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

Summary: 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.l‐Cβ1) and the SAG staphylococcal enterotoxin d (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 cleft 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 die antigen in 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 die 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 die affinity of a SAG for MHC can overcome a large decrease in the SAG's affinity for die 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 diese positions show different Vβ‐binding specificities.