Serum antibodies against Panton-Valentine leukocidin in a normal population and during Staphylococcus aureus infection

Severe Staphylococcus aureus infections in children: Case reports and management of positive Panton-Valentine leucocidin cases

BACKGROUND

Staphylococcus aureus is a well-known bacterium associated with carriage and responsible for different types of infections. The Panton-Valentine leucocidin (PVL) is a key virulence factor causing tissue necrosis. PVL can, however, be present in both benign and life-threatening infections.

CASE REPORTS AND MANAGEMENT

We present three pediatric severe infections occurring over a period of only three weeks, in February 2021, and caused by genetically unrelated methicillin-sensitive Staphylococcus aureus producing PVL in a tertiary children's hospital in Belgium. The first one presented with necrotizing pneumonia, the second one with a neck abscess extended to the mediastinum, and the last one had sacral osteomyelitis complicated by endocarditis. The management of these infections is mostly based on expert opinions. The most appropriate treatment seems to be the combination of early surgical drainage of infected collections with an antibiotic regimen associating two antibiotics; beta-lactams and either clindamycin or linezolid. Human immunoglobulins also appear to be useful as adjunctive therapy.

CONCLUSION

PVL-producing Staphylococcus aureus is associated with life-threatening infections in children. Prompt management is needed including surgery and appropriate antibiotic regimens.

Human Memory B Cells Targeting Staphylococcus aureus Exotoxins Are Prevalent with Skin and Soft Tissue Infection

Staphylococcus aureus is a Gram-positive opportunistic pathogen that causes superficial and invasive infections in the hospital and community. High mortality from infection emphasizes the need for improved methods for prevention and treatment. Although S. aureus possesses an arsenal of virulence factors that contribute to evasion of host defenses, few studies have examined long-term humoral and B-cell responses. Adults with acute-phase skin and soft tissue infections were recruited; blood samples were obtained; and S. aureus isolates, including methicillin-resistant strains, were subjected to genomic sequence analysis. In comparisons of acute-phase sera with convalescent-phase sera, a minority (37.5%) of patients displayed 2-fold or greater increases in antibody titers against three or more S. aureus antigens, whereas nearly half exhibited no changes, despite the presence of toxin genes in most infecting strains. Moreover, enhanced antibody responses waned over time, which could reflect a defect in B-cell memory or long-lived plasma cells. However, memory B cells reactive with a range of S. aureus antigens were prevalent at both acute-phase and convalescent-phase time points. While some memory B cells exhibited toxin-specific binding, those cross-reactive with structurally related leucocidin subunits were dominant across patients, suggesting the targeting of conserved epitopes. Memory B-cell reactivity correlated with serum antibody levels for selected S. aureus exotoxins, suggesting a relationship between the cellular and humoral compartments. Overall, although there was no global defect in the representation of anti-S. aureus memory B cells, there was evidence of restrictions in the range of epitopes recognized, which may suggest potential therapeutic approaches for augmenting host defenses.

The contribution of B-cell memory and long-term antibody responses to host defenses against S. aureus exotoxins remains poorly understood. Our studies confirmed that infection did not commonly lead to enhanced long-term humoral responses. Whereas circulating memory B cells against S. aureus secreted exotoxins were prevalent, they were dominated by cross-reactivity with structurally related leucocidin subunits, consistent with recognition of conserved epitopes. These findings also provide the first evidence of a relationship between the reactivity of antistaphylococcal circulating memory B cells and serum antibody levels. In general, infection was not associated with a global defect in B-cell memory for S. aureus secreted factors, and responses were highly dominated by cross-reactivity to structurally related exotoxins, which arguably may alone be suboptimal in providing host defenses. Our studies illuminate aspects of the S. aureus-host relationship that may better inform strategies for the development of an effective protective vaccine.

