Structure-function analysis of the C3 binding region of Staphylococcus aureus immune subversion protein Sbi

Virulence attributes of successful methicillin-resistant Staphylococcus aureus lineages

Methicillin-resistant Staphylococcus aureus (MRSA) is a leading cause of severe and often fatal infections. MRSA epidemics have occurred in waves, whereby a previously successful lineage has been replaced by a more fit and better adapted lineage. Selection pressures in both hospital and community settings are not uniform across the globe, which has resulted in geographically distinct epidemiology. This review focuses on the mechanisms that trigger the establishment and maintenance of current, dominant MRSA lineages across the globe. While the important role of antibiotic resistance will be mentioned throughout, factors which influence the capacity of S. aureus to colonize and cause disease within a host will be the primary focus of this review. We show that while MRSA possesses a diverse arsenal of toxins including alpha-toxin, the success of a lineage involves more than just producing toxins that damage the host. Success is often attributed to the acquisition or loss of genetic elements involved in colonization and niche adaptation such as the arginine catabolic mobile element, as well as the activity of regulatory systems, and shift metabolism accordingly (e.g., the accessory genome regulator, agr). Understanding exactly how specific MRSA clones cause prolonged epidemics may reveal targets for therapies, whereby both core (e.g., the alpha toxin) and acquired virulence factors (e.g., the Panton-Valentine leukocidin) may be nullified using anti-virulence strategies.

Novel method for detecting complement C3 deposition on Staphylococcus aureus

The primary host response to Staphylococcus aureus infection occurs via complement. Complement is an elegant evolutionarily conserved system, playing essential roles in early defences by working in concert with immune cells to survey, label and destroy microbial intruders and coordinate inflammation. Currently the exact mechanisms employed by S. aureus to manipulate and evade complement is not clear and is hindered by the lack of accurate molecular tools that can report on complement deposition on the bacterial surface. Current gold-standard detection methods employ labelled complement-specific antibodies and flow cytometry to determine complement deposited on bacteria. These methods are restricted by virtue of the expression of the S. aureus immunoglobulin binding proteins, Protein A and Sbi. In this study we describe the use of a novel antibody-independent C3 probe derived from the staphylococcal Sbi protein, specifically Sbi-IV domain. Here we show that biotin-labelled Sbi-IV interacts specifically with deposited C3 products on the staphylococcal surface and thus can be used to measure complement fixation on wild-type cells expressing a full repertoire of immune evasion proteins. Lastly, our data indicates that genetically diverse S. aureus strains restrict complement to different degrees suggesting that complement evasion is a variable virulence trait among S. aureus isolates.

Cloning and structural analysis of complement component 3d in wild birds provides insight into its functional evolution

Complement component 3 d (C3d) is the final cleavage product of the complement component C3 and serves as a crucial role in link innate and adaptive immunity, and increase B-cell sensitivity to an antigen by 1000-10000 fold. The crystal structure of human C3d revealed there are two distinct surfaces, a convex surface containing the thioester-constituting residues that mediate covalent binding to the target antigen, and a concave surface with an acidic pocket responsible for interaction with CR2. In this study, we cloned and sequenced cDNA fragment encoding C3d region from 15 wild bird species. Then, the C3d sequences from wild birds, chicken and mammals were aligned to construct phylogenetic trees. Phylogenetic tree displayed two main branches, indicating mammals and birds, but the bird C3d branch was divided into two main parts, with five wild birds (Ardeola bacchus, Zoothera, Bubo, Crossoptilon mantchuricum and Caprimulgus europaeus) clustering much closer to mammals. In addition, the C3d proteins of Ardeola bacchus, Bubo, Crossoptilon mantchuricum and Caprimulgus europaeus contained a Glu163 residue at the position at which Lys163 was found in other birds. However, Glu163 have the same charge polarity as Asp163, which is the key amino acid residue comprising the acidic pocket combined with CR2 found at this position in mammals, and Zoothera also possessed Asp163 at this position. Structure modeling analyses also verified that the C3ds of these five wild bird species exhibited the amino acid sequence and structure comprising the typical acidic pocket found in mammals that is required for combination with B cell surface receptors, which contribute electrostatic forces to interact with CR2. Our investigations indicate that some bird C3ds may already have the ability to bind with CR2 by electrostatic force, like mammals. As Ardeola bacchus, Zoothera, Bubo, Crossoptilon mantchuricum and Caprimulgus europaeus have more typical C3d concave acid pockets and thus a stronger ability to bind CR2, we speculate that these five wild birds may have a solider immunity against pathogens. Our phylogenetic and structural analyses of bird C3ds provide insights on the evolutionary divergence in the function of immune factors of avian and mammalian.

