Superantigens: a tool to gain new insight into cellular immunity

Staphylococcal Superantigens: Pyrogenic Toxins Induce Toxic Shock

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

Characterization of the interaction of staphylococcal enterotoxin B with CD1d expressed in human renal proximal tubule epithelial cells

Background

Participation of renal cells in the pathogenesis of staphylococcal enterotoxin B (SEB) is critical for late cleansing and sequestration of the antigens facilitated by CD1d mediated antigen sensing and recognition. This is a noted deviation from the typical antigen recognition process that recruits the major histocompatibility complex class II (MHC II) molecules. The immunological importance of CD1d is underscored by its influences on the performances of natural killer T-cells and thereby mediates the innate and adaptive immune systems.

Results

Using diffraction-based dotReady™ immunoassays, the present study showed that SEB directly and specifically conjugated to CD1d. The specificity of the conjugation between SEB and CD1d expressed on human renal proximal tubule epithelial cells (RPTEC) was further established by selective inhibition of CD1d prior to its exposure to SEB. We found that SEB induced the expression of CD1d on the cell surface prompting a rapid conjugation between them. The mRNA transcripts encoding CD1d remained elevated potentially after completing the antigen cleansing process.

Conclusion

Molecular targets associated with the delayed pathogenic response have essential therapeutic values. Particularly in the event of bioterrorism, the caregivers are typically able to intervene much later than the toxic exposures. Given circumstances mandate a paradigm shift from the conventional therapeutic strategy that counts on targeting the host markers responding to the early assault of pathogens. We demonstrated the role of CD1d in the late stage of pathogen recognition and cleansing, and thereby underscored its clinical potential in treating bioweaponizable antigens, such as Staphylococcal enterotoxin B (SEB).

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-015-0344-5) contains supplementary material, which is available to authorized users.

CD1d-independent activation of invariant natural killer T cells by staphylococcal enterotoxin B through major histocompatibility complex class II/T cell receptor interaction results in acute lung injury

There are two important mechanisms of activation of invariant natural killer T cells (iNKT cells) by microbes: direct activation of the invariant T-cell receptor (TCR) by microbial glycolipids presented by CD1d and indirect activation, mediated by the responses of antigen-presenting cells to microbes. In this study, we provide evidence for a novel CD1d-independent direct activation of iNKT cells involving a microbial protein superantigen presented in the context of major histocompatibility complex class II (MHC-II), which plays a critical role in pathogenesis, thereby redefining the role of iNKT cells. Intranasal exposure to staphylococcal enterotoxin B (SEB) in C57BL/6 wild-type mice caused acute lung injury (ALI) characterized by vascular leak, cytokine storm, and infiltration of mononuclear cells in the lungs. In contrast, the vascular leak and inflammation were decreased by ~50% in NKT cell-deficient Jα18(-/-) and CD1d(-/-) mice following SEB exposure, which was reversed following adoptive transfer of iNKT cells into CD1d(-/-) mice. In vitro, SEB could directly stimulate iNKT cells in a CD1d-independent manner via MHC-II/TCR interaction, specifically involving Vβ8. These studies not only demonstrate that iNKT cells can be activated directly by a bacterial protein superantigen independent of CD1d but also indicate that in addition to the conventional T cells, iNKT cells play a critical role in SEB-mediated ALI.

Staphylococcus aureus-superantigen decreases FKBP51 mRNA expression and cell-response to suppressive efficacy of a glucocorticoid in human peripheral blood mononuclear cells: possible implication of mitogen-activated protein kinase pathways

Glucocorticoids are commonly used for treatment of chronic inflammatory diseases, while a number of patients show insensitivity to glucocorticoid treatment. The molecular basis of these individual differences in glucocorticoid pharmacodynamics has little been taken into account. Here we focus on the implication of Staphylococcus aureus-producing superantigen, such as toxic shock syndrome toxin-1 (TSST-1), in the glucocorticoid sensitivity of human peripheral blood mononuclear cells and cell-response to glucocorticoid to produce a transcript for FK506-binding protein (FKBP51). Peripheral blood mononuclear cell-sensitivity to glucocorticoid was assessed by a cell proliferation test. FKBP51mRNA expressions were determined by real-time reverse transcription-polymerase chain reaction (RT-PCR). We also compared concentrations of various cytokines produced in culture supernatant between concanavalin A- and TSST-1-stimulated peripheral blood mononuclear cells using a cytometric beads array. Mitogen-activated protein kinase (MAPK) phosphorylation activity in peripheral blood mononuclear cells stimulated with concanavalin A and TSST-1 was analyzed by a cell-based ELISA. Prednisolone markedly inhibited concanavalin A-induced peripheral blood mononuclear cell proliferation, but they scarcely inhibited TSST-1-induced peripheral blood mononuclear cell proliferation. The mean (S.D.) of immunosuppressant concentrations that would give 50% (IC(50)) values for prednisolone in concanavalin A-stimulated peripheral blood mononuclear cells was 52.6 (54.2) ng/ml, which was significantly lower than that in TSST-1-stimulated peripheral blood mononuclear cells, i.e., 574.2 (817.0) ng/ml (P<0.001). TSST-1-stimulated peripheral blood mononuclear cells for 48 h attenuated prednisolone-induced FKBP51mRNA expressions concomitantly with decreased sensitivity to the anti-proliferative effects of prednisolone. Concentrations of interleukin-2 (IL-2) produced from TSST-1-stimulated peripheral blood mononuclear cells were significantly higher than that from peripheral blood mononuclear cells stimulated with concanavalin A (P<0.0001). Both concanavalin A and TSST-1 enhanced the phosphorylation of extracellular signal-regulated kinase (ERK) and p38, whereas the level of c-jun terminal kinase (JNK) phosphorylation was only increased by TSST-1-stimulation in peripheral blood mononuclear cells. Furthermore, the decreased FKBP51mRNA by TSST-1was found to be recovered by JNK and mitogen-activated protein kinase (MEK)/ERK inhibitors. Our data suggest that TSST-1 reduces activity of glucocorticoid in peripheral blood mononuclear cells by JNK activation and subsequent production of IL-2. Therefore, JNK might be an attractive target for overcoming glucocorticoid insensitivity induced by TSST-1 in peripheral blood mononuclear cells.

