Disruption of the alternative pathway convertase occurs at the staphylococcal surface via the acquisition of factor H by Staphylococcus aureus

A Factor H-Fc fusion protein increases complement-mediated opsonophagocytosis and killing of community associated methicillin-resistant Staphylococcus aureus

Staphylococcus aureus employs a multitude of immune-evasive tactics to circumvent host defenses including the complement system, a component of innate immunity central to controlling bacterial infections. With antibiotic resistance becoming increasingly common, there is a dire need for novel therapies. Previously, we have shown that S. aureus binds the complement regulator factor H (FH) via surface protein SdrE to inhibit complement. To address the need for novel therapeutics and take advantage of the FH:SdrE interaction, we examined the effect of a fusion protein comprised of the SdrE-interacting domain of FH coupled with IgG Fc on complement-mediated opsonophagocytosis and bacterial killing of community associated methicillin-resistant S. aureus. S. aureus bound significantly more FH-Fc compared to Fc-control proteins and FH-Fc competed with serum FH for S. aureus binding. FH-Fc treatment increased C3-fragment opsonization of S. aureus for both C3b and iC3b, and boosted generation of the anaphylatoxin C5a. In 5 and 10% serum, FH-Fc treatment significantly increased S. aureus killing by polymorphonuclear cells. This anti-staphylococcal effect was evident in 75% (3/4) of clinical isolates tested. This study demonstrates that FH-Fc fusion proteins have the potential to mitigate the protective effects of bound serum FH rendering S. aureus more vulnerable to the host immune system. Thus, we report the promise of virulence-factor-targeted fusion-proteins as an avenue for prospective anti-staphylococcal therapeutic development.

New potential role of serum apolipoprotein E mediated by its binding to clumping factor A during Staphylococcus aureus invasive infections to humans

Staphylococcus aureus is a crucial human pathogen expressing various immune-evasion proteins that interact with the host-cell molecules. Clumping factor A (ClfA) is a microbial surface protein that promotes S. aureus binding to fibrinogen, and is associated with septic arthritis and infective endocarditis. In order to identify the major human serum proteins that bind the ClfA, we utilized recombinant ClfA region A in a plate-based assay. SDS-PAGE analysis of the bound proteins yielded five prominent bands, which were analysed by MS yielding apolipoprotein E (ApoE) as the predominant protein. ClfA-sufficient S. aureus bound purified ApoE by more than one log greater than an isogenic ClfA-deficient mutant. An immunodot-blot assay yielded a linearity model for ClfA binding to human ApoE with a stoichiometric-binding ratio of 1.702 at maximal Pearson's correlation coefficient (0.927). These data suggest that ApoE could be a major and novel binding target for the S. aureus virulence factor ClfA. Thus, ClfA recruitment of serum ApoE to the S. aureus surface may sequester ApoE and blunt its host defence function against S. aureus-invasive infections to humans. In this context, compounds that can block or suppress ClfA binding to ApoE might be utilized as prophylactic or therapeutic agents.

Identification of peptidic inhibitors of the alternative complement pathway based on Staphylococcus aureus SCIN proteins

The complement system plays a central role in a number of human inflammatory diseases, and there is a significant need for development of complement-directed therapies. The discovery of an arsenal of anti-complement proteins secreted by the pathogen Staphylococcus aureus brought with it the potential for harnessing the powerful inhibitory properties of these molecules. One such family of inhibitors, the SCINs, interact with a functional "hot-spot" on the surface of C3b. SCINs not only stabilize an inactive form of the alternative pathway (AP) C3 convertase (C3bBb), but also overlap the C3b binding site of complement factors B and H. Here we determined that a conserved Arg residue in SCINs is critical for function of full-length SCIN proteins. Despite this, we also found SCIN-specific differences in the contributions of other residues found at the C3b contact site, which suggested that a more diverse repertoire of residues might be able to recognize this region of C3b. To investigate this possibility, we conducted a phage display screen aimed at identifying SCIN-competitive 12-mer peptides. In total, seven unique sequences were identified and all exhibited direct C3b binding. A subset of these specifically inhibited the AP in assays of complement function. The mechanism of AP inhibition by these peptides was probed through surface plasmon resonance approaches, which revealed that six of the seven peptides disrupted C3bBb formation by interfering with factor B/C3b binding. To our knowledge this study has identified the first small molecules that retain inhibitory properties of larger staphylococcal immune evasion proteins.

