Dependence of bacterial protein adhesins on toll-like receptors for proinflammatory cytokine induction

The intriguing strategies of Tannerella forsythia's host interaction

Tannerella forsythia, a member of the "red complex" bacteria implicated in severe periodontitis, employs various survival strategies and virulence factors to interact with the host. It thrives as a late colonizer in the oral biofilm, relying on its unique adaptation mechanisms for persistence. Essential to its survival are the type 9 protein secretion system and O-glycosylation of proteins, crucial for host interaction and immune evasion. Virulence factors of T. forsythia, including sialidase and proteases, facilitate its pathogenicity by degrading host glycoproteins and proteins, respectively. Moreover, cell surface glycoproteins like the S-layer and BspA modulate host responses and bacterial adherence, influencing colonization and tissue invasion. Outer membrane vesicles and lipopolysaccharides further induce inflammatory responses, contributing to periodontal tissue destruction. Interactions with specific host cell types, including epithelial cells, polymorphonuclear leukocytes macrophages, and mesenchymal stromal cells, highlight the multifaceted nature of T. forsythia's pathogenicity. Notably, it can invade epithelial cells and impair PMN function, promoting dysregulated inflammation and bacterial survival. Comparative studies with periodontitis-associated Porphyromonas gingivalis reveal differences in protease activity and immune modulation, suggesting distinct roles in disease progression. T. forsythia's potential to influence oral antimicrobial defense through protease-mediated degradation and interactions with other bacteria underscores its significance in periodontal disease pathogenesis. However, understanding T. forsythia's precise role in host-microbiome interactions and its classification as a keystone pathogen requires further investigation. Challenges in translating research data stem from the complexity of the oral microbiome and biofilm dynamics, necessitating comprehensive studies to elucidate its clinical relevance and therapeutic implications in periodontitis management.

A systematic review of the impact of Porphyromonas gingivalis on foam cell formation: Implications for the role of periodontitis in atherosclerosis

BACKGROUND

The current literature suggests the significant role of foam cells in the initiation of atherosclerosis through the formation of a necrotic core in atherosclerotic plaques. Moreover, an important periodontal pathogen called Porphyromonas gingivalis (P. gingivalis) is indicated to play a significant role in this regard. Thus, the aim of this systematic review was to comprehensively study the pathways by which P. gingivalis as a prominent bacterial species in periodontal disease, can induce foam cells that would initiate the process of atherosclerosis formation.

METHODS

An electronic search was undertaken in three databases (Pubmed, Scopus, and Web of Science) to identify the studies published from January 2000 until March 2023. The risk of bias in each study was also assessed using the QUIN risk of bias assessment tool.

RESULTS

After the completion of the screening process, 11 in-vitro studies met the inclusion criteria and were included for further assessments. Nine of these studies represented a medium risk of bias, while the other two had a high risk of bias. All of the studies have reported that P. gingivalis can significantly induce foam cell formation by infecting the macrophages and induction of oxidized low-density lipoprotein (oxLDL) uptake. This process is activated through various mediators and pathways. The most important factors in this regard are the lipopolysaccharide of P. gingivalis and its outer membrane vesicles, as well as the changes in the expression rate of transmembrane lipid transportation channels, including transient receptor potential channel of the vanilloid subfamily 4 (TRPV4), lysosomal integral protein 2 (LIMP2), CD36, etc. The identified molecular pathways involved in this process include but are not limited to NF-κB, ERK1/2, p65.

CONCLUSION

Based on the results of this study, it can be concluded that P. gingivalis can effectively promote foam cell formation through various pathogenic elements and this bacterial species can affect the expression rate of various genes and the function of specific receptors in the cellular and lysosomal membranes. However, due to the moderate to high level of risk of bias among the studies, further studies are required in this regard.

A computational method for the prediction and functional analysis of potential Mycobacterium tuberculosis adhesin-related proteins

OBJECTIVES

Mycobacterial adherence plays a major role in the establishment of infection within the host. Adhesin-related proteins attach to host receptors and cell-surface components. The current study aimed to utilize in-silico strategies to determine the adhesin potential of conserved hypothetical (CH) proteins.

METHODS

Computational analysis was performed on the whole Mycobacterium tuberculosis H37Rv proteome using a software program for the prediction of adhesin and adhesin-like proteins using neural networks (SPAAN) to determine the adhesin potential of CH proteins. A robust pipeline of computational analysis tools: Phyre2 and pFam for homology prediction; Mycosub, PsortB, and Loctree3 for subcellular localization; SignalP-5.0 and SecretomeP-2.0 for secretory prediction, were utilized to identify adhesin candidates.

RESULTS

SPAAN revealed 776 potential adhesins within the whole MTB H37Rv proteome. Comprehensive analysis of the literature was cross-tabulated with SPAAN to verify the adhesin prediction potential of known adhesin (n = 34). However, approximately a third of known adhesins were below the probability of adhesin (Pad) threshold (Pad ≥0.51). Subsequently, 167 CH proteins of interest were categorized using essential in-silico tools.

CONCLUSION

The use of SPAAN with supporting in-silico tools should be fundamental when identifying novel adhesins. This study provides a pipeline to identify CH proteins as functional adhesin molecules.

Shut-Down of Type IX Protein Secretion Alters the Host Immune Response to Tannerella forsythia and Porphyromonas gingivalis

Tannerella forsythia and Porphyromonas gingivalis target distinct virulence factors bearing a structurally conserved C-terminal domain (CTD) to the type IX protein secretion system (T9SS). The T9SS comprises an outer membrane translocation complex which works in concert with a signal peptidase for CTD cleavage. Among prominent T9SS cargo linked to periodontal diseases are the TfsA and TfsB components of T. forsythia’s cell surface (S-) layer, the bacterium’s BspA surface antigen and a set of cysteine proteinases (gingipains) from P. gingivalis. To assess the overall role of the bacterial T9SS in the host response, human macrophages and human gingival fibroblasts were stimulated with T. forsythia and P. gingivalis wild-type bacteria and T9SS signal peptidase-deficient mutants defective in protein secretion, respectively. The immunostimulatory potential of these bacteria was compared by analyzing the mRNA expression levels of the pro-inflammatory mediators IL-6, IL-8, MCP-1 and TNF-α by qPCR and by measuring the production of the corresponding proteins by ELISA. Shot-gun proteomics analysis of T. forsythia and P. gingivalis outer membrane preparations confirmed that several CTD-bearing virulence factors which interact with the human immune system were depleted from the signal peptidase mutants, supportive of effective T9SS shut-down. Three and, more profoundly, 16 hours post stimulation, the T. forsythia T9SS mutant induced significantly less production of cytokines and the chemokine in human cells compared to the corresponding parent strain, while the opposite was observed for the P. gingivalis T9SS mutant. Our data indicate that T9SS shut-down translates into an altered inflammatory response in periodontal pathogens. Thus, the T9SS as a potential novel target for periodontal therapy needs further evaluation.

Porphyromonas gingivalis Mfa1 Induces Chemokine and Cell Adhesion Molecules in Mouse Gingival Fibroblasts via Toll-Like Receptors

Porphyromonas gingivalis Mfa1 fimbriae are thought to act as adhesion factors and to direct periodontal tissue destruction but their immunomodulatory actions are poorly understood. Here, we investigated the effect of Mfa1 stimulation on the immune and metabolic mechanisms of gingival fibroblasts from periodontal connective tissue. We also determined the role of Toll-like receptor (TLR) 2 and TLR4 in Mfa1 recognition. Mfa1 increased the expression of genes encoding chemokine (C-X-C motif) ligand (CXCL) 1, CXCL3, intercellular adhesion molecule (ICAM) 1 and Selectin endothelium (E) in gingival fibroblasts, but did not have a significant effect on genes that regulate metabolism. Mfa1-stimulated up-regulation of genes was significantly suppressed in Tlr4 siRNA-transfected cells compared with that in control siRNA-transfected cells, which indicates that recognition by TLR4 is essential for immunomodulation by Mfa1. Additionally, suppression of Tlr2 expression partially attenuated the stimulatory effect of Mfa1. Overall, these results help explain the involvement of P. gingivalis Mfa1 fimbriae in the progression of periodontal disease.

