Delineation of the Common Developmental Pathway for Granulocyte/Monocyte and Lymphoid Lineages by Using an Expression Reporter for PU.1.

Understanding how multipotent cells commit to each of their terminal fate potentials is an important aspect of stem cell biology. Hematopoietic stem cells (HSCs) of Lin ⁻Sca-1⁺c-Kit⁺ (LSK) phenotype have been purified, which were further divided into CD34-long-term and CD34+ short-term (ST)-HSCs. The existence of phenotypically isolatable common lymphoid progenitors (CLPs) and common myeloid progenitors (CMPs) downstream of ST-HSCs suggests that the first commitment step after the HSC stage is the bifurcation of lymphoid vs. myeloid pathway. Recent studies, however, suggest that the loss of MegE potential could be an early event in HSC stage. For example, LSK cells activating RAG-1 or Flt-3 expression retained granulocyte/monocyte (GM) but not megakaryocyte/erythrocyte (MegE) potential together with lymphoid potential, suggesting the existence of common progenitor for GM and lymphoid lineages. Here we report that a fraction of ST-LSK cells expressing high levels of PU.1, a transcription factor necessary for GM and lymphoid development, represents GM/lymphoid bipotent progenitors. In mice harboring knock-in GFP reporter for PU.1, LSK cells were divided into GFP high and GFP low subpopulations. Although PU.1low LSK cells were multipotent, PU.1high LSK cells differentiated only into GM and lymphoid cells in vitro and in vivo. We also found that single PU.1high LSK cells differentiated into GM, T and B cells in vivo. These data formally prove the existence of the third major early progenitor population activating PU.1 at a high level, the granulocyte/monocyte/lymphoid progenitor (GMLP). The existence of prospectively isolatable GMLPs strongly suggests that HSCs sequentially lose MegE then GM potential during their lymphoid commitment.