Immune control of Staphylococcus aureus – Regulation and counter-regulation of the adaptive immune response

Vaccines and monoclonal antibodies to prevent healthcare-associated bacterial infections

SUMMARYHealthcare-associated infections (HAIs) represent a burden for public health with a high prevalence and high death rates associated with them. Pathogens with a high potential for antimicrobial resistance, such as ESKAPE pathogens (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species) and Clostridioides difficile, are responsible for most HAIs. Despite the implementation of infection prevention and control intervention, globally, HAIs prevalence is stable and they are mainly due to endogenous pathogens. It is undeniable that complementary to infection prevention and control measures, prophylactic approaches by active or passive immunization are needed. Specific groups at-risk (elderly people, chronic condition as immunocompromised) and also healthcare workers are key targets. Medical procedures and specific interventions are known to be at risk of HAIs, in addition to hospital environmental exposure. Vaccines or monoclonal antibodies can be seen as attractive preventive approaches for HAIs. In this review, we present an overview of the vaccines and monoclonal antibodies in clinical development for prevention of the major bacterial HAIs pathogens. Based on the current state of knowledge, we look at the challenges and future perspectives to improve prevention by these means.

The extracellular serine protease from Staphylococcus epidermidis elicits a type 2-biased immune response in atopic dermatitis patients

Background

Atopic dermatitis (AD) is a chronic, relapsing inflammatory skin disease with skin barrier defects and a misdirected type 2 immune response against harmless antigens. The skin microbiome in AD is characterized by a reduction in microbial diversity with a dominance of staphylococci, including Staphylococcus epidermidis (S. epidermidis).

Objective

To assess whether S. epidermidis antigens play a role in AD, we screened for candidate allergens and studied the T cell and humoral immune response against the extracellular serine protease (Esp).

Methods

To identify candidate allergens, we analyzed the binding of human serum IgG4, as a surrogate of IgE, to S. epidermidis extracellular proteins using 2-dimensional immunoblotting and mass spectrometry. We then measured serum IgE and IgG1 binding to recombinant Esp by ELISA in healthy and AD individuals. We also stimulated T cells from AD patients and control subjects with Esp and measured the secreted cytokines. Finally, we analyzed the proteolytic activity of Esp against IL-33 and determined the cleavage sites by mass spectrometry.

Results

We identified Esp as the dominant candidate allergen of S. epidermidis. Esp-specific IgE was present in human serum; AD patients had higher concentrations than controls. T cells reacting to Esp were detectable in both AD patients and healthy controls. The T cell response in healthy adults was characterized by IL-17, IL-22, IFN-γ, and IL-10, whereas the AD patients' T cells lacked IL-17 production and released only low amounts of IL-22, IFN-γ, and IL-10. In contrast, Th2 cytokine release was higher in T cells from AD patients than from healthy controls. Mature Esp cleaved and activated the alarmin IL-33.

Conclusion

The extracellular serine protease Esp of S. epidermidis can activate IL-33. As an antigen, Esp elicits a type 2-biased antibody and T cell response in AD patients. This suggests that S. epidermidis can aggravate AD through the allergenic properties of Esp.

Antigen specific activation of cytotoxic CD8+ T cells by Staphylococcus aureus infected dendritic cells

Staphylococcus aureus (S. aureus) is a pathogen associated with a wide variety of diseases, from minor to life-threatening infections. Antibiotic-resistant strains have emerged, leading to increasing concern about the control of S. aureus infections. The development of vaccines may be one way to overcome these resistant strains. However, S. aureus ability to internalize into cells - and thus to form a reservoir escaping humoral immunity - is a challenge for vaccine development. A role of T cells in the elimination of persistent S. aureus has been established in mice but it remains to be established if CD8+ T cells could display a cytotoxic activity against S. aureus infected cells. We examined in vitro the ability of CD8+ T cells to recognize and kill dendritic cells infected with S. aureus. We first evidenced that both primary mouse dendritic cells and DC2.4 cell line can be infected with S. aureus. We then generated a strain of S. aureus expressing a model CD8 epitope and transgenic F5 CD8+ T cells recognizing this model epitope were used as reporter T cells. In response to S. aureus-infected dendritic cells, F5 CD8+ T cells produced IFN-γ in an antigen-specific manner and displayed an increased ability to kill infected cells. Altogether, these results demonstrate that cells infected by S. aureus display bacteria-derived epitopes at their surface that are recognized by CD8+ T cells. This paves the way for the development of CD8+ T cell-based therapies against S. aureus.

Novel antimicrobial strategies to treat multi-drug resistant Staphylococcus aureus infections

Antimicrobial resistance is a major obstacle for the treatment of infectious diseases and currently represents one of the most significant threats to global health. Staphylococcus aureus remains a formidable human pathogen with high mortality rates associated with severe systemic infections. S. aureus has become notorious as a multidrug resistant bacterium, which when combined with its extensive arsenal of virulence factors that exacerbate disease, culminates in an incredibly challenging pathogen to treat clinically. Compounding this major health issue is the lack of antibiotic discovery and development, with only two new classes of antibiotics approved for clinical use in the last 20 years. Combined efforts from the scientific community have reacted to the threat of dwindling treatment options to combat S. aureus disease in several innovative and exciting developments. This review describes current and future antimicrobial strategies aimed at treating staphylococcal colonization and/or disease, examining therapies that show significant promise at the preclinical development stage to approaches that are currently being investigated in clinical trials.

Exploring epigenetic reprogramming during central nervous system infection

Epigenetics involves the study of various modes of adaptable transcriptional regulation, contributing to cell identity, characteristics, and function. During central nervous system (CNS) infection, epigenetic mechanisms can exert pronounced control over the maturation and antimicrobial properties of nearly every immune cell type. Epigenetics is a relatively new field, with the first mention of these marks proposed only a half-century ago and a substantial body of immunological epigenetic research emerging only in the last few decades. Here, we review the best-characterized epigenetic marks and their functions as well as illustrate how various immune cell populations responding to CNS infection utilize these marks to organize their activation state and inflammatory processes. We also discuss the metabolic and clinical implications of epigenetic marks and the rapidly expanding set of tools available to researchers that are enabling elucidation of increasingly detailed genetic regulatory pathways. These considerations paint an intricate picture of inflammatory regulation, where epigenetic marks influence genetic, signaling, and environmental elements to orchestrate a tailored immunological response to the threat at hand, cementing epigenetics as an important player in immunity.

