Capsule production and growth phase influence binding of complement to Staphylococcus aureus

Microbial Mysteries: Staphylococcus aureus and the Enigma of Carcinogenesis

Staphylococcus aureus, a common human pathogen, has long been the focus of scientific investigation due to its association with various infections. However, recent research has unveiled a tantalizing enigma surrounding this bacterium and its potential involvement in carcinogenesis. Chronic S. aureus infections have been linked to an elevated risk of certain cancers, including skin cancer and oral cancer. This review explores the current state of knowledge regarding this connection, examining epidemiological evidence, pathogenic mechanisms, and biological interactions that suggest a correlation. Although initial studies point to a possible link, the precise mechanisms through which S. aureus may contribute to cancer development remain elusive. Emerging evidence suggests that the chronic inflammation induced by persistent S. aureus infections may create a tumor-promoting environment. This inflammation can lead to DNA damage, disrupt cellular signaling pathways, and generate an immunosuppressive microenvironment conducive to cancer progression. Additionally, S. aureus produces a variety of toxins and metabolites that can directly interact with host cells, potentially inducing oncogenic transformations. Despite these insights, significant gaps remain in our understanding of the exact biological processes involved. This review emphasizes the urgent need for more comprehensive research to clarify these microbiological mysteries. Understanding the role of S. aureus in cancer development could lead to novel strategies for cancer prevention and treatment, potentially transforming therapeutic approaches.

The human factor H protein family - an update

Complement is an ancient and complex network of the immune system and, as such, it plays vital physiological roles, but it is also involved in numerous pathological processes. The proper regulation of the complement system is important to allow its sufficient and targeted activity without deleterious side-effects. Factor H is a major complement regulator, and together with its splice variant factor H-like protein 1 and the five human factor H-related (FHR) proteins, they have been linked to various diseases. The role of factor H in inhibiting complement activation is well studied, but the function of the FHRs is less characterized. Current evidence supports the main role of the FHRs as enhancers of complement activation and opsonization, i.e., counter-balancing the inhibitory effect of factor H. FHRs emerge as soluble pattern recognition molecules and positive regulators of the complement system. In addition, factor H and some of the FHR proteins were shown to modulate the activity of immune cells, a non-canonical function outside the complement cascade. Recent efforts have intensified to study factor H and the FHRs and develop new tools for the distinction, quantification and functional characterization of members of this protein family. Here, we provide an update and overview on the versatile roles of factor H family proteins, what we know about their biological functions in healthy conditions and in diseases.

Platelet factor 4 improves survival in a murine model of antibiotic-susceptible and methicillin-resistant Staphylococcus aureus peritonitis

The complement receptor CR3, also known as integrin Mac-1 (CD11b/CD18), is one of the major phagocytic receptors on the surface of neutrophils and macrophages. We previously demonstrated that in its protein ligands, Mac-1 binds sequences enriched in basic and hydrophobic residues and strongly disfavors negatively charged sequences. The avoidance by Mac-1 of negatively charged surfaces suggests that the bacterial wall and bacterial capsule possessing net negative electrostatic charge may repel Mac-1 and that the cationic Mac-1 ligands can overcome this evasion by acting as opsonins. Indeed, we previously showed that opsonization of Gram-negative Escherichia coli with several cationic peptides, including PF4 (Platelet Factor 4), strongly augmented phagocytosis by macrophages. Here, we investigated the effect of recombinant PF4 (rPF4) on phagocytosis of Gram-positive Staphylococcus aureus in vitro and examined its impact in a mouse model of S. aureus peritonitis. Characterization of the interaction of rPF4 with nonencapsulated and encapsulated S. aureus showed that rPF4 localizes on the bacterial surface, thus making it available for Mac-1. Furthermore, rPF4 did not have direct bactericidal and bacteriostatic activity and was not toxic to host cells. rPF4 enhanced phagocytosis of S. aureus bioparticles by various primary and cultured Mac-1-expressing leukocytes by several folds. It also increased phagocytosis of live nonencapsulated and encapsulated bacteria. Notably, the augmentation of phagocytosis by rPF4 did not compromise the intracellular killing of S. aureus by macrophages. Using a murine S. aureus peritonitis model, we showed that treatment of infected mice with rPF4 caused a significant increase in the clearance of antibiotic-susceptible S. aureus and its methicillin-resistant (MRSA) variant and markedly improved survival. These findings indicate that rPF4 binding to the bacterial surface circumvents its antiphagocytic properties, improving host defense against antibiotic-susceptible and antibiotic-resistant bacteria.

PLATELET FACTOR 4 (PF4) IMPROVES SURVIVAL IN A MURINE MODEL OF ANTIBIOTIC-SUSCEPTIBLE AND METHICILLIN-RESISTANT STAPHYLOCOCCUS AUREUS PERITONITIS

The complement receptor CR3, also known as integrin Mac-1 (CD11b/CD18), is one of the major phagocytic receptors on the surface of neutrophils and macrophages. We previously demonstrated that in its protein ligands, Mac-1 binds sequences enriched in basic and hydrophobic residues and strongly disfavors negatively charged sequences. The avoidance by Mac-1 of negatively charged surfaces suggests that the bacterial wall and bacterial capsule possessing net negative electrostatic charge may repel Mac-1 and that the cationic Mac-1 ligands can overcome this evasion by acting as opsonins. Indeed, we previously showed that opsonization of Gram-negative Escherichia coli with several cationic peptides, including PF4 (Platelet Factor 4), strongly augmented phagocytosis by macrophages. Here, we investigated the effect of recombinant PF4 (rPF4) on phagocytosis of Gram-positive Staphylococcus aureus in vitro and examined its impact in a mouse model of S. aureus peritonitis. Characterization of the interaction of rPF4 with nonencapsulated and encapsulated S. aureus showed that rPF4 localizes on the bacterial surface, thus making it available for Mac-1. Furthermore, rPF4 did not have direct bactericidal and bacteriostatic activity and was not toxic to host cells. rPF4 enhanced phagocytosis of S. aureus bioparticles by various primary and cultured Mac-1-expressing leukocytes by several folds. It also increased phagocytosis of live nonencapsulated and encapsulated bacteria. Notably, the augmentation of phagocytosis by rPF4 did not compromise the intracellular killing of S. aureus by macrophages. Using a murine S. aureus peritonitis model, we showed that treatment of infected mice with rPF4 caused a significant increase in the clearance of antibiotic-susceptible S. aureus and its methicillin-resistant (MRSA) variant and markedly improved survival. These findings indicate that rPF4 binding to the bacterial surface circumvents its antiphagocytic properties, improving host defense against antibiotic-susceptible and antibiotic-resistant bacteria.

Contribution of Extracellular Membrane Vesicles To the Secretome of Staphylococcus aureus

The microbial secretome modulates how the organism interacts with its environment. Included in the Staphylococcus aureus secretome are extracellular membrane vesicles (MVs) that consist of cytoplasmic and membrane proteins, as well as exoproteins, some cell wall-associated proteins, and glycopolymers. The extent to which MVs contribute to the diverse composition of the secretome is not understood. We performed a proteomic analysis of MVs purified from the S. aureus strain MRSA252 along with a similar analysis of the whole secretome (culture supernatant) before and after depletion of MVs. The MRSA252 secretome was comprised of 1,001 proteins, of which 667 were also present in MVs. Cell membrane-associated proteins and lipoteichoic acid in the culture supernatant were highly associated with MVs, followed by cytoplasmic and extracellular proteins. Few cell wall-associated proteins were contained in MVs, and capsular polysaccharides were found both in the secretome and MVs. When MVs were removed from the culture supernatant by ultracentrifugation, 54 of the secretome proteins were significantly depleted in abundance. Proteins packaged in MVs were characterized by an isoelectric point that was significantly higher than that of proteins excluded from MVs. Our data indicate that the generation of S. aureus MVs is a mechanism by which lipoteichoic acid, cytoplasmic, and cell membrane-associated proteins are released into the secretome. IMPORTANCE The secretome of Staphylococcus aureus includes soluble molecules and nano-sized extracellular membrane vesicles (MVs). The protein composition of both the secretome and MVs includes cytoplasmic and membrane proteins, as well as exoproteins, some cell wall-associated proteins, and glycopolymers. How the MV cargo differs from the protein composition of the secretome has not yet been addressed. Although the compositions of the secretome and MVs were strikingly similar, we identified 54 proteins that were specifically packaged in MVs. Proteins highly associated with MVs were characterized by their abundance in the secretome, an association with the bacterial membrane, and a basic isoelectric point. This study deepens our limited understanding about the contribution of MVs to the secretome of S. aureus.

