Staphylococcus Aureus: Structure and Functions
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1.1 Staphylococcus aureus
Staphylococcus aureus is an opportunistic yet versatile pathogen that can infect almost all types of tissue in the human body. 33-50% of healthy individuals were reported to be S. aureus carriers. The diseases resulting from S. aureus infection range from superficial infections; invasive infections such as endocarditis to the life threatening septic shock and toxic shock. The presence of foreign material greatly increases the risk of infection by providing a base for attachment and biofilm formation. S. aureus appears as clustered gram positive cocci under the microscope. Criteria used to identify this organism include the production of golden pigment on nutrient agar; being positive for coagulase; mannitol fermentation and the production of deoxyribonuclease. The virulence of the infecting strain and the nature of the host immune response are important determinants for the outcome of severe S. aureus infection .
1.1.1 S. aureus genome and regulation of gene expression
S. aureus has a 2.8-2.9 Mbp circular genome. House-keeping genes and genes associated with central metabolism and some virulence determinants (e.g. protein A) are highly conserved among strains and make up the majority of S. aureus genome. The rest more variable regions mainly consists of mobile elements such as pathogenicity islands (SaPI, 7 identified), genomic islands (νSaα and νSaβ), Staphylococcal chromosomal cassettes (SCC) and bacteriophages. S. aureus expresses an array of adhesins, immunomodulatory molecules; anti-inflammatory proteins and as many as 30 toxins to invade, evade and cause host tissue damage. Many of the virulence factors discussed below are encoded by genes located in the variable region. Their expression is under the control of a complicated and interconnected regulatory network .
Four main gene regulators have been identified, including the two component regulatory system (agr, sae); the transcription factors (SarA and its homologous); the nutrient regulated CodY regulator and alternative transcription factor σB. agr is an auto-inducible quorum-sensing system, promoting expression of extracellular virulence factors and down-regulating cell surface proteins. σB has been shown to participate in the general stress response, and may be involved in antibiotic resistance; pigmentation; biofilm and micro-colony formation (referenced in ). Many surface proteins/adhesin have been shown to be positively influenced by σB, while the expression of most exoproteins and toxins were down-regulated. Activity ofthe agr system is influenced by other gene regulators and may be regulated by cell density. The transcription of agr components were found to be promoted by SarA and gene products of the sar locus and repressed by CodY. To add up the complexity, transcription of the Sar locus was controlled by three promoters that can be differently activated during in vitro growth. One of these promoters was demonstrated to be σB dependent .
It is generally regarded that exponential growth in vitro is correlated with the colonization phase of infection. Correspondingly, genes encoding surface proteins were found to be expressed earlier, starting during the transition from stationary to exponential growth while the expression of secreted proteins started at late-exponential growth phase. The in vivo situation of infection is likely to be more dynamic. Different stimuli trigger combined and coordinated action of the regulatory network, reflecting the particular state of bacterial growth, host defense and environmental nutrition. For example, phagocytosis by neutrophil triggered differential expression of 21.8-39.1% of S. aureus genes “at any time” following the event. The suppressive effect of CodY on S. aureus virulence factors lead to the possible suggestion that S. aureus may maintain its virulence factor to low level to ‘co-exist' harmlessly with the host until a invading opportunity arise. In any case, it would be interesting to see how host immunological activity ‘feedback' on S. aureus regulation of its virulence factors, especially immune evasion molecules.
1.1.2 Antibiotic resistance
The rapid spreading of antibiotic resistant S. aureus strains through human communities presents a major challenge for conventional treatment. In 2005, it was reported that methicillin-resistant S. aureus (MRSA) infection caused more death than AIDS in USA. S. aureus demonstrated amazing ability to “co-evolve” with the development of antibiotics. The first generation penicillin uses β-lactam to bind penicillin-binding proteins (PBP, bacterial transpeptidase) and inhibits bacterial cell wall synthesis. S. aureus resists this class of antibiotics by producing β-lactamases (aka penicillinase) that cleaves β-lactam. Second generation of penicillin, such as methicillin was developed. The structure of their β-lactam has been modified to resist β-lactamases. S. aureus soon acquired the mecA operon that encodes a modified PBP (PBP 2a), which showed reduced susceptibility for β-lactam and thus granted resistance to all β-lactam containing antibiotics. mecA is located on the staphylococcal chromosome cassette mec (SCCmec), a mobile genetic element that allows horizontal transfer of mecA between strains. MRSA infection shows increasing prevalence in all continents, primarily in healthcare-environments (HA-MRSA). Alarmingly, these strains have been shown to exhibit resistance to other types of commonly used non-lactam antibiotics such as ciprofloxacin and erythromycin. Vancomycin, a peptidoglycan polymerization inhibitor, is now considered as the last-line antibiotics for MRSA treatment. However, strains of vancomycin-intermediate S. aureus (VISA) have emerged. Notably, these strains have developed thicker cell walls that require higher concentrations of vancomycin to be effective (minimal inhibitory concentration MIC over 4ug/ml). A vancomycin resistant S. aureus (VRSA) strain has been reported recently (MIC >32 μg/ml). This complete resistance is most likely due to the horizontal transfer of the vancomycin resistant gene (VanA) from Enterococcus faecalis. Development of effective new antibiotics against S. aureus has met with only limited success, urging the development of alternative therapies based on better understanding of the pathogenicityof S. aureus .
