Staphylococcus aureus alpha toxin suppresses effective innate and adaptive immune responses in a murine dermonecrosis model

The Prospect of Biomimetic Immune Cell Membrane-Coated Nanomedicines for Treatment of Serious Bacterial Infections and Sepsis

Invasive bacterial infections and sepsis are persistent global health concerns, complicated further by the escalating threat of antibiotic resistance. Over the past 40 years, collaborative endeavors to improve the diagnosis and critical care of septic patients have improved outcomes, yet grappling with the intricate immune dysfunction underlying the septic condition remains a formidable challenge. Anti-inflammatory interventions that exhibited promise in murine models failed to manifest consistent survival benefits in clinical studies through recent decades. Novel therapeutic approaches that target bacterial virulence factors, for example with monoclonal antibodies, aim to thwart pathogen-driven damage and restore an advantage to the immune system. A pioneering technology addressing this challenge is biomimetic nanoparticles-a therapeutic platform featuring nanoscale particles enveloped in natural cell membranes. Borne from the quest for a durable drug delivery system, the original red blood cell-coated nanoparticles showcased a broad capacity to absorb bacterial and environmental toxins from serum. Tailoring the membrane coating to immune cell sources imparts unique characteristics to the nanoparticles suitable for broader application in infectious disease. Their capacity to bind both inflammatory signals and virulence factors assembles the most promising sepsis therapies into a singular, pathogen-agnostic therapeutic. This review explores the ongoing work on immune cell-coated nanoparticle therapeutics for infection and sepsis. SIGNIFICANCE STATEMENT: Invasive bacterial infections and sepsis are a major global health problem made worse by expanding antibiotic resistance, meaning better treatment options are urgently needed. Biomimetic cell-membrane-coated nanoparticles are an innovative therapeutic platform that deploys a multifaceted mechanism to action to neutralize microbial virulence factors, capture endotoxins, and bind excessive host proinflammatory cytokines, seeking to reduce host tissue injury, aid in microbial clearance, and improve patient outcomes.

From Host Defense to Metabolic Signatures: Unveiling the Role of γδ T Cells in Bacterial Infections

The growth of antibiotic-resistant bacterial infections necessitates focusing on host-derived immunotherapies. γδ T cells are an unconventional T cell subset, making up a relatively small portion of healthy circulating lymphocytes but a substantially increased proportion in mucosal and epithelial tissues. γδ T cells are activated and expanded in response to bacterial infection, having the capability to produce proinflammatory cytokines to recruit neutrophils and clear infection. They also play a significant role in dampening immune response to control inflammation and protecting the host against secondary challenge, making them promising targets when developing immunotherapy. Importantly, γδ T cells have differential metabolic states influencing their cytokine profile and subsequent inflammatory capacity. Though these differential metabolic states have not been well studied or reviewed in the context of bacterial infection, they are critical in understanding the mechanistic underpinnings of the host's innate immune response. Therefore, this review will focus on the context-specific host defense conferred by γδ T cells during infection with Staphylococcus aureus, Streptococcus pneumoniae, Listeria monocytogenes, and Mycobacterium tuberculosis.

Mammary γδ T cells promote IL-17A-mediated immunity against Staphylococcus aureus-induced mastitis in a microbiota-dependent manner

Mastitis, a common disease for female during lactation period that could cause a health risk for human or huge economic losses for animals, is mainly caused by S. aureus invasion. Here, we found that neutrophil recruitment via IL-17A-mediated signaling was required for host defense against S. aureus-induced mastitis in a mouse model. The rapid accumulation and activation of Vγ4+ γδ T cells in the early stage of infection triggered the IL-17A-mediated immune response. Interestingly, the accumulation and influence of γδT17 cells in host defense against S. aureus-induced mastitis in a commensal microbiota-dependent manner. Overall, this study, focusing on γδT17 cells, clarified innate immune response mechanisms against S. aureus-induced mastitis, and provided a specific response to target for future immunotherapies. Meanwhile, a link between commensal microbiota community and host defense to S. aureus mammary gland infection may unveil potential therapeutic strategies to combat these intractable infections.

Staphylococcus aureus α-toxin impairs early neutrophil localization via electrogenic disruption of store-operated calcium entry

The pore-forming S. aureus α-toxin (Hla) contributes to virulence and disease pathogenesis. While high concentrations of toxin induce cell death, neutrophils exhibit relative resistance to lysis, suggesting that the action of Hla may not be solely conferred by lytic susceptibility. Using intravital microscopy, we observed that Hla disrupts neutrophil localization and clustering early in infection. Hla forms a narrow, ion-selective pore, suggesting that Hla may dysregulate calcium or other ions to impair neutrophil function. We found that sub-lytic Hla did not permit calcium influx but caused rapid membrane depolarization. Depolarization decreases the electrogenic driving force for calcium, and concordantly, Hla suppressed calcium signaling in vitro and in vivo and calcium-dependent leukotriene B4 (LTB4) production, a key mediator of neutrophil clustering. Thus, Hla disrupts the early patterning of the neutrophil response to infection, in part through direct impairment of neutrophil calcium signaling. This early mis-localization of neutrophils may contribute to establishment of infection.

Understanding mechanisms of virulence in MRSA: implications for antivirulence treatment strategies

INTRODUCTION

Methicillin-resistant Staphylococcus aureus (MRSA) is a widespread pathogen, often causing recurrent and deadly infections in the hospital and community. Many S. aureus virulence factors have been suggested as potential targets for antivirulence therapy to decrease the threat of diminishing antibiotic availability. Antivirulence methods hold promise due to their adjunctive and prophylactic potential and decreased risk for selective pressure.

AREAS COVERED

This review describes the dominant virulence mechanisms exerted by MRSA and antivirulence therapeutics that are currently undergoing testing in clinical or preclinical stages. We also discuss the advantages and downsides of several investigational antivirulence approaches, including the targeting of bacterial transporters, host-directed therapy, and quorum-sensing inhibitors. For this review, a systematic search of literature on PubMed, Google Scholar, and Web of Science for relevant search terms was performed in April and May 2023.

EXPERT OPINION

Vaccine and antibody strategies have failed in clinical trials and could benefit from more basic science-informed approaches. Antivirulence-targeting approaches need to be set up better to meet the requirements of drug development, rather than only providing limited results to provide 'proof-of-principle' translational value of pathogenesis research. Nevertheless, there is great potential of such strategies and potential particular promise for novel probiotic approaches.

Monoclonal antibodies for prophylaxis and therapy of respiratory syncytial virus, SARS-CoV-2, human immunodeficiency virus, rabies and bacterial infections: an update from the World Association of Infectious Diseases and Immunological Disorders and the Italian Society of Antinfective Therapy

Monoclonal antibodies (mABs) are safe and effective proteins produced in laboratory that may be used to target a single epitope of a highly conserved protein of a virus or a bacterial pathogen. For this purpose, the epitope is selected among those that play the major role as targets for prevention of infection or tissue damage. In this paper, characteristics of the most important mABs that have been licensed and used or are in advanced stages of development for use in prophylaxis and therapy of infectious diseases are discussed. We showed that a great number of mABs effective against virus or bacterial infections have been developed, although only in a small number of cases these are licensed for use in clinical practice and have reached the market. Although some examples of therapeutic efficacy have been shown, not unlike more traditional antiviral or antibacterial treatments, their efficacy is significantly greater in prophylaxis or early post-exposure treatment. Although in many cases the use of vaccines is more effective and cost-effective than that of mABs, for many infectious diseases no vaccines have yet been developed and licensed. Furthermore, in emergency situations, like in epidemics or pandemics, the availability of mABs can be an attractive adjunct to our armament to reduce the impact. Finally, the availability of mABs against bacteria can be an important alternative, when multidrug-resistant strains are involved.

Staphylococcal Enterotoxins Promote Virulence in Bacterial Keratitis

Purpose

Staphylococcus aureus is an important cause of corneal infections (keratitis). To better understand the virulence mechanisms mediating keratitis, a recent comparative genomics study revealed that a set of secreted enterotoxins were found with higher prevalence among ocular versus non-ocular S. aureus clinical infection isolates, suggesting a key role for these toxins in keratitis. Although well known to cause toxic shock syndrome and S. aureus food poisoning, enterotoxins have not yet been shown to mediate virulence in keratitis.

