Enterotoxin residues determining T-cell receptor V beta binding specificity

Molecular Characteristics and Pathogenicity of Staphylococcus aureus Exotoxins

Staphylococcus aureus stands as one of the most pervasive pathogens given its morbidity and mortality worldwide due to its roles as an infectious agent that causes a wide variety of diseases ranging from moderately severe skin infections to fatal pneumonia and sepsis. S. aureus produces a variety of exotoxins that serve as important virulence factors in S. aureus-related infectious diseases and food poisoning in both humans and animals. For example, staphylococcal enterotoxins (SEs) produced by S. aureus induce staphylococcal foodborne poisoning; toxic shock syndrome toxin-1 (TSST-1), as a typical superantigen, induces toxic shock syndrome; hemolysins induce cell damage in erythrocytes and leukocytes; and exfoliative toxin induces staphylococcal skin scalded syndrome. Recently, Panton-Valentine leucocidin, a cytotoxin produced by community-associated methicillin-resistant S. aureus (CA-MRSA), has been reported, and new types of SEs and staphylococcal enterotoxin-like toxins (SEls) were discovered and reported successively. This review addresses the progress of and novel insights into the molecular structure, biological activities, and pathogenicity of both the classic and the newly identified exotoxins produced by S. aureus.

The tumor targeted superantigen ABR-217620 selectively engages TRBV7-9 and exploits TCR-pMHC affinity mimicry in mediating T cell cytotoxicity

The T lymphocytes are the most important effector cells in immunotherapy of cancer. The conceptual objective for developing the tumor targeted superantigen (TTS) ABR-217620 (naptumomab estafenatox, 5T4Fab-SEA/E-120), now in phase 3 studies for advanced renal cell cancer, was to selectively coat tumor cells with cytotoxic T lymphocytes (CTL) target structures functionally similar to natural CTL pMHC target molecules. Here we present data showing that the molecular basis for the anti-tumor activity by ABR-217620 resides in the distinct interaction between the T cell receptor β variable (TRBV) 7-9 and the engineered superantigen (Sag) SEA/E-120 in the fusion protein bound to the 5T4 antigen on tumor cells. Multimeric but not monomeric ABR-217620 selectively stains TRBV7-9 expressing T lymphocytes from human peripheral blood similar to antigen specific staining of T cells with pMHC tetramers. SEA/E-120 selectively activates TRBV7-9 expressing T lymphocytes resulting in expansion of the subset. ABR-217620 selectively triggers TRBV7-9 expressing cytotoxic T lymphocytes to kill 5T4 positive tumor cells. Furthermore, ABR-217620 activates TRBV7-9 expressing T cell line cells in the presence of cell- and bead-bound 5T4 tumor antigen. Surface plasmon resonance analysis revealed that ABR-217620 binds to 5T4 with high affinity, to TRBV7-9 with low affinity and to MHC class II with very low affinity. The T lymphocyte engagement by ABR-217620 is constituted by displaying high affinity binding to the tumor cells (K_D approximately 1 nM) and with the mimicry of natural productive immune TCR-pMHC contact using affinities of around 1 µM. This difference in kinetics between the two components of the ABR-217620 fusion protein will bias the binding towards the 5T4 target antigen, efficiently activating T-cells via SEA/E-120 only when presented by the tumor cells.

Assessment of the functional regions of the superantigen staphylococcal enterotoxin B

The functional activity of superantigens is based on capacity of these microbial proteins to bind to both the β-chain of the T cell receptor (TcR) and the major histocompatibility complex (MHC) class II dimer. We have previously shown that a subset of the bacterial superantigens also binds to a membrane protein, designated p85, which is expressed by renal epithelial cells. This binding activity is a property of SEB, SEC1, 2 and 3, but not SEA, SED, SEE or TSST. The crystal structure of the tri-molecular complex of the superantigen staphylococcal enterotoxin B (SEB) with both the TcR and class II has previously been reported. However, the relative contributions of regions of the superantigen to the overall functional activity of this superantigen remain undefined. In an effort to better define the molecular basis for the interaction of SEB with the TcR β-chain, we report studies here which show the comparative contributions of amino- and carboxy-terminal regions in the superantigen activity of SEB. Recombinant fusion proteins composed of bacterial maltose-binding protein linked to either full-length or truncated toxins in which the 81 N-terminal, or 19 or 34 C-terminal amino acids were deleted, were generated for these studies. This approach provides a determination of the relative strength of the functional activity of the various regions of the superantigen protein.

Human TGFalpha-derived peptide TGFalphaL3 fused with superantigen for immunotherapy of EGFR-expressing tumours

Background

Monoclonal antibodies have been employed as targeting molecules of superantigen for the preclinical treatment of a variety of tumours. However, other targeting molecules, such as tumour-related ligands or peptides, are less exploited. Here, we tested other targeting molecules by genetically fusing the third loop of transforming growth factor alpha (TGFalphaL3) to mutant staphylococcal enterotoxin A (SEA_D227A).

Results

The resultant fusion proteins were expressed in E. coli and purified to homogeneity through a Ni-NTA affinity column. Fusion protein TGFalphaL3SEA_D227A can promote splenocyte proliferation to a level comparable to recombinant SEA (rSEA) and bind to EGFR-expressing tumour cells in an EGFR-dependent way. Consistent with these observations, TGFalphaL3SEA_D227A exerted an inhibitory effect on the growth of EGFR-expressing tumour cells both in vitro and in vivo. Notably, significant infiltrations of CD8⁺ and CD4⁺ T cells were detected in the tumour tissues of these C57BL/6 mice treated with TGFalphaL3SEA_D227A, suggesting the involvement of T cells in this tumour-inhibitory process.

Conclusions

The data here showed that TGFαL3 is capable of targeting superantigen to tumours and exerting an inhibitory effect on tumour growth, which enables TGFαL3SEA_D227A to be an attractive candidate for the immunotherapy of EGFR-expressing tumours.

