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

The Regulation of Neutrophil Migration in Patients with Sepsis: The Complexity of the Molecular Mechanisms and Their Modulation in Sepsis and the Heterogeneity of Sepsis Patients

Sepsis is defined as life-threatening organ dysfunction caused by a dysregulated host response to infection. Common causes include gram-negative and gram-positive bacteria as well as fungi. Neutrophils are among the first cells to arrive at an infection site where they function as important effector cells of the innate immune system and as regulators of the host immune response. The regulation of neutrophil migration is therefore important both for the infection-directed host response and for the development of organ dysfunctions in sepsis. Downregulation of CXCR4/CXCL12 stimulates neutrophil migration from the bone marrow. This is followed by transmigration/extravasation across the endothelial cell barrier at the infection site; this process is directed by adhesion molecules and various chemotactic gradients created by chemotactic cytokines, lipid mediators, bacterial peptides, and peptides from damaged cells. These mechanisms of neutrophil migration are modulated by sepsis, leading to reduced neutrophil migration and even reversed migration that contributes to distant organ failure. The sepsis-induced modulation seems to differ between neutrophil subsets. Furthermore, sepsis patients should be regarded as heterogeneous because neutrophil migration will possibly be further modulated by the infecting microorganisms, antimicrobial treatment, patient age/frailty/sex, other diseases (e.g., hematological malignancies and stem cell transplantation), and the metabolic status. The present review describes molecular mechanisms involved in the regulation of neutrophil migration; how these mechanisms are altered during sepsis; and how bacteria/fungi, antimicrobial treatment, and aging/frailty/comorbidity influence the regulation of neutrophil migration.

Systemic Metabolomic Profiles in Adult Patients with Bacterial Sepsis: Characterization of Patient Heterogeneity at the Time of Diagnosis

Sepsis is a dysregulated host response to infection that causes potentially life-threatening organ dysfunction. We investigated the serum metabolomic profile at hospital admission for patients with bacterial sepsis. The study included 60 patients; 35 patients fulfilled the most recent 2016 Sepsis-3 criteria whereas the remaining 25 patients only fulfilled the previous Sepsis-2 criteria and could therefore be classified as having systemic inflammatory response syndrome (SIRS). A total of 1011 identified metabolites were detected in our serum samples. Ninety-seven metabolites differed significantly when comparing Sepsis-3 and Sepsis-2/SIRS patients; 40 of these metabolites constituted a heterogeneous group of amino acid metabolites/peptides. When comparing patients with and without bacteremia, we identified 51 metabolites that differed significantly, including 16 lipid metabolites and 11 amino acid metabolites. Furthermore, 42 metabolites showed a highly significant association with the maximal total Sequential Organ Failure Assessment (SOFA )score during the course of the disease (i.e., Pearson's correlation test, p-value < 0.005, and correlation factor > 0.6); these top-ranked metabolites included 23 amino acid metabolites and a subset of pregnenolone/progestin metabolites. Unsupervised hierarchical clustering analyses based on all 42 top-ranked SOFA correlated metabolites or the subset of 23 top-ranked amino acid metabolites showed that most Sepsis-3 patients differed from Sepsis-2/SIRS patients in their systemic metabolic profile at the time of hospital admission. However, a minority of Sepsis-3 patients showed similarities with the Sepsis-2/SIRS metabolic profile even though several of them showed a high total SOFA score. To conclude, Sepsis-3 patients are heterogeneous with regard to their metabolic profile at the time of hospitalization.

Temperate Bacteriophages-The Powerful Indirect Modulators of Eukaryotic Cells and Immune Functions

Bacteriophages are natural biological entities that limit the growth and amplification of bacteria. They are important stimulators of evolutionary variability in bacteria, and currently are considered a weapon against antibiotic resistance of bacteria. Nevertheless, apart from their antibacterial activity, phages may act as modulators of mammalian immune responses. In this paper, we focus on temperate phages able to execute the lysogenic development, which may shape animal or human immune response by influencing various processes, including phagocytosis of bacterial invaders and immune modulation of mammalian host cells.

Monoclonal Antibodies Targeting Surface-Exposed and Secreted Proteins from Staphylococci

Staphylococci (specifically Staphylococcus aureus and Staphylococcus epidermidis) are the causative agents of diseases ranging from superficial skin and soft tissue infections to severe conditions such as fatal pneumonia, bacteremia, sepsis and endocarditis. The widespread and indiscriminate use of antibiotics has led to serious problems of resistance to staphylococcal disease and has generated a renewed interest in alternative therapeutic agents such as vaccines and antibodies. Staphylococci express a large repertoire of surface and secreted virulence factors, which provide mechanisms (adhesion, invasion and biofilm development among others) for both bacterial survival in the host and evasion from innate and adaptive immunity. Consequently, the development of antibodies that target specific antigens would provide an effective protective strategy against staphylococcal infections. In this review, we report an update on efforts to develop anti-staphylococci monoclonal antibodies (and their derivatives: minibodies, antibody–antibiotic conjugates) and the mechanism by which such antibodies can help fight infections. We also provide an overview of mAbs used in clinical trials and highlight their therapeutic potential in various infectious contexts.

The Surge of Hypervirulent ST398 MRSA Lineage With Higher Biofilm-Forming Ability Is a Critical Threat to Clinics

The global increase of community-associated (CA) infections with methicillin-resistant Staphylococcus aureus (MRSA) is a major healthcare problem. Although sequence type (ST) 398 MRSA was first described as a livestock-associated (LA) lineage, human-adapted MRSA (HO-MRSA) ST398 without livestock contact has subsequently been reported from China in our previous study and other later research. The proportion of ST398 HO-MRSA has also remarkably increased in recent years in China. Based on 3878 S. aureus isolates that were collected in a general hospital between 2008 and 2018, we identified 56 ST398 HO-MRSA isolates. The four early appearing isolates of them have been sequenced by whole-genome sequencing (WGS) in our previous study. Here, by usage of WGS on the later-appearing 52 isolates and analyzing the phylogenetic dynamics of the linage, we found that 50 isolates clustered together with the former 4 isolates, making it a main clade out of MSSA clones and other MRSA clones, although ST398 HO-MRSA evolved with multiple origins. Drug resistance and virulence gene analysis based on the WGS data demonstrated that ST398 HO-MRSA main clade exhibited a similar pattern in both parts. Furthermore, they all carried a conserved variant of prophage 3 to guarantee virulence and a short SCCmec type V element of class D to maintain considerable lower methicillin resistance. Further phenotypical research verified that the epidemic HO-MRSA ST398 displayed enhanced biofilm formation ability when keeping high virulence. The dual advantages of virulence and biofilm formation in the HO-MRSA ST398 subtype promote their fitness in the community and even in the healthcare environment, which poses a serious threat in clinical S. aureus infections. Therefore, further surveillance is required to prevent and control the problematic public health impact of HO-MRSA ST398 in the future.