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.

Staphylococcal and streptococcal superantigens: molecular, biological and clinical aspects

Superantigens (SAgs) include a class of certain bacterial and viral proteins exhibiting highly potent lymphocyte-transforming (mitogenic) activity towards human and or other mammalian T lymphocytes. Unlike conventional antigens, SAgs bind to certain regions of major histocompatibility complex (MHC) class II molecules of antigen-presenting cells (APCs) outside the classical antigen-binding groove and concomitantly bind in their native form to T cells at specific motifs of the variable region of the beta chain (Vbeta) of the T cell receptor (TcR). This interaction triggers the activation (proliferation) of the targeted T lymphocytes and leads to the in vivo or in vitro release of high amounts of various cytokines and other effectors by immune cells. Each SAg interacts specifically with a characteristic set of Vbeta motifs. The review summarizes our current knowledge on S. aureus and S. pyogenes superantigen proteins. The repertoire of the staphylococcal and streptococcal SAgs comprises 24 and 8 proteins, respectively. The staphylococcal SAgs include (i) the classical enterotoxins A, B, C (and antigenic variants), D, E, and the recently discovered enterotoxins G to Q, (ii) toxic shock syndrome toxin-1, (iii) exfoliatins A and B. The streptococcal SAgs include the classical pyrogenic exotoxins A and C and the newly identified pyrogenic toxins, G, H, I, J, SMEZ, and SSA. The structural and genomic aspects of these toxins and their molecular relatedness are described as well as the available 3-D crystal structure of some of them and that of certain of their complexes with MHC class II molecules and the TcR, respectively. The pathophysiological properties and clinical disorders related to these SAgs are reviewed.

Superantigen bacterial toxins: state of the art

Superantigens (SAgs) are viral and bacterial proteins exhibiting a highly potent polyclonal lymphocyte-proliferating activity for CD4(+), CD8(+) and sometimes gammadelta(+) T cells of human and (or) various animal species. Unlike conventional antigens, SAgs bind as unprocessed proteins to invariant regions of major histocompatibility complex (MHC) class II molecules on the surface of antigen-presenting cells (APCs) and to particular motifs of the variable region of the beta chain (Vbeta) of T-cell receptor (TcR) outside the antigen-binding groove. As a consequence, SAgs stimulate at nano-to picogram concentrations up to 10 to 30% of host T-cell repertoire while only one in 10(5)-10(6) T cells (0.01-0.0001%) are activated upon conventional antigenic peptide binding to TcR. SAg activation of an unusually high percentage of T lymphocytes initiates massive release of pro-inflammatory and other cytokines which play a pivotal role in the pathogenesis of the diseases provoked by SAg-producing microorganisms. We briefly describe in this review the molecular and biological properties of the bacterial superantigen toxins and mitogens identified in the past decade.

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.

Active ion secretion and permeability of rabbit maxillary sinus epithelium, impact of staphylococcal enterotoxin B

The aim of this study was to investigate the impact of staphylococcal enterotoxin B (SEB), a model superantigen, on the physiologic functions of rabbit maxillary sinus epithelium. Rabbit sinus mucosae were separated under a surgical microscope and mounted in Ussing chambers to record short-circuit current, conductance, and permeability to horseradish peroxidase. The results showed that SEB evoked increases in sinus epithelial cell baseline short-circuit current, conductance, and permeability to horseradish peroxidase. When tumor necrosis factor-alpha (TNF-alpha) was added to the Ussing chambers, we got results similar to those obtained by SEB stimulation in vitro; the effects of SEB on sinus epithelial cells could be blocked by pretreatment with anti-TNF-alpha antibody. These results demonstrate that SEB is able to alter the function of sinus epithelial cells and to affect the capability of the epithelial defensive barrier, which may be mediated by TNF-alpha.

In vivo staphylococcal enterotoxin B (SEB)-primed murine splenocytes secrete mediators which suppress CD25(hi) expression and cell cycle progression of naive splenocytes in response to SEB in vitro

Administration of bacterial superantigen results in clonal activation of T cells followed by a state of hyporesponsiveness to subsequent antigen stimulation. Using a coculture system, we showed that the splenocytes from staphylococcal enterotoxin B (SEB)-injected BALB/c mice suppressed the proliferative response of naive splenocytes to SEB stimulation. The suppressive effect also occurred in Fas-deficient MRL-lpr/lpr mice. When naive responder cells were separated by a semipermeable membrane from SEB-primed effector cells, the suppressive effect remained apparent. The hyporesponsiveness of responder cells did not result from excessive induction of apoptosis, but rather from prevention of entering the S and G2/M phases of the cell cycle. The IL-2 levels in culture supernatants were low with the presence of SEB-primed effector cells. However, addition of IL-2 to the cocultures only partially reversed the inhibitory effect. Further studies revealed a reduced level of the CD25(hi) subpopulation in responder cells when cultured in the transwell with the presence of SEB-primed effector cells compared to that with saline-primed controls. This inhibitory effect was not observed for SEB-induced activation of CD25(int) and CD69 expression. Taken together, using a transwell culture system, we show in this study an inhibition of CD25(hi) expression and cell cycle arrest in target cells, which may serve at least in part the mechanisms of SEB-induced hyporesponsiveness.

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.