Chemotaxis inhibitory protein of Staphylococcus aureus, a bacterial antiinflammatory agent

Characterisation of receptor binding by the chemotaxis inhibitory protein of Staphylococcus aureus and the effects of the host immune response

The chemotaxis inhibitory protein of Staphylococcus aureus (CHIPS) is reported to bind to the receptors for C5a and formylated peptides and has been proposed as a promising lead for the development of new anti-inflammatory compounds. Here we have examined the receptor specificity and mode of action of recombinant CHIPS_28–149 and also the immune response to CHIPS_28–149 in patients with S. aureus infections and in uninfected controls. Recombinant CHIPS_28–149 bound with high affinity to the human C5a receptor (C5aR), but had low affinity for the second C5a receptor, C5L2, and the formyl peptide receptor, FPR. Although ligand binding to C5aR was potently inhibited, CHIPS_28–149 had much weaker effects on ligand binding to C5L2 and FPR. Similarly, CHIPS_28–149 potently inhibited the ligand-induced activation of C5aR but was less potent at inhibition via FPR. NMR studies showed that CHIPS_28–149 bound directly to the N-terminus of C5aR but not C5L2, and CHIPS_28–149 residues involved in the interaction were identified by chemical shift analysis. All human sera examined contained high titres of IgG and IgA reactivity against CHIPS_28–149, and no correlation was observed between infection status at the time of serum collection and antibody titre. Individual serum samples promoted or inhibited the binding of CHIPS_28–149 to C5aR, or had no effect. IgG depletion of serum samples abrogated the effects on CHIPS binding, demonstrating that these were antibody mediated. Sera from infected individuals were more likely to inhibit CHIPS_28–149 binding than sera from healthy controls. However, high antibody titres correlated well with both inhibition and enhancement of CHIPS_28–149 binding to C5aR; this suggests that the inhibitory effect relates to epitope specificity rather than greater antibody binding. We conclude that CHIPS is likely to be too immunogenic to be used as an anti-inflammatory treatment but that some antibodies against CHIPS may be useful in the treatment of S. aureus infections.

A new staphylococcal anti-inflammatory protein that antagonizes the formyl peptide receptor-like 1

Bacteria have developed mechanisms to escape the first line of host defense, which is constituted by the recruitment of phagocytes to the sites of bacterial invasion. We previously described the chemotaxis inhibitory protein of Staphylococcus aureus, a protein that blocks the activation of neutrophils via the formyl peptide receptor (FPR) and C5aR. We now describe a new protein from S. aureus that impaired the neutrophil responses to FPR-like1 (FPRL1) agonists. FPRL1 inhibitory protein (FLIPr) inhibited the calcium mobilization in neutrophils stimulated with MMK-1, WKYMVM, prion-protein fragment PrP(106-126), and amyloid beta(1-42). Stimulation with low concentrations of fMLP was partly inhibited. Directed migration was also completely prevented toward MMK-1 and partly toward fMLP. Fluorescence-labeled FLIPr efficiently bound to neutrophils, monocytes, B cells, and NK cells. HEK293 cells transfected with human C5aR, FPR, FPRL1, and FPRL2 clearly showed that FLIPr directly bound to FPRL1 and, at higher concentrations, also to FPR but not to C5aR and FPRL2. FLIPr can reveal unknown inflammatory ligands crucial during S. aureus infections. As a novel described FPRL1 antagonist, it might lead to the development of therapeutic agents in FPRL1-mediated inflammatory components of diseases such as systemic amyloidosis, Alzheimer's, and prion disease.

Staphylococcal superantigen-like 5 binds PSGL-1 and inhibits P-selectin–mediated neutrophil rolling

Staphylococcus aureus secretes several virulence factors interfering with host-cell functions. Staphylococcal superantigen-like (SSL) proteins are a family of 11 exotoxins with structural homology to superantigens but with generally unknown functions. Recently, we described that chemotaxis inhibitory protein of Staphylococcus aureus (CHIPS31-121), a potent inhibitor of C5a-induced responses, is structurally homologous to the C-terminal domain of SSL5. Here, we identify P-selectin glycoprotein ligand-1 (PSGL-1), involved in the initial rolling of neutrophils along the endothelium, as a target for SSL5. SSL5 specifically bound to Chinese hamster ovary cells stably expressing PSGL-1 (CHO–PSGL-1), which was dependent of sulfation and sialylation. Furthermore, SSL5 bound to PSGL-1/Ig fusion protein immobilized on a biosensor chip. SSL5 affected binding of soluble P-selectin/Fc chimera, the principle ligand of PSGL-1, to CHO–PSGL-1 cells and inhibited adhesion of neutrophils to immobilized P-selectin under static conditions. Under flow conditions SSL5 strongly decreased neutrophil rolling on immobilized P-selectin/Fc and activated human endothelial cells. In conclusion, SSL5 interferes with the interaction between PSGL-1 and P-selectin, suggesting that S aureus uses SSL5 to prevent neutrophil extravasation toward the site of infection. This makes SSL5 a potential lead for the development of new anti-inflammatory compounds for disorders characterized by excessive recruitment of leukocytes.