Panton-Valentine Leukocidin associated with S. aureus osteomyelitis activates platelets via neutrophil secretion products

Globalization and migration promote the spread of Panton-Valentine leukocidin (PVL)-positive Staphylococcus aureus strains. The toxin PVL is linked to the development of thrombosis in association with osteomyelitis. The mechanisms by which PVL drives thrombosis development are however still unknown. We demonstrate that PVL-damaged neutrophils activate platelets via neutrophil secretion products, such as α-defensins and the myeloperoxidase product HOCl, as well as the formation of HOCl-modified proteins. Neutrophil damage by PVL is blocked by anti-PVL-antibodies, explaining why especially young osteomyelitis patients with a low antibody titre against PVL suffer from thrombotic complications. Platelet activation in the presence of PVL-damaged neutrophils is prevented by α-defensin inhibitors and by glutathione and resveratrol, which are both inhibitors of HOCl-modified protein-induced platelet activation. Remarkably, intravenously infused glutathione also prevents activation of human platelets in an ex vivo assay. We here describe a new mechanism of PVL-neutrophil-platelet interactions, which might be extrapolated to other toxins that act on neutrophils. Our observations may make us think about new approaches to treat and/or prevent thrombotic complications in the course of infections with PVL-producing S. aureus strains.

Safety and immunogenicity of a recombinant Staphylococcus aureus α-toxoid and a recombinant Panton-Valentine leukocidin subunit, in healthy adults

We conducted a randomized, double-blind, placebo-controlled dose-escalation study in healthy adults to evaluate the safety and immunogenicity of recombinant Staphylococcus aureus candidate vaccine antigens, recombinant α-toxoid (rAT) and a sub-unit of Panton-Valentine leukocidin (rLukS-PV). 176 subjects were enrolled and randomized within 1 of 11 treatment cohorts: monovalent rAT or rLukS-PV dosages of 10, 25, 50, and 100 μg; bivalent rAT:rLukS dosages of 10:10, 25:25, and 50:50 μg; and alum or saline placebo. All subjects were assessed at Days 0, 7, 14, 28, and 84. Subjects in the 50:50 μg bivalent cohort received a second injection on Day 84 and were assessed on Days 98 and 112. Incidence and severity of reactogenicity and adverse events (AEs) were compared. Geometric mean serum concentrations (GMC) and neutralizing activity of anti-rAT and anti-rLukS-PV IgG were assessed. Reactogenicity incidence was significantly higher in vaccine than placebo recipients (77% versus 55%, respectively; p = 0.006). However, 77% of reactogenicity events were mild and 19% were moderate in severity. The AE incidence and severity were similar between the cohorts. All monovalent and bivalent rAT dosages resulted in a significant increase in the anti-rAT IgG and anti- rLukS-PV GMCs between day 0 and 28 compared with placebo, and persisted through Day 84. Exploratory subgroup analyses suggested a higher GMC and neutralizing antibody titers for the 50 μg monovalent or bivalent rAT and rLukS-PV dose as compared to the other doses. No booster effect was observed after administration of the second dose. We conclude that the rAT and rLukS-PV vaccine formulations were well-tolerated and had a favorable immunogenicity profile, producing antibody with neutralizing activity through day 84. There was no benefit observed with a booster dose of the vaccine.

The levels of antibodies to Panton-Valentine leukocidin (PVL) vary with PVL prevalence along a north-to-south gradient

Recent research on Staphylococcus aureus vaccine development has focused on active immunization against Panton-Valentine leukocidin (PVL), a potent leukotoxin associated with both superficial and severe deep-seated infections. PVL prevalence is highly variable worldwide, but it is unknown to what extent immunity to PVL varies between patients from geographic areas with different PVL-positive S. aureus prevalences. We conducted a retrospective multicentric study of anti-PVL and anti-alpha-toxin (Hla) antibody levels in uninfected adult patients from France (low PVL prevalence; n = 200), Algeria (moderate prevalence; n = 143), and Senegal (high prevalence; n = 228). The antibody levels were quantified by an enzyme-linked immunosorbent assay (ELISA) procedure. Because Hla is present in virtually all S. aureus strains, its corresponding antibody levels were considered to reflect population exposure to S. aureus. Compared with French participants, the average anti-PVL antibody levels were 2.5-fold and 8.2-fold higher in Algerian and Senegalese participants, respectively (p < 0.001). Conversely, anti-Hla antibody levels did not differ between participants from the three countries, suggesting that the observed differences in anti-PVL antibody levels were not biased by variations in population exposure to S. aureus. Hence, anti-PVL antibody levels in the general populations of France, Algeria, and Senegal vary widely and match variations in PVL-positive S. aureus strain prevalence, with an increasing north-to-south gradient. To conclude, immunity to PVL in a given population correlates with local PVL prevalence. This finding can help to inform PVL vaccine strategies.