Molecular Targets for Antibody-Based Anti-Biofilm Therapy in Infective Endocarditis

Infective endocarditis (IE) is a heart disease caused by the infection of heart valves, majorly caused by Staphilococcus aureus. IE is initiated by bacteria entering the blood circulation in favouring conditions (e.g., during invasive procedures). So far, the conventional antimicrobial strategies based on the usage of antibiotics remain the major intervention for treating IE. Nevertheless, the therapeutic efficacy of antibiotics in IE is limited not only by the bacterial drug resistance, but also by the formation of biofilms, which resist the penetration of antibiotics into bacterial cells. To overcome these drawbacks, the development of anti-biofilm treatments that can expose bacteria and make them more susceptible to the action of antibiotics, therefore resulting in reduced antimicrobial resistance, is urgently required. A series of anti-biofilm strategies have been developed, and this review will focus in particular on the development of anti-biofilm antibodies. Based on the results previously reported in the literature, several potential anti-biofilm targets are discussed, such as bacterial adhesins, biofilm matrix and bacterial toxins, covering their antigenic properties (with the identification of potential promising epitopes), functional mechanisms, as well as the antibodies already developed against these targets and, where feasible, their clinical translation.

Vaccines for healthcare associated infections without vaccine prevention to date

In spite of the widespread implementation of preventive strategies, the prevalence of healthcare-associated infections (HAIs) remains high. The prevalence of multidrug resistant organisms is high in HAIs. In 2019, the World Health Organization retained antimicrobial resistance as one of the ten issues for global health. The development of vaccines may contribute to the fight against antimicrobial resistance to reduce the burden of HAIs. Staphylococcus aureus, Gram negative bacteria and Clostridium difficile are the most frequent pathogens reported in HAIs. Consequently, the development of vaccines against these pathogens is crucial. At this stage, the goal of obtaining effective vaccines against S.aureus and Gram negative bacteria has not yet been achieved. However, we can expect in the near future availability of a vaccine against C. difficile. In addition, identifying populations who may benefit from these vaccines is complex, as at-risk patients are not great responders to vaccines, or as vaccination may occur too late, when they are already confronted to the risk. Vaccinating healthcare workers (HCWs) against these pathogens may have an impact only if HCWs play a role in the transmission and in the pathogens acquisition in patients, if the vaccine is effective to reduce pathogens carriage and if vaccine coverage is sufficient to protect patients. Acceptance of these potential vaccines should be evaluated and addressed in patients and in HCWs.

S. aureus and IgE-mediated diseases: pilot or copilot? A narrative review

INTRODUCTION

S. aureus is a major opportunistic pathogen that has been implicated in the pathogenesis of several chronic inflammatory diseases including bronchial asthma, chronic rhinosinusitis with nasal polyps (CRSwNP), chronic spontaneous urticaria (CSU), and atopic dermatitis. S. aureus can induce the production of both polyclonal and specific IgE that can elicit an inflammatory cascade.

AREAS COVERED

The link between the sensitization to S. aureus enterotoxins and the severity of several chronic inflammatory diseases is reviewed in detail, as well as its therapeutic implications.

EXPERT OPINION

An anti-IgE strategy to inhibit S. aureus enterotoxins would be a valid approach to treat several endotypes of severe asthma, CRSwNP and CSU in which IgE against S. aureus enterotoxins should represent, not only a marker of severity of the diseases but also a target of a treatment.

Progress towards the Elusive Mastitis Vaccines

Mastitis is a major problem in dairy farming. Vaccine prevention of mammary bacterial infections is of particular interest in helping to deal with this issue, all the more so as antibacterial drug inputs in dairy farms must be reduced. Unfortunately, the effectiveness of current vaccines is not satisfactory. In this review, we examine the possible reasons for the current shortcomings of mastitis vaccines. Some reasons stem from the peculiarities of the mammary gland immunobiology, others from the pathogens adapted to the mammary gland niche. Infection does not induce sterilizing protection, and recurrence is common. Efficacious vaccines will have to elicit immune mechanisms different from and more effective than those induced by infection. We propose focusing our research on a few points pertaining to either the current immune knowledge or vaccinology approaches to get out of the current deadlock. A possible solution is to focus on the contribution of cell-mediated immunity to udder protection based on the interactions of T cells with the mammary epithelium. On the vaccinology side, studies on the orientation of the immune response by adjuvants, the route of vaccine administration and the delivery systems are among the keys to success.

Vancomycin-decorated microbubbles as a theranostic agent for Staphylococcus aureus biofilms

Bacterial biofilms are a huge burden on our healthcare systems worldwide. The lack of specificity in diagnostic and treatment possibilities result in difficult-to-treat and persistent infections. The aim of this in vitro study was to investigate if microbubbles targeted specifically to bacteria in biofilms could be used both for diagnosis as well for sonobactericide treatment and demonstrate their theranostic potential for biofilm infection management. The antibiotic vancomycin was chemically coupled to the lipid shell of microbubbles and validated using mass spectrometry and high-axial resolution 4Pi confocal microscopy. Theranostic proof-of-principle was investigated by demonstrating the specific binding of vancomycin-decorated microbubbles (vMB) to statically and flow grown Staphylococcus aureus (S. aureus) biofilms under increasing shear stress flow conditions (0-12 dyn/cm²), as well as confirmation of microbubble oscillation and biofilm disruption upon ultrasound exposure (2 MHz, 250 kPa, and 5,000 or 10,000 cycles) during flow shear stress of 5 dyn/cm² using time-lapse confocal microscopy combined with the Brandaris 128 ultra-high-speed camera. Vancomycin was successfully incorporated into the microbubble lipid shell. vMB bound significantly more often than control microbubbles to biofilms, also in the presence of free vancomycin (up to 1000 µg/mL) and remained bound under increasing shear stress flow conditions (up to 12 dyn/cm²). Upon ultrasound insonification biofilm area was reduced of up to 28%, as confirmed by confocal microscopy. Our results confirm the successful production of vMB and support their potential as a new theranostic tool for S. aureus biofilm infections by allowing for specific bacterial detection and biofilm disruption.

Superantigen Recognition and Interactions: Functions, Mechanisms and Applications

Superantigens are unconventional antigens which recognise immune receptors outside their usual recognition sites e.g. complementary determining regions (CDRs), to elicit a response within the target cell. T-cell superantigens crosslink T-cell receptors and MHC Class II molecules on antigen-presenting cells, leading to lymphocyte recruitment, induction of cytokine storms and T-cell anergy or apoptosis among many other effects. B-cell superantigens, on the other hand, bind immunoglobulins on B-cells, affecting opsonisation, IgG-mediated phagocytosis, and driving apoptosis. Here, through a review of the structural basis for recognition of immune receptors by superantigens, we show that their binding interfaces share specific physicochemical characteristics when compared with other protein-protein interaction complexes. Given that antibody-binding superantigens have been exploited extensively in industrial antibody purification, these observations could facilitate further protein engineering to optimize the use of superantigens in this and other areas of biotechnology.

