Major T Cell Progenitor Activity in Bone Marrow–derived Spleen Colonies

Common lymphoid progenitors (CLP) are generated in adult bone marrow (BM), but the intermediate steps leading to T cell commitment are unknown, and so is the site at which this commitment occurs. Here, we show that colonies arising in the spleen 12 days after BM injection harbor T cell precursors that are undetectable in BM. These precursors did not generate myeloid cells in vivo but repopulated the thymus and the peripheral T cell compartment much faster than did CLP. Two lineage negative (Lin(−)) subpopulations were distinguished, namely CD44(+) Thy1(−) cells still capable of natural killer generation and transient low-level B cell generation, and T cell–restricted CD44(−) Thy1(+) cells. At a molecular level, frequency of CD3ɛ and preTα mRNA was very different in each subset. Furthermore, only the CD44(−) Thy1(+) subset have initiated rearrangements in the T cell receptor β locus. Thus, this study identifies extramedullary T cell progenitors and will allow easy approach to T cell commitment studies.