High affinity of interaction between superantigen and T cell receptor Vbeta molecules induces a high level and prolonged expansion of superantigen-reactive CD4+ T cells

In mice implanted with an osmotic pump filled with the superantigen (SAG) staphylococcal enterotoxin A (SEA), the Vβ3(+)CD4(+) T cells exhibited a high level of expansion whereas the Vβ11(+)CD4(+) T cells exhibited a mild level of expansion. In contrast, in mice implanted with an osmotic pump filled with SE-like type P (SElP, 78.1% homologous with SEA), the Vβ11(+)CD4(+) T cells exhibited a high level of expansion while the Vβ3(+)CD4(+) T cells exhibited a low level of expansion, suggesting that the level of the SAG-induced response is determined by the affinities between the TCR Vβ molecules and SAG. Analyses using several hybrids of SEA and SElP showed that residue 206 of SEA determines the response levels of Vβ3(+)CD4(+) and Vβ11(+)CD4(+) T cells both in vitro and in vivo. Analyses using the above-mentioned hybrids showed that the binding affinities between SEA and the Vβ3/Vβ11 β chains and between SEA-MHC class II-molecule complex and Vβ3(+)/Vβ11(+) CD4(+) T cells determines the response levels of the SAG-reactive T cells both in vitro and in vivo.

Phase I dose escalation, pharmacokinetic and pharmacodynamic study of naptumomab estafenatox alone in patients with advanced cancer and with docetaxel in patients with advanced non-small-cell lung cancer

PURPOSE

Two phase I studies were conducted of ABR-217620 alone or in combination with docetaxel. This is a recombinant fusion protein consisting of a mutated variant of the superantigen staphylococcal enterotoxin E (SEA/E-120) linked to fragment antigen binding moiety of a monoclonal antibody recognizing the tumor-associated antigen 5T4.

PATIENTS AND METHODS

Patients with non-small-cell lung cancer (NSCLC), pancreatic cancer (PC), and renal cell cancer (RCC) received 5 daily boluses of ABR-217620 (3-month cycles) in escalating doses to determine the maximum-tolerated dose (MTD; ABR-217620 dose escalation monotherapy [MONO] study). Doses were selected based on individual patient anti-SEA/E-120 titers pretreatment. Patients with NSCLC received 4 daily, escalating doses of ABR-217620 followed by docetaxel in 21-day cycles (ABR-217620 dose escalation combination with docetaxel [COMBO] study).

RESULTS

Thirty-nine patients were enrolled in the MONO study and 13 were enrolled in the COMBO study. The monotherapy MTD was 26 microg/kg (NSCLC and PC) and 15 microg/kg (RCC). Dose-limiting toxicities (DLTs) in the MONO study were fever, hypotension, acute liver toxicity, and vascular leak syndrome. In the COMBO study, the MTD was 22 microg/kg (neutropenic sepsis). Adverse events included grade 1 to 2 fever, hypotension, nausea, and chills. Treatment caused a systemic increase of inflammatory cytokines and selective expansion of SEA/E-120 reactive T-cells. Tumor biopsies demonstrated T-cell infiltration after therapy. Fourteen patients (36%) had stable disease (SD) on day 56 of the MONO study. Two patients (15%) in the COMBO study had partial responses, one in a patient with progressive disease on prior docetaxel, and five patients (38%) had SD on day 56.

CONCLUSION

ABR-217620 was well tolerated with evidence of immunological activity and antitumor activity.

Inactivated parapoxvirus ovis activates canine blood phagocytes and T lymphocytes

Inactivated parapoxvirus ovis (iPPVO) shows strong immunomodulatory activities in several species and is used in veterinary medicine as an immunostimulatory biological for the prevention and/or treatment of infectious diseases. In this study the immunostimulatory capacity of iPPVO on the innate immune system was investigated in vitro by the evaluation of induction of the oxidative burst and modulation of phagocytosis by canine blood leukocytes (polymorphonuclear cells and monocytes) of dogs. In addition, the activation of canine T lymphocytes was also studied. After stimulation with iPPVO the phagocytosis of FITC-labeled Listeria monocytogenes was increased in canine blood monocytes and neutrophils. Enhanced burst rates by canine monocytes stimulated with iPPVO were observed and the MHC-II expression on canine CD14+ monocytes was elevated following stimulation with iPPVO compared to the stabiliser control. Canine CD4+ T cells were activated for oligoclonal proliferation in response to iPPVO. This study shows that iPPVO is able to stimulate both phagocytotic and T-cell-dependent immune mechanisms in canine blood leukocytes.

Differential diagnosis of eosinophilic chronic rhinosinusitis

Eosinophilic chronic rhinosinusitis (ECRS) encompasses a wide variety of etiologies. To date, a unifying pathophysiologic mechanism remains elusive. Eosinophilia is frequently, but not exclusively, caused by immunoglobulin (Ig)E-mediated hypersensitivity and is dominated by the associated cytokine milieu of Th2 inflammation. The provisional subcategories of ECRS include superantigen-induced eosinophilic chronic rhinosinusitis, allergic fungal sinusitis, nonallergic fungal eosinophilic chronic rhinosinusitis, and aspirin-exacerbated eosinophilic chronic rhinosinusitis. Within each subcategory, recent findings supporting distinct mechanisms that promote eosinophilic infiltration are presented, and, therefore, targeted therapeutic interventions with specific antibacterial, antifungal, or immune modulation may be indicated.

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

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

HIV-1 Viral Protein-R (VPR) Protects against Lethal Superantigen Challenge While Maintaining Homeostatic T Cell Levels in Vivo

The HIV-1 accessory protein Vpr exhibits many interesting features related to macrophage and T cell biology. As a viral protein or as a soluble molecule it can suppress immune cell activation and cytokine production in vitro in part by targeted inhibition of NF-kappaB. In this regard we sought to test its effects in vivo on an NF-kappaB-dependent immune pathway. We examined the activity of Vpr in a lethal toxin-mediated challenge model in mice. Intravenous delivery of Vpr was sufficient to protect mice from lethal challenge with staphylococcal endotoxin B (SEB). Furthermore, Vpr protected host CD4+ T cells from in vivo depletion likely by preventing induction of AICD of SEB-exposed cells in a post-toxin-binding fashion. Understanding the biology of Vpr's activities in this model may allow for new insight into potential mechanisms of hyperinflammatory disease and into Vpr pathobiology in the context of HIV infection.