Discovery of Staphylococcus aureus Adhesion Inhibitors by Automated Imaging and Their Characterization in a Mouse Model of Persistent Nasal Colonization

Due to increasing mupirocin resistance, alternatives for Staphylococcus aureus nasal decolonization are urgently needed. Adhesion inhibitors are promising new preventive agents that may be less prone to induce resistance, as they do not interfere with the viability of S. aureus and therefore exert less selection pressure. We identified promising adhesion inhibitors by screening a library of 4208 compounds for their capacity to inhibit S. aureus adhesion to A-549 epithelial cells in vitro in a novel automated, imaging-based assay. The assay quantified DAPI-stained nuclei of the host cell; attached bacteria were stained with an anti-teichoic acid antibody. The most promising candidate, aurintricarboxylic acid (ATA), was evaluated in a novel persistent S. aureus nasal colonization model using a mouse-adapted S. aureus strain. Colonized mice were treated intranasally over 7 days with ATA using a wide dose range (0.5–10%). Mupirocin completely eliminated the bacteria from the nose within three days of treatment. In contrast, even high concentrations of ATA failed to eradicate the bacteria. To conclude, our imaging-based assay and the persistent colonization model provide excellent tools to identify and validate new drug candidates against S. aureus nasal colonization. However, our first tested candidate ATA failed to induce S. aureus decolonization.

Analysis of Staphylococcus aureus Transcriptome in Pediatric Soft Tissue Abscesses and Comparison to Murine Infections

A comprehensive understanding of how Staphylococcus aureus adapts to cause infections in humans can inform development of diagnostic, therapeutic, and preventive approaches. Expression analysis of clinical strain libraries depicts in vitro conditions that differ from those in human infection, but low bacterial burden and the requirement for reverse transcription or nucleic acid amplification complicate such analyses of bacteria causing human infection. We developed methods to evaluate the mRNA transcript signature of S. aureus in pediatric skin and soft tissue infections (SSTI) directly ex vivo Abscess drainage from 47 healthy pediatric patients undergoing drainage of a soft tissue infection was collected, and RNA was extracted from samples from patients with microbiologically confirmed S. aureus abscesses (42% due to methicillin-resistant S. aureus [MRSA]). Using the NanoString platform and primers targeting S. aureus mRNA transcripts encoding surface-expressed or secreted proteins, we measured direct counts of 188 S. aureus mRNA transcripts in abscess drainage. We further evaluated this mRNA signature in murine models of S. aureus SSTI and nasal colonization where the kinetics of the transcriptome could be determined. Heat maps of the S. aureus mRNA signatures from pediatric abscesses demonstrated consistent per-target expression across patients. While there was significant overlap with the profiles from murine SSTI and nasal colonization, important differences were noted, which can inform efforts to develop therapeutic and vaccine approaches.

Disruption of Phosphate Homeostasis Sensitizes Staphylococcus aureus to Nutritional Immunity

To control infection, mammals actively withhold essential nutrients, including the transition metal manganese, by a process termed nutritional immunity. A critical component of this host response is the manganese-chelating protein calprotectin. While many bacterial mechanisms for overcoming nutritional immunity have been identified, the intersection between metal starvation and other essential inorganic nutrients has not been investigated. Here, we report that overexpression of an operon encoding a highly conserved inorganic phosphate importer, PstSCAB, increases the sensitivity of Staphylococcus aureus to calprotectin-mediated manganese sequestration. Further analysis revealed that overexpression of pstSCAB does not disrupt manganese acquisition or result in overaccumulation of phosphate by S. aureus However, it does reduce the ability of S. aureus to grow in phosphate-replete defined medium. Overexpression of pstSCAB does not aberrantly activate the phosphate-responsive two-component system PhoPR, nor was this two-component system required for sensitivity to manganese starvation. In a mouse model of systemic staphylococcal disease, a pstSCAB-overexpressing strain is significantly attenuated compared to wild-type S. aureus This defect is partially reversed in a calprotectin-deficient mouse, in which manganese is more readily available. Given that expression of pstSCAB is regulated by PhoPR, these findings suggest that overactivation of PhoPR would diminish the ability of S. aureus to resist nutritional immunity and cause infection. As PhoPR is also necessary for bacterial virulence, these findings imply that phosphate homeostasis represents a critical regulatory node whose activity must be precisely controlled in order for S. aureus and other pathogens to cause infection.

Staphylococcus aureus and its IgE-inducing enterotoxins in asthma: current knowledge

While immunoglobulin (Ig) E is a prominent biomarker for early-onset, its levels are often elevated in non-allergic late-onset asthma. However, the pattern of IgE expression in the latter is mostly polyclonal, with specific IgEs low or below detection level albeit with an increased total IgE. In late-onset severe asthma patients, specific IgE to Staphylococcal enterotoxins (se-IgE) can frequently be detected in serum, and has been associated with asthma, with severe asthma defined by hospitalisations, oral steroid use and decrease in lung function. Recently, se-IgE was demonstrated to even predict the development into severe asthma with exacerbations over the next decade. Staphylococcus aureus manipulates the airway mucosal immunology at various levels via its proteins, including superantigens, serine-protease-like proteins (Spls), or protein A (SpA) and possibly others. Release of IL-33 from respiratory epithelium and activation of innate lymphoid cells (ILCs) via its receptor ST2, type 2 cytokine release from those ILCs and T helper (Th) 2 cells, mast cell degranulation, massive local B-cell activation and IgE formation, and finally eosinophil attraction with consequent release of extracellular traps, adding to the epithelial damage and contributing to disease persistence via formation of Charcot-Leyden crystals are the most prominent hallmarks of the manipulation of the mucosal immunity by S. aureus In summary, S. aureus claims a prominent role in the orchestration of severe airway inflammation and in current and future disease severity. In this review, we discuss current knowledge in this field and outline the needs for future research to fully understand the impact of S. aureus and its proteins on asthma.

Allergy-A New Role for T Cell Superantigens of Staphylococcus aureus?

Staphylococcus aureus superantigens (SAgs) are among the most potent T cell mitogens known. They stimulate large fractions of T cells by cross-linking their T cell receptor with major histocompatibility complex class-II molecules on antigen presenting cells, resulting in T cell proliferation and massive cytokine release. To date, 26 different SAgs have been described in the species S. aureus; they comprise the toxic shock syndrome toxin (TSST-1), as well as 25 staphylococcal enterotoxins (SEs) or enterotoxin-like proteins (SEls). SAgs can cause staphylococcal food poisoning and toxic shock syndrome and contribute to the clinical symptoms of staphylococcal infection. In addition, there is growing evidence that SAgs are involved in allergic diseases. This review provides an overview on recent epidemiological data on the involvement of S. aureus SAgs and anti-SAg-IgE in allergy, demonstrating that being sensitized to SEs—in contrast to inhalant allergens—is associated with a severe disease course in patients with chronic airway inflammation. The mechanisms by which SAgs trigger or amplify allergic immune responses, however, are not yet fully understood. Here, we discuss known and hypothetical pathways by which SAgs can drive an atopic disease.

