CD34-POSITIVE EARLY STAGES OF HUMAN T-CELL DIFFERENTIATION

Thymus, the main organ for T lymphopoiesis, requires a permanent influ x of progenitors from bone marrow (BM) or fetal liver. An essential qu estion relating to early T-cell development is the identification of t he progenitor population which actually homes to the thymus. Recent fi ndings have shown that human multipotent progenitor/stem cells express ing CD34 have the capacity to differentiate into T cells when introduc ed into a thymic environment. More mature CD34(+) bone marrow cells co expressing CD7 and having a poor myeloid differentiation capacity can also efficiently differentiate into T cells in vitro. These lymphoid c ommitted precursors might be the true thymic repopulating cells. In th e thymus, cells with a similar CD34(+)7(+) phenotype include the most primitive thymocyte precursors. CD34(+) thymocytes have no myeloid dif ferentiation potential, but may include precursors for natural killer (NK) cells. Interleukin-7 (IL7) is a potent in vitro growth factor for CD34(+) thymocytes. Whereas current data do not support a crucial rol e for IL2, patients with IL2 receptor gamma chain (IL2R gamma) deficie ncy lack T- and NK cells. The recent demonstration that IL2R gamma is part of the receptor for IL7 strongly suggests that this cytokine play s an essential role in in vivo T lymphocyte and NK development.