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

Enhancement of antitumor response of staphylococcal enterotoxin C2 mutant 2M-118 by promoting cell-mediated antitumor immunity

BACKGROUND

Staphylococcal enterotoxin C2 (SEC2) is used as an immunotherapeutic drug in China. However, SEC2 are limited due to its immunosuppressive and toxic effects. A SEC2 2M-118 (H118A/T20L/G22E) mutant generated by site-directed mutagenesis was studied to elucidate the underlying antitumor mechanism.

METHODS

The effects of 2M-118 on mouse fibrosarcoma (Meth-A) cells and cytokine responses were tested in vitro using a transwell assay and ELISA, respectively. 2M-118 effect on immune function in tumor-bearing mice was tested. Cytokine levels and antitumor responses were measured using ELISA and flow cytometry, respectively. TUNEL staining and immunohistochemistry were employed to detect the tumor apoptosis and CD4+ and CD8+ tumor infiltrating lymphocytes (TILs) in tumor tissue.

RESULTS

2M-118 demonstrated the growth inhibition on tumor cells, increase of cytokines production (IL-2, IFN-γ, and TNF-α) and splenocyte proliferation in vitro. 2M-118 effectively inhibited tumor development and increased lymphocytes and cytokines in a tumor-bearing mouse model. Additionally, 2M-118 regulated the tumormicroenvironment by reducing the number of myeloid-derived suppressor cells (MDSCs), increasing the number of TILs, and inducing tumorcell apoptosis.

CONCLUSION

2M-118 promotes immune function and enhances antitumor response. This indicates that 2M-118 could potentially be developed as a novel anti-tumor drug with-highefficiencyandlowtoxicity.

Identification of a novel fully human anti-toxic shock syndrome toxin (TSST)-1 single-chain variable fragment antibody averting TSST-1-induced mitogenesis and cytokine secretion

Background

Staphylococcal superantigens are virulence factors that help the pathogen escape the immune system and develop an infection. Toxic shock syndrome toxin (TSST)-1 is one of the most studied superantigens whose role in toxic shock syndrome and some particular disorders have been demonstrated. Inhibiting TSST-1 production with antibiotics and targeting TSST-1 with monoclonal antibodies might be one of the best strategies to prevent TSST-1-induced cytokines storm followed by lethality.

Results

A novel single-chain variable fragment (scFv), MS473, against TSST-1 was identified by selecting an scFv phage library on the TSST-1 protein. The MS473 scFv showed high affinity and specificity for TSST-1. Moreover, MS473 could significantly prevent TSST-1-induced mitogenicity (the IC₅₀ value: 1.5 µM) and cytokine production.

Conclusion

Using traditional antibiotics with an anti-TSST-1 scFv as a safe and effective agent leads to deleting the infection source and preventing the detrimental effects of the toxin disseminated into the whole body.

Supplementary information

The online version contains supplementary material available at 10.1186/s12896-022-00760-8.

Staphylococcal Enterotoxin C-An Update on SEC Variants, Their Structure and Properties, and Their Role in Foodborne Intoxications

Staphylococcal enterotoxins are the most common cause of foodborne intoxications (staphylococcal food poisoning) and cause a wide range of diseases. With at least six variants staphylococcal enterotoxin C (SEC) stands out as particularly diverse amongst the 25 known staphylococcal enterotoxins. Some variants present unique and even host-specific features. Here, we review the role of SEC in human and animal health with a particular focus on its role as a causative agent for foodborne intoxications. We highlight structural features unique to SEC and its variants, particularly, the emetic and superantigen activity, as well as the roles of SEC in mastitis and in dairy products. Information about the genetic organization as well as regulatory mechanisms including the accessory gene regulator and food-related stressors are provided.

Structural Refinement of Proteins by Restrained Molecular Dynamics Simulations with Non-interacting Molecular Fragments

The knowledge of multiple conformational states is a prerequisite to understand the function of membrane transport proteins. Unfortunately, the determination of detailed atomic structures for all these functionally important conformational states with conventional high-resolution approaches is often difficult and unsuccessful. In some cases, biophysical and biochemical approaches can provide important complementary structural information that can be exploited with the help of advanced computational methods to derive structural models of specific conformational states. In particular, functional and spectroscopic measurements in combination with site-directed mutations constitute one important source of information to obtain these mixed-resolution structural models. A very common problem with this strategy, however, is the difficulty to simultaneously integrate all the information from multiple independent experiments involving different mutations or chemical labels to derive a unique structural model consistent with the data. To resolve this issue, a novel restrained molecular dynamics structural refinement method is developed to simultaneously incorporate multiple experimentally determined constraints (e.g., engineered metal bridges or spin-labels), each treated as an individual molecular fragment with all atomic details. The internal structure of each of the molecular fragments is treated realistically, while there is no interaction between different molecular fragments to avoid unphysical steric clashes. The information from all the molecular fragments is exploited simultaneously to constrain the backbone to refine a three-dimensional model of the conformational state of the protein. The method is illustrated by refining the structure of the voltage-sensing domain (VSD) of the Kv1.2 potassium channel in the resting state and by exploring the distance histograms between spin-labels attached to T4 lysozyme. The resulting VSD structures are in good agreement with the consensus model of the resting state VSD and the spin-spin distance histograms from ESR/DEER experiments on T4 lysozyme are accurately reproduced.

