Recognition of Staphylococcus aureus by the innate immune system

Pleiotropic roles of polyglycerolphosphate synthase of lipoteichoic acid in growth of Staphylococcus aureus cells

Lipoteichoic acid (LTA) is one of two anionic polymers on the surface of the gram-positive bacterium Staphylococcus aureus. LTA is critical for the bacterium-host cell interaction and has recently been shown to be required for cell growth and division. To determine additional biological roles of LTA, we found it necessary to identify permissive conditions for the growth of an LTA-deficient mutant. We found that an LTA-deficient S. aureus Delta ltaS mutant could grow at 30 degrees C but not at 37 degrees C. Even at the permissive temperature, Delta ltaS mutant cells had aberrant cell division and separation, decreased autolysis, and reduced levels of peptidoglycan hydrolases. Upshift of Delta ltaS mutant cells to a nonpermissive temperature caused an inability to exclude Sytox green dye. A high-osmolarity growth medium remarkably rescued the colony-forming ability of the Delta ltaS mutant at 37 degrees C, indicating that LTA synthesis is required for growth under low-osmolarity conditions. In addition, the Delta ltaS mutation was found to be synthetically lethal with the Delta tagO mutation, which disrupts the synthesis of the other anionic polymer, wall teichoic acid (WTA), at 30 degrees C, suggesting that LTA and WTA compensate for one another in an essential function.

Host polymorphisms in interleukin 4, complement factor H, and C-reactive protein associated with nasal carriage of Staphylococcus aureus and occurrence of boils

BACKGROUND

Staphylococcus aureus is capable of persistently colonizing the vestibulum nasi. We hypothesized that polymorphisms in host inflammatory response genes and genetic variation in S. aureus contribute to susceptibility to S. aureus carriage and infection.

METHODS

The prevalence of persistent nasal carriage of S. aureus in 3851 participants aged 61-101 years was 18% (678 of 3851 participants), whereas 73% of volunteers (2804 of 3851) were not colonized. A total of 1270 individuals had boils. Polymorphisms in TNFA (C -863T), IL4 (C -542T), CFH (Tyr402His), and CRP (C1184T, C2042T, and C2911G) were determined. Genetic similarity among 428 S. aureus isolates was determined by use of amplified fragment length polymorphism analysis (AFLP)-mediated genotyping.

RESULTS

The IL4 -524 C/C host genotype was associated with an increased risk of persistent S. aureus carriage, irrespective of S. aureus AFLP genotype. The CRP haplotype 1184C; 2042C; 2911C was overrepresented in individuals who were not colonized . In individuals with boils, carriers of the CFH Tyr402 variant, and the CRP 2911 C/C genotype were overrepresented.

CONCLUSION

Persistent carriage of S. aureus is influenced by genetic variation in host inflammatory response genes. As would be expected in multifactorial host-microbe interactions, these effects are limited. Interestingly, host genotype was associated with the carriage of certain S. aureus genotypes. Apparently, a close interaction between host and bacterial determinants are prerequisites for long-term colonization.

Human mast cell activation by Staphylococcus aureus: interleukin-8 and tumor necrosis factor alpha release and the role of Toll-like receptor 2 and CD48 molecules

The ability of Staphylococcus aureus to invade and survive within host cells is believed to contribute to its propensity to cause persistent and metastatic infections. In addition, S. aureus infections often are associated with atopic diseases such as dermatitis, rhinitis, and asthma. Mast cells, the key cells of allergic diseases, have a pivotal role in innate immunity and have the capacity of phagocytosis, and they can destroy some pathogenic bacteria. However, little is known about the ability of some other bacteria to survive and overcome mast cell phagocytosis. Therefore, we were interested in evaluating the interplay between mast cells and S. aureus. In this study, we show that human cord blood-derived mast cells (CBMC) can be infected by pathogenic S. aureus. S. aureus displayed a high adherence to mast cells as well as invasive and survival abilities within them. However, when infections were performed in the presence of cytochalasin D or when CBMC were preincubated with anti-Toll-like receptor 2 (TLR2) or anti-CD48 antibodies, the invasiveness and the inflammatory response were abrogated, respectively. Furthermore, we observed an increase of TLR2 and CD48 molecules on CBMC after S. aureus infection. The infection of CBMC with S. aureus also caused the release of tumor necrosis factor alpha (TNF-alpha) and interleukin-8 (IL-8). Both live and killed S. aureus organisms were found to trigger TNF-alpha and IL-8 release by CBMC in a time-dependent manner. Cumulatively, these findings suggest that S. aureus internalizes and survives in mast cells. This may play an important role in infections and in atopic diseases associated with S. aureus.