Surface Proteins of Staphylococcus aureus

The surface of Staphylococcus aureus is decorated with over 20 proteins that are covalently anchored to peptidoglycan by the action of sortase A. These cell wall-anchored (CWA) proteins can be classified into several structural and functional groups. The largest is the MSCRAMM family, which is characterized by tandemly repeated IgG-like folded domains that bind peptide ligands by the dock lock latch mechanism or the collagen triple helix by the collagen hug. Several CWA proteins comprise modules that have different functions, and some individual domains can bind different ligands, sometimes by different mechanisms. For example, the N-terminus of the fibronectin binding proteins comprises an MSCRAMM domain which binds several ligands, while the C-terminus is composed of tandem fibronectin binding repeats. Surface proteins promote adhesion to host cells and tissue, including components of the extracellular matrix, contribute to biofilm formation by stimulating attachment to the host or indwelling medical devices followed by cell-cell accumulation via homophilic interactions between proteins on neighboring cells, help bacteria evade host innate immune responses, participate in iron acquisition from host hemoglobin, and trigger invasion of bacteria into cells that are not normally phagocytic. The study of genetically manipulated strains using animal infection models has shown that many CWA proteins contribute to pathogenesis. Fragments of CWA proteins have the potential to be used in multicomponent vaccines to prevent S. aureus infections.

Staphylococcus pseudintermedius Sbi paralogs inhibit complement and bind IgM, IgG Fc and Fab

The success of staphylococci as pathogens has been attributed, in part, to their ability to evade their hosts’ immune systems. Although the proteins involved in evasion have been extensively studied in staphylococci affecting humans little characterization has been done with Staphylococcus pseudintermedius, an important cause of pyoderma in dogs. Staphylococcus aureus binder of immunoglobulin (Sbi) interferes with innate immune recognition by interacting with multiple host proteins. In this study, a S. pseudintermedius gene that shares 38% similarity to S. aureus Sbi was cloned from S. pseudintermedius strains representative of major clonal lineages bearing two paralogs of the protein. Binding of immunoglobulins and Fab and Fc fragments as well as interaction with complement was measured. S. pseudintermedius Sbi protein bound IgG from multiple species and canine complement C3, neutralized complement activity and bound to canine IgM and B cells. Evidence from this work suggests Sbi may play an important role in S. pseudintermedius immune evasion.

A novel tandem repeat cloning technique for creation of multiple short peptide repeats to differentiate closely related antigens

Antibody cross-reactivity is a problem often associated with closely related antigens. This study was aimed to develop a method enabling differentiation of closely related toxins based on antigen designing strategy. The method involves identification of disparate amino acids (AA) confined to target antigen in comparison with two or more closely related antigens, their assembly into a DNA oligomer and further cloning as six tandem repeats (TR) using restriction and ligation strategy into a desired vector and finally generation of antigen specific antibodies. The practical utility of this method was demonstrated by generating and testing the specificity of polyclonal antibodies against staphylococcal enterotoxin C (SEC). Cross-reactivity is a problem often associated with SEC in immunoassays due to its amino acid sequence identity with staphylococcal enterotoxin B (SEB) (40-60%). To circumvent the same, the above-mentioned strategy was applied. Unique AA of SEC (36 AA) in comparison to SEB were selected, reassembled and with deduced corresponding nucleotides, an oligomer of 117 bases was designed. Using primers with restriction overhangs, three constructs were created each with two repeats using a common restriction site. The resulting three constructs were sequentially cloned into alternating restriction sites of pRSET A vector in directional orientation, expressed in E. coli for rTR/SEC protein which was used to generate specific polyclonal antibodies against SEC. Specificity was compared with antibody raised against whole SEC recombinant protein using Western blot and dot blot assays. High specificity was achieved through the developed strategy signifying its possible application to address cross-reactivity problem associated with closely related antigens.

Utilization of Staphylococcal Immune Evasion Protein Sbi as a Novel Vaccine Adjuvant

Co-ligation of the B cell antigen receptor with complement receptor 2 on B-cells via a C3d-opsonised antigen complex significantly lowers the threshold required for B cell activation. Consequently, fusions of antigens with C3d polymers have shown great potential in vaccine design. However, these linear arrays of C3d multimers do not mimic the natural opsonisation of antigens with C3d. Here we investigate the potential of using the unique complement activating characteristics of Staphylococcal immune-evasion protein Sbi to develop a pro-vaccine approach that spontaneously coats antigens with C3 degradation products in a natural way. We show that Sbi rapidly triggers the alternative complement pathway through recruitment of complement regulators, forming tripartite complexes that act as competitive antagonists of factor H, resulting in enhanced complement consumption. These functional results are corroborated by the structure of the complement activating Sbi-III-IV:C3d:FHR-1 complex. Finally, we demonstrate that Sbi, fused with Mycobacterium tuberculosis antigen Ag85b, causes efficient opsonisation with C3 fragments, thereby enhancing the immune response significantly beyond that of Ag85b alone, providing proof of concept for our pro-vaccine approach.

Staphylococcus aureus: setting its sights on the human innate immune system

Staphylococcus aureus has colonized humans for at least 10 000 years, and today inhabits roughly a third of the population. In addition, S. aureus is a major pathogen that is responsible for a significant disease burden, ranging in severity from mild skin and soft-tissue infections to life-threatening endocarditis and necrotizing pneumonia, with treatment often hampered by resistance to commonly available antibiotics. Underpinning its versatility as a pathogen is its ability to evade the innate immune system. S. aureus specifically targets innate immunity to establish and sustain infection, utilizing a large repertoire of virulence factors to do so. Using these factors, S. aureus can resist phagosomal killing, impair complement activity, disrupt cytokine signalling and target phagocytes directly using proteolytic enzymes and cytolytic toxins. Although most of these virulence factors are well characterized, their importance during infection is less clear, as many display species-specific activity against humans or against animal hosts, including cows, horses and chickens. Several staphylococcal virulence factors display species specificity for components of the human innate immune system, with as few as two amino acid changes reducing binding affinity by as much as 100-fold. This represents a major issue for studying their roles during infection, which cannot be examined without the use of humanized infection models. This review summarizes the major factors S. aureus uses to impair the innate immune system, and provides an in-depth look into the host specificity of S. aureus and how this problem is being approached.

