Superantigens as immunomodulators: recent structural insights

Blocking Superantigen-Mediated Diseases: Challenges and Future Trends

Superantigens are virulence factors secreted by microorganisms that can cause various immune diseases, such as overactivating the immune system, resulting in cytokine storms, rheumatoid arthritis, and multiple sclerosis. Some studies have demonstrated that superantigens do not require intracellular processing and instated bind as intact proteins to the antigen-binding groove of major histocompatibility complex II on antigen-presenting cells, resulting in the activation of T cells with different T-cell receptor Vβ and subsequent overstimulation. To combat superantigen-mediated diseases, researchers have employed different approaches, such as antibodies and simulated peptides. However, due to the complex nature of superantigens, these approaches have not been entirely successful in achieving optimal therapeutic outcomes. CD28 interacts with members of the B7 molecule family to activate T cells. Its mimicking peptide has been suggested as a potential candidate to block superantigens, but it can lead to reduced T-cell activity while increasing the host's infection risk. Thus, this review focuses on the use of drug delivery methods to accurately target and block superantigens, while reducing the adverse effects associated with CD28 mimic peptides. We believe that this method has the potential to provide an effective and safe therapeutic strategy for superantigen-mediated diseases.

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.

Streptococcal toxins: role in pathogenesis and disease

Group A Streptococcus (Streptococcus pyogenes), group B Streptococcus (Streptococcus agalactiae) and Streptococcus pneumoniae (pneumococcus) are host-adapted bacterial pathogens among the leading infectious causes of human morbidity and mortality. These microbes and related members of the genus Streptococcus produce an array of toxins that act against human cells or tissues, resulting in impaired immune responses and subversion of host physiological processes to benefit the invading microorganism. This toxin repertoire includes haemolysins, proteases, superantigens and other agents that ultimately enhance colonization and survival within the host and promote dissemination of the pathogen.

Engineering a soluble high-affinity receptor domain that neutralizes staphylococcal enterotoxin C in rabbit models of disease

Superantigens (SAgs) are a class of immunostimulatory exotoxins that activate large numbers of T cells, leading to overproduction of cytokines and subsequent inflammatory reactions and systemic toxicity. Staphylococcal enterotoxin C (SEC), a SAg secreted by Staphylococcus aureus, has been implicated in various illnesses including non-menstrual toxic shock syndrome (TSS) and necrotizing pneumonia. SEC has been shown to cause TSS illness in rabbits and the toxin contributes to lethality associated with methicillin-resistant S.aureus (MRSA) in a rabbit model of pneumonia. With the goal of reducing morbidity and mortality associated with SEC, a high-affinity variant of the extracellular variable domain of the T-cell receptor beta-chain for SEC (~14 kDa) was generated by directed evolution using yeast display. This protein was characterized biochemically and shown to cross-react with the homologous (65% identical) SAg staphylococcal enterotoxin B (SEB). The soluble, high-affinity T-cell receptor protein neutralized SEC and SEB in vitro and also significantly reduced the bacterial burden of an SEC-positive strain of MRSA (USA400 MW2) in an infective endocarditis model. The neutralizing agent also prevented lethality due to MW2 in a necrotizing pneumonia rabbit model. These studies characterize a soluble high-affinity neutralizing agent against SEC, which is cross-reactive with SEB, and that has potential to be used intravenously with antibiotics to manage staphylococcal diseases that involve these SAgs.

Oral vaccine formulations stimulate mucosal and systemic antibody responses against staphylococcal enterotoxin B in a piglet model

Despite the potential for its use as an agent of biowarfare or bioterrorism, no approved vaccine against staphylococcal enterotoxin B (SEB) exists. Nontoxic, mutant forms of SEB have been developed; however, it has been difficult to determine the efficacy of such subunit vaccine candidates due to the lack of superantigen activity of native SEB in rodents and due to the limitations of primate models. Since pigs respond to SEB in a manner similar to that of human subjects, we utilized this relevant animal model to investigate the safety and immunogenicity of a triple mutant of SEB carrying the amino acid changes L45R, Y89A, and Y94A. This recombinant mutant SEB (rmSEB) did not possess superantigen activity in pig lymphocyte cultures. Furthermore, rmSEB was unable to compete with native SEB for binding to pig leukocytes. These in vitro studies suggested that rmSEB could be a safe subunit vaccine. To test this possibility, piglets immunized orally with rmSEB formulations experienced no significant decrease in food consumption and no weight loss during the vaccination regimen. Oral vaccination with 1-mg doses of rmSEB on days 0, 7, 14, and 24 resulted in serum IgG and fecal IgA levels by day 36 that cross-reacted with native SEB. Surprisingly, the inclusion of cholera toxin adjuvant in vaccine formulations containing rmSEB did not result in increased antibody responses compared to formulations using the immunogen alone. Taken together, these studies provide additional evidence for the potential use of nontoxic forms of SEB as vaccines.