Methods

A set of clinical isolate test strains, including a keratitis isolate that encodes five enterotoxins (sed, sej, sek, seq, ser), its corresponding enterotoxin deletion mutant and complementation strain, a keratitis isolate devoid of enterotoxins, and the non-ocular S. aureus strain USA300 along with its corresponding enterotoxin deletion and complementation strains, were evaluated for cellular adhesion, invasion and cytotoxicity in a primary corneal epithelial model as well as with microscopy. Additionally, strains were evaluated in an in vivo model of keratitis to quantify enterotoxin gene expression and measure disease severity.

Results

We demonstrate that, although enterotoxins do not impact bacterial adhesion or invasion, they do elicit direct cytotoxicity in vitro toward corneal epithelial cells. In an in vivo model, sed, sej, sek, seq, ser were found to have variable gene expression across 72 hours of infection and test strains encoding enterotoxins resulted in increased bacterial burden as well as a reduced host cytokine response.

Conclusions

Our results support a novel role for staphylococcal enterotoxins in promoting virulence in S. aureus keratitis.

Toxin expression during Staphylococcus aureus infection imprints host immunity to inhibit vaccine efficacy

Staphylococcus aureus infections are a major public health issue, and a vaccine is urgently needed. Despite a considerable promise in preclinical models, all vaccines tested thus far have failed to protect humans against S. aureus. Unlike laboratory mice, humans are exposed to S. aureus throughout life. In the current study, we hypothesized that prior exposure to S. aureus "imprints" the immune response to inhibit vaccine-mediated protection. We established a mouse model in which S. aureus skin and soft tissue infection (SSTI) is followed by vaccination and secondary SSTI. Unlike naïve mice, S. aureus-sensitized mice were incompletely protected against secondary SSTI by vaccination with the inactivated α-hemolysin (Hla) mutant Hla_H35L. Inhibition of protection was specific for the Hla_H35L vaccine and required hla expression during primary SSTI. Surprisingly, inhibition occurred at the level of vaccine-elicited effector T cells; hla expression during primary infection limited the expansion of T cells and dendritic cells and impaired vaccine-specific T cell responses. Importantly, the T cell-stimulating adjuvant CAF01 rescued inhibition and restored vaccine-mediated protection. Together, these findings identify a potential mechanism for the failure of translation of promising S. aureus vaccines from mouse models to clinical practice and suggest a path forward to prevent these devastating infections.

Neutralization of Staphylococcus aureus Protein A Prevents Exacerbated Osteoclast Activity and Bone Loss during Osteomyelitis

Osteomyelitis caused by Staphylococcus aureus is an important and current health care problem worldwide. Treatment of this infection frequently fails not only due to the increasing incidence of antimicrobial-resistant isolates but also because of the ability of S. aureus to evade the immune system, adapt to the bone microenvironment, and persist within this tissue for decades. We have previously demonstrated the role of staphylococcal protein A (SpA) in the induction of exacerbated osteoclastogenesis and increased bone matrix degradation during osteomyelitis. The aim of this study was to evaluate the potential of using anti-SpA antibodies as an adjunctive therapy to control inflammation and bone damage. By using an experimental in vivo model of osteomyelitis, we demonstrated that the administration of an anti-SpA antibody by the intraperitoneal route prevented excessive inflammatory responses in the bone upon challenge with S. aureus. Ex vivo assays indicated that blocking SpA reduced the priming of osteoclast precursors and their response to RANKL. Moreover, the neutralization of SpA was able to prevent the differentiation and activation of osteoclasts in vivo, leading to reduced expression levels of cathepsin K, reduced expression of markers associated with abnormal bone formation, and decreased trabecular bone loss during osteomyelitis. Taken together, these results demonstrate the feasibility of using anti-SpA antibodies as an antivirulence adjunctive therapy that may prevent the development of pathological conditions that not only damage the bone but also favor bacterial escape from antimicrobials and the immune system.

Lysine Inhibits Hemolytic Activity of Staphylococcus aureus and Its Application in Food Model Contaminated with Staphylococcus aureus

Alpha-hemolysin (Hla) is one of the important exotoxins of Staphylococcus aureus (S. aureus) and can be used as a target to reduce the virulence of S. aureus. This study explored the inhibitory effect of Lysine (Lys) on Hla and its application in food safety. Lys significantly inhibited the expression of Hla at sub-inhibitory concentrations and directly interacted with Hla to interfere with its oligomerization and thus significantly inhibited its hemolytic activity. Notably, Lys attenuated S. aureus damage to mouse small intestine and Caco-2 cells and delayed mouse mortality. In the food model, Lys inhibited the expression of Hla of S. aureus and had no significant effect on the sensory score. Moreover, Lys had no obvious damage effect on the main organs of mice, which indicated that Lys has good biocompatibility and has the potential to be used in the food industry as an anti-S. aureus preparation.

Keratinocyte-derived defensins activate neutrophil-specific receptors Mrgpra2a/b to prevent skin dysbiosis and bacterial infection

Healthy skin maintains a diverse microbiome and a potent immune system to fight off infections. Here, we discovered that the epithelial-cell-derived antimicrobial peptides defensins activated orphan G-protein-coupled receptors (GPCRs) Mrgpra2a/b on neutrophils. This signaling axis was required for effective neutrophil-mediated skin immunity and microbiome homeostasis. We generated mutant mouse lines lacking the entire Defensin (Def) gene cluster in keratinocytes or Mrgpra2a/b. Def and Mrgpra2 mutant animals both exhibited skin dysbiosis, with reduced microbial diversity and expansion of Staphylococcus species. Defensins and Mrgpra2 were critical for combating S. aureus infections and the formation of neutrophil abscesses, a hallmark of antibacterial immunity. Activation of Mrgpra2 by defensin triggered neutrophil release of IL-1β and CXCL2 which are vital for proper amplification and propagation of the antibacterial immune response. This study demonstrated the importance of epithelial-neutrophil signaling via the defensin-Mrgpra2 axis in maintaining healthy skin ecology and promoting antibacterial host defense.

A novel fully human recombinant antibody neutralizing the α-hemolysin of Staphylococcus aureus

BACKGROUND

Methicillin-resistant Staphylococcus aureus (MRSA) is resistant to almost all β-lactam antibiotics. Hence, new ways to control MRSA infection, such as antibacterial antibodies, need to be explored. α-hemolysin is the most important virulence factor widely expressed in S. aureus. This study aimed to develop a new fully human antibody against α-hemolysin of S. aureus and research its neutralizing effect.

RESULTS

The single-chain antibody fragments(scFvs)against S. aureus were screened from a fully human scFv library using phage display technology. The selected scFvs had good binding affinities to α-hemolysin and S. aureus. The IgG-like scFv-Fc inserted into the pcDNA3.1 or pMH3 vector was expressed in HEK293F suspension cells to extend the half-life and restore Fc function. The size of purified scFv-Fc was about 55 kDa. The functions of expressed scFv-Fcs against α-hemolysin were validated. The cytotoxicity assays showed that scFv555-Fc had better protective effects on A549 cells than other scFv-Fcs. The results of anti-rabbit erythrocyte lysis and A549 cell apoptosis assay confirmed that scFv555-Fc had a significant neutralizing effect on α-hemolysin. The scFv555-Fc was used to construct the docking model of antigen-antibody complexes using Discovery Studio software. It predicted that the key binding sites of α-hemolysin were TYR28, LYS37, PHE39, ARG56, and LYS58, which might be the key toxic sites of α-hemolysin.

CONCLUSION

A novel fully human scFv-Fc antibody neutralizing the α-hemolysin toxin of S. aureus was successfully developed. The findings might provide a new theoretical basis and treatment method for preventing MRSA infection.

Kinetic Characterization of the Immune Response to Methicillin-Resistant Staphylococcus aureus Subcutaneous Skin Infection

Staphylococcus aureus is a leading cause of skin and soft tissue infections (SSTIs). Studies examining the immune response to S. aureus have been conducted, yet our understanding of the kinetic response to S. aureus subcutaneous skin infection remains incomplete. In this study, we used C57BL/6J mice and USA300 S. aureus to examine the host-pathogen interface from 8 h postinfection to 15 days postinfection (dpi), with the following outcomes measured: lesion size, bacterial titers, local cytokine and chemokine levels, phenotype of the responding leukocytes, and histopathology and Gram staining of skin tissue. Lesions were largest at 1 dpi, with peak necrotic tissue areas at 3 dpi, and were largely resolved by 15 dpi. During early infection, bacterial titers were high, neutrophils were the most abundant immune cell type, there was a decrease in most leukocyte populations found in uninfected skin, and many different cytokines were produced. Histopathological analysis demonstrated swift and extensive keratinocyte death and robust and persistent neutrophil infiltration. Gram staining revealed subdermal S. aureus colonization and, later, limited migration into upper skin layers. Interleukin-17A/F (IL-17A/F) was detected only starting at 5 dpi and coincided with an immediate decrease in bacterial numbers in the following days. After 9 days, neutrophils were no longer the most abundant immune cell type present as most other leukocyte subsets returned, and surface wounds resolved coincident with declining bacterial titers. Collectively, these data illustrate a dynamic immune response to S. aureus skin infection and suggest a key role for precisely timed IL-17 production for infection clearance and healthy tissue formation.