Knowledge of multiple conformational states of membrane transport proteins is a prerequisite to understand their function. However, the determination of atomic structures for all these states with conventional high-resolution approaches can be very challenging due to inherent difficulties in high yield purification of functional membrane transport proteins. Various complementary structural information of proteins in their native states can be obtained by a variety of biophysical and biochemical methods with site-directed mutations. Here, a novel restrained molecular dynamics structural refinement method is developed to help derive a structural model that is consistent with experimental data by incorporating all the experimental constraints simultaneously through the use of non-interacting all-atom molecular fragments. The method can be easily and effectively extended to incorporate many kinds of structural constraints from a variety of biophysical and biochemical experiments, and should be very useful in generating and refining models of proteins in specific functional states.

Structural basis for the neutralization and specificity of Staphylococcal enterotoxin B against its MHC Class II binding site

Staphylococcal enterotoxin (SE) B is among the potent toxins produced by Staphylococcus aureus that cause toxic shock syndrome (TSS), which can result in multi-organ failure and death. Currently, neutralizing antibodies have been shown to be effective immunotherapeutic agents against this toxin, but the structural basis of the neutralizing mechanism is still unknown. In this study, we generated a neutralizing monoclonal antibody, 3E2, against SEB, and analyzed the crystal structure of the SEB-3E2 Fab complex. Crystallographic analysis suggested that the neutralizing epitope overlapped with the MHC II molecule binding site on SEB, and thus 3E2 could inhibit SEB function by preventing interaction with the MHC II molecule. Mutagenesis studies were done on SEB, as well as the related Staphylococcus aureus toxins SEA and SEC. These studies revealed that tyrosine (Y)46 and lysine (K)71 residues of SEB are essential to specific antibody-antigen recognition and neutralization. Substitution of Y at SEA glutamine (Q)49, which corresponds to SEB Y46, increased both 3E2's binding to SEA in vitro and the neutralization of SEA in vivo. These results suggested that SEB Y46 is responsible for distinguishing SEB from SEA. These findings may be helpful for the development of antibody-based therapy for SEB-induced TSS.

Functional characterization of the twin ZIP/SLC39 metal transporters, NpunF3111 and NpunF2202 in Nostoc punctiforme

The ZIP family of metal transporters is involved in the transport of Zn(2+) and other metal cations from the extracellular environment and/or organelles into the cytoplasm of prokaryotes, eukaryotes and archaeotes. In the present study, we identified twin ZIP transporters, Zip11 (Npun_F3111) and Zip63 (Npun_F2202) encoded within the genome of the filamentous cyanobacterium, Nostoc punctiforme PCC73120. Sequence-based analyses and structural predictions confirmed that these cyanobacterial transporters belong to the SLC39 subfamily of metal transporters. Quantitative real-time (QRT)-PCR analyses suggested that the enzymes encoded by zip11 and zip63 have a broad allocrite range that includes zinc as well as cadmium, cobalt, copper, manganese and nickel. Inactivation of either zip11 or zip63 via insertional mutagenesis in N. punctiforme resulted in reduced expression of both genes, highlighting a possible co-regulation mechanism. Uptake experiments using (65)Zn demonstrated that both zip mutants had diminished zinc uptake capacity, with the deletion of zip11 resulting in the greatest overall reduction in (65)Zn uptake. Over-expression of Zip11 and Zip63 in an E. coli mutant strain (ZupT736::kan) restored divalent metal cation uptake, providing further evidence that these transporters are involved in Zn uptake in N. punctiforme. Our findings show the functional role of these twin metal uptake transporters in N. punctiforme, which are independently expressed in the presence of an array of metals. Both Zip11 and Zip63 are required for the maintenance of homeostatic levels of intracellular zinc N. punctiforme, although Zip11 appears to be the primary zinc transporter in this cyanobacterium, both ZIP's may be part of a larger metal uptake system with shared regulatory elements.

Increased T-cell stimulating activity by mutated SEC2 correlates with its improved antitumour potency

AIMS

To investigate the improved antitumour activity of SAM-3 compared with recombinant staphylococcal enterotoxins C2 (rSEC2).

METHODS AND RESULTS

Methylthiazol tetrazolium and flow cytometry assays showed that the antitumour activity of SAM-3 in vivo was improved because of enhanced T-cell stimulating potency, resulting in massive activation of T cells, particularly CD4(+) and CD8(+) T cells, and subsequent cytokine release. Quantitative real-time PCR assay showed that despite similar Vβ specificities induced by rSEC2 and SAM-3, the quantities of activated T cells bearing specific Vβin vitro were different.

