Immature human thymocytes can be driven to differentiate into nonlymphoid lineages by cytokines from thymic epithelial cells.

The signals and cellular interactions required for hematopoietic stem-cell commitment to the T lineage are unknown, yet are central to understanding the early stages of normal T-cell development. To study the differentiative capacity of T-cell precursors, we isolated CD4-, CD8-, surface(s) CD3- thymocytes from postnatal human thymuses and determined their capacity to differentiate into lymphoid and nonlymphoid lineages in vitro. We found that CD4-, CD8-, sCD3- thymocytes, which differentiated in the presence of T-cell conditioned medium plus interleukin 2 into T cells expressing the gamma delta receptor for antigen, were capable of differentiating into myeloid or erythroid lineages in the presence of either 5637 bladder carcinoma cell line conditioned medium plus recombinant human erythropoietin or human thymic epithelial cell conditioned medium. Thymic epithelial cell conditioned medium was as effective as 5637 supernatant plus erythropoietin in inducing myeloerythroid differentiation in the CD4-, CD8-, sCD3- thymocytes. Sixty-eight +/- 14% of CD4-, CD8-, sCD3- thymocytes underwent nonlymphoid differentiation within 4 days in culture with 5637 supernatant plus erythropoietin. Twenty-six +/- 4% of freshly isolated CD4-, CD8-, sCD3- cells were CD34+, and clonal granulocyte/macrophage, granulocyte/erythrocyte/monocyte/megakaryocyte, and T-cell progenitors were found in both CD34+ and CD34- subsets of CD4-, CD8-, sCD3- thymocytes. Thus, cells within the human CD4-, CD8-, sCD3- thymocyte subset can give rise to gamma delta+ T cells as well as to cells of myeloerythroid lineages. Moreover, CD34+, CD4-, CD8-, sCD3- cells can give rise to clonal T-cell progenitors as well as to clonal myeloid progenitors.