Sonobactericide: An Emerging Treatment Strategy for Bacterial Infections

Ultrasound has been developed as both a diagnostic tool and a potent promoter of beneficial bio-effects for the treatment of chronic bacterial infections. Bacterial infections, especially those involving biofilm on implants, indwelling catheters and heart valves, affect millions of people each year, and many deaths occur as a consequence. Exposure of microbubbles or droplets to ultrasound can directly affect bacteria and enhance the efficacy of antibiotics or other therapeutics, which we have termed sonobactericide. This review summarizes investigations that have provided evidence for ultrasound-activated microbubble or droplet treatment of bacteria and biofilm. In particular, we review the types of bacteria and therapeutics used for treatment and the in vitro and pre-clinical experimental setups employed in sonobactericide research. Mechanisms for ultrasound enhancement of sonobactericide, with a special emphasis on acoustic cavitation and radiation force, are reviewed, and the potential for clinical translation is discussed.

Staphylococcus aureus colonization in hemodialysis patients: a prospective 25 months observational study

Background

Dialysis patients are frequently exposed to Staphylococcus aureus due to stays in dialysis centers, hospitals or rest homes. The hemodialysis vascular access is a potential entry site for S. aureus, in particular when using a central venous catheter (CVC) which increases the risk of sepsis compared to arteriovenous (AV) fistula. We prospectively followed a cohort of 86 hemodialysis patients from an outpatient dialysis center over 25 months analyzing S. aureus carrier status, S. aureus infection rates and mortality.

Methods

Demographic data and patients´ medical histories were collected and followed from all hemodialysis patients. Blood samples, nasal swabs and swabs from the hemodialysis vascular access site were taken every six months for a period of 25 months and tested for S. aureus. Strains were cultured and further characterized by spa PCR and microarray-based genotyping. Resulting data were compared with those from the general population.

Results

In cross-sectional analyses, an average of 40% of hemodialysis patients were S. aureus carriers compared to 27% in the general population. Longitudinally, a total of 65% were S. aureus carriers: 16% were persistent carriers, 43% were intermittently colonized. The most common S. aureus lineage in the dialysis patient cohort was the clonal complex (CC) 8 and the spa type t008, while in the general population, the clonal complex CC30 dominates. During the study period, we observed six S. aureus-associated blood stream infections with one S. aureus attributable death. S. aureus carriers with an AV fistula were more densely colonized in the nasal mucosa compared to patients with a CVC. Overall mortality was lower for hemodialysis patients with a positive S. aureus carrier status compared to non-carriers (hazard ratio of 0.19).

Conclusions

Compared to the general population, hemodialysis patients were more frequently colonized with S. aureus and displayed both different S. aureus colonization densities as well as lineages, possibly explained by more frequent exposure to health care environments. The lower overall mortality in carriers compared to non-carriers is intriguing and will be investigated in detail in the future.

Trial registration

ISRCTN 14385893, 2. October 2018, retrospectively registered.

Electronic supplementary material

The online version of this article (10.1186/s12882-019-1332-z) contains supplementary material, which is available to authorized users.

Staphylococcus aureus and Host Immunity in Recurrent Furunculosis

Staphylococcus aureus is one of the severest and most persistent bacterial pathogens. The most frequent S. aureus infections include impetigo, folliculitis, furuncles, furunculosis, abscesses, hidradenitis suppurativa, and mastitis. S. aureus produces a great variety of cellular and extracellular factors responsible for its invasiveness and ability to cause pathological lesions. Their expression depends on the growth phase, environmental factors, and location of the infection. Susceptibility to staphylococcal infections is rooted in multiple mechanisms of host immune responses and reactions to bacterial colonization. Immunological and inflammatory processes of chronic furunculosis are based on the pathogenicity of S. aureus as well as innate and acquired immunity. In-depth knowledge about them may help to discover the whole pathomechanism of the disease and to develop effective therapeutic options. In this review, we focus on the S. aureus-host immune interactions in the pathogenesis of recurrent furunculosis according to the most recent experimental and clinical findings.

Fighting Staphylococcus aureus Biofilms with Monoclonal Antibodies

Staphylococcus aureus (S. aureus) is a notorious pathogen and one of the most frequent causes of biofilm-related infections. The treatment of S. aureus biofilms is hampered by the ability of the biofilm structure to shield bacteria from antibiotics as well as the host's immune system. Therefore, new preventive and/or therapeutic interventions, including the use of antibody-based approaches, are urgently required. In this review, we describe the mechanisms by which anti-S. aureus antibodies can help in combating biofilms, including an up-to-date overview of monoclonal antibodies currently in clinical trials. Moreover, we highlight ongoing efforts in passive vaccination against S. aureus biofilm infections, with special emphasis on promising targets, and finally indicate the direction into which future research could be heading.

Messing with the Sentinels-The Interaction of Staphylococcus aureus with Dendritic Cells

Staphylococcus aureus (S. aureus) is a dangerous pathogen as well as a frequent colonizer, threatening human health worldwide. Protection against S. aureus infection is challenging, as the bacteria have sophisticated strategies to escape the host immune response. To maintain equilibrium with S. aureus, both innate and adaptive immune effector mechanisms are required. Dendritic cells (DCs) are critical players at the interface between the two arms of the immune system, indispensable for inducing specific T cell responses. In this review, we highlight the importance of DCs in mounting innate as well as adaptive immune responses against S. aureus with emphasis on their role in S. aureus-induced respiratory diseases. We also review what is known about mechanisms that S. aureus has adopted to evade DCs or manipulate these cells to its advantage.

Manipulation of Innate and Adaptive Immunity by Staphylococcal Superantigens

Staphylococcal superantigens (SAgs) constitute a family of potent exotoxins secreted by Staphylococcus aureus and other select staphylococcal species. SAgs function to cross-link major histocompatibility complex (MHC) class II molecules with T cell receptors (TCRs) to stimulate the uncontrolled activation of T lymphocytes, potentially leading to severe human illnesses such as toxic shock syndrome. The ubiquity of SAgs in clinical S. aureus isolates suggests that they likely make an important contribution to the evolutionary fitness of S. aureus. Although the apparent redundancy of SAgs in S. aureus has not been explained, the high level of sequence diversity within this toxin family may allow for SAgs to recognize an assorted range of TCR and MHC class II molecules, as well as aid in the avoidance of humoral immunity. Herein, we outline the major diseases associated with the staphylococcal SAgs and how a dysregulated immune system may contribute to pathology. We then highlight recent research that considers the importance of SAgs in the pathogenesis of S. aureus infections, demonstrating that SAgs are more than simply an immunological diversion. We suggest that SAgs can act as targeted modulators that drive the immune response away from an effective response, and thus aid in S. aureus persistence.

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.

