Staphylococcal enterotoxin H induces V alpha-specific expansion of T cells

Genomic analysis and identification of a novel superantigen, SargEY, in Staphylococcus argenteus isolated from atopic dermatitis lesions

During surveillance of Staphylococcus aureus in lesions from patients with atopic dermatitis (AD), we isolated Staphylococcus argenteus, a species registered in 2011 as a new member of the genus Staphylococcus and previously considered a lineage of S. aureus. Genome sequence comparisons between S. argenteus isolates and representative S. aureus clinical isolates from various origins revealed that the S. argenteus genome from AD patients closely resembles that of S. aureus causing skin infections. We previously reported that 17%-22% of S. aureus isolated from skin infections produce staphylococcal enterotoxin Y (SEY), which predominantly induces T-cell proliferation via the T-cell receptor (TCR) Vα pathway. Complete genome sequencing of S. argenteus isolates revealed a gene encoding a protein similar to superantigen SEY, designated as SargEY, on its chromosome. Population structure analysis of S. argenteus revealed that these isolates are ST2250 lineage, which was the only lineage positive for the SEY-like gene among S. argenteus. Recombinant SargEY demonstrated immunological cross-reactivity with anti-SEY serum. SargEY could induce proliferation of human CD4+ and CD8+ T cells, as well as production of TNF-α and IFN-γ. SargEY showed emetic activity in a marmoset monkey model. SargEY and SET (a phylogenetically close but uncharacterized SE) revealed their dependency on TCR Vα in inducing human T-cell proliferation. Additionally, TCR sequencing revealed other previously undescribed Vα repertoires induced by SEH. SargEY and SEY may play roles in exacerbating the respective toxin-producing strains in AD.

IMPORTANCE

Staphylococcus aureus is frequently isolated from active lesions of atopic dermatitis (AD) patients. We reported that 17%-22% of S. aureus isolated from AD patients produced a novel superantigen staphylococcal enterotoxin Y (SEY). Unlike many S. aureus superantigens that activate T cells via T-cell receptor (TCR) Vß, SEY activates T cells via TCR Vα and stimulates cytokine secretion. Staphylococcus argenteus was isolated from AD patients during the surveillance for S. aureus. Phylogenetic comparison of the genome indicated that the isolate was very similar to S. aureus causing skin infections. The isolate encoded a SEY-like protein, designated SargEY, which, like SEY, activated T cells via the TCR Vα. ST2250 is the only lineage positive for SargEY gene. ST2250 S. argenteus harboring a superantigen SargEY gene may be a novel staphylococcal clone that infects human skin and is involved in the exacerbation of AD.

Novel insights into the immune response to bacterial T cell superantigens

Bacterial T cell superantigens (SAgs) are a family of microbial exotoxins that function to activate large numbers of T cells simultaneously. SAgs activate T cells by direct binding and crosslinking of the lateral regions of MHC class II molecules on antigen-presenting cells with T cell receptors (TCRs) on T cells; these interactions alter the normal TCR-peptide-MHC class II architecture to activate T cells in a manner that is independent of the antigen specificity of the TCR. SAgs have well-recognized, central roles in human diseases such as toxic shock syndrome and scarlet fever through their quantitative effects on the T cell response; in addition, numerous other consequences of SAg-driven T cell activation are now being recognized, including direct roles in the pathogenesis of endocarditis, bloodstream infections, skin disease and pharyngitis. In this Review, we summarize the expanding family of bacterial SAgs and how these toxins can engage highly diverse adaptive immune receptors. We highlight recent findings regarding how SAg-driven manipulation of the adaptive immune response may operate in multiple human diseases, as well as contributing to the biology and life cycle of SAg-producing bacterial pathogens.

Toxin exposure and HLA alleles determine serum antibody binding to toxic shock syndrome toxin 1 (TSST-1) of Staphylococcus aureus

Life-threatening toxic shock syndrome is often caused by the superantigen toxic shock syndrome toxin-1 (TSST-1) produced by Staphylococcus aureus. A well-known risk factor is the lack of neutralizing antibodies. To identify determinants of the anti-TSST-1 antibody response, we examined 976 participants of the German population-based epidemiological Study of Health in Pomerania (SHIP-TREND-0). We measured anti-TSST-1 antibody levels, analyzed the colonization with TSST-1-encoding S. aureus strains, and performed a genome-wide association analysis of genetic risk factors. TSST-1-specific serum IgG levels varied over a range of 4.2 logs and were elevated by a factor of 12.3 upon nasal colonization with TSST-1-encoding S. aureus. Moreover, the anti-TSST-1 antibody levels were strongly associated with HLA class II gene loci. HLA-DRB1*03:01 and HLA-DQB1*02:01 were positively, and HLA-DRB1*01:01 as well as HLA-DQB1*05:01 negatively associated with the anti-TSST-1 antibody levels. Thus, both toxin exposure and HLA alleles affect the human antibody response to TSST-1.

Superantigen Recognition and Interactions: Functions, Mechanisms and Applications

Superantigens are unconventional antigens which recognise immune receptors outside their usual recognition sites e.g. complementary determining regions (CDRs), to elicit a response within the target cell. T-cell superantigens crosslink T-cell receptors and MHC Class II molecules on antigen-presenting cells, leading to lymphocyte recruitment, induction of cytokine storms and T-cell anergy or apoptosis among many other effects. B-cell superantigens, on the other hand, bind immunoglobulins on B-cells, affecting opsonisation, IgG-mediated phagocytosis, and driving apoptosis. Here, through a review of the structural basis for recognition of immune receptors by superantigens, we show that their binding interfaces share specific physicochemical characteristics when compared with other protein-protein interaction complexes. Given that antibody-binding superantigens have been exploited extensively in industrial antibody purification, these observations could facilitate further protein engineering to optimize the use of superantigens in this and other areas of biotechnology.

Activation of human γδ T cells and NK cells by Staphylococcal enterotoxins requires both monocytes and conventional T cells

Staphylococcal enterotoxins (SE) pose a great threat to human health due to their ability to bypass antigen presentation and activate large amounts of conventional T cells resulting in a cytokine storm potentially leading to toxic shock syndrome. Unconventional T- and NK cells are also activated by SE but the mechanisms remain poorly understood. In this study, the authors aimed to explore the underlying mechanism behind SE-mediated activation of MAIT-, γδ T-, and NK cells in vitro. CBMC or PBMC were stimulated with the toxins SEA, SEH, and TSST-1, and cytokine and cytotoxic responses were analyzed with ELISA and flow cytometry. All toxins induced a broad range of cytokines, perforin and granzyme B, although SEH was not as potent as SEA and TSST-1. SE-induced IFN-γ expression in MAIT-, γδ T-, and NK cells was clearly reduced by neutralization of IL-12, while cytotoxic compounds were not affected at all. Kinetic assays showed that unconventional T cell and NK cell-responses are secondary to the response in conventional T cells. Furthermore, co-cultures of isolated cell populations revealed that the ability of SEA to activate γδ T- and NK cells was fully dependent on the presence of both monocytes and αβ T cells. Lastly, it was found that SE provoked a reduced and delayed cytokine response in infants, particularly within the unconventional T and NK cell populations. This study provides novel insights regarding the activation of unconventional T- and NK cells by SE, which contribute to understanding the vulnerability of young children towards Staphylococcus aureus infections.

