Staphylococcal exotoxins deliver activation signals to human T-cell clones via major histocompatibility complex class II molecules

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

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

The adjuvant-like activity of staphylococcal enterotoxin B in a murine asthma model is independent of IL-1R signaling

BACKGROUND

Staphylococcal enterotoxin B (SEB) is a superantigen known to be a modulator of chronic airway inflammation in mice and humans, yet little is known about the mechanisms that regulate its interaction with the innate immune system. We investigated this mechanism in a murine model of allergic airway inflammation induced by OVA (ovalbumin) in the presence of SEB.

METHODS

Superantigen-induced allergic inflammation was studied in IL-1R knockout (KO) mice exposed to OVA+SEB. Multicolor flow cytometry was used to analyze the inflammatory cell profile in airways and lymph nodes. Production of IL-4, IL-5, IL-10, and IL-13 in lymph nodes was assessed by Luminex technology.

RESULTS

In wild-type mice, endonasal instillation of OVA+SEB induced a pulmonary inflammation, characterized by an increase in the number of eosinophils, T cells, and dendritic cells and in the production of Th2 cytokines and OVA-specific IgE. In IL-1R KO mice exposed to OVA+SEB, attraction of CD4+ cells and production of Th2 cytokines were reduced. However, knocking out IL-1R did not affect any of the features of allergic airway inflammation, such as bronchial eosinophilia, OVA-specific IgE production and goblet cell metaplasia.

CONCLUSION

We provide new insights into the mechanisms of airways allergy development in the presence of bacterial superantigen. The asthma features induced by OVA+SEB, such as bronchial eosinophilia, goblet cell proliferation, production of OVA-specific IgE and increase in inflammatory dendritic cells, are IL-1R independent. Yet, IL-1R signaling is crucial for CD4 cell accumulation and Th2 cytokine production.

Effective activation alleviates the replication block of CCR5-tropic HIV-1 in chimpanzee CD4+ lymphocytes

Human immunodeficiency virus type 1 (HIV-1) originated in chimpanzees; yet, several previous studies have shown that primary HIV-1 isolates replicate poorly in chimpanzee CD4+ T lymphocytes in vitro and in vivo. The reasons for this apparent restriction are not understood. Here, we describe a new activation protocol that led to a reproducible expansion and activation of chimpanzee CD4+ T lymphocytes in vitro. Using this protocol, we uncovered species-specific differences in the activation profiles of human and chimpanzee CD4+ T-cells, including HLA-DR and CD62L. Moreover, we found that improved activation facilitated the replication of both CXCR4 and CCR5-tropic HIV-1 in CD4+ T-cell cultures from over 30 different chimpanzees. Thus, the previously reported "replication block" of CCR5-tropic HIV-1 in chimpanzee lymphocytes appears to be due, at least in large part, to suboptimal T-cell activation.

MHC class II exacerbates demyelination in vivo independently of T cells

We have shown previously the importance of MHC class II for central nervous system remyelination; however, the function of MHC class II during cuprizone-induced demyelination has not been examined. Here, we show that I-A(beta)-/- mice exhibit significantly reduced inflammation and demyelination. RAG-1(1/1) mice are indistinguishable from controls, indicating T cells may not play a role. The role of MHC class II depends on an intact cytoplasmic tail that leads to the production of IL-1beta, TNF-alpha, and nitric oxide, and oligodendrocyte apoptosis. Thus, the function of MHC class II cytoplasmic tail appears to increase microglial proliferation and activation that exacerbates demyelination.

Induction of tumor necrosis factor alpha and interleukin-8 gene expression in bronchial epithelial cells by toxic shock syndrome toxin 1

Major histocompatibility complex (MHC) class II engagement by toxic shock syndrome toxin 1 (TSST-1) transduces signals leading to proinflammatory cytokine gene expression (tumor necrosis factor alpha [TNF-alpha]) in human monocytes. To study the proinflammatory role of MHC class II molecules expressed by bronchial epithelial cells (BEC), primary human BEC were isolated from surgical bronchial samples, expanded in vitro, and cultured in the presence or absence of gamma interferon (IFN-gamma) for 48 h. (125)I-TSST-1 binding to BEC pretreated with IFN-gamma was inhibited up to 97% by anti-MHC class II monoclonal antibody 3B12, indicating that in BEC also MHC class II molecules were targets for the staphylococcal exotoxin. As analyzed by a quantitative reverse transcriptase PCR, a 1-h stimulation of BEC with TSST-1 resulted in a vigorous expression of TNF-alpha and interleukin-8 (IL-8) genes. TNF-alpha and IL-8 expression was optimal in BEC pretreated with 50 IU of IFN-gamma/ml, whereas TSST-1 stimulation of BEC pretreated with 200 IU of IFN-gamma/ml failed to enhance either TNF-alpha or IL-8 transcripts. In a time course study, peak expression of TNF-alpha and IL-8 mRNA was reached 6 h after TSST-1 stimulation. These results demonstrate that bacterial superantigen TSST-1 binds to MHC molecules on BEC and induces TNF-alpha and IL-8 gene expression upon engagement of MHC class II molecules on BEC, thus contributing to the perpetuation of bronchial mucosa inflammation via chemokine or cytokine gene expression.