Peptides from the Mycobacterium tuberculosis Rv1980c protein involved in human cell infection: insights into new synthetic subunit vaccine candidates

Mycobacterium tuberculosis infection continues to be a major cause of morbidity and mortality throughout the world. The vast complexity of the intracellular pathogen M. tuberculosis and the diverse mechanisms by which it can invade host cells highlight the importance of developing a fully protective vaccine. Our vaccine development strategy consists of including fragments from multiple mycobacterial proteins involved in cell invasion. The aim of this study was to identify high activity binding peptides (HABPs) in the immunogenic protein Rv1980c from M. tuberculosis H37Rv with the ability to inhibit mycobacterial invasion into U937 monocyte-derived macrophages and A549 cells. The presence and transcription of the Rv1980c gene was assessed in members belonging to the M. tuberculosis complex and other nontuberculous mycobacteria by PCR and RT-PCR, respectively. Cell surface localization was confirmed by immuno-electron microscopy. Three peptides binding with high activity to U937 cells and one to A549 cells were identified. HABPs 31100, 31101, and 31107 inhibited invasion of M. tuberculosis into A549 and U937 cells and therefore could be promising candidates for the design of a subunit-based antituberculous vaccine.

Characteristics of carbohydrate antigen binding to the presentation protein HLA-DR

Zwitterionic polysaccharide antigens (ZPSs) were recently shown to activate T cells in a class II major histocompatibility complex (MHCII)-dependent fashion requiring antigen presenting cell (APC)-mediated oxidative processing although little is known about the mechanism or affinity of carbohydrate presentation (Cobb BA, Wang Q, Tzianabos AO, Kasper DL. 2004. Polysaccharide processing and presentation by the MHCII pathway. Cell. 117:677-687). A recent study showed that the helical conformation of ZPSs (Wang Y, Kalka-Moll WM, Roehrl MH, Kasper DL. 2000. Structural basis of the abscess-modulating polysaccharide A2 from Bacteroides fragilis. Proc Natl Acad Sci USA. 97:13478-13483; Choi YH, Roehrl MH, Kasper DL, Wang JY. 2002. A unique structural pattern shared by T-cell-activating and abscess-regulating zwitterionic polysaccharides. Biochemistry. 41:15144-15151) is closely linked with immunogenic activity (Tzianabos AO, Onderdonk AB, Rosner B, Cisneros RL, Kasper DL. 1993. Structural features of polysaccharides that induce intra-abdominal abscesses. Science. 262:416-419) and is stabilized by a zwitterionic charge motif (Kreisman LS, Friedman JH, Neaga A, Cobb BA. 2007. Structure and function relations with a T-cell-activating polysaccharide antigen using circular dichroism. Glycobiology. 17:46-55), suggesting a strong carbohydrate structure-function relationship. In this study, we show that PSA, the ZPS from Bacteroides fragilis, associates with MHCII at high affinity and 1:1 stoichiometry through a mechanism mirroring peptide presentation. Interestingly, PSA binding was mutually exclusive with common MHCII antigens and showed significant allelic differences in binding affinity. The antigen exchange factor HLA-DM that catalyzes peptide antigen association with MHCII also increased the rate of ZPS association and was required for APC presentation and ZPS-mediated T cell activation. Finally, the zwitterionic nature of these antigens was required only for MHCII binding, and not endocytosis, processing, or vesicular trafficking to MHCII-containing vesicles. This report is the first quantitative analysis of the binding mechanism of carbohydrate antigens with MHCII and leads to a novel model for nontraditional MHCII antigen presentation during bacterial infections.

