Toxicity of staphylococcal enterotoxins potentiated by lipopolysaccharide: major histocompatibility complex class II molecule dependency and cytokine release

Anti-inflammatory and anti-angiogenic effects of Withania somnifera extract on liver toxicity induced by silver nanoparticles in vivo

The liver is a critical organ in the human body and is frequently exposed to numerous exogenous toxic substances, including silver nanoparticles (AgNPs). This study aimed to examine the anti-inflammatory, anti-angiogenic, and hepatoprotective effects of Withania somnifera (W. somnifera) extract on AgNP-induced liver toxicity in Swiss mice. Fifty mice were divided into five groups. Group I (negative control) consisted of ten mice. Group II received oral W. somnifera extracts (80 mg/kg/bw) for 14 days. Group III was injected intraperitoneally (i.p.) with AgNPs at a daily dose of 35 mg/kg/bw for 3 days. Group IV received i.p. AgNPs for 3 days, followed by saline for 14 days. Group V received i.p. AgNPs for 3 days, followed by oral W. somnifera (80 mg/kg/bw) for 14 days. Liver function tests, pro and anti-inflammatory cytokines, antioxidant activities, protein carbonyl (PC) levels, liver histopathological analysis, immunohistochemical expressions of transcription factor (NF-κB), and vascular endothelial growth factor (VEGF) were examined. Group III had elevated levels of liver function, a significant increase of pro and anti-inflammatory cytokines, antioxidant activity, and PC levels. Histological observations revealed congested sinusoids filled with red blood cells (RBCs) and hepatocyte necrosis. Also, positive expressions of NF-κB and VEGF were detected compared with Group I. However, the administration of W. somnifera to Group V revealed significant changes with evident improvements in liver function biomarkers, pro and anti-inflammatory cytokines, antioxidant activities, oxidative stress markers (PC), and histopathological and immunohistochemical parameters compared to Group III. The results revealed that W. somnifera has promising and potential hepatoprotective, anti-inflammatory, and anti-angiogenic effects against liver toxicity. Further detailed studies are recommended to explore the potential of W. somnifera as a treatment for human liver ailments.

MyD88 and beyond: a perspective on MyD88-targeted therapeutic approach for modulation of host immunity

The continuous emergence of infectious pathogens along with antimicrobial resistance creates a need for an alternative approach to treat infectious diseases. Targeting host factor(s) which are critically involved in immune signaling pathways for modulation of host immunity offers to treat a broad range of infectious diseases. Upon pathogen-associated ligands binding to the Toll-like/ IL-1R family, and other cellular receptors, followed by recruitment of intracellular signaling adaptor proteins, primarily MyD88, trigger the innate immune responses. But activation of host innate immunity strongly depends on the correct function of MyD88 which is tightly regulated. Dysregulation of MyD88 may cause an imbalance that culminates to a wide range of inflammation-associated syndromes and diseases. Furthermore, recent reports also describe that MyD88 upregulation with many viral infections is linked to decreased antiviral type I IFN response, and MyD88-deficient mice showed an increase in survivability. These reports suggest that MyD88 is also negatively involved via MyD88-independent pathways of immune signaling for antiviral type I IFN response. Because of its expanding role in controlling host immune signaling pathways, MyD88 has been recognized as a potential drug target in a broader drug discovery paradigm. Targeting BB-loop of MyD88, small molecule inhibitors were designed by structure-based approach which by blocking TIR-TIR domain homo-dimerization have shown promising therapeutic efficacy in attenuating MyD88-mediated inflammatory impact, and increased antiviral type I IFN response in experimental mouse model of diseases. In this review, we highlight the reports on MyD88-linked immune response and MyD88-targeted therapeutic approach with underlying mechanisms for controlling inflammation and antiviral type I IFN response. HIGHLIGHTS: • Host innate immunity is activated upon PAMPs binding to PRRs followed by immune signaling through TIR domain-containing adaptor proteins mainly MyD88. • Structure-based approach led to develop small-molecule inhibitors which block TIR domain homodimerization of MyD88 and showed therapeutic efficacy in limiting severe inflammation-associated impact in mice. • Therapeutic intervention of MyD88 also showed an increase in antiviral effect with strong type I IFN signaling linked to increased phosphorylation of IRFs via MyD88-independent pathway. • MyD88 inhibitors might be potentially useful as a small-molecule therapeutics for modulation of host immunity against inflammatory diseases and antiviral therapy. • However, prior clinical use of more in-depth efforts should be focused for suitability of the approach in deploying to complex diseases including COPD and COVID-19 in limiting inflammation-associated syndrome to infection.

Staphylococcal enterotoxin B- and lipopolysaccharide-induced toxic shock syndrome in a burn patient

Toxic shock syndrome (TSS) is caused by toxic shock syndrome toxin 1 or enterotoxins secreted by Staphylococcus aureus. Lipopolysaccharide (LPS) has also been shown to play a major role in the development of sepsis. Staphylococcal superantigens and LPS operate synergistically in conditioning cytokine release and lethal shock in mice. An 80-year-old woman was admitted because of a 20% mixed-depth flame burn. Despite two excisions and grafts, necrotic ulcers with methicillin-resistant Staphylococcus aureus (MRSA) colonization remained. On the 7th day after the operation, she developed shock with an erythematous rash. Blood examination revealed evidence of disseminated intravascular coagulation, and liver and renal dysfunction. A blood culture revealed a staphylococcal enterotoxin B (SEB)-producing strain of MRSA and Klebsiella pneumoniae. The septic symptoms were prolonged, but the condition gradually improved with extensive treatment. T-cell receptor analysis demonstrated a marked accumulation of SEB-mediated Vβ T cells. Stimulation of peripheral blood mononuclear cells in the recovery phase with SEB and LPS induced additive effects on tumor necrosis factor-α, interferon-γ, and interleukin-6 production. Although the present case did not fulfill the clinical criteria for TSS, the additive effects of SEB and LPS might have caused the severe septic shock.

Effective Treatment of Staphylococcal Enterotoxin B Aerosol Intoxication in Rhesus Macaques by Using Two Parenterally Administered High-Affinity Monoclonal Antibodies

Staphylococcal enterotoxin B (SEB) is a protein exotoxin found on the cell surface of Staphylococcus aureus that is the source for multiple pathologies in humans. When purified and concentrated in aerosol form, SEB can cause an acute and often fatal intoxication and thus is considered a biological threat agent.

Staphylococcal enterotoxin B (SEB) is a protein exotoxin found on the cell surface of Staphylococcus aureus that is the source for multiple pathologies in humans. When purified and concentrated in aerosol form, SEB can cause an acute and often fatal intoxication and thus is considered a biological threat agent. There are currently no vaccines or treatments approved for human use. Studies with rodent models of SEB intoxication show that antibody therapy may be a promising treatment strategy; however, many have used antibodies only prophylactically or well before any clinical signs of intoxication are apparent. We assessed and compared the protective efficacies of two monoclonal antibodies, Ig121 and c19F1, when administered after aerosol exposure in a uniformly lethal nonhuman primate model of SEB intoxication. Rhesus macaques were challenged using small-particle aerosols of SEB and then were infused intravenously with a single dose of either Ig121 or c19F1 (10 mg/kg of body weight) at either 0.5, 2, or 4 h postexposure. Onset of clinical signs and hematological and cytokine response in untreated controls confirmed the acute onset and potency of the toxin used in the challenge. All animals administered either Ig121 or c19F1 survived SEB challenge, whereas the untreated controls succumbed to SEB intoxication 30 to 48 h postexposure. These results represent the successful therapeutic in vivo protection by two investigational drugs against SEB in a severe nonhuman primate disease model and punctuate the therapeutic value of monoclonal antibodies when faced with treatment options for SEB-induced toxicity in a postexposure setting.