Wortmannin inhibits translation of tumor necrosis factor-alpha in superantigen-activated T cells

The superantigen toxic shock syndrome toxin (TSST)-1 can induce tumor necrosis factor (TNF)-alpha expression in T cells and monocytes, through different signaling pathways. We have stimulated peripheral blood mononuclear cells with TSST-1 and found that the major cell producers of TNF-alpha as detected by cytofluorimetry and immunocytochemistry were CD4(+) T lymphocytes. The expression of TNF-alpha by CD4(+) T cells can be inhibited by either, wortmannin (WN) or LY 294002, two phosphatidylinositol 3-kinase (PI 3-K) inhibitors. The inhibitory effect is not transcriptional as WN does not change the mRNA steady state of TNF-alpha at any of the concentrations tested and LY 294002 when preincubated with mononuclear cells at its median inhibitory concentration (IC(50) = 1. 4 microM) significantly inhibited the expression of TNF-alpha but not its mRNA. Immunoprecipitation of pulse-labeled intracellular TNF-alpha showed a specific decrease in the synthesis of this cytokine on cells treated with PI 3-K inhibitors. The p38 mitogen-activated protein kinase (MAPK) is involved in control of TNF-alpha translation in human macrophages. In T cells, we have found that the p38 MAPK inhibitor SB 203580 significantly decreased the secretion of TNF-alpha but not its mRNA. In addition, the combined use of WN and SB 203580 had an additive inhibitory effect on secretion of TNF-alpha. Therefore, both PI 3-K and p38 MAPK signaling pathways control TNF-alpha production in T cells.

The overlooked "nonclassical" functions of major histocompatibility complex (MHC) class II antigens in immune and nonimmune cells

Besides their "classical" antigenic peptide-presenting activity, major histocompatibility complex (MHC) class II antigens can activate different cellular functions in immune and nonimmune cells. However, this "nonclassical" role and its functional consequences are still substantially overlooked. In this review, we will focus on these alternative functional properties of MHC class II antigens, to reawaken attention to their present and foreseeable immunobiologic and pathogenetic implications. The main issues that will be addressed concern 1) the role of MHC class II molecules as basic components of exchangeable oligomeric protein complexes with intracellular signaling ability; 2) the nonclassical functions of MHC class II antigens in immune cells; 3) the pathogenetic role of MHC class II antigens in inflammatory/autoimmune and infectious disease; and 4) the functional role of MHC class II antigens in solid malignancies.

Immunological study on CD3 defective cutaneous T cell lymphoma cells from a patient with Sézary syndrome

Here we investigated the nature of cutaneous T cell lymphoma (CTCL) cells lacking surface CD3. A large number of CD3- CD4 T cells were found in the peripheral blood and lesional skin of a patient with Sézary syndrome, which is a variant of CTCL. Southern blot analysis revealed that a clonal rearrangement of T cell receptor (TCR) genes was detected in the separated CD3- CD4 cells, whereas CD3+ CD4 cells showed no clonal rearrangement, indicating that the CD3- CD4 cells represented CTCL cells. However, the CTCL cells expressed TCR with a particular Vbeta apart from CD3. The CTCL cells showed significant responses to staphylococcal enterotoxins (SEs) in vitro, although they hardly responded to phytohaemagglutinin, Mycobacterium tuberculosis antigen, and alloantigen. They required antigen-presenting cells (APC) to respond to SEB. Blocking analyses with MoAbs revealed that they recognized SEB through TCR depending on HLA-DR and intercellular adhesion molecule-1 (ICAM-1). Taken collectively, these results indicate that the CTCL cells lacking surface CD3 could proliferate in response to bacterial superantigens, whereas the responses to conventional antigens were generally suppressed. These results also implied that CTCL could be exacerbated by bacterial infection.

In vitro cell death of activated lymphocytes in Omenn's syndrome

Omenn's syndrome (OS) is characterized by erythrodermia, hepatosplenomegaly, lymphadenopathy, hypereosinophilia and elevated IgE levels associated with increased susceptibility to severe infections. Peripheral blood T cells, though usually present in normal number, show an activated phenotype (including an increased expression of CD95/Fas), a Th2 pattern of cytokine secretion and defective proliferative response to mitogens. In this report, we demonstrate that T cells from patients with OS undergo an excessive cell death in vitro resulting from two mechanisms. First, a substantial number of peripheral blood lymphocytes from OS patients die in unstimulated cultures (p = 0.009 vs. healthy controls). This spontaneous apoptosis is associated with reduced expression of bcl-2 gene product (p < 0.05) and can be prevented by addition of interleukin (IL)-2 (which also prevents down-modulation of bcl-2), while is independent from CD95 signaling. Second, lymphocytes from OS patients are highly susceptible to activation-induced cell death (AICD) induced with mitogens. This mechanism is largely independent from IL-2, while it can be significantly inhibited blocking CD95 with an IgG2b monoclonal antibody (mAb). The dependence of AICD from signals transduced via CD95 was confirmed showing that cross-linking CD95 with an IgM mAb induces a higher cell death in purified CD4+ CD45R0+ cells from OS patients than in controls (comparable for CD95 expression). Both mechanisms of cell death observed in this study result from lymphocyte hyperactivation occurring in vivo in these patients and may contribute to functional T cell defects of OS.

