A peptide antagonist of CD28 signaling attenuates toxic shock and necrotizing soft-tissue infection induced by Streptococcus pyogenes

CD8+ T cells are necessary for improved sepsis survival induced by CD28 agonism in immunologically experienced mice

Introduction

A hallmark of T cell dysregulation during sepsis is the downregulation of costimulatory molecules. CD28 is one of T cell costimulatory molecules significantly altered on memory T cells during sepsis. We recently showed that treatment with a αCD28 agonist in septic immunologically experienced mice led to improved survival. Therefore, here we aimed to identify the cell subset(s) necessary for the survival benefit observed in the context of CD28 agonism, and to further investigate the mechanism by which CD28 agonism improves sepsis survival in immunologically experienced mice. Methods: Mice received specific pathogen inoculation to generate memory T cell populations similar in frequency to that of adult humans. Once these infections were cleared and the T cell response had transitioned to the memory phase, animals were rendered septic via cecal ligation and puncture in the presence or absence of an agonistic anti-CD28 mAb.

Results

Results demonstrated that CD8+ T cells, and not bulk CD4+ T cells or CD25+ regulatory T cells, were necessary for the survival benefit observed in CD28 agonist-treated septic immunologically experienced mice. Upon examination of these CD8+ T cells, we found that CD28 agonism in septic immunologically experienced mice was associated with an increase in Foxp3+ CD8+ T cells as compared to vehicle-treated controls. When CD8+ T cells were depleted in septic immunologically experienced mice in the setting of CD28 agonism, a significant increase in levels of inflammatory cytokines in the blood was observed.

Discussion

Taken together, these results indicate that CD28 agonism in immunologically experienced mice effectively suppresses inflammation via a CD8+-dependent mechanism to decrease mortality during sepsis.

Targeting staphylococcal enterotoxin B binding to CD28 as a new strategy for dampening superantigen-mediated intestinal epithelial barrier dysfunctions

Staphylococcus aureus is a gram-positive bacterium that may cause intestinal inflammation by secreting enterotoxins, which commonly cause food-poisoning and gastrointestinal injuries. Staphylococcal enterotoxin B (SEB) acts as a superantigen (SAg) by binding in a bivalent manner the T-cell receptor (TCR) and the costimulatory receptor CD28, thus stimulating T cells to produce large amounts of inflammatory cytokines, which may affect intestinal epithelial barrier integrity and functions. However, the role of T cell-mediated SEB inflammatory activity remains unknown. Here we show that inflammatory cytokines produced by T cells following SEB stimulation induce dysfunctions in Caco-2 intestinal epithelial cells by promoting actin cytoskeleton remodelling and epithelial cell-cell junction down-regulation. We also found that SEB-activated inflammatory T cells promote the up-regulation of epithelial-mesenchymal transition transcription factors (EMT-TFs) in a nuclear factor-κB (NF-κB)- and STAT3-dependent manner. Finally, by using a structure-based design approach, we identified a SEB mimetic peptide (pSEB116-132) that, by blocking the binding of SEB to CD28, dampens inflammatory-mediated dysregulation of intestinal epithelial barrier.

Antistaphylococcal discovery pipeline; where are we now?

The serious spread of antibiotic-resistant Staphylococcal aureus strains is alarming. This is reflected by the measures governments and health-related bodies are offering to ease antibiotic drug development. Finding new active agents, preferably with novel mechanism of action, or even finding new targets for drug development are essential. In this review, we summarize the current status of novel antistaphylococcal agents undergoing clinical trials. We mainly discuss antistaphylococcal small molecules and peptides in the text with a special focus on their chemistry, while antistaphylococcal immunotherapy (antibodies) are mentioned in a summative table. This review shall serve as a summary that influences future synthetic efforts in the antistaphyloccocals development field.

Antimicrobial resistance crisis: could artificial intelligence be the solution?

Antimicrobial resistance is a global public health threat, and the World Health Organization (WHO) has announced a priority list of the most threatening pathogens against which novel antibiotics need to be developed. The discovery and introduction of novel antibiotics are time-consuming and expensive. According to WHO's report of antibacterial agents in clinical development, only 18 novel antibiotics have been approved since 2014. Therefore, novel antibiotics are critically needed. Artificial intelligence (AI) has been rapidly applied to drug development since its recent technical breakthrough and has dramatically improved the efficiency of the discovery of novel antibiotics. Here, we first summarized recently marketed novel antibiotics, and antibiotic candidates in clinical development. In addition, we systematically reviewed the involvement of AI in antibacterial drug development and utilization, including small molecules, antimicrobial peptides, phage therapy, essential oils, as well as resistance mechanism prediction, and antibiotic stewardship.