Staphylococcus aureus enterotoxin C2 mutants: biological activity assay in vitro

Staphylococcal enterotoxin C2 (SEC2) is one member of bacterial superantigens produced by Staphylococcus aureus. It can be attributed to its superantigenic activity to cross-link major histocompatibility complex class II molecules with T-cell receptors and activate a large number of resting T cells resulting in release of massive cytokines, which will produce significant tumor inhibition in vivo and in vitro. However, it could be not broadly applied to cure malignant tumors in clinic because of emetic activity of SEC2. The aim of this study was to inactivate emetic activity of SEC2 through site-directed mutagenesis. Cys93, Cys110 and His118 were selected as substitutional sites based on the functional sites responsible for emesis. The mutated proteins were used to determine Peripheral blood mononuclear cell proliferation activity and anti-tumor activity in vitro. Results showed that these mutated proteins efficiently stimulated T cell and exhibited the same tumor-inhibition effect as SEC2. It is possible to inactivate emetic activity of SEC2 through site-directed mutagenesis and provide satisfying agents for tumor treatment in clinic.

The Staphylococcus aureus extracellular adherence protein (Eap) adopts an elongated but structured conformation in solution

The extracellular adherence protein (Eap) of Staphylococcus aureus participates in a wide range of protein-protein interactions that facilitate the initiation and dissemination of Staphylococcal disease. In this report, we describe the use of a multidisciplinary approach to characterize the solution structure of full-length Eap. In contrast to previous reports suggesting that a six-domain isoform of Eap undergoes multimerization, sedimentation equilibrium analytical ultracentrifugation data revealed that a four-domain isoform of Eap is a monomer in solution. In vitro proteolysis and solution small angle X-ray scattering studies both indicate that Eap adopts an extended conformation in solution, where the linkers connecting sequential EAP modules are solvent exposed. Construction of a low-resolution model of full-length Eap using a combination of ab initio deconvolution of the SAXS data and rigid body modeling of the EAP domain crystal structure suggests that full-length Eap may present several unique concave surfaces capable of participating in ligand binding. These results also raise the possibility that such surfaces may be held together by additional interactions between adjacent EAP modules. This hypothesis is supported by a comparative Raman spectroscopic analysis of full-length Eap and a stoichiometric solution of the individual EAP modules, which indicates the presence of additional secondary structure and a greater extent of hydrogen/deuterium exchange protection in full-length Eap. Our results provide the first insight into the solution structure of full-length Eap and an experimental basis for interpreting the EAP domain crystal structures within the context of the full-length molecule. They also lay a foundation for future studies into the structural and molecular bases of Eap-mediated protein-protein interactions with its many ligands.

The Solution Structure of Antigen MPT64 from Mycobacterium tuberculosis Defines a New Family of Beta-Grasp Proteins

The MPT64 protein and its homologs form a highly conserved family of secreted proteins with unknown function that are found within the pathogenic Mycobacteria genus. The founding member of this family from Mycobacterium tuberculosis (MPT64 or protein Rv1980c) is expressed only when Mycobacteria cells are actively dividing. By virtue of this relatively unique expression profile, Rv1980c is currently under phase III clinical trials to evaluate its potential to replace tuberculin, or purified protein derivative, as the rapid diagnostic of choice for detection of active tuberculosis infection. We describe here the NMR solution structure of Rv1980c. This structure reveals a previously undescribed fold that is based upon a variation of a beta-grasp motif most commonly found in protein-protein interaction domains. Examination of this structure in conjunction with multiple sequence alignments of MPT64 homologs identifies a candidate ligand-binding site, which may help guide future studies of Rv1980c function. The work presented here also suggests structure-based approaches for increasing the antigenic potency of a Rv1980c-based diagnostic.

Staphylococcal superantigens of the enterotoxin gene cluster (egc) for treatment of stage IIIb non-small cell lung cancer with pleural effusion

There has been renewed interest in the superantigens as antitumor agents with the discovery of a group of bacterial superantigens known as the enterotoxin gene cluster (egc staphylococcal enterotoxins [SEs]). This article discusses the mechanisms by which egc SEs induce tumor killing and pleurodesis. The application of SE homolog and nucleic acid compositions as vaccines and for treatment of established tumors is reviewed. Finally, the use of native SEs ex vivo-intratumorally and intravesicularly administered superantigens against established tumors-is described and the interrelation between superantigen therapy and chemoradiotherapy.