Transcriptional and translational control of TNF-alpha gene expression in human monocytes by major histocompatibility complex class II ligands

While non-stimulated primary human monocytes exhibit very low levels of tumor necrosis factor (TNF)-alpha mRNA, direct binding of the staphylococcal exotoxin toxic shock syndrome toxin-1 (TSST-1) to major histocompatibility complex (MHC) class II molecules results in a fast (peak 1 h after stimulation), transient induction (sevenfold) of TNF-alpha mRNA. This induction correlates with a fourfold increase in transcription rates of the TNF-alpha gene, as detected by run-on assays, and does not require de novo protein synthesis. Mapping of DNase-I hypersensitive sites (DHS) discloses two constitutive DHS, one located far upstream (within the TNF-beta promoter) and the other centered at -39 +/- 40 bp relative to the major TNF-alpha transcription start site, suggesting that the TNF-alpha gene was transcriptionally competent even prior to MHC class II engagement. Furthermore, stimulation of human monocytes with either TSST-1 or lipopolysaccharide increases the translational efficiency of TNF-alpha mRNA, as shown by a shift in the distribution of this mRNA species in polysome gradients and the translation rates of TNF-alpha measured by immunoprecipitation from cells pulsed with [35S] methionine. The increase in translation efficiency of TNF-alpha mRNA is independent of the half-life of TNF-alpha transcripts, which under the conditions used is unchanged. Taken together, our data indicate that TNF-alpha expression is tightly regulated by MHC class II ligands, both at the transcriptional and translational levels.

Epitope-specific engagement of the protein tyrosine phosphatase CD45 induces tumor necrosis factor-alpha gene expression via transcriptional mechanisms

The common leukocyte antigen CD45 plays a central role in T cell activation in coupling the T cell receptor (TCR) to the phosphatidylinositol pathway via interactions with TCR-associated protein tyrosine kinases lck and fyn. We here demonstrate that engagement of CD45 by monoclonal antibodies (mAb) on activated T cells induces tumor necrosis factor (TNF)-alpha as well as TNF-beta, interleukin (IL)-2 and IL-3 gene expression. When human alloreactive T cells are stimulated with mAb 4B2, which recognizes a determinant common to all CD45 isoforms, a vigorous production of TNF-alpha mRNA was detected, which peaked 2 h later. Anti-CD45 mAb cross-linking was required. In contrast, neither mAb 10G10, which recognizes an epitope distinct from the one recognized by mAb 4B2, nor mAb UCHL-1, a CD45RO-specific antibody, induced any significant increase in TNF-alpha transcription. Nuclear run-on transcription assays demonstrated that CD45 cross-linking caused transcriptional activation of the TNF-alpha gene. De novo protein synthesis was not required, since incubation with cycloheximide (CHX) did not block transcriptional activation. CHX in contrast up-regulated TNF-alpha gene expression and increased transcript half-life, an effect that was under control of post-transcriptional mechanisms. Engagement of CD45 by itself did not affect transcript stability. CD45 ligation resulted in TNF-alpha secretion. These results indicate that in addition to its role in TCR/CD3-mediated T cell activation, CD45, in an epitope-specific manner, may act as a primary signaling molecule, leading to the transcriptional regulation and secretion of a major pro-inflammatory cytokine.

Cutaneous exposure to the superantigen staphylococcal enterotoxin B elicits a T-cell-dependent inflammatory response

We analyzed the impact of superantigens secreted by skin-colonizing Staphylococci on the skin and the associated lymphoid tissue following epicutaneous application and intracutaneous injection of small amounts of staphylococcal enterotoxin B (SEB). A single intracutaneous injection of 50 ng of SEB elicited a strong inflammatory response in the skin of BALB/c mice. Three to 6 h later, we observed langerhans cell activation, mast cell degranulation, vasodilation, upregulation of ICAM-1, and induction of VCAM-1 on dermal blood vessels, with vascular adhesion of granulocytes. by 12 to 24 h, cell infiltration of the dermis increased, reaching the epidermis. Among the infiltrating leukocytes, a substantial number of eosinophils was found. After 48 h, the infiltrate was dominated by mononuclear cells. The response to SEB was dose-dependent, and signs of inflammation slowly disappeared over 5 to 7 days. Although the induction of VCAM-1 on dermal blood vessels suggested a role for interleukin-1/tumor necrosis factor-alpha in this reaction, the activation of monocytes/macrophages was not able to substitute for lymphocytes, as severe combined immunodeficiency (SCID) mice (which are lymphocyte-deficient) did not mount an inflammatory skin response to intradermal injection of SEB. The fact that nude mice (T-cell-deficient) also did not mount an inflammatory response to SEB indicated the T-cell dependency of the response. The V beta specificity of the SEB effect was demonstrated by the fact that SJL/J mice, which lack V beta 8+ T cells (the major SEB-reactive T cell population in mice), exhibited much weaker responses. Deletion or tolerization of SEB-reactive V beta T cells was not observed after a single intradermal injection of such minute amounts of SEB.

Reactivity of mouse T-cell hybridomas expressing human Vbeta gene segments with staphylococcal and streptococcal superantigens

A panel of 15 mouse T-cell hybridomas, each expressing a different human Vbeta gene segment (hVbeta) in an otherwise mouse T-cell receptor (i.e., mouse alpha chain and CD3 complex), was constructed by transfection of hVbeta/mouse Cbeta chimeric T-cell receptor (TCR)-beta genes into a mouse T-cell hybridoma recipient lacking the endogenous TCR-beta chain. Several qualities that are conferred by the hVbeta chain of the TCR are retained in the chimeric human-mouse TCR complex: a large panel of hVbeta-specific antibodies specifically stained the hVbeta expressed by the mouse T-cell hybridomas. Moreover, hVbeta-transfected mouse cells could readily produce interleukin 2 when stimulated by superantigens presented by antigen-presenting cells. These characteristics made it possible to refine the reactivity of 17 superantigen preparations with the available transfected Vbetas. Each superantigen gave a characteristic pattern of reactivity on the transfectants. Positive reactivities with some of these transfectants, which differ only by the expressed hVbeta, demonstrate unambiguously the superantigenic character of a protein or fraction and its potential to react with the corresponding Vbetas. Therefore, these hVbeta-transfected cells constituted a valuable tool for determining "specificity fingerprints" of known or putative superantigens. First, commonly used, commercially available superantigens such as staphylococcal enterotoxin B and toxic shock syndrome toxin-1 (TSST-1) showed additional Vbeta reactivities, compared with those of their recombinant counterparts. This stresses the importance of using defined preparations of superantigens for the definition of Vbeta specificities. Second, the stimulatory pattern of a strain of Streptococcus pyogenes demonstrated that this strain, unlike others, produces a potent Vbeta 8-specific superantigen that is an yet undefined at the molecular level.

