Thymic stroma is required for the development of human T cell lineages in vitro

Development of the T cell lineage is characterized by the homing of hematopoietic precursors to thymus, followed by their acquisition of receptors for antigen. T cell receptors are αβ or γδ heterodimers associated with CD3 (TCR-CD3). Very early T cell precursors in humans have been characterized as CD7+45+ cells which lack the T cell differentiation antigens CD1, CD2, CD3, CD4, and CD8. A phenotypically equivalent early thymocyte population also occurs in postnatal life, and we have previously shown that interleukin 2 (IL2) promotes the development in vitro of both the αβ and the γδ T cells from these early thymocytes. Here we have analyzed the requirements of the induction of the IL2 pathway in early thymocytes, and their developmental potential. We show that: (I) thymic stromal cells, which are present in thymocyte suspensions, are necessary to induce the IL2 pathway and the development of αβ or γδ T cell lineages from early thymocytes in vitro; and (II) when removed from the in vivo environment, early thymocytes can develop in vitro into TCR-CD3− cells of the natural killer (NK) lineage. We conclude that CD7+45+, CD1-2-3-4-8- early thymocytes are multipotential progenitors that, at least, have the capacity to develop into αβ or γδ T cell and NK lineages. The analysis of the mechanisms of generation and selection of human T and NK cell diversity, not feasible in bone marrow cultures, is now possibl