Flow cytometric determination of Panton-Valentine leucocidin S component binding

A bibliometric analysis: Ca2+ fluxes and inflammatory phenotyping by flow cytometry in peripheral blood mononuclear cells

Background

The immune system, composed of organs, tissues, cells, and proteins, is the key to protecting the body from external biological attacks and inflammation. The latter occurs in several pathologies, such as cancers, type 1 diabetes, and human immunodeficiency virus infection. Immunophenotyping by flow cytometry is the method of choice for diagnosing these pathologies. Under inflammatory conditions, the peripheral blood mononuclear cells (PBMCs) are partially activated and generate intracellular pathways involving Ca2+-dependent signaling cascades leading to transcription factor expression. Ca2+ signaling is typically studied by microscopy in cell lines but can present some limitations to explore human PBMCs, where flow cytometry can be a good alternative.

Objective

In this review, we dived into the research field of inflammation and Ca2+ signaling in PBMCs. We aimed to investigate the structure and evolution of this field in a physio-pathological context, and then we focused our review on flow cytometry analysis of Ca2+ fluxes in PBMCs.

Methods

From 1984 to 2022, 3865 articles on inflammation and Ca2+ signaling in PBMCs were published, according to The Clarivate Web of Science (WOS) database used in this review. A bibliometric study was designed for this collection and consisted of a co-citation and bibliographic coupling analysis.

Results

The co-citation analysis was performed on 133 articles: 4 clusters highlighted the global context of Ca2+ homeostasis, including chemical probe development, identification of the leading players in Ca2+ signaling, and the link with chemokine production in immune cell function. Next, the bibliographic coupling analysis combined 998 articles in 8 clusters. This analysis outlined the mechanisms of PBMC activation, from signal integration to cellular response. Further explorations of the bibliographic coupling network, focusing on flow cytometry, revealed 21 articles measuring cytosolic Ca2+ in PBMCs, with only 5 since 2016. This final query showed that Ca2+ signaling analysis in human PBMCs using flow cytometry is still underdeveloped and investigates mainly the cytosolic Ca2+ compartment.

Conclusion

Our review uncovers remaining knowledge gaps of intracellular players involved in Ca2+ signaling in PBMCs, such as reticulum and mitochondria, and presents flow cytometry as a solid option to supplement gold-standard microscopy studies.

Recent insights into the role of defensins in diabetic wound healing

Diabetic wound, one of the most common serious complications of diabetic patients, is an important factor in disability and death. Much of the research on the pathophysiology of diabetic wound healing has long focused on mechanisms mediated by hyperglycemia, chronic inflammation, microcirculatory and macrocirculatory dysfunction. However, recent evidence suggests that defensins may play a crucial role in the development and perpetuation of diabetic wound healing. The available findings suggest that defensins exert a beneficial influence on diabetic wound healing through antimicrobial, immunomodulatory, angiogenic, tissue regenerator effects, and insulin resistance improvement. Therefore, summarizing the existing research progress on defensins in the diabetic wound may present a promising strategy for diabetic patients.

Bi-component HlgC/HlgB and HlgA/HlgB γ-hemolysins from S. aureus: Modulation of Ca2+ channels activity through a differential mechanism

Staphylococcal bi-component leukotoxins known as *pore-forming toxins* induce upon a specific binding to membrane receptors, two independent cellular events in human neutrophils. First, they provoke the opening of pre-existing specific ionic channels including Ca²⁺ channels. Then, they form membrane pores specific to monovalent cations leading to immune cells death. Among these leukotoxins, HlgC/HlgB and HlgA/HlgB γ-hemolysins do act in synergy to induce the opening of different types of Ca²⁺ channels in the absence as in the presence of extracellular Ca²⁺. Here, we investigate the mechanism underlying the modulation of Ca²⁺-independent Ca²⁺ channels in response to both active leukotoxins in human neutrophils. In the absence of extracellular Ca²⁺, the Mn²⁺ has been used as a Ca²⁺ surrogate to determine the activity of Ca²⁺-independent Ca²⁺ channels. Our findings provide new insights about different mechanisms involved in the staphylococcal γ-hemolysins activity to regulate three different types of Ca²⁺-independent Ca²⁺ channels. We conclude that (i) HlgC/HlgB stimulates the opening of La³⁺-sensitive Ca²⁺ channels, through a cholera toxin-sensitive G protein, (ii) HlgA/HlgB stimulates the opening of Ca²⁺ channels not sensitive to La³⁺, through a G protein-independent process, and (iii) unlike HlgA/HlgB, HlgC/HlgB toxins prevent the opening of a new type of Ca²⁺ channels by phosphorylation/de-phosphorylation-dependent mechanisms.

Panton-Valentine Leukocidin Induces Cytokine Release and Cytotoxicity Mediated by the C5a Receptor on Rabbit Alveolar Macrophages

Necrotizing pneumonia caused by Panton-Valentine leukocidin (PVL)-positive community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) has high mortality rates and is currently a serious clinical issue. PVL is a two-component toxin (LukS-PV and LukF-PV). It can cause necrosis in target cells by forming pores consisting of an octamer comprised of LukS-PV and LukF-PV. However, considering the specificity of PVL towards several target cells and species, the specific effect of PVL remains controversial. Therefore, we focused on necrotizing pneumonia caused by PVL-positive S. aureus and clarified the effect of PVL on alveolar macrophages, which play a central role in innate immunity in the alveolar space. We constructed recombinant PVL (rPVL) components and stimulated alveolar macrophages isolated from rabbits to evaluate cytotoxicity and pro-inflammatory cytokine release. Recombinant LukS-PV (rLukS-PV), but not recombinant LukF-PV (rLukF-PV), induced pro-inflammatory cytokine release. Specifically, tumor necrosis factor (TNF)-α release was mediated by the C5a receptor (C5aR) expressed on rabbit alveolar macrophages, and the toxicity of rPVL, consisting of rLukS-PV and rLukF-PV, towards rabbit alveolar macrophages was mediated by the same receptor. Overall, our findings shed light on the C5aR-mediated cytotoxic effect of PVL on alveolar macrophages, which may be useful for understanding the mechanism of necrotizing pneumonia caused by PVL.

