Delineation by use of specific monoclonal antibodies of the T-cell receptor and major histocompatibility complex interaction sites on the superantigen toxic shock syndrome toxin 1

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.

Inhibition of staphylococcal enterotoxin B-induced lymphocyte proliferation and tumor necrosis factor alpha secretion by MAb5, an anti-toxic shock syndrome toxin 1 monoclonal antibody

Toxic shock syndrome (TSS) is primarily caused by toxic shock syndrome toxin-1 (TSST-1) and staphylococcal enterotoxin B (SEB). These toxins belong to a family of pyrogenic toxin superantigens (PTSAgs) produced by Staphylococcus aureus and exhibit several shared biological properties, including the induction of massive cytokine release and Vbeta-specific T-cell proliferation. The crystal structures of most PTSAgs are now published, and they demonstrate a striking similarity in conformational architecture even though their primary protein sequences are different. Despite these structural and immunobiological similarities, no cross-reactivity between TSST-1 and other PTSAgs has been demonstrated in serological or neutralization assays. Our laboratory has developed a neutralizing murine anti-TSST-1 monoclonal antibody (MAb5) which displayed cross-reactivity with SEB by enzyme-linked immunosorbent assay. The aim of the present study was to evaluate whether MAb5 can also cross-neutralize SEB-induced superantigenic activities in vitro. MAb5 was found to partially inhibit SEB-induced T-cell mitogenesis (63%) and tumor necrosis factor alpha (TNF-alpha) secretion (70%) in human peripheral blood mononuclear cells (PBMC) in a dose-dependent manner, while an isotypic anti-TSST-1 monoclonal antibody showed no effect. Epitope mapping revealed that MAb5 bound to TSST-1 residues 47 to 56 ((47)FPSPYYSPAF(56)) and to SEB residues 83 to 92 ((83)DVFGANYYYQ(92)), sequences that located in different regions of these toxins and are structurally dissimilar. SEB peptide (83)DVFGANYYYQ(92) was synthesized and found to also inhibit SEB-induced mitogenesis and TNF-alpha secretion in human PBMC. Our results demonstrate for the first time that MAb5 binds to different epitopes on TSST-1 and SEB that appear functionally important in inducing T-cell mitogenesis and TNF-alpha secretion in vitro.

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.

The effect of site-specific monoclonal antibodies directed to toxic shock syndrome toxin-1 in experimental Staphylococcus aureus arthritis

Staphylococcus aureus produces a large number of potential virulence factors, among these the superantigen toxic shock syndrome toxin-1 (TSST-1). We have recently demonstrated that TSST-1 is involved in the pathogenesis of septic arthritis. Recent data show that the TSST-1 molecule is composed of two distinct domains, one proposed to interact with T cell receptor (TCR) and one with the MHC class II. The aim of this study was to assess if interaction between TSST-1-specific MoAbs directed to sites on the MHC and/or TCR Vbeta affects the development of experimental S. aureus-induced arthritis. For that purpose we used a panel of seven MoAbs, which were injected intraperitoneally before and after inoculation with a TSST-1-producing S. aureus strain. Administration of antibodies did not affect the development of arthritis, suggesting inefficacy of such a procedure in neutralization of exotoxin-mediated disease manifestations.

Antitumor Response Elicited by a Superantigen- Transmembrane Sequence Fusion Protein Anchored onto Tumor Cells

Superantigens stimulate T cells bearing certain TCR β-chain variable regions when bound to MHC II molecules. We investigated whether the superantigen toxic shock syndrome toxin-1 (TSST1) could induce an antitumor immune response when anchored onto MHC II-negative tumor cells. Our approach was to facilitate association of TSST1 with cell membranes by fusing its coding region to the transmembrane region (TM) sequence of the proto-oncogene c-erb-B-2. TSST1-TM was expressed in bacteria with an N-terminal histidine tag and purified using nickel-agarose affinity chromatography. Purified TSST1-TM added to cultures of several different MHC II-negative tumor cells spontaneously associated with cell membranes, as detected by flow cytometry. Because superantigens can direct cell-mediated cytotoxicity against MHC II-positive cells, a TM fusion protein lacking the TSST1 MHC II binding domain (TSST88–194-TM) was also constructed. Tumor cells precoated with TSST1-TM or TSST88–194-TM stimulated proliferation of human peripheral blood lymphocytes in vitro whereas uncoated tumor cells did not. Mice preimmunized with TSST1-TM- or TSST88–194-TM-coated tumor cells mounted a systemic response that resulted in significant antitumor immunity as measured by regression of a parental tumor challenge. TSST1-TM and TSST88–194-TM fusion proteins represent a useful new strategy for attaching superantigens or potentially other proteins onto tumor cell surfaces without genetic manipulation.

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.

Transcytosis of staphylococcal superantigen toxins

Staphylococcus aureus produces a set of proteins (e.g., staphylococcal enterotoxin A [SEA], SEB, toxic shock syndrome toxin 1 [TSST-1]) which act both as superantigens (SAgs) and toxins. Although their mode of action as SAgs is well understood, little is known about how they enter the body via the intestine and cause food poisoning. To examine this problem we used an in vitro culture system to study the capacity of class II MHC-negative human intestinal epithelial cells (Caco-2) to transcytose several staphylococcal toxins. We found that Caco-2 cells are capable of dose-dependent, facilitated transcytosis of SEB and TSST-1, but not SEA. We extended these studies in vivo in mice by showing that ingested SEB appears in the blood more efficiently than SEA. Our data suggest that these toxins can cross the epithelium in an immunologically intact form. These results may have important implications for the pathogenesis of food poisoning.