The homodimer interfaces of costimulatory receptors B7 and CD28 control their engagement and pro-inflammatory signaling

BACKGROUND

The inflammatory response is indispensable for protective immunity, yet microbial pathogens often trigger an excessive response, 'cytokine storm', harmful to the host. Full T-cell activation requires interaction of costimulatory receptors B7-1(CD80) and B7-2(CD86) expressed on antigen-presenting cells with CD28 expressed on the T cells. We created short peptide mimetics of the homodimer interfaces of the B7 and CD28 receptors and examined their ability to attenuate B7/CD28 coligand engagement and signaling through CD28 for inflammatory cytokine induction in human immune cells, and to protect from lethal toxic shock in vivo.

METHODS

Short B7 and CD28 receptor dimer interface mimetic peptides were synthesized and tested for their ability to attenuate the inflammatory cytokine response of human peripheral blood mononuclear cells, as well as for their ability to attenuate B7/CD28 intercellular receptor engagement. Mice were used to test the ability of such peptides to protect from lethal superantigen toxin challenge when administered in molar doses far below the toxin dose.

RESULTS

B7 and CD28 homodimer interfaces are remote from the coligand binding sites, yet our finding is that by binding back into the receptor dimer interfaces, short dimer interface mimetic peptides inhibit intercellular B7-2/CD28 as well as the tighter B7-1/CD28 engagement, attenuating thereby pro-inflammatory signaling. B7 mimetic peptides exhibit tight selectivity for the cognate receptor in inhibiting intercellular receptor engagement with CD28, yet each diminishes signaling through CD28. In a prominent example of inflammatory cytokine storm, by attenuating formation of the B7/CD28 costimulatory axis, B7-1 and CD28 dimer interface mimetic peptides protect mice from lethal toxic shock induced by a bacterial superantigen even when administered in doses far submolar to the superantigen.

CONCLUSIONS

Our results reveal that the B7 and CD28 homodimer interfaces each control B7/CD28 costimulatory receptor engagement and highlight the protective potential against cytokine storm of attenuating, yet not ablating, pro-inflammatory signaling via these receptor domains.

Fighting antibiotic resistance-strategies and (pre)clinical developments to find new antibacterials

Antibacterial resistance is one of the greatest threats to human health. The development of new therapeutics against bacterial pathogens has slowed drastically since the approvals of the first antibiotics in the early and mid-20^th century. Most of the currently investigated drug leads are modifications of approved antibacterials, many of which are derived from natural products. In this review, we highlight the challenges, advancements and current standing of the clinical and preclinical antibacterial research pipeline. Additionally, we present novel strategies for rejuvenating the discovery process and advocate for renewed and enthusiastic investment in the antibacterial discovery pipeline.

Superantigens, a Paradox of the Immune Response

Staphylococcal enterotoxins are a wide family of bacterial exotoxins with the capacity to activate as much as 20% of the host T cells, which is why they were called superantigens. Superantigens (SAgs) can cause multiple diseases in humans and cattle, ranging from mild to life-threatening infections. Almost all S. aureus isolates encode at least one of these toxins, though there is no complete knowledge about how their production is triggered. One of the main problems with the available evidence for these toxins is that most studies have been conducted with a few superantigens; however, the resulting characteristics are attributed to the whole group. Although these toxins share homology and a two-domain structure organization, the similarity ratio varies from 20 to 89% among different SAgs, implying wide heterogeneity. Furthermore, every attempt to structurally classify these proteins has failed to answer differential biological functionalities. Taking these concerns into account, it might not be appropriate to extrapolate all the information that is currently available to every staphylococcal SAg. Here, we aimed to gather the available information about all staphylococcal SAgs, considering their functions and pathogenicity, their ability to interact with the immune system as well as their capacity to be used as immunotherapeutic agents, resembling the two faces of Dr. Jekyll and Mr. Hyde.

