Using the Traditional Ex Vivo Whole Blood Model to Discriminate Bacteria by Their Inducible Host Responses

Whole blood models are rapid and versatile for determining immune responses to inflammatory and infectious stimuli, but they have not been used for bacterial discrimination. Staphylococcus aureus, S. epidermidis and Escherichia coli are the most common causes of invasive disease, and rapid testing strategies utilising host responses remain elusive. Currently, immune responses can only discriminate between bacterial 'domains' (fungi, bacteria and viruses), and very few studies can use immune responses to discriminate bacteria at the species and strain level. Here, whole blood was used to investigate the relationship between host responses and bacterial strains. Results confirmed unique temporal profiles for the 10 parameters studied: IL-6, MIP-1α, MIP-3α, IL-10, resistin, phagocytosis, S100A8, S100A8/A9, C5a and TF3. Pairwise analysis confirmed that IL-6, resistin, phagocytosis, C5a and S100A8/A9 could be used in a discrimination scheme to identify to the strain level. Linear discriminant analysis (LDA) confirmed that (i) IL-6, MIP-3α and TF3 could predict genera with 95% accuracy; (ii) IL-6, phagocytosis, resistin and TF3 could predict species at 90% accuracy and (iii) phagocytosis, S100A8 and IL-10 predicted strain at 40% accuracy. These data are important because they confirm the proof of concept that host biomarker panels could be used to identify bacterial pathogens.

Toll-like Receptors, NOD1, and NOD2 in Oral Epithelial Cells

Oral epithelium might be the first barrier against oral bacteria in periodontal tissue. We hypothesized that oral epithelium is endowed with innate immune receptors for bacterial components, which play roles in host defense against bacterial infection without being accompanied by excessive inflammatory responses. We found clear expression of Toll-like receptor (TLR)4 as well as TLR2, and strong expression of NOD1 and NOD2 in normal oral epithelial tissues by immunohistochemical analysis. We also showed that primary oral epithelial cells in culture expressed these molecules using PCR, flow cytometry, and immunostaining. In inflamed oral epithelium, cell-surface localizations of TLR2 and TLR4 were more clearly observed than in healthy tissue. Upon stimulation with synthetic ligands for these receptors, the expression of β-defensin 2 was markedly up-regulated. These findings indicate that these molecules in oral epithelial cells are functional receptors that induce antibacterial responses.

A polymer-type water-soluble peptidoglycan exhibited both Toll-like receptor 2- and NOD2-agonistic activities, resulting in synergistic activation of human monocytic cells

Bacterial peptidoglycan (PGN) has been reported to be sensed by cell-surface Toll-like receptor (TLR)2. On the other hand, intracellular NOD-like receptors recognize PGN partial structures: NOD1 and NOD2 recognize the peptide moiety containing diaminopimelic acid, and the muramyldipeptide (MDP) moiety, respectively. In this study, we examined in human monocytic THP-1 cells the pro-inflammatory cytokine-inducing abilities of PGNs and their fragments enzymatically prepared from Staphylococcus epidermidis ATCC 155: a polymer-type water-soluble PGN possessing an intact glycan chain (SEPS) and a monomer-type PGN (SEPS-M). The water-soluble PGN polymer, SEPS, exhibited considerably stronger activities to induce pro-inflammatory cytokines than parent PGNs and the PGN monomer, SEPS-M. Short interference RNA targeting TLR2 and NOD2 markedly reduced the activities of SEPS. In the same experiments, the activities of PGNs were mainly reduced in TLR2-silenced cells, whereas the activities of SEPS-M as well as a synthetic MDP were markedly reduced in NOD2-silenced cells. Furthermore, the PGNs and a reference PGN from Staphylococcus aureus in combination with MDP synergistically induced interleukin-8 in THP-1 cells. These findings strongly suggested that a polymer-type water-soluble PGN fragment, SEPS, exhibits both TLR2-and NOD2-agonistic activities, which induced the synergistic activation of human monocytic cells.