The Streptococcus agalactiae complement interfering protein combines multiple complement-inhibitory mechanisms by interacting with both C4 and C3 ligands

Group B Streptococcus (GBS) colonizes the human lower intestinal and genital tracts and constitutes a major threat to neonates from pregnant carrier mothers and to adults with underlying morbidity. The pathogen expresses cell-surface virulence factors that enable cell adhesion and penetration and that counteract innate and adaptive immune responses. Among these, the complement interfering protein (CIP) was recently described for its capacity to interact with the human C4b ligand and to interfere with the classical- and lectin-complement pathways. In the present study, we provide evidence that CIP can also interact with C3, C3b, and C3d. Immunoassay-based competition experiments showed that binding of CIP to C3d interferes with the interaction between C3d and the complement receptor 2/cluster of differentiation 21 (CR2/CD21) receptor on B cells. By B-cell intracellular signaling assays, CIP was confirmed to down-regulate CR2/CD21-dependent B-cell activation. The CIP domain involved in C3d binding was mapped via hydrogen deuterium exchange–mass spectrometry. The data obtained reveal a new role for this GBS polypeptide at the interface between the innate and adaptive immune responses, adding a new member to the growing list of virulence factors secreted by gram-positive pathogens that incorporate multiple immunomodulatory functions.—Giussani, S., Pietrocola, G., Donnarumma, D., Norais, N., Speziale, P., Fabbrini, M., Margarit, I. The Streptococcus agalactiae complement interfering protein combines multiple complement-inhibitory mechanisms by interacting with both C4 and C3 ligands.

Antibody-Based Agents in the Management of Antibiotic-Resistant Staphylococcus aureus Diseases

Staphylococcus aureus is a human pathogen that can cause a wide spectrum of diseases, including sepsis, pneumonia, arthritis, and endocarditis. Ineffective treatment of a number of staphylococcal infections with antibiotics is due to the development and spread of antibiotic-resistant strains following decades of antibiotic usage. This has generated renewed interest within the scientific community in alternative therapeutic agents, such as anti-S. aureus antibodies. Although the role of antibodies in the management of S. aureus diseases is controversial, the success of this pathogen in neutralizing humoral immunity clearly indicates that antibodies offer the host extensive protection. In this review, we report an update on efforts to develop antibody-based agents, particularly monoclonal antibodies, and their therapeutic potential in the passive immunization approach to the treatment and prevention of S. aureus infections.

Epic Immune Battles of History: Neutrophils vs. Staphylococcus aureus

Neutrophils are the most abundant leukocytes in human blood and the first line of defense after bacteria have breached the epithelial barriers. After migration to a site of infection, neutrophils engage and expose invading microorganisms to antimicrobial peptides and proteins, as well as reactive oxygen species, as part of their bactericidal arsenal. Ideally, neutrophils ingest bacteria to prevent damage to surrounding cells and tissues, kill invading microorganisms with antimicrobial mechanisms, undergo programmed cell death to minimize inflammation, and are cleared away by macrophages. Staphylococcus aureus (S. aureus) is a prevalent Gram-positive bacterium that is a common commensal and causes a wide range of diseases from skin infections to endocarditis. Since its discovery, S. aureus has been a formidable neutrophil foe that has challenged the efficacy of this professional assassin. Indeed, proper clearance of S. aureus by neutrophils is essential to positive infection outcome, and S. aureus has developed mechanisms to evade neutrophil killing. Herein, we will review mechanisms used by S. aureus to modulate and evade neutrophil bactericidal mechanisms including priming, activation, chemotaxis, production of reactive oxygen species, and resolution of infection. We will also highlight how S. aureus uses sensory/regulatory systems to tailor production of virulence factors specifically to the triggering signal, e.g., neutrophils and defensins. To conclude, we will provide an overview of therapeutic approaches that may potentially enhance neutrophil antimicrobial functions.

The red edge excitation shift phenomenon can be used to unmask protein structural ensembles: implications for NEMO-ubiquitin interactions

To understand complex molecular interactions, it is necessary to account for molecular flexibility and the available equilibrium of conformational states. Only a small number of experimental approaches can access such information. Potentially steady-state red edge excitation shift (REES) spectroscopy can act as a qualitative metric of changes to the protein free energy landscape (FEL) and the equilibrium of conformational states. First, we validate this hypothesis using a single Trp-containing protein, NF-κB essential modulator (NEMO). We provide detailed evidence from chemical denaturation studies, macromolecular crowding studies, and the first report of the pressure dependence of the REES effect. Combination of these data demonstrate that the REES effect can report on the 'ruggedness' of the FEL and we present a phenomenological model, based on realistic physical interpretations, for fitting steady-state REES data to allow quantification of this aspect of the REES effect. We test the conceptual framework we have developed by correlating findings from NEMO ligand-binding studies with the REES data in a range of NEMO-ligand binary complexes. Our findings shed light on the nature of the interaction between NEMO and poly-ubiquitin, suggesting that NEMO is differentially regulated by poly-ubiquitin chain length and that this regulation occurs via a modulation of the available equilibrium of conformational states, rather than gross structural change. This study therefore demonstrates the potential of REES as a powerful tool for tackling contemporary issues in structural biology and biophysics and elucidates novel information on the structure-function relationship of NEMO and key interaction partners.