Potent Neutralization of Staphylococcal Enterotoxin B In Vivo by Antibodies that Block Binding to the T-Cell Receptor

To develop an antibody (Ab) therapeutic against staphylococcal enterotoxin B (SEB), a potential incapacitating bioterrorism agent and a major cause of food poisoning, we developed a "class T" anti-SEB neutralizing Ab (GC132) targeting an epitope on SEB distinct from that of previously developed "class M" Abs. A systematic engineering approach was applied to affinity-mature Ab GC132 to yield an optimized therapeutic candidate (GC132a) with sub-nanomolar binding affinity. Mapping of the binding interface by hydrogen-deuterium exchange coupled to mass spectrometry revealed that the class T epitope on SEB overlapped with the T-cell receptor binding site, whereas other evidence suggested that the class M epitope overlapped with the binding site for the major histocompatibility complex. In the IgG format, GC132a showed ∼50-fold more potent toxin-neutralizing efficacy than the best class M Ab in vitro, and fully protected mice from lethal challenge in a toxic shock post-exposure model. We also engineered bispecific Abs (bsAbs) that bound tetravalently by utilizing two class M binding sites and two class T binding sites. The bsAbs displayed enhanced toxin neutralization efficacy compared with the respective monospecific Ab subunits as well as a mixture of the two, indicating that enhanced efficacy was due to heterotypic tetravalent binding to two non-overlapping epitopes on SEB. Together, these results suggest that class T anti-SEB Ab GC132a is an excellent candidate for clinical development and for bsAb engineering.

Staphylococcal Superantigens: Pyrogenic Toxins Induce Toxic Shock

Staphylococcal enterotoxin B (SEB) and related superantigenic toxins produced by Staphylococcus aureus are potent activators of the immune system. These protein toxins bind to major histocompatibility complex (MHC) class II molecules and specific Vβ regions of T-cell receptors (TCRs), resulting in the activation of both monocytes/macrophages and T lymphocytes. The bridging of TCRs with MHC class II molecules by superantigens triggers an early “cytokine storm” and massive polyclonal T-cell proliferation. Proinflammatory cytokines, tumor necrosis factor α, interleukin 1 (IL-1), IL-2, interferon γ (IFNγ), and macrophage chemoattractant protein 1 elicit fever, inflammation, multiple organ injury, hypotension, and lethal shock. Upon MHC/TCR ligation, superantigens induce signaling pathways, including mitogen-activated protein kinase cascades and cytokine receptor signaling, which results in NFκB activation and the phosphoinositide 3-kinase/mammalian target of rapamycin pathways. In addition, gene profiling studies have revealed the essential roles of innate antimicrobial defense genes in the pathogenesis of SEB. The genes expressed in a murine model of SEB-induced shock include intracellular DNA/RNA sensors, apoptosis/DNA damage-related molecules, endoplasmic reticulum/mitochondrial stress responses, immunoproteasome components, and IFN-stimulated genes. This review focuses on the signaling pathways induced by superantigens that lead to the activation of inflammation and damage response genes. The induction of these damage response genes provides evidence that SEB induces danger signals in host cells, resulting in multiorgan injury and toxic shock. Therapeutics targeting both host inflammatory and cell death pathways can potentially mitigate the toxic effects of staphylococcal superantigens.

TBA225, a fusion toxoid vaccine for protection and broad neutralization of staphylococcal superantigens

Superantigens (SAgs) play a major role in the pathogenesis of Staphylococcus aureus and are associated with several diseases, including food poisoning, bacterial arthritis, and toxic shock syndrome. Monoclonal antibodies to these SAgs, primarily TSST-1, SEB and SEA have been shown to provide protection in animal studies and to reduce clinical severity in bacteremic patients. Here we quantify the pre-existing antibodies against SAgs in many human plasma and IVIG samples and demonstrate that in a major portion of the population these antibody titers are suboptimal and IVIG therapy only incrementally elevates the anti-SAg titers. Our in vitro neutralization studies show that a combination of antibodies against SEA, SEB,and TSST-1 can provide broad neutralization of staphylococcal SAgs. We report a single fusion protein (TBA₂₂₅) consisting of the toxoid versions of TSST-1, SEB and SEA and demonstrate its immunogenicity and protective efficacy in a mouse model of toxic shock. Antibodies raised against this fusion vaccine provide broad neutralization of purified SAgs and culture supernatants of multiple clinically relevant S. aureus strains. Our data strongly supports the use of this fusion protein as a component of an anti-virulence based multivalent toxoid vaccine against S. aureus disease.

Maternal immune activation with staphylococcal enterotoxin A produces unique behavioral changes in C57BL/6 mouse offspring

Stimulation of the immune system during pregnancy, known as maternal immune activation (MIA), can cause long-lasting neurobiological and behavioral changes in the offspring. This phenomenon has been implicated in the etiology of developmental psychiatric disorders, such as autism and schizophrenia. Much of this evidence is predicated on animal models using bacterial agents such as LPS and/or viral mimics such as Poly I:C, both of which act through toll-like receptors. However, fewer studies have examined the role of direct activation of maternal T-cells during pregnancy using microbial agents. Bacterial superantigens, such as Staphylococcal Enterotoxin A and B (SEA; SEB), are microbial proteins that activate CD4+ T-cells and cause prominent T-cell proliferation and cytokine production. We injected pregnant and non-pregnant adult female C57BL/6 mice with 200 μg/Kg of SEA, SEB, or 0.9% saline, and measured splenic T-cell-derived cytokine concentrations (viz., IL-2, IFN-γ, IL-6, and IL-4) 2 h later; animals injected with SEA were also measured for splenic concentrations of TNF-α and IL-17A. Half of the injected pregnant animals were brought to term, and their offspring were tested on a series of behavioral tasks starting at six weeks of age (postnatal day 42 [P42]). These tasks included social interaction, the elevated plus maze (EPM), an open field and object recognition (OR) task, prepulse inhibition (PPI) of sensorimotor gating, and the Morris water maze (MWM). Results showed that SEA and SEB induced significant concentrations of all measured cytokines, and in particular IFN-γ, although cytokine responses were greater following SEA exposure. In addition, pregnancy induced an inhibitory effect on cytokine production. Behavioral results showed distinct phenotypes among offspring from SEA- or SEB-injected mothers, very likely due to differences in the magnitude of cytokines generated in response to each toxin. Offspring from SEA-injected mothers displayed modest decreases in social behavior, but increased anxiety, locomotion, interest in a novel object, and short-term spatial memory, while offspring of SEB-injected mothers only exhibited increased anxiety and locomotion. There were no deficits in PPI, which was actually pronounced in SEA and SEB offspring. Overall, the novel use of SEA and SEB as prenatal immune challenges elicited distinct behavioral profiles in the offspring that both mirrors and diverges from previous models of maternal immune activation in important ways. We conclude that superantigen-induced T-cell-mediated maternal immune activation is a valid and valuable model for studying and expanding our understanding of the effects of prenatal immune challenge on neurodevelopmental and behavioral alterations in offspring.

Systemic cytokine and chemokine responses in immunized mice challenged with staphylococcal enterotoxin B

The cytokine storm induced by staphylococcal enterotoxin B (SEB) describes the rapid and dramatic induction of mediators which are likely responsible for the toxin's deleterious effects. However despite the use of numerous animal models for investigating SEB related illness in humans, mechanisms of toxicity and correlates of protection remain unclear. In the present study, we used an LPS-potentiated model of SEB lethality to investigate the toxin-induced cytokine and chemokine responses in untreated and immunized mice. Of 30 separate mediators analyzed, serum levels for 28 or 27 of these cytokines and chemokines were elevated following administration of dosages of 3 or 30 LD50 of native SEB, respectively. Mice immunized with a non-toxic SEB vaccine candidate expressed in either E. coli or transgenic soy expression systems were protected from lethality when challenged with potentiated SEB. The majority of SEB-induced cytokines and chemokines (21 of 28 or 23 of 27 following challenge with dosages of 3 or 30 LD50 of native SEB, respectively) were significantly decreased in mice immunized with an SEB vaccine candidate when compared to control animals. Together, these studies provide the most comprehensive evaluation of the cytokine storm induced in this LPS-potentiated model of SEB lethality to date. As with other animal models, the identification of those mediators which are necessary and sufficient for SEB-induced toxicity remains unclear.

