Positive selecting cell type determines the phenotype of MHC class Ib-restricted CD8 ⁺ T cells

Several studies have demonstrated an apparent link between positive selection on hematopoietic cells (HCs) and an "innate"T-cell phenotype. Whereas conventional CD8 ⁺ T cells are primarily selected on thymic epithelial cells (TECs) and certain innate T cells are exclusively selected on HCs, MHC class Ib-restricted CD8 ⁺ T cells appear to be selected on both TECs and HCs. However, whether TEC- and HC-selected T cells represent distinct lineages or whether the same T-cell precursors have the capacity to be selected on either cell type is unknown. Using an M3-restricted T-cell receptor transgenic mouse model, we demonstrate that not only are MHC class Ib-restricted CD8 ⁺ T cells capable of being selected on either cell type but that selecting cell type directly affects the phenotype of the resulting CD8 ⁺ T cells. M3-restricted CD8 ⁺ T cells selected on HCs acquire a more activated phenotype and possess more potent effector functions than those selected on TECs. Additionally, these two developmental pathways are active in the generation of the natural pool of M3-restricted CD8 ⁺ T cells. Our results suggest that these two distinct populations may allow MHC class Ib-restricted CD8 ⁺ T cells to occupy different immunological niches playing unique roles in immune responses to infection.