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Laboratoire d'Immunologie, Institut de Recherches Cliniques de Montréal, Montréal, Québec H2W 1R7, Canada; the Department of Microbiology, Molecular Biology, and Biochemistry, University of Idaho, Moscow, Idaho 83844; the || Department of Microbiology, University of Minnesota Medical School, Minneapolis, Minnesota 55455; and the ¶ Basel Institute for Immunology, Postfach CH-4005, Basel, Switzerland
The three-dimensional structure of the complex between a T cell receptor (TCR) β chain (mouse Vβ8.2Jβ2.1Cβ1) and the superantigen (SAG) staphylococcal enterotoxin C3 (SEC3) has been recently determined to 3.5 Å resolution. To evaluate the actual contribution of individual SAG residues to stabilizing the β–SEC3 complex, as well as to investigate the relationship between the affinity of SAGs for TCR and MHC and their ability to activate T cells, we measured the binding of a set of SEC3 and staphylococcal enterotoxin B (SEB) mutants to soluble recombinant TCR β chain and to the human MHC class II molecule HLA-DR1. Affinities were determined by sedimentation equilibrium and/or surface plasmon detection, while mitogenic potency was assessed using T cells from rearrangement-deficient TCR transgenic mice. We show that there is a clear and simple relationship between the affinity of SAGs for the TCR and their biological activity: the tighter the binding of a particular mutant of SEC3 or SEB to the TCR β chain, the greater its ability to stimulate T cells. We also find that there is an interplay between TCR–SAG and SAG–MHC interactions in determining mitogenic potency, such that a small increase in the affinity of a SAG for MHC can overcome a large decrease in the SAG's affinity for the TCR. Finally, we observe that those SEC3 residues that make the greatest energetic contribution to stabilizing the β–SEC3 complex ("hot spot" residues) are strictly conserved among enterotoxins reactive with mouse Vβ8.2, thereby providing a basis for understanding why SAGs having other residues at these positions show different Vβ-binding specificities.
This research was supported by National Institutes of Health (NIH) grant 36900 and National Multiple Sclerosis Society grant RG2747 (R.A. Mariuzza); NIH grant HL-36611 (P.M. Schlievert); NIH grant AI-28401 and USDA grant 9402399 (G.A. Bohach); and grants from the National Cancer Institute of Canada (R.-P. Sékaly). Support from the Lucille P. Markey Charitable Trust is also gratefully acknowledged. The Basel Institute for Immunology was founded and is supported by F. Hoffmann-LaRoche Ltd., Basel, Switzerland. L. Leder is a Fellow of the Swiss National Science Foundation. R.-P. Sékaly holds a Medical Research Council of Canada Scientist Award.
Abbreviations used: HA, hemaglutinin peptide; HBS, Hepes-buffered saline; Kd, dissociation constant; RU, resonance unit; SAG, superantigen; SEA, staphylococcal enterotoxin A; SEB, staphylococcal enterotoxin B; SEC, staphylococcal enterotoxin C; SED, staphylococcal enterotoxin D; SEE, staphylococcal enterotoxin E; SPEA, streptococcal pyrogenic exotoxin A.
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