T cells made deficient in interleukin-2 production by exposure to staphylococcal enterotoxin B in vivo are primed for interferon-gamma and interleukin-10 secretion

Staphylococcus aureus Enterotoxin B Is a Cofactor of Food Allergy beyond a Superantigen

Staphylococcus aureus enterotoxin B (SEB), one of the most common bacterial toxins in food contamination, has been poorly understood in relationship to food allergy outcomes. To investigate whether the ingestion of enterotoxins in food allergens could affect the development of food allergy, OVA-sensitized female BALB/c mice were challenged with OVA added with different doses of SEB or LPS. Allergic symptoms, such as diarrhea rate and hypothermia, could be aggravated in mice challenged with OVA and a low dose of SEB. The increased differentiation of Th2 and reduced expression of CD103 in dendritic cells was found in mice coexposed to SEB and OVA. Additionally, there was an increasing differentiation of Th1 induced by a high dose of SEB. The expression of ST2+ in intestinal mast cells was also increased in mice sensitized with a low dose of SEB and OVA. Employing several in vitro cell culture models showed that the secretion of IL-33 from intestinal epithelial cells and IL-4 from group 2 innate lymphoid cells, activation of bone marrow-derived dendritic cells, and differentiation of naive T cells were induced by SEB and OVA. Our work proved that challenge with low-dose SEB and OVA partly aggravated the food allergy, suggesting a (to our knowledge) new finding of the potential cofactor of food allergy and that the contamination of SEB in food allergens deserves attention for allergic and normal individuals.

Multisystem Inflammatory Syndrome in Children and Long COVID: The SARS-CoV-2 Viral Superantigen Hypothesis

Multisystem inflammatory syndrome in children (MIS-C) is a febrile pediatric inflammatory disease that may develop weeks after initial SARS-CoV-2 infection or exposure. MIS-C involves systemic hyperinflammation and multiorgan involvement, including severe cardiovascular, gastrointestinal (GI) and neurological symptoms. Some clinical attributes of MIS-C-such as persistent fever, rashes, conjunctivitis and oral mucosa changes (red fissured lips and strawberry tongue)-overlap with features of Kawasaki disease (KD). In addition, MIS-C shares striking clinical similarities with toxic shock syndrome (TSS), which is triggered by bacterial superantigens (SAgs). The remarkable similarities between MIS-C and TSS prompted a search for SAg-like structures in the SARS-CoV-2 virus and the discovery of a unique SAg-like motif highly similar to a Staphylococcal enterotoxin B (SEB) fragment in the SARS-CoV-2 spike 1 (S1) glycoprotein. Computational studies suggest that the SAg-like motif has a high affinity for binding T-cell receptors (TCRs) and MHC Class II proteins. Immunosequencing of peripheral blood samples from MIS-C patients revealed a profound expansion of TCR β variable gene 11-2 (TRBV11-2), which correlates with MIS-C severity and serum cytokine levels, consistent with a SAg-triggered immune response. Computational sequence analysis of SARS-CoV-2 spike further identified conserved neurotoxin-like motifs which may alter neuronal cell function and contribute to neurological symptoms in COVID-19 and MIS-C patients. Additionally, autoantibodies are detected during MIS-C, which may indicate development of post-SARS-CoV-2 autoreactive and autoimmune responses. Finally, prolonged persistence of SARS-CoV-2 RNA in the gut, increased gut permeability and elevated levels of circulating S1 have been observed in children with MIS-C. Accordingly, we hypothesize that continuous and prolonged exposure to the viral SAg-like and neurotoxin-like motifs in SARS-CoV-2 spike may promote autoimmunity leading to the development of post-acute COVID-19 syndromes, including MIS-C and long COVID, as well as the neurological complications resulting from SARS-CoV-2 infection.

Staphylococcal Enterotoxin Superantigens Induce Prophylactic Antiviral Activity Against Encephalomyocarditis Virus In Vivo and In Vitro

The staphylococcal enterotoxins (SEs) are classified as superantigens due to their potent stimulation of the immune system resulting in T cell activation and prodigious cytokine production and toxicity. This study examined the ability of superantigens to induce prophylactic antiviral activity in vivo and in vitro and evaluated potential superantigen mimetic peptides. Prophylactic treatment of mice in vivo with intraperitoneal injections of SE superantigens SEA and SEB (both at 20 μg/day for 3 days) prevented encephalomyocarditis virus (EMCV)-induced lethality in 100% and 80% of mice, respectively, as compared with control saline-treated groups in which EMCV was lethal to all mice. Furthermore, SEA (2 μg/mL) and SEB (1 μg/mL) induced antiviral activity in mouse splenocytes to produce an antiviral factor since their supernatant prevented EMCV lysis of L929 cells in tissue culture. It was found that superantigens do not directly prevent EMCV infection, but rather indirectly through inducing interferon gamma (IFNγ) production in cells as the antiviral factor. Evaluation of various superantigen mimetic peptides showed that one peptide (SEA3) had superantigen-like activity by inducing IFNγ production in cells but without the cellular proliferation, as associated with superantigens. However, the induction of IFNγ activation by the SEA3 peptide was not as pronounced, and took a much higher peptide concentration, when compared with the parent superantigen. If the negative side effects of superantigens can be eliminated, their beneficial properties can be harnessed for prophylactic treatment of viral infections and other pathologies requiring a robust immune response.

Dual delivery of IL-10 and AT-RvD1 from PEG hydrogels polarize immune cells towards pro-regenerative phenotypes

Inflammation after traumatic injury or surgical intervention is both a protective tissue response leading to regeneration and a potential cause of wound complications. One potentially successful strategy to harness to pro-regenerative roles of host inflammation is the localized delivery of bioactive materials to induce immune suppressive cellular responses by cells responding to injury. In this study, we designed a fully synthetic poly (ethylene) glycol (PEG)-based hydrogel to release the specialized pro-resolving lipid mediator aspirin-triggered resolvin-D1 (AT-RvD1) and recombinant human interleukin 10 (IL-10). We utilized a unique side-by-side internally controlled implant design wherein bioactive hydrogels were implanted adjacent to control hydrogels devoid of immune modulatory factors in the dorsal skinfold window chamber. We also explored single-immune cell data with unsupervised approaches such as SPADE. First, we show that RGD-presenting hydrogel delivery results in enhanced immune cell recruitment to the site of injury. We then use intra-vital imaging to assess cellular recruitment and microvascular remodeling to show an increase in the caliber and density of local microvessels. Finally, we show that the recruitment and re-education of mononuclear phagocytes by combined delivery IL-10 and AT-RvD1 localizes immune suppressive subsets to the hydrogel, including CD206+ macrophages (M2a/c) and IL-10 expressing dendritic cells in the context of chronic inflammation following surgical tissue disruption. These data demonstrate the potential of combined delivery on the recruitment of regenerative cell subsets involved in wound healing complications.

