Structure-function studies of T-cell receptor-superantigen interactions

Superantigens (SAGs) are a class of disease-causing and immunostimulatory proteins of bacterial or viral origin that activate T cells by binding to the Vβ domain of the T-cell antigen receptor (TCR). The three-dimensional structure of the complex between a TCR β chain (mouse Vβ8.2-Jβ2.1-Cβ1) and the SAG staphylococcal enterotoxin C3 (SEC3) has been recently determined. The complementarity-determining region 2 (CDR2) of the β chain and, to lesser extents, CDR1 and hypervariable region 4 (HV4) bind in a deft between the small and large domains of the SAG. A model of the TCR-SAG-peptide/MHC complex constructed from available crystal structures reveals how the SAG acts as a wedge between the TCR and MHC, thereby displacing the antigenic peptide away from the TCR and circumventing the normal mechanism for T-cell activation by peptide/MHC. To evaluate the actual contribution of individual SAG residues to stabilizing the VβCβ-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 mutants to a soluble recombinant TCR β chain and to the human MHC class II molecule HLA-DR1. We show that there is direct correlation between affinity and ability to stimulate T cells, with SAGs having the highest affinity for the TCR being the most biologically active. 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 VβCβ-SEC3 complex are strictly conserved among enterotoxins reactive with mouse Vβ8.2, thereby explaining why SAGs having other residues at these positions show different Vβ-binding specificities.