TLR2 transmodulates monocyte adhesion and transmigration via Rac1- and PI3K-mediated inside-out signaling in response to Porphyromonas gingivalis fimbriae

Porphyromonas gingivalis fimbriae

Marginal periodontitis is not a homogeneous disease but is rather influenced by an intricate set of host susceptibility differences as well as diversities in virulence among the harbored organisms. It is likely that clonal heterogeneity of subpopulations with both high and low levels of pathogenicity exists among organisms harbored by individuals with negligible, slight, or even severe periodontal destruction. Therefore, specific virulent clones of periodontal pathogens may cause advanced and/or aggressive periodontitis. Porphyromonas gingivalis is a predominant periodontal pathogen that expresses a number of potential virulence factors involved in the pathogenesis of periodontitis, and accumulated evidence shows that its expression of heterogenic virulence properties is dependent on clonal diversity. Fimbriae are considered to be critical factors that mediate bacterial interactions with and invasion of host tissues, with P. gingivalis shown to express two distinct fimbria-molecules, long and short fimbriae, on the cell surface, both of which seem to be involved in development of periodontitis. Long fimbriae are classified into six types (I to V and Ib) based on the diversity of fimA genes encoding FimA (a subunit of long fimbriae). Studies of clones with type II fimA have revealed their significantly greater adhesive and invasive capabilities as compared to other fimA type clones. Long and short fimbriae induce various cytokine expressions such as IL-1α, IL-β, IL-6, and TNF-α, which result in alveolar bone resorption. Although the clonal diversity of short fimbriae is unclear, distinct short fimbria-molecules have been found in different strains. These fimbriae variations likely influence the development of periodontal disease.

Complement and dysbiosis in periodontal disease

Signaling crosstalk between complement and Toll-like receptors (TLRs) normally serves to coordinate host immunity. However, the periodontal bacterium Porphyromonas gingivalis expresses C5 convertase-like enzymatic activity and adeptly exploits complement-TLR crosstalk to subvert host defenses and escape elimination. Intriguingly, this defective immune surveillance leads to the remodeling of the periodontal microbiota to a dysbiotic state that causes inflammatory periodontitis. Understanding the mechanisms by which P. gingivalis modulates complement function to cause dysbiosis offers new targets for complement therapeutics.

Mechanism and implications of CXCR4-mediated integrin activation by Porphyromonas gingivalis

In monocytes and macrophages, the interaction of Porphyromonas gingivalis with Toll-like receptor 2 (TLR2) leads to the activation of a MyD88-dependent antimicrobial pathway and a phosphatidylinositol-3 kinase (PI3K) -dependent pro-adhesive pathway, which activates the β2 -integrin complement receptor 3 (CR3). By means of its fimbriae, P. gingivalis binds CXC-chemokine receptor 4 (CXCR4) and induces crosstalk with TLR2 that inhibits the MyD88-dependent antimicrobial pathway. In this paper, we investigated the impact of the P. gingivalis-CXCR4 interaction on the pro-adhesive pathway. Using human monocytes, mouse macrophages, or receptor-transfected cell lines, we showed that the binding of P. gingivalis fimbriae to CXCR4 induces CR3 activation via PI3K, albeit in a TLR2-independent manner. An isogenic strain of P. gingivalis expressing mutant fimbriae that do not interact with CXCR4 failed to efficiently activate CR3, leading to enhanced susceptibility to killing in vivo compared with the wild-type organism. This in vivo observation is consistent with previous findings that activated CR3 mediates safe entry of P. gingivalis into macrophages. Taken together with our previous work, these results indicate that the interaction of P. gingivalis with CXCR4 leads to inhibition of antimicrobial responses and enhancement of pro-adhesive responses, thereby maximizing its adaptive fitness in the mammalian host.

Butyric acid induces apoptosis in both human monocytes and lymphocytes equivalently

Short-chain fatty acids (SCFAs) are metabolites from anaerobic periodontopathic bacteria that induce apoptosis in immune cells such as lymphocytes, monocytes and macrophages. However, it remains unclear if SCFAs from pathogens induce apoptosis in monocytes/macrophages similarly with lymphocytes. This study investigated whether SCFAs-induced apoptosis is equal among the immunoregulatory cells. Cell apoptosis of the employed human cells was evaluated after treatment with culture supernatants from various periodontopathic bacteria or sodium butyrate. Apoptosis and viability were determined by detection of DNA fragmentation and using an MTS assay kit, respectively. Porphyromonas gingivalis and Fusobacterium nucleatum culture filtrates strongly induced apoptosis whereas Prevotella nigrescens and Prevotella intermedia culture filtrates failed to induce apoptosis in the THP-1 and U937 human monocyte and macrophage cell lines. Healthy gingival fibroblasts and oral epithelial cells were resistant to all the culture filtrates. Gas-liquid chromatography detected butyric acid in P. gingivalis (21.0-34.0 mM) and F. nucleatum (36.0 mM) in culture filtrates, whereas, only trace levels were seen in P. nigrescens and P. intermedia. These results suggest that butyric acid produced by periodontopathic bacteria severely damages immunoregulatory cells in a consistent manner and, likewise, could be involved in mediating periodontal chronic inflammation.

Integrin αXβ₂ is a leukocyte receptor for Candida albicans and is essential for protection against fungal infections

The opportunistic fungus Candida albicans is one of the leading causes of infections in immunocompromised patients, and innate immunity provides a principal mechanism for protection from the pathogen. In the present work, the role of integrin α(X)β₂ in the pathogenesis of fungal infection was assessed. Both purified α(X)β₂ and α(X)β₂-expressing human epithelial kidney 293 cells recognized and bound to the fungal hyphae of SC5314 strain of C. albicans but not to the yeast form or to hyphae of a strain deficient in the fungal mannoprotein, Pra1. The binding of the integrin to the fungus was inhibited by β-glucans but not by mannans, implicating a lectin-like activity in recognition but distinct in specificity from that of α(M)β₂. Mice deficient in α(X)β₂ were more prone to systemic infection with the LD₅₀ fungal inoculum decreasing 3-fold in α(X)β₂-deficient mice compared with wild-type mice. After challenging i.v. with 1.5 × 10⁴ cell/g, 60% of control C57BL/6 mice died within 14 d compared with 100% mortality of α(X)β₂-deficient mice within 9 d. Organs taken from α(X)β₂-deficient mice 16 h postinfection revealed a 10-fold increase in fungal invasion into the brain and a 2-fold increase into the liver. These data indicate that α(X)β₂ is important for protection against systemic C. albicans infections and macrophage subsets in the liver, Kupffer cells, and in the brain, microglial cells use α(X)β₂ to control fungal invasion.