FDA-approved immunosuppressants targeting staphylococcal superantigens: mechanisms and insights

Immunostimulating staphylococcal enterotoxin B (SEB) and related superantigenic toxins cause diseases in human beings and laboratory animals by hyperactivating cells of the immune system. These protein toxins bind to the major histocompatibility complex class II (MHC II) molecules and specific Vβ regions of T-cell receptors (TCRs), resulting in the stimulation of both monocytes/macrophages and T lymphocytes. The bridging of TCR with MHC II molecules by superantigens triggers intracellular signaling cascades, resulting in excessive release of proinflammatory mediators and massive polyclonal T-cell proliferation. The early induction of tumor necrosis factor α, interleukin 1 (IL-1), interleukin 2 (IL-2), interferon gamma (IFNγ), and macrophage chemoattractant protein 1 promotes fever, inflammation, and multiple organ injury. The signal transduction pathways for staphylococcal superantigen-induced toxicity downstream from TCR/major histocompatibility complex (MHC) ligation and interaction of cell surface co-stimulatory molecules include the mitogen-activated protein kinase cascades and cytokine receptor signaling, activating nuclear factor κB (NFκB) and the phosphoinositide 3-kinase/mammalian target of rapamycin pathways. Knowledge of host regulation within these activated pathways and molecules initiated by SEB and other superantigens enables the selection of US Food and Drug Administration (FDA)-approved drugs to interrupt and prevent superantigen-induced shock in animal models. This review focuses on the use of FDA-approved immunosuppressants in targeting the signaling pathways induced by staphylococcal superantigens.

Rapid and Rigorous IL-17A Production by a Distinct Subpopulation of Effector Memory T Lymphocytes Constitutes a Novel Mechanism of Toxic Shock Syndrome Immunopathology

Toxic shock syndrome (TSS) is caused by staphylococcal and streptococcal superantigens (SAgs) that provoke a swift hyperinflammatory response typified by a cytokine storm. The precipitous decline in the host's clinical status and the lack of targeted therapies for TSS emphasize the need to identify key players of the storm's initial wave. Using a humanized mouse model of TSS and human cells, we herein demonstrate that SAgs elicit in vitro and in vivo IL-17A responses within hours. SAg-triggered human IL-17A production was characterized by remarkably high mRNA stability for this cytokine. A distinct subpopulation of CD4+ effector memory T (TEM) cells that secrete IL-17A, but not IFN-γ, was responsible for early IL-17A production. We found mouse "TEM-17" cells to be enriched within the intestinal epithelium and among lamina propria lymphocytes. Furthermore, interfering with IL-17A receptor signaling in human PBMCs attenuated the expression of numerous inflammatory mediators implicated in the TSS-associated cytokine storm. IL-17A receptor blockade also abrogated the secondary effect of SAg-stimulated PBMCs on human dermal fibroblasts as judged by C/EBP δ expression. Finally, the early IL-17A response to SAgs was pathogenic because in vivo neutralization of IL-17A in humanized mice ameliorated hepatic and intestinal damage and reduced mortality. Together, our findings identify CD4+ TEM cells as a key effector of TSS and reveal a novel role for IL-17A in TSS immunopathogenesis. Our work thus elucidates a pathogenic, as opposed to protective, role for IL-17A during Gram-positive bacterial infections. Accordingly, the IL-17-IL-17R axis may provide an attractive target for the management of SAg-mediated illnesses.

Lipopolysaccharide is required for the lethal effects of enterotoxin B after D-galactosamine sensitization

We tested the D-galactosamine sensitization model with staphylococcal enterotoxin B (SEB). LPS was required for the lethal effects of SEB in D-galactosamine sensitized mice. Only two (2/62) among the C3HeB/FeJ (H-2k), Balb/c (H-2d) and C57BL/6J (H-2b) mice died in response to SEB in the absence of LPS whereas injection of SEB and minimally lethal concentrations of LPS became highly toxic. Similar to LPS, the lethal effect of SEB was dependent on the mouse strain used. Mouse strains more sensitive to the effects of LPS (Balb/c and C57BL/6J) were also more sensitive to the effects of SEB in comparison to C3H mice when equivalent doses of LPS and SEB were used. Among Balb/c and C3HeB/FeJ but not C57BL/6J mice, SEB (20 μg) potentiated the lethal effects of LPS at low doses (0.1 μg LPS), but had an apparent protective effect at high doses (1 μg LPS). Lastly, there was an inverse correlation between pathogenesis and serum IL-2 concentrations and splenic T cell activation in C3H mice. However, macrophage mobilization did correlate with lethality. Therefore, some questions remain about the mechanisms involved in the D-galactosamine/SEB pathogenesis model. We conclude that when sensitizing mice with D-galactosamine and assessing the lethal effects of SEB, endotoxin contamination must be assessed.

Staphylococcal Superantigens Spark Host-Mediated Danger Signals

Staphylococcal enterotoxin B (SEB) of Staphylococcus aureus, and related superantigenic toxins produced by myriad microbes, are potent stimulators of the immune system causing a variety of human diseases from transient food poisoning to lethal toxic shock. These protein toxins bind directly to specific Vβ regions of T-cell receptors (TCR) and major histocompatibility complex (MHC) class II on antigen-presenting cells, resulting in hyperactivation of T lymphocytes and monocytes/macrophages. Activated host cells produce excessive amounts of proinflammatory cytokines and chemokines, especially tumor necrosis factor α, interleukin 1 (IL-1), IL-2, interferon γ (IFNγ), and macrophage chemoattractant protein 1 causing clinical symptoms of fever, hypotension, and shock. Because of superantigen-induced T cells skewed toward TH1 helper cells, and the induction of proinflammatory cytokines, superantigens can exacerbate autoimmune diseases. Upon TCR/MHC ligation, pathways induced by superantigens include the mitogen-activated protein kinase cascades and cytokine receptor signaling, resulting in activation of NFκB and the phosphoinositide 3-kinase/mammalian target of rapamycin pathways. Various mouse models exist to study SEB-induced shock including those with potentiating agents, transgenic mice and an “SEB-only” model. However, therapeutics to treat toxic shock remain elusive as host response genes central to pathogenesis of superantigens have only been identified recently. Gene profiling of a murine model for SEB-induced shock reveals novel molecules upregulated in multiple organs not previously associated with SEB-induced responses. The pivotal genes include intracellular DNA/RNA sensors, apoptosis/DNA damage-related molecules, immunoproteasome components, as well as antiviral and IFN-stimulated genes. The host-wide induction of these, and other, antimicrobial defense genes provide evidence that SEB elicits danger signals resulting in multi-organ damage and toxic shock. Ultimately, these discoveries might lead to novel therapeutics for various superantigen-based diseases.

Microbial influences on hormesis, oncogenesis, and therapy: A review of the literature

Utilization of environmental stimuli for growth is the main factor contributing to the evolution of prokaryotes and eukaryotes, independently and mutualistically. Epigenetics describes an organism's ability to vary expression of certain genes based on their environmental stimuli. The diverse degree of dose-dependent responses based on their variances in expressed genetic profiles makes it difficult to ascertain whether hormesis or oncogenesis has or is occurring. In the medical field this is shown where survival curves used in determining radiotherapeutic doses have substantial uncertainties, some as large as 50% (Barendsen, 1990). Many in-vitro radiobiological studies have been limited by not taking into consideration the innate presence of microbes in biological systems, which have either grown symbiotically or pathogenically. Present in-vitro studies neglect to take into consideration the varied responses that commensal and opportunistic pathogens will have when exposed to the same stimuli and how such responses could act as stimuli for their macro/microenvironment. As a result many theories such as radiation carcinogenesis explain microscopic events but fail to describe macroscopic events (Cohen, 1995). As such, this review shows how microorganisms have the ability to perturb risks of cancer and enhance hormesis after irradiation. It will also look at bacterial significance in the microenvironment of the tumor before and during treatment. In addition, bacterial systemic communication after irradiation and the host's immune responses to infection could explain many of the phenomena associated with bystander effects. Therefore, the present literature review considers the paradigms of hormesis and oncogenesis in order to find a rationale that ties them all together. This relationship was thus characterized to be the microbiome.

Bacterial Toxins-Staphylococcal Enterotoxin B

Staphylococcal enterotoxin B is one of the most potent bacterial superantigens that exerts profound toxic effects upon the immune system, leading to stimulation of cytokine release and inflammation. It is associated with food poisoning, nonmenstrual toxic shock, atopic dermatitis, asthma, and nasal polyps in humans. Currently, there is no treatment or vaccine available. Passive immunotherapy using monoclonal antibodies made in several different species has shown significant inhibition in in vitro studies and reduction in staphylococcal enterotoxin B-induced lethal shock in in vivo studies. This should encourage future endeavors to develop these antibodies as therapeutic reagents.