Neutralizing Staphylococcus aureus Virulence with AZD6389, a Three mAb Combination, Accelerates Closure of a Diabetic Polymicrobial Wound

Nonhealing diabetic foot ulcers (DFU), a major complication of diabetes, are associated with high morbidity and mortality despite current standard of care. Since Staphylococcus aureus is the most common pathogen isolated from nonhealing and infected DFU, we hypothesized that S. aureus virulence factors would damage tissue, promote immune evasion and alter the microbiome, leading to bacterial persistence and delayed wound healing. In a diabetic mouse polymicrobial wound model with S. aureus, Pseudomonas aeruginosa, and Streptococcus pyogenes, we report a rapid bacterial proliferation, prolonged pro-inflammatory response and large necrotic lesions unclosed for up to 40 days. Treatment with AZD6389, a three-monoclonal antibody combination targeting S. aureus alpha toxin, 4 secreted leukotoxins, and fibrinogen binding cell-surface adhesin clumping factor A resulted in full skin re-epithelization 21 days after inoculation. By neutralizing multiple virulence factors, AZD6389 effectively blocked bacterial agglutination and S. aureus-mediated cell killing, abrogated S. aureus-mediated immune evasion and targeted the bacteria for opsonophagocytic killing. Neutralizing S. aureus virulence not only facilitated S. aureus clearance in lesions, but also reduced S. pyogenes and P. aeruginosa numbers, damaging inflammatory mediators and markers for neutrophil extracellular trap formation 14 days post initiation. Collectively, our data suggest that AZD6389 holds promise as an immunotherapeutic approach against DFU complications.

IMPORTANCE Diabetic foot ulcers (DFU) represent a major complication of diabetes and are associated with poor quality of life and increased morbidity and mortality despite standard of care. They have a complex pathogenesis starting with superficial skin lesions, which often progress to deeper tissue structures up to the bone and ultimately require limb amputation. The skin microbiome of diabetic patients has emerged as having an impact on DFU occurrence and chronicity. DFU are mostly polymicrobial, and the Gram-positive bacterium Staphylococcus aureus detected in more than 95% of cases. S. aureus possess a collection of virulence factors which participate in disease progression and may facilitate growth of other pathogens. Here we show in a diabetic mouse wound model that targeting some specific S. aureus virulence factors with a multimechanistic antibody combination accelerated wound closure and promoted full skin re-epithelization. This work opens promising new avenues for the treatment of DFU.

Inhibitory Effect of Andrographis paniculata Lactone on Staphylococcus aureus α-Hemolysin

We investigated the effect of andrographolide (AP) on the hemolytic capacity of Staphylococcus aureus (S. aureus) isolated from our region. AP is a labdane diterpenoid isolated from the stem and leaves of Andrographis paniculata. The hla gene from 234 S. aureus strains and the quality control standard strain ATCC29213 in dairy cows in some areas of Ningxia was analyzed. Evolutionary analysis, homology modeling, and functional enrichment annotation of α-hemolysin Hla detected from our region were performed through bioinformatics. The hemolytic ability of S. aureus isolates from the region was examined using the hemolysis test, and the effect of AP on S. aureus was quantified. Moreover, the effect of AP on the transcript levels of hla and genes highly related to hla (i.e., clfA and fnbA) was examined through fluorescence quantitative PCR. The mode of action of AP on the detected Hla was analyzed through molecular docking and dynamic simulation. The results showed that S. aureus in our region has a high rate of hla carriage. The hemolytic activity of strains NM98 and XF10 was significant, and ATCC29213 also exhibited some hemolytic activity. AP could inhibit the expression of Hla and its related proteins by downregulating hla, clfA, and fnbA transcript levels, which in turn attenuated the S. aureus hemolytic activity. Meanwhile, the AP molecule can form three hydrogen bonds with residues ASN105, SER106, and THR155 of Hla protein; bind with PRO103 through alkyl intermolecular forces; and form carbon hydrogen bonds with LYS154, reflecting that the AP molecule has a comparatively ideal theoretical binding activity with Hla protein. Among them, PRO103 and LYS154 are highly conserved in Hla protein molecules and play pivotal roles in the biological functions of Hla, and their binding may affect these functions. Their binding may also prevent the conformational transition of Hla from a monomer to an oligomer, thus inhibiting Hla hemolytic activity. This study offers a molecular basis for use of AP as an antivirulence drug and new ideas for developing novel drugs against S. aureus infection.

Evaluation of the immune response to a multi-epitope vaccine candidate in comparison with HlaH35L, MntC, and SACOL0723 proteins against MRSA infection

Staphylococcus aureus is an important human opportunistic pathogen that can have a major influence on public health. Here, we aimed to evaluate different aspects of the immune response to a novel multi-epitope fusion protein (HMS) based on Hla_H35L, MntC, and SACOL0723 proteins in comparison to the individual antigens. For this purpose, specific total IgG, IgG1, and IgG2a isotypes and the cytokines related to Th1, Th2, and Th17 were assessed. The Bio-efficiency of the fusion protein was evaluated by opsonic killing activity. The HMS fusion protein elicited a high specific IgG level and also induced a higher level of Th1, Th2, and Th17-related cytokines which were more polarized towards the Th1 and Th17 compared to individual antigens. The HMS-specific antisera also significantly promoted phagocytosis of S. aureus COL strain by mouse macrophages. In conclusion, the fusion protein might be an effective vaccine for potential protective immunity against a lethal infection of S. aureus in mice.

Old problems and new solutions: antibiotic alternatives in food animal production

The antimicrobial resistance crisis is a Global Health challenge that impacts humans, animals, and the environment alike. In response to increased demands for animal protein and by-products, there has been a substantial increase in the use of antimicrobial agents in the animal industry. Indeed, they are extensively used to prevent, control, and (or) treat disease in animals. In addition to infection control, in-feed supplementation with antimicrobials became common practice for growth promotion of livestock. Unfortunately, the global overuse of antimicrobials has contributed to the emergence and spread of resistance. As such, many countries have implemented policies and approaches to eliminate the use of antimicrobials as growth promoters in food animals, which necessitates the need for alternate and One Health strategies to maintain animal health and welfare. This review summarizes the antimicrobial resistance crisis from Global Health and One Health perspectives. In addition, we outline examples of potential alternate strategies to circumvent antimicrobial use in animal husbandry practices, including antivirulence agents, bacteriophages, and nutritional measures to control bacterial pathogens. Overall, these alternate strategies require further research and development efforts, including assessment of efficacy and the associated development, manufacturing, and labor costs.

Staphylococcus aureus toxin suppresses antigen-specific T cell responses

Staphylococcus aureus remains a leading cause of human infection. These infections frequently recur when the skin is a primary site of infection, especially in infants and children. In contrast, invasive staphylococcal disease is less commonly associated with reinfection, suggesting that tissue-specific mechanisms govern the development of immunity. Knowledge of how S. aureus manipulates protective immunity has been hampered by a lack of antigen-specific models to interrogate the T cell response. Using a chicken egg OVA-expressing S. aureus strain to analyze OVA-specific T cell responses, we demonstrated that primary skin infection was associated with impaired development of T cell memory. Conversely, invasive infection induced antigen-specific memory and protected against reinfection. This defect in adaptive immunity following skin infection was associated with a loss of DCs, attributable to S. aureus α-toxin (Hla) expression. Gene- and immunization-based approaches to protect against Hla during skin infection restored the T cell response. Within the human population, exposure to α-toxin through skin infection may modulate the establishment of T cell-mediated immunity, adversely affecting long-term protection. These studies prompt consideration that vaccination targeting S. aureus may be most effective if delivered prior to initial contact with the organism.