The Multifaceted Nature of Streptococcal Antigen I/II Proteins in Colonization and Disease Pathogenesis

Streptococci are Gram-positive bacteria that belong to the natural microbiota of humans and animals. Certain streptococcal species are known as opportunistic pathogens with the potential to cause severe invasive disease. Antigen I/II (AgI/II) family proteins are sortase anchored cell surface adhesins that are nearly ubiquitous across streptococci and contribute to many streptococcal diseases, including dental caries, respiratory tract infections, and meningitis. They appear to be multifunctional adhesins with affinities to various host substrata, acting to mediate attachment to host surfaces and stimulate immune responses from the colonized host. Here we will review the literature including recent work that has demonstrated the multifaceted nature of AgI/II family proteins, focusing on their overlapping and distinct functions and their important contribution to streptococcal colonization and disease.

Porphyromonas gingivalis: Immune subversion activities and role in periodontal dysbiosis

Purpose of review

This review summarizes mechanisms by which Porphyromonas gingivalis interacts with community members and the host so that it can persist in the periodontium under inflammatory conditions that drive periodontal disease.

Recent findings

Recent advances indicate that, in great part, the pathogenicity of P. gingivalis is dependent upon its ability to establish residence in the subgingival environment and to subvert innate immunity in a manner that uncouples the nutritionally favorable (for the bacteria) inflammatory response from antimicrobial pathways. While the initial establishment of P. gingivalis is dependent upon interactions with early colonizing bacteria, the immune subversion strategies of P. gingivalis in turn benefit co-habiting species.

Summary

Specific interspecies interactions and subversion of the host response contribute to the emergence and persistence of dysbiotic communities and are thus targets of therapeutic approaches for the treatment of periodontitis.

Topical treatment with probiotic Lactobacillus brevis CD2 inhibits experimental periodontal inflammation and bone loss

BACKGROUND AND OBJECTIVE

An increasing body of evidence suggests that the use of probiotic bacteria is a promising intervention approach for the treatment of inflammatory diseases with a polymicrobial etiology. The objective of this study was to determine whether Lactobacillus brevis CD2 could inhibit periodontal inflammation and bone loss in experimental periodontitis.

MATERIAL AND METHODS

Periodontitis was induced by placing a silk ligature around the second maxillary molar of mice treated with L. brevis CD2 (8 × 10(5)  CFU in 1 mm(2) lyopatch) or placebo, which were placed between the gingiva and the buccal mucosa near the ligated teeth. The mice were killed after 5 d and bone loss was measured morphometrically, gingival expression of proinflammatory cytokines was determined by quantitative real-time polymerase chain reaction, and CFU counts of periodontitis-associated bacteria were determined after aerobic and anaerobic culture. To determine the role of arginine deiminase released by L. brevis CD2, soluble extracts with or without formamidine (arginine deiminase inhibitor) were tested in in vitro cellular activation assays.

RESULTS

Mice topically treated with L. brevis CD2 displayed significantly decreased bone loss and lower expression of tumor necrosis factor, and interleukin-1β, -6 and -17A as compared to placebo-treated mice. Moreover, L. brevis CD2-treated mice displayed lower counts of anaerobic bacteria but higher counts of aerobic bacteria than placebo-treated mice. In in vitro assays, the anti-inflammatory effects of soluble L. brevis CD2 extracts were heavily dependent on the presence of functional arginine deiminase, an enzyme that can inhibit nitric oxide synthesis.

CONCLUSION

These data provide proof-of-concept that the probiotic L. brevis CD2 can inhibit periodontitis through modulatory effects on the host response and the periodontal microbiota.

Activation of NLRP3 and AIM2 inflammasomes by Porphyromonas gingivalis infection

Porphyromonas gingivalis, a major periodontopathogen, is involved in the pathogenesis of periodontitis. Interleukin-1β (IL-1β), a proinflammatory cytokine, regulates innate immune responses and is critical for the host defense against bacterial infection. However, excessive IL-1β is linked to periodontal destruction. IL-1β synthesis, maturation, and secretion are tightly regulated by Toll-like receptor (TLR) signaling and inflammasome activation. We found much higher levels of inflammasome components in the gingival tissues from patients with chronic periodontitis than in those from healthy controls. To investigate the molecular mechanisms by which P. gingivalis infection causes IL-1β secretion, we examined the characteristics of P. gingivalis-induced signaling in differentiated THP-1 cells. We found that P. gingivalis induces IL-1β secretion and inflammatory cell death via caspase-1 activation. We also found that P. gingivalis-induced IL-1β secretion and pyroptic cell death required both NLRP3 and AIM2 inflammasome activation. The activation of the NLRP3 inflammasome was mediated by ATP release, the P2X7 receptor, and lysosomal damage. In addition, we found that the priming signal via TLR2 and TLR4 activation precedes P. gingivalis-induced IL-1β release. Our study provides novel insight into the innate immune response against P. gingivalis infection which could potentially be used for the prevention and therapy of periodontitis.

Effects of high fat feeding on liver gene expression in diabetic goto-kakizaki rats

Effects of high fat diet (HFD) on obesity and, subsequently, on diabetes are highly variable and modulated by genetics in both humans and rodents. In this report, we characterized the response of Goto-Kakizaki (GK) rats, a spontaneous polygenic model for lean diabetes and healthy Wistar-Kyoto (WKY) controls, to high fat feeding from weaning to 20 weeks of age. Animals fed either normal diet or HFD were sacrificed at 4, 8, 12, 16 and 20 weeks of age and a wide array of physiological measurements were made along with gene expression profiling using Affymetrix gene array chips. Mining of the microarray data identified differentially regulated genes (involved in inflammation, metabolism, transcription regulation, and signaling) in diabetic animals, as well as the response of both strains to HFD. Functional annotation suggested that HFD increased inflammatory differences between the two strains. Chronic inflammation driven by heightened innate immune response was identified to be present in GK animals regardless of diet. In addition, compensatory mechanisms by which WKY animals on HFD resisted the development of diabetes were identified, thus illustrating the complexity of diabetes disease progression.

Identification of a unique TLR2-interacting peptide motif in a microbial leucine-rich repeat protein

Pathogenesis of many bacterially-induced inflammatory diseases is driven by Toll-like receptor (TLR) mediated immune responses following recognition of bacterial factors by different TLRs. Periodontitis is a chronic inflammation of the tooth supporting apparatus often leading to tooth loss, and is caused by a Gram-negative bacterial consortium that includes Tannerella forsythia. This bacterium expresses a virulence factor, the BspA, which drives periodontal inflammation by activating TLR2. The N-terminal portion of the BspA protein comprises a leucine-rich repeat (LRR) domain previously shown to be involved in the binding and activation of TLR2. The objective of the current study was to identify specific epitopes in the LRR domain of BspA that interact with TLR2. Our results demonstrate that a sequence motif GC(S/T)GLXSIT is involved in mediating the interaction of BspA with TLR2. Thus, our study has identified a peptide motif that mediates the binding of a bacterial protein to TLR2 and highlights the promiscuous nature of TLR2 with respect to ligand binding. This work could provide a structural basis for designing peptidomimetics to modulate the activity of TLR2 in order to block bacterially-induced inflammation.

Porphyromonas gingivalis Fimbria-Induced Expression of Inflammatory Cytokines and Cyclooxygenase-2 in Mouse Macrophages and Its Inhibition by the Bioactive Compounds Fibronectin and Melatonin

Porphyromonas gingivalis (Pg) fimbriae, in addition to lipopolysaccharide, are involved in the pathogenesis of periodontal disease. At the same time, bioactive compounds such as fibronectin (FN) and melatonin in saliva and gingival crevicular fluid have been reported to exert a preventive effect against periodontitis. Here, we review current knowledge regarding the potent inhibitory effects of FN and melatonin against Pg fimbria-induced induction of proinflammatory cytokines, cyclooxygenase-2 (COX-2) expression, and NF-kappa B activation in mouse macrophages and discuss their possible clinical application for prevention of periodontal diseases induced by oral bacteria.