The Role of Bacterial Superantigens in the Immune Response: From Biology to Cancer Treatment

Aims:

Encouraging results have been indicated preclinically and in patients using the bacterial superantigen. This review article intends to summarize the role of the superantigens that have been recently used in the treatment of cancer. In addition, the vector systems, including lentiviral vectors, adeno-associated vector systems and retroviral vectors that are increasingly being used in basic and applied research, were discussed. Most importantly, the new CRISPR technique has also been discussed in this literature review.

Discussion:

More successful therapies can be achieved by manipulating bacterial vector systems through incorporating genes related to the superantigens and cytokines. The products of SAg and cytokine genes contribute to the strong stimulation of the immune system against tumor cells. They bind to MHC II molecules as well as the V beta regions of TCR and lead to the production of IL2 and other cytokines, the activation of antigen-presenting cells and T lymphocytes. Additionally, superantigens can be used to eradicate tumor cells. Better results in cancer treatment can be achieved by transferring superantigen genes and subsequent strong immune stimulation along with other cancer immunotherapy agents.

Conclusion:

Superantigens induce the proliferation of T lymphocytes and antigen-presenting cells by binding to MHCII molecules and V beta regions in T cell receptors. Therefore, the presentation of tumor cell antigens is increased. Additionally, the production of important cytokines by T cells and APCs contributes to the stimulation of immune response against tumor cells. The manipulation of bacterial vector systems through incorporating genesrelated to SAgs and other immune response factors is a good strategy for the immune system stimulating and eradicating tumor cells along with other immunotherapy agents.

T Cell Immunity to Bacterial Pathogens: Mechanisms of Immune Control and Bacterial Evasion

The human body frequently encounters harmful bacterial pathogens and employs immune defense mechanisms designed to counteract such pathogenic assault. In the adaptive immune system, major histocompatibility complex (MHC)-restricted αβ T cells, along with unconventional αβ or γδ T cells, respond to bacterial antigens to orchestrate persisting protective immune responses and generate immunological memory. Research in the past ten years accelerated our knowledge of how T cells recognize bacterial antigens and how many bacterial species have evolved mechanisms to evade host antimicrobial immune responses. Such escape mechanisms act to corrupt the crosstalk between innate and adaptive immunity, potentially tipping the balance of host immune responses toward pathological rather than protective. This review examines the latest developments in our knowledge of how T cell immunity responds to bacterial pathogens and evaluates some of the mechanisms that pathogenic bacteria use to evade such T cell immunosurveillance, to promote virulence and survival in the host.

Epidemiological and Clinical Evidence for the Role of Toxins in S. aureus Human Disease

Staphylococcus aureus asymptomatically colonizes approximately 30–50% of the population and is a leading cause of bacteremia, bone/joint infections, and skin infections in the US. S. aureus has become a major public health threat due to antibiotic resistance and an increasing number of failed vaccine attempts. To develop new anti-staphylococcal preventive therapies, it will take a more thorough understanding of the current role S. aureus virulence factors play in contributing to human disease. This review focuses on the clinical association of individual toxins with S. aureus infection as well as attempted treatment options. Further understanding of these associations will increase understanding of toxins and their importance to S. aureus pathogenesis.

Allergy-A New Role for T Cell Superantigens of Staphylococcus aureus?

Staphylococcus aureus superantigens (SAgs) are among the most potent T cell mitogens known. They stimulate large fractions of T cells by cross-linking their T cell receptor with major histocompatibility complex class-II molecules on antigen presenting cells, resulting in T cell proliferation and massive cytokine release. To date, 26 different SAgs have been described in the species S. aureus; they comprise the toxic shock syndrome toxin (TSST-1), as well as 25 staphylococcal enterotoxins (SEs) or enterotoxin-like proteins (SEls). SAgs can cause staphylococcal food poisoning and toxic shock syndrome and contribute to the clinical symptoms of staphylococcal infection. In addition, there is growing evidence that SAgs are involved in allergic diseases. This review provides an overview on recent epidemiological data on the involvement of S. aureus SAgs and anti-SAg-IgE in allergy, demonstrating that being sensitized to SEs—in contrast to inhalant allergens—is associated with a severe disease course in patients with chronic airway inflammation. The mechanisms by which SAgs trigger or amplify allergic immune responses, however, are not yet fully understood. Here, we discuss known and hypothetical pathways by which SAgs can drive an atopic disease.

Staphylococcus aureus Isolated from Skin from Atopic-Dermatitis Patients Produces Staphylococcal Enterotoxin Y, Which Predominantly Induces T-Cell Receptor Vα-Specific Expansion of T Cells

While investigating the virulence traits of Staphylococcus aureus adhering to the skin of atopic-dermatitis (AD) patients, we identified a novel open reading frame (ORF) with structural similarity to a superantigen from genome sequence data of an isolate from AD skin. Concurrently, the same ORF was identified in a bovine isolate of S. aureus and designated SElY (H. K. Ono, Y. Sato'o, K. Narita, I. Naito, et al., Appl Environ Microbiol 81:7034-7040, 2015, https://doi.org/10.1128/AEM.01873-15). Recombinant SElY_bov had superantigen activity in human peripheral blood mononuclear cells. It further demonstrated emetic activity in a primate animal model, and it was proposed that SElY be renamed SEY (H. K. Ono, S. Hirose, K. Narita, M. Sugiyama, et al., PLoS Pathog 15:e1007803, 2019, https://doi.org/10.1371/journal.ppat.1007803). Here, we investigated the prevalence of the sey gene in 270 human clinical isolates of various origins in Japan. Forty-two strains were positive for the sey gene, and the positive isolates were from patients with the skin diseases atopic dermatitis and impetigo/staphylococcal scalded skin syndrome (SSSS), with a detection rate of ∼17 to 22%. There were three variants of SEY (SEY₁, SEY₂, and SEY₃), and isolates producing SEY variants formed three distinct clusters corresponding to clonal complexes (CCs) 121, 59, and 20, respectively. Most sey ⁺ isolates produced SEY in broth culture. Unlike SEY_bov, the three recombinant SEY variants exhibited stability against heat treatment. SEY predominantly activated human T cells with a particular T-cell receptor (TCR) Vα profile, a unique observation since most staphylococcal enterotoxins exert their superantigenic activities through activating T cells with specific TCR Vβ profiles. SEY may act to induce localized inflammation via skin-resident T-cell activation, facilitating the pathogenesis of S. aureus infection in disrupted epithelial barriers.

Staphylococcus aureus and Host Immunity in Recurrent Furunculosis

Staphylococcus aureus is one of the severest and most persistent bacterial pathogens. The most frequent S. aureus infections include impetigo, folliculitis, furuncles, furunculosis, abscesses, hidradenitis suppurativa, and mastitis. S. aureus produces a great variety of cellular and extracellular factors responsible for its invasiveness and ability to cause pathological lesions. Their expression depends on the growth phase, environmental factors, and location of the infection. Susceptibility to staphylococcal infections is rooted in multiple mechanisms of host immune responses and reactions to bacterial colonization. Immunological and inflammatory processes of chronic furunculosis are based on the pathogenicity of S. aureus as well as innate and acquired immunity. In-depth knowledge about them may help to discover the whole pathomechanism of the disease and to develop effective therapeutic options. In this review, we focus on the S. aureus-host immune interactions in the pathogenesis of recurrent furunculosis according to the most recent experimental and clinical findings.