1.2 Adhesin / ECM interaction as key to S. aureus colonization/invasion
Colonization is always the first step in bacterial infection and is an important component in pathogenesis. The wide range of tissues that S. aureus can infect reflects its ability to adhere and therefore colonize many different biological niches. S. aureus is now recognized as an invasive organism, targeting non-professional phagocytes such as keratinocytes, epithelial cells, endothelial cells and osteoblasts. The tasks of initial colonization and invasion are assisted by S. aureus surface anchored and secreted adhesive molecules known as MSCRAMM, microbial surface components recognizing adhesive matrix molecules and SERAM, secretable expanded repertoire adhesive molecules. The majority of MSCRAMM and SERAM bind extracellular matrix (ECM) proteins such as fibrinogen and fibronectin. In general, the interaction with ECM proteins is regarded as providing a ‘foothold' for S. aureus to establish and to exacerbate infection. However, it has been reported that in some animal models loss of fibronectin binding proteins (FnBPs) led to an increase in bacterial virulence, indicating a role of the ECM protein in limiting bacterial dissemination or enhancing bacterial clearance. Due to overlap in target specificity; functional redundancy and the difference in experiment settings, the exact contribution of each isolated staphylococcal adhensin protein remains ambiguous. Nevertheless, fibronectin (Fn) and fibrinogen (Fg) are the most popular targets of known S. aureus adhesins.
The interaction between FnBPs and Fn is thought to play a key role in S. aureus internalization by non-phagocytic cells, as FnBP-coated latex beads and non-invasive bacteria expressing FnBPs were readily taken into the human cells (293 cells). Internalization provides several benefits for the bacterium: firstly, protection from host serum defense mechanisms and antibiotics ; secondly, access to nutrient-rich environments after escape from phagocytic vessels; thirdly, facilitating the crossing of the endothelial/epithelial layer and spreading of the infection; and lastly, establishing intracellular bacterial reservoirs for recurrent infection. S. aureus produces two related FnBPs (namely A and B) that contain almost identical Fn-binding domains in the close proximity to their cell-wall-spanning domain. The Fn-binding repeats of FnBPA were crystallized in complex with the N-terminal F1 modules of Fn. FnBPA peptide is inserted along the triple-stranded β sheets of the Fn F1 module to form an antiparallel tandem β zipper. Since the C-terminus of Fn interacts with α5β1 integrin that are expressed on most host cell surfaces, S. aureus can attach to the host cell through a Fn bridge. The attachment was shown to trigger the accumulation of actin and focal-contact-associated proteins (e.g. tensin) at the bacteria contact site and initiate internalization in a protein tyrosin kinase FAK-dependent manner. It was predicted that one FnBP can bind six to nine Fn molecules. This multivalent interaction was suggested to be important in mediating internalization .
The bindings of Fg by the clumping factors (ClfA and ClfB) and FnBPA are mediated by a shared protein structure called the A domain and by a “dock, lock and latch” mechanism. Interestingly, Clf A is expressed during stationary phase and binds to the γ chain of Fg. ClfB is expressed at the exponential growth phase and binds to the C-terminus of Fg Aα chain. This portion of Fg has been reported to be involved in the coagulation process and wound healing .
Fn and Fg interactions have been blamed particularly in S. aureus wound infection and infective endocarditis in humans. Fg binding alone could initiate experimental endocarditis in mice and has been correlated with valve colonization. However, cell invasion and persistence of the infection only occurred when the Fn-binding ability was enabled (via FnBPs). Both interactions were statistically correlated to disease severity. Binding to Fg and Fn simultaneously significantly accelerated the rate of internalization into cultured endothelial cells. In this regard FnBPA that is capable of binding both proteins may play an important role in establishing endocarditis .
Platelets accumulation on the heart valve is another critical factor for the development of infective endocarditis. S. aureus-induced platelet aggregation is a complicated and multifactorial process and was suggested to be dependent on Fg or fibrin. FnBPA, ClfA, ClfB and SdrE (serine-aspartate repeat protein) were shown to be able to cause platelet aggregation independently of other S. aureus surface proteins. ClfA displayed the strongest aggregation effect among the last three. SdrE required the presence of plasma to cause aggregation, although the mediator was not identified. ClfA was proposed to interact with platelet indirectly through an Fg bridge or directly with platelet surface protein p118. Other studies have argued that ClfA-specific IgG also participated in platelet activation by cross-linking ClfA to platelet FcγRIIa receptor .
1.3 S. aureus toxins--direct damage of host cells
S. aureus produces a range of cytotoxins, including the β barrel pore forming toxin (e.g. α hemolysin); the two component pore forming leukocidins and the exfoliative toxin. Besides the effects of reducing viable phagocytes and weakening host immune system, S. aureus cytotoxins are currently believed to contribute to bacterial dissemination. Lysis of host cells might also provide nutrients for proliferating bacteria, especially iron from hemolysis .