Identification of Methicillin-Resistant Staphylococcus Aureus From Methicillin-Sensitive Staphylococcus Aureus and Molecular Characterization in Quanzhou, China

To distinguish Methicillin-Resistant Staphylococcus aureus (MRSA) from Methicillin-Sensitive Staphylococcus aureus (MSSA) in the protein sequences level, test the susceptibility to antibiotic of all Staphylococcus aureus isolates from Quanzhou hospitals, define the virulence factor and molecular characteristics of the MRSA isolates. MRSA and MSSA Pfam protein sequences were used to extract feature vectors of 188D, n-gram and 400D. Weka software was applied to classify the two Staphylococcus aureus and performance effect was evaluated. Antibiotic susceptibility testing of the 81 Staphylococcus aureus was performed by the Mérieux Microbial Analysis Instrument. The 65 MRSA isolates were characterized by Panton-Valentine leukocidin (PVL), X polymorphic region of Protein A (spa), multilocus sequence typing test (MLST), staphylococcus chromosomal cassette mec (SCCmec) typing. After comparing the results of Weka six classifiers, the highest correctly classified rates were 91.94, 70.16, and 62.90% from 188D, n-gram and 400D, respectively. Antimicrobial susceptibility test of the 81 Staphylococcus aureus: Penicillin-resistant rate was 100%. No resistance to teicoplanin, linezolid, and vancomycin. The resistance rate of the MRSA isolates to clindamycin, erythromycin and tetracycline was higher than that of the MSSAs. Among the 65 MRSA isolates, the positive rate of PVL gene was 47.7% (31/65). Seventeen sequence types (STs) were identified among the 65 isolates, and ST59 was the most prevalent. SCCmec type III and IV were observed at 24.6 and 72.3%, respectively. Two isolates did not be typed. Twenty-one spa types were identified, spa t437 (34/65, 52.3%) was the most predominant type. MRSA major clone type of molecular typing was CC59-ST59-spa t437-IV (28/65, 43.1%). Overall, 188D feature vectors can be applied to successfully distinguish MRSA from MSSA. In Quanzhou, the detection rate of PVL virulence factor was high, suggesting a high pathogenic risk of MRSA infection. The cross-infection of CA-MRSA and HA-MRSA was presented, the molecular characteristics were increasingly blurred, HA-MRSA with typical CA-MRSA molecular characteristics has become an important cause of healthcare-related infections. CC59-ST59-spa t437-IV was the main clone type in Quanzhou, which was rare in other parts of mainland China.

Staphylococcus aureus Panton-Valentine Leukocidin triggers an alternative NETosis process targeting mitochondria

Panton-Valentine Leukocidin (PVL) is a bicomponent leukotoxin produced by 3%-10% of clinical Staphylococcus aureus (SA) strains involved in the severity of hospital and community-acquired infections. Although PVL was long known as a pore-forming toxin, recent studies have challenged the formation of a pore at the plasma membrane, while its endocytosis and the exact mode of action remain to be defined. In vitro immunolabeling of human neutrophils shows that Neutrophil Extracellular Traps (NETosis) is triggered by the action of purified PVL, but not by Gamma hemolysin CB (HlgCB), a structurally similar SA leukotoxin. PVL causes the ejection of chromatin fibers (NETs) decorated with antibacterial peptides independently of the NADPH oxidase oxidative burst. Leukotoxin partially colocalizes with mitochondria and enhances the production of reactive oxygen species from these organelles, while showing an increased autophagy, which results unnecessary for NETs ejection. PVL NETosis is elicited through Ca²⁺ -activated SK channels and Myeloperoxidase activity but is abolished by Allopurinol pretreatment of neutrophils. Moreover, massive citrullination of the histone H3 is performed by peptidyl arginine deiminases. Inhibition of this latter enzymes fails to abolish NET extrusion. Unexpectedly, PVL NETosis does not seem to involve Src kinases, which is the main kinase family activated downstream the binding of PVL F subunit to CD45 receptor, while the specific kinase pathway differs from the NADPH oxidase-dependent NETosis. PVL alone causes a different and specific form of NETosis that may rather represent a bacterial strategy conceived to disarm and disrupt the immune response, eventually allowing SA to spread.

Structure-based discovery of a small-molecule inhibitor of methicillin-resistant Staphylococcus aureus virulence

The rapid emergence and dissemination of methicillin-resistant Staphylococcus aureus (MRSA) strains poses a major threat to public health. MRSA possesses an arsenal of secreted host-damaging virulence factors that mediate pathogenicity and blunt immune defenses. Panton-Valentine leukocidin (PVL) and α-toxin are exotoxins that create lytic pores in the host cell membrane. They are recognized as being important for the development of invasive MRSA infections and are thus potential targets for antivirulence therapies. Here, we report the high-resolution X-ray crystal structures of both PVL and α-toxin in their soluble, monomeric, and oligomeric membrane-inserted pore states in complex with n-tetradecylphosphocholine (C₁₄PC). The structures revealed two evolutionarily conserved phosphatidylcholine-binding mechanisms and their roles in modulating host cell attachment, oligomer assembly, and membrane perforation. Moreover, we demonstrate that the soluble C₁₄PC compound protects primary human immune cells in vitro against cytolysis by PVL and α-toxin and hence may serve as the basis for the development of an antivirulence agent for managing MRSA infections.

Studying Staphylococcal Leukocidins: A Challenging Endeavor

Staphylococcus aureus is a well-known colonizer of the human skin and nose, but also a human pathogen that causes a wide spectrum of diseases. It is well established that S. aureus secretes an arsenal of virulence factors that have evolved to circumvent the human immune system. A major group of S. aureus virulence factors is the bi-component β-barrel pore-forming toxins, also known as leukocidins. These pore-forming toxins target specific cells of the innate and adaptive immune system by interacting with specific receptors expressed on the cell membrane. Even though still heavily debated, clinical and epidemiological studies suggest the involvement of one of the bi-component toxin, Panton-Valentine Leukocidin (PVL), as an important factor contributing to the epidemic spread and increased virulence of CA-MRSA strains. However, the host- and cell-specificity of PVL and other leukocidins, and the lack of adequate in vivo models, fuels the controversy and impairs the appropriate assessment of their role in S. aureus pathophysiology. Currently, the mechanisms of pore-formation and the contribution of PVL and other leukocidins to S. aureus pathophysiology are incompletely understood. This review summarizes our current understanding of leukocidin pore-formation, knowledge gaps, and highlights recent findings identifying novel host-factors involved in the toxin-host interface. As a result, this review furthers emphasizes the complexity behind S. aureus leukocidin cytotoxicity and the challenges associated in the quest to study and understand these major virulence factors.

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.

Aspartic Acid Residue 51 of SaeR Is Essential for Staphylococcus aureus Virulence

Staphylococcus aureus is a common Gram-positive bacteria that is a major cause of human morbidity and mortality. The SaeR/S two-component sensory system of S. aureus is important for virulence gene transcription and pathogenesis. However, the influence of SaeR phosphorylation on virulence gene transcription is not clear. To determine the importance of potential SaeR phosphorylation sites for S. aureus virulence, we generated genomic alanine substitutions at conserved aspartic acid residues in the receiver domain of the SaeR response regulator in clinically significant S. aureus pulsed-field gel electrophoresis (PFGE) type USA300. Transcriptional analysis demonstrated a dramatic reduction in the transcript abundance of various toxins, adhesins, and immunomodulatory proteins for SaeR with an aspartic acid to alanine substitution at residue 51. These findings corresponded to a significant decrease in cytotoxicity against human erythrocytes and polymorphonuclear leukocytes, the ability to block human myeloperoxidase activity, and pathogenesis during murine soft-tissue infection. Analysis of SaeR sequences from over 8,000 draft S. aureus genomes revealed that aspartic acid residue 51 is 100% conserved. Collectively, these results demonstrate that aspartic acid residue 51 of SaeR is essential for S. aureus virulence and underscore a conserved target for novel antimicrobial strategies that treat infection caused by this pathogen.

