The Staphylococcus aureus superantigen SElX is a bifunctional toxin that inhibits neutrophil function

Bacterial superantigens (SAgs) cause Vβ-dependent T-cell proliferation leading to immune dysregulation associated with the pathogenesis of life-threatening infections such as toxic shock syndrome, and necrotizing pneumonia. Previously, we demonstrated that staphylococcal enterotoxin-like toxin X (SElX) from Staphylococcus aureus is a classical superantigen that exhibits T-cell activation in a Vβ-specific manner, and contributes to the pathogenesis of necrotizing pneumonia. Here, we discovered that SElX can also bind to neutrophils from human and other mammalian species and disrupt IgG-mediated phagocytosis. Site-directed mutagenesis of the conserved sialic acid-binding motif of SElX abolished neutrophil binding and phagocytic killing, and revealed multiple glycosylated neutrophil receptors for SElX binding. Furthermore, the neutrophil binding-deficient mutant of SElX retained its capacity for T-cell activation demonstrating that SElX exhibits mechanistically independent activities on distinct cell populations associated with acquired and innate immunity, respectively. Finally, we demonstrated that the neutrophil-binding activity rather than superantigenicity is responsible for the SElX-dependent virulence observed in a necrotizing pneumonia rabbit model of infection. Taken together, we report the first example of a SAg, that can manipulate both the innate and adaptive arms of the human immune system during S. aureus pathogenesis.

Staphylococcus aureus is a bacterial pathogen responsible for an array of disease types in healthcare and community settings. One of the keys to the success of this pathogen is its ability to subvert the immune system of the host. Here we demonstrate that the superantigen (SAg) staphylococcal enterotoxin-like toxin X (SElX) contributes to immune evasion by inducing unregulated T-cell proliferation, and by inhibition of phagocytosis by neutrophils. We observed that the capacity to bind neutrophils appears to be central to the SElX-dependent toxicity observed in a necrotising pneumonia infection model in rabbits. We report the first example of a staphylococcal SAg with two independent immunomodulatory functions acting on distinct immune cell types.

The ESX-5 System of Pathogenic Mycobacteria Is Involved In Capsule Integrity and Virulence through Its Substrate PPE10

Mycobacteria produce a capsule layer, which consists of glycan-like polysaccharides and a number of specific proteins. In this study, we show that, in slow-growing mycobacteria, the type VII secretion system ESX-5 plays a major role in the integrity and stability of the capsule. We have identified PPE10 as the ESX-5 substrate responsible for this effect. Mutants in esx-5 and ppe10 both have impaired capsule integrity as well as reduced surface hydrophobicity. Electron microscopy, immunoblot and flow cytometry analyses demonstrated reduced amounts of surface localized proteins and glycolipids, and morphological differences in the capsular layer. Since capsular proteins secreted by the ESX-1 system are important virulence factors, we tested the effect of the mutations that cause capsular defects on virulence mechanisms. Both esx-5 and ppe10 mutants of Mycobacterium marinum were shown to be impaired in ESX-1-dependent hemolysis. In agreement with this, the ppe10 and esx5 mutants showed reduced recruitment of ubiquitin in early macrophage infection and intermediate attenuation in zebrafish embryos. These results provide a pivotal role for the ESX-5 secretion system and its substrate PPE10, in the capsular integrity of pathogenic mycobacteria. These findings open up new roads for research on the mycobacterial capsule and its role in virulence and immune modulation.

Neutrophil-Mediated Phagocytosis of Staphylococcus aureus

Initial elimination of invading Staphylococcus aureus from the body is mediated by professional phagocytes. The neutrophil is the major phagocyte of the innate immunity and plays a key role in the host defense against staphylococcal infections. Opsonization of the bacteria with immunoglobulins and complement factors enables efficient recognition by the neutrophil that subsequently leads to intracellular compartmentalization and killing. Here, we provide a review of the key processes evolved in neutrophil-mediated phagocytosis of S. aureus and briefly describe killing. As S. aureus is not helpless against the professional phagocytes, we will also highlight its immune evasion arsenal related to phagocytosis.