Complement System Part II: Role in Immunity

The complement system has been considered for a long time as a simple lytic cascade, aimed to kill bacteria infecting the host organism. Nowadays, this vision has changed and it is well accepted that complement is a complex innate immune surveillance system, playing a key role in host homeostasis, inflammation, and in the defense against pathogens. This review discusses recent advances in the understanding of the role of complement in physiology and pathology. It starts with a description of complement contribution to the normal physiology (homeostasis) of a healthy organism, including the silent clearance of apoptotic cells and maintenance of cell survival. In pathology, complement can be a friend or a foe. It acts as a friend in the defense against pathogens, by inducing opsonization and a direct killing by C5b–9 membrane attack complex and by triggering inflammatory responses with the anaphylatoxins C3a and C5a. Opsonization plays also a major role in the mounting of an adaptive immune response, involving antigen presenting cells, T-, and B-lymphocytes. Nevertheless, it can be also an enemy, when pathogens hijack complement regulators to protect themselves from the immune system. Inadequate complement activation becomes a disease cause, as in atypical hemolytic uremic syndrome, C3 glomerulopathies, and systemic lupus erythematosus. Age-related macular degeneration and cancer will be described as examples showing that complement contributes to a large variety of conditions, far exceeding the classical examples of diseases associated with complement deficiencies. Finally, we discuss complement as a therapeutic target.

Application of the C3-binding motif of streptococcal pyrogenic exotoxin B to protect mice from invasive group a streptococcal infection

Group A streptococcus (GAS) is an important human pathogen that produces several extracellular exotoxins to facilitate invasion and infection. Streptococcal pyrogenic exotoxin B (SPE B) has been demonstrated to be an important virulence factor of GAS. Our previous studies indicate that SPE B cleaves complement 3 (C3) and inhibits the activation of complement pathways. In this study, we constructed and expressed recombinant fragments of SPE B to examine the C3-binding site of SPE B. Using enzyme-linked immunosorbent assays and pull-down assays, we found that the C-terminal domain, containing amino-acid residues 345–398, of SPE B was the major binding site of human serum C3. We further identified a major, Ala³⁷⁶-Pro³⁹⁸, and a minor C3-binding motif, Gly³⁴⁶-Gly³⁶⁰, that both mediated the binding of C3 complement. Immunization with the C3-binding motifs protected mice against challenge with a lethal dose of non-invasive M49 strain GAS but not invasive M1 strains. To achieve higher efficiency against invasive M1 GAS infection, a combination of synthetic peptides derived from C-terminal epitope of streptolysin S (SLSpp) and from the major C3-binding motif of SPE B (PP6, Ala³⁷⁶-Pro³⁹⁸) was used to elicit specific immune response to those two important streptococcal exotoxins. Death rates and the severity of skin lesions decreased significantly in PP6/SLSpp-immunized mice that were infected with invasive M1 strains of GAS. These results indicate a combination of the C3-binding motif of SPE B and the protective epitope of SLS could be used as a subunit vaccine against invasive M1 strains group A streptococcal infection.

Molecular analysis of the interaction between staphylococcal virulence factor Sbi-IV and complement C3d

Staphylococcus aureus expresses numerous virulence factors that aid in immune evasion. The four-domain staphylococcal immunoglobulin binding (Sbi) protein interacts with complement component 3 (C3) and its thioester domain (C3d)-containing fragments. Recent structural data suggested two possible modes of binding of Sbi domain IV (Sbi-IV) to C3d, but the physiological binding mode remains unclear. We used a computational approach to provide insight into the C3d-Sbi-IV interaction. Molecular dynamics (MD) simulations showed that the first binding mode (PDB code 2WY8) is more robust than the second (PDB code 2WY7), with more persistent polar and nonpolar interactions, as well as conserved interfacial solvent accessible surface area. Brownian dynamics and steered MD simulations revealed that the first binding mode has faster association kinetics and maintains more stable intermolecular interactions compared to the second binding mode. In light of available experimental and structural data, our data confirm that the first binding mode represents Sbi-IV interaction with C3d (and C3) in a physiological context. Although the second binding mode is inherently less stable, we suggest a possible physiological role. Both binding sites may serve as a template for structure-based design of novel complement therapeutics.

Staphylococcus aureus: the multi headed hydra resists and controls human complement response in multiple ways

The Gram positive human pathogen Staphylococcus aureus causes a spectrum of human diseases including pneumonia, tissue and skin infections, endocarditis, pneumonia and sepsis. The increasing number of resistant bacteria and the threat of methicillin resistant S. aureus (MRSA) urge for the need to develop new antibacterial compounds. A prerequisite for development of such anti microbial compounds is a better understanding of the complex immune crosstalk between the pathogenic bacterium and its human host. To this end proteins staphylococcal proteins that contribute to innate immune evasion especially to complement control need to be identified and their mode of action needs to be analyzed in order to provide new targets for immune interference.

