TCR signaling for initiation and completion of thymocyte positive selection has distinct requirements for ligand quality and presenting cell type

Thymocyte selection involves signaling by TCR engaging diverse self-peptide :MHC molecule ligands on various cell types in the cortex and medulla, Here we separately analyze early and late stages of selection to better underst and how presenting cell type, ligand quality, and the timing of TCR signali ng contribute to intrathymic differentiation. TCR transgenic CD4(+)CD8(+) t hymocytes (double positive (DP)) from MHC-deficient mice were stimulated us ing various presenting cells and ligands, The resulting CD69(high) cells we re isolated and evaluated for maturation in reaggregate cultures with wild- type or MHC molecule-deficient thymic stroma with or without added hemopoie tic dendritic cells (DC), Production of CD4(+) T cells required TCR signali ng in the reaggregates, indicating that transient recognition of self-ligan ds by DP is inadequate for full differentiation. DC bearing a potent agonis t ligand could initiate positive selection, producing activated thymocytes that matured into agonist-responsive T cells in reaggregates lacking the sa me ligand, DC could also support the TCR signaling necessary for late matur ation. These results argue that despite the negative role assigned to DC in past studies, neither the peptide:MHC molecule complexes present on DC nor any other signals provided by these cells stimulate only thymocyte death. These findings also indicate that unique epithelial ligands are not necessa ry for positive selection. They provide additional insight into the role of ligand quality in selection events and support the concept that following initiation of maturation from the DP state, persistent TCR signaling is cha racteristic of and perhaps required by T cells.