Recognition of Staphylococcus aureus by the innate immune system

Purified Staphylococcus aureus leukotoxin LukM/F' does not trigger inflammation in the bovine mammary gland

An early recruitment of neutrophils in mammary tissue and milk is considered an important component of the defense of the mammary gland against Staphylococcus aureus. We investigated whether the leukotoxin LukM/F', which is produced by a proportion of mastitis-causing strains of S. aureus, would be able to trigger inflammation in the udder. Infusion of purified LukM/F' toxin in lactating mammary glands did not cause neutrophil influx in milk, showing that the toxin was not able to cause mastitis on its own. Purified LukM/F' did not kill or stimulate mammary epithelial cells in culture. As expected, LukM bound to mammary macrophages and the complete LukM/F' toxin killed these cells, but subcytotoxic LukM/F' concentrations did not induce secretion of IL-8, TNF-α, IL-1β or IL-6 by macrophages. On the contrary, the production of these pro-inflammatory mediators by adhesion-stimulated macrophages was reduced. Overall, these results indicate that purified leukotoxin LukM/F' is not likely to contribute to the initiation of the inflammatory response and could even play an anti-inflammatory role in the mammary gland by inactivating macrophages.

Adenosine derived from Staphylococcus aureus-engulfed macrophages functions as a potent stimulant for the induction of inflammatory cytokines in mast cells

In this study, we attempted to isolate novel mast cell-stimulating molecules from Staphylococcus aureus. Water-soluble extract of S. aureus cell lysate strongly induced human interleukin- 8 in human mast cell line-1 and mouse interleukin-6 in mouse bone marrow-derived mast cells. The active molecule was purified to homogeneity through a C(18) reverse phase HPLC column. By determination of its structure by MALDITOF and (1)H- and (13)C-NMR, adenosine was revealed to be responsible for the observed cytokine induction activities. Further studies using 8-sulfophenyl theophylline, a selective adenosine receptor blocker, verified that purified adenosine can induce interleukin-8 production via adenosine receptors on mast cells. Moreover, adenosine was purified from S. aureusengulfed RAW264.7 cells, a murine macrophage cell line, used to induce phagocytosis of S. aureus. These results show a novel view of the source of exogenous adenosine in vivo and provide a mechanistic link between inflammatory disease and bacterial infection.

Enhancement of cutaneous immune response to bacterial infection after low-level light therapy with 1072 nm infrared light: a preliminary study

We investigated the photobiomodulation effects of 1072 nm infrared light on the natural immune response involved in anti-bacterial and wound healing processes. Thirty mice infected with MRSA on the skin were divided into two groups. The experimental group was treated with 1072 nm infrared light (irradiance: 20 mW/cm(2), fluence: 12 J/cm(2) for 10 min) at 2, 4, 8, 12, 24 h, 3 and 5 days after inoculation and the control group with sham light. Serial changes of the mRNA levels of TLR2, IL-1β, TNF-α, IL-6, iNOS, MCP-1, TGF-β, bFGF and VEGF were studied by real time RT-PCR and those of the expression level of VEGF, bFGF, TGF-β and NF-κB by immunohistochemistry. The mRNA levels of the cytokines involved in the early phase of anti-bacterial immune response (IL-1β, TNF-α, IL-6, MCP-1) increased significantly in the 1072 nm group, peaking between 12 and 24 h post-inoculation. These levels normalized after 3-5 days. Immunohistochemistry revealed a notably stronger expression of VEGF in the 1072 nm group from 8-h post-inoculation to 5-day post-inoculation. We concluded that 1072 nm infrared light had a photobiomodulation effect which resulted in an enhanced biological immune response to the bacterial infection by MRSA and also increased the expression of VEGF to a significant level.

Staphylococcus aureus protein A mediates invasion across airway epithelial cells through activation of RhoA GTPase signaling and proteolytic activity

Staphyococcus aureus and especially the epidemic methicillin-resistant S. aureus strains cause severe necrotizing pneumonia. The mechanisms whereby these organisms invade across the mucosal epithelial barrier to initiate invasive infection are not well understood. Protein A (SpA), a highly conserved and abundant surface protein of S. aureus, activates TNF receptor 1 and EGF receptor (EGFR) signaling cascades that can perturb the cytoskeleton. We demonstrate that wild-type S. aureus, but not spa mutants, invade across polarized airway epithelial cell monolayers via the paracellular junctions. SpA stimulated a RhoA/ROCK/MLC cascade, resulting in the contraction of the cytoskeleton. SpA(+) but not SpA(-) mutants stimulated activation of EGFR and along with subsequent calpain activity cleaved the membrane-spanning junctional proteins occludin and E-cadherin, facilitating staphylococcal transmigration through the cell-cell junctions. Treatment of polarized human airway epithelial monolayers with inhibitors of ROCK, EGFR, MAPKs, or calpain prevented staphylococcal penetration through the monolayers. In vivo, blocking calpain activity impeded bacterial invasion into the lung parenchyma. Thus, S. aureus exploits multiple receptors available on the airway mucosal surface to facilitate invasion across epithelial barriers.

Immunity against Staphylococcus aureus cutaneous infections

Complications arising from cutaneous and soft tissue infections with Staphylococcus aureus are a major clinical problem owing to the high incidence of these infections and the widespread emergence of antibiotic-resistant bacterial strains. If prophylactic vaccines or immunotherapy for certain patient populations are to be developed as an alternative to antibiotics, it will be essential to better understand the immune mechanisms that provide protection against S. aureus skin infections. Recent discoveries have identified a key role for interleukin-1 (IL-1)- and IL-17-mediated immune responses in promoting neutrophil recruitment to the site of infection in the skin, a process that is required for host defence and bacterial clearance. This Review describes these new insights and discusses their potential impact on immune-based therapies and vaccination strategies.

TLR2 mediates the innate response of retinal Muller glia to Staphylococcus aureus

Muller cells, the principal glia of the retina, play several key roles in normal and various retinal diseases. To date, their direct involvement in retinal innate defense against bacterial pathogens has not been investigated. In this article, we show that Muller cells express TLR2, a key sensor implicated in recognizing Gram-positive bacteria. We found that intravitreal injection of TLR2 agonist Pam3Cys and Staphylococcus aureus activated Muller glia in C57BL/6 mouse retina. Similarly, Pam3Cys or S. aureus elicited the expression of TLR2 and activated the NF-κB and p38 MAPK signaling cascade. Concomitant with the activation of signaling pathways, transcriptional expression and secretion of various proinflammatory cytokines (IL-6, TNF-α, and IL-1β), chemokines (IL-8), and antimicrobial peptide (LL-37) were also induced in Muller glia. Importantly, the culture media derived from TLR2-activated Muller glia exhibited robust bactericidal activity against S. aureus. Furthermore, use of neutralizing Ab, small interfering RNA, and pharmacological inhibitors revealed that Muller glial innate response to S. aureus is mediated via the TLR2-NF-κB axis. Collectively, this study for the first time, to our knowledge, establishes that the retinal Muller glia senses pathogens via TLR2 and contributes directly to retinal innate defense via production of inflammatory mediators and antimicrobial peptides.