Post-translational modification in the archaea: structural characterization of multi-enzyme complex lipoylation

Lipoylation, the covalent attachment of lipoic acid to 2-oxoacid dehydrogenase multi-enzyme complexes, is essential for metabolism in aerobic bacteria and eukarya. In Escherichia coli, lipoylation is catalysed by LplA (lipoate protein ligase) or by LipA (lipoic acid synthetase) and LipB [lipoyl(octanoyl) transferase] combined. Whereas bacterial and eukaryotic LplAs comprise a single two-domain protein, archaeal LplA function typically involves two proteins, LplA-N and LplA-C. In the thermophilic archaeon Thermoplasma acidophilum, LplA-N and LplA-C are encoded by overlapping genes in inverted orientation (lpla-c is upstream of lpla-n). The T. acidophilum LplA-N structure is known, but the LplA-C structure is unknown and LplA-C's role in lipoylation is unclear. In the present study, we have determined the structures of the substrate-free LplA-N-LplA-C complex and E2lipD (dihydrolipoyl acyltransferase lipoyl domain) that is lipoylated by LplA-N-LplA-C, and carried out biochemical analyses of this archaeal lipoylation system. Our data reveal the following: (i) LplA-C is disordered but folds upon association with LplA-N; (ii) LplA-C induces a conformational change in LplA-N involving substantial shortening of a loop that could repress catalytic activity of isolated LplA-N; (iii) the adenylate-binding region of LplA-N-LplA-C includes two helices rather than the purely loop structure of varying order observed in other LplA structures; (iv) LplAN-LplA-C and E2lipD do not interact in the absence of substrate; (v) LplA-N-LplA-C undergoes a conformational change (the details of which are currently undetermined) during lipoylation; and (vi) LplA-N-LplA-C can utilize octanoic acid as well as lipoic acid as substrate. The elucidated functional inter-dependence of LplA-N and LplA-C is consistent with their evolutionary co-retention in archaeal genomes.

Regulation of humoral immunity by complement

The complement system of innate immunity is important in regulating humoral immunity largely through the complement receptor CR2, which forms a coreceptor on B cells during antigen-induced activation. However, CR2 also retains antigens on follicular dendritic cells (FDCs). Display of antigen on FDCs is critical for clonal selection and affinity maturation of activated B cells. This review will discuss the role of complement in adaptive immunity in general with a focus on the interplay between CR2-associated antigen on B cells with CR2 expressed on FDCs. This latter interaction provides an opportunity for memory B cells to sample antigen over prolonged periods. The cocrystal structure of CR2 with its ligand C3d provides insight into how the complement system regulates access of antigen by B cells with implications for therapeutic manipulations to modulate aberrant B cell responses in the case of autoimmunity.

Staphylococcus aureus proteins Sbi and Efb recruit human plasmin to degrade complement C3 and C3b

Upon host infection, the human pathogenic microbe Staphylococcus aureus (S. aureus) immediately faces innate immune reactions such as the activated complement system. Here, a novel innate immune evasion strategy of S. aureus is described. The staphylococcal proteins surface immunoglobulin-binding protein (Sbi) and extracellular fibrinogen-binding protein (Efb) bind C3/C3b simultaneously with plasminogen. Bound plasminogen is converted by bacterial activator staphylokinase or by host-specific urokinase-type plasminogen activator to plasmin, which in turn leads to degradation of complement C3 and C3b. Efb and to a lesser extend Sbi enhance plasmin cleavage of C3/C3b, an effect which is explained by a conformational change in C3/C3b induced by Sbi and Efb. Furthermore, bound plasmin also degrades C3a, which exerts anaphylatoxic and antimicrobial activities. Thus, S. aureus Sbi and Efb comprise platforms to recruit plasmin(ogen) together with C3 and its activation product C3b for efficient degradation of these complement components in the local microbial environment and to protect S. aureus from host innate immune reactions.

The immune evasion protein Sbi of Staphylococcus aureus occurs both extracellularly and anchored to the cell envelope by binding lipoteichoic acid

The Sbi protein of Staphylococcus aureus comprises two IgG-binding domains similar to those of protein A and a region that triggers the activation of complement C3. Sbi is expressed on the cell surface but its C-terminal domain lacks motifs associated with wall or membrane anchoring of proteins in Gram-positive bacteria. Cell-associated Sbi fractionates with the cytoplasmic membrane and is not solubilized during protoplast formation. S. aureus expressing Sbi truncates of the C-terminal Y domain allowed identification of residues that are required for association of Sbi with the membrane. Recombinant Sbi bound to purified cytoplasmic membrane material in vitro and to purified lipoteichoic acid. This explains how Sbi partitions with the membrane in fractionation experiments yet is partially exposed on the cell surface. An LTA-defective mutant of S. aureus had reduced levels of Sbi in the cytoplasmic membrane.

