Positive Selection by Purified MHC Class II / Thymic Epithelial Cells In Vitro: Costimulatory Signals Mediated by B7 Are Not Involved

We have investigated the possibility that the costimulatory signals required for activation ofmature T cells also play a role in providing differentiation signals for positive selectionduring T-cell development. We show that purified MHC Class II⁺ thymic epithelial cells areable to support positive selection in vitro but lack both the functional capacity to delivercostimulatory signals and expression of the costimulatory ligand B7. Our results suggest thatthe additional signals provided by costimulatory ligands are not required for TCR-mediatedpositive selection, although other ancillary signals provided by thymic epithelial cells maybe involved.