Levels of alpha-toxin correlate with distinct phenotypic response profiles of blood mononuclear cells and with agr background of community-associated Staphylococcus aureus isolates

Epidemiological studies of Staphylococcus aureus have shown a relation between certain clones and the presence of specific virulence genes, but how this translates into virulence-associated functional responses is not fully elucidated. Here we addressed this issue by analyses of community-acquired S. aureus strains characterized with respect to antibiotic resistance, ST types, agr types, and virulence gene profiles. Supernatants containing exotoxins were prepared from overnight bacterial cultures, and tested in proliferation assays using human peripheral blood mononuclear cells (PBMC). The strains displayed stable phenotypic response profiles, defined by either a proliferative or cytotoxic response. Although, virtually all strains elicited superantigen-mediated proliferative responses, the strains with a cytotoxic profile induced proliferation only in cultures with the most diluted supernatants. This indicated that the superantigen-response was masked by a cytotoxic effect which was also confirmed by flow cytometry analysis. The cytotoxic supernatants contained significantly higher levels of α-toxin than did the proliferative supernatants. Addition of α-toxin to supernatants characterized as proliferative switched the response into cytotoxic profiles. In contrast, no effect of Panton Valentine Leukocidin, δ-toxin or phenol soluble modulin α-3 was noted in the proliferative assay. Furthermore, a significant association between agr type and phenotypic profile was found, where agrII and agrIII strains had predominantly a proliferative profile whereas agrI and IV strains had a predominantly cytotoxic profile. The differential response profiles associated with specific S. aureus strains with varying toxin production could possibly have an impact on disease manifestations, and as such may reflect specific pathotypes.

Clinical and molecular epidemiology of community-onset invasive Staphylococcus aureus infection in New Zealand children

Our aim was to describe the epidemiology and incidence of community-onset invasive S. aureus disease in children presenting to our hospital, and to compare the clonal complexes and virulence genes of S. aureus strains causing invasive and non-invasive disease. The virulence gene repertoire of invasive disease isolates was characterized using DNA microarray and compared with the virulence gene repertoire of non-invasive S. aureus isolates. Over the study period, 163 children had an invasive S. aureus infection. There was no difference in the distribution of clonal complexes or in the prevalence of genes encoding virulence factors between invasive and non-invasive isolates. Future research should include a strong focus on identifying the host and environmental factors that, along with organism virulence factors, are contributing to the patterns of invasive S. aureus disease observed in New Zealand.

Children with invasive Staphylococcus aureus disease exhibit a potently neutralizing antibody response to the cytotoxin LukAB

Despite the importance of Staphylococcus aureus as a common invasive bacterial pathogen, the humoral response to infection remains inadequately defined, particularly in children. The purpose of this study was to assess the humoral response to extracellular staphylococcal virulence factors, including the bicomponent leukotoxins, which are critical for the cytotoxicity of S. aureus toward human neutrophils. Children with culture-proven S. aureus infection were prospectively enrolled and stratified by disease type. Fifty-three children were enrolled in the study, of which 90% had invasive disease. Serum samples were obtained during the acute (within 48 h) and convalescent (4 to 6 weeks postinfection) phases, at which point both IgG titers against S. aureus exotoxins were determined, and the functionality of the generated antibodies was evaluated. Molecular characterization of clinical isolates was also performed. We observed a marked rise in antibody titer from acute-phase to convalescent-phase sera for LukAB, the most recently described S. aureus bicomponent leukotoxin. LukAB production by the isolates was strongly correlated with cytotoxicity in vitro, and sera containing anti-LukAB antibodies potently neutralized cytotoxicity. Antibodies to S. aureus antigens were detectable in healthy pediatric controls but at much lower titers than in sera from infected subjects. The discovery of a high-titer, neutralizing antibody response to LukAB during invasive infections suggests that this toxin is produced in vivo and that it elicits a functional humoral response.

Dose-related regulatory effect of intravenous immunoglobulin on dendritic cells-mediated immune response

CONTEXT

Intravenous immunoglobulin (IVIG) has been successfully applied in immune-related diseases of adults and neonates, such as human immunodeficiency virus (HIV) infection and systemic lupus erythematosus (SLE).

OBJECTIVE

This study aims to investigate the distinct impacts of IVIG on cultured dendritic cells (DCs) from newborn and healthy adult.