Human Immunoglobulin G Cannot Inhibit Fibrinogen Binding by the Genetically Diverse A Domain of Staphylococcus aureus Fibronectin-Binding Protein A

The fibronectin-binding protein A (FnBPA) is a cell surface-associated protein of Staphylococcus aureus which mediates adherence to the host extracellular matrix and is important for bacterial virulence. Previously, substantial sequence diversity was found among strains in the fibrinogen-binding A domain of this protein, and 7 different isotypes were described. The effect of this sequence diversity on the human antibody response, in terms of both antibody production and antibody function, remains unclear. In this study, we identify five different FnBPA A domain isotypes based on the sequence results of 22 clinical S. aureus isolates, obtained from the same number of patients suffering from bacteremia. Using a bead-based Luminex technique, we measure the patients' total immunoglobulin G (IgG) against the 7 FnBPA isotypes at the onset and during the time course of bacteremia (median of 10 serum samples per patient over a median of 35 days). A significant increase in IgG against the FnBPA A domain, including the isotype carried by the infecting strain, is observed in only three out of 22 patients (14%) after the onset of bacteremia. Using a Luminex-based FnBPA-fibrinogen-binding assay, we find that preincubation of recombinant FnBPA isotypes with IgG from diverse patients does not interfere with binding to fibrinogen. This observation is confirmed using an alternative Luminex-based assay and enzyme-linked immunosorbent assay (ELISA). IMPORTANCE Despite the many in vitro and murine in vivo studies involving FnBPA, the actual presence of this virulence factor during human infection is less well established. Furthermore, it is currently unknown to what extent sequence variation in such a virulence factor affects the human antibody response and the ability of antibodies to interfere with FnBPA function. This study sheds new light on these issues. First, the uniform presence of a patient's IgG against FnBPA indicates the presence and importance of this virulence factor during S. aureus pathogenesis. Second, the absence of an increase in antibody production in most patients following bacteremia indicates the complexity of S. aureus-host interactions, possibly involving immune evasion or lack of expression of FnBPA during invasive infection. Finally, we provide new insights into the inability of human antibodies to interfere with FnBPA-fibrinogen binding. These observations should be taken into account during the development of novel vaccination approaches.

Expression of Tumor Necrosis Factor Receptor 2 Characterizes TLR9-Driven Formation of Interleukin-10-Producing B Cells

B cell-derived interleukin-10 (IL-10) production has been described as a hallmark for regulatory function in B lymphocytes. However, there is an ongoing debate on the origin of IL-10-secreting B cells and lack of specific surface markers has turned into an important obstacle for studying human B regulatory cells. In this study, we propose that tumor necrosis factor receptor 2 (TNFR2) expression can be used for enrichment of IL-10-secreting B cells. Our data confirm that IL-10 production can be induced by TLR9 stimulation with CpG ODN and that IL-10 secretion accompanies differentiation of peripheral blood B cells into plasma blasts. We further show that CpG ODN stimulation induces TNFR2 expression, which correlates with IL-10 secretion and terminal differentiation. Indeed, flow cytometric sorting of TNFR2⁺ B cells revealed that TNFR2⁺ and TNFR2⁻ fractions correspond to IL-10⁺ and IL-10⁻ fractions, respectively. Furthermore, CpG-induced TNFR2⁺ B cells were predominantly found in the IgM⁺ CD27⁺ B cell subset and spontaneously released immunoglobulin. Finally, our data corroborate the functional impact of TNFR2 by demonstrating that stimulation with a TNFR2 agonist significantly augments IL-10 and IL-6 production in B cells. Altogether, our data highlight a new role for TNFR2 in IL-10-secreting human B lymphocytes along with the potential to exploit this finding for sorting and isolation of this currently ill-defined B cell subset.

Les vaccins dans la prévention des infections associées aux soins

Les infections associées aux soins (IAS) constituent un véritable problème de santé publique. Escherichia coli, Staphylococcus aureus, Clostridium difficile sont les plus souvent à l’origine des IAS. L’antibiorésistance fréquente complique encore la prise en charge et des impasses thérapeutiques existent à présent. Les mesures d’hygiène hospitalière bien qu’essentielles sont insuffisantes pour diminuer drastiquement les IAS. Ainsi, des stratégies alternatives à l’antibiothérapie s’avèrent nécessaires pour prévenir et traiter les IAS. Parmi celles-ci, la vaccination et l’immunisation passive sont probablement les plus prometteuses. Nous avons fait une mise au point sur les vaccins disponibles et en développement clinique pour lutter contre les IAS, chez les patients à risque d’IAS et les soignants. L’intérêt de la vaccination grippale et rotavirus chez les patients pour prévenir ces IAS virales a été examiné. Le développement d’un vaccin anti-S. aureus, déjà émaillé de 2 échecs est complexe. Toutefois, ces échecs ont permis d’améliorer les connaissances sur l’immunité anti-S. aureus. La mise à disposition d’un vaccin préventif anti-C. difficile semble plus proche. Pour les autres bactéries gram négatif responsables d’IAS, le développement est moins avancé. La vaccination des patients à risques d’IAS pose également des problèmes de réponse vaccinale qu’il faudra résoudre pour utiliser cette stratégie. Ainsi, la vaccination des soignants, de par l’effet de groupe permet également de prévenir les IAS. Nous faisons ici le point sur l’intérêt de la vaccination des soignants contre la rougeole, la coqueluche, la grippe, la varicelle, l’hépatite B pour réduire les IAS avec des vaccins déjà disponibles.

Staphylococcus aureus pathogenesis in diverse host environments

Staphylococcus aureus is an eminent human pathogen that can colonize the human host and cause severe life-threatening illnesses. This bacterium can reside in and infect a wide range of host tissues, ranging from superficial surfaces like the skin to deeper tissues such as in the gastrointestinal tract, heart and bones. Due to its multifaceted lifestyle, S. aureus uses complex regulatory networks to sense diverse signals that enable it to adapt to different environments and modulate virulence. In this minireview, we explore well-characterized environmental and host cues that S. aureus responds to and describe how this pathogen modulates virulence in response to these signals. Lastly, we highlight therapeutic approaches undertaken by several groups to inhibit both signaling and the cognate regulators that sense and transmit these signals downstream.

Global spread of mouse-adapted Staphylococcus aureus lineages CC1, CC15, and CC88 among mouse breeding facilities

We previously reported that laboratory mice from all global vendors are frequently colonized with Staphylococcus aureus (S. aureus). Genotyping of a snap sample of murine S. aureus isolates from Charles River, US, showed that mice were predominantly colonized with methicillin-sensitive CC88 strains. Here, we expanded our view and investigated whether laboratory mice from other global animal facilities are colonized with similar strains or novel S. aureus lineages, and whether the murine S. aureus isolates show features of host adaptation. In total, we genotyped 230 S. aureus isolates from various vendor facilities of laboratory mice around the globe (Charles River facilities in the USA, Canada, France, and Germany; another US facility) and university- or company-associated breeding facilities in Germany, China and New Zealand. Spa typing was performed to analyse the clonal relationship of the isolates. Moreover, multiplex PCRs were performed for human-specific virulence factors, the immune-evasion cluster (IEC) and superantigen genes (SAg). We found a total of 58 different spa types that clustered into 15 clonal complexes (CCs). Three of these S. aureus lineages had spread globally among laboratory mice and accounted for three quarters of the isolates: CC1 (13.5%), CC15 (14.3%), and CC88 (47.0%). Compared to human colonizing isolates of the same lineages, the murine isolates frequently lacked IEC genes and SAg genes on mobile genetic elements, implying long-term adaptation to the murine host. In conclusion, laboratory mice from various vendors are colonized with host-adapted S. aureus-strains of a few lineages, predominantly the CC88 lineage. S. aureus researchers must be cautioned that S. aureus colonization might be a relevant confounder in infection and vaccination studies and are therefore advised to screen their mice before experimentation.