CONCLUSIONS

The results strongly suggested that the increased SAM-3-T-cell receptor (TCR) binding affinity contributed to massive T-cell activation and cytokine release, substantially amplifying antitumour immune response in vivo.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study provided evidence for the mechanism of SAM-3 antitumour activity improvement compared with rSEC2. Results indicated that SAM-3 could be used as a potent powerful candidate agent for tumour treatment in clinics.

Staphylococcus aureus isolates encode variant staphylococcal enterotoxin B proteins that are diverse in superantigenicity and lethality

Staphylococcus aureus produces superantigens (SAgs) that bind and cross-link T cells and APCs, leading to activation and proliferation of immune cells. SAgs bind to variable regions of the β-chains of T cell receptors (Vβ-TCRs), and each SAg binds a unique subset of Vβ-TCRs. This binding leads to massive cytokine production and can result in toxic shock syndrome (TSS). The most abundantly produced staphylococcal SAgs and the most common causes of staphylococcal TSS are TSS toxin-1 (TSST-1), and staphylococcal enterotoxins B and C (SEB and SEC, respectively). There are several characterized variants of humans SECs, designated SEC1-4, but only one variant of SEB has been described. Sequencing the seb genes from over 20 S. aureus isolates show there are at least five different alleles of seb, encoding forms of SEB with predicted amino acid substitutions outside of the predicted immune-cell binding regions of the SAgs. Examination of purified, variant SEBs indicates that these amino acid substitutions cause differences in proliferation of rabbit splenocytes in vitro. Additionally, the SEBs varied in lethality in a rabbit model of TSS. The SEBs were diverse in their abilities to cause proliferation of human peripheral blood mononuclear cells, and differed in their activation of subsets of T cells. A soluble, high-affinity Vβ-TCR, designed to neutralize the previously characterized variant of SEB (SEB1), was able to neutralize the variant SEBs, indicating that this high-affinity peptide may be useful in treating a variety of SEB-mediated illnesses.

An engineered superantigen SEC2 exhibits promising antitumor activity and low toxicity

Recent studies suggested that the histidine residues at 118 and 122 play an important role for the toxicity of staphylococcal enterotoxin C subtype 2 (SEC2), and the substitutions of both histidines with alanine can severely impair the fever activity of SEC2. We hypothesized that promising SEC2 antitumor agent with low toxicity and enhanced superantigen activity can be constructed by introducing related mutations at protein functional sites of SEC2. We showed that the SEC2 mutants H122A and H118A/H122A exhibited improved superantigen activity after introducing the point mutations at Thr20 and Gly22. A resultant mutant, named as SAM-3, has considerable abilities to inhibit the growth of H22 and Hepa1-6 tumor cells in vitro and colon 26 solid tumor in vivo. Furthermore, SAM-3 also exhibits significantly reduced toxicity compared with native SEC2. The study provides a novel strategy for designing promising superantigen immunotherapeutic agent. The constructed SEC2 mutant SAM-3 can be used as a powerful candidate for cancer immunotherapy and could compensate the deficiency caused by toxicity of native SEC2 in clinic.

Crystal structure of staphylococcal enterotoxin G (SEG) in complex with a mouse T-cell receptor {beta} chain

Superantigens (SAgs) are bacterial or viral toxins that bind MHC class II (MHC-II) molecules and T-cell receptor (TCR) in a nonconventional manner, inducing T-cell activation that leads to inflammatory cytokine production, which may result in acute toxic shock. In addition, the emerging threat of purpura fulminans and community-associated meticillin-resistant Staphylococcus aureus emphasizes the importance of a better characterization of SAg binding to their natural ligands that may allow the development of reagents to neutralize their action. The three-dimensional structure of the complex between a mouse TCR β chain (mVβ8.2) and staphylococcal enterotoxin G (SEG) at 2.0 Å resolution revealed a binding site that does not conserve the "hot spots" present in mVβ8.2-SEC2, mVβ8.2-SEC3, mVβ8.2-SEB, and mVβ8.2-SPEA complexes. Analysis of the mVβ8.2-SEG interface allowed us to explain the higher affinity of this complex compared with the others, which may account for the early activation of T-cells bearing mVβ8.2 by SEG. This mode of interaction between SEG and mVβ8.2 could be an adaptive advantage to bestow on the pathogen a faster rate of colonization of the host.