Staphylococcus aureus Fatty Acid Kinase FakA Modulates Pathogenesis during Skin Infection via Proteases

Staphylococcus aureus fatty acid kinase FakA is necessary for the incorporation of exogenous fatty acids into the lipid membrane. We previously demonstrated that the inactivation of fakA leads to decreased α-hemolysin (Hla) production but increased expression of the proteases SspAB and aureolysin in vitro, and that the ΔfakA mutant causes larger lesions than the wild type (WT) during murine skin infection. As expected, necrosis is Hla dependent in the presence or absence of FakA, as both hla and hla ΔfakA mutants are unable to cause necrosis of the skin. At day 4 postinfection, while the ΔfakA mutant maintains larger and more necrotic abscesses, bacterial numbers are similar to those of the WT, indicating the enhanced tissue damage of mice infected with the ΔfakA mutant is not due to an increase in bacterial burden. At this early stage of infection, skin infected with the ΔfakA mutant has decreased levels of proinflammatory cytokines, such as interleukin-17A (IL-17A) and IL-1α, compared to those of WT-infected skin. At a later stage of infection (day 7), abscess resolution and bacterial clearance are hindered in ΔfakA mutant-infected mice. The paradoxical findings of decreased Hla in vitro but increased necrosis in vivo led us to investigate the role of the proteases regulated by FakA. Utilizing Δaur and ΔsspAB mutants in both the WT and fakA mutant backgrounds, we found that the absence of these proteases in a fakA mutant reduced dermonecrosis to levels similar to those of the WT strain. These studies suggest that the overproduction of proteases is one factor contributing to the enhanced pathogenesis of the ΔfakA mutant during skin infection.

Pathogenesis of Intradermal Staphylococcal Infections: Rabbit Experimental Approach to Natural Staphylococcus aureus Skin Infections

Despite the enormous efforts made to achieve effective tools that fight against Staphylococcus aureus, the results have not been successful. This failure may be due to the absence of truly representative experimental models. To overcome this deficiency, the present work describes and immunologically characterizes the infection for 28 days, in an experimental low-dose (300 colony-forming units) intradermal model of infection in rabbits, which reproduces the characteristic staphylococcal abscess. Surprisingly, when mutant strains in the genes involved in virulence (JΔagr, JΔcoaΔvwb, JΔhla, and JΔpsmα) were inoculated, no strong effect on the severity of lesions was observed, unlike other models that use high doses of bacteria. The inoculation of a human rabbitized (FdltB^r) strain demonstrated its capacity to generate a similar inflammatory response to a wild-type rabbit strain and, therefore, validated this model for conducting these experimental studies with human strains. To conclude, this model proved reproducible and may be an option of choice to check both wild-type and mutant strains of different origins.

Preparation and preclinical evaluation of two novel Staphylococcus aureus capsular polysaccharide 5 and 8-fusion protein (Hla-MntC-SACOL0723) immunoconjugates

Staphylococcus aureus is one of the most common pathogens in the hospital and the community. The emergence of broad-spectrum antibiotic resistance in S. aureus has made the treatment process more difficult. Therefore, it is obvious that an effective prevention strategy against the pathogen could significantly reduce costs related to care in hospitals. In this report, we describe a simple approach to conjugate S. aureus capsular polysaccharide 5 (CP5) from S. aureus Reynolds strain and 8 (CP8) from S. aureus Becker strain to a fusion protein (Hla-MntC-SACOL0723) and investigation of its bioactivity. The conjugation was done by using ADH (as a bridge) and EDAC (as a coupling agent). The immunoconjugates were characterized by routine polysaccharide/protein contents assays followed by reverse phase chromatography and FTIR spectroscopy. The groups of mice were immunized with conjugate vaccines, capsular polysaccharides, and phosphate-buffered saline (PBS) as a control group. The functional activity of the vaccine candidates was evaluated by ELISA, opsonophagocytosis tests, and determination of bacterial load in challenge study. The results showed that the specific antibody (total IgG) titers raised against conjugate molecules were higher than those of the nonconjugated capsular polysaccharides. The opsonic activity of the conjugate vaccines antisera was significantly higher than polysaccharides alone (58% reduction in the number of bacteria versus 16.3% at 1:2 dilution, p < .05), Further, the conjugate vaccine group had a significant reduction in bacterial load after challenge with S. aureus COL strain cells as compared to the PBS and nonconjugated controls. In conclusion, the immunoconjugates could be developed as a potential vaccine candidate against S. aureus.

Contribution of hla Regulation by SaeR to Staphylococcus aureus USA300 Pathogenesis

The SaeRS two-component system in Staphylococcus aureus is critical for regulation of many virulence genes, including hla, which encodes alpha-toxin. However, the impact of regulation of alpha-toxin by Sae on S. aureus pathogenesis has not been directly addressed. Here, we mutated the SaeR-binding sequences in the hla regulatory region and determined the contribution of this mutation to hla expression and pathogenesis in strain USA300 JE2. Western blot analyses revealed drastic reduction of alpha-toxin levels in the culture supernatants of SaeR-binding mutant in contrast to the marked alpha-toxin production in the wild type. The SaeR-binding mutation had no significant effect on alpha-toxin regulation by Agr, MgrA, and CcpA. In animal studies, we found that the SaeR-binding mutation did not contribute to USA300 JE2 pathogenesis using a rat infective endocarditis model. However, in a rat skin and soft tissue infection model, the abscesses on rats infected with the mutant were significantly smaller than the abscesses on those infected with the wild type but similar to the abscesses on those infected with a saeR mutant. These studies indicated that there is a direct effect of hla regulation by SaeR on pathogenesis but that the effect depends on the animal model used.

The good side of inflammation: Staphylococcus aureus proteins SpA and Sbi contribute to proper abscess formation and wound healing during skin and soft tissue infections

Staphylococcus aureus is the most prominent cause of skin and soft tissue infections (SSTI) worldwide. Mortality associated with invasive SSTI is a major threat to public health considering the incidence of antibiotic resistant isolates in particular methicillin resistant S. aureus both in the hospital (HA-MRSA) and in the community (CA-MRSA). To overcome the increasing difficulties in the clinical management of SSTI due to MRSA, new prophylactic and therapeutic approaches are urgently needed and a preventive vaccine would be welcome. The rational design of an anti-S. aureus vaccine requires a deep knowledge of the role that the different bacterial virulence factors play according to the type of infection. In the present study, using a set of isogenic deficient mutants and their complemented strains we determined that the staphylococcal surface proteins SpA and Sbi play an important role in the induction of inflammatory cytokines and chemokines in the skin during SSTI. SpA and Sbi initiate signaling cascades that lead to the early recruitment of neutrophils, modulate their lifespan in the skin milieu and contribute to proper abscess formation and bacterial eradication. Moreover, the expression of SpA and Sbi appear critical for skin repair and wound healing. Thus, these results indicate that SpA and Sbi can promote immune responses in the skin that are beneficial for the host and therefore, should not be neutralized with vaccine formulations designed to prevent SSTI.

Staphylococcus aureus SaeR/S-Regulated Factors Decrease Monocyte-Derived Tumor Necrosis Factor-α to Reduce Neutrophil Bactericidal Activity

Background

The ability of Staphylococcus aureus to evade killing by human neutrophils significantly contributes to disease progression. In this study, we characterize an influential role for the S. aureus SaeR/S 2-component gene regulatory system in suppressing monocyte production of tumor necrosis factor alpha (TNF-α) to subsequently influence human neutrophil priming.

Methods

Using flow cytometry and TNF-α specific enzyme-linked immunosorbent assays we identify the primary cellular source of TNF-α in human blood and in purified peripheral blood mononuclear cells (PBMCs) during interaction with USA300 and an isogenic saeR/S deletion mutant (USA300∆saeR/S). Assays with conditioned media from USA300 and USA300∆saeR/S exposed PBMCs were used to investigate priming on neutrophil bactericidal activity.

Results

TNF-α production from monocytes was significantly reduced following challenge with USA300 compared to USA300∆saeR/S. We observed that priming of neutrophils using conditioned medium from peripheral blood mononuclear cells stimulated with USA300∆saeR/S significantly increased neutrophil bactericidal activity against USA300 relative to unprimed neutrophils and neutrophils primed with USA300 conditioned medium. The increased neutrophil bactericidal activity was associated with enhanced reactive oxygen species production that was significantly influenced by elevated TNF-α concentrations.

Conclusions

Our findings identify an immune evasion strategy used by S. aureus to impede neutrophil priming and subsequent bactericidal activity.