Adipose Tissue-Derived Mesenchymal Stem Cells Attenuate Staphylococcal Enterotoxin A-Induced Toxic Shock

Adipose tissue-derived stem cells (ASCs), which are mesenchymal stromal cells isolated from adipose tissues, exhibit immunomodulatory effects that are promising for several applications, including the therapeutics of inflammatory diseases. In the present study, the effect of ASCs on bacterial toxin-induced inflammation was investigated. Intraperitoneal administration of ASCs rescued mice from lethal shock induced by staphylococcal enterotoxin A (SEA) potentiated with lipopolysaccharide. In the sera and/or spleens of mice administered ASCs, the production of proinflammatory cytokines, including interferon gamma, tumor necrosis factor alpha, interleukin-6 (IL-6), and IL-2 was reduced. By quantitative real-time PCR, the expression of Foxp3 in the mice administered ASCs was not altered. On the other hand, the expression of IL-12 receptor and STAT4 was decreased with ASC administration. These results imply that the effect of ASCs is not involved in the lineage of regulatory T cells but that these cells may modulate TH1 differentiation. This information provides evidence that ASCs have properties that are effective to attenuate SEA-induced toxic shock and should prompt further exploration on other inflammatory diseases caused by bacterial toxins or bacterial infections.

Mechanisms mediating enhanced neutralization efficacy of staphylococcal enterotoxin B by combinations of monoclonal antibodies

Staphylococcal enterotoxin B (SEB) is a superantigen that cross-links the major histocompatibility complex class II and specific V-β chains of the T-cell receptor, thus forming a ternary complex. Developing neutralizing mAb to disrupt the ternary complex and abrogate the resulting toxicity is a major therapeutic challenge because SEB is effective at very low concentrations. We show that combining two SEB-specific mAbs enhances their efficacy, even though one of the two mAbs by itself has no effect on neutralization. Crystallography was employed for fine-mapping conformational epitopes in binary and ternary complexes between SEB and Fab fragments. NMR spectroscopy was used to validate and identify subtle allosteric changes induced by mAbs binding to SEB. The mapping of epitopes established that a combination of different mAbs can enhance efficacy of mAb-mediated protection from SEB induced lethal shock by two different mechanisms: one mAb mixture promoted clearance of the toxin both in vitro and in vivo by FcR-mediated cross-linking and clearance, whereas the other mAb mixture induced subtle allosteric conformational changes in SEB that perturbed formation of the SEB·T-cell receptor·major histocompatibility complex class II trimer. Finally structural information accurately predicted mAb binding to other superantigens that share conformational epitopes with SEB. Fine mapping of conformational epitopes is a powerful tool to establish the mechanism and optimize the action of synergistic mAb combinations.

Structure-Based Design and Synthesis of a Small Molecule that Exhibits Anti-inflammatory Activity by Inhibition of MyD88-mediated Signaling to Bacterial Toxin Exposure

Both Gram-positive and Gram-negative pathogens or pathogen-derived components, such as staphylococcal enterotoxins (SEs) and endotoxin (LPS) exposure, activate MyD88-mediated pro-inflammatory cellular immunity for host defense. However, dysregulated MyD88-mediated signaling triggers exaggerated immune response that often leads to toxic shock and death. Previously, we reported a small molecule compound 1 mimicking BB-loop structure of MyD88 was capable of inhibiting pro-inflammatory response to SEB exposure in mice. In this study, we designed a dimeric structure compound 4210 covalently linked with compound 1 by a non-polar cyclohexane linker which strongly inhibited the production of pro-inflammatory cytokines in human primary cells to SEB (IC50 1-50 μm) or LPS extracted from Francisella tularensis, Escherichia coli, or Burkholderia mallei (IC50 10-200 μm). Consistent with cytokine inhibition, in a ligand-induced cell-based reporter assay, compound 4210 inhibited Burkholderia mallei or LPS-induced MyD88-mediated NF-kB-dependent expression of reporter activity (IC50 10-30 μm). Furthermore, results from a newly expressed MyD88 revealed that 4210 inhibited MyD88 dimer formation which is critical for pro-inflammatory signaling. Importantly, a single administration of compound 4210 in mice showed complete protection from lethal toxin challenge. Collectively, these results demonstrated that compound 4210 inhibits toxin-induced inflated pro-inflammatory immune signaling, thus displays a potential bacterial toxin therapeutic.

Staphylococcal superantigens and toxins are detectable in the serum of adult burn patients

Bacterial infection in burn patients is still a devastating contributor to morbidity and mortality. Little is known regarding the presence of staphylococcal toxins in the burn-injured patient. The aim of this study was to characterize the prevalence of several of these toxins and their relationship to clinical metrics and mortality in burn patients. Levels of exotoxins staphylococcal enterotoxin A (SEA), staphylococcal enterotoxin B, toxic shock syndrome toxin 1 (TSST-1), and α-hemolysin were assayed from the serum of 207 adult burn patients aged 16-92 years. Clinical, demographic, and microbiological data from these patients were then compared to toxin levels. Staphylococcal exotoxins α-hemolysin and SEA were present in 45% and 25% of the population, respectively. Bacterial cultures concomitantly showed a high prevalence of Staphylococcus aureus in 48% of patients, of which 59% were methicillin resistant. Several metrics may be predictive of high toxin concentrations of α-hemolysin and TSST-1 and SEA including burn size, length of stay, and bacteremia. Mortality associations indicated that burn size, bacteremia, age, and the presence of α-hemolysin and SEA may be predictors of mortality. A high prevalence of staphylococcal toxin α-hemolysin and superantigens TSST-1 and SEA can be found in the circulation of the adult burn population. The presence of these toxins may contribute to the morbidity and mortality of the burn patient.

Late multiple organ surge in interferon-regulated target genes characterizes staphylococcal enterotoxin B lethality

Background

Bacterial superantigens are virulence factors that cause toxic shock syndrome. Here, the genome-wide, temporal response of mice to lethal intranasal staphylococcal enterotoxin B (SEB) challenge was investigated in six tissues.

Results

The earliest responses and largest number of affected genes occurred in peripheral blood mononuclear cells (PBMC), spleen, and lung tissues with the highest content of both T-cells and monocyte/macrophages, the direct cellular targets of SEB. In contrast, the response of liver, kidney, and heart was delayed and involved fewer genes, but revealed a dominant genetic program that was seen in all 6 tissues. Many of the 85 uniquely annotated transcripts participating in this shared genomic response have not been previously linked to SEB. Nine of the 85 genes were subsequently confirmed by RT-PCR in every tissue/organ at 24 h. These 85 transcripts, up-regulated in all tissues, annotated to the interferon (IFN)/antiviral-response and included genes belonging to the DNA/RNA sensing system, DNA damage repair, the immunoproteasome, and the ER/metabolic stress-response and apoptosis pathways. Overall, this shared program was identified as a type I and II interferon (IFN)-response and the promoters of these genes were highly enriched for IFN regulatory matrices. Several genes whose secreted products induce the IFN pathway were up-regulated at early time points in PBMCs, spleen, and/or lung. Furthermore, IFN regulatory factors including Irf1, Irf7 and Irf8, and Zbp1, a DNA sensor/transcription factor that can directly elicit an IFN innate immune response, participated in this host-wide SEB signature.

Conclusion

Global gene-expression changes across multiple organs implicated a host-wide IFN-response in SEB-induced death. Therapies aimed at IFN-associated innate immunity may improve outcome in toxic shock syndromes.

Mesenchymal stromal (stem) cells suppress pro-inflammatory cytokine production but fail to improve survival in experimental staphylococcal toxic shock syndrome

Background

Toxic shock syndrome (TSS) is caused by an overwhelming host-mediated response to bacterial superantigens produced mainly by Staphylococcus aureus and Streptococcus pyogenes. TSS is characterized by aberrant activation of T cells and excessive release of pro-inflammatory cytokines ultimately resulting in capillary leak, septic shock, multiple organ dysfunction and high mortality rates. No therapeutic or vaccine has been approved by the U.S. Food and Drug Administration for TSS, and novel therapeutic strategies to improve clinical outcome are needed. Mesenchymal stromal (stem) cells (MSCs) are stromal cells capable of self-renewal and differentiation. Moreover, MSCs have immunomodulatory properties, including profound effects on activities of T cells and macrophages in specific contexts. Based on the critical role of host-derived immune mediators in TSS, we hypothesized that MSCs could modulate the host-derived proinflammatory response triggered by Staphylococcal enterotoxin B (SEB) and improve survival in experimental TSS.