Free lipid A isolated from Porphyromonas gingivalis lipopolysaccharide is contaminated with phosphorylated dihydroceramide lipids: recovery in diseased dental samples

Recent reports indicate that Porphyromonas gingivalis mediates alveolar bone loss or osteoclast modulation through engagement of Toll-like receptor 2 (TLR2), though the factors responsible for TLR2 engagement have yet to be determined. Lipopolysaccharide (LPS) and lipid A, lipoprotein, fimbriae, and phosphorylated dihydroceramides of P. gingivalis have been reported to activate host cell responses through engagement of TLR2. LPS and lipid A are the most controversial in this regard because conflicting evidence has been reported concerning the capacity of P. gingivalis LPS or lipid A to engage TLR2 versus TLR4. In the present study, we first prepared P. gingivalis LPS by the Tri-Reagent method and evaluated this isolate for contamination with phosphorylated dihydroceramide lipids. Next, the lipid A prepared from this LPS was evaluated for the presence of phosphorylated dihydroceramide lipids. Finally, we characterized the lipid A by the matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) and electrospray-MS methods in order to quantify recovery of lipid A in lipid extracts from diseased teeth or subgingival plaque samples. Our results demonstrate that both the LPS and lipid A derived from P. gingivalis are contaminated with phosphorylated dihydroceramide lipids. Furthermore, the lipid extracts derived from diseased teeth or subgingival plaque do not contain free lipid A constituents of P. gingivalis but contain substantial amounts of phosphorylated dihydroceramide lipids. Therefore, the free lipid A of P. gingivalis is not present in measurable levels at periodontal disease sites. Our results also suggest that the TLR2 activation of host tissues attributed to LPS and lipid A of P. gingivalis could actually be mediated by phosphorylated dihydroceramides.

Mechanistic role of microRNA-146a in endotoxin-induced differential cross-regulation of TLR signaling

Human TLRs are critical sensors for microbial components leading to the production of proinflammatory cytokines that are controlled by various mechanisms. Monocytes pretreated with LPS exhibit a state of hyporesponsiveness, referred to as cross-tolerance, to both homologous and heterologous ligands, which play a broader role in innate immunity. To date, LPS-induced cross-tolerance has not been examined regarding microRNA expression kinetics. In this study, THP-1 monocytes treated with various inflammatory ligands showed a continuous amplification of microRNA (miR)-146a over 24 h that is inversely correlated to TNF-α production. In contrast, inhibition of miR-146a showed a reciprocal effect. Thus, the characteristic upregulation of miR-146a in LPS-exposed THP-1 monocytes was studied for cross-tolerance. Strikingly, in LPS-tolerized THP-1 monocytes, only miR-146a showed a continuous overexpression, suggesting its crucial role in cross-tolerance. Similarly, peptidoglycan-primed THP-1 cells showed homologous tolerance associated with miR-146a upregulation. Subsequently, interchangeable differential cross-regulation was observed among non-LPS ligands. TLR2 and TLR5 ligands showed both homologous and heterologous tolerance correlated to miR-146a overexpression. More importantly, inflammatory responses to TLR4, TLR2, and TLR5 ligands were reduced due to knockdown of miR-146a targets IL-1R-associated kinase 1 or TNFR-associated factor 6, suggesting the regulatory effect of miR-146a on these TLRs signaling. Transfection of miR-146a into THP-1 cells caused reduction of TNF-α production, mimicking LPS-induced cross-tolerance. Aside from individual ligands, a whole bacterial challenge in LPS-primed THP-1 monocytes was accompanied by less TNF-α production, which is conversely correlated to miR-146a expression. Our studies have thus demonstrated that miR-146a plays a crucial role for in vitro monocytic cell-based endotoxin-induced cross-tolerance.

Functional characterization of the IlpA protein of Vibrio vulnificus as an adhesin and its role in bacterial pathogenesis

Vibrio vulnificus is a Gram-negative bacterium that causes a fatal septicemia. One of its virulence factors is a membrane-bound lipoprotein, IlpA, which can induce cytokine production in human immune cells. In the present study, the role of IlpA as an adhesion molecule was investigated. An ilpA-deleted V. vulnificus mutant showed significantly decreased adherence to INT-407 human intestinal epithelial cells, which in turn resulted in reduced cytotoxicity. The DeltailpA mutant recovered the adherence ability of the wild type by complementation in trans with the intact ilpA gene. In addition, pretreatment of V. vulnificus with anti-IlpA polyclonal antibodies resulted in a significant reduction of bacterial adherence. To localize the domain of IlpA required for cytoadherence, three truncated recombinant IlpA polypeptides were constructed and tested for the ability to adhere to human cells by a ligand-binding immunoblot assay and fluorescence microscopy. The polypeptide containing the carboxy (C)-terminal hydrophilic domain exhibited direct binding to INT-407 cells. Therefore, the C-terminal domain of IlpA allows this protein to be an adhesion molecule of V. vulnificus.

Trichomonas vaginalis vast BspA-like gene family: evidence for functional diversity from structural organisation and transcriptomics

Background

Trichomonas vaginalis is the most common non-viral human sexually transmitted pathogen and importantly, contributes to facilitating the spread of HIV. Yet very little is known about its surface and secreted proteins mediating interactions with, and permitting the invasion and colonisation of, the host mucosa. Initial annotations of T. vaginalis genome identified a plethora of candidate extracellular proteins.

Results

Data mining of the T. vaginalis genome identified 911 BspA-like entries (TvBspA) sharing TpLRR-like leucine-rich repeats, which represent the largest gene family encoding potential extracellular proteins for the pathogen. A broad range of microorganisms encoding BspA-like proteins was identified and these are mainly known to live on mucosal surfaces, among these T. vaginalis is endowed with the largest gene family. Over 190 TvBspA proteins with inferred transmembrane domains were characterised by a considerable structural diversity between their TpLRR and other types of repetitive sequences and two subfamilies possessed distinct classic sorting signal motifs for endocytosis. One TvBspA subfamily also shared a glycine-rich protein domain with proteins from Clostridium difficile pathogenic strains and C. difficile phages. Consistent with the hypothesis that TvBspA protein structural diversity implies diverse roles, we demonstrated for several TvBspA genes differential expression at the transcript level in different growth conditions. Identified variants of repetitive segments between several TvBspA paralogues and orthologues from two clinical isolates were also consistent with TpLRR and other repetitive sequences to be functionally important. For one TvBspA protein cell surface expression and antibody responses by both female and male T. vaginalis infected patients were also demonstrated.

Conclusions

The biased mucosal habitat for microbial species encoding BspA-like proteins, the characterisation of a vast structural diversity for the TvBspA proteins, differential expression of a subset of TvBspA genes and the cellular localisation and immunological data for one TvBspA; all point to the importance of the TvBspA proteins to various aspects of T. vaginalis pathobiology at the host-pathogen interface.

TLR cross-talk specifically regulates cytokine production by B cells from chronic inflammatory disease patients

Chronic systemic inflammation links periodontal disease and diabetes to increased incidence of serious comorbidities. Activation of TLRs, particularly TLR2 and TLR4, promotes chronic systemic inflammation. Human B cells have been generally thought to lack these TLRs. However, recent work showed that an increased percentage of circulating B cells from inflammatory disease patients express TLR2 and TLR4, and that TLR engagement on B cells resulted in unexpected changes in gene expression. New data show that B cells from inflammatory disease patients secrete multiple cytokines in response to different classes of TLR ligands. Furthermore, the B cell response to combinations of TLR ligands is cytokine- and ligand-specific. Some cytokines (IL-1beta and IL-10) are predominantly regulated by TLR4, but others (IL-8 and TNF-alpha) are predominantly regulated by TLR2, due in part to TLR-dictated changes in transcription factor/promoter association. TLR2 and TLR9 also regulate B cell TLR4 expression, demonstrating that TLR cross-talk controls B cell responses at multiple levels. Parallel examination of B cells from periodontal disease and diabetes patients suggested that outcomes of TLR cross-talk are influenced by disease pathology. We conclude that disease-associated alteration of B cell TLR responses specifically regulates cytokine production and may influence chronic inflammation.