Human-specific staphylococcal virulence factors enhance pathogenicity in a humanised zebrafish C5a receptor model

Staphylococcus aureus infects ∼30% of the human population and causes a spectrum of pathologies ranging from mild skin infections to life-threatening invasive diseases. The strict host specificity of its virulence factors has severely limited the accuracy of in vivo models for the development of vaccines and therapeutics. To resolve this, we generated a humanised zebrafish model and determined that neutrophil-specific expression of the human C5a receptor conferred susceptibility to the S. aureus toxins PVL and HlgCB, leading to reduced neutrophil numbers at the site of infection and increased infection-associated mortality. These results show that humanised zebrafish provide a valuable platform to study the contribution of human-specific S. aureus virulence factors to infection in vivo that could facilitate the development of novel therapeutic approaches and essential vaccines.

Increasing prevalence of hypervirulent ST5 methicillin susceptible Staphylococcus aureus subtype poses a serious clinical threat

Staphylococcus aureus (S. aureus) is a clinical pathogen of great significance causing metastatic or complicated infections. ST5 clonotype isolates have dominated S. aureus infections for more than 10 years in Shanghai, China, and the proportion of methicillin-susceptible S. aureus (MSSA) has remarkably increased in the past decades. By whole-genome sequencing (WGS) 121 ST5 clonotype S. aureus isolates using next-generation sequencing (NGS) platforms and characterizing the evolutionary dynamics of ST5 linages, we found that MSSA evolved independently, making it a subtype differed from other MRSA clones. Drug resistance gene analysis by using the NGS data demonstrated that ST5 clonotype MRSA might be more tolerant under the threat of antimicrobials, which was confirmed in further in vitro susceptibility tests. However, MSSA subtype isolates exhibited relatively high virulence upon the analysis of virulence factors. Furthermore, MSSA subtype isolates displayed higher hemolysis capacity and higher ability to adhere to epithelial cells including A549 human alveolar epithelial cells and HaCaT human skin keratinocytes, caused more severe infections in murine abscess model. With its high virulence and enhanced magnitude in the past decades, the ST5 MSSA subtype poses a serious clinical threat hence more attention should be paid to the prevention and control.

Pre-existing antibody-mediated adverse effects prevent the clinical development of a bacterial anti-inflammatory protein

Bacterial pathogens have evolved to secrete strong anti-inflammatory proteins that target the immune system. It was long speculated whether these virulence factors could serve as therapeutics in diseases in which abnormal immune activation plays a role. We adopted the secreted chemotaxis inhibitory protein of Staphylococcus aureus (CHIPS) as a model virulence factor-based therapeutic agent for diseases in which C5AR1 stimulation plays an important role. We show that the administration of CHIPS in human C5AR1 knock-in mice successfully dampens C5a-mediated neutrophil migration during immune complex-initiated inflammation. Subsequent CHIPS toxicology studies in animal models were promising. However, during a small phase I trial, healthy human volunteers showed adverse effects directly after CHIPS administration. Subjects showed clinical signs of anaphylaxis with mild leukocytopenia and increased C-reactive protein concentrations, which are possibly related to the presence of relatively high circulating anti-CHIPS antibodies and suggest an inflammatory response. Even though our data in mice show CHIPS as a potential anti-inflammatory agent, safety issues in human subjects temper the use of CHIPS in its current form as a therapeutic candidate. The use of staphylococcal proteins, or other bacterial proteins, as therapeutics or immune-modulators in humans is severely hampered by pre-existing circulating antibodies.

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.

Staphylococcus aureus toxin LukSF dissociates from its membrane receptor target to enable renewed ligand sequestration

Staphylococcus aureus Panton-Valentine leukocidin is a pore-forming toxin targeting the human C5a receptor (hC5aR), enabling this pathogen to battle the immune response by destroying phagocytes through targeted lysis. The mechanisms that contribute to rapid cell lysis are largely unexplored. Here, we show that cell lysis may be enabled by a process of toxins targeting receptor clusters and present indirect evidence for receptor "recycling" that allows multiple toxin pores to be formed close together. With the use of live cell single-molecule super-resolution imaging, Förster resonance energy transfer and nanoscale total internal reflection fluorescence colocalization microscopy, we visualized toxin pore formation in the presence of its natural docking ligand. We demonstrate disassociation of hC5aR from toxin complexes and simultaneous binding of new ligands. This effect may free mobile receptors to amplify hyperinflammatory reactions in early stages of microbial infections and have implications for several other similar bicomponent toxins and the design of new antibiotics.-Haapasalo, K., Wollman, A. J. M., de Haas, C. J. C., van Kessel, K. P. M., van Strijp, J. A. G., Leake, M. C. Staphylococcus aureus toxin LukSF dissociates from its membrane receptor target to enable renewed ligand sequestration.

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 influence of etiological factors on immunoreactivity in patients with community-acquired pneumonia

 The aim of this study was to uncover the characteristics of immunoreactivity in patients with community-acquired pneumonia, depending on the pathogen. The inflammatory process caused by various pathogens, has its own characteristics and affects the course of the disease. The study of the mechanisms of these complex interactions can improve the understanding of the processes occurring in community-acquired pneumonia, and, therefore, develop individual approaches to the therapy depending on the etiological factor.

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.

Molecular analysis of immune evasion cluster (IEC) genes and intercellular adhesion gene cluster (ICA) among methicillin-resistant and methicillin-sensitive isolates of Staphylococcus aureus

Introduction

Resistance to antibiotics and presence of virulence factors play an important role in increased mortality associated with infection due to Staphylococcus aureus. In this study, we determine antibiotic resistance pattern, presence of the icaADBC locus as well as biofilm formation and distribution and diversity the immune evasion cluster (IEC) genes in clinical isolate of S. aureus from Kerman, Iran.