Staphylococcus aureus biofilms prevent macrophage phagocytosis and attenuate inflammation in vivo

Biofilms are complex communities of bacteria encased in a matrix composed primarily of polysaccharides, extracellular DNA, and protein. Staphylococcus aureus can form biofilm infections, which are often debilitating due to their chronicity and recalcitrance to antibiotic therapy. Currently, the immune mechanisms elicited during biofilm growth and their impact on bacterial clearance remain to be defined. We used a mouse model of catheter-associated biofilm infection to assess the functional importance of TLR2 and TLR9 in the host immune response during biofilm formation, because ligands for both receptors are present within the biofilm. Interestingly, neither TLR2 nor TLR9 impacted bacterial density or inflammatory mediator secretion during biofilm growth in vivo, suggesting that S. aureus biofilms circumvent these traditional bacterial recognition pathways. Several potential mechanisms were identified to account for biofilm evasion of innate immunity, including significant reductions in IL-1β, TNF-α, CXCL2, and CCL2 expression during biofilm infection compared with the wound healing response elicited by sterile catheters, limited macrophage invasion into biofilms in vivo, and a skewing of the immune response away from a microbicidal phenotype as evidenced by decreases in inducible NO synthase expression concomitant with robust arginase-1 induction. Coculture studies of macrophages with S. aureus biofilms in vitro revealed that macrophages successful at biofilm invasion displayed limited phagocytosis and gene expression patterns reminiscent of alternatively activated M2 macrophages. Collectively, these findings demonstrate that S. aureus biofilms are capable of attenuating traditional host proinflammatory responses, which may explain why biofilm infections persist in an immunocompetent host.

A peptidoglycan monomer with the glutamine to serine change and basic peptides bind in silico to TLR-2 (403-455)

Bacterial cell wall polysaccharides, such as PGN, bind and activate TLR-2, NOD2 and PGRP on monocytes/macrophages and activate inflammation. We found that the peptides containing basic amino acids (cations) at N -terminus and tyrosine at C-terminus interfered with activating ability of PGN. This finding is significant because the ECD of TLR-2 in vivo encounters a large number of proteins or peptides. Some should bind ECD and "pre-form" TLR-2 to respond or not to its activators, although they cannot activate TLR-2 alone. TLR-2 is receptor for a large number of ligands, including lipopeptides and bacterial cell wall glycoproteins. A binding site for lipopeptides has been identified; however, a binding site for soluble or multimeric PGN has not been proposed. To identify the candidate binding sites of peptides and PGN on TLR-2, we modeled docking of peptides and of the PGN monomer (PGN-S-monomer) to extracellular domain (ECD-TLR-2) of the unbound TLR-2. Quantification, in silico, of free energy of binding (DG) identified 2 close sites for peptides and PGN. The PGN-S-monomer binding site is between amino acids TLR-2, 404-430 or more closely TLR-2, 417-428. The peptide-binding site is between amino acids TLR-2, 434-455. Molecular models show PGN-S-monomer inserts its N -acetyl-glucosamine (NAG) deep in the TLR-2 coil, while its terminal lysine interacts with inside (Glu(403)) and outside pocket (Tyr(378)). Peptides insert their two N -terminal arginines or their C-terminal tyrosines in the TLR-2 coil. PGN did not bind the lipopeptide-binding site in the TLR-2. It can bind the C-terminus, 572-586 (DG = 0.026 kcal), of "lipopeptide-bound" TLR-2. An additional, low-affinity PGN-binding site is TLR-2 (227-237). MTP, MDP, and lysine-less PGN bind to TLR-2, 87-113. This is the first report identifying candidate binding sites of monomer PGN and peptides on TLR-2. Experimental verification of our findings is needed to create synthetic adjuvant for vaccines. Such synthetic PGN can direct both adjuvant and cancer antigen to TLR-2.

Barrier dysfunction caused by environmental proteases in the pathogenesis of allergic diseases

Skin barrier dysfunction has emerged as a critical driving force in the initiation and exacerbation of atopic dermatitis and the "atopic march" in allergic diseases. The genetically determined barrier deficiency and barrier disruption by environmental and endogenous proteases in skin and epithelium are considered to increase the risk of sensitization to allergens and contribute to the exacerbation of allergic diseases. Sources of allergens such as mites, cockroaches, fungi, and pollen, produce or contain proteases, which are frequently themselves allergens. Staphylococcus aureus, which heavily colonizes the lesions of atopic dermatitis patients and is known to trigger a worsening of the disease, also produces extracellular proteases. Environmental proteases can cause barrier breakdown in the skin, not only in the epithelium, and stimulate various types of cells through IgE-independent mechanisms. Endogenous protease inhibitors control the functions of environmental and endogenous proteases. In this review, we focus on the barrier dysfunction caused by environmental proteases and roles of endogenous protease inhibitors in the pathogenesis of allergic diseases. Additionally, we examine the subsequent innate response to Th2-skewed adaptive immune reactions.

Green tea polyphenol epigallocatechin-3-gallate inhibits TLR2 signaling induced by peptidoglycan through the polyphenol sensing molecule 67-kDa laminin receptor

Here we show the molecular basis for the inhibition of peptidoglycan (PGN)-induced TLR2 signaling by a major green tea polyphenol epigallocatechin-3-gallate (EGCG). Recently, we identified the 67-kDa laminin receptor (67LR) as the cell-surface EGCG receptor. Anti-67LR antibody treatment or silencing of 67LR resulted in abrogation of the inhibitory action of EGCG on PGN-induced production of pro-inflammatory mediators and activation of mitogen-activated protein kinases. Silencing of Toll-interacting protein (Tollip), a negative regulator of TLR signaling impaired the TLR2 signaling inhibitory activity of EGCG, suggesting that TLR2 response could be inhibited by EGCG via 67LR and Tollip.