Anchoring and length regulation of Porphyromonas gingivalis Mfa1 fimbriae by the downstream gene product Mfa2

Porphyromonas gingivalis, a causative agent of periodontitis, has at least two types of thin, single-stranded fimbriae, termed FimA and Mfa1 (according to the names of major subunits), which can be discriminated by filament length and by the size of their major fimbrilin subunits. FimA fimbriae are long filaments that are easily detached from cells, whereas Mfa1 fimbriae are short filaments that are tightly bound to cells. However, a P. gingivalis ATCC 33277-derived mutant deficient in mfa2, a gene downstream of mfa1, produced long filaments (10 times longer than those of the parent), easily detached from the cell surface, similar to FimA fimbriae. Longer Mfa1 fimbriae contributed to stronger autoaggregation of bacterial cells. Complementation of the mutant with the wild-type mfa2 allele in trans restored the parental phenotype. Mfa2 is present in the outer membrane of P. gingivalis, but does not co-purify with the Mfa1 fimbriae. However, co-immunoprecipitation demonstrated that Mfa2 and Mfa1 are associated with each other in whole P. gingivalis cells. Furthermore, immunogold microscopy, including double labelling, confirmed that Mfa2 was located on the cell surface and likely associated with Mfa1 fimbriae. Mfa2 may therefore play a role as an anchor for the Mfa1 fimbriae and also as a regulator of Mfa1 filament length. Two additional downstream genes (pgn0289 and pgn0290) are co-transcribed with mfa1 (pgn0287) and mfa2 (pgn0288), and proteins derived from pgn0289, pgn0290 and pgn0291 appear to be accessory fimbrial components.

Leucine-rich repeats of bacterial surface proteins serve as common pattern recognition motifs of human scavenger receptor gp340

Scavenger receptors are innate immune molecules recognizing and inducing the clearance of non-host as well as modified host molecules. To recognize a wide pattern of invading microbes, many scavenger receptors bind to common pathogen-associated molecular patterns, such as lipopolysaccharides and lipoteichoic acids. Similarly, the gp340/DMBT1 protein, a member of the human scavenger receptor cysteine-rich protein family, displays a wide ligand repertoire. The peptide motif VEVLXXXXW derived from its scavenger receptor cysteine-rich domains is involved in some of these interactions, but most of the recognition mechanisms are unknown. In this study, we used mass spectrometry sequencing, gene inactivation, and recombinant proteins to identify Streptococcus pyogenes protein Spy0843 as a recognition receptor of gp340. Antibodies against Spy0843 are shown to protect against S. pyogenes infection, but no function or host receptor have been identified for the protein. Spy0843 belongs to the leucine-rich repeat (Lrr) family of eukaryotic and prokaryotic proteins. Experiments with truncated forms of the recombinant proteins confirmed that the Lrr region is needed in the binding of Spy0843 to gp340. The same motif of two other Lrr proteins, LrrG from the Gram-positive S. agalactiae and BspA from the Gram-negative Tannerella forsythia, also mediated binding to gp340. Moreover, inhibition of Spy0843 binding occurred with peptides containing the VEVLXXXXW motif, but also peptides devoid of the XXXXW motif inhibited binding of Lrr proteins. These results thus suggest that the conserved Lrr motif in bacterial proteins serves as a novel pattern recognition motif for unique core peptides of human scavenger receptor gp340.

B cells from periodontal disease patients express surface Toll-like receptor 4

Chronic systemic inflammation links periodontal disease (PD) to increased incidence of cardiovascular disease. Activation of TLRs, particularly TLR4, promotes chronic inflammation in PD by stimulating myeloid cells. B cells from healthy individuals are generally refractory to TLR4 agonists as a result of low surface TLR4 expression. Unexpectedly, a significantly increased percentage of gingival and peripheral blood B cells from patients with PD expressed surface TLR4. Surface expression correlated with an active TLR4 promoter that mimicked the TLR4 promoter in neutrophils. B cells from PD patients were surface myeloid differentiation protein 2-positive and also packaged the enhancer of a proinflammatory cytokine, IL-1 beta, into an active structure, demonstrating that these cells harbor key characteristics of proinflammatory cell types. Furthermore, B cells lacked activating signatures of a natural IL-1 beta inhibitor, IL-1 receptor antagonist. Surprisingly, despite multiple signatures of proinflammatory cells, freshly isolated B cells from PD patients had decreased expression of TLR pathway genes compared with B cells from healthy individuals. Decreases in inflammatory gene expression were even more dramatic in B cells stimulated with a TLR4 ligand from a periodontal pathogen, Porphyromonas gingivalis LPS 1690. In contrast, B cell TLR4 was not activated by the prototypic TLR4 ligand Escherichia coli LPS. These findings raise the unexpected possibility that TLR4 engagement modulates B cell activation in PD patients.

Bacterial fimbriae stimulate proinflammatory activation in the endothelium through distinct TLRs

The major and minor fimbriae proteins produced by the human pathogen Porphyromonas gingivalis are required for invasion of human aortic endothelial cells and for the stimulation of potent inflammatory responses. In this study, we report that native forms of both the major and minor fimbriae proteins bind to and signal through TLR2 for this response. Major and minor fimbriae bound to a human TLR2:Fc chimeric protein with an observed K(d) of 28.9 nM and 61.7 nM, respectively. Direct binding of the major and minor fimbriae to a human chimeric CD14-Fc protein also established specific binding of the major and minor fimbriae to CD14 with classic saturation kinetics. Using a P. gingivalis major and minor fimbriae mutant, we confirmed that TLR2 binding in whole cells is dependent on the expression of the major and minor fimbriae. Although we did not observe binding with the major or minor fimbriae to the TLR4-Fc chimeric protein, signaling through TLR4 for both proteins was demonstrated in human embryonic kidney 293 cells transfected with TLR4 and only in the presence MD-2. Transient transfection of dominant-negative forms of TLR2 or TLR4 reduced IL-8 production by human aortic endothelial cells following stimulation with major or minor fimbriae. The ability of two well-defined microbe-associated molecular patterns to select for innate immune recognition receptors based on accessory proteins may provide a novel way for a pathogen to sense and signal in appropriate host environments.

Toll-like receptor 2-mediated interleukin-8 expression in gingival epithelial cells by the Tannerella forsythia leucine-rich repeat protein BspA

Tannerella forsythia is a gram-negative anaerobe strongly associated with chronic human periodontitis. This bacterium expresses a cell surface-associated and secreted protein, designated BspA, which has been recognized as an important virulence factor. The BspA protein belongs to the leucine-rich repeat (LRR) and bacterial immunoglobulin-like protein families. BspA is, moreover, a multifunctional protein which interacts with a variety of host cells, including monocytes which appear to respond to BspA through Toll-like receptor (TLR) signaling. Since gingival epithelium forms a barrier against periodontal pathogens, this study was undertaken to determine if gingival epithelial cells respond to BspA challenge and if TLRs play any role in BspA recognition. This study was also directed towards identifying the BspA domains responsible for cellular activation. We provide direct evidence for BspA binding to TLR2 and demonstrate that the release of the chemokine interleukin-8 from human gingival epithelial cells by BspA is TLR2 dependent. Furthermore, the LRR domain of BspA is involved in activation of TLR2, while TLR1 serves as a signaling partner. Thus, our findings suggest that BspA is an important modulator of host innate immune responses through activation of TLR2 in cooperation with TLR1.

Transcriptional organization of the Porphyromonas gingivalis fimA locus

Two different transcriptional start sites, as well as promoter regions and translational starts, have been proposed for the fimA gene encoding a long fimbriae subunit protein of Porphyromonas gingivalis. In this study, the fimA promoter regions and organization of the fimA operon were characterized. The two putative promoter regions for fimA were fused with a lacZ reporter gene, cloned into the shuttle plasmid vector pT-COW, and the recombinant plasmids were introduced to P. gingivalis 33277. Reverse transcriptase-polymerase chain reaction demonstrated mRNA production from the promoter proximal to the translational start. LacZ activities of P. gingivalis containing the recombinant plasmids showed that maximal expression of fimA was promoted by the proximal promoter in combination with distal regulatory sequences. A polycistronic message spanning PG2130, PG2131 and fimA (PG2132) was observed, thus fimA transcripts may also be generated by processing of the polycistronic message.

