Evlambia Harokopakis

Differential interactions of fimbriae and lipopolysaccharide from Porphyromonas gingivalis with the Toll-like receptor 2-centred pattern recognition apparatus

The lipopolysaccharide (LPS) and fimbriae of Porphyromonas gingivalis play important roles in periodontal inflammation and pathogenesis. We investigated fimbriae and LPS from several P. gingivalis strains in terms of relative dependence on Toll‐like receptor (TLR) signalling partners or accessory pattern‐recognition molecules mediating ligand transfer to TLRs, and determined induced assembly of receptor complexes in lipid rafts. Fimbriae could utilize TLR1 or TLR6 for cooperative TLR2‐dependent activation of transfected cell lines, in contrast to LPS and a mutant version of fimbriae which displayed preference for TLR1. Whether used to activate human cell lines or mouse macrophages, fimbriae exhibited strong dependence on membrane‐expressed CD14 (mCD14), which could not be substituted for by soluble CD14 (sCD14). In contrast, sCD14 efficiently substituted for mCD14 in LPS‐induced cellular activation. LPS‐binding protein was more important for LPS‐ than for fimbria‐induced cell activation, whereas the converse was true for CD11b/CD18. Cell activation by LPS or fimbriae required lipid raft function and formation of heterotypic receptor complexes (TLR1‐2/CD14/CD11b/CD18), although wild‐type fimbriae additionally recruited TLR6. In summary, TLR2 activation by P. gingivalis LPS or fimbriae involves differential dependence on accessory signalling or ligand‐binding receptors, which may differentially influence innate immune responses.

Lipopolysaccharides from atherosclerosis-associated bacteria antagonize TLR4, induce formation of TLR2/1/CD36 complexes in lipid rafts and trigger TLR2-induced inflammatory responses in human vascular endothelial cells

Infection with bacteria such as Chlamydia pneumonia, Helicobacter pylori or Porphyromonas gingivalis may be triggering the secretion of inflammatory cytokines that leads to atherogenesis. The mechanisms by which the innate immune recognition of these pathogens could lead to atherosclerosis remain unclear. In this study, using human vascular endothelial cells or HEK‐293 cells engineered to express pattern‐recognition receptors (PRRs), we set out to determine Toll‐like receptors (TLRs) and functionally associated PRRs involved in the innate recognition of and response to lipopolysaccharide (LPS) from H. pylori or P. gingivalis. Using siRNA interference or recombinant expression of cooperating PRRs, we show that H. pylori and P. gingivalis LPS‐induced cell activation is mediated through TLR2. Human vascular endothelial cell activation was found to be lipid raft‐dependent and to require the formation of heterotypic receptor complexes comprising of TLR2, TLR1, CD36 and CD11b/CD18. In addition, we report that LPS from these bacterial strains are able to antagonize TLR4. This antagonistic activity of H. pylori or P. gingivalis LPS, as well as their TLR2 activation capability may be associated with their ability to contribute to atherosclerosis.

TLR2 Transmodulates Monocyte Adhesion and Transmigration via Rac1- and PI3K-Mediated Inside-Out Signaling in Response to Porphyromonas gingivalis Fimbriae

We present evidence for a novel TLR2 function in transmodulating the adhesive activities of human monocytes in response to the fimbriae of Porphyromonas gingivalis, a pathogen implicated in chronic periodontitis and atherosclerosis. Monocyte recruitment into the subendothelium is a crucial step in atherosclerosis, and we investigated the role of P. gingivalis fimbriae in stimulating monocyte adhesion to endothelial cells and transendothelial migration. Fimbriae induced CD11b/CD18-dependent adhesion of human monocytes or mouse macrophages to endothelial receptor ICAM-1; these activities were inhibited by TLR2 blockade or deficiency or by pharmacological inhibitors of PI3K. Moreover, this inducible adhesive activity was sensitive to the action of Clostridium difficile toxin B, but was not affected by Clostridium botulinum C3 exoenzyme, pertussis toxin, or cholera toxin. Accordingly, we subsequently showed through the use of dominant negative signaling mutants of small GTPases, that Rac1 mediates the ability of fimbria-stimulated monocytes to bind ICAM-1. A dominant negative mutant of Rac1 also inhibited the lipid kinase activity of PI3K suggesting that Rac1 acts upstream of PI3K in this proadhesive pathway. Furthermore, fimbriae stimulated monocyte adhesion to HUVEC and transmigration across HUVEC monolayers; both activities required TLR2 and Rac1 signaling and were dependent upon ICAM-1 and the high-affinity state of CD11b/CD18. P. gingivalis-stimulated monocytes displayed enhanced transendothelial migration compared with monocytes stimulated with nonfimbriated isogenic mutants. Thus, P. gingivalis fimbriae activate a novel proadhesive pathway in human monocytes, involving TLR2, Rac1, PI3K, and CD11b/CD18, which may constitute a mechanistic basis linking P. gingivalis to inflammatory atherosclerotic processes.