Transcriptome and Proteome of Methicillin-Resistant Staphylococcus aureus Small-Colony Variants Reveal Changed Metabolism and Increased Immune Evasion

Methicillin-resistant Staphylococcus aureus (MRSA) infection has become a public health crisis. Recently, we isolated small-colony variants (SCVs) of MRSA, which are characterized by slow growth, decreased virulence, increased antibiotic resistance, and immune evasion. In the present study, we provided proteomic and transcriptomic profiles of clinical MRSA sequence type 239 (ST239) normal strains and SCVs and attempted to identify the key genes or pathways closely related to SCV formation and survival. RNAs and proteins were extracted and subjected to RNA sequencing and mass spectrometry, and the transcriptome and proteome were evaluated via bioinformatic analysis. The results were verified by functional assays. In total, 822 differentially expressed genes (DEGs) and 773 differentially expressed proteins (DEPs) were identified; of these, 286 DEGs and DEPs were correlated and subjected to Kyoto Encyclopedia Genes and Genomes analysis. Some pathways were significant, including ABC transporters, ribosome biogenesis, and metabolic pathways such as glycolysis/gluconeogenesis and the citrate cycle (tricarboxylic acid [TCA] cycle). Based on these results, we found that the downregulation of ABC transporters and the TCA cycle pathway resulted in electron transport chain deficiencies and reduced ATP production in SCVs, leading to a dependence on glycolysis and its upregulation. In addition, the upregulation of capsule polysaccharides and the downregulation of surface proteins prevented phagocytosis and reduced the adhesion of host cells, contributing to immune evasion by SCVs. These findings contribute to a better understanding of the mechanisms of SCV formation and survival. IMPORTANCE Small-colony variants (SCVs) of Staphylococcus aureus have drawn increasing research attention. Owing to their slow growth, atypical colony morphology, and unusual metabolic characteristics, SCVs often cause confusion in the laboratory. Furthermore, clinical treatment of SCVs is challenging owing to their antibiotic resistance and immune evasion, leading to persistent and recurrent infections. However, the mechanisms underlying their formation remain unclear. In this study, we isolated SCVs of methicillin-resistant S. aureus and provided transcriptomic and proteomic profiles of normal strains and SCVs. Based on our analysis, glycolysis upregulation and TCA cycle downregulation affected the electron transport chain and energy supply, leading to slower metabolism. Moreover, capsular biosynthesis was increased, while the number of surface proteins decreased, thus promoting immune evasion by SCVs.

Vaccination against Bacterial Mastitis in Sheep

The objective of this review is to discuss the application of vaccination for the prevention of bacterial mastitis in ewes, performed within the frame of health management schemes in sheep flocks. Mastitis is a multi-faceted infection, caused most often by staphylococci; hence, special emphasis is given to staphylococcal mastitis, also given that most relevant studies refer to vaccinations against that infection. Studies regarding various vaccines have been performed; most studies refer to vaccination by using a vaccine making use of cell-free surface polysaccharides in various vehicles, bacterial unbound cells or bacterial cells embedded in their biofilm matrix. Vaccination against mastitis should be better performed during the final stage of pregnancy to allow protection of ewes from lambing and should be considered as one of many control measures for the prevention of the disease. The expected benefits of mastitis vaccination in sheep flocks include the following: (a) reduced incidence risk of clinical and subclinical mastitis, (b) reduced somatic cell counts, optimum chemical composition, absence of staphylococci in milk, (c) increased milk production, (d) reduced dissemination of mastitis-causing pathogens and (e) reduction of antibiotic use in flocks.

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.

LPS modifications and AvrA activity of Salmonella enterica serovar Typhimurium are required to prevent Perforin-2 expression by infected fibroblasts and intestinal epithelial cells

Cellular Perforin-2 (MPEG1) is a pore-forming MACPF family protein that plays a critical role in the defense against bacterial pathogens. Macrophages, neutrophils, and several other cell types that are part of the front line of innate defenses constitutively express high levels of Perforin-2; whereas, most other cell types must be induced to express Perforin-2 by interferons (α, β and γ) and/or PAMPs such as LPS. In this study, we demonstrate that many bacterial pathogens can limit the expression of Perforin-2 in cells normally inducible for Perforin-2 expression, while ordinarily commensal or non-pathogenic bacteria triggered high levels of Perforin-2 expression in these same cell types. The mechanisms by which pathogens suppress Perforin-2 expression was explored further using Salmonella enterica serovar Typhimurium and cultured MEFs as well as intestinal epithelial cell lines. These studies identified multiple factors required to minimize the expression of Perforin-2 in cell types inducible for Perforin-2 expression. These included the PmrAB and PhoPQ two-component systems, select LPS modification enzymes and the Type III secretion effector protein AvrA.

Antiseptic quaternary ammonium compound tolerance by gram-negative bacteria can be rapidly detected using an impermeant fluorescent dye-based assay

Biocides such as quaternary ammonium compounds (QACs) are potentially important contributors towards bacterial antimicrobial resistance development, however, their contributions are unclear due to a lack of internationally recognized biocide testing standards. Methods to detect QAC tolerance are limited to laborious traditional antimicrobial susceptibility testing (AST) methods. Here, we developed a rapid fluorescent dye-based membrane impermeant assay (RFDMIA) to discriminate QAC susceptibility among Gram-negative Enterobacterales and Pseudomonadales species. RFDMIA uses a membrane impermeant fluorescent dye, propidium iodide, in a 30-min 96-well fluorescent microplate-based assay where cell suspensions are exposed to increasing QAC concentrations. Our results demonstrate that RFDMIA can discriminate between QAC-susceptible and QAC-adapted Escherichia coli tolerant phenotypes and predict benzalkonium and cetrimide tolerance in all species tested except for intrinsically fluorescent Pseudomonas aeruginosa. RFDMIA identified a close association to minimum inhibitory concentration values determined by broth microdilution AST and increasing fluorescent dye emission values. RFDMIA emission values and scanning electron microscopy results also suggest that CET-adapted E. coli isolates have a CET dependence, where cells require sub-inhibitory CET concentrations to maintain bacilliform cell integrity. Overall, this study generates a new, rapid, sensitive fluorescent assay capable of detecting QAC-susceptible Gram-negative bacteria phenotypes and cell membrane perturbations.

Classical complement activation on human erythrocytes in subjects with systemic lupus erythematosus and a history of autoimmune hemolytic anemia

INTRODUCTION

Autoimmune hemolytic anemia (AIHA) is a serious manifestation of systemic lupus erythematosus (SLE) associated with significant morbidity and mortality. In order to more fully understand the causative pathways, we utilized sera from subjects with SLE and active AIHA, or a history of AIHA, to evaluate the classical complement pathway, anti-erythrocyte antibodies, and immune complexes.

METHODS

To evaluate antibody-mediated complement activation on the surface of erythrocytes, as occurs in AIHA, blood type O erythrocytes were incubated with sera from 19 subjects with SLE and a history of AIHA. Circulating anti-erythrocyte antibodies and immune complexes were measured with ELISA-based assays.