Proteomic mapping of stimulus-specific signaling pathways involved in THP-1 cells exposed to Porphyromonas gingivalis or its purified components

Periodontitis is an inflammatory disease initiated by host-parasite interactions which contributes to connective tissue destruction and alveolar bone resorption. Porphyromonas gingivalis (P.g.), a black-pigmented Gram-negative anaerobic bacterium, is a major pathogen in the development and progression of periodontitis. To characterize the role that P. gingivalis and its cell surface components play in disease processes, we investigated the differential expression of proteins induced by live P.g., P.g. LPS, and P.g. FimA, using two-dimensional gel electrophoresis in combination with mass spectrometry. We have tested whether, at the level of protein expression, unique signaling pathways are differentially induced by the bacterial components P.g. LPS and P.g. FimA, as compared to live P.g. We found that P.g. LPS stimulation of THP-1 up-regulated the expression of a set of proteins compared to control: deoxyribonuclease, actin, carbonic anhydrase 2, alpha enolase, adenylyl cyclase-associated protein (CAP1), protein disulfide isomerase (PDI), glucose regulated protein (grp78), and 70-kDa heat shock protein (HSP70), whereas FimA treatment did not result in statistically significant changes to protein levels versus the control. Live P.g. stimulation resulted in 12 differentially expressed proteins: CAP1, tubulin beta-2 chain, ATP synthase beta chain, tubulin alpha-6 chain, PDI, vimentin, 60-kDa heat shock protein, and nucleolin were found to be up-regulated, while carbonic anhydrase II, beta-actin, and HSP70 were down-regulated relative to control. These differential changes by the bacteria and its components are interpreted as preferential signal pathway activation in host immune/inflammatory responses to P.g. infection.

Up-regulation of gamma interferon receptor expression due to Chlamydia-toll-like receptor interaction does not enhance signal transducer and activator of transcription 1 signaling

Gamma interferon (IFN-gamma)-induced indoleamine dioxygenase (IDO), which inhibits chlamydial replication by reducing the availability of tryptophan, is up-regulated by interleukin-1beta (IL-1beta) and tumor necrosis factor alpha (TNF-alpha). The mechanisms by which this occurs include an increase in the synthesis of interferon regulatory factor-1 as well as a nuclear factor-kappaB (NF-kappaB)-dependent increase in the expression of IFN-gamma receptors (IFN-gammaR). Although Chlamydia is susceptible to IDO, it up-regulates IFN-gammaR expression to a greater degree than either IL-1beta or TNF-alpha, perhaps through interaction with Toll-like receptors (TLR). The purpose of this study was to determine the mechanism by which Chlamydia psittaci up-regulates IFN-gammaR expression and evaluate this effect on IDO induction. Infection of HEK 293 cells with C. psittaci increased IFN-gammaR expression only in cells expressing either TLR2 or TLR4 and the adaptor protein MD-2. In addition, up-regulation of IFN-gammaR expression in Chlamydia-infected HeLa cells could be blocked either by neutralizing TLRs with anti-TLR2 and/or anti-TLR4 or by inhibiting NF-kappaB transactivation with a proteasome inhibitor. Although the newly expressed IFN-gammaR in Chlamydia-infected cells were capable of binding IFN-gamma, they did not enhance IFN-gamma-induced IDO activity in a manner similar to those observed for IL-1beta and TNF-alpha. Instead, IDO activation in Chlamydia-infected cells was no different than that induced in uninfected cells, despite the increase in IFN-gammaR expression. Furthermore, the amount of IFN-gamma-induced signal transducer and activator of transcription 1 (STAT-1) activation in infected cells paralleled that observed in uninfected cells, suggesting that STAT-1 activation by these newly expressed receptors was impaired.

Porphyromonas gingivalis vesicles enhance attachment, and the leucine-rich repeat BspA protein is required for invasion of epithelial cells by "Tannerella forsythia"

The human oral cavity harbors more than 500 species of bacteria. Periodontitis, a bacterially induced inflammatory disease that leads to tooth loss, is believed to result from infection by a select group of gram-negative periodontopathogens that includes Porphyromonas gingivalis, Treponema denticola, and "Tannerella forsythia" (opinion on name change from Tannerella forsythensis pending; formerly Bacteroides forsythus). Epithelial cell invasion by periodontopathogens is considered to be an important virulence mechanism for evasion of the host defense responses. Further, the epithelial cells with invading bacteria also serve as reservoirs important in recurrent infections. The present study was therefore undertaken to address the epithelial cell adherence and invasion properties of T. forsythia and the role of the cell surface-associated protein BspA in these processes. Further, we were interested in determining if P. gingivalis, one of the pathogens frequently found associated in disease, or its outer membrane vesicles (OMVs) could modulate the epithelial cell adherence and invasion abilities of T. forsythia. Here we show that epithelial cell attachment and invasion by T. forsythia are dependent on the BspA protein. In addition, P. gingivalis or its OMVs enhance the attachment and invasion of T. forsythia to epithelial cells. Thus, interactions between these two bacteria may play important roles in virulence by promoting host cell attachment and invasion.

Innate immune signaling and Porphyromonas gingivalis-accelerated atherosclerosis

Periodontal diseases are a group of diseases that lead to erosion of the hard and soft tissues of the periodontium, which, in severe cases, can result in tooth loss. Anecdotal clinical observations have suggested that poor oral health may be associated with poor systemic health; however, only recently have appropriate epidemiological studies been initiated, with defined clinical endpoints of periodontal disease, to address the association of periodontal disease with increased risk for cardiovascular and cerebrovascular disease. Although conflicting reports exist, these epidemiological studies support this connection. Paralleling these epidemiological studies, emerging basic scientific studies also support that infection may represent a risk factor for atherosclerosis. With P. gingivalis as a model pathogen, in vitro studies support that this organism can activate host innate immune responses associated with atherosclerosis, and in vivo studies demonstrate that this organism can accelerate atheroma deposition in animal models. In this review, we focus primarily on the basic scientific studies performed to date which support that infection with bacteria, most notably P. gingivalis, accelerates atherosclerosis. Furthermore, we attempt to bring together these studies to provide an up-to-date framework of emerging theories into the mechanisms underlying periodontal disease and increased risk for atherosclerosis, as well as identify intervention strategies to reduce the incidence of periodontal disease in humans, in an attempt to decrease risk for systemic complications of periodontal disease such as atherosclerotic cardiovascular disease.

Fimbria-dependent activation of pro-inflammatory molecules in Porphyromonas gingivalis infected human aortic endothelial cells

Epidemiological studies support that chronic periodontal infections are associated with an increased risk of cardiovascular disease. Previously, we reported that the periodontal pathogen Porphyromonas gingivalis accelerated atherosclerotic plaque formation in hyperlipidemic apoE-/- mice, while an isogenic fimbria-deficient (FimA-) mutant did not. In this study, we utilized 41 kDa (major) and 67 kDa (minor) fimbria mutants to demonstrate that major fimbria are required for efficient P. gingivalis invasion of human aortic endothelial cells (HAEC). Enzyme-linked immunosorbent assay (ELISA) revealed that only invasive P. gingivalis strains induced HAEC production of pro-inflammatory molecules interleukin (IL)-1beta, IL-8, monocyte chemoattractant protein (MCP)-1, intracellular adhesion molecule (ICAM)-1, vascular cellular adhesion molecule (VCAM)-1 and E-selectin. The purified native forms of major and minor fimbria induced chemokine and adhesion molecule expression similar to invasive P. gingivalis, but failed to elicit IL-1beta production. In addition, the major and minor fimbria-mediated production of MCP-1 and IL-8 was inhibited in a dose-dependent manner by P. gingivalis lipopolysaccharide (LPS). Both P. gingivalis LPS and heat-killed organisms failed to stimulate HAEC. Treatment of endothelial cells with cytochalasin D abolished the observed pro-inflammatory MCP-1 and IL-8 response to invasive P. gingivalis and both purified fimbria, but did not affect P. gingivalis induction of IL-1beta. These results suggest that major and minor fimbria elicit chemokine production in HAEC through actin cytoskeletal rearrangements; however, induction of IL-1beta appears to occur via a separate mechanism. Collectively, these data support that invasive P. gingivalis and fimbria stimulate endothelial cell activation, a necessary initial event in the development of atherogenesis.

