose‑dependent manner. The phosphorylation of IκB‑α and MAPKs (p38, extracellular signal‑regulated kinases and c‑Jun N‑terminal kinases) induced by F. nucleatum and A. actinomycetemcomitans was also inhibited by WA. F. nucleatum and A. actinomycetemcomitans induced iNOS expression and NO production in BMDMs, which was inhibited by WA in a dose‑dependent manner. WA also reduced endogenous and induced expression of TLR2 and TLR4 in these cells. These results suggest that WA may be a potential therapeutic agent or preventive additive for periodontitis control.

E-selectin expression induced by Porphyromonas gingivalis in human endothelial cells via nucleotide-binding oligomerization domain-like receptors and Toll-like receptors

Porphyromonas gingivalis, an important periodontal pathogen, has been proved to actively invade cells, induce endothelial cell activation, and promote development of atherosclerosis. Innate immune surveillance, which includes the activity of nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs) and Toll-like receptors (TLRs), are essential for the control of microbial infections; however, the roles of receptor families in P. gingivalis infections remain unclear. Here, we examined the roles of NLRs and TLRs in endothelial cell activation caused by P. gingivalis. Live P. gingivalis and whole cell sonicates were used to stimulate endothelial cells, and both showed upregulation of E-selectin as well as NOD1, NOD2, and TLR2. In addition, silencing of these genes in endothelial cells infected with P. gingivalis led to a reduction in E-selectin expression. Porphyromonas gingivalis also induced nuclear factor-κB (NF-κB) and P38 mitogen-activated protein kinase (MAPK) activity in endothelial cells, whereas small interfering RNA targeting NOD1 significantly reduced these signals. Moreover, inhibition of either NOD2 or TLR2 inhibited NF-κB significantly, but had only a weak inhibitory effect on P38 MAPK signaling. Direct inhibition of NF-κB and P38 MAPK significantly attenuated E-selectin expression induced by P. gingivalis in endothelial cells. Taken together, these findings suggest that NOD1, NOD2, and TLR2 play important, non-redundant roles in endothelial cell activation following P. gingivalis infection.

NADPH oxidases: an overview from structure to innate immunity-associated pathologies

Oxygen-derived free radicals, collectively termed reactive oxygen species (ROS), play important roles in immunity, cell growth, and cell signaling. In excess, however, ROS are lethal to cells, and the overproduction of these molecules leads to a myriad of devastating diseases. The key producers of ROS in many cells are the NOX family of NADPH oxidases, of which there are seven members, with various tissue distributions and activation mechanisms. NADPH oxidase is a multisubunit enzyme comprising membrane and cytosolic components, which actively communicate during the host responses to a wide variety of stimuli, including viral and bacterial infections. This enzymatic complex has been implicated in many functions ranging from host defense to cellular signaling and the regulation of gene expression. NOX deficiency might lead to immunosuppression, while the intracellular accumulation of ROS results in the inhibition of viral propagation and apoptosis. However, excess ROS production causes cellular stress, leading to various lethal diseases, including autoimmune diseases and cancer. During the later stages of injury, NOX promotes tissue repair through the induction of angiogenesis and cell proliferation. Therefore, a complete understanding of the function of NOX is important to direct the role of this enzyme towards host defense and tissue repair or increase resistance to stress in a timely and disease-specific manner.