Extensive phage dynamics in Staphylococcus aureus contributes to adaptation to the human host during infection

Bacteriophages serve as a driving force in microbial evolution, adaptation to new environments and the pathogenesis of human bacterial infections. In Staphylococcus aureus phages encoding immune evasion molecules (SAK, SCIN, CHIPS), which integrate specifically into the beta-haemolysin (Hlb) gene, are widely distributed. When comparing S. aureus strain collections from infectious and colonizing situations we could detect a translocation of sak-encoding phages to atypical genomic integration sites in the bacterium only in the disease-related isolates. Additionally, significantly more Hlb producing strains were detected in the infectious strain collection. Extensive phage dynamics (intragenomic translocation, duplication, transfer between hosts, recombination events) during infection was shown by analysing cocolonizing and consecutive isolates of patients. This activity leads to the splitting of the strain population into various subfractions exhibiting different virulence potentials (Hlb-production and/or production of immune evasion molecules). Thus, phage-inducing conditions and strong selection for survival of the bacterial host after phage movement are typical for the infectious situation. Further in vitro characterization of phages revealed that: (i) SAK is encoded not only on serogroup F phages showing a conserved tropism for hlb but also on serogroup B phages which always integrate in a distinct intergenic region, (ii) the level of sak transcription correlates to phage inducibility but is independent of the phage localization in the chromosome, and (iii) phages can be stabilized extra-chromosomally during their life cycle.

How do microbes evade neutrophil killing?

Many microbial pathogens evolved to circumvent the attack of neutrophils, which are essential effector cells of the innate immune system. Here we review six major strategies that pathogenic bacteria and fungi use to evade neutrophil defences: (i) turning on survival and stress responses, (ii) avoiding contact, (iii) preventing phagocytosis, (iv) surviving intracellularly, (v) inducing cell death and (vi) evading killing by neutrophil extracellular traps. For each category we give examples and further focus on one particular pathogenic microbe in more detail. Pathogens include Candida albicans, Cryptococcus neoformans, Yersinia ssp., Helicobacter pylori, Staphylococcus aureus, Streptococcus pyogenes and Streptococcus pneumoniae.

Pathogen-derived immunomodulatory molecules: future immunotherapeutics?

The identification of molecules from various pathogens that modulate innate and/or adaptive immunity is a dynamic and rapidly developing area of research. These immunomodulatory molecules (IM) have been optimized during pathogen-host co-evolution, and have a potential application as novel immunotherapeutics. In this review, we illustrate the use of pathogen IM that have been produced as recombinant proteins, with different modes of modulatory activity, and discuss their potential to modulate undesirable immune responses in human diseases.

Early expression of SCIN and CHIPS drives instant immune evasion by Staphylococcus aureus

Chemotaxis inhibitory protein of staphylococci (CHIPS) and Staphylococcal complement inhibitor (SCIN) are small, excreted molecules that play a crucial role in the staphylococcal defence against the human innate immune system. Here we show that they both counteract crucial acute responses of our immune system such as complement activation, neutrophil chemotaxis and neutrophil activation. By studying gene expression via promoter-green fluorescent protein fusions, Northern blots and protein expression analyses, we show that SCIN and CHIPS are produced during the early (exponential) growth stages. Although the SCIN and CHIPS genes are expressed simultaneously, they are differently regulated by various Staphylococcus aureus regulatory loci. However, the sae locus is crucial for upregulation of both SCIN and CHIPS. This is the first study that presents the expression of two extracellular S. aureus proteins early during growth. Because SCIN and CHIPS are both efficient modulators of neutrophil chemotaxis, phagocytosis and killing, their early expression is necessary for efficient modulation of the early immune response.

Bacterial complement evasion

The human complement system is elemental to recognize bacteria, opsonize them for handling by phagocytes, or kill them by direct lysis. However, successful bacterial pathogens have in turn evolved ingenious strategies to overcome this part of the immune system. In this review we discuss the different stages of complement activation sequentially and illustrate the immune evasion strategies that various bacteria have developed to evade each subsequent step. The focus is on bacterial proteins, either surface-bound or excreted, that block complement activation. The underlying molecular mechanism of action and the possible role in pathophysiology of bacterial infections are discussed.