These toxins have different prevalence in different diseases. Exfoliative toxin (ET) operates at the epidermal layer of the skin and causes staphylococcal scalded-skin syndrome. ET-A acts as serine protease and specifically cleaves Desmoglian-1 (Dsg-1). Dsg-1 is a cell-cell adhesion molecule expressed on epidermal keratinocytes. Cleavage of Dsg-1 disrupts the superficial layer of epidermis and helps bacterial invasion. α-hemolysin (Hla) is strongly implicated in S. aureus lung infections. Active and passive immunization of Hla strongly protected mice against S. aureus pneumonia. Hla is released as a water soluble monomer and oligomerises on the host cell membrane to insert the hydrophobic stem domains. Seven Hla monomers are required to form a pore which eventually leads to cell lysis. At sub-lethal concentrations of Hla, host cells produce pro-inflammatory cytokines like IL-8; IL-6; vasoregulators (PGI2, PGE2 and thromboxane), which could have detrimental systemic effect. The expression of twenty S. aureus virulence factors were disrupted individually or in combination and the virulence of the resultant deletion mutants were compared in a mice pneumonia model. Deletion of argA, which encodes a component of the arg gene regulation system, resulted in almost complete loss of virulence. Only the Hla deletion mutant produced comparable effects, indicating the importance of Hla in causing lung tissue damage. Human neutrophils can resist Hla lysis but these cells are targeted by Leukocidins. Leukocidins are thought to act in the similar way as Hla. Among the leukocidins, Panton-valentine leukocidin (PVL) is found in all CA-MRSA (community acquired-MRSA) isolates and strongly associated with CA-MRSA infection. However in mouse models of abscess, sepsis, and pneumonia, the severity of diseases caused by PVL- deletion strains were not significantly different from that caused by the wt strains. More surprisingly, PVL-expressing S. aureus strains did not lyse human neutrophils better than that without PVL. However, it is not clear how these experimental conditions resemble the amount and time of PVL production during infection in human. Another significant problem is that PVL might be human specific and might not work in mice. Nevertheless, a new class of leukocidal molecules, the α type phenol-soluble modulin (PSMα), was produced at a considerably high level by CA-MRSA. They were shown to contribute to the increased neutrophil lysis, skin lesion formation and mortality rate of experimental animals caused by CA-MRSA infection .
1.4 Interaction of S. aureus with host immune system
1.4.1 Brief overview of host defense against S. aureus
Host defense against S. aureus mainly relies on the innate immune system, in particular, neutrophil mediated killing. The integrity of skin and mucosal layer is the first line of defense against invading bacteria. They also encounter antimicrobial substances released by epithelial cells and phagocytes (e.g. defensins; cathelicidins and lysosome). Defensins and cathelicidin are positively charged antimicrobial peptides that could permeate the bacterial membrane. Lysozyme is produced by many cell types and secreted into various tissues. It cleaves bacterial cell wall peptidoglycan at β 1-4 glycosidic linkage between N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG). Innate pattern recognition receptors (e.g. Toll-like receptor 2) and immunoglobulin also detect the presence of S. aureus. The latter is recognized by C1q and initiates the complement classical pathway (CP) or directly activates phagocytosis through Fc receptor expressed on neutrophils or monocytes. The human complement system is discussed in more detail below. One outcome of complement activation is the production of anaphylatoxin C3a and C5a. Together with formyl methionine peptides produced by bacteria, these chemoattractants are sensed by their receptors expressed on leukocytes and attract leukocytes to the sites of infection. Leukocyte recruitment is accomplished through highly coordinated interactions between adhesins on leukocytes and endothelial cells. Leukocytes change from rolling in the blood stream to firm adhesion to the endothelium and then transmigrate through the endothelial layer. Subsequent migration of leukocytes to infection sites is mediated by integrin interacting with ECM proteins. Once they reach the infection site, the phagocytes recognize antibodies or complement fragments deposited on the bacterial surface through their respective receptors. Phagocytosis then occurs. The phagosome is fused to the lysosome to form the phagolysosome for enzyme-mediated and oxygen free radical mediated destruction .
1.4.2 The human complement system
The human complement system is a sequentially activated proteolytic cascade that involves more than 30 fluid phase and surface bound proteins. It is one of the key elements of the innate immune system that connects bacterial recognition, leukocyte chemotaxis; phagocytosis as well as adaptive immunity .
Three main activation pathways are utilized to recognize foreign and danger signals. The classical pathway (CP) senses antigen bound antibody by C1q. Bacterial carbohydrate ligand is recognized by mannose binding lectin (MBL) or ficolins and initiates the lectin pathway (LP). Upon ligand recognition, both pathways use their specific proteases to cleave C4 and then C2. The resultant C4b2a (the C3 convertase) converts C3 to C3a and C3b. The Alternative pathway (AP) starts by direct binding of C3b to the bacterial surface generated by spontaneous lysis of C3 to C3b called “tick-over”. Factor B that associates with surface-bound C3b is subsequently cleaved to Bb and forms the alternative pathway C3 convertase C3bBb. The amplification loop of C3 activation is started. C3 cleavage is the central event and merging point in the three pathways. Accumulation of surface bound C3b changes the substrate specificity of C3 convertase to C5. C5 cleavage produces C5a and C5b. The latter mediates the formation of the membrane attack complex C5b-9 (MAC), which inserts into the target cell membrane and ultimately causes lysis of target cells. C3a and C5a are powerful anaphylotoxin that trigger neutrophil homing. The overall picture of this complex system is depicted in Fig. 1.1
MASP: mannose-binding lectin–associated serine proteases. Figure adapted from Walport et al 2001 (Ref.).
Three main outcomes of complement activation are: 1, chemotaxis of leukocytes via C5a and C3a; 2, C3b mediated phagocytosis via complement receptors on phagocytes; and 3, lysis of bacteria by MAC. It is also suggested by recent studies that the C3b presented on bacterial surface are critical factors for B cell activation and the production of specific antibodies. Although the MAC complex cannot function on gram positive bacteria like S. aureus, leukocytes mediated killing is critical for fighting against S. aureus infection .
1.4.3 S. aureus Immune evasion molecules
126.96.36.199 Resistance to antimicrobial substances
S. aureus circumvents antimicrobial substances by three main strategies: 1, changing surface charges; 2, modifying the substrates of antimicrobial substances and 3, inactivating antimicrobial substances. S. aureus employs two enzymes to change the highly negatively charged teichoic acid in its cell wall (WTA). DltABCD (Dlt operon) adds D-alanine to WTA and the multiple peptide resistance factor F (Mprf) participates in the modification of membrane phosphatidylglycerol with L-lysine. Both modifications increase bacterial surface charges and thus reduce the attraction to the cationic defensins. dlt or MprF defective strains were killed more efficiently by neutrophil oxygen-independent killing. The former was much less able to cause arthritis and mortality in mice sepsis and arthritis models .