Human CD45 is an F-component-specific receptor for the staphylococcal toxin Panton-Valentine leukocidin

The staphylococcal bi-component leukocidins Panton-Valentine leukocidin (PVL) and γ-haemolysin CB (HlgCB) target human phagocytes. Binding of the toxins' S-components to human complement C5a receptor 1 (C5aR1) contributes to cellular tropism and human specificity of PVL and HlgCB. To investigate the role of both leukocidins during infection, we developed a human C5aR1 knock-in (hC5aR1^KI) mouse model. HlgCB, but unexpectedly not PVL, contributed to increased bacterial loads in tissues of hC5aR1^KI mice. Compared to humans, murine hC5aR1^KI neutrophils showed a reduced sensitivity to PVL, which was mediated by the toxin's F-component LukF-PV. By performing a genome-wide CRISPR-Cas9 screen, we identified CD45 as a receptor for LukF-PV. The human-specific interaction between LukF-PV and CD45 provides a molecular explanation for resistance of hC5aR1^KI mouse neutrophils to PVL and probably contributes to the lack of a PVL-mediated phenotype during infection in these mice. This study demonstrates an unsuspected role of the F-component in driving the sensitivity of human phagocytes to PVL.

Incidence, prevalence, and management of MRSA bacteremia across patient populations-a review of recent developments in MRSA management and treatment

Methicillin-resistant Staphylococcus aureus (MRSA) infection is still a major global healthcare problem. Of concern is S. aureus bacteremia, which exhibits high rates of morbidity and mortality and can cause metastatic or complicated infections such as infective endocarditis or sepsis. MRSA is responsible for most global S. aureus bacteremia cases, and compared with methicillin-sensitive S. aureus, MRSA infection is associated with poorer clinical outcomes. S. aureus virulence is affected by the unique combination of toxin and immune-modulatory gene products, which may differ by geographic location and healthcare- or community-associated acquisition. Management of S. aureus bacteremia involves timely identification of the infecting strain and source of infection, proper choice of antibiotic treatment, and robust prevention strategies. Resistance and nonsusceptibility to first-line antimicrobials combined with a lack of equally effective alternatives complicates MRSA bacteremia treatment. This review describes trends in epidemiology and factors that influence the incidence of MRSA bacteremia. Current and developing diagnostic tools, treatments, and prevention strategies are also discussed.

Staphylococcus aureus pore-forming toxins: The interface of pathogen and host complexity

Staphylococcus aureus is a prominent human pathogen capable of infecting a variety of host species and tissue sites. This versatility stems from the pathogen's ability to secrete diverse host-damaging virulence factors. Among these factors, the S. aureus pore-forming toxins (PFTs) α-toxin and the bicomponent leukocidins, have garnered much attention for their ability to lyse cells at low concentrations and modulate disease severity. Although many of these toxins were discovered nearly a century ago, their host cell specificities have only been elucidated over the past five to six years, starting with the discovery of the eukaryotic receptor for α-toxin and rapidly followed by identification of the leukocidin receptors. The identification of these receptors has revealed the species- and cell type-specificity of toxin binding, and provided insight into non-lytic effects of PFT intoxication that contribute to disease pathogenesis.

Leukocidins: staphylococcal bi-component pore-forming toxins find their receptors

Staphylococcus aureus is a major bacterial pathogen that causes disease worldwide. The emergence of strains that are resistant to commonly used antibiotics and the failure of vaccine development have resulted in a renewed interest in the pathophysiology of this bacterium. Staphylococcal leukocidins are a family of bi-component pore-forming toxins that are important virulence factors. During the past five years, cellular receptors have been identified for all of the bi-component leukocidins. The identification of the leukocidin receptors explains the cellular tropism and species specificity that is exhibited by these toxins, which has important biological consequences. In this Review, we summarize the recent discoveries that have reignited interest in these toxins and provide an outlook for future research.

Above and beyond C5a Receptor Targeting by Staphylococcal Leucotoxins: Retrograde Transport of Panton-Valentine Leucocidin and γ-Hemolysin

Various membrane receptors associated with the innate immune response have recently been identified as mediators of the cellular action of Staphylococcus aureus leucotoxins. Two of these, the Panton–Valentine leucotoxin LukS-PV/LukF-PV and the γ-hemolysin HlgC/HlgB, bind the C5a complement-derived peptide receptor. These leucotoxins utilize the receptor to induce intracellular Ca²⁺ release from internal stores, other than those activated by C5a. The two leucotoxins are internalized with the phosphorylated receptor, but it is unknown whether they divert retrograde transport of the receptor or follow another pathway. Immunolabeling and confocal microscopic techniques were used to analyze the presence of leucotoxins in endosomes, lysosomes, endoplasmic reticulum, and Golgi. The two leucotoxins apparently followed retrograde transport similar to that of the C5a peptide-activated receptor. However, HlgC/HlgB reached the Golgi network very early, whereas LukS-PV/LukF-PV followed slower kinetics. The HlgC/HlgB leucotoxin remained in neutrophils 6 h after a 10-min incubation of the cells in the presence of the toxin with no signs of apoptosis, whereas apoptosis was observed 3 h after neutrophils were incubated with LukS-PV/LukF-PV. Such retrograde transport of leucotoxins provides a novel understanding of the cellular effects initiated by sublytic concentrations of these toxins.

Is LukS-PV a novel experimental therapy for leukemia?

Although the studies on the pathogenesis and prognosis of leukemia have made revolutionary progress, the long-term survival remains unsatisfactory. Alternative techniques are being developed to target leukemia. Several decades after researchers' work, a variety of bacteria toxins are being explored as potential anti-leukemia agents, either to provide direct effects or to deliver therapeutic proteins to leukemia. LukS-PV, a component of Panton-Valentine Leukocidin secreted by S. aureus, has been tested in acute myeloid leukemia as a novel experimental strategy. Further researches about the targeting mechanisms of LukS-PV are required to make it a complete therapeutic approach for leukemia treatment. The function of this article is to provide clinicians and experimentalists with a chronological and comprehensive appraisal of use of LukS-PV as an experimental strategy for leukemia therapy.

Innate Immune Signaling Activated by MDR Bacteria in the Airway

Health care-associated bacterial pneumonias due to multiple-drug resistant (MDR) pathogens are an important public health problem and are major causes of morbidity and mortality worldwide. In addition to antimicrobial resistance, these organisms have adapted to the milieu of the human airway and have acquired resistance to the innate immune clearance mechanisms that normally prevent pneumonia. Given the limited efficacy of antibiotics, bacterial clearance from the airway requires an effective immune response. Understanding how specific airway pathogens initiate and regulate innate immune signaling, and whether this response is excessive, leading to host-induced pathology may guide future immunomodulatory therapy. We will focus on three of the most important causes of health care-associated pneumonia, Staphylococcus aureus, Pseudomonas aeruginosa, and Klebsiella pneumoniae, and review the mechanisms through which an inappropriate or damaging innate immune response is stimulated, as well as describe how airway pathogens cause persistent infection by evading immune activation.