Alexidine and chlorhexidine bind to lipopolysaccharide and lipoteichoic acid and prevent cell activation by antibiotics

OBJECTIVES

Many antibiotics used to treat infections cause release of immunostimulatory cell wall components from bacteria. Therefore, a combination of antimicrobial and endotoxin-neutralizing activity is desired to prevent inflammation induced by destroyed bacteria. Chlorhexidine and alexidine are amphipathic bisbiguanides and could neutralize bacterial membrane components as stimulators of Toll-like receptors (TLRs).

METHODS

Binding of chlorhexidine and alexidine to lipopolysaccharide (LPS) and lipoteichoic acid (LTA) was determined by fluorescence displacement assay and isothermal calorimetric titration. Neutralization of the biological effect of LPS and LTA on TLR-activated cellular activation was determined by NF-kappaB reporter luciferase activation on cells transfected with specific TLRs and NO production of murine macrophages in the presence of isolated agonists and antibiotic-treated bacteria.

RESULTS

Alexidine and chlorhexidine bind not only to LPS but also to LTA from Gram-positive bacteria. Alexidine has a higher affinity than chlorhexidine for both compounds. Calorimetric titration shows an initial endothermic contribution indicating participation of hydrophobic interactions in LPS binding, while binding to LTA displayed initial exothermic contribution. Both compounds prevent cell activation of TLR4 and TLR2 by LPS and LTA, respectively. The addition of both compounds suppressed NO production by macrophages in the presence of bacteria treated with different types of antibiotics.

CONCLUSIONS

Chlorhexidine and alexidine suppress bacterial membrane-induced cell activation at concentrations two orders of magnitude lower than that used in topical applications. Combining biocides with different types of antibiotics prevented macrophage activation in the presence of bacteria and demonstrated the potential of chlorhexidine and alexidine to suppress inflammatory responses caused by activation of TLRs.

Dual mechanism of bacterial lethality for a cationic sequence-random copolymer that mimics host-defense antimicrobial peptides

Flexible sequence-random polymers containing cationic and lipophilic subunits that act as functional mimics of host-defense peptides have recently been reported. We used bacteria and lipid vesicles to study one such polymer, having an average length of 21 residues, that is active against both Gram-positive and Gram-negative bacteria. At low concentrations, this polymer is able to permeabilize model anionic membranes that mimic the lipid composition of Escherichia coli, Staphylococcus aureus, or Bacillus subtilis but is ineffective against model zwitterionic membranes, which explains its low hemolytic activity. The polymer is capable of binding to negatively charged vesicles, inducing segregation of anionic lipids. The appearance of anionic lipid-rich domains results in formation of phase-boundary defects through which leakage can occur. We had earlier proposed such a mechanism of membrane disruption for another antimicrobial agent. Experiments with the mutant E. coli ML-35p indicate that permeabilization is biphasic: at low concentrations, the polymer permeabilizes the outer and inner membranes; at higher polymer concentrations, permeabilization of the outer membrane is progressively diminished, while the inner membrane remains unaffected. Experiments with wild-type E. coli K12 show that the polymer blocks passage of solutes into the intermembrane space at high concentrations. Cell membrane integrity in E. coli K12 and S. aureus exhibits biphasic dependence on polymer concentration. Isothermal titration calorimetry indicates that the polymer associates with the negatively charged lipopolysaccharide of Gram-negative bacteria and with the lipoteichoic acid of Gram-positive bacteria. We propose that this polymer has two mechanisms of antibacterial action, one predominating at low concentrations of polymer and the other predominating at high concentrations.

Different bacterial gene expression patterns and attenuated host immune responses are associated with the evolution of low-level vancomycin resistance during persistent methicillin-resistant Staphylococcus aureus bacteraemia

BACKGROUND

Low-level vancomycin resistance in Staphylococcus aureus (vancomycin-intermediate S. aureus (VISA) and hetero-VISA [hVISA]) emerges during persistent infection and failed vancomycin therapy. Up-regulation of genes associated with the "cell wall stimulon" and mutations in the vraSR operon have both been implicated in the development of resistance, however the molecular mechanisms of resistance are not completely understood. To further elucidate the mechanisms leading to resistance transcriptome comparisons were performed using multiple clinical pairs of vancomycin-susceptible S. aureus (VSSA) and hVISA/VISA (n = 5), and three VSSA control pairs from hospitalized patients with persistent bacteraemia that did not develop hVISA/VISA. Based on the transcriptome results multiple genes were sequenced and innate immune system stimulation was assessed in the VSSA and hVISA/VISA pairs.