High prevalence of edin-C encoding RhoA-targeting toxin in clinical isolates of Staphylococcus aureus

Staphylococcus aureus, a major causative agent of human infection, produces a large array of virulence factors, including various toxins. Among them, the host RhoA GTPase ADP-ribosylating EDIN toxins are considered as potential virulence factors. Using the polymerase chain reaction (PCR) assay, we analyzed the virulence profile of 256 S. aureus isolates from various clinical sites of infections. We developed specific primers to detect the three isoforms of edin-encoding genes. We found a prevalence of 14% (36 bacteria) of edin-encoding genes among these clinical isolates. Strikingly, we found that 90% of all edin-bearing S. aureus isolates carried the type-C allele. Both the spa types and the profile of virulence factors of these edin-positive isolates are highly variable. Notably, we show for the first time that edin-C-positive isolates were more frequently recovered from deep-seated infections than other types of infections. Our present work, thus, strongly suggests that the presence of edin-C is a risk factor of S. aureus dissemination in tissues and, thus, represents a predictive marker for a pejorative evolution of staphylococcal infections.

TLR2-dependent pathway of heterologous down-modulation for the CC chemokine receptors 1, 2, and 5 in human blood monocytes

During innate immune responses, the inflammatory CC chemokine receptors CCR1, CCR2, and CCR5 mediate the recruitment of blood monocytes to infected tissues by promoting cell migration in response to chemokines CCL2-5. Toll-like receptors also play an essential role, allowing pathogen recognition by the recruited monocytes. Here, we demonstrate that Toll-like receptor 2 (TLR2) stimulation by lipoteichoic acid (LTA) from Staphylococcus aureus leads to gradual down-modulation of CCR1, CCR2, and CCR5 from the plasma membrane of human blood-isolated monocytes and inhibits chemotaxis. Interestingly, LTA does not promote rapid desensitization of chemokine-mediated calcium responses. We found that the TLR2 crosstalk with chemokine receptors is not dependent on the Toll/interleukin-1 receptor domain-containing adaptor protein, but instead involves phospholipase C, the small G protein Rac1, and is phorbol ester sensitive. Activation of this pathway by LTA lead to β-arrestin-mediated endocytosis of Ser349-phosphorylated CCR5 into recycling endosomes, as does CCL5 treatment. However, LTA-induced internalization of CCR5 is a slower process associated with phospholipase C-mediated and phorbol ester-sensitive phosphorylation. Overall, our data indicate that TLR2 negatively regulates CCR1, CCR2, and CCR5 on human blood monocytes by activating the machinery used to support chemokine-dependent down-modulation and provide a molecular mechanism for inhibiting monocyte migration after pathogen recognition.

Two genes on A/J chromosome 18 are associated with susceptibility to Staphylococcus aureus infection by combined microarray and QTL analyses

Although it has recently been shown that A/J mice are highly susceptible to Staphylococcus aureus sepsis as compared to C57BL/6J, the specific genes responsible for this differential phenotype are unknown. Using chromosome substitution strains (CSS), we found that loci on chromosomes 8, 11, and 18 influence susceptibility to S. aureus sepsis in A/J mice. We then used two candidate gene selection strategies to identify genes on these three chromosomes associated with S. aureus susceptibility, and targeted genes identified by both gene selection strategies. First, we used whole genome transcription profiling to identify 191 (56 on chr. 8, 100 on chr. 11, and 35 on chr. 18) genes on our three chromosomes of interest that are differentially expressed between S. aureus-infected A/J and C57BL/6J. Second, we identified two significant quantitative trait loci (QTL) for survival post-infection on chr. 18 using N₂ backcross mice (F₁ [C18A]×C57BL/6J). Ten genes on chr. 18 (March3, Cep120, Chmp1b, Dcp2, Dtwd2, Isoc1, Lman1, Spire1, Tnfaip8, and Seh1l) mapped to the two significant QTL regions and were also identified by the expression array selection strategy. Using real-time PCR, 6 of these 10 genes (Chmp1b, Dtwd2, Isoc1, Lman1, Tnfaip8, and Seh1l) showed significantly different expression levels between S. aureus-infected A/J and C57BL/6J. For two (Tnfaip8 and Seh1l) of these 6 genes, siRNA-mediated knockdown of gene expression in S. aureus–challenged RAW264.7 macrophages induced significant changes in the cytokine response (IL-1 β and GM-CSF) compared to negative controls. These cytokine response changes were consistent with those seen in S. aureus-challenged peritoneal macrophages from CSS 18 mice (which contain A/J chromosome 18 but are otherwise C57BL/6J), but not C57BL/6J mice. These findings suggest that two genes, Tnfaip8 and Seh1l, may contribute to susceptibility to S. aureus in A/J mice, and represent promising candidates for human genetic susceptibility studies.

Staphylococcus aureus has a wide spectrum of human infection, ranging from asymptomatic nasal carriage to overwhelming sepsis and death. Mouse models offer an attractive strategy for investigating complex diseases such as S. aureus infections. A/J mice are highly susceptible to S. aureus infection compared with C57BL/6J mice. We showed that genes on chromosomes 8, 11, and 18 in A/J are responsible for susceptibility to S. aureus by using chromosome substitution strains (CSS). From the ∼4200 genes on these three chromosomes, we identified 191 which were differentially expressed between A/J and C57BL/6J when challenged with S. aureus. Next, we identified two significant QTLs on chromosome 18 that are associated with susceptibility to S. aureus infection in N₂ backcross mice. Ten genes (March3, Cep120, Chmp1b, Dcp2, Dtwd2, Isoc1, Lman1, Spire1, Tnfaip8, and Seh1l) mapped to the two significant QTLs and were differentially expressed between A/J and C57BL/6J. One gene on each QTL, Tnfaip8 and Seh1l, affected expression of cytokines in mouse macrophages exposed to S. aureus. These cytokine response patterns were consistent with those seen in S. aureus-challenged peritoneal macrophages from CSS 18, but not C57BL/6J. Tnfaip8 and Seh1l are strong candidates for genes influencing susceptibility to S. aureus of A/J mice.

