In vivo responses of CD4+ and CD8+ cells to bacterial superantigens

Discordant rearrangement of primary and anamnestic CD8+ T cell responses to influenza A viral epitopes upon exposure to bacterial superantigens: Implications for prophylactic vaccination, heterosubtypic immunity and superinfections

Infection with (SAg)-producing bacteria may precede or follow infection with or vaccination against influenza A viruses (IAVs). However, how SAgs alter the breadth of IAV-specific CD8⁺ T cell (T_CD8) responses is unknown. Moreover, whether recall responses mediating heterosubtypic immunity to IAVs are manipulated by SAgs remains unexplored. We employed wild-type (WT) and mutant bacterial SAgs, SAg-sufficient/deficient Staphylococcus aureus strains, and WT, mouse-adapted and reassortant IAV strains in multiple in vivo settings to address the above questions. Contrary to the popular view that SAgs delete or anergize T cells, systemic administration of staphylococcal enterotoxin B (SEB) or Mycoplasma arthritidis mitogen before intraperitoneal IAV immunization enlarged the clonal size of ‘select’ IAV-specific T_CD8 and reshuffled the hierarchical pattern of primary T_CD8 responses. This was mechanistically linked to the TCR Vβ makeup of the impacted clones rather than their immunodominance status. Importantly, SAg-expanded T_CD8 retained their IFN-γ production and cognate cytolytic capacities. The enhancing effect of SEB on immunodominant T_CD8 was also evident in primary responses to vaccination with heat-inactivated and live attenuated IAV strains administered intramuscularly and intranasally, respectively. Interestingly, in prime-boost immunization settings, the outcome of SEB administration depended strictly upon the time point at which this SAg was introduced. Accordingly, SEB injection before priming raised CD127^highKLRG1^low memory precursor frequencies and augmented the anamnestic responses of SEB-binding T_CD8. By comparison, introducing SEB before boosting diminished recall responses to IAV-derived epitopes drastically and indiscriminately. This was accompanied by lower Ki67 and higher Fas, LAG-3 and PD-1 levels consistent with a pro-apoptotic and/or exhausted phenotype. Therefore, SAgs can have contrasting impacts on anti-IAV immunity depending on the naïve/memory status and the TCR composition of exposed T_CD8. Finally, local administration of SEB or infection with SEB-producing S. aureus enhanced pulmonary T_CD8 responses to IAV. Our findings have clear implications for superinfections and prophylactic vaccination.

Exposure to bacterial superantigens (SAgs) is often a consequence of infection with common Gram-positive bacteria causing septic and toxic shock or food poisoning. How SAgs affect the magnitude, breadth and quality of infection/vaccine-elicited CD8⁺ T cell (T_CD8) responses to respiratory viral pathogens, including influenza A viruses (IAVs), is far from clear. Also importantly, superinfections with IAVs and SAg-producing bacteria are serious clinical occurrences during seasonal and pandemic flu and require urgent attention. We demonstrate that two structurally distinct SAgs, including staphylococcal enterotoxin B (SEB), unexpectedly enhance primary T_CD8 responses to ‘select’ IAV-derived epitopes depending on the TCR makeup of the responding clones. Intriguingly, the timing of exposure to SEB dictates the outcome of prime-boost immunization. Seeing a SAg before priming raises memory precursor frequencies and augments anamnestic T_CD8 responses. Conversely, a SAg encounter before boosting renders T_CD8 prone to death or exhaustion and impedes recall responses, thus likely compromising heterosubtypic immunity to IAVs. Finally, local exposure to SEB increases the pulmonary response of immunodominant IAV-specific T_CD8. These findings shed new light on how bacterial infections and SAgs influence the effectiveness of anti-IAV T_CD8 responses, and have, as such, wide-ranging implications for preventative vaccination and infection control.

Streptococcal Pyrogenic Exotoxin A-Stimulated Monocytes Mediate Regulatory T-Cell Accumulation through PD-L1 and Kynurenine

Bacterial superantigens (SAgs) are exotoxins that promote a fulminant activation of the immune system. The subsequent intense release of inflammatory cytokines often results in hypotension, shock, and organ failure with high mortality rates. In the current paradigm, the direct and simultaneous binding of SAgs with T-cell receptor (TCR)-bearing Vβ regions and conserved structures on major histocompatibility complex class II (MHC class II) on antigen-presenting cells (APCs) induces the activation of both cell types. However, by crosslinking MHC class II molecules, APCs can be activated by SAgs independently of T lymphocytes. Recently, we showed that streptococcal pyrogenic exotoxin A (SPEA) of Streptococcus pyogenes stimulates an immunogenic APC phenotype with upregulated costimulatory molecules and inflammatory cytokines. Additionally, we revealed that SPEA triggers immunosuppressive programs in monocytes that facilitate the accumulation of regulatory T cells (Tregs) in in vitro monocyte/CD4⁺ T-cell cocultures. Immunosuppressive factors include anti-inflammatory interleukin 10 (IL-10), co-inhibitory surface molecule programmed cell death 1 ligand 1 (PD-L1), and the inhibitory indoleamine 2,3-dioxygenase (IDO)/kynurenine effector system. In the present study, we investigated the underlying mechanism of SPEA-stimulated monocyte-mediated accumulation of Tregs. Blood-derived monocytes from healthy donors were stimulated with SPEA for 48 h (SPEA-monocytes). For the evaluation of SPEA-monocyte-mediated modulation of CD4⁺ T lymphocytes, SPEA was removed from the culture through extensive washing of cells before adding allogeneic CD3/CD28-activated T cells. Results: In coculture with allogeneic CD4⁺ T cells, SPEA-monocytes mediate apoptosis of CD4⁺Foxp3⁻ lymphocytes and accumulation of CD4⁺Foxp3⁺ Tregs. PD-L1 and kynurenine are critically involved in the mediated cell death because blocking both factors diminished apoptosis and decreased the proportion of the CD25⁺/Foxp3⁺ Treg subpopulation significantly. Upregulation of PD-L1 and kynurenine as well as SPEA-monocyte-mediated effects on T cells depend on inflammatory IL-1β. Our study shows that monocytes activated by SPEA mediate apoptosis of CD4⁺Foxp3⁻ T effector cells through PD-L1 and kynurenine. CD4⁺Foxp3⁺ T cells are resistant to apoptosis and accumulate in SPEA-monocyte/CD4⁺ T-cell coculture.

Staphylococcal Exotoxins and Nasal Polyposis: Analysis of Systemic and Local Responses

Background

Staphylococcal exotoxins have been implicated in the pathogenesis of several chronic inflammatory diseases including atopic dermatitis (AD), asthma, and, most recently, chronic rhinosinusitis with nasal polyposis (CRS/NP). In severe AD, these toxins act both as superantigens (SAg), triggering massive T-cell activation, and as conventional allergens, triggering toxin-specific immunoglobulin E (IgE) in the serum. In CRS/NP, evidence for both processes has been reported but it is unclear whether these processes are linked. The aim of this study was to correlate SAg activity as inferred by staphylococcal-specific T-cell receptor (TCR) V-β expansion in the polyp and blood of CRS/NP patients with staphylococcal-specific anti-IgE antibodies in the serum.

Methods

IgE antibodies to staphylococcal exotoxin A (SEA), staphylococcal exotoxin B (SEB), and toxic shock syndrome toxin (TSST) 1 were measured in the serum of 12 individuals with CRS/NP before functional endoscopic sinus surgery. Flow cytometry was used to analyze the SEA, SEB, and TSST-1–specific TCR V-β domains on the T cells from the polyp and blood of these patients.

