TCR-α/β CD4^- CD8^- double negative T cells arise from CD8⁺ T cells

The cellular origin of CD4^- CD8^- (double negative, DNT) TCR-α/β⁺ T cells remains unknown. Available evidence indicates that they may derive from CD8⁺ T cells, but most published data have been obtained using cells that bear an invariant transgenic T cell receptor that recognizes an Ag that is not present in normal mice. Here, we have used complementary fate mapping and adoptive transfer experiments to identify the cellular lineage of origin of DNT cells in wild-type mice with a polyclonal T cell repertoire. We show that TCR-α/β⁺ DNT cells can be traced back to CD8⁺ and CD4⁺ CD8⁺ double positive cells in the thymus. We also demonstrate that polyclonal DNT cells generated in secondary lymphoid organs proliferate upon adoptive transfer and can regain CD8 expression in lymphopenic environment. These results demonstrate the cellular origin of DNT cells and provide a conceptual framework to understand their presence in pathological circumstances.