Materials and methods

During 15 months, 100 clinical isolates S. aureus recovered from different patients were admitted to Kerman University affiliated hospitals. Resistance to different antibiotic agents was determined by disk diffusion method. Phenotypic method was used to the determination of biofilm formation ability and methicillin-resistance S. aureus (MRSA). Polymerase chain reaction technique (PCR) was used to the detection of nuc, mecA, icaA, icaD, icaB, icaC, scn, sea, sak, sep and chp genes.

Results

Forty-four isolates were considered as MRSA and all of isolates were sensitive to vancomycin and linezolid. Our results showed, 77.2% (34/44) of MRSA and 8.9 % (5/56) of MSSA isolates were multidrug resistant. The predominant IEC variant was type B and our results displayed that 77.7% of the MRSA isolates harbor loci icaD and mecA. There was no significant difference in production biofilm between MSSA and MRSA isolates (P ≥ 0.05). There was significant difference in presence IEC types between MSSA and MRSA isolates (P = 0.000).

Conclusions

The presence of icaADBC locus may not be a determining factor for biofilm formation in Staphtlocooci and other mechanisms might be involved in this process.The high prevalence IEC types in MSSA isolates can indicate that the presence of these genes can be an advantage for pathogenesis of these isolates in different infections.

The Sbi Protein Contributes to Staphylococcus aureus Inflammatory Response during Systemic Infection

Staphylococcus aureus is an important human pathogen that causes infections that may present high morbidity and mortality. Among its many virulence factors protein A (SpA) and Staphylococcal binding immunoglobulin protein (Sbi) bind the Fc portion of IgG interfering with opsonophagocytosis. We have previously demonstrated that SpA interacts with the TNF-α receptor (TNFR) 1 through each of the five IgG binding domains and induces the production of pro-inflammatory cytokines and chemokines. The IgG binding domains of Sbi are homologous to those of SpA, which allow us to hypothesize that Sbi might also have a role in the inflammatory response induced by S. aureus. We demonstrate that Sbi is a novel factor that participates in the induction of the inflammatory response during staphylococcal infections via TNFR1 and EGFR mediated signaling as well as downstream MAPKs. The expression of Sbi significantly contributed to IL-6 production and modulated CXCL-1 expression as well as neutrophil recruitment to the site of infection, thus demonstrating for the first time its relevance as a pro-inflammatory staphylococcal antigen in an in vivo model.

Differential Interaction of the Staphylococcal Toxins Panton-Valentine Leukocidin and γ-Hemolysin CB with Human C5a Receptors

Staphylococcus aureus is well adapted to the human host. Evasion of the host phagocyte response is critical for successful infection. The staphylococcal bicomponent pore-forming toxins Panton-Valentine leukocidin LukSF-PV (PVL) and γ-hemolysin CB (HlgCB) target human phagocytes through interaction with the complement receptors C5aR1 and C5aR2. Currently, the apparent redundancy of both toxins cannot be adequately addressed in experimental models of infection because mice are resistant to PVL and HlgCB. The molecular basis for species specificity of the two toxins in animal models is not completely understood. We show that PVL and HlgCB feature distinct activity toward neutrophils of different mammalian species, where activity of PVL is found to be restricted to fewer species than that of HlgCB. Overexpression of various mammalian C5a receptors in HEK cells confirms that cytotoxicity toward neutrophils is driven by species-specific interactions of the toxins with C5aR1. By taking advantage of the species-specific engagement of the toxins with their receptors, we demonstrate that PVL and HlgCB differentially interact with human C5aR1 and C5aR2. In addition, binding studies illustrate that different parts of the receptor are involved in the initial binding of the toxin and the subsequent formation of lytic pores. These findings allow a better understanding of the molecular mechanism of pore formation. Finally, we show that the toxicity of PVL, but not of HlgCB, is neutralized by various C5aR1 antagonists. This study offers directions for the development of improved preclinical models for infection, as well as for the design of drugs antagonizing leukocidin toxicity.

The bicomponent pore-forming leucocidins of Staphylococcus aureus

The ability to produce water-soluble proteins with the capacity to oligomerize and form pores within cellular lipid bilayers is a trait conserved among nearly all forms of life, including humans, single-celled eukaryotes, and numerous bacterial species. In bacteria, some of the most notable pore-forming molecules are protein toxins that interact with mammalian cell membranes to promote lysis, deliver effectors, and modulate cellular homeostasis. Of the bacterial species capable of producing pore-forming toxic molecules, the Gram-positive pathogen Staphylococcus aureus is one of the most notorious. S. aureus can produce seven different pore-forming protein toxins, all of which are believed to play a unique role in promoting the ability of the organism to cause disease in humans and other mammals. The most diverse of these pore-forming toxins, in terms of both functional activity and global representation within S. aureus clinical isolates, are the bicomponent leucocidins. From the first description of their activity on host immune cells over 100 years ago to the detailed investigations of their biochemical function today, the leucocidins remain at the forefront of S. aureus pathogenesis research initiatives. Study of their mode of action is of immediate interest in the realm of therapeutic agent design as well as for studies of bacterial pathogenesis. This review provides an updated perspective on our understanding of the S. aureus leucocidins and their function, specificity, and potential as therapeutic targets.

Parasitic scabies mites and associated bacteria joining forces against host complement defence

Scabies is a ubiquitous and contagious skin disease caused by the parasitic mite Sarcoptes scabiei Epidemiological studies have identified scabies as a causative agent for secondary skin infections caused by Staphylococcus aureus and Streptococcus pyogenes. This is an important notion, as such bacterial infections can lead to serious downstream life-threatening complications. As the complement system is the first line of host defence that confronts invading pathogens, both the mite and bacteria produce a large array of molecules that inhibit the complement cascades. It is hypothesised that scabies mite complement inhibitors may play an important role in providing a favourable micro-environment for the establishment of secondary bacterial infections. This review aims to bring together the current literature on complement inhibition by scabies mites and bacteria associated with scabies and to discuss the proposed molecular link between scabies and bacterial co-infections.

