In vitro assay for SEB-induced Vβ17 deletions. We developed an in vitro assay capable of detecting negative selection of freshly isolated human thymocytes to bacterial SAgs. (A) Thymocytes were cultured with different ratios of vector-transfected A20APCs in the presence (▪) or absence (□) of the soluble bacterial SAg SEB. After culture, percentages of Vβ2 (1), Vβ7 (2), and the SEB reactive Vβ17 (3) were determined on DP, SPCD4, and SPCD8 thymocytes. (B) SEB was added directly to thymocytes (▪) or not (□) without exogenous APCs.