Proinflammatory Activity of Cell-wall Constituents from Gram-positive Bacteria

Innate immunity reacts to conserved bacterial molecules. The outermost lipopolysaccharide (LPS) of Gram-negative organisms is highly inflammatory. It activates responsive cells via specific CD14 and toll-like receptor-4 (TLR4) surface receptor and co-receptors. Gram-positive bacteria do not contain LPS, but carry surface teichoic acids, lipoteichoic acids and peptidoglycan instead. Among these, the thick peptidoglycan is the most conserved. It also triggers cytokine release via CD14, but uses the TLR2 co-receptor instead of TLR4 used by LPS. Moreover, whole peptidoglycan is 1000-fold less active than LPS in a weight-to-weight ratio. This suggests either that it is not important for inflammation, or that only part of it is reactive while the rest acts as ballast. Biochemical dissection of Staphylococcus aureus and Streptococcus pneumoniae cell walls indicates that the second assumption is correct. Long, soluble peptidoglycan chains (approximately 125 kDa) are poorly active. Hydrolysing these chains to their minimal unit (2 sugars and a stem peptide) completely abrogates inflammation. Enzymatic dissection of the pneumococcal wall generated a mixture of highly active fragments, constituted of trimeric stem peptides, and poorly active fragments, constituted of simple monomers and dimers or highly polymerized structures. Hence, the optimal constraint for activation might be 3 cross-linked stem peptides. The importance of structural constraint was demonstrated in additional studies. For example, replacing the first L-alanine in the stem peptide with a D-alanine totally abrogated inflammation in experimental meningitis. Likewise, modifying the D-alanine decorations of lipoteichoic acids with L-alanine, or deacylating them from their diacylglycerol lipid anchor also decreased the inflammatory response. Thus, although considered as a broad-spectrum pattern-recognizing system, innate immunity can detect very subtle differences in Gram-positive walls. This high specificity underlines the importance of using well-characterized microbial material in investigating the system.

Intracameral Toxicity of Bacterial Components Muramyl Dipeptide and Staurosporine: Ciliary Cyst Formation, Epithelial Cell Apoptosis and Necrosis

The uveitogenic bacterial cell wall component muramyl dipeptide (MDP) is apoptogenic in rabbit kidney cells. The purpose of this investigation was to assess the cytotoxic activity of MDP and staurosporine (STSP; induces cultured corneal and lens cells apoptosis) in rabbit ciliary body tissue. Anterior uveitis was determined by clinical symptoms and increased aqueous humor (AH) protein. Ciliary body tissue was assessed for histological changes, caspase-3 activity, dye uptake, distribution of immunoreactive caspase-3 and DNA ladders at 4 and 6 hours postinjection. Increases in caspase-3 activity, APOPercentage dye uptake, and localization of immunoreactive caspase-3 in ciliary epithelial cells were associated with ciliary cysts of detached nonpigmented epithelial (NPE) cells, as well as apoptotic and necrotic DNA ladders in ciliary body tissues from eyes injected with MDP and/or STSP. The results suggest that intracameral injection of the bacterial components MDP and STSP can induce acute endophthalmic changes in uveal tissue including formation of ciliary body, NPE and pigmented epithelial (PE) cell apoptosis, and ciliary body tissue necrosis.

TLR2 - promiscuous or specific? A critical re-evaluation of a receptor expressing apparent broad specificity

Of all pattern recognition receptors (PRR) in innate immunity, Toll-like receptor 2 (TLR2) recognizes the structurally broadest range of different bacterial compounds known as pathogen-associated molecular patterns (PAMPs). TLR2 agonists identified so far are lipopolysaccharides (LPSs) from different bacterial strains, lipoproteins, (synthetic) lipopeptides, lipoarabinomannans, lipomannans, glycosylphosphatidylinositol, lipoteichoic acids (LTA), various proteins including lipoproteins and glycoproteins, zymosan, and peptidoglycan (PG). Because these molecules are structurally diverse, it seems unlikely that TLR2 has the capability to react with all agonists to the same degree. The aim of this review is to identify and describe well-defined structure-function relationships for TLR2. Because of its biomedical importance and because its genetics and biochemistry are presently most completely known among all Gram-positive bacteria, we have chosen Staphylococcus aureus as a focus. Our data together with those reported by other groups reveal that only lipoproteins/lipopeptides are sensed at physiologically concentrations by TLR2 at picomolar levels. This finding implies that the activity of all other putative bacterial compounds so far reported as TLR2 agonists was most likely due to contaminating highly active natural lipoproteins and/or lipopeptides.