MATERIALS AND METHODS

Blood samples were collected from eight full-term newborns and eight healthy adult volunteers. DCs from cord blood and peripheral blood were both cultured in the RPMI 1640 medium containing 10% fetal calf serum, 50 ng/ml granulocyte/macrophage colony-stimulating factor (GM-CSF) and 10 ng/ml recombinant human interleukin-4 (rhIL-4) for 5 d with therapeutic IVIG (20 mg/ml) or physiological IVIG (10 mg/ml). Lipopolysaccharides (LPSs, 1 μg/ml) were added on the fifth day to induce the maturation of immature DCs. The phagocytosis of monocytes, expression of MR (mannose receptor), CD14, CD1a, CD80, CD83, CD86 and MHC II were examined by flow cytometry. The expression of IL-4 mRNA was detected by RT-PCR, while IFN-γ, IL-12 and IL-10 were analyzed by enzyme-linked immunosorbent assay (ELISA) commercial kits.

RESULTS

IVIG of therapeutic dose inhibited the phagocytosis, differentiation and maturation of DCs, whereas physiological dose exhibited an accelerated role in vitro, especially on DCs from neonates, but aroused different effects on cytokine secretion.

DISCUSSION AND CONCLUSION

The different responses are generally due to immature immune system of neonate, which has a limit capacity to maintain immunity homeostasis. Modulation of DCs phagocytosis, differentiation, maturation and cytokine secretion by IVIG is of potential relevance to its dosage and immune status of patients.

A serologic correlate of protective immunity against community-onset Staphylococcus aureus infection

BACKGROUND

Staphylococcus aureus is among the leading causes of human infection. Widespread drug resistance, emergence of highly virulent strains, and the ability of S. aureus to colonize >30% of the human population contribute to this organism's pathogenic success. Human serologic responses to S. aureus and their relationship to protective immunity remain incompletely defined, challenging the strategic development of efficacious vaccines.

METHODS

We measured humoral responses to 2 staphylococcal exotoxins, α-hemolysin (Hla) and Panton-Valentine leukocidin (PVL; LukF-PV/LukS-PV subunits), both premier targets of current vaccine and immunotherapy development. We correlated acute and convalescent serum antibody levels with incidence of recurrent infection over 12 months follow-up in 235 children with S. aureus colonization, primary or recurrent skin and soft tissue infection, or invasive disease.

RESULTS

Cutaneous infection elicited transient increases in anti-Hla and anti-PVL antibodies; however, subsequent infection risk was similar between primary and recurrent cutaneous infection cohorts. Patients with invasive infections had the lowest preexisting titers against Hla and LukF but displayed the highest convalescent titers. Across cohorts, convalescent anti-Hla titers correlated with protection against subsequent S. aureus infection.

CONCLUSIONS

Cutaneous S. aureus infection does not reliably provoke durable, protective immune responses. This study provides the first link between protection from disease recurrence and the humoral response to Hla, a virulence factor already implicated in disease pathogenesis. These observations can be utilized to refine ongoing vaccine and immunotherapy efforts and inform the design of clinical trials.

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.

The potential use of toxin antibodies as a strategy for controlling acute Staphylococcus aureus infections

INTRODUCTION

The pandemic human pathogen, Staphylococcus aureus, displays high levels of antibiotic resistance and is a major cause of hospital- and community-associated infections. S. aureus disease manifestation is to a great extent due to the production of a large arsenal of virulence factors, which include a series of secreted toxins. Antibodies to S. aureus toxins are found in people who are infected or asymptomatically colonized with S. aureus. Immunotherapies consisting of neutralizing anti-toxin antibodies could provide immediate aid to patients with impaired immune systems or in advanced stages of disease.

AREAS COVERED

Important S. aureus toxins, their roles in pathogenesis, rationales for selecting S. aureus toxins for immunization efforts, and caveats associated with monoclonal antibody-based passive immunization are discussed. This review will focus on hyper-virulent community-associated methicillin-resistant S. aureus because of their recent surge and clinical importance.

EXPERT OPINION

Antibodies against genome-encoded toxins may be more broadly applicable than those directed against toxins found only in a sub-population of S. aureus isolates. Furthermore, there is substantial functional redundancy among S. aureus toxins. Thus, an optimal anti-S. aureus formulation may consist of multiple antibodies directed against a series of key S. aureus genome-encoded toxins.