RESULTS

In total, 90% of subjects with SLE and a history of AIHA, but not active clinical hemolysis, had measurable anti-erythrocyte antibodies. Of those with anti-erythrocyte antibody, 53% demonstrated complement opsonization on the erythrocyte surface >twofold above negative control and 29% generated the anaphylatoxin C5a.

CONCLUSIONS

For subjects with SLE and a history of AIHA, the persistence of circulating anti-erythrocyte antibodies and resultant erythrocyte complement opsonization and anaphylatoxin generation suggests the possibility that these complement effectors contribute to chronic morbidity and risk of AIHA relapse.

A complement-mediated rat xenotransfusion model of platelet refractoriness

BACKGROUND

Platelet refractoriness remains a challenging clinical dilemma although significant advancements have been made in identifying human leukocyte antigen (HLA) matched or HLA compatible units. Antiplatelet antibodies are the major risk factor for immune-mediated platelet refractoriness, yet the role of antibody-initiated complement-mediated platelet destruction remains poorly understood.

STUDY DESIGN AND METHODS

Human complement-mediated opsonization and killing of platelets was assayed ex vivo using antibody-sensitized human platelets incubated with complement-sufficient human sera. A new animal model of platelet refractoriness utilizing Wistar rats transfused with human platelets is described.

RESULTS

Human platelets sensitized with anti-platelet antibodies were rapidly opsonized with iC3b upon incubation in human sera. This opsonization could be completely blocked with a classical pathway complement inhibitor, PA-dPEG24. Complement activation decreased platelet viability, which was also reversible with complement inhibitor PA-dPEG24. A new rat model of platelet refractoriness was developed that demonstrated some platelet removal from the blood stream was complement mediated.

CONCLUSIONS

Complement activation initiated by anti-platelet antibodies leads to complement opsonization and decreased platelet viability. A new rat model of platelet refractoriness was developed that adds a new tool for elucidating the mechanisms of platelet refractoriness.

Collectins: Innate Immune Pattern Recognition Molecules

Collectins are collagen-containing C-type (calcium-dependent) lectins which are important pathogen pattern recognising innate immune molecules. Their primary structure is characterised by an N-terminal, triple-helical collagenous region made up of Gly-X-Y repeats, an a-helical coiled-coil trimerising neck region, and a C-terminal C-type lectin or carbohydrate recognition domain (CRD). Further oligomerisation of this primary structure can give rise to more complex and multimeric structures that can be seen under electron microscope. Collectins can be found in serum as well as in a range of tissues at the mucosal surfaces. Mannanbinding lectin can activate the complement system while other members of the collectin family are extremely versatile in recognising a diverse range of pathogens via their CRDs and bring about effector functions designed at the clearance of invading pathogens. These mechanisms include opsonisation, enhancement of phagocytosis, triggering superoxidative burst and nitric oxide production. Collectins can also potentiate the adaptive immune response via antigen presenting cells such as macrophages and dendritic cells through modulation of cytokines and chemokines, thus they can act as a link between innate and adaptive immunity. This chapter describes the structure-function relationships of collectins, their diverse functions, and their interaction with viruses, bacteria, fungi and parasites.

Inhibition of complement activation, myeloperoxidase, NET formation and oxidant activity by PIC1 peptide variants

Background

A product of rational molecular design, PA-dPEG24 is the lead derivative of the PIC1 family of peptides with multiple functional abilities including classical complement pathway inhibition, myeloperoxidase inhibition, NET inhibition and antioxidant activity. PA-dPEG24 is composed of a sequence of 15 amino acid, IALILEPICCQERAA, and contains a monodisperse 24-mer PEGylated moiety at its C terminus to increase aqueous solubility. Here we explore a sarcosine substitution scan of the PA peptide to evaluate impacts on solubility in the absence of PEGylation and functional characteristics.

Methods

Sixteen sarcosine substitution variants were synthesized and evaluated for solubility in water. Aqueous soluble variants were then tested in standard complement, myeloperoxidase, NET formation and antioxidant capacity assays.

Results

Six sarcosine substitution variants were aqueous soluble without requiring PEGylation. Substitution with sarcosine of the isoleucine at position eight yielded a soluble peptide that surpassed the parent molecule for complement inhibition and myeloperoxidase inhibition. Substitution with sarcosine of the cysteine at position nine improved solubility, but did not otherwise change the functional characteristics compared with the parent compound. However, replacement of both vicinal cysteine residues at positions 9 and 10 with a single sarcosine residue reduced functional activity in most of the assays tested.

Conclusions

Several of the sarcosine PIC1 variant substitutions synthesized yielded improved solubility as well as a number of unanticipated structure-function findings that provide new insights. Several sarcosine substitution variants demonstrate increased potency over the parent peptide suggesting enhanced therapeutic potential for inflammatory disease processes involving complement, myeloperoxidase, NETs or oxidant stress.

The Nature of Antibacterial Adaptive Immune Responses against Staphylococcus aureus Is Dependent on the Growth Phase and Extracellular Peptidoglycan

Staphylococcus aureus has evolved different strategies to evade the immune response, which play an important role in its pathogenesis. The bacteria express and shed various cell wall components and toxins during different stages of growth that may affect the protective T cell responses to extracellular and intracellular S. aureus However, if and how the dendritic cell (DC)-mediated T cell response against S. aureus changes during growth of the bacterium remain elusive. In this study, we show that exponential-phase (EP) S. aureus bacteria were endocytosed very efficiently by human DCs, and these DCs strongly promoted production of the T cell polarizing factor interleukin-12 (IL-12). In contrast, stationary-phase (SP) S. aureus bacteria were endocytosed less efficiently by DCs, and these DCs produced small amounts of IL-12. The high level of IL-12 production induced by EP S. aureus led to the development of a T helper 1 (Th1) cell response, which was inhibited after neutralization of IL-12. Furthermore, preincubation with the staphylococcal cell wall component peptidoglycan (PGN), characteristically shed during the exponential growth phase, modulated the DC response to EP S. aureus PGN preincubation appeared to inhibit IL-12p35 expression, leading to downregulation of IL-12 and an increase of IL-23 production by DCs, enhancing Th17 cell development. Taken together, our data indicate that exponential-phase S. aureus bacteria induce a stronger IL-12-dependent Th1 cell response than stationary-phase S. aureus and that this Th1 cell response shifted toward a Th17 cell response in the presence of PGN.

Mammary Defences and Immunity against Mastitis in Sheep

The objectives of this review paper are to present udder defences, including teat of the udder, mammary epithelial cells, leucocytes, immunoglobulins, complement system and chemical antibacterial agents, to describe cooperation and interactions between them and to elaborate on potentials regarding their significance in mammary immunisation strategies. The teat of the udder provides initial protection to the mammary gland. The mammary epithelial cells synthesise antibacterial proteins and the leucocytes produce various inflammation mediators (cytokines or chemokines), phagocytose bacteria and recognise antigenic structures. In the mammary gland, four immunoglobulins (IgG1, IgG2, IgM and IgA) have important roles against bacterial pathogens. The complement system is a collection of proteins, participating in the inflammatory process through various pathways. Other components contributing to humoral mammary defence include lactoferrin, lysozyme and the lactoperoxidase/myeloperoxidase systems, as well as oligosaccharides, gangliosides, reactive oxygen species, acute phase proteins (e.g., haptoglobin and serum amyloid A), ribonucleases and a wide range of antimicrobial peptides. Management practices, genetic variations and nutrition can influence mammary defences and should be taken into account in the formulation of prevention strategies against ovine mastitis.