NOD2 contributes to Porphyromonas gingivalis-induced bone resorption

The NOD-like receptors are cytoplasmic proteins that sense microbial by-products released by invasive bacteria. Although NOD1 and NOD2 are functionally expressed in cells from oral tissues and play a role triggering immune responses, the role of NOD2 receptor in the bone resorption and in the modulation of osteoclastogenesis is still unclear. We show that in an experimental model of periodontitis with Porphyromonas gingivalis W83, NOD2(-/-) mice showed lower bone resorption when compared to wild type. Quantitative polymerase chain reaction analysis revealed that wild-type infected mice showed an elevated RANKL/OPG ratio when compared to NOD2(-/-) infected mice. Moreover, the expression of 2 osteoclast activity markers-cathepsin K and matrix metalloproteinase 9-was significantly lower in gingival tissue from NOD2(-/-) infected mice compared to WT infected ones. The in vitro study reported an increase in the expression of the NOD2 receptor 24 hr after stimulation of hematopoietic bone marrow cells with M-CSF and RANKL. We also evaluated the effect of direct activation of NOD2 receptor on osteoclastogenesis, by the activation of this receptor in preosteoclasts culture, with different concentrations of muramyl dipeptide. The results show no difference in the number of TRAP-positive cells. Although it did not alter the osteoclasts differentiation, the activation of NOD2 receptor led to a significant increase of cathepsin K expression. We confirm that this enzyme was active, since the osteoclasts resorption capacity was enhanced by muramyl dipeptide stimulation, evaluated in osteoassay plate. These results show that the lack of NOD2 receptor impairs the bone resorption, suggesting that NOD2 receptor could contribute to the progression of bone resorption in experimental model of periodontitis. The stimulation of NOD2 by its agonist, muramyl dipeptide, did not affect osteoclastogenesis, but it does favor the bone resorption capacity identified by increased osteoclast activity.

Porphyromonas gingivalis infection increases osteoclastic bone resorption and osteoblastic bone formation in a periodontitis mouse model

Background

Porphyromonas gingivalis has been shown to invade osteoblasts and inhibit their differentiation and mineralization in vitro. However, it is unclear if P. gingivalis can invade osteoblasts in vivo and how this would affect alveolar osteoblast/osteoclast dynamics. This study aims to answer these questions using a periodontitis mouse model under repetitive P. gingivalis inoculations.

Methods

For 3-month-old BALB/cByJ female mice, 10⁹ CFU of P. gingivalis were inoculated onto the gingival margin of maxillary molars 4 times at 2-day intervals. After 2 weeks, another 4 inoculations at 2-day intervals were applied. Calcein was injected 7 and 2 days before sacrificing animals to label the newly formed bone. Four weeks after final inoculation, mice were sacrificed and maxilla collected. Immunohistochemistry, micro-CT, and bone histomorphometry were performed on the specimens. Sham infection with only vehicle was the control.

Results

P. gingivalis was found to invade gingival epithelia, periodontal ligament fibroblasts, and alveolar osteoblasts. Micro-CT showed alveolar bone resorption and significant reduction of bone mineral density and content in the infected mice compared to the controls. Bone histomorphometry showed a decrease in osteoblasts, an increase in osteoclasts and bone resorption, and a surprisingly increased osteoblastic bone formation in the infected mice compared to the controls.

Conclusions

P. gingivalis invades alveolar osteoblasts in the periodontitis mouse model and cause alveolar bone loss. Although P. gingivalis appears to suppress osteoblast pool and enhance osteoclastic bone resorption, the bone formation capacity is temporarily elevated in the infected mice, possibly via some anti-microbial compensational mechanisms.

Aggregatibacter actinomycetemcomitans outer membrane vesicles are internalized in human host cells and trigger NOD1- and NOD2-dependent NF-κB activation

Aggregatibacter actinomycetemcomitans is an oral and systemic pathogen associated with aggressive forms of periodontitis and with endocarditis. We recently demonstrated that outer membrane vesicles (OMVs) disseminated by A. actinomycetemcomitans could deliver multiple proteins, including biologically active cytolethal distending toxin (CDT), into the cytosol of HeLa cells and human gingival fibroblasts (HGF). In the present work, we have used immunoelectron and confocal microscopy analysis and fluorescently labeled vesicles to further investigate mechanisms for A. actinomycetemcomitans OMV-mediated delivery of bacterial antigens to these host cells. Our results supported that OMVs were internalized into the perinuclear region of HeLa cells and HGF. Colocalization analysis revealed that internalized OMVs colocalized with the endoplasmic reticulum and carried antigens, detected using an antibody specific to whole A. actinomycetemcomitans serotype a cells. Consistent with OMV internalization mediating intracellular antigen exposure, the vesicles acted as strong inducers of cytoplasmic peptidoglycan sensor NOD1- and NOD2-dependent NF-κB activation in human embryonic kidney cells. Moreover, NOD1 was the main sensor of OMV-delivered peptidoglycan in myeloid THP1 cells, contributing to the overall inflammatory responses induced by the vesicles. This work reveals a role of A. actinomycetemcomitans OMVs as a trigger of innate immunity via carriage of NOD1- and NOD2-active pathogen-associated molecular patterns (PAMPs).