A host-directed macrocyclic peptide therapeutic for MDR gram negative bacterial infections

The emergence of infections by carbapenem resistant Enterobacteriaceae (CRE) pathogens has created an urgent public health threat, as carbapenems are among the drugs of last resort for infections caused by a growing fraction of multi-drug resistant (MDR) bacteria. There is global consensus that new preventive and therapeutic strategies are urgently needed to combat the growing problem of MDR bacterial infections. Here, we report on the efficacy of a novel macrocyclic peptide, minimized theta-defensin (MTD)-12813 in CRE sepsis. MTD12813 is a theta-defensin inspired cyclic peptide that is highly effective against CRE pathogens K. pneumoniae and E. coli in vivo. In mouse septicemia models, single dose administration of MTD12813 significantly enhanced survival by promoting rapid host-mediated bacterial clearance and by modulating pathologic cytokine responses, restoring immune homeostasis, and preventing lethal septic shock. The peptide lacks direct antibacterial activity in the presence of mouse serum or in peritoneal fluid, further evidence for its indirect antibacterial mode of action. MTD12813 is highly stable in biological matrices, resistant to bacterial proteases, and nontoxic to mice at dose levels 100 times the therapeutic dose level, properties which support further development of the peptide as a first in class anti-infective therapeutic.

Binding of Staphylococcal Enterotoxin B (SEB) to B7 Receptors Triggers TCR- and CD28-Mediated Inflammatory Signals in the Absence of MHC Class II Molecules

The inflammatory activity of staphylococcal enterotoxin B (SEB) relies on its capacity to trigger polyclonal T-cell activation by binding both T-cell receptor (TCR) and costimulatory receptor CD28 on T cells and MHC class II and B7 molecules on antigen presenting cells (APC). Previous studies highlighted that SEB may bind TCR and CD28 molecules independently of MHC class II, yet the relative contribution of these interactions to the pro-inflammatory function of SEB remained unclear. Here, we show that binding to MHC class II is dispensable for the inflammatory activity of SEB, whereas binding to TCR, CD28 and B7 molecules is pivotal, in both human primary T cells and Jurkat T cell lines. In particular, our finding is that binding of SEB to B7 molecules suffices to trigger both TCR- and CD28-mediated inflammatory signalling. We also provide evidence that, by strengthening the interaction between CD28 and B7, SEB favours the recruitment of the TCR into the immunological synapse, thus inducing lethal inflammatory signalling.

Resolution of organ dysfunction as a predictor of long-term survival in necrotizing soft tissue infections: Analysis of the AB103 Clinical Composite Endpoint Study in Necrotizing Soft Tissue Infections trial and a retrospective claims database-linked chart study

BACKGROUND

Necrotizing soft tissue infections (NSTIs) are an acute surgical condition with high morbidity and mortality. Timely identification, resuscitation, and aggressive surgical management have significantly decreased inpatient mortality. However, reduced inpatient mortality has shifted the burden of disease to long-term mortality associated with persistent organ dysfunction.

METHODS

We performed a combined analysis of NSTI patients from the AB103 Clinical Composite Endpoint Study in Necrotizing Soft Tissue Infections randomized-controlled interventional trial (ATB-202) and comprehensive administrative database (ATB-204) to determine the association of persistent organ dysfunction on inpatient and long-term outcomes. Persistent organ dysfunction was defined as a modified Sequential Organ Failure Assessment (mSOFA) score of 2 or greater at Day 14 (D14) after NSTI diagnosis, and resolution of organ dysfunction defined as mSOFA score of 1 or less.

RESULTS

The analysis included 506 hospitalized NSTI patients requiring surgical debridement, including 247 from ATB-202, and 259 from ATB-204. In both study cohorts, age and comorbidity burden were higher in the D14 mSOFA ≥2 group. Patients with D14 mSOFA score of 1 or less had significantly lower 90-day mortality than those with mSOFA score of 2 or higher in both ATB-202 (2.4% vs. 21.5%; p < 0.001) and ATB-204 (6% vs. 16%: p = 0.008) studies. In addition, in an adjusted covariate analysis of the combined study data sets D14 mSOFA score of 1 or lesss was an independent predictor of lower 90-day mortality (odds ratio, 0.26; 95% confidence interval, 0.13-0.53; p = 0.001). In both studies, D14 mSOFA score of 1 or less was associated with more favorable discharge status and decreased resource utilization.

CONCLUSION

For patients with NSTI undergoing surgical management, persistent organ dysfunction at 14 days, strongly predicts higher resource utilization, poor discharge disposition, and higher long-term mortality. Promoting the resolution of acute organ dysfunction after NSTI should be considered as a target for investigational therapies to improve long-term outcomes after NSTI.

LEVEL OF EVIDENCE

Prognostic/epidemiology study, level III.