Results

Serum SEA-, SEB-, and TSST-1-specific IgE antibodies were detected in 0/12 (0%), 6/12 (50.0%), and 9/12 (75%) of CRS/NP patients, respectively. Evidence of SAg effect in the polyp lymphocytes (TCR V-β expansion in both CD4+ and CD8+ subsets) was noted in 7/12 (58.3%) patients. Five of 6 CRS/NP patients had overlapping evidence of a systemic IgE response and TCR V-β expansion, suggestive of exposure to the same exotoxin. No patients had evidence a SAg effect in blood lymphocytes. Nine of 12 subjects also had coexistent asthma.

Conclusion

These results provide evidence for a local SAg effect in 7/12 (58.3%) polyp patients and establish a positive correlation of V-β expansion with the presence of corresponding toxin-specific IgE in the serum.

Nasopharyngeal infection by Streptococcus pyogenes requires superantigen-responsive Vβ-specific T cells

The globally prominent pathogen Streptococcus pyogenes secretes potent immunomodulatory proteins known as superantigens (SAgs), which engage lateral surfaces of major histocompatibility class II molecules and T-cell receptor (TCR) β-chain variable domains (Vβs). These interactions result in the activation of numerous Vβ-specific T cells, which is the defining activity of a SAg. Although streptococcal SAgs are known virulence factors in scarlet fever and toxic shock syndrome, mechanisms by how SAgs contribute to the life cycle of S. pyogenes remain poorly understood. Herein, we demonstrate that passive immunization against the Vβ8-targeting SAg streptococcal pyrogenic exotoxin A (SpeA), or active immunization with either wild-type or a nonfunctional SpeA mutant, protects mice from nasopharyngeal infection; however, only passive immunization, or vaccination with inactive SpeA, resulted in high-titer SpeA-specific antibodies in vivo. Mice vaccinated with wild-type SpeA rendered Vβ8⁺ T cells poorly responsive, which prevented infection. This phenotype was reproduced with staphylococcal enterotoxin B, a heterologous SAg that also targets Vβ8⁺ T cells, and rendered mice resistant to infection. Furthermore, antibody-mediated depletion of T cells prevented nasopharyngeal infection by S. pyogenes, but not by Streptococcus pneumoniae, a bacterium that does not produce SAgs. Remarkably, these observations suggest that S. pyogenes uses SAgs to manipulate Vβ-specific T cells to establish nasopharyngeal infection.

T-cell activation or tolerization: the Yin and Yang of bacterial superantigens

Bacterial superantigens (SAg) are exotoxins from pathogens which interact with innate and adaptive immune cells. The paradox that SAgs cause activation and inactivation/anergy of T-cells was soon recognized. The structural and molecular events following SAg binding to antigen presenting cells (APCs) followed by crosslinking of T-cell receptors were characterized in detail. Activation, cytokine burst and T-cell anergy have been described in vitro and in vivo. Later it became clear that SAg-induced T-cell anergy is in part caused by SAg-dependent activation of T-regulatory cells (Tregs). Although the main focus of analyses was laid on T-cells, it was also shown that SAg binding to MHC class II molecules on APCs induces a signal, which leads to activation and secretion of pro-inflammatory cytokines. Accordingly APCs are mandatory for T-cell activation. So far it is not known, whether APCs play a role during SAg-triggered activation of Tregs. We therefore tested whether in SAg (Streptococcal pyrogenic exotoxin A) -treated APCs an anti-inflammatory program is triggered in addition. We show here that not only the anti-inflammatory cytokine IL-10 and the co-inhibitory surface molecule PD-L1 (CD274) but also inhibitory effector systems like indoleamine 2,3-dioxygenase (IDO) or intracellular negative feedback loops (suppressor of cytokine signaling molecules, SOCS) are induced by SAgs. Moreover, cyclosporine A completely prevented induction of this program. We therefore propose that APCs triggered by SAgs play a key role in T-cell activation as well as inactivation and induction of Treg cells.

Superantigenic Yersinia pseudotuberculosis induces the expression of granzymes and perforin by CD4+ T cells

Bacterial superantigens (SAgs) are immunostimulatory toxins that induce acute diseases mainly through the massive release of inflammatory cytokines. Yersinia pseudotuberculosis is the only Gram-negative bacterium known to produce a SAg (Y. pseudotuberculosis-derived mitogen [YPM]). This SAg binds major histocompatibility complex class II molecules on antigen-presenting cells and T cell receptors (TcR) bearing the variable region Vβ3, Vβ9, Vβ13.1, or Vβ13.2 (in humans) and Vβ7 or Vβ8 (in mice). We have previously shown that YPM exacerbates the virulence of Y. pseudotuberculosis in mice. With a view to understanding the mechanism of YPM's toxicity, we compared the immune response in BALB/c mice infected with a YPM-producing Y. pseudotuberculosis or the corresponding isogenic, SAg-deficient mutant. Five days after infection, we observed strong CD4(+) Vβ7(+) T cell expansion and marked interleukin-4 (IL-4) production in mice inoculated with SAg-producing Y. pseudotuberculosis. These phenomena were correlated with the activation of ypm gene transcription in liver and spleen. A transcriptomic analysis revealed that the presence of YPM also increased expression of granzyme and perforin genes in the host's liver and spleen. This expression was attributed to a CD4(+) T cell subset, rather than to natural killer T (NKT) cells that display a TcR with a Vβ region that is potentially recognized by YPM. Increased production of cytotoxic molecules was correlated with hepatotoxicity, as demonstrated by an increase in plasma alanine aminotransferase activity. Our results demonstrate that YPM activates a potentially hepatotoxic CD4(+) T cell population.

Control of established colon cancer xenografts using a novel humanized single chain antibody-streptococcal superantigen fusion protein targeting the 5T4 oncofetal antigen

Superantigens (SAgs) are microbial toxins that cross-link T cell receptors with major histocompatibility class II (MHC-II) molecules leading to the activation of large numbers of T cells. Herein, we describe the development and preclinical testing of a novel tumor-targeted SAg (TTS) therapeutic built using the streptococcal pyrogenic exotoxin C (SpeC) SAg and targeting cancer cells expressing the 5T4 tumor-associated antigen (TAA). To inhibit potentially harmful widespread immune cell activation, a SpeC mutation within the high-affinity MHC-II binding interface was generated (SpeC_D203A) that demonstrated a pronounced reduction in mitogenic activity, yet this mutant could still induce immune cell-mediated cancer cell death in vitro. To target 5T4⁺ cancer cells, we engineered a humanized single chain variable fragment (scFv) antibody to recognize 5T4 (scFv5T4). Specific targeting of scFv5T4 was verified. SpeC_D203A fused to scFv5T4 maintained the ability to activate and induce immune cell-mediated cytotoxicity of colorectal cancer cells. Using a xenograft model of established human colon cancer, we demonstrated that the SpeC-based TTS was able to control the growth and spread of large tumors in vivo. This required both TAA targeting by scFv5T4 and functional SAg activity. These studies lay the foundation for the development of streptococcal SAgs as ‘next-generation’ TTSs for cancer immunotherapy.

Detection of Staphylococcal Exotoxins in Antrochoanal Polyps and Chronic Rhinosinusitis with Nasal Polyps

Objective

Staphylococcal exotoxins have been reported in the pathogenesis of many chronic inflammatory diseases. Recent reports have hypothesized that staphylococcal exotoxins might be related to inflammatory mucosal changes seen in chronic sinusitis with nasal polyps (CRS-NPs). Staphylococcal exotoxins have the capacity to act as superantigens (SAgs), bypassing normal antigen processing and directly stimulating a massive inflammatory response. The objective of this study was to analyze polyp tissue samples from patients with antrochoanal polyps (ACPs) and CRS-NPs for the presence of staphylococcal exotoxins.

Study Design

Prospective cohort study.

Setting

Tertiary medical center.

Subjects and Methods

Tissue samples were obtained from 29 patients and 16 controls. Thirteen of 29 patients had ACPs, and 16 had CRS-NPs. Specimens were analyzed for the presence of 5 staphylococcal exotoxins (SEA, SEB, SEC, SED, and toxic shock syndrome toxin–1) using enzyme-linked immunosorbent assay (ELISA).