The crystal structures of EAP domains from Staphylococcus aureus reveal an unexpected homology to bacterial superantigens

The Eap (extracellular adherence protein) of Staphylococcus aureus functions as a secreted virulence factor by mediating interactions between the bacterial cell surface and several extracellular host proteins. Eap proteins from different Staphylococcal strains consist of four to six tandem repeats of a structurally uncharacterized domain (EAP domain). We have determined the three-dimensional structures of three different EAP domains to 1.8, 2.2, and 1.35 A resolution, respectively. These structures reveal a core fold that is comprised of an alpha-helix lying diagonally across a five-stranded, mixed beta-sheet. Comparison of EAP domains with known structures reveals an unexpected homology with the C-terminal domain of bacterial superantigens. Examination of the structure of the superantigen SEC2 bound to the beta-chain of a T-cell receptor suggests a possible ligand-binding site within the EAP domain (Fields, B. A., Malchiodi, E. L., Li, H., Ysern, X., Stauffacher, C. V., Schlievert, P. M., Karjalainen, K., and Mariuzza, R. (1996) Nature 384, 188-192). These results provide the first structural characterization of EAP domains, relate EAP domains to a large class of bacterial toxins, and will guide the design of future experiments to analyze EAP domain structure/function relationships.

Individualized Patient Dosing in Phase I Clinical Trials: The Role of Escalation With Overdose Control in PNU-214936

Purpose

A patient-specific dose-escalation scheme using a Bayesian model of Escalation with Overdose Control (EWOC) was conducted to establish the maximum tolerated dose (MTD) of PNU-214936 in advanced non–small-cell lung cancer (NSCLC). PNU-214936 is a murine Fab fragment of the monoclonal antibody 5T4 fused to a mutated superantigen staphylococcal enterotoxin A (SEA).

Patients and Methods

Seventy-eight patients with NSCLC were treated with an individualized dose of PNU-214936 calculated using EWOC, based on their anti-SEA antibody level, and given as a 3-hour infusion on 4 consecutive days.

Results

Fever (82%; grade 3 to 4, 2.6%) and hypotension (57%; grade 3 to 4, 9%) were the most common toxicities. Eight dose-limiting toxicities occurred, as defined as any grade 4 toxicity occurring within the first 5 days. The MTD was defined as a function of pretreatment anti-SEA antibody level. MTD ranged from 103 ng/kg for patients with anti-SEA concentrations ≤ 10 pmol/mL, to 601 ng/kg for patients with anti-SEA concentrations of 91 to 150 pmol/mL. A minor tumor response was demonstrated in five of 66 assessable patients.

Conclusion

EWOC determined phase I doses of PNU-214936 that were adjusted for patient anti-SEA antibody level, while safeguarding against overdose. Furthermore, the method permitted the construction of a dosing algorithm that would allow patients in subsequent clinical investigations to be treated with a dose of PNU-214936 that is tailored to their specific tolerance for the agent, as reflected by their pretreatment anti-SEA.

Crystal structure of a SEA variant in complex with MHC class II reveals the ability of SEA to crosslink MHC molecules

Although the biological properties of staphylococcal enterotoxin A (SEA) have been well characterized, structural insights into the interaction between SEA and major histocompatibilty complex (MHC) class II have only been obtained by modeling. Here, the crystal structure of the D227A variant of SEA in complex with human MHC class II has been determined by X-ray crystallography. SEA(D227A) exclusively binds with its N-terminal domain to the alpha chain of HLA-DR1. The ability of one SEA molecule to crosslink two MHC molecules was modeled. It shows that this SEA molecule cannot interact with the T cell receptor (TCR) while a second SEA molecule interacts with MHC. Because of its relatively low toxicity, the D227A variant of SEA is used in tumor therapy.

Identification of key amino acids of the mouse mammary tumor virus superantigen involved in the specific interaction with T-cell receptor V(beta) domains

Mouse mammary tumor virus (MMTV) is a retrovirus encoding a superantigen that is recognized in association with major histocompatibility complex class II by the variable region of the beta chain (V(beta)) of the T-cell receptor. The C-terminal 30 to 40 amino acids of the superantigen of different MMTVs display high sequence variability that correlates with the recognition of particular T-cell receptor V(beta) chains. Interestingly, MMTV(SIM) and mtv-8 superantigens are highly homologous but have nonoverlapping T-cell receptor V(beta) specificities. To determine the importance of these few differences for specific V(beta) interaction, we studied superantigen responses in mice to chimeric and mutant MMTV(SIM) and mtv-8 superantigens expressed by recombinant vaccinia viruses. We show that only a few changes (two to six residues) within the C terminus are necessary to modify superantigen recognition by specific V(beta)s. Thus, the introduction of the MMTV(SIM) residues 314-315 into the mtv-8 superantigen greatly decreased its V(beta)12 reactivity without gain of MMTV(SIM)-specific function. The introduction of MMTV(SIM)-specific residues 289 to 295, however, induced a recognition pattern that was a mixture of MMTV(SIM)- and mtv-8-specific V(beta) reactivities: both weak MMTV(SIM)-specific V(beta)4 and full mtv-8-specific V(beta)11 recognition were observed while V(beta)12 interaction was lost. The combination of the two MMTV(SIM)-specific regions in the mtv-8 superantigen established normal MMTV(SIM)-specific V(beta)4 reactivity and completely abolished mtv-8-specific V(beta)5, -11, and -12 interactions. These new functional superantigens with mixed V(beta) recognition patterns allowed us to precisely delineate sites relevant for molecular interactions between the SIM or mtv-8 superantigen and the T-cell receptor V(beta) domain within the 30 C-terminal residues of the viral superantigen.

Staphylococcal enterotoxins

Staphylococcus aureus is a major human pathogen that produces a wide array of toxins, thus causing various types of disease symptoms. Staphylococcal enterotoxins (SEs), a family of nine major serological types of heat stable enterotoxins, are a leading cause of gastroenteritis resulting from consumption of contaminated food. In addition, SEs are powerful superantigens that stimulate non-specific T-cell proliferation. SEs share close phylogenetic relationships, with similar structures and activities. Here we review the structure and function of each known enterotoxin.