CD28 Agonism Improves Survival in Immunologically Experienced Septic Mice via IL-10 Released by Foxp3+ Regulatory T Cells

Immune dysregulation during sepsis is mediated by an imbalance of T cell costimulatory and coinhibitory signaling. CD28 is downregulated during sepsis and is significantly altered on memory versus naive T cells. Thus, to study the role of CD28 during sepsis in a more physiologically relevant context, we developed a "memory mouse" model in which animals are subjected to pathogen infections to generate immunologic memory, followed by sepsis induction via cecal ligation and puncture. Using this system, we show that agonistic anti-CD28 treatment resulted in worsened survival in naive septic animals but conferred a significant survival advantage in immunologically experienced septic animals. Mechanistically, this differential response was driven by the ability of CD28 agonism to elicit IL-10 production from regulatory T cells uniquely in memory but not naive mice. Moreover, elevated IL-10 released by activated regulatory T cells in memory mice inhibited sepsis-induced T cell apoptosis via the antiapoptotic protein Bcl-xL. Together, these data demonstrate that immunologic experience is an important parameter that affects sepsis pathophysiology and can fundamentally change the outcome of modulating the CD28 pathway during sepsis. This study suggests that testing therapeutic strategies in immunologically experienced hosts may be one way to increase the physiologic relevance of rodent models in sepsis research.

Host Defense Peptides as Templates for Antifungal Drug Development

Current treatment for invasive fungal diseases is limited to three classes of antifungal drugs: azoles, polyenes, and echinocandins. The most recently introduced antifungal class, the echinocandins, was first approved nearly 30 years ago. The limited antifungal drug portfolio is rapidly losing its clinical utility due to the inexorable rise in the incidence of invasive fungal infections and the emergence of multidrug resistant (MDR) fungal pathogens. New antifungal therapeutic agents and novel approaches are desperately needed. Here, we detail attempts to exploit the antifungal and immunoregulatory properties of host defense peptides (HDPs) in the design and evaluation of new antifungal therapeutics and discuss historical limitations and recent advances in this quest.

Acute Kidney Injury: From Diagnosis to Prevention and Treatment Strategies

Acute kidney injury (AKI) is characterized by an acute decrease in renal function that can be multifactorial in its origin and is associated with complex pathophysiological mechanisms. In the short term, AKI is associated with an increased length of hospital stay, health care costs, and in-hospital mortality, and its impact extends into the long term, with AKI being associated with increased risks of cardiovascular events, progression to chronic kidney disease (CKD), and long-term mortality. Given the impact of the prognosis of AKI, it is important to recognize at-risk patients and improve preventive, diagnostic, and therapy strategies. The authors provide a comprehensive review on available diagnostic, preventive, and treatment strategies for AKI.

Staging and Personalized Intervention for Infection and Sepsis

Background: Sepsis is defined as a dysregulated host response to infection, resulting in life-threatening organ dysfunction. It is now understood that this dysregulation not only constitutes excessive inflammation, but also sustained immune suppression. Immune-modulatory therapies thus have great potential for novel sepsis therapies. Here, we provide a review of biomarkers and functional assays designed to immunologically stage patients with sepsis as well as therapies designed to alter the innate and adaptive immune systems of patients with sepsis beneficially. Methods: A search of PubMed/MEDLINE and clinicaltrials.gov was performed between October 1, 2019 and December 22, 2019 using search terms such as "sepsis immunotherapy," "sepsis biomarkers," "sepsis clinical trials," and variations thereof. Results: Despite more than 30 years of research, there is still no Food and Drug Administration (FDA)-cleared biomarker that has proven to be effective in either identifying patients with sepsis who are at an increased risk of adverse outcomes or responsive to specific interventions. Similarly, past clinical trials investigating new treatment strategies have rarely stratified patients with sepsis. Overall, the results of these trials have been disappointing. Novel efforts to properly gauge an individual patient's immune response and choose an appropriate immunomodulatory agent based on the results are underway. Conclusion: Our evolving understanding of the different mechanisms perturbing immune homeostasis during sepsis strongly suggests that future successes will depend on finding the right therapy for the right patient and administering it at the right time. For such a personalized medicine approach, novel biomarkers and functional assays to properly stage the patient with sepsis will be crucial. The growing repertoire of immunomodulatory agents at our disposal, as well as re-appraisal of agents that have already been tested in unstratified cohorts of patients with sepsis, may finally translate into successful treatment strategies for sepsis.

egc Superantigens Impair Monocytes/Macrophages Inducing Cell Death and Inefficient Activation