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.

Staphylococcal enterotoxin B-induced microRNA-155 targets SOCS1 to promote acute inflammatory lung injury

Staphylococcal enterotoxin B (SEB) causes food poisoning in humans. It is considered a biological weapon, and inhalation can trigger lung injury and sometimes respiratory failure. Being a superantigen, SEB initiates an exaggerated inflammatory response. While the role of microRNAs (miRNAs) in immune cell activation is getting increasing recognition, their role in the regulation of inflammatory disease induced by SEB has not been studied. In this investigation, we demonstrate that exposure to SEB by inhalation results in acute inflammatory lung injury accompanied by an altered miRNA expression profile in lung-infiltrating cells. Among the miRNAs that were significantly elevated, miR-155 was the most overexpressed. Interestingly, miR-155(-/-) mice were protected from SEB-mediated inflammation and lung injury. Further studies revealed a functional link between SEB-induced miR-155 and proinflammatory cytokine gamma interferon (IFN-γ). Through the use of bioinformatics tools, suppressor of cytokine signaling 1 (SOCS1), a negative regulator of IFN-γ, was identified as a potential target of miR-155. While miR-155(-/-) mice displayed increased expression of Socs1, the overexpression of miR-155 led to its suppression, thereby enhancing IFN-γ levels. Additionally, the inhibition of miR-155 resulted in restored Socs1expression. Together, our data demonstrate an important role for miR-155 in promoting SEB-mediated inflammation in the lungs through Socs1 suppression and suggest that miR-155 may be an important target in preventing SEB-mediated inflammation and tissue injury.

A model of an integrated immune system pathway in Homo sapiens and its interaction with superantigen producing expression regulatory pathway in Staphylococcus aureus: comparing behavior of pathogen perturbed and unperturbed pathway

Response of an immune system to a pathogen attack depends on the balance between the host immune defense and the virulence of the pathogen. Investigation of molecular interactions between the proteins of a host and a pathogen helps in identifying the pathogenic proteins. It is necessary to understand the dynamics of a normally behaved host system to evaluate the capacity of its immune system upon pathogen attack. In this study, we have compared the behavior of an unperturbed and pathogen perturbed host system. Moreover, we have developed a formalism under Flux Balance Analysis (FBA) for the optimization of conflicting objective functions. We have constructed an integrated pathway system, which includes Staphylococcal Superantigen (SAg) expression regulatory pathway and TCR signaling pathway of Homo sapiens. We have implemented the method on this pathway system and observed the behavior of host signaling molecules upon pathogen attack. The entire study has been divided into six different cases, based on the perturbed/unperturbed conditions. In other words, we have investigated unperturbed and pathogen perturbed human TCR signaling pathway, with different combinations of optimization of concentrations of regulatory and signaling molecules. One of these cases has aimed at finding out whether minimization of the toxin production in a pathogen leads to the change in the concentration levels of the proteins coded by TCR signaling pathway genes in the infected host. Based on the computed results, we have hypothesized that the balance between TCR signaling inhibitory and stimulatory molecules can keep TCR signaling system into resting/stimulating state, depending upon the perturbation. The proposed integrated host-pathogen interaction pathway model has accurately reflected the experimental evidences, which we have used for validation purpose. The significance of this kind of investigation lies in revealing the susceptible interaction points that can take back the Staphylococcal Enterotoxin (SE)-challenged system within the range of normal behavior.

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.

Relationship of varying patterns of cytokine production to the anorexic and neuroendocrine effects of repeated Staphylococcal enterotoxin A exposure

Staphylococcal enterotoxin A (SEA) is a superantigen that stimulates T cells and induces the production of multiple cytokines. Previous studies have shown that SEA augments gustatory neophobia and activates the hypothalamic-pituitary-adrenal (HPA) axis. This study aimed to determine if the cytokine response, behavioral effects, and HPA axis activation persisted after repeated SEA treatment. Male C57BL/6J mice were given 1-4 intraperitoneal injections of 5 microg SEA, after which food intake, corticosterone, or peripheral cytokines were measured. In a series of experiments, it was found that secondary exposure to SEA two or three days after priming increased corticosterone, but attenuated splenic TNFalpha, while augmenting IL-1beta, IL-2, and IFNgamma. The anorexic response was intact after secondary exposure, but absent after a third injection, which was still able to elevate corticosterone. It is unlikely that IL-1 mediated the persistent effects on corticosterone, since this was increased in groups lacking corticosterone elevations. Similarly, TNFalpha was only modestly elevated under repeated SEA conditions that elevated plasma corticosterone. This attenuation appeared to be inversely related to the levels of IL-10, the production of which incrementally rose with each successive injection. In conclusion, repeated exposure to SEA activates the HPA axis and alters behavior. However, there may be dissociation between the behavioral and endocrine effects of SEA with increased SEA exposure. Furthermore, it is possible that while TNFalpha was previously shown to be important in response to acute SEA-induced HPA axis activation, further exposure to SEA elicits other cytokines that may exert neuromodulatory effects through sensitization and/or synergistic mechanisms.

Staphylococcal enterotoxin B in vivo modulates both gamma interferon receptor expression and ligand-induced activation of signal transducer and activator of transcription 1 in T cells

Superantigens (SAg) are bacterial exotoxins that provoke extreme responses in the immune system; for example, the acute hyperactivation of SAg-reactive T cells that leads to toxic shock syndrome is followed within days by strong immunosuppression. The gamma interferon (IFN-gamma) response is deeply affected in both extremes. The implication of IFN-gamma in the pathophysiology of lethal shock induced in mice after a secondary challenge with the SAg staphylococcal enterotoxin B (SEB) prompted us to study the regulation of IFN-gamma secretion and the intracellular response. We demonstrate in this study that a rechallenge with SEB becomes lethal only when given inside a critical time window after SEB priming and is associated with an increase of IFN-gamma serum release 72 h after priming. However, at this time, a selective blockade of IFN-gamma/STAT1 signaling develops in spleen cells, correlating with a lack of expression of the IFN-gamma receptor beta subunit and STAT1 in the T-cell population. Selective blockade of the STAT1 signaling pathway--while simultaneously maintaining STAT3 signaling and expression--may be a protective mechanism that shortens IFN-gamma production during the Th1 effector response. This blockade may also have consequences on switching towards a suppressor phenotype with chronic exposure to the superantigen.

The Role of Immune Response to Staphylococcus aureus Superantigens and Disease Severity in Relation to the Sensitivity to Tacrolimus in Atopic Dermatitis

BACKGROUND

Staphylococcus aureus-producing superantigens (SAgs), such as staphylococcal enterotoxin B (SEB) or toxic shock syndrome toxin-1 (TSST-1), are frequently observed in atopic dermatitis (AD). However, little has been done to establish the association of immune responses to SAgs and the therapeutic response to immunosuppressive drugs in AD. Therefore, we investigated the prevalence and role of SAgs in the pathophysiology and immunosuppressive drug sensitivity in AD patients.