The T cell receptor beta-chain second complementarity determining region loop (CDR2beta governs T cell activation and Vbeta specificity by bacterial superantigens

Superantigens (SAgs) are microbial toxins defined by their ability to activate T lymphocytes in a T cell receptor (TCR) β-chain variable domain (Vβ)-specific manner. Although existing structural information indicates that diverse bacterial SAgs all uniformly engage the Vβ second complementarity determining region (CDR2β) loop, the molecular rules that dictate SAg-mediated T cell activation and Vβ specificity are not fully understood. Herein we report the crystal structure of human Vβ2.1 (hVβ2.1) in complex with the toxic shock syndrome toxin-1 (TSST-1) SAg, and mutagenesis of hVβ2.1 indicates that the non-canonical length of CDR2β is a critical determinant for recognition by TSST-1 as well as the distantly related SAg streptococcal pyrogenic exotoxin C. Frame work (FR) region 3 is uniquely critical for TSST-1 function explaining the fine Vβ-specificity exhibited by this SAg. Furthermore, domain swapping experiments with SAgs, which use distinct domains to engage both CDR2β and FR3/4β revealed that the CDR2β contacts dictate T lymphocyte Vβ-specificity. These findings demonstrate that the TCR CDR2β loop is the critical determinant for functional recognition and Vβ-specificity by diverse bacterial SAgs.

Epithelial barrier resistance is increased by the divalent cation zinc in cultured MDCKII epithelial monolayers

Topical zinc applications promote wound healing and epithelialization. "Leaky" MDCKII epithelia exposed to apical ZnCl₂ (10 mM) showed a time-dependent increase (t (0.5) 22.2 ± 2.7 min) of transepithelial resistance (R (t)) from 82.3 ± 2.4 Ω cm² to 1,551 ± 225.6 Ω cm²; the increase was dose-dependent, being observed at 3 mM but not at 1 mM. Basal Zn²+ applications also increased epithelial resistance (at 10 mM to 323 ± 225.6 Ω cm²). The linear current-voltage relationship in control epithelia changed after apical 10 mM ZnCl₂ to show rectification. Voltage deflections resulting from inward currents showed time-dependent relaxation (basal potential difference (p.d.)-positive), with outward currents being time-independent. Cation selectivity was tested after apical ZnCl₂ elevated resistance; both the NaCl:mannitol (basal replacement) dilution p.d. and the choline:Na bi-ionic p.d. decreased (P(Na)/P(Cl) from 4.9 to 2.3 and P(Na)/P(choline) from 3.8 to 2.1, respectively). Transepithelial paracellular basal to apical ⁴⁵Ca fluxes increased approximately twofold when driven by a basal positive Na:NMDG bi-ionic p.d., but with basal 10 mM ZnCl₂, ⁴⁵Ca fluxes decreased approximately twofold. Neither ZO-1 nor occludin distribution was altered after ~2-h exposure to apical 10 mM ZnCl₂. However, claudin-2, though present at the tight junction, increased within the cell. Increased epithelial barrier resistance by Zn²+ is due to modification of the paracellular pathway, most probably by multiple mechanisms.

Generation of Fab fragment-like molecular recognition proteins against staphylococcal enterotoxin B by phage display technology

Antigen-binding fragments (Fab fragments) and single-chain variable fragments (scFv) against staphylococcal enterotoxin B (SEB) were produced by phage display technology. SEB epitopes were first identified by phage display approach using the commercial anti-SEB monoclonal antibody ab53981 as the target. Heptamer and dodecamer mimotope peptides recognized by ab53981 were screened from Ph.D-7 or Ph.D-12 random peptide phage libraries expressed in Escherichia coli. The isolated 7-mer and 12-mer mimotopes were shown to share a sequence homologous to ⁸PDELHK¹⁴S in the amino acid sequence of SEB. The N-terminal 15-mer peptide of SEB was determined to be an epitope of ab53981. After immunization of mice with maltose-binding protein-tagged N-terminal 15-mer peptide, a phage display Fab library was constructed using cDNA prepared from the mRNAs of spleen cells. Three phage clones displaying the Fab molecule which recognized SEB were isolated through three rounds of panning. Only one of them produced a soluble Fab fragment from the transformed cells, and the fragment fused with a histidine tag sequence was produced in E. coli cells and converted into scFv. Surface plasmon resonance analysis showed that the dissociation constants of these proteins with SEB were (4.1 ± 1.1) × 10⁻⁹ M and (8.4 ± 2.3) × 10⁻¹⁰ M, respectively. The produced molecule was applied to the determination of SEB by enzyme-linked immunosorbent assay and Western blot analysis.

Staphylococcal enterotoxins

Staphylococcus aureus (S. aureus) is a Gram positive bacterium that is carried by about one third of the general population and is responsible for common and serious diseases. These diseases include food poisoning and toxic shock syndrome, which are caused by exotoxins produced by S. aureus. Of the more than 20 Staphylococcal enterotoxins, SEA and SEB are the best characterized and are also regarded as superantigens because of their ability to bind to class II MHC molecules on antigen presenting cells and stimulate large populations of T cells that share variable regions on the β chain of the T cell receptor. The result of this massive T cell activation is a cytokine bolus leading to an acute toxic shock. These proteins are highly resistant to denaturation, which allows them to remain intact in contaminated food and trigger disease outbreaks. A recognized problem is the emergence of multi-drug resistant strains of S. aureus and these are a concern in the clinical setting as they are a common cause of antibiotic-associated diarrhea in hospitalized patients. In this review, we provide an overview of the current understanding of these proteins.

