A mutation at histidine residue 135 of toxic shock syndrome toxin yields an immunogenic protein with minimal toxicity

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

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

Interleukin-10 (IL-10) Produced by Mutant Toxic Shock Syndrome Toxin 1 Vaccine-Induced Memory T Cells Downregulates IL-17 Production and Abrogates the Protective Effect against Staphylococcus aureus Infection

Development of long-term memory is crucial for vaccine-induced adaptive immunity against infectious diseases such as Staphylococcus aureus infection. Toxic shock syndrome toxin 1 (TSST-1), one of the superantigens produced by S. aureus, is a possible vaccine candidate against infectious diseases caused by this pathogen. We previously reported that vaccination with less toxic mutant TSST-1 (mTSST-1) induced T helper 17 (Th17) cells and elicited interleukin-17A (IL-17A)-mediated protection against S. aureus infection 1 week after vaccination. In the present study, we investigated the host immune response induced by mTSST-1 vaccination in the memory phase, 12 weeks after the final vaccination. The protective effect and IL-17A production after vaccination with mTSST-1 were eliminated because of IL-10 production. In the presence of IL-10-neutralizing monoclonal antibody (mAb), IL-17A production was restored in culture supernatants of CD4⁺ T cells and macrophages sorted from the spleens of vaccinated mice. Vaccinated mice treated with anti-IL-10 mAb were protected against systemic S. aureus infection in the memory phase. From these results, it was suggested that IL-10 produced in the memory phase suppresses the IL-17A-dependent vaccine effect through downregulation of IL-17A production.

Review: D-Galactosamine lethality model: scope and limitations

D-Galactosamine (D-galN) is well established as sensitizing mice and other animals to the lethal effects of TNF, specifically, and by several orders of magnitude. Protection by anti-TNF neutralizing antibody is complete, as is (metabolically-based) protection by uridine. Sensitization occurs regardless of the origin of the released TNF, whether it is released from macrophages and/or T-cells. The same is true for the challenging agent which leads to the release of TNF, whether it is endotoxin, a superantigen, lipoprotein, bacterial DNA, or bacteria, either killed or proliferating. Most studies have utilized endotoxin as the challenging agent, and more than 70 agents have been reported to confer protection against LPS and/or TNF challenge in the model. The model has provided new insight regarding modes of protection, including from dexamethasone, which protects against challenge from LPS but not from challenge by TNF. The D-galN lethality model has also been used to test for synergistic behavior between different bacterial components, and to test for lethality when only small amounts of the challenging agent are available (lipid A chemistry).

Vaccination with non-toxic mutant toxic shock syndrome toxin-1 induces IL-17-dependent protection against Staphylococcus aureus infection

Toxic shock syndrome toxin-1 (TSST-1) is one of superantigens produced by Staphylococcus aureus. We have previously demonstrated that vaccination with non-toxic mutant TSST-1 (mTSST-1) develops host protection to lethal S. aureus infection in mice. However, the detailed mechanism underlying this protection is necessary to elucidate because the passive transfer of antibodies against TSST-1 fails to provide complete protection against S. aureus infection. In this study, the results showed that interleukin-17A (IL-17A)-producing cells were increased in the spleen cells of mTSST-1-vaccinated mice. The main source of IL-17A in mTSST-1-vaccinated mice was T-helper 17 (Th17) cells. The protective effect of vaccination was induced when the vaccinated wild type but not IL-17A-deficient mice were challenged with S. aureus. Gene expression of chemokines, CCL2 and CXCL1, and infiltration of neutrophils and macrophages were increased in spleens and livers of vaccinated mice after infection. The IL-17A-dependent immune response was TSST-1 specific because TSST-1-deficient S. aureus failed to induce the response. The present study suggests that mTSST-1 vaccination is able to provide the IL-17A-dependent host defense against S. aureus infection which promotes chemokine-mediated infiltration of phagocytes into the infectious foci.