S. aureus avoids lysozyme cell wall cleavage by modifying its cell wall peptidoglycan. O-acetyltransferase (OatA) was proposed to mediate this response. OatA catalyses the acetylation of muramic acid, which results in the addition of an acetyl group on S. aureus peptidoglycan. An oatA- strain was sensitive to lysozyme, while complementary expression of the enzyme restored its resistance .
Two enzymes have been reported to directly inactivate antimicrobial peptides. Aureolysin (a metalloproteinase) could cleave and therefore inactivate cathelicidin LL-37. Staphylokinase (SAK), a secreted plasminogen binding protein, was reported to form a complex with α defensin and almost completely blocked its antimicrobial activity. This activity was independent of plasminogen binding. S. aureus strains that produce SAK had a higher survival rate in vitro and higher virulence in a mouse arthritis model .
188.8.131.52 Prevent phagocytosis and opsonision
184.108.40.206.1 General anti-opsonin molecules
S. aureus clinical isolates produce a capsular polysaccharide outer cell wall. Serotypes 5 and 8 of the capsular polysaccharide are associated with increased virulence. The capsule is anti-opsonic via blocking of surface deposition of opsonins and their receptor/ligand interaction. Immunoglobulins are targeted by protein A (SpA) and its homologues Sbi (S. aureus IgG-binding protein), SSL7 (Staphylococcal superantigen-like protein 7) and SAK. SpAis a 42 kDa surface anchored molecule. It has four to five IgG binding units that could interact with IgG Fc portion. This interaction presents IgG to leukocytes Fc receptor in the wrong orientation and therefore prevents recognition. In the present of IgG, strains expressing high level of SpA were shown to be more resistant to leukocyte phagocytosis than SpA-poor strains, especially when the sole source of opsonin was purified IgG. Opposite results occurred with IgG-deficient serum, indicating SpA confers protection against neutrophil through IgG interaction. Sbi contains two IgG binding sites at the N-terminal domain. The significance of the Sbi/IgG interaction remains to be fully established. SAK forms a complex with plasminogen and converts plasminogen to plasmin. Plasmin cleaves IgG and C3b deposited on S. aureus resulting in reduced recognition by phagocytes and likely impaired initiation of C1q mediated CP activation .
220.127.116.11 Complement inhibition
S. aureus is now recognized as ‘a master of complement evasion'. Unlike other bacteria which use host factors to ‘disguise' the bacteria or inhibit complement activation, S. aureus produces a group of proteins to attack the key elements of the complement cascades. The list of anti-complement virulence factors is growing. So far, five complement inhibitors have been demonstrated and well characterized: SCIN; Efb; Ehp; SSL7 and Sbi. Interestingly, SCIN, Efb, Ehp and Sbi all use a triple α helix bundle as their functional domain while their modes of action are markedly different (Fig. 1.2). SSL7 belongs to the SSL protein family. Its function will be discussed in context with other members of the family in a later section.
SCIN (staphylococcal complement inhibitor) is a human-specific 9.8 kDa secreted protein with a trip α helix structure (Fig. 1.2) and is produced by 90% of S. aureus strains. SCIN was shown to inhibit all three complement activation pathways. In the presence of SCIN, the deposition of C3b and MAC were prevented, so was the C2b and Ba release. However the amount of surface bound C3 convertases (both forms) were increased, suggesting SCIN had a stabilizing effect on this complex. SCIN bound to surface-immobilized C3b directly and the binding site was predicted to involve the C3b MG7 and MG8 domain and the area nearby. This was confirmed by the co-crystallized SCIN-C3bBb complex. In vitro binding studies suggested the complex was likely to form in a 1:1 or 2:2 ratio. Indeed in the crystal structure, the complex appeared as a SCIN2C3b2Bb2 heterodimer that was bridged by the two SCIN molecules cross-linking the two C3b fragments. However this cross-linkage was not critical in SCIN complement inhibition as monomeric interaction also resulted in stable and inhibited C3 convertase. Native C3 could still associate with the SCIN-C3 convertase complex, but the production of C3b was largely inhibited, suggesting the convertase was likely trapped in a dysfunctional yet stable state by SCIN. Moreover, SCIN exerted a partial competition for factor B binding to C3b and a complete competition for factor H/C3b binding. The significance of the inhibition on factor H was not clear although reducing iC3b (inactive product of the cleavage of C3b) mediated downstream signaling could be beneficial for the bacterium. Physiologically, C3 convertase decays shortly after assembly. This disassociation is required for further cleavage of C2 and factor B. Thus SCIN functions by preventing C3 convertase decay; hindering conformational activation of C3 convertase and possibly by blocking C3b/iC3b mediated signaling .