Different types of cell death induced by enterotoxins

The infection of bacterial organisms generally causes cell death to facilitate microbial invasion and immune escape, both of which are involved in the pathogenesis of infectious diseases. In addition to the intercellular infectious processes, pathogen-produced/secreted enterotoxins (mostly exotoxins) are the major weapons that kill host cells and cause diseases by inducing different types of cell death, particularly apoptosis and necrosis. Blocking these enterotoxins with synthetic drugs and vaccines is important for treating patients with infectious diseases. Studies of enterotoxin-induced apoptotic and necrotic mechanisms have helped us to create efficient strategies to use against these well-characterized cytopathic toxins. In this article, we review the induction of the different types of cell death from various bacterial enterotoxins, such as staphylococcal enterotoxin B, staphylococcal alpha-toxin, Panton-Valentine leukocidin, alpha-hemolysin of Escherichia coli, Shiga toxins, cytotoxic necrotizing factor 1, heat-labile enterotoxins, and the cholera toxin, Vibrio cholerae. In addition, necrosis caused by pore-forming toxins, apoptotic signaling through cross-talk pathways involving mitochondrial damage, endoplasmic reticulum stress, and lysosomal injury is discussed.

Bacterial peptidoglycan degrading enzymes and their impact on host muropeptide detection

Peptidoglycan (PGN) is a major component of the bacterial cell envelope in both Gram-positive and Gram-negative bacteria. These muropeptides can be produced or modified by the activity of bacterial glycolytic and peptidolytic enzymes referred to as PGN hydrolases and autolysins. Some of these bacterial enzymes are crucial for bacterial pathogenicity and have been shown to modulate muropeptide release and/or host innate immune responses. The ability of muropeptides to modulate host responses is due to the fact that eukaryotes do not produce PGN and have instead evolved numerous strategies to detect intact PGN and PGN fragments (muropeptides). Here we review the structure of PGN and introduce the various bacterial enzymes known to degrade or modify bacterial PGN. Host factors involved in PGN and muropeptide detection are also briefly discussed, as are examples of how specific bacterial pathogens use PGN degradation and modification to subvert host innate immunity.

Divergent roles of Toll-like receptor 2 in response to lipoteichoic acid and Staphylococcus aureus in vivo

The response of leukocytes to lipoteichoic acid (LTA), a TLR2-dependent major cell wall component of Staphylococcus aureus, is linked to the outcome of an infection. In this study we investigated the role of nonhematopoietic TLR2 in response to LTA and S. aureus by creating bone marrow chimeras. Significant leukocyte recruitment in response to LTA required IFN-gamma priming in WT C57BL/6 and TLR2(-/-)-->WT mice, but was not observed in TLR2(-/-) or WT-->TLR2(-/-) animals. LTA also induced a proinflammatory response in IFN-gamma primed primary human microvascular endothelial cells leading to leukocyte recruitment in vitro. When mice were infected with S. aureus, the most profound elevation of TNF-alpha and IL-6 was seen in TLR2(-/-) and TLR2(-/-)-->WT mice. TLR2(-/-), but not chimeric mice, demonstrated increased IL-17, blood leukocytosis and pulmonary neutrophilia compared to WT mice. Collectively, the results suggest an essential role for IFN-gamma and nonhematopoietic TLR2 for leukocyte recruitment in response to LTA. In contrast, TLR2 on both hematopoietic and nonhematopoietic cells appears to orchestrate an inhibitory response to S. aureus such that in complete TLR2 deficiency, there is an exaggerated proinflammatory response and/or skewing of the immune response towards a Th17 phenotype that may contribute to the decreased survival of TLR2(-/-) mice.

The infectious aspects of atopic dermatitis

Atopic dermatitis is characterized by Staphylococcus aureus colonization and recurrent skin infections. In addition to an increased risk of invasive infections by herpes simplex or vaccinia viruses, there is ample evidence that microbial pathogens, particularly S aureus and fungi, contribute to the cutaneous inflammation of atopic dermatitis. The authors describe recent developments in the pathogenesis of atopic dermatitis in relation to the role of microbial pathogens. Understanding how microbial pathogens interact or evade the cutaneous immunity of atopic dermatitis may be crucial in preventing infections or cutaneous inflammation in this disease.

Novel targeted immunotherapy approaches for staphylococcal infection

IMPORTANCE OF THE FIELD

Staphylococcus aureus is a leading human pathogen in the hospital and the community. Many S. aureus strains are resistant to antibiotics, making treatment of S. aureus infections often very complicated. In contrast to many other bacterial pathogens, a working vaccine has never been found for S. aureus despite considerable efforts in academia and pharmaceutical companies.

AREAS COVERED IN THIS REVIEW

The latest strategies aimed at finding a working vaccine against S. aureus, including active and passive immunization efforts in pre-clinical and clinical stages, and the molecular reasons for why it may be difficult to develop a vaccine are discussed.

WHAT THE READER WILL GAIN

In addition to receiving an overview of current efforts in S. aureus vaccine research, the reader will understand that vaccine development for S. aureus may be difficult owing to the facts that S. aureus is a commensal microorganism and produces toxins that lyse white blood cells, thereby undermining a vaccine's role as a facilitator of opsonophagocytosis.

TAKE HOME MESSAGE

As a result of failed clinical trials with monovalent traditional vaccines, recent developments include a shift towards the potential use of polyvalent formulas and therapeutic antibodies and more systematic selection of optimal antigens.

NOD2 mediates inflammatory responses of primary murine glia to Streptococcus pneumoniae

It is now widely accepted that resident central nervous system (CNS) cells such as microglia and astrocytes initiate and/or augment inflammation following trauma or infection. However, the mechanisms by which glial cells perceive microbial challenges are only now becoming apparent. We have recently demonstrated that microglia and astrocytes constitutively express nucleotide-binding oligomerization domain-2 (NOD2), a member of the novel nucleotide-binding domain leucine-rich repeat region-containing family of proteins (NLR) that functions as an intracellular receptor for a minimal motif present in all bacterial peptidoglycans. Furthermore, we have shown that this NLR is essential for glial responses to gram-negative pathogens and in vivo CNS inflammation elicited by these organisms. In the present study, we have established that intact Streptococcus pneumoniae, the major causative agent for gram-positive bacterial meningitis in adults, is a potent stimulus for the activation of the pivotal inflammatory transcription factor NF-kB and production of inflammatory cytokines in primary murine microglia and astrocytes. We demonstrate that NOD2 is essential for the maximal responses of these cells to intact S. pneumoniae but not cellular lysates. Finally, we have shown that this cytosolic pattern recognition receptor is required for the elevated inflammatory mediator levels, astrogliosis, and demyelination, following in vivo administration of this gram-positive CNS pathogen. As such, we suggest that NOD2 plays a critical role in the establishment of the lethal inflammation associated with streptococcal meningitis.