RESULTS

Here we show that up-regulation of vraS and the "cell wall stimulon" is not essential for acquisition of low-level vancomycin resistance and that different transcriptional responses occur, even between closely related hVISA/VISA strains. DNA sequencing of vraSR, saeSR, mgrA, rot, and merR regulatory genes and upstream regions did not reveal any differences between VSSA and hVISA/VISA despite transcriptional changes suggesting mutations in these loci may be linked to resistance in these strains. Enhanced capsule production and reduced protein A expression in hVISA/VISA were confirmed by independent bioassays and fully supported the transcriptome data. None of these changes were observed in the three control pairs that remained vancomycin-susceptible during persistent bacteremia. In a macrophage model of infection the changes in cell surface structures in hVISA/VISA strains were associated with significantly reduced NF-kappaB activation resulting in reduced TNF-alpha and IL-1beta expression.

CONCLUSION

We conclude that there are multiple pathways to low-level vancomycin resistance in S. aureus, even among closely related clinical strains, and these can result in an attenuated host immune response. The persistent infections associated with hVISA/VISA strains may be a consequence of changes in host pathogen interactions in addition to the reduced antibiotic susceptibility.

Noninvasive optical imaging of staphylococcus aureus bacterial infection in living mice using a Bis-dipicolylamine-Zinc(II) affinity group conjugated to a near-infrared fluorophore

Optical imaging of bacterial infection in living animals is usually conducted with genetic reporters such as light-emitting enzymes or fluorescent proteins. However, there are many circumstances where genetic reporters are not applicable, and there is a need for exogenous synthetic probes that can selectively target bacteria. The focus of this study is a fluorescent imaging probe that is composed of a bacterial affinity group conjugated to a near-infrared dye. The affinity group is a synthetic zinc (II) coordination complex that targets the anionic surfaces of bacterial cells. The probe allows detection of Staphylococcus aureus infection (5 x 10 (7) cells) in a mouse leg infection model using whole animal near-infrared fluorescence imaging. Region of interest analysis showed that the signal ratio for infected leg to uninfected leg reaches 3.9 +/- 0.5 at 21 h postinjection of the probe. Ex vivo imaging of the organs produced a signal ratio of 8 for infected to uninfected leg. Immunohistochemical analysis confirmed that the probe targeted the bacterial cells in the infected tissue. Optimization of the imaging filter set lowered the background signal due to autofluorescence and substantially improved imaging contrast. The study shows that near-infrared molecular probes are amenable to noninvasive optical imaging of localized S. aureus infection.

Microbial patterns signaling via Toll-like receptors 2 and 5 contribute to epithelial repair, growth and survival

Epithelial cells (ECs) continuously interact with microorganisms and detect their presence via different pattern-recognition receptors (PRRs) including Toll-like receptors (TLRs). Ligation of epithelial TLRs by pathogens is usually associated with the induction of pro-inflammatory mediators and antimicrobial factors. In this study, using human airway ECs as a model, we found that detection of microbial patterns via epithelial TLRs directly regulates tissue homeostasis. Staphylococcus aureus (S. aureus) and microbial patterns signaling via TLR2 and TLR5 induce a set of non-immune epithelial responses including cell migration, wound repair, proliferation, and survival of primary and cancerous ECs. Using small interfering RNA (siRNA) gene targeting, receptor-tyrosine kinase microarray and inhibition studies, we determined that TLR and the epidermal growth factor receptor (EGFR) mediate the stimulating effect of microbial patterns on epithelial repair. Microbial patterns signaling via Toll-like receptors 2 and 5 contribute to epithelial repair, growth and survival. This effect is independent of hematopoietic and other cells as well as inflammatory cytokines suggesting that epithelia are able to regulate their integrity in an autonomous non-inflammatory manner by sensing microbes directly via TLRs.

Airway epithelial cell signaling in response to bacterial pathogens

The airway epithelium represents a primary site for the introduction and deposition of potentially pathogenic microorganisms into the body, through inspired air. The epithelial mucosa is an important component of the innate immune system that recognizes conserved structures in microorganisms and initiates appropriate signaling to recruit and activate phagocytic cells to the airways. This review focuses on how airway epithelial cells sense and respond to the presence of bacterial pathogens. The major signaling cascades initiated by epithelial receptors that lead to phagocyte recruitment to the airways as well as the ability of the epithelium to regulate inflammation are discussed.

Mitochondrial biogenesis restores oxidative metabolism during Staphylococcus aureus sepsis

RATIONALE

The extent, timing, and significance of mitochondrial injury and recovery in bacterial sepsis are poorly characterized, although oxidative and nitrosative mitochondrial damage have been implicated in the development of organ failure.