Identification of proteins differentially expressed in human monocytes exposed to Porphyromonas gingivalis and its purified components by high-throughput immunoblotting

To characterize the roles of Porphyromonas gingivalis and its components in disease processes, we investigated the cytokine profiles induced by live P. gingivalis, its lipopolysaccharide (LPS), and its major fimbrial protein, fimbrillin (FimA). A cytokine antibody array revealed that human monocyte-derived macrophages were induced to produce chemokines (e.g., monocyte chemoattractant protein 1, macrophage inflammatory protein 1beta [MIP-1beta], and MIP-3alpha) as early as 1 h after exposure to P. gingivalis, with production declining after 24 h of exposure. As expected, an extensive repertoire of inflammatory mediators increased subsequent to infection, most predominantly tumor necrosis factor alpha (TNF-alpha), interleukin 1beta (IL-1beta), IL-6, IL-10, and granulocyte-macrophage colony-stimulating factor. The induction of cytokines by P. gingivalis was not triggered simply by bacterial cell surface components, since purified P. gingivalis LPS and FimA induced similar patterns of cytokines, while the pattern of cytokines induced by live P. gingivalis was significantly different, indicating that the host defense system senses live bacteria differently than it does the cell surface components LPS and FimA. To further understand the mechanisms by which live P. gingivalis and its components exert their effects, we used a high-throughput immunoblot screening approach (Becton-Dickinson PowerBlot) to analyze intracellular proteins involved in P. gingivalis infection in human macrophages. Exposure of human macrophages to either live P. gingivalis, its LPS, or its FimA protein led to the up-regulation of 12, 8, and 10 proteins and the down-regulation of 15, 8, and 17 proteins, respectively. The expression of proteins involved in gene transcription (e.g., monocyte enhancer factor 2D [MEF2D], signal transducer and activator of transcription 1 [STAT1], STAT3, STAT6, and IL enhancer binding factors [ILF3]), of protein kinases (e.g., mitogen-activated protein kinase 3 [MAPK3], MAP3K8, double-stranded RNA-activated protein kinase [PRKR], and MAP2K4), and of proteins involved in immune responses (e.g., TNF super family member 6 [TNFSF6] and interferon-induced protein with tetratricopeptide repeat 4 [IFIT4]), apoptosis (e.g., genes associated with retinoid interferon-induced mortality 19 [GRIM19]), and other fundamental cellular processes (e.g., clathrin heavy-chain polypeptide, culreticulin, and Ras-associated protein RAB27A) was found to be modulated differentially by P. gingivalis, LPS, and FimA. These differential changes are interpreted as preferential signal pathway activation in host immune/inflammatory responses to P. gingivalis infection.

Sensitization of human aortic endothelial cells to lipopolysaccharide via regulation of Toll-like receptor 4 by bacterial fimbria-dependent invasion

Toll-like receptors (TLRs) are differentially up-regulated in response to microbial infection and chronic inflammatory diseases such as atherosclerosis. Epidemiological data support the idea that periodontal disease may be a risk factor for acceleration of atherosclerosis. Porphyromonas gingivalis, the etiological agent of periodontal disease, invades endothelium, has been detected in human atheromatous tissue, and accelerates atheroma formation in apolipoprotein E-/- mice with concurrent induction of TLRs in the aorta. As endothelial cells can present antigen via TLRs and play an important role in the development of atherosclerosis, we examined TLR expression in human aortic endothelial cells (HAEC) cultured with wild-type P. gingivalis, a fimbria-deficient mutant, and purified antigens. We observed increased TLR expression in HAEC infected with wild-type P. gingivalis by fluorescence-activated cell sorter, but not with noninvasive, fimbria-deficient mutant or purified P. gingivalis antigens. Following a wild-type P. gingivalis challenge, functional TLR2 and TLR4 activation was assessed by subsequent stimulation with TLR agonists Staphylococcus aureus lipoteichoic acid (SLTA; TLR2 ligand) and Escherichia coli lipopolysaccharide (LPS; TLR4 ligand). Unchallenged HAEC failed to elicit monocyte chemoattractant protein 1 (MCP-1) in response to LPS or SLTA but did so when cultured with wild-type P. gingivalis. P. gingivalis-induced TLR2 and -4 expression on HAEC functionally reacted to SLTA and E. coli LPS as measured by a further increase in MCP-1 production. Furthermore, MCP-1 expression elicited by E. coli LPS was inhibitable with TLR4-specific antibody and polymyxin B. These results indicate that invasive P. gingivalis stimulates TLR expression on the surface of endothelium and these primed cells respond to defined TLR-specific ligands.

Interactions of oral pathogens with toll-like receptors: possible role in atherosclerosis

Toll-like receptors (TLR) function as important signal transducers that mediate innate immune and inflammatory responses to pathogens through pattern recognition of virulence molecules. Although TLRs mediate protection against infection, it is also likely that they may have a pathophysiologic role in certain inflammatory diseases, such as atherosclerosis. In atherosclerotic lesions, endothelial cells and macrophages have been shown to upregulate TLR expression and may respond to TLR agonists of microbial origin, resulting in detrimental inflammatory reactions. Some of these potential TLR-activating virulence factors may be of oral origin. The detection in atherosclerotic plaques of DNA specific for Porphyromonas gingivalis and other periodontal pathogens suggests that these pathogens disseminate into the systemic circulation and localize in atheromas. The potential of periodontal and some other oral pathogens to activate TLRs in vivo is suggested by findings from cell culture experiments on interactions of selected virulence protein adhesins with TLRs and their coreceptors. Specifically, we have shown that proinflammatory cytokine induction by P. gingivalis fimbriae was inhibited by monoclonal antibodies to TLR2, TLR4, CD14, and beta2 integrins, but not by immunoglobulin isotype controls. Cytokine induction by Bacteroides forsythus protein A depended heavily on CD14 and TLR2. We also found that the ability of Streptococcus mutans protein AgI/II to stimulate cytokine release was partially dependent on CD14 and TLR4. Moreover, P. gingivalis fimbriae induced TLR-dependent activation of nuclear factor-kappaB and upregulation of costimulatory molecules in monocytic cells. These proinflammatory activities have been implicated in the pathogenesis of periodontitis, and similar inflammatory mechanisms could potentially operate in atherosclerosis. Studies by other groups have shown that P. gingivalis is capable of stimulating low-density lipoprotein oxidation, foam cell formation, and rupture of atherosclerotic plaque through induction of matrix metalloproteinases. Interestingly, at least some of these activities can be induced by TLR agonists (lipopolysaccharide and heat-shock protein-60) from Chlamydia pneumoniae, a major risk factor in atherosclerosis. Future research in animal models and in vitro cellular systems with defined mutations in TLRs may implicate TLR participation in oral pathogen-mediated atherosclerotic processes, thereby providing a mechanistic basis for the epidemiological findings linking oral pathogens to atherosclerotic disease.

Tannerella forsythia-induced alveolar bone loss in mice involves leucine-rich-repeat BspA protein

Tannerella forsythia (formerly Bacteroides forsythus) is one of the periodontal pathogens recently implicated in the development of periodontal disease. The cell-surface-associated, as well as the secreted, leucine-rich-repeat protein (BspA) of this bacterium have been suggested to play roles in bacterial adherence, and also in inflammation, by triggering release of pro-inflammatory cytokines from monocytes and chemokines from osteoblasts, leading to inflammation and bone resorption. In this study, we sought to determine the pathogenic potential of T. forsythia and the in vivo role of the BspA protein in pathogenesis in the mouse model of infection-induced alveolar bone loss. The results showed alveolar bone loss in mice infected with the T. forsythia wild-type strain, whereas the BspA mutant was impaired. In conclusion, evidence is presented in support of T. forsythia as an important organism involved in inducing alveolar bone loss, and the BspA protein is an important virulence factor of this bacterium.

