Figure 3.

Effect of the Ii on the binding of SEA, SEB, and TSST-1 on HeLa DR1 cells. (A) Specific binding of ¹²⁵I-SEA, -SEB, or -TSST-1 on HeLa transfectants in presence (+) or absence (−) of a 10-fold excess of cold competitor. (B) Surface staining confirm the expression of Ii at the surface of HeLa DR1/Ii or LG2, but not DAP or HeLa DR1. The control antibody (dotted line) is fluorescein isothiocyanate-coupled goat anti-mouse Ig. Cell surface Ii expression (solid line) was determined using the anti-Ii antibody BU45 (which recognizes the extracellular C-terminal end of Ii). (C) SEA, but not TSST-1, bind to DR1/Ii complexes. The cell surface immunoprecipitation was carried out by incubating intact LG2 cells with biotinylated SEA or TSST-1. After washing, cells were lysed and incubated with streptavidin-coupled magnetic beads. (Upper) Samples blotted using XD5, an anti-DRβ antibody, and the same samples (Lower) were blotted with an anti-CLIP antibody (obtained from Peter Cresswell, Yale University).