Staphylococcal cutaneous infections: invasion, evasion and aggression

Staphylococcal infections cause a variety of cutaneous and systemic infections, including impetigo, furuncle, subcutaneous abscess, staphylococcal scalded skin syndrome (SSSS), toxic shock syndrome (TSS) and neonatal toxic shock syndrome-like exanthematous disease (NTED), in association with microbial virulence factors. The virulence factors produced by Staphylococcus aureus have a wide array of biological properties, including disruption of the epithelial barrier, inhibition of opsonization by antibody and complement, interference with neutrophil chemotaxis, cytolysis of neutrophils, and inactivation of antimicrobial peptides. Exfoliative toxins (ETs) induce the 'acantholytic' infection of S. aureus due to the disruption of cell-to-cell cohesion, which allows the pathogenic organisms to spread within the epithelium. Furthermore, S. aureus expresses exotoxins with biological properties of superantigens that induce T-cell activation with subsequent anergy and immunosuppression. Of the S. aureus leukotoxins, Panton-Valentine leukocidin (PVL) is involved in the development of multiple furuncles with more intense erythema, particularly in healthy young adults. TSS is an acute life-threatening illness caused by TSS toxin-1 (TSST-1) and is usually classified into two categories; menstrual TSS, originally described in association with tampon use, and nonmenstrual TSS with a variety of clinical settings. NTED is a neonatal disease induced by TSST-1 although clinical symptoms are much milder than those of TSS. In TSS and NTED, the expansion of TSST-1-reactive Vbeta2-positive T cells is observed. The production of pathogenic S. aureus exotoxins and biofilm formation is regulated by the accessory gene regulator (agr) locus in the quorum-sensing signaling pathway. There is no doubt that targeting the quorum-sensing signaling pathway or anti-toxin therapy is a promising therapeutic approach supportive of primary antibiotic therapy.

The extracellular adherence protein (Eap) of Staphylococcus aureus inhibits wound healing by interfering with host defense and repair mechanisms

Staphylococcus aureus is a major human pathogen interfering with host-cell functions. Impaired wound healing is often observed in S aureus-infected wounds, yet, the underlying mechanisms are poorly defined. Here, we identify the extracellular adherence protein (Eap) of S aureus to be responsible for impaired wound healing. In a mouse wound-healing model wound closure was inhibited in the presence of wild-type S aureus and this effect was reversible when the wounds were incubated with an isogenic Eap-deficient strain. Isolated Eap also delayed wound closure. In the presence of Eap, recruitment of inflammatory cells to the wound site as well as neovascularization of the wound were prevented. In vitro, Eap significantly reduced intercellular adhesion molecule 1 (ICAM-1)-dependent leukocyte-endothelial interactions and diminished the consequent activation of the proinflammatory transcription factor nuclear factor kappaB (NFkappaB) in leukocytes associated with a decrease in expression of tissue factor. Moreover, Eap blocked alphav-integrin-mediated endothelial-cell migration and capillary tube formation, and neovascularization in matrigels in vivo. Collectively, the potent anti-inflammatory and antiangiogenic properties of Eap provide an underlying mechanism that may explain the impaired wound healing in S aureus-infected wounds. Eap may also serve as a lead compound for new anti-inflammatory and antiangiogenic therapies in several pathologies.

The innate immune modulators staphylococcal complement inhibitor and chemotaxis inhibitory protein of Staphylococcus aureus are located on beta-hemolysin-converting bacteriophages

Two newly discovered immune modulators, chemotaxis inhibitory protein of Staphylococcus aureus (CHIPS) and staphylococcal complement inhibitor (SCIN), cluster on the conserved 3' end of beta-hemolysin (hlb)-converting bacteriophages (betaC-phis). Since these betaC-phis also carry the genes for the immune evasion molecules staphylokinase (sak) and enterotoxin A (sea), this 8-kb region at the 3' end of betaC-phi represents an innate immune evasion cluster (IEC). By PCR and Southern analyses of 85 clinical Staphylococcus aureus strains and 5 classical laboratory strains, we show that 90% of S. aureus strains carry a betaC-phi with an IEC. Seven IEC variants were discovered, carrying different combinations of chp, sak, or sea (or sep), always in the same 5'-to-3' orientation and on the 3' end of a betaC-phi. From most IEC variants we could isolate active bacteriophages by mitomycin C treatment, of which lysogens were generated in S. aureus R5 (broad phage host). All IEC-carrying bacteriophages integrated into hlb, as was measured by Southern blotting of R5 lysogens. Large quantities of the different bacteriophages were obtained by mitomycin C treatment of the lysogens, and bacteriophages were collected and used to reinfect all lysogenic R5 strains. In total, five lytic families were found. Furthermore, phage DNA was isolated and digested with EcoR1, revealing that one IEC variant can be found on different betaI-phis. In conclusion, the four human-specific innate immune modulators SCIN, CHIPS, SAK, and SEA form an IEC that is easily transferred among S. aureus strains by a diverse group of beta-hemolysin-converting bacteriophages.