IgG-binding proteins of bacteria

Proteins capable of non-immune binding of immunoglobulins G (IgG) of various mammalian species, i.e. without the involvement of the antigen-binding sites of the immunoglobulins, are widespread in bacteria. These proteins are located on the surface of bacterial cells and help them to evade the host's immune response due to protection against the action of complement and to decrease in phagocytosis. This review summarizes data on the structure of immunoglobulin-binding proteins (IBP) and their complexes with IgG. Common and distinctive structural features of IBPs of gram-positive bacteria (staphylococci, streptococci, peptostreptococci) are discussed. Conditions for IBP expression by bacteria and their functional heterogeneity are considered. Data on IBPs of gram-negative bacteria are presented.

The Sbi protein is a multifunctional immune evasion factor of Staphylococcus aureus

The second immunoglobulin-binding protein (Sbi) of Staphylococcus aureus has two N-terminal domains that bind the Fc region of IgG in a fashion similar to that of protein A and two domains that can bind to the complement protein C3 and promote its futile consumption in the fluid phase. It has been proposed that Sbi helps bacteria to avoid innate immune defenses. By comparing a mutant defective in Sbi with mutants defective in protein A, clumping factor A, iron-regulated surface determinant H, and capsular polysaccharide, it was shown that Sbi is indeed an immune evasion factor that promotes bacterial survival in whole human blood and the avoidance of neutrophil-mediated opsonophagocytosis. Sbi is present in the culture supernatant and is also associated with the cell envelope. S. aureus strains that expressed truncates of Sbi lacking N-terminal domains D1 and D2 (D1D2) or D3 and D4 (D3D4) or a C-terminal truncate that was no longer retained in the cell envelope were analyzed. Both the secreted and envelope-associated forms of Sbi contributed to immune evasion. The IgG-binding domains contributed only when Sbi was attached to the cell, while only the secreted C3-binding domains were biologically active.

Immune evasion of Moraxella catarrhalis involves ubiquitous surface protein A-dependent C3d binding

The complement system plays an important role in eliminating invading pathogens. Activation of complement results in C3b deposition (opsonization), phagocytosis, anaphylatoxin (C3a, C5a) release, and consequently cell lysis. Moraxella catarrhalis is a human respiratory pathogen commonly found in children with otitis media and in adults with chronic obstructive pulmonary disease. The species has evolved multiple complement evasion strategies, which among others involves the ubiquitous surface protein (Usp) family consisting of UspA1, A2, and A2 hybrid. In the present study, we found that the ability of M. catarrhalis to bind C3 correlated with UspA expression and that C3 binding contributed to serum resistance in a large number of clinical isolates. Recombinantly expressed UspA1 and A2 inhibit both the alternative and classical pathways, C3b deposition, and C3a generation when bound to the C3 molecule. We also revealed that the M. catarrhalis UspA-binding domain on C3b was located to C3d and that the major bacterial C3d-binding domains were within UspA1(299-452) and UspA2(165-318). The interaction with C3 was not species specific since UspA-expressing M. catarrhalis also bound mouse C3 that resulted in inhibition of the alternative pathway of mouse complement. Taken together, the binding of C3 to UspAs is an efficient strategy of Moraxella to block the activation of complement and to inhibit C3a-mediated inflammation.

A structural basis for Staphylococcal complement subversion: X-ray structure of the complement-binding domain of Staphylococcus aureus protein Sbi in complex with ligand C3d

The structure of the complement-binding domain of Staphylococcus aureus protein Sbi (Sbi-IV) in complex with ligand C3d is presented. The 1.7Å resolution structure reveals the molecular details of the recognition of thioester-containing fragment C3d of the central complement component C3, involving interactions between residues of Sbi-IV helix α2 and the acidic concave surface of C3d. The complex provides a structural basis for the binding preference of Sbi for native C3 over C3b and explains how Sbi-IV inhibits the interaction between C3d and complement receptor 2. A second C3d binding site on Sbi-IV is identified in the crystal structure that is not observed in related S. aureus C3 inhibitors Efb-C and Ehp. This binding mode perhaps hints as to how Sbi-IV, as part of Sbi, forms a C3b-Sbi adduct and causes futile consumption of C3, an extraordinary aspect of Sbi function that is not shared by any other known Staphylococcal complement inhibitor.

Delineation of the complement receptor type 2-C3d complex by site-directed mutagenesis and molecular docking

The interactions between the complement receptor type 2 (CR2) and the C3 complement fragments C3d, C3dg, and iC3b are essential for the initiation of a normal immune response. A crystal-derived structure of the two N-terminal short consensus repeat (SCR1-2) domains of CR2 in complex with C3d has previously been elucidated. However, a number of biochemical and biophysical studies targeting both CR2 and C3d appear to be in conflict with these structural data. Previous mutagenesis and heteronuclear NMR spectroscopy studies directed toward the C3d-binding site on CR2 have indicated that the CR2-C3d cocrystal structure may represent an encounter/intermediate or nonphysiological complex. With regard to the CR2-binding site on C3d, mutagenesis studies by Isenman and coworkers [Isenman, D. E., Leung, E., Mackay, J. D., Bagby, S. & van den Elsen, J. M. H. (2010). Mutational analyses reveal that the staphylococcal immune evasion molecule Sbi and complement receptor 2 (CR2) share overlapping contact residues on C3d: Implications for the controversy regarding the CR2/C3d cocrystal structure. J. Immunol. 184, 1946-1955] have implicated an electronegative "concave" surface on C3d in the binding process. This surface is discrete from the CR2-C3d interface identified in the crystal structure. We generated a total of 18 mutations targeting the two (X-ray crystallographic- and mutagenesis-based) proposed CR2 SCR1-2 binding sites on C3d. Using ELISA analyses, we were able to assess binding of mutant forms of C3d to CR2. Mutations directed toward the concave surface of C3d result in substantially compromised CR2 binding. By contrast, targeting the CR2-C3d interface identified in the cocrystal structure and the surrounding area results in significantly lower levels of disruption in binding. Molecular modeling approaches used to investigate disparities between the biochemical data and the X-ray structure of the CR2-C3d cocrystal result in highest-scoring solutions in which CR2 SCR1-2 is docked within the concave surface of C3d.