Staphylococcus aureus Pore-Forming Toxins

Staphylococcus aureus (S. aureus) is a formidable foe equipped with an armamentarium of virulence factors to thwart host defenses and establish a successful infection. Among these virulence factors, S. aureus produces several potent secreted proteins that act as cytotoxins, predominant among them the beta-barrel pore-forming toxins. These toxins play several roles in pathogenesis, including disruption of cellular adherens junctions at epithelial barriers, alteration of intracellular signaling events, modulation of host immune responses, and killing of eukaryotic immune and non-immune cells. This chapter provides an updated overview on the S. aureus beta-barrel pore-forming cytotoxins, the identification of toxin receptors on host cells, and their roles in pathogenesis.

Serious Complications from Staphylococcal aureus in Atopic Dermatitis

Colonization with Staphylococcal aureus is markedly more frequent in individuals with atopic dermatitis (AD) than in unaffected individuals. Chronic scratching leads to worsening of an existing defect in the epidermal barrier, which can allow S. aureus invasion into the bloodstream and subsequent systemic infections. We report two unusual cases of systemic illness in individuals with AD. One developed infective endocarditis followed by a stroke and the other developed septic arthritis and osteomyelitis. We performed an extensive literature review of reported systemic complications caused by S. aureus in patients with AD. Although reports are rare, practitioners should be aware of these important, albeit unlikely, complications of staphylococcal superinfections in individuals with AD.

Antibodies to S. aureus LukS-PV Attenuated Subunit Vaccine Neutralize a Broad Spectrum of Canonical and Non-Canonical Bicomponent Leukotoxin Pairs

S. aureus vaccine development has proven particularly difficult. The conventional approach to achieve sterile immunity through opsonophagocytic killing has been largely unsuccessful. S. aureus is highly toxigenic and a great body of evidence suggests that a successful future vaccine for this organism should target extracellular toxins which are responsible for host tissue destruction and immunosuppression. Major staphylococcal toxins are alpha toxin (a single subunit hemolysin) along with a group of bicomponent pore-forming toxins (BCPFT), namely Panton-Valentine leukocidin (PVL), gamma hemolysins (HlgCB and AB), LukAB and LukED. In our previous report, an attenuated mutant of LukS-PV (PVL- S subunit) named as “LukS-mut9” elicited high immunogenic response as well as provided a significant protection in a mouse sepsis model. Recent discovery of PVL receptors shows that mice lack receptors for this toxin, thus the reported protection of mice with the PVL vaccine may relate to cross protective responses against other homologous toxins. This manuscript addresses this issue by demonstrating that polyclonal antibody generated by LukS-mut9 can neutralize other canonical and non-canonical leukotoxin pairs. In this report, we also demonstrated that several potent toxins can be created by non-canonical pairing of subunits. Out of 5 pairs of canonical and 8 pairs of non-canonical toxins tested, anti-LukS-mut9 polyclonal antibodies neutralized all except for LukAB. We also studied the potential hemolytic activities of canonical and noncanonical pairs among biocomponent toxins and discovered that a novel non-canonical pair consisting of HlgA and LukD is a highly toxic combination. This pair can lyse RBC from different species including human blood far better than alpha hemolysin. Moreover, to follow-up our last report, we explored the correlation between the levels of pre-existing antibodies to new sets of leukotoxins subunits and clinical outcomes in adult patients with S. aureus bacteremia. We found that there is an inversed correlation between the antibody titer to sepsis for leukotoxins LukS-mut9, LukF-PV, HlgC, LukE and LukAB, suggesting the risk of sepsis was significantly lower in the patients with higher antibody titer against those toxins.

α-Defensins partially protect human neutrophils against Panton-Valentine leukocidin produced by Staphylococcus aureus

α-Defensins produced by neutrophils are important effector molecules of the innate immune system. In addition to their microbicidal effects, α-defensins have the ability to neutralize bacterial toxins. Panton-Valentine leukocidin (PVL) is the hallmark of community-acquired methicillin-resistant Staphylococcus aureus. Staphylococcus aureus that produce PVL are responsible for severe diseases, including necrotizing pneumonia. Polymorphonuclear neutrophils (PMNs) are the target cells of PVL action. The goal of this study was to elucidate the effect of a group of α-defensins known as the human neutrophil peptides (HNPs) on the interactions between LukS-PV and LukF-PV, which compose PVL, and human PMNs. We observed that HNPs bound to both subunits of PVL and significantly decreased PVL pore formation in PMNs, with a maximum inhibition of 27%. When various HNP molecules were tested individually under the same conditions, we observed that HNP3, but not HNP1 or 2, decreased pore formation. Similarly, HNP3 significantly decreased PVL-induced PMN lysis, with a maximum inhibition of 31%. Interestingly, HNPs did not affect LukS-PV LukF-PV oligomerization, LukS-PV LukF-PV binding to PMNs or calcium influx induced by PVL in PMNs. Our results suggest that HNP3 partially protects neutrophils against PVL by interfering with the conformational changes of PVL required to form a functional pore.

SIGNIFICANCE AND IMPACT OF THE STUDY

Panton-Valentine leukocidin (PVL) is a pore-forming toxin produced by Staphylococcus aureus, responsible for neutrophil damage and key player of severe staphylococcal diseases. Antimicrobial peptides produced by neutrophils (HNP1-3) neutralize several other bacterial cytotoxins. We examined the impact of human neutrophil peptides (HNPs) on PVL cytotoxicity against human neutrophils and we found that HNPs bind to both LukS and LukF components of PVL, thereby inhibiting pore formation and neutrophil lysis. Our results suggest that HNP3 may impair PVL conformational changes required to form a functional pore and provide insight into the pathogenesis of PVL-related staphylococcal infection, with potential impact on the disease outcome.

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.

The staphylococcal toxins γ-haemolysin AB and CB differentially target phagocytes by employing specific chemokine receptors

Evasion of the host phagocyte response by Staphylococcus aureus is crucial to successful infection with the pathogen. γ-haemolysin AB and CB (HlgAB, HlgCB) are bicomponent pore-forming toxins present in almost all human S. aureus isolates. Cellular tropism and contribution of the toxins to S. aureus pathophysiology are poorly understood. Here we identify the chemokine receptors CXCR1, CXCR2 and CCR2 as targets for HlgAB, and the complement receptors C5aR and C5L2 as targets for HlgCB. The receptor expression patterns allow the toxins to efficiently and differentially target phagocytic cells. Murine neutrophils are resistant to HlgAB and HlgCB. CCR2 is the sole murine receptor orthologue compatible with γ-haemolysin. In a murine peritonitis model, HlgAB contributes to S. aureus bacteremia in a CCR2-dependent manner. HlgAB-mediated targeting of CCR2(+) cells highlights the involvement of inflammatory macrophages during S. aureus infection. Functional quantification identifies HlgAB and HlgCB as major secreted staphylococcal leukocidins.

