Despite years of research, the identity of the bone marrow progenitor cells that seed the thymus, as well as the place where they ultimately commit to being T cells, have remained elusive. On page 21, Benz and Bleul identify the first multipotent progenitor cell in the adult thymus. The new data suggest that, for at least some progenitor cells, commitment to a T cell fate occurs after arrival in the thymus.

Populations of thymic precursor cells have been shown to generate multiple cell lineages, but it remains a matter of debate whether single precursor cells can give rise to all the possible thymic cell lineages—T cells, B cells, and dendritic cells (DCs).

Benz and Bleul now show that these cell types can indeed arise from a single precursor cell. By fusing EGFP with a marker of T cell development sites (CCR9), they pinpointed a small population of cells in the bone marrow that was able to populate the thymus.

Single-cell sorting of these precursor cells from the thymus revealed that a single cell could generate T cells, B cells and DCs, suggesting that these progenitor cells commit to a T cell fate in the thymus. Thymic cells with the same surface markers but lower levels of EGFP—presumably representing a slightly more mature population of cells (with decaying EGFP)—were no longer able to generate B cells, suggesting that the EGFP+ population marks the branching point of the T and B cell lineages.