METHODS

We classified 29 patients into two groups on the basis of their clinical AD scores: a low-score group (n = 14) corresponding to mild to moderate patients and a high-score group (n = 15) corresponding to severe patients. We estimated the plasma anti-SEB or TSST-1 IgE of these patients and healthy subjects by ELISA. We also estimated individual drug sensitivity by determining drug concentrations that would give 50% inhibition (IC(50)) of peripheral-blood mononuclear cell (PBMC) proliferation in vitro.

RESULTS

The levels of plasma anti-SEB or TSST-1 IgE in the severe patients were significantly higher than those in the mild to moderate patients (p < 0.05 and p < 0.01, respectively). When stimulated with concanavalin A in vitro, PBMCs in the severe patients exhibited low sensitivity to the suppressive efficacy of tacrolimus (FK506) as compared to the mild to moderate patients (p < 0.01). Furthermore, there was a significant correlation between the IC(50)s of FK506 and plasma anti-TSST-1 IgE levels (p < 0.01).

CONCLUSIONS

We showed that PBMCs in severe AD patients exhibited lower sensitivity to FK506, and had higher plasma levels of anti-TSST-1 IgE as compared to the mild AD patients. SAgs appear to be one of the causes of decreased PBMC sensitivity to FK506, and therefore an alternative treatment would be useful based on the individual drug sensitivity data and anti-TSST-1 IgE levels.

Endothelial nitric oxide synthase regulates T cell receptor signaling at the immunological synapse

The role of nitric oxide (NO) in T cells remains controversial, and the origin and localization of endogenous NO and whether it regulates lymphocyte activation are unclear. We show here that, within minutes of binding to antigen, T cells produce NO via endothelial nitric oxide synthase (eNOS). This process required increased intracellular Ca2+ and phosphoinositide3-kinase activity. By using an eNOS-green fluorescent fusion protein and fluorescent probes to detect NO, we show that eNOS translocates with the Golgi apparatus to the immune synapse of T helper cells engaged with antigen-presenting cells (APC), where it was fully activated. Overexpression of eNOS prevented the central coalescence of CD3 at the T cell-APC contact site, which was accompanied by increased phosphorylation of CD3zeta chain, ZAP-70, and extracellular signal-regulated kinases and increased IFN-gamma synthesis, but reduced production of IL-2. Therefore, eNOS-derived NO selectively potentiates T cell receptor signaling to antigen at the immunological synapse.

Cytokine-induced IL-10-secreting CD8 T cells represent a phenotypically distinct suppressor T-cell lineage

Populations of regulatory T cells (Tregs) control autoimmune and allergic immunopathology induced by self or foreign antigens. Several types of CD4(+) MHC class II-restricted Treg populations have been characterized, but the biology of CD8(+), MHC class I-restricted Tregs is less understood. We show here that CD8(+) Tregs are rapidly generated in the presence of IL-4 and IL-12, produce IL-10, and exhibit a unique cell-surface phenotype with coexpression of activation and naive cell-associated markers. They block activation of naive or effector T cells and suppress IgG/IgE antibody responses and graft-versus-host disease in vivo. Suppression is dependent on cell contact and mediated by direct T-cell-T-cell interaction that antagonizes T-cell-receptor (TCR) signals. The data establish the existence of a CD8 T-cell suppressor effector subset distinct in both phenotype and function from T cytotoxic 1 (Tc1) and Tc2 cells. Production of such CD8 Tregs has potential for cell-based therapy of CD4 or CD8 T-cell-mediated disease.

The mechanism of superantigen-mediated toxic shock: not a simple Th1 cytokine storm

The profound clinical consequences of Gram-positive toxic shock are hypothesized to stem from excessive Th1 responses to superantigens. We used a new superantigen-sensitive transgenic model to explore the role of TCRalphabeta T cells in responses to staphylococcal enterotoxin B (SEB) in vitro and in two different in vivo models. The proliferative and cytokine responses of HLA-DR1 spleen cells were 100-fold more sensitive than controls and were entirely dependent on TCRalphabeta T cells. HLA-DR1 mice showed greater sensitivity in vivo to two doses of SEB with higher mortality and serum cytokines than controls. When d-galactosamine was used as a sensitizing agent with a single dose of SEB, HLA-DR1 mice died of toxic shock whereas controls did not. In this sensitized model of toxic shock there was a biphasic release of cytokines, including TNF-alpha, at 2 h and before death at 7 h. In both models, mortality and cytokine release at both time points were dependent on TCRalphabeta T cells. Anti-TNF-alpha pretreatment was protective against shock whereas anti-IFN gamma pretreatment and delayed anti-TNF-alpha treatment were not. Importantly, anti-TNF-alpha pretreatment inhibited the early TNF-alpha response but did not inhibit the later TNF-alpha burst, to which mortality has previously been attributed. Splenic T cells were shown definitively to be the major source of TNF-alpha during the acute cytokine response. Our results demonstrate unequivocally that TCRalphabeta T cells are critical for lethality in toxic shock but it is the early TNF-alpha response and not the later cytokine surge that mediates lethal shock.

Implication of STAT1 and STAT3 transcription factors in the response to superantigens

The activation of STAT1 and STAT3 in response to SEB was analyzed in spleen of Balb/c mice. The intraperitoneal injection of the superantigen SEB activated STAT1 and STAT3 in spleen. Activated STAT1 almost completely disappeared in 24 h even though activated STAT3 was present for more than 48 h after SEB injection. Cyclosporine A was able to block the initial STAT1 activation, but STAT3 activation was only partially affected. SEB also increased the mRNA levels for STAT1, STAT3 and SOCS1. When a second injection with SEB was given 72 h after the first stimulus, STAT1 activation was much lower than that observed after the first stimulation with SEB and no increase in the STAT1 mRNA level was observed. Nevertheless, after this second injection, STAT3 was again activated without any significant interference from the first stimulus and the STAT3 and SOCS1 mRNA levels again increased. These data indicate that a first stimulation with superantigen re-programs cells so that they respond to a second stimulation in a different way. Understanding the mechanisms implicated in this re-programming is basic for designing therapeutic strategies in processes such as septic shock.

