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

Although much is known about the identity of progenitor stages in mouse lymphopoiesis, considerably less is understood about the critical stages of lymphoid commitment of human hematopoietic cells. Early models developed from mouse studies assumed strictly dichotomous pathways of lineage commitment1. Those ideas have evolved into models of gradual loss of lineage potential that can occur via multiple alternative pathways, although the physiological relevance of lineage potential demonstrated in certain in vitro assays continues to be debated2–5. A stage at which mouse bone marrow progenitors are ‘lymphoid primed’ before complete loss of myeloid potential has been defined on the basis of expression of the cell-surface receptor Flt3, and cells at this stage have been called ‘lymphoid-primed multipotent progenitors’ (LMPPs)2. Critical species-specific differences create challenges for the ‘translation’ of knowledge about cellular hierarchies derived from mouse studies to the specifics of human hematopoiesis6. In addition, the source and stage in ontogeny of human hematopoiesis can influence the functional abilities, surface immunophenotypes and transcriptional profiles of the cells under study6–8. Most studies of the earliest progenitor stages of human hematopoiesis have used neonatal umbilical cord blood (UCB) as the source of hematopoietic cells. However, understanding how lymphopoiesis is regulated during steady-state adult hematopoiesis requires direct study of hematopoietic stem cells and progenitors from postnatal human bone marrow8,9. The stepwise process of the lymphoid differentiation of multipotent hematopoietic stem cells (HSCs) in human bone marrow has been assumed to begin with expression of the cell surface antigen CD10 (CALLA or MME) on CD34+ progenitors, based on the finding that CD10+ progenitors lack myeloid and erythroid potential but are able to generate all lymphoid lineages10. However, subsequent studies have shown that CD34+CD10+ cells, even those without expression of lineage markers (Lin−: CD3−CD14−CD15−CD19−CD56−CD235a−), show a strong bias toward B cell potential with relatively little T cell or natural killer (NK) cell potential11,12. CD34+Lin−CD10+ cells that lack expression of CD24 are precursors of the CD34+Lin−CD10+CD24+ population but nonetheless show molecular evidence of commitment to the B cell lineage, with expression of several B cell–specific genes12. Therefore, to understand the progenitor hierarchy of the lymphoid commitment of human cells, we sought to identify a stage of lymphoid priming that precedes commitment to the B lymphoid lineage, either before or independently of CD10 expression. L-selectin (CD62L) is expressed on lymphocytes and mediates homing to peripheral lymphoid organs13. Studies have reported that upregulation of CD62L expression on c-Kit+Lin−Sca-1+ mouse bone marrow cells correlates with loss of erythroid and megakaryocyte potential and efficient thymic engraftment14–16. In this study we have identified a CD34+Lin−CD10− progenitor subpopulation in human bone marrow that had high expression of CD62L and that was devoid of clonogenic myeloid or erythroid potential. In stromal cultures, these cells were able to generate B cells, NK cells and T cells, as well as monocytic and dendritic cells, similar to the LMPPs in mouse bone marrow that