NOD2 contributes to the inflammatory responses of primary murine microglia and astrocytes to Staphylococcus aureus

We have recently demonstrated that microglia and astrocytes express nucleotide-binding oligomerization domain-2 (NOD2), a novel cytosolic pattern recognition receptor for bacterial motifs, and we have shown that this intracellular receptor is essential for glial responses to Gram-negative pathogens. Here, we demonstrate that intact Staphylococcus aureus, a major Gram-positive causative agent of brain abscesses, activates the transcription factor NF-kappaB and is a potent stimulus for inflammatory cytokine production in primary murine microglia and astrocytes. Interestingly, we demonstrate that NOD2 is essential for maximal glial responses to intact S. aureus, but not cellular lysates. As such, this data indicates that NOD2 plays an important role in initiating inflammatory mediator production by resident brain cells following S. aureus infection and we suggest that this cytosolic receptor acts in conjunction with cell surface pattern recognition receptors to elicit maximal glial responses.

Staphylococcus aureus evades lysozyme-based peptidoglycan digestion that links phagocytosis, inflammasome activation, and IL-1beta secretion

IL-1beta produced by phagocytes is important for protection against the mucosal pathogen Staphylococcus aureus. Processing and maturation of this cytokine requires activation of the multiprotein inflammasome complex. We observed that the bacterial cell wall component peptidoglycan (PGN) must be particulate and internalized via phagocytosis to activate NLRP3 inflammasomes and IL-1beta secretion. In the context of S. aureus infection of macrophages, we find that phagocytosis and lysozyme-based bacterial cell wall degradation are necessary to induce IL-1beta secretion. Further, an S. aureus enzyme, PGN O-acetyltransferase A, previously demonstrated to make cell wall PGN resistant to lysozyme, strongly suppresses inflammasome activation and inflammation in vitro and in vivo. These observations demonstrate that phagocytosis and lysozyme-based cell wall degradation of S. aureus are functionally coupled to inflammasome activation and IL-1beta secretion and illustrate a case whereby a bacterium specifically subverts IL-1beta secretion through chemical modification of its cell wall PGN.

Regulation of heme oxygenase-1 gene by peptidoglycan involves the interaction of Elk-1 and C/EBPalpha to increase expression

Heme oxygenase (HO)-1 is a cytoprotective enzyme with anti-inflammatory properties. HO-1 is induced during a systemic inflammatory response, and expression of HO-1 is beneficial during sepsis of a Gram-positive source. Systemic infection from Gram-positive organisms has emerged as an important cause of sepsis, with Staphylococcus aureus as a common etiology. An important mediator of Gram-positive infections is peptidoglycan (PGN), a cell wall component of these organisms. Here, we demonstrate that HO-1 played an important, protective role in vivo, as mice deficient in HO-1 were very sensitive to the lethal effects of PGN derived from S. aureus. PGN induced HO-1 protein and mRNA levels, and this regulation occurred at the level of gene transcription. The PGN-responsive region of the HO-1 promoter (from -117 to -66 bp) contains a functional EBS, and Ets proteins are known to be involved in the regulation of inflammatory responses. We showed previously that Ets factors (activators Ets-2 and Ets-1 and repressor Elk-3) regulate HO-1 expression by Gram-negative endotoxin. However, during exposure to a Gram-positive stimulus in the present study, Elk-1 was a potent activator of HO-1 in conjunction with PGN. The ability of Elk-1 to induce HO-1 promoter activity was independent of direct DNA binding, but rather occurred by interacting with the CCAAT/enhancer-binding protein-alpha (C/EBPalpha), which binds to DNA. Moreover, silencing of C/EBPalpha in macrophages prevented induction of HO-1 promoter activity by either Elk-1 or PGN. These data provide further insight into the regulation and function of HO-1 by a mediator of Gram-positive bacteria.

Relationship between S. aureus gene pattern and dairy herd mastitis prevalence

The importance to evaluate Staphylococcus aureus virulence analysing the combination of virulence genes is largely recognised, and the recent availability of simplified microarray tools allows performing these analyses also in the dairy field. The combined availability of herd-specific S. aureus mastitis prevalence data, isolates from these herds, and microarray technology offered the opportunity to investigate the relationship between S. aureus genetic pattern and their prevalence among dairy herds. Eleven commercial dairy herds following a S. aureus control programme were enrolled in the study, and 33 S. aureus isolates were collected from these herds. Diagnostic DNA microarrays based on the array-tube platform were used for genotyping of staphylococcal DNA. The genetic analysis of the 157-genes microarray showed as only 19 genes were present in all the isolates considered, and among them the genes coding for the leukocidin subunits (LukF, LukS and LukY), haemolysins (hla, hld and an unnamed haemolysin) and enterotoxin X. Several genes considered in the arrays were absent in all the isolates, including the ones encoding the resistance to most of the antimicrobials, except for tetracycline. In our isolates, some agr alleles were never identified (B-III, C-III, D-III, C-IV and D-IV). The comparison of epidemiological data with the genetic pattern suggests that agr type II is associated to the most diffusive isolates, being recovered from the largest number of herds and with the highest frequency. Microarray technique showed to be a useful method to assess the characteristics of virulence of S. aureus isolated in dairy herds and to investigate the relationship with the prevalence of the microorganism. These results support previous evidence that specific gene patterns could be associated to S. aureus mastitis.

Staphylococcus aureus stimulates neutrophil targeting chemokine expression in keratinocytes through an autocrine IL-1alpha signaling loop

Staphylococcus aureus is a significant human pathogen that can colonize the skin. Neutrophils are well known to be involved in clearance of the bacterium. This study focused on exploring the role that human keratinocytes have as first responders to bacterial challenges. IL-1alpha and IL-1beta increased mRNA production and protein secretion of the neutrophil chemotactic CXCL1, CXCL2, and IL-8 in keratinocytes. S. aureus and the bacterial cell wall components lipoteichoic acid (LTA) and peptidoglycan (PGN) induced similar expression profiles in a Toll-like receptor (TLR)-2-dependent manner. Interestingly, the S. aureus-induced mRNA levels peaked at later time points than those induced by IL-1. The S. aureus-activated chemokine production was preceded by significant IL-1alpha and IL-1beta secretion. Expression of IL-1alpha was significantly higher than that of IL-1beta. Inhibition of IL-1RI using neutralizing antibodies revealed that S. aureus-derived LTA and PGN-induced chemokine expression requires IL-1RI engagement. Surprisingly, we further found that chemokine secretion is dependent upon endocrine IL-1alpha, but not IL-1beta, signaling. Our data show that the innate immune response of keratinocytes is regulated differently than those of other cell types. This may represent a fail-safe system that protects the host against genetic variation and immune evasion mechanisms developed by pathogens.