The systemic and pulmonary immune response to staphylococcal enterotoxins

In response to environmental cues the human pathogen Staphylococcus aureus synthesizes and releases proteinaceous enterotoxins. These enterotoxins are natural etiologic entities of severe food poisoning, toxic shock syndrome, and acute diseases. Staphylococcal enterotoxins are currently listed as Category B Bioterrorism Agents by the Center for Disease Control and Prevention. They are associated with respiratory illnesses, and may contribute to exacerbation of pulmonary disease. This likely stems from the ability of Staphylococcal enterotoxins to elicit powerful episodes of T cell stimulation resulting in release of pro-inflammatory cytokines. Here, we discuss the role of the immune system and potential mechanisms of disease initiation and progression.

Food poisoning and Staphylococcus aureus enterotoxins

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

Antitumour response of a double mutant of staphylococcal enterotoxin C2 with the decreased affinity for MHC class II molecule

Staphylococcal enterotoxin C2 (SEC2) is one of the most potent known activators of human T lymphocytes, and recombinant SEC2 shows promising clinical values, but SEC2 can cause food poisoning and toxic shock syndrome in vivo. In this study, site-directed mutagenesis has been used to introduce alanine substitutions at Phe144 and Leu45 in the molecule. The mutant genes were cloned and expressed, and the corresponding proteins were purified by nickel agarose affinity chromatography. We found that the SEC2 mutant proteins could stimulate the proliferation of human peripheral blood lymphocytes and inhibit the growth of tumour cells as native SEC2. Furthermore, flow cytometry assay showed that mSEC2(F44A, L45A) drastically reduced the ability of the toxin to bind to MHC class II. Physiological parameters revealed that mSEC2(F44A, L45A) reduced significantly rat temperature compared with native SEC2 in vivo. Our results clearly suggest that this genetically modified SEC2 protein is less toxic and justifies its further development as a new, safer antitumour superantigen to prevent SEC2 intoxication.

Biological characterization of the zinc site coordinating histidine residues of staphylococcal enterotoxin C2

The bacterial toxin staphylococcal enterotoxin C2 (SEC2) can cause staphylococcal toxic shock syndrome and food poisoning. Although the previously determined crystal structure of SEC2 revealed that some histidine residues (His47, His118 and His122) contribute to the binding of zinc ions, little is known about their biological roles in SEC2. This prompted us to investigate the role of the zinc site coordinating histidine residues in the biological activities of SEC2. The mutants with substitutions at positions 118 and 122 all retained T-cell stimulatory activity, whereas the histidine mutants at position 47 were defective in the ability to stimulate T-cell proliferation. Further toxicity assays in vivo indicated that mutants SEC2-H118A and SEC2-H122A were defective in emetic and febrile activities. However, mutant SEC2-H47A could cause significant emetic and febrile responses in comparison with the other two histidine mutants. These findings suggested that the zinc-coordinating histidine residues play significant roles in superantigen and toxic activities of SEC2 and further implied that superantigen and febrile activities could be separable in staphylococcal enterotoxins. The results also show that it should be possible to design new SEC2 immunotherapeutic agents that have superantigen activity and low toxicity.

Overview of protein folds in the immune system

The rapid advancement of X-ray crystallography and nuclear magnetic resonance techniques in recent years has resulted in the solution of macromolecular structures at an unprecedented rate. This review aims at providing a comprehensive description of structures and folds related to the function of the immune system. Focus is placed on immunologically relevant proteins such as immunoreceptors and major histocompatibility complexes. Information is also provided regarding protein structure data banks.

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.

Superantigen natural affinity maturation revealed by the crystal structure of staphylococcal enterotoxin G and its binding to T-cell receptor Vβ8.2

The illnesses associated with bacterial superantigens (SAgs) such as food poisoning and toxic shock syndrome, as well as the emerging threat of purpura fulminans and community-associated methicillin-resistant S. aureus producer of SAgs, emphasize the importance of a better characterization of SAg binding to their natural ligands, which would allow the development of drugs or biological reagents able to neutralize their action. SAgs are toxins that bind major histocompatibility complex class II molecules (MHC-II) and T-cell receptors (TCR), in a nonconventional manner, inducing T-cell activation that leads to production of cytokines such as tumor necrosis factor and interleukin-2, which may result in acute toxic shock. Previously, we cloned and expressed a new natural variant of staphylococcal enterotoxin G (SEG) and evaluated its ability to stimulate in vivo murine T-cell subpopulations. We found an early, strong, and widespread stimulation of mouse Vbeta8.2 T-cells when compared with other SAgs member of the SEB subfamily. In search for the reason of the strong mitogenic potency, we determined the SEG crystal structure by X-ray crystallography to 2.2 A resolution and analyzed SEG binding to mVbeta8.2 and MHC-II. Calorimetry and SPR analysis showed that SEG has an affinity for mVbeta8.2 40 to 100-fold higher than that reported for other members of SEB subfamily, and the highest reported for a wild type SAg-TCR couple. We also found that mutations introduced in mVbeta8.2 to produce a high affinity mutant for other members of the SEB subfamily do not greatly affect binding to SEG. Crystallographic analysis and docking into mVbeta8.2 in complex with SEB, SEC3, and SPEA showed that the deletions and substitution of key amino acids remodeled the putative surface of the mVbeta8.2 binding site without affecting the binding to MHC-II. This results in a SAg with improved binding to its natural ligands, which may confer a possible evolutionary advantage for bacterial strains expressing SEG.