Results

At least 1 toxin was detected in 7 of 13 patients with ACPs and in 13 of 16 patients with CRS-NPs, whereas it was detected in only 4 controls. There were no statistically significant differences between ACP and control groups ( P = .063). Our results showed a statistically significant association between toxin detection and patients with CRS-NPs ( P = .003).

Conclusion

Inflammatory triggers in ACP remain a subject of debate, and this study does not support the hypothesis that staphylococcal exotoxins may play a role in ACP ethiopathogenesis. Our research is consistent with the possibility of SAgs as etiological agents in the development of bilateral nasal polyposis.

Staphylococcal superantigens in colonization and disease

Superantigens (SAgs) are a family of potent immunostimulatory exotoxins known to be produced by only a few bacterial pathogens, including Staphylococcus aureus. More than 20 distinct SAgs have been characterized from different S. aureus strains and at least 80% of clinical strains harbor at least one SAg gene, although most strains encode many. SAgs have been classically associated with food poisoning and toxic shock syndrome (TSS), for which these toxins are the causative agent. TSS is a potentially fatal disease whereby SAg-mediated activation of T cells results in overproduction of cytokines and results in systemic inflammation and shock. Numerous studies have also shown a possible role for SAgs in other diseases such as Kawasaki disease (KD), atopic dermatitis (AD), and chronic rhinosinusitis (CRS). There is also now a rich understanding of the mechanisms of action of SAgs, as well as their structures and function. However, we have yet to discover what purpose SAgs play in the life cycle of S. aureus, and why such a wide array of these toxins exists. This review will focus on recent developments within the SAg field in terms of the molecular biology of these toxins and their role in both colonization and disease.

CD1d-independent activation of mouse and human iNKT cells by bacterial superantigens

Invariant NKT (iNKT) cells are infrequent but important immunomodulatory lymphocytes that exhibit CD1d-restricted reactivity with glycolipid Ags. iNKT cells express a unique T-cell receptor (TCR) composed of an invariant α-chain, paired with a limited range of β-chains. Superantigens (SAgs) are microbial toxins defined by their ability to activate conventional T cells in a TCR β-chain variable domain (Vβ)-specific manner. However, whether iNKT cells are directly activated by bacterial SAgs remains an open question. Herein, we explored the responsiveness of mouse and human iNKT cells to a panel of staphylococcal and streptococcal SAgs and examined the contribution of major histocompatibility complex (MHC) class II and CD1d to these responses. Bacterial SAgs that target mouse Vβ8, such as staphylococcal enterotoxin B (SEB), were able to activate mouse hybridoma and primary hepatic iNKT cells in the presence of mouse APCs expressing human leukocyte antigen (HLA)-DR4. iNKT cell-mediated cytokine secretion in SEB-challenged HLA-DR4-transgenic mice was CD1d-independent and accompanied by a high interferon-γ:interleukin-4 ratio consistent with an in vivo Th1 bias. Furthermore, iNKT cells from SEB-injected HLA-DR4-transgenic mice, and iNKT cells from SEB-treated human PBMCs, showed early activation by intracellular cytokine staining and CD69 expression. Unlike iNKT cell stimulation by α-galactosylceramide, stimulation by SEB did not induce TCR downregulation of either mouse or human iNKT cells. We conclude that Vβ8-targeting bacterial SAgs can activate iNKT cells by utilizing a novel pathway that requires MHC class II interactions, but not CD1d. Therefore, iNKT cells fulfill important effector functions in response to bacterial SAgs and may provide attractive targets in the management of SAg-induced illnesses.

Superantigen presentation by airway smooth muscle to CD4+ T lymphocytes elicits reciprocal proasthmatic changes in airway function

Microbial products serving as superantigens (SAgs) have been implicated in triggering various T cell-mediated chronic inflammatory disorders, including severe asthma. Given earlier evidence demonstrating that airway smooth muscle (ASM) cells express MHC class II molecules, we investigated whether ASM can present SAg to resting CD4(+) T cells, and further examined whether this action reciprocally elicits proasthmatic changes in ASM responsiveness. Coincubation of CD4(+) T cells with human ASM cells pulsed with the SAg, staphylococcal enterotoxin A (SEA), elicited adherence and clustering of class II and CD3 molecules at the ASM/T cell interface, indicative of immunological synapse formation, in association with T cell activation. This ASM/T cell interaction evoked up-regulated mRNA expression and pronounced release of the Th2-type cytokine, IL-13, into the coculture medium, which was MHC class II dependent. Moreover, when administering the conditioned medium from the SEA-stimulated ASM/T cell cocultures to isolated naive rabbit ASM tissues, the latter exhibited proasthmatic-like changes in their constrictor and relaxation responsiveness that were prevented by pretreating the tissues with an anti-IL-13 neutralizing Ab. Collectively, these observations are the first to demonstrate that ASM can present SAg to CD4(+) T cells, and that this MHC class II-mediated cooperative ASM/T cell interaction elicits release of IL-13 that, in turn, evokes proasthmatic changes in ASM constrictor and relaxant responsiveness. Thus, a new immuno-regulatory role for ASM is identified that potentially contributes to the pathogenesis of nonallergic (intrinsic) asthma and, accordingly, may underlie the reported association between microbial SAg exposure, T cell activation, and severe asthma.

Antigen challenge inhibits thymic emigration

T cell development in the thymus involves a series of TCR-mediated control points including TCR-beta selection and positive and negative selection. Approximately half of the thymic sojourn is spent in the medulla, where thymocytes undergo final maturation before emigrating to the periphery. Although it is acknowledged that thymic emigration is an active process, relatively little is known about how this is regulated, why it takes so long, and whether TCR-mediated signaling can influence this step. Using wild-type and TCR transgenic mice, we found that Ag injected i.v. or intrathymically led to a striking reduction in the number of recent thymic emigrants (RTE) in the periphery. This was caused by inhibition of T cell export rather than peripheral deletion, because a cohort of RTE that was already released before in vivo Ag challenge was not depleted, and similar results were observed in Bim-deficient mice, which have impaired T cell deletion. Within the thymus, the loss of RTE was associated with retention of medullary thymocytes rather than increased negative selection. In addition to Ag-specific inhibition of export, some TCR-independent suppression of emigration was also observed that appeared to be partly the result of the inflammatory cytokine TNF. Thus, in addition to its accepted role in intrathymic selection events, TCR signaling can also play an important role in the regulation of thymic emigration.

Topical superantigen exposure induces epidermal accumulation of CD8+ T cells, a mixed Th1/Th2-type dermatitis and vigorous production of IgE antibodies in the murine model of atopic dermatitis

Patients with atopic dermatitis (AD) have repeated cutaneous exposure to both environmental allergens and superantigen-producing strains of Staphylococcus aureus. We used a murine model of AD to investigate the role of staphylococcal enterotoxin B (SEB) in the modulation of allergen-induced skin inflammation. Mice were topically exposed to SEB, OVA, a combination of OVA and SEB (OVA/SEB), or PBS. Topical SEB and OVA/SEB exposure induced epidermal accumulation of CD8+ T cells and TCRVbeta8+ cells in contrast to OVA application, which induced a mainly dermal infiltration of CD4+ cells. SEB and OVA/SEB exposure elicited a mixed Th1/Th2-associated cytokine and chemokine expression profile within the skin. Restimulation of lymph node cells from OVA- and OVA/SEB-exposed mice with OVA elicited strong production of IL-13 protein, whereas substantial amounts of IFN-gamma protein were detected after SEB stimulation of cells derived from SEB- or OVA/SEB-exposed mice. Topical SEB treatment elicited vigorous production of SEB-specific IgE and IgG2a Abs and significantly increased the production of OVA-specific IgE and IgG2a Abs. The present study shows that topical exposure to SEB provokes epidermal accumulation of CD8+ T cells, a mixed Th2/Th1 type dermatitis and vigorous production of specific IgE and IgG2a Abs, which can be related to the chronic phase of atopic skin inflammation.