New perspectives on inflammation in atopic dermatitis

Recent information implicates the stimulation of T cells by Staphylococcus aureus antigens and exotoxins as a likely factor in provoking the inflammatory response in atopic dermatitis. S. aureus secrets exotoxins called superantigens, which stimulate a large proportion of T cells. In addition, protein A, a component of the cell wall of S. aureus, is a potent B cell mitogen. This understanding provides a rationale for attempting to reduce the staphylococcal skin colonization of patients with severe atopic dermatitis and correlates with the clinical observation in a number of situations of marked improvement in atopic dermatitis following antibiotic treatment.

Mycoplasma arthritidis mitogen up-regulates human NK cell activity

While the effects of superantigens on T lymphocytes are well characterized, how superantigens interact with other immune cells is less clear. This report examines the effects of Mycoplasma arthritidis mitogen (MAM) on human natural killer (NK) cell activity. Incubation of peripheral blood mononuclear cells (PBMC) with MAM for 16 to 20 h augmented NK cytotoxicity (against K562) in a dose-dependent manner (P < or = 0.05). Superantigen-dependent cellular cytotoxicity, an activity of superantigen-activated cytotoxic T cells, was not involved in lysis of K562 cells because the erythroleukemic tumor target cells expressed no class II major histocompatibility complex by fluorescence-activated cell sorter analysis. Kinetic experiments showed that the largest increase in NK activity induced by MAM occurred within 48 h. Incubation with MAM caused a portion of NK cells to become adherent to tissue culture flasks, a quality associated with activation, and augmented NK activity was found in both adherent and nonadherent subpopulations. Experiments using cytokine-specific neutralizing antibodies showed that interleukin-2 contributed to enhancement of the NK activity observed in superantigen-stimulated PBMC. Interestingly, MAM was able to augment NK lysis of highly purified NK (CD56+) cells in the absence of other immune cells in 9 of 12 blood specimens, with the augmented lytic activity ranging from 110 to 170% of unstimulated NK activity. In summary, data presented in this report show for the first time that MAM affects human NK cells directly by increasing their lytic capacity and indirectly in PBMC as a consequence of cytokines produced by T cells. Results of this work suggest that, in vivo, one consequence of interaction with superantigen-secreting microorganisms may be up-regulation of NK lytic activity. These findings may have clinical application as a means of generating augmented NK effector cells useful in the immunotherapy of parasitic infections or neoplasms.

Staphylococcal enterotoxin A has two cooperative binding sites on major histocompatibility complex class II

The superantigen staphylococcal enterotoxin A (SEA) binds to major histocompatibility complex (MHC) class II molecules at two sites on either side of the peptide groove. Two separate but cooperative interactions to the human class II molecule HLA-DR1 were detected. The first high affinity interaction to the DR1 beta chain is mediated by a zinc atom coordinated by H187, H225, and D227 in SEA and H81 in the polymorphic DR1 beta chain. The second low affinity site is to the DR1 alpha chain analogous to SEB binding and is mediated by residue F47 in SEA. Binding of one SEA to the DR1 beta chain enhances the binding of a second SEA molecule to the DR1 alpha chain. The zinc site is on the opposite side of the SEA molecule from residue F47 so that one SEA molecule can readily bind two class II molecules. Both binding sites on SEA are required for maximal activity. Thus, unlike, SEB, SEA requires two separate binding sites for optimal activity, which may allow it to stabilize SEA interaction with T cell receptors, as well as to activate the antigen-presenting cell by cross-linking MHC class II.

Modulation of contact sensitivity responses by bacterial superantigen

Superantigens are potent modulators of the immune system, especially T cells. Therefore, we determined the influence of superantigens on the T-cell-mediated immune response, contact sensitivity. We chose the combination of staphylococcal enterotoxin B (SEB) as superantigen and 2,4-dinitrofluorbenzene (DNFB) as the contact sensitizer, because in BALB/c mice SEB reacts almost exclusively with V beta 8+ T cells, and these cells are capable of transferring contact sensitivity to DNFB from sensitized donors to naive syngeneic recipients. Pretreatment with a single intradermal injection of 50 ng SEB 24 h before DNFB exposure at the same site on the lower abdomen enhanced the induction of contact sensitivity: its intradermal injection permitted sensitization with non-sensitizing concentrations of DNFB as assessed by ear swelling responses after challenge with DNFB. In contrast, pretreatment with repeated intradermal injections of 50 ng SEB every other day over at least 1 week inhibited the induction of contact sensitivity following sensitization. The enhancing effect of SEB may be explained by the creation of a proinflammatory milieu in the skin after a single intradermal injection of the bacterial toxin, whereas the inhibitory effect may be due to tolerization of V beta 8+ T cells. The data indicate that products of skin-colonizing bacteria that can serve as superantigens are able to augment or inhibit the development of contact sensitivity.