Zinc induces dimerization of the class II major histocompatibility complex molecule that leads to cooperative binding to a superantigen

Dimerization of class II major histocompatibility complex (MHC) plays an important role in the MHC biological function. Mycoplasma arthritidis-derived mitogen (MAM) is a superantigen that can activate large fractions of T cells bearing specific T cell receptor Vbeta elements. Here we have used structural, sedimentation, and surface plasmon resonance detection approaches to investigate the molecular interactions between MAM and the class II MHC molecule HLA-DR1 in the context of a hemagglutinin peptide-(306-318) (HA). Our results revealed that zinc ion can efficiently induce the dimerization of the HLA-DR1/HA complex. Because the crystal structure of the MAM/HLA-DR1/hemagglutinin complex in the presence of EDTA is nearly identical to the structure of the complex crystallized in the presence of zinc ion, Zn(2+) is evidently not directly involved in the binding between MAM and HLA-DR1. Sedimentation and surface plasmon resonance studies further revealed that MAM binds the HLA-DR1/HA complex with high affinity in a 1:1 stoichiometry, in the absence of Zn(2+). However, in the presence of Zn(2+), a dimerized MAM/HLA-DR1/HA complex can arise through the Zn(2+)-induced DR1 dimer. In the presence of Zn(2+), cooperative binding of MAM to the DR1 dimer was also observed.

Mutagenesis, biochemical, and biophysical characterization of Mycoplasma arthritidis-derived mitogen

Mycoplasma arthritidis-derived mitogen (MAM) is a superantigen (SAg) that can activate large fractions of T cells bearing particular TCR Vbeta elements. Here we report the mutagenesis, biochemical and biophysical studies on the dimerization of MAM in solution. Our studies showed that although MAM mainly exists as a monomer in solution, a small percentage of MAM molecules form homodimer at high protein concentration, regardless of the presence of Zn2+. A distinct peak corresponding to a MAM homodimer was detected in the presence of EDTA, using both chemical cross-linking and analytical ultracentrifugation methods. Further mutagenesis studies revealed that single mutation of residues at the interface of the crystallographic dimer of MAM does not significantly affect the dimerization of MAM in solution. Circular dichroism (CD) analysis indicated that addition of Zn2+ does not induce conformational changes of MAM from its apo-state. Thermal denaturation experiments indicated that addition of Zn2+ to MAM solution resulted in a decrease of melting point (Tm), whereas addition of EDTA did not affect the Tm of MAM. These results imply that there is no defined Zn2+-binding site on MAM.

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.

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.

Superantigens: structure-function relationships

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

Interplay Between Superantigens and Immunoreceptors

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

Identification of 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.

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

Staphylococcal enterotoxin H (SEH) is a bacterial superantigen secreted by Staphylococcus aureus. Superantigens are presented on the MHC class II and activate large amounts of T cells by cross-linking APC and T cells. In this study, RT-PCR was used to show that SEH stimulates human T cells via the Valpha domain of TCR, in particular Valpha10 (TRAV27), while no TCR Vbeta-specific expansion was seen. This is in sharp contrast to all other studied bacterial superantigens, which are highly specific for TCR Vbeta. It was further confirmed by flow cytometry that SEH stimulation does not alter the levels of certain TCR Vbeta. In a functional assay addressing cross-reactivity, Vbeta binding superantigens were found to form one group, whereas SEH has different properties that fit well with Valpha reactivity. As SEH binds on top of MHC class II, an interaction between MHC and TCR upon SEH binding is not likely. This concludes that the specific expansion of TCR Valpha is not due to contacts between MHC and TCR, instead we suggest that SEH directly interacts with the TCR Valpha domain.

Staphylococcal and streptococcal superantigens: molecular, biological and clinical aspects

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

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.