Increased sensitivity to staphylococcal enterotoxin B following adenoviral infection

Staphylococcal enterotoxin B induces toxic shock and is a major virulence factor of staphylococcal diseases. We examined the effects of systemic adenoviral infection on responses to staphylococcal enterotoxin B in a murine model. We found that adenoviral infection markedly increases the severity of liver injury following exposure to staphylococcal enterotoxin B without d-galactosamine sensitization. In adenovirus-infected mice, staphylococcal enterotoxin B triggered a more profound hypothermia and increased apoptosis in the liver. Consistent with these observations, we also found that adenoviral infection primed for an increased production of gamma interferon in vivo and in vitro following stimulation with staphylococcal enterotoxin B. Gamma-interferon-knockout mice did not show increased sensitivity to staphylococcal enterotoxin B following adenoviral infection. These data suggest that a preexisting viral infection primes mice for subsequent staphylococcal enterotoxin B exposure, possibly via a gamma-interferon-mediated mechanism.

Burn Injury Initiates a Shift in Superantigen-Induced T Cell Responses and Host Survival

Severe injury induces a temporal shift in immune reactivity that can cause serious complications or even death. We previously reported that mice exposed to bacterial superantigen (SAg) early after injury undergo a strong SAg response with lethal consequences. This study compares the early and late effects of burn injury on SAg reactivity in vivo to establish how injury influences adaptive immune responses. We found that mice challenged with ordinarily sublethal doses of staphylococcal enterotoxin A or staphylococcal enterotoxin B at 1 day after burn injury exhibited high mortality, whereas no mortality occurred at 7 days after injury. This shift in mortality correlated with higher Th2-type cytokines (IL-4 and IL-10) being expressed by CD4(+) and CD8(+) T cells from burn as opposed to sham mice at 7 days after injury. Lymph node cells from burn-injured mice also produced higher levels of Th2-type cytokines at 7 days after injury. The results of cell-mixing studies using CD4(+) and CD8(+) T cells mixed with APCs from sham or burn mice suggested that changes in both T cells and APCs are involved in the altered SAg response. Finally, the biological significance of altered SAg reactivity following injury was shown by demonstrating that blocking IL-10 activity in vivo caused higher SAg-induced mortality at 7 days after injury. These findings support the idea that injury promotes a Th2-type shift in adaptive immune reactivity. Although prior studies link this counterinflammatory-type response to lowered resistance to infection, the present results suggest it may sometimes benefit the injured host.

Vbeta-restricted T cell adherence to endothelial cells: a mechanism for superantigen-dependent vascular injury

OBJECTIVE

To investigate the potential for endothelial cells to operate as superantigen-presenting cells for T cells and the potential for such an interaction to cause endothelial cell activation and injury.

METHODS

Class II major histocompatibility complex (MHC)-positive human umbilical vein endothelial cells (HUVECs) were cocultured for 4 hours with purified T cells and the superantigens staphylococcal enterotoxin B (SEB) or toxic shock syndrome toxin 1 (TSST-1). After staining with fluorescence-conjugated monoclonal antibodies, flow cytometric analysis was performed on the HUVECs and T cells to examine V(beta)-restricted T cell adherence to the endothelial cell monolayer, V(beta)-restricted T cell activation (CD69 up-regulation), surface expression of endothelial cell activation markers, and generation of endothelial microparticles (EMPs).

RESULTS

Coculture of purified T cells with class II MHC-positive HUVECs and either TSST-1 or SEB resulted in V(beta)-restricted CD69 up-regulation by CD4 and CD8 cells (V(beta)2 activation for TSST-1; V(beta)3, V(beta)5.1, and V(beta)12 activation for SEB). Additionally, there was CD4 and CD8 T cell V(beta)-restricted adherence to the HUVEC monolayer at 4 hours. Expression of intercellular adhesion molecule 1, E-selectin, and vascular cell adhesion molecule 1 was up-regulated on the class II MHC-positive HUVECs following exposure to superantigen in the presence of T cells, and there was increased EMP release from activated HUVECs, which occurred earlier and was of greater magnitude than that observed in response to tumor necrosis factor alpha.

CONCLUSION

Class II MHC-positive endothelial cells operate as competent superantigen-presenting cells for CD4 and CD8 lymphocytes in vitro. Dual signaling between endothelial cells and T cells results in V(beta)-restricted activation and adherence to endothelial monolayers and endothelial cell activation and release of EMPs expressing inducible cell adhesion molecules. It is proposed that this mechanism could account in part for the vascular injury associated with superantigen-mediated diseases including Kawasaki disease.

Staphylococcal superantigens and T cell expansions in Wegener's granulomatosis

In Wegener's granulomatosis (WG), a form of autoimmune systemic vasculitis, chronic carriage of Staphylococcus aureus constitutes a risk factor for the development of exacerbations. Circulating T cells in this disease are persistently activated, suggesting the presence of a chronic stimulus. A causal link between chronic carriage of S. aureus and chronic T cell activation in WG is conceivable, because S. aureus produces superantigens (SAg), which are potent T cell stimulators. Superantigenic stimulation of T cells results in expansion of T cell subsets expressing SAg-binding T cell receptor V-beta (Vbeta) chains. In the present study we hypothesized that in WG the presence of staphylococcal SAg is accompanied by expansion of SAg-reacting T cell subsets. We tested our hypothesis in a cross-sectional and a longitudinal study in which the association between seven staphylococcal SAg genes [typed by poplymerase chain reaction (PCR)], eight SAg-binding Vbeta chains and four SAg-non-binding Vbeta chains (assessed by flow-cytometry) was assessed. Both studies showed that T cell expansions were present at a significantly higher rate in WG patients than in healthy individuals, but were not associated with the presence of either S. aureus or its SAg. Moreover, T cell expansions were generally of small extent, and did not appear simultaneously in both CD4 and CD8 subsets. We conclude that in WG S. aureus effects its supposed pathogenic function by a mechanism other than superantigenic T cell activation.

Activation of human T cells with NK cell markers by staphylococcal enterotoxin A via IL-12 but not via IL-18

We have reported recently that mouse liver NK cells and NK1 x 1+ T cells were activated by bacterial superantigens via the IL-12 production from Kupffer cells. In the present study, we examined the effect of staphyloccoccal enterotoxin A (SEA) on human T cells with NK cell markers, CD56 or CD57 (NK-type T cells). After stimulating peripheral blood mononuclear cells (PBMC) with SEA, PBMC produced a large amount of IFN- and acquired a potent antitumour cytotoxicity. The in vitro depletion of either CD56+ TCR NK cells, CD56+ T cells or 57+ T cells from PBMC significantly inhibited the IFN- production from PBMC. When purified NK-type T cells, NK cells and regular T cells were cultured with monocytes and SEA they all produced IFN-, while the IFN- amounts produced by both NK-type T cells were greater than those produced by NK cells. NK cells as well as CD56+ T cells showed cytotoxicity against NK-sensitive K562 cells, whereas both NK-type T cells showed a more potent cytotoxicity against NK-resistant Raji cells than did NK cells. The IFN- production from each population as well as from whole PBMC was greatly inhibited by anti-IL-12 antibody but not by anti-IL-18 antibody. The antitumour cytotoxicity of whole PBMC was also significantly inhibited by anti-IL-12 antibody while the SEA-induced proliferation of PBMC was not affected by anti-IL-12 antibody. Furthermore, SEA-activated NK-type T cells as well as NK cells showed cytotoxicities against vascular endothelial cells. Our findings suggest that human NK-type T cells are thus involved in bacterial superantigen-induced immune response.