The crystal structure of staphylococcal enterotoxin H: implications for binding properties to MHC class II and TcR molecules

The X-ray structure of the superantigen staphylococcal enterotoxin H (SEH) has been determined at 1.69 A resolution. In this paper we present two structures of zinc-free SEH (apoSEH) and one zinc-loaded form of SEH (ZnSEH). SEH exhibits the conventional superantigen (SAg) fold with two characteristic domains. In ZnSEH one zinc ion per SEH molecule is bound to the C-terminal beta-sheet in the region implicated for major histocompatibility complex class II (MHC class II) binding in SEA, SED and SEE. Surprisingly, the zinc ion has only two ligating amino acid residues His206 and Asp208. The other ligands to the zinc ion are two water molecules. An extensive packing interaction between two symmetry-related molecules in the crystal, 834 A(2)/molecule, forms a cavity that buries the zinc ions of the molecules. This dimer-like interaction is found in two crystal forms. Nevertheless, zinc-dependent dimerisation is not observed in solution, as seen in the case of SED. A unique feature of SEH as compared to other staphylococcal enterotoxins is a large negatively charged surface close to the Zn(2+) site. The interaction of SEH with MHC class II is the strongest known among the staphylococcal enterotoxins. However, SEH seems to lack a SEB-like MHC class II binding site, since the side-chain properties of structurally equivalent amino acid residues in SEH and those in SEB-binding MHC class II differ dramatically. There is also a structural flexibility between the domains of SEH. The domains of two apoSEH structures are related by a 5 degrees rotation leading to at most 3 A difference in C(alpha) positions. Since the T-cell receptor probably interacts with both domains, SEH by this rotation may modulate its binding to different TcR Vbeta-chains.

The spectral and thermodynamic properties of staphylococcal enterotoxin A, E, and variants suggest that structural modifications are important to control their function

The superantigens staphylococcal enterotoxin A and E (SEA and SEE) can activate a large number of T-cells. SEA and SEE have approximately 80% sequence identity but show some differences in their biological function. Here, the two superantigens and analogues were characterized biophysically. SEE was shown to have a substantially higher thermal stability than SEA. Both SEA and SEE were thermally stabilized by 0.1 mM Zn(2+) compared with Zn(2+)-reduced conditions achieved using 1 mM EDTA or specific replacements that affect Zn(2+) coordination. The higher stability of SEE was only partly caused by the T-cell receptor (TCR) binding regions, whereas regions in the vicinity of the major histocompatibility complex class II binding sites affected the stability to a greater extent. SEE exhibited a biphasic denaturation between pH 5.0-6.5, influenced by residues in the TCR binding regions. Interestingly, enzyme-linked immunosorbent assay, isoelectric focusing, and circular dichroism analysis indicated that conformational changes had occurred in the SEA/E chimerical constructs relative to SEA and SEE. Thus, it is proposed that the Zn(2+) binding site is very important for the stability and potency of SEA and SEE, whereas residues in the TCR binding site have a substantial influence on the molecular conformation to control specificity and function.

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

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

Cutting Edge: Trimolecular Interaction of TCR with MHC Class II and Bacterial Superantigen Shows a Similar Affinity to MHC:Peptide Ligands

Bacterial superantigens such as Staphylococcus aureus enterotoxin A (SEA) are very potent stimulators of T cells. They bind to the Vβ region of the TCR and to MHC class II, stimulating T cells at nanomolar concentrations. Using surface plasmon resonance measurements, we find that binding between the individual components of the complex (TCR-class II, TCR-SEA, SEA-class II) is very weak, but that the stability of the trimolecular complex is considerably enhanced, reaching an affinity similar to that found for TCR interactions with MHC:peptide ligand. Thus, the potency of SEA in stimulation of T cells is not due to particularly strong affinities between the proteins, but to a cooperative effect of interactions in the TCR-SEA-MHC class II trimolecular complex that brings the kinetics into a similar range to binding of conventional Ags. This range may be the optimum for T cell activation.

Understanding the mechanism of action of bacterial superantigens from a decade of research

In the face of the unique diversity and plasticity of the immune system pathogenic organisms have developed multiple mechanisms in adaptation to their hosts, including the expression of a particular class of molecules called superantigens. Bacterial superantigens are the most potent stimulators of T cells. The functional consequences of the expression of superantigens by bacteria can be extended not only to T lymphocytes, but also to B lymphocytes and to cells of the myeloid compartment, including antigen-presenting cells and phagocytes. The biological effects of bacterial superantigens as well as their molecular aspects have now been studied for a decade. Although there is still a long way to go to clearly understand the role these molecules play in the establishment of disease, recently acquired knowledge of their biochemistry now offers unique experimental opportunities in defining the molecular rules of T-cell activation. Here, we present some of the most recent functional and molecular aspects of the interaction of bacterial superantigens with MHC class II molecules and the T-cell receptor.

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

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

Molecular characterization of the putative T-cell receptor cavity of the superantigen staphylococcal enterotoxin B

A number of investigators have utilized a variety of methods to identify the structural basis for the interaction of superantigens with the T-cell receptor beta-chain. The previous studies strongly suggest that a region of the toxin near residues N23, Y61, Y91 and D209 is important for this binding activity. Examination of crystal structure data shows that these residues line the rim of one side of a shallow cavity in the toxin. In an attempt further to define the face of the staphylococcal enterotoxin B (SEB) molecule involved in the interaction with the beta-chain, we have employed a polymerase chain reaction (PCR)-based, site-specific mutagenesis method to generate amino acid substitutions of residues on the opposite side of this putative T-cell receptor interaction cavity. Our results show that Y175 and N179 appear to be involved in the function of this superantigen, since each of several substitutions at this position exhibits a significantly reduced ability to induce T-cell proliferation. At the same time, mutation of the proximal Y186 does not alter the superantigen activity of SEB. Binding analysis of these mutants shows that class II binding activity is not significantly altered. Analysis of the responding T cells shows that the mutant toxins maintain T-cell receptor V beta selectivity. However, responses of T cells bearing the V beta 8.1 allele appear to be particularly diminished. When viewed in the context of other results reported in the literature, our results suggest that the T-cell receptor interaction site involves SEB residues which ring both the Y175/N179-side and the N23-side of a cavity on one side of the toxin molecule.