Manipulation of Innate and Adaptive Immunity by Staphylococcal Superantigens

Staphylococcal superantigens (SAgs) constitute a family of potent exotoxins secreted by Staphylococcus aureus and other select staphylococcal species. SAgs function to cross-link major histocompatibility complex (MHC) class II molecules with T cell receptors (TCRs) to stimulate the uncontrolled activation of T lymphocytes, potentially leading to severe human illnesses such as toxic shock syndrome. The ubiquity of SAgs in clinical S. aureus isolates suggests that they likely make an important contribution to the evolutionary fitness of S. aureus. Although the apparent redundancy of SAgs in S. aureus has not been explained, the high level of sequence diversity within this toxin family may allow for SAgs to recognize an assorted range of TCR and MHC class II molecules, as well as aid in the avoidance of humoral immunity. Herein, we outline the major diseases associated with the staphylococcal SAgs and how a dysregulated immune system may contribute to pathology. We then highlight recent research that considers the importance of SAgs in the pathogenesis of S. aureus infections, demonstrating that SAgs are more than simply an immunological diversion. We suggest that SAgs can act as targeted modulators that drive the immune response away from an effective response, and thus aid in S. aureus persistence.

Immunochemical assay with monoclonal antibodies for detection of staphylococcal enterotoxin H

Staphylococcal enterotoxins cause food poisoning of various degrees of severity. For milk and meat products, there is a high probability of contamination with staphylococcal enterotoxin H (SEH). In this regard specific and sensitive methods are required to be developed for its detection and monitoring. In this work, the gene seh was expressed and a preparation of recombinant toxin was obtained. Using hybridoma technology, a panel of high-affinity monoclonal antibodies (mAbs) to SEH was produced. The antibodies were characterized and shown to have no cross-reactivity towards the main staphylococcal enterotoxins (A, B, C1, D, E, G and I). Based on these mAbs, a method for specific and quantitative detection of SEH was developed in the format of sandwich enzyme immunoassay (linear range, 0.2–3 ng/ml). All the mAbs produced revealed SEH by immunoblotting. Immunochemical analysis of the culture fluids of staphylococcal isolates obtained from the milk of mastitis-infected cows by immunoblotting and sandwich enzyme immunoassay demonstrated the conformity of these methods. Using the developed method, the toxin was revealed in blood serum and liquid food products practically to 100%. From non-liquid foods, it was shown to be extracted to a maximum with a buffer of pH 4.0–4.5.

Probiotic Lactobacilli Modulate Staphylococcus aureus-Induced Activation of Conventional and Unconventional T cells and NK Cells

Lactobacilli are probiotic commensal bacteria and potent modulators of immunity. When present in the gut or supplemented as probiotics, they beneficially modulate ex vivo immune responsiveness. Further, factors derived from several lactobacilli strains act immune regulatory in vitro. In contrast, Staphylococcus aureus (S. aureus) is known to induce excessive T cell activation. In this study, we aimed to investigate S. aureus-induced activation of human mucosal-associated invariant T cells (MAIT cells), γδ T cells, NK cells, as well as of conventional CD4⁺ and CD8⁺ T cells in vitro. Further, we investigated if lactobacilli-derived factors could modulate their activation. PBMC were cultured with S. aureus 161:2 cell-free supernatants (CFS), staphylococcal enterotoxin A or CD3/CD28-beads alone, or in combination with Lactobacillus rhamnosus GG-CFS or Lactobacillus reuteri DSM 17938-CFS and activation of T and NK cells was evaluated. S. aureus-CFS induced IFN-γ and CD107a expression as well as proliferation. Costimulation with lactobacilli-CFS dampened lymphocyte-activation in all cell types analyzed. Preincubation with lactobacilli-CFS was enough to reduce subsequent activation, and the absence of APC or APC-derived IL-10 did not prevent lactobacilli-mediated dampening. Finally, lactate selectively dampened activation of unconventional T cells and NK cells. In summary, we show that molecules present in the lactobacilli-CFS are able to directly dampen in vitro activation of conventional and unconventional T cells and of NK cells. This study provides novel insights on the immune-modulatory nature of probiotic lactobacilli and suggests a role for lactobacilli in the modulation of induced T and NK cell activation.

Staphylococcus aureus-derived factors induce IL-10, IFN-γ and IL-17A-expressing FOXP3+CD161+ T-helper cells in a partly monocyte-dependent manner

Staphylococcus aureus (S. aureus) is a human pathogen as well as a frequent colonizer of skin and mucosa. This bacterium potently activates conventional T-cells through superantigens and it is suggested to induce T-cell cytokine-production as well as to promote a regulatory phenotype in T-cells in order to avoid clearance. This study aimed to investigate how S. aureus impacts the production of regulatory and pro-inflammatory cytokines and the expression of CD161 and HELIOS by peripheral CD4(+)FOXP3(+) T-cells. Stimulation of PBMC with S. aureus 161:2-cell free supernatant (CFS) induced expression of IL-10, IFN-γ and IL-17A in FOXP3(+) cells. Further, CD161 and HELIOS separated the FOXP3(+) cells into four distinct populations regarding cytokine-expression. Monocyte-depletion decreased S. aureus 161:2-induced activation of FOXP3(+) cells while pre-stimulation of purified monocytes with S. aureus 161:2-CFS and subsequent co-culture with autologous monocyte-depleted PBMC was sufficient to mediate activation of FOXP3(+) cells. Together, these data show that S. aureus potently induces FOXP3(+) cells and promotes a diverse phenotype with expression of regulatory and pro-inflammatory cytokines connected to increased CD161-expression. This could indicate potent regulation or a contribution of FOXP3(+) cells to inflammation and repression of immune-suppression upon encounter with S. aureus.

Superantigen-Producing Staphylococcus aureus Elicits Systemic Immune Activation in a Murine Wound Colonization Model

Staphylococcus aureus, the most common cause of wound infection, produces several exotoxins, including superantigens (SAgs). SAgs are the potent activators of the immune system. Given this unique property, we hypothesized that SAgs produced by S. aureus in wounds would have local, as well as systemic immunologic effects. We tested our hypothesis using a novel staphylococcal skin wound infection model in transgenic mice expressing HLA-DR3. Skin wounds were left uninfected or colonized with S. aureus strains producing SAgs or an isogenic strain not producing any SAg. Animals with wounds challenged with SAg-producing S. aureus had increased morbidity and lower serum IL-17 levels compared to those challenged with the SAg non-producing S. aureus (p = 0.027 and p = 0.032, respectively). At Day 8 following microbial challenge, compared to mice with uninfected wounds, the proportion of Vβ8⁺CD4⁺ T cells was increased, while the proportion of Vβ8⁺CD8⁺ T cells was decreased only in the spleens of mice challenged with SAg-producing S. aureus (p < 0.001). No such changes were measured in mice challenged with SAg non-producing S. aureus. Lungs, livers and kidneys from mice challenged with SAg-producing, but not SAg non-producing, S. aureus showed inflammatory changes. Overall, SAg-mediated systemic immune activation in wounds harboring S. aureus may have clinical implications.

Structure of the superantigen staphylococcal enterotoxin B in complex with TCR and peptide-MHC demonstrates absence of TCR-peptide contacts

Superantigens are immune-stimulatory toxins produced by Staphylococcus aureus, which are able to interact with host immune receptors to induce a massive release of cytokines, causing toxic shock syndrome and possibly death. In this article, we present the x-ray structure of staphylococcal enterotoxin B (SEB) in complex with its receptors, the TCR and MHC class II, forming a ternary complex. The structure, in combination with functional analyses, clearly shows how SEB adopts a wedge-like position when binding to the β-chain of TCR, allowing for an interaction between the α-chain of TCR and MHC. Furthermore, the binding mode also circumvents contact between TCR and the peptide presented by MHC, which enables SEB to initiate a peptide-independent activation of T cells.