Bacterial superantigens induce the proliferation of resting gamma/delta receptor bearing T cells

We observe that highly purified (> or = 97% pure) gamma/delta T cells isolated from normal peripheral blood proliferate to bacterial toxin supperantigens SEA, SEB, SED, and TSST-1. MHC class II molecules were necessary and sufficient for the recognition of superantigens by gamma/delta T cells because MHC Class II deficient B cell line failed to support the proliferation of gamma/delta T cells to toxins and murine L cells transfected with HLA-DR but not untransfected cells were capable of presenting toxins to gamma/delta T cells. As in the case with alpha/beta T cells, bacterial superantigens synergized with PMA in causing the proliferation of purified gamma/delta T cells rigorously depleted of accessory cells. Together, our findings suggest that gamma/delta T cells recognize and respond to bacterial superantigens in a manner similar to alpha/beta T cells.

The role of superantigens in skin disease

Staphylococcus aureus and streptococci secrete a large family of exotoxins involved in the pathogenesis of toxic-shock-like syndromes and have been implicated in several autoimmune disorders. These toxins act as prototypic superantigens capable of binding to major histocompatibility complex proteins on antigen-presenting cells outside the antigen peptide-binding groove and can thereby stimulate cytokine release from macrophages. The superantigen-major histocompatibility complex unit is recognized primarily by the variable region of the T-cell receptor beta chain, and by engaging this region, can activate a large portion of the T-cell repertoire. It is thought that the capacity of these toxins to cause the massive stimulation of T cells and accessory cells such as macrophages, Langerhans cells, and activated keratinocytes accounts for most of their pathologic effects. The current review examines the evidence that implicates a role for these superantigens in the pathogenesis of certain skin diseases.

Bacterial pyrogenic exotoxins as superantigens

The recent discovery of the mode of interaction between a group of microbial proteins known as superantigens and the immune system has opened a wide area of investigation into the possible role of these molecules in human diseases. Superantigens produced by certain viruses and bacteria, including Mycoplasma species, are either secreted or membrane-bound proteins. A unique feature of these proteins is that they can interact simultaneously with distinct receptors on different types of cells, resulting in enhanced cell-cell interaction and triggering a series of biochemical reactions that can lead to excessive cell proliferation and the release of inflammatory cytokines. However, although superantigens share many features, they can have very different biological effects that are potentiated by host genetic and environmental factors. This review focuses on a group of secreted pyrogenic toxins that belong to the superantigen family and highlights some of their structural-functional features and their roles in diseases such as toxic shock and autoimmunity. Deciphering the biological activities of the various superantigens and understanding their role in the pathogenesis of microbial infections and their sequelae will enable us to devise means by which we can intervene with their activity and/or manipulate them to our advantage.

Regulation of staphylococcal enterotoxin B-elicited nitric oxide production by endothelial cells

The effect of staphylococcal enterotoxin B (SEB)-elicited inducible nitric oxide synthase (iNOS) in mouse endothelial cells was investigated. Results showed that SEB stimulated the same level of NO production in gamma interferon (IFN-gamma)-primed cells as did trichloroacetic acid-extracted lipopolysaccharide. The kinetics of induced NO production and expression of mRNA for iNOS differed markedly in endothelial and macrophage cells. Induced endothelial nitrite production was transient and was 15 to 20% of that generated by macrophage cells; mRNA levels peaked by 2 h and then steadily declined, whereas macrophage message levels continually increased. The ability of endothelial cells to produce SEB-induced NO depended on priming with IFN-gamma, although detectable mRNA could be elicited by SEB alone. Induction of endothelial iNOS mRNA was inhibited by cycloheximide, which indicated a requirement for de novo protein synthesis. Niacinamide and interleukin-10 significantly reduced SEB-induced endothelial NO production. Both are reported to affect IFN-gamma-induced class II major histocompatibility complex (MHC) expression on antigen-presenting cells. Niacinamide reduced iNOS mRNA levels and markedly reduced IFN-gamma induction of endothelial class II MHC surface antigen. Interleukin-10 did not consistently reduce iNOS mRNA expression and had no effect on IFN-gamma induction of endothelial class II MHC surface antigen. These results suggest that SEB interacts with IFN-gamma-primed endothelial cells to elicit induced NO and that this induction can be effectively modulated at the receptor or transcriptional level.

Inhibition of the development of immediate hypersensitivity by staphylococcal enterotoxin B

We investigated the ability of staphylococcal enterotoxin B (SEB) to modify the immediate hypersensitivity response induced in BALB/c mice following sensitization to ovalbumin (OVA), a response mediated by OVA-reactive V beta 8 T cells. Mice were sensitized by skin painting with OVA every second day over a period of 2 weeks. SEB, a potent activator of V beta 8+ T cells, was administered at the same site where OVA was applied (skin of the lower abdomen) following two different protocols. In protocol (A) SEB was injected intradermally 1 day before painting with OVA and on day 7; in protocol B, SEB was injected each time OVA was applied to the skin (eight times). SEB (but not SEA) altered the development of immediate hypersensitivity to OVA, as demonstrated by the reduction in allergen-specific IgE, decreased OVA-specific immediate skin test responsiveness, and prevented the development of increased airways responsiveness after bronchial challenge with OVA. Injections of SEB did not alter the proliferative responses of local draining lymph node cells or spleen mononuclear cells to OVA, indicating that administration of SEB did not inhibit the sensitization of OVA, but shifted the immune response away from an immediate type response (IgE/IgG1) to IgG2a, IgG2b and IgG3. Although both protocols of SEB treatment did not lead to a major deletion of the V beta 8 T cell population, they did reduce the proliferative response of V beta 8+ T cells to OVA. These data indicate that the bacterial toxin SEB is capable of modifying the immediate hypersensitivity response induced by OVA by altering the functional capacity of antigen-reactive V beta 8 T cells.