Staphylococcus aureus proteins SSL6 and SElX interact with neutrophil receptors as identified using secretome phage display

In order to cause colonization and invasive disease, pathogenic bacteria secrete proteins that modulate host immune defences. Identification and characterization of these proteins leads to a better understanding of the pathological processes underlying infectious and inflammatory diseases and is essential in the development of new strategies for their prevention and treatment. Current techniques to functionally characterize these proteins are laborious and inefficient. Here we describe a high-throughput functional selection strategy using phage display in order to identify immune evasion proteins. Using this technique we identified two previously uncharacterized proteins secreted by Staphylococcus aureus, SElX and SSL6 that bind to neutrophil surface receptors. SElX binds PSGL-1 on neutrophils and thereby inhibits the interaction between PSGL-1 and P-selectin, a crucial step in the recruitment of neutrophils to the site of infection. SSL6 is the first bacterial protein identified that binds CD47, a widely expressed cell surface protein recently described as an interesting target in anti-cancer therapy. Our findings provide new insights into the pathogenesis of S. aureus infections and support phage display as an efficient method to identify bacterial secretome proteins interacting with humoral or cellular immune components.

Hidden behind autophagy: the unconventional roles of ATG proteins

Macroautophagy (hereafter referred to as autophagy) is an evolutionarily conserved intracellular catabolic transport route that generally allows the lysosomal degradation of cytoplasmic components, including bulk cytosol, protein aggregates, damaged or superfluous organelles and invading microbes. Target structures are sequestered by double‐membrane vesicles called autophagosomes, which are formed through the concerted action of the autophagy (ATG)‐related proteins. Until recently it was assumed that ATG proteins were exclusively involved in autophagy. A growing number of studies, however, have attributed functions to some of them that are distinct from their classical role in autophagosome biogenesis. Autophagy‐independent roles of the ATG proteins include the maintenance of cellular homeostasis and resistance to pathogens. For example, they assist and enhance the turnover of dead cells and microbes upon their phagocytic engulfment, and inhibit murine norovirus replication. Moreover, bone resorption by osteoclasts, innate immune regulation triggered by cytoplasmic DNA and the ER‐associated degradation regulation all have in common the requirement of a subset of ATG proteins. Microorganisms such as coronaviruses, Chlamydia trachomatis or Brucella abortus have even evolved ways to manipulate autophagy‐independent functions of ATG proteins in order to ensure the completion of their intracellular life cycle. Taken together these novel mechanisms add to the repertoire of functions and extend the number of cellular processes involving the ATG proteins.

Evasion of Toll-like receptor 2 activation by staphylococcal superantigen-like protein 3

Toll-like receptors (TLRs) are crucial for our host defense against microbial infections. TLR2 is especially important to fight bacterial infections, as it specifically recognizes bacterial lipoproteins of both Gram-positive and Gram-negative origin. Present on a variety of immune cells, TLR2 is critical for host protection against several bacterial infections, including those caused by Staphylococcus aureus. This major human pathogen causes increasing health care problems due to its increased resistance to antibiotics. S. aureus secretes a wide variety of proteins that inhibit innate immune responses. Recently, several staphylococcal superantigen-like proteins (SSLs) have been described to mediate immune evasive properties. Here, we describe that SSL3 specifically binds and inhibits TLR2 activation on human and murine neutrophils and monocytes. Through binding of the extracellular TLR2 domain, SSL3 inhibits IL-8 production by HEK cells expressing TLR1/2 and TLR2/6 dimers, stimulated with their specific ligands. The SSL3-TLR2 interaction is partially glycan dependent as binding of SSL3 to TLR2 is affected upon removal of sialic acid residues. Moreover, the SSL3(R308A) mutant lacking glycan-binding properties shows lower TLR2 inhibition. An SSL3 mutant, lacking the N-terminal 126 amino acids, still retains full TLR2 inhibiting activity. Of other SSLs tested, only SSL4, which shares the highest homology with SSL3, blocks TLR2 activation. SSL3 is the first-described bacterial protein that blocks TLR2 activation through direct extracellular interaction with the receptor. This unique function of SSL3 adds to the arsenal of immune evasive molecules that S. aureus can employ to subvert both innate and adaptive immunity.