Overview of clinical trials employing antibody-targeted superantigens

Two Phase I clinical trials have been conducted using PNU-214565, a recombinant fusion protein of C242Fab and staphylococcal enterotoxin A (SEA). The initial escalating single dose trial was performed to determine safety and define toxicities. Cumulative doses determined to be safe as single doses were incorporated in an escalating, repeated dose regimen. Twenty-one patients were treated in the single dose and 27 in the repeated dose trials. Patient demographics were equivalent in both, as were the toxicities encountered - primarily fever and hypotension. Three patients in the single dose regimen treated at 0.5 ng/kg experienced grade 3 fever and/or hypotension, and one patient in the repeated dose trial had a dose-limiting grade 4 hypotension (2.75 ng/kg). TNFalpha and IL-2 induction in circulating blood preceded the development of clinical symptoms. One partial response was observed in the repeated dose trial. Pre-existing anti-SEA plasma antibodies protect patients against toxicity at a given drug dose. Based on these findings, a pharmacodynamically-based dosing scheme is currently being tested in a new repeated dose trial.

A mutation of F47 to A in staphylococcus enterotoxin A activates the T-cell receptor Vbeta repertoire in vivo

The bacterial superantigen staphylococcal enterotoxin A (SEA) binds with high affinity to major histocompatibility complex (MHC) class II molecules and subsequently activates T cells bearing particular T-cell receptor (TCR) Vbeta chains. Structural and mutational studies have defined two distinct MHC class II binding sites located in the N-terminal and C-terminal domains of SEA. The N-terminal F47 amino acid is critically involved in a low-affinity interaction to the MHC class II alpha-chain, while the C-terminal residues H187, H225, and D227 coordinate a Zn2+ ion and bind with moderate affinity to the beta-chain. In order to analyze whether the SEA-MHC class II alpha-chain interaction plays a role in dictating the in vivo repertoire of T-cell subsets, we studied distinct Vbeta populations after stimulation with wild-type SEA [SEA(wt)] and SEA with an F47A mutation [SEA(F47A)]. Injections of SEA(wt) in C57BL/6 mice induced cytokine release in serum, strong cytotoxic T-lymphocyte activity, expansion of T-cell subsets, and modulated expression of the T-cell activation antigens CD25, CD11a, CD44, CD62L, and CD69. SEA-reactive TCR Vbeta3+ and Vbeta11+ T cells were activated, while TCR Vbeta8+ T cells remained unaffected. The SEA(F47A) mutant protein induced a weaker T-cell response and failed to induce substantial interleukin-6 production compared to SEA(wt). Notably, SEA(F47A) failed to activate TCR Vbeta11+ T cells, whereas in vivo expansion and modulation of T-cell activation markers on TCR Vbeta3+ T cells were similar to those for SEA(wt). A similar response to SEA(F47A) was seen among CD4+ and CD8+ T cells. Activation of TCR Vbeta3+ and TCR Vbeta11+ T-cell hybridomas confirmed that SEA(F47A) activates TCR Vbeta3+ but not TCR Vbeta11+ T cells. The data support the view that the SEA-N-terminal MHC class II alpha-chain interaction defines a topology that is required for engagement of certain TCR Vbeta chains in vivo.

Refined structures of three crystal forms of toxic shock syndrome toxin-1 and of a tetramutant with reduced activity

The structure of toxic shock syndrome toxin-1 (TSST-1), the causative agent in toxic shock syndrome, has been determined in three crystal forms. The three structural models have been refined to R-factors of 0.154, 0.150, and 0.198 at resolutions of 2.05 A, 2.90 A, and 2.75 A, respectively. One crystal form of TSST-1 contains a zinc ion bound between two symmetry-related molecules. Although not required for biological activity, zinc dramatically potentiates the mitogenicity of TSST-1 at very low concentrations. In addition, the structure of the tetramutant TSST-1H [T69I, Y80W, E132K, I140T], which is nonmitogenic and does not amplify endotoxin shock, has been determined and refined in a fourth crystal form (R-factor = 0.173 to 1.9 A resolution).

Selection of phage-displayed superantigen by binding to cell-surface MHC class II

We have expressed the superantigen staphylococcal enterotoxin A (SEA) on the surface of bacteriophage as a fusion with the gene VIII protein (gVIIIp). This phage-displayed superantigen retains the properties inherent in the natural protein. It binds to MHC class II and activates T-cells bearing appropriate V beta regions. A flexible 5-amino acid linker sequence between the SEA molecule and the phage coat protein improved the production of functional phage-displayed SEA. Binding to MHC class II-expressing cells effectively selected SEA-phage from non-SEA-phage background. This indicates that this will be an effective method for selecting new specificities of superantigen from libraries of SEA mutants and for cloning of novel superantigens.

Mutations affecting the superantigen activity of staphylococcal enterotoxin B

As a superantigen, staphylococcal enterotoxin B (SEB) possesses the ability to bind to major histocompatibility complex class II molecules and be recognized by T cells bearing certain T-cell receptor (TCR) V beta alleles. Other investigators have utilized site-specific mutagenesis to generate amino acid substitutions to identify residues that may be involved in the interaction with the TCR beta-chain. In an attempt further to define the face of the SEB molecule involved in the interaction with the beta-chain, we have employed a polymerase chain reaction (PCR)-based, site-specific mutagenesis method to generate amino acid substitutions with altered superantigen activity. Our results show that valine at position 169 appears to be involved in the function of this superantigen, since each of several substitutions at this position exhibit a significantly reduced ability to induce T-cell proliferation. Analysis of the responding T cells to the residue 169 substitution shows that the mutant toxins maintain TCR V beta selectivity. At the same time, mutation of the proximal histidine at position 166 does not alter the superantigen activity of SEB. Radiolabelled binding analysis of these H166 and V169 mutants shows that class II-binding activity is not significantly altered. When viewed in the context of other results reported in the literature, combined with the crystal structure of the toxin, our results suggest that the interaction with the TCR probably involves SEB residues which ring a cavity along one side of the toxin molecule.