Binding of natural variants of staphylococcal superantigens SEG and SEI to TCR and MHC class II molecule

SEG and SEI are staphylococcal superantigens (SAgs) identified recently that belong to the egc operon and whose genes are in tandem orientation. Only a few allelic variants of SEG and SEI have been reported. Here we analyzed four Staphylococcus aureus strains with genotypic variation in both SAgs. However, both SAgs retain key residues in their putative TCR and MHC binding sites and, accordingly, their superantigenic properties. Thus, SEI significantly stimulates mouse T-cells bearing Vbeta3, 5 and 13, while SEG stimulates Vbeta7 and 9 in the draining node when inoculated in the footpad. As another member of the SEB subfamily, SEG also stimulates mouse Vbeta8.1+2. However, the increase in Vbeta8.1+2 T-cells observed at day 2 after inoculation reverts to normal values at day 4, whereas it remains high at day 4 following inoculation with SEC3 or SSA. T-cell stimulation assays in the mouse and analysis of the putative Vbeta8.2 binding site on SEG, which includes three non-conserved residues, suggest a possibly unique interaction between Vbeta8.2 and SEG. We also analyzed biochemical and biophysical characteristics of SEI and SEG binding to their cognate human beta chains by surface plasmon resonance, and binding to the HLA-DR1 MHC class II molecule by gel filtration. SEI binds human Vbeta5.2 and Vbeta1 with apparent K(D)'s of 23 and 118 microM, respectively; SEG binds Vbeta13.6 with a K(D) of 5 microM. As suggested by sequence homology, SEI requires Zn2+ for strong binding to DR1, which goes undetected in the presence of EDTA. SEG and SEI have characteristics such as co-expression, different interaction with MHC class II and stimulation of completely different subsets of human and mouse T-cells, which indicate complementary superantigenic activity and suggest an important advantage to staphylococcal strains in producing them both.

Preparation of murine B7.1-glycosylphosphatidylinositol and transmembrane-anchored staphylococcal enterotoxin. A dual-anchored tumor cell vaccine and its antitumor effect

BACKGROUND

The authors have previously reported a tumor cell vaccine modified with superantigen staphylococcal enterotoxin A (SEA) and its antitumor effect. The tumor cell vaccines modified with multiple immune activators frequently elicited stronger immune responses against established tumors than single-modified vaccines.

METHODS

The authors explored the effectiveness of a tumor cell vaccine transduced with immune activators, dual-modified using the protein transfer technique. First, a glycosylphosphatidylinositol (GPI)-anchored murine B7.1 (mB7.1-GPI) and a transmembrane-anchored SEA (TM-SEA) were genetically generated. Then, the murine lymphoma EL4 cells were dual modified with the incorporation of mB7.1-GPI and TM-SEA onto the cell surface. Flow cytometry and laser confocal microscopy showed that the incorporation of B7.1 and SEA molecules onto EL4 cells was quite stable.

RESULTS

The dual-modified tumor cell vaccine EL4/mB7.1-GPI + TM-SEA elicited significantly stronger antitumor immune responses both in vitro and in vivo when compared with the single-modified tumor cell vaccines EL4/mB7.1-GPI and EL4/TM-SEA.

CONCLUSIONS

The results of the current study validated the novel approach for preparing tumor cell vaccines modified with dual immune active molecules using the protein transfer technique, and supported the feasibility and effectiveness of the dual-modified tumor cell vaccine.

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.

Crystallographic and Mutational Data Show That the Streptococcal Pyrogenic Exotoxin J Can Use a Common Binding Surface for T-cell Receptor Binding and Dimerization

The protein toxins known as superantigens (SAgs), which are expressed primarily by the pathogenic bacteria Staphylococcus aureus and Streptococcus pyogenes, are highly potent immunotoxins with the ability to cause serious human disease. These SAgs share a conserved fold but quite varied activities. In addition to their common role of cross-linking T-cell receptors (TCRs) and major histocompatibility complex class II (MHC-II) molecules, some SAgs can cross-link MHC-II, using diverse mechanisms. The crystal structure of the streptococcal superantigen streptococcal pyrogenic exotoxin J (SPE-J) has been solved at 1.75 A resolution (R = 0.209, R(free) = 0.240), both with and without bound Zn(2+). The structure displays the canonical two-domain SAg fold and a zinc-binding site that is shared by a subset of other SAgs. Most importantly, in concentrated solution and in the crystal, SPE-J forms dimers. These dimers, which are present in two different crystal environments, form via the same face that is used for TCR binding in other SAgs. Site-directed mutagenesis shows that this face is also used for TCR binding SPE-J. We infer that SPE-J cross-links TCR and MHC-II as a monomer but that dimers may form on the antigen-presenting cell surface, cross-linking MHC-II and eliciting intracellular signaling.