Genetic variation in Staphylococcus aureus surface and immune evasion genes is lineage associated: implications for vaccine design and host-pathogen interactions

Background

S. aureus is a coloniser and pathogen of humans and mammals. Whole genome sequences of 58 strains of S. aureus in the public domain and data from multi-strain microarrays were compared to assess variation in the sequence of proteins known or putatively interacting with host.

Results

These included 24 surface proteins implicated in adhesion (ClfA, ClfB, Cna, Eap, Ebh, EbpS, FnBPA, FnBPB, IsaB, IsdA, IsdB, IsdH, SasB, SasC, SasD, SasF, SasG, SasH, SasK, SdrC, SdrD, SdrE, Spa and SraP) and 13 secreted proteins implicated in immune response evasion (Coa, Ecb, Efb, Emp, EsaC, EsxA, EssC, FLIPr, FLIPr like, Sbi, SCIN-B, SCIN-C, VWbp) located on the stable core genome. Many surface protein genes were missing or truncated, unlike immune evasion genes, and several distinct variants were identified. Domain variants were lineage specific. Unrelated lineages often possess the same sequence variant domains proving that horizontal transfer and recombination has contributed to their evolution. Surprisingly, sequenced strains from four animal S. aureus strains had surface and immune evasion proteins remarkably similar to those found in human strains, yet putative targets of these proteins vary substantially between different hosts. This suggests these proteins are not essential for virulence. However, the most variant protein domains were the putative functional regions and there is biological evidence that variants can be functional, arguing they do play a role.

Conclusion

Surface and immune evasion genes are candidates for S. aureus vaccines, and their distribution and functionality is key. Vaccines should contain cocktails of antigens representing all variants or they will not protect against naturally occurring S. aureus populations.

A Staphylococcus aureus small RNA is required for bacterial virulence and regulates the expression of an immune-evasion molecule

Staphylococcus aureus, a pathogen responsible for hospital and community-acquired infections, expresses many virulence factors under the control of numerous regulatory systems. Here we show that one of the small pathogenicity island RNAs, named SprD, contributes significantly to causing disease in an animal model of infection. We have identified one of the targets of SprD and our in vivo data demonstrate that SprD negatively regulates the expression of the Sbi immune-evasion molecule, impairing both the adaptive and innate host immune responses. SprD interacts with the 5′ part of the sbi mRNA and structural mapping of SprD, its mRNA target, and the ‘SprD-mRNA’ duplex, in combination with mutational analysis, reveals the molecular details of the regulation. It demonstrates that the accessible SprD central region interacts with the sbi mRNA translational start site. We show by toeprint experiments that SprD prevents translation initiation of sbi mRNA by an antisense mechanism. SprD is a small regulatory RNA required for S. aureus pathogenicity with an identified function, although the mechanism of virulence control by the RNA is yet to be elucidated.

Bacteria possess numerous and diverse means of gene regulation using RNA molecules, including small RNAs (sRNAs). Here we show that one sRNA is essential for a major human bacterial pathogen, Staphylococcus aureus, to cause a disease in an animal model of infection. Our study provides evidence that this RNA regulates the expression of an immune evasion molecule secreted by the bacterium to impair the host immune responses, and we have solved the mechanism of the RNA-based regulation at molecular level. So far, the mechanism of bacterial virulence controlled by SprD is unrevealed, but that small RNA has a huge impact in the course of a bacterial infection. It implies possible new strategies in fighting against that major human and animal bacterial pathogen in preventing the expression of this regulatory RNA.

Convertase inhibitory properties of Staphylococcal extracellular complement-binding protein

The human pathogen Staphylococcus aureus secretes several complement evasion molecules to combat the human immune response. Extracellular complement-binding protein (Ecb) binds to the C3d domain of C3 and thereby blocks C3 convertases of the alternative pathway and C5 convertases via all complement pathways. Inhibition of C5 convertases results in complete inhibition of C5a generation and subsequent neutrophil migration. Here, we show that binding of Ecb to the C3d domain of C3b is crucial for inhibition of C5 convertases. Ecb does not interfere with substrate binding to convertases but prevents formation of an active convertase enzyme.

Grading the commercial optical biosensor literature-Class of 2008: 'The Mighty Binders'

Optical biosensor technology continues to be the method of choice for label-free, real-time interaction analysis. But when it comes to improving the quality of the biosensor literature, education should be fundamental. Of the 1413 articles published in 2008, less than 30% would pass the requirements for high-school chemistry. To teach by example, we spotlight 10 papers that illustrate how to implement the technology properly. Then we grade every paper published in 2008 on a scale from A to F and outline what features make a biosensor article fabulous, middling or abysmal. To help improve the quality of published data, we focus on a few experimental, analysis and presentation mistakes that are alarmingly common. With the literature as a guide, we want to ensure that no user is left behind.