TCR binding differs for a bacterial superantigen (SEE) and a viral superantigen (Mtv-9)

Both superantigens (SAG) and many anti-TCR monoclonal antibodies (mAb) have specificity for the V beta region of the TCR encoded by TCRBV genes. For instance the bacterial SAG staphylococcal enterotoxin E (SEE), the retroviral SAG MTV-9 and the mAb OT145 each react with human T cells expressing BV6S7. This BV gene encodes two common alleles. We found that SEE and the mAb preferentially activate T cells expressing BV6S7*1 as opposed to BV6S7*2, but Mtv-9 activates T cells expressing either allele. Thus binding to the TCR differs between the two SAGs. A mutation in the TCR HVR-4 region of BV6S7*1 (G72E), where the two BV6S7 alleles differ, indicated that HVR-4 is a component of the binding site for SEE and for the mAb OT145. BV6S7*2 has a charged E72 which may result in electrostatic repulsion of SEE, as SEE contains a similarly acidic aspartic acid residue at a TCR interaction site (204D).

Immune response during tumor therapy with antibody-superantigen fusion proteins

To engineer superantigens (SAg) to express tumor reactivity, we genetically fused the Fab-part of the tumor-reactive MAb C215 and the bacterial SAg staphylococcal enterotoxin A (SEA). Treatment of mice carrying established lung micrometastases of the C215-transfected syngeneic B16 melanoma with 3-4 daily injections of C215Fab-SEA resulted in strong antitumor effects, while only moderate effects were seen when treatment was given every 4th day (intermittent treatment). High serum levels of IL-2, TNF-alpha, IFN-gamma and strong induction of CTLs (cytotoxic T lymphocytes) were noted after priming with the fusion protein. T cells responded well to 3 daily injections of C215Fab-SEA and then gradually entered a hyporesponsive state, characterized by a reduced ability to produce IL-2, TNF-alpha and IFN-gamma and failure to mediate cytotoxicity in vitro. Intermittent treatment was characterized by increased levels of IL-10, concomitant with accentuated loss of IL-2, TNF-alpha and IFN-gamma production. A 10-fold increase in SEA-reactive TCR V(beta)3+ CD4+ cells was observed in the spleen, while a loss of TCR V(beta)3+ CD8+ and V(beta)11+ CD8+ cells was noted. This is in striking contrast to injections of native SEA which induced a marked deletion of TCR V(beta)3+ CD4+ T cells, but not of CD8+ cells. Recovery of the TH1 cytokine profile occurred within 1-2 weeks, while restoration of cytotoxicity required several months and correlated with recovery of TCR V(beta)3+ CD8+ and TCR V(beta)11+ CD8+ T cells. These results show that the temporal relationship of SAg stimulations dictates the cytokine profile. Moreover, different mechanisms appear to regulate hyporesponsiveness in CD4+ and CD8+ T cells.

The J beta segment of the T cell receptor contributes to the V beta-specific T cell expansion caused by staphylococcal enterotoxin B and Urtica dioica superantigens

We have used a new polymerase chain reaction-based technique to analyze at the clonal level the CDR3 diversity and the J beta usage associated with the V beta-dependent T cell receptor (TCR) recognition of two superantigens: the staphylococcal enterotoxin B and the Urtica dioica agglutinin. Our results show that subset of J beta elements is preferentially expanded in a given V beta family, independently of the nature of the superantigen. By contrast, the CDR3 loop does not contribute significantly to the T cell expansion induced by the superantigens. We conclude that the J beta segment of the TCR beta chain, but not the CDR3 region, participates in superantigen binding, presumably by influencing the quaternary structure of the TCR beta chain.

Bifunctional antibody retargeting in vivo-activated T lymphocytes: simplifying clinical application

For antitumor x anti-CD3 bifunctional antibody (BFA) therapy to be clinically relevant in solid tumors, activated lymphocytes must be present within tumors. Toward that end, three uniquely different in vivo activation approaches were investigated in a p97 human antigen expressing syngeneic murine melanoma model. beta-Glucan (200 micrograms), staphylococcal enterotoxin B (SEB) (50 micrograms), and F(ab')2 BFA (10 micrograms) were tested for their ability to activate lymphocytes, neutralize pulmonary metastases, and treat established tumors. Systemic activation, measured as the ability of splenocytes to lyse tumor cells in vitro in the presence of BFA, was enhanced by the in vivo administration of SEB but not by beta-glucan or F(ab')2 BFA. Despite lacking a systemic effect, F(ab')2 BFA increased both direct and BFA-mediated cytotoxicity in fresh tumor infiltrating lymphocytes. beta-Glucan did not increase systemic or intratumor T cell activation. However, it significantly enhanced the ability of splenocytes to lyse NK-sensitive YAC-1 cells. When tested in a pulmonary metastases model, all three forms of immune modulation combined with F(ab')2 BFA significantly reduced the number of metastases. BFA were more effective at tumor neutralization when combined with SEB compared with adoptively transferred, in vitro-activated splenocytes. These studies demonstrate that immune modulators when combined with F(ab')2 BFA can provide effective antitumor therapy. Several clinical obstacles may be overcome by the application of these reagents.

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.

Crystal structure of the superantigen enterotoxin C2 from Staphylococcus aureus reveals a zinc-binding site

BACKGROUND

Staphylococcus aureus enterotoxin C2 (SEC2) belongs to a family of proteins, termed 'superantigens', that form complexes with class II MHC molecules enabling them to activate a substantial number of T cells. Although superantigens seem to act by a common mechanism, they vary in many of their specific interactions and biological properties. Comparison of the structure of SEC2 with those of two other superantigens--staphylococcal enterotoxin B (SEB) and toxic shock syndrome toxin-1 (TSST-1)--may provide insight into their mode of action.

RESULTS

The crystal structure of SEC2 has been determined at 2.0 A resolution. The overall topology of the molecule resembles that of SEB and TSST-1, and the regions corresponding to the MHC class II and T-cell receptor binding sites on SEB are quite similar in SEC2. A unique feature of SEC2 is the presence of a zinc ion located in a solvent-exposed region at the interface between the two domains of the molecule. The zinc ion is coordinated to Asp83, His118, His122 and Asp9* (from the neighbouring molecule in the crystal lattice). Atomic absorption spectrometry demonstrates that zinc is also bound to SEC2 in solution.

