Cloning, expression and interaction of human T-cell receptors with the bacterial superantigen SSA

Abstract

Superantigens (SAgs) are a class of disease-causing and immunostimulatory proteins of bacterial or viral origin that activate a large number of T-cells through interaction with the Vβ domain of T-cell receptors (TCRs). In this study, recombinant TCR βchains were constructed with human variable domains Vβ5.2, Vβl and Vβ2.1, expressed as inclusion bodies, refolded and purified. The Streptococcus pyogenes SAg SSA-1 was cloned and expressed as a soluble periplasmic protein. SSA-1 was obtained both as a monomer and a dimer that has an intermolecular disulfide bond. We analyzed the biological activity of the recombinant SAgs by proliferation assays. The results suggest that SSA dimerization occludes the TCR interaction site. Naturally occurring SSA dimerization was also observed in supernatants of S. pyogenes isolates. An SSA mutant [SSA(C26S)] was produced to eliminate the Cys responsible for dimerization. Affinity assays using a resonant biosensor showed that both the mutant and monomeric wild type SSA have affinity for human Vβ5.2 and Vβ1 with Kd of 9-11 μM with a fast kass and a moderately fast kdiss. In spite of the reported stimulation of Vβ2.1 bearing T-cells by SSA, we observed no measurable interaction.