Localization of binding sites of staphylococcal enterotoxin B (SEB), a superantigen, for HLA-DR by inhibition with synthetic peptides of SEB

Staphylococcal enterotoxins are major causes of food poisoning and toxic shock syndrome. Their ability to bind to major histocompatibility complex (MHC) class II molecules has been suggested to be the first step in the mechanism whereby they cause illness. By flow cytometric analysis, the sites of interaction of staphylococcal enterotoxin B (SEB) with HLA-DR molecules were probed in the present study by inhibiting the binding of biotinylated SEB to a human T-cell line (HUT-78) with synthetic peptides of SEB. Five peptides of SEB gave significant inhibition of binding: a peptide containing amino acids 9 to 20 [SEB(9-20)], SEB(30-38), SEB(61-70), SEB(90-114), and SEB(169-181). One peptide, SEB(39-51), enhanced binding. Among the inhibitory peptides, SEB(90-114), a peptide spanning the entire disulfide loop, showed the most efficient inhibition of binding. Peptides SEB(9-20) and SEB(39-51) include amino acid residues that have been identified by previous mutation studies (J.W. Kappler, A. Herman, J. Clements, and P. Marrack, J. Exp. Med. 175:387-396, 1992) as being important in binding to MHC class II. Amino acids lining the alpha 5 groove of SEB have also been postulated to be involved in binding to MHC class II molecules. However, only two of the residues that line the alpha 5 groove of SEB, His-12 and Tyr-17, are on peptide SEB(9-20) that inhibits binding. These results confirm previous studies that implicated the amino-terminal portion of the molecule in binding to MHC class II molecules and further indicate an important role for residues in other regions, particularly the disulfide loop.

Interactions between staphylococcal superantigens and human T-cell clones are predominantly but not exclusively governed by their T-cell receptor V beta usage

Staphylococcal exotoxins (SE) are potent mitogens for human and murine T cells. Extensive studies in mice have demonstrated strict correlations between T-cell responses to individual SE and TCR V beta expression. Studies examining the TCR V beta expression of SE-activated human peripheral blood T cells also suggest close correlations, whereas the data reported using human T-cell clones (TCC) are conflicting. We have determined the cDNA TCRB sequences of 52 different human TCC, expressing 35 different T-cell receptor V beta (TCRBV)-encoded sequences. The TCC were tested in proliferative assays using nine different SE. Most of these TCC express V beta s which have not been tested previously in studies examining interaction between TCC and SE. The SE stimulated a variable fraction (1/48-31/52) of the TCC. The ability of a given SE to stimulate TCC in many cases correlated with V beta expression, but several exceptions were found. With one possible exception, comparisons between deduced amino-acid sequences within the 'fourth hypervariable region' of the TCR beta chain and SE responsiveness did not reveal potential SE binding motifs. We conclude that the reactivity of T cells towards SE is governed mainly by their TCR V beta expression. However, the authors' results also suggest that the interaction between SE and human T cells may involve elements unidentified as yet which are in addition to the beta chain of the TCR.

T cell influence on superantigen-induced arthritis in MRL-lpr/lpr mice

OBJECTIVE

To define the influence of the T cell receptor (TCR) and the lpr autoimmune gene on the induction and progression of superantigen-induced arthritis in V beta 8 transgenic MRL-lpr/lpr mice.

METHODS

The time to onset and the extent of synovial hyperplasia after the induction of arthritis by intraarticular injection of staphylococcal enterotoxin B (SEB) were compared in mice having T cells that bear the V beta 8 transgene alone (V beta 8 TCR transgenic MRL-+/+), the lpr gene without the V beta 8 gene (nontransgenic MRL-lpr/lpr), both the V beta 8 gene and the lpr gene (V beta 8 transgenic MRL-lpr/lpr), or neither gene (nontransgenic MRL-+/+). Synovial hyperplasia was compared in SEB-injected V beta 8 transgenic MRL-lpr/lpr mice after treatment with cyclosporin A (CSA), anti-V beta 8 and anti-CD4 monoclonal antibodies, and in V beta 8 transgenic MRL-lpr/lpr mice after injection of a non-V beta 8-reactive superantigen, staphylococcal enterotoxin A (SEA).

RESULTS

At day 30, increased synovial cells were observed in all SEB-treated mice, but the increase was greatest in the V beta 8 transgenic MRL-lpr/lpr mice. T cell involvement was indicated by the inability of either heat-denatured SEB or SEA to induce severe arthritis, the reduction in the severity of the arthritis on systemic treatment with CSA or anti-V beta 8, and the correlation of synovial hyperplasia with in vitro SEB reactivity of T cells.

CONCLUSION

These observations suggest that superantigens can induce chronic arthritis and that the induction and progression of the arthritis requires an underlying T cell defect in anergy induction in addition to exposure to the superantigen.