Staphylococcal superantigen (TSST-1) mutant analysis reveals that t cell activation is required for biological effects in the rabbit including the cytokine storm

Staphylococcal superantigens (sAgs), such as toxic shock syndrome toxin 1 (TSST-1), induce massive cytokine production, which may result in toxic shock syndrome (TSS) and sepsis. Recently, we reported that in vitro studies in human peripheral blood mononuclear cells (PBMC) do not reflect the immunological situation of the host, because after exposure to superantigens (sAgs) in vivo, mononuclear cells (MNC) leave the circulation and migrate to organs, e.g., the spleen, liver and lung. Our experimental model of choice is the rabbit because it is comparable to humans in its sensitivity to sAg. T cell activation has been assessed by lymphocyte proliferation and IL-2 gene expression after in vivo challenge with TSST-1 and the mutant antigens; expression of the genes of proinflammatory cytokines were taken as indicators for the inflammatory reaction after the combined treatment with TSST-1 and LPS. The question as to whether the biological activities of TSST-1, e.g., lymphocyte extravasation, toxicity and increased sensitivity to LPS, are mediated by T cell activation or activation by MHC II-only, are unresolved and results are contradictory. We have addressed this question by studying these reactions in vivo, with two TSST-1 mutants: one mutated at the MHC binding site (G31R) with reduced MHC binding with residual activity still present, and the other at the T cell binding site (H135A) with no residual function detectable. Here, we report that the mutant G31R induced all the biological effects of the wild type sAg, while the mutant with non-functional TCR binding did not retain any of the toxic effects, proving the pivotal role of T cells in this system.

Aldehyde modification of peptide immunogen enhances protein-reactive antibody response to toxic shock syndrome toxin-1

Introduction of aldehyde groups into protein conjugates enhanced the immune response to a coupled peptide without the use of strong adjuvants. Synthetic peptides representing the N-terminal (residues 1-16) and internal (residues 53-65) epitopes of toxic shock syndrome toxin-1 (TSST-1) were coupled to carrier protein, and carbonyl tags were introduced by Amadori reaction with glycolaldehyde. Modified and unmodified antigens in alum were used to immunize rabbits and the reactivities of antisera were compared. Aldehyde modification augmented the response detected by ELISA, which included enhanced binding to peptides and to native TSST-1. In western blot, TSST-1 was detected by antiserum elicited to the N-terminal peptide, but not that generated to the peptide representing the internal sequence. The same antiserum also neutralized TSST-1 activity in a lymphocyte proliferation assay. The circular dichroism spectrum of the N-terminal peptide indicated a propensity for helical conformation, similar to the structure at the corresponding sequence of the native protein. These data suggest that aldehyde modification can boost immunogenicity of peptide-based vaccines, generating epitope-specific immune responses against the cognate protein antigens without using potent adjuvants.

Immunization with glutathioneS-transferase and mutant toxic shock syndrome toxin 1 fusion protein protects againstStaphylococcus aureusinfection

To investigate whether immunization with glutathione S-transferase (GST) and mutant toxic shock syndrome toxin 1 (mTSST-1) fusion protein can protect against Staphylococcus aureus infection, we purified a non-toxic mutant GST-mTSST-1 fusion protein. Mice were immunized with the GST-mTSST-1 plus alum adjuvant and then challenged with viable S. aureus. The results showed that the survival rate of GST-mTSST-1-immunized group was higher and the bacteria counts in the organs were significantly lower than those of the non-immunized mice. Immunization with GST-mTSST-1 induced strongly the production of TSST-1 specific antibodies, especially immunoglobulin G1 and immunoglobulin G2b. Furthermore, the serum samples from GST-mTSST-1-immunized mice also significantly inhibited interferon-gamma and tumor necrosis factor-alpha production from murine spleen cells by TSST-1. These results suggest that vaccination with GST-mTSST-1 provides protection against S. aureus infection and that the protection might be mediated by TSST-1-neutralizing antibody.