Candida albicans Augments Staphylococcus aureus Virulence by Engaging the Staphylococcal agr Quorum Sensing System

Relatively little is known about the complex interactions and signaling events that occur between microbes and even less so about how microbial “cross talk” shapes human health and disease. Candida albicans (a fungus) and Staphylococcus aureus (a bacterium) are formidable human nosocomial pathogens, causing severe morbidity and mortality. Moreover, they are frequently coisolated from central venous catheters and deep-seated infections, including intra-abdominal sepsis. In this work, we have shown that coinfection with C. albicans and S. aureus is highly lethal, leading to >80% mortality by day 1 postinfection, whereas monoinfection with C. albicans or S. aureus does not cause mortality. This infectious synergism is dependent on the expression of staphylococcal alpha-toxin, and secretion of this potent virulence factor is actually augmented by C. albicans via an agr-dependent mechanism. Moreover, prophylactic neutralization of alpha-toxin with a monoclonal antibody is sufficient to elicit protection during coinfection. Therefore, we have demonstrated that a pathogenic fungus can enhance virulence determinants of a bacterium in vivo with devastating consequences to the host. These results have important implications in the surveillance and treatment of polymicrobial disease and highlight the dynamic intersection of environment, pathogens, and host.

Candida albicans and Staphylococcus aureus are among the most prevalent nosocomial pathogens that are responsible for severe morbidity and mortality, even with appropriate treatment. Using a murine model of polymicrobial intra-abdominal infection (IAI), we have previously shown that coinfection with these pathogens results in synergistic lethality that is partially dependent on exacerbated prostaglandin signaling, while monomicrobial infection is nonlethal. Therefore, the objective of this study was to identify staphylococcal virulence determinants that drive lethal synergism during polymicrobial IAI. Using the toxigenic S. aureus strain JE2, we observed that coinfection with C. albicans led to a striking 80 to 100% mortality rate within 20 h postinoculation (p.i.) while monomicrobial infections were nonlethal. Use of a green fluorescent protein (GFP)-P3 promoter S. aureus reporter strain revealed enhanced activation of the staphylococcal agr quorum sensing system during in vitro polymicrobial versus monomicrobial growth. Analyses by quantitative real-time PCR (qPCR), Western blot, and toxin functional assays confirmed enhanced agr-associated gene transcription and increases in secreted alpha- and delta-toxins. C. albicans-mediated elevated toxin production and hemolytic activity were determined to be agrA dependent, and genetic knockout and complementation of hla identified alpha-toxin as the key staphylococcal virulence factor driving lethal synergism. Analysis of mono- and polymicrobial infections 8 h p.i. demonstrated equivalent bacterial burdens in the peritoneal cavity but significantly elevated levels of alpha-toxin (3-fold) and the eicosanoid prostaglandin E₂ (PGE₂) (4-fold) during coinfection. Importantly, prophylactic passive immunization using the monoclonal anti-alpha-toxin antibody MEDI4893* led to significantly improved survival rates compared to those following treatment with isotype control antibody. Collectively, these results define alpha-toxin as an essential virulence determinant during C. albicans-S. aureus IAI and describe a novel mechanism by which a human-pathogenic fungus can augment the virulence of a highly pathogenic bacterium in vivo.

Clonal Vγ6+Vδ4+ T cells promote IL-17-mediated immunity against Staphylococcus aureus skin infection

T cell cytokines contribute to immunity against Staphylococcus aureus, but the predominant T cell subsets involved are unclear. In an S. aureus skin infection mouse model, we found that the IL-17 response was mediated by γδ T cells, which trafficked from lymph nodes to the infected skin to induce neutrophil recruitment, proinflammatory cytokines IL-1α, IL-1β, and TNF, and host defense peptides. RNA-seq for TRG and TRD sequences in lymph nodes and skin revealed a single clonotypic expansion of the encoded complementarity-determining region 3 amino acid sequence, which could be generated by canonical nucleotide sequences of TRGV5 or TRGV6 and TRDV4 However, only TRGV6 and TRDV4 but not TRGV5 sequences expanded. Finally, Vγ6⁺ T cells were a predominant γδ T cell subset that produced IL-17A as well as IL-22, TNF, and IFNγ, indicating a broad and substantial role for clonal Vγ6⁺Vδ4⁺ T cells in immunity against S. aureus skin infections.

Comparison of livestock-associated and community-associated Staphylococcus aureus pathogenicity in a mouse model of skin and soft tissue infection

Industrial hog operation (IHO) workers are at increased risk of carrying Staphylococcus aureus in their nares, particularly strains that are livestock-associated (LA) and multidrug-resistant. The pathogenicity of LA-S. aureus strains remains unclear, with some prior studies suggesting reduced transmission and virulence in humans compared to community-associated methicillin-resistant (CA-MRSA) S. aureus. The objective of this study was to determine the degree to which LA-S. aureus strains contracted by IHO workers cause disease relative to a representative CA-MRSA strain in a mouse model of skin and soft tissue infection (SSTI). Mice infected with CC398 LA-S. aureus strains (IHW398-1 and IHW398-2) developed larger lesion sizes with higher bacterial burden than mice infected with CA-MRSA (SF8300) (p < 0.05). The greatest lesion size and bacterial burden was seen with a CC398 strain that produced a recurrent SSTI in an IHO worker. The LA-S. aureus infected mice had decreased IL-1β protein levels compared with CA-MRSA-infected mice (p < 0.05), suggesting a suboptimal host response to LA-S. aureus SSTIs. WGSA revealed heterogeneity in virulence factor and antimicrobial resistance genes carried by LA-S. aureus and CA-MRSA strains. The observed pathogenicity suggest that more attention should be placed on preventing the spread of LA-S. aureus into human populations.

Staphylococcus aureus Biofilm-Conditioned Medium Impairs Macrophage-Mediated Antibiofilm Immune Response by Upregulating KLF2 Expression

Staphylococcus aureus infections associated with the formation of biofilms on medical implants or host tissue play a critical role in the persistence of chronic infections. One critical mechanism of biofilm infection that leads to persistent infection lies in the capacity of biofilms to evade the macrophage-mediated innate immune response. It is now increasingly apparent that microorganisms exploit the negative regulatory mechanisms of the pattern recognition receptor (PRR)-mediated inflammatory response to subvert host cell functions by using various virulence factors. However, the detailed molecular mechanism, along with the identity of a target molecule, underlying the evasion of the macrophage-mediated innate immune response against S. aureus infection associated with biofilm formation remains to be elucidated. Here, using an in vitro culture model of murine macrophage-like RAW 264.7 cells, we demonstrate that S. aureus biofilm-conditioned medium significantly attenuated the capacity for macrophage bactericidal and proinflammatory responses. Importantly, the responses were associated with attenuated activation of NF-κB and increased expression of Kruppel-like factor 2 (KLF2) in RAW 264.7 cells. Small interfering RNA (siRNA)-mediated silencing of KLF2 in RAW 264.7 cells could restore the activation of NF-κB toward the bactericidal activity and generation of proinflammatory cytokines in the presence of S. aureus biofilm-conditioned medium. Collectively, our results suggest that factors secreted from S. aureus biofilms might exploit the KLF2-dependent negative regulatory mechanism to subvert macrophage-mediated innate immune defense against S. aureus biofilms.

Alanine Scanning Mutagenesis of the MEDI4893 (Suvratoxumab) Epitope Reduces Alpha Toxin Lytic Activity In Vitro and Staphylococcus aureus Fitness in Infection Models

Alpha toxin (AT) is a cytolytic pore-forming toxin that plays a key role in Staphylococcus aureus pathogenesis; consequently, extensive research was undertaken to understand the AT mechanism of action and its utility as a target for novel prophylaxis and treatment strategies against S. aureus infections. MEDI4893 (suvratoxumab) is a human anti-AT IgG1 monoclonal antibody (MAb) that targets AT and is currently in phase 2 clinical development.

Alpha toxin (AT) is a cytolytic pore-forming toxin that plays a key role in Staphylococcus aureus pathogenesis; consequently, extensive research was undertaken to understand the AT mechanism of action and its utility as a target for novel prophylaxis and treatment strategies against S. aureus infections. MEDI4893 (suvratoxumab) is a human anti-AT IgG1 monoclonal antibody (MAb) that targets AT and is currently in phase 2 clinical development. As shown previously, the MEDI4893-binding epitope on AT is comprised of the highly conserved amino acid regions 177 to 200 and 261 to 271, suggesting these amino acids are important for AT function. To test this hypothesis and gain insight into the effect of mutations in the epitope on AT neutralization by MEDI4893, nine MEDI4893 contact residues in AT were individually mutated to alanine. Consistent with our hypothesis, 8 out of 9 mutants exhibited >2-fold loss in lytic activity resulting from a defect in cell binding and pore formation. MEDI4893 binding affinity was reduced >2-fold (2- to 27-fold) for 7 out of 9 mutants, and no binding was detected for the W187A mutant. MEDI4893 effectively neutralized all of the lytic mutants in vitro and in vivo. When the defective mutants were introduced into an S. aureus clinical isolate, the mutant-expressing strains exhibited less severe disease in mouse models and were effectively neutralized by MEDI4893. These results indicate the MEDI4893 epitope is highly conserved due in part to its role in AT pore formation and bacterial fitness, thereby decreasing the likelihood for the emergence of MAb-resistant variants.