Methods

Effects of MSCs on proinflammatory cytokines in peripheral blood were measured in wild-type C57BL/6 mice injected with 50 μg of SEB. Effects of MSCs on survival were monitored in fatal experimental TSS induced by consecutive doses of D-galactosamine (10 mg) and SEB (10 μg) in HLA-DR4 transgenic mice.

Results

Despite significantly decreasing serum levels of IL-2, IL-6 and TNF induced by SEB in wild-type mice, human MSCs failed to improve survival in experimental TSS in HLA-DR4 transgenic mice. Similarly, a previously described downstream mediator of human MSCs, TNF-stimulated gene 6 (TSG-6), did not significantly improve survival in experimental TSS. Furthermore, murine MSCs, whether unstimulated or pre-treated with IFNγ, failed to improve survival in experimental TSS.

Conclusions

Our results suggest that the immunomodulatory effects of MSCs are insufficient to rescue mice from experimental TSS, and that mediators other than IL-2, IL-6 and TNF are likely to play critical mechanistic roles in the pathogenesis of experimental TSS.

Expression and bioactivity analysis of staphylococcal enterotoxin G and staphylococcal enterotoxin I

CONTEXT

The filtrate of Staphylococcus aureus culture, named staphylococcal enterotoxin C injection, has been used for 10 years in China. SEC2 has been claimed to be the only staphylococcal enterotoxins (SEs) without certifiable evidence.

OBJECTIVES

To present an efficient procedure for the expression and purification of recombinant proteins SEG and SEI, from S. aureus.

MATERIALS AND METHODS

In present work, we extracted total DNA from S. aureus (FRI 1230) and the recombinant proteins of SEG and SEI were then cloned, expressed and purified using E. coli. Splenic lymphocytes were used as effector cells and K562 and B16 cells were used as target cells to evaluate the inhibitory and stimulatory abilities of purified rSEG and rSEI on in vitro proliferation.

RESULTS

The size of amplified products of SEG and SEI genes were found to be about 400 and 467 bp, respectively. pGEX-SEG and pGEX-SEI were constructed successfully. SEG and SEI were demonstrated to be active stimulators of T-cell proliferation; moreover, they inhibited the proliferation of K562 cells and B16 cells.

DISCUSSION

The current findings suggest that SEC2 might not be the only active component of staphylococcal enterotoxin C injection and may involve other essential proteins like SEG and SEI in its clinical efficacy.

CONCLUSION

This efficient procedure for the expression and purification of SEG and SEI and may be useful for mass production of therapeutically important proteins. In the future, proteins acting as active stimulators of T-cell proliferation may help in developing effective cancer therapy.

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.

The staphylococcal enterotoxin (SE) family: SEB and siblings

Staphylococcus aureus plays an important role in numerous human cases of food poisoning, soft tissue, and bone infections, as well as potentially lethal toxic shock. This common bacterium synthesizes various virulence factors that include staphylococcal enterotoxins (SEs). These protein toxins bind directly to major histocompatibility complex class II on antigen-presenting cells and specific Vβ regions of T-cell receptors, resulting in potentially life-threatening stimulation of the immune system. Picomolar concentrations of SEs ultimately elicit proinflammatory cytokines that can induce fever, hypotension, multi-organ failure, and lethal shock. Various in vitro and in vivo models have provided important tools for studying the biological effects of, as well as potential vaccines/therapeutics against, the SEs. This review succinctly presents known physical and biological properties of the SEs, including various intervention strategies. In particular, SEB will often be portrayed as per biodefense concerns dating back to the 1960s.

Determination of C5a in murine models of staphylococcal enterotoxin B-induced toxic shock

Robust host innate immune response to staphylococcal enterotoxin B (SEB) and structurally related superantigens causes toxic shock and various autoimmune diseases. While proinflammatory cytokines are known for mediating SEB-induced toxicity, the role of complement C5a in SEB-mediated shock is less well-understood. An ELISA was developed to measure the complement activation product, C5a, in different murine models of toxic shock. This assay provides easy, quantifiable data for complement activation and its role in various SEB-induced toxic shock models.

Sublethal staphylococcal enterotoxin B challenge model in pigs to evaluate protection following immunization with a soybean-derived vaccine

In an effort to develop a sustainable platform for manufacturing protein-based vaccine candidates, we expressed a triple mutant of staphylococcal enterotoxin B carrying the L45R, Y89A, and Y94A modifications in transgenic soybean seeds (soy-mSEB). Soy-mSEB possessed no detectable superantigen activity in vitro. We found that this soybean-derived, nontoxic mutant of SEB could be stably expressed, stored in seeds for extended periods at room temperature without degradation, and easily purified from contaminating soy proteins. Vaccination of pigs with purified soy-mSEB, or the identical triple mutant expressed in Escherichia coli (E. coli-mSEB), resulted in high antibody titers against the native toxin in immunized animals. In fact, titers were indistinguishable regardless of the immunogen used, demonstrating the equivalence of soy-mSEB and E. coli-mSEB vaccinations. Antisera from either immunized group were able to block native SEB superantigen activity in an in vitro neutralization assay. Similar results were obtained when immunized animals were challenged with a sublethal dose of native toxin. Significant reductions in toxin-induced serum cytokine levels were observed in soy-mSEB- and E. coli-mSEB-immunized pigs compared to control animals. The reductions in SEB-induced cytokine responses were similar regardless of the immunogen used for vaccination. Surprisingly, however, some clinical symptoms, such as prostration, lethargy, emesis, and/or diarrhea, were still observed in all immunized animals. These studies demonstrate the potential for soybean-derived proteins as a platform technology for sustainable vaccine manufacturing and the usefulness of a sublethal challenge model in pigs for evaluating the efficacy of potential SEB vaccine candidates.

PI3K/Akt/mTOR, a pathway less recognized for staphylococcal superantigen-induced toxicity

Immunostimulating staphylococcal enterotoxin B (SEB) and related superantigenic toxins cause diseases in humans and laboratory animals by activating cells of the immune system. These toxins bind directly to the major histocompatibility complex (MHC) class II molecules on antigen-presenting cells and specific Vβ regions of T-cell receptors (TCR), resulting in hyperactivation of both T lymphocytes and monocytes/macrophages. Activated host cells produce excessive amounts of proinflammatory cytokines and chemokines, especially tumor necrosis factor α, interleukin 1 (IL-1), IL-2, interferon γ (IFNγ), and macrophage chemoattractant protein 1 causing clinical symptoms of fever, hypotension, and shock. The well-explored signal transduction pathways for SEB-induced toxicity downstream from TCR/MHC ligation and interaction of cell surface co-stimulatory molecules include the mitogen-activated protein kinase cascades and cytokine receptor signaling, culminating in NFκB activation. Independently, IL-2, IFNγ, and chemokines from activated T cells signal via the phosphoinositide 3-kinase (PI3K), the serine/threonine kinases, Akt and mammalian target of rapamycin (mTOR) pathways. This article reviews the signaling molecules induced by superantigens in the activation of PI3K/Akt/mTOR pathways leading to staphylococcal superantigen-induced toxicity and updates potential therapeutics against superantigens.

Intranasal rapamycin rescues mice from staphylococcal enterotoxin B-induced shock

Staphylococcal enterotoxin B (SEB) and related exotoxins produced by Staphylococcus aureus are potent activators of the immune system and cause toxic shock in humans. Currently there is no effective treatment except for the use of intravenous immunoglobulins administered shortly after SEB exposure. Intranasal SEB induces long-lasting lung injury which requires prolonged drug treatment. We investigated the effects of rapamycin, an immunosuppressive drug used to prevent graft rejection, by intranasal administration in a lethal mouse model of SEB-induced shock. The results show that intranasal rapamycin alone delivered as late as 17 h after SEB protected 100% of mice from lethal shock. Additionally, rapamycin diminished the weight loss and temperature fluctuations elicited by SEB. Intranasal rapamycin attenuated lung MCP-1, IL-2, IL-6, and IFNγ by 70%, 30%, 64%, and 68% respectively. Furthermore, short courses (three doses) of rapamycin were sufficient to block SEB-induced shock. Intranasal rapamycin represents a novel use of an immunosuppressant targeting directly to site of toxin exposure, reducing dosages needed and allowing a wider therapeutic window.