CD4 T cell antigens from Staphylococcus aureus Newman strain identified following immunization with heat-killed bacteria

Staphylococcus aureus is a commensal bacterium associated with the skin and mucosal surfaces of humans and animals that can also cause chronic infection. The emergence of antibiotic-resistant strains such as methicillin-resistant S. aureus (MRSA) and strains causing chronic intramammary infections (IMI) in cows results in severe human and livestock infections. Conventional approaches to vaccine development have yielded only a few noneffective vaccines against MRSA or IMI strains, so there is a need for improved vaccine development. CD4 T lymphocytes are required for promoting gamma interferon (IFN-γ) mediated immunoglobulin isotype switching in B lymphocytes to produce high-affinity IgG antibodies and IFN-γ-mediated phagocyte activation for an effective resolution of bacterial infection. However, the lack of known CD4 T cell antigens from S. aureus has made it difficult to design effective vaccines. The goal of this study was to identify S. aureus proteins recognized by immune CD4 T cells. Using a reverse genetics approach, 43 antigens were selected from the S. aureus Newman strain. These included lipoproteins, proteases, transcription regulators, an alkaline shock protein, conserved-domain proteins, hemolysins, fibrinogen-binding protein, staphylokinase, exotoxin, enterotoxin, sortase, and protein A. Screening of expressed proteins for recall T cell responses in outbred, immune calves identified 13 proteins that share over 80% sequence identity among MRSA or IMI strains. These may be useful for inclusion in a broadly protective multiantigen vaccine against MRSA or IMI.

Do differences in Panton-Valentine leukocidin production among international methicillin-resistant Staphylococcus aureus clones affect disease presentation and severity?

Panton-Valentine leukocidin (PVL) production by methicillin-resistant Staphylococcus aureus (MRSA) was determined in vitro using the enzyme-linked immunosorbent assay (ELISA), and associations with clinical presentation and bacterial genetic characteristics were examined. PVL production ranged from 0.02 to 4.865 μg/ml and correlated with a multilocus sequence type (MLST) clonal complex associated with specific PVL phage types. A relationship between PVL production and clinical presentation or patient demographics could not be demonstrated.

Panton-Valentine leukocidin antibodies for the treatment of MRSA skin infections?

Methicillin-resistant Staphylococcus aureus (MRSA) has recently caused epidemic outbreaks of community-associated (CA) skin infections. The infecting strains frequently contain the genes encoding the staphylococcal toxin, Panton-Valentine leukocidin (PVL). On that ground, the use of PVL-based vaccines has been proposed for the treatment of CA-MRSA infections, despite experimental and epidemiological evidence that does not support a major role of the PVL toxin in CA-MRSA skin disease. Hermos et al. show that antibodies to PVL do not protect from CA-MRSA skin infections in children, strongly suggesting that PVL-based immunization is of little benefit for this most frequent disease caused by CA-MRSA.

Panton-Valentine leukocidin does play a role in the early stage of Staphylococcus aureus skin infections: a rabbit model

Despite epidemiological data linking necrotizing skin infections with the production of Panton-Valentine leukocidin (PVL), the contribution of this toxin to the virulence of S. aureus has been highly discussed as a result of inconclusive results of in vivo studies. However, the majority of these results originate from experiments using mice, an animal species which neutrophils - the major target cells for PVL - are highly insensitive to the action of this leukocidin. In contrast, the rabbit neutrophils have been shown to be as sensitive to PVL action as human cells, making the rabbit a better experimental animal to explore the PVL role. In this study we examined whether PVL contributes to S. aureus pathogenicity by means of a rabbit skin infection model. The rabbits were injected intradermally with 10⁸ cfu of either a PVL positive community-associated methicillin-resistant S. aureus isolate, its isogenic PVL knockout or a PVL complemented knockout strain, and the development of skin lesions was observed. While all strains induced skin infection, the wild type strain produced larger lesions and a higher degree of skin necrosis compared to the PVL knockout strain in the first week after the infection. The PVL expression in the rabbits was indirectly confirmed by a raise in the serum titer of anti-LukS-PV antibodies observed only in the rabbits infected with PVL positive strains. These results indicate that the rabbit model is more suitable for studying the role of PVL in staphylococcal diseases than other animal models. Further, they support the epidemiological link between PVL producing S. aureus strains and necrotizing skin infections.