Soluble T cell receptor Vβ domains engineered for high-affinity binding to staphylococcal or streptococcal superantigens

Staphylococcus aureus and group A Streptococcus secrete a collection of toxins called superantigens (SAgs), so-called because they stimulate a large fraction of an individual’s T cells. One consequence of this hyperactivity is massive cytokine release leading to severe tissue inflammation and, in some cases, systemic organ failure and death. The molecular basis of action involves the binding of the SAg to both a T cell receptor (TCR) on a T cell and a class II product of the major histocompatibility complex (MHC) on an antigen presenting cell. This cross-linking leads to aggregation of the TCR complex and signaling. A common feature of SAgs is that they bind with relatively low affinity to the variable region (V) of the beta chain of the TCR. Despite this low affinity binding, SAgs are very potent, as each T cell requires only a small fraction of their receptors to be bound in order to trigger cytokine release. To develop high-affinity agents that could neutralize the activity of SAgs, and facilitate the development of detection assays, soluble forms of the Vβ regions have been engineered to affinities that are up to 3 million-fold higher for the SAg. Over the past decade, six different Vβ regions against SAgs from S. aureus (SEA, SEB, SEC3, TSST-1) or S. pyogenes (SpeA and SpeC) have been engineered for high-affinity using yeast display and directed evolution. Here we review the engineering of these high-affinity Vβ proteins, structural features of the six different SAgs and the Vβ proteins, and the specific properties of the engineered Vβ regions that confer high-affinity and specificity for their SAg ligands.

Pfit is a structurally novel Crohn's disease-associated superantigen

T cell responses to enteric bacteria are important in inflammatory bowel disease. I2, encoded by the pfiT gene of Pseudomonas fluorescens, is a T-cell superantigen associated with human Crohn's disease. Here we report the crystal structure of pfiT at 1.7Å resolution and provide a functional analysis of the interaction of pfiT and its homolog, PA2885, with human class II MHC. Both pfiT and PA2885 bound to mammalian cells and stimulated the proliferation of human lymphocytes. This binding was greatly inhibited by anti-class II MHC HLA-DR antibodies, and to a lesser extent, by anti HLA-DQ and DP antibodies, indicating that the binding was class II MHC-specific. GST-pfiT efficiently precipitated both endogenous and in vitro purified recombinant HLA-DR1 molecules, indicating that pfiT directly interacted with HLA-DR1. Competition studies revealed that pfiT and the superantigen Mycoplasma arthritidis mitogen (MAM) competed for binding to HLA-DR, indicating that their binding sites overlap. Structural analyses established that pfiT belongs to the TetR-family of DNA-binding transcription regulators. The distinct structure of pfiT indicates that it represents a new family of T cell superantigens.

Human inflammatory bowel disease (IBD) is a family of chronic inflammatory disorders of the gastrointestinal tract which affect genetically susceptible individuals. IBD is a lifelong disease involving mostly young people, often severely. Crohn's disease (CD) and ulcerative colitis are the two major forms of IBD. Although the exact cause of these diseases remains unknown, both genetic and environmental factors together play significant roles in the disease pathogenesis. Several lines of evidence implicate commensal bacteria as an important pathogenic element in clinical disease, particularly in CD. We recently identified a novel microbial gene, I2, encoded by Pseudomonas fluorescens, a gram-negative commensal, which may be involved in the pathogenesis of CD. Both molecular and immunological approaches were used to identify the human receptor for the microbial antigen encoded by I2, to characterize the ligand-receptor interactions, and to determine the three-dimensional structure of the microbial gene product. In particular, we show that the pfiT is a T cell superantigen, which may help to explain how microbial flora can trigger immune activation in IBD, and may provide the groundwork for novel therapies to treat CD.

T-large granular lymphocyte leukemia: current molecular concepts

T-large granular lymphocyte (T-LGL) leukemia is a chronic and often indolent T cell lymphoproliferation characterized by extreme expansion of a semi-autonomous cytotoxic T lymphocyte (CTL) clone. Clinically, T-LGL can be associated with various cytopenias; neutropenia constitutes the most frequent manifestation. LGL clone represents a pathologic counterpart of the cytotoxic effector T cell but an abnormal memory CD8 cell seems to provide the supply of the matured LGL population. Analysis of clonal T cell receptor (TCR) rearrangement and complementarity determining region 3 (CDR3) of the TCR beta-chain is a useful tool to investigate clonal expansions, track the frequency of expanded clones and also clinically useful to monitor the response to therapy. The lessons learned from molecular analysis of clonal repertoire support a clinically-derived conclusion that the LGL clone arises in the context of an initially polyclonal immune response or an autoimmune process. Consequently, specific manifestations of T-LGL may be a result of the recognition spectrum of the transformed clone and the cytokines it produces. Due to the often monoclonal manifestation, T-LGL constitutes a suitable model to investigate polyclonal CTL-mediated processes. Application of new technologies, including TCR repertoire analysis by sequencing, clonotypic quantitative PCR and VB flow cytometry facilitate clinical diagnosis and may allow insights into the regulation of TCR repertoire and consequences resulting from the contraction of clonal diversity.

Staphylococcus aureus toxins--their functions and genetics

The outcome of encounters between Staphylococcus (S.) aureus and its human host ranges from life-threatening infection through allergic reactions to symptom-free colonization. The pan-genome of this bacterial species encodes numerous toxins, known or strongly suspected to cause specific diseases or symptoms. Three toxin families are in the focus of this review, namely (i) pore-forming toxins, (ii) exfoliative toxins and (iii) superantigens. The majority of toxin-encoding genes are located on mobile genetic elements (MGEs), resulting in a pronounced heterogeneity in the endowment with toxin genes of individual S. aureus strains. Recent population genomic analysis have provided a framework for an improved understanding of the temporal and spatial scales of the motility of MGEs and their associated toxin genes. The distribution of toxin genes among clonal lineages within the species S. aureus is not random, and phylogenetic (sub-)lineages within clonal complexes feature characteristic toxin signatures. When studying pathogenesis, this lineage association, which is caused by the clonal nature of S. aureus makes it difficult to discriminate effects of specific toxins from contributions of the genetic background and/or other associated genetic factors.

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.

Induction of contact-dependent CD8(+) regulatory T cells through stimulation with staphylococcal and streptococcal superantigens

The bacterial superantigen exotoxins of Staphylococcus aureus and Streptococcus pyogenes are potent stimulators of polyclonal T-cell proliferation. They are the causes of toxic shock syndrome but also induce CD25(+)  FOXP3(+) regulatory cells in the CD4 compartment. Several studies have recently described different forms of antigen-induced regulatory CD8(+) T cells in the context of inflammatory diseases and chronic viral infections. In this paper we show that bacterial superantigens are potent inducers of human regulatory CD8(+) T cells. We used four prototypic superantigens of S. aureus (toxic shock syndrome toxin-1 and staphylococcal enterotoxin A) and Str. pyogenes (streptococcal pyrogenic exotoxins A and K/L). At concentrations below 1 ng/ml each toxin triggers concentration-dependent T-cell receptor Vβ-specific expression of CD25 and FOXP3 on CD8(+) T cells. This effect is independent of CD4(+) T-cell help but requires antigen-presenting cells for maximum effect. The cells also express the activation/regulatory markers cytotoxic T-lymphocyte antigen-4 and glucocorticoid-induced tumour necrosis factor receptor-related protein and skin homing adhesins CD103 and cutaneous lymphocyte-associated antigen. Superantigen-induced CD25(+)  FOXP3(+)  CD8(+) T cells were as potent as freshly prepared naturally occurring CD4(+) regulatory T cells in suppressing proliferation of CD4(+)  CD25(-) T cells in response to anti-CD3 stimulation. Although superantigen-induced CD8(+)  CD25(+) FOXP3(+) express interleukin-10 and interferon-γ their suppressive function is cell contact dependent. Our findings indicate that regulatory CD8(+) T cells may be a feature of acute bacterial infections contributing to immune evasion by the microbe and disease pathogenesis. The presence and magnitude of regulatory CD8(+) T-cell responses may represent a novel biomarker in such infections. Superantigen-induced regulatory CD8(+) T cells also have therapeutic potential.