Zinc-mediated dimerization and its effect on activity and conformation of staphylococcal enterotoxin type C

Staphylococcal enterotoxins are superantigen exotoxins that mediate food poisoning and toxic shock syndrome in humans. Despite their structural and functional similarities, superantigens display subtle differences in biological properties and modes of receptor binding as a result of zinc atoms bound differently in their crystal structures. For example, the crystal structures of the staphylococcal enterotoxins in the type C serogroup (SECs) contain a zinc atom coordinated by one aspartate and two histidine residues from one molecule and another aspartate residue from the next molecule, thus forming a dimer. This type of zinc ligation and zinc-mediated dimerization occurs in several SECs, but not in most other staphylococcal enterotoxin serogroups. This prompted us to investigate the potential importance of zinc in SEC-mediated pathogenesis. Site-directed mutagenesis was used to replace SEC zinc binding ligands with alanine. SEC mutants unable to bind zinc did not have major conformational alterations although they failed to form dimers. Zinc binding was not essential for T cell stimulation, emesis, or lethality although in general the mutants were less pyrogenic. Thus the zinc atom in SECs might represent a non-functional heavy atom in an exotoxin group that has diverged from related bacterial toxins containing crucial zinc atoms.

Staphylococcal enterotoxin H contrasts closely related enterotoxins in species reactivity

Staphylococcus aureus enterotoxin H (SEH) belongs to the staphylococcal enterotoxin (SE) family of superantigens (SAgs). SEH has structural similarities to other SE; however, its biological properties are less well characterized. SEH binds with high affinity to human major histocompatibility complex (MHC) class II and exhibits strong mitogenic activity in human T cells, although it was found to be less potent than the related SEA. Surprisingly and in sharp contrast to related SEs, SEH did not possess superantigen activity in murine T cells and T cells from three investigated rat strains. However, SEH bound to a high extent to murine MHC class II expressing cells and when presented by these cells SEH stimulated human T cells to proliferate. Thus, SEH interacts with the murine MHC class II molecule in a functional manner. Notably, SEH had an inhibitory effect on murine SEA response, demonstrating that SEH interferes with the SEA interactions with murine cells. Despite this, murine T cells did not proliferate regardless of whether SEH was presented on human or murine MHC class II expressing cells. Consequently, SEH differs in species reactivity as compared to related SEs and lacks critical properties for T-cell activation in mice. We propose that unlike other SEs, SEH does not interact with murine T cells since it is not recognized by murine T-cell receptors.

Toxic shock syndrome and bacterial superantigens: an update

Toxic shock syndrome (TSS) is an acute onset illness characterized by fever, rash formation, and hypotension that can lead to multiple organ failure and lethal shock, as well as desquamation in patients that recover. The disease is caused by bacterial superantigens (SAGs) secreted from Staphylococcus aureus and group A streptococci. SAGs bypass normal antigen presentation by binding to class II major histocompatibility complex molecules on antigen-presenting cells and to specific variable regions on the beta-chain of the T-cell antigen receptor. Through this interaction, SAGs activate T cells at orders of magnitude above antigen-specific activation, resulting in massive cytokine release that is believed to be responsible for the most severe features of TSS. This review focuses on clinical and epidemiological aspects of TSS, as well as important developments in the genetics, biochemistry, immunology, and structural biology of SAGs. From the evolutionary relationships between these important toxins, we propose that there are five distinct groups of SAGs.

Superantigen bacterial toxins: state of the art

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

Structural features of a zinc binding site in the superantigen strepococcal pyrogenic exotoxin A (SpeA1): implications for MHC class II recognition

Streptococcal pyrogenic exotoxin A (SpeA) is produced by Streptococcus pyogenes, and has been associated with severe infections such as scarlet fever and Streptococcal Toxic Shock Syndrome (STSS). In this study, the crystal structure of SpeA1 (the product of speA allele 1) in the presence of 2.5 mM zinc was determined at 2.8 A resolution. The protein crystallizes in the orthorhombic space group P2(1)2(1)2, with four molecules in the crystallographic asymmetric unit. The final structure has a crystallographic R-factor of 21.4% for 7,031 protein atoms, 143 water molecules, and 4 zinc atoms (one zinc atom per molecule). Four protein ligands-Glu 33, Asp 77, His 106, and His 110-form a zinc binding site that is similar to the one observed in a related superantigen, staphylococcoal enterotoxin C2. Mutant toxin forms substituting Ala for each of the zinc binding residues were generated. The affinity of these mutants for zinc ion confirms the composition of this metal binding site. The implications of zinc binding to SpeA1 for MHC class II recognition are explored using a molecular modeling approach. The results indicate that, despite their common overall architecture, superantigens appear to have multiple ways of complex formation with MHC class II molecules.

Role of the T cell receptor ligand affinity in T cell activation by bacterial superantigens

Similar to native peptide/MHC ligands, bacterial superantigens have been found to bind with low affinity to the T cell receptor (TCR). It has been hypothesized that low ligand affinity is required to allow optimal TCR signaling. To test this, we generated variants of Staphylococcus enterotoxin C3 (SEC3) with up to a 150-fold increase in TCR affinity. By stimulating T cells with SEC3 molecules immobilized onto plastic surfaces, we demonstrate that increasing the affinity of the SEC3/TCR interaction caused a proportional increase in the ability of SEC3 to activate T cells. Thus, the potency of the SEC3 variants correlated with enhanced binding without any optimum in the binding range covered by native TCR ligands. Comparable studies using anti-TCR antibodies of known affinity confirmed these observations. By comparing the biological potency of the two sets of ligands, we found a significant correlation between ligand affinity and ligand potency indicating that it is the density of receptor-ligand complexes in the T cell contact area that determines TCR signaling strength.