The extracellular adherence protein from Staphylococcus aureus inhibits the classical and lectin pathways of complement by blocking formation of the C3 proconvertase

The pathogenic bacterium Staphylococcus aureus actively evades many aspects of human innate immunity by expressing a series of small inhibitory proteins. A number of these proteins inhibit the complement system, which labels bacteria for phagocytosis and generates inflammatory chemoattractants. Although the majority of staphylococcal complement inhibitors act on the alternative pathway to block the amplification loop, only a few proteins act on the initial recognition cascades that constitute the classical pathway (CP) and lectin pathway (LP). We screened a collection of recombinant, secreted staphylococcal proteins to determine whether S. aureus produces other molecules that inhibit the CP and/or LP. Using this approach, we identified the extracellular adherence protein (Eap) as a potent, specific inhibitor of both the CP and LP. We found that Eap blocked CP/LP-dependent activation of C3, but not C4, and that Eap likewise inhibited deposition of C3b on the surface of S. aureus cells. In turn, this significantly diminished the extent of S. aureus opsonophagocytosis and killing by neutrophils. This combination of functional properties suggested that Eap acts specifically at the level of the CP/LP C3 convertase (C4b2a). Indeed, we demonstrated a direct, nanomolar-affinity interaction of Eap with C4b. Eap binding to C4b inhibited binding of both full-length C2 and its C2b fragment, which indicated that Eap disrupts formation of the CP/LP C3 proconvertase (C4b2). As a whole, our results demonstrate that S. aureus inhibits two initiation routes of complement by expression of the Eap protein, and thereby define a novel mechanism of immune evasion.

Staphylococcal Ecb protein and host complement regulator factor H enhance functions of each other in bacterial immune evasion

Staphylococcus aureus is a major human pathogen causing more than a tenth of all septicemia cases and often superficial and deep infections in various tissues. One of the immune evasion strategies of S. aureus is to secrete proteins that bind to the central complement opsonin C3b. One of these, extracellular complement binding protein (Ecb), is known to interfere directly with functions of C3b. Because C3b is also the target of the physiological plasma complement regulator, factor H (FH), we studied the effect of Ecb on the complement regulatory functions of FH. We show that Ecb enhances acquisition of FH from serum onto staphylococcal surfaces. Ecb and FH enhance mutual binding to C3b and also the function of each other in downregulating complement activation. Both Ecb and the C-terminal domains 19-20 of FH bind to the C3d part of C3b. We show that the mutual enhancing effect of Ecb and FH on binding to C3b depends on binding of the FH domain 19 to the C3d part of C3b next to the binding site of Ecb on C3d. Our results show that Ecb, FH, and C3b form a tripartite complex. Upon exposure of serum-sensitive Haemophilus influenzae to human serum, Ecb protected the bacteria, and this effect was enhanced by the addition of the C-terminal domains 19-20 of FH. This finding indicates that the tripartite complex formation could give additional protection to bacteria and that S. aureus is thereby able to use host FH and bacterial Ecb in a concerted action to eliminate C3b at the site of infection.

Staphylococcus aureus surface protein SdrE binds complement regulator factor H as an immune evasion tactic

Similar to other highly successful invasive bacterial pathogens, Staphylococcus aureus recruits the complement regulatory protein factor H (fH) to its surface to inhibit the alternative pathway of complement. Here, we report the identification of the surface-associated protein SdrE as a fH-binding protein using purified fH overlay of S. aureus fractionated cell wall proteins and fH cross-linking to S. aureus followed by mass spectrometry. Studies using recombinant SdrE revealed that rSdrE bound significant fH whether from serum or as a purified form, in both a time- and dose-dependent manner. Furthermore, rSdrE-bound fH exhibited cofactor functionality for factor I (fI)-mediated cleavage of C3b to iC3b which correlated positively with increasing amounts of fH. Expression of SdrE on the surface of the surrogate bacterium Lactococcus lactis enhanced recruitment of fH which resulted in increased iC3b generation. Moreover, surface expression of SdrE led to a reduction in C3-fragment deposition, less C5a generation, and reduced killing by polymorphonuclear cells. Thus, we report the first identification of a S. aureus protein associated with the staphylococcal surface that binds factor H as an immune evasion mechanism.