Immuno-pathogenesis of Periodontal Disease: Current and Emerging Paradigms

Periodontal disease (PD) is a highly complex disease involving many factors; however, two principal facets central to initiation and progression of the majority of PD are the composition of the microbes in the sub-gingival plaque, and the host immune response to these organisms. Numerous studies point to the complexity of PD, and to the fact that despite innate and adaptive immune activation, and resultant inflammation, our immune response fails to cure disease. Stunning new findings have begun to clarify several complexities of the host-pathogen interaction of PD pointing to key roles for microbial dysboisis and immune imbalance in the pathogenesis of disease. Furthermore, these investigations have identified novel translational opportunities to intercede in PD treatment. In this review we will highlight a select few recent findings in innate and adaptive immunity, and host pathogen interactions of PD at a micro-environmental level that may have profound impact on PD progression.

Activation of NOD receptors by Neisseria gonorrhoeae modulates the innate immune response

NOD1 and NOD2 are members of the NOD-like receptor family of cytosolic pattern recognition receptors that recognize specific fragments of the bacterial cell wall component peptidoglycan. Neisseria species are unique amongst Gram-negative bacteria in that they turn over large amounts of peptidoglycan during growth. We examined the ability of NOD1 and NOD2 to recognize Neisseria gonorrhoeae, and determined the role of NOD-dependent signaling in regulating the immune response to gonococcal infection. Gonococci, as well as conditioned medium from mid-logarithmic phase grown bacteria, were capable of activating both human NOD1 and NOD2, as well as mouse NOD2, leading to the activation of the transcription factor NF-κB and polyubiquitination of the adaptor receptor-interacting serine-threonine kinase 2. We identified a number of cytokines and chemokines that were differentially expressed in wild type versus NOD2-deficient macrophages in response to gonococcal infection. Moreover, NOD2 signaling up-regulated complement pathway components and cytosolic nucleic acid sensors, suggesting a broad impact of NOD activation on innate immunity. Thus, NOD1 and NOD2 are important intracellular regulators of the immune response to infection with N. gonorrhoeae. Given the intracellular lifestyle of this pathogen, we believe these cytosolic receptors may provide a key innate immune defense mechanism for the host during gonococcal infection.

Intracellular adhesion molecule-1 is regulated by porphyromonas gingivalis through nucleotide binding oligomerization domain-containing proteins 1 and 2 molecules in periodontal fibroblasts

BACKGROUND

The mechanism by which Porphyromonas gingivalis regulates intracellular adhesion molecule 1 (ICAM-1) expression in human periodontal ligament cells (hPDLCs) and human gingival fibroblasts (hGFs) is unknown. The aim of this study is to investigate whether nucleotide binding oligomerization domain-containing protein (NOD) 1 and NOD2 are involved in this process and the clinical significance of ICAM-1 in periodontitis.

METHODS

hPDLCs and hGFs were treated with P. gingivalis, l-Ala-γ-d-glutamyl-mesodiaminopimelic acid (an agonist for NOD1), and muramyl dipeptide (an agonist for NOD2). Alternatively, cells transfected with small interfering RNA targeting NOD1and NOD2 were treated with P. gingivalis. ICAM-1, NOD1, and NOD2 were detected at mRNA and protein levels. In addition, clinical examinations were performed in 30 healthy controls and 40 patients with chronic periodontitis (CP) before and after treatment, and serum-soluble ICAM-1 (sICAM-1) levels in these individuals were detected by enzyme-linked immunosorbent assay.

RESULTS

This study shows that P. gingivalis caused an increase in ICAM-1, NOD1, and NOD2 expression in periodontal fibroblasts. There was a linear correlation between ICAM-1 and NOD1 and NOD2 levels. Activation of NOD1 and NOD2 by the specific agonist led to the upregulation of ICAM-1, whereas knocking down NOD1 and NOD2 caused a reduction in P. gingivalis-induced ICAM-1 production. Furthermore, sICAM-1 levels were higher in patients with CP than in healthy controls and were positively related to the clinical periodontal parameters. After periodontal treatment, sICAM-1 levels decreased significantly.