Identification of a transcytosis epitope on staphylococcal enterotoxins

Staphylococcal enterotoxins (SE) are exoproteins produced by Staphylococcus aureus that act as superantigens and have been implicated as a leading cause of food-borne disease and toxic shock. Little is known about how these molecules penetrate the gut lining and gain access to both local and systemic immune tissues. To model movement in vitro of staphylococcal enterotoxins, we have employed a monolayer system composed of crypt-like human colonic T-84 cells. SEB and SEA showed comparable dose-dependent transcytosis in vitro, while toxic shock syndrome toxin (TSST-1) exhibited increased movement at lower doses. Synthetic peptides corresponding to specific regions of the SEB molecule were tested in vitro to identify the domain of the protein involved in the transcytosis of SE. A toxin peptide of particular interest contains the amino acid sequence KKKVTAQELD, which is highly conserved across all SE. At a toxin-to-peptide ratio of 1:10, movement of SEB across the monolayers was reduced by 85%. Antisera made against the SEB peptide recognized native SEB and also inhibited SEB transcytosis. Finally, the conserved 10-amino-acid peptide inhibited transcytosis of multiple staphylococcal enterotoxins, SEA, SEE, and TSST-1. These data demonstrate that this region of the staphylococcal enterotoxins plays a distinct role in toxin movement across epithelial cells. It has implications for the prevention of staphylococcal enterotoxin-mediated disease by design of a peptide vaccine that could reduce systemic exposure to oral or inhaled superantigens. Since the sequence identified is highly conserved, it allows for a single epitope blocking the transcytosis of multiple SE.

Signaling through CD28 and CTLA-4 controls two distinct forms of T cell anergy

Primary T cell proliferative responses to TCR ligation plus CD28 costimulation are surprisingly heterogeneous. Many cells that enter G1 fail to progress further through the cell cycle, and some of these cells subsequently fail to divide upon restimulation, even in the presence of IL-2. Such IL-2-refractory anergy is distinct from the IL-2-reversible anergy induced by TCR occupancy in the absence of CD28 costimulation. Here, we focus on the contributions of cell cycle progression and costimulatory (CD28/CTLA-4) signals in the regulation of anergy. We show that CD28 costimulation is not sufficient for anergy avoidance and that activated T cells must progress through the cell cycle in order to escape anergy. Induction of this "division-arrest" form of anergy requires CTLA-4 signaling during the primary response. Also, cell division per se is not sufficient for anergy avoidance: the few T cells that undergo multiple rounds of cell division during overt CD28 costimulatory blockade do not escape the ultimate induction of clonal anergy. Anergy avoidance by primary T cells is thus a multistep process: in order to participate in a productive immune response, an individual T cell activated through its antigen receptor must receive CD28 costimulation and progress through the cell cycle. Anergy may be induced either through a combination of CTLA-4 signaling and the failure of cell cycle progression, or through a proliferation-independent mechanism in which TCR ligation occurs in the absence of CD28.

Cutting edge: the related molecules CD28 and inducible costimulator deliver both unique and complementary signals required for optimal T cell activation

Optimal T cell activation requires engagement of CD28 with its counterligands B7-1 and B7-2. Inducible costimulator (ICOS) is the third member of the CD28/CTLA4 family that binds a B7-like protein, B7RP-1. Administration of ICOS-Ig attenuates T cell expansion following superantigen (SAg) administration, but fails to regulate either peripheral deletion or anergy induction. ICOS-Ig, but not CTLA4-Ig, uniquely regulates SAg-induced TNF-alpha production, whereas IL-2 secretion is modulated by CTLA4-Ig, but not ICOS-Ig. In contrast, both ICOS and CD28 are required for complete attenuation of IL-4 production. Our data suggest that ICOS and CD28 regulate T cell expansion and that ligation of either CD28 or ICOS can either uniquely regulate cytokine production (IL-2/TNF-alpha) or synergize for optimal cytokine production (IL-4) after SAg administration.

Inhibition of thymidine synthesis by folate analogues induces a Fas-Fas ligand-independent deletion of superantigen-reactive peripheral T cells

Methotrexate (MTX), a folate antagonist with multiple enzymatic targets, is used in the treatment of malignancies as well as in autoimmune and chronic inflammatory diseases, and ZD1694 (tomudex), a water-soluble quinazoline specific inhibitor of thymidylate synthase (TS), is used in the treatment of adenocarcinomas. In this study, we investigated the effects of these folate analogues on superantigen (SAg)-reactive peripheral T cells in vivo. In BALB/c mice, staphylococcal enterotoxin B (SEB)-induced cytokine secretion, IL-2R (CD25) expression and early deletion of a fraction of SEB-reactive V(beta)8(+) T cells were not impaired by either MTX (7 mg/kg/day) or tomudex (5 mg/kg/day). However, both MTX and tomudex prevented V(beta)8-selective T cell expansion and accelerated their peripheral elimination. Administration of thymidine (500 mg/kg/12 h) completely abrogated this effect, indicating that inhibition of TS but not that of other folate-dependent enzymes was the main mechanism involved. Furthermore, a marked increase of apoptotic cells restricted to the V(beta)8(+) T cell subset indicated that proliferation inhibition was associated with apoptosis. In contrast with peripheral V(beta)8(+) T cell deletion, MTX and tomudex did not prevent the increase of V(beta)8(+) thymocytes triggered by SEB. Experiments in C57BL/6-lpr/lpr mice further demonstrated that deletion of V(beta)8(+) T cells induced by folate analogues was independent of Fas-Fas ligand interaction. Our results provide evidence that folate analogues may selectively delete dividing peripheral T cells through TS inhibition, but do not interfere with other events triggered by SAg.

Staphylococcal enterotoxin B induces potent cytotoxic activity by intraepithelial lymphocytes

In food poisoning, Staphylococcus aureus secretes staphylococcal enterotoxin B (SEB), a superantigen that causes intense T-cell proliferation and cytotoxicity. The effects of SEB on lytic activity by human intestinal intraepithelial lymphocytes (IEL) were investigated. Jejunal IEL, from morbidly obese individuals undergoing gastric bypass operations, were tested for SEB-induced cytotoxicity against C1R B-lymphoblastoid cells, HT-29 adenocarcinoma cells, or CD1d-transfected cells using the 51Cr-release assay. Fas and Fas ligand expression were detected by immunofluorescence and flow cytometry and soluble ligand by enzyme-linked immunosorbent assay (ELISA). In the presence of SEB, IEL became potently cytotoxic against C1R cells and interferon-gamma (IFN-gamma)-precultured HT-29 cells, causing 55+/-10% and 31+/-6% lysis, respectively, greater than that by phytohaemagglutinin (PHA)-, interleukin-2 (IL-2)-, or anti-T-cell receptor (TCR)-activated IEL. SEB-stimulated peripheral blood (PB) CD8+ T cells lysed similar numbers of C1R cells but fewer HT-29 cells (53+/-13% and 8+/-5%, respectively). IEL killing of C1R cells involved interaction of major histocompatibility complex (MHC) class II with TCR, CD2 with CD58, and CD11a with CD54, and was perforin mediated. SEB-induced IEL lysis of HT-29 cells, in contrast, was caused by an unknown target cell structure, not MHC class II or CD1d, and resulted from a combination of perforin and Fas-mediated events. The potent cytotoxic activities of IEL promoted by SEB utilize two different mechanisms, depending on the surface receptors expressed by the target cells.