Staphylococcal Immune Evasion Proteins: Structure, Function, and Host Adaptation

Staphylococcus aureus is a successful human and animal pathogen. Its pathogenicity is linked to its ability to secrete a large amount of virulence factors. These secreted proteins interfere with many critical components of the immune system, both innate and adaptive, and hamper proper immune functioning. In recent years, numerous studies have been conducted in order to understand the molecular mechanism underlying the interaction of evasion molecules with the host immune system. Structural studies have fundamentally contributed to our understanding of the mechanisms of action of the individual factors. Furthermore, such studies revealed one of the most striking characteristics of the secreted immune evasion molecules: their conserved structure. Despite high-sequence variability, most immune evasion molecules belong to a small number of structural categories. Another remarkable characteristic is that S. aureus carries most of these virulence factors on mobile genetic elements (MGE) or ex-MGE in its accessory genome. Coevolution of pathogen and host has resulted in immune evasion molecules with a highly host-specific function and prevalence. In this review, we explore how these shared structures and genomic locations relate to function and host specificity. This is discussed in the context of therapeutic options for these immune evasion molecules in infectious as well as in inflammatory diseases.

Safety and immunogenicity of an investigational 4-component Staphylococcus aureus vaccine with or without AS03B adjuvant: Results of a randomized phase I trial

We assessed the safety, reactogenicity and immunogenicity of a staphylococcal vaccine combining capsular polysaccharides types 5 and 8 (CPS5/8), conjugated to tetanus toxoid (TT), with mutated detoxified α-toxin (AT) and clumping factor A (ClfA). In this phase I, randomized, placebo-controlled, observer-blind trial (NCT01160172), 88 healthy 18- to 40-year-olds received CPS5-TT/CPS8-TT/AT/ClfA vaccine (5/5/10/10 μg or 10/10/30/30 μg dose, each with or without AS03B adjuvant) or saline, at months 0, 1, 6. Solicited and unsolicited adverse events (AEs) were recorded for 7 and 30 d post-vaccination, respectively; potential immune-mediated diseases (pIMDs) and serious AEs (SAEs) were recorded throughout the study. Humoral and antigen-specific CD4(+)/CD8(+) T-cell immunity were assessed from Day (D) 0 to D540 post-vaccination. The most frequently reported solicited local and general AEs were pain (78.6%-100% of subjects), fatigue (36.4%-93.3% of subjects post-dose 1-2) and headache (20%-44.4% of subjects post-dose 3). Overall, 4 SAEs and 2 potential immune-mediated diseases (pIMDs) (none fatal or vaccine-related) were reported. For each antigen, pre-vaccination seropositivity rates were high (85.7%-100%) and geometric mean concentrations (GMCs) in vaccine recipients sharply increased from D0 to D14, then plateaued to study end. Exploratory group comparisons suggested higher GMCs with higher dosage, without AS03B effect. Vaccine-induced antibodies were functional (CPS5 opsonophagocytic assays, and AT/ClfA inhibition assays). AT- and ClfA-specific CD4(+) T-cells with Th0/Th1 cytokine profile were induced at low levels (median <0.05%) by each formulation (intracellular cytokine staining). In conclusion, no safety concerns were identified and each vaccine formulation induced robust humoral immune responses after the first vaccine dose.

The staphylococcal toxin Panton-Valentine Leukocidin targets human C5a receptors

Panton-Valentine Leukocidin (PVL) is a staphylococcal bicomponent pore-forming toxin linked to severe invasive infections. Target-cell and species specificity of PVL are poorly understood, and the mechanism of action of this toxin in Staphylococcus aureus virulence is controversial. Here, we identify the human complement receptors C5aR and C5L2 as host targets of PVL, mediating both toxin binding and cytotoxicity. Expression and interspecies variations of the C5aR determine cell and species specificity of PVL. The C5aR binding PVL component, LukS-PV, is a potent inhibitor of C5a-induced immune cell activation. These findings provide insight into leukocidin function and staphylococcal virulence and offer directions for future investigations into individual susceptibility to severe staphylococcal disease.

Neutrophils versus Staphylococcus aureus: a biological tug of war

The pathogen Staphylococcus aureus is well adapted to its human host. Neutrophil-mediated killing is a crucial defense system against S. aureus; however, the pathogen has evolved many strategies to resist killing. We first describe the discrete steps of neutrophil activation and migration to the site of infection and the killing of microbes by neutrophils in general. We then highlight the different approaches utilized by S. aureus to resist the different steps of neutrophil attack. Various molecules are discussed in their evolutionary context. Most of the molecules secreted by S. aureus to combat neutrophil attacks at the site of infection show clear human specificity. Many elements of human neutrophil defenses appear redundant, and so the evasion strategies of staphylococci display redundant functions as well. All efforts by S. aureus to resist neutrophil-mediated killing stress the importance of these mechanisms in the pathophysiology of staphylococcal diseases. However, the highly human-specific nature of most host-pathogen interactions hinders the in vivo establishment of their contribution to staphylococcal pathophysiology.

International Union of Basic and Clinical Pharmacology. [corrected]. LXXXVII. Complement peptide C5a, C4a, and C3a receptors

The activation of the complement cascade, a cornerstone of the innate immune response, produces a number of small (74-77 amino acid) fragments, originally termed anaphylatoxins, that are potent chemoattractants and secretagogues that act on a wide variety of cell types. These fragments, C5a, C4a, and C3a, participate at all levels of the immune response and are also involved in other processes such as neural development and organ regeneration. Their primary function, however, is in inflammation, so they are important targets for the development of antiinflammatory therapies. Only three receptors for complement peptides have been found, but there are no satisfactory antagonists as yet, despite intensive investigation. In humans, there is a single receptor for C3a (C3a receptor), no known receptor for C4a, and two receptors for C5a (C5a₁ receptor and C5a₂ receptor). The most recently characterized receptor, the C5a₂ receptor (previously known as C5L2 or GPR77), has been regarded as a passive binding protein, but signaling activities are now ascribed to it, so we propose that it be formally identified as a receptor and be given a name to reflect this. Here, we describe the complex biology of the complement peptides, introduce a new suggested nomenclature, and review our current knowledge of receptor pharmacology.

Immunization With a Combination of 2 Peptides Derived From the C5a Receptor Significantly Reduces Early Atherosclerotic Lesion in Ldlr tm1Her Apob tm2Sgy J Mice

Objective—

The goal of this study was to assess whether immunization of Ldlr tm1Her Apob tm2Sgy J mice with 2 peptides located at the N-terminus of the C5a receptor (C5aR), either alone or in combination, is effective in reducing atherosclerotic lesions.