Porphyromonas gingivalis Fimbriae Proactively Modulate β2 Integrin Adhesive Activity and Promote Binding to and Internalization by Macrophages

ABSTRACT In monocytes, the fimbriae of the oral pathogen Porphyromonas gingivalis activate cross talk signaling from Toll-like receptor 2 (TLR2) to the β2 integrin CD11b/CD18, leading to the induction of the high-affinity state of the latter receptor. CD14 plays an important role in this “inside-out” proadhesive pathway by binding fimbriae and facilitating the activation of TLR2 and phosphatidylinositol 3-kinase signaling. In its high-affinity state, CD11b/CD18 mediates monocyte adhesion to endothelial cells and transmigration to sites of infection. We have now shown that P. gingivalis fimbriae function as both an activator and a ligand of CD11b/CD18; thus, fimbriae proactively promote their own binding to monocytes. Indeed, treatments that interfered with fimbria-induced activation of CD11b/CD18 (i.e., blockade of CD14, TLR2, or phosphatidylinositol 3-kinase signaling) also suppressed the cell binding activity of fimbriae, which was largely inducible and CD11b/CD18 dependent. Development of a recombinant inside-out signaling system in Chinese hamster ovary cells confirmed the ability of fimbriae to activate CD14/TLR2 signaling and induce their own CD11b/CD18-dependent binding. Induction of this proadhesive pathway by P. gingivalis fimbriae appeared to take place in lipid rafts. Indeed, methyl-β-cyclodextrin, a cholesterol-sequestering agent that disrupts lipid raft organization, was found to inhibit the fimbria-induced assembly of CD14/TLR2 signaling complexes and the activation of the high-affinity state of CD11b/CD18. Experiments using macrophages from mice deficient in various pattern recognition receptors indicated that the receptors involved in the inside-out proadhesive pathway (CD14, TLR2, and CD11b/CD18) are important for mediating P. gingivalis internalization within macrophages. It therefore appears that P. gingivalis proactively modulates β2 integrin adhesive activity for intracellular uptake.

Peptide Mapping of Bacterial Fimbrial Epitopes Interacting with Pattern Recognition Receptors

The fimbriae of the oral pathogen Porphyromonas gingivalis induce Toll-like receptor 2 (TLR2)-dependent macrophage activation upon their recognition by CD14 and the β2 integrin CD11b/CD18. To map functional epitopes of fimbriae that interact with these pattern recognition receptors (PRRs), we examined 20 synthetic peptides covering the entire length of the 41-kDa fimbrillin subunit. Using direct or competitive inhibition assays for receptor binding or cell activation, the CD14 binding activity of fimbriae was localized to residues 69–90 and was essential for TLR2-dependent cytokine induction. The CD11b/CD18 binding activity of fimbriae was localized to two neighboring epitopes defined by residues 166–185 and 206–225. Unlike epitope 69–90 that constitutively bound CD14, the CD11b/CD18 binding activity of epitopes 166–185 and 206–225 was inducible by integrin activators. The CD11b/CD18 binding activity played a contributory role to TLR2-dependent induction of tumor necrosis factor-α by fimbriae but was involved in specific down-regulation of interleukin-12. Cell activation by a combination of fimbrillin peptides corresponding to the CD14 and CD11b/CD18 binding activities resulted in higher tumor necrosis factor-α responses than would be expected from a simply additive effect, attributable to CD14-dependent inside-out signaling leading to enhanced binding interactions with CD11b/CD18. These data suggest that P. gingivalis fimbriae display a modular structure that interacts through discrete epitopes and in a regulated mode with distinct PRRs, which in turn differentially modulate the state of cell activation. Elucidation of pathogen interactions with PRRs at the molecular level may glean insight into host defense mechanisms as well as into microbial strategies that subvert innate immunity.