CD4+CD25+ and CD4+CD25− T Cells Act Respectively as Inducer and Effector T Suppressor Cells in Superantigen-Induced Tolerance

The repeated injection of low doses of bacterial superantigens (SAg) is known to induce specific T cell unresponsiveness. We show in this study that the spleen of BALB/c mice receiving chronically, staphylococcal enterotoxin B (SEB) contains SEB-specific CD4(+) TCRBV8(+) T cells exerting an immune regulatory function on SEB-specific primary T cell responses. Suppression affects IL-2 and IFN-gamma secretion as well as proliferation of T cells. However, the suppressor cells differ from the natural CD4(+) T regulatory cells, described recently in human and mouse, because they do not express cell surface CD25. They are CD152 (CTLA-4)-negative and their regulatory activity is not associated with expression of the NF Foxp3. By contrast, after repeated SEB injection, CD4(+)CD25(+) splenocytes were heterogenous and contained both effector as well as regulatory cells. In vivo, CD4(+)CD25(-) T regulatory cells prevented SEB-induced death independently of CD4(+)CD25(+) T cells. Nevertheless, SEB-induced tolerance could not be achieved in thymectomized CD25(+) cell-depleted mice because repeated injection of SEB did not avert lethal toxic shock in these animals. Collectively, these data demonstrate that, whereas CD4(+)CD25(+) T regulatory cells are required for the induction of SAg-induced tolerance, CD4(+)CD25(-) T cells exert their regulatory activity at the maintenance stage of SAg-specific unresponsiveness.

Trypanosoma cruzi sensitizes mice to fulminant SEB-induced shock: overrelease of inflammatory cytokines and independence of Chagas' disease or TCR Vbeta-usage

Trypanosoma cruzi-infected mice display increased susceptibility to shock induced by injection of lipopolysaccharide (LPS), anti-CD3, or resulting from interleukin (IL)-10-defective response to the parasite itself, but the basis of such susceptibility remains unknown. Herein, we tested the susceptibility of mice inoculated with virulent and avirulent T. cruzi to staphylococcal enterotoxins (SE), potent inducers of inflammatory cytokine secretion. Mice infected with T. cruzi CL-strain or inoculated with the avirulent clone CL-14, a clone that does not induce disease or polyclonal lymphocyte activation, succumb suddenly to low doses of staphylococcal enterotoxin B (SEB), but not to staphylococcal enterotoxin A (SEA). High plasma levels of TNF, IFN-gamma, and liver transaminases alanine aminotransferase (ALT) and aspartate aminotransferase (AST) were found in these mice, indicating lethal toxic shock. Sensitization to shock required inoculation of live avirulent trypomastigotes and a time interval before challenge with SEB. We found no prior skewing of T cell receptor (TCR) Vbeta-repertoire in CL-14-inoculated mice that could be responsible for sensitization. Splenocytes from CL-14-inoculated mice proliferated more under anti-Vbeta8 than anti-TCRbeta stimulation when compared with normal mice, but were suppressed to SEB stimulation. Both SEB and anti-Vbeta8 antibodies stimulated splenocytes from T. cruzi-inoculated mice to secrete higher levels of inflammatory cytokines than normal controls. Taken together, our results show that T. cruzi inoculation can sensitize mice to lethal SEB-induced shock even in the absence of tissue damage, polyclonal lymphocyte activation, or previously increased levels of inflammatory cytokines, and they suggest that altered reactivity of Vbeta8 lymphocytes may be involved in the phenomenon.

Role of endogenous interleukin-12 in immune response to staphylococcal enterotoxin B in mice

In the present study, the roles of interleukin 12 (IL-12) and IL-18 and their possible interaction during superantigen-induced responses were studied by injection of staphylococcal enterotoxin B (SEB) into mice. These data suggest that the role of IL-12 in SEB-induced responses is limited to sustaining gamma interferon release by an IL-18-independent mechanism.

Cellular and cytokine responses in the circulation and tissue reactions in the lung of rhesus monkeys (Macaca mulatta) pretreated with cyclosporin A and challenged with staphylococcal enterotoxin B

Cyclosporin A (CsA), an inhibitor of T cell cytokine production, protects mice against staphylococcal enterotoxin B (SEB) intoxication. To determine whether CsA treatment would work in a species closer to humans. 4 rhesus monkeys were given 50 mg/kg CsA followed by an intratracheal challenge with approximately 6 LD50 of SEB. The CsA was not protective: one of the monkeys died and the other three had to be euthanised when they became moribund. All monkeys made IL-2, TNF, and IFN-gamma in response to SEB. In addition, there was about a 10-fold increase in ACTH levels 2 hr after SEB challenge. CsA significantly suppressed in vitro proliferation of lymphocytes from treated monkeys. Both CsA-treated monkeys and monkeys that had been challenged in a previous experiment with a lethal dose of SEB but had received no cyclosporin had pathologic changes in several organs. The most prominent changes were marked edema and leukocytic infiltration of the bronchial and bronchiolar mucosa. The CsA treatment appeared to reduce the intensity of lung inflammation, but this effect was not sufficient to protect the monkeys. The results suggest that CsA alone may not be an effective therapeutic agent for humans suffering from SEB intoxication or gram-positive septic shock.

Chronic exposure to superantigen induces regulatory CD4(+) T cells with IL-10-mediated suppressive activity

The repeated injection of bacterial superantigens (SAg), such as staphylococcus enterotoxin (SE) A or B, has been shown in mice to induce a state of unresponsiveness characterized by the lack of secretion of Th1 lymphokines, such as IL-2 and IFN-gamma, following subsequent SAg challenge. We made the observation, in vivo as well as in vitro, that unresponsiveness to SAg could be transferred from SEA- to SEB-reactive T cells (and reversibly from SEB- to SEA-specific T cells) in C57BL/6 mice but not in BALB/c mice. Since C57BL/6 mice, unlike BALB/c mice, possess TCR V(beta)3+ and V(beta)11+ T cells able to react with both SEA and SEB, we hypothesized that SAg-unresponsive V(beta)3(+) and V(beta)11+ T cells could mediate linked suppression of other SAg-reactive T cells. To analyze further this possibility, spleen cells from BALB/c mice made unresponsive to SEB were tested for their capacity to suppress the response of normal BALB/c cells to SEB. The production of both IFN-gamma and IL-2 following SEB stimulation was greatly impaired in co-cultures containing CD4(+) T cells, but not CD8(+) T cells, isolated from unresponsive animals. In vivo, the production of both IFN-gamma and IL-2 responses to SEB was dramatically reduced in animals adoptively transferred with unresponsive spleen cells. This suppression was abrogated in recipients injected with neutralizing anti-IL-10 antibodies. Moreover, in animals made unresponsive to SEB, SAg-reactive CD4(+) T cells were found to express high levels of CTLA-4, a molecule recently described to play an essential role in the suppressive function of regulatory T cells. Taken together these results demonstrate that the repetitive injection of SAg induces the differentiation of regulatory CD4(+) T cells capable of suppressing SAg-reactive naive T cells.