Immunological and biochemical characterization of streptococcal pyrogenic exotoxins I and J (SPE-I and SPE-J) from Streptococcus pyogenes

Recently, we described the identification of novel streptococcal superantigens (SAgs) by mining the Streptococcus pyogenes M1 genome database at Oklahoma University. Here, we report the cloning, expression, and functional analysis of streptococcal pyrogenic exotoxin (SPE)-J and another novel SAg (SPE-I). SPE-I is most closely related to SPE-H and staphylococcal enterotoxin I, whereas SPE-J is most closely related to SPE-C. Recombinant forms of SPE-I and SPE-J were mitogenic for PBL, both reaching half maximum responses at 0.1 pg/ml. Evidence from binding studies and cell aggregation assays using a human B-lymphoblastoid cell line (LG-2) suggests that both toxins exclusively bind to the polymorphic MHC class II beta-chain in a zinc-dependent mode but not to the generic MHC class II alpha-chain. The results from analysis by light scattering indicate that SPE-J exists as a dimer in solution above concentrations of 4.0 mg/ml. Moreover, SPE-J induced a rapid homotypic aggregation of LG-2 cells, suggesting that this toxin might cross-link MHC class II molecules on the cell surface by building tetramers of the type HLA-DRbeta-SPE-J-SPE-J-HLA-DRbeta. SPE-I preferably stimulates T cells bearing the Vbeta18.1 TCR, which is not targeted by any other known SAG: SPE-J almost exclusively stimulates Vbeta2.1 T cells, a Vbeta that is targeted by several other streptococcal SAgs, suggesting a specific role for this T cell subpopulation in immune defense. Despite a primary sequence diversity of 51%, SPE-J is functionally indistinguishable from SPE-C and might play a role in streptococcal disease, which has previously been addressed to SPE-C.

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.

Structure of streptococcal pyrogenic exotoxin A reveals a novel metal cluster

The streptococcal pyrogenic toxins A, B, and C (SPEA, SPEB, and SPEC) are responsible for the fever, rash, and other toxicities associated with scarlet fever and streptococcal toxic shock syndrome. This role, together with the ubiquity of diseases caused by Streptococcus pyogenes, have prompted structural analyses of SPEA by several groups. Papageorgiou et al. (1999) have recently reported the structure of SPEA crystallized in the absence of zinc. Zinc has been shown to be important in the ability of some staphylococcal and streptococcal toxins to stimulate proliferation of CD4+ T-cells. Since cadmium is more electron dense than zinc and typically binds interchangeably, we grew crystals in the presence of 10 mM CdCl2. Crystals have been obtained in three space groups, and the structure in the P2(1)2(1)2(1) crystal form has been refined to 1.9 A resolution. The structural analysis revealed an identical tetramer as well as a novel tetrahedral cluster of cadmium in all three crystal forms on a disulfide loop encompassing residues 87-98. No cadmium was bound at the site homologous to the zinc site in staphylococcal enterotoxins C (SECs) despite the high structural homology between SPEA and SECs. Subsequent soaking of crystals grown in the presence of cadmium in 10 mM ZnCl2 showed that zinc binds in this site (indicating it can discriminate between zinc and cadmium ions) using the three ligands (Asp77, His106, and His110) homologous to the SECs plus a fourth ligand (Glu33).

Development of streptococcal pyrogenic exotoxin C vaccine toxoids that are protective in the rabbit model of toxic shock syndrome

Streptococcal pyrogenic exotoxin C (SPE C) is a superantigen produced by many strains of Streptococcus pyogenes that (along with streptococcal pyrogenic exotoxin A) is highly associated with streptococcal toxic shock syndrome (STSS) and other invasive streptococcal diseases. Based on the three-dimensional structure of SPE C, solvent-exposed residues predicted to be important for binding to the TCR or the MHC class II molecule, or important for dimerization, were generated. Based on decreased mitogenic activity of various single-site mutants, the double-site mutant Y15A/N38D and the triple-site mutant Y15A/H35A/N38D were constructed and analyzed for superantigenicity, toxicity (lethality), immunogenicity, and the ability to protect against wild-type SPE C-induced STSS. The Y15A/N38D and Y15A/H35A/N38D mutants were nonmitogenic for rabbit splenocytes and human PBMCs and nonlethal in two rabbit models of STSS, yet both mutants were highly immunogenic. Animals vaccinated with the Y15A/N38D or Y15A/H35A/N38D toxoids were protected from challenge with wild-type SPE C. Collectively, these data indicate that the Y15A/N38D and Y15A/H35A/N38D mutants may be useful as toxoid vaccine candidates.

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.