Complete genome sequence of USA300, an epidemic clone of community-acquired meticillin-resistant Staphylococcus aureus

BACKGROUND

USA300, a clone of meticillin-resistant Staphylococcus aureus, is a major source of community-acquired infections in the USA, Canada, and Europe. Our aim was to sequence its genome and compare it with those of other strains of S aureus to try to identify genes responsible for its distinctive epidemiological and virulence properties.

METHODS

We ascertained the genome sequence of FPR3757, a multidrug resistant USA300 strain, by random shotgun sequencing, then compared it with the sequences of ten other staphylococcal strains.

FINDINGS

Compared with closely related S aureus, we noted that almost all of the unique genes in USA300 clustered in novel allotypes of mobile genetic elements. Some of the unique genes are involved in pathogenesis, including Panton-Valentine leucocidin and molecular variants of enterotoxin Q and K. The most striking feature of the USA300 genome is the horizontal acquisition of a novel mobile genetic element that encodes an arginine deiminase pathway and an oligopeptide permease system that could contribute to growth and survival of USA300. We did not detect this element, termed arginine catabolic mobile element (ACME), in other S aureus strains. We noted a high prevalence of ACME in S epidermidis, suggesting not only that ACME transfers into USA300 from S epidermidis, but also that this element confers a selective advantage to this ubiquitous commensal of the human skin.

INTERPRETATION

USA300 has acquired mobile genetic elements that encode resistance and virulence determinants that could enhance fitness and pathogenicity.

Understanding the rise of the superbug: investigation of the evolution and genomic variation of Staphylococcus aureus

The bacterium Staphylococcus aureus is a common cause of human infection, and it is becoming increasingly virulent and resistant to antibiotics. Our understanding of the evolution of this species has been greatly enhanced by the recent sequencing of the genomes of seven strains of S. aureus. Comparative genomic analysis allows us to identify variation in the chromosomes and understand the mechanisms by which this versatile bacterium has accumulated diversity within its genome structure.

Neutrophil chemotaxis by pathogen-associated molecular patterns - formylated peptides are crucial but not the sole neutrophil attractants produced by Staphylococcus aureus

The chemotactic migration of phagocytes to sites of infection, guided by gradients of microbial molecules, plays a key role in the first line of host defence. Bacteria are distinguished from eukaryotes by initiation of protein synthesis with formyl methionine. Synthetic formylated peptides (FPs) have been shown to be chemotactic for phagocytes, leading to the concept of FPs as pathogen-associated molecular patterns (PAMPs). However, it remains unclear whether FPs are major chemoattractants released by bacteria and whether further chemoattractants are produced. A Staphylococcus aureus mutant whose formyltransferase gene was inactivated (Deltafmt) produced no FPs and the in vitro and in vivo ability of Deltafmt culture supernatants to recruit neutrophils was considerably reduced compared with those of the parental strain. However, some chemotactic activity was retained, indicating that bacteria produce also unknown, non-FP chemoattractants. The activity of these novel PAMPs was sensitive to pertussis toxin but insensitive to the formyl peptide receptor inhibitor CHIPS. Deltafmt culture supernatants caused reduced calcium ion fluxes and reduced CD11b upregulation in neutrophils compared with wild-type supernatants. These data demonstrate an important role of FPs in innate immunity against bacterial infections and indicate that host chemotaxis receptors recognize a larger set of bacterial molecules than previously thought.

The innate immune modulators staphylococcal complement inhibitor and chemotaxis inhibitory protein of Staphylococcus aureus are located on beta-hemolysin-converting bacteriophages