Crystal structure of a dimeric form of streptococcal pyrogenic exotoxin A (SpeA1)

Streptococcal pyrogenic exotoxin A (SpeA1) is a bacterial superantigen associated with scarlet fever and streptococcal toxic shock syndrome (STSS). SpeA1 is found in both monomeric and dimeric forms, and previous work suggested that the dimer results from an intermolecular disulfide bond between the cysteines at positions 90 of each monomer. Here, we present the crystal structure of the dimeric form of SpeA1. The toxin crystallizes in the orthorhombic space group P212121, with two dimers in the crystallographic asymmetric unit. The final structure has a crystallographic R-factor of 21.52% for 7248 protein atoms, 136 water molecules, and 4 zinc atoms (one zinc atom per molecule). The implications of SpeA1 dimer on MHC class II and T-cell receptor recognition are discussed.

Identification of a secondary zinc-binding site in staphylococcal enterotoxin C2. Implications for superantigen recognition

The previously determined crystal structure of the superantigen staphylococcal enterotoxin C2 (SEC2) showed binding of a single zinc ion located between the N- and C-terminal domains. Here we present the crystal structure of SEC2 determined to 2.0 A resolution in the presence of additional zinc. The structure revealed the presence of a secondary zinc-binding site close to the major histocompatibility complex (MHC)-binding site of the toxin and some 28 A away from the primary zinc-binding site of the toxin found in previous studies. T cell stimulation assays showed that varying the concentration of zinc ions present affected the activity of the toxin and we observed that high zinc concentrations considerably inhibited T cell responses. This indicates that SEC2 may have multiple modes of interaction with the immune system that are dependent on serum zinc levels. The potential role of the secondary zinc-binding site and that of the primary one in the formation of the TCR.SEC2.MHC complex are considered, and the possibility that zinc may regulate the activity of SEC2 as a toxin facilitating different T cell responses is discussed.

Toxicity of the staphylococcal enterotoxin B mutants with histidine-to-tyrosine substitutions

In this study we made a series of site-directed mutants of staphylococcal enterotoxin B (SEB), in which histidine residues in the molecule were replaced by tyrosine. The mutant genes were cloned and expressed, and the corresponding proteins were purified. These mutant proteins were tested for binding to human HLA-DR4 and for mitogenetic activity in mouse splenocyte culture. Toxicity of the proteins in vivo was evaluated in the actinomycin D-primed C3H/HeJ mouse model. We found that SEB mutant proteins with fewer than four histidine-to-tyrosine (his-to-tyr) substitutions retained toxic properties similar to wild-type SEB. However, studies showed that his-to-tyr substitution of four consecutive histidine residues eliminated SEB toxicity. Our results clearly show that this genetically modified SEB protein is non-toxic and justifies its further development as a component of a new, safer vaccine to prevent SEB intoxication.

So many ways of getting in the way: diversity in the molecular architecture of superantigen-dependent T-cell signaling complexes

Superantigens (SAGs) elicit massive T-cell proliferation through simultaneous interaction with MHC and TCR molecules. SAGs have been implicated in toxic shock syndrome and food poisoning, and they may also play a pathogenic role in autoimmune diseases. The best-characterized group of SAGs are the pyrogenic bacterial SAGs, which utilize a high degree of genetic variation on a common structural scaffold to achieve a wide range of MHC-binding and T-cell-stimulating effects while assisting pathogen evasion of the adaptive immune response. Several new structures of SAG-MHC and SAG-TCR complexes have significantly increased understanding of the molecular bases for high-affinity peptide/MHC binding by SAGs and for TCR Vbeta domain specificity of SAGs. Using the currently available SAG-MHC and SAG-TCR complex structures, models of various trimolecular MHC-SAG-TCR complexes may be constructed that reveal wide diversity in the architecture of SAG-dependent T-cell signaling complexes, which nevertheless may result in similar signaling outcomes.