Efb (extracellular fibrinogen-binding protein) and the recently discovered Ehp (Efb-homologous protein aka Ecb) have been shown to inhibit C3b deposition in AP and C5 convertase activity in all three pathways. The efb gene is present in 85% of S. aureus isolates sequenced to date. Its inhibitory effects on complement mediated lysis and neutrophil activation in response to C5a have been demonstrated. The Efb complement binding domain (Efb-c) is also a triple α helix huddle, although the arrangement is different from SCIN (Fig.1.2). It binds both C3 and C3b via the C3d fragment. However, which of the C3 forms is the primary target of Efb is debated. Structural-biochemical analysis suggested the binding affinity of Efb-C to native C3 was higher than that to C3b. The binding was proposed to induce conformational changes in C3 and prevent proper cleavage of the molecule. This view was challenged by Jongerius et al based on the observation that C3 cleavage was not influenced by Efb-C in CP/LP. Instead, they suggested Efb-C attacked complement intermediates that contain C3b. Therefore, in CP/LP, C3 convertase (C4b2a) was not affected by Efb but C5 convertase (C4b2aC3b) was inhibited, as evidenced by the decreased C5a production. Ehp was found to contain two C3d binding sites and showed higher levels of inhibition compared to Efb, though the mechanism of action was proposed to be similar. Nevertheless, the C3d fragment carries another important task: mediating interaction with complement receptor 2 (CR2) on B cells and facilitating B cell activation. Both Efb-C and Ehp were shown to completely inhibit C3d:CR2 interaction and its stimulatory effect on a B lymphoma cell line. The predicted binding site of CR2 on C3d was in close proximity to the residues that have been shown to be involved in Efb-C:C3d and Ehp:C3d binding. The exact competition mechanism remains to be confirmed .
Sbi is another secreted S. aureus protein that targets C3 activation. Sbi binds C3b but has a stronger affinity to C3dg. The binding was shown to be improved when the C3a domain was also present. These observations suggested the C3dg and C3a portion of C3 were important for Sbi/C3 interaction. By comparing binding profiles of different domains of Sbi, the C3 binding site was proposed to be located in its fourth domain (Sib IV). Interestingly, Sbi IV alone inhibited AP activation in a dose-dependent manner. However when the third domain was also present, incubation of human serum with this recombinant protein induced activation of complement, as shown by the degradation of native C3. A distinctive C3 derivative was present in Sbi-III-IV treated serum, whose molecular weight and migration behavior on two dimensional SDS-PAGE correspond to a transacylation product of C3b and Sbi. The cleavage of C3 activates an internal thioester moiety in the C3b fragment, which allows C3b to form covalent bonds with hydroxyl groups in close proximity (e.g. bacterial surface). Thus it was proposed the Sbi III-IV provided a fluid-phase transacylation target for C3 cleavage and cause non-effective activation of the alternative pathway. Since mutational study confirmed the Sbi-IV was responsible for the complement inhibitiory effect, Sbi-III was proposed to be important in consumptive complement activation. Once again, Sbi IV adopts the triple helix structure, arranged in a similar fashion as Efb-c and Ehp (Fig. 1.2). Just like Efb and Ehp, the binding of C3dg to CR2 was inhibited by Sbi-III-IV, implying that Sbi may also influence adaptive immunity .
18.104.22.168 Preventing phagocyte extravasation and chemotaxis
22.214.171.124.1 CHIPS; FLIPr and FLIPr-like
CHIPS (the chemotaxis inhibitory protein of S. aureus) was identified for its ability to prevent neutrophil chemotaxis to formylated peptide and C5a. CHIPS encodes a 121-amino acids (aa) secreted protein and presents in 62% of S. aureus clinical isolates. Postma et al demonstrated that CHIPS selectively and directly bound to C5a receptor (C5aR) and formylated peptide receptor (FPR) expressed on cell surface and competed with their respective ligands for binding. Receptor activation, as measured by the Ca2+ influx and murine neutrophil migration to the injury site, was inhibited. The two receptors were shown to interact with different areas on CHIPS. The N-terminal 6 residues of CHIPS, F1 and F3 in particular, were identified as the FPR binding sites. While the C5aR-binding domain was mapped to residues 31-121, which forms a packed structure similar to the β grasp domain. Arginine 44 and lysine 95 were shown to be critical for its antagonizing activity. C5aR binds CHIPS through its N-terminus residues 10-18. These residues were suggested to be either involved directly in C5a binding or were required to stabilize the interaction. Therefore, CHIPS may function by direct competing with C5a or by disrupting the formation of the stable ligand/receptor complex .
Two more S. aureus proteins were found to inhibit neutrophil chemotaxis. The secreted FPR-like 1 inhibitory protein (FLIPr) and its homologue FLIPr-like bound FPR and FPR-like 1 receptor (FPRL1). The N-terminal 6 residues of both proteins were important in both interactions but the phenylalanine was not critical for FRP binding. The inhibition of FPRL-1 by both proteins occurred in the nanomolar range. FLIPr-like was shown to be a potent inhibitor for FPR with activity comparable to CHIPS. However, animal infection models with chips or flipr isogenic deletion strains has not been reported. The contribution of CHIPS in bacterial infection is yet to be determined .
Map (MHC class II analog protein a.k.a. Eap) is a multifunctional 60-70 kDa secreted protein expressed by 97% of S. aureus isolates. It is an anti-inflammatory molecule that reduces leukocyte availability and function. Map can interact with a wide range of host ligands, including Fg, Fn and ICAM-1 (intercellular adhesion molecule-1). Map largely blocked ICAM-1 mediated firm adhesion and transmigration of neutrophil. In a peritonitis mice model, intraperitoneal injection of Map or pre-treatment of mice with Map showed 50-75% inhibition of neutrophil recruitment. Neutrophil infiltration in mice infected with a map- strain was 2-3 folds higher than that seen with a map+ strain, demonstrating Map inhibits ICAM-1 mediated neutrophil migration in vivo. Interaction of Map with ICAM-1 may modify signal transduction in leukocytes. NF-κB activation and tissue factor release in THP-1 cells were diminished by Map treatment. A substantial amount of IL-6 and TNF-α and a smaller amount of IL-4 were released from Map treated human CD14+ PBMC in vitro. Anti-ICAM-1 antibody blocked the action, suggesting the induction was also mediated by ICAM-1. It is not clear if cellular processes other than cytokine production are also influenced, although Map's effect on PBMCs appears to be dose dependent .