Bacterial superantigens (SAgs) are enterotoxins that bind to MHC-II and TCR molecules, activating as much as 20% of the T cell population and promoting a cytokine storm which enhances susceptibility to endotoxic shock, causing immunosuppression, and hindering the immune response against bacterial infection. Since monocytes/macrophages are one of the first cells SAgs find in infected host and considering the effect these cells have on directing the immune response, here, we investigated the effect of four non-classical SAgs of the staphylococcal egc operon, namely, SEG, SEI, SEO, and SEM on monocytic-macrophagic cells, in the absence of T cells. We also analyzed the molecular targets on APCs which could mediate SAg effects. We found that egc SAgs depleted the pool of innate immune effector cells and induced an inefficient activation of monocytic-macrophagic cells, driving the immune response to an impaired proinflammatory profile, which could be mediated directly or indirectly by interactions with MHC class II. In addition, performing surface plasmon resonance assays, we demonstrated that non-classical SAgs bind the gp130 molecule, which is also present in the monocytic cell surface, among other cells.

Staphylococcal and Streptococcal Superantigens Trigger B7/CD28 Costimulatory Receptor Engagement to Hyperinduce Inflammatory Cytokines

Staphylococcal and streptococcal superantigens are virulence factors that cause toxic shock by hyperinducing inflammatory cytokines. Effective T-cell activation requires interaction between the principal costimulatory receptor CD28 and its two coligands, B7-1 (CD80) and B7-2 (CD86). To elicit an inflammatory cytokine storm, bacterial superantigens must bind directly into the homodimer interfaces of CD28 and B7-2. Recent evidence revealed that by engaging CD28 and B7-2 directly at their dimer interface, staphylococcal enterotoxin B (SEB) potently enhances intercellular synapse formation mediated by B7-2 and CD28, resulting in T-cell hyperactivation. Here, we addressed the question, whether diverse bacterial superantigens share the property of triggering B7-2/CD28 receptor engagement and if so, whether they are capable of enhancing also the interaction between B7-1 and CD28, which occurs with an order-of-magnitude higher affinity. To this end, we compared the ability of distinct staphylococcal and streptococcal superantigens to enhance intercellular B7-2/CD28 engagement. Each of these diverse superantigens promoted B7-2/CD28 engagement to a comparable extent. Moreover, they were capable of triggering the intercellular B7-1/CD28 interaction, analyzed by flow cytometry of co-cultured cell populations transfected separately to express human CD28 or B7-1. Streptococcal mitogenic exotoxin Z (SMEZ), the most potent superantigen known, was as sensitive as SEB, SEA and toxic shock syndrome toxin-1 (TSST-1) to inhibition of inflammatory cytokine induction by CD28 and B7-2 dimer interface mimetic peptides. Thus, superantigens act not only by mediating unconventional interaction between MHC-II molecule and T-cell receptor but especially, by strongly promoting engagement of CD28 by its B7-2 and B7-1 coligands, a critical immune checkpoint, forcing the principal costimulatory axis to signal excessively. Our results show that the diverse superantigens use a common mechanism to subvert the inflammatory response, strongly enhancing B7-1/CD28 and B7-2/CD28 costimulatory receptor engagement.

Bacterial Superantigen Toxins, CD28, and Drug Development

During severe bacterial infections, death and disease are often caused by an overly strong immune response of the human host. Acute toxic shock is induced by superantigen toxins, a diverse set of proteins secreted by Gram-positive staphylococcal and streptococcal bacterial strains that overstimulate the inflammatory response by orders of magnitude. The need to protect from superantigen toxins led to our discovery that in addition to the well-known MHC class II and T cell receptors, the principal costimulatory receptor, CD28, and its constitutively expressed coligand, B7-2 (CD86), previously thought to have only costimulatory function, are actually critical superantigen receptors. Binding of the superantigen into the homodimer interfaces of these costimulatory receptors greatly enhances B7-2/CD28 engagement, leading to excessive pro-inflammatory signaling. This finding led to the design of short receptor dimer interface mimetic peptides that block the binding of superantigen and thus protect from death. It then turned out that such a peptide will protect also from Gram-negative bacterial infection and from polymicrobial sepsis. One such CD28 mimetic peptide is advancing in a Phase 3 clinical trial to protect from lethal wound infections by flesh-eating bacteria. These host-oriented therapeutics target the human immune system itself, rendering pathogens less likely to become resistant.