Epitope mapping of neutralizing TSST-1 specific antibodies induced by immunization with toxin or toxoids

Toxic shock syndrome toxin-1 (TSST-1), a superantigen produced by Staphylococcus aureus, is a potent stimulator of the immune system. T-cells are activated by crosslinking of MHC class II molecules on antigen presenting cells with T-cell receptors (TCR). TSST-1 is associated with the majority of the cases of menstrual staphylococcal toxic shock, a severe and life-threatening multisystem disorder. Even though antibody mediated protection has been studied, information on antibody specificity directed to individual antigenic determinants of the protein is incomplete. To obtain immunogens with low toxicity, we generated a double-site mutant (dmTSST-1), modified at solvent-exposed residues predicted to be important for both MHC class II and TCR binding, and detoxified recombinantly expressed TSST-1 (rTSST-1) as well as native TSST-1 (nTSST-1) isolated from Staphylococcus aureus by treatment with formaldehyde. Rabbits were immunized with rTSST-1, nTSST-1, dmTSST-1, and formaldehyde inactivated toxoids. The sera obtained were used to map the antigen-reactive regions of the molecule and to identify specificities of antibodies induced by immunization with the different antigens. To detect linear antigenic epitopes of TSST-1 the reactivity of the sera with 11-meric peptides having an overhang of four residues, covering the entire molecule of TSST-1, have been studied. We found that sera of TSST-1 immunized rabbits predominantly reacted with N-terminal residues 1-15, while sera generated with formaldehyde inactivated toxoid recognized a total of 7 regions located at the N- and C-terminus and internal sites of TSST-1. Despite different specificities all sera were able to inhibit TSST-1 induced proliferation of human mononuclear cells.

Double mutant and formaldehyde inactivated TSST-1 as vaccine candidates for TSST-1-induced toxic shock syndrome

Up to now there is no treatment for staphylococcal toxic shock syndrome, a disease mainly induced by toxic shock syndrome toxin-1(TSST-1). There is great demand in finding means to control the disease, one of them is the development of an effective and safe vaccine against TSST-1. In this study we constructed a series of vaccine candidates and investigated their biological activity, toxicity, and potential to invoke an immune response. TSST-1 was isolated from Stahylococcus aureus supernatants and recombinantly expressed as a N-terminal 6x histidine-tagged protein in Escherichia coli. In order to obtain molecules with minimal toxicity we constructed single mutants (G31R and H135A) and one double mutant (G31R/H135A) with both residues exchanged. We also detoxified native TSST-1 isolated from S. aureus, and recombinantly expressed TSST-1 by treatment with formaldehyde. Functional activity of native and recombinant TSST-1 and grade of inocuity of mutants and toxoids was determined by investigating mitogenity, T-cell activation, and cytokine release upon stimulation of human mononuclear cells with the vaccine candidates. All substances were tested in a rabbit immunization study. After primary immunization and three additional boosts all vaccinated animals developed antibody titers against TSST-1 and were protected against challenge with a lethal doses of superantigen potentiated with lipopolysaccharide.

Temporal sequence and kinetics of proinflammatory and anti-inflammatory cytokine secretion induced by toxic shock syndrome toxin 1 in human peripheral blood mononuclear cells

