Identification of human T‐lymphoid progenitor cells in CD34⁺CD38low and CD34⁺CD38⁺ subsets of human cord blood and bone marrow cells using NOD‐SCID fetal thymus organ cultures

In contrast to myeloid and B-lymphoid differentiation, which take place in the marrow environment, development of T cells requires the presence of thymic stromal cells. We demonstrate in this study that human CD34+, CD34+ CD38+ and CD34+ CD38(low) cells from both cord blood and adult bone marrow reproducibly develop into CD4+ CD8+ T cells when introduced into NOD-SCID embryonic thymuses and further cultured in organotypic cultures. Such human/mouse FTOC fetal thymic organ culture) thus represents a reproducible and sensitive system to assess the T-cell potential of human primitive progenitor cells. The frequency of T-cell progenitors among cord-blood-derived CD34+ cells was estimated to be 1/500. Furthermore, the differentiation steps classically observed in human thymus were reproduced in NOD-SCID FTOC initiated with cord blood and human marrow CD34+ cells: immature human CD41(low) CD8- sCD3- TCR alphabeta- CD5+ CD1a+ T cells were mixed with CD4+ CD8+ cells and more mature CD4+ CD8- TCR alphabeta+ cells. However, in FTOC initiated with bone marrow T progenitors, <10% double-positive cells were observed, whereas this proportion increased to 50% when cord blood CD34+ cells were used, and most CD4+ cells were immature T cells. These differences may be explained by a lower frequency of T-cell progenitors in adult samples, but may also suggest differences in the thymic signals required by bone marrow versus cord blood T progenitors. Finally, since cytokine-stimulated CD34+ CD38(low) cells retained their ability to generate T cells, these FTOC assays will be of value to monitor, when combined with other biological assays, the influence of different expansion protocols on the potential of human stem cells.