Synthetic immunomodulatory peptide IDR-1002 enhances monocyte migration and adhesion on fibronectin

Regulation of the immune system by immunomodulatory agents, such as the synthetic innate defense regulator (IDR) peptides, has been proposed as a potential strategy to strengthen host immune responses against infection. IDR peptides confer protection in vivo against a range of bacterial infections and have been developed as components of single-dose vaccine adjuvants due to their ability to modulate innate immunity, correlating with an increased recruitment of monocytes to sites of infection or immunization. However, the mechanisms by which IDR peptides augment monocyte recruitment remain poorly defined. Anti-infective peptide IDR-1002 was demonstrated here to lack direct monocyte chemoattractive activity yet enhance, by up to 5-fold, the ability of human monocytes to migrate on fibronectin towards chemokines. This effect correlated with an increased adhesion of monocytes and THP-1 cells to fibronectin by IDR-1002 and other IDR peptides and the adhesion of THP-1 cells to fibronectin occurred in a β(1)-integrin-dependent manner, corresponding with an increased activation of β(1)-integrins and the phosphoinositide 3-kinase (PI3K)-Akt pathway. PI3K- and Akt-specific inhibitors abrogated IDR-1002-induced adhesion and activation of β(1)-integrins, whereas p38 and MEK1 inhibitors did not affect, or moderately inhibited, adhesion, respectively. Furthermore, IDR-1002 enhancement of monocyte migration towards chemokines and activation of β(1)-integrins was abrogated in the presence of PI3K- and Akt-specific inhibitors. In summary, IDR-1002 enhanced monocyte migration on fibronectin through promotion of β(1)-integrin-mediated interactions regulated by the PI3K-Akt pathway, revealing a mechanism by which IDR-1002 promotes monocyte recruitment.

Toll-like receptor 2 gene polymorphisms associated with aggressive periodontitis in Japanese

BACKGROUND AND OBJECTIVE

Aggressive periodontitis is a rare and very severe periodontal disease of early onset, which is closely associated with Porphyromonas.gingivalis (P.g.) infection in the Japanese population. TLR2 encodes Toll-like receptor 2, which plays an important role in the protective response to P.g. infection. We investigated a possible association between TLR2 and aggressive periodontitis.

MATERIAL AND METHODS

Of 2,460 Japanese patients with periodontitis, 38 patients with aggressive periodontitis were enrolled in this study. These 38 aggressive periodontitis patients and 190 Japanese healthy controls were examined for an insertion/deletion (Ins/Del) polymorphism in exon 1, a polymorphism in intron 1 (rs7696323), and a synonymous polymorphism in exon 3 (rs3804100) in TLR2.

RESULTS

We found significant associations of resistance to aggressive periodontitis with the Ins allele (allele frequency in the patients versus controls, 0.540 vs. 0.676, OR=0.56, 95% confidence interval (CI); 0.34-0.92, p=0.022) and the T allele of rs3804100 (0.579 vs. 0.716, OR=0.55, 95% CI; 0.33-0.91, p=0.018), although the C allele of rs7696323 showed no significant association (0.733 vs. 0.829, OR=0.58). A permutation test of Ins/Del-rs7696323-rs3804100 haplotype revealed a significant association between Ins-C-T haplotype (0.252 vs. 0.479, p=0.0003) and resistance to aggressive periodontitis.

CONCLUSIONS

The TLR2 polymorphisms were suggested to confer protection against aggressive periodontitis in a Japanese population. The association should be replicated in other cohorts to further identify the responsible TLR polymorphism(s) involved in the pathogenesis of aggressive periodontitis.

Porphyromonas gingivalis promotes monocyte migration by activating MMP-9

BACKGROUND AND OBJECTIVE

The migration of monocytes into the local environment is crucial for their maturation into macrophages or osteoclasts in the pathogenesis of periodontal disease. The objective of this study was to investigate the role and mechanisms mediated by Porphyromonas gingivalis in promoting the migration of monocytes by regulating MMP-9 and TIMP-1 expression.

MATERIAL AND METHODS

Human THP1 monocytes were treated with culture supernatant derived from P. gingivalis (ATCC 33277) for 24 h. Zymography, western blot analysis and quantitative PCRs were performed to analyse protein and mRNA levels of MMP-9. Protein and mRNA levels of TIMP-1 from monocytes treated with or without P. gingivalis were determined as well. Transwell migration assay was carried out to analyse the effect of P. gingivalis on the migration of human peripheral blood CD14-positive monocytes. An MMP inhibitor (GM6001) and a proteinase inhibitor (leupeptin) were used to determine the role of MMP-9 in P. gingivalis supernatant- and lipopolysaccharide-induced monocyte migration.

RESULTS

In zymography and western blot, an 82 kDa band of active MMP-9 emerged in P. gingivalis-treated monocyte culture media in a dose-dependent manner, in addition to the MMP-9 proenzyme (92 kDa) band expressed in control cell culture media. P. gingivalis supernatant increased both the protein and the mRNA levels of MMP-9 and TIMP-1. P. gingivalis supernatant, but not its lipopolysaccharide, increased the migratory ability of CD14-positive monocytes. The increased migratory ability of P. gingivalis-treated monocytes was partly inhibited by leupeptin (200 μg/mL) and completely antagonized by the MMP inhibitor GM6001 (100 nm). Lipopolysaccharide of P. gingivalis increased protein and mRNA levels of MMP-9 in monocytes, but had no effect on the migratory ability or MMP-9 activation.

CONCLUSION

P. gingivalis supernatant increased the migratory ability of monocytes, in part, by increasing activation and expression of MMP-9.

How has neutrophil research improved our understanding of periodontal pathogenesis?

BACKGROUND

Neutrophils are the predominant cells responsible for host defence against bacterial infection. Loss of neutrophil defence, due either to deficient number or function, strongly predisposes to bacterial infections such as periodontitis. Yet, the neutrophil oxidative and proteolytic arsenal has also been implicated in perpetrating periodontal tissue damage in periodontitis.

AIM

In this review, we focus on recent developments that shed light on these two aspects of neutrophil function in periodontitis.

METHODS

Primary search: using PubMed search for "neutophil", "periodontal", and "periodontitis". Secondary search: using references from the articles found in the first stage.

RESULTS

Early histological studies showed that infiltrating neutrophils form a wall of cells abutting the junctional epithelium in periodontal inflammatory lesions. The chronic standoff between these neutrophils and the bacterial community suggests that bacterial evasion of neutrophil clearance is a major characteristic of periodontitis. Indeed, not all functional neutrophil deficiencies increase the risk of periodontitis, an observation that points the way towards identification of particular anti-bacterial pathways essential for protection against periodontal pathogens. The net result in the majority of periodontitis patients who exhibit normal neutrophil number and function, is that neutrophils accumulate in the periodontal tissue where they are available to participate in tissue destruction. Diminished neutrophil clearance further contributes to the persistence of activated neutrophils in the periodontal tissue.

CONCLUSIONS

Data on the role of neutrophils in the pathogenesis of periodontitis are mixed. Neutrophils are a critical arm of the defence against periodontitis, but bacterial evasion of the neutrophil microbicidal machinery coupled with delayed neutrophil apoptosis may transform the neutrophil from defender to perpetrator. At this stage of knowledge, attempts to induce host modulation through neutrophil suppression or activation are premature.

Microbial manipulation of receptor crosstalk in innate immunity

In the arms race of host-microbe co-evolution, successful microbial pathogens have evolved ingenious ways to evade host immune responses. In this Review, we focus on 'crosstalk manipulation' - the microbial strategies that instigate, subvert or disrupt the molecular signalling crosstalk between receptors of the innate immune system. This proactive interference undermines host defences and contributes to microbial adaptive fitness and persistent infections. Understanding how pathogens exploit host receptor crosstalk mechanisms and infiltrate the host signalling network is essential for developing interventions to redirect the host response and achieve protective immunity.