Pragmatic management of Panton-Valentine leukocidin-associated staphylococcal diseases

Panton-Valentine leukocidin (PVL)-producing Staphylococcus aureus is associated with a broad spectrum of diseases, ranging from common uncomplicated soft tissue infections to severe diseases such as complicated soft tissue infections, extensive bone and joint infections, and necrotising pneumonia. Specialised management of infection based on the presence of PVL may not be required for mild infections, whereas it could be lifesaving in other settings. Moreover, most severe PVL diseases are recently identified entities and a 'gold standard' treatment from comparatives studies of different therapeutic options is lacking. Thus, recommendations are based on expert opinions, which are elaborated based on theory, in vitro data and analogies with other toxin-mediated diseases. In this review, we consider the potential need for specialised PVL-based management and, if required, which tools should be used to achieve optimal management.

Future challenges and treatment of Staphylococcus aureus bacteremia with emphasis on MRSA

Staphylococcus aureus bacteremia (SAB) is an urgent medical problem due to its growing frequency and its poor associated outcome. As healthcare delivery increasingly involves invasive procedures and implantable devices, the number of patients at risk for SAB and its complications is likely to grow. Compounding this problem is the growing prevalence of methicillin-resistant S. aureus (MRSA) and the dwindling efficacy of vancomycin, long the treatment of choice for this pathogen. Despite the recent availability of several new antibiotics for S. aureus, new strategies for treatment and prevention are required for this serious, common cause of human infection.

High levels of antibody to panton-valentine leukocidin are not associated with resistance to Staphylococcus aureus-associated skin and soft-tissue infection

BACKGROUND

Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) expressing Panton-Valentine leukocidin (PVL) causes severe skin and soft-tissue infection (SSTI), necrotizing pneumonia, and other invasive infections. The PVL toxin has been implicated as a virulence factor, and antibody to a component of this toxin is under investigation as a vaccine candidate. The role of PVL in pathogenesis remains controversial, and it is unknown whether human serum antibody to PVL modulates infection.

METHODS

We determined antibody levels to PVL in serum samples from children aged 0-18 years presenting with polymerase chain reaction-confirmed, PVL-positive MRSA-associated SSTI (with or without prior MRSA infection or SSTI), PVL-positive MRSA invasive infection, and PVL-negative MRSA infection, as well as uninfected control subjects. We also measured antibody-mediated neutralization of PVL-induced lysis of human polymorphonuclear cells.

RESULTS

Antibody to PVL was present in healthy children reaching adult levels by 4-6 years, with a nadir at 3-11 months likely due to loss of maternal antibody. Children with a primary PVL-positive MRSA infection had moderate levels of antibody to PVL that increased after infection. Children with prior MRSA infection or SSTI had high levels of antibody to PVL after the onset of PVL-positive MRSA infection. There was no increase in antibody to PVL in this population's serum samples after the onset of infection. Serum samples from children with PVL-positive MRSA-associated SSTIs, particularly those with prior MRSA infection or SSTI, and convalescent-phase serum samples from children with invasive PVL-positive MRSA infection potently inhibited PVL-induced lysis of polymorphonuclear cells.

CONCLUSIONS

Neutralizing antibody to PVL does not protect children against primary or recurrent CA-MRSA-associated SSTI.

Animal and human antibodies to distinct Staphylococcus aureus antigens mutually neutralize opsonic killing and protection in mice

New prophylactic approaches are needed to control infection with the Gram-positive bacterium Staphylococcus aureus, which is a major cause of nosocomial and community-acquired infections. To develop these, greater understanding of protective immunity against S. aureus infection is needed. Human immunity to extracellular Gram-positive bacterial pathogens is primarily mediated by opsonic killing (OPK) via antibodies specific for surface polysaccharides. S. aureus expresses two such antigens, capsular polysaccharide (CP) and poly-N-acetyl glucosamine (PNAG). Here, we have shown that immunization-induced polyclonal animal antisera and monoclonal antibodies specific for either CP or PNAG antigens have excellent in vitro OPK activity in human blood but that when mixed together they show potent interference in OPK activity. In addition, reductions in antibody binding to the bacterial surface, complement deposition, and passive protection were seen in two mouse models of S. aureus infection. Electron microscopy, isothermal calorimetry, and surface plasmon resonance indicated that antibodies to CP and PNAG bound together via an apparent idiotype-anti-idiotype interaction. This interaction was also found in sera from humans with S. aureus bacteremia. These findings suggest that the lack of effective immunity to S. aureus infections in humans could be due, in part, to interference in OPK when antibodies to CP and PNAG antigens are both present. This information could be used to better design S. aureus vaccine components.