Thinking Outside the Box-Novel Antibacterials To Tackle the Resistance Crisis

The public view on antibiotics as reliable medicines changed when reports about "resistant superbugs" appeared in the news. While reasons for this resistance development are easily spotted, solutions for re-establishing effective antibiotics are still in their infancy. This Review encompasses several aspects of the antibiotic development pipeline from very early strategies to mature drugs. An interdisciplinary overview is given of methods suitable for mining novel antibiotics and strategies discussed to unravel their modes of action. Select examples of antibiotics recently identified by using these platforms not only illustrate the efficiency of these measures, but also highlight promising clinical candidates with therapeutic potential. Furthermore, the concept of molecules that disarm pathogens by addressing gatekeepers of virulence will be covered. The Review concludes with an evaluation of antibacterials currently in clinical development. Overall, this Review aims to connect select innovative antimicrobial approaches to stimulate interdisciplinary partnerships between chemists from academia and industry.

Bacterial superantigen toxins induce a lethal cytokine storm by enhancing B7-2/CD28 costimulatory receptor engagement, a critical immune checkpoint

Formation of the costimulatory axis between the B7-2 and CD28 coreceptors is critical for T-cell activation. Superantigens, Gram-positive bacterial virulence factors, cause toxic shock and sepsis by hyperinducing inflammatory cytokines. We report a novel role for costimulatory receptors CD28 and B7-2 as obligatory receptors for superantigens, rendering them therapeutic targets. We show that by engaging not only CD28 but also its coligand B7-2 directly, superantigens potently enhance the interaction between B7-2 and CD28, inducing thereby T-cell hyperactivation. Using a conserved twelve amino-acid domain, superantigens engage both B7-2 and CD28 at their homodimer interfaces, sites far removed from where these receptors interact, implying that inflammatory signaling can be controlled through the receptor homodimer interfaces. Short B7-2 and CD28 dimer interface mimetic peptides bind diverse superantigens, prevent superantigen binding to cell-surface B7-2 or CD28, attenuate inflammatory cytokine overexpression, and protect mice from lethal superantigen challenge. Thus, superantigens induce a cytokine storm by mediating not only the interaction between MHC-II molecule and T-cell receptor but critically, by promoting B7-2/CD28 coreceptor engagement, forcing the principal costimulatory axis to signal excessively. Our findings highlight the B7/CD28 interaction as a bottleneck in signaling for expression of inflammatory cytokines. B7-2 and CD28 homodimer interface mimetic peptides prevent superantigen lethality by blocking the superantigen-host costimulatory receptor interaction.

Superantigens hyperinduce inflammatory cytokines by enhancing the B7-2/CD28 costimulatory receptor interaction

Full T-cell activation requires interaction between the costimulatory receptors B7-2 and CD28. By binding CD28, bacterial superantigens elicit harmful inflammatory cytokine overexpression through an unknown mechanism. We show that, by engaging not only CD28 but also its coligand B7-2 directly, superantigens potently enhance the avidity between B7-2 and CD28, inducing thereby T-cell hyperactivation. Using the same 12-aa β-strand-hinge-α-helix domain, superantigens engage both B7-2 and CD28 at their homodimer interfaces, areas remote from where these coreceptors interact, implying that inflammatory signaling can be controlled through the receptor homodimer interfaces. Short B7-2 dimer interface mimetic peptides bind diverse superantigens, prevent superantigen binding to cell-surface B7-2 or CD28, attenuate inflammatory cytokine overexpression, and protect mice from lethal superantigen challenge. Thus, superantigens induce a cytokine storm not only by mediating the interaction between MHC-II molecule and T-cell receptor but also, critically, by promoting B7-2/CD28 coreceptor engagement, forcing the principal costimulatory axis to signal excessively. Our results reveal a role for B7-2 as obligatory receptor for superantigens. B7-2 homodimer interface mimotopes prevent superantigen lethality by blocking the superantigen-host costimulatory receptor interaction.

CD28 Costimulation: From Mechanism to Therapy

Ligation of the CD28 receptor on T cells provides a critical second signal alongside T cell receptor (TCR) ligation for naive T cell activation. Here, we discuss the expression, structure, and biochemistry of CD28 and its ligands. CD28 signals play a key role in many T cell processes, including cytoskeletal remodeling, production of cytokines, survival, and differentiation. CD28 ligation leads to unique epigenetic, transcriptional, and post-translational changes in T cells that cannot be recapitulated by TCR ligation alone. We discuss the function of CD28 and its ligands in both effector and regulatory T cells. CD28 is critical for regulatory T cell survival and the maintenance of immune homeostasis. We outline the roles that CD28 and its family members play in human disease and we review the clinical efficacy of drugs that block CD28 ligands. Despite the centrality of CD28 and its family members and ligands to immune function, many aspects of CD28 biology remain unclear. Translation of a basic understanding of CD28 function into immunomodulatory therapeutics has been uneven, with both successes and failures. Such real-world results might stem from multiple factors, including complex receptor-ligand interactions among CD28 family members, differences between the mouse and human CD28 families, and cell-type specific roles of CD28 family members.