Two newly discovered immune modulators, chemotaxis inhibitory protein of Staphylococcus aureus (CHIPS) and staphylococcal complement inhibitor (SCIN), cluster on the conserved 3' end of beta-hemolysin (hlb)-converting bacteriophages (betaC-phis). Since these betaC-phis also carry the genes for the immune evasion molecules staphylokinase (sak) and enterotoxin A (sea), this 8-kb region at the 3' end of betaC-phi represents an innate immune evasion cluster (IEC). By PCR and Southern analyses of 85 clinical Staphylococcus aureus strains and 5 classical laboratory strains, we show that 90% of S. aureus strains carry a betaC-phi with an IEC. Seven IEC variants were discovered, carrying different combinations of chp, sak, or sea (or sep), always in the same 5'-to-3' orientation and on the 3' end of a betaC-phi. From most IEC variants we could isolate active bacteriophages by mitomycin C treatment, of which lysogens were generated in S. aureus R5 (broad phage host). All IEC-carrying bacteriophages integrated into hlb, as was measured by Southern blotting of R5 lysogens. Large quantities of the different bacteriophages were obtained by mitomycin C treatment of the lysogens, and bacteriophages were collected and used to reinfect all lysogenic R5 strains. In total, five lytic families were found. Furthermore, phage DNA was isolated and digested with EcoR1, revealing that one IEC variant can be found on different betaI-phis. In conclusion, the four human-specific innate immune modulators SCIN, CHIPS, SAK, and SEA form an IEC that is easily transferred among S. aureus strains by a diverse group of beta-hemolysin-converting bacteriophages.

Schistosoma mansoni secretes a chemokine binding protein with antiinflammatory activity

The coevolution of humans and infectious agents has exerted selective pressure on the immune system to control potentially lethal infections. Correspondingly, pathogens have evolved with various strategies to modulate and circumvent the host's innate and adaptive immune response. Schistosoma species are helminth parasites with genes that have been selected to modulate the host to tolerate chronic worm infections, often for decades, without overt morbidity. The modulation of immunity by schistosomes has been shown to prevent a range of immune-mediated diseases, including allergies and autoimmunity. Individual immune-modulating schistosome molecules have, therefore, therapeutic potential as selective manipulators of the immune system to prevent unrelated diseases. Here we show that S. mansoni eggs secrete a protein into host tissues that binds certain chemokines and inhibits their interaction with host chemokine receptors and their biological activity. The purified recombinant S. mansoni chemokine binding protein (smCKBP) suppressed inflammation in several disease models. smCKBP is unrelated to host proteins and is the first described chemokine binding protein encoded by a pathogenic human parasite and may have potential as an antiinflammatory agent.

DltABCD- and MprF-mediated cell envelope modifications of Staphylococcus aureus confer resistance to platelet microbicidal proteins and contribute to virulence in a rabbit endocarditis model

The DltABCD and MprF proteins contribute a net positive charge to the Staphylococcus aureus surface envelope by alanylating and lysinylating teichoic acids and membrane phosphatidylglycerol, respectively. These surface charge modifications are associated with increased in vitro resistance profiles of S. aureus to a number of endogenous cationic antimicrobial peptides (CAPs), such as alpha-defensins. The current study investigated the effects of dltA and mprF mutations on the following host factors relevant to endovascular infections: (i) in vitro susceptibility to the CAP thrombin-induced platelet microbicidal protein 1 (tPMP-1), (ii) in vitro adherence to endothelial cells (EC) and matrix proteins, and (iii) in vivo virulence in an endovascular infection model (rabbit endocarditis) in which tPMP-1 is felt to play a role in limiting S. aureus pathogenesis. Both mutations resulted in substantial increases in the in vitro susceptibility to tPMP-1 compared to that of the parental strain. The dltA (but not the mprF) mutation resulted in a significantly reduced capacity to bind to EC in vitro, while neither mutation adversely impacted in vitro binding to fibronectin, fibrinogen, or platelets. In vivo, both mutations significantly attenuated virulence in terms of early colonization of sterile vegetations and subsequent proliferation at this site (versus the parental strain). However, only the dltA mutation significantly reduced metastatic infections in kidneys and spleens compared to those in animals infected with the parental strain. These data underscore the importance of resistance to distinct CAPs and of teichoic acid-dependent EC interactions in the context of endovascular infection pathogenesis.

Capsule-negative Staphylococcus aureus induces chronic experimental mastitis in mice

Staphylococcus aureus capsular polysaccharides (CP) have been shown to enhance staphylococcal virulence in numerous animal models of infection. Although serotype 5 CP (CP5) and CP8 predominate among S. aureus isolates from humans, most staphylococcal isolates from bovines with mastitis in Argentina are capsule negative. This study was designed to evaluate the effects of CP5 and CP8 expression on the pathogenesis of experimental murine mastitis. Lactating mice were challenged by the intramammary route with one of three isogenic S. aureus strains producing CP5, CP8, or no capsule. Significantly greater numbers of acapsular mutant cells were recovered from the infected glands 12 days after bacterial challenge compared with the encapsulated strains. Histopathological analyses revealed greater polymorphonuclear and mononuclear leukocyte infiltration and congestion in the mammary glands of mice infected with the encapsulated strains compared with the acapsular mutant, and the serotype 5 strain elicited more inflammation than the serotype 8 strain. In vitro experiments revealed that the acapsular S. aureus strain was internalized by MAC-T bovine epithelial cells in significantly greater numbers than the CP5- or CP8-producing strain. Taken together, the results suggest that S. aureus lacking a capsule was able to persist in the murine mammary gland, whereas encapsulated strains elicited more inflammation and were eliminated faster. Loss of CP5 or CP8 expression may enhance the persistence of staphylococci in the mammary glands of chronically infected hosts.

