SUPERANTIGEN BINDING TO A T-CELL RECEPTOR-BETA CHAIN OF KNOWN 3-DIMENSIONAL STRUCTURE

The three-dimensional structure of an unglycosylated T cell antigen re ceptor (TCR) beta chain has recently been determined to 1.7 Angstrom r esolution. To investigate whether this soluble beta chain (murine V be ta 8.2J beta 2.1C beta 1) retains superantigen (SAG)-binding activity, we measured its affinity for various bacterial SAGs in the absence of MHC class II molecules. Dissociation constants (K(D)s) were determine d using two independent techniques: surface plasmon resonance detectio n and sedimentation equilibrium. Specific binding was demonstrated to staphylococcal enterotoxins (SEs) B, C1, C2, and C3 and to streptococc al pyrogenic exotoxin A (SPEA), cansistent with the known proliferativ e effects of these SAGs on T cells expressing V beta 8.2. In contrast, SEA, which does not stimulate V beta 8.2-bearing cells, does not bind the recombinant beta chain. Binding of the beta chain to SAGs was cha racterized by extremely fast dissociation rates (>0.1 s(-1)), similar to those reported for certain leukocyte adhesion molecules. Whereas th e beta chain bound SEC1, 2, and 3 with K(D)s of 0.9-2,5 mu M, the corr esponding value for SEB was similar to 140 mu M. The much weaker bindi ng to SEB than to SEC1, 2, or 3 was surprising, especially since SEB w as found to actually be 3- to 10-fold more effective, on a molar basis , than the other toxins in stimulating the parental T cell hybridoma. We interpret these results in terms of the ability of SEC to activate T cells independently of MHC, in contrast to SEB. We have also measure d SE binding to the glycosylated form of the beta chain and found that carbohydrate apparently does not contribute to recognition, even thou gh the N-linked glycosylation sites at V beta 8.2 residues Asn24 and A sn74 are at or near the putative SAG-binding site. This result, along with the structural basis for the V beta specificity of SEs, are discu ssed in relation to the crystal structure of the unglycosylated beta c hain.