Prostaglandin E2 is variably induced by bacterial superantigens in bovine mononuclear cells and has a regulatory role for the T cell proliferative response

Signal transduction in antigen presenting cells via MHC class II molecules induces production of prostaglandin E2 (PGE2) known to possess immunoregulatory potential. Since Staphylococcus aureus superantigens (SAgs) utilize MHC class II molecules as primary ligands, we wanted to know whether PGE2 is induced after in vitro SAg stimulation of bovine blood mononuclear cells (boMNC), and whether this arachidonic acid metabolite modulates the preferential SAg-induced proliferative response of bovine CD8+ T cells. SEB as well as SEA induced maximal amounts of PGE2 on day 2 of culture (1-2.5 x 10(-8) mol/l per 2 x 10(5) boMNC). PGE2 production could be inhibited completely by indomethacin (10(-5) mol/l) causing enhanced proliferation of boCD4+ T cells (174%) as well as of boCD8+ T cells (122%) between day 4 and 6 of the in vitro culture, however, only in a subset of the tested animals. Notably, the striking preference of proliferation of boCD8+ over boCD4+ T cells following SAg stimulation remained largely unchanged after inhibition of endogenous PGE2 synthesis or after addition of exogenous PGE2. Higher concentrations of exogenously added PGE2 (> or = 10(-8) mol/l) inhibited the proliferation reaction, mainly due to an increased death rate of both CD4+ and CD8+ blasts. In contrast, lower PGE2 concentrations between 10(-8)-10(-9) mol/l even slightly enhanced the proliferation of both T cell subsets, depending on the individual cell donor. Summing up: These data show that SAgs, indeed, can induce PGE2 production in boMNC which can enhance or reduce the proliferative response of bovine CD4+ and CD8+ T cells.

Herpes simplex virus type 1 infection of activated cytotoxic T cells: Induction of fratricide as a mechanism of viral immune evasion

Herpes simplex virus type 1 (HSV1), a large DNA-containing virus, is endemic in all human populations investigated. After infection of mucocutaneous surfaces, HSV1 establishes a latent infection in nerve cells. Recently, it was demonstrated that HSV1 can also infect activated T lymphocytes. However, the consequences of T cell infection for viral pathogenesis and immunity are unknown. We have observed that in contrast to the situation in human fibroblasts, in human T cell lines antigen presentation by major histocompatibility complex class I molecules is not blocked after HSV1 infection. Moreover, HSV1 infection of T cells results in rapid elimination of antiviral T cells by fratricide. To dissect the underlying molecular events, we used a transgenic mouse model of HSV1 infection to demonstrate that CD95 (Apo-1, Fas)-triggered apoptosis is essential for HSV1-induced fratricide, whereas tumor necrosis factor (TNF) also contributes to this phenomenon but to a lesser extent. By contrast, neither TRAIL (TNF-related apoptosis-inducing ligand) nor perforin were involved. Finally, we defined two mechanisms associated with HSV1-associated fratricide of antiviral T cells: (a) T cell receptor-mediated upregulation of CD95 ligand and (b) a viral "competence-to-die" signal that renders activated T lymphocytes susceptible to CD95 signaling. We propose that induction of fratricide is an important immune evasion mechanism of HSV1, helping the virus to persist in the host organism throughout its lifetime.

Distribution of Cycling T Lymphocytes in Blood and Lymphoid Organs During Immune Responses

Proliferation of murine T lymphocytes in blood, lymph nodes, and spleen was studied in four in vivo stimulation systems, using BrdU pulse-labeling of DNA-synthesizing cells. The T cell response to the superantigen Staphylococcus enterotoxin B (SEB) was studied in detail. Vβ8+ T cells showed a peak of DNA synthesis 16–24 h after SEB injection, and the percentage of BrdU+ CD4 and CD8 T cells was higher in blood than in lymph nodes and spleen. DNA synthesis was preceded by massive migration of Vβ8+ cells from blood to lymphoid organs, in which the early activation marker CD69 was first up-regulated. SEB-nonspecific Vβ6+ cells showed minimal stimulation but, when cycling, also expressed a high level of CD69. The other systems studied were injection of the IFN-γ inducer polyinosinic:polycytidylic acid, infection by the BM5 variants of murine leukemia virus (the causative agent of murine AIDS), and T cell expansion after transfer of normal bone marrow and lymph node cells into recombinase-activating gene-2-deficient mice. In each case, a peak of T cell proliferation was observed in blood. These data demonstrate the extensive redistribution of cycling T cells in the first few hours after activation. Kinetic studies of blood lymphocyte status appear crucial for understanding primary immune responses because cycling and redistributing T lymphocytes are enriched in the circulating compartment.

Requirement of I-E molecule for thymocyte apoptosis induced by staphylococcal enterotoxin B in vivo

In vivo administration of bacterial superantigen staphylococcal enterotoxin B (SEB) to BALB/c mice led to thymus atrophy resulting from thymocyte apoptosis. In this study, we demonstrated that SEB induced a substantial reduction in thymocyte numbers in BALB/c, B10. D2 (H-2(d) haplotype), B10.BR, C3H/HeJ, C3H/HeN (H-2(k)), and (BALB/c x B6)F1 (H-2(dxb)), but caused little or no effect in I-E- strains such as B6, B10, A.BY (H-2(b)), and A.SW (H-2(s)) mice. Elimination of CD4(+)CD8(+) cells predominantly accounted for the thymocyte loss, although the numbers of other subpopulations may also be reduced. Thymocyte apoptosis was shown by an increase in the level of DNA fragmentation in BALB/c but not in B6 mice after SEB administration. Treatment with anti-I-Ed monoclonal antibody to BALB/c mice blocked SEB-induced thymocyte apoptosis when anti-I-Ad exerted less effect. In contrast to SEB, staphylococcal enterotoxin A led to comparable levels of thymus atrophy in BALB/c and B6 mice. Studies on the surface marker expression indicated that CD25 expression was upregulated on BALB/c mouse thymocytes but with only a moderate increase in B6 mice. The CD4(+)CD8(+) cells were the major (>90%) population that expressed elevated levels of CD25 in BALB/c mice. An increase in the expression of TCRalphabeta, CD3, and CD69 surface markers was also observed on thymocytes from BALB/c mice, but not from I-E- strains. The differential response of I-E+ and I-E- mice to SEB may be exploited as a model for the study of apoptosis in the thymus.

SEA-scFv as a bifunctional antibody: construction of a bacterial expression system and its functional analysis

A SEA-antibody single chain Fv (SEA-scFv) fusion protein was produced by bacterial expression system in this study. SEA-scFv has both staphylococcal enterotoxin A (SEA) effects and antibody activity directed at the epithelial mucin core protein MUC1, a cancer associated antigen. It was expressed mostly in the cytoplasm as an insoluble form. The gene product was solubilized by guanidine hydrochloride, refolded by conventional dilution method, and purified using metal-chelating chromatography. The resulting SEA-scFv fusion protein preparation was found to react with MUC1 and MHC class II antigens and had the ability to enhance cytotoxicity of lymphokine activated killer cells with a T cell phenotype against a human bile duct carcinoma cell line, TFK-1, expressing MUC1. This genetically engineered SEA-scFv fusion protein promises to be an important reagent for cancer immunotherapy.

Airway exposure to bacterial superantigen (SEB) induces lymphocyte-dependent airway inflammation associated with increased airway responsiveness--a model for non-allergic asthma

Although immunological consequences of systemic superantigen administration have been extensively studied, the effects of local mucosal exposure to superantigens are not well defined. The purpose of this study was to delineate the type of immune response triggered by superantigen exposure to the airway mucosa in mice. In dose-response experiments we determined a low dose of staphylococcal enterotoxin B (SEB) that triggered an inflammatory response characterized by mucosal and airway recruitment of lymphocytes, eosinophils and neutrophils together with elevated levels of IL-4, but not IFN-gamma, in bronchoalveolar lavage (BAL) fluids. TCR Vbeta analysis revealed that superantigen-responsive and -non-responsive T cells were equally recruited into the airways. SEB markedly enhanced the frequency of TNF-alpha-positive BAL macrophages as well as the amount of TNF-alpha in BAL fluids. These responses were associated with the development of increased airway responsiveness (AR) in SEB-treated mice. This effect occurred in an antibody-independent fashion. Furthermore, this type of response was observed in IgE-high responder BALB/c as well as in IgE-low/intermediate responder C57BL/6 mice. The development of increased AR was CD4+ T cell dependent as shown by transfer experiments into BALB/c nu/nu mice. These results suggest that the local immune response following mucosal superantigen administration triggers a unique inflammatory response in the airways resembling many features of "intrinsic asthma".