In addition to the cytokine changes, T cell function was impaired by Map. Mice treated with Map showed significantly reduced T cell mediated delayed-type hypersensitivity (DTH) and T cell proliferation. Adoptive transfer of T cells from Map-treated mice to untreated recipients prevented the recipients from developing DTH when challenged with allergen. The Map-induced T cell functional defect was speculated to contribute to S. aureus persistence. Indeed, mice infected with Map deficient S. aureus showed stronger resistance to secondary S. aureus (Map+) infection compared with mice infected with Map-producing strain initially. This difference was lost in T cell deficient mice, confirming the effect is mediated by T cells. Although Map appeared to have an apoptotic effect on cultured T lymphocytes, the exact mechanism of its modulation on T cell function and how it is related to cytokine changes remains unclear. Moreover Map was not essential in some infection models and did not provide bacterial survival advantage in the mouse infection models. Its role in in vivo infection still needs further investigation .
126.96.36.199 Survival in the phagocytes
S. aureus demonstrated significant survival after being phagocytosed by isolated human neutrophils. Intracellular S. aureus caused neutrophil destruction 3-6 hrs after phagocytosis. Significant net growth were evidenced in all of the S. aureus strains used in this study, indicating a natural ability to survive neutrophil killing. The main effector of killing is the production of radical forms of oxygen such as peroxide, hyperchloric acid. The oxygen free radicals are removed by several bacterial enzymes, including two superoxide dismutases and three methionine sulphoxide reductases and catalase. The expression of these enzymes was up-regulated after phagocytosis. Interestingly, the yellow carotonoid pigment of S. aureus also has antioxidant activity. Bacterial resistance to hydrogen peroxides and survival in nuetrophils were reduced when the carotonogenesis pathway was mutated .
Genes encoding cytotoxins were found to be up-regulated shortly following phagocytosis. One of the toxins, Hla, was shown to be important for S. aureus escaping from the phogosome in cystic fibrosis epithelial cells (CFT-1). Hla null S. aureus were trapped in LAMP-1 (lysosome marker) positive vesicles whereas the majority of wild type bacteria had escaped. The defect was similar to that found in agr– S. aureus strain. Complementary expression of Hla in the agr - strain only partially restored lysosome escaping, suggesting Hla participates in the process but is not the sole contributor. Hla may form pores in the membrane of the vesicles and cause neutralization of the acidic endo/lysosomal environment. That in turn could delay the delivery of hydrolytic enzymes and allow translation of other S. aureus proteins that contribute to escaping .
1.4.4 S. aureus superantigens (Sags)
The S. aureus superantigens are a group (20 members to date) of secreted proteins that have potent mitogenic effect on T cells. They can stimulate massive T cell activation and subsequently elevate cytokine level with the most profound effects on IL-2, IFNγ and TNFα production. The concentration to achieve half maximal proliferation of human T cells in vitro is as low as 0.1 pg/ml (SEA e.g.). Resolved Sag crystal structures all exhibit a packed two domain arrangement: the C-terminal β-grasp domain and the smaller N-terminal OB domain. A small portion of the N-terminus reaching over the β grasp domain further stabilizes the structure and contributes to T cell receptor (TCR) binding (Fig. 1.3). Currently it is thought Sags function by cross linking TCR and MHC class II molecules independent of antigen stimulation and providing strong signal for T cell activation, which may lead to activation induced cell death (AICD) .
Light grey: N-terminal OB domain. Dark grey: C-terminal β grasp domain. Their respective ligand binding sites are indicated on the structures. Figure modified from Fraser and Proft, 2008 (Ref. ).
Sags are linked with allergic diseases, nasal polyp disease, food poisoning and toxic shock syndrome (TSS), as reviewed in. Most of these conditions involve elevated/biased inflammation as a result of altered T cell function and activation. The infiltration of immune cells and the release of inflammatory as well as vasoregulatory mediators contribute to host tissue damage. In extreme conditions it could progress to hypotension, shock and multiple organ failure. The significance of Sag in S. aureus immune evasion is not clear, although it has been suggested Sag produced early in infection at low concentrations may favor an immune-suppressive local environment by activating local T cells and perhaps altering the cytokine milieu. Interestingly, Sags encoded by the enterotoxin gene cluster (egc) were found to be reversely associated with the severity of S. aureus infection, yet the neutralizing effects of human serum on egc-Sags were significantly weaker than that on the non-egc Sags (e.g. toxic shock syndrome toxin-1 and SEA). Cytokine production; transcriptional changes in PBMC and T cell proliferation in response to the two groups of Sags were found to be similar. The only difference noted was that, in cell culture, egc-Sags were detected during the log phase of bacterial growth whereas the production of non-egc Sags only occurred at high cell density when the release of egc-Sags had stopped. Thus the observed differences in host responses and disease outcome were not likely to be caused by the intrinsic differences between the Sags but may be related to the time of their releases. Although the in vivo situation needs to be confirmed, this study still supports the view that the amount and time of Sag production could largely influence the effect on the host .
Besides the anti-opsonisation effect, protein A (SpA) may also function as a B cell ‘superantigen'. SpA treatment resulted in B cell deletion in transgenic mice expressing human VHIII. Deletion of B cells occurred in the lymph nodes, blood stream and spleen and this effect was long lasting. Four months after treatment, spleen margin zone B cells still exhibited 51% depletion. When challenged with antigen later those SpA-reactive B cells showed no sign of antigen specific antibody production, indicating SpA could induce B cell function defects. Expression of Bim2, a proapoptotic Bcl-2 family molecule, was also increased in SpA affected B cells. When Bim2 was knocked out, SpA treatment induced expansion of B cell population instead of deletion and these B cell responded vigorously to stimulation. The B cell receptors (BCRs) were found to be tightly clustered together when treated with SpA. It was suggested that SpA may deliver very strong activation signals that could trigger AICD. Thus, SpA could induce Bim2-dependent B cell deletion and functional impairment in both circulation and lymphoid organs and therefore compromise the host immune system .
