Intracellular expression of toxic shock syndrome toxin 1 in Saccharomyces cerevisiae

Identification of novel staphylococcal virulence genes by in vivo expression technology

We have applied in vivo expression technology (IVET) to the study of staphylococcal virulence. Using a promoter trap that relies on genetic recombination as a reporter of gene expression, we identified 45 staphylococcal genes that are induced during infection in a murine renal abscess model. Of these, only six were known previously; 11 others have homology to known non-staphylococcal genes. The known staphylococcal genes include agrA, part of a key locus regulating numerous virulence products, and a glycerol ester hydrolase, which may enhance staphylococcal survival in abscesses. We constructed 11 strains containing mutations in previously unknown ivi genes. Of these strains, seven were significantly attenuated in virulence compared with the wild-type parent. The mutagenized ivi genes may encode novel staphylococcal virulence factors.

Expression of bacterial superantigen genes in mice induces localized mononuclear cell inflammatory responses

Bacterial superantigens are potent T cell activators, and superantigen proteins have been injected into mice and other animals to study T cell responses in vivo. When superantigen proteins are injected, however, the T cell stimulatory effects cannot be confined to specific tissues. Therefore, to target superantigen expression to specific tissues, we used gene transfer techniques to express bacterial superantigen genes in mammalian cells in vitro and in tissues in vivo. Murine, human, and canine cells transfected with superantigen genes in vitro all produced superantigen proteins both intracellularly and extracellularly, as assessed by bioassay, immunocytochemistry, and antigen ELISA. Superantigens produced by transfected eukaryotic cells retained their biologic specificity for T cell receptor binding. Intramuscular injection of superantigen plasmid DNA in vivo induced an intense intramuscular mononuclear cell infiltrate, an effect that could not be reproduced by intramuscular injection of superantigen protein. Intradermal and intravenous injection of superantigen DNA induced cutaneous and intrapulmonary mononuclear cell inflammatory responses, respectively. Thus, superantigen genes can be expressed by mammalian cells in vivo. Superantigen gene therapy represents a novel method of targeting localized T cell inflammatory reactions, with potential application to treatment of cancer and certain infectious diseases.

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

Structure-function studies have revealed that the region between amino acids 115 and 141 of toxic shock syndrome toxin 1 (TSST-1) constitutes a biologically active domain. A critical residue appears to be histidine 135, since a site-directed mutation that alters the histidine to alanine (H135A) results in a loss of mitogenic activity and an absence of toxicity as measured in a rabbit infection model of toxic shock syndrome. We have characterized the mutant toxin further and report here on its immunogenic activity in rabbits and on the protective ability of mutant-specific antibodies in two animal models of toxin-mediated shock. Antibodies raised in rabbits by immunization with the purified H135A are fully cross-reactive with staphylococcal TSST-1 and wild-type recombinant TSST-1 (rTSST-1) expressed in Escherichia coli. The H135A antibodies neutralized the mitogenic activity for murine splenic T cells equally well as did TSST-1-specific polyclonal and monoclonal antibodies. In addition, the H135A antibodies blocked the production of tumor necrosis factor by spleen cells stimulated with rTSST-1. The toxicities of rTSST-1 and H135A were compared in D-galactosamine (D-GalNH2)-sensitized MRL-lpr/lpr mice. The nontoxicity of H135A was confirmed in this murine model of superantigen-induced septic shock. No toxicity of H135A was demonstrable at doses of 60 micrograms, while doses of rTSST-1 as low as 2 micrograms caused significant mortality within 24 to 72 h after challenge. Furthermore, subsequent to challenge of mice with H135A, no elevation in the serum levels of interleukin-2 or tumor necrosis factor was measurable. Passive immunization with H135A antibodies also protected MRL-lpr/lpr mice against lethal challenge with rTSST-1. Finally, rabbits actively immunized with purified H135A did not succumb to infection with a transformed strain of Staphylococcus aureus expressing rTSST-1. Additional animal studies will be required to confirm the immunizing potential of H135A and the efficacy of H135A antibodies as a neutralizing antitoxin.

Mutations affecting the activity of toxic shock syndrome toxin-1

Toxic shock syndrome toxin-1 (TSST-1), the potent staphylococcal exoprotein linked to most cases of the toxic shock syndrome, is a V beta-restricted T-cell mitogen (a so-called "superantigen"). TSST-ovine (TSST-O) is a natural variant of TSST-1, and is produced by certain ovine mastitis-associated strains of Staphylococcus aureus. Compared to TSST-1, TSST-O is only weakly mitogenic for leporine or murine splenocytes. It differs from TSST-1 at 7 amino acid residues over its 194 amino acid length. Terminus shuffling between the two proteins has suggested that their C-terminal differences (T69, Y80, E132, and I140 in TSST-1; 169, W80, K132, and T140 in TSST-O) are in part responsible for their discrepant mitogenic properties. In order to explore further the functional consequences of altering TSST-1 at residues 132 and 140, we engineered point mutants of TSST-1 at those positions. The mutant proteins were purified to homogeneity from culture supernants of a nontoxigenic strain of S. aureus using a combination of ultrafiltration, liquid-phase isoelectric focusing, and ion-exchange chromatography. The mutants retained global structural integrity as evidenced by circular dichroism spectroscopy, their preserved resistance to trypsin digestion, and their preserved binding to a neutralizing murine monoclonal antibody. The mutants were then tested for mitogenicity for human T-cells: The mutant I140T was approximately as active as wild-type TSST-1, while the mutant E132D was about 10-fold attenuated. On the other hand, the mutants E132A or E132K were each at least 1000-fold attenuated.(ABSTRACT TRUNCATED AT 250 WORDS)