OBJECTIVES

To define the relationships between mitochondrial biogenesis, oxidative metabolism, and recovery from Staphylococcus aureus sepsis.

METHODS

We developed a murine model of fibrin clot peritonitis, using S. aureus. The model yielded dose-dependent decreases in survival and resting energy expenditure, allowing us to study recovery from sublethal sepsis.

MEASUREMENTS AND MAIN RESULTS

Peritonitis caused by 10(6) colony-forming units of S. aureus induced a low tumor necrosis factor-alpha state and minimal hepatic cell death, but activated prosurvival protein kinase A, B, and C sequentially over 3 days. Basal metabolism by indirect calorimetry was depressed because of selective mitochondrial oxidative stress and subsequent loss of mitochondrial DNA copy number. During recovery, mitochondrial biogenesis was strongly activated by regulated expression of the requisite nuclear respiratory factors 1 and 2 and the coactivator peroxisome proliferator-activated receptor gamma coactivator-1alpha, as well as by repression of the biogenesis suppressor nuclear receptor interacting protein-140. Biogenesis reconstituted mitochondrial DNA copy number and transcription, and restored basal metabolism without significant hepatocellular proliferation. These events dramatically increased hepatic mitochondrial density in transgenic mice expressing mitochondrially targeted green fluorescent protein.

CONCLUSIONS

This is the first demonstration that mitochondrial biogenesis restores oxidative metabolism in bacterial sepsis and is therefore an early and important prosurvival factor.

Syk-dependent ERK activation regulates IL-2 and IL-10 production by DC stimulated with zymosan

Zymosan is a particulate yeast preparation that elicits high levels of IL-2 and IL-10 from dendritic cells (DC) and engages multiple innate receptors, including the Syk-coupled receptor dectin-1 and the MyD88-coupled receptor TLR2. Here, we show that induction of IL-2 and IL-10 by zymosan requires activation of ERK MAP kinase in murine DC. Surprisingly, ERK activation in response to zymosan is completely blocked in Syk-deficient DC and unaffected by MyD88 deficiency. Conversely, ERK activation in response to the TLR2 agonist Pam3Cys is completely MyD88 dependent and unaffected by Syk deficiency. The inability of TLR2 ligands in zymosan to couple to ERK may explain the Syk dependence of the IL-2 and IL-10 response in DC and emphasises the importance of Syk-coupled pattern recognition receptors such as dectin-1 in the detection of yeasts. Furthermore, the lack of receptor compensation observed here suggests that responses induced by complex innate stimuli cannot always be predicted by the signalling pathways downstream of individual receptors.

The anti-inflammatory activities of Staphylococcus aureus

Staphylococcus aureus is a versatile and harmful pathogen in both hospital- and community-associated infections that range from superficial to systemic infections. S. aureus engages a multitude of mechanisms to subvert the innate immune response of the host, including inhibition of complement activation and neutralization of anti-microbial peptides. In addition, inflammatory cell and phagocyte recruitment is an integral part of the innate defense to staphylococcal infection and comprises a well-coordinated multi-step cascade of adhesive events. Recent and rapidly growing experimental evidence indicates the existence of a machinery of anti-adhesive and anti-chemotactic moieties of S. aureus that allow the bacterium to interfere with specific adhesive steps of the homing mechanism of leukocytes. Understanding the functions of these S. aureus-derived anti-inflammatory agents could also provide the platform for designing new therapies in several inflammatory and autoimmune diseases.

Role of sugars in surface microbe?host interactions and immune reaction modulation

Sugars in the form of monosaccharides, oligosaccharides, polysaccharides and glycoconjugates (glycoproteins, glycolipids) are vital components of infecting microbes and host cells, and are involved in cell signalling associated with modulation of inflammation in all integumental structures. Indeed, sugars are the molecules most commonly involved in cell recognition and communication. In skin, they are essential to epidermal development and homeostasis. They play important roles in microbial adherence, colonization and biofilm formation, and in virulence. Two groups of pathogen recognition receptors, C-type lectins (CTL) and their receptors (CTLR), and the Toll-like receptors enable the host to recognize pathogen-associated molecular patterns (PAMPs), which are mainly glycolipids. The CTLs can recognize a wide variety of bacteria, fungi and parasites and are important in phagocytosis and endocytosis. TLRs are expressed on the surfaces of a variety of cells, including keratinocytes, dendritic cells, monocytes and macrophages; they play a major role in innate immunity. Interaction of TLRs with PAMPs initiates a cascade of events leading to production of reactive oxygen intermediates, cytokines and chemokines, and promotes inflammation. Exogenous sugars can block carbohydrate receptors and competitively displace bacteria from attachment to cells, including keratinocytes. Thus sugars may provide valuable adjunctive anti-inflammatory and/or antimicrobial treatment. A promising approach is the use of a panel of carbohydrate derivatives with anti-adhesive efficacy against bacteria frequently involved in diseases affecting skin and other epithelia. More complete characterization of sugar receptors and their ligands will provide further keys to use of carbohydrates in immunomodulation and infection control in skin.

