Oncogene (2007) 26, 6687–6696 & 2007 Nature Publishing Group All rights reserved 0950-9232/07 $30.00 www.nature.com/onc REVIEW Hematopoietic developmental pathways: on cellular basis H Iwasaki1 and K Akashi1,2 1Center for Cellular and Molecular Medicine, Kyushu University Hospital, Fukuoka, Japan and 2Department of Cancer Immunology and AIDS, Harvard Medical School, Dana-Farber Cancer Institute, Boston, MA, USA To elucidate the molecular mechanisms underlying normal counterpart, the common myeloid progenitor (CMP) and malignant hematopoietic development, it is critical to that can be a source of all myeloid cell types, supports identify developmental intermediates for each lineage the concept that lymphoid and myeloid lineages develop downstream of hematopoietic stem cells. Recent advances independently downstream of HSCs (Kondo et al., 1997; in prospective isolation of hematopoietic stem and Akashi et al., 2000). progenitor cells, and efficient xenogeneic transplantation Recent progresses in the fluorescence-activated cell systems have provided a detailed developmental map in sorting analysis using additional surface markers for both mouse and human hematopoiesis, demonstrating that early hematopoiesis, however, have provided more surface phenotypes of mouse stem–progenitor cells and detailed developmental map downstream of HSCs. their human counterparts are considerably different. Importantly, prior to proceed into the classical myeloid Here, we summarize the phenotype and functional vs lymphoid pathways, HSCs appear to form myelo- properties and their differences of hematopoietic stem erythroid vs myelo-lymphoid progenitors (Arinobu and progenitor cell populations between mouse and et al., 2007). Furthermore, it is now clear that there human. are considerable differences in distribution of surface Oncogene (2007) 26, 6687–6696; doi:10.1038/sj.onc.1210754 markers between human and mouse hematopoiesis, which makes identification of human counterparts of Keywords: hematopoietic stem cell; lineage commitment; mouse progenitors difficult. In this review, we summar- transcription factor; leukemic stem cell ize what we have learned from recent studies concerning the developmental sequences in murine and human hematopoiesis. Introduction Hematopoietic stem cells (HSCs) can self-renew, and Developmental pathways in murine hematopoiesis differentiate into all types of blood cells throughout life. Mature hematopoietic cells are traditionally categorized In murine hematopoiesis, cells with multipotent activity into two distinct lineages: the lymphoid and the myeloid. reside in a small fraction of bone marrow, which lacks The lymphoid lineage consists of T, B and natural killer the expression of lineage-affiliated surface markers (Lin) (NK) cells, while the myeloid lineage includes a number but expresses high levels of Sca-1 and c-Kit (Spangrude of morphologically, phenotypically and functionally et al., 1988; Ikuta and Weissman, 1992). Within the LSK distinct cell types such as different subclasses of population, the most primitive self-renewing HSCs with granulocytes (neutrophils, eosinophils and basophils), long-term reconstituting activity (LT-HSCs) reside in lo À À monocytes–macrophages, erythrocytes, megakaryocytes the Thy1.1 , CD34 , fms-like tyrosine kinase-3 (Flt3) low and mast cells. These two lineages have been thought to or Hoechst (side population; SP) fraction (Morrison use separate differentiation pathways. and Weissman, 1994; Osawa et al., 1996; Goodell et al., lo À HSCs were identified within the ‘LSK’ (LinÀSca-1 þ c-Kit þ ) 1997; Adolfsson et al., 2001). The Thy1.1 or the CD34 B population of the mouse bone marrow (Spangrude LSK fraction constitutes only 0.01% of total bone B lo et al., 1988; Morrison and Weissman, 1994; Osawa marrow cells. At the single cell level, 20% of Thy1.1 B À et al., 1996), and subsequently lineage-restricted pro- or 35% of CD34 LSK cells display multilineage genitors at various developmental stages have been isolated long-term reconstitution in competitive reconstitution downstream of HSCs by using the multicolor fluorescence- assays (Osawa et al., 1996; Wagers et al., 2002). À þ þ activated cell sorting system. The successful isolation of On the other hand, the Thy1.1 , CD34 or Flt3 LSK the common lymphoid progenitor (CLP) that can generate fraction is capable of only transient reconstitution, all lymphoid types but not any myeloid cells, and its thereby contains short-term (ST) HSCs or multipotent progenitors (MPPs) (Morrison and Weissman, 1994; Osawa et al., 1996; Adolfsson et al., 2001; Christensen Correspondence: Professor K Akashi, Department of Cancer Immuno- logy and AIDS, Harvard Medical School, Dana-Farber Cancer Institute, and Weissman, 2001). Several reports have