HLA class II molecule-mediated signal transduction mechanism responsible for the expression of interleukin-1 beta and tumor necrosis factor-alpha genes induced by a staphylococcal superantigen

Superantigens including staphylococcal enterotoxins (SE) bind to major histocompatibility complex class II molecules and interact with T cells bearing particular V beta chains. SEB was shown to induce the expression of interleukin (IL)-1 beta and tumor necrosis factor (TNF)-alpha genes in human peripheral blood monocytes bearing HLA class II molecules. Monoclonal antibodies directed against HLA-DR and -DQ abolished the SEB-induced expression of both the IL-1 beta and TNF-alpha genes, suggesting that the HLA class II molecules mediated the gene expression. Therefore, we investigated the signal transduction mechanism responsible for the expression of IL-1 beta and TNF-alpha genes induced by binding of SEB to the HLA class II molecules. Three protein tyrosine kinase (PTK) inhibitors, genistein, herbimycin A, and tyrphostin, each of which has a different mechanism of action, strongly inhibited the expression of the monokine mRNA induced by SEB. Analyses of PTK activity revealed that SEB induced a rapid increase of membrane-associated PTK activity and this was blocked by tyrphostin. Furthermore, H-7 inhibited the expression of the monokine mRNA induced by SEB, suggesting the involvement of protein kinase C (PKC) in the signaling pathway. The involvement of PKC was confirmed by the observations that phorbol 12-myristate 13-acetate (PMA), a direct activator of PKC, induced the expression of the monokine mRNA and that SEB evoked the activation of membrane-associated PKC. Both activation of PKC and expression of the monokine mRNA induced by SEB appeared to be inhibited by tyrphostin, but those induced by PMA were not. Taken together, these findings indicate that both PTK and PKC play essential roles in HLA class II molecule-mediated signal transduction elicited by SEB and that PTK activation may precede PKC activation in the signaling pathway.

Activator protein-1 (AP-1) is stimulated by microbial superantigens in human monocytic cells

Microbial superantigens bind to major histocompatibility complex (MHC) class II molecules and activate gene transcription in monocytes. In search of transcription factors that potentially mediate the effects of superantigens at the nuclear level, we examined the capacity of staphylococcal superantigens to stimulate the activity of the transcriptional promoter factor activator protein-1 (AP-1). Electrophoretic mobility shift assays showed an increase in nuclear proteins that bound to the consensus AP-1 motif within 5 min following the stimulation of the monocytic cell line THP-1 with toxic shock syndrome toxin-1 (TSST-1) or staphylococcal endotoxin A. We show that mRNA levels for the subunits that compose AP-1, the protooncogenes c-fos and c-jun, are upregulated by stimulation of THP-1 cells with TSST-1. The activated AP-1 complexes were functional, as evidenced by the capacity of TSST-1 to stimulate the expression of an AP-1-driven reporter gene construct transfected into THP-1 cells. These results establish that the engagement of MHC class II molecules by superantigens increases the activity of functional AP-1 complexes and that this may proceed in part by transcriptional activation of c-fos and c-jun protooncogenes.

Presence of IgE antibodies to staphylococcal exotoxins on the skin of patients with atopic dermatitis. Evidence for a new group of allergens

In the current study, we investigated whether Staphylococcus aureus grown from affected skin of atopic dermatitis (AD) patients secreted identifiable toxins that could act as allergens to induce IgE-mediated basophil histamine release. The secreted toxins of S. aureus grown from AD patients were identified by ELISA using antibodies specific for staphylococcal enterotoxin (SE) exfoliative toxin (ET), or toxic shock syndrome toxin (TSST-1). S. aureus isolates from 24 of 42 AD patients secreted identifiable toxins with SEA, SEB, and TSST accounting for 92% of the isolates. 32 of 56 AD sera (57%) tested contained significant levels of IgE primarily to SEA, SEB, and/or TSST. In contrast, although SEA, SEB, or TSST secreting S. aureus could be recovered from the skin of psoriasis patients, their sera did not contain IgE antitoxins. Freshly isolated basophils from 10 AD patients released 5-59% of total histamine in response to SEA, SEB, or TSST-1 but only with toxins to which patients had specific IgE. Basophils from eight other AD patients and six normal controls who had no IgE antitoxin failed to demonstrate toxin-induced basophil histamine release. Stripped basophils sensitized with three AD sera containing IgE to toxin released 15-41% of total basophil histamine only when exposed to the relevant toxin, but not to other toxins. Sensitization of basophils with AD sera lacking IgE antitoxin did not result in release of histamine to any of the toxins tested. These data indicate that a subset of patients with AD mount an IgE response to SEs that can be grown from their skin. These toxins may exacerbate AD by activating mast cells, basophils, and/or other Fc epsilon-receptor bearing cells armed with the relevant IgE antitoxin.

Activation with superantigens induces programmed death in antigen-primed CD4+ class II+ major histocompatibility complex T lymphocytes via a CD11a/CD18-dependent mechanism