Lipoxin A₄ inhibits porphyromonas gingivalis-induced aggregation and reactive oxygen species production by modulating neutrophil-platelet interaction and CD11b expression

Porphyromonas gingivalis is an etiological agent that is strongly associated with periodontal disease, and it correlates with numerous inflammatory disorders, such as cardiovascular disease. Circulating bacteria may contribute to atherogenesis by promoting CD11b/CD18-mediated interactions between neutrophils and platelets, causing reactive oxygen species (ROS) production and aggregation. Lipoxin A₄ (LXA₄) is an endogenous anti-inflammatory and proresolving mediator that is protective of inflammatory disorders. The aim of this study was to investigate the effect of LXA₄ on the P. gingivalis-induced activation of neutrophils and platelets and the possible involvement of Rho GTPases and CD11b/CD18 integrins. Platelet/leukocyte aggregation and ROS production was examined by lumiaggregometry and fluorescence microscopy. Integrin activity was studied by flow cytometry, detecting the surface expression of CD11b/CD18 as well as the exposure of the high-affinity integrin epitope, whereas the activation of Rac2/Cdc42 was examined using a glutathione S-transferase pulldown assay. The study shows that P. gingivalis activates Rac2 and Cdc42 and upregulates CD11b/CD18 and its high-affinity epitope on neutrophils, and that these effects are diminished by LXA₄. Furthermore, we found that LXA₄ significantly inhibits P. gingivalis-induced aggregation and ROS generation in whole blood. However, in platelet-depleted blood and in isolated neutrophils and platelets, LXA₄ was unable to inhibit either aggregation or ROS production, respectively. In conclusion, this study suggests that LXA₄ antagonizes P. gingivalis-induced cell activation in a manner that is dependent on leukocyte-platelet interaction, likely via the inhibition of Rho GTPase signaling and the downregulation of CD11b/CD18. These findings may contribute to new strategies in the prevention and treatment of periodontitis-induced inflammatory disorders, such as atherosclerosis.

Regulation of innate immune response to Candida albicans infections by αMβ2-Pra1p interaction

Candida albicans is a common opportunistic fungal pathogen and is the leading cause of invasive fungal diseases in immunocompromised individuals. The induction of cell-mediated immunity to C. albicans is one of the main tasks of cells of the innate immune system, and in vitro evidence suggests that integrin α(M)β₂ (CR3, Mac-1, and CD11b/CD18) is the principal leukocyte receptor involved in recognition of the fungus. Using α(M)β₂-KO mice and mutated strains of C. albicans in two models of murine candidiasis, we demonstrate that neutrophils derived from mice deficient in α(M)β₂ have a reduced ability to kill C. albicans and that the deficient mice themselves exhibit increased susceptibility to fungal infection. Disruption of the PRA1 gene of C. albicans, the primary ligand for α(M)β₂, protects the fungus against leukocyte killing in vitro and in vivo, impedes the innate immune response to the infection, and increases fungal virulence and organ invasion in vivo. Thus, recognition of pH-regulated antigen 1 protein (Pra1p) by α(M)β₂ plays a pivotal role in determining fungal virulence and host response and protection against C. albicans infection.

Polybacterial challenge effects on cytokine/chemokine production by macrophages and dendritic cells

OBJECTIVE

To investigate the polymicrobial infection of periodontal disease, which elicits inflammatory mediators/cytokines/chemokines in the local gingival tissues, and a polybacterial challenge of antigen-presenting cells, e.g. macrophages and dendritic cells (DCs), at the mucosal surface.

MATERIALS AND METHODS

The cytokine/chemokine profiles of human macrophages and DCs in response to polybacterial challenges were investigated.

RESULTS

Oral Gram-negative bacteria elicited significantly greater IL-8 levels from macrophages, compared to Gram-positive bacteria. Gram-positive bacteria did not show synergism in inducing this chemokine from macrophages. In contrast, pairs of oral Gram-negative bacteria elicited synergistic production of IL-8 by macrophages. Similar results were not observed with TNFα, which only appeared additive with the polybacterial challenge. Selected Gram-negative bacterial pairs synergized in IL-6 production by immature DCs. In mature DCs (mDCs), a Porphyromonas gingivalis/Fusobacterium nucleatum and Porphyromonas intermedia/F. nucleatum polybacterial challenge resulted in significant synergism for IL-6 and TNFα levels. However, only the Pi/Fn combination synergized for IL-12 production and there appeared to be no polybacterial effect on IL-10 production by the mDCs.

CONCLUSIONS

These results indicate that a polybacterial challenge of cells linking innate and adaptive immune responses results in varied response profiles that are dependent upon the characteristics of the microorganisms that are components of the polybacterial complex.

Complement: a key system for immune surveillance and homeostasis

Nearly a century after the significance of the human complement system was recognized, we have come to realize that its functions extend far beyond the elimination of microbes. Complement acts as a rapid and efficient immune surveillance system that has distinct effects on healthy and altered host cells and foreign intruders. By eliminating cellular debris and infectious microbes, orchestrating immune responses and sending 'danger' signals, complement contributes substantially to homeostasis, but it can also take action against healthy cells if not properly controlled. This review describes our updated view of the function, structure and dynamics of the complement network, highlights its interconnection with immunity at large and with other endogenous pathways, and illustrates its multiple roles in homeostasis and disease.

Role of arginine deiminase of Streptococcus cristatus in Porphyromonas gingivalis colonization

The ability to attach to a variety of oral surfaces is an important characteristic of Porphyromonas gingivalis. Previous studies have demonstrated that expression and production of FimA, a major subunit protein of the long fimbriae, is required for P. gingivalis colonization. Here we report that a surface protein, arginine deiminase (ArcA) of Streptococcus cristatus, represses FimA production and inhibits biofilm formation of P. gingivalis. This inhibitory function of ArcA is also observed in the formation of heterotypic P. gingivalis-Streptococcus gordonii biofilms. P. gingivalis is released from streptococcal substrates in the presence of ArcA, likely due to an inhibition of FimA production. This work suggests that ArcA may have the potential to be a specific antibiofilm agent to fight P. gingivalis infections.

Porphyromonas gingivalis lipids inhibit osteoblastic differentiation and function

Porphyromonas gingivalis produces unusual sphingolipids that are known to promote inflammatory reactions in gingival fibroblasts and Toll-like receptor 2 (TLR2)-dependent secretion of interleukin-6 from dendritic cells. The aim of the present study was to examine whether P. gingivalis lipids inhibit osteoblastic function. Total lipids from P. gingivalis and two fractions, phosphoglycerol dihydroceramides and phosphoethanolamine dihydroceramides, were prepared free of lipid A. Primary calvarial osteoblast cultures derived from 5- to 7-day-old CD-1 mice were used to examine the effects of P. gingivalis lipids on mineralized nodule formation, cell viability, apoptosis, cell proliferation, and gene expression. P. gingivalis lipids inhibited osteoblast differentiation and fluorescence expression of pOBCol2.3GFP in a concentration-dependent manner. However, P. gingivalis lipids did not significantly alter osteoblast proliferation, viability, or apoptosis. When administered during specific intervals of osteoblast growth, P. gingivalis total lipids demonstrated inhibitory effects on osteoblast differentiation only after the proliferation stage of culture. Reverse transcription-PCR confirmed the downregulation of osteoblast marker genes, including Runx2, ALP, OC, BSP, OPG, and DMP-1, with concurrent upregulation of RANKL, tumor necrosis factor alpha, and MMP-3 genes. P. gingivalis total lipids and lipid fractions inhibited calvarial osteoblast gene expression and function in vivo, as determined by the loss of expression of another osteoblast differentiation reporter, pOBCol3.6GFPcyan, and reduced uptake of Alizarin complexone stain. Finally, lipid inhibition of mineral nodule formation in vitro was dependent on TLR2 expression. Our results indicate that inhibition of osteoblast function and gene expression by P. gingivalis lipids represents a novel mechanism for altering alveolar bone homeostasis at periodontal disease sites.