Role of Toll-like receptor 2 in innate resistance to Group B Streptococcus

The Gram-positive bacterium Group B Streptococcus (GBS) is an important cause of serious neonatal and adult infections. Toll-like receptor 2 (TLR2) recognizes components of the cell wall of Gram-positive bacteria and is critical for defense against certain invasive pathogens. In GBS, penicillin-binding protein 1a (PBP1a), encoded by ponA, is required for virulence. PBPs participate in cell wall synthesis and in previous studies; the absence of PBP1a was shown to result in subtle changes in the cell wall ultrastructure. Here, we examine the role of TLR2 in defense against GBS infection and the impact of mutation of ponA on TLR2-mediated host responses. We demonstrate TLR2-recognition of both wild-type (WT) GBS and the ponA mutant in vitro. TLR2(-/-) mice were significantly more susceptible than WT mice to infection with either strain of GBS, indicating a crucial role for TLR2 in defense against GBS. Additionally, the ponA mutant was severely attenuated for virulence in both strains of mice. The mutation in ponA did not affect cytokine expression by WT or TLR2(-/-) mice. These data indicate that TLR2 is required for host defense against GBS and this response is unaffected by the absence of PBP1a and the resultant changes in cell wall ultrastructure.

Thrombospondin-1 promotes cellular adherence of gram-positive pathogens via recognition of peptidoglycan

Thrombospondin-1 (TSP1) is a matricellular glycoprotein that has key roles in interactions between human cells and components of the extracellular matrix. Here we report a novel role for the lectin TSP1 in pathogen-host interactions. Binding assays and flow cytometric analysis demonstrate that Streptococcus pneumoniae and other gram-positive pathogens including S. pyogenes, Staphylococcus aureus, and Listeria monocytogenes interact specifically with human TSP1. We also show for the first time that host cell-bound TSP1 promotes adherence of gram-positive pathogens to human epithelial and endothelial cell lines. Pretreatment of bacteria with sodium periodate but not Pronase E substantially reduced TSP1-mediated bacterial adherence to host cells, suggesting that a glycoconjugate on the bacterial cell surface functions as the receptor for TSP1. Lipoteichoic acids did not affect TSP1-mediated adherence of S. pneumoniae to host cells. In contrast, attachment of S. pneumoniae and other gram-positive pathogens to host cells via TSP1 was blocked by soluble peptidoglycan, indicating recognition of bacterial peptidoglycan by TSP1. In conclusion, our results demonstrate that recognition of gram-positive pathogens by TSP1 promotes bacterial colonization of host tissue cells. In this scenario, peptidoglycan functions as adhesin and TSP1 acts as a molecular bridge linking gram-positive bacteria with receptors on the host cell.

Molecular mechanisms underlying anti-inflammatory phenotype of neonatal splenic macrophages

Neonatal humans and rodents are susceptible to infection with encapsulated bacteria as a result of an inability to make antibodies to capsular polysaccharides. This is partly a result of decreased production of proinflammatory cytokines by splenic macrophages (MPhi) from neonates. In this study, we show that when stimulated with a variety of agonists to TLR2, -4, and -9, neonatal MPhi make less proinflammatory cytokines and more IL-10 than adult MPhi. IL-10 appears to have a role in the decreased proinflammatory cytokine production, as neonatal MPhi treated with anti-IL-10 receptor antibody or from IL-10(-/-) mice produced levels of proinflammatory cytokines at a level comparable with that produced by adult MPhi. A microarray analysis of RNA from resting and LPS-stimulated MPhi from neonatal and adult mice showed that expression of a large number of genes encoding cytokines, chemokines, and their receptors was decreased dramatically in the neonatal MPhi, although some cytokines, including IL-10 and IL-16, were enhanced. Several genes in the TLR signaling pathway leading to NF-kappaB activation were down-regulated, which may account for the decreased chemokine and cytokine synthesis. It is surprising that p38alpha MAPK, known to be required for TLR-induced cytokine secretion, was enhanced in the neonatal MPhi. Our studies with the p38 MAPK inhibitor SB203580 suggested that excess p38 MAPK activity can be inhibitory for TLR2-, -4-, and -9-induced production of proinflammatory cytokines but not IL-10. The anti-inflammatory phenotype of the neonatal Mphi may be unique to the developing organism, although it compromises the neonate's ability to respond to encapsulated bacteria.