Staphylococcal enterotoxin superantigens (SAg) bind class II major histocompatibility complex (MHC) molecules on antigen-presenting cells (APC) and upon cell-to-cell contact stimulate proliferation of T cells expressing appropriate V beta gene products. In addition, SAg can also deliver negative signals to Ag-specific T cells resulting in a state of unresponsiveness or a loss of viability. The present study examines the functional consequences of a direct interaction of SAg with alloAg-specific class II MHC+ CD4+ T cell lines (TCL). Our results demonstrate that SAg induce programmed death (apoptosis) in a majority of Ag-specific CD4+ T cells accompanied by genomic DNA fragmentation. SAg binding to Ag-specific TCL resulted in a rapid mobilization of intracellular free calcium ([Ca2+]i) and transcription of a number of cytokine genes including interleukin-2(IL-2), IL-4, interferon-gamma (IFN-gamma), granulocyte-macrophage colony-stimulating factor (GM-CSF), and granzyme B indicating the activation of primed T cells. Both SAg-induced cytokine gene expression as well as subsequent death were significantly inhibited by a tyrosine kinase inhibitor herbimycin A and also by cyclosporin A. SAg-induced death of primed T cells was also inhibited by monoclonal antibodies (mAb) directed at the CD11a/CD18 molecule but not those reactive with other T cell surface molecules such as CD2, CD7, CD28, CD29 or CD49d. None of these mAb, including anti-CD11a/CD18, had any effect on SAg-induced expression of IL-2 and IL-4 genes or SAg-induced [Ca2+]i response. Addition of cytokines such as IL-1 alpha, IL-2, IL-4, IL-6, GM-CSF, IFN-gamma, tumor necrosis factor (TNF-alpha, or TNF-beta), or neutralizing Ab to these cytokines had no effect on SAg-induced death of Ag-specific TCL. The T cells which survived the death-inducing effects of SAg showed down-regulation of the CD3/T cell receptor and up-regulation of CD2 and HLA-DR expression, and upon re-exposure to the same SAg upregulated expression of mRNA for IL-2 and IFN-gamma. Presentation of SAg by B7+ ICAM-1+ LFA-3+ DR+ professional APC was also able to induce the death of Ag-specific TCL. Together these results suggest that the activation with SAg causes programmed death of Ag-specific TCL cells via a mechanism that requires late participation of the CD11a/CD18 molecule.

Reactivity of human gamma delta T cells to staphylococcal enterotoxins: a restricted reaction pattern mediated by two distinct recognition pathways

Staphylococcal enterotoxins (SEs) are known superantigens for T cells expressing the alpha beta T-cell receptor (TCR). They bind to MHC class II molecules on antigen-presenting cells and can subsequently trigger T-cell responses by binding to V beta-gene products. The reactivity of gamma delta T cells with enterotoxins is less well defined although both proliferative and cytotoxic responses have been described. In the present study we have tested the cytotoxic reactivity of a panel of 41 gamma delta T-cell clones against target cells coated with the enterotoxins SEA, SEB, SEC1, SEC2, SEC3, SED, SEE or TSST. Three reaction patterns were observed with the gamma delta T-cell clones: (1) clones that specifically lysed SEA-coated target cells only; (2) clones that specifically lysed SEE-coated target cells only, and (3) clones that specifically lysed SEA-coated target cells only in the presence of certain human sera. The presence of SEA-specific antibodies in such human sera could be demonstrated. Moreover, gamma delta T-cell clones of this third category expressed the IgG FcRIII (CD16) which indicates that these clones are capable of mediating antibody-dependent cellular cytotoxicity towards SEA-coated target cells. Thus, the cytotoxic response of gamma delta T cells to SEs is mediated by two distinct pathways: an antibody-independent and an antibody-dependent pathway. The antibody-independent reactivity of gamma delta T cells was directed to either SEA or SEE, whereas antibody-dependent reactivity was found only towards SEA. The capacity of gamma delta T-cell clones to respond to stimulation with SEs, combined with their high cytolytic capacity in vitro, suggests that these cells can be involved in SE-directed immune responses and efficiently kill SE-coated target cells in vivo.

A potential role for superantigens in the pathogenesis of psoriasis

Psoriasis is a complex inflammatory skin disease in which local vascular changes, T-cell activation, abnormal keratinocyte proliferation and differentiation, and neutrophil activation all contribute to the ongoing disease process. Because of recent interest in T-cell activation as a trigger for psoriatic lesions, we hypothesized that psoriasis may be triggered by superantigens, e.g., toxins of microbial origin that stimulate T cells expressing particular T-cell receptor (TCR) beta chain variable (V beta) gene segments. Lesional skin biopsies and peripheral blood from two patients with acute exacerbations of their psoriasis that appeared to be triggered by infection were analyzed for TCR V beta gene expression using monoclonal antibodies directed against V beta 5.1, 5.2, 6.7, 8.1, and 12. Skin biopsies from both patients demonstrated a different pattern of V beta expansion that correspond to the V beta pattern expected to be induced by the type of superantigen expressed during the infection. In contrast, using immunofluorescence and flow cytometry, peripheral blood T cells from these patients did not demonstrate any expansion of the 5 V beta subsets studied. These observations support the hypothesis that local activation of cutaneous T cells in psoriasis may be caused by a superantigen and provides a new direction for investigating the pathogenesis of this complex and fascinating skin disorder.

Superantigen staphylococcal enterotoxin B-induced T-helper cell activation is independent of CD4 molecules and phosphatidylinositol hydrolysis

The role of the CD4 molecule in activation of T-helper cells was examined by investigating the effect of an anti-CD4 monoclonal antibody (Leu3a) in conventional peptide antigen-specific cloned T-helper cells that are also reactive to staphylococcal enterotoxin B (SEB). These T-helper cell clones are CD4+/CD45RO+/T-cell antigen receptor beta-chain variable region 12-positive and can respond to nominal peptide antigens and SEB by proliferation in the presence of class II major histocompatibility complex-expressing accessory cells. Although antigen and SEB were comparable in their ability to induce proliferative responses, interleukin 2 (IL-2) production, and IL-2 receptor alpha-chain expression, stimulation with SEB failed to trigger phosphatidylinositol hydrolysis or a rise in the intracellular free calcium ion concentration. Leu3a treatment inhibited antigen-induced proliferative responses of T cells with concomitant suppression of IL-2 production and IL-2 receptor expression. In contrast, SEB-induced responses were unaffected by Leu3a. These findings indicate that the functional consequences of binding (ligation) of conventional antigen and of superantigen with the T-cell receptor are distinct in the context of both signal transduction pathways and participation of CD4 molecules.