Toll-like receptor 2 ligand-induced protection against bacterial endophthalmitis

BACKGROUND

Activation of innate immunity plays a key role in determining the outcome of an infection. Here, we investigated whether Toll-like receptors (TLRs) are involved in retinal innate response and explored the prophylactic use of TLR2 ligand in preventing bacterial endophthalmitis.

METHODS

C57BL/6 mice were given intravitreal injections of Pam3Cys, a synthetic ligand of TLR2, or vehicle (phosphate-buffered saline) 24 h prior to Staphylococcus aureus inoculation. The severity of endophthalmitis was graded by slit lamp, electroretinography, histological examinations, and determination of bacterial load in the retina. The expression of cytokines/chemokines and cathelicidin-related antimicrobial peptide was assessed by enzyme-linked immunosorbent assay and Western blot, respectively.

RESULTS

Intravitreal injections of Pam3Cys up-regulated TLR2 expression in the retina of C57BL/6 mice, and Pam3Cys pretreatment significantly improved the outcome of S. aureus endophthalmitis, preserved retinal structural integrity, and maintained visual function as assessed by electroretinography in C57BL/6 mice. Furthermore, Pam3Cys pretreatment activated retinal microglia cells, induced the expression of cathelicidin-related antimicrobial peptide, and remarkably reduced the bacterial load.

CONCLUSIONS

This is the first report that highlights the existence and role of TLR2 in retinal innate immune response to S. aureus infection and suggests that modulation of TLR activation provides a novel prophylactic approach to prevent bacterial endophthalmitis.

Modulation of paired immunoglobulin-like type 2 receptor signaling alters the host response to Staphylococcus aureus-induced pneumonia

Paired immunoglobulin-like type 2 receptors (PILRs) inhibitory PILRalpha and activating PILRbeta are predominantly expressed on myeloid cells. Their functions in host defense and inflammation are largely unknown, and in this study, we evaluated their roles in an acute Staphylococcus aureus pneumonia model. Compared to their respective controls, Pilrb(-/-) mice or mice in which PILRalpha was activated with an agonistic antibody showed improved clearance of pulmonary staphylococci and improved survival. These mice had reduced serum or bronchoalveolar lavage fluid levels of interleukin-1beta (IL-1beta), tumor necrosis factor alpha (TNF-alpha), and IL-6 and elevated levels of gamma interferon (IFN-gamma), IL-12, and IL-10. In contrast, mice in which PILRbeta was activated had increased lung bacterial burdens and higher mortality coupled with an intense proinflammatory response with highly elevated levels of IL-1beta, TNF-alpha, and IL-6. Treatment groups with reduced bacterial burdens had higher levels of Keratinocyte-derived chemokine (KC), macrophage inflammatory protein 2 (MIP-2), and MIP-1alpha in bronchoalveolar lavage fluid and an increased influx of neutrophils and macrophages to the lungs. Consistent with our in vivo findings, bone marrow-derived macrophages from Pilrb(-/-) mice released significantly less IL-1beta and TNF-alpha and more IFN-gamma and IL-12 than did the wild-type macrophages when directly stimulated with heat-killed S. aureus. To our knowledge, this is the first evidence that S. aureus directly interacts with PILRbeta. It provides a mechanism by which manipulating the balance in favor of an inhibitory PILR signal, by activation of PILRalpha or deletion of PILRbeta, helps to control acute S. aureus-mediated pneumonia and attenuate the inflammatory response. These results highlight the importance of PILRs in innate immunity and the control of inflammation.

Artesunate protects sepsis model mice challenged with Staphylococcus aureus by decreasing TNF-alpha release via inhibition TLR2 and Nod2 mRNA expressions and transcription factor NF-kappaB activation

Gram-positive bacteria have become the most common organisms responsible for the development of sepsis. Staphylococcus aureus (S. aureus) is the major gram-positive pathogen in both community-acquired and nosocomial infections. The Mortality associated with nosocomial infections caused by S. aureus may vary but are generally high. In the present study, we found that artesunate (AS) could protect mice against a lethal challenge with heat-killed S. aureus in a dose-dependent manner, and AS in combination with ampicillin sodium-sulbactam sodium (AMPS) could further increase survival of mice challenged with live S. aureus than AMPS alone. This protection was associated with reductions of serum at TNF-alpha level. In in vitro experiments, AS-pretreatment strongly inhibited TNF-alpha release from murine peritoneal macrophage induced by heat-killed S. aureus or peptidoglycan in a dose-dependent manner. AS reduced the Toll like receptor 2 (TLR2) and nucleotide-binding oligomerization domain containing 2 (Nod2) mRNA expressions up-regulated by heat-killed S. aureus and inhibited NF-kappaB activation induced by heat-killed S. aureus. In conclusion, our results demonstrated that AS-mediated protection on septic mice challenged with S. aureus was associated with its reduction on TNF-alpha release via inhibition of TLR2 and Nod2 mRNA expressions and transcription factor NF-kappaB activation.

Infected atopic dermatitis lesions contain pharmacologic amounts of lipoteichoic acid

BACKGROUND

Bacterial infection with Staphylococcus aureus is a known trigger for worsening of atopic dermatitis (AD); the exact mechanisms by which bacterial infection worsens dermatitis are unknown.

OBJECTIVE

We sought to characterize the amounts of the biologically active bacterial lipoprotein lipoteichoic acid (LTA) in infected AD lesions.

METHODS

Eighty-nine children with clinically impetiginized lesions of AD were enrolled in this study. A lesion was graded clinically by using the Eczema Area and Severity Index (EASI), wash fluid obtained from the lesion for quantitative bacterial culture, and measurement of LTA and cytokines. The staphylococcal isolate was tested for antibiotic susceptibilities. The patients were treated with a regimen that included topical corticosteroids and systemic antibiotics, and the lesion was reanalyzed after 2 weeks.

RESULTS

S aureus was identified in 79 of 89 children enrolled in the study. The bacterial colony-forming unit (CFU) counts correlated with the EASI lesional score (P = .04). LTA levels as high as 9.8 mug/mL were measured in the wash fluid samples, and the amounts correlated with the lesional EASI scores (P = .01) and S aureus CFU (P < .001). Approximately 30% of clinically impetiginized AD lesions contained greater than 1 mug/mL LTA, amounts that exert effects on various cell types in vitro. Moreover, injection of skin tissue ex vivo with amounts of LTA found in AD lesions resulted in epidermal cytokine gene expression.

CONCLUSION

Pharmacologic levels of LTA are found in many infected atopic dermatitis lesions.