Immune and Inflammatory Reponses to Staphylococcus aureus Skin Infections

Purpose of Review

There have been recent advances in our understanding of cutaneous immune responses to the important human skin pathogen, Staphylococcus aureus (S. aureus). This review will highlight these insights into innate and adaptive immune mechanisms in host defense and cutaneous inflammation in response to S. aureus skin infections.

Recent Findings

Antimicrobial peptides, pattern recognition receptors and inflammasome activation function in innate immunity as well as T cells and their effector cytokines play a key role in adaptive immunity against S. aureus skin infections. In addition, certain mechanisms by which S. aureus contributes to aberrant cutaneous inflammation, such as in flares of the inflammatory skin disease atopic dermatitis have also been identified.

Summary

These cutaneous immune mechanisms could provide new targets for future vaccines and immune-based therapies to combat skin infections and cutaneous inflammation caused by S. aureus.

Liposomal Therapy Attenuates Dermonecrosis Induced by Community-Associated Methicillin-Resistant Staphylococcus aureus by Targeting α-Type Phenol-Soluble Modulins and α-Hemolysin

Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA), typified by the pulse-field type USA300, is an emerging endemic pathogen that is spreading rapidly among healthy people. CA-MRSA causes skin and soft tissue infections, life-threatening necrotizing pneumonia and sepsis, and is remarkably resistant to many antibiotics. Here we show that engineered liposomes composed of naturally occurring sphingomyelin were able to sequester cytolytic toxins secreted by USA300 and prevent necrosis of human erythrocytes, peripheral blood mononuclear cells and bronchial epithelial cells. Mass spectrometric analysis revealed the capture by liposomes of phenol-soluble modulins, α-hemolysin and other toxins. Sphingomyelin liposomes prevented hemolysis induced by pure phenol-soluble modulin-α3, one of the main cytolytic components in the USA300 secretome. In contrast, sphingomyelin liposomes harboring a high cholesterol content (66 mol/%) were unable to protect human cells from phenol-soluble modulin-α3-induced lysis, however these liposomes efficiently sequestered the potent staphylococcal toxin α-hemolysin. In a murine cutaneous abscess model, a single dose of either type of liposomes was sufficient to significantly decrease tissue dermonecrosis. Our results provide further insights into the promising potential of tailored liposomal therapy in the battle against infectious diseases.

Deciphering the Pathological Role of Staphylococcal α-Toxin and Panton-Valentine Leukocidin Using a Novel Ex Vivo Human Skin Model

Staphylococcus aureus alpha-toxin and Panton-Valentine leukocidin (PVL) have been reported to play critical roles in different animal models of skin infection. These models, however, do not completely recapitulate the human disease due to the host specificity of these toxins as well as the intrinsic anatomical and immunological differences between animals and humans. Human skin explants represent a valid alternative to animal models for studying skin infections. Herein, we developed a human skin explant wound model to study the pathogenic role of alpha-toxin and PVL; inflammatory responses elicited by these toxins; and the neutralizing ability of antibodies to mitigate skin damage. Different concentrations of alpha-toxin and/PVL were applied to superficial wounds on human skin explants. Treatment with alpha-toxin resulted in high tissue toxicity and loss of skin epithelial integrity. PVL induced a milder but significant toxicity with no loss of skin structural integrity. The combination of both toxins resulted in increased tissue toxicity as compared with the individual toxins alone. Treatment of the skin with these toxins also resulted in a decrease of CD45-positive cells in the epidermis. In addition, both toxins induced the release of pro-inflammatory cytokines and chemokines. Finally, antibodies raised against alpha-toxin were able to mitigate tissue toxicity in a concentration-dependent manner. Results from this study confirm the key role of α-toxin in staphylococcal infection of the human skin and suggest a possible cooperation of the two toxins in tissue pathology.

Staphylococcus Aureus Surface Protein G is An Immunodominant Protein and a Possible Target in An Anti-Biofilm Drug Development

Background

Staphylococcus aureus is a Gram-positive bacterium that causes severe illnesses in the human population. The capacity of S. aureus strains to form biofilms on biotic and abiotic surfaces creates serious problems for treatment of hospital infections and has stimulated efforts to develop new means of specific protection or immunotherapy.

Material and Methods

We found that rabbit serum raised against crude concentrated S. aureus liquid culture significantly decreased the development of staphylococcal biofilm in vitro. To discover the corresponding staphylococcal antigen, we used mass-spectrometry and molecular cloning and identified three major immunodominant proteins. They included α-haemolysin, serine proteinase SplB and S. aureus surface protein G, known as adhesin SasG.

Results

Although according to literature data, all these proteins represent virulence factors of S. aureus and play diverse and important roles in the pathogenesis of staphylococcal diseases, only SasG can be directly implicated into the biofilm formation because of its surface location on a staphylococcal cell. Indeed, rabbit serum directed against purified recombinant SasG, similar to serum against crude staphylococcal liquid culture, prevented the formation of a biofilm.

Conclusion

SasG can be considered as a target in an anti-biofilm drug development and a component of the vaccine or immunotherapeutic preparations directed against staphylococcal infections in humans.

Alpha-Toxin Contributes to Biofilm Formation among Staphylococcus aureus Wound Isolates

Biofilms complicate treatment of Staphylococcus aureus (SA) wound infections. Previously, we determined alpha-toxin (AT)-promoted SA biofilm formation on mucosal tissue. Therefore, we evaluated SA wound isolates for AT production and biofilm formation on epithelium and assessed the role of AT in biofilm formation. Thirty-eight wound isolates were molecularly typed by pulsed-field gel electrophoresis (PFGE), multilocus sequence typing (ST), and spa typing. We measured biofilm formation of these SA isolates in vitro and ex vivo and quantified ex vivo AT production. We also investigated the effect of an anti-AT monoclonal antibody (MEDI4893*) on ex vivo biofilm formation by methicillin-resistant SA (USA 300 LAC) and tested whether purified AT rescued the biofilm defect of hla mutant SA strains. The predominant PFGE/ST combinations were USA100/ST5 (50%) and USA300/ST8 (33%) for methicillin-resistant SA (MRSA, n = 18), and USA200/ST30 (20%) for methicillin-susceptible SA (MSSA, n = 20). Ex vivo AT production correlated significantly with ex vivo SA wound isolate biofilm formation. Anti-alpha-toxin monoclonal antibody (MEDI4893*) prevented ex vivo biofilm formation by MRSA USA300 strain LAC. Wild-type AT rescued the ex vivo biofilm defect of non-AT producing SA strains. These findings provide evidence that AT plays a role in SA biofilm formation on epithelial surfaces and suggest that neutralization of AT may be useful in preventing and treating SA infections.

Individual variation is the key to the development of a vaccine against Staphylococcus aureus: a comparative study between mice lineages

Bacterial infections occur worldwide and are a major public health problem. Among pathogens, Staphylococcus aureus is the main causative agent of bacterial diseases in the world. This study aimed to evaluate which components of the immune system could act protectively against a S. aureus infection in intradermally immunized mice. C57BL/6 and A/j mice were immunized intradermally with S. aureus inactivated by heat and then challenged with viable strains in an air pouch model. At 6, 12, and 24 h after the challenge, euthanasia was performed, and the cellular profile of the inflammatory infiltrate, cytokines, and the bacterial load were evaluated in the air pouch lavages. Immunized mice demonstrated that the intradermal immunization with S. aureus promoted protection in C57BL/6 mice by reducing the bacterial, which was correlated with increased serum concentration of IgG antibodies (IgG1 and IgG2a) against S. aureus. The increase in IgG2a antibody levels was correlated with a decrease of bacterial load in intradermally immunized C57BL/6 mice, along with production of IL-17A at the inflammation site, as well as IgG1consumption. Similar results were not found in the A/j lineage. In conclusion, a vaccine against S. aureus should focus more on the individual characteristics of the host because it is a determinant factor for the success of the immunization.