Host- and microbe determinants that may influence the success of S. aureus colonization

Staphylococcus aureus may cause serious skin and soft tissue infections, deep abscesses, endocarditis, osteomyelitis, pneumonia, and sepsis. S. aureus persistently colonizes 25–30% of the adult human population, and S. aureus carriers have an increased risk for infections caused by the bacterium. The major site of colonization is the nose, i.e., the vestibulum nasi, which is covered with ordinary skin and hair follicles. Several host and microbe determinants are assumed to be associated with colonization. These include the presence and expression level of bacterial adhesins, which can adhere to various proteins in the extracellular matrix or on the cellular surface of human skin. The host expresses several antimicrobial peptides and lipids. The level of β-defensin 3, free sphingosine, and cis-6-hexadecenoic acid are found to be associated with nasal carriage of S. aureus. Other host factors are certain polymorphisms in Toll-like receptor 2, mannose-binding lectin, C-reactive protein, glucocorticoid-, and vitamin D receptor. Additional putative determinants for carriage include genetic variation and expression of microbial surface components recognizing adhesive matrix molecules and their interaction partners, as well as variation among humans in the ability of recognizing and responding appropriately to the bacteria. Moreover, the available microflora may influence the success of S. aureus colonization. In conclusion, colonization is a complex interplay between the bacteria and its host. Several bacterial and host factors are involved, and an increased molecular understanding of these are needed.

Complement regulator C4BP binds to Staphylococcus aureus and decreases opsonization

Staphylococcus aureus is the major cause of human skin and soft-tissue infections as well as invasive infections like post-operative wound infections, septic arthritis, and osteomyelitis. The complement system plays an important role in the immunological control of many bacteria, but can be inhibited by a variety of strategies including recruitment of complement regulatory proteins like C4b-binding protein (C4BP). These experiments demonstrate that S. aureus opsonization with C4b occurs rapidly in serum and is predominantly initiated by anti-staphylococcal antibodies. Much of the S. aureus-bound C4b is quickly cleaved to the inactive forms iC4b and C4d. Clinical S. aureus strains rapidly bind significant amounts of the complement regulator C4BP from serum. S. aureus also binds purified C4BP. S. aureus-bound C4BP functions as a cofactor for factor I-mediated C4b cleavage to iC4b and C4d. In the absence of factor I, C4BP decreases classical pathway-mediated deposition of C3b on the S. aureus surface by inhibiting the classical pathway C3-convertase. In summary, C4BP is recruited to the S. aureus surface where it functions to inhibit C4 complement effectors, suggesting a previously undescribed immune evasion strategy for this pathogen.

Surface shaving as a versatile tool to profile global interactions between human serum proteins and the Staphylococcus aureus cell surface

The human commensal bacterium Staphylococcus aureus is renowned as a causative agent of severe invasive diseases. Upon entering the bloodstream, S. aureus can infect almost every tissue and organ system in the human body. To withstand insults from the immune system upon invasion, several immune-evasive mechanisms have evolved in S. aureus, such as complement inhibition by secreted proteins and IgG-binding by surface-exposed protein A. While it is generally accepted that S. aureus cells bind a range of host factors for various purposes, no global analyses to profile staphylococcal host factor binding have so far been performed. Therefore, we explored the possibility to profile the binding of human serum proteins to S. aureus cells by "surface shaving" with trypsin and subsequent MS analysis of liberated peptides. This resulted in the identification of several components of the complement system, the platelet factor 4 and the isoform 1 of the inter-α-trypsin inhibitor heavy chain H4 on the staphylococcal cell surface. We conclude that surface shaving is a versatile tool to profile global interactions between human serum proteins and the S. aureus cell surface.