Structure-function analysis of heterodimer formation, oligomerization, and receptor binding of the Staphylococcus aureus bi-component toxin LukGH

The bi-component leukocidins of Staphylococcus aureus are important virulence factors that lyse human phagocytic cells and contribute to immune evasion. The γ-hemolysins (HlgAB and HlgCB) and Panton-Valentine leukocidin (PVL or LukSF) were shown to assemble from soluble subunits into membrane-bound oligomers on the surface of target cells, creating barrel-like pore structures that lead to cell lysis. LukGH is the most distantly related member of this toxin family, sharing only 30-40% amino acid sequence identity with the others. We observed that, unlike other leukocidin subunits, recombinant LukH and LukG had low solubility and were unable to bind to target cells, unless both components were present. Using biolayer interferometry and intrinsic tryptophan fluorescence we detected binding of LukH to LukG in solution with an affinity in the low nanomolar range and dynamic light scattering measurements confirmed formation of a heterodimer. We elucidated the structure of LukGH by x-ray crystallography at 2.8-Å resolution. This revealed an octameric structure that strongly resembles that reported for HlgAB, but with important structural differences. Structure guided mutagenesis studies demonstrated that three salt bridges, not found in other bi-component leukocidins, are essential for dimer formation in solution and receptor binding. We detected weak binding of LukH, but not LukG, to the cellular receptor CD11b by biolayer interferometry, suggesting that in common with other members of this toxin family, the S-component has the primary contact role with the receptor. These new insights provide the basis for novel strategies to counteract this powerful toxin and Staphylococcus aureus pathogenesis.

Mass spectrometry and multiplex antigen assays to assess microbial quality and toxin production of Staphylococcus aureus strains isolated from clinical and food samples

The aim of our study was to investigate the microbial quality of meat products and on some clinical samples in Abidjan focused on Staphylococcus genus and the toxin production profile of Staphylococcus aureus (S. aureus) isolated. Bacteria were collected from 240 samples of three meat products sold in Abidjan and 180 samples issued from clinical infections. The strains were identified by both microbiological and MALDI-TOF-MS methods. The susceptibility to antibiotics was determined by the disc diffusion method. The production of Panton-Valentine Leukocidin, LukE/D, and epidermolysins was screened using radial gel immunodiffusion. The production of staphylococcal enterotoxins and TSST-1 was screened by a Bio-Plex Assay. We observed that 96/240 of meat samples and 32/180 of clinical samples were contaminated by Staphylococcus. Eleven species were isolated from meats and 4 from clinical samples. Forty-two S. aureus strains were isolated from ours samples. Variability of resistance was observed for most of the tested antibiotics but none of the strains displays a resistance to imipenem and quinolones. We observed that 89% of clinical S. aureus were resistant to methicillin against 58% for those issued from meat products. All S. aureus isolates issued from meat products produce epidermolysins whereas none of the clinical strains produced these toxins. The enterotoxins were variably produced by both clinical and meat product samples.

Residues essential for Panton-Valentine leukocidin S component binding to its cell receptor suggest both plasticity and adaptability in its interaction surface

Panton-Valentine leukocidin (PVL), a bicomponent staphylococcal leukotoxin, is involved in the poor prognosis of necrotizing pneumonia. The present study aimed to elucidate the binding mechanism of PVL and in particular its cell-binding domain. The class S component of PVL, LukS-PV, is known to ensure cell targeting and exhibits the highest affinity for the neutrophil membrane (K_d∼10⁻¹⁰ M) compared to the class F component of PVL, LukF-PV (K_d∼10⁻⁹ M). Alanine scanning mutagenesis was used to identify the residues involved in LukS-PV binding to the neutrophil surface. Nineteen single alanine mutations were performed in the rim domain previously described as implicated in cell membrane interactions. Positions were chosen in order to replace polar or exposed charged residues and according to conservation between leukotoxin class S components. Characterization studies enabled to identify a cluster of residues essential for LukS-PV binding, localized on two loops of the rim domain. The mutations R73A, Y184A, T244A, H245A and Y250A led to dramatically reduced binding affinities for both human leukocytes and undifferentiated U937 cells expressing the C5a receptor. The three-dimensional structure of five of the mutants was determined using X-ray crystallography. Structure analysis identified residues Y184 and Y250 as crucial in providing structural flexibility in the receptor-binding domain of LukS-PV.

Staphylococcal bicomponent pore-forming toxins: targets for prophylaxis and immunotherapy

Staphylococccus aureus represents one of the most challenging human pathogens as well as a common colonizer of human skin and mucosal surfaces. S. aureus causes a wide range of diseases from skin and soft tissue infection (SSTI) to debilitating and life-threatening conditions such as osteomyelitis, endocarditis, and necrotizing pneumonia. The range of diseases reflects the remarkable diversity of the virulence factors produced by this pathogen, including surface antigens involved in the establishment of infection and a large number of toxins that mediate a vast array of cellular responses. The staphylococcal toxins are generally believed to have evolved to disarm the innate immune system, the first line of defense against this pathogen. This review focuses on recent advances on elucidating the biological functions of S. aureus bicomponent pore-forming toxins (BCPFTs) and their utility as targets for preventive and therapeutic intervention. These toxins are cytolytic to a variety of immune cells, primarily neutrophils, as well as cells with a critical barrier function. The lytic activity of BCPFTs towards immune cells implies a critical role in immune evasion, and a number of epidemiological studies and animal experiments relate these toxins to clinical disease, particularly SSTI and necrotizing pneumonia. Antibody-mediated neutralization of this lytic activity may provide a strategy for development of toxoid-based vaccines or immunotherapeutics for prevention or mitigation of clinical diseases. However, certain BCPFTs have been proposed to act as danger signals that may alert the immune system through an inflammatory response. The utility of a neutralizing vaccination strategy must be weighed against such immune-activating potential.

Staphylococcus aureus interaction with phospholipid vesicles--a new method to accurately determine accessory gene regulator (agr) activity

The staphylococcal accessory gene regulatory (agr) operon is a well-characterised global regulatory element that is important in the control of virulence gene expression for Staphylococcus aureus, a major human pathogen. Hence, accurate and sensitive measurement of Agr activity is central in understanding the virulence potential of Staphylococcus aureus, especially in the context of Agr dysfunction, which has been linked with persistent bacteraemia and reduced susceptibility to glycopeptide antibiotics. Agr function is typically measured using a synergistic haemolysis CAMP assay, which is believe to report on the level of expression of one of the translated products of the agr locus, delta toxin. In this study we develop a vesicle lysis test (VLT) that is specific to small amphipathic peptides, most notably delta and Phenol Soluble Modulin (PSM) toxins. To determine the accuracy of this VLT method in assaying Agr activity, we compared it to the CAMP assay using 89 clinical Staphylococcus aureus isolates. Of the 89 isolates, 16 were designated as having dysfunctional Agr systems by the CAMP assay, whereas only three were designated as such by VLT. Molecular analysis demonstrated that of these 16 isolates, the 13 designated as having a functional Agr system by VLT transcribed rnaIII and secreted delta toxin, demonstrating they have a functional Agr system despite the results of the CAMP assay. The agr locus of all 16 isolates was sequenced, and only the 3 designated as having a dysfunctional Agr system contained mutations, explaining their Agr dysfunction. Given the potentially important link between Agr dysfunction and clinical outcome, we have developed an assay that determines this more accurately than the conventional CAMP assay.