Revisiting the regulation of the capsular polysaccharide biosynthesis gene cluster in Staphylococcus aureus

Capsular polysaccharide (CP) biosynthesis in Staphylococcus aureus is tightly controlled resulting in a heterogeneous phenotype within a population and CP being mainly detectable in nongrowing cells. Expression of the corresponding biosynthesis gene cluster is driven by one promoter element (P_cap ). Here, we demonstrate that P_cap contains a main SigB-dependent promoter. The SigB consensus motif overlaps with a previously described inverted repeat (IR) that is crucial for cap expression. The essentiality of the IR is derived from this region acting as a SigB binding site rather than as an operator site for the proposed cap activators RbsR and MsaB. Furthermore, P_cap contains an extensive upstream region harboring a weak SigA-dependent promoter and binding sites for cap repressors such as SaeR, CodY and Rot. Heterogeneous CP synthesis is determined by SigB activity and repressor binding to the upstream region. SigB dependency and regulation by the upstream repressors are also sufficient to explain the temporal gene expression pattern at the transcriptional level. However, CP synthesis remains growth phase-dependent even when transcription is rendered constitutive, suggesting additional posttranscriptional regulatory circuits. Thus, the interference of multiple repressors with SigB-dependent promoter activity as well as post-transcriptional mechanisms ensure the appropriate regulation of CP synthesis.

Immune Evasion by Staphylococcus aureus

Staphylococcus aureus has become a serious threat to human health. In addition to having increased antibiotic resistance, the bacterium is a master at adapting to its host by evading almost every facet of the immune system, the so-called immune evasion proteins. Many of these immune evasion proteins target neutrophils, the most important immune cells in clearing S. aureus infections. The neutrophil attacks pathogens via a plethora of strategies. Therefore, it is no surprise that S. aureus has evolved numerous immune evasion strategies at almost every level imaginable. In this review we discuss step by step the aspects of neutrophil-mediated killing of S. aureus, such as neutrophil activation, migration to the site of infection, bacterial opsonization, phagocytosis, and subsequent neutrophil-mediated killing. After each section we discuss how S. aureus evasion molecules are able to resist the neutrophil attack of these different steps. To date, around 40 immune evasion molecules of S. aureus are known, but its repertoire is still expanding due to the discovery of new evasion proteins and the addition of new functions to already identified evasion proteins. Interestingly, because the different parts of neutrophil attack are redundant, the evasion molecules display redundant functions as well. Knowing how and with which proteins S. aureus is evading the immune system is important in understanding the pathophysiology of this pathogen. This knowledge is crucial for the development of therapeutic approaches that aim to clear staphylococcal infections.

Pathogenomics of Virulence Traits of Plesiomonas shigelloides That Were Deemed Inconclusive by Traditional Experimental Approaches

One of the major challenges of modern medicine includes the failure of conventional protocols to characterize the pathogenicity of emerging pathogens. This is particularly apparent in the case of Plesiomonas shigelloides. Although a number of infections have been linked to this microorganism, experimental evidence of its virulence factors (VFs), obtained by traditional approaches, is somewhat inconclusive. Hence, it remains unclear whether P. shigelloides is a true or opportunistic one. In the current study, four publicly available whole-genome sequences of P. shigelloides (GN7, NCTC10360, 302-73, and LS1) were profiled using bioinformatics platforms to determine the putative candidate VFs to characterize the bacterial pathogenicity. Overall, 134 unique open reading frames (ORFs) were identified that were homologous or orthologous to virulence genes identified in other pathogens. Of these, 52.24% (70/134) were jointly shared by the strains. The numbers of strain-specific virulence traits were 4 in LS1; 7 in NCTC10360; 10 in 302-73; and 15 in GN7. The pathogenicity islands (PAIs) common to all the strains accounted for 24.07% ORFs. The numbers of PAIs exclusive to each strain were 8 in 302-73; 11 in NCTC10360; 14 in GN7; and 18 in LS1. A PAI encoding Vibrio cholerae ToxR-activated gene d protein was specific to 302-73, GN7, and NCTC10360 strains. Out of 33 antibiotic multi-resistance genes identified, 16 (48.48%) genes were intrinsic to all strains. Further, 17 (22.08%) of 77 antibiotic resistance islands were found in all the strains. Out of 23 identified distinct insertion sequences, 13 were only harbored by strain LS1. The number of intact prophages identified in the strains was 1 in GN7; 2 in 302-73; and 2 in NCTC10360. Further, 1 CRISPR element was identified in LS1; 2 in NCTC10360; and 8 in 302-73. Fifteen (78.95%) of 19 secretion systems and secretion effector variants were identified in all the strains. In conclusion, certain P. shigelloides strains might possess VFs associated with gastroenteritis and extraintestinal infections. However, the role of host factors in the onset of infections should not be undermined.

Complement and Bacterial Infections: From Molecular Mechanisms to Therapeutic Applications

Complement is a complex protein network of plasma, and an integral part of the innate immune system. Complement activation results in the rapid clearance of bacteria by immune cells, and direct bacterial killing via large pore-forming complexes. Here we review important recent discoveries in the complement field, focusing on interactions relevant for the defense against bacteria. Understanding the molecular interplay between complement and bacteria is of great importance for future therapies for infectious and inflammatory diseases. Antibodies that support complement-dependent bacterial killing are of interest for the development of alternative therapies to treat infections with antibiotic-resistant bacteria. Furthermore, a variety of novel therapeutic complement inhibitors have been developed to prevent unwanted complement activation in autoimmune inflammatory diseases. A better understanding of how such inhibitors may increase the risk of bacterial infections is essential if such therapies are to be successful.

MsaB and CodY Interact To Regulate Staphylococcus aureus Capsule in a Nutrient-Dependent Manner

Staphylococcus aureus has a complex regulatory network for controlling the production of capsule polysaccharide. In S. aureus, capsule production is controlled by several regulators in response to various environmental stimuli. Previously, we described MsaB as a new regulator that specifically binds to the cap promoter in a growth phase- or nutrient-dependent manner. In addition to MsaB, several other regulators have also been shown to bind the same region. In this study, we examined the interactions between MsaB and other nutrient-sensing regulators (CodY and CcpE) with respect to binding to the cap promoter in a nutrient-dependent manner. We observed that msaABCR and ccpE interact in a complex fashion to regulate capsule production. However, we confirmed that ccpE does not bind cap directly. We also defined the regulatory relationship between msaABCR and CodY. When nutrients (branched-chain amino acids) are abundant, CodY binds to the promoter region of the cap operon and represses its transcription. However, when nutrient concentrations decrease, MsaB, rather than CodY, binds to the cap promoter. Binding of MsaB to the cap promoter activates transcription of the cap operon. We hypothesize that this same mechanism may be used by S. aureus to regulate other virulence factors.IMPORTANCE Findings from this study define the mechanism of regulation of capsule production in Staphylococcus aureus Specifically, we show that two key regulators, MsaB and CodY, coordinate their functions to control the expression of capsule in response to nutrients. S. aureus fine-tunes the production of capsule by coordinating the activity of several regulators and by sensing nutrient levels. This study demonstrates the importance of incorporating multiple inputs prior to the expression of costly virulence factors, such as capsule.

Staphylococcus aureus virulence attenuation and immune clearance mediated by a phage lysin-derived protein

New anti-infective approaches are much needed to control multi-drug-resistant (MDR) pathogens, such as methicillin-resistant Staphylococcus aureus (MRSA). Here, we found for the first time that a recombinant protein derived from the cell wall binding domain (CBD) of the bacteriophage lysin PlyV12, designated as V12CBD, could attenuate S. aureus virulence and enhance host immune defenses via multiple manners. After binding with V12CBD, S. aureus became less invasive to epithelial cells and more susceptible to macrophage killing. The expressions of multiple important virulence genes of S. aureus were reduced 2.4- to 23.4-fold as response to V12CBD More significantly, V12CBD could activate macrophages through NF-κB pathway and enhance phagocytosis against S. aureus As a result, good protections of the mice from MRSA infections were achieved in therapeutic and prophylactic models. These unique functions of V12CBD would render it a novel alternative molecule to control MDRS. aureus infections.