1.5 The staphylococcal superantigen-like proteins
The staphylococcal superantigen-like proteins (SSLs) are a group of newly identified S. aureus virulence factors that exhibit similar structural organization to S. aureus superantigens. The genes of SSLs were discovered during an attempt to identify new S. aureus exoproteins. Because of their sequence homology to the staphylococcal and streptococcal Sags (25%) and that they all contain the staphylococcal/streptococcal exotoxin consensus signature 2 (K-X2-[LIV]-X2-[LIV]-D-X3-R-X2-L-X5-[LIV]-Y), they were thought to be “novel exotoxin-like proteins” and hence the name SETs (the staphylococcal exotoxin-like toxins). However to avoid the potential confusion with the staphylococcal enterotoxin T (SET), the International Nomenclature Committee for Staphylococcal Superantigen Nomenclature renamed this family as SSLs and standardized the numbering of the genes to follow the “clockwise order from the origin of replication based on homology to the full complement of genes found in strain MW2”. 14 genes were identified in total that belong to this family. ssl1-11 forms the main cluster located in the SaPIn2 while ssl12-14 locates downstream and in the reverse orientation. To date, all the sequenced S. aureus genomes contain the ssls, highlighting their importance in S. aureus virulence
The crystal structures of SSL5; SSL7; SSL9 and SSL11 have been resolved. All four proteins exhibited the same basic structural architecture like the superantigens: an N-terminal OB fold and C-terminal β grasp domain with a small section of the N-terminal peptides extending to the β grasp domain (Fig. 1.3, SSL7). SSLs do not possess the conserved residues that are important for superantigen interaction with TCR and MHC class II and correspondingly, they do not function as T cell mitogens. Instead, they appear to target the central events in innate immunity against S. aureus. The Sags and SSLs could be reviewed as another example of how S. aureus uses one basic protein structural to suit remarkably variable target requirements (Fig. 1.3). The target specificity and functional differences are evidenced even among the SSLs. According to their function, the SSLs can be classified into two groups: one favors the interaction with glycans (the glycan binders) and the other binds to complement proteins (the complement binders).
1.5.1 The glycan binders
The best characterized SSLs of this subgroup are SSL5 and SSL11. SSL5 and SSL11 both bind specifically to glycans with terminal trisaccharide sialy-lactosamine (sLacNac), including IgA receptor (FcαRI) and P-selectin Glycoprotein Ligand-1 (PSGL-1). Both molecules potently inhibit neutrophil attachment to P-selectin coated surface. The involvement of terminal sialic acid was verified by the observation that neuraminidase treatment reduced their binding and abolished the inhibitory effects .The binding site to glycans on SSL5 and SSL11 are highly conserved. The same binding feature is also present in SSL2-6, suggesting their potential for glycan interactions. The affinity and binding orientation of SSL5 and SSL11 to sialyl Lewis X (sLex), a sLacNac containing tetrasaccharide often found on PSGL-1, strongly resembled P-selectin binding of sLex with one major difference. Fucose is involved in P-selectin binding whereas it can be dispensed in the binding of SSL5 without changing the affinity of the interaction. Interestingly despite the wide expression of sLacNac on the cell surfaces, SSL11 targets granulocytes and monocytes, while SSL5 binds to neutrophils and monocytes. Interaction of SSL11 with sialyated glycoporoteins on neutrophils resulted in internalization of the protein, however the cellular target is yet to be identified. Recently, the repertoire of SSL5 ligands has been expanded to all classes of G protein coupled receptors. The interaction was shown to occur through their glycosylated N-termini so activations that involve the N-termini of the receptors were inhibited by SSL5. The leukocyte chemokine and anaphylatoxin receptors all belong to this class. Thus SSL5 inhibits neutrophil chemotaxis at two levels: rolling to infection sites and sensing chemokines and anaphylatoxins .
1.5.2 The complement binders
This subgroup includes SSL7, 9, 10 and SSL12-14. SSL7 (Fig. 1.3) binds both C5 and IgA independent of each other. SSL7 interacts with both serum and secretory IgA via the IgA heavy chain Cα2/Cα3 domain. Leukocyte IgA receptor (FcαRI) also binds IgA through this region and this binding was competitively inhibited by SSL7. SSL7 showed nonomolar affinity to both IgA1 and IgA2 (the main mucosal IgA subclass) and the residues recognized by SSL7 are present in all human IgA allotypes. Thus this interaction may be important for S. aureus survival, especially in the mucosal layer. C5 selectively bound to immobilized SSL7 in serum affinity pull-down assay and biosensor analysis. The binding site for C5 was predicated to locate in the β grasp domain. The SSL7/C5 interaction inhibited complement mediated hemolysis and enhanced bacterial survival (Escherichia coli) in serum. 14% of E.coli survived in the presence of 2.1nM SSL7. SSL9, 10, 12-14 were all reported to inhibit both the classical and lectin complement pathways. SSL9 was proposed to directly interact with the CP and LP C3 convertase. Interestingly, like SSL7, SSL10 and SSL12-14 all bind human immunoglobins. SSL10 specific targets human IgG isotype 1 although whether the interaction is solely responsible for classical pathway inhibition is yet to be determined (Deepa Patel, personal communication, the University of Auckland, New Zealand). IgM was captured by immobilized SSL12-14 in affinity pull down assays. The meanings of these interactions remain to be investigated.