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.

Cell targeting by the Staphylococcus aureus pore-forming toxins: it's not just about lipids

Staphylococcus aureus employs numerous pore-forming cytotoxins to injure host immune cells and promote infection. Until recently, it was unclear how these cytotoxins targeted specific cell types for lysis. Membrane lipids were initially postulated to be cytotoxin receptor candidates. However, the cell-type specificity and species-dependent targeting of these toxins did not support lipids as sole receptors. The recent identification of proteinaceous receptors for several S. aureus cytotoxins now provides an explanation for the observed tropism. These findings also have important implications for the implementation of animal models to study S. aureus pathogenesis, and for the development of novel therapeutics.

The effects of Staphylococcus aureus leukotoxins on the host: cell lysis and beyond

The success of Staphylococcus aureus as a leading cause of deadly hospital-acquired and community-acquired infections is attributed to its high-level resistance to most antibiotics, and the multitude of virulence factors it elaborates. Most clinical isolates produce up to four bi-component pore-forming toxins capable of lysing cells of the immune system. Subtle differences in activity and target range of each leukotoxin suggest that these toxins are not redundant, but instead may have specialized functions in attacking and/or evading host defenses. In turn, the host has developed countermeasures recognizing sublytic levels of leukotoxins as signals to activate protective immune defenses. The opposing cytotoxic and immune-activating effects of leukotoxins on host cells make for a complex dynamic between S. aureus and the host.

Variability of antibiotic susceptibility and toxin production of Staphylococcus aureus strains isolated from skin, soft tissue, and bone related infections

Background

Staphylococcus aureus is an opportunistic commensal bacterium that mostly colonizes the skin and soft tissues. The pathogenicity of S. aureus is due to both its ability to resist antibiotics, and the production of toxins. Here, we characterize a group of genes responsible for toxin production and antibiotic resistance of S. aureus strains isolated from skin, soft tissue, and bone related infections.

Results

A total of 136 S. aureus strains were collected from five different types of infection: furuncles, pyomyositis, abscesses, Buruli ulcers, and osteomyelitis, from hospital admissions and out-patients in Benin. All strains were resistant to benzyl penicillin, while 25% were resistant to methicillin, and all showed sensitivity to vancomycin. Panton-Valentine leukocidin (PVL) was the most commonly produced virulence factor (70%), followed by staphylococcal enterotoxin B (44%). Exfoliative toxin B was produced by 1.3% of the strains, and was only found in isolates from Buruli ulcers. The tsst-1, sec, and seh genes were rarely detected (≤1%).

Conclusions

This study provides new insight into the prevalence of toxin and antibiotic resistance genes in S. aureus strains responsible for skin, soft tissue, and bone infections. Our results showed that PVL was strongly associated with pyomyositis and osteomyelitis, and that there is a high prevalence of PVL-MRSA skin infections in Benin.

Therapeutic antibodies and infectious diseases, Tours, France, November 20-22, 2012

The Therapeutic Antibodies and Infectious Diseases international congress was held in Tours, France on November 20−22, 2012. The first session was devoted to the development of antibodies directed against bacterial toxins or viruses that could be used in a potential bioterrorist threat situation. The second session dealt with the effector functions of anti-microbial antibodies, while the third was oriented toward anti-viral antibodies, with a special emphasis on antibodies directed against the human immunodeficiency and hepatitis C viruses. After a lecture by a speaker from the US Food and Drug Administration on antibody cocktails, the second day ended with a special session dedicated to discussions regarding the involvement of French biotechnology industries in the field. On the last day, the congress concluded with talks about current antibody treatments for infectious diseases, with a particular focus on their adverse events. Participants enjoyed this very stimulating and convivial meeting, which gathered scientists from various countries who had different scientific research interests.

p-Sulfonato-calix[n]arenes inhibit staphylococcal bicomponent leukotoxins by supramolecular interactions

PVL (Panton-Valentine leukocidin) and other Staphylococcus aureus β-stranded pore-forming toxins are important virulence factors involved in various pathologies that are often necrotizing. The present study characterized leukotoxin inhibition by selected SCns (p-sulfonato-calix[n]arenes): SC4, SC6 and SC8. These chemicals have no toxic effects on human erythrocytes or neutrophils, and some are able to inhibit both the activity of and the cell lysis by leukotoxins in a dose-dependent manner. Depending on the type of leukotoxins and SCns, flow cytometry revealed IC50 values of 6-22 μM for Ca2+ activation and of 2-50 μM for cell lysis. SCns were observed to affect membrane binding of class S proteins responsible for cell specificity. Electrospray MS and surface plasmon resonance established supramolecular interactions (1:1 stoichiometry) between SCns and class S proteins in solution, but not class F proteins. The membrane-binding affinity of S proteins was Kd=0.07-6.2 nM. The binding ability was completely abolished by SCns at different concentrations according to the number of benzenes (30-300 μM; SC8>SC6≫SC4). The inhibitory properties of SCns were also observed in vivo in a rabbit model of PVL-induced endophthalmitis. These calixarenes may represent new therapeutic avenues aimed at minimizing inflammatory reactions and necrosis due to certain virulence factors.

Accumulation of staphylococcal Panton-Valentine leukocidin in the detergent-resistant membrane microdomains on the target cells is essential for its cytotoxicity

The mechanisms for the cytotoxicity of staphylococcal Panton-Valentine leukocidin (PVL), a pore-forming toxin consisting of LukS-PV and LukF-PV, in human immune cells are still unclear. Because LukS-PV binds to ganglioside GM1, a constituent of detergent-resistant membrane microdomains (DRMs) of the plasma membrane, the role of DRMs in PVL cytotoxicity was examined in human polymorphonuclear neutrophils (PMNs), monocytes, HL-60 cells, and THP-1 cells. PVL binding capacities in HL-60 and THP-1 cells were higher than those in PMNs and monocytes; however, the PVL concentration to obtain more than 80% cell lysis in HL-60 cells was 10 times higher than that in PMNs and PVL even at such concentration induced < 10% cell lysis in THP-1 cells. After incubation of PMNs with LukS-PV, more than 90% of LukS-PV bound to the detergent-soluble membranes. Subsequent incubation with LukF-PV at 4 °C induced the accumulation of more than 70% of PVL components and 170- to 220-kDa complex formation in DRMs in an actin-independent manner. However, only 30% of PVL was found, and complex formation was under detectable level in DRMs in HL-60 cells. PVL did not accumulate in DRMs in THP-1 cells. Our observations strongly indicate that PVL accumulation in DRMs is essential for PVL cytotoxicity.