The staphylococcal superantigen toxic shock syndrome toxin 1 (TSST-1) induces massive cytokine production, which is believed to be the key factor in the pathogenesis of TSS. The temporal sequence and kinetics of both proinflammatory and anti-inflammatory cytokines induced by TSST-1 in human peripheral blood mononuclear cells were investigated. A panel of loss-of-function single-amino-acid-substitution mutants of TSST-1, previously demonstrated to be defective in either major histocompatibility complex (MHC) class II binding (G31R) or T-cell receptor (TCR) interaction (H135A, S14N), was studied in parallel to further elucidate the mechanisms of cytokine secretion. Wild-type recombinant (WT r) TSST-1 induced a biphasic pattern of cytokine secretion: an early phase with rapid release of proinflammatory cytokines (especially gamma interferon, interleukin-2 [IL-2], and tumor necrosis factor alpha [TNF-alpha]) within 3 to 4 h poststimulation, and a later phase with more gradual production of both proinflammatory (IL-1beta, IL-12, and TNF-beta) and anti-inflammatory (IL-6, IL-10) cytokines within 16 to 72 h poststimulation. G31R, which is defective in MHC class II binding, induced a cytokine profile similar to that of WT rTSST-1, except that secretion of the early-phase proinflammatory cytokines was delayed and production of IL-1beta and IL-12 was markedly reduced. In contrast, mutant toxins defective in TCR interaction either demonstrated complete absence of any cytokine secretion during the entire observation period (H135A) or resulted in complete abolishment of IL-2 and other early-phase proinflammatory cytokines, while secretion of IL-10 appeared unaffected (S14N). Neither WT rTSST-1 nor the mutant toxins induced IL-4 or transforming growth factor beta. Our data indicate that effective TCR interaction is critical for the induction of the early-phase proinflammatory cytokine response, thus underscoring the importance of T-cell signaling in TSS.

Exotoxins of Staphylococcus aureus

This article reviews the literature regarding the structure and function of two types of exotoxins expressed by Staphylococcus aureus, pyrogenic toxin superantigens (PTSAgs) and hemolysins. The molecular basis of PTSAg toxicity is presented in the context of two diseases known to be caused by these exotoxins: toxic shock syndrome and staphylococcal food poisoning. The family of staphylococcal PTSAgs presently includes toxic shock syndrome toxin-1 (TSST-1) and most of the staphylococcal enterotoxins (SEs) (SEA, SEB, SEC, SED, SEE, SEG, and SEH). As the name implies, the PTSAgs are multifunctional proteins that invariably exhibit lethal activity, pyrogenicity, superantigenicity, and the capacity to induce lethal hypersensitivity to endotoxin. Other properties exhibited by one or more staphylococcal PTSAgs include emetic activity (SEs) and penetration across mucosal barriers (TSST-1). A detailed review of the molecular mechanisms underlying the toxicity of the staphylococcal hemolysins is also presented.

Defining a novel domain of staphylococcal toxic shock syndrome toxin-1 critical for major histocompatibility complex class II binding, superantigenic activity, and lethality

Staphylococcal toxic shock syndrome toxin-1 (TSST-1) is implicated in the pathogenesis of superantigen-mediated shock. We previously identified TSST-1 residues G31/S32 to be important for major histocompatibility complex (MHC) class II binding, as well as superantigenic and lethal activities. However, the site-directed TSST-1 mutant toxin, G31R, could still induce mitogenesis and low-level TNFalpha secretion, suggesting that additional MHC class II binding sites other than G31/S32 may exist. In the current study, a TSST-1-neutralizing monoclonal antibody, MAb5, was found to inhibit TSST-1 binding to human peripheral blood mononuclear cells, neutralize TSST-1-induced mitogenesis and cytokine secretion, and protect against TSST-1-induced lethality in vivo. Epitope mapping revealed that MAb5 bound to TSST-1 residues 51-56 (T(51-56);51YYSPAF56). Peptide T(51-56) was synthesized and found to also inhibit TSST-1 binding to human monocytes as well as TSST-1-induced mitogenesis, cytokine secretion, and lethality in vivo. This T(51-56) epitope, located within the beta3/beta4 loop, and the previously identified G31/S32 epitope, within the beta1/beta2 loop of TSST-1, are separated within the primary sequence, but spatially juxtaposed to each other. Collectively, these findings suggest that a discontinuous epitope comprising of regions within both the beta1/beta2 and beta3/beta4 loops, are critical for MHC class II binding, and the consequent superantigenic and lethal activities of TSST-1.