Temporal expression of staphylococcal enterotoxin h in comparison with accessory gene regulator-dependent and -independent enterotoxins

Using sandwich enzyme-linked immunosorbent assay (ELISA), the production of staphylococcal enterotoxin (SE) H was determined in 22 Staphylococcus aureus isolates bearing the seh gene. Samples of supernatants were taken at four time points corresponding to exponential phase (optical density at 600 nm [OD(600)] 0.3 to 0.6), late exponential phase (OD(600) 2 to 4), early stationary phase (OD(600) 4 to 6), and late stationary phase (OD(600) 7 to 12). In four isolates, SEH was detectable at a very low level at the first time point. In 18 isolates, the earliest SEH production was detected in the late exponential phase. For all isolates, there was an increase of SEH concentration with time. Western blot analysis revealed that SEH production, similar to SEA, started in the early exponential phase (OD(600) ∼ 0.5). Isolates with high SEH productivity, as measured by ELISA, demonstrated a higher seh transcription as well. sec transcription was induced in the stationary phase. An induction in the sea transcript was observed during mid- to late exponential phase. Expression profile of seh was similar to that of sea. We showed that the seh expression profile is similar to that of Agr-independent sea and not to that of Agr-dependent sec genes. SEH can be effectively expressed at low bacterial counts, meaning that even in an environment not favorable for S. aureus growth, seh-bearing strains can pose a risk for food safety.

Narcolepsy with hypocretin/orexin deficiency, infections and autoimmunity of the brain

The loss of hypothalamic hypocretin/orexin (hcrt) producing neurons causes narcolepsy with cataplexy. An autoimmune basis for the disease has long been suspected and recent results have greatly strengthened this hypothesis. Narcolepsy with hcrt deficiency is now known to be associated with a Human Leukocyte Antigen (HLA) and T-cell receptor (TCR) polymorphisms, suggesting that an autoimmune process targets a single peptide unique to hcrt-cells via specific HLA-peptide-TCR interactions. Recent data have shown a robust seasonality of disease onset in children and associations with Streptococcus Pyogenes, and influenza A H1N1-infection and H1N1-vaccination, pointing towards processes such as molecular mimicry or bystander activation as crucial for disease development. We speculate that upper airway infections may be common precipitants of a whole host of CNS autoimmune complications including narcolepsy.

Gram-positive bacterial superantigen outside-in signaling causes toxic shock syndrome

Staphylococcus aureus and Streptococcus pyogenes (group A streptococci) are Gram-positive pathogens capable of producing a variety of bacterial exotoxins known as superantigens. Superantigens interact with antigen-presenting cells (APCs) and T cells to induce T cell proliferation and massive cytokine production, which leads to fever, rash, capillary leak and subsequent hypotension, the major symptoms of toxic shock syndrome. Both S. aureus and group A streptococci colonize mucosal surfaces, including the anterior nares and vagina for S. aureus, and the oropharynx and less commonly the vagina for group A streptococci. However, due to their abilities to secrete a variety of virulence factors, the organisms can also cause illnesses from the mucosa. This review provides an updated discussion of the biochemical and structural features of one group of secreted virulence factors, the staphylococcal and group A streptococcal superantigens, and their abilities to cause toxic shock syndrome from a mucosal surface. The main focus of this review, however, is the abilities of superantigens to induce cytokines and chemokines from epithelial cells, which has been linked to a dodecapeptide region that is relatively conserved among all superantigens and is distinct from the binding sites required for interactions with APCs and T cells. This phenomenon, termed outside-in signaling, acts to recruit adaptive immune cells to the submucosa, where the superantigens can then interact with those cells to initiate the final cytokine cascades that lead to toxic shock syndrome.

The structure of superantigen complexed with TCR and MHC reveals novel insights into superantigenic T cell activation

Superantigens (SAgs) are bacterial toxins that interact with immunoreceptors, T cell receptor (TCR) and major histocompatibility complex (MHC) class II, conventionally through the variable β-domain of TCR (TCRVβ). They induce a massive release of cytokines, which can lead to diseases such as food poisoning and toxic shock syndrome. In this study, we report the X-ray structure of the ternary complex between staphylococcal enterotoxin H (SEH) and its human receptors, MHC class II and TCR. The structure demonstrates that SEH predominantly interacts with the variable α-domain of TCR (TCRVα), which is supported by nuclear magnetic resonance (NMR) analyses. Furthermore, there is no contact between MHC and TCR upon complex formation. Structural analyses suggest that the major contact points to TCRVα are conserved among other bacterial SAgs. Consequently, a new dimension of SAg biology emerges, suggesting that in addition to the conventional interactions with the TCRVβ domain, SAgs can also activate T cells through the TCRVα domain.

Superantigen-induced proliferation of human CD4+CD25- T cells is followed by a switch to a functional regulatory phenotype

Bacterial superantigens are potent T cell activators. In humans they cause toxic shock and scarlet fever, and they are implicated in Kawasaki's disease, autoimmunity, atopy, and sepsis. Their function remains unknown, but it may be to impair host immune responses increasing bacterial carriage and transmission. Regulatory (CD25(+)FOXP3(+)) T cells (Tregs) play a role in controlling inflammatory responses to infection. Approximately 2% of circulating T cells are naturally occurring Tregs (nTregs). Conventional Ag stimulation of naive FOXP3(-) T cells induces Ag-specific Tregs. Polyclonal T cell activation has been shown to produce non-Ag-specific Tregs. Because superantigens are unique among microbial virulence factors in their ability to trigger polyclonal T cell activation, we wanted to determine whether superantigen stimulation of T cells could induce non-Ag-specific Tregs. We assessed the effect of superantigen stimulation of human T cells on activation, regulatory markers, and cytokine production by flow cytometry and T cell suppression assays. Stimulation of PBMCs with staphylococcal exotoxin A and streptococcal pyrogenic exotoxins A and K/L resulted in dose-dependent FOXP3 expression. Characterization of this response for streptococcal pyrogenic exotoxin K/L confirmed its Vβ specificity, that CD25(+)FOXP3(+) cells arose from CD25(-) T cells and required APCs. These cells had increased CTLA-4 and CD127 expression, typical of the recently described activated converted Treg-like cells, and exhibited functional suppressor activity comparable to nTregs. Superantigen-stimulated CD25(+)FOXP3(+) T cells expressed IL-10 at lower superantigen concentrations than was required to trigger IFN-γ production. This study provides a mechanism for bacterial evasion of the immune response through the superantigen induction of Tregs.