Inhibition of Immune Complex Complement Activation and Neutrophil Extracellular Trap Formation by Peptide Inhibitor of Complement C1

Two major aspects of systemic lupus erythematosus (SLE) pathogenesis that have yet to be targeted therapeutically are immune complex-initiated complement activation and neutrophil extracellular trap (NET) formation by neutrophils. Here, we report in vitro testing of peptide inhibitor of complement C1 (PIC1) in assays of immune complex-mediated complement activation in human sera and assays for NET formation by human neutrophils. The lead PIC1 derivative, PA-dPEG24, was able to dose-dependently inhibit complement activation initiated by multiple types of immune complexes (IC), including C1-anti-C1q IC, limiting the generation of pro-inflammatory complement effectors, including C5a and membrane attack complex (sC5b-9). In several instances, PA-dPEG24 achieved complete inhibition with complement effector levels equivalent to background. PA-dPEG24 was also able to dose-dependently inhibit NET formation by human neutrophils stimulated by PMA, MPO, or immune complex activated human sera. In several instances PA-dPEG24 achieved complete inhibition with NETosis with quantitation equivalent to background levels. These results suggest that PA-dPEG24 inhibition of NETs occurs by blocking the MPO pathway of NET formation. Together these results demonstrate that PA-dPEG24 can inhibit immune complex activation of the complement system and NET formation. This provides proof of concept that peptides can potentially be developed to inhibit these two important contributors to rheumatologic pathology that are currently untargeted by available therapies.

[Effects of recombinant fusion protein interleukin-18 on expression of immune-inflammatory factors in mice infected with Staphylococcus aureus]

OBJECTIVE

To observe the effects of recombinant fusion protein interleukin (IL)-18 on the expression of immune-inflammatory factors in the mice infected with Staphylococcus aureus (SA), and to investigate the mechanism of action of IL-18 in defense of SA infection in vivo.

METHODS

A total of 40 specific pathogen-free female BLAB/c mice were randomly divided into four groups: control, SA infection, immunized, and intervention. A mouse model of SA infection was established by nasal inoculation with SA liquid. The immunized group and the intervention group were intranasally given IL-18 before SA modeling, and then the SA infection group and the intervention group received the nasal inoculation with SA liquid; the control group was treated with phosphate buffered saline instead. The levels of IL-4, interferon (IFN)-γ, tumor necrosis factor (TNF), granulocyte colony-stimulating factor (G-CSF), IgM in the serum and bronchoalveolar lavage fluid (BALF) of mice were measured by enzyme-linked immunosorbent assay. The expression of macrophage inflammatory protein (MIP)-1α mRNA and MIP-2β mRNA in the lung tissue of mice were determined by real-time fluorescent quantitative PCR.

RESULTS

Compared with the control group, the SA infection group and the immunized group had significantly higher levels of IL-4, G-CSF, and IgM in the serum and BALF and expression of MIP-1α mRNA and MIP-2β mRNA in the lung tissue (P<0.05); the SA infection group had a significantly lower level of IFN-γ and a significantly higher level of TNF in the serum and BALF (P<0.05); the immunized group had a significantly higher level of IFN-γ in the serum and BALF (P<0.05). Compared with the SA infection group, the intervention group had significantly higher levels of IL-4, IFN-γ, G-CSF, and IgM in the serum and BALF and expression of MIP-1α mRNA in the lung tissue. In contrast, the intervention group showed a significantly lower level of TNF in the serum and BALF and expression of MIP-2β mRNA in the lung tissue (P<0.05). All the above indicators in the intervention group were significantly higher than those in the control group (P<0.05), except the serum level of IFN-γ.

CONCLUSIONS

In the mice infected with SA, the recombinant fusion protein IL-18 by mucosal immunity can affect inflammatory factors in the serum and BALF and the expression of MIP-1α mRNA and MIP-2β mRNA in the lung tissue to promote the anti-infective immune response and enhance the ability to clear pathogens.

The immunopathogenesis of staphylococcal skin infections - A review

Staphylococcus aureus and S. pseudintermedius are the major causes of bacterial skin disease in humans and dogs. These organisms can exist as commensals on the skin, but they can also cause severe or even devastating infections. The immune system has evolved mechanisms to deal with pathogenic microorganisms and has strategies to combat bacteria of this type. What emerges is a delicate "peace" between the opposing sides, but this balance can be disrupted leading to a full blown "war". In the ferocious battle that ensues, both sides attempt to get the upper hand, using strategies that are comparable to those used by modern day armies. In this review article, the complex interactions between the immune system and the organisms are described using such military analogies. The process is described in a sequential manner, starting with the invasion itself, and progressing to the eventual battlezone in which there are heavy casualties on both sides. By the end, the appearance of a simple pustule on the skin surface will take on a whole new meaning.

The Staphylococcus aureus polysaccharide capsule and Efb-dependent fibrinogen shield act in concert to protect against phagocytosis

Staphylococcus aureus has developed many mechanisms to escape from human immune responses. To resist phagocytic clearance, S. aureus expresses a polysaccharide capsule, which effectively masks the bacterial surface and surface-associated proteins, such as opsonins, from recognition by phagocytic cells. Additionally, secretion of the extracellular fibrinogen binding protein (Efb) potently blocks phagocytic uptake of the pathogen. Efb creates a fibrinogen shield surrounding the bacteria by simultaneously binding complement C3b and fibrinogen at the bacterial surface. By means of neutrophil phagocytosis assays with fluorescently labelled encapsulated serotype 5 (CP5) and serotype 8 (CP8) strains we compare the immune-modulating function of these shielding mechanisms. The data indicate that, in highly encapsulated S. aureus strains, the polysaccharide capsule is able to prevent phagocytic uptake at plasma concentrations <10 %, but loses its protective ability at higher concentrations of plasma. Interestingly, Efb shows a strong inhibitory effect on both capsule-negative and encapsulated strains at all tested plasma concentrations. Furthermore, the results suggest that both shielding mechanisms can exist simultaneously and collaborate to provide optimal protection against phagocytosis at a broad range of plasma concentrations. As opsonizing antibodies will be shielded from recognition by either mechanism, incorporating both capsular polysaccharides and Efb in future vaccines could be of great importance.

IBI302, a promising candidate for AMD treatment, targeting both the VEGF and complement system with high binding affinity in vitro and effective targeting of the ocular tissue in healthy rhesus monkeys

Uncontrolled activation of complement and upregulation of vascular endothelial growth factor (VEGF) play fundamental roles in age-related macular degeneration (AMD). However, most drugs used to treat AMD focus on a single target, and the percentage of effectively treated patients in clinical practice needs to be improved. Therefore, novel AMD treatment approaches are needed. IBI302 is a novel bispecific decoy receptor fusion protein designed with both a VEGF inhibition domain and a complement cascade inhibition domain, which are connected by the Fc region of human immunoglobulin. In this study, we systematically evaluated the binding affinity between IBI302 and VEGF isoforms and complement proteins by using surface plasmon resonance (SPR) technology. Anti-VEGF blockers (aflibercept and bevacizumab) and complement receptor 1 were used as references. The SPR assay results indicated that IBI302 could bind different VEGF isoforms and complement proteins with high affinity. The biological activity of IBI302 was also studied. IBI302 showed an inhibitory effect on human primary umbilical vein endothelial cell proliferation and the activation of complement pathways in vitro. Finally, the pharmacokinetic (PK) properties of IBI302 were evaluated in rhesus monkeys. The PK results showed that after a 0.5 mg/eye intravitreal dosage, IBI302 became rapidly distributed from the vitreous humor into targeted tissues and remained active over 504 h. Overall, the favorable anti-angiogenic and anti-complement effects of IBI302 along with the good PK profiles in rhesus monkeys support the selection and development of IBI302 as a promising candidate for AMD treatment.