T cell deletion and unresponsiveness induced by intrathymic injection of staphylococcal enterotoxin B

Intrathymic injection of alloantigens appears to be the most efficient route to induce alterations of T cell reactivity. In the present study, we explored the modifications of Vbeta8.1, 8.2 T cell population and T cell reactivity in the thymus and in the spleen induced by intrathymic injection of staphylococcal enterotoxin B to adult mice. Vbeta8 antigen expression was investigated by flow cytometry analysis. T Cell reactivity was studied in vitro by the proliferative response to SEB. SEB induced a significant reduction in the percentage of mature Vbeta8+ T cells in the thymus (days 7-14), and in the spleen (days 7-28). Interestingly, this depletion occurs in the CD4- CD8+ cells in the thymus whereas in the CD4+ CD8- cells in the spleen. In parallel, the proliferative response to SEB but not to SEA was significantly decreased in the thymus on days 7 and 14, and in the spleen from day 7 to day 28. Moreover, this unresponsiveness was more pronounced in the spleen than in the thymus. Anergy was SEB-specific and fully reversed by exogenous IL-2. SEB injected intrathymically induced significantly more pronounced and more durable T cell alterations than intraperitoneal and subcutaneous injections. This may be related to the observation that after i.t. injection, SEB was detected both at a higher amount and for a longer period in the central and peripheral compartments. Our results clearly demonstrate that the intrathymic route is definitely the most efficient to induce not only thymic but also peripheral pivotal immune alterations in our model.

Superantigens: mechanisms by which they may induce, exacerbate and control autoimmune diseases

Superantigens are polypeptide molecules produced by a broad range of infectious microorganisms which elicit excessive and toxic T-cell responses in mammalian hosts. In light of this property and the fact that autoimmune diseases are frequently the sequelae of microbial infections, it has been suggested that superantigens may be etiologic agents of autoreactive immunological responses resulting in initiation, exacerbation or relapse of autoimmune diseases. This article relates the biology of superantigens to possible mechanisms by which they may exert these activities and reviews the evidence for their roles in various human and animal models of autoimmune disease. Finally, a mechanism of active suppression by superantigen-activated CD4+ T-cells that could be exploited for therapy as well as prophylaxis of human autoimmune diseases is proposed.

Immune response to staphylococcal superantigens

Staphylococcal exotoxins, staphylococcal enterotoxins A-E (SEA-SEE), and toxic shock syndrome toxin- (TSST-1) are potent activators of the immune system and cause a variety of diseases in humans, ranging from food poisoning to shock. These toxins are called superantigens because of their ability to polyclonally activate T cells at picromolar concentrations. Superantigens bind to both MHC class II molecules and specific Vbeta regions of the T cell receptor, leading to the activation of both antigen-presenting cells and T lymphocytes. These interactions lead to excessive production of proinflammatory cytokines and T cell proliferation, causing clinical symptoms that include fever, hypotension, and shock. Recent studies suggest that staphylococcal superantigens may also be involved in the pathogenesis of arthritis and other autoimmune disorders. This review summarizes the in vitro and in vivo effects of staphylococcal enterotoxins and TSST-1, recent progress with the use of transgenic knockout mice to identify key mediators and receptors involved in superantigen-induced shock, and therapeutic agents to mitigate the toxic effects of staphylococcal superantigens.

Effect of staphylococcal enterotoxin B injection on the development of experimental autoimmune encephalomyelitis: influence of cytokine and inducible nitric oxide synthase production

Possible mechanisms involved in the protective effect of staphylococcal enterotoxin B (SEB) injection on the subsequent development of experimental autoimmune encephalomyelitis (EAE) were investigated. Only partial clonal deletion and anergy of Vbeta8 + T-lymphocytes were documented after myelin basic protein immunization in SEB injected mice. Brain permeability was not influenced. Within the brain or during in vitro rechallenge assays SEB protected mice produced significantly more IL-10, IL-4, TNF-alpha and iNOS. It is suggested that the immune deviating effect of SEB may be involved in its EAE protective effect.

Down-regulation of interleukin-2 and interferon-gamma and maintenance of interleukin-4 and interleukin-10 production after administration of an anti-CD3 monoclonal antibody in mice

BACKGROUND

Activating anti-CD3 monoclonal antibodies (mAbs), such as OKT3, are potent immunosuppressive agents that are widely used in clinical transplantation. We investigated whether the in vivo induction of T cell unresponsiveness contributes to the immunosuppressive properties of the anti-mouse-CD3 mAb 145-2C11.

METHODS

After a single in vivo administration of 145-2C11 residual T cells were restimulated in vivo and in vitro to assess cytokine production. Mice were also transplanted with allogeneic skin 9 days after 145-2C11 administration to investigate whether the immunosuppressive properties of the antibody persist after the reexpression of the T cell receptor.

RESULTS

Pretreatment with anti-CD3 mAbs caused a profound deficit in both interleukin- (IL) 2 and interferon- (IFN) y secretion upon restimulation in vivo, whereas IL-4 was only partially inhibited and IL-10 production was significantly increased. Purified T cells obtained from mice injected with anti-CD3 mAb also displayed deficient IL-2 and IFN-gamma production together with persisting IL-4 and IL-10 secretion. 145-2C11 had immunosuppressive properties that per sisted after the reexpression of the T cell receptor because mice transplanted with allogeneic skin 9 days after a single anti-CD3 mAb injection still had significantly prolonged graft survival (14.1+/-0.6 days vs. 10.7+/-0.4 days in controls, P<0.02). Blocking IL-4 and IL-10 by neutralizing mAbs further prolonged skin graft survival in mice injected with 145-2C11 (18.3+/-0.7 vs. 14.8+/-0.6 days, P<0.02).

CONCLUSION

The in vivo administration of the 145-2C11 anti-CD3 mAb results in the selective inhibition of Thl-type cytokine secretion upon restimulation, which correlates with a state of immunosuppression. The persistent production of Th2-type cytokines does not contribute to the anti-CD3 mAb-mediated prolonged survival of skin allografts in our experimental model.

Role and regulation of IL-12 in the in vivo response to staphylococcal enterotoxin B

Injection of a staphylococcal superantigen (SAg) such as staphylococcal enterotoxin B (SEB) in adult mice results in cytokine production and cell proliferation which can lead to septic shock. The aim of the present work was to identify the cytokines and co-stimulatory molecules regulating the in vivo systemic release of IFN-gamma, a cytokine known to play an important role in the pathophysiology associated with bacterial infections. We demonstrate in this study that (i) in contrast to lipopolysaccharide (LPS), SEB administration induces high levels of the p70, bioactive form, of IL-12; (ii) IL-12 production in response to SEB requires both CD40-dependent signals and IFN-gamma secretion; (iii) the early systemic release of IFN-gamma (3 h post-treatment) in response to SEB is IL-12 independent, while the sustained, late response (6-9 h post-treatment) requires endogenous IL-12 production; (iv) IL-12 produced during the primary SEB response (day 0) is responsible for priming cells in vivo to high IFN-gamma production upon secondary challenge (day 2); (v) the priming effect of IL-12 is TCR unrelated, as SEB-primed animals secrete high levels of IFN-gamma in response to both staphylococcal enterotoxin A and LPS administered 48 h later. The ability of bacterial SAg to induce septic shock and to modulate the immune response to unrelated antigens may therefore be related to their unique capacity to induce systemic IL-12 production in vivo. These observations also help to explain why SEB-primed animals, known to express an anergic phenotype 48 h post-treatment (as judged by defective IL-2 production and proliferation), nevertheless display an increased capacity to secrete the inflammatory cytokine IFN-gamma.