Food poisoning and Staphylococcus aureus enterotoxins

Staphylococcus aureus produces a wide variety of toxins including staphylococcal enterotoxins (SEs; SEA to SEE, SEG to SEI, SER to SET) with demonstrated emetic activity, and staphylococcal-like (SEl) proteins, which are not emetic in a primate model (SElL and SElQ) or have yet to be tested (SElJ, SElK, SElM to SElP, SElU, SElU2 and SElV). SEs and SEls have been traditionally subdivided into classical (SEA to SEE) and new (SEG to SElU2) types. All possess superantigenic activity and are encoded by accessory genetic elements, including plasmids, prophages, pathogenicity islands, vSa genomic islands, or by genes located next to the staphylococcal cassette chromosome (SCC) implicated in methicillin resistance. SEs are a major cause of food poisoning, which typically occurs after ingestion of different foods, particularly processed meat and dairy products, contaminated with S. aureus by improper handling and subsequent storage at elevated temperatures. Symptoms are of rapid onset and include nausea and violent vomiting, with or without diarrhea. The illness is usually self-limiting and only occasionally it is severe enough to warrant hospitalization. SEA is the most common cause of staphylococcal food poisoning worldwide, but the involvement of other classical SEs has been also demonstrated. Of the new SE/SEls, only SEH have clearly been associated with food poisoning. However, genes encoding novel SEs as well as SEls with untested emetic activity are widely represented in S. aureus, and their role in pathogenesis may be underestimated.

Backbone resonance assignment of Staphylococcal Enterotoxin H

The staphylococcal enterotoxin H (SEH; 217 aa, 25 kD) belongs to a family of superantigens that cause a massive immune response upon simultaneous binding to the T cell receptor (TCR) and the major histocompatibility complex class II. The SEH-TCR interaction is weak and amenable to studies using NMR methodology. Essentially, 2 mg of U{(2)H, (13)C,(15)N}-labeled SEH was used for the complete sequential backbone assignment of SEH at 900 MHz. The protein secondary structure inferred from the chemical shift index (C(alpha) and C(beta)) is in very good agreement with the secondary structure elements of the X-ray structure.

Staphylococcus aureus superantigens elicit redundant and extensive human Vbeta patterns

Staphylococcus aureus can produce a wide variety of exotoxins, including toxic shock syndrome toxin 1 (TSST-1), staphylococcal enterotoxins, and staphylococcal enterotoxin-like toxins. These toxins share superantigenic activity. To investigate the beta chain (Vbeta) specificities of each of these toxins, TSST-1 and all known S. aureus enterotoxins and enterotoxin-like toxins were produced as recombinant proteins and tested for their ability to induce the selective in vitro expansion of human T cells bearing particular Vbeta T-cell receptors (TCR). Although redundancies were observed between the toxins and the Vbeta populations, each toxin induced the expansion of distinct Vbeta subsets, including enterotoxin H and enterotoxin-like toxin J. Surprisingly, the Vbeta signatures were not associated with a specific phylogenic group of toxins. Interestingly, each human Vbeta analyzed in this study was stimulated by at least one staphylococcal superantigen, suggesting that the bacterium derives a selective advantage from targeting the entire human TCR Vbeta panel.

Skew in T cell receptor usage with polyclonal expansion in lesions of oral lichen planus without hepatitis C virus infection

Oral lichen planus (OLP) is a refractory disorder of the oral mucosa. Its predominant symptoms are pain and haphalgesia that impair the quality of life of patients. OLP develops via a T cell-mediated immune process. Here, we examined the characteristics of the infiltrating T cells in terms of the T cell receptor (TCR) repertoires, T cell clonality, T cell phenotypes and cytokine production profiles. TCR repertoire analyses and CDR3 size spectratyping were performed using peripheral blood mononuclear cells (PBMCs) and tissue specimens of OLP biopsies from 12 patients. The cytokine expression profiles and T cell phenotypes were measured by real-time quantitative polymerase chain reaction. We observed that there were skewed TCR repertoires in the tissue samples (TCRVA8-1, VA22-1, VB2-1, VB3-1 and VB5-1) and PBMCs (TCRVA8-1, VB2-1, VB3-1 and VB5-1) from OLP patients. Furthermore, the CDR3 distributions in the skewed TCR subfamilies exhibited polyclonal patterns. We observed increases in CD4(+) T lymphocytes, interleukin (IL)-5, tumour necrosis factor (TNF)-alpha and human leucocyte antigen D-related in the OLP tissue specimens. Taken together, the present results suggest that T cells bearing these TCRs are involved in the pathogenesis of OLP, and that IL-5 and TNF-alpha may participate in its inflammatory process.

Identification and characterization of two novel staphylococcal enterotoxins, types S and T

In addition to two known staphylococcal enterotoxin-like genes (selj and selr), two novel genes coding for two superantigens, staphylococcal enterotoxins S and T (SES and SET), were identified in plasmid pF5, which is harbored by food poisoning-related Staphylococcus aureus strain Fukuoka 5. This strain was implicated in a food poisoning incident in Fukuoka City, Japan, in 1997. Recombinant SES (rSES) specifically stimulated human T cells in a T-cell receptor Vbeta9- and Vbeta16-specific manner in the presence of major histocompatibility complex (MHC) class II(+) antigen-presenting cells (APC). rSET also stimulated T cells in the presence of MHC class II(+) APC, although its Vbeta skewing was not found in reactive T cells. Subsequently, we examined the emetic activity of SES and SET. We also studied SElR to determine emetic activity in primates. This toxin was identified in previous studies but was not examined in terms of possession of emetic activity for primates. rSES induced emetic reactions in two of four monkeys at a dose of 100 microg/kg within 5 h of intragastric administration. In one monkey, rSET induced a delayed reaction (24 h postadministration) at a dose of 100 microg/kg, and in the other one, the reaction occurred 5 days postadministration. rSElR induced a reaction in two of six animals within 5 h at 100 microg/kg. On this basis, we speculate that the causative toxins of vomiting in the Fukuoka case are SES and SER. Additionally, SES, SER, and SET also induced emesis in house musk shrews as in the monkeys.

Comprehensive analysis of the functional TCR repertoire at the single-cell level

A Vbeta TCR repertoire is analyzed for understanding the T-cell population in the immune response. However, the TCR repertoire of the Valpha-Vbeta pair is difficult to analyze because no suitable analytical method is available. Here, we have applied the single-cell 5'-RACE method for amplifying TCR cDNAs from single T-cells and analyzed the repertoire of Valpha-Vbeta pairs in human T-cells that responded to a superantigen, SEB. We found that the TCR Vbeta profile of the SEB-stimulated CD4(+) T-cells was in accordance with the previous reports, that the TCR Valpha profile also exhibited a prominent difference, and that the TCR Valpha-Vbeta pairs of the SEB-responding T-cells were promiscuous. We have also found a significant dual TCRalpha expression in single T-cells. This is the first report of a comprehensive analysis of the functional repertoire of Valpha-Vbeta pairs at the single T-cell level. This novel method may contribute to TCR-based immunotherapeutics.

Staphylococcal enterotoxins G and I, a cause of severe but reversible neonatal enteropathy

BACKGROUND & AIMS

Staphylococcus aureus is recognized to produce toxins A-E and toxic shock syndrome toxin-1 associated with food poisoning and toxic shock syndrome. Enterotoxins G and I co-exist in the same S aureus strains (staphylococcal enterotoxin G and staphylococcal enterotoxin I) and are implicated in scarlet fever and toxic shock. We report these enterotoxins as causative agents of 2 cases of neonatal intractable diarrhea with enteropathy.