CONCLUSIONS

The present results indicate that sICAM-1 levels are correlated to the severity of periodontitis. NOD1 and NOD2 mediate P. gingivalis-induced ICAM-1 production in periodontal fibroblasts. NOD1 and NOD2 could be considered potential targets for periodontal therapy.

Lipopolysaccharide-induced indoleamine 2,3-dioxygenase expression in the periodontal ligament

BACKGROUND AND OBJECTIVE

Indoleamine 2,3-dioxygenase (IDO) is a tryptophan-oxidizing enzyme with immune-inhibitory effects. The aim of this study was to investigate the expression of IDO by lipopolysaccharide (LPS), a component of gram-negative bacteria, in human periodontal ligament (PDL) cells.

MATERIAL AND METHODS

Human PDL cells and gingival fibroblasts (GFs) were prepared from explants of human PDLs and from gingival tissues of clinically healthy donors, respectively. Real-time RT-PCR, western blotting and the IDO enzyme assay were performed to determine the expression of IDO following LPS treatment of cells. LPS was injected into mice tail veins to evaluate the effects of LPS in vivo in the maxillary first molar. Immunofluorescence staining and histological analysis were followed to localize IDO in mouse PDL.

RESULTS

The level of expression of IDO mRNA in primary human PDL cells after LPS treatment was increased in a dose-dependent manner, reaching a peak 8 h after LPS treatment. The expression and activities of IDO protein were significantly increased in comparison with those of the control. In addition, the increased production of kynurenine in culture medium was observed 72 h after LPS treatment. In the immunofluorescence findings, stronger immunoreactivities were shown in PDL than in gingival tissues in the maxillae. In accordance with the immunofluorescence findings, LPS treatment induced a strong up-regulation of IDO mRNA in human PDL cells, whereas human GFs showed only a weak response to LPS.

CONCLUSION

These results clearly show that IDO was induced by LPS in primary human PDL cells, suggesting that PDL might be involved in the regulation of oral inflammatory disease.

Porphyromonas gingivalis mediates inflammasome repression in polymicrobial cultures through a novel mechanism involving reduced endocytosis

The interleukin (IL)-1β-processing inflammasome has recently been identified as a target for pathogenic evasion of the inflammatory response by a number of bacteria and viruses. We postulated that the periodontal pathogen, Porphyromonas gingivalis may suppress the inflammasome as a mechanism for its low immunogenicity and pathogenic synergy with other, more highly immunogenic periodontal bacteria. Our results show that P. gingivalis lacks signaling capability for the activation of the inflammasome in mouse macrophages. Furthermore, P. gingivalis can suppress inflammasome activation by another periodontal bacterium, Fusobacterium nucleatum. This repression affects IL-1β processing, as well as other inflammasome-mediated processes, including IL-18 processing and cell death, in both human and mouse macrophages. F. nucleatum activates IL-1β processing through the Nlrp3 inflammasome; however, P. gingivalis repression is not mediated through reduced levels of inflammasome components. P. gingivalis can repress Nlrp3 inflammasome activation by Escherichia coli, and by danger-associated molecular patterns and pattern-associated molecular patterns that mediate activation through endocytosis. However, P. gingivalis does not suppress Nlrp3 inflammasome activation by ATP or nigericin. This suggests that P. gingivalis may preferentially suppress endocytic pathways toward inflammasome activation. To directly test whether P. gingivalis infection affects endocytosis, we assessed the uptake of fluorescent particles in the presence or absence of P. gingivalis. Our results show that P. gingivalis limits both the number of cells taking up beads and the number of beads taken up for bead-positive cells. These results provide a novel mechanism of pathogen-mediated inflammasome inhibition through the suppression of endocytosis.