Structural and functional role of threonine 112 in a superantigen Staphylococcus aureus enterotoxin B

Bacterial superantigens are potent T-cell stimulatory protein molecules produced by Staphylococcus aureus and Streptococcus pyogenes. Their superantigenic activity can be attributed to their ability to cross-link major histocompatibility complex class II molecules with T-cell receptors (TCRs) to form a tri-molecular complex. Each superantigen is known to interact with a specific V(beta) element of TCR. Staphylococcal enterotoxin B (SEB, a superantigen), a primary cause of food poisoning, is also responsible for a significant percentage of non-menstrual associated toxic shock syndrome in patients with a variety of staphylococcal infections. Structural studies have elucidated a binding cavity on the toxin molecule essential for TCR binding. To understand the crucial residues involved in binding, mutagenesis analysis was performed. Our analysis suggest that mutation of a conserved residue Thr(112) to Ser (T112S) in the binding cavity induces a selective reduction in the affinity for binding one TCR V(beta) family and can be attributed to the structural differences in the native and mutant toxins. We present a detailed comparison of the mutant structure determined at 2.0 A with the previously reported native SEB and SEB-TCR V(beta) complex structures.

Crystal structure of a superantigen bound to the high-affinity, zinc-dependent site on MHC class II

MHC class II molecules possess two binding sites for bacterial superantigens (SAGs): a low-affinity site on the alpha chain and a high-affinity, zinc-dependent site on the beta chain. Only the former has been defined crystallographically. We report the structure of streptococcal pyrogenic exotoxin C (SPE-C) complexed with HLA-DR2a (DRA*0101, DRB5*0101) bearing a self-peptide from myelin basic protein (MBP). SPE-C binds the beta chain through a zinc bridge that links the SAG and class II molecules. Surprisingly, SPE-C also makes extensive contacts with the MBP peptide, such that peptide accounts for one third of the surface area of the MHC molecule buried in the complex, similar to TCR-peptide/MHC complexes. Thus, SPE-C may optimize T cell responses by mimicking the peptide dependence of conventional antigen presentation and recognition.

Microbial superantigens: from structure to function

Superantigens are highly potent immune stimulators with a unique ability to interact simultaneously with MHC class II molecules and T cell receptors, forming a trimolecular complex that induces profound T-cell proliferation and massive cytokine production. Recent structural studies have provided a wealth of information regarding these complex interactions, and it is now emerging that, despite their common 3-D architecture, superantigens are able to crosslink MHC class II molecules and T cell receptors in a variety of ways.

Purification and biochemical characterization of a basic superantigen (SPEX/SMEZ3) from Streptococcus pyogenes

A potent basic superantigen (designated streptococcal pyrogenic exotoxin X, SPEX/SMEZ3) was purified to homogeneity from culture supernatants of a Streptococcus pyogenes scarlatina strain of type 12 (genotype speA(-), speC(-)) and characterized. Sequence alignments revealed SPEX to be an allele of the streptococcal mitogens type Z (SMEZ). The N-terminal amino acid sequence of SPEX was found with LEVDNNSLLR to be identical to the recently described acidic superantigen SMEZ. Although SPEX/SMEZ genes were present in all of the streptococcal strains tested, a toxin production could only be detected in a small number of strains. The produced toxin concentration in the culture supernatants of positive strains differed between 0 and 20 ng ml(-1). The purified SPEX stimulated human T-lymphocytes with Vbeta8 specificity at extremely low concentrations (lower than 100 pg ml(-1)).

Conservation and variation in superantigen structure and activity highlighted by the three-dimensional structures of two new superantigens from Streptococcus pyogenes

Bacterial superantigens (SAgs) are a structurally related group of protein toxins secreted by Staphylococcus aureus and Streptococcus pyogenes. They are implicated in a range of human pathologies associated with bacterial infection whose symptoms result from SAg-mediated stimulation of a large number (2-20%) of T-cells. At the molecular level, bacterial SAgs bind to major histocompatability class II (MHC-II) molecules and disrupt the normal interaction between MHC-II and T-cell receptors (TCRs). We have determined high-resolution crystal structures of two newly identified streptococcal superantigens, SPE-H and SMEZ-2. Both structures conform to the generic bacterial superantigen folding pattern, comprising an OB-fold N-terminal domain and a beta-grasp C-terminal domain. SPE-H and SMEZ-2 also display very similar zinc-binding sites on the outer concave surfaces of their C-terminal domains. Structural comparisons with other SAgs identify two structural sub-families. Sub-families are related by conserved core residues and demarcated by variable binding surfaces for MHC-II and TCR. SMEZ-2 is most closely related to the streptococcal SAg SPE-C, and together they constitute one structural sub-family. In contrast, SPE-H appears to be a hybrid whose N-terminal domain is most closely related to the SEB sub-family and whose C-terminal domain is most closely related to the SPE-C/SMEZ-2 sub-family. MHC-II binding for both SPE-H and SMEZ-2 is mediated by the zinc ion at their C-terminal face, whereas the generic N-terminal domain MHC-II binding site found on many SAgs appears not to be present. Structural comparisons provide evidence for variations in TCR binding between SPE-H, SMEZ-2 and other members of the SAg family; the extreme potency of SMEZ-2 (active at 10(-15) g ml-1 levels) is likely to be related to its TCR binding properties. The smez gene shows allelic variation that maps onto a considerable proportion of the protein surface. This allelic variation, coupled with the varied binding modes of SAgs to MHC-II and TCR, highlights the pressure on SAgs to avoid host immune defences.