Gram-positive sepsis. Mechanisms and differences from gram-negative sepsis

This article has reviewed the mechanisms by which gram-positive bacteria lead to septic shock, with regard to bacterial structure and toxicology and the host responses elicited both in animal models and in the clinical setting. Gram-positive organisms are better suited to invade host tissues and elicit, in general, a brisker phagocytic response than gram-negative organisms. The lack of endotoxin in the outer cell wall is compensated for by the presence of exposed peptidoglycan and a range of other toxic secreted products. It appears that cell wall components of gram-positive bacteria may signal via the same receptor as gram-negative endotoxin, although the type of signal and coreceptor may differ. Both animal and clinical data suggest that, unlike endotoxin-mediated shock, gram-positive infection produces a modest TNF response only and does not respond well to anti-TNF therapies. This leads one to conclude that the mechanisms leading to shock in gram-positive infection may be multifactorial and perhaps more difficult to treat. A thorough review of gram-positive mechanisms of sepsis is hampered by a lack of basic research in this field. Understanding of gram-negative bacterial structure and the regulation of virulence genes is at an advanced stage, yet the molecular tools to analyse virulence factors in the gram-positive genome have only recently become available. There is a paucity of good animal models of gram-positive infection and a lack of microbiologic data from some of the major trials in sepsis that might have given greater insight into the mechanisms leading to shock in various infections.

The role of IL-4 in the staphylococcal enterotoxin B-triggered immune response: increased susceptibility to shock and deletion of CD8Vbeta8+ T cells in IL-4 knockout mice

Administration of superantigens in vivo triggers responding T cells into clonal expansion and subsequent activation of the programmed cell death pathway, as well as into anergy. We examined the possibility that Th1 cytokines are involved in rescue from superantigen-induced programmed cell death and prevention of anergy by studying the Staphylococcus aureus enterotoxin B (SEB) immune response in mice in which the IL-4 gene was deleted (IL-4-/-). In these mice, Th1 cell activation triggers increased IFN-gamma and reduced IL-5 production as compared to IL-4+/+ mice. The primary anti-SEB antibody response in IL-4-/- mice is thus dominated by immunoglobulins of the IgG2a isotype, whereas the IgG1 isotype prevails in IL-4+/+ mice. Our results also show that, in contrast to expectations, IL4-/- mice are more susceptible to SEB plus low-dose D-galactosamine-induced shock and that this response is TNF-alpha-dependent. In vivo treatment induces partial deletion and anergy of remaining SEB-reactive T cells. During the SEB-induced response, CD4Vbeta8+ T cells are deleted in IL-4-/- mice, but not in IL-4+/+ mice, suggesting a function for IL-4 in CD8+ T cell rescue from apoptosis. We show that IL-4 efficiently protects CD8+ T cells from in vitro starvation-induced apoptosis, and conclude that IL-4 has an important role in Th1 immune response regulation.

Superantigen activation and kinetics of cytokines in the Long-Evans rat

This study was conducted to identify and quantify, over time, selected cytokine responses in Long-Evans rats that were exposed to staphylococcus enterotoxin B (SEB). The kinetics of selected cytokines [interleukin-2 (IL-2), IL-6, interferon-gamma (IFN-gamma) and tumour necrosis factor (TNF)] and phenotype and cell cycle analysis of T lymphocytes were determined in Long-Evans rats administered a single intraperitoneal (i.p.) dose of either 50 microg or 500 microg of SEB. Rats injected with 50 microg SEB had significantly elevated levels of IL-2, IL-6 and IFN-gamma in their serum 2 hr post-injection. IL-2 serum levels were significantly elevated at 2 hr and returned to near control values by 12 hr while both IL-6 and IFN-gamma peaked at 6 hr but remained significantly increased at 24 hr post SEB exposure. A 500 microg dose of SEB did not further enhance these cytokine responses. When spleen cells were collected for culture 2 hr after rats were injected i.p. with 50 microg SEB and cocultured with SEB, TNF and IL-6 levels were significantly increased after 2 hr incubation, while IL-2 and IL-6 were significantly elevated at 6 hr. Production of all these cytokines in spleen cell cultures continued to increase over the 24 hr sampled. Peritoneal cells were collected for culture either at 1 hr or 2 hr after injection of either 50 microg or 500 microg of SEB. IL-6 was significantly increased after 1 hr in culture while TNF was significantly increased by 2 hr regardless of whether the cells were harvested 1 or 2 hr after SEB injection. The greatest response for both IL-6 and TNF occurred when cells from animals injected with 50 microg SEB were restimulated in vitro with SEB. The peak levels for IL-6 were at 12 hr post SEB exposure while TNF peaked at 6 hr. The percentage of CD4+ cells was significantly increased at 48 hr and 72 hr post SEB (50 microg) administration while the percentage of CD8+ cells remained similar to control values for the 168-hr test period. A similar pattern was observed in cell cycling where the CD4+ cells proliferated up to 2 days post SEB injection and then were significantly suppressed at day 3. The CD8+ cells were comparable to control values. These studies demonstrate that the cytokine responses in Long-Evans rats exposed to a superantigen are somewhat similar to those that occur in mice and humans, e.g. a rapid short increase in the production of IFN-gamma and TNF that was accompanied by an increase in the production of IL-2. Additional responses noted in this species, however, were a marked increase in IL-6 production, as well as an early increase in the number and cycling of CD4+ cells followed by a down-regulation of these events. These activities occurred in the absence of notable histopathological alteration of lymphoid organs. The results indicate that the Long-Evans rat is an acceptable animal model to investigate the pathogenesis of superantigen-induced disease and that IL-6 may be an active mediator of this process.