Knowledge gaps and research priorities in Staphylococcus aureus mastitis control

This study assessed knowledge gaps and suggested research priorities in the field of Staphylococcus aureus mastitis. Staphylococcus aureus infecting the mammary gland remains a major problem to the dairy industry worldwide because of its pathogenicity, contagiousness, persistence in the cow environment, colonization of skin or mucosal epithelia, and the poor curing efficacy of treatments. Staphylococcus aureus also constitutes a threat to public health due to food safety and antibiotic usage issues and the potential for bidirectional transmission of strains between humans and dairy animals (cows and small ruminants). Gaps have been identified in (i) understanding the molecular basis for pathogenesis of S. aureus mastitis, (ii) identifying staphylococcal antigens inducing protection and (iii) determining the cell-mediated immune responses to infection and vaccination. The recommended priorities for research are (i) improved diagnostic methods for early detection of infection and intervention through treatment or management, (ii) development of experimental models to investigate the strategies used by S. aureus to survive within the mammary gland and resist treatment with anti-microbials, (iii) investigation of the basis for cow-to-cow variation in response to S. aureus mastitis, (iv) identification of the immune responses (adaptive and innate) induced by infection or vaccination and (v) antibacterial discovery programmes to develop new, more effective, narrow spectrum antibacterial agents for the treatment of S. aureus mastitis. With the availability and ongoing improvement of molecular research tools, these objectives may not be out of reach in the future.

Mixed Lactobacillus plantarum Strains Inhibit Staphylococcus aureus Induced Inflammation and Ameliorate Intestinal Microflora in Mice

Objective. Staphylococcus aureus is an important pathogen that causes intestinal infection. We examined the immunomodulatory function of single and mixed Lactobacillus plantarum strains, as well as their impacts on the structure of the microbiome in mice infected with Staphylococcus aureus. The experiment was divided into three groups: protection, treatment, and control. Serum IFN-γ and IL-4 levels, as well as intestinal sIgA levels, were measured during and 1 week after infection with Staphylococcus aureus with and without Lactobacillus plantarum treatment. We used 16s rRNA tagged sequencing to analyze microbiome composition. IFN-γ/IL-4 ratio decreased significantly from infection to convalescence, especially in the mixed Lactobacillus plantarum group. In the mixed Lactobacillus plantarum group the secretion of sIgA in the intestine of mice (9.4-9.7 ug/mL) was significantly higher than in the single lactic acid bacteria group. The dominant phyla in mice are Firmicutes, Bacteroidetes, and Proteobacteria. Treatment with mixed lactic acid bacteria increased the anti-inflammatory factor and the secretion of sIgA in the intestine of mice infected with Staphylococcus aureus and inhibited inflammation.

Immunoglobulins and their receptors, and subversion of their protective roles by bacterial pathogens

Immunoglobulins (Igs) play critical roles in immune defence against infectious disease. They elicit potent elimination processes such as triggering complement activation and engaging specific Fc receptors present on immune cells, resulting in phagocytosis and other killing mechanisms. Many important pathogens have evolved mechanisms to subvert or evade Ig-mediated defence. One such mechanism used by several pathogenic bacteria features proteins that bind the Ig Fc region and compromise engagement of host effector molecules. Examples include different IgA-binding proteins produced by Staphylococcus aureus, Streptococcus pyogenes, and group B streptococci, all of which interact with the same interdomain region on IgA Fc. Since this region also forms the interaction site for the major human IgA-specific Fc receptor CD89, the bacteria are able to evade CD89-mediated clearance mechanisms. Similar disruption of Ig effector function by pathogen Ig-binding proteins is evident in other species. Remarkably, all the Ig-binding proteins studied in detail to date are seen to target the CH2-CH3 domain interface in the Ig Fc region, suggesting a common mode of immune evasion. A second Ig subversion mechanism that has evolved independently in numerous pathogens involves proteases that cleave Ig molecules within their hinge regions, uncoupling the antigen recognition capability of the Fab region from clearance mechanisms elicited by the Fc region. The emerging understanding of the structural basis for the recognition of Igs as substrates for these proteases and as interaction partners for Ig-binding proteins may open up new avenues for treatment or vaccination.

Photodynamic therapy controls of Staphylococcus aureus intradermal infection in mice

Infections caused by Staphylococcus aureus lead to skin infections, as well as soft tissues and bone infections. Given the communal resistance to antibiotics developed by strains of this bacterium, photodynamic therapy emerges as a promising alternative treatment to control and cure infections. Females of the Balb/C mice were infected with 10⁸ CFU of methicillin-resistant S. aureus (MRSA) and divided into four distinct groups: P-L- (negative control group), P+L- (group exposed only to curcumin), P-L+ (group exposed only to LED incidence of 450 nm, 75 mW/cm², and 54 J/cm² for 10 min), and P+L+ (group exposed to curcumin followed by 10 min of LED irradiation) (n = 24). The mice were euthanized 48 and 72 h after infection, and biologic materials were collected for analysis of the bacterial load, peripheral blood leukocyte counts, and draining lymph nodes cell counts. The normalization of data was checked and the ANOVA test was applied. The bacterial load in the draining lymph node of P+L+ group was lower when compared to the control groups 72 h post infection (p < 0.0001), indicating that the LED incidence associated with curcumin controls of the staphylococci intradermal infection. The number of the total lymph node cells shows to be lower than control groups in the two availed times (p < 0.01). The histological analysis and the counting of white blood cells did not show differences among cells in the blood and in the tissue of infection. This is the first report showing that photodynamic therapy may be effective against MRSA infection in a murine model of intradermal infection.