Mapping the energy of superantigen Staphylococcus enterotoxin C3 recognition of an alpha/beta T cell receptor using alanine scanning mutagenesis

Binding of the T cell receptor (TCR) to a bacterial superantigen (SAG) results in stimulation of a large population of T cells and subsequent inflammatory reactions. To define the functional contribution of TCR residues to SAG recognition, binding by 24 single-site alanine substitutions in the TCR Vbeta domain to Staphylococcus aureus enterotoxin (SE) C3 was measured, producing an energy map of the TCR-SAG interaction. The results showed that complementarity determining region 2 (CDR2) of the Vbeta contributed the majority of binding energy, whereas hypervariable region 4 (HV4) and framework region 3 (FR3) contributed a minimal amount of energy. The crystal structure of the Vbeta8.2-SEC3 complex suggests that the CDR2 mutations act by disrupting Vbeta main chain interactions with SEC3, perhaps by affecting the conformation of CDR2. The finding that single Vbeta side chain substitutions had significant effects on binding and that other SEC3-reactive Vbeta are diverse at these same positions indicates that SEC3 binds to other TCRs through compensatory mechanisms. Thus, there appears to be strong selective pressure on SAGs to maintain binding to diverse T cells.

Structural similarities and differences in Staphylococcus aureus exfoliative toxins A and B as revealed by their crystal structures

Staphylococcal aureus epidermolytic toxins (ETs) A and B are responsible for the induction of staphylococcal scalded skin syndrome, a disease of neonates and young children. The clinical features of this syndrome vary from localized blisters to severe exfoliation affecting most of the body surface. Comparison of the crystal structures of two subtypes of ETs-rETA (at 2.0 A resolution), rETB (at 2.8 A resolution), and an active site variant of rETA, Ser195Ala at 2.0 A resolution has demonstrated that their overall topology resembles that of a "trypsin-like" serine protease, but with significant differences at the N- and C-termini and loop regions. The details of the catalytic site in both ET structures are very similar to those in glutamate-specific serine proteases, suggesting a common catalytic mechanism. However, the "oxyanion hole," which is part of the catalytic sites of glutamate specific serine proteases, is in the closed or inactive conformation for rETA, yet in the open or active conformation for rETB. The ETs contain a unique amphipathic helix at the N-terminus, and it appears to be involved in optimizing the conformation of the catalytic site residues. Determination of the structure of the rETA catalytic site variant, Ser195Ala, showed no significant perturbation at the active site, establishing that the loss of biological and esterolytic activity can be attributed solely to disruption of the catalytic serine residue. Finally, the crystal structure of ETs, together with biochemical data and mutagenesis studies, strongly confirms the classification of these molecules as "serine proteases" rather than "superantigens."

The Extracellular Domain of the ζ-Chain Is Essential for TCR Function

The ζ-chain homodimer is a key component in the TCR complex and exerts its function through its cytoplasmic immunoreceptor-tyrosine activation motif (1). The ζ-chain extracellular (EC) domain is highly conserved; however, its functional and structural contributions to the TCR signaling have not been elucidated. We show that the EC domain of the ζ homodimer is essential for TCR surface expression. To gain a more detailed structural and functional information about the ζ-chain EC domain, we applied a cysteine scanning mutagenesis to conserved amino acids of the short domain. The results showed that the interchain disulfide bridge can be displaced by seven or eight amino acids along the EC domain. The TCR signaling efficacy was dramatically reduced during peptide/MHC engagement in the ζ mutants containing the displaced disulfide bond. These signaling defective ζ mutants produced an unconventional early tyrosine phosphorylation pattern. While the tyrosine phosphorylated forms of ζ (p21 and p23) could be observed during Ag stimulation, downstream signaling events such as the generation of phospho-p36, higher m.w. forms of phospho-ζ, and phospho-ζ/ZAP-70 complexes were impaired. Together these results suggest an important function of the phylogenetically conserved ζ-EC domain.