Absence of SpeB Production in Virulent Large Capsular Forms of Group A Streptococcal Strain 64

ABSTRACT Passage in human blood of group A streptococcal isolate 64p was previously shown to result in the enhanced expression of M and M-related proteins. Similarly, when this isolate was injected into mice via an air sac model for skin infection, organisms recovered from the spleens showed both increased expression of M and M-related proteins and increased skin-invasive potential. We show that these phenotypic changes were not solely the result of increased transcription of the mRNAs encoding the M and M-related gene products. Rather, the altered expression was associated with posttranslational modifications of the M and M-related proteins that occur in this strain, based on the presence or absence of another virulence protein, the streptococcal cysteine protease SpeB. The phenotypic variability also correlates with colony size variation. Large colonies selected by both regimens expressed more hyaluronic acid, which may explain differences in colony morphology. All large-colony variants were SpeB negative and expressed three distinct immunoglobulin G (IgG)-binding proteins in the M and M-related protein family. Small-colony variants were SpeB positive and bound little IgG through their M and M-related proteins because these proteins, although made, were degraded or altered in profile by the SpeB protease. We conclude that passage in either human blood or a mouse selects for a stable, phase-varied strain of group A streptococci which is altered in many virulence properties.

Conjugation of cholera toxin or its B subunit to liposomes for targeted delivery of antigens

Several immunoadjuvant systems have been proposed to enhance mucosal immune responses of orally administered purified antigens. Cholera toxin (CT) or its B subunit (CTB) have been found to promote immune responses to antigens when they are co-administered via mucosal routes. Oral administration of antigens incorporated into liposomes has also been shown to result in enhanced mucosal immune responses. Here, we describe the covalent coupling of CT and CTB to small unilamellar liposomes for targeting these vesicles to Peyers patch M cells, following their oral administration. Conjugation was done by means of a thioether bond using succinimidyl(4-N-maleimidomethyl)cyclohexane-1-carboxylate to modify the dipalmitoylphosphatidyl-ethanolamine constituent of liposomes and N-succinimidyl-3-(2-pyridyldithio)propionate to thiolate CT or CTB. The biological activity of CT or CTB bound to liposomes was confirmed by a hemagglutination assay using GM1-enriched human erythrocytes. Furthermore, oral administration of CT-conjugated liposomes to rats resulted in the induction of serum IgG and salivary IgA anti-CT responses. CT-conjugated liposomes may prove to be a useful system for targeted delivery and immunoenhancement of weakly immunogenic antigens.

Construction and oral immunogenicity of a Salmonella typhimurium strain expressing a streptococcal adhesin linked to the A2/B subunits of cholera toxin.

A major adhesin from the oral pathogen Streptococcus mutans has been shown to be mucosally immunogenic upon genetic fusion with the cholera toxin A2/B subunits. To take advantage of the ability of Salmonella typhimurium to deliver cloned antigens to the mucosal inductive sites that would obviate the need for antigen purification, we expressed this chimeric construct in an attenuated S. typhimurium strain under the control of bacteriophage T7 transcription. Residual expression of the temperature-regulated T7 RNA polymerase at 30 degrees C allowed production of the chimeric protein at 2-3% of the total soluble protein, but it was increased five to six times following induction at 37 degrees C. Oral administration of a single dose of 10(9) recombinant Salmonella to mice resulted in serum IgG and salivary IgA antibody responses to Salmonella, cholera toxin, and the streptococcal adhesin, which were generally enhanced after a booster immunization.