Therapeutic inhibition of pro-inflammatory signaling and toxicity to staphylococcal enterotoxin B by a synthetic dimeric BB-loop mimetic of MyD88

Staphylococcal enterotoxin B (SEB) exposure triggers an exaggerated pro-inflammatory cytokine response that often leads to toxic shock syndrome (TSS) associated with organ failure and death. MyD88 mediates pro-inflammatory cytokine signaling induced by SEB exposure and MyD88(-/-) mice are resistant to SEB intoxication, suggesting that MyD88 may be a potential target for therapeutic intervention. We targeted the BB loop region of the Toll/IL-1 receptor (TIR) domain of MyD88 to develop small-molecule therapeutics. Here, we report that a synthetic compound (EM-163), mimic to dimeric form of BB-loop of MyD88 attenuated tumor necrosis factor (TNF)- α, interferon (IFN)-γ, interleukin (IL)-1β, IL-2 and IL-6 production in human primary cells, whether administered pre- or post-SEB exposure. Results from a direct binding assay, and from MyD88 co-transfection/co-immunoprecipitation experiments, suggest that EM-163 inhibits TIR-TIR domain interaction. Additional results indicate that EM-163 prevents MyD88 from mediating downstream signaling. In an NF-kB-driven reporter assay of lipopolysaccharide-stimulated MyD88 signaling, EM-163 demonstrated a dose-dependent inhibition of reporter activity as well as TNF-α and IL-1β production. Importantly, administration of EM-163 pre- or post exposure to a lethal dose of SEB abrogated pro-inflammatory cytokine responses and protected mice from toxic shock-induced death. Taken together, our results suggest that EM-163 exhibits a potential for therapeutic use against SEB intoxication.

Synthetic human monoclonal antibodies toward staphylococcal enterotoxin B (SEB) protective against toxic shock syndrome

Staphylococcal enterotoxin B (SEB) is a potent toxin that can cause toxic shock syndrome and act as a lethal and incapacitating agent when used as a bioweapon. There are currently no vaccines or immunotherapeutics available against this toxin. Using phage display technology, human antigen-binding fragments (Fabs) were selected against SEB, and proteins were produced in Escherichia coli cells and characterized for their binding affinity and their toxin neutralizing activity in vitro and in vivo. Highly protective Fabs were converted into full-length IgGs and produced in mammalian cells. Additionally, the production of anti-SEB antibodies was explored in the Nicotiana benthamiana plant expression system. Affinity maturation was performed to produce optimized lead anti-SEB antibody candidates with subnanomolar affinities. IgGs produced in N. benthamiana showed characteristics comparable with those of counterparts produced in mammalian cells. IgGs were tested for their therapeutic efficacy in the mouse toxic shock model using different challenge doses of SEB and a treatment with 200 μg of IgGs 1 h after SEB challenge. The lead candidates displayed full protection from lethal challenge over a wide range of SEB challenge doses. Furthermore, mice that were treated with anti-SEB IgG had significantly lower IFNγ and IL-2 levels in serum compared with mock-treated mice. In summary, these anti-SEB monoclonal antibodies represent excellent therapeutic candidates for further preclinical and clinical development.

Antibodies for biodefense

Potential bioweapons are biological agents (bacteria, viruses, and toxins) at risk of intentional dissemination. Biodefense, defined as development of therapeutics and vaccines against these agents, has seen an increase, particularly in the US following the 2001 anthrax attack. This review focuses on recombinant antibodies and polyclonal antibodies for biodefense that have been accepted for clinical use. These antibodies aim to protect against primary potential bioweapons, or category A agents as defined by the Centers for Disease Control and Prevention (Bacillus anthracis, Yersinia pestis, Francisella tularensis, botulinum neurotoxins, smallpox virus, and certain others causing viral hemorrhagic fevers) and certain category B agents. Potential for prophylactic use is presented, as well as frequent use of oligoclonal antibodies or synergistic effect with other molecules. Capacities and limitations of antibodies for use in biodefense are discussed, and are generally applicable to the field of infectious diseases.

A small molecule that mimics the BB-loop in the Toll interleukin-1 (IL-1) receptor domain of MyD88 attenuates staphylococcal enterotoxin B-induced pro-inflammatory cytokine production and toxicity in mice

Toxic shock syndrome (TSS) is a clinical consequence of the profound amplification of host pro-inflammatory cytokine signaling that results from staphylococcal enterotoxin (SE) exposure. We recently reported that MyD88(-/-) mice were resistant to SEA or SEB toxic shock and displayed reduced levels of pro-inflammatory cytokines in their serum. Here we report that SEB stimulation of total mononuclear cells up-regulated MyD88 in monocytes and T cells. Further, MyD88 gene silencing in primary human cells using siRNA prevented SEB or SEB plus lipopolysaccharide (LPS) induction of interleukin-1β (IL-1β) transcriptional activation, suggesting that MyD88-mediated signaling is an essential component of SEB toxicity. We synthesized small molecules that mimic the conserved BB-loop in the Toll/IL-1 receptor (TIR) domain of MyD88. In primary human cells, these mimetics attenuated SEB-induced pro-inflammatory cytokine production. SEB stimulation of primary cells with mimetic affected newly synthesized MyD88 and downstream signaling components. Furthermore, LPS-induced MyD88 signaling was likewise inhibited in a cell-based reporter assay. More importantly, administration of mimetic reduced cytokine responses and increased survivability in a murine SEB challenge model. Collectively, these results suggest that MyD88 BB-loop mimetics interfere with SEB-induced pro-inflammatory signaling and toxicity, thus offering a potential approach in the therapy of toxic shock.

Nasal immunity to staphylococcal toxic shock is controlled by the nasopharynx-associated lymphoid tissue

The nasopharynx-associated lymphoid tissue (NALT) of humans and other mammals is associated with immunity against airborne infections, though it is generally considered to be a secondary component of the mucosa-associated lymphoid system. We found that protective immunity to a virulence factor of nasal mucosa-colonizing Staphylococcus aureus, staphylococcal enterotoxin B (SEB), requires a functional NALT. We examined the role of NALT using intranasal (IN) vaccination with a recombinant SEB vaccine (rSEBv) combined with an adjuvant in a mouse model of SEB-induced toxic shock. The rSEBv was rapidly internalized by NALT cells at the mucosal barrier, and transport into NALT was accelerated by inclusion of a Toll-like receptor 4 (TLR4) agonist. Vaccine-induced germinal centers of B cells formed within NALT, accompanied by elevated levels of IgA(+) and IgG(+) cells, and these were further increased by TLR4 activation. The NALT was the site of specific anti-rSEBv IgA and IgG production but was also influenced by intraperitoneal (IP) inoculation and perhaps other isolated lymphoid follicles observed within the nasal cavity. Vaccination by the IN route generated robust levels of anti-rSEBv IgA in saliva, nasal secretions, and blood compared to much lower levels after IP vaccination. IN vaccination also induced secretion of anti-rSEBv IgG in the blood and nasal secretions. Significantly, the efficacy of IN vaccination was dependent on NALT, as surgical removal resulted in greater sensitivity to IN challenge with wild-type SEB. Thus, protective immunity to SEB within the nasal sinuses was elicited by responses originating in NALT.

MyD88-dependent pro-inflammatory cytokine response contributes to lethal toxicity of staphylococcal enterotoxin B in mice

An elevated pro-inflammatory cytokine response is the primary cause of death by toxic shock after exposure to staphylococcal enterotoxin B (SEB). Identifying an intracellular signal mediator that predominantly controls the pro-inflammatory response is important for developing a therapeutic strategy. We examined the role of the signaling adaptor MyD88 in cell culture and in a mouse model of toxic shock. Our results indicated that elevated tumor necrosis factor-α, interferon-γ, interleukin (IL)-1α/β and IL-6 production from mouse spleen cells treated with SEB alone or in combination with lipopolysaccharide (LPS) was regulated by MyD88. Elevated levels of MyD88 protein in spleen cells, as well as in CD11c+ or Mac3+ cells, and activation of nuclear factor-κB in spleen cells were observed in mice treated with SEB. An SEB-dose dependent lethality was observed in LPS-potentiated and in D-galactosamine-sensitized mice. D-Galactosamine treatment of spleen cells had no effect in cytokine induction but rather increased the sensitivity to toxic shock in mice. Our results demonstrated an impaired pro-inflammatory cytokine production by spleen cells of MyD88–/– mice in response to SEB or SEB plus LPS. Most importantly, MyD88–/– mice were resistant to SEB-induced death. These results demonstrate that MyD88-dependent pro-inflammatory signaling is responsible for SEB intoxication. In addition, our studies also demonstrated that LPS potentiation, in comparison to D-galactosamine sensitization, contributes to a stronger SEB–induced lethality. This is due to the pro-inflammatory cytokine response elicited by MyD88 after exposure to SEB and LPS. These findings offer an important insight upon SEB intoxication and subsequent therapy targeting MyD88.

