Identification of an IL-7-dependent pre-T committed population in the spleen.

Several extrathymic T cell progenitors have been described but their various contributions to the T cell lineage puzzle are unclear. In this study, we provide evidence for a splenic Lin(-)Thy1.2(+) T cell-committed population, rare in B6 mice, abundant in TCRalpha(-/-), CD3epsilon(-/-), and nude mice, and absent in IL-7- and Rag-2-deficient mice. Neither B nor myeloid cells are generated in vivo and in vitro. The incidence of these pre-T cells is under the control of thymus and/or mature T cells, as revealed by graft experiments. Indeed, IL-7 consumption by mature T cells inhibits the growth of these pre-T cells. Moreover, the nude spleen contains an additional Lin(-)Thy1.2(+)CD25(+) subset which is detected in B6 mice only after thymectomy. We establish that the full pre-T cell potential and proliferation capacity are only present in the c-kit(low) fraction of progenitors. We also show that most CCR9(+) progenitors are retained in the spleen of nude mice, but present in the blood of B6 mice. Thus, our data describe a new T cell lineage restricted subset that accumulates in the spleen before migration to the thymus.