Methods and Results—

Five- to 6-week-old female Ldlr tm1Her Apob tm2Sgy J mice were immunized using a repetitive immunization multiple sites strategy with keyhole limpet hemocyanin-conjugated peptides derived from the C5aR, either alone (designated as C5aR-P1 [aa 1–21] and C5aR-P2 [aa 19–31]) or in combination (designated as C5aR-P1+C5aR-P2). Mice were fed a high-fat diet for 10 weeks. Lesions were evaluated histologically; local and systemic immune responses were analyzed by immunohistochemistry of aorta samples and cytokine measurements in plasma samples and splenocyte supernatants. Immunization of Ldlr tm1Her Apob tm2Sgy J mice with these peptides elicited high concentrations of antibodies against each peptide. Immunization with the single peptide inhibited plaque development. Combined inoculation with C5aR-P1+C5aR-P2 had an additive effect on reducing the lesion in the aorta sinus and descending aortas when compared with controls. This effect correlated with cellular infiltration and cytokine/chemokine secretion in the serum or in stimulated spleen cells as well as specific cellular immune responses when compared with controls.

Conclusion—

Immunization of mice with C5aR-P1 and C5aR-P2, either alone or in combination, was effective in reducing early atherosclerotic lesion development. The combined peptide is more potential than either epitope alone to reduce atherosclerotic lesion formation through the induction of a specific Treg cell response as well as blockage of monocyte differentiation into macrophages.

Exploring Staphylococcus aureus pathways to disease for vaccine development

Staphylococcus aureus is a commensal of the human skin or nares and a pathogen that frequently causes skin and soft tissue infections as well as bacteremia and sepsis. Recent efforts in understanding the molecular mechanisms of pathogenesis revealed key virulence strategies of S. aureus in host tissues: bacterial scavenging of iron, induction of coagulation pathways to promote staphylococcal agglutination in the vasculature, and suppression of innate and adaptive immune responses. Advances in all three areas have been explored for opportunities in vaccine design in an effort to identify the critical protective antigens of S. aureus. Human clinical trials with specific subunit vaccines have failed, yet provide important insights for the design of future trials that must address the current epidemic of S. aureus infections with drug-resistant isolates (MRSA, methicillin-resistant S. aureus).

Recurrent infections and immune evasion strategies of Staphylococcus aureus

Staphylococcus aureus causes purulent skin and soft tissue infections (SSTIs) that frequently reoccur. Staphylococal SSTIs can lead to invasive disease and sepsis, which are among the most significant causes of infectious disease mortality in both developed and developing countries. Human or animal infections with S. aureus do not elicit protective immunity against staphylococcal diseases. Here we review what is known about the immune evasive strategies of S. aureus that enable the pathogen's escape from protective immune responses. Three secreted products are discussed in detail, staphylococcal protein A (SpA), staphylococcal binder of immunoglobulin (Sbi) and adenosine synthase A (AdsA). By forming a complex with V(H)3-type IgM on the surface of B cells, SpA functions as a superantigen to modulate antibody responses to staphylococcal infection. SpA also captures pathogen-specific antibodies by binding their Fcγ portion. The latter activity of SpA is shared by Sbi, which also associates with complement factors 3d and factor H to promote the depletion of complement. AdsA synthesizes the immune signaling molecule adenosine, thereby dampening innate and adaptive immune responses during infection. We discuss strategies how the three secreted products of staphylococci may be exploited for the development of vaccines and therapeutics.

Tyrosine sulfation in N-terminal domain of human C5a receptor is necessary for binding of chemotaxis inhibitory protein of Staphylococcus aureus

AIM

Staphylococcus aureus evades host defense through releasing several virulence proteins, such as chemotaxis inhibitory protein of staphylococcus aureus (CHIPS). It has been shown that extracellular N terminus of C5a receptor (C5aR) forms the binding domain for CHIPS, and tyrosine sulfation is emerging as a key factor in determining protein-protein interaction. The aim of this study was to evaluate the role of tyrosine sulfation of N-terminal of C5aR in its binding with CHIPS.

METHODS

Expression plasmids encoding C5aR and its mutants were prepared using PCR and site-directed mutagenesis and were used to transfect HEK 293T cells using calcium phosphate. Recombinant CHIPS protein was purified. Western blotting was used to examine the binding efficiency of CHIPS to C5aR or its mutants.

RESULTS

CHIPS exclusively binds to C5aR, but not to C5L2 or C3aR. A nonspecific sulfation inhibitor, sodium chlorate (50 nmol/L), diminishes the binding ability of C5aR with CHIPS. Blocking sulfation by mutation of tyrosine to phenylalanine at positions 11 and 14 of C5aR N terminus, which blocked sulfation, completely abrogates CHIPS binding. When tyrosine 14 alone was mutated to phenylalanine, the binding efficiency of recombinant CHIPS was substantially decreased.

CONCLUSION

The results demonstrate a structural basis of C5aR-CHIPS association, in which tyrosine sulfation of N-terminal C5aR plays an important role. Our data may have potential significance in development of novel drugs for therapeutic intervention.

Staphylococcus aureus metalloprotease aureolysin cleaves complement C3 to mediate immune evasion

Complement is one of the first host defense barriers against bacteria. Activated complement attracts neutrophils to the site of infection and opsonizes bacteria to facilitate phagocytosis. The human pathogen Staphylococcus aureus has successfully developed ways to evade the complement system, for example by secretion of specific complement inhibitors. However, the influence of S. aureus proteases on the host complement system is still poorly understood. In this study, we identify the metalloprotease aureolysin as a potent complement inhibitor. Aureolysin effectively inhibits phagocytosis and killing of bacteria by neutrophils. Furthermore, we show that aureolysin inhibits the deposition of C3b on bacterial surfaces and the release of the chemoattractant C5a. Cleavage analyses show that aureolysin cleaves the central complement protein C3. Strikingly, there was a clear difference between the cleavages of C3 in serum versus purified conditions. Aureolysin cleaves purified C3 specifically in the α-chain, close to the C3 convertase cleavage site, yielding active C3a and C3b. However, in serum we observe that the aureolysin-generated C3b is further degraded by host factors. We pinpointed these factors to be factor H and factor I. Using an aureolysin mutant in S. aureus USA300, we show that aureolysin is essential and sufficient for C3 cleavage by bacterial supernatant. In short, aureolysin acts in synergy with host regulators to inactivate C3 thereby effectively dampening the host immune response.