ASC-dependent RIP2 kinase regulates reduced PGE2 production in chronic periodontitis

Levels of prostaglandin E(2) (PGE(2)) and its processing enzyme, prostaglandin-endoperoxide-synthase-2/ cyclooxygenase-2 (PTGS2/COX-2), are elevated in actively progressing periodontal lesions, but suppressed in chronic disease. COX-2 expression is regulated through inflammatory signaling that converges on the mitogen-activated protein kinase (MAPK) pathway. Emerging evidence suggests a role for the inflammatory adaptor protein, ASC/Pycard, in MAPK activation. We postulated that ASC may represent a mediator of the MAPK-mediated regulatory network of PGE(2) production. Using RNAi-mediated gene slicing, we demonstrated that ASC regulates COX-2 expression and PGE(2) production in THP1 monocytic cells following infection with Porphyromonas gingivalis (Pg). Production of PGE(2) did not require the inflammasome adaptor function of ASC, but was dependent on MAPK activation. Furthermore, the MAP kinase kinase kinase CARD domain-containing protein RIPK2 was induced by Pg in an ASC-dependent manner. Reduced ASC and RIPK2 levels were revealed by orthogonal comparison of the expression of the RIPK family in ASC-deficient THP1 cells with that in chronic periodontitis patients. We show that pharmacological inhibition of RIPK2 represses PGE(2) secretion, and RNAi-mediated silencing of RIPK2 leads to diminished MAPK activation and PGE(2) secretion. These findings identify a novel ASC-RIPK2 axis in the generation of PGE(2) that is repressed in patients diagnosed with chronic adult periodontitis.

Pathogen-mediated proteolysis of the cell death regulator RIPK1 and the host defense modulator RIPK2 in human aortic endothelial cells

Porphyromonas gingivalis is the primary etiologic agent of periodontal disease that is associated with other human chronic inflammatory diseases, including atherosclerosis. The ability of P. gingivalis to invade and persist within human aortic endothelial cells (HAEC) has been postulated to contribute to a low to moderate chronic state of inflammation, although how this is specifically achieved has not been well defined. In this study, we demonstrate that P. gingivalis infection of HAEC resulted in the rapid cleavage of receptor interacting protein 1 (RIPK1), a mediator of tumor necrosis factor (TNF) receptor-1 (TNF-R1)-induced cell activation or death, and RIPK2, a key mediator of both innate immune signaling and adaptive immunity. The cleavage of RIPK1 or RIPK2 was not observed in cells treated with apoptotic stimuli, or cells stimulated with agonists to TNF-R1, nucleotide oligomerization domain receptor 1(NOD1), NOD2, Toll-like receptor 2 (TLR2) or TLR4. P. gingivalis-induced cleavage of RIPK1 and RIPK2 was inhibited in the presence of a lysine-specific gingipain (Kgp) inhibitor. RIPK1 and RIPK2 cleavage was not observed in HAEC treated with an isogenic mutant deficient in the lysine-specific gingipain, confirming a role for Kgp in the cleavage of RIPK1 and RIPK2. Similar proteolysis of poly (ADP-ribose) polymerase (PARP) was observed. We also demonstrated direct proteolysis of RIPK2 by P. gingivalis in a cell-free system which was abrogated in the presence of a Kgp-specific protease inhibitor. Our studies thus reveal an important role for pathogen-mediated modification of cellular kinases as a potential strategy for bacterial persistence within target host cells, which is associated with low-grade chronic inflammation, a hallmark of pathogen-mediated chronic inflammatory disorders.

Peptidoglycan and lipopolysaccharide synergistically enhance bone resorption and osteoclastogenesis

BACKGROUND AND OBJECTIVE

Peptidoglycan (PGN) and lipopolysaccharide (LPS) are bacterial cell wall constituents that are able to induce bone resorption by stimulating Toll-like receptor (TLR) 2 and TLR4, respectively. The fragments of PGN also stimulate inflammatory responses via nucleotide-binding oligomerization domain (NOD) 1 and NOD2, although there are differences in the NOD-stimulatory activities between gram-positive and gram-negative PGNs. The TLR and NOD signaling pathways are known to engage in cross-talk to enhance the production of inflammatory cytokines. In the present study, we investigated the effects of gram-negative and gram-positive PGNs on bone resorption and osteoclastogenesis in the presence or absence of LPS.