Therapeutic down-modulators of staphylococcal superantigen-induced inflammation and toxic shock

Staphylococcal enterotoxin B (SEB) and related superantigenic toxins are potent stimulators of the immune system and cause a variety of diseases in humans, ranging from food poisoning to toxic shock. These toxins bind directly to major histocompatibility complex (MHC) class II molecules on antigen-presenting cells and specific Vβ regions of T-cell receptors (TCR), resulting in hyperactivation of both monocytes/macrophages and T lymphocytes. Activated host cells produce massive amounts of proinflammatory cytokines and chemokines, activating inflammation and coagulation, causing clinical symptoms that include fever, hypotension, and shock. This review summarizes the in vitro and in vivo effects of staphylococcal superantigens, the role of pivotal mediators induced by these toxins in the pathogenic mechanisms of tissue injury, and the therapeutic agents to mitigate the toxic effects of superantigens.

Proinflammatory mediators of toxic shock and their correlation to lethality

Bacterial exotoxins and endotoxins both stimulate proinflammatory mediators but the contribution of each individual toxin in the release of mediators causing lethal shock is incompletely understood. This study examines the cytokine response and lethality of mice exposed to varying doses of staphylococcal enterotoxin B (SEB) or lipopolysaccharide (LPS) and their combinations. In vivo, SEB alone induced moderate levels of IL-2 and MCP-1 and all mice survived even with a high dose of SEB (100 μg/mouse). LPS (80 μg/mouse) caused 48% lethality and induced high levels of IL-6 and MCP-1. SEB induced low levels of TNFα, IL-1, IFNγ, MIP-2, and LPS synergized with SEB in the expression of these cytokines and that of IL-6 and MCP-1. Importantly, the synergistic action of SEB and LPS resulted in lethal shock and hypothermia. ANOVA of cytokine levels by survival status of SEB-plus-LPS groups revealed significantly higher levels of TNFα, IL-6, MIP-2, and MCP-1 in nonsurvivors measured at 8 hours. Significantly higher levels of IFNγ and IL-2 were observed at 21 hours in nonsurvivors of toxic shock compared to those in survivors. Overall, synergistic action of SEB and LPS resulted in higher and prolonged levels of these key cytokines leading to toxic shock.

Oral vaccine formulations stimulate mucosal and systemic antibody responses against staphylococcal enterotoxin B in a piglet model

Despite the potential for its use as an agent of biowarfare or bioterrorism, no approved vaccine against staphylococcal enterotoxin B (SEB) exists. Nontoxic, mutant forms of SEB have been developed; however, it has been difficult to determine the efficacy of such subunit vaccine candidates due to the lack of superantigen activity of native SEB in rodents and due to the limitations of primate models. Since pigs respond to SEB in a manner similar to that of human subjects, we utilized this relevant animal model to investigate the safety and immunogenicity of a triple mutant of SEB carrying the amino acid changes L45R, Y89A, and Y94A. This recombinant mutant SEB (rmSEB) did not possess superantigen activity in pig lymphocyte cultures. Furthermore, rmSEB was unable to compete with native SEB for binding to pig leukocytes. These in vitro studies suggested that rmSEB could be a safe subunit vaccine. To test this possibility, piglets immunized orally with rmSEB formulations experienced no significant decrease in food consumption and no weight loss during the vaccination regimen. Oral vaccination with 1-mg doses of rmSEB on days 0, 7, 14, and 24 resulted in serum IgG and fecal IgA levels by day 36 that cross-reacted with native SEB. Surprisingly, the inclusion of cholera toxin adjuvant in vaccine formulations containing rmSEB did not result in increased antibody responses compared to formulations using the immunogen alone. Taken together, these studies provide additional evidence for the potential use of nontoxic forms of SEB as vaccines.

Staphylococcal enterotoxin A induction of pro-inflammatory cytokines and lethality in mice is primarily dependent on MyD88

Staphylococcal enterotoxin (SE) -induced toxic shock is triggered by inflammatory cytokine signal amplification after SE binding to major histocompatibility complex class II molecules on antigen-presenting cells and T-cell receptors. Identifying host cellular elements contributing to this pro-inflammatory signal amplification is critical for developing a strategy for therapeutic intervention. Myeloid differentiation primary-response protein 88 (MyD88) is an intracellular signalling adaptor protein primarily known for mediating pro-inflammatory cytokine responses. We investigated the role of MyD88 in staphylococcal enterotoxin A (SEA) -treated cell cultures and mouse models of toxic shock. Our results demonstrated that elevated levels of tumour necrosis factor-alpha, interferon-gamma, interleukin-1alpha/beta (IL-1alpha/beta), IL-2 and IL-6 production correlated with up-regulation of MyD88 after treatment of spleen cells and mice with SEA alone or in combination with lipopolysaccharide (LPS). The SEA-induced lethality was also observed in (LPS-independent) D-galactosamine-sensitized mice. While LPS potentiated SEA-induced cytokine responses, D-galactosamine treatment had no additive effect. Most importantly, our results demonstrated that MyD88(-/-) mice were resistant to SEA-induced toxic shock and had reduced pro-inflammatory cytokine responses. These results suggest that SEA-induced lethality is primarily dependent on MyD88. Our findings offer an important insight on potential therapeutic treatment of SEA-induced toxic shock targeting MyD88.

Rapamycin protects mice from staphylococcal enterotoxin B-induced toxic shock and blocks cytokine release in vitro and in vivo

Staphylococcal enterotoxins are potent activators for human T cells and cause lethal toxic shock. Rapamycin, an immunosuppressant, was tested for its ability to inhibit staphylococcal enterotoxin B (SEB)-induced activation of human peripheral blood mononuclear cells (PBMC) in vitro and toxin-mediated shock in mice. Stimulation of PMBC by SEB was effectively blocked by rapamycin as evidenced by the inhibition of tumor necrosis factor alpha (TNF-alpha), interleukin 1beta (IL-1beta), IL-6, IL-2, gamma interferon (IFN-gamma), monocyte chemoattractant protein 1 (MCP-1), macrophage inflammatory protein 1alpha (MIP-1alpha), MIP-1beta, and T-cell proliferation. In vivo, rapamycin protected 100% of mice from lethal shock, even when administered 24 h after intranasal SEB challenge. The serum levels of MCP-1 and IL-6, after intranasal exposure to SEB, were significantly reduced in mice given rapamycin versus controls. Additionally, rapamycin diminished the weight loss and temperature fluctuations elicited by SEB.

Critical timing, location and duration of glucocorticoid administration rescue mice from superantigen-induced shock and attenuate lung injury

Bacterial superantigens, such as staphylococcal enterotoxin B (SEB), are major virulence factors implicated in the pathogenesis of toxic shock. In this study we investigated the efficacy of glucocorticoid therapy in preventing SEB-induced lethal shock initiated through the respiratory route in mice. Dexamethasone, a potent anti-inflammatory steroid, administrated intranasally on the first day, followed by intraperitoneal doses on the subsequent 4 days, was effective in attenuating SEB-induced hypothermia, and reduction in systemic and pulmonary proinflammatory mediator release. This optimal dosing and schedule of glucocorticoid treatment mitigated lung inflammation and resulted in 100% survival in this intranasal mouse model of SEB-mediated shock.