D-Alanylation of Teichoic Acids and Loss of Poly-N-Acetyl Glucosamine in Staphylococcus aureus during Exponential Growth Phase Enhance IL-12 Production in Murine Dendritic Cells

Staphylococcus aureus is a major human pathogen that has evolved very efficient immune evading strategies leading to persistent colonization. During different stages of growth, S. aureus express various surface molecules, which may affect the immune stimulating properties, but very little is known about their role in immune stimulation and evasion. Depending on the growth phase, S. aureus may affect antigen presenting cells differently. Here, the impact of growth phases and the surface molecules lipoteichoic acid, peptidoglycan and poly-N-acetyl glucosamine on the induction of IL-12 imperative for an efficient clearance of S. aureus was studied in dendritic cells (DCs). Exponential phase (EP) S. aureus was superior to stationary phase (SP) bacteria in induction of IL-12, which required actin-mediated endocytosis and endosomal acidification. Moreover, addition of staphylococcal cell wall derived peptidoglycan to EP S. aureus stimulated cells increased bacterial uptake but abrogated IL-12 induction, while addition of lipoteichoic acid increased IL-12 production but had no effect on the bacterial uptake. Depletion of the capability to produce poly-N-acetyl glucosamine increased the IL-12 inducing activity of EP bacteria. Furthermore, the mutant dltA unable to produce D-alanylated teichoic acids failed to induce IL-12 but like peptidoglycan and the toll-like receptor (TLR) ligands LPS and Pam3CSK4 the mutant stimulated increased macropinocytosis. In conclusion, the IL-12 response by DCs against S. aureus is highly growth phase dependent, relies on cell wall D-alanylation, endocytosis and subsequent endosomal degradation, and is abrogated by receptor induced macropinocytosis.

MsaB activates capsule production at the transcription level in Staphylococcus aureus

Staphylococcus aureus produces several virulence factors that allow it to cause a variety of infections. One of the major virulence factors is the capsule, which contributes to the survival of the pathogen within the host as a way to escape phagocytosis. The production of the capsular polysaccharide is encoded in a 16 gene operon, which is regulated in response to several environmental stimuli including nutrient availability. For instance, the capsule is produced in the late- and post-exponential growth phases, but not in the early- or mid-exponential growth phase. Several regulators are involved in capsule production, but the regulation of the cap operon is still poorly understood. In this study, we show that MsaB activates the cap operon by binding directly to a 10 bp repeat in the promoter region. We show that despite the fact that MsaB is expressed throughout four growth phases, it only activates capsule production in the late- and post-exponential growth phases. Furthermore, we find that MsaB does not bind to its target site in the early and mid-exponential growth phases. This correlates with decreased nutrient availability and capsule production. These data suggest either that MsaB binding ability changes in response to nutrients or that other cap operon regulators interfere with the binding of MsaB to its target site. This study increases our understanding of the regulation of capsule production and the mechanism of action of MsaB.

Bacterial Surfaces: Front Lines in Host-Pathogen Interaction

All bacteria are bound by at least one membrane that acts as a barrier between the cell's interior and the outside environment. Surface components within and attached to the cell membrane are essential for ensuring that the overall homeostasis of the cell is maintained. However, many surface components of the bacterial cell also have an indispensable role mediating interactions of the bacteria with their immediate environment and as such are essential to the pathogenesis of infectious disease. During the course of an infection, bacterial pathogens will encounter many different ecological niches where environmental conditions such as salinity, temperature, pH, and the availability of nutrients fluctuate. It is the bacterial cell surface that is at the front-line of these host-pathogen interactions often protecting the bacterium from hostile surroundings but at the same time playing a critical role in the adherence to host tissues promoting colonization and subsequent infection. To deal effectively with the changing environments that pathogens may encounter in different ecological niches within the host many of the surface components of the bacterial cell are subject to phenotypic variation resulting in heterogeneous subpopulations of bacteria within the clonal population. This dynamic phenotypic heterogeneity ensures that at least a small fraction of the population will be adapted for a particular circumstance should it arise. Diversity within the clonal population has often been masked by studies on entire bacterial populations where it was often assumed genes were expressed in a uniform manner. This chapter, therefore, aims to highlight the non-uniformity in certain cell surface structures and will discuss the implication of this heterogeneity in bacterial-host interaction. Some of the recent advances in studying bacterial surface structures at the single cell level will also be reviewed.

Complement Effectors of Inflammation in Cystic Fibrosis Lung Fluid Correlate with Clinical Measures of Disease

In cystic fibrosis (CF), lung damage is mediated by a cycle of obstruction, infection, and inflammation. Here we explored complement inflammatory effectors in CF lung fluid. In this study soluble fractions (sols) from sputum samples of 15 CF patients were assayed for complement effectors and analyzed with clinical measurements. The pro-inflammatory peptide C5a was increased 4.8-fold (P = 0.04) in CF sols compared with controls. Incubation of CF sols with P. aeruginosa or S. aureus increased C5a concentration 2.3-fold (P = 0.02). A peptide inhibitor of complement C1 (PIC1) completely blocked the increase in C5a concentration from P. aeruginosa in CF sol in vitro (P = 0.001). C5a concentration in CF sol correlated inversely with body mass index (BMI) percentile in children (r = -0.77, P = 0.04). C3a, which has anti-inflammatory effects, correlated positively with FEV1% predicted (r_s = 0.63, P = 0.02). These results suggest that complement effectors may significantly impact inflammation in CF lung fluid.

Surface Glycopolymers Are Crucial for In Vitro Anti-Wall Teichoic Acid IgG-Mediated Complement Activation and Opsonophagocytosis of Staphylococcus aureus

The cell envelopes of many Gram-positive bacteria contain wall teichoic acids (WTAs). Staphylococcus aureus WTAs are composed of ribitol phosphate (RboP) or glycerol phosphate (GroP) backbones substituted with D-alanine and N-acetyl-D-glucosamine (GlcNAc) or N-acetyl-D-galactosamine (GalNAc). Two WTA glycosyltransferases, TarM and TarS, are responsible for modifying the RboP WTA with α-GlcNAc and β-GlcNAc, respectively. We recently reported that purified human serum anti-WTA IgG specifically recognizes β-GlcNAc of the staphylococcal RboP WTA and then facilitates complement C3 deposition and opsonophagocytosis of S. aureus laboratory strains. This prompted us to examine whether anti-WTA IgG can induce C3 deposition on a diverse set of clinical S. aureus isolates. To this end, we compared anti-WTA IgG-mediated C3 deposition and opsonophagocytosis abilities using 13 different staphylococcal strains. Of note, the majority of S. aureus strains tested was recognized by anti-WTA IgG, resulting in C3 deposition and opsonophagocytosis. A minority of strains was not recognized by anti-WTA IgG, which correlated with either extensive capsule production or an alteration in the WTA glycosylation pattern. Our results demonstrate that the presence of WTAs with TarS-mediated glycosylation with β-GlcNAc in clinically isolated S. aureus strains is an important factor for induction of anti-WTA IgG-mediated C3 deposition and opsonophagocytosis.

Peptide Inhibitor of Complement C1 (PIC1) Rapidly Inhibits Complement Activation after Intravascular Injection in Rats

The complement system has been increasingly recognized to play a pivotal role in a variety of inflammatory and autoimmune diseases. Consequently, therapeutic modulators of the classical, lectin and alternative pathways of the complement system are currently in pre-clinical and clinical development. Our laboratory has identified a peptide that specifically inhibits the classical and lectin pathways of complement and is referred to as Peptide Inhibitor of Complement C1 (PIC1). In this study, we determined that the lead PIC1 variant demonstrates a salt-dependent binding to C1q, the initiator molecule of the classical pathway. Additionally, this peptide bound to the lectin pathway initiator molecule MBL as well as the ficolins H, M and L, suggesting a common mechanism of PIC1 inhibitory activity occurs via binding to the collagen-like tails of these collectin molecules. We further analyzed the effect of arginine and glutamic acid residue substitution on the complement inhibitory activity of our lead derivative in a hemolytic assay and found that the original sequence demonstrated superior inhibitory activity. To improve upon the solubility of the lead derivative, a pegylated, water soluble variant was developed, structurally characterized and demonstrated to inhibit complement activation in mouse plasma, as well as rat, non-human primate and human serum in vitro. After intravenous injection in rats, the pegylated derivative inhibited complement activation in the blood by 90% after 30 seconds, demonstrating extremely rapid function. Additionally, no adverse toxicological effects were observed in limited testing. Together these results show that PIC1 rapidly inhibits classical complement activation in vitro and in vivo and is functional for a variety of animal species, suggesting its utility in animal models of classical complement-mediated diseases.

