The thymus is seeded via the blood, but the identity of hematopoietic progenitors with access to the circulation in adult mice is unknown. We report here that the only progenitors in blood with efficient T lineage potential were lineage negative with high expression of stem cell antigen 1 and c-Kit (LSK cells). The blood LSK population, like its counterpart in the bone marrow, contained hematopoietic stem cells and nonrenewing, multipotent progenitors, including early lymphoid progenitors and CD62L+ cells previously described as efficient T lineage progenitors. Common lymphoid progenitors could not be identified in the circulation, suggesting they are not physiological T lineage precursors. We conclude that blood LSK cells are the principal circulating progenitors with T lineage potential.
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