Mycobacterial secretion systems ESX-1 and ESX-5 play distinct roles in host cell death and inflammasome activation

During infection of humans and animals, pathogenic mycobacteria manipulate the host cell causing severe diseases such as tuberculosis and leprosy. To understand the basis of mycobacterial pathogenicity, it is crucial to identify the molecular virulence mechanisms. In this study, we address the contribution of ESX-1 and ESX-5--two homologous type VII secretion systems of mycobacteria that secrete distinct sets of immune modulators--during the macrophage infection cycle. Using wild-type, ESX-1- and ESX-5-deficient mycobacterial strains, we demonstrate that these secretion systems differentially affect subcellular localization and macrophage cell responses. We show that in contrast to ESX-1, the effector proteins secreted by ESX-5 are not required for the translocation of Mycobacterium tuberculosis or Mycobacterium marinum to the cytosol of host cells. However, the M. marinum ESX-5 mutant does not induce inflammasome activation and IL-1β activation. The ESX-5 system also induces a caspase-independent cell death after translocation has taken place. Importantly, by means of inhibitory agents and small interfering RNA experiments, we reveal that cathepsin B is involved in both the induction of cell death and inflammasome activation upon infection with wild-type mycobacteria. These results reveal distinct roles for two different type VII secretion systems during infection and shed light on how virulent mycobacteria manipulate the host cell in various ways to replicate and spread.

Functional basis for complement evasion by staphylococcal superantigen-like 7

The human pathogen Staphylococcus aureus has a plethora of virulence factors that promote its colonization and survival in the host. Among such immune modulators are staphylococcal superantigen-like (SSL) proteins, comprising a family of 14 small, secreted molecules that seem to interfere with the host innate immune system. SSL7 has been described to bind immunoglobulin A (IgA) and complement C5, thereby inhibiting IgA-FcαRI binding and serum killing of Escherichia coli. As C5a generation, in contrast to C5b-9-mediated lysis, is crucial for immune defence against staphylococci, we investigated the impact of SSL7 on staphylococcal-induced C5a-mediated effects. Here, we show that SSL7 inhibits C5a generation induced by staphylococcal opsonization, slightly enhanced by its IgA-binding capacity. Moreover, we demonstrate a strong protective activity of SSL7 against staphylococcal clearance in human whole blood. SSL7 strongly inhibited the C5a-induced phagocytosis of S. aureus and oxidative burst in an in vitro whole-blood inflammation model. Furthermore, we found that SSL7 affects all three pathways of complement activation and inhibits the cleavage of C5 by interference of its binding to C5 convertases. Finally, SSL7 effects were also demonstrated in vivo. In a murine model of immune complex peritonitis, SSL7 abrogated the C5a-driven influx of neutrophils in mouse peritoneum.

Staphylococcal SSL5 binding to human leukemia cells inhibits cell adhesion to endothelial cells and platelets

Bacterial proteins provide promising tools for novel anticancer therapies. Staphylococcal superantigen-like 5 (SSL5) was recently described to bind P-selectin glycoprotein ligand-1 (PSGL-1) on leukocytes and to inhibit neutrophil rolling on a P-selectin surface. As leukocytes and tumor cells share many characteristics in migration and dissemination, we explored the potential of SSL5 as an antagonist of malignant cell behavior. Previously, it was demonstrated that rolling of human HL-60 leukemia cells on activated endothelial cells was mediated by P-selectin. In this study, we show that SSL5 targets HL-60 cells. Binding of SSL5 was rapid and without observed toxicity. Competition of SSL5 with the binding of three anti-PSGL-1 antibodies and P-selectin to HL-60 cells identified PSGL-1 as the ligand on HL-60 cells. Presence of sialyl Lewis x epitopes on PSGL-1 was crucial for its interaction with SSL5. Importantly, SSL5 not only inhibited the interaction of HL-60 cells with activated endothelial cells but also with platelets, which both play an important role in growth and metastasis of cancers. These data support the concept that SSL5 could be a lead in the search for novel strategies against hematological malignancies.

