According to the current model of adult hematopoiesis, differentiation of pluripotential hematopoietic stem cells into common myeloid- and lymphoid-committed progenitors establishes an early separation between the myeloid and lymphoid lineages. This report describes a rare and previously unidentified CD45R-CD19+ B cell progenitor population in postnatal bone marrow that can also generate macrophages. In addition to the definition of this B-lineage intermediate, the data indicate that a developmental relationship between the B and macrophage lineages is retained during postnatal hematopoiesis.