1.5.3 SSl14 as a potential S. aureus virulence factor
Several facts indicate SSL14 is a potential virulence factor of S. aureus. ssl14 is present in all sequenced S. aureus strains andclinical isolates tested and the sequences are highly conserved among strains. ssl12-14 are located in the 2nd immune evasion cluster (IEC-2) along with genes encoding FLIPr, Efb , Ehp, SCIN-B and SCIN-C (functional isoforms of SCIN) and are up-regulated under stress conditions. SSL14 antibodies were detected in patients infected with S. aureus as well as healthy individuals, indicating it is expressed duringencounters with the host. It was found that SSL14 was able to interact with endothelial cells and, most interestingly, T cells and B cells, though the surface receptors for binding and the biological significance of these interactions were not clear. Both guinea pig and human complement activities were significantly inhibited by SSL14 in a dose-dependent fashion. The effect was shown to be specific for the classical and lectin pathways (Fig. 1.4). C1q was found to be the complement component targeted by SSL14, as evidenced by both affinity pull down and direct binding ELISA. The plasma C4 and C2 level were not affected by SSL14 treatment, suggesting that the C1q binding is likely to contribute to SSL14 mediated complement inhibition .
A: Classical pathway. B: Lectin pathway. C: Alternative pathway. Fig.ure adapted from Taylor, A. 2008 (Ref. ).
1.6 C1q and C1 complex
C1q is the recognition molecule of the C1 complex. It is made up of 18 peptides, organized in a ‘bouquet of tulips' fashion. Three peptides (the A, B and C chains) form a triple-helical collagen-like fiber with a C-terminal globular head. Six of these hetero-trimers are joined at the N-terminal parts by disulfide bonds to form the C1q ‘stalk'. The serine protease C1r2-C1s2 tetramer folds around the C1q stems, completing the complex. Ca2+ is required to stabilize the structure. Binding studies with truncated C1r and C1s fragments indicated the protease tetramer bound to C1q through four separate sites on C1r and two on C1s. Each site was proposed to interact with one C1q stem (Fig. 1.5). This proposition was in agreement with an independent mutational study, which further suggested that each site involved acidic residues that participate in Ca2+ binding and the residues in close proximity to the Ca2+ binding sites. However it was argued those residues could indirectly contribute to the interaction through altered Ca2+ binding. Several models of the C1 complex activation have been proposed and no conclusion has yet been reached as to which one best represents the action. In general these models suggest that the C1r and C1s components are protected in the C1q “core”. The binding of C1q to its targets resulted in movement of the heads and the ‘stems'. This change altered the relative position of C1r and C1s sub-domains and led to activation of the C1 complex. The conformational changes occurring during activation also make the complex less stable .
A and B: side and top view of the interaction between C1q (white) and the CUB1-EGF-CUB2 domains of C1r (orange and dark green) and C1s (red and pale green). C1r and C1s pair to form two antiparellel heterodimers, each interacting with three C1q stems (indicated by arrows in B). C1r binds to C1q through both CUB domains and only the CUB1 domain of C1s is involved. The predicated interfaces are highlighted in blue on C1q and purple on C1rs. C: Side view of the catalytic domain (CCP1-CCP2-CP) dimer confirmation (purple) in active C1r, aligned with the C1rs C1q binding portions (CUB1-EGF-CUB2). CUB: domain found in complement component C1r/C1s, Uegf and bone morphogenic protein 1. EGF: epidermal growth factor-like domain. CCP: complement control protein. SP: serine protease. Figure is modified from Phillips et al 2009 (Ref. ).
C1q has been shown to recognize a diverse range of ligands including altered or misfolded endogenous molecules through its globular heads. Charge and hydrophobicity distribution on the A, B and C subunits of the C1q head are different and provide specificity of ligand recognition. Besides activation of the classical complement pathway, C1q was also reported to have chemotaxis effects on neutrophil and mast cells and to directly enhance phagocytosis via interacting with the CD91 receptor on phagocytes. Not surprisingly, C1 activation and down-stream signal transduction is a carefully maintained balance that leads to disease development once disturbed. Some host proteins (e.g.fibromodulin) were found to have dual binding capacity for C1q and endogenous complement inhibitors (e.g factor H), thus they could activate the C1 complex but inhibit the terminal activation of complement. Humans produce the endogenous C1 inhibitor (C1-inh) that directly down-regulates C1 activity. It acts as a suicidal substrate for C1s, by forming a stable C1r-C1s-C1-inh complex following cleavage and promoting the disassembly of the C1 complex .
Direct inhibition of C1q is not common in human pathogens. SSL14 is the only S. aureus protein that has been reported to interact with C1q. Given the range of C1q functions, this binding may have significances beyond inhibiting complement activation. Understanding the SSL14/C1q interaction and SSL14 mediated complement inhibition will contribute to our knowledge of S. aureus immune evasion strategies and the function of β grasp domain in complement inhibition. It also offers valuable information on C1 biology and the importance of the classical activation pathway to host defense against S. aureus.
1.7 Project aim
We hypothesize that SSL14 is a virulence factor of Staphylococcus aureus and functions by inhibiting complement mediated immunity.
The aim of this study is to locate the binding site on SSL14 involved in C1q binding and complement inhibition. We intend to map the binding site via site-directed mutagenesis by overlap PCR. Residues of interest will be selected based on the structure and sequence alignments to other identified SSL complement inhibitors. SSL14 mutants will be screened for complement inhibition in hemolytic assays. Residues indentified will be tested for their potential roles in C1q binding.
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