Paired Ig-like receptors bind to bacteria and shape TLR-mediated cytokine production

The innate immune system uses a wide variety of pattern recognition receptors including TLRs, scavenger receptors, and lectins to identify potential pathogens. A carefully regulated balance between activation and inhibition must be kept to avoid detrimental and inappropriate inflammatory responses. In this study, we identify murine-paired Ig-like receptor (PIR)-B, and its human orthologs Ig-like transcript 2 and Ig-like transcript 5 as novel receptors for Staphylococcus aureus. PIR-B contains four ITIM motifs and is thought to be an inhibitory receptor. Expression of these receptors enables NIH3T3 cells to bind S. aureus. In mouse bone marrow-derived macrophages, masking of PIR-B by anti-PIR mAb or genetic deletion of PIR-B shows significantly impaired recognition of S. aureus and enhanced TLR-mediated inflammatory responses to the bacteria. These data suggest a novel mechanism for innate immune regulation by paired Ig-like receptor family members.

Innate immune response of bovine mammary gland to pathogenic bacteria responsible for mastitis

Mastitis (mammary gland inflammation) is one of the most important bovine diseases causing economic losses to dairy producers. Mammary gland inflammation is a consequence of the activity of a number of cell and soluble factors that function together to eliminate invading microorganisms. The factors involved in this inflammatory response differ depending on the infectious agent. This review analyzes the factors involved in the immunologic mechanisms against the main pathogenic bacteria causing mastitis, and emphasizes the innate immune response of the mammary gland. Knowledge, at the molecular level, of the mammary gland immune response during infection by pathogenic bacteria is fundamental to the design of effective therapies to control and eradicate bovine mastitis.

Cell wall assembly in Bacillus subtilis: how spirals and spaces challenge paradigms

Although the bacterial cell wall has been the subject of decades of investigation, recent studies continue to generate novel and controversial models of its synthesis and assembly. Here we compare and contrast the transcompartmental biosyntheses of peptidoglycan and teichoic acid in Bacillus subtilis. In addition, the current paradigms of B. subtilis wall assembly and structure are distinguished from emerging models of murein insertion and organization. We discuss evidence for the directed, cytoskeleton-dependent insertion of nascent peptidoglycan and the existence of a periplasmic compartment. Furthermore, we summarize the challenges these findings represent to the existing paradigm of murein insertion. Finally, motivated by these new developments, we discuss outstanding issues that remain to be addressed and suggest research directions that may contribute to a better understanding of cell wall assembly in B. subtilis.

Mammalian antimicrobial proteins and peptides: overview on the RNase A superfamily members involved in innate host defence

The review starts with a general outlook of the main mechanisms of action of antimicrobial proteins and peptides, with the final aim of understanding the biological function of antimicrobial RNases, and identifying the key events that account for their selective properties. Although most antibacterial proteins and peptides do display a wide-range spectrum of action, with a cytotoxic activity against bacteria, fungi, eukaryotic parasites and viruses, we have only focused on their bactericidal activity. We start with a detailed description of the main distinctive structural features of the bacteria target and on the polypeptides, which act as selective host defence weapons.Following, we include an overview of all the current available information on the mammalian RNases which display an antimicrobial activity. There is a wealth of information on the structural, catalytic mechanism and evolutionary relationships of the RNase A superfamily. The bovine pancreatic RNase A (RNase A), the reference member of the mammalian RNase family, has been the main research object of several Nobel laureates in the 60s, 70s and 80s. A potential antimicrobial function was only recently suggested for several members of this family. In fact, the recent evolutionary studies indicate that this protein family may have started off with a host defence function. Antimicrobial RNases constitute an interesting example of proteins involved in the mammalian innate immune defence system. Besides, there is wealth of available information on the mechanism of action of short antimicrobial peptides, but little is known on larger polypeptides, that is, on proteins. Therefore, the identification of the mechanisms of action of antimicrobial RNases would contribute to the understanding of the proteins involved in the innate immunity.

Muropeptide modification-amidation of peptidoglycan D-glutamate does not affect the proinflammatory activity of Staphylococcus aureus

Peptidoglycan muropeptides, potent proinflammatory components, are amidated in Staphylococcus aureus for unknown reasons. To study whether this modification may modulate proinflammatory capacity, cytokine induction by isogenic S. aureus strains with different amidation levels and by synthetic amidated/nonamidated muramyldipeptides was evaluated. However, amidation did not significantly affect cytokine induction. This finding contributes to defining peptidoglycan receptor specificities and indicates that further rationales for muropeptide amidation have to be considered.