A peptide mimic of the chemotaxis inhibitory protein of Staphylococcus aureus: towards the development of novel anti-inflammatory compounds

Complement factor C5a is one of the most powerful pro-inflammatory agents involved in recruitment of leukocytes, activation of phagocytes and other inflammatory responses. C5a triggers inflammatory responses by binding to its G-protein-coupled C5a-receptor (C5aR). Excessive or erroneous activation of the C5aR has been implicated in numerous inflammatory diseases. The C5aR is therefore a key target in the development of specific anti-inflammatory compounds. A very potent natural inhibitor of the C5aR is the 121-residue chemotaxis inhibitory protein of Staphylococcus aureus (CHIPS). Although CHIPS effectively blocks C5aR activation by binding tightly to its extra-cellular N terminus, it is not suitable as a potential anti-inflammatory drug due to its immunogenic properties. As a first step in the development of an improved CHIPS mimic, we designed and synthesized a substantially shorter 50-residue adapted peptide, designated CHOPS. This peptide included all residues important for receptor binding as based on the recent structure of CHIPS in complex with the C5aR N terminus. Using isothermal titration calorimetry we demonstrate that CHOPS has micromolar affinity for a model peptide comprising residues 7–28 of the C5aR N terminus including two O-sulfated tyrosine residues at positions 11 and 14. CD and NMR spectroscopy showed that CHOPS is unstructured free in solution. Upon addition of the doubly sulfated model peptide, however, the NMR and CD spectra reveal the formation of structural elements in CHOPS reminiscent of native CHIPS.

Complement inhibition by gram-positive pathogens: molecular mechanisms and therapeutic implications

The plasma proteins of the complement system are essential in the innate immune response against bacteria. Complement labels bacteria with opsonins to support phagocytosis and generates chemoattractants to attract phagocytes to the site of infection. In turn, bacterial human pathogens have evolved different strategies to specifically impair the complement response. Here, we review the large arsenal of complement inhibitors produced by the gram-positive pathogens Staphylococcus aureus and Group A Streptococcus. We discuss how these bacterial molecules provide us with new tools to treat both infectious and inflammatory disease conditions in humans.

Colonization and infection of the human host by staphylococci: adhesion, survival and immune evasion

The natural habitat of Staphylococcus aureus in humans is the moist squamous epithelium of the anterior nares. Several bacterial surface proteins are implicated in promoting adhesion to desquamated epithelial cells. Clumping factor B (ClfB) and iron-regulated surface determinant A both promote nasal colonization in rodent models, and in the case of ClfB, humans. One of the ligands involved in adhesion is cytokeratin 10. Reduction in nasal colonization can be achieved by active and passive immunization. S. aureus is well endowed with secreted and surface components that compromise innate immune responses, particularly the function of neutrophils. S. aureus secretes proteins that reduce migration of neutrophils from the bloodstream to the site of infection by impeding diapedesis and receptors for chemotactic molecules. Several secreted proteins interfere with complement C3 and C5 convertases, thus reducing the level of C3b opsonin and the chemotactic peptide C5a. Host proteases are recruited to the cell surface to enhance destruction of opsonic C3b and IgG. Surface components ClfA, protein A and polysaccharide capsule compromise the recognition of opsonins on the bacterial cell surface. If engulfed by neutrophils the intracellular bacterium can resist reactive oxygen intermediates, nitric oxide radicals, defensin peptides and bactericidal proteins. A prior infection by S. aureus does not induce complete protective immunity. This could be due to immunosuppression caused by expression of superantigen proteins that disrupt normal activation of T cells and B cells during antigen presentation. By studying the molecular pathogenesis of S. aureus infections markers might be found for investigating S. pseudintermedius infections of dogs.

CHIPS binds to the phosphorylated N-terminus of the C5a-receptor

Replacement of the sulfate groups, present in vivo on the N-terminus of the C5a-receptor (C5aR), by phosphate groups is explored. Phosphorylated mimics of the C5a-receptor N-terminus are synthesized and their binding to Chemotaxis Inhibitory Protein of Staphylococcus aureus (CHIPS) is studied by ITC and NMR. The phosphorylated C5aR mimics showed comparable binding affinity and a similar binding mode towards CHIPS compared to their sulfated forms. The activities of the phosphorylated peptides in a biological assay, however, were significantly lower compared to their sulfated counterparts.

Directed evolution of chemotaxis inhibitory protein of Staphylococcus aureus generates biologically functional variants with reduced interaction with human antibodies

Chemotaxis inhibitory protein of Staphylococcus aureus (CHIPS) is a protein that binds and blocks the C5a receptor (C5aR) and formylated peptide receptor, thereby inhibiting the immune cell recruitment associated with inflammation. If CHIPS was less reactive with existing human antibodies, it would be a promising anti-inflammatory drug candidate. Therefore, we applied directed evolution and computational/rational design to the CHIPS gene in order to generate new CHIPS variants displaying lower interaction with human IgG, yet retaining biological function. The optimization was performed in four rounds: one round of random mutagenesis to add diversity into the CHIPS gene and three rounds of DNA recombination by Fragment INduced Diversity (FIND). Every round was screened by phage selection and/or ELISA for decreased interaction with human IgG and retained C5aR binding. The mean binding of human anti-CHIPS IgG decreased with every round of evolution. For further optimization, new amino acid substitutions were introduced by rational design, based on the mutations identified during directed evolution. Finally, seven CHIPS variants with low interaction with human IgG and retained C5aR blocking capacity could be identified.

A homolog of formyl peptide receptor-like 1 (FPRL1) inhibitor from Staphylococcus aureus (FPRL1 inhibitory protein) that inhibits FPRL1 and FPR

The members of the formyl peptide receptor (FPR) family are involved in the sensing of chemoattractant substances, including bacteria-derived N-formylated peptides and host-derived peptides and proteins. We have recently described two chemoattractant receptor inhibitors from Staphylococcus aureus. Chemotaxis inhibitory protein of S. aureus (CHIPS) blocks the formyl peptide receptor (FPR) and the receptor for complement C5a (C5aR), while FPR-like 1 (FPRL1) inhibitory protein (FLIPr) blocks the FPRL1. Here, we describe another staphylococcal chemoattractant-inhibiting protein with 73% overall homology to FLIPr and identical first 25 aa, which we termed FLIPr-like. This protein inhibits neutrophil calcium mobilization and chemotaxis induced by the FPRL1-ligand MMK-1 and FPR-ligand fMLP. While its FPRL1-inhibitory activity lies in the comparable nanomolar range of FLIPr, its antagonism of the FPR is approximately 100-fold more potent than that of FLIPr and comparable to that of CHIPS. The second N-terminal phenylalanine was required for its inhibition of the FPR, but it was dispensable for the FPRL1. Furthermore, the deletion of the first seven amino acids reduced its antagonism of the FPRL1, and the exchange of the first six amino acids with that of CHIPS-conferred receptor specificity. Finally, studies with cells transfected with several chemoattractant receptors confirmed that FLIPr-like specifically binds to the FPR and FPRL1. In conclusion, the newly described excreted protein from S. aureus, FLIPr-like, is a potent inhibitor of the FPR- and FPRL1-mediated neutrophil responses and may be used to selectively modulate these chemoattractant receptors.