Chemically synthesized pathogen-associated molecular patterns increase the expression of peptidoglycan recognition proteins via toll-like receptors, NOD1 and NOD2 in human oral epithelial cells

Peptidoglycan recognition proteins (PGRPs), a novel family of pattern recognition molecules (PRMs) in innate immunity conserved from insects to mammals, recognize bacterial cell wall peptidoglycan (PGN) and are suggested to act as anti-bacterial factors. In humans, four kinds of PGRPs (PGRP-L, -Ialpha, -Ibeta and -S) have been cloned and all four human PGRPs bind PGN. In this study, we examined the possible regulation of the expression of PGRPs in oral epithelial cells upon stimulation with chemically synthesized pathogen-associated molecular patterns (PAMPs) in bacterial cell surface components: Escherichia coli-type tryacyl lipopeptide (Pam3CSSNA), E. coli-type lipid A (LA-15-PP), diaminopimelic acid containing desmuramyl peptide (gamma-D-glutamyl-meso-DAP; iE-DAP), and muramyldipeptide (MDP). These synthetic PAMPs markedly upregulated the mRNA expression of the four PGRPs and cell surface expression of PGRP-Ialpha and -Ibeta, but did not induce either mRNA expression or secretion of inflammatory cytokines, in oral epithelial cells. Suppression of the expression of Toll-like receptor (TLR)2, TLR4, nucleotide-binding oligomerization domain (NOD)1 and NOD2 by RNA interference specifically inhibited the upregulation of PGRP mRNA expression induced by Pam3CSSNA, LA-15-PP, iE-DAP and MDP respectively. These PAMPs definitely activated nuclear factor (NF)-kappaB in the epithelial cells, and suppression of NF-kappaB activation clearly prevented the induction of PGRP mRNA expression induced by these PAMPs in the cells. These findings suggested that bacterial PAMPs induced the expression of PGRPs, but not proinflammatory cytokines, in oral epithelial cells, and the PGRPs might be involved in host defence against bacterial invasion without accompanying inflammatory responses.

Recognition of Staphylococcus aureus by the innate immune system

The gram-positive bacterium Staphylococcus aureus is a major pathogen responsible for a variety of diseases ranging from minor skin infections to life-threatening conditions such as sepsis. Cell wall-associated and secreted proteins (e.g., protein A, hemolysins, and phenol-soluble modulin) and cell wall components (e.g., peptidoglycan and alanylated lipoteichoic acid) have been shown to be inflammatory, and these staphylococcal components may contribute to sepsis. On the host side, many host factors have been implicated in the innate detection of staphylococcal components. One class of pattern recognition molecules, Toll-like receptor 2, has been shown to function as the transmembrane component involved in the detection of staphylococcal lipoteichoic acid and phenol-soluble modulin and is involved in the synthesis of inflammatory cytokines by monocytes/macrophages in response to these components. Nod2 (nucleotide-binding oligomerization domain 2) is the intracellular sensor for muramyl dipeptide, the minimal bioactive structure of peptidoglycan, and it may contribute to the innate immune defense against S. aureus. The staphylococcal virulence factor protein A was recently shown to interact directly with tumor necrosis factor receptor 1 in airway epithelium and to reproduce the effects of tumor necrosis factor alpha. Finally, peptidoglycan recognition protein L is an amidase that inactivates the proinflammatory activities of peptidoglycan. However, peptidoglycan recognition protein L probably plays a minor role in the innate immune response to S. aureus. Thus, several innate immunity receptors may be implicated in host defense against S. aureus.