Innate Immune Signaling Activated by MDR Bacteria in the Airway

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

CD28 homodimer interface mimetic peptide acts as a preventive and therapeutic agent in models of severe bacterial sepsis and gram-negative bacterial peritonitis

BACKGROUND

Severe gram-negative bacterial infections and sepsis are major causes of morbidity and mortality. Dysregulated, excessive proinflammatory cytokine expression contributes to the pathogenesis of sepsis. A CD28 mimetic peptide (AB103; previously known as p2TA) that attenuates CD28 signaling and T-helper type 1 cytokine responses was tested for its ability to increase survival in models of polymicrobial infection and gram-negative sepsis.

METHODS

Mice received AB103, followed by an injection of Escherichia coli 0111:B4 lipopolysaccharide (LPS); underwent induction E. coli 018:K1 peritonitis induction, followed by treatment with AB103; or underwent cecal ligation and puncture (CLP), followed by treatment with AB103. The effects of AB103 on factors associated with and the lethality of challenge infections were analyzed.

RESULTS

AB103 strongly attenuated induction of tumor necrosis factor α and interleukin 6 (IL-6) by LPS in human peripheral blood mononuclear cells. Receipt of AB103 following intraperitoneal injection of LPS resulted in survival among 73% of CD1 mice (11 of 15), compared with 20% of controls (3 of 15). Suboptimal doses of antibiotic alone protected 20% of mice (1 of 5) from E. coli peritonitis, whereas 100% (15 of 15) survived when AB103 was added 4 hours following infection. Survival among mice treated with AB103 12 hours after CLP was 100% (8 of 8), compared with 17% among untreated mice (1 of 6). In addition, receipt of AB103 12 hours after CLP attenuated inflammatory cytokine responses and neutrophil influx into tissues and promoted bacterial clearance. Receipt of AB103 24 hours after CLP still protected 63% of mice (5 of 8).

CONCLUSIONS

Single-dose AB103 reduces mortality in experimental models of polymicrobial and gram-negative bacterial infection and sepsis, warranting further studies of this agent in clinical trials.

CD4+ T cells promote the pathogenesis of Staphylococcus aureus pneumonia

We postulated that the activation of proinflammatory signaling by methicillin-resistant Staphylococcus aureus (MRSA) strain USA300 is a major factor in the pathogenesis of severe pneumonia and a target for immunomodulation. Local activation of T cells in the lung was a conserved feature of multiple strains of S. aureus, in addition to USA300. The pattern of Vβ chain activation was consistent with known superantigens, but deletion of SelX or SEK and SEQ was not sufficient to prevent T-cell activation, indicating the participation of multiple genes. Using Rag2(-/-), Cd4(-/-), and Cd28(-/-) mice, we observed significantly improved clearance of MRSA from the airways and decreased lung pathology, compared with findings for wild-type controls. The improved outcome correlated with decreased production of proinflammatory cytokines (tumor necrosis factor, KC, interleukin 6, and interleukin 1β). Our data suggest that T-cell-mediated hypercytokinemia induced by infection with MRSA strain USA300 contributes to pathogenesis and may be a therapeutic target for improving outcomes of this common infection in a clinical setting.

Strategies to improve drug development for sepsis

Sepsis, a common and potentially fatal systemic illness, is triggered by microbial infection and often leads to impaired function of the lungs, kidneys or other vital organs. Since the early 1980s, a large number of therapeutic agents for the treatment of sepsis have been evaluated in randomized controlled clinical trials. With few exceptions, the results from these trials have been disappointing, and no specific therapeutic agent is currently approved for the treatment of sepsis. To improve upon this dismal record, investigators will need to identify more suitable therapeutic targets, improve their approaches for selecting candidate compounds for clinical development and adopt better designs for clinical trials.

