LINEAGE RELATIONSHIPS OF THE FETAL THYMOCYTE SUBSET THAT EXPRESSES THE BETA-CHAIN OF THE INTERLEUKIN-2 RECEPTOR

The beta chain (p75) of the interleukin-2 (IL-2) receptor (IL-2R) is e xpressed on up to 5-7% of fetal thymocytes on day 16 of gestation, dec lining thereafter to a minute proportion of less than 1% around birth, and of 1-2% of adult thymocytes. A significant part of fetal IL-2Rbet a+ thymocytes are gammadelta cells. The precursor-progeny relationship s of fetal IL-2Rbeta+ thymocytes to the alphabeta T cell lineage have not been previously studied, nor has their position within the develop mental sequence been determined. Here we show that IL-2Rbeta is expres sed on a subset of very immature cells, along with high amounts of Pgp 1 and FcgammaRII/III, partially preceding the expression of intracellu lar CD3epsilon. IL-2-Rbeta disappears before expression of IL-2Ralpha. IL-2Rbeta+ cells, purified by sorting on day 15 of gestation, efficie ntly reconstituted fetal thymic lobes depleted of lymphoid cells by tr eatment with desoxyguanosine. They developed into T cell receptor (TCR )alphabeta+, TCRgammadelta+, and CD4/CD8 double- and single-positive c ells in similar proportions as did sorted IL-2Ralpha+ day 15 fetal thy mocytes. These data suggest that IL-2Rbeta expression marks a short pe riod of very early thymocyte development, perhaps immediately after en try into the thymus.