Panton-Valentine leukocidin-producing Staphylococcal aureus: report of four siblings

Panton-Valentine leukocidin (PVL)-producing Staphylococcus aureus results in leukocyte destruction and tissue necrosis (Pediatric Dermatology 2007;24:401). It can be associated with a spectrum of clinical manifestations that range from localized staphylococcal skin infections to sometimes severe necrotizing pneumonia (Clin Infect Dis 1999;29:1128). We report a case of four siblings, three brothers whose atopic dermatitis was complicated by cutaneous lesions and furunculosis, while their 21-month-old sister had a fatal PVL positive staphylococcal pneumonia.

Staphylococcus aureus Panton-Valentine leukocidin induces an inflammatory response in human phagocytes via the NLRP3 inflammasome

The Staphylococcus aureus pore-forming toxin PVL is most likely causative for life-threatening necrotizing infections, which are characterized by massive tissue inflammation and necrosis. Whereas the cytotoxic action of PVL on human neutrophils is already well established, the PVL effects on other sensitive cell types, such as monocytes and macrophages, are less clear. In this study, we used different types of human leukocytes (neutrophils, monocytes, macrophages, lymphocytes) to investigate cell-specific binding of PVL subunits and subsequent proinflammatory and cytotoxic effects. In all PVL-sensitive cells, we identified the binding of the subunit LukS-PV as the critical factor for PVL-induced cytotoxicity, which was followed by binding of LukF-PV. LukS-PV binds to monocytes, macrophages, and neutrophils but not to lymphocytes. Additionally, we showed that PVL binding to monocytes and macrophages leads to release of caspase-1-dependent proinflammatory cytokines IL-1β and IL-18. PVL activates the NLRP3 inflammasome, a signaling complex of myeloid cells that is involved in caspase-1-dependent IL-1β processing in response to pathogens and endogenous danger signals. Specific inhibition of this pathway at several steps significantly reduced inflammasome activation and subsequent pyronecrosis. Furthermore, we found that PAMPs and DAMPs derived from dying neutrophils can dramatically enhance this response by up-regulating pro-IL-1β in monocytes/macrophages. This study analyzes a specific host signaling pathway that mediates PVL-induced inflammation and cytotoxicity, which has high relevance for CA-MRSA-associated and PVL-mediated pathogenic processes, such as necrotizing infections.

Sublytic concentrations of Staphylococcus aureus Panton-Valentine leukocidin alter human PMN gene expression and enhance bactericidal capacity

CA-MRSA infections are often caused by strains encoding PVL, which can cause lysis of PMNs and other myeloid cells in vitro, a function considered widely as the primary means by which PVL might contribute to disease. However, at sublytic concentrations, PVL can function as a PMN agonist. To better understand this phenomenon, we investigated the ability of PVL to alter human PMN function. PMNs exposed to PVL had enhanced capacity to produce O(2)(-) in response to fMLF, but unlike priming by LPS, this response did not require TLR signal transduction. On the other hand, there was subcellular redistribution of NADPH oxidase components in PMNs following exposure of these cells to PVL--a finding consistent with priming. Importantly, PMNs primed with PVL had an enhanced ability to bind/ingest and kill Staphylococcus aureus. Priming of PMNs with other agonists, such as IL-8 or GM-CSF, altered the ability of PVL to cause formation of pores in the plasma membranes of these cells. Microarray analysis revealed significant changes in the human PMN transcriptome following exposure to PVL, including up-regulation of molecules that regulate the inflammatory response. Consistent with the microarray data, mediators of the inflammatory response were released from PMNs after stimulation with PVL. We conclude that exposure of human PMNs to sublytic concentrations of PVL elicits a proinflammatory response that is regulated in part at the level of gene expression. We propose that PVL-mediated priming of PMNs enhances the host innate immune response.

Staphylococcus aureus hemolysins, bi-component leukocidins, and cytolytic peptides: a redundant arsenal of membrane-damaging virulence factors?

One key aspect of the virulence of Staphylococcus aureus lies in its ability to target the host cell membrane with a large number of membrane-damaging toxins and peptides. In this review, we describe the hemolysins, the bi-component leukocidins (which include the Panton Valentine leukocidin, LukAB/GH, and LukED), and the cytolytic peptides (phenol soluble modulins). While at first glance, all of these factors might appear redundant, it is now clear that some of these factors play specific roles in certain S. aureus life stages and diseases or target specific cell types or species. In this review, we present an update of the literature on toxin receptors and their cell type and species specificities. Furthermore, we review epidemiological studies and animal models illustrating the role of these membrane-damaging factors in various diseases. Finally, we emphasize the interplay of these factors with the host immune system and highlight all their non-lytic functions.

Immunopathogenesis of Staphylococcus aureus pulmonary infection

Staphylococcus aureus is a common human pathogen highly evolved as both a component of the commensal flora and as a major cause of invasive infection. Severe respiratory infection due to staphylococci has been increasing due to the prevalence of more virulent USA300 CA-MRSA strains in the general population. The ability of S. aureus to adapt to the milieu of the respiratory tract has facilitated its emergence as a respiratory pathogen. Its metabolic versatility, the ability to scavenge iron, coordinate gene expression, and the horizontal acquisition of useful genetic elements have all contributed to its success as a component of the respiratory flora, in hospitalized patients, as a complication of influenza and in normal hosts. The expression of surface adhesins facilitates its persistence in the airways. In addition, the highly sophisticated interactions of the multiple S. aureus virulence factors, particularly the α-hemolysin and protein A, with diverse immune effectors in the lung such as ADAM10, TNFR1, EGFR, immunoglobulin, and complement all contribute to the pathogenesis of staphylococcal pneumonia.

Staphylococcal Panton-Valentine leucocidin as a major virulence factor associated to furuncles

Panton-Valentine Leucocidin (PVL), one of the β-barrel pore-forming staphylococcal leucotoxins, is known to be associated to furuncles and some severe community pneumonia. However, it is still uncertain how many other virulence factors are also associated to furuncles and what the risk factors of furuncles are in immuno-compromised status of patients, especially the HIV (+) patients. In this paper, we use antigen immunoprecipitation and multiplex PCR approach to determine the presence of 19 toxins, 8 adhesion factors and the PFGE profiles associated to furuncles in three independent patient study groups of S. aureus (SA) isolates collected from the Cayenne General Hospital (French Guiana). The patient groups were made of: 16 isolates from HIV (−) patients, 9 from HIV (+) patients suffering from furuncles, and 30 control isolates from patients with diverse secondary infected dermatitis. Our data reveals that the majority (96%) of SA strains isolated from HIV patient-derived furuncles significantly produced PVL (p<10⁻⁷), whereas only 10% of SA strains produced this toxin in secondary infected dermatosis. A high prevalence of LukE-LukD-producing isolates (56 to 78%) was recorded in patient groups. Genes encoding clumping factor B, collagen- and laminin-binding proteins (clfB, cna, lbp, respectively) were markedly frequent (30 to 55%), without being associated to a specific group. Pulse field gel electrophoresis evidenced 24 overall pulsotypes, whereas the 25 PVL-producing isolates were distributed into 15 non clonal fingerprints. These pulsotypes were not specific PVL-producing isolates. PVL appears to be the major virulence factor associated to furuncles in Europe and in South America regardless of the immune status of the HIV patients.