Single administration of p2TA (AB103), a CD28 antagonist peptide, prevents inflammatory and thrombotic reactions and protects against gastrointestinal injury in total-body irradiated mice

The goal of this study was to elucidate the action of the CD28 mimetic peptide p2TA (AB103) that attenuates an excessive inflammatory response in mitigating radiation-induced inflammatory injuries. BALB/c and A/J mice were divided into four groups: Control (C), Peptide (P; 5 mg/kg of p2TA peptide), Radiation (R; total body irradiation with 8 Gy γ-rays), and Radiation + Peptide (RP; irradiation followed by p2TA peptide 24 h later). Gastrointestinal tissue damage was evaluated by analysis of jejunum histopathology and immunohistochemistry for cell proliferation (Cyclin D1) and inflammation (COX-2) markers, as well as the presence of macrophages (F4/80). Pro-inflammatory cytokines IL-6 and KC as well as fibrinogen were quantified in plasma samples obtained from the same mice. Our results demonstrated that administration of p2TA peptide significantly reduced the irradiation-induced increase of IL-6 and fibrinogen in plasma 7 days after exposure. Seven days after total body irradiation with 8 Gy of gamma rays numbers of intestinal crypt cells were reduced and villi were shorter in irradiated animals compared to the controls. The p2TA peptide delivery 24 h after irradiation led to improved morphology of villi and crypts, increased Cyclin D1 expression, decreased COX-2 staining and decreased numbers of macrophages in small intestine of irradiated mice. Our study suggests that attenuation of CD28 signaling is a promising therapeutic approach for mitigation of radiation-induced tissue injury.

Functional genomic characterization of virulence factors from necrotizing fasciitis-causing strains of Aeromonas hydrophila

The genomes of 10 Aeromonas isolates identified and designated Aeromonas hydrophila WI, Riv3, and NF1 to NF4; A. dhakensis SSU; A. jandaei Riv2; and A. caviae NM22 and NM33 were sequenced and annotated. Isolates NF1 to NF4 were from a patient with necrotizing fasciitis (NF). Two environmental isolates (Riv2 and -3) were from the river water from which the NF patient acquired the infection. While isolates NF2 to NF4 were clonal, NF1 was genetically distinct. Outside the conserved core genomes of these 10 isolates, several unique genomic features were identified. The most virulent strains possessed one of the following four virulence factors or a combination of them: cytotoxic enterotoxin, exotoxin A, and type 3 and 6 secretion system effectors AexU and Hcp. In a septicemic-mouse model, SSU, NF1, and Riv2 were the most virulent, while NF2 was moderately virulent. These data correlated with high motility and biofilm formation by the former three isolates. Conversely, in a mouse model of intramuscular infection, NF2 was much more virulent than NF1. Isolates NF2, SSU, and Riv2 disseminated in high numbers from the muscular tissue to the visceral organs of mice, while NF1 reached the liver and spleen in relatively lower numbers on the basis of colony counting and tracking of bioluminescent strains in real time by in vivo imaging. Histopathologically, degeneration of myofibers with significant infiltration of polymorphonuclear cells due to the highly virulent strains was noted. Functional genomic analysis provided data that allowed us to correlate the highly infectious nature of Aeromonas pathotypes belonging to several different species with virulence signatures and their potential ability to cause NF.

Update on staphylococcal superantigen-induced signaling pathways and therapeutic interventions

Staphylococcal enterotoxin B (SEB) and related bacterial toxins cause diseases in humans and laboratory animals ranging from food poisoning, acute lung injury to toxic shock. These superantigens bind directly to the major histocompatibility complex class II molecules on antigen-presenting cells and specific Vβ regions of T-cell receptors (TCR), resulting in rapid hyper-activation of the host immune system. In addition to TCR and co-stimulatory signals, proinflammatory mediators activate signaling pathways culminating in cell-stress response, activation of NFκB and mammalian target of rapamycin (mTOR). This article presents a concise review of superantigen-activated signaling pathways and focuses on the therapeutic challenges against bacterial superantigens.

CD28: direct and critical receptor for superantigen toxins

Every adaptive immune response requires costimulation through the B7/CD28 axis, with CD28 on T-cells functioning as principal costimulatory receptor. Staphylococcal and streptococcal superantigen toxins hyperstimulate the T-cell-mediated immune response by orders of magnitude, inducing a lethal cytokine storm. We show that to elicit an inflammatory cytokine storm and lethality, superantigens must bind directly to CD28. Blocking access of the superantigen to its CD28 receptor with peptides mimicking the contact domains in either toxin or CD28 suffices to protect mice effectively from lethal shock. Our finding that CD28 is a direct receptor of superantigen toxins broadens the scope of microbial pathogen recognition mechanisms.