Vaccine based on a ubiquitous cysteinyl protease and streptococcal pyrogenic exotoxin A protects against Streptococcus pyogenes sepsis and toxic shock

Background

The gram-positive bacterium Streptococcus pyogenes is a common pathogen of humans that causes invasive infections, toxic-shock syndrome, rheumatic fever, necrotizing fasciitis and other diseases. Detection of antibiotic resistance in clinical isolates has renewed interest in development of new vaccine approaches for control S. pyogenes sepsis. In the study presented, a novel protein vaccine was examined. The vaccine was based on a recombinant protein fusion between streptococcal pyrogenic exotoxin B (SpeB), a cysteinyl protease expressed by all clinical isolates, and streptococcal pyrogenic exotoxin A (SpeA), a superantigen produced by a large subset of isolates.

Results

A novel protein was produced by mutating the catalytic site of SpeB and the receptor binding surface of SpeA in a fusion of the two polypeptides. Vaccination of HLA-DQ8 transgenic mice with the SpeA-SpeB fusion protein protected against a challenge with the wild-type SpeA that was lethal to naïve controls, and vaccinated mice were protected from an otherwise lethal S. pyogenes infection.

Conclusion

These results suggest that the genetically attenuated SpeA-SpeB fusion protein may be useful for controlling S. pyogenes infections. Vaccination with the SpeA-SpeB fusion protein described in this study may potentially result in protective immunity against multiple isolates of S. pyogenes due to the extensive antibody cross-reactivity previously observed among all sequence variants of SpeB and the high frequency of SpeA-producing strains.

Central roles for IL-2 and MCP-1 following intranasal exposure to SEB: a new mouse model

Murine models for bacterial superantigens like staphylococcal enterotoxin B (SEB) have to date been rather cumbersome. The reasons include: (1) necessary use of potentiating agents such as actinomycin D, d-galactosamine, lipopolysaccharide (LPS), or viruses; (2) high toxin amounts required to elicit effects; and/or (3) generation of phenotypic-stable transgenic animals. Our study employed readily available C3H/HeJ (TLR4 negative, LPS-nonresponsive) mice with intranasal and intraperitoneal administration of low microgram quantities of SEB. These animals responded to SEB with severe lung inflammation and hypothermia, culminating in death. A survey of cytokines/chemokines in sera and lungs after lethal intoxication revealed that monocyte chemoattractant protein-1 and interleukin-2 were associated with effects in this model. In contrast, SEB had minimal effects upon congenic (TLR4 positive, LPS-responsive) C3H/OuJ mice. Lethality of SEB in C3H/HeJ mice was neutralized with SEB-specific antibodies, suggesting potential utility of this model for future therapeutic studies.

Interleukin-15 increases vaccine efficacy through a mechanism linked to dendritic cell maturation and enhanced antibody titers

Interleukin-15 (IL-15) is generally considered to sustain T-cell memory and to be a growth factor for natural killer cells. Previous data from our laboratory demonstrated that IL-15 is also an important factor for developing human dendritic cells. For this study, we investigated the effects of IL-15 on antibody responses in mice to a recombinant staphylococcal enterotoxin B (SEB) vaccine (STEBVax) in a preclinical model of toxic shock syndrome induced by SEB. We observed that mouse spleen cells treated with IL-15 in ex vivo culture gained a dendritic cell-like phenotype. Administration of IL-15 to mice also resulted in an increased number of mature CD11c+ dendritic cells in mouse spleens. A significant, IL-15 dose-dependent increase in antigen-specific antibody was observed after coadministration with the vaccine and an aluminum-based adjuvant (alhydrogel). Furthermore, the coadministration of IL-15 with STEBVax and alhydrogel also protected mice from lethal toxic shock above the levels that obtained without IL-15. Thus, the vaccine response enhanced by IL-15 appears to be mediated by mature dendritic cells and results in prevalent seroconversion to Th2-dependent antibodies. This suggests a potential use of IL-15 as an adjuvant for antibody-dependent responses to vaccines.

Superantigen-induced cytokine release from whole-blood cell culture as a functional measure of drug efficacy after oral dosing in nonhuman primates

Evaluation of drug efficacy for human diseases is routinely performed in animal models for efficiency and in accordance with FDA regulations. Rhesus macaques have been used as models for various lethal diseases and correlates of immunity, as nonhuman primates (NHP) closely resemble humans. We examined the ex vivo cytokine response of superantigen-stimulated whole-blood cells as a first step to therapeutic efficacy testing for bacterial superantigen-induced shock in NHP after oral dosing of pentoxifylline. Doses of 120mg/kg of pentoxifylline effectively attenuated staphylococcal enterotoxin B-induced tumor necrosis factor alpha (TNFalpha), gamma interferon (IFNgamma) and interleukin 2 (IL-2) in ex vivo culture of NHP whole-blood cells by 88%, 81%, and 76%, respectively, whereas lower doses of 48 or 72mg/kg had no inhibitory effect. Thus cytokine release of stimulated peripheral blood cells provides a convenient biological measurement of the anti-inflammatory potency of pentoxifylline and has the advantage of assessing functional responses to a specific biotoxin of interest.

HLA Transgenic Mice Provide Evidence for a Direct and Dominant Role of HLA Class II Variation in Modulating the Severity of Streptococcal Sepsis

Our epidemiologic studies on invasive Group A Streptococci (GAS) infections identified specific HLA class II haplotypes/alleles conferring high-risk or protection from streptococcal toxic shock syndrome with a strong protection conferred by the DRB1*15/DQB1*06 haplotype. We used HLA-transgenic mice to provide an in vitro and in vivo validation for the direct role of HLA class II allelic variation in streptococcal toxic shock syndrome. When splenocytes from mice expressing the protective HLA-DQB1*06 (DQ6) allele were stimulated with a mixture of streptococcal superantigens (SAgs), secreted by the prevalent M1T1 strain, both proliferative and cytokine responses were significantly lower than those of splenocytes from mice expressing the neutral DRB1*0402/DQB1*0302 (DR4/DQ8) alleles (p < 0.001). In crisscross experiments, the presentation of SAgs to pure T cells from either the DQ6 or the DR4/DQ8 mice resulted in significantly different levels of response depending on the HLA type expressed on the APCs. Presentation by HLA-DQ6 APCs elicited significantly lower responses than the presentation by HLA-DR4/DQ8 APCs. Our in vitro data were supported by in vivo findings, as the DQ6 mice showed significantly longer survival post-i.v. infection with live M1T1 GAS (p < 0.001) and lower inflammatory cytokine responses as compared with the DR4/DQ8 mice (p < 0.01). The data presented here provide evidence for a direct role of HLA class II molecules in modulating responses to GAS SAgs and underscore the dominant role of HLA class II allelic variation in potentiating the severity of GAS systemic infections.

Staphylococcus aureus enterotoxins induce IL-8 secretion by human nasal epithelial cells

Background

Staphylococcus aureus produces a set of proteins which act both as superantigens and toxins. Although their mode of action as superantigens is well understood, little is known about their effects on airway epithelial cells.

Methods

To investigate this problem, primary nasal epithelial cells derived from normal and asthmatic subjects were stimulated with staphylococcal enterotoxin A and B (SEA and SEB) and secreted (supernatants) and cell-associated (cell lysates) IL-8, TNF-α, RANTES and eotaxin were determined by specific ELISAs.

Results

Non-toxic concentrations of SEA and SEB (0.01 μg/ml and 1.0 μg/ml) induced IL-8 secretion after 24 h of culture. Pre-treatment of the cells with IFN-γ (50 IU/ml) resulted in a further increase of IL-8 secretion. In cells from healthy donors pretreated with IFN-γ, SEA at 1.0 μg/ml induced release of 1009 pg/ml IL-8 (733.0–1216 pg/ml, median (range)) while in cells from asthmatic donors the same treatment induced significantly higher IL-8 secretion – 1550 pg/ml (1168.0–2000.0 pg/ml p = 0.04). Normal cells pre-treated with IFN-γ and then cultured with SEB at 1.0 μg/ml released 904.6 pg/ml IL-8 (666.5–1169.0 pg/ml). Cells from asthmatics treated in the same way produced significantly higher amounts of IL-8 – 1665.0 pg/ml (1168.0–2000.0 pg/ml, p = 0.01). Blocking antibodies to MHC class II molecules added to cultures stimulated with SEA and SEB, reduced IL-8 secretion by about 40% in IFN-γ unstimulated cultures and 75% in IFN-γ stimulated cultures. No secretion of TNF-α, RANTES and eotaxin was noted.

Conclusion

Staphylococcal enterotoxins may have a role in the pathogenesis of asthma.