High affinity T cell receptors from yeast display libraries block T cell activation by superantigens

The αβ T cell receptor (TCR) can be triggered by a class of ligands called superantigens. Enterotoxins secreted by bacteria act as superantigens by simultaneously binding to an MHC class II molecule on an antigen-presenting cell and to a TCR β-chain, thereby causing activation of the T cell. The cross-reactivity of enterotoxins with different Vβ regions can lead to stimulation of a large fraction of T cells. To understand the molecular details of TCR-enterotoxin interactions and to generate potential antagonists of these serious hyperimmune reactions, we engineered soluble TCR mutants with improved affinity for staphylococcal enterotoxin C3 (SEC3). A library of randomly mutated, single-chain TCRs (Vβ-linker-Vα) were expressed as fusions to the Aga2p protein on the surface of yeast cells. Mutants were selected by flow cytometric cell sorting with a fluorescent-labeled SEC3. Various mutations were identified, primarily in Vβ residues that are located at the TCR:SEC3 interface. The combined mutations created a remodeled SEC3-binding surface and yielded a Vβ domain with an affinity that was increased by 1000-fold (K_D = 7 nM). A soluble form of this Vβ mutant was a potent inhibitor of SEC3-mediated T cell activity, suggesting that these engineered proteins may be useful as antagonists.