Structural basis for the recognition of superantigen streptococcal pyrogenic exotoxin A (SpeA1) by MHC class II molecules and T-cell receptors

Streptococcal pyrogenic exotoxin A (SpeA) is a superantigen produced by Streptococcus pyogenes and is associated with severe infections characterized by rash, hypotension, multiorgan failure and a high mortality rate. In this study, an allelic form of this toxin, SpeA1, was crystallized with four molecules in the crystallographic asymmetric unit and its crystal structure was determined at 2.6 A resolution. The crystallographic R-factor was 19.4% (33 497 reflections) for 7031 protein atoms and 88 water molecules. The overall structure of SpeA1 is considerably similar to that of other prototype microbial superantigens, either of staphylococcal or streptococcal origin, but has greatest similarity to staphylococcal enterotoxin C (SEC). Based on structural and mutagenesis data, we have mapped several important residues on the toxin molecule, which are involved in the recognition of major histocompatibility complex (MHC) class II molecules and T-cell receptors. Also, the toxin appears to possess a potential zinc-binding site which may have implications in binding to particular MHC class II molecules. Finally, we propose models for SpeA1-MHC class II and SpeA1-T-cell receptor association and the relevance of this phenomenon to the superantigenic action of this toxin is considered.

Bacterial toxins: friends or foes?

Many emerging and reemerging bacterial pathogens synthesize toxins that serve as primary virulence factors. We highlight seven bacterial toxins produced by well-established or newly emergent pathogenic microbes. These toxins, which affect eukaryotic cells by a variety of means, include Staphylococcus aureus alpha-toxin, Shiga toxin, cytotoxic necrotizing factor type 1, Escherichia coli heat-stable toxin, botulinum and tetanus neurotoxins, and S. aureus toxic-shock syndrome toxin. For each, we discuss the information available on its synthesis and structure, mode of action, and contribution to virulence. We also review the role certain toxins have played in unraveling signal pathways in eukaryotic cells and summarize the beneficial uses of toxins and toxoids. Our intent is to illustrate the importance of the analysis of bacterial toxins to both basic and applied sciences.

Crystal structure of microbial superantigen staphylococcal enterotoxin B at 1.5 A resolution: implications for superantigen recognition by MHC class II molecules and T-cell receptors

Staphylococcal enterotoxin B is a member of a family of toxins known as superantigens that activate a large number of T-cells (up to 20%) by cross-linking MHC class II molecules with T-cell receptors in a Vbeta-restricted fashion. The crystal structure of staphylococcal enterotoxin B presented here has been determined at 1.5 A resolution, the highest resolution so far for a superantigen. The final model contains 1948 protein atoms and 177 water molecules and has excellent geometry with root-mean-square (rms) deviation of 0.007 A and 1.73 degrees in bond lengths and bond angles, respectively. The overall fold is similar to that of other microbial superantigens, but as it lacks the zinc-binding site found in other members of this family, such as staphylococcal enterotoxin A, C2 and D, this enterotoxin possesses only one MHC class II binding site. Comparison of the crystal structure of free SEB and in complex with an MHC class II molecule revealed no major changes in the MHC-binding site upon complex formation. However, a number of water molecules found in the free SEB may be displaced in the complex or contribute further to its stability. Detailed analysis of the TcR-binding site of SEB, SEA and SEC2 shows significant differences which may account for the ability of each superantigen to bind specific Vbeta sequences.

Superantigens of gram-positive bacteria: structure-function analyses and their implications for biological activity

Just as we thought that we know everything about superantigens, new molecular and structural studies indicate that we have only just begun to unravel the secrets of these fascinating molecules. Recent structure-function analysis of superantigens from Gram-positive bacteria, with emphasis on their interaction with major histocompatibility complex molecules, could help us decipher the role of superantigens in disease, identify host factors that potentiate their effects and design drugs that specifically block their activity.