Mutational analyses reveal that the staphylococcal immune evasion molecule Sbi and complement receptor 2 (CR2) share overlapping contact residues on C3d: implications for the controversy regarding the CR2/C3d cocrystal structure

We recently characterized an interaction between the Staphylococcus aureus immune evasion molecule Staphylococcus aureus binder of Ig (Sbi) and complement C3, an interaction mediated primarily through the binding of C3d(g) to Sbi domain IV. Events related to these studies prompted us to investigate via mutagenesis the binding interface of C3d for Sbi domain IV (Sbi-IV), as well as to revisit the controversial issue of the complement receptor 2 (CR2) binding site of C3d. Specifically, we had shown that Sbi domains III and IV fragment binding to C3dg inhibited the latter's binding to CR2. Moreover, a published cocrystal structure of C3d bound to complement inhibitory C-terminal domain of extracellular fibrinogen-binding protein (Efb-C), a structural and functional homolog of Sbi-IV, showed Efb-C binding to a region on the concave face of C3d previously implicated in CR2 binding by our mutagenesis data but not confirmed in the CR2(short consensus repeat [SCR]1-2):C3d cocrystal structure. We have now analyzed by surface plasmon resonance the binding of a series of variant C3dg molecules to biosensor-bound Sbi-IV or CR2(SCR1-2). We found that mutations to the concave face acidic pocket of C3d significantly affected binding to both Sbi-IV and CR2, although there was divergence in which residues were most important in each case. By contrast, no binding defects were seen for mutations made to the sideface of C3d implicated from the cocrystal structure to be involved in binding CR2(SCR1-2). The results with Sbi-IV suggest a mode of binding highly similar to that visualized in the Efb-C:C3d complex. The results with CR2 confirm our earlier mapping studies and cast even further doubt on the physiologic relevance of the complex visualized in the C3d:CR2 cocrystal.

A molecular insight into complement evasion by the staphylococcal complement inhibitor protein family

Staphylococcus aureus possesses an impressive arsenal of complement evasion proteins that help the bacterium escape attack of the immune system. The staphylococcal complement inhibitor (SCIN) protein exhibits a particularly high potency and was previously shown to block complement by acting at the level of the C3 convertases. However, many details about the exact binding and inhibitory mechanism remained unclear. In this study, we demonstrate that SCIN directly binds with nanomolar affinity to a functionally important area of C3b that lies near the C terminus of its beta-chain. Direct competition of SCIN with factor B for C3b slightly decreased the formation of surface-bound convertase. However, the main inhibitory effect can be attributed to an entrapment of the assembled convertase in an inactive state. Whereas native C3 is still able to bind to the blocked convertase, no generation and deposition of C3b could be detected in the presence of SCIN. Furthermore, SCIN strongly competes with the binding of factor H to C3b and influences its regulatory activities: the SCIN-stabilized convertase was essentially insensitive to decay acceleration by factor H and the factor I- and H-mediated conversion of surface-bound C3b to iC3b was significantly reduced. By targeting a key area on C3b, SCIN is able to block several essential functions within the alternative pathway, which explains the high potency of the inhibitor. Our findings provide an important insight into complement evasion strategies by S. aureus and may act as a base for further functional studies.

Complement evasion strategies of pathogens-acquisition of inhibitors and beyond

Activation of the complement system and resulting opsonisation with C3b are key events of the innate immune defense against infections. However, a wide variety of bacterial pathogens subvert complement attack by binding host complement inhibitors such as C4b-binding protein, factor H and vitronectin, which results in diminished opsonophagocytosis and killing of bacteria by lysis. Another widely used strategy is production of proteases, which can effectively degrade crucial complement components. Furthermore, bacterial pathogens such as Moraxella catarrhalis and Staphylococcus aureus capture and incapacitate the key complement component C3. The current review describes examples of these three strategies. Targeting binding sites for complement inhibitors on bacterial surfaces and complement-degrading proteases with vaccine-induced antibodies may be used to enhance a common vaccine design strategy that depends on the generation of complement-dependent bactericidal and opsonophagocytic antibody activities.

Microbial complement inhibitors as vaccines

Complement inhibiting surface proteins of pathogenic bacteria provide candidates for vaccines because of two reasons. First, an immune response against them would recognize the microbes and secondly, it would neutralize the key bacterial virulence mechanism. Prerequisites for a vaccine protein include the following: (i) it should show limited variability, (ii) it should be immunogenic and the immune response against it should cover a sufficiently broad range of microbial strains, (iii) it should not be hidden beneath a capsule, long LPS O-polysaccharide side chains or a protein coat and (iv) it should not raise unwanted immune responses against host structures. Bacterial complement inhibitors often act by binding the soluble inhibitors factor H or C4 bp, by blocking C3 or C5 activation or by enzymatically cleaving key complement components. Inhibitors have been found from all major types of pathogens and may offer promise as rational vaccine candidates for preventing diseases such as meningococcal meningitis, systemic pneumococcal or group B streptococcal disease and Lyme borreliosis.