Laboratory Mice Are Frequently Colonized with Staphylococcus aureus and Mount a Systemic Immune Response-Note of Caution for In vivo Infection Experiments

Whether mice are an appropriate model for S. aureus infection and vaccination studies is a matter of debate, because they are not considered as natural hosts of S. aureus. We previously identified a mouse-adapted S. aureus strain, which caused infections in laboratory mice. This raised the question whether laboratory mice are commonly colonized with S. aureus and whether this might impact on infection experiments. Publicly available health reports from commercial vendors revealed that S. aureus colonization is rather frequent, with rates as high as 21% among specific-pathogen-free mice. In animal facilities, S. aureus was readily transmitted from parents to offspring, which became persistently colonized. Among 99 murine S. aureus isolates from Charles River Laboratories half belonged to the lineage CC88 (54.5%), followed by CC15, CC5, CC188, and CC8. A comparison of human and murine S. aureus isolates revealed features of host adaptation. In detail, murine strains lacked hlb-converting phages and superantigen-encoding mobile genetic elements, and were frequently ampicillin-sensitive. Moreover, murine CC88 isolates coagulated mouse plasma faster than human CC88 isolates. Importantly, S. aureus colonization clearly primed the murine immune system, inducing a systemic IgG response specific for numerous S. aureus proteins, including several vaccine candidates. Phospholipase C emerged as a promising test antigen for monitoring S. aureus colonization in laboratory mice. In conclusion, laboratory mice are natural hosts of S. aureus and therefore, could provide better infection models than previously assumed. Pre-exposure to the bacteria is a possible confounder in S. aureus infection and vaccination studies and should be monitored.

Staphylococcus aureus Protein A Disrupts Immunity Mediated by Long-Lived Plasma Cells

Infection with Staphylococcus aureus does not induce long-lived protective immunity for reasons that are not completely understood. Human and murine vaccine studies support a role for Abs in protecting against recurring infections, but S. aureus modulates the B cell response through expression of staphylococcus protein A (SpA), a surface protein that drives polyclonal B cell expansion and induces cell death in the absence of costimulation. In this murine study, we show that SpA altered the fate of plasmablasts and plasma cells (PCs) by enhancing the short-lived extrafollicular response and reducing the pool of bone marrow (BM)-resident long-lived PCs. The absence of long-lived PCs was associated with a rapid decline in Ag-specific class-switched Ab. In contrast, when previously inoculated mice were challenged with an isogenic SpA-deficient S. aureus mutant, cells proliferated in the BM survival niches and sustained long-term Ab titers. The effects of SpA on PC fate were limited to the secondary response, because Ab levels and the formation of B cell memory occurred normally during the primary response in mice inoculated with wild-type or SpA-deficient S. aureus mutant. Thus, failure to establish long-term protective Ab titers against S. aureus was not a consequence of diminished formation of B cell memory; instead, SpA reduced the proliferative capacity of PCs that entered the BM, diminishing the number of cells in the long-lived pool.

Adaptive immune response to lipoproteins of Staphylococcus aureus in healthy subjects

Staphylococcus aureus is a frequent commensal but also a dangerous pathogen, causing many forms of infection ranging from mild to life‐threatening conditions. Among its virulence factors are lipoproteins, which are anchored in the bacterial cell membrane. Lipoproteins perform various functions in colonization, immune evasion, and immunomodulation. These proteins are potent activators of innate immune receptors termed Toll‐like receptors 2 and 6. This study addressed the specific B‐cell and T‐cell responses directed to lipoproteins in human S. aureus carriers and non‐carriers. 2D immune proteomics and ELISA approaches revealed that titers of antibodies (IgG) binding to S. aureus lipoproteins were very low. Proliferation assays and cytokine profiling data showed only subtle responses of T cells; some lipoproteins did not elicit proliferation. Hence, the robust activation of the innate immune system by S. aureus lipoproteins does not translate into a strong adaptive immune response. Reasons for this may include inaccessibility of lipoproteins for B cells as well as ineffective processing and presentation of the antigens to T cells.

Staphylococcal serine protease-like proteins are pacemakers of allergic airway reactions to Staphylococcus aureus

BACKGROUND

A substantial subgroup of asthmatic patients have "nonallergic" or idiopathic asthma, which often takes a severe course and is difficult to treat. The cause might be allergic reactions to the gram-positive pathogen Staphylococcus aureus, a frequent colonizer of the upper airways. However, the driving allergens of S aureus have remained elusive.

OBJECTIVE

We sought to search for potentially allergenic S aureus proteins and characterize the immune response directed against them.

METHODS

S aureus extracellular proteins targeted by human serum IgG₄ were identified by means of immunoblotting to screen for potential bacterial allergens. Candidate antigens were expressed as recombinant proteins and used to analyze the established cellular and humoral immune responses in healthy adults and asthmatic patients. The ability to induce a type 2 immune response in vivo was tested in a mouse asthma model.

RESULTS

We identified staphylococcal serine protease-like proteins (Spls) as dominant IgG₄-binding S aureus proteins. SplA through SplF are extracellular proteases of unknown function expressed by S aureus in vivo. Spls elicited IgE antibody responses in most asthmatic patients. In healthy S aureus carriers and noncarriers, peripheral blood T cells elaborated T_H2 cytokines after stimulation with Spls, as is typical for allergens. In contrast, T_H1/T_H17 cytokines, which dominated the response to S aureus α-hemolysin, were of low concentration or absent. In mice inhalation of SplD without adjuvant induced lung inflammation characterized by T_H2 cytokines and eosinophil infiltration.

CONCLUSION

We identify Spls as triggering allergens released by S aureus, opening prospects for diagnosis and causal therapy of asthma.

Global antibody response to Staphylococcus aureus live-cell vaccination

The pathogen Staphylococcus aureus causes a broad range of severe diseases and is feared for its ability to rapidly develop resistance to antibiotic substances. The increasing number of highly resistant S. aureus infections has accelerated the search for alternative treatment options to close the widening gap in anti-S. aureus therapy. This study analyses the humoral immune response to vaccination of Balb/c mice with sublethal doses of live S. aureus. The elicited antibody pattern in the sera of intravenously and intramuscularly vaccinated mice was determined using of a recently developed protein array. We observed a specific antibody response against a broad set of S. aureus antigens which was stronger following i.v. than i.m. vaccination. Intravenous but not intramuscular vaccination protected mice against an intramuscular challenge infection with a high bacterial dose. Vaccine protection was correlated with the strength of the anti-S. aureus antibody response. This study identified novel vaccine candidates by using protein microarrays as an effective tool and showed that successful vaccination against S. aureus relies on the optimal route of administration.

The T Cell Response to Staphylococcus aureus

Staphylococcus aureus (S. aureus) is a dangerous pathogen and a leading cause of both nosocomial and community acquired bacterial infection worldwide. However, on the other hand, we are all exposed to this bacterium, often within the first hours of life, and usually manage to establish equilibrium and coexist with it. What does the adaptive immune system contribute toward lifelong control of S. aureus? Will it become possible to raise or enhance protective immune memory by vaccination? While in the past the S. aureus-specific antibody response has dominated this discussion, the research community is now coming to appreciate the role that the cellular arm of adaptive immunity, the T cells, plays. There are numerous T cell subsets, each with differing functions, which together have the ability to orchestrate the immune response to S. aureus and hence to tip the balance between protection and pathology. This review summarizes the state of the art in this dynamic field of research.