In vivo elimination of viral superantigen-activated CD4+ T cells: apoptosis occurs at a distance from the activation site

Local injection of mouse mammary tumor virus (MMTV) induces a local immune response, with activation of viral superantigen (vSAG)-specific T cell subsets followed by their clonal deletion. We investigated the fate of vSAG-reactive T cells following footpad injection of MMTV(SW) to mice. Activated T cells accumulated in draining lymph nodes. However, we demonstrated that apoptosis did not occur at the activation site, on the contrary of what has been shown after bacterial SAG activation. Although activated T cells were already shown to have the capacity to migrate to the gut, the fate of gut homing cells remains unclear. We demonstrate that the number of vSAG-specific T cells activated in the periphery was increasing in the follicles of gut-associated lymphoid organs, together with the number of apoptotic cell clusters. These results strongly suggested that gut-associated lymphoid tissue was the specific graveyard for apoptotic vSAG-activated CD4 T cells.

Antitumor Response Elicited by a Superantigen- Transmembrane Sequence Fusion Protein Anchored onto Tumor Cells

Superantigens stimulate T cells bearing certain TCR β-chain variable regions when bound to MHC II molecules. We investigated whether the superantigen toxic shock syndrome toxin-1 (TSST1) could induce an antitumor immune response when anchored onto MHC II-negative tumor cells. Our approach was to facilitate association of TSST1 with cell membranes by fusing its coding region to the transmembrane region (TM) sequence of the proto-oncogene c-erb-B-2. TSST1-TM was expressed in bacteria with an N-terminal histidine tag and purified using nickel-agarose affinity chromatography. Purified TSST1-TM added to cultures of several different MHC II-negative tumor cells spontaneously associated with cell membranes, as detected by flow cytometry. Because superantigens can direct cell-mediated cytotoxicity against MHC II-positive cells, a TM fusion protein lacking the TSST1 MHC II binding domain (TSST88–194-TM) was also constructed. Tumor cells precoated with TSST1-TM or TSST88–194-TM stimulated proliferation of human peripheral blood lymphocytes in vitro whereas uncoated tumor cells did not. Mice preimmunized with TSST1-TM- or TSST88–194-TM-coated tumor cells mounted a systemic response that resulted in significant antitumor immunity as measured by regression of a parental tumor challenge. TSST1-TM and TSST88–194-TM fusion proteins represent a useful new strategy for attaching superantigens or potentially other proteins onto tumor cell surfaces without genetic manipulation.

Oral Administration of the Bacterial Superantigen Staphylococcal Enterotoxin B Induces Activation and Cytokine Production by T Cells in Murine Gut-Associated Lymphoid Tissue

The toxicity of the staphylococcal enterotoxins (SEs) has been linked to the activation of large numbers of T cells in the peripheral lymphoid tissues. Because the primary manifestations of foodborne enterotoxic poisoning are associated with the gastrointestinal tract, we have compared the responses of T cells in the gut-associated lymphoid tissue and in the periphery to intragastric (i.g.) and i.p. administration of SEB. Intraperitoneal SEB results in an early expansion of peripheral Vβ8+ T cells and Th1 cytokine secretion followed by deletion at 7–10 days. We found that i.g. SEB rapidly (within 4 h) leads to the expansion and activation of Vβ8+ T cells in the Peyer’s patch and mesenteric lymph nodes. Analysis of cytokine mRNA in purified Vβ8+ T cells by competitive RT-PCR showed that, 4 h after i.g. SEB, the induction of mRNA for IL-2 and IFN-γ is about 10-fold greater in mucosal than in peripheral lymphoid tissue. Our results show that activated mucosal T cells expand and up-regulate cytokine mRNA in response to luminal exposure to SEB, suggesting a role for the gut-associated lymphoid tissue in the gastrointestinal manifestations of enterotoxic poisoning.

Selective Induction of CD8+ Cytotoxic T Lymphocyte Effector Function by Staphylococcus Enterotoxin B

Upon encounter with its antigenic stimulus, CTL characteristically proliferate, produce cytokines, and lyse the Ag-presenting cell in an attempt to impede further infection. Superantigens are extremely efficient immunostimulatory proteins that promote high levels of proliferation and massive cytokine production in reactive T cells. We compared the activation of murine influenza-specific CD8+ CTL clones stimulated with either influenza peptide or the superantigen staphylococcus enterotoxin B (SEB). We found that influenza peptide/MHC and SEB appeared equally capable of eliciting proliferation and IFN-γ production. However, while influenza peptide/MHC elicited both perforin- and Fas ligand (FasL)/Fas (CD95L/CD95)-mediated cytolytic mechanisms, SEB was unable to trigger perforin-mediated cytolysis or serine esterase release. Examination of intracellular Ca2+ mobilization events revealed that the ability to trigger intracellular Ca2+ flux was not comparable between influenza peptide and SEB. SEB stimulated only a small rise in levels of intracellular Ca2+, at times indistinguishable from background. These findings indicate that the short-term cytolytic potential of superantigen-activated CD8+ CTL clones appears to be restricted to FasL/Fas (CD95L/CD95) mediated cytolysis.

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.

T cell- and perforin-dependent depletion of B cells in vivo by staphylococcal enterotoxin A

Bacterial superantigens bind to major histocompatibility complex (MHC) class II and subsequently activate both CD4+ and CD8+ T lymphocytes expressing certain T-cell receptor (TCR)-Vbeta chains. In response to superantigen exposure these subsets proliferate, produce large amounts of proinflammatory cytokines and in addition CD8+ cytotoxic T lymphocytes (CTL) are induced. Previous studies in vitro have shown that these CTL effectively lyse MHC class II-expressing cells presenting the proper superantigen. However, it is unknown whether superantigens induce a similar response towards MHC class II+ antigen-presenting cells in vivo. In this study we demonstrate that administration of repeated injections of the superantigen staphylococcal enterotoxin A (SEA) to TCR-Vbeta3 transgenic mice results in a loss of MHC class II-expressing cells in the spleen. Analysis of different MHC class II+ subsets revealed a selective depletion of CD19+ B cells, while F4/80+ macrophages increased in number. Depletion of T cells with anti-CD4 or anti-CD8 monoclonal antibody indicated that CD8+ T cells were crucial for SEA-induced cytotoxicity in vivo. Repeated injections of SEA to perforin-deficient mice resulted in significantly less B-cell depletion compared with control mice. This suggests that superantigen-activated CD8+ T cells lyse MHC class II+ antigen-presenting cells in a perforin-dependent manner in vivo. It is suggested that this represents a novel bacterial immune escape mechanism, which may particularly impair local humoral immune responses.

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.

In Vivo Effects of a Bacterial Superantigen on Macaque TCR Repertoires

A macaque model was employed to explore staphylococcal enterotoxin B (SEB) superantigen-driven T lymphocyte responses. The SEB-reactive Vβ+ cell subpopulations demonstrated a striking tri-phase response in rhesus monkeys following an SEB challenge in vivo. The hyperacute down-regulation, seen as early as 2 h through 2 days after SEB injection, was characterized by a disappearance of the reactive Vβ-restricted PBL subpopulations from the circulation and decreased expression of these cell subpopulations in lymphoid tissues. Following this, a dominant expansion of reactive Vβ-expressing CD4+ cell subpopulations occurred in lymph nodes and spleens, whereas in the peripheral blood a preferential expansion of reactive Vβ-expressing CD8+ cell subpopulations was seen. An exhaustion of this response was then seen, with a prolonged decrease in the number of the reactive Vβ+ CD4+ lymphocyte subpopulations. Interestingly, monoclonal or oligoclonal dominance was seen in the reactive Vβ+ cell subpopulations in the period of the transition from the polyclonal cellular expansion to the exhaustion of the response, suggesting that some Vβ+ cell clones may be more resistant than others to superantigen-mediated depletion. These results indicate that in vivo SEB superantigen-mediated effect on lymphocyte subpopulations in macaques is complex, suggesting that profound dynamics in the TCR repertoires may in part account for the susceptibility of higher primates to SEB-induced diseases.