tried to Boston, MA 02115, USA. discriminate ST-HSCs and MPPs by using low or E-mail: [email protected] negative expression of CD4, CD11b, and/or Thy1.1, Hematopoietic developmental pathways H Iwasaki and K Akashi 6688 and showed some difference in duration and magnitude FcgRII/IIIloCD34 þ CMPs, FcgRII/IIIhiCD34 þ granu- of reconstitution (Morrison and Weissman, 1994; Morrison locyte–monocyte progenitors (GMPs) and FcgRII/ et al., 1997). These reports, however, did not provide IIIloCD34À megakaryocyte–erythrocyte progenitors clear-cut definition or functional difference of ‘ST-HSC’ (MEPs) are isolatable (Akashi et al., 2000). The CMP and ‘MPP’. We will therefore use MPPs instead of can generate all types of myeloid colonies, while the ST-HSCs throughout this review. GMP or the MEP produces only granulocyte macro- phage (GM) or megakaryocyte erythrocyte (MegE) The myeloid vs lymphoid pathways downstream of MPP lineage cells, respectively. The CMP differentiates into within the LSK fraction the GMP and the MEP in vitro, upregulating the Because the LSK population contains all multipotent FcgRII/III and downregulating the CD34 expression, precursors such as LT-HSCs and MPPs, lineage-restrict- respectively, indicating that the CMP is the precursor ed progenitor populations were first sought outside for the GMP and the MEP (Akashi et al., 2000). Upon the LSK fraction. The CLP is the earliest population in vivo transfer, these populations display short-term that upregulates the receptor for interleukin-7 (IL-7) production of lineages corresponding to their in vitro (Kondo et al., 1997), an essential cytokine for both activities, indicating that they do not appreciably self- T- and B-cell development (Peschon et al., 1994; von renew (Na Nakorn et al., 2002). Freeden-Jeffry et al., 1995). The IL-7 receptor signaling The existence of the CLP and the CMP outside the plays a critical role in thymocyte survival through LSK fraction leads to the idea that lymphoid and maintenance of Bcl-2 (Akashi et al., 1997), and in myeloid development starts at the CLP and the CMP the rearrangement of immunoglobulin heavy chain V stage, respectively (Figure 1a). These stem and progeni- segments through the activation of the Pax5 gene tor populations are widely used for targeted analysis and (Corcoran et al., 1998). In human, the genetic loss of manipulation of cells at a specific hematopoietic stage. IL-7Ra chain is found in patients with severe combined immunodeficiency (SCID) (Puel et al., 1998). The Initial hematopoietic lineage commitment starts within the IL-7Ra þ fraction concentrates lymphoid potential in LSK ‘MPP’ fraction prior to the conventional CMP or the bone marrow, and IL-7Ra þ LinÀSca-1loc-Kitlo cells, CLP stages termed as the CLP, possess clonogenic T- and B-cell, Although the CMP and the CLP were found outside the and NK-cell potential, but lack myelo-erythroid differ- LSK fraction, recent studies have revealed that the MPP entiation activity (Kondo et al., 1997). population is heterogeneous, and contains progenitor The CMP was also isolated outside the LSK fraction. subsets committed to myelo-erythroid or myelo-lym- The IL-7RaÀ LinÀSca-1Àc-Kit þ population, which con- phoid lineages (Figures 1b and 2) (Arinobu et al., 2007). tains the majority of myelo-erythroid colony-forming The question regarding the simple myeloid vs activity in the bone marrow, is further fractionated lymphoid diversification model initially arose from the based on the expression of FcgRII/III and CD34. finding that a fraction of MPPs have primed to the Three distinct myeloid progenitor subsets such as lymphoid lineage (Iwasaki and Akashi, 2007). In mice abCMP vs. CLP model CMP vs. GMLP model HSC HSC Flt3+ LMPP MPP MPP CMP CLP CMP GMLP MEP GMP proT proB MEP GMP CLP proT proB Figure 1 Cellular pathways in adult murine hematopoiesis based on prospective purification of lineage-restricted progenitors. (a) The conventional myeloid vs lymphoid developmental pathways. (b) The new developmental pathway with integration of newly identified GMLP within the MPP population. CLP, common lymphoid progenitor; CMP, common myeloid progenitor; GMLP, granulocyte– monocyte–lymphoid progenitor; GMP, granulocyte–monocyte progenitor; LMPP, lymphoid-primed multipotent progenitor; MEP, megakaryocyte–erythrocyte progenitor; MPP, multipotent progenitor. Oncogene Hematopoietic developmental pathways H Iwasaki and K Akashi 6689 percent of ELPs give rise to GM and rarely MegE colonies. In our hands, the ELP resides within the CD34 þ HSC CD34 - MPP population (Osawa et al., 1996), and almost 40% of ELPs can form GM but not MegE colonies (Arinobu et al., 2007). These data suggest that the ELP population contains cells with GM and lymphoid but not MegE lineage potential. Additional