Epidemiology of non-multiresistant methicillin-resistant Staphylococcus aureus infection in Queensland, Australia: associations with indigenous populations and Panton-Valentine leukocidin

The purpose of this study was to determine the extent of the spread of epidemic clones of non-multiresistant methicillin-resistant Staphylococcus aureus (nmMRSA) and the epidemiology of resultant infections throughout the state of Queensland. We collected a sample of clinical isolates of nmMRSA from laboratories serving public hospitals and clinics throughout the state. Three hundred isolates were typed and tested for the presence of Panton-Valentine leukocidin (PVL) genes and demographic and clinical data were collected from associated cases. Fifteen percent of S. aureus isolates were nmMRSA and 69% of these belonged to PVL-positive clones, predominantly ST93 and CC30. Low numbers of USA300- and USA400-like isolates were also present. Infections due to PVL-positive strains were much less frequently acquired in hospital (3.4%) than those due to PVL-negative nmMRSA (23.7%). Thirty-seven percent of cases were in indigenous people who make up only 3.6% of the general population. The proportion of cases with PVL-positive, but non-negative isolates decreased progressively with age, suggesting that immunity to PVL might be an important determinant of protection. nmMRSA strains are present throughout Queensland and cause infections in both community and healthcare settings.

Rapid detection of Staphylococcus aureus Panton-Valentine leukocidin in clinical specimens by enzyme-linked immunosorbent assay and immunochromatographic tests

Staphylococcus aureus strains producing Panton-Valentine leukocidin (PVL) have been epidemiologically linked to specific human infections. To evaluate immunological tests that may be used to diagnose infections with PVL-producing strains, we prospectively collected pus, respiratory tract specimens, and joint fluid specimens from which S. aureus had been isolated in clinical laboratories in six countries. An enzyme-linked immunosorbent assay (ELISA) and an immunochromatographic test (ICT) targeting LukS-PV were performed directly with clinical samples for the detection of PVL. The same tests were applied to S. aureus culture supernatants. The corresponding S. aureus isolates were characterized by PCR for the presence of the PVL locus (lukS-PV and lukF-PV) and the mecA gene. A total of 185 samples from 144 skin infections, 23 bone and joint infections, and 18 lower respiratory tract infections were analyzed. By PCR, 72/185 S. aureus isolates were PVL locus positive (PVL(+)); 28 of these were also mecA positive. PVL was detected in the supernatants of all PVL(+) strains by both ELISA and an ICT, while no signal was observed with PVL-negative strains. The PVL concentrations in human clinical samples that grew PVL(+) strains ranged from 0 to 399 microg/ml by ELISA. By the use of 0.015 microg/ml of PVL as a cutoff value, PVL was detected in 65/72 (90%) of the clinical samples by ELISA. The sensitivity and specificity of the ELISA test were 90% and 100%, respectively. By the ICT, PVL was detected in 57/72 (79%) of the samples, and the sensitivity and specificity of ICT were 79% and 100%, respectively. PVL is expressed by S. aureus during human infection, and a PVL-specific ELISA and ICT could be reliable tests for the diagnosis of infections caused by PVL-producing strains.

Antibody-mediated enhancement of community-acquired methicillin-resistant Staphylococcus aureus infection

Community-acquired infections caused by methicillin-resistant Staphylococcus aureus (MRSA) expressing the Panton-Valentine leukocidin (PVL) are rampant, but the contribution of PVL to bacterial virulence remains controversial. While PVL is usually viewed as a cytotoxin, at sublytic amounts it activates protective innate immune responses. A leukotoxic effect might predominate in high inoculum studies, whereas protective proinflammatory properties might predominate in settings with lower bacterial inocula that more closely mimic what initially occurs in humans. However, these protective effects might possibly be neutralized by antibodies to PVL, which are found in normal human sera and at increased levels following PVL(+) S. aureus infections. In a low-inoculum murine skin abscess model including a foreign body at the infection site, strains deleted for the pvl genes replicated more efficiently within abscesses than isogenic PVL(+) strains. Coinfection of mice at separate sites with isogenic PVL(+) and PVL(-) MRSA abrogated the differences in bacterial burdens, indicating a systemic effect on host innate immunity from production of PVL. Mice given antibody to PVL and then infected with seven different PVL(+) strains also had significantly higher bacterial counts in abscesses compared with mice given nonimmune serum. Antibody to PVL had no effect on MRSA strains that did not produce PVL. In vitro, antibody to PVL incapacitated PVL-mediated activation of PMNs, indicating that virulence of PVL(+) MRSA is enhanced by the interference of PVL-activated innate immune responses. Given the high rates of primary and recurring MRSA infections in humans, it appears that antibodies to PVL might contribute to host susceptibility to infection.