Complement and periodontitis

Although the complement system is centrally involved in host defense, its overactivation or deregulation (e.g., due to inherent host genetic defects or due to pathogen subversion) may excessively amplify inflammation and contribute to immunopathology. Periodontitis is an oral infection-driven chronic inflammatory disease which exerts a systemic impact on health. This paper reviews evidence linking complement to periodontal inflammation and pathogenesis. Clinical and histological observations show a correlation between periodontal inflammatory activity and local complement activation. Certain genetic polymorphisms or deficiencies in specific complement components appear to predispose to increased susceptibility to periodontitis. Animal model studies and in vitro experiments indicate that periodontal bacteria can either inhibit or activate distinct components of the complement cascade. Porphyromonas gingivalis, a keystone species in periodontitis, subverts complement receptor 3 and C5a anaphylatoxin receptor signaling in ways that promote its adaptive fitness in the presence of non-productive inflammation. Overall, available evidence suggests that complement activation or subversion contributes to periodontal pathogenesis, although not all complement pathways or functions are necessarily destructive. Effective complement-targeted therapeutic intervention in periodontitis would require determining the precise roles of the various inductive or effector complement pathways. This information is essential as it may reveal which specific pathways need to be blocked to counteract microbial evasion and inflammatory pathology or, conversely, kept intact to promote host immunity.

14-3-3 mediates apelin-13-induced enhancement of adhesion of monocytes to human umbilical vein endothelial cells

To investigate whether apelin-13 induced THP-1 monocytes (MCs) adhesion to ECV304 human umbilical vein endothelial cells (HUVECs) via 14-3-3 signaling transduction pathway and the potential novel physiological function and signaling transduction pathway of apelin-APJ, HUVECs ECV304 were cultured in DMEM and MCs THP-1 were cultured in RPMI 1640 medium. Monocyte adhesion and the expression of vascular cell adhesion molecule-1 (VCAM-1) and 14-3-3 were measured with monocyte adhesion assay and western blot analysis. Data showed that apelin-13 increased adhesion of MCs to HUVECs in a concentration- and time-dependent manner, which reached their peaks at 1 mM and 12 h, respectively. Similarly, apelin-13 induced the expression of HUVECs adhesion molecule, VCAM-1, in a concentration- and time-dependent manner, reached their peaks at 1 microM and 12 h, respectively. Apelin-13 induced the expression of 14-3-3 in a concentration- and timedependent manner, which reached their peaks at 1 mM and 5 min, respectively. Furthermore, the potent 14-3-3 inhibitor difopein significantly reduced the expression of 14-3-3 and VCAM-1 in apelin-13 stimulated HUVECs, and difopein significantly inhibited the effect of apelin-13 on induction of MCs adhesion to HUVECs. These data suggested that 14-3-3 mediated the induction of adhesion of MCs to HUVECs by Apelin-13.

Lipoproteins and lipoprotein metabolism in periodontal disease

A growing body of evidence indicates that the incidence of atherosclerosis is increased in subjects with periodontitis - a chronic infection of the oral cavity. This article summarizes the evidence that suggests periodontitis shifts the lipoprotein profile to be more proatherogenic. LDL-C is elevated in periodontitis and most studies indicate that triglyceride levels are also increased. By contrast, antiatherogenic HDL tends to be low in periodontitis. Periodontal therapy tends to shift lipoprotein levels to a healthier profile and also reduces subclinical indices of atherosclerosis. In summary, periodontal disease alters lipoprotein metabolism in ways that could promote atherosclerosis and cardiovascular disease.

Binding to gangliosides containing N-acetylneuraminic acid is sufficient to mediate the immunomodulatory properties of the nontoxic mucosal adjuvant LT-IIb(T13I)

By use of a mouse mucosal immunization model, LT-IIb(T13I), a nontoxic mutant type II heat-labile enterotoxin, was shown to have potent mucosal and systemic adjuvant properties. In contrast to LT-IIb, which binds strongly to ganglioside receptors decorated with either N-acetylneuraminic acid (NeuAc) or N-glycolylneuraminic acid (NeuGc), LT-IIb(T13I) binds NeuAc gangliosides much less well. Rather, LT-IIb(T13I) binds preferentially to NeuGc gangliosides. To determine if the adjuvant properties of LT-IIb(T13I) are altered in the absence of NeuGc ganglioside receptors, experiments were conducted using a Cmah-null mouse line which is deficient in the synthesis of NeuGc gangliosides. Several immunomodulatory properties of LT-IIb(T13I) were shown to be dependent on NeuGc gangliosides. LT-IIb(T13I) had reduced binding activity for NeuGc-deficient B cells and macrophages; binding to NeuGc-deficient T cells and dendritic cells (DC) was essentially undetectable. Treatment of Cmah-null macrophages with LT-IIb(T13I), however, upregulated the transcription of interleukin-4 (IL-4), IL-6, IL-17, and gamma interferon (IFN-gamma), four cytokines important for promoting immune responses. The production of mucosal IgA and serum IgG against an immunizing antigen was augmented in NeuGc-deficient mice administered LT-IIb(T13I) as a mucosal adjuvant. Notably, NeuGc gangliosides are not expressed in humans. Still, treatment of human monocytes with LT-IIb(T13I) induced the secretion of IL-6, an inflammatory cytokine that mediates differential control of leukocyte activation. These results suggested that NeuAc gangliosides are sufficient to mediate the immunomodulatory properties of LT-IIb(T13I) in mice and in human cells. The nontoxic mutant enterotoxin LT-IIb(T13I), therefore, is potentially a new and safe human mucosal adjuvant.

Toxins-useful biochemical tools for leukocyte research

Leukocytes are a heterogeneous group of cells that display differences in anatomic localization, cell surface phenotype, and function. The different subtypes include e.g., granulocytes, monocytes, dendritic cells, T cells, B cells and NK cells. These different cell types represent the cellular component of innate and adaptive immunity. Using certain toxins such as pertussis toxin, cholera toxin or clostridium difficile toxin, the regulatory functions of Gα_i, Gαs and small GTPases of the Rho family in leukocytes have been reported. A summary of these reports is discussed in this review.

Modulation of pattern recognition receptor-mediated inflammation and risk of chronic diseases by dietary fatty acids

Chronic inflammation is known to promote the development of many chronic diseases. Pattern recognition receptors (PRRs), Toll-like receptors (TLRs), and nucleotide-binding oligomerization domain proteins (NODs) mediate both infection-induced inflammation and sterile inflammation by recognizing pathogen- associated molecular patterns and endogenous molecules, respectively. PRR-mediated inflammation is an important determinant in altering the risk of many chronic diseases. Saturated fatty acids (SFAs) can activate PRRs, leading to enhanced expression of pro-inflammatory target gene products. However, n-3 polyunsaturated fatty acids (PUFAs) inhibit agonist-induced activation of PRRs. These results suggest that SFAs and n-3 PUFAs can reciprocally modulate PRR-mediated inflammation, and that PRRs and their downstream signaling components are molecular targets for dietary strategies to reduce chronic inflammation and subsequent risk of chronic diseases. This advancement in knowledge provides a new paradigm for understanding the mechanism by which different dietary fatty acids modify risk of chronic diseases including insulin resistance, atherosclerosis, and cancer.