Role of chemokines and their receptors in cancer

Metastases are the cause of 90% of human cancer deaths. The current treatment of cancer with chemo,- and/or radiotherapy is based on cell death by DNA damage neglecting the fact that cancer cell invasion into surrounding tissues and metastasizing are fundamental features of neoplasms and the major reason for treatment failure. Metastasis is the result of several sequential steps and represents a highly organized, non-random, and organ-selective process. A number of in vitro and in vivo models show that tumor cells use chemokine-mediated mechanisms during this metastasizing process, comparable to those observed in the regulation of leukocyte trafficking. Furthermore, chemokines modulate tumor behavior such as the regulation of tumor-associated angiogenesis, activation of host tumor-specific immunological responses, and direct stimulation of tumor cell proliferation in an autocrine fashion. These findings may lead to new drugs that target chemokines or their receptors and will likely be of great additional value for treatment of cancer patients.

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.

A new staphylococcal anti-inflammatory protein that antagonizes the formyl peptide receptor-like 1

Bacteria have developed mechanisms to escape the first line of host defense, which is constituted by the recruitment of phagocytes to the sites of bacterial invasion. We previously described the chemotaxis inhibitory protein of Staphylococcus aureus, a protein that blocks the activation of neutrophils via the formyl peptide receptor (FPR) and C5aR. We now describe a new protein from S. aureus that impaired the neutrophil responses to FPR-like1 (FPRL1) agonists. FPRL1 inhibitory protein (FLIPr) inhibited the calcium mobilization in neutrophils stimulated with MMK-1, WKYMVM, prion-protein fragment PrP(106-126), and amyloid beta(1-42). Stimulation with low concentrations of fMLP was partly inhibited. Directed migration was also completely prevented toward MMK-1 and partly toward fMLP. Fluorescence-labeled FLIPr efficiently bound to neutrophils, monocytes, B cells, and NK cells. HEK293 cells transfected with human C5aR, FPR, FPRL1, and FPRL2 clearly showed that FLIPr directly bound to FPRL1 and, at higher concentrations, also to FPR but not to C5aR and FPRL2. FLIPr can reveal unknown inflammatory ligands crucial during S. aureus infections. As a novel described FPRL1 antagonist, it might lead to the development of therapeutic agents in FPRL1-mediated inflammatory components of diseases such as systemic amyloidosis, Alzheimer's, and prion disease.

Staphylococcal superantigen-like 5 binds PSGL-1 and inhibits P-selectin–mediated neutrophil rolling

Staphylococcus aureus secretes several virulence factors interfering with host-cell functions. Staphylococcal superantigen-like (SSL) proteins are a family of 11 exotoxins with structural homology to superantigens but with generally unknown functions. Recently, we described that chemotaxis inhibitory protein of Staphylococcus aureus (CHIPS31-121), a potent inhibitor of C5a-induced responses, is structurally homologous to the C-terminal domain of SSL5. Here, we identify P-selectin glycoprotein ligand-1 (PSGL-1), involved in the initial rolling of neutrophils along the endothelium, as a target for SSL5. SSL5 specifically bound to Chinese hamster ovary cells stably expressing PSGL-1 (CHO–PSGL-1), which was dependent of sulfation and sialylation. Furthermore, SSL5 bound to PSGL-1/Ig fusion protein immobilized on a biosensor chip. SSL5 affected binding of soluble P-selectin/Fc chimera, the principle ligand of PSGL-1, to CHO–PSGL-1 cells and inhibited adhesion of neutrophils to immobilized P-selectin under static conditions. Under flow conditions SSL5 strongly decreased neutrophil rolling on immobilized P-selectin/Fc and activated human endothelial cells. In conclusion, SSL5 interferes with the interaction between PSGL-1 and P-selectin, suggesting that S aureus uses SSL5 to prevent neutrophil extravasation toward the site of infection. This makes SSL5 a potential lead for the development of new anti-inflammatory compounds for disorders characterized by excessive recruitment of leukocytes.