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.

Structure and Function of Surface Polysaccharides of Staphylococcus aureus

The major surface polysaccharides of Staphylococcus aureus include the capsular polysaccharide (CP), cell wall teichoic acid (WTA), and polysaccharide intercellular adhesin/poly-β(1-6)-N-acetylglucosamine (PIA/PNAG). These glycopolymers are important components of the staphylococcal cell envelope, but none of them is essential to S. aureus viability and growth in vitro. The overall biosynthetic pathways of CP, WTA, and PIA/PNAG have been elucidated, and the functions of most of the biosynthetic enzymes have been demonstrated. Because S. aureus CP and WTA (but not PIA/PNAG) utilize a common cell membrane lipid carrier (undecaprenyl-phosphate) that is shared by the peptidoglycan biosynthesis pathway, there is evidence that these processes are highly integrated and temporally regulated. Regulatory elements that control glycopolymer biosynthesis have been described, but the cross talk that orchestrates the biosynthetic pathways of these three polysaccharides remains largely elusive. CP, WTA, and PIA/PNAG each play distinct roles in S. aureus colonization and the pathogenesis of staphylococcal infection. However, they each promote bacterial evasion of the host immune defences, and WTA is being explored as a target for antimicrobial therapeutics. All the three glycopolymers are viable targets for immunotherapy, and each (conjugated to a carrier protein) is under evaluation for inclusion in a multivalent S. aureus vaccine. Future research findings that increase our understanding of these surface polysaccharides, how the bacterial cell regulates their expression, and their biological functions will likely reveal new approaches to controlling this important bacterial pathogen.

Clinical hypothermia temperatures increase complement activation and cell destruction via the classical pathway

Background

Therapeutic hypothermia is a treatment modality that is increasingly used to improve clinical neurological outcomes for ischemia-reperfusion injury-mediated diseases. Antibody-initiated classical complement pathway activation has been shown to contribute to ischemia-reperfusion injury in multiple disease processes. However, how therapeutic hypothermia affects complement activation is unknown. Our goal was to measure the independent effect of temperature on complement activation, and more specifically, examine the relationship between clinical hypothermia temperatures (31–33°C), and complement activation.

Methods

Antibody-sensitized erythrocytes were used to assay complement activation at temperatures ranging from 0-41°C. Individual complement pathway components were assayed by ELISA, Western blot, and quantitative dot blot. Peptide Inhibitor of complement C1 (PIC1) was used to specifically inhibit activation of C1.

Results

Antibody-initiated complement activation resulting in eukaryotic cell lysis was increased by 2-fold at 31°C compared with 37°C. Antibody-initiated complement activation in human serum increased as temperature decreased from 37°C until dramatically decreasing at 13°C. Quantitation of individual complement components showed significantly increased activation of C4, C3, and C5 at clinical hypothermia temperatures. In contrast, C1s activation by heat-aggregated IgG decreased at therapeutic hypothermia temperatures consistent with decreased enzymatic activity at lower temperatures. However, C1q binding to antibody-coated erythrocytes increased at lower temperatures, suggesting that increased classical complement pathway activation is mediated by increased C1 binding at therapeutic hypothermia temperatures. PIC1 inhibited hypothermia-enhanced complement-mediated cell lysis at 31°C by up to 60% (P = 0.001) in a dose dependent manner.

Conclusions

In summary, therapeutic hypothermia temperatures increased antibody-initiated complement activation and eukaryotic cell destruction suggesting that the benefits of therapeutic hypothermia may be mediated via other mechanisms. Antibody-initiated complement activation has been shown to contribute to ischemia-reperfusion injury in several animal models, suggesting that for diseases with this mechanism hypothermia-enhanced complement activation may partially attenuate the benefits of therapeutic hypothermia.

Pertussis toxin improves immune responses to a combined pneumococcal antigen and leads to enhanced protection against Streptococcus pneumoniae

Pneumococcal surface protein A (PspA) is a candidate antigen for the composition of protein-based vaccines against Streptococcus pneumoniae. While searching for efficient adjuvants for PspA-based vaccines, our group has described the potential of combining PspA with the whole-cell pertussis vaccine (wP). When given to mice through the nasal route, a formulation composed of PspA from clade 5 (PspA5) and wP (PspA5-wP) induced high levels of antibodies and protection against challenges with different pneumococcal strains. PspA5-wP also induced the secretion of interleukin 17 (IL-17) by splenocytes and the infiltration of leukocytes in the lungs after challenge. Here, we show that protection against a pneumococcal invasive challenge was completely abrogated in μMT(-/-) mice, which are deficient in the maturation of B cells, illustrating the importance of antibodies in the survival elicited by the PspA5-wP vaccine. Moreover, passive immunization showed that IgG purified from the sera of mice immunized with PspA5-wP conferred significant protection to naive mice, whereas the respective F(ab')2 did not. Additionally, in vivo depletion of complement abolished protection against the pneumococcal challenge. The combination of PspA5 with wild-type or mutant Bordetella pertussis strains or with purified components showed that the pertussis toxin (PT)-containing formulations induced the highest levels of antibodies and protection. This suggests that the adjuvant activity of wP in the PspA5 model is mediated at least in part by PT. The sera from mice immunized with such formulations displayed high IgG binding and induction of complement deposition on the pneumococcal surface in vitro, which is consistent with the in vivo results.

Hyperglycemia inhibits complement-mediated immunological control of S. aureus in a rat model of peritonitis

Hyperglycemia from diabetes is associated with increased risk of infection from S. aureus and increased severity of illness. Previous work in our laboratory demonstrated that elevated glucose (>6 mM) dramatically inhibited S. aureus-initiated complement-mediated immune effectors. Here we report in vivo studies evaluating the extent to which a hyperglycemic environment alters complement-mediated control of S. aureus infection in a rat peritonitis model. Rats were treated with streptozocin to induce diabetes or sham-treated and then inoculated i.p. with S. aureus. Rats were euthanized and had peritoneal lavage at 2 or 24 hours after infection to evaluate early and late complement-mediated effects. Hyperglycemia decreased the influx of IgG and complement components into the peritoneum in response to S. aureus infection and decreased anaphylatoxin generation. Hyperglycemia decreased C4-fragment and C3-fragment opsonization of S. aureus recovered in peritoneal fluids, compared with euglycemic or insulin-rescued rats. Hyperglycemic rats showed decreased phagocytosis efficiency compared with euglycemic rats, which correlated inversely with bacterial survival. These results suggest that hyperglycemia inhibited humoral effector recruitment, anaphylatoxin generation, and complement-mediated opsonization of S. aureus, suggesting that hyperglycemic inhibition of complement effectors may contribute to the increased risk and severity of S. aureus infections in diabetic patients.