The role of the complement system in innate immunity

Complement is a major component of innate immune system involved in defending against all the foreign pathogens through complement fragments that participate in opsonization, chemotaxis, and activation of leukocytes and through cytolysis by C5b-9 membrane attack complex. Bacterias and viruses have adapted in various ways to escape the complement activation, and they take advantage of the complement system by using the host complement receptors to infect various cells. Complement activation also participates in clearance of apoptotic cells and immune complexes. Moreover, at sublytic dose, C5b-9 was shown to promote cell survival. Recently it was also recognized that complement plays a key role in adaptive immunity by modulating and modifying the T cell responses. All these data suggest that complement activation constitutes a critical link between the innate and acquired immune responses.

Immune evasion by staphylococci

Staphylococcus aureus can cause superficial skin infections and, occasionally, deep-seated infections that entail spread through the blood stream. The organism expresses several factors that compromise the effectiveness of neutrophils and macrophages, the first line of defence against infection. S. aureus secretes proteins that inhibit complement activation and neutrophil chemotaxis or that lyse neutrophils, neutralizes antimicrobial defensin peptides, and its cell surface is modified to reduce their effectiveness. The organism can survive in phagosomes, express polysaccharides and proteins that inhibit opsonization by antibody and complement, and its cell wall is resistant to lysozyme. Furthermore, S. aureus expresses several types of superantigen that corrupt the normal humoral immune response, resulting in anergy and immunosuppression. In contrast, Staphylococcus epidermidis must rely primarily on cell-surface polymers and the ability to form a biolfilm to survive in the host.

Staphylococcal innate immune evasion

Upon entering the human body, bacteria are confronted with the sophisticated innate defense mechanisms of the human host. From work in recent years it has become obvious that a new and growing family of small and excreted proteins can counteract the antibacterial effects of innate immunity. These highly selective proteins pick out crucial elements of our immune system and inhibit their function. In Staphylococcus aureus these proteins act on specific cellular receptors, on antimicrobial peptides and especially on the complement system. The combined action of this growing group of essential virulence factors ascertains efficient innate immune evasion.

The structure of the C5a receptor-blocking domain of chemotaxis inhibitory protein of Staphylococcus aureus is related to a group of immune evasive molecules

The chemotaxis inhibitory protein of Staphylococcus aureus (CHIPS) is a 121 residue excreted virulence factor. It acts by binding the C5a- (C5aR) and formylated peptide receptor (FPR) and thereby blocks specific phagocyte responses. Here, we report the solution structure of a CHIPS fragment consisting of residues 31-121 (CHIPS31-121). CHIPS31-121 has the same activity in blocking the C5aR compared to full-length CHIPS, but completely lacks FPR antagonism. CHIPS31-121 has a compact fold comprising an alpha-helix (residues 38-51) packed onto a four-stranded anti-parallel beta-sheet. Strands beta2 and beta3 are joined by a long loop with a relatively well-defined conformation. Comparison of CHIPS31-121 with known structures reveals striking homology with the C-terminal domain of staphylococcal superantigen-like proteins (SSLs) 5 and 7, and the staphyloccocal and streptococcal superantigens TSST-1 and SPE-C. Also, the recently reported structures of several domains of the staphylococcal extracellullar adherence protein (EAP) show a high degree of structural similarity with CHIPS. Most of the conserved residues in CHIPS and its structural homologues are present in the alpha-helix. A conserved arginine residue (R46 in CHIPS) appears to be involved in preservation of the structure. Site-directed mutagenesis of all positively charged residues in CHIPS31-121 reveals a major involvement of arginine 44 and lysine 95 in C5aR antagonism. The structure of CHIPS31-121 will be vital in the further unraveling of its precise mechanism of action. Its structural homology to S.aureus SSLs, superantigens, and EAP might help the design of future experiments towards an understanding of the relationship between structure and function of these proteins.