Surface calreticulin mediates muramyl dipeptide-induced apoptosis in RK13 cells

Calreticulin (CRT) is a binding protein for apoptotic N-acetylmuramyl-L-alanyl-D-isoglutamine (L,D-MDP) or peptidoglycan in RK(13) cells. CRT on RK(13) cell surface (srCRT) forms complex(es) with tumor necrosis factor receptor 1 (TNFR1) and TNFR-associated death domain (TRADD) protein of the cell membrane. CRT polyclonal or monoclonal antibody binding to RK(13) srCRT dose-dependently inhibited L,D-MDP-induced apoptosis. In RK(13) cells, L,D-MDP up-regulated the TNFR1.TRADD complex of the plasma membrane and subsequently induced cytosolic TRADD-Fas-associated death domain protein complex. Biotinylated srCRT was capable of calcium-dependent binding of Sepharose-immobilized L,D-MDP or peptidoglycan. However, Toll-like receptors TLR-2 and TLR-4, Nod2, and CD14 of RK(13) cells did not specifically bind Sepharose-immobilized L,D-MDP. High concentrations (5-40 mm) of EGTA dose-dependently inhibited free L,D-MDP binding to purified RK(13) cell CRT and promoted free L,D-MDP dissociation from RK(13) cell CRT.MDP complex. Different concentrations of EGTA (0-40 mm) added to Dulbecco's modified essential medium with 1.8 mm calcium or phosphate-buffered saline with 0.18 mm calcium have different effects on medium free calcium concentrations but have identical inhibiting effects on L,D-MDP-induced apoptosis. More inhibition of the L,D-MDP-induced apoptotic DNA ladders and caspase-3 activity in RK(13) cells was obtained with EGTA pretreatment (83%) than just EGTA + L,D-MDP (47%). The knocking down of srCRT by antisense oligonucleotide CRTAS121 (250 nmol/ml) and stealth small interfering RNA CRT_siR479 (150 pm/ml) for 2 days (44 and 66%, respectively), resulted in the inhibition of L,D-MDP-induced caspase-3 activity (47 and 65%, respectively). The results suggest that (a) the binding of L,D-MDP to srCRT is calcium-dependent, i.e. on srCRT-bound calcium, and (b) it is srCRT, not TLR-2, TLR-4, Nod2 or CD14, that mediates L,D-MDP-induced RK(13) cell apoptosis through activating the TNFR1. TRADD-Fas-associated death domain protein apoptotic pathway.

The role of peptidoglycan in pathogenesis

Bacterial pathogens rely on a variety of virulence factors to establish the colonization of a new niche. Although peptidoglycan and its muropeptide derivatives have been known to possess potent biological properties, until recently the molecular bases were poorly understood. With the identification of the cytosolic surveillance mechanism mediated by the nucleotide-binding oligomerization domain (Nod)1 and Nod2 proteins, which detect unique peptidoglycan-derived muropeptides, these muropeptides should be considered as potential virulence factors. Recent research highlights the role of peptidoglycan in the pathogenesis of different human pathogens such as Streptococcus pneumoniae, Listeria monocytogenes or Helicobacter pylori.

Calreticulin Is a Binding Protein for Muramyl Dipeptide and Peptidoglycan in RK13Cells†

Calreticulin (CRT) was isolated and identified as a protein in rabbit kidney RK(13) cells that binds the apoptogenic bacterial cell wall (BCW) components, muramyl dipeptide (MDP) and peptidoglycan (PG). Mannan-agarose purified RK(13) cell CRT (rCRT) selectively bound sepharose-immobilized L,D-MDP and PG, but not L,L-MDP or D,D-MDP. Purified rCRT and bovine CRT (bCRT) also bound free PG and L,D-MDP demonstrated in bioassays of RK(13) cell apoptosis. The results suggest that, in RK(13) cells, (a) CRT is a specific binding protein for both L,D-MDP and PG and (b) CRT binding L,D-MDP or PG is dependent on the stereoisomeric configuration of the dipeptide (L-alanyl-D-isoglutamine) moiety. In addition, the results also suggest that, in RK(13) cells, the binding of L,D-MDP, L,L-MDP, D,D-MDP, or PG to CRT correlates with their capacities of inducing apoptosis.

The stereochemistry of the amino acid side chain influences the inflammatory potential of muramyl dipeptide in experimental meningitis

Intrathecal injections of 50 to 100 micro g of (N-acetylmuramyl-L-alanyl-D-isoglutamine) muramyl dipeptide (MDP)/rabbit dose-dependently triggered tumor necrosis factor alpha (TNF-alpha) secretion (12 to 40,000 pg/ml) preceding the influx of leukocytes in the subarachnoid space of rabbits. Intrathecal instillation of heat-killed unencapsulated R6 pneumococci produced a comparable leukocyte influx but only a minimal level of preceding TNF-alpha secretion. The stereochemistry of the first amino acid (L-alanine) of the MDP played a crucial role with regard to its inflammatory potential. Isomers harboring D-alanine in first position did not induce TNF-alpha secretion and influx of leukocytes. This stereospecificity of MDPs was also confirmed by measuring TNF-alpha release from human peripheral mononuclear blood cells stimulated in vitro. These data show that the inflammatory potential of MDPs depends on the stereochemistry of the first amino acid of the peptide side chain and suggest that intact pneumococci and MDPs induce inflammation by different pathways.