Exotoxins of Staphylococcus aureus

This article reviews the literature regarding the structure and function of two types of exotoxins expressed by Staphylococcus aureus, pyrogenic toxin superantigens (PTSAgs) and hemolysins. The molecular basis of PTSAg toxicity is presented in the context of two diseases known to be caused by these exotoxins: toxic shock syndrome and staphylococcal food poisoning. The family of staphylococcal PTSAgs presently includes toxic shock syndrome toxin-1 (TSST-1) and most of the staphylococcal enterotoxins (SEs) (SEA, SEB, SEC, SED, SEE, SEG, and SEH). As the name implies, the PTSAgs are multifunctional proteins that invariably exhibit lethal activity, pyrogenicity, superantigenicity, and the capacity to induce lethal hypersensitivity to endotoxin. Other properties exhibited by one or more staphylococcal PTSAgs include emetic activity (SEs) and penetration across mucosal barriers (TSST-1). A detailed review of the molecular mechanisms underlying the toxicity of the staphylococcal hemolysins is also presented.

Generated single point-mutations can considerably dismantle the lymphocyte overstimulation induced by Yersinia pseudotuberculosis superantigen

The superantigenic Yersinia pseudotuberculosis-derived mitogen (YPM) may contribute to severe complications in Y. pseudotuberculosis infections. Since the pathogenic mechanism of a superantigen (SAg) is based on its capability for T-cell overstimulation, by introducing point mutations into YPM an attempt was made to abrogate this effect. Six mutants studied exhibited a variety of T-cell proliferating responses. Two had activity reduced by 80-90%, three had activity reduced by approximately 50%, and one mutant showed almost no attenuation. The SAg-associated in vitro pathogenic functions, cytotoxic activation and the production of proinflammatory cytokines, were also diminished, in parallel. Since these mutants were confirmed to be defective in TCR Vbeta binding, it was possible to compare them with native YPM. Our results suggested that the intensity of TCR Vbeta binding is a crucial factor determining the severity of pathogenesis and that single amino acid alterations might be useful for producing immunotherapeautical agents from native YPM.

Localization of a T-cell epitope of superantigen toxic shock syndrome toxin 1 to residues 125 to 158

Toxic shock syndrome toxin 1 (TSST-1) is a member of the staphylococcal enterotoxin superantigen family. So far, little is known about T-cell epitopes on superantigens. In this study, we developed an improved method for localizing T-cell epitopes on superantigens that involved synthetic peptides plus costimulation by CD28 or phorbol myristate acetate. Using this method, we localized a T-cell epitope to a 34-residue region, TSST-1 (residues 125 to 158), which possessed only two of four TSST-1-targeted beta-chain variable element (Vbeta) specificities of T-cell receptors in humans and mice, human Vbeta2 and murine Vbeta15.

Separation of Function Between the Domains of Toxic Shock Syndrome Toxin-1

Toxic shock syndrome toxin-1 (TSST1) is a superantigenic exotoxin produced by certain strains of Staphylococcus aureus. Structurally, TSST1 is composed of two domains: residues determined by crystallography to directly interact with MHC II molecules reside within the N-terminal domain, while TSST1 residues critical for superantigenicity are within the C-terminal domain. In this study, we expressed the individual N- and C-terminal domains of TSST1 in Escherichia coli and studied their biologic activities. The TSST1 N-terminal domain (TSST(1–87)) did not induce proliferation of human PBLs or release of TNF-β, but did induce TNF-α release. However, TSST1-elicited proliferation and release of both TNF isoforms were inhibited by a molar excess of TSST(1–87). The TSST1 C-terminal domain (TSST(88–194)) did not bind MHC II molecules, yet it elicited production of TNF-α and TNF-β, and induced TCR Vβ-specific proliferation similarly to intact TSST1. When covalently cross-linked to tumor cells, TSST(88–194) elicited a local in vivo antitumor response indistinguishable from TSST1. Although intact TSST1 causes lethal shock in vivo, the individual domains of this molecule may have therapeutic potential: the N-terminal domain to antagonize lymphocyte activation and TNF release during acute TSST1-precipitated toxic shock syndrome, and the C-terminal domain to stimulate antitumor responses without MHC II binding.

