Frontiers in Bioscience-Scholar (FBS) is published by IMR Press from Volume 13 Issue 1 (2021). Previous articles were published by another publisher on a subscription basis, and they are hosted by IMR Press on imrpress.com as a courtesy and upon agreement with Frontiers in Bioscience.
*Author to whom correspondence should be addressed.
T cells are unique in that they begin their development as a progenitor within the bone marrow but complete their differentiation within the thymus. Furthermore, long-term T-lymphopoiesis requires a continuous supply of thymus-bound progenitors derived from the bone marrow. The critical role for T cells is clearly observed in individuals with genetic or acquired immunodeficiencies or those having undergone hematopoietic stem cell transplantation. Here, we review the work done by several groups aimed at characterizing the earliest T-lineage progenitors (ETPs), in mouse and human, found within the thymus, in addition to the long-sought after thymus-colonizing progenitor, which makes its journey from the bone marrow via the bloodstream into thymus. The characterization of these progenitors may herald therapeutic insight into the restoration of T cells in immunodeficient individuals.