CONCLUSIONS

SEC2 appears to be capable of binding to MHC class II molecules in much the same manner as SEB. However, structure-function studies have suggested an alternative binding mode that involves a different site on the toxin. The zinc ion of SEC2 lies within this region and thus may be important for complex formation, for example by acting as a bridge between the two molecules. Other possible roles for the metal cation, including a catalytic one, are also considered.

Superantigen engineering

Bacterial superantigens are extremely potent activators of the immune system. Their ability to efficiently cross-link molecules of the major histocompatibility complex class II and T-cell receptors causes the normal antigen specificity of each receptor to be bypassed. Two well characterized superantigens are the staphylococcal enterotoxins A and B. Data from mutagenesis studies in combination with recent structural information allow the definition of the surfaces on these superantigens involved in the binding of either type of receptor. Wild-type and engineered mutants of these superantigens have been used to modulate the activity of cells in the immune system, in an attempt to develop therapeutics applications.

Residues defining V beta specificity in staphylococcal enterotoxins

The three-dimensional structure of staphylococcal enterotoxin C2 has been determined at 2.7 A resolution by x-ray diffraction, while the structures of enterotoxins A and E have been modelled based on their sequence homology to other staphylococcal enterotoxins. The T-cell receptor-binding sites of staphylococcal enterotoxin (SE) B and SEC2 are compared and the stereochemical interactions likely to be responsible for their differing V beta specificities are identified. A similar comparison is made between SEA and SEE.

A natural mutation of the amino acid residue at position 60 destroys staphylococcal enterotoxin A murine T-cell mitogenicity

A variety of techniques have been used to identify the amino acid residues of bacterial superantigens involved in their interactions with major histocompatibility complex (MHC) class II and T-cell receptor (TCR). In this study, we isolated a naturally mutated staphylococcal enterotoxin A (SEA) from three different Staphylococcus aureus strains, in which the amino acid at position 60 has been changed from aspartic acid (D) to asparagine (N). We then studied the influence of this change on the immunological activities of SEA. Our results demonstrated that this mutation does not affect the capacity of SEA to bind MHC class II molecules and consequently activates human monocytes and peripheral blood lymphocytes. In contrast, mutated SEA failed to stimulate the proliferation of murine splenic lymphocytes of two different strains, and when presented by human MHC class II molecules, it also failed to activate murine cell line 3DT, which expresses the SEA-specific TCR V beta element (V beta 1). These results indicate that this mutation alters the interaction between SEA and murine TCR. The reactivity patterns of the mutated SEA with two specific anti-SEA monoclonal antibodies suggested that the observed effect of the isolated mutation in the murine system might be due to certain conformational changes in the SEA molecule introduced upon changing the D at position 60 to N. Site-directed mutagenesis of the N residue to D or to glycine reconstituted the ability of SEA to stimulate murine splenic lymphocytes. The different effects of this natural mutation at position 60 on the immunological activities of SEA with murine and human cells highlight the relevance of the affinity and avidity in SEA-TCR interactions in the function of different species or may reflect a difference in epitope specificity.

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.

Biological activities of staphylococcal enterotoxin type A mutants with N-terminal substitutions

The purpose of this study was to examine the importance of certain N-terminal amino acid residues of staphylococcal enterotoxin type A (SEA) for biological activity. The results confirm our previous observation that Asn-25, Phe-47, and Leu-48 are important for SEA's emetic and superantigen activities. Substitutions at six other sites (Leu-12, Lys-14, Ser-16, Asp-45, Gln-46, and Thr-51) did not reveal any additional residues required for biological activity. Mutant SEAs with substitutions at 25, 47, or 48 all had decreased T-cell stimulatory activity, with the mutants at position 47 being the most defective. Results of a competition assay for binding to the major histocompatibility complex (MHC) class II-expressing cell line Raji suggested that the decreased superantigen activities of the mutants with substitutions at positions 47 and 48 are due to poor interactions with MHC class II molecules, whereas the defects of the mutants at position 25 are a consequence of faulty interactions with T-cell receptors. With respect to emetic activity in rhesus monkeys, the mutants at position 25 or 48 exhibited decreased but significant activity. Interestingly, the two mutants at position 47 had different emetic activities; SEA-F47G was nonemetic when administered intragastrically at 500 micrograms per animal, whereas SEA-F47S was emetic at this dosage. Since the mutants at position 47 were equally defective for superantigen activity, this further supports our previous suggestion of an incomplete correlation between SEA's emetic and superantigen activities.

Staphylococcal enterotoxin A and toxic shock syndrome toxin compete with CD4 for human major histocompatibility complex class II binding

We have examined the role of the CD4 molecule in primary T-lymphocyte responses to the staphylococcal enterotoxins SEA, SEB, SEC1, and the toxic shock syndrome toxin TSST-1. Proliferating cells were predominantly CD4+; however, the responses to SEA and TSST-1 were most sensitive to inhibition by the anti-CD4 antibody Leu-3a. T-lymphocyte responses to the bacterial superantigens were inhibited by site-directed mutations of residues in the DR beta membrane-proximal domain (DR beta 2) that are also known to be important for interactions with CD4. SEA and TSST-1 binding to DR was reduced by the DR beta 2 mutations and by competition with soluble recombinant CD4. We propose that bacterial superantigens sequentially, or simultaneously with CD4, stabilize complexes of T-cell antigen receptors and major histocompatibility complex class II molecules. The superantigen qualities of these toxins may be due, in part, to a molecular mimicry of CD4 and other adhesion molecules.

Superantigens produced by infectious pathogens: molecular mechanism of action and biological significance

"Superantigens" have in common an extremely potent stimulatory activity for CD4+, CD8+, and some gamma delta+ T lymphocytes. Superantigens use a unique mechanism: they crosslink variable parts of the T cell receptor with MHC class II molecules on accessory or target cells. The interaction site on the T cell receptor is the variable part of the beta-chain (V beta). There are several reasons why these molecules have aroused such tremendous interest in recent years. First, they have provided key information on tolerance mechanisms, both on the deletion of T cells in the thymus and on the induction of peripheral tolerance by anergy and apoptosis. Second, of all polyclonal T cell stimulators they are the ones that most closely mimic the recognition of specific antigen. Finally, they have been recognized as important factors in the pathogenicity of the producing pathogens, inducing shock and immunosuppression. Moreover, it has been postulated that superantigens could be involved in the pathogenesis of certain human diseases.

