Spontaneous generation of human CD8+ TCRαβ+ cells derived from precursors within the double negative compartment

Summary Flow cytometric analysis demonstrated that fresh human thymocytes contain only a low level of mature CD8αβ+ TCRαβ+ or CD8+ TCRγδ+ cells and that they consist of − 70% double positive (DP) and − 10% double negative (DN) cells. These unfractionated thymocytes could be selectively expanded in vitro by stimulation with 12‐o‐tetradecanoylphorbol 13‐acetate (TPA) and PHA in the presence of IL‐2. The majority of the cells expanded from unfractionated thymocytes expressed CD3, TCRαβ and CD8 molecules after long‐term culture (18 days). When highly purified DN thymocytes were expanded over a period of 18 days in the presence of DP cells, they also co‐expressed CD3, TCRγδ and CD8 molecules on their surface. However, when pruified DN thymocytes were expanded alone, that is, in the absence of DP cells for 18 days, they expressed CD3‐assoeiated TCRαβ, but not CD8 or TCRαβ. Despite the expression of measurable levels of IL‐2 α and β receptors, as well as a significant level of TCRαβ, purified DP cells failed to proliferate. These findings provide the first evidence, in humans, that the progression of precursor cells in the DN compartment to a later stage of differentiation can be induced outside the thymus and that DP cells can affect the development of TCR expression in proliferating DN thymocytes.