Structural models for the complex of chemotaxis inhibitory protein of Staphylococcus aureus with the C5a receptor

The study presents structural models for the complex of the chemotaxis inhibitory protein of Staphylococcus aureus, CHIPS, and receptor for anaphylotoxin C5a, C5aR. The models are based on the recently found NMR structure of the complex between CHIPS fragment 31-121 and C5aR fragment 7-28, as well as on previous results of molecular modeling of C5aR. Simple and straightforward modeling procedure selected low-energy conformations of the C5aR fragment 8-41 that simultaneously fit the NMR structure of the C5aR 10-18 fragment and properly orient the NMR structure of CHIPS(31-121) relative to C5aR. Extensive repacking of the side chains of CHIPS(31-121) and C5aR(8-41) predicted specific residue-residue interactions on the interface between CHIPS and C5aR. Many of these interactions were rationalized with experimental data obtained by site-directed mutagenesis of CHIPS and C5aR. The models correctly showed that CHIPS binds only to the first binding site of C5a to C5aR not competing with C5a fragment 59-74, which binds the second binding site of C5aR. The models also predict that two elements of CHIPS, fragments 48-58 and 97-111, may be used as structural templates for potential inhibitors of C5a.

Staphylococcal superantigen-like 10 inhibits CXCL12-induced human tumor cell migration

PURPOSE

Tumor cell migration and metastasis share many similarities with leukocyte trafficking, which is critically regulated by chemokines and their receptors. CXCR4 is the most widely expressed chemokine receptor in many different types of cancer and has been linked to tumor dissemination and poor prognosis. Several CXCR4 antagonists have been synthesized. A totally novel approach to discover chemokine receptor antagonists is the use of bacteria. Bacteria produce chemokine receptor inhibitors to prevent neutrophil extravasation and migration toward the infection site to escape clearance by innate immune cells. The aim of the current study was to find and identify the mechanism of a bacterial protein that specifically targets CXCR4, a chemokine receptor shared by neutrophils and cancer cells.

EXPERIMENTAL DESIGN

Several staphylococcal proteins were screened for their capacity to prevent binding of a function-blocking antibody against CXCR4.

RESULTS

Staphylococcal superantigen-like 10 was found to bind CXCR4 expressed on human T acute lymphoblastic leukemia, lymphoma, and cervical carcinoma cell lines. It potently inhibited CXCL12-induced calcium mobilization and cell migration.

CONCLUSIONS

Staphylococcal superantigen-like 10 is a potential lead in the development of new anticancer compounds preventing metastasis by targeting CXCR4.

Structure of the tyrosine-sulfated C5a receptor N terminus in complex with chemotaxis inhibitory protein of Staphylococcus aureus

Complement component C5a is a potent pro-inflammatory agent inducing chemotaxis of leukocytes toward sites of infection and injury. C5a mediates its effects via its G protein-coupled C5a receptor (C5aR). Although under normal conditions highly beneficial, excessive levels of C5a can be deleterious to the host and have been related to numerous inflammatory diseases. A natural inhibitor of the C5aR is chemotaxis inhibitory protein of Staphylococcus aureus (CHIPS). CHIPS is a 121-residue protein excreted by S. aureus. It binds the N terminus of the C5aR (residues 1-35) with nanomolar affinity and thereby potently inhibits C5a-mediated responses in human leukocytes. Therefore, CHIPS provides a starting point for the development of new anti-inflammatory agents. Two O-sulfated tyrosine residues located at positions 11 and 14 within the C5aR N terminus play a critical role in recognition of C5a, but their role in CHIPS binding has not been established so far. By isothermal titration calorimetry, using synthetic Tyr-11- and Tyr-14-sulfated and non-sulfated C5aR N-terminal peptides, we demonstrate that the sulfate groups are essential for tight binding between the C5aR and CHIPS. In addition, the NMR structure of the complex of CHIPS and a sulfated C5aR N-terminal peptide reveals the precise binding motif as well as the distinct roles of sulfated tyrosine residues sY11 and sY14. These results provide a molecular framework for the design of novel CHIPS-based C5aR inhibitors.

Staphylococcal SSL5 inhibits leukocyte activation by chemokines and anaphylatoxins

Staphylococcus aureus secretes several virulence factors modulating immune responses. Staphylococcal superantigen-like (SSL) proteins are a family of 14 exotoxins with homology to superantigens, but with generally unknown function. Recently, we showed that SSL5 binds to P-selectin glycoprotein ligand 1 dependently of sialyl Lewis X and inhibits P-selectin-dependent neutrophil rolling. Here, we show that SSL5 potently and specifically inhibits leukocyte activation by anaphylatoxins and all classes of chemokines. SSL5 inhibited calcium mobilization, actin polymerization, and chemotaxis induced by chemokines and anaphylatoxins but not by other chemoattractants. Antibody competition experiments showed that SSL5 targets several chemokine and anaphylatoxin receptors. In addition, transfection studies showed that SSL5 binds glycosylated N-termini of all G protein-coupled receptors (GPCRs) but only inhibits stimuli of protein nature that require the receptor N-terminus for activation. Furthermore, SSL5 increased binding of chemokines to cells independent of chemokine receptors through their common glycosaminoglycan-binding site. Importance of glycans was shown for both GPCR and chemokine binding. Thus, SSL5 is an important immunomodulatory protein of S aureus that targets several crucial, initial stages of leukocyte extravasation. It is therefore a potential new antiinflammatory compound for diseases associated with chemoattractants and their receptors and disorders characterized by excessive recruitment of leukocytes.