Staphylococcus aureus isolates from blood and anterior nares induce similar innate immune responses in endothelial cells

To evaluate the possibility to distinguish virulent from non-virulent isolates, gene expression in human umbilical vein endothelial cells (HUVEC) induced by invasive and colonizing isolates of Staphylococcus aureus was compared. Gene expression in HUVEC was analyzed by microarray analysis after 4 h of infection with Staphylococcus aureus, isolated from healthy nasal carriers (n = 5) and from blood of septic patients (n = 5), to explore possible differences between the groups of bacteria in interaction with HUVEC. All isolates were spa-typed to disclose strain relatedness. Moreover, the isolates were characterized with DNA microarray to determine the presence of virulence genes and to investigate the potential genes of importance in HUVEC interaction. The expression of 41 genes was up-regulated, and four were down-regulated in HUVEC by all isolates. Most of the up-regulated genes encode cytokines, chemokines, interferon-induced proteins, proteins regulating apoptosis and cell proliferation. There was no difference in the gene expression pattern between HUVEC infected with invasive or colonizing isolates. Furthermore, there was no difference in the presence of bacterial virulence genes between the two groups. In conclusion, our data indicate that S. aureus isolates induce comparable expression patterns in HUVEC, irrespective of invasiveness or presence of virulence genes.

Expression of NOD2 is increased in inflamed human dental pulps and lipoteichoic acid-stimulated odontoblast-like cells

Human odontoblasts trigger immune response s to oral bacteria that invade dental tissues during the caries process. To date, their ability to regulate the expression of the nucleotide-binding domain leucine-rich repeat containing receptor NOD2 when challenged by Gram-positive bacteria is unknown. In this study, we investigated NOD2 expression in healthy and inflamed human dental pulps challenged by bacteria, and in cultured odontoblast-like cells stimulated with lipoteichoic acid (LTA), a Toll-like receptor (TLR) 2 agonist which is specific for Gram-positive bacteria. We found that NOD2 gene expression was significantly up-regulated in pulps with acute inflammation compared to healthy ones. In vitro, LTA augmented NOD2 gene expression and protein level in odontoblast-like cells. The increase was more pronounced in odontoblast-like cells compared to dental pulp fibroblasts. Blocking experiments in odontoblast-like cells with anti-TLR2 antibody strongly reduced the NOD2 gene expression increase, whereas stimulation with the synthetic TLR2 ligand Pam2CSK4 confirmed NOD2 gene up-regulation following TLR2 engagement. These data suggest that NOD2 up-regulation is part of the odontoblast immune response to Gram-positive bacteria and might be important in protecting human dental pulp from the deleterious effects of cariogenic pathogens.

Eukaryotic vs. prokaryotic chemosensory systems

In the last decades, microbiologists demonstrated that microorganisms possess chemosensory capabilities and communicate with each other via chemical signals. In parallel, it was demonstrated that solitary eukaryotic chemosensory cells are diffusely located on the mucosae of digestive and respiratory apparatuses. It is now evident that on the mucosal surfaces of vertebrates, two chemoreceptorial systems (i.e. eukaryotic and prokaryotic) coexist in a common microenvironment. To date, it is not known if the two chemosensory systems reciprocally interact and compete for detection of chemical cues. This appears to be a fruitful field of study and future researches must consider that the mucosal epithelia possess more chemosensory capabilities than previously supposed.

Non-spa-typeable clinical Staphylococcus aureus strains are naturally occurring protein A mutants

Staphylococcus aureus is a major human pathogen responsible for increasing the prevalence of community- and hospital-acquired infections. Protein A (SpA) is a key virulence factor of S. aureus and is highly conserved. Sequencing of the variable-number tandem-repeat region of SpA (spa typing) provides a rapid and reliable method for epidemiological studies. Rarely, non-spa-typeable S. aureus strains are encountered. The reason for this is not known. In this study, we characterized eight non-spa-typeable bacteremia isolates. Sequencing of the entire spa locus was successful for five strains and revealed various mutations of spa, all of which included a deletion of immunoglobulin G binding domain C, in which the upper primer for spa typing is located, while two strains were truly spa negative. This is the first report demonstrating that nontypeability of S. aureus by spa sequencing is due either to mutation or to a true deficiency of spa.

Staphylococcus aureus induces microglial inflammation via a glycogen synthase kinase 3beta-regulated pathway

A proinflammatory role for glycogen synthase kinase 3beta (GSK-3beta) has been demonstrated. Here, we addressed its roles on heat-inactivated Staphylococcus aureus-induced microglial inflammation. Heat-inactivated S. aureus induced tumor necrosis factor alpha (TNF-alpha) and nitric oxide (NO) production, at least in part, via a Toll-like receptor 2-regulated pathway. Neutralization of TNF-alpha largely blocked heat-inactivated S. aureus-induced NO. Heat-inactivated S. aureus activated GSK-3beta, and inhibiting GSK-3beta reduced TNF-alpha production as well as inducible NO synthase (iNOS)/NO biosynthesis. While activation of NF-kappaB was essential for heat-inactivated S. aureus-induced TNF-alpha and NO, inhibiting GSK-3beta blocked heat-inactivated S. aureus-induced NF-kappaB p65 nuclear translocation. Additionally, inhibiting GSK-3beta enhanced heat-inactivated S. aureus-induced interleukin-10 (IL-10) production (IL-10 is an anti-inflammatory cytokine which inhibits TNF-alpha production). Neutralization of IL-10 reduced TNF-alpha downregulation caused by GSK-3beta inhibition. These results suggest that GSK-3beta regulates heat-inactivated S. aureus-induced TNF-alpha and NO production in microglia mainly by activating NF-kappaB and probably by inhibiting IL-10.