Crosstalk pathways between Toll-like receptors and the complement system

The Toll-like receptors (TLRs) and complement are key innate defense systems that are triggered rapidly upon infection. Although both systems have been investigated primarily as separate entities, an emerging body of evidence indicates extensive crosstalk between complement and TLR signaling pathways. Analysis of these data suggests that the complement-TLR interplay reinforces innate immunity or regulates excessive inflammation, through synergistic or antagonistic interactions, respectively. However, the facility of complement and TLRs for communication is exploited by certain pathogens as a means to modify the host response in ways that favor the persistence of the pathogens. Further elucidation of regulatory links between complement and TLRs is essential for understanding their complex roles in health and disease.

Scavenger receptor A is expressed by macrophages in response to Porphyromonas gingivalis, and participates in TNF-alpha expression

INTRODUCTION

Porphyromonas gingivalis is a periodontopathic bacterium closely associated with generalized aggressive periodontal disease. Pattern recognition receptors (PRRs) participate in host response to this organism. It is likely that PRRs not previously recognized as part of the host response to P. gingivalis also participate in host response to this organism.

METHODS AND RESULTS

Employing qRT-PCR, we observed increased msr1 gene expression at 2, 6, and 24 h of culture with P. gingivalis strain 381. Flow cytometry revealed increased surface expression of SR-A protein by the 24 h time point. Macrophages cultured with an attachment impaired P. gingivalis fimA- mutant (DPG3) expressed intermediate levels of SR-A expression. Heat-killed P. gingivalis stimulated SR-A expression similar to live bacteria, and purified P. gingivalis capsular polysaccharide stimulated macrophage SR-A expression, indicating that live whole organisms are not necessary for SR-A protein expression in macrophage response. As SR-A is known to play a role in lipid uptake by macrophages, we tested the ability of low-density lipoprotein (LDL) to influence the SR-A response of macrophages to P. gingivalis, and observed no effect of LDL on P. gingivalis-elicited SR-A expression. Lastly, we observed that SR-A knockout (SR-A(-/-)) mouse macrophages produced significantly more tumor necrosis factor (TNF)-alpha than wild type mouse macrophages cultured with P. gingivalis.

CONCLUSION

These data identify that SR-A is expressed by macrophages in response to P. gingivalis, and support that this molecule plays a role in TNF-alpha production by macrophages to this organism.

TLR2-dependent inflammatory response to Porphyromonas gingivalis is MyD88 independent, whereas MyD88 is required to clear infection

Porphyromonas gingivalis is a gram-negative anaerobe considered to be a major periodontal pathogen. TLR2 plays a central role in the response to P. gingivalis infection in vivo. In its absence there is a weak inflammatory response; however, bacteria are cleared rapidly compared with wild-type mice. We examined the role of the TLR adaptor proteins MyD88 and TLR/IL-1R-domain-containing adaptor-inducing IFN-beta in the inflammatory response to P. gingivalis in vivo and in the ability to clear the bacterial infection. Proinflammatory cytokine production in response to P. gingivalis infection depends on TLR2, but it does not require MyD88 or TLR/IL-1R-domain-containing adaptor-inducing IFN-beta. In contrast, the generation of intracellular toxic oxygen species and the ultimate clearance of P. gingivalis infection depend critically on MyD88, independent of TLR2. Thus, robust cytokine production and bacterial clearance are independent events mediated by distinct signaling pathways following infection with P. gingivalis.

Enhanced monocyte migration and pro-inflammatory cytokine production by Porphyromonas gingivalis infection

BACKGROUND AND OBJECTIVE

Porphyromonas gingivalis, a major periodontal pathogen, has been reported to be involved in atherogenesis. In order to further understand this pathogen's link with systemic inflammation and vascular disease, we investigated its influence on murine monocytes and macrophages from three different sources.

MATERIAL AND METHODS

Concanavalin A-elicited peritoneal macrophages, peripheral blood monocyte-derived macrophages and WEHI 274.1 monocytes were infected with either P. gingivalis 381 or its non-invasive fimbriae-deficient mutant, DPG3.

RESULTS

Infection with P. gingivalis 381 markedly induced monocyte migration and significantly enhanced production of the pro-inflammatory cytokines, tumor necrosis factor-alpha and interleukin-6. Consistent with a role for this pathogen's major fimbriae and/or its invasive capacity, infection with DPG3 had a minimal effect on both monocyte attraction and pro-inflammatory cytokine production.

CONCLUSION

Since monocyte recruitment and activation are important steps in the development of vascular inflammation and atherosclerosis, these results suggest that P. gingivalis infection may be involved in these processes.

Toll-like receptor signaling in cell proliferation and survival

Toll-like receptors (TLRs) are important sensors of foreign microbial components as well as products of damaged or inflamed self tissues. Upon sensing these molecules, TLRs initiate a series of downstream signaling events that drive cellular responses including the production of cytokines, chemokines, and other inflammatory mediators. This outcome results from the intracellular assembly of protein complexes that drive phosphorylation and other signaling cascades ultimately leading to chromatin remodeling and transcription factor activation. In addition to driving inflammatory responses, TLRs also regulate cell proliferation and survival which serves to expand useful immune cells and integrate inflammatory responses and tissue repair processes. In this context, central TLR signaling molecules, such as the mitogen-activated protein kinases (MAPK) and phosphoinositide 3-kinase (PI3K), play key roles. In addition, four major groups of transcription factors which are targets of TLR activation also control cell fate. This review focuses on the role of TLR signaling as it relates to cell proliferation and survival. This topic not only has important implications for understanding host defense and tissue repair, but also cancer which is often associated with conditions of chronic inflammation.

Postischemic vascular permeability requires both TLR-2 and TLR-4, but only TLR-2 mediates the transendothelial migration of leukocytes

Ischemia-reperfusion (I/R) activates innate immunity involving Toll-like receptor (TLR) 2 and TLR-4 signaling. Leukocyte migration and vascular permeability contribute to postischemic tissue damage. We hypothesized that TLR-2 and TLR-4 directly mediate leukocyte migration and vascular permeability during I/R. We used in vivo microscopy on postischemic murine cremaster muscle to quantify leukocyte adhesion as well as transendothelial and interstitial migration in sham-operated wild-type mice and in wild-type, TLR-2(-/-), and TLR-4-mutant mice 30 and 120 min after I/R. Alterations in fluorescein isothiocyanate-dextran leakage across cremasteric venules were determined as a measure of endothelial permeability. I/R-induced leukocyte adhesion in TLR-2(-/-) and TLR-4-mutant mice was comparable to that in wild-type mice. The number of transmigrated leukocytes was increased upon I/R in wild-type mice as compared with the sham-operated group. In contrast, leukocyte transmigration was significantly attenuated in TLR-2(-/-) but not in TLR-4-mutant mice. Motility and polarization of interstitially migrating leukocytes did not significantly differ in TLR-2(-/-) and TLR-4-mutant mice from wild-type mice. Postischemic vascular leakage was significantly lower in both TLR-2(-/-) and TLR-4-mutant than in wild-type mice. We conclude that both TLR-2 signaling and TLR-4 signaling enhance postischemic vascular permeability and that TLR-2 has additional effects on the transendothelial migration of leukocytes at the postischemic vascular wall.