Staphylococcal enterotoxin B primes cytokine secretion and lytic activity in response to native bacterial antigens

Superantigens stimulate T-lymphocyte proliferation and cytokine production, but the effects of superantigen exposure on cell function within a complex, highly regulated immune response remain to be determined. In this study, we demonstrate that superantigen exposure significantly alters the murine host response to bacterial antigens in an in vitro coculture system. Two days after exposure to the superantigen staphylococcal enterotoxin B, splenocytes cultured with Streptococcus mutans produced significantly greater amounts of gamma interferon (IFN-gamma) and interleukin-12 than did sham-injected controls. The majority of IFN-gamma production appeared to be CD8(+) T-cell derived since depletion of this cell type dramatically reduced the levels of IFN-gamma. To study host cell damage that may occur following superantigen exposure, we analyzed cytotoxicity to "bystander" fibroblast cells cultured with splenocytes in the presence of bacterial antigens. Prior host exposure to staphylococcal enterotoxin B significantly enhanced fibroblast cytotoxicity in the presence of bacteria. Neutralization of IFN-gamma decreased the amount of cytotoxicity observed. However, a greater reduction was evident when splenocyte-bacterium cocultures were separated from the bystander cell monolayer via a permeable membrane support. Increased cytotoxicity appears to be primarily dependent upon cell-cell contact. Collectively, these data indicate that overproduction of inflammatory cytokines may alter the activity of cytotoxic immune cells. Superantigen exposure exacerbates cytokine production and lytic cell activity when immune cells encounter bacteria in vitro and comparable activities could possibly occur in vivo.

CD4+ T cells mediate superantigen-induced abnormalities in murine jejunal ion transport

The immunomodulatory properties of bacterial superantigens (SAgs) have been defined, yet comparatively little is known of how SAgs may affect enteric physiology. Staphylococcus aureus enterotoxin B (SEB) was used to examine the ability of SAgs to alter epithelial ion transport. BALB/c mice, severe combined immunodeficient (SCID, lack T cells) mice, or SCID mice reconstituted with lymphocytes or CD4+ T cells received SEB intraperitoneally, and jejunal segments were examined in Ussing chambers; controls received saline only. Baseline short-circuit current (Isc, indicates net ion transport) and Isc responses evoked by electrical nerve stimulation, histamine, carbachol, or forskolin were recorded. Serum levels of interleukin-2 (IL-2) and interferon-gamma (IFN-gamma) were measured. SEB-treated BALB/c mice showed elevated serum IL-2 and IFN-gamma levels, and jejunal segments displayed a time- and dose-dependent increase in baseline Isc compared with controls. Conversely, evoked ion secretion was selectively reduced in jejunum from SEB-treated mice. Elevated cytokine levels and changes in jejunal Isc were not observed in SEB-treated SCID mice. In contrast, SCID mice reconstituted with T cells were responsive to SEB challenge as shown by increased cytokine production and altered jejunal Isc responses that were similar to those observed in jejunum from SEB-treated BALB/c mice. We conclude that exposure to a model bacterial SAg causes distinct changes in epithelial physiology and that these events can be mediated by CD4+ T cells.

Sequential production of IL-2, IFN-gamma and IL-10 by individual staphylococcal enterotoxin B-activated T helper lymphocytes

Upon primary activation, T helper (Th) cell populations express different cytokines transiently and with different kinetics. Stimulation of naive murine splenic Th cells with the bacterial superantigen Staphylococcus aureus enterotoxin B (SEB) in vitro results in expression of IL-2, IFN-gamma and IL-10 with fast, intermediate and slow kinetics, respectively. This first report of a functional analysis of cells separated alive according to cytokine expression shows that these cytokines are not produced by different Th cell subpopulations, but can be expressed sequentially by individual Th cells. Th cells, activated with SEB for 1 day and isolated according to expression of IL-2, using the cellular affinity matrix technology, upon continued stimulation with SEB later secrete most of the IFN-gamma and IL-10. Likewise, after 2 days of SEB culture, cells expressing IFN-gamma, separated according to specific surface-associated IFN-gamma as detected by magnetofluorescent liposomes, 1 day later secrete IL-10. Thus, individual Th1 cells can contribute to the control of their own IFN-gamma expression by sequential expression of first IL-2, supporting their proliferation, and later IL-10, down-regulating the production of IFN-gamma-inducing monokines and limiting the pro-inflammatory effects of IFN-gamma.

The crucial role of IL-10 in the suppression of the immunological response in mice exposed to staphylococcal enterotoxin B

Staphylococcal enterotoxin B (SEB), a bacterial superantigen, activates the immune system resulting in a burst of pro- and anti-inflammatory cytokines. A central anti-inflammatory mediator in this process is IL-10. Using IL-10-deficient C57BL/6 (IL-10 KO) mice, we studied the role of endogenous IL-10 in the regulation of the immune response to SEB. SEB (100 microg) induced the release of IL-10 in control C57BL/6 [IL-10 wild type (WT)] mice, but not in their IL-10 KO counterparts. SEB-evoked plasma levels of TNF-alpha, IL-1beta, IL-2, IL-6, IL-12 and IFN-gamma were significantly higher in the IL-10 KO mice than in the WT animals. The release of macrophage inflammatory proteins-1alpha and -2 was also enhanced in the IL-10 KO mice. Further, upon SEB challenge, mice deficient in IL-10 produced higher levels of nitric oxide than the WT animals. IL-10 deficiency resulted in a marked enhancement of the SEB-induced apoptosis of thymocytes. Finally, IL-10 KO mice were more susceptible to SEB-induced lethal shock than their WT controls. These results show that IL-10 plays an important immunoregulatory role in the response to a superantigenic stimulus, by dampening of the shock-inducing inflammatory response and early activation-induced cell death elicited by SEB.

Development of a novel in vitro co-culture system for studying host response to native bacterial antigens

We have developed a novel co-culture system in which murine splenocytes are cultured with live bacteria in the presence of a bacteriostatic antibiotic. Superantigens, like staphylococcal enterotoxin B (SEB) are important factors in bacterial pathogenicity. Research has shown that superantigens affect numerous immune cell types, either directly or indirectly, yet their involvement in pathogenic mechanisms remains poorly defined. In these studies, we utilize the co-culture system to study how superantigen pretreatment affects interferon-gamma (IFN-gamma) production by splenocytes co-cultured with gram-positive bacteria. Streptococcus mutans, S. sanguis and Bacillus subtilis were tested for susceptibility to a panel of antibiotics. Spectinomycin was found to maintain a bacteriostatic state of approximately 10(5) bacteria ml(-1) at optimal concentrations for each bacterial strain. Co-culturing splenocytes with bacteria did not affect splenocyte viability and cultured splenocytes responded to mitogenic stimulation as expected. Two days after SEB pretreatment, isolated splenocytes cultured with either Streptococcus species produced 10-15 times more IFN-gamma than splenocytes from sham-injected controls; however, no differences in CD4+ or CD8+ T cell populations appeared in cultures with or without bacteria. Splenocytes isolated four days after SEB treatment did not produce significant amounts of IFN-gamma in co-culture. Co-cultures containing live bacteria produced four times more IFN-gamma than cultures containing heat-killed bacteria. Splenocytes depleted of natural killer (NK) cells prior to SEB treatment produced 25% less IFN-gamma after 20 h co-culturing with S. mutans. T lymphocytes were identified to be the major producer of IFN-gamma at this time point by intracellular cytokine staining. Apparently SEB exposure primes a response to live bacteria and the response is evident two days after initial exposure. The in vitro co-culture system allows us to observe host responses to bacteria in the context of the multicellular interdependent immune response. With this assay we can more closely 'mimic' in vivo events, particularly immune cell interactions in microfloral environments, to study how the pathogenic effects of superantigens alter this response.