Molecular analysis of the bovine anaphylatoxin C5a receptor

Recruitment of phagocytes to inflammatory sites involves the coordinated action of several chemoattractants, including the anaphylatoxin C5a. While the C5a receptor (C5aR) has been well characterized in humans and rodents, little is known about the bovine C5aR. Here, we report cloning of bovine C5R1, the gene encoding bovine C5aR. We also analyzed genomic sequence upstream of the C5R1 translation start site. Although the bovine C5aR amino acid sequence was well conserved among species, significant differences in conserved features were found, including major differences in the N terminus, intracellular loop 3, and transmembrane domain VII. Analysis of C5aR expression by flow cytometry and confocal microscopy demonstrated high levels of C5aR on all bovine neutrophils and a subset of bovine monocytes. C5aR was not expressed on resting or activated bovine lymphocytes, although C5aR message was present in these cells. C5aR was also expressed on a small subset of bovine mammary epithelial cells. Pharmacological analysis of bovine C5aR-mediated responses showed that bovine C5a and C5adesArg both induced dose-dependent calcium fluxes and chemotaxis in bovine neutrophils, with similar efficacy for both agonists. Treatment of bovine neutrophils with C5a or C5adesArg resulted in homologous desensitization of bovine C5aR and cross-desensitization to interleukin 8 (IL-8) and platelet-activating factor (PAF); whereas, treatment with IL-8 or PAF did not cross-desensitize the cells to C5a or C5adesArg. Overall, these studies provide important information regarding distinct structural and functional features that may contribute to the unique pharmacological properties of bovine C5aR.

Staphylococcal complement inhibitor: structure and active sites

The pathogenic bacterium Staphylococcus aureus counteracts the host immune defense by excretion of the 85 residue staphylococcal complement inhibitor (SCIN). SCIN inhibits the central complement convertases; thereby, it reduces phagocytosis following opsonization and efficiently blocks all downstream effector functions. In this study, we present the crystal structure of SCIN at 1.8 A resolution and the identification of its active site. Functional characterization of structure based chimeric proteins, consisting of SCIN and the structurally but nonfunctional homologue open reading frame-D, indicate an 18-residue segment (Leu-31-Gly-48) crucial for SCIN activity. In all complement activation pathways, chimeras lacking these SCIN residues completely fail to inhibit production of the potent mediator of inflammation C5a. Inhibition of alternative pathway-mediated opsonization (C3b deposition) and formation of the lytic membrane attack complex (C5b-9 deposition) are strongly reduced for these chimeras as well. For inhibition of the classical/lectin pathway-mediated C3b and C5b-9 deposition, the same residues are critical although additional sites are involved. These chimeras also display reduced capacity to stabilize the C3 convertases of both the alternative and the classical/lectin pathway indicating the stabilizing effect is pivotal for the complement inhibitory activity of SCIN. Because SCIN specifically and efficiently inhibits complement, it has a high potential in anti-inflammatory therapy. Our data are a first step toward the development of a second generation molecule suitable for such therapeutic complement intervention.

Anaphylatoxins: their role in bacterial infection and inflammation

Activation of the complement system plays a crucial role in the pathogenesis of infection and inflammation. Especially the complement activation products C3a and C5a, known as the anaphylatoxins, are potent proinflammatory mediators. In addition to their evident role in innate immunity, it is clear that the anaphylatoxins also play a role in regulation of adaptive immune responses. The anaphylatoxins play a role in a variety of infectious and inflammatory diseases like sepsis, ischemia-reperfusion injury, immune complex diseases, and hypersensitivity diseases like asthma. In this review we discuss the role of anaphylatoxins in infection and inflammation. Furthermore, we focus on bacterial complement evasion strategies that can provide tools for further research on pathogenesis of infectious diseases and a better understanding of the role of complement and anaphylatoxins in infection and inflammation.

The anti-inflammatory activities of Staphylococcus aureus

Staphylococcus aureus is a versatile and harmful pathogen in both hospital- and community-associated infections that range from superficial to systemic infections. S. aureus engages a multitude of mechanisms to subvert the innate immune response of the host, including inhibition of complement activation and neutralization of anti-microbial peptides. In addition, inflammatory cell and phagocyte recruitment is an integral part of the innate defense to staphylococcal infection and comprises a well-coordinated multi-step cascade of adhesive events. Recent and rapidly growing experimental evidence indicates the existence of a machinery of anti-adhesive and anti-chemotactic moieties of S. aureus that allow the bacterium to interfere with specific adhesive steps of the homing mechanism of leukocytes. Understanding the functions of these S. aureus-derived anti-inflammatory agents could also provide the platform for designing new therapies in several inflammatory and autoimmune diseases.

Differential activation of polymorphisms of the formyl peptide receptor by formyl peptides

We have investigated the role of two polymorphic sites (R190W and N192K) on the binding and activation of the formyl peptide receptor (FPR) by viral and formyl peptides. WEDWVGWI, a peptide with antiviral activity derived from the membrane proximal region of feline immunodeficiency virus, binds with high affinity to FPR. The three tryptophans in the peptide are all essential for FPR binding, just as they were essential for antiviral activity [S. Giannecchini, A. Di Fenza, A.M. D'Ursi, D. Matteucci, P. Rovero, M. Bendinelli, Antiviral activity and conformational features of an octapeptide derived from the membrane-proximal ectodomain of the feline immunodeficiency virus transmembrane glycoprotein, J. Virol. 77 (2003) 3724]. Formyl-NleWEDWVGWI behaved as a weak partial agonist with FPR W190/N192 but a stronger partial agonist with FPR R190/K192 and FPR R190/N192. Formyl-NleNleWEDWVGWI behaved as a full agonist toward all three FPRs but exhibited a much higher EC(50) with W190/N192 FPR (300+/-45 nM) than for R190/K192 FPR (40+/-3 nM) or R190/N192 (60+/-8 nM). Formyl-MYKWPWYVWL preferentially activated R190/K192 and R190/N192 FPRs by>5 fold compared to W190/N192 FPR. Formyl-MFEDAVAWF, a peptide derived from a protein in Mycobacterium avium subsp. paratuberculosis and formyl-MFTFEPFPTN, a peptide derived from the N-terminus of chemotaxis inhibitory protein of Staphylococcus aureus with an added N-terminal formyl-methionine exhibited the greatest selectivity for R190/K192 and R190/N192 FPRs with approximately 10 fold lower EC(50)s than that observed with FPR W190/N192. Thus, individuals with the W190 polymorphism may display a reduced ability to detect certain formyl peptides.

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

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