Staphylococcus aureus protein A induced inflammatory response in human corneal epithelial cells

In the present study, we examined the role of Staphylococcus aureus protein A (SpA) in inducing inflammatory response in human corneal epithelial cells (HCECs). Exposure of HCECs to SpA induces rapid NF-kappaB activation and secretion of proinflammatory cytokine/chemokines (TNF-alpha and IL-8) in both concentration and time-dependent manner. Challenge of HCECs with live SpA(-/-) mutant S. aureus strains resulted in significantly reduced production of the cytokines when compared to the wild-type S. aureus strain. SpA also elicited the activation of MAP Kinases P38, ERK, but not JNK, in HCECs. SpA-induced production of proinflammatory cytokine were completely blocked by the NF-kappaB and p38 inhibitors and partially inhibited by the Jnk inhibitor. Pretreatment with anti-TLR2 neutralizing antibody had no effect on SpA-induced inflammatory response in HCECs, suggesting that this response is independent of TLR2 signaling. Moreover, unlike TLR2 ligands, SpA failed to induce the expression of antimicrobial peptides (hBD2 and LL-37) in HCECs. These studies indicate that SpA is a S. aureus virulence factor that stimulates HCEC inflammatory response through a pathway distinct from TLR2 in HCECs.

Increased levels of circulating soluble CD14 but not CD83 in infants are associated with early intestinal colonization with Staphylococcus aureus

BACKGROUND

Soluble forms of the monocyte marker CD14 and the mature dendritic cell marker CD83 are plasma proteins with immunoregulatory functions. The physiological stimulus for their production is unclear and their possible role in allergy development is unknown.

METHODS

We measured the plasma levels of soluble CD14 (sCD14) and soluble CD83 (sCD83) in 64 Swedish children in relation to intestinal bacterial colonization pattern in a prospective birth cohort. Soluble CD14 and sCD83 levels were quantified by enzyme linked immunosorbent assay in plasma obtained at birth and at 4, 18 and 36 months of age. All major aerobic and anaerobic bacteria were quantified in faecal samples obtained regularly over the first 8 weeks of life. Clinical allergy and IgE levels were evaluated at 18 months of age.

RESULTS

Soluble CD14 in plasma increased during the first 18 months of life while sCD83 peaked at 4 months of age. Children who were perinatally colonized with Staphylococcus aureus had significantly higher levels of sCD14 in plasma at 4 months of age relative to non-colonized children. The levels of sCD14 were unrelated to colonization with Escherichia coli, other enterobacteria, enterococci, clostridia, Bacteroides, bifidobacteria or lactobacilli. Further, children with food allergy by 18 months tended to have lower levels of sCD14 than healthy children. Plasma levels of sCD83 were not related to either bacterial colonization pattern or allergy development.

CONCLUSIONS

Perinatal colonization with S. aureus may trigger the occurrence of sCD14 in plasma, which may influence development of the infantile immune system and risk of allergy development.

Contribution of Toll-like receptors to the innate immune response to Gram-negative and Gram-positive bacteria

Innate recognition of bacteria is a key step in the activation of inflammation and coagulation, and it is dependent on pathogen-associated molecular pattern (PAMP) ligation to Toll-like receptors (TLRs) and CD14. The dominant receptors activated when cells encounter a whole bacterium, which express several PAMPs, are poorly defined. Herein, we have stimulated various human cells with prototypic Gram-negative and Gram-positive bacteria. Receptor-dependent responses to whole bacteria were assessed using both TLR-transfected cells and specific monoclonal antibodies against TLRs, MD-2, and CD14. Enterobacteria-activated leukocytes and endothelial cells in a TLR4/MD-2-dependent manner, most likely via lipopolysaccharide (LPS). TLR2 activation was observed with a high bacterial inoculum, and in epithelial cells expressing TLR2 but not TLR4. Pseudomonas aeruginosa stimulated cells by both TLR2 and TLR4/MD-2. Gram-positive bacteria activated cells only at high concentrations, in a partially TLR2-dependent but TLR4/MD-2-independent manner. Either TLR or CD14 neutralization blocked activation to all bacterial strains tested with the exception of some Gram-positive strains in whole blood in which partial inhibition was noted. This study identifies dominant TLRs involved in responses to whole bacteria. It also validates the concept that host cell activation by bacterial pathogens can be therapeutically reduced by anti-TLR4, -TLR2, and -CD14 mAbs.

