Lineage relationships of the fetal thymocyte subset that expresses the β chain of the interleukin‐2 receptor

The beta chain (p75) of the interleukin-2 (IL-2) receptor (IL-2R) is expressed on up to 5-7% of fetal thymocytes on day 16 of gestation, declining thereafter to a minute proportion of less than 1% around birth, and of 1-2% of adult thymocytes. A significant part of fetal IL-2R beta+ thymocytes are gamma delta cells. The precursor-progeny relationships of fetal IL-2R beta+ thymocytes to the alpha beta T cell lineage have not been previously studied, nor has their position within the developmental sequence been determined. Here we show that IL-2R beta is expressed on a subset of very immature cells, along with high amounts of Pgp1 and Fc gamma RII/III, partially preceding the expression of intracellular CD3 epsilon. IL-2-R beta disappears before expression of IL-2R alpha. IL-2R beta+ cells, purified by sorting on day 15 of gestation, efficiently reconstituted fetal thymic lobes depleted of lymphoid cells by treatment with desoxyguanosine. They developed into T cell receptor (TCR) alpha beta+, TCR gamma delta+, and CD4/CD8 double- and single-positive cells in similar proportions as did sorted IL-2R alpha+ day 15 fetal thymocytes. These data suggest that IL-2R beta expression marks a short period of very early thymocyte development, perhaps immediately after entry into the thymus.