Lymphoid priming in human bone marrow begins before expression of CD10 with upregulation of L-selectin

'Lymphoid priming' in human bone marrow is traditionally thought to begin with the expression of CD10 on CD34 + progenitors. Crooks and colleagues now demonstrate lymphoid priming in a subset of CD10 – CD34 + progenitors that are CD62L hi . Expression of the cell-surface antigen CD10 has long been used to define the lymphoid commitment of human cells. Here we report a unique lymphoid-primed population in human bone marrow that was generated from hematopoietic stem cells (HSCs) before onset of the expression of CD10 and commitment to the B cell lineage. We identified this subset by high expression of the homing molecule L-selectin (CD62L). CD10 − CD62L hi progenitors had full lymphoid and monocytic potential but lacked erythroid potential. Gene-expression profiling placed the CD10 − CD62L hi population at an intermediate stage of differentiation between HSCs and lineage-negative (Lin − ) CD34 + CD10 + progenitors. CD62L was expressed on immature thymocytes, and its ligands were expressed at the cortico-medullary junction of the thymus, which suggested a possible role for this molecule in homing to the thymus. Our studies identify the earliest stage of lymphoid priming in human bone marrow.