MATERIAL AND METHODS

We injected Escherichia coli PGN or Staphylococcus aureus PGN with or without LPS into mouse gingiva, and histopathologically assessed alveolar bone resorption by tartrate-resistant acid phosphatase staining. We also stimulated osteoclast precursors from mouse bone marrow macrophages with these PGNs in vitro and assessed osteoclastogenesis. The cells were also stimulated with synthetic ligands for NOD1; γ-D-glutamyl-meso-DAP NOD2; muramyl dipeptide or TLR2; Pam(3) CSK(4) with or without LPS to analyse the signaling cross-talk.

RESULTS

S. aureus PGN, but not E. coli PGN, induced alveolar bone resorption, as did LPS. However, PGN from both sources significantly enhanced the bone resorption in the mice co-injected with LPS. Both types of PGNs induced osteoclastogenesis and accelerated osteoclastogenesis when the cells were co-stimulated with LPS in vitro. All synthetic ligands synergistically induced osteoclastogenesis by co-stimulation with LPS.

CONCLUSION

Gram-positive or gram-negative PGN worked synergistically with LPS to induce bone resorption and osteoclastogenesis, possibly by co-ordinating the effects of TLR2, NOD1, NOD2 and TLR4 signaling.

The macrophage pattern recognition scavenger receptors SR-A and CD36 protect against microbial induced pregnancy loss

OBJECTIVES AND DESIGN

Microbial products can act via stress-induced signaling cascades to link dysregulated endogenous microbiota to immune activation (e.g., macrophages) and pregnancy loss. Our previous studies demonstrated that mice deficient in the macrophage pattern recognition scavenger receptors, SR-A and CD36, are more susceptible to inflammatory complications including gut leakiness and experimental colitis. We hypothesized that bacterial penetration of the maternal mucosal surfaces and replication in embryonic fluids compromise the fetal status and can result in miscarriage.

MATERIALS AND METHODS

Eighty pregnant ICR and SR-A/CD36-deficient mice were injected via tail vein or intraperitoneally with commensal bacteria (Streptococcus cricetus and/or Actinobacillus sp.) or sham controls. Dams were monitored daily for physical distress, pain and abortion.

RESULTS

Dams injected with single dose bacterial inoculum did not develop clinical symptoms. Day old pups injected with bacteria developed internal focal abscesses, lost weight but recovered after 1 week. Dams receiving a second bacterial inoculum delivered dead fetuses. However, SR-A/CD36-deficnet dams demonstrated 100% fetal death via aborted fetuses, and significant up-regulation of the proinflammatory markers (IL-6, serum Amyloid A) 24-74 h after single inoculum.

CONCLUSIONS

These data indicate that macrophage scavenger receptors are required for the fetal protection against microbial attack and support that maternal transfer of innate immunity contributes to this protection.

Destructive and protective roles of cytokines in periodontitis: a re-appraisal from host defense and tissue destruction viewpoints

Periodontal diseases (PD) are chronic infectious inflammatory diseases characterized by the destruction of tooth-supporting structures, being the presence of periodontopathogens required, but not sufficient, for disease development. As a general rule, host inflammatory mediators have been associated with tissue destruction, while anti-inflammatory mediators counteract and attenuate disease progression. With the discovery of several T-cell subsets bearing distinct immunoregulatory properties, this pro- vs. anti-inflammatory scenario became more complex, and a series of studies has hypothesized protective or destructive roles for Th1, Th2, Th17, and Treg subpopulations of polarized lymphocytes. Interestingly, the "protective vs. destructive" archetype is usually considered in a framework related to tissue destruction and disease progression. However, it is important to remember that periodontal diseases are infectious inflammatory conditions, and recent studies have demonstrated that cytokines (TNF-α and IFN-γ) considered harmful in the context of tissue destruction play important roles in the control of periodontal infection. Therefore, in this review, the state-of-the-art knowledge concerning the protective and destructive roles of host inflammatory immune response will be critically evaluated and discussed from the tissue destruction and control-of-infection viewpoints.