Bacterial adhesins in host-microbe interactions

Most commensal and pathogenic bacteria interacting with eukaryotic hosts express adhesive molecules on their surfaces that promote interaction with host cell receptors or with soluble macromolecules. Even though bacterial attachment to epithelial cells may be beneficial for bacterial colonization, adhesion may come at a cost because bacterial attachment to immune cells can facilitate phagocytosis and clearing. Many pathogenic bacteria have solved this dilemma by producing an antiphagocytic surface layer usually consisting of polysaccharide and by expressing their adhesins on polymeric structures that extend out from the cell surface. In this review, we will focus on the interaction between bacterial adhesins and the host, with an emphasis on pilus-like structures.

Induction of distinct TLR2-mediated proinflammatory and proadhesive signaling pathways in response to Porphyromonas gingivalis fimbriae

The oral pathogen Porphyromonas gingivalis, as well as its purified fimbriae, are known to activate TLR2 and induce proinflammatory and proadhesive effects. The TLR2 proinflammatory pathway induces NF-kappaB-dependent inflammatory cytokines, whereas the TLR2 proadhesive pathway is characterized by inside-out signaling that transactivates beta(2) integrin adhesive activities. In this article, using dominant-negative or pharmacological approaches, we show that the two pathways bifurcate and proceed independently downstream of TLR2. Whereas the proinflammatory pathway is dependent on the adaptor molecules Toll/IL-1 receptor domain-containing adaptor protein (also known as Mal) and MyD88, the proadhesive pathway is Toll/IL-1 receptor domain-containing adaptor protein/MyD88-independent and proceeds through PI3K-mediated signaling. Although the Ser/Thr kinase Akt is a major downstream target of PI3K and was activated by P. gingivalis fimbriae in a TLR2- and PI3K-dependent way, Akt was shown not to play a role in the proadhesive pathway. In contrast, another PI3K downstream target, cytohesin-1, was shown to mediate P. gingivalis fimbria-induced activation of beta(2) integrin for ICAM-1 binding. Therefore, P. gingivalis fimbriae activate two distinct TLR2 pathways mediating proinflammatory or proadhesive effects. The delineation of these signaling pathways may provide appropriate targets for selectively inhibiting or enhancing specific activities, depending on whether they undermine or promote the host defense.

Inhibitory effect of estrogen on Rac1-expression in monocytes

Recruitment of circulating monocytes into the vasculature and release of reactive oxygen species (ROS) promote atherogenesis. Rac1-GTPase is an essential component of the superoxide-producing NADPH-oxidase complex. Estrogens inhibit production of vascular reactive oxygen species. Angiotensin II as well as overexpression of the constitutively active mutant RacL61 increased ROS production in monocytes. AngII-mediated ROS release was completely inhibited by overexpression of the dominant negative mutant RacN17 or treatment with 17beta-estradiol. 17beta-Estradiol reduced Rac1-expression concentration- and time-dependently and decreased basal, as well as AngII-induced Rac1 activity. The effects of 17beta-estradiol were receptor-mediated. In vivo, down-regulation of Rac1 by 17beta-estradiol was observed in human mononuclear cells of women with elevated 17beta-estradiol levels after controlled ovarian hyperstimulation. In summary, the data show that down-regulation of Rac1-GTPase contributes to the inhibition of angiotensin II-mediated superoxide release by 17beta-estradiol in monocytes.

Toll gates to periodontal host modulation and vaccine therapy

Toll-like receptors (TLRs) are central mediators of innate antimicrobial and inflammatory responses and play instructive roles in the development of the adaptive immune response. Thus when stimulated by certain agonists, TLRs serve as adjuvant receptors that link innate and adaptive immunity. However, when excessively activated or inadequately controlled during an infection, TLRs may contribute to immunopathology associated with inflammatory diseases, such as periodontitis. Moreover, certain microbial pathogens appear to exploit aspects of TLR signalling in ways that enhance their adaptive fitness. The diverse and important roles played by TLRs suggest that therapeutic manipulation of TLR signalling may have implications in the control of infection, attenuation of inflammation, and the development of vaccine adjuvants for the treatment of periodontitis. Successful application of TLR-based therapeutic modalities in periodontitis would require highly selective and precisely targeted intervention. This would in turn necessitate precise characterization of TLR signalling pathways in response to periodontal pathogens, as well as development of effective and specific agonists or antagonists of TLR function and signalling. This review summarizes the current status of TLR biology as it relates to periodontitis, and evaluates the potential of TLR-based approaches for host-modulation therapy in this oral disease.

Treatment of periodontal disease results in improvements in endothelial dysfunction and reduction of the carotid intima-media thickness

Several cohort studies reported a relation of cardiovascular events and periodontal disease. In particular, Porphyromonas gingivalis is associated with the development of atherosclerotic plaques. We verified in a longitudinal study whether inflammation biomarkers, endothelial adhesion molecules, leukocyte activation markers, and intima-media thickness could be beneficially modified by periodontal treatment alone. Thirty-five otherwise healthy individuals affected by mild to moderate parodontopathy were enrolled in the study. Echo-Doppler cardiography of the carotid artery, fluorescence-activated cell sorting analyses on lymphocytes and monocytes, and plasma inflammatory indices were evaluated at baseline and at multiple time points after the periodontal treatment. Results showed that inflammation biomarkers were abnormally increased at baseline. Periodontal treatment resulted in a significant reduction of the total oral bacterial load that was associated with a significant amelioration of inflammation biomarkers and of adhesion and activation proteins. Notably, intima-media thickness was significantly diminished after treatment. Inflammatory alterations associated with the genesis of atherosclerotic plaques are detected in otherwise healthy individuals affected by parodontopathy and are positively influenced by periodontal treatment. Reduction of oral bacterial load results in a modification of an anatomical parameter directly responsible for atherosclerosis. These results shed light on the pathogenesis of atherosclerosis and could have practical implications for public health.

Toll-like receptor 2 mediates apolipoprotein CIII-induced monocyte activation

Apolipoprotein (apo)CIII predicts risk for coronary heart disease. We recently reported that apoCIII directly activates human monocytes. Recent evidence indicates that toll-like receptor (TLR)2 can contribute to atherogenesis through transduction of inflammatory signals. Here, we tested the hypothesis that apoCIII activates human monocytoid THP-1 cells through TLR2. ApoCIII induced the association of TLR2 with myeloid differentiation factor 88, activated nuclear factor (NF)-kappaB in THP-1 cells, and increased their adhesion to human umbilical vein endothelial cells (HUVECs). Anti-TLR2 blocking antibody, but not anti-TLR4 blocking antibody or isotype-matched IgG, inhibited these processes (P<0.05). ApoCIII bound with high affinity to human recombinant TLR2 protein and showed a significantly higher (P<0.05) and saturable binding to 293 cells overexpressing human TLR2 than to parental 293 cells with no endogenous TLR2. Overexpression of TLR2 in 293 cells augmented apoCIII-induced NF-kappaB activation and beta(1) integrin expression, processes inhibited by anti-apoCIII antibody as well as anti-TLR2 antibody. Exposure of peripheral blood monocytes isolated from C57BL/6 (wild-type) mice to apoCIII activated their NF-kappaB and increased their adhesiveness to HUVECs. In contrast, apoCIII did not activate monocytes from TLR2-deficient mice. Finally, intravenous administration to C57BL/6 mice of apoCIII-rich very-low-density lipoprotein (VLDL), but not of apoCIII-deficient VLDL, activated monocytes and increased their adhesiveness to HUVECs, processes attenuated by anti-TLR2 or anti-apoCIII antibody. ApoCIII-rich VLDL did not activate monocytes from TLR2-deficient mice. In conclusion, apoCIII activated monocytes at least partly through a TLR2-dependent pathway. The present study identifies a novel mechanism for proinflammatory and proatherogenic effects of apoCIII and a role for TLR2 in atherosclerosis induced by atherogenic lipoproteins.

