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Localization of a site on bacterial superantigens that determines T cell receptor beta chain specificity
J A Mollick1, R L McMasters, D Grossman
1Department of Microbiology and Immunology, Baylor College of Medicine, Houston, Texas 77030.
Abstract:
A defining characteristic of superantigens is their ability to stimulate T cells based predominantly on the type of variable segment of the T cell receptor (TCR) beta chain (V beta). The V beta specificity of these toxins most likely results from direct contact between the toxin and the TCR, although the low affinity nature of this binding has prevented direct assessment of this interaction. To identify important functional sites on the toxin, we created chimeric enterotoxin genes between staphylococcal enterotoxins A and E (SEA and SEE) and tested the V beta specificity of the chimeric toxins. This approach allowed us to identify three amino acid residues in the extreme COOH terminus of these toxins that are largely responsible for their ability to stimulate either human V beta 5- or V beta 8-bearing T cells, or mouse V beta 3 or V beta 11. We also found that residues in the NH2 terminus were required for wild-type levels of V beta-specific T cell activation, suggesting that the NH2 and COOH ends of these superantigens may come together to form the full TCR V beta contact site. SEA and SEE also differ with respect to their class II binding characteristics. Using the same chimeric molecules, we demonstrate that the first third of the molecule controls the class II binding phenotype. These data lead us to propose that for SEA and SEE, and perhaps for all bacterial-derived superantigens, the COOH and NH2 termini together form the contact sites for the TCR and therefore largely determine the V beta specificity of the toxin, while the NH2 terminus alone binds major histocompatibility complex class II molecules. The predominant role of the COOH terminus of bacterial superantigens in determining V beta specificity resembles current models being proposed for virally encoded superantigens, suggesting that these molecules may demonstrate some structural relationship not seen at the amino acid level.
Insights
Superantigens stimulate T cells by binding to T cell receptors (TCRs). Researchers identified key toxin regions responsible for V beta specificity and class II binding, revealing structural similarities between bacterial and viral superantigens.
Area of Science:
- Immunology
- Molecular Biology
- Microbial Pathogenesis
Background:
- Superantigens are potent immune activators that bind to T cell receptors (TCRs), primarily through variable beta (V beta) chain interactions.
- The precise molecular interactions between superantigens and TCRs, particularly the role of specific amino acid residues, remain incompletely understood due to low binding affinities.
- Staphylococcal enterotoxins A (SEA) and E (SEE) exhibit distinct V beta specificities and major histocompatibility complex class II (MHC II) binding properties.
Purpose of the Study:
- To delineate the functional regions of staphylococcal enterotoxins responsible for T cell receptor (TCR) V beta specificity.
- To identify the molecular determinants governing the differential binding of SEA and SEE to MHC class II molecules.
- To elucidate the structural basis for superantigen activity and explore potential similarities with viral superantigens.
Main Methods:
- Construction of chimeric genes between staphylococcal enterotoxins A and E (SEA and SEE).
- Assessment of V beta specificity of chimeric toxins using T cell stimulation assays.
- Analysis of MHC class II binding characteristics of chimeric molecules.
Main Results:
- Three amino acid residues at the carboxyl (COOH) terminus of SEA and SEE were identified as critical for determining V beta specificity.
- Residues in the amino (NH2) terminus were also found to be necessary for achieving wild-type levels of V beta-specific T cell activation.
- The amino (NH2) terminus of the superantigens was shown to control MHC class II binding, while the COOH terminus primarily dictates V beta specificity.
Conclusions:
- The T cell receptor (TCR) contact site for SEA and SEE is likely formed by the cooperative interaction of both the NH2 and COOH termini.
- The NH2 terminus is responsible for binding major histocompatibility complex class II (MHC II) molecules, while the COOH terminus largely determines V beta specificity.
- The findings suggest a conserved structural motif for TCR interaction across bacterial and viral superantigens, despite differences at the amino acid level.