Omics Approaches for the Study of Adaptive Immunity to Staphylococcus aureus and the Selection of Vaccine Candidates

Staphylococcus aureus is a dangerous pathogen both in hospitals and in the community. Due to the crisis of antibiotic resistance, there is an urgent need for new strategies to combat S. aureus infections, such as vaccination. Increasing our knowledge about the mechanisms of protection will be key for the successful prevention or treatment of S. aureus invasion. Omics technologies generate a comprehensive picture of the physiological and pathophysiological processes within cells, tissues, organs, organisms and even populations. This review provides an overview of the contribution of genomics, transcriptomics, proteomics, metabolomics and immunoproteomics to the current understanding of S. aureus‑host interaction, with a focus on the adaptive immune response to the microorganism. While antibody responses during colonization and infection have been analyzed in detail using immunoproteomics, the full potential of omics technologies has not been tapped yet in terms of T-cells. Omics technologies promise to speed up vaccine development by enabling reverse vaccinology approaches. In consequence, omics technologies are powerful tools for deepening our understanding of the “superbug” S. aureus and for improving its control.

Staphylococcus aureus-derived lipoteichoic acid induces temporary T-cell paralysis independent of Toll-like receptor 2

BACKGROUND

The interplay between microbes and surface organs, such as the skin, shapes a complex immune system with several checks and balances. The first-line defense is mediated by innate immune pathways leading to inflammation. In the second phase specific T cells invade the infected organ, amplifying inflammation and defense. Consecutively, termination of inflammation is crucial to avoid chronic inflammation triggered by microbes, such as in patients with atopic dermatitis.

OBJECTIVE

We aimed to elucidate how the Staphylococcus aureus-derived cell-wall component lipoteichoic acid (LTA) governs the second phase of immune responses when high concentrations of LTA access T cells directly through disrupted skin.

METHODS

We analyzed the direct exposure of T cells to LTA in vitro. For in vivo analyses, we used fluorescein isothiocyanate contact hypersensitivity and ovalbumin-induced dermatitis as models for TH2-mediated cutaneous inflammation.

RESULTS

We observed that LTA potently suppressed T-lymphocyte activation in a Toll-like receptor 2-independent manner. LTA-exposed T cells did not proliferate and did not produce cytokines. Importantly, these T cells remained completely viable and were responsive to consecutive activation signals on subsequent removal of LTA. Thus LTA exposure resulted in temporary functional T-cell paralysis. In vivo experiments revealed that T-cell cytokine production and cutaneous recall responses were significantly suppressed by LTA.

CONCLUSION

We identified a new mechanism through which bacterial compounds directly but temporarily modulate adaptive immune responses.

A Multiple Antigenic Peptide Mimicking Peptidoglycan Induced T Cell Responses to Protect Mice from Systemic Infection with Staphylococcus aureus

Due to the enormous capacity of Staphylococcus aureus to acquire antibiotic resistance, it becomes imperative to develop vaccines for decreasing the risk of its life-threatening infections. Peptidoglycan (PGN) is a conserved and major component of S. aureus cell wall. However, it has not been used as a vaccine candidate since it is a thymus-independent antigen. In this study, we synthesized a multiple antigenic peptide, named MAP27, which comprised four copies of a peptide that mimics the epitope of PGN. After immunization with MAP27 five times and boosting with heat-inactivated bacterium one time, anti-MAP27 serum bound directly to S. aureus or PGN. Immunization with MAP27 decreased the bacterial burden in organs of BALB/c mice and significantly prolonged their survival time after S. aureus lethal-challenge. The percentage of IFN-γ⁺CD3⁺ T cells and IL-17⁺CD4⁺ T cells in spleen, as well as the levels of IFN-γ, IL-17A/F and CCL3 in spleen and lung, significantly increased in the MAP27-immunized mice after infection. Moreover, in vitro incubation of heat-inactivated S. aureus with splenocytes isolated from MAP27-immunized mice stimulated the production of IFN-γ and IL-17A/F. Our findings demonstrated that MAP27, as a thymus-dependent antigen, is efficient at eliciting T cell-mediated responses to protect mice from S. aureus infection. This study sheds light on a possible strategy to design vaccines against S. aureus.

A bacterial regulatory RNA attenuates virulence, spread and human host cell phagocytosis

Staphylococcus aureus pathogenesis is directed by regulatory proteins and RNAs. We report the case of an RNA attenuating virulence and host uptake, possibly to sustain commensalism. A S. aureus sRNA, SprC (srn_3610), reduced virulence and bacterial loads in a mouse infection model. S. aureus deleted for sprC became more virulent and increased bacterial dissemination in colonized animals. Conversely, inducing SprC expression lowered virulence and the bacterial load. Without sprC, S. aureus phagocytosis by monocytes and macrophages was higher, whereas bacteria were internalized at lower yields when SprC expression was stimulated. Without sprC, higher internalization led to a greater number of extracellular bacteria, facilitating colonization. SprC expression decreased after phagocytosis, concurring with the facilitated growth of bacteria lacking the sRNA in the presence of an oxidant. The major staphylococcal autolysin facilitates S. aureus uptake by human phagocytes. ATL proved to be negatively regulated by SprC. The SprC domains involved in pairing with atl mRNA were analyzed. The addition of ATL reduced phagocytosis of bacteria lacking sprC with no effects on wild-type bacterial uptake, implying that SprC influences phagocytosis, at least in part, by controlling ATL. Since the control of SprC on ATL was modest, other factors must contribute to atl regulation.

Microarray-based identification of human antibodies against Staphylococcus aureus antigens

PURPOSE

The mortality rate of patients with Staphylococcus aureus infections is alarming and urgently demands new strategies to attenuate the course of these infections or to detect them at earlier stages.

EXPERIMENTAL DESIGN

To study the adaptive immune response to S. aureus antigens in healthy human volunteers, a protein microarray containing 44 S. aureus proteins was developed using the ArrayStrip platform technology.

RESULTS

Testing plasma samples from 15 S. aureus carriers and 15 noncarriers 21 immunogenic S. aureus antigens have been identified. Seven antigens were recognized by antibodies present in at least 60% of the samples, representing the core S. aureus immunome of healthy individuals. S. aureus-specific serum immunoglobulin G (IgG) levels were significantly lower in noncarriers than in carriers specifically anti-IsaA, anti-SACOL0479, and anti-SACOL0480 IgGs were found at lower frequencies and quantities. Twenty-two antigens present on the microarray were encoded by all S. aureus carrier isolates. Nevertheless, the immune system of the carriers was responsive to only eight of them and with different intensities.

CONCLUSION AND CLINICAL RELEVANCE

The established protein microarray allows a broad profiling of the S. aureus-specific antibody response and can be used to identify S. aureus antigens that might serve as vaccines or diagnostic markers.