Geographic distribution of Staphylococcus aureus causing invasive infections in Europe: a molecular-epidemiological analysis

BACKGROUND

Staphylococcus aureus is one of the most important human pathogens and methicillin-resistant variants (MRSAs) are a major cause of hospital and community-acquired infection. We aimed to map the geographic distribution of the dominant clones that cause invasive infections in Europe.

METHODS AND FINDINGS

In each country, staphylococcal reference laboratories secured the participation of a sufficient number of hospital laboratories to achieve national geo-demographic representation. Participating laboratories collected successive methicillin-susceptible (MSSA) and MRSA isolates from patients with invasive S. aureus infection using an agreed protocol. All isolates were sent to the respective national reference laboratories and characterised by quality-controlled sequence typing of the variable region of the staphylococcal spa gene (spa typing), and data were uploaded to a central database. Relevant genetic and phenotypic information was assembled for interactive interrogation by a purpose-built Web-based mapping application. Between September 2006 and February 2007, 357 laboratories serving 450 hospitals in 26 countries collected 2,890 MSSA and MRSA isolates from patients with invasive S. aureus infection. A wide geographical distribution of spa types was found with some prevalent in all European countries. MSSA were more diverse than MRSA. Genetic diversity of MRSA differed considerably between countries with dominant MRSA spa types forming distinctive geographical clusters. We provide evidence that a network approach consisting of decentralised typing and visualisation of aggregated data using an interactive mapping tool can provide important information on the dynamics of MRSA populations such as early signalling of emerging strains, cross border spread, and importation by travel.

CONCLUSIONS

In contrast to MSSA, MRSA spa types have a predominantly regional distribution in Europe. This finding is indicative of the selection and spread of a limited number of clones within health care networks, suggesting that control efforts aimed at interrupting the spread within and between health care institutions may not only be feasible but ultimately successful and should therefore be strongly encouraged.

Panton-valentine leukocidin enhances the severity of community-associated methicillin-resistant Staphylococcus aureus rabbit osteomyelitis

Background

Extensive spread of community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) in the United States, and the concomitant increase in severe invasive staphylococcal infections, including osteomyelitis, in healthy children, has led to renewed interest in Panton-Valentine leukocidin (PVL). However, the pathogenetic role of PVL in staphylococcal infections remains controversial, possibly because it depends on the site of infection.

Methodology/Principal Findings

We compared the course of experimental rabbit osteomyelitis due to the PVL-positive CA-MRSA strain USA 300 (LAC) and its PVL-negative isogenic derivative (LACΔpvl), using a low and a high inoculum (8×10⁵ and 4×10⁸ CFU). With the low inoculum, bone infection was less frequent on day 7 (D7) and day 28 (D28) with LACΔpvl than with LAC (respectively 12/19 and 18/19 animals, p = 0.042). With the high inoculum of both strains, all the animals were infected on D7 and the infection persisted on D28 in almost every case. However, tibial bacterial counts and the serum CRP concentration fell significantly between D7 and D28 with LACΔpvl but not with LAC. Respectively 67% and 60% of LAC-infected rabbits had bone deformation and muscle/joint involvement on D7, compared to 0% and 7% of LACΔpvl-infected rabbits (p = 0.001 and p = 0.005 respectively). Between D0 and D28, the anti-PVL antibody titer increased significantly only with the high inoculum of LAC.

Conclusions/Significance

PVL appears to play a role in the persistence and rapid local extension of rabbit osteomyelitis, in keeping with the greater severity of human bone infections due to PVL-positive S. aureus. The possible therapeutic implications of these findings are discussed.