Staphylococcal enterotoxin B mutants (N23K and F44S): biological effects and vaccine potential in a mouse model

Superantigens produced by Staphylococcus aureus can cause food poisoning and toxic shock syndrome. The biological activities and vaccine potential of mutant staphylococcal enterotoxin B (SEB) proteins, N23K and F44S, were studied in a lipopolysaccharide-potentiated mouse model. Although 10 micrograms of SEB per mouse is equivalent to 30 LD50, the same intraperitoneal dose of either mutant protein was nonlethal and did not elevate serum levels of tumor necrosis factors (TNF). N23K, F44S, and SEB were serologically identical in an enzyme-linked immunosorbent assay with polyclonal anti-SEB. Immunization with alum containing N23K, F44S, or SEB elicited an anti-SEB response that protected 80-87% of the mice against a 10 micrograms SEB challenge. Controls lacking an anti-SEB titer did not survive. Pooled sera from immunized mice effectively blocked SEB-induced T-cell proliferation in vitro. Naive mice survived a lethal SEB challenge when given pooled antisera 1, 2, or 4 h later, whereas the antisera failed to protect animals when administered 6 or 8 h after the toxin. Lethality at the later times was consistent with increased serum levels of TNF observed 6 h after SEB injection. These studies suggest that the N23K and F44S mutant proteins of SEB are less biologically active than the wild-type toxin, yet retain epitopes useful for eliciting a protective antibody response.

Crystal structure of a biologically inactive mutant of toxic shock syndrome toxin-1 at 2.5 A resolution

Toxic shock syndrome toxin-1 (TSST-1) is one of a family of staphylococcal exotoxins recognized as microbial superantigens. The toxin plays a dominant role in the genesis of toxic shock in humans through a massive activation of the immune system. This potentially lethal illness occurs as a result of the interaction of TSST-1 with a significant proportion of the T-cell repertoire. TSST-1, like other superantigens, can bind directly to class II major histocompatibility (MHC class II) molecules prior to its interaction with entire families of V beta chains of the T-cell receptor (TCR). The three-dimensional structure of a mutant (His-135-Ala) TSST-1 was compared with the structure of the native (wild-type) TSST-1 at 2.5 A resolution. The replacement of His 135 of TSST-1 with an Ala residue results in the loss of T-cell mitogenicity and toxicity in experimental animals. This residue, postulated to be directly involved in the toxin-TCR interactions, is located on the major helix alpha 2, which forms the backbone of the molecule and is exposed to the solvent. In the molecular structure of the mutant toxin, the helix alpha 2 remains unaltered, but the His to Ala modification causes perturbations on the neighboring helix alpha 1 by disrupting helix-helix interactions. Thus, the effects on TCR binding of the His 135 residue could actually be mediated, wholly or in part, by the alpha 1 helix.

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

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

A toxic shock syndrome toxin 1 mutant that defines a functional site critical for T-cell activation

Toxic shock syndrome toxin 1 (TSST-1), a superantigen produced by Staphylococcus aureus, is a causative agent of toxic shock syndrome (TSS). This superantigen is a potent stimulator of T cells and macrophages/monocytes, resulting in the release of cytokines that are implicated in the pathogenesis of TSS. This study characterizes a mutant TSST-1, derived by site-directed mutagenesis, that has an alanine substitution at histidine 135 (mutant 135). This single-amino-acid change results in a mutant toxin that has lost mitogenic activity for T cells. In contrast to wild-type TSST-1, this mutant does not induce T cells to express interleukin-2, gamma interferon, or tumor necrosis factor beta (TNF-beta). The inability of mutant 135 to activate T cells is not due to a lack of binding to the class II major histocompatibility complex receptor. In addition, the mutant TSST-1 does not induce expression of TNF-alpha, which plays a role in the development of lethal shock. The lack of TNF-alpha induction by mutant 135 is likely due to its inability to activate T cells. These data suggest that the mutation at histidine 135 in TSST-1 affects toxin interactions with the T-cell receptor rather than the class II major histocompatibility complex receptor.