Toll-like receptor 2 mediates cellular activation by the B subunits of type II heat-labile enterotoxins

The type II heat-labile enterotoxins (LT-IIa and LT-IIb) of Escherichia coli have an AB5 subunit structure similar to that of cholera toxin (CT) and other type I enterotoxins, despite significant differences in the amino acid sequences of their B subunits and different ganglioside receptor specificities. LT-II holotoxins and their nontoxic B subunits display unique properties as immunological adjuvants distinct from those of CT and its B subunits. In contrast to type II holotoxins, the corresponding pentameric B subunits, LT-IIaB and LT-IIbB, stimulated cytokine release in both human and mouse cells dependent upon Toll-like receptor 2 (TLR2). Induction of interleukin-1beta (IL-1beta), IL-6, IL-8, or tumor necrosis factor alpha in human THP-1 cells by LT-IIaB or LT-IIbB was inhibited by anti-TLR2 but not by anti-TLR4 antibody. Furthermore, transient expression of TLR1 and TLR2 in human embryonic kidney 293 cells resulted in activation of a nuclear factor-kappaB-dependent luciferase gene in response to LT-IIaB or LT-IIbB. Moreover, peritoneal macrophages from TLR2-deficient mice failed to respond to LT-IIaB or LT-IIbB, in contrast to wild-type or TLR4-deficient cells. These results demonstrate that besides their established binding to gangliosides, the B subunits of type II enterotoxins also interact with TLR2. Although a ganglioside-nonbinding mutant (T34I) of LT-IIaB effectively induced cytokine release, a phenotypically similar point mutation (T13I) in LT-IIbB abrogated cytokine induction, suggesting a variable requirement for gangliosides as coreceptors in TLR2 agonist activity. TLR2-dependent activation of mononuclear cells by type II enterotoxin B subunits appears to be a novel mechanism whereby these molecules may exert their immunomodulatory and adjuvant activities.

Cytokine profiling of macrophages exposed to Porphyromonas gingivalis, its lipopolysaccharide, or its FimA protein

To characterize the roles of Porphyromonas gingivalis and its components in the disease processes, we investigated the cytokine profile induced by live P. gingivalis, its lipopolysaccharides (LPS), and its major fimbrial protein, fimbrillin (FimA). Using cytokine antibody arrays, we found that P. gingivalis LPS and FimA induced a similar profile of cytokine expression when exposed to mouse peritoneal macrophages but that this profile differed significantly in response to live P. gingivalis. In vitro, mouse peritoneal macrophages were stimulated to produce interleukin-6 (IL-6), granulocyte colony-stimulating factor, and lymphotactin by live P. gingivalis, but not by P. gingivalis LPS or FimA, while RANTES, gamma interferon, IL-17, vascular cell adhesion molecule 1 (VCAM-1), and vascular endothelial growth factor were induced by P. gingivalis LPS or FimA, but not by live P. gingivalis. In vivo, IL-6 mRNA was strongly induced only by live P. gingivalis while monocyte chemoattractant protein 1 mRNA was strongly induced only by P. gingivalis LPS and FimA in mouse calvarial scalp, further confirming the differences of cytokine profile induced in vitro. Cytokine antibody arrays using toll-like receptor 2 (TLR2)- and TLR4-deficient macrophages revealed that most of the cytokines induced by P. gingivalis LPS or FimA signal through TLR2, while most of cytokines induced by live P. gingivalis signal through both TLR2 and TLR4. Interestingly, the activation of TLR2 by live P. gingivalis inhibited the release of RANTES, VCAM-1, and IL-1alpha from mouse peritoneal macrophages. A tumor necrosis factor alpha enzyme-linked immunosorbent assay further confirmed that P. gingivalis LPS and FimA activate mouse peritoneal macrophages via TLR2. These results indicate that host immune cells sense live P. gingivalis and its components differently, which translates into the expression of different inflammatory cytokine profiles.

The Type II heat-labile enterotoxins LT-IIa and LT-IIb and their respective B pentamers differentially induce and regulate cytokine production in human monocytic cells

The type II heat-labile enterotoxins, LT-IIa and LT-IIb, exhibit potent adjuvant properties. However, little is known about their immunomodulatory activities upon interaction with innate immune cells, unlike the widely studied type I enterotoxins that include cholera toxin (CT). We therefore investigated interactions of LT-IIa and LT-IIb with human monocytic THP-1 cells. We found that LT-II enterotoxins were inactive in stimulating cytokine release, whereas CT induced low levels of interleukin-1beta (IL-1beta) and IL-8. However, all three enterotoxins potently regulated cytokine induction in cells activated by bacterial lipopolysaccharide or fimbriae. Induction of proinflammatory (tumor necrosis factor alpha [TNF-alpha]) or chemotactic (IL-8) cytokines was downregulated, whereas induction of cytokines with anti-inflammatory (IL-10) or mucosal adjuvant properties (IL-1beta) was upregulated by the enterotoxins. These effects appeared to depend on their A subunits, because isolated B-pentameric subunits lacked regulatory activity. Enterotoxin-mediated inhibition of proinflammatory cytokine induction in activated cells was partially attributable to synergism for endogenous production of IL-10 and to an IL-10-independent inhibition of nuclear factor kappaB (NF-kappaB) activation. In sharp contrast to the holotoxins, the B pentamers (LT-IIaB and, to a greater extent, LT-IIbB) stimulated cytokine production, suggesting a link between the absence of the A subunit and increased proinflammatory properties. In this regard, the ability of LT-IIbB to activate NF-kappaB and induce TNF-alpha and IL-8 was antagonized by the LT-IIb holotoxin. These findings support distinct immunomodulatory roles for the LT-II holotoxins and their respective B pentamers. Moreover, the anti-inflammatory properties of the holotoxins may serve to suppress innate immunity and promote the survival of the pathogen.

Characterization of monoclonal antibody HTA125 with specificity for human TLR4

Binding of monoclonal antibody HTA125 to human toll-like receptor 4 (TLR4) was characterized by flow cytometry using MonoMac6 human monocytic cells. Data were obtained using direct binding to cell surface TLR4 by labeled HTA125, as well as inhibition of direct binding using purified reagents, and by two-step binding. HTA125 bound weakly to human TLR4, and could be inhibited by mouse Ig, mouse IgG Fc, and mouse IgG2a. In addition, purified human IgG Fc and purified human immunoglobulin of isotypes IgG1 and IgG4 could block binding of HTA125 to MonoMac6 cells. Furthermore, a mouse IgG1 monoclonal antibody possessing specificity for human CD64, which is a high affinity IgG Fc receptor, partially inhibited binding of HTA125 to MonoMac6 cells. Finally, co-stimulation via TLR4 and Fc receptor, resulted in cytokine production by MonoMac6 cells different than that induced via TLR4 alone. Therefore, the utility of HTA125 remains as a weak detector of human TLR4, and as an agent to block TLR4 ligands with an understanding that Fc receptor may be engaged also.

Fimbriated Porphyromonas gingivalis is more efficient than fimbria-deficient P. gingivalis in entering human dendritic cells in vitro and induces an inflammatory Th1 effector response

Porphyromonas gingivalis is a fimbriated mucosal pathogen implicated in chronic periodontitis (CP). The fimbriae are required for invasion of the gingival mucosa and for induction of CP in animal models of periodontitis. CP is associated with infection of immature dendritic cells (DCs) by P. gingivalis in situ and with increased numbers of dermal DCs (DDCs) and mature DCs in the lamina propria. The role of fimbriae in gaining entry into human DCs and how this modulates the inflammatory and effector immune responses, however, have not been explored. To address this, we generated monocyte-derived DCs (MDDCs) in vitro which phenotypically and functionally resemble DDCs. We show here that virulent fimbriated P. gingivalis 381, in contrast to its fimbria-deficient mutant, P. gingivalis DPG3, efficiently gains entry to MDDCs in a manner dependent on active cell metabolism and cytoskeletal rearrangement. In addition, uptake of 381, unlike DPG3, induces DCs to undergo maturation, upregulate costimulatory molecules, and secrete inflammation cytokines interleukin-1beta (IL-1beta), IL-6, tumor necrosis factor alpha, IL-10, and IL-12. Moreover, MDDCs pulsed with 381 also stimulated a higher autologous mixed lymphocyte reaction and induced a Th1-type response, with gamma interferon (IFN-gamma) being the main cytokine. Monocytes used as controls demonstrated fimbria-dependent uptake of 381 as well but produced low levels of inflammatory cytokines compared to MDDCs. When MDDCs were pulsed with recombinant fimbrillin of P. gingivalis (10 micro g/ml), maturation of MDDCs was also induced; moreover, matured MDDCs induced proliferation of autologous CD4(+) T cells and release of IFN-gamma. Thus, these results establish the significance of P. gingivalis fimbriae in the uptake of P. gingivalis by MDDCs and in induction of immunostimulatory Th1 responses.