Propofol inhibits lipoteichoic acid-induced iNOS gene expression in macrophages possibly through downregulation of toll-like receptor 2-mediated activation of Raf-MEK1/2-ERK1/2-IKK-NFkappaB

Our previous study showed that propofol suppressed Gram-negative bacterial LPS-induced NO biosynthesis. Lipoteichoic acid (LTA), an outer membrane component of Gram-positive bacteria, can induce septic shock. This study was further aimed to evaluate the effects of propofol on LTA-induced iNOS gene expression in macrophages and its possible molecular mechanisms. Exposure of macrophages to LTA increased production of nitrite and intracellular reactive oxygen species, but propofol reduced such enhancements in concentration- and time-dependent manners. Treatment of macrophages with LTA-induced iNOS mRNA and protein productions. Meanwhile, propofol at a clinically relevant concentration of 50 microM significantly inhibited LTA-caused augmentations of iNOS mRNA and protein syntheses. In parallel, exposure to LTA increased translocation of nuclear factor-kappa B (NFkappaB) from the cytoplasm to nuclei. Propofol at 50 microM decreased such translocation. Analyses by an electrophoretic mobility shift and reporter gene further showed that propofol could alleviate LTA-induced transactivation of NFkappaB. Sequentially, propofol decreased phosphorylation of IKK, ERK1/2, MEK1/2, and Raf in LTA-stimulated macrophages. Application of toll-like receptor 2 (TLR2) small interference (si)RNA decreased the translation of this receptor and Raf phosphorylation in LTA-stimulated macrophages. Co-treatment with propofol and TLR2 siRNA synergistically ameliorated LTA-induced iNOS mRNA expression and nitrite production. Thus, this study shows that propofol can downregulate NO biosynthesis via inhibiting iNOS gene expression. The suppressive mechanism occurs possibly through reduction of TLR2-mediated sequential activation of Raf-MEK1/2-ERK1/2-IKK-NFkappaB.

Chitin is a size-dependent regulator of macrophage TNF and IL-10 production

Chitin is a ubiquitous polysaccharide in fungi, insects, and parasites. We hypothesized that chitin is a size-dependent regulator of innate immunity. To test this hypothesis, we characterized the effects of chitins of different sizes on murine bronchoalveolar or peritoneal macrophages. In these studies, large chitin fragments were inert, while both intermediate-sized chitin (40-70 microm) and small chitin (SC; <40 microm, largely 2-10 microm) stimulated TNF elaboration. In contrast, only SC induced IL-10 elaboration. The effects of intermediate-sized chitin were mediated by pathways that involve TLR2, dectin-1, and NF-kappaB. In contrast, the effects of SC were mediated by TLR2-dependent and -independent, dectin-1-dependent pathways that involved the mannose receptor and spleen tyrosine kinase. Chitin contains size-dependent pathogen-associated molecular patterns that stimulate TLR2, dectin-1, and the mannose receptor, differentially activate NF-kappaB and spleen tyrosine kinase, and stimulate the production of pro- and anti-inflammatory cytokines.

Enhanced monocyte response and decreased central memory T cells in children with invasive Staphylococcus aureus infections

Staphylococcus aureus has emerged as a significant pathogen causing severe invasive disease in otherwise healthy people. Despite considerable advances in understanding the epidemiology, resistance mechanisms, and virulence factors produced by the bacteria, there is limited knowledge of the in vivo host immune response to acute, invasive S. aureus infections. Herein, we report that peripheral blood mononuclear cells from patients with severe S. aureus infections demonstrate a distinctive and robust gene expression profile which is validated in a distinct group of patients and on a different microarray platform. Application of a systems-wide modular analysis framework reveals significant over-expression of innate immunity genes and under-expression of genes related to adaptive immunity. Simultaneous flow cytometry analyses demonstrated marked alterations in immune cell numbers, with decreased central memory CD4 and CD8 T cells and increased numbers of monocytes. CD14+ monocyte numbers significantly correlated with the gene expression levels of genes related to the innate immune response. These results demonstrate the value of applying a systems biology approach that reveals the significant alterations in the components of circulating blood lymphocytes and monocytes in invasive S. aureus infections.

Molecular pathogenesis of Staphylococcus aureus infection

S. aureus has evolved a comprehensive strategy to address the challenges posed by the human immune system. The emergence of community-associated methicillin-resistant S. aureus (CA-MRSA) infections in individuals with no predisposing conditions suggests an increased pathogenicity of the bacterium, which may be related to acquisition of novel genetic elements. Remarkably, despite an abundance of research, the underlying cause of the epidemic is not known. Here, the various strategies used by S. aureus to evade obstacles laid out by the human host during colonization and infection were reviewed. The controversies surrounding MRSA research were described, and how acquisition of the novel genes could explain the increased incidence and severity of CA-MRSA diseases was described.

NOD2 plays an important role in the inflammatory responses of microglia and astrocytes to bacterial CNS pathogens

While glial cells are recognized for their roles in maintaining neuronal function, there is growing appreciation that resident central nervous system (CNS) cells initiate and/or augment inflammation following trauma or infection. We have recently demonstrated that microglia and astrocytes constitutively express nucleotide-binding oligomerization domain-2 (NOD2), a member of the novel nucleotide-binding domain leucine-rich repeat region containing a family of proteins (NLR) that functions as an intracellular receptor for a minimal motif present in all bacterial peptidoglycans. In this study, we have confirmed the functional nature of NOD2 expression in astrocytes and microglia and begun to determine the relative contribution that this NLR makes in inflammatory CNS responses to clinically relevant bacterial pathogens. We demonstrate the increased association of NOD2 with its downstream effector molecule, Rip2 kinase, in primary cultures of murine microglia and astrocytes following exposure to bacterial antigens. We show that this cytosolic receptor underlies the ability of muramyl dipeptide to augment the production of inflammatory cytokines by glia following exposure to specific ligands for disparate Toll-like receptor homologues. In addition, we demonstrate that NOD2 is an important component in the in vitro inflammatory responses of resident glia to N. meningitidis and B. burgdorferi antigens. Finally, we have established that NOD2 is required, at least in part, for the astrogliosis, demyelination, behavioral changes, and elevated inflammatory cytokine levels observed following in vivo infection with these pathogens. As such, we have identified NOD2 as an important component in the generation of damaging CNS inflammation following bacterial infection.

TLR2-induced calpain cleavage of epithelial junctional proteins facilitates leukocyte transmigration

Recruitment of polymorphonuclear leukocytes (PMNs) into the lungs in response to inhaled pathogens is initiated by epithelial signaling, the activation of toll-like receptors (TLRs), and the production of the chemokine interleukin-8. To reach the site of infection, PMNs must be mobilized through epithelial junctions. Here, we demonstrate that Ca(2+) fluxes generated by TLR2 signals activate calpains, Ca(2+)-dependent cysteine proteases. These activated calpains cleave the transmembrane junctional proteins occludin and E-cadherin without breaching the integrity of the epithelial barrier. Calpain inhibitors decrease PMN transepithelial migration in response to TLR2 agonists both in vitro and in a mouse model of P. aeruginosa infection. Thus, TLR2 signaling in the airway not only induces chemokine expression to recruit PMNs, but also initiates cleavage of junctional proteins to accommodate transmigration of the recruited PMNs.