Beta-PIX and Rac1 GTPase mediate trafficking and negative regulation of NOD2

The nucleotide-binding domain and leucine-rich repeat containing protein NOD2 serves as a cytoplasmic pattern recognition molecule sensing bacterial muramyl dipeptide (MDP), whereas TLR2 mediates cell surface recognition of bacterial lipopeptides. In this study, we show that NOD2 stimulation activated Rac1 in human THP-1 cells and primary human monocytes. Rac1 inhibition or knock-down, or actin cytoskeleton disruption increased MDP-stimulated IL-8 secretion and NF-kappaB activation, whereas TLR2-dependent cell activation was suppressed by Rac1 inhibition. p21-activated kinase [Pak]-interacting exchange factor (beta-PIX) plays a role in this negative regulation, because knock-down of beta-PIX also led to increased NOD2-mediated but not TLR2-mediated IL-8 secretion, and coimmunoprecipitation experiments demonstrated that NOD2 interacted with beta-PIX as well as Rac1 upon MDP stimulation. Moreover, knock-down of beta-PIX or Rac1 abrogated membrane recruitment of NOD2, and interaction of NOD2 with its negative regulator Erbin. Overall, our data indicate that beta-PIX and Rac1 mediate trafficking and negative regulation of NOD2-dependent signaling which is different from Rac1's positive regulatory role in TLR2 signaling.

Pathogen induction of CXCR4/TLR2 cross-talk impairs host defense function

We report a mechanism of microbial evasion of Toll-like receptor (TLR)-mediated immunity that depends on CXCR4 exploitation. Specifically, the oral/systemic pathogen Porphyromonas gingivalis induces cross-talk between CXCR4 and TLR2 in human monocytes or mouse macrophages and undermines host defense. This is accomplished through its surface fimbriae, which induce CXCR4/TLR2 co-association in lipid rafts and interact with both receptors: Binding to CXCR4 induces cAMP-dependent protein kinase A (PKA) signaling, which in turn inhibits TLR2-mediated proinflammatory and antimicrobial responses to the pathogen. This outcome enables P. gingivalis to resist clearance in vitro and in vivo and thus to promote its adaptive fitness. However, a specific CXCR4 antagonist abrogates this immune evasion mechanism and offers a promising counterstrategy for the control of P. gingivalis periodontal or systemic infections.

Phosphoinositide-3 kinase-Akt pathway controls cellular entry of Ebola virus

The phosphoinositide-3 kinase (PI3K) pathway regulates diverse cellular activities related to cell growth, migration, survival, and vesicular trafficking. It is known that Ebola virus requires endocytosis to establish an infection. However, the cellular signals that mediate this uptake were unknown for Ebola virus as well as many other viruses. Here, the involvement of PI3K in Ebola virus entry was studied. A novel and critical role of the PI3K signaling pathway was demonstrated in cell entry of Zaire Ebola virus (ZEBOV). Inhibitors of PI3K and Akt significantly reduced infection by ZEBOV at an early step during the replication cycle. Furthermore, phosphorylation of Akt-1 was induced shortly after exposure of cells to radiation-inactivated ZEBOV, indicating that the virus actively induces the PI3K pathway and that replication was not required for this induction. Subsequent use of pseudotyped Ebola virus and/or Ebola virus-like particles, in a novel virus entry assay, provided evidence that activity of PI3K/Akt is required at the virus entry step. Class 1A PI3Ks appear to play a predominant role in regulating ZEBOV entry, and Rac1 is a key downstream effector in this regulatory cascade. Confocal imaging of fluorescently labeled ZEBOV indicated that inhibition of PI3K, Akt, or Rac1 disrupted normal uptake of virus particles into cells and resulted in aberrant accumulation of virus into a cytosolic compartment that was non-permissive for membrane fusion. We conclude that PI3K-mediated signaling plays an important role in regulating vesicular trafficking of ZEBOV necessary for cell entry. Disruption of this signaling leads to inappropriate trafficking within the cell and a block in steps leading to membrane fusion. These findings extend our current understanding of Ebola virus entry mechanism and may help in devising useful new strategies for treatment of Ebola virus infection.

HMG-CoA reductase inhibition induces IL-1beta release through Rac1/PI3K/PKB-dependent caspase-1 activation

Mevalonate kinase deficiency (MKD) is an autoinflammatory disorder characterized by recurring fever episodes and results from disturbed isoprenoid biosynthesis. Lipopolysaccharide-stimulated peripheral blood mononuclear cells from MKD patients secrete high levels of interleukin-1beta (IL-1beta) because of the presence of hyperactive caspase-1, and this has been proposed to be the primary cause of recurring inflammation. Here we show that inhibition of HMG-CoA reductase by simvastatin treatment, mimicking MKD, results in increased IL-1beta secretion in a Rac1/PI3K-dependent manner. Simvastatin treatment was found to activate protein kinase B (PKB)/c-akt, a primary effector of PI3K, and ectopic expression of constitutively active PKB was sufficient to induce IL-1beta release. The small GTPase Rac1 was activated by simvastatin, and this was required for both PKB activation and IL-1beta secretion. IL-1beta release is mediated by caspase-1, and simvastatin treatment resulted in increased caspase-1 activity in a Rac1/PI3K-dependent manner. These data suggest that, in MKD, dysregulated isoprenoid biosynthesis activates Rac1/PI3K/PKB, resulting in caspase-1 activation with increased IL-1beta release. Importantly, inhibition of Rac1 in peripheral blood mononuclear cells isolated from MKD patients resulted in a dramatic reduction in IL-1beta release. These data suggest that pharmacologic inhibition of Rac1 could provide a novel therapeutic strategy for treatment of MKD.

Lipid raft-dependent uptake, signalling and intracellular fate of Porphyromonas gingivalis in mouse macrophages

Lipid rafts are cholesterol-enriched microdomains involved in cellular trafficking and implicated as portals for certain pathogens. We sought to determine whether the oral pathogen Porphyromonas gingivalis enters macrophages via lipid rafts, and if so, to examine the impact of raft entry on its intracellular fate. Using J774A.1 mouse macrophages, we found that P. gingivalis colocalizes with lipid rafts in a cholesterol-dependent way. Depletion of cellular cholesterol using methyl-beta-cyclodextrin resulted in about 50% inhibition of P. gingivalis uptake, although this effect was reversed by cholesterol reconstitution. The intracellular survival of P. gingivalis was dramatically inhibited in cholesterol-depleted cells relative to untreated or cholesterol-reconstituted cells, even when infections were adjusted to allow equilibration of the initial intracellular bacterial load. P. gingivalis thus appeared to exploit raft-mediated uptake for promoting its survival. Consistent with this, lipid raft disruption enhanced the colocalization of internalized P. gingivalis with lysosomes. In contrast, raft disruption did not affect the expression of host receptors interacting with P. gingivalis, although it significantly inhibited signal transduction. In summary, P. gingivalis uses macrophage lipid rafts as signalling and entry platforms, which determine its intracellular fate to the pathogen's own advantage.