Changes in murine jejunal morphology evoked by the bacterial superantigen Staphylococcus aureus enterotoxin B are mediated by CD4+ T cells

Bacterial superantigens (SAgs) are potent T-cell stimuli that have been implicated in the pathophysiology of autoimmune and inflammatory disease. We used Staphylococcus aureus enterotoxin B (SEB) as a model SAg to assess the effects of SAg exposure on gut form and cellularity. BALB/c, SCID (lacking T cells) and T-cell-reconstituted SCID mice were treated with SEB (5 or 100 microg intraperitoneally), and segments of the mid-jejunum were removed 4, 12, or 48 h later and processed for histochemical or immunocytochemical analysis of gut morphology and major histocompatibility complex class II (MHC II) expression and the enumeration of CD3+ T cells and goblet cells. Control mice received saline only. SEB treatment of BALB/c mice caused a time- and dose-dependent enteropathy that was characterized by reduced villus height, increased crypt depth, and a significant increase in MHC II expression. An increase in the number of CD3+ T cells was observed 48 h after exposure to 100 microg of SEB. Enteric structural alterations were not apparent in SEB-treated SCID mice compared to saline-treated SCID mice. In contrast, SEB challenge of SCID mice reconstituted with a mixed lymphocyte population or purified murine CD4+ T cells resulted in enteric histopathological changes reminiscent of those observed in SEB-treated BALB/c mice. These findings implicate CD4+ T cells in this SEB-induced enteropathy. Our results show that SAg immune activation causes significant changes in jejunal villus-crypt architecture and cellularity that are likely to impact on normal physiological processes. We speculate that the elevated MHC II expression and increased number of T cells could allow for enhanced immune responsiveness to other SAgs or environmental antigens.

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.

T helper 1 (Th1) and Th2 characteristics start to develop during T cell priming and are associated with an immediate ability to induce immunoglobulin class switching

The respective production of specific immunoglobulin (Ig)G2a or IgG1 within 5 d of primary immunization with Swiss type mouse mammary tumor virus [MMTV(SW)] or haptenated protein provides a model for the development of T helper 1 (Th1) and Th2 responses. The antibody-producing cells arise from cognate T cell B cell interaction, revealed by the respective induction of Cgamma2a and Cgamma1 switch transcript production, on the third day after immunization. T cell proliferation and upregulation of mRNA for interferon gamma in response to MMTV(SW) and interleukin 4 in response to haptenated protein also starts during this day. It follows that there is minimal delay in these responses between T cell priming and the onset of cognate interaction between T and B cells leading to class switching and exponential growth. The Th1 or Th2 profile is at least partially established at the time of the first cognate T cell interaction with B cells in the T zone. The addition of killed Bordetella pertussis to the hapten-protein induces nonhapten-specific IgG2a and IgG1 plasma cells, whereas the anti-hapten response continues to be IgG1 dominated. This indicates that a Th2 response to hapten-protein can proceed in a node where there is substantial Th1 activity.

CTLA-4 Regulates Tolerance Induction and T Cell Differentiation In Vivo

Cytotoxic T lymphocyte Ag-4 (CTLA-4; CD152) is an important T cell regulatory molecule. In vitro experiments have shown that the blockade of signals through CTLA-4 augments T cell expansion, while CTLA-4 cross-linking results in decreased T cell proliferation due to decreased IL-2 production. However, less is known about the role of CTLA-4 in regulating an ongoing immune response. In this study, we examined the role of CTLA-4 in the expansion, decline, tolerization, and differentiation of T cells following treatment with staphylococcal enterotoxin B (SEB). Anti-CTLA-4 treatment resulted in increased numbers of SEB-reactive T cells and blockade of subsequent tolerance induction. Further examination of the SEB-reactive cells from anti-CTLA-4-treated mice demonstrated that both the CD4+ and CD8+ Vβ8+ T cells produced IL-4, providing evidence that not only do signals through CTLA-4 regulate T cell-tolerizing events, but they also play an important role in the differentiation of T cells in vivo.

Repeated treatment with antibody-targeted superantigens strongly inhibits tumor growth

Superantigens (SAg) are microbial proteins with the capacity to activate a large proportion of T cells. We have developed a novel approach for cancer immunotherapy by genetically fusing the SAg staphylococcal enterotoxin A (SEA) to a Fab-fragment of a tumor-specific antibody. Repeated exposure to SEA induces a state of unresponsiveness including cell deletion and functional hyporesponsiveness, i.e., anergy. In this study we have developed improved therapeutic schedules to allow repeated injections of Fab-SEA, limit development of immunological unresponsiveness and promote maximal anti-tumor response. Four daily injections of Fab-SEA to mice carrying B 16-C215 lung metastases resulted in 90-95% reduction in the number of metastases. However, the animals did retain a minimal residual tumor disease. The immune system was in a hyporesponsive state after 4 daily Fab-SEA injections, and further injections did not improve therapy. Two repeated cycles, each comprising 4 daily injections of Fab-SEA, significantly prolonged the survival and resulted in complete cure of a fraction of the animals. A rest period of 10 days between the cycles was required to mount an efficient secondary anti-tumor response. This secondary immune response was characterized by partial recovery of cytokine production i.e., interleukin-2, interferon-gamma and tumor necrosis factor-alpha. Strong CTL activity was detected in animals that had rested for 8 weeks between the 2 cycles. Interestingly, irrespective of the resting period, the CD4+ SEA-reactive T cells expanded in response to all 4 additional Fab-SEA injections both locally and in spleen. In contrast, only marginal expansion of CD8+ T cells was seen if restimulation was given within 1 month. Our data show that potent anti-tumor effector functions can be induced after repeated stimulation cycles with a SAg-monoclonal antibody fusion protein resulting in a CD4+ T cell-dependent cytokine release, prolonged survival and induction of complete cures.

Activation of bovine lymphocyte subpopulations by staphylococcal enterotoxin C

Staphylococcus aureus is a major mastitis-causing pathogen in cattle. The chronic nature of bovine staphylococcal mastitis suggests that some products or components of S. aureus may interfere with the development of protective immunity. One class of molecules that could be involved are superantigens (SAgs). Although a significant number of mastitis isolates produce SAgs, the effect of these molecules on the bovine immune system is unresolved. To determine if immunosuppression caused by SAgs could play a role in pathogenesis, we monitored bovine lymphocytes exposed to staphylococcal enterotoxin C1 (SEC1). Activation of bovine lymphocytes by either SEC1 or concanavalin A (ConA) was influenced by the gammadelta/alphabeta T-cell ratio in the culture. Compared to ConA-induced stimulation, cultures stimulated with SEC1 generated small numbers of CD4+ alphabeta T cells expressing high levels of interleukin-2 receptor alpha chain (IL-2R alpha) and major histocompatibility complex class II (MHCII), suggesting that SAg exposure does not lead to full activation of these cells. This state of partial activation was most pronounced in cultures with a high gammadelta/alphabeta ratio. In contrast, significant numbers of CD8+ alphabeta T cells expressed high levels of IL-2R alpha and MHCII, regardless of the gammadelta/alphabeta ratio and the stimulant used. CD8+ blasts in cultures stimulated with SEC1 also expressed another activation marker, ACT3, previously detected predominantly on thymocytes and CD4+ T cells. Although gammadelta CD2- and CD2+ T cells expressed MHCII and IL-2R alpha following stimulation with SEC1, only a few cells increased to blast size, suggesting that they were only partially activated. The results suggest ways in which SAgs might facilitate immunosuppression that promotes the persistence of bacteria in cattle and contributes to chronic intramammary infection.

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.