Neutralizing Alpha-Toxin Accelerates Healing of Staphylococcus aureus-Infected Wounds in Nondiabetic and Diabetic Mice

Staphylococcus aureus wound infections delay healing and result in invasive complications such as osteomyelitis, especially in the setting of diabetic foot ulcers. In preclinical animal models of S. aureus skin infection, antibody neutralization of alpha-toxin (AT), an S. aureus-secreted pore-forming cytolytic toxin, reduces disease severity by inhibiting skin necrosis and restoring effective host immune responses. However, whether therapeutic neutralization of alpha-toxin is effective against S. aureus-infected wounds is unclear. Herein, the efficacy of prophylactic treatment with a human neutralizing anti-AT monoclonal antibody (MAb) was evaluated in an S. aureus skin wound infection model in nondiabetic and diabetic mice. In both nondiabetic and diabetic mice, anti-AT MAb treatment decreased wound size and bacterial burden and enhanced reepithelialization and wound resolution compared to control MAb treatment. Anti-AT MAb had distinctive effects on the host immune response, including decreased neutrophil and increased monocyte and macrophage infiltrates in nondiabetic mice and decreased neutrophil extracellular traps (NETs) in diabetic mice. Similar therapeutic efficacy was achieved with an active vaccine targeting AT. Taken together, neutralization of AT had a therapeutic effect against S. aureus-infected wounds in both nondiabetic and diabetic mice that was associated with differential effects on the host immune response.

Innate Sex Bias of Staphylococcus aureus Skin Infection Is Driven by α-Hemolysin

Numerous studies have reported sex bias in infectious diseases, with bias direction dependent on pathogen and site of infection. Staphylococcus aureus is the most common cause of skin and soft tissue infections (SSTIs), yet sex bias in susceptibility to S. aureus SSTI has not been described. A search of electronic health records revealed an odds ratio of 2.4 for S. aureus SSTI in males versus females. To investigate the physiological basis of this bias, we compared outcomes between male and female mice in a model of S. aureus dermonecrosis. Consistent with the epidemiological data, female mice were better protected against SSTI, with reduced dermonecrosis followed later by increased bacterial clearance. Protection in females was disrupted by ovariectomy and restored by short-term estrogen administration. Importantly, this sex bias was mediated by a sex-specific response to the S. aureus-secreted virulence factor α-hemolysin (Hla). Infection with wild-type S. aureus suppressed inflammatory cytokine production in the skin of female, but not male, mice when compared with infection with an isogenic hla deletion mutant. This differential response was conserved following injection with Hla alone, demonstrating a direct response to Hla independent of bacterial burden. Additionally, neutrophils, essential for clearing S. aureus, demonstrated sex-specific S. aureus bactericidal capacity ex vivo. This work suggests that sex-specific skin innate responsiveness to Hla and neutrophil bactericidal capacity play important roles in limiting S. aureus SSTI in females. Understanding the molecular mechanisms controlling this sex bias may reveal novel targets to promote host innate defense against S. aureus skin infection.

Pharmacokinetic and Pharmacodynamic Considerations for the Use of Monoclonal Antibodies in the Treatment of Bacterial Infections

Antibiotic-resistant bacterial pathogens are increasingly implicated in hospital- and community-acquired infections. Recent advances in monoclonal antibody (mAb) production and engineering have led to renewed interest in the development of antibody-based therapies for treatment of drug-resistant bacterial infections. Currently, there are three antibacterial mAb products approved by the Food and Drug Administration (FDA) and at least nine mAbs are in clinical trials. Antibacterial mAbs are typically developed to kill bacteria or to attenuate bacterial pathological activity through neutralization of bacterial toxins and virulence factors. Antibodies exhibit distinct pharmacological mechanisms from traditional antimicrobials and, hence, cross-resistance between small molecule antimicrobials and antibacterial mAbs is unlikely. Additionally, the long biological half-lives typically found for mAbs may allow convenient dosing and vaccine-like prophylaxis from infection. However, the high affinity of mAbs and the involvement of the host immune system in their pharmacological actions may lead to complex and nonlinear pharmacokinetics and pharmacodynamics. In this review, we summarize the pharmacokinetics and pharmacodynamics of the FDA-approved antibacterial mAbs and those are currently in clinical trials. Challenges in the development of antibacterial mAbs are also discussed.

Skin Wound Healing Is Accelerated by a Lipid Mixture Representing Major Lipid Components of Chamaecyparis obtusa Plant Extract

In chronic nonhealing wounds, the healing process is disrupted and wounds are often infected with bacteria. About 85% of lower extremity amputations in diabetes are attributed to deep infection of foot ulcers. Therefore, infection control is critical for wound care. In this study, we analyzed lipid composition of Chamaecyparis obtusa extract, and we describe the wound-healing properties of its combination of 10 major lipid components. A 10-lipid mixture up-regulated HBD-3 and LL-37 through the olfactory receptor 2AT4 and induced phosphorylation of extracellular signal-regulated kinases and p38 mitogen-activated protein kinases in primary human keratinocytes. In addition, the 10-lipid mixture had direct bactericidal effects against Staphylococcus aureus and Streptococcus pyogenes and protected against staphylococcal α-toxin-induced keratinocyte cell death. In an animal model, the 10-lipid mixture accelerated skin wound healing and was also effective in healing wounds superinfected with S. aureus. We suggest that the 10-lipid mixture, because of its wound-healing and antimicrobial properties, can be beneficial for wound treatment.

α-Toxin Regulates Local Granulocyte Expansion from Hematopoietic Stem and Progenitor Cells in Staphylococcus aureus-Infected Wounds

The immune response to Staphylococcus aureus infection in skin involves the recruitment of polymorphonuclear neutrophils (PMNs) from the bone marrow via the circulation and local granulopoiesis from hematopoietic stem and progenitor cells (HSPCs) that also traffic to infected skin wounds. We focus on regulation of PMN number and function and the role of pore-forming α-toxin (AT), a virulence factor that causes host cell lysis and elicits inflammasome-mediated IL-1β secretion in wounds. Infection with wild-type S. aureus enriched in AT reduced PMN recruitment and resulted in sustained bacterial burden and delayed wound healing. In contrast, PMN recruitment to wounds infected with an isogenic AT-deficient S. aureus strain was unimpeded, exhibiting efficient bacterial clearance and hastened wound resolution. HSPCs recruited to infected wounds were unaffected by AT production and were activated to expand PMN numbers in proportion to S. aureus abundance in a manner regulated by TLR2 and IL-1R signaling. Immunodeficient MyD88-knockout mice infected with S. aureus experienced lethal sepsis that was reversed by PMN expansion mediated by injection of wild-type HSPCs directly into wounds. We conclude that AT-induced IL-1β promotes local granulopoiesis and effective resolution of S. aureus-infected wounds, revealing a potential antibiotic-free strategy for tuning the innate immune response to treat methicillin-resistant S. aureus infection in immunodeficient patients.

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

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

VLP-based vaccine induces immune control of Staphylococcus aureus virulence regulation

Staphylococcus aureus is the leading cause of skin and soft tissue infections (SSTIs) and mounting antibiotic resistance requires innovative treatment strategies. S. aureus uses secreted cyclic autoinducing peptides (AIPs) and the accessory gene regulator (agr) operon to coordinate expression of virulence factors required for invasive infection. Of the four agr alleles (agr types I-IV and corresponding AIPs1-4), agr type I isolates are most frequently associated with invasive infection. Cyclization via a thiolactone bond is essential for AIP function; therefore, recognition of the cyclic form of AIP1 may be necessary for antibody-mediated neutralization. However, the small sizes of AIPs and labile thiolactone bond have hindered vaccine development. To overcome this, we used a virus-like particle (VLP) vaccine platform (PP7) for conformationally-restricted presentation of a modified AIP1 amino acid sequence (AIP1S). Vaccination with PP7-AIP1S elicited AIP1-specific antibodies and limited agr-activation in vivo. Importantly, in a murine SSTI challenge model with a highly virulent agr type I S. aureus isolate, PP7-AIP1S vaccination reduced pathogenesis and increased bacterial clearance compared to controls, demonstrating vaccine efficacy. Given the contribution of MRSA agr type I isolates to human disease, vaccine targeting of AIP1-regulated virulence could have a major clinical impact in the fight against antibiotic resistance.

