Commitment of Common T/Natural Killer (Nk) Progenitors to Unipotent T and Nk Progenitors in the Murine Fetal Thymus Revealed by a Single Progenitor Assay

We have established a new clonal assay system that can evenly support the development of T and natural killer (NK) cells. With this system, we show that all T cell progenitors in the earliest CD44⁺CD25⁻FcγRII/III⁻ fetal thymus (FT) cell population retain NK potential, and that the NK lineage–committed progenitors (p-NK) also exist in this population. T cell lineage–committed progenitors (p-T), which are unable to generate NK cells, first appear at the CD44⁺CD25⁻ FcγRII/III⁺ stage in day 12 FT. The proportion of p-T markedly increases during the transition from the CD44⁺CD25⁻ stage to the CD44⁺CD25⁺ stage in day 14 FT. On the other hand, p-NK preferentially increase in number at the CD44⁺CD25⁻ stage between days 12 and 14 of gestation. The production of p-NK continues up to the CD44⁺CD25⁺ stage, but ceases before the rearrangement of T cell receptor β chain genes. It was further shown that the CD44⁺CD25⁻ CD122⁺ population of day 14 FT exclusively contains p-NK. These results indicate that the earliest T cell progenitor migrating into the FT is T/NK bipotent, and strongly suggest that the bipotent progenitor continuously produces p-NK and p-T until the CD44⁺CD25⁺ stage.