Complement regulator C4BP binds to Staphylococcus aureus surface proteins SdrE and Bbp inhibiting bacterial opsonization and killing

Staphylococcus aureus is a premier human pathogen and the most common cause of osteoarticular, wound, and implanted device infections. We recently demonstrated S. aureus efficiently binds the classical complement regulator C4b-binding protein (C4BP) inhibiting antibody-initiated complement-mediated opsonization. Here we identify S. aureus surface protein SdrE as a C4BP-binding protein. Recombinant SdrE and recombinant bone sialoprotein-binding protein (Bbp), an allelic variant of SdrE, both efficiently bound to C4BP in heat-inactivated human serum. We previously described SdrE as binding alternative pathway regulator factor H. Recombinant SdrE and Bbp efficiently bound C4BP and factor H in serum without apparent interference. Gain of function studies utilizing Lactococcus lactis clones expressing SdrE or Bbp increased serum C4BP and factor H binding, compared with empty-vector control (WT) approximately 2-fold. Correspondingly, classical pathway-mediated C3-fragment opsonization and bacterial killing by human neutrophils decreased by half for L. lactis clones expressing SdrE or Bbp compared with WT. In summary, we identify SdrE and allelic variant Bbp as S. aureus surface proteins that bind the complement regulator C4BP inhibiting classical pathway-mediated bacterial opsonization and killing.

Lightning-triggered electroporation and electrofusion as possible contributors to natural horizontal gene transfer

Phylogenetic studies show that horizontal gene transfer (HGT) is a significant contributor to genetic variability of prokaryotes, and was perhaps even more abundant during the early evolution. Hitherto, research of natural HGT has mainly focused on three mechanisms of DNA transfer: conjugation, natural competence, and viral transduction. This paper discusses the feasibility of a fourth such mechanism--cell electroporation and/or electrofusion triggered by atmospheric electrostatic discharges (lightnings). A description of electroporation as a phenomenon is followed by a review of experimental evidence that electroporation of prokaryotes in aqueous environments can result in release of non-denatured DNA, as well as uptake of DNA from the surroundings and transformation. Similarly, a description of electrofusion is followed by a review of experiments showing that prokaryotes devoid of cell wall can electrofuse into hybrids expressing the genes of their both precursors. Under sufficiently fine-tuned conditions, electroporation and electrofusion are efficient tools for artificial transformation and hybridization, respectively, but the quantitative analysis developed here shows that conditions for electroporation-based DNA release, DNA uptake and transformation, as well as for electrofusion are also present in many natural aqueous environments exposed to lightnings. Electroporation is thus a plausible contributor to natural HGT among prokaryotes, and could have been particularly important during the early evolution, when the other mechanisms might have been scarcer or nonexistent. In modern prokaryotes, natural absence of the cell wall is rare, but it is reasonable to assume that the wall has formed during a certain stage of evolution, and at least prior to this, electrofusion could also have contributed to natural HGT. The concluding section outlines several guidelines for assessment of the feasibility of lightning-triggered HGT.

Rapid and reliable identification of Staphylococcus aureus capsular serotypes by means of artificial neural network-assisted Fourier transform infrared spectroscopy

Staphylococcus aureus capsular polysaccharides (CP) are important virulence factors and represent putative targets for vaccine development. Therefore, the purpose of this study was to develop a high-throughput method to identify and discriminate the clinically important S. aureus capsular serotypes 5, 8, and NT (nontypeable). A comprehensive set of clinical isolates derived from different origins and control strains, representative for each serotype, were used to establish a CP typing system based on Fourier transform infrared (FTIR) spectroscopy and chemometric techniques. By combining FTIR spectroscopy with artificial neuronal network (ANN) analysis, a system was successfully established, allowing a rapid identification and discrimination of all three serotypes. The overall accuracy of the ANN-assisted FTIR spectroscopy CP typing system was 96.7% for the internal validation and 98.2% for the external validation. One isolate in the internal validation and one isolate in the external validation failed in the classification procedure, but none of the isolates was incorrectly classified. The present study demonstrates that ANN-assisted FTIR spectroscopy allows a rapid and reliable discrimination of S. aureus capsular serotypes. It is suitable for diagnostic as well as large-scale epidemiologic surveillance of S. aureus capsule expression and provides useful information with respect to chronicity of infection.

Capsular polysaccharides are an important immune evasion mechanism for Staphylococcus aureus

Staphylococcus aureus can cause severe life threatening invasive diseases. The principal immune effector mechanism by which humans are protected from Gram positive bacteria such as S. aureus is antigen specific antibody- and complement-dependent opsonophagocytosis. This process can be measured in vitro using the opsonophagocytic antibody assay (OPA), which is a complex assay composed of live S. aureus bacteria, a complement source, phagocytic effector cells such as differentiated HL-60 cells, and test serum. In this report, we investigated the impact on the OPA of S. aureus surface antigens capsular polysaccharides (CP) and protein A (SpA). We demonstrated that higher CP expression renders bacteria more resistant to non-specific opsonophagocytic killing than increased SpA expression, suggesting that the expression of capsular polysaccharides may be the more important immune evasion strategy for S. aureus. Bacteria that were not fully encapsulated were highly susceptible to non-specific killing in the assay in the absence of immune serum. This non-specific killing was prevented by growing the bacteria under conditions that increased capsular polysaccharide levels on the surface of the bacteria. In contrast, the level of SpA expression had no detectable effect on non-specific killing in OPA. Using anti-CP antibodies we demonstrated type-specific killing in OPA of both MRSA and MSSA clinical isolates. SpA expression on the cell surface did not interfere with OPA activity, providing evidence that despite the role of SpA in sequestering antibodies by their Fc region, killing is easily accomplished in the presence of high titered anti-capsular polysaccharide antibodies. This highlights the role of CP as an important immune evasion mechanism and supports the inclusion of capsular polysaccharide antigens in the formulation of multi-component prophylactic vaccines against S. aureus.

A bactericidal guanidinomethyl biaryl that alters the dynamics of bacterial FtsZ polymerization

The prevalence of multidrug resistance among clinically significant bacterial pathogens underscores a critical need for the development of new classes of antibiotics with novel mechanisms of action. Here we describe the synthesis and evaluation of a guanidinomethyl biaryl compound {1-((4'-(tert-butyl)-[1,1'-biphenyl]-3-yl)methyl)guanidine} that targets the bacterial cell division protein FtsZ. In vitro studies with various bacterial FtsZ proteins reveal that the compound alters the dynamics of FtsZ self-polymerization via a stimulatory mechanism, while minimally impacting the polymerization of tubulin, the closest mammalian homologue of FtsZ. The FtsZ binding site of the compound is identified through a combination of computational and mutational approaches. The compound exhibits a broad spectrum of bactericidal activity, including activity against the multidrug-resistant pathogens methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococcus (VRE), while also exhibiting a minimal potential to induce resistance. Taken together, our results highlight the compound as a promising new FtsZ-targeting bactericidal agent.

A novel peptide inhibitor of classical and lectin complement activation including ABO incompatibility

Previous experiments from our laboratories have identified peptides derived from the human astrovirus coat protein (CP) that bind C1q and mannose binding lectin (MBL) inhibiting activation of the classical and lectin pathways of complement, respectively. The purpose of this study was to evaluate the function of these coat protein peptides (CPPs) in an in vitro model of complement-mediated disease (ABO incompatibility), preliminarily assess their in vivo complement suppression profile and develop more highly potent derivatives of these molecules. E23A, a 30 amino acid CPP derivative previously demonstrated to inhibit classical pathway activation was able to dose-dependently inhibit lysis of AB erythrocytes treated with mismatched human O serum. Additionally, when injected into rats, E23A inhibited the animals' serum from lysing antibody-sensitized erythrocytes, providing preliminary in vivo functional evidence that this CPP can cross the species barrier to inhibit serum complement activity in rodents. A rational drug design approach was implemented to identify more potent CPP derivatives, resulting in the identification and characterization of a 15 residue peptide (polar assortant (PA)), which demonstrated both superior inhibition of classical complement pathway activation and robust binding to C1q collagen-like tails. PA also inhibited ABO incompatibility in vitro and demonstrated in vivo complement suppression up to 24h post-injection. CPP's ability to inhibit ABO incompatibility in vitro, proof of concept in vivo inhibitory activity in rats and the development of the highly potent PA derivative set the stage for preclinical testing of this molecule in small animal models of complement-mediated disease.

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.