Resolution of Unique Sca-1highc-Kit− Lymphoid-Biased Progenitors in Adult Bone Marrow

We have identified a distinctive lymphoid-restricted progenitor population in adult mouse bone marrow based on a unique c-Kit−Sca-1highFlt3+ AA4+ surface phenotype. These cells are highly lymphoid biased and rapidly generate B and T cells after adoptive transfer. However, whereas previously described lymphoid progenitors such as common lymphoid progenitors express TdT and relatively high levels of RAG2, and are enriched for cells with an active V(D)J recombinase, Flt3+ AA4+ cells within the c-Kit−Sca-1high bone marrow fraction are TdT−, are RAG2low, and do not display evidence for ongoing or past recombinase activity. Furthermore, unlike common lymphoid progenitors that readily generate B cells upon stimulation with IL-7, c-Kit−Sca-1highFlt3+ precursors do not express abundant levels of the IL-7R, and require costimulation with Flt3 ligand and IL-7 to generate B cells in vitro. Moreover, these findings suggest that hematopoietic stem cells in adults generate an array of lymphoid-biased progenitor populations characterized by distinct gene expression and cytokine response profiles.