METHODS

We used a note review for this study. Stool culture, multiplex polymerase chain reaction for enterotoxin, duodenal biopsy specimens for H&E, periodic acid-Schiff staining, and electron microscopy were used.

RESULTS

Infant 1 had diarrhea from age 2 weeks and was referred at age 5 weeks with weight less than the 0.4th percentile. Infant 2 was referred at age 7 weeks with 4 weeks' of diarrhea, weight less than the 0.4th percentile. Both infants were severely malnourished. Elemental feeds were not tolerated and total parenteral nutrition was required. S aureus producing staphylococcal enterotoxin G and staphylococcal enterotoxin I was isolated in stools from both infants. Clinical improvement occurred after intravenous flucloxacillin and parenteral nutrition. Histology showed subtotal villous atrophy (H&E) with abnormal brush border (periodic acid-Schiff). Electron microscopy showed severe microvilli destruction, dilated mitochondria, and lysosomes containing cellular debris. Repeat histology was normal in infant 2, age 3 months, off parenteral nutrition, showed return to normal. Currently, both infants are 2 years of age and are thriving on a normal diet.

CONCLUSIONS

Staphylococcal enterotoxin G- and I-induced enteropathy is a life-threatening condition, causing reversible disruption of enterocyte ultrastructure that responds well to supportive treatment with flucloxacillin and parenteral nutrition This condition should be a differential diagnosis of neonatal early onset diarrhea.

HLA class II transgenic mice mimic human inflammatory diseases

Population studies have shown that among all the genetic factors linked with autoimmune disease development, MHC class II genes on chromosome 6 accounts for majority of familial clustering in the common autoimmune diseases. Despite the highly polymorphic nature of HLA class II genes, majority of autoimmune diseases are linked to a limited set of class II-DR or -DQ alleles. Thus a more detailed study of these HLA-DR and -DQ alleles were needed to understand their role in genetic predisposition and pathogenesis of autoimmune diseases. Although in vitro studies using class-II restricted CD4 T cells and purified class II molecules have helped us in understanding some aspects of HLA class-II association with disease, it is difficult to study the role of class II genes in vivo because of heterogeneity of human population, complexity of MHC, and strong linkage disequilibrium among different class II genes. To overcome this problem, we pioneered the generation of HLA-class II transgenic mice to study role of these molecule in inflammatory disease. These HLA class II transgenic mice were used to develop novel in vivo disease model for common autoimmune diseases such as rheumatoid arthritis, multiple sclerosis, insulin-dependent diabetes mellitus, myasthenia gravis, celiac disease, autoimmune relapsing polychondritis, autoimmune myocarditis, thyroiditis, uveitis, as well as other inflammatory disease such as allergy, tuberculosis and toxic shock syndrome. As the T-cell repertoire in these humanized HLA transgenic mice are shaped by human class II molecules, they show the same HLA restriction as humans, implicate potential triggering mechanism and autoantigens, and identify similar antigenic epitopes seen in human. This review describes the value of these humanized transgenic mice in deciphering role of HLA class II molecules in immunopathogenesis of inflammatory diseases.

Distribution of enterotoxin genes among carriage- and infection-associated isolates of Staphylococcus aureus

AIMS

To compare the distribution of genes encoding classical and newly described enterotoxins among Staphylococcus aureus, associated with carriage and infection.

METHODS AND RESULTS

Forty-five nasal isolates from carriers and 42 clinical isolates were included. The genes sea to see and seg to sei as well as sem, sen, seo and seu were tested using multiplex and conventional PCR. The most frequently found toxin genes were egc-related genes, in particular the combination seg and sei (n = 55, 63.1%), followed by sen and seu (n = 54, 62.1%), sem (n = 51, 58.6%) and seo (n = 48, 55.2%). Significant differences were found for seg and sei combination (33 of the nasal vs 22 of the infection isolates, P = 0.048) as well as for the genes sem (P = 0.004), sen (P = 0.029) and seo (P = 0.032). Regarding the classical toxin genes no significant differences between the two groups of isolates were found.

CONCLUSIONS

Significant differences between infection and carriage strains were found only for the egc-related genes, which were more common in the nasal isolates.

SIGNIFICANCE AND IMPACT OF THE STUDY

The egc-related enterotoxin genes seem to be more prevalent in carriage- than in infection-associated S. aureus isolates. The possible contribution of egc-related genes in determining the potential for nasal carriage requires further investigation.

Cutting Edge: Evidence of Direct TCR α-Chain Interaction with Superantigen

Superantigens are known to activate a large number of T cells. The SAg is presented by MHC class II on the APC and its classical feature is that it recognizes the variable region of the beta-chain of the TCR. In this article, we report, by direct binding studies, that staphylococcal enterotoxin (SE) H (SEH), a bacterial SAg secreted by Staphylococcus aureus, instead recognizes the variable alpha-chain (TRAV27) of TCR. Furthermore, we show that different SAgs (e.g., SEH and SEA) can simultaneously bind to one TCR by binding the alpha-chain and the beta-chain, respectively. Theoretical three-dimensional models of the penta complexes are presented. Hence, these findings open up a new dimension of the biology of the staphylococcal enterotoxins.

TCR recognition of peptide/MHC class II complexes and superantigens

Major histocompatibility complex (MHC) class II molecules display peptides to the T cell receptor (TCR). The ability of the TCR to discriminate foreign from self-peptides presented by MHC molecules is a requirement of an effective adaptive immune response. Dysregulation of this molecular recognition event often leads to a disease state. Recently, a number of structural studies have provided significant insight into several such dysregulated interactions between peptide/MHC complexes and TCR molecules. These include TCR recognition of self-peptides, which results in autoimmune reactions, and of mutant self-peptides, common in the immunosurveillance of tumors, as well as the engagement of TCRs by superantigens, a family of bacterial toxins responsible for toxic shock syndrome.

Crystal structure of a complete ternary complex of TCR, superantigen and peptide-MHC

'Superantigens' (SAgs) trigger the massive activation of T cells by simultaneous interactions with MHC and TCR receptors, leading to human diseases. Here we present the first crystal structure, at 2.5-A resolution, of a complete ternary complex between a SAg and its two receptors, HLA-DR1/HA and TCR. The most striking finding is that the SAg Mycoplasma arthritidis mitogen, unlike others, has direct contacts not only with TCR Vbeta but with TCR Valpha.

Molecular Basis of TCR Selectivity, Cross-Reactivity, and Allelic Discrimination by a Bacterial Superantigen: Integrative Functional and Energetic Mapping of the SpeC-Vβ2.1 Molecular Interface

Superantigens activate large fractions of T cells through unconventional interactions with both TCR beta-chain V domains (Vbetas) and MHC class II molecules. The bacterial superantigen streptococcal pyrogenic exotoxin C (SpeC) primarily stimulates human Vbeta2(+) T cells. Herein, we have analyzed the SpeC-Vbeta2.1 interaction by mutating all SpeC residues that make contact with Vbeta2.1 and have determined the energetic and functional consequences of these mutations. Our comprehensive approach, including mutagenesis, functional readouts from both bulk T cell populations, and an engineered Vbeta2.1(+) Jurkat T cell, as well as surface plasmon resonance binding analysis, has defined the SpeC "functional epitope" for TCR engagement. Although only two SpeC residues (Tyr(15) and Arg(181)) are critical for activation of virtually all human CD3(+) T cells, a larger cluster of four hot spot residues are required for interaction with Vbeta2.1. Three of these residues (Tyr(15), Phe(75), and Arg(181)) concentrate their binding energy on the CDR2 loop residue Ser(52a), a noncanonical residue insertion found only in Vbeta2 and Vbeta4 chains. Plasticity of this loop is important for recognition by SpeC. Although SpeC interacts with the Vbeta2.1 hypervariable CDR3 loop, our data indicate these contacts have little to no influence on the functional interaction with Vbeta2.1. These studies also provide a molecular basis for selectivity and cross-reactivity of SpeC-TCR recognition and reveal a degree of fine specificity in these interactions, whereby certain SpeC mutants are capable of distinguishing between different alleles of the same Vbeta domain subfamily.