Heavy chain-only antibodies and tetravalent bispecific antibody neutralizing Staphylococcus aureus leukotoxins

Panton-Valentine leukocidin (PVL) is a pore-forming toxin associated with current outbreaks of community-associated methicillin-resistant strains and implicated directly in the pathophysiology of Staphylococcus aureus-related diseases. Humanized heavy chain-only antibodies (HCAb) were generated against S. aureus PVL from immunized transgenic mice to neutralize toxin activity. The active form of PVL consists of the two components, LukS-PV and LukF-PV, which induce osmotic lysis following pore formation in host defense cells. One anti-LukS-PV HCAb, three anti-LukF-PV HCAbs with affinities in the nanomolar range, and one engineered tetravalent bispecific HCAb were tested in vitro and in vivo, and all prevented toxin binding and pore formation. Anti-LukS-PV HCAb also binds to γ-hemolysin C (HlgC) and inhibits HlgC/HlgB pore formation. Experiments in vivo in a toxin-induced rabbit endophthalmitis model showed that these HCAbs inhibit inflammatory reactions and tissue destruction, with the tetravalent bispecific HCAb performing best. Our findings show the therapeutic potential of HCAbs, and in particular, bispecific antibodies.

Binding of the Staphylococcus aureus leucotoxin LukM to its leucocyte targets

The range of leucocytes susceptible to the leucotoxin LukM/F', a two-component pore-forming toxin of Staphylococcus aureus causing mastitis in ruminants, had not been defined. We used fluorescent-labeled LukM to investigate the binding of this toxin to bovine cells and to identify its cellular targets among bovine, human and murine leucocytes. LukM bound to bovine blood neutrophils from all the individuals tested with similar affinity, with an apparent dissociation constant (Kd) of 1.81 ± 0.14 nM and 13 3100 ± 506 binding sites. The amount of LukM bound to bovine neutrophils did not depend on the presence of the complementary component LukF', suggesting that the binding of LukM to its ligand does not depend on the formation of pore-forming oligomers, and that the number of bound LukM molecules corresponds to the number of available cell membrane ligands. Other staphylococcal class S components of bipartite leucotoxins (HlgA, HlgC, LukE, LukS-PV) were inefficient competitors of LukM for the binding to bovine neutrophils, indicating that LukM has a distinct ligand on target cells. Bovine blood neutrophils bound slightly more LukM than did milk neutrophils, and much more than did ovine and caprine blood neutrophils. Bovine monocytes and milk macrophages readily bound LukM, whereas blood lymphocytes did not. Human neutrophils bound little LukM and were resistant to LukM/F' at the highest tested concentration (40 nM). Murine neutrophils bound LukM and were susceptible to the toxicity of LukM/F', exhibiting flattening and nucleus alteration beginning at 0.3 nM concentration. Among murine peritoneal exudate cells, T lymphocytes (CD3+) and monocytes/macrophages (F4/80+) bound LukM, whereas binding to B lymphocytes (CD19+) was not detected. These results indicate that cells of the myeloid lineage are the main targets of LukM/F' in dairy ruminants, and that resident or inflammatory migrated phagocytes are susceptible to this toxin.

Polymorphonuclear leukocytes mediate Staphylococcus aureus Panton-Valentine leukocidin-induced lung inflammation and injury

Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) is epidemic in the United States, even rivaling HIV/AIDS in its public health impact. The pandemic clone USA300, like other CA-MRSA strains, expresses Panton-Valentine leukocidin (PVL), a pore-forming toxin that targets polymorphonuclear leukocytes (PMNs). PVL is thought to play a key role in the pathogenesis of necrotizing pneumonia, but data from rodent infection models are inconclusive. Rodent PMNs are less susceptible than human PMNs to PVL-induced cytolysis, whereas rabbit PMNs, like those of humans, are highly susceptible to PVL-induced cytolysis. This difference in target cell susceptibility could affect results of experimental models. Therefore, we developed a rabbit model of necrotizing pneumonia to compare the virulence of a USA300 wild-type strain with that of isogenic PVL-deletion mutant and -complemented strains. PVL enhanced the capacity of USA300 to cause severe lung necrosis, pulmonary edema, alveolar hemorrhage, hemoptysis, and death, hallmark clinical features of fatal human necrotizing pneumonia. Purified PVL instilled directly into the lung caused lung inflammation and injury by recruiting and lysing PMNs, which damage the lung by releasing cytotoxic granule contents. These findings provide insights into the mechanism of PVL-induced lung injury and inflammation and demonstrate the utility of the rabbit for studying PVL-mediated pathogenesis.

Relative contribution of Panton-Valentine leukocidin to PMN plasma membrane permeability and lysis caused by USA300 and USA400 culture supernatants

Panton-Valentine leukocidin (PVL) is a cytolytic toxin associated with severe community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) infections. However, the relative contribution of PVL to host cell lysis during CA-MRSA infection remains unknown. Here we investigated the relative contribution of PVL to human polymorphonuclear leukocyte (PMN) plasma membrane permeability and lysis in vitro by using culture supernatants from wild-type and isogenic lukS/F-PV negative (Deltapvl) USA300 and USA400 strains. Using S. aureus culture conditions that favor selective high production of PVL (CCY medium), there was on average more PMN plasma membrane permeability and cell lysis caused by supernatants derived from wild-type strains compared with those from Deltapvl strains. Unexpectedly, plasma membrane permeability did not necessarily correlate with ultimate cell lysis. Moreover, the level of pore formation caused by culture supernatants varied dramatically (e.g., range was 0.32-99.09% for wild-type USA300 supernatants at 30 min) and was not attributable to differences in PMN susceptibility to PVL among human blood donors. We conclude that PMN pore formation assays utilizing S. aureus culture supernatants have limited ability to estimate the relative contribution of PVL to pathogenesis (or cytolysis in vitro or in vivo), especially when assayed using culture media that promote selective high production of PVL.

Panton-Valentine leukocidin-producing Staphylococcus aureus infections: Report of 4 French cases

We report 4 cases of community-acquired infections due to Staphylococcus aureus producing Panton-Valentin leukocidin (SA-PVL) with uncommon multivisceral localizations. These cases highlight the need to screen for PVL in patients with serious staphylococcal infections. All patients were cured. Two of them received intravenous immunoglobulins in addition to antibiotics.