Staphylococcal colonization and infection: homeostasis versus disbalance of human (innate) immunity and bacterial virulence

PURPOSE OF REVIEW

This review identifies trends in the study of interactions between Staphylococcus aureus and humans. When nasal colonization is in the neutral state, infection clearly represents a state of host-pathogen disbalance. The features leading from apparent homeostatic colonization to pathogenesis are identified at an increasing rate.

RECENT FINDINGS

Persistent carriage of S. aureus predisposes to infection but limits bacteraemia-associated mortality. Intermittent carriage is usually imposed and of lesser clinical relevance. The nature and function of several staphylococcal virulence factors have been elucidated and near complete gene catalogues have been established. There does not seem to be a difference in virulence, however, between methicillin-susceptible and resistant S. aureus. Biological selection takes place in the nose and innate immune features relevant to colonization have been discovered. Acquired immunity remains underexposed, but the host factors involved in the host-pathogen interaction have been identified.

SUMMARY

Virulence assessment of S. aureus has been facilitated by novel technology: genome-wide inventories of virulence potential can be made and new pathogenic mechanisms have been presumptively identified. These involve invasion procedures but also (innate) immune evasion strategies. These cross-fertilizing developments shed light on the feasibility of novel prophylactic or therapeutic strategies for combating staphylococcal carriage and disease.

Staphyloxanthin plays a role in the fitness of Staphylococcus aureus and its ability to cope with oxidative stress

Staphyloxanthin is a membrane-bound carotenoid of Staphylococcus aureus. Here we studied the interaction of staphyloxanthin with reactive oxygen substances (ROS) and showed by comparative analysis of the wild type (WT) and an isogenic crtM mutant that the WT is more resistant to hydrogen peroxide, superoxide radical, hydroxyl radical, hypochloride, and neutrophil killing.

CD4+ T cells and CXC chemokines modulate the pathogenesis of Staphylococcus aureus wound infections

T cells are critical for the formation of intraabdominal abscesses by Staphylococcus aureus. We hypothesized that T cells modulate the development of experimental staphylococcal infections by controlling polymorphonuclear leukocyte (PMN) trafficking. In models of staphylococcal s.c. abscess formation, hindpaw infection, and surgical wound infection, S. aureus multiplied in the tissues of WT C57BL/6J mice and elicited a marked inflammatory response. CD4(+) alphabeta T cells homed to the surgical wound infection site of WT animals. In contrast, significantly fewer S. aureus were recovered from the tissues of mice deficient in alphabeta T cells, and the inflammatory response was considerably diminished compared with that of WT animals. Alphabeta T cell receptor (-/-) mice had significantly lower concentrations of PMN-specific CXC chemokines in wound tissue than did WT mice. The severity of the wound infection was enhanced by administration of a CXC chemokine and abrogated by antibodies that blocked the CXC receptor. An acapsular mutant was less virulent than the parental S. aureus strain in both the s.c. abscess and the surgical wound infection models in WT mice. These data reveal an important and underappreciated role for CD4(+) alphabeta T cells in S. aureus infections in controlling local CXC chemokine production, neutrophil recruitment to the site of infection, and subsequent bacterial replication.

Effect of d-Alanylation of (Lipo)Teichoic Acids ofStaphylococcus aureuson Host Secretory Phospholipase A2Action before and after Phagocytosis by Human Neutrophils

Invading bacteria such as Staphylococcus aureus induce mobilization of professional phagocytes (e.g., neutrophils) and extracellular antibacterial proteins (e.g., group IIA phospholipase A2 (gIIA PLA2)). Accumulation of gIIA PLA2 in inflammatory fluids confers potent extracellular antistaphylococcal activity and at lower concentrations promotes bacterial phospholipid degradation during phagocytosis of S. aureus by human neutrophils. D-alanylation of (lipo) teichoic acids of S. aureus increases bacterial resistance to gIIA PLA2 approximately 100-fold, raising the possibility that the resistance of ingested S. aureus to related gV and gX secretory PLA2 present in human neutrophil granules depends on D-alanylation mediated by the dlt operon. However, we show that isogenic wild-type and dltA S. aureus are equally resistant to gV/X PLA2 during phagocytosis and when exposed to the purified enzymes. The fates of wild-type and dltA S. aureus exposed to serum and human neutrophils differed significantly only when extracellular gIIA PLA2 was also present before phagocytosis. The extreme potency of the gIIA PLA2 toward dltA S. aureus suggests that even small amounts of this extracellular enzyme mobilized early in inflammation could contribute substantially to the overall cytotoxicity of acute inflammatory exudates toward S. aureus when D-alanylation of (lipo)teichoic acids is limiting.