Intranasal exposure to bacterial superantigens induces airway inflammation in HLA class II transgenic mice

Staphylococcus aureus is widely prevalent in the nasopharynges of healthy individuals (carriers) but can also cause serious infections. S. aureus can elaborate a variety of superantigen exotoxins in "carrier" or "pathogenic" states. Streptococcus pyogenes can also colonize the nasopharynx and elaborate superantigens. Unlike the acute effects of superantigen exotoxins absorbed through the gut or vaginal mucosa, little is known regarding the pathogenesis of superantigens entering through the intranasal route. In the current study, we evaluated the local and systemic effects of staphylococcal enterotoxin B (SEB) and streptococcal pyrogenic exotoxin A (SPEA) delivered through the intranasal route. Superantigens were administered intranasally on multiple occasions, and experimental animals were sacrificed on day 8 for experimental analyses. SEB-induced airway inflammation was more pronounced for HLA-DR3 transgenic mice than for BALB/c mice, consistent with bacterial superantigens binding more efficiently to human than murine major histocompatibility complex class II. The nature of the airway inflammation in HLA-DR3 mice was determined by the concentration of SEB applied intranasally. Low concentrations (20 ng) induced eosinophilic airway inflammation as well as eosinophil degranulation, whereas intranasal exposure to higher concentrations (2,000 ng) resulted in neutrophilic airway inflammation, permanent airway destruction, toxic shock, and mortality. SEB-induced eosinophilic inflammatory response was enhanced in signal transducer and activator of transcription (STAT)-4-deficient HLA-DQ8 transgenic mice with defective interleukin-12 signaling. Intranasal administration of SPEA induced airway inflammation and systemic immune activation in HLA-DQ8 transgenic mice. In conclusion, repeated chronic intranasal exposure to bacterial superantigens causes airway inflammation and systemic immune activation.

Evidence for Restricted Vβ Usage in the Leukemic Phase of Cutaneous T Cell Lymphoma

Antibodies directed against the beta chain of the T cell receptor (anti-Vbeta antibodies) are useful to identify the Vbeta repertoire of T cells in various diseases and to quantify numbers of Vbeta-bearing T cells. The goals of this study were to identify Vbeta+ cases of leukemic phase cutaneous T cell lymphoma (CTCL) and to compare the percentage of positive calls with other measures of blood tumor burden, i.e., lymphocyte subsets with a CD4+CD7- and CD4+CD26- phenotype and Sezary cell counts. Thirty-three of 49 (67%) cases of leukemic CTCL reacted with an anti-Vbeta antibody. When combined with reports from the literature, the frequency of Vbeta5 (probably Vbeta5.1) usage was relatively high when compared with Vbeta2 that is also frequently expressed by normal CD4+ T cells. The percentage of Vbeta+ cells correlated to the percentage of CD4+CD7- and CD4+CD26- cells for cases in which the neoplastic cells were deficient in expression of CD7 and CD26, respectively, but not the Sezary cell count. We hypothesize that the increased Vbeta5.1 usage in CTCL may be the result of depletion of Vbeta2 and other Vbeta-bearing T cells by staphylococcal superantigens prior to neoplastic transformation, resulting in a relative increase in the frequency of Vbeta5.1 usage in CTCL.

Superantigens: structure-function relationships

Superantigens are a class of highly potent immuno-stimulatory molecules produced by Staphylococcus aureus and Streptococcus pyogenes. These toxins possess the unique ability to interact simultaneously with MHC class II molecules and T-cell receptors, forming a trimolecular complex that induces profound T-cell proliferation. The resultant massive cytokine release causes epithelial damage and leads to capillary leak and hypotension. The staphylococcal superantigens are designated staphylococcal enterotoxins A, B, C (and antigenic variants), D, E, and the recently discovered enterotoxins G to Q, and toxic shock syndrome toxin-1. The streptococcal superantigens include the pyrogenic exotoxins A (and antigenic variants), C, G-J, SMEZ, and SSA. Superantigens are implicated in several diseases including toxic shock syndrome, scarlet fever and food poisoning; and their function appears primarily to debilitate the host sufficiently to permit the causation of disease. Structural studies over the last 10 years have provided a great deal of information regarding the complex interactions of these molecules with their receptors. This, combined with the wealth of new information from genomics initiatives, have shown that, despite their common molecular architecture, superantigens are able to crosslink MHC class II molecules and T-cell receptors by a variety of subtly different ways through the use of various structural regions within each toxin.

Interplay Between Superantigens and Immunoreceptors

Superantigens (SAGs) cause a massive T-cell proliferation by simultaneously binding to major histocompatibility complex (MHC) class II on antigen-presenting cells and T-cell receptors (TCRs) on T cells. These T-cell mitogens can cause disease in host, such as food poisoning or toxic shock. The best characterized groups of SAGs are the bacterial SAGs secreted by Staphylococcus aureus and Streptococcus pyogenes. Despite a common overall three-dimensional fold of these SAGs, they have been shown to bind to MHC class II in different ways. Recently, it has also been shown that SAGs have individual preferences in their binding to the TCRs. They can interact with various regions of the variable beta-chain of TCRs and at least one SAG seems to bind to the alpha-chain of TCRs. In this review, different subclasses of SAGs are classified based upon their binding mode to MHC class II, and models of trimolecular complexes of MHC-SAG-TCR molecules are described in order to reveal and understand the complexity of SAG-mediated T-cell activation.

Identification of the antigenic epitopes in staphylococcal enterotoxins A and E and design of a superantigen for human cancer therapy

Monoclonal antibodies have a potential for cancer therapy that may be further improved by linking them to effector molecules such as superantigens. Tumor targeting of a superantigen leads to a powerful T cell attack against the tumour tissue. Encouraging results have been observed preclinically and in patients using the superantigen staphylococcal enterotoxin A, SEA. To further improve the concept, we have reduced the reactivity to antibodies against superantigens, which is found in all individuals. Using epitope mapping, antibody binding sites in SEA and SEE were found around their MHC class II binding sites. These epitopes were removed genetically and a large number of synthetic superantigens were produced in an iterative engineering procedure. Properties such as decreased binding to anti-SEA as well as higher selectivity to induce killing of tumour cells compared to MHC class II expressing cells, were sequentially improved. The lysine residues 79, 81, 83 and 84 are all part of major antigenic epitopes, Gln204, Lys74, Asp75 and Asn78 are important for optimal killing of tumour cells while Asp45 affects binding to MHC class II. The production properties were optimised by further engineering and a novel synthetic superantigen, SEA/E-120, was designed. It is recognised by approximately 15% of human anti-SEA antibodies and have more potent tumour cell killing properties than SEA. SEA/E-120 is likely to have a low toxicity due to its reduced capacity to mediate killing of MHC class II expressing cells. It is produced as a Fab fusion protein at approximately 35 mg/l in Escherichia coli.