Biological activities of Bacteroides forsythus lipoproteins and their possible pathological roles in periodontal disease

Bacteroides forsythus is a gram-negative, anaerobic, fusiform bacterium and is considered to be an etiological agent in periodontal disease. A lipoprotein fraction prepared from B. forsythus cells by Triton X-114 phase separation (BfLP) activated human gingival fibroblasts and a human monocytic cell line, THP-1, to induce interleukin-6 production and tumor necrosis factor alpha production. BfLP was found to be capable of inducing nuclear factor-kappaB translocation in human gingival fibroblasts and THP-1 cells. By using Chinese hamster ovary K1 cells transfected with Toll-like receptor genes together with a nuclear factor-kappaB-dependent CD25 reporter plasmid, it was found that signaling by BfLP was mediated by Toll-like receptor 2 but not by CD14 or Toll-like receptor 4. BfLP induced apoptotic cell death in human gingival fibroblasts, KB cells (an oral epithelial cell line), HL-60 cells (a human myeloid leukemia cell line), and THP-1 cells but not in MOLT4 cells (a T-cell leukemia cell line). Caspase-8, an initiator caspase in apoptosis, was found to be activated in these cells in response to BfLP stimulation. Thus, this study suggested that BfLP plays some etiological roles in oral infections, especially periodontal disease, by induction of cell activation or apoptosis.

Downregulation of the DNA-binding activity of nuclear factor-kappaB p65 subunit in Porphyromonas gingivalis fimbria-induced tolerance

Porphyromonas gingivalis fimbriae induce high levels of nuclear factor-kappaB (NF-kappaB)-dependent cytokine release upon primary but not secondary stimulation of monocytic cells (FimA tolerance). In this study, fimbriae induced Toll-like receptor-mediated activation of both p50 and p65 subunits of NF-kappaB upon primary cellular activation. However, activation of the transactivating p65 subunit (but not of the transcriptionally inactive p50 subunit) was significantly inhibited in fimbria-restimulated cells. Moreover, expression of a NF-kappaB-dependent reporter gene was inhibited upon secondary stimulation with fimbriae. NF-kappaB p65 downregulation may thus contribute to induction of FimA tolerance.

Role of the phosphatidylinositol 3 kinase-Akt pathway in the regulation of IL-10 and IL-12 by Porphyromonas gingivalis lipopolysaccharide

Stimulation of the APC by Porphyromonas gingivalis LPS has been shown to result in the production of certain pro- and anti-inflammatory cytokines. However, the signaling pathways that regulate these processes are currently unknown. In the present study, the role of the phosphatidylinositol 3 kinase (PI3K)-Akt pathway in regulating P. gingivalis LPS-induced production of IL-10, IL-12 p40, and IL-12 p70 by human monocytes was investigated. P. gingivalis LPS selectively activates the PI3K-Akt pathway via Toll-like receptor 2, and inhibition of this pathway results in an abrogation of extracellular signal-regulated kinase 1/2 phosphorylation, whereas the activation of p38 and c-Jun N-terminal kinase 1/2 kinases were unaffected. Analysis of cytokine production following stimulation of monocytes with P. gingivalis LPS revealed that inhibition of the PI3K pathway differentially regulated IL-10 and IL-12 synthesis. IL-10 production was suppressed, whereas IL-12 levels were enhanced. Inhibition of P. gingivalis LPS-mediated activation of the PI3K-Akt pathway resulted in a pronounced augmentation of NF-kappaB p65 that was independent of IkappaB-alpha degradation. Furthermore, the ability of the PI3K-Akt pathway to modulate IL-10 and IL-12 production appears to be mediated by the selective suppression of extracellular signal-regulated kinase 1/2 activity, as the MEK1 inhibitor PD98059 closely mimicked the effects of wortmannin and LY294002 to differentially regulate IL-10 and IL-12 production by P. gingivalis LPS-stimulated monocytes. These studies provide new insight into how engagement of the PI3K-Akt pathway by P. gingivalis LPS affects the induction of key immunoregulatory cytokines that control both qualitative and quantitative aspects of innate and adaptive immunity.

Role of innate immune factors in the adjuvant activity of monophosphoryl lipid A

Monophosphoryl lipid A (MPL) is a nontoxic derivative of lipopolysaccharide (LPS) that exhibits adjuvant properties similar to those of the parent LPS molecule. However, the mechanism by which MPL initiates its immunostimulatory properties remains unclear. Due to the involvement of Toll-like receptors in recognizing and transducing intracellular signals in response to LPS, the aim of the present study was to determine the ability of MPL to utilize the Toll-like receptor 2 (TLR2) and TLR4. We provide evidence that MPL differentially utilizes TLR2 and TLR4 for the induction of tumor necrosis factor alpha, interleukin 10 (IL-10), and IL-12 by purified human monocytes as well as by human peripheral blood mononuclear cells. Assessment of NF-kappa B activity demonstrated that MPL utilized TLR2 and especially TLR4 for the activation of NF-kappa B p65 by human monocytes. In addition, stimulation of human monocytes by MPL led to an up-regulation of the costimulatory molecules CD80 and CD86, an effect that could be reduced by pretreatment of cells with a monoclonal antibody to TLR2 or TLR4. Analysis of MPL-induced activation of the extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein (MAP) kinases revealed that MPL utilized both TLR2 and TLR4 for the phosphorylation of ERK1/2, while TLR4 was the predominant receptor involved in the ability of MPL to phosphorylate p38. Moreover, using selective inhibitors for MAP kinase kinase (PD98059) and p38 (SB203580), we show that ERK1/2 exhibited differential effects on production of TNF-alpha and IL-12 p40 by human monocytes, whereas MPL-induced activation of p38 appeared to be predominantly involved in production of IL-10 and IL-12 p40 by MPL-stimulated monocytes. Taken together, these findings aid in understanding the cellular mechanisms by which MPL induces host cell activation and subsequent adjuvant properties.

Counteracting interactions between lipopolysaccharide molecules with differential activation of toll-like receptors

We investigated counteracting interactions between the lipopolysaccharides (LPS) from Escherichia coli (Ec-LPS) and Porphyromonas gingivalis (Pg-LPS), which induce cellular activation through Toll-like receptor 4 (TLR4) and TLR2, respectively. We found that Ec-LPS induced tolerance in THP-1 cells to subsequent tumor necrosis factor alpha (TNF-alpha) and interleukin 1 beta (IL-1beta) induction by Pg-LPS, though the reverse was not true, and looked for explanatory differential effects on the signal transduction pathway. Cells exposed to Pg-LPS, but not to Ec-LPS, displayed persisting expression of IL-1 receptor-associated kinase without apparent degradation, presumably allowing prolonged relay of downstream signals. Accordingly, cells pretreated with Pg-LPS, but not with Ec-LPS, were effectively activated in response to subsequent exposure to either LPS molecule, as evidenced by assessing nuclear factor (NF)-kappaB activity. In fact, Pg-LPS primed THP-1 cells for enhanced NF-kappaB activation and TNF-alpha release upon restimulation with the same LPS. This was a dose-dependent effect and correlated with upregulation of surface TLR2 expression. Furthermore, we observed inhibition of NF-kappaB-dependent transcription in a reporter cell line pretreated with Ec-LPS and restimulated with Pg-LPS (compared to cells pretreated with medium only and restimulated with Pg-LPS), but not when the reverse treatment was made. Although Pg-LPS could not make cells tolerant to subsequent activation by Ec-LPS, Pg-LPS inhibited Ec-LPS-induced TNF-alpha and IL-6 release when the two molecules were added simultaneously into THP-1 cell cultures. Pg-LPS also suppressed P. gingivalis FimA protein-induced NF-kappaB-dependent transcription in the 3E10/huTLR4 reporter cell line, which does not express TLR2. This rules out competition for common signaling intermediates, suggesting that Pg-LPS may block component(s) of the TLR4 receptor complex. Interactions between TLR2 and TLR4 agonists may be important in the regulation of inflammatory reactions.