Virulence determinants in Staphylococcus aureus and their involvement in clinical syndromes

Staphylococcus aureus is a major human pathogen responsible for a variety of toxin-mediated and suppurative diseases. About 50 staphylococcal virulence factors have been described to date. In this review, we examine the clinical implications of key staphylococcal virulence factors in toxin-mediated diseases, septic shock, and severe focal infections such as arthritis, infective endocarditis, pneumonia acquired during mechanical ventilation, and necrotizing pneumonia. Staphylococcal pathogenicity is sometimes due principally to a single virulence factor, as in toxic shock syndrome and necrotizing pneumonia. In contrast, several virulence factors are involved in other staphylococcal disease, such as septic shock. A better knowledge of the mechanism of action of each virulence factor involved in the different staphylococcal diseases could open the way to the use of specific inhibitors in the clinical setting.

Both IL-12p70 and IL-23 are synthesized during active Crohn's disease and are down-regulated by treatment with anti-IL-12 p40 monoclonal antibody

BACKGROUND

Interleukin (IL)-12p70 and IL-23 are key T helper-1 (TH1) cytokines that drive the inflammation seen in numerous models of intestinal inflammation. These molecules contain an identical p40 chain that is bound to a p35 chain in IL-12 and a p19 chain in IL-23, making both potentially susceptible to modulation by an anti-IL-12p40 monoclonal antibody (mAb).

METHODS

In the present study, we sought to determine whether active inflammation in Crohn's disease (CD) is associated with the increased synthesis of both of these cytokines and whether patients treated with an anti-IL-12p40 mAb down-regulate IL-23 as well as IL-12p70 as previous reported.

RESULTS

To this end we initially determined that IL-12p70 secretion by control and CD antigen-presenting cells (macrophages) in lamina propria mononuclear populations is optimized by stimulation with CD40L and interferon-gamma. In subsequent studies using these stimulation conditions we found that patients with CD manifested both increased IL-12p70 and IL-23 secretion before anti-IL-12p40 mAb treatment and normal levels of secretion of these cytokines following cessation of treatment. Antigen-presenting cells in lamina propria mononuclear cells from ulcerative colitis patients, in contrast, produced only baseline levels of IL-23. Finally, we found that IL-23-induced T cell production of IL-17 and IL-6 are also greatly reduced after antibody treatment. The latter data are parallel to those from previous studies showing that anti-IL-12p40 down-regulates IFN-gamma and tumor necrosis factor-alpha secretion.

CONCLUSIONS

We conclude that CD but not ulcerative colitis is associated with high levels of both IL-12p70 and IL-23 secretion as well as the secretion of downstream effector cytokines, and that this cytokine production is down-regulated following administration of IL-12p40 mAb.

NOD2/CARD15 mediates induction of the antimicrobial peptide human beta-defensin-2

Production of inducible antimicrobial peptides offers a first and rapid defense response of epithelial cells against invading microbes. Human beta-defensin-2 (hBD-2) is an antimicrobial peptide induced in various epithelia upon extracellular as well as intracellular bacterial challenge. Nucleotide-binding oligomerization domain protein 2 (NOD2/CARD15) is a cytosolic protein involved in intracellular recognition of microbes by sensing peptidoglycan fragments (e.g. muramyl dipeptide). We used luciferase as a reporter gene for a 2.3-kb hBD-2 promoter to test the hypothesis that NOD2 mediates the induction of hBD-2. Activation of NOD2 in NOD2-overexpressing human embryonic kidney 293 cells through its ligand muramyl dipeptide (MDP) induced hBD-2 expression. In contrast, overexpression of NOD2 containing the 3020insC frame-shift mutation, the most frequent NOD2 variant associated with Crohn disease, resulted in defective induction of hBD-2 through MDP. Luciferase gene reporter analyses and site-directed mutagenesis experiments demonstrated that functional binding sites for NF-kappaB and AP-1 in the hBD-2 promoter are required for NOD2-mediated induction of hBD-2 through MDP. Moreover, the NF-kappaB inhibitor Helenalin as well as a super-repressor form of the NF-kappaB inhibitor IkappaB strongly inhibited NOD2-mediated hBD-2 promoter activation. Expression of NOD2 was detected in primary keratinocytes, and stimulation of these cells with MDP induced hBD-2 peptide release. In contrast, small interference RNA-mediated down-regulation of NOD2 expression in primary keratinocytes resulted in a defective induction of hBD-2 upon MDP treatment. Together, these data suggest that NOD2 serves as an intracellular pattern recognition receptor to enhance host defense by inducing the production of antimicrobial peptides such as hBD-2.