The diverse functions of Src family kinases in macrophages

Macrophages are key components of the innate immune response. These cells possess a diverse repertoire of receptors that allow them to respond to a host of external stimuli including cytokines, chemokines, and pathogen-associated molecules. Signals resulting from these stimuli activate a number of macrophage functional responses such as adhesion, migration, phagocytosis, proliferation, survival, cytokine release and production of reactive oxygen and nitrogen species. The cytoplasmic tyrosine kinase Src and its family members (SFKs) have been implicated in many intracellular signaling pathways in macrophages, initiated by a diverse set of receptors ranging from integrins to Toll-like receptors. However, it has been difficult to implicate any given member of the family in any specific pathway. SFKs appear to have overlapping and complementary functions in many pathways. Perhaps the function of these enzymes is to modulate the overall intracellular signaling network in macrophages, rather than operating as exclusive signaling switches for defined pathways. In general, SFKs may function more like rheostats, influencing the amplitude of many pathways.

Convergence and amplification of toll-like receptor (TLR) and receptor for advanced glycation end products (RAGE) signaling pathways via high mobility group B1 (HMGB1)

Sustained proinflammatory responses in rheumatoid arthritis, atherosclerosis, and diabetic retinopathy, as well as in cancer, are often associated with increased angiogenesis that contributes to tissue disruption and disease progression. High mobility group B1 (HMGB1) has been recognized as a proinflammatory cytokine and more recently, as a proangiogenic factor. HMGB1 can either be passively released from necrotic cells or actively secreted in response to angiogenic and inflammatory signals. HMGB1 itself may signal through the receptor for advanced glycation end products (RAGE), and via toll-like receptors, TLR2 and TLR4. Activation of these receptors results in the activation of NFkappaB, which induces the upregulation of leukocyte adhesion molecules and the production of proinflammatory cytokines and angiogenic factors in both hematopoietic and endothelial cells, thereby promoting inflammation. Interestingly, HMGB1 seems to be involved in a positive feedback mechanism, that may help to sustain inflammation and angiogenesis in several pathological conditions, thereby contributing to disease progression. Endothelial cells express HMGB1, as well as the receptors RAGE, TLR2, and TLR4, and in diverse pathologies HMGB1 and its receptors are overexpressed. Furthermore, HMGB1-induced signaling can activate NFkappaB, which can subsequently induce the expression of HMGB1 receptors. Thus, HMGB1 can mediate amplification of inflammation and angiogenesis through increased secretion of HMGB1 and increased expression of the receptors it can interact with. In this review, we discuss signaling cascades that HMGB1 can induce via TLRs and RAGE, as well as its contribution to pathologies involving endothelial cells.

Subversion of innate immunity by periodontopathic bacteria via exploitation of complement receptor-3

The capacity of certain pathogens to exploit innate immune receptors enables them to undermine immune clearance and persist in their host, often causing disease. Here we review subversive interactions of Porphyromonas gingivalis, a major periodontal pathogen, with the complement receptor-3 (CR3; CD11b/CD18) in monocytes/macrophages. Through its cell surface fimbriae, P. gingivalis stimulates Toll-like receptor-2 (TLR2) inside-out signaling which induces the high-affinity conformation of CR3. Although this activates CR3-dependent monocyte adhesion and transendothelial migration, P. gingivalis has co-opted this TLR2 proadhesive pathway for CR3 binding and intracellular entry. In CR3-deficient macrophages, the internalization of P. gingivalis is reduced twofold but its ability to survive intracellularly is reduced 1,000-fold, indicating that CR3 is exploited by the pathogen as a relatively safe portal of entry. The interaction of P. gingivalis fimbriae with CR3 additionally inhibits production of bioactive (p70) interleukin-12, which mediates immune clearance. In vivo blockade of CR3 leads to reduced persistence of P. gingivalis in the mouse host and diminished ability to cause periodontal bone loss, the hallmark of periodontal disease. Strikingly, the ability of P. gingivalis to interact with and exploit CR3 depends upon quantitatively minor components (FimCDE) of its fimbrial structure, which predominantly consists of polymerized fimbrillin (FimA). Indeed, isogenic mutants lacking FimCDE but expressing FimA are dramatically less persistent and virulent than the wildtype organism both in vitro and in vivo. This model of immune evasion through CR3 exploitation by P. gingivalis supports the concept that pathogens evolved to manipulate innate immune function for promoting their adaptive fitness.

Periodontal therapy alters gene expression of peripheral blood monocytes

AIMS

We investigated the effects of periodontal therapy on gene expression of peripheral blood monocytes.

METHODS

Fifteen patients with periodontitis gave blood samples at four time points: 1 week before periodontal treatment (#1), at treatment initiation (baseline, #2), 6-week (#3) and 10-week post-baseline (#4). At baseline and 10 weeks, periodontal status was recorded and subgingival plaque samples were obtained. Periodontal therapy (periodontal surgery and extractions without adjunctive antibiotics) was completed within 6 weeks. At each time point, serum concentrations of 19 biomarkers were determined. Peripheral blood monocytes were purified, RNA was extracted, reverse-transcribed, labelled and hybridized with AffymetrixU133Plus2.0 chips. Expression profiles were analysed using linear random-effects models. Further analysis of gene ontology terms summarized the expression patterns into biologically relevant categories. Differential expression of selected genes was confirmed by real-time reverse transcriptase-polymerase chain reaction in a subset of patients.

RESULTS

Treatment resulted in a substantial improvement in clinical periodontal status and reduction in the levels of several periodontal pathogens. Expression profiling over time revealed more than 11,000 probe sets differentially expressed at a false discovery rate of <0.05. Approximately 1/3 of the patients showed substantial changes in expression in genes relevant to innate immunity, apoptosis and cell signalling.

CONCLUSIONS

The data suggest that periodontal therapy may alter monocytic gene expression in a manner consistent with a systemic anti-inflammatory effect.

Complement receptor 3 blockade promotes IL-12-mediated clearance of Porphyromonas gingivalis and negates its virulence in vivo

The ability of certain pathogens to exploit innate immune function allows them to undermine immune clearance and thereby increase their persistence and capacity to cause disease. Porphyromonas gingivalis is a major pathogen in periodontal disease and is associated with increased risk of systemic conditions. We have previously shown that the fimbriae of P. gingivalis interact with complement receptor 3 (CR3; CD11b/CD18) in monocytes/macrophages, resulting in inhibition of IL-12p70 production in vitro. The in vivo biological implications of this observation were investigated in this study using a CR3 antagonist (XVA143). XVA143 was shown to block CR3 binding of P. gingivalis fimbriae and reverse IL-12p70 inhibition; specifically, CR3 blockade resulted in inhibition of ERK1/2 phosphorylation and up-regulation of IL-12 p35 and p40 mRNA expression. Importantly, mice pretreated with XVA143 elicited higher IL-12p70 and IFN-gamma levels in response to P. gingivalis i.p. infection and displayed enhanced pathogen clearance, compared with similarly infected controls. The notion that CR3 is associated with reduced IL-12p70 induction and impaired P. gingivalis clearance was confirmed using i.p. infected wild-type and CR3-deficient mice. Moreover, XVA143 dramatically attenuated the persistence and virulence of P. gingivalis in experimental mouse periodontitis, as evidenced by reduced induction of periodontal bone loss. Therefore, CR3 blockade may represent a promising immunomodulatory approach for controlling human periodontitis and possibly associated systemic diseases.