Staphylococcus aureus-dependent septic arthritis in murine knee joints: local immune response and beneficial effects of vaccination

Staphylococcus aureus is the major cause of human septic arthritis and osteomyelitis, which deserve special attention due to their rapid evolution and resistance to treatment. The progression of the disease depends on both bacterial presence in situ and uncontrolled disruptive immune response, which is responsible for chronic disease. Articular and bone infections are often the result of blood bacteremia, with the knees and hips being the most frequently infected joints showing the worst clinical outcome. We report the development of a hematogenous model of septic arthritis in murine knees, which progresses from an acute to a chronic phase, similarly to what occurs in humans. Characterization of the local and systemic inflammatory and immune responses following bacterial infection brought to light specific signatures of disease. Immunization of mice with the vaccine formulation we have recently described (4C-Staph), induced a strong antibody response and specific CD4+ effector memory T cells, and resulted in reduced bacterial load in the knee joints, a milder general inflammatory state and protection against bacterial-mediated cellular toxicity. Possible correlates of protection are finally proposed, which might contribute to the development of an effective vaccine for human use.

Critical Role of Alpha-Toxin and Protective Effects of Its Neutralization by a Human Antibody in Acute Bacterial Skin and Skin Structure Infections

Methicillin-resistant Staphylococcus aureus (MRSA) causes large-scale epidemics of acute bacterial skin and skin structure infections (ABSSSI) within communities across the United States. Animal models that reproduce ABSSSI as they occur in humans are urgently needed to test new therapeutic strategies. Alpha-toxin plays a critical role in a variety of staphylococcal infection models in mice, but its role in the pathogenesis of ABSSSI remains to be elucidated in rabbits, which are similar to humans in their susceptibility to S. aureus superantigens and certain bicomponent pore-forming leukocidins. We report here a new rabbit model of ABSSSI and show that those infected with a mutant deficient in expression of alpha-toxin (Δhla) developed a small dermonecrotic lesion, whereas those infected with isogenic USA300 MRSA wild-type or complemented Δhla strains developed ABSSSI that mimic the severe infections that occur in humans, including the large central dermonecrotic core surrounded by erythema, induration, and marked subcutaneous hemorrhage. More importantly, immunoprophylaxis with MEDI4893*, an anti-alpha-toxin human monoclonal antibody, significantly reduced the severity of disease caused by a USA300 wild-type strain to that caused by the Δhla mutant, indicating that this toxin could be completely neutralized during infection. Thus, this study illustrates a potential high standard for the development of new immunotherapeutic agents in which a toxin-neutralizing antibody provides protection to the same degree achieved with a toxin gene knockout. When MEDI4893* was administered as adjunctive therapy with a subtherapeutic dose of linezolid, the combination was significantly more efficacious than either agent alone in reducing the severity of ABSSSI.

Staphylococcus aureus Alpha-Toxin Is Conserved among Diverse Hospital Respiratory Isolates Collected from a Global Surveillance Study and Is Neutralized by Monoclonal Antibody MEDI4893

Staphylococcus aureus infections lead to an array of illnesses ranging from mild skin infections to serious diseases, such endocarditis, osteomyelitis, and pneumonia. Alpha-toxin (Hla) is a pore-forming toxin, encoded by the hla gene, that is thought to play a key role in S. aureus pathogenesis. A monoclonal antibody targeting Hla, MEDI4893, is in clinical development for the prevention of S. aureus ventilator-associated pneumonia (VAP). The presence of the hla gene and Hla protein in 994 respiratory isolates collected from patients in 34 countries in Asia, Europe, the United States, Latin America, the Middle East, Africa, and Australia was determined. Hla levels were correlated with the geographic location, age of the subject, and length of stay in the hospital. hla gene sequence analysis was performed, and mutations were mapped to the Hla crystal structure. S. aureus supernatants containing Hla variants were tested for susceptibility or resistance to MEDI4893. The hla gene was present and Hla was expressed in 99.0% and 83.2% of the isolates, respectively, regardless of geographic region, hospital locale, or age of the subject. More methicillin-susceptible than methicillin-resistant isolates expressed Hla (86.9% versus 78.8%; P = 0.0007), and S. aureus isolates from pediatric patients expressed the largest amounts of Hla. Fifty-seven different Hla subtypes were identified, and 91% of the isolates encoded an Hla subtype that was neutralized by MED4893. This study demonstrates that Hla is conserved in diverse S. aureus isolates from around the world and is an attractive target for prophylactic monoclonal antibody (MAb) or vaccine development.

Host factors that contribute to recurrent staphylococcal skin infection

PURPOSE OF REVIEW

Staphylococcus aureus is the most common cause of skin and soft tissue infections (SSTI) in the United States and elsewhere. Recurrent infections occur frequently in patients with S. aureus SSTI, underscoring the need to better understand the nature of protective immunity against these infections. Here, we review recent findings concerning the host factors that predispose to S. aureus SSTI.

RECENT FINDINGS

Recurrent infections occur in nearly half of all patients with S. aureus SSTI. Epidemiologic and environmental factors, such as exposure to healthcare, age, and household contacts with S. aureus SSTI, and contaminated household fomites are associated with recurrence. The majority of the population has evidence of antistaphylococcal antibodies, but whether these are protective remains enigmatic. In contrast, recent clinical and experimental findings clearly highlight the critical roles of innate and T cell-mediated immunity in defense against these infections. S. aureus interferes with innate and adaptive immunity by a number of recently elucidated mechanisms.

SUMMARY

Recurrent S. aureus SSTIs are common, suggesting incomplete or absent protective immunity among these patients. Our understanding of protective immunity against recurrent infections is incomplete, and further basic and translational investigation is urgently needed to design strategies to prevent and treat these infections.

Detection of Alpha-Toxin and Other Virulence Factors in Biofilms of Staphylococcus aureus on Polystyrene and a Human Epidermal Model

BACKGROUND & AIM

The ability of Staphylococcus aureus to successfully colonize (a)biotic surfaces may be explained by biofilm formation and the actions of virulence factors. The aim of the present study was to establish the presence of 52 proteins, including virulence factors such as alpha-toxin, during biofilm formation of five different (methicillin resistant) S. aureus strains on Leiden human epidermal models (LEMs) and polystyrene surfaces (PS) using a competitive Luminex-based assay.

RESULTS

All five S. aureus strains formed biofilms on PS, whereas only three out of five strains formed biofilms on LEMs. Out of the 52 tested proteins, six functionally diverse proteins (ClfB, glucosaminidase, IsdA, IsaA, SACOL0688 and nuclease) were detected in biofilms of all strains on both PS and LEMs. At the same time, four toxins (alpha-toxin, gamma-hemolysin B and leukocidins D and E), two immune modulators (formyl peptide receptor-like inhibitory protein and Staphylococcal superantigen-like protein 1), and two other proteins (lipase and LytM) were detectable in biofilms by all five S. aureus strains on LEMs, but not on PS. In contrast, fibronectin-binding protein B (FnbpB) was detectable in biofilms by all S. aureus biofilms on PS, but not on LEMs. These data were largely confirmed by the results from proteomic and transcriptomic analyses and in case of alpha-toxin additionally by GFP-reporter technology.

CONCLUSION

Functionally diverse virulence factors of (methicillin-resistant) S. aureus are present during biofilm formation on LEMs and PS. These results could aid in identifying novel targets for future treatment strategies against biofilm-associated infections.

The Innovative Medicines Initiative's New Drugs for Bad Bugs programme: European public-private partnerships for the development of new strategies to tackle antibiotic resistance

Antibiotic resistance (ABR) is a global public health threat. Despite the emergence of highly resistant organisms and the huge medical need for new drugs, the development of antibacterials has slowed to an unacceptable level worldwide. Numerous government and non-government agencies have called for public-private partnerships and innovative funding mechanisms to address this problem. To respond to this public health crisis, the Innovative Medicines Initiative Joint Undertaking programme has invested more than €660 million, with a goal of matched contributions from the European Commission and the European Federation of Pharmaceutical Industries and Associations, in the development of new antibacterial strategies. The New Drugs for Bad Bugs (ND4BB) programme, an Innovative Medicines Initiative, has the ultimate goal to boost the fight against ABR at every level from basic science and drug discovery, through clinical development to new business models and responsible use of antibiotics. Seven projects have been launched within the ND4BB programme to achieve this goal. Four of them will include clinical trials of new anti-infective compounds, as well as epidemiological studies on an unprecedented scale, which will increase our knowledge of ABR and specific pathogens, and improve the designs of the clinical trials with new investigational drugs. The need for rapid concerted action has driven the funding of seven topics, each of which should add significantly to progress in the fight against ABR. ND4BB unites expertise and provides a platform where the commitment and resources required by all parties are streamlined into a joint public-private partnership initiative of unprecedented scale.