Biological activity of toxic shock syndrome toxin 1 and a site-directed mutant, H135A, in a lipopolysaccharide-potentiated mouse lethality model

A recombinant of toxic shock syndrome toxin 1 (TSST-1) which contains a single histidine-to-alanine mutation at residue 135 (H135A) was analyzed for toxicity and vaccine potential in a lipopolysaccharide (LPS)-potentiated mouse lethality model. The 50% lethal dose (LD50) of TSST-1 in BALB/c mice was 47.2 micrograms/kg, but H135A was not lethal when tested at a dose equivalent to 10 LD50s of TSST-1. Levels of tumor necrosis factor (TNF) and gamma interferon (IFN-gamma) in serum were, respectively, 10- and 50-fold higher in LPS-potentiated mice injected with 15 LD50s of TSST-1 than in mice given H135A. Mice injected with only TSST-1 did not have elevated levels of TNF or IFN-gamma in serum, while H135A plus LPS or LPS alone elicited identical, yet very low, levels of TNF and IFN-gamma. An enzyme-linked immunosorbent assay of H135A and TSST-1 with anti-TSST-1 serum yielded very similar dose-response curves, which strongly suggests that H135A serologically and conformationally resembles the native toxin. Mice immunized with H135A developed antibodies that recognized TSST-1 in an enzyme-linked immunosorbent assay and afforded protection against a 15-LD50 challenge of TSST-1 plus LPS. The pooled sera of mice immunized with either TSST-1 or H135A also prevented lymphocyte proliferation due to TSST-1.

Role of a carboxy-terminal site of toxic shock syndrome toxin 1 in eliciting immune responses of human peripheral blood mononuclear cells

Staphylococcus aureus toxic shock syndrome toxin 1 (TSST-1) is involved in the pathogenesis of toxic shock syndrome and perhaps other staphylococcal diseases. Recently, the C-terminal part of the TSST-1 toxin has been shown to be responsible for mitogenic activity in animal models. We studied the role of the C-terminal structural unit of TSST-1 with regard to proliferation, cytokine release (tumor necrosis factor alpha [TNF-alpha], interleukin-6 [IL-6], and IL-8), mRNA expression for IL-6, IL-8, IL-10, TNF-alpha, and CD40 ligand (CD40L), synthesis of immunoglobulin E (IgE), IgA, IgG, and IgM, CD23 expression, and soluble CD23 (sCD23) release from human peripheral blood mononuclear cells (PBMC). For this purpose, we used the recombinant wild-type TSST-1 (p17) mutant toxin Y115A (tyrosine residue modified to alanine) and toxin H135A (histidine residue modified to alanine). Unmodified toxin p17 and mutant toxin Y115A, at a concentration below 5 ng, to a lesser degree, induced a strong proliferation. Toxin p17 followed by toxin Y115A was the most pronounced inducer for mRNA expression for IL-10 and CD40L and cytokine generation (mRNA and protein) for TNF-alpha, IL-6, and IL-8. Mutant protein H135A failed to activate human PBMC. Both toxins p17 and, to a lesser degree, Y115A significantly suppressed IL-4- and anti-CD40-induced synthesis of all four Igs as well as IL-4-induced CD23 expression and sCD23 release. Mutant toxin H135A failed to do so. Thus, our data show that a region in the C terminus of TSST-1 is responsible not only for mitogenic activity but also for additional immunomodulating biological activities of TSST-1. More specifically, histidine residue H135A of the 194-amino-acid toxin appears to be critical for the expression of biological activities in a human in vitro model.