Immune evasion by a staphylococcal complement inhibitor that acts on C3 convertases

The complement system is pivotal in host defense but also contributes to tissue injury in several diseases. The assembly of C3 convertases (C4b2a and C3bBb) is a prerequisite for complement activation. The convertases catalyze C3b deposition on activator surfaces. Here we describe the identification of staphylococcal complement inhibitor, an excreted 9.8-kilodalton protein that blocks human complement by specific interaction with C4b2a and C3bBb. Staphylococcal complement inhibitor bound and stabilized C3 convertases, interfering with additional C3b deposition through the classical, lectin and alternative complement pathways. This led to a substantial decrease in phagocytosis and killing of Staphylococcus aureus by human neutrophils. As a highly active and small soluble protein that acts exclusively on surfaces, staphylococcal complement inhibitor may represent a promising anti-inflammatory molecule.

Bacterial evasion of innate host defenses--the Staphylococcus aureus lesson

Bacterial pathogens such as Staphylococcus aureus use highly efficient mechanisms to evade recognition and elimination by the innate immune system. S. aureus produces sophisticated anti-inflammatory molecules and it employs several mechanisms protecting the bacteria against host cationic antimicrobial molecules such as defensin-like peptides and bacteriolytic enzymes such as lysozyme. Cell wall teichoic acids and lipoteichoic acids, complex Gram-positive surface polymers, and modified membrane lipids such as lysylphosphatidylglycerol are crucial in defensin resistance and other important aspects of staphylococcal virulence such as nasal colonization and biofilm formation on biomaterials. Certain S. aureus genes conferring escape from innate host defenses are conserved in many human pathogens suggesting that the underlying mechanisms are of general significance in bacterial virulence.

Anti-opsonic properties of staphylokinase

Recently we described a novel bacteriophage-encoded pathogenicity island in Staphylococcus aureus that harbors a number of virulence factors that are all involved in the evasion of innate immunity. Here we describe a mechanism by which staphylokinase (SAK), frequently present on this pathogenicity island, interferes with innate immune defenses: SAK is anti-opsonic. By activating human plasminogen (PLG) into plasmin (PL) at the bacterial surface, it creates bacterium-bound serine protease activity that leads to degradation of two major opsonins: human immunoglobulin G (IgG) and human C3b. Incubation of opsonized bacteria with PLG and SAK resulted in removal of anti-staphylococcal IgGs and C3b from the bacterial surface. In phagocytosis assays this proved to be a very efficient mechanism to reduce the opsonic activity of human IgG and serum. The fact that SAK activates human PLG at the bacterial surface and removes IgG as well as C3b makes this protein a unique anti-opsonic molecule.

The Staphylococcus aureus "superbug"

There has been some debate about the disease-invoking potential of Staphylococcus aureus strains and whether invasive disease is associated with particularly virulent genotypes, or "superbugs." A study in this issue of the JCI describes the genotyping of a large collection of nonclinical, commensal S. aureus strains from healthy individuals in a Dutch population. Extensive study of their genetic relatedness by amplified restriction fragment typing and comparison with strains that are associated with different types of infections revealed that the S. aureus population is clonal and that some strains have enhanced virulence. This is discussed in the context of growing interest in the mechanisms of bacterial colonization, antibiotic resistance, and novel vaccines.

N-terminal residues of the chemotaxis inhibitory protein of Staphylococcus aureus are essential for blocking formylated peptide receptor but not C5a receptor

Staphylococcus aureus excretes a factor that specifically and simultaneously acts on the C5aR and the formylated peptide receptor (FPR). This chemotaxis inhibitory protein of S. aureus (CHIPS) blocks C5a- and fMLP-induced phagocyte activation and chemotaxis. Monoclonal anti-CHIPS Abs inhibit CHIPS activity against one receptor completely without affecting the other receptor, indicating that two distinct sites are responsible for both actions. A CHIPS-derived N-terminal 6 aa peptide is capable of mimicking the anti-FPR properties of CHIPS but has no effect on the C5aR. Synthetic peptides in which the first 6 aa are substituted individually for all other naturally occurring amino acids show that the first and third residue play an important role in blocking the FPR. Using an Escherichia coli expression system, we created mutant CHIPS proteins in which these amino acids are substituted. These mutant proteins have impaired or absent FPR- but still an intact C5aR-blocking activity, indicating that the loss of the FPR-blocking activity is not caused by any structural impairment. This identifies the first and third amino acid, both a phenylalanine, to be essential for CHIPS blocking the fMLP-induced activation of phagocytes. The unique properties of CHIPS to specifically inhibit the FPR with high affinity (kd=35.4 +/- 7.7 nM) could be an important new tool to further stimulate the fundamental research on the mechanisms underlying the FPR and its role in disease processes.