Predictions of T-cell receptor- and major histocompatibility complex-binding sites on staphylococcal enterotoxin C1

We have focused on regions of staphylococcal enterotoxin C1 (SEC1) causing immunomodulation. N-terminal deletion mutants lacking residues 6 through 13 induced T-cell proliferation similar to that induced by native toxin. However, mutants with residues deleted between positions 19 and 33, although nonmitogenic themselves, were able to inhibit both SEC1-induced T-cell proliferation and binding of the native toxin to major histocompatibility complex (MHC) class II. Presumably, these deletions define a part of SEC1 that interacts with the T-cell receptor. Three synthetic peptides containing residues located in a region analogous to the alpha 5 groove of SEC3 had residual mitogenic activity or blocked T-cell proliferation induced by SEC1 and appear to recognize the same site as SEC1 on a receptor for the toxin, presumably MHC class II. We conclude that isolated portions of the SEC1 molecule can retain residual mitogenic activity but that the entire protein is needed to achieve maximal superantigenic stimulation. Our results, together with the results of other investigators, support a model in which SEC1 binds to an alpha helix of MHC class II through a central groove in the toxin and thereby promotes or stabilizes the interaction between antigen-presenting cells and T cells.

T-cell activation by superantigens

Superantigens stimulate powerful T-cell responses that can have marked effects in vivo, sometimes leading to shock or even death. The demonstration that strong T-cell responses to superantigens in vivo can be followed by tolerance, reflecting either clonal elimination or anergy, has provided important insights into how mature T cells can be regulated. Further progress in understanding the factors that control these responses relies heavily on defining the specific interactions between T-cell receptors, superantigens and major histocompatibility complex molecules which lead to T-cell activation as well as on the characterization of the specific signal transduction events and molecules involved in this activation. Significant progress has been made, during the past year, in the first area and these findings are summarized below; though less information is available in the latter area, recent observations relevant to this issue are discussed.

Molecular characterization and phylogenetic distribution of the streptococcal superantigen gene (ssa) from Streptococcus pyogenes

A striking increase in the frequency and severity of invasive infections caused by Streptococcus pyogenes has occurred in recent years. Among these diseases is streptococcal toxic-shock-like syndrome (TSLS), a condition characterized by fulminant soft-tissue destruction and multiorgan failure. Streptococcal superantigen (SSA), a superantigen isolated from a TSLS-inducing, serotype M3 S. pyogenes strain, has recently been identified. We here describe the cloning, sequencing, and phylogenetic distribution of the SSA structural gene. The 783-bp open reading frame encodes a predicted 260-amino-acid protein that is similar in size to several other bacterial superantigens. The deduced sequence of the mature protein is 60.2% identical to that of staphylococcal enterotoxin B but only 49% identical to that of streptococcal pyrogenic exotoxin A. Southern blot and PCR analysis of 138 group A streptococcal strains representing 65 M protein serotypes and 15 nontypeable isolates identified ssa in 68 strains from 10 distinct clonal lineages. All ssa-positive clones expressed SSA. Of the two clones associated with TSLS, the ET 2-M3 lineage, but not the ET 1-M1 lineage, carried the SSA gene. Further analysis of the ET 2-M3 lineage found evidence for temporal variation in ssa association. Contemporary ET 2-M3 disease isolates had ssa, but two older isolates of this clone recovered in 1910 and 1920 lacked the gene. The clonal and temporal distribution patterns of ssa suggest a relatively recent acquisition of this superantigen-encoding gene by the ET 2-M3 lineage, perhaps by horizontal transfer and recombination.

Structural basis of superantigen action inferred from crystal structure of toxic-shock syndrome toxin-1

Superantigens stimulate T cells bearing particular T-cell receptor V beta sequences, so they are extremely potent polyclonal T-cell mitogens. T-cell activation is preceded by binding of superantigens to class II major histocompatibility complex (MHC) molecules. To further the structural characterization of these interactions, the crystal structure of a toxin associated with toxic-shock syndrome, TSST-1, which is a microbial superantigen, has been determined at 2.5 A resolution. The N- and C-terminal domains of the structure both contain regions involved in MHC class II association; the C-terminal domain is also implicated in binding the T-cell receptor. Despite low sequence conservation, the TSST-1 topology is similar to the structure reported for the superantigen staphylococcal enterotoxin B4. But TSST-1 lacks several of the structural features highlighted as central to superantigen activity in the staphylococcal enterotoxin B and we therefore reappraise the structural basis of superantigen action.

Superantigens

"Superantigens" is the term for a group of molecules that have in common an extremely potent stimulatory activity for T lymphocytes of several species. They stimulate CD4+, CD8+ and gamma delta + T cells by a unique mechanism: they cross-link variable parts of the T-cell receptor (TCR) with MHC class II molecules on accessory or target cells. The interaction site on the class II molecule and on the TCR is different from the peptide binding site; on the TCR it is the variable part of the beta chain (V beta). The prototype superantigen is the staphylococcal enterotoxin B (SEB), member of a family of genetically related proteins produced by Staphylococcus aureus and Streptococcus pyogenes. These are soluble exotoxins of approximately 27 kd molecular mass. It is intriguing that this molecular mechanism of T-cell stimulation has been independently produced at least three times in evolution. Other pathogens producing superantigens are retroviruses (the Mouse Mammary Tumor Viruses) and a mycoplasma (Mycoplasma arthritidis). Many additional candidate superantigens have been proposed, but in most cases unequivocal evidence for superantigen activity is still missing. There are several reasons why these molecules have aroused such tremendous interest in recent years. First, they have provided key information on tolerance mechanisms, both on the deletion of T cells in the thymus and on the induction of peripheral tolerance by anergy and apoptosis. Second, of all polyclonal T-cell stimulators they are the ones that most closely mimic the recognition of specific antigen. Finally, they have been recognized as important factors in the pathogenicity of the producing pathogens, inducing shock and immunosuppression. Whilst there is evidence that superantigens could be involved in the pathogenesis of certain human diseases, in most cases this is still very preliminary and indirect.