T cell dependent B cell activation occurs during the induction of T cell anergy by staphylococcal enterotoxin B in mice

Staphylococcal enterotoxin B (SEB) can activate specific T cell clones bearing specific TcR V beta domains together with MHC class II ligands on accessory cells. The release of proinflammatory cytokines is the consequence of this activation as well as the main pathological aspect involved in SEB infection. This current study looked at the active role of both T and B cells during the induction of anergy by SEB in vivo. Euthymic and nude BALB/c mice were injected with SEB and over a period of 8 days, cells from the spleen and sera from the blood were collected. After a single injection with SEB (50 micrograms/mouse), a transient increase of CD4+V beta 8+ T cells were detected after 2 days followed by a decrease after 4 days, which persisted until day 8. These clones were rendered anergic upon restimulation in vitro with SEB. Interestingly, cells taken out 2 days after SEB injection, exhibited reduced proliferation in response to Con A. However, this response gradually recovered on days 4, 6 and 8. Furthermore, early IgM antibody production (day 2) was observed after SEB injection. SEB-induced IgM antibody production in euthymic BALB/c was found to have specificity against SEB, cardiolipin (CL) and phosphatidylethanolamine (PE). SEB-treated nude mice did not produce antibody secreting cells in response to SEB, indicating that this process is T cell dependent.

2,3,7,8-Tetrachlorodibenzo-p-dioxin co-stimulates staphylococcal enterotoxin beta (SEB) cytokine production and phenotypic cell cycling in Long-Evans rats

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is an environmental contaminant that is considered to be a potent immunotoxicant. In the present study, we examined the effect of 25 micrograms/kg TCDD on cytokine production and T lymphocyte phenotype, cell cycling and receptor activity in female Long-Evans rats that had been injected with 50 micrograms of Staphylococcal Enterotoxin B (SEB). In the SEB-injected rats, TCDD increased the serum levels of interleukin-2 (IL-2) but did not affect the serum levels of interleukin-1 (IL-1), interleukin-6 (IL-6) or tumor necrosis factor (TNF). The ability of spleen cells and peritoneal cells to produce cytokines in response to SEB restimulation in vitro was also evaluated. TCDD exposure significantly enhanced IL-2 production by spleen cells from SEB-primed rats after 6 h or 24 h in cultures co-stimulated with SEB in vitro. However, TCDD treatment did not alter the production of IL-6 and TNF in these cultures. Although TCDD did not influence the production of IL-6 and TNF in peritoneal cells from SEB-primed rats with SEB restimultion in vitro, IL-1 production was significantly increased at 2 h. Both the kinetics and extent of SEB-induced IL-2 receptor (IL-2R) and T-cell receptor (TCR) expression on CD4+ cells was unaffected by TCDD. TCDD did not significantly alter the percentage or the total numbers of CD4+ and CD8+ subpopulations at various times after SEB injection. However, flow cytometric analysis showed that TCDD exposure increased the percentage of both CD4+ and CD8+ cells cycling in the S and G2M phase. TCDD, in the absence of SEB priming, did not affect any of the immune parameters tested. Nevertheless, collectively these results showed that TCDD can enhance the production of IL-2 and the percentage of CD4+ and CD8+ cells cycling in SEB-exposed Long-Evans rats. Histopatholgically, there were not observable effects of SEB on lymphoid organs while thymic atrophy and diffuse hepatocellular hypertrophy was observed in the TCDD-treated animals.

Further clonal expansion of T cells upon rechallenge of superantigen staphylococcal enterotoxin A

Superantigens are known to induce clonal anergy and/or deletion in reactive T cells peripherally. This study was undertaken to investigate the T-cell status early after exposure to staphylococcal enterotoxin A (SEA) in vivo and in vitro. At the peak of clonal expansion following the administration of 5 microg SEA (i.e., 2 days after the injection), C57BL/6 mice were rechallenged with the same dose of SEA in vivo. The secondary stimulation augmented clonal expansion of the T cells bearing Vbeta3 and Vbeta11 in both CD4+ and CD8+ populations. In vitro restimulation of the spleen cells taken from the SEA-primed mice also induced further expansion of the Vbeta3+ T cells during 2 days of culturing, whereas without restimulation, a marked reduction of Vbeta3+ T cells occurred. The spleen cells from the SEA-primed mice were hyper-reactive to in vitro restimulation with SEA as measured by 3H-TdR uptake on day 1 of culturing, but augmented proliferation leveled off thereafter. By day 3, the values of 3H-TdR uptake were less than 20% of those of the controls in which spleen cells from native mice were stimulated with SEA in vitro. These results suggest that T cells exposed to SEA in vivo are still capable of proliferating upon SEA rechallenge, but subsequently, the proliferation starts to wane.

Induction of unresponsiveness and impaired T cell expansion by staphylococcal enterotoxin B in CD28-deficient mice

We used CD28-deficient mice to analyze the importance of CD28 costimulation for the response against Staphylococcal enterotoxin B (SEB) in vivo. CD28 was necessary for the strong expansion of V beta 8+ T cells, but not for deletion. The lack of expansion was not due to a failure of SEB to activate V beta 8+ T cells, as V beta 8+ T cells from both CD28-/- and CD28+/+ mice showed similar phenotypic changes within the first 24 h after SEB injection and cell cycle analysis showed that an equal percentage of V beta 8+ T cells started to proliferate. However, the phenotype and the state of proliferation of V beta 8+ T cells was different at later time points. Furthermore, in CD28-/- mice injection with SEB led to rapid induction of unresponsiveness in SEB responsive T cells, indicated by a drastic reduction of proliferation after secondary SEB stimulation in vitro. Unresponsiveness could also be demonstrated in vivo, as CD28-/- mice produced only marginal amounts of TNF alpha after rechallenge with SEB. In addition CD28-/- mice were protected against a lethal toxic shock induced by a second injection with SEB. Our results indicate that CD28 costimulation is crucial for the T cell-mediated toxicity of SEB and demonstrate that T cell stimulation in the absence of CD28 costimulation induces unresponsiveness in vivo.

Bacterial superantigens in human disease: structure, function and diversity

All bacterial superantigens use common structural strategies to bind to major histocompatibility complex class II receptors, while binding the T cell antigen receptor in different ways. Overstimulation of the immune response is responsible for the acute pathological effects, while reactivation of developmentally silenced T cells might result in autoimmune disease. Certain diseases might be controlled with superantigens or genetically attenuated vaccines.