Residues 10-18 within the C5a receptor N terminus compose a binding domain for chemotaxis inhibitory protein of Staphylococcus aureus

Chemotaxis inhibitory protein of Staphylococcus aureus (CHIPS) is excreted by the majority of S. aureus strains and is a potent inhibitor of C5a- and formylated peptide-mediated chemotaxis of neutrophils and monocytes. Recently, we reported that CHIPS binds to the C5a receptor (C5aR) and the formylated peptide receptor, thereby blocking activation by C5a and formylated peptides, respectively. The anaphylatoxin C5a plays an important role in host immunity and pathological inflammatory processes. For C5a a two-site binding model is proposed in which C5a initially binds the C5aR N terminus, followed by interaction of the C5a C-terminal tail with an effector domain on the receptor. We have shown here that CHIPS does not affect activation of the C5aR by a peptide mimic of the C5a C terminus. Moreover, CHIPS was found to bind human embryonic kidney 293 cells expressing only the C5aR N terminus. Deletion and mutation experiments within this C5aR N-terminal expression system revealed that the binding site of CHIPS is contained in a short stretch of 9 amino acids (amino acids 10-18), of which the aspartic acid residues at positions 10, 15, and 18 plus the glycine at position 12 are crucial. Binding studies with C5aR/C3aR and C5aR/IL8RA chimeras confirmed that CHIPS binds only to the C5aR N terminus without involvement of its extracellular loops. CHIPS may provide new strategies to block the C5aR, which may lead to the development of new C5aR antagonists.

The extracellular adherence protein from Staphylococcus aureus inhibits neutrophil binding to endothelial cells

Extracellular adherence protein (Eap) from Staphylococcus aureus inhibits the adherence of neutrophils to nonstimulated and tumor necrosis factor alpha-stimulated endothelial cells in both static adhesion assays and flow adhesion assays. Consequently, Eap also impaired their transendothelial migration. During an S. aureus infection, Eap may thus serve to reduce inflammation by inhibiting neutrophil adhesion and extravasation.

Identification and characterization of the C3 binding domain of the Staphylococcus aureus extracellular fibrinogen-binding protein (Efb)

The secreted Staphylococcus aureus extracellular fibrinogen-binding protein (Efb) is a virulence factor that binds to both the complement component C3b and fibrinogen. Our laboratory previously reported that by binding to C3b, Efb inhibited complement activation and blocked opsonophagocytosis. We have now located the Efb binding domain in C3b to the C3d fragment and determined a disassociation constant (Kd) of 0.24 microM for the Efb-C3d binding using intrinsic fluorescence quenching assays. Using truncated, recombinant forms of Efb, we also demonstrate that the C3b binding region of Efb is located within the C terminus, in contrast to the fibrinogen binding domains that are located at the N-terminal end of the protein. Enzyme-linked immunosorbent assay-type binding assays demonstrated that recombinant Efb could bind to both C3b and fibrinogen simultaneously, forming a trimolecular complex and that the C-terminal region of Efb could inhibit complement activity in vitro. In addition, secondary structure analysis using circular dichroism spectroscopy revealed that the C-terminal, C3b binding region of Efb is composed primarily of alpha-helices, suggesting that this domain of Efb represents a novel type of C3b-binding protein.

Chemotaxis inhibitory protein of Staphylococcus aureus binds specifically to the C5a and formylated peptide receptor

Chemotaxis inhibitory protein of Staphylococcus aureus (CHIPS) is an exoprotein produced by several strains of S. aureus, and a potent inhibitor of neutrophil and monocyte chemotaxis toward C5a and formylated peptides like fMLP. These chemoattractants act on their target cells by binding and activating the C5aR and formylated peptide receptor (FPR), respectively. In the present report, we examined the mechanism by which CHIPS affects both of these receptors. We showed that CHIPS blocked binding of anti-C5aR mAb and formylated peptide to human neutrophils as efficiently at temperatures of 0 and 37 degrees C, implying that it is independent of signal transducing systems. This was confirmed by showing that CHIPS acts completely independently of ATP. Additionally, CHIPS was not internalized upon binding to neutrophils. Furthermore, we showed that CHIPS binds specifically to the C5aR and FPR expressed on U937 cells. This binding was functional in blocking C5a- and fMLP-induced calcium mobilization in these cell lines. These results suggest that CHIPS binds directly to the C5aR and FPR, thereby preventing the natural ligands from activating these receptors. The apparent K(d) values of CHIPS for the C5aR and FPR were 1.1 +/- 0.2 nM and 35.4 +/- 7.7 nM, respectively. Moreover, after screening a wide variety of other G protein-coupled receptors, CHIPS was found to affect exclusively the C5aR and FPR. This selectivity and high-affinity binding with potent antagonistic effects makes CHIPS a promising lead for the development of new anti-inflammatory compounds for diseases in which damage by neutrophils plays a key role.