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The Complex Cartography of Stem Cell Commitment

2005; Cell Press; Volume: 121; Issue: 2 Linguagem: Inglês

10.1016/j.cell.2005.04.005

1097-4172

Koichi Akashi, David Traver, Leonard I. Zon,

Hematopoietic Stem Cell Transplantation

In this issue of Cell, a study by Adolfsson and coworkers (Adolfsson et al., 2005Adolfsson J. Mansson R. Buza-Vidas N. Hultquist A. Liuba K. Jensen C.T. Bryder D. Yang L. Borge O.-J. Thoren L. et al.Cell. 2005; 121 (this issue): 295-306Abstract Full Text Full Text PDF PubMed Scopus (916) Google Scholar) provides insight into the early lineage commitment events of multipotent hematopoietic stem cells (HSCs). These studies demonstrate the importance of the Flt3 receptor tyrosine kinase as the earliest marker of hematopoietic cell fate commitment in that erythrocyte and megakaryocyte potentials are lost first as HSCs differentiate to lymphocyte progenitors. In this issue of Cell, a study by Adolfsson and coworkers (Adolfsson et al., 2005Adolfsson J. Mansson R. Buza-Vidas N. Hultquist A. Liuba K. Jensen C.T. Bryder D. Yang L. Borge O.-J. Thoren L. et al.Cell. 2005; 121 (this issue): 295-306Abstract Full Text Full Text PDF PubMed Scopus (916) Google Scholar) provides insight into the early lineage commitment events of multipotent hematopoietic stem cells (HSCs). These studies demonstrate the importance of the Flt3 receptor tyrosine kinase as the earliest marker of hematopoietic cell fate commitment in that erythrocyte and megakaryocyte potentials are lost first as HSCs differentiate to lymphocyte progenitors. Understanding how multipotent cells commit to each of their terminal fate potentials is an important aspect of stem cell biology. Within the hematopoietic system, the prospective isolation of stem and progenitor cell subsets by cell surface phenotype has identified the branch points at which major lineage decisions occur. This approach has been instrumental in creating the fate maps necessary to understand the hierarchical loss of lineage potential as multipotent hematopoietic stem cells (HSCs) differentiate (Traver and Akashi, 2004Traver D. Akashi K. Adv. Immunol. 2004; 83: 1-54Crossref PubMed Scopus (14) Google Scholar). HSCs with long-term (LT) self-renewal potential have been purified from within the cell population known as Lin−Sca-1+c-Kit+ (LSK) by using additional cell surface markers such as Thy1.1 expression or lack of Flt3 and CD34 expression (Figure 1A ) (Adolfsson et al., 2001Adolfsson J. Borge O.J. Bryder D. Theilgaard-Monch K. Astrand-Grundstrom I. Sitnicka E. Sasaki Y. Jacobsen S.E. Immunity. 2001; 15: 659-669Abstract Full Text Full Text PDF PubMed Scopus (531) Google Scholar, Osawa et al., 1996Osawa M. Hanada K. Hamada H. Nakauchi H. Science. 1996; 273: 242-245Crossref PubMed Scopus (1718) Google Scholar). Importantly, these cells can reconstitute multilineage hematopoiesis for more than six months following single cell transplantation. In contrast, Flt3+ or CD34+ LSK cells do not appreciably self renew, being capable of multilineage reconstitution for only short-term (ST) periods of time. Upon loss of Sca-1 expression by ST-HSCs, myeloid-committed progenitor cells, including common myeloid progenitors (CMPs), granulocyte/monocyte progenitors (GMPs), and megakaryocyte/erythrocyte progenitors (MEPs), have been purified by cell surface phenotype (Figure 1A) (Akashi et al., 2000Akashi K. Traver D. Miyamoto T. Weissman I.L. Nature. 2000; 404: 193-197Crossref PubMed Scopus (1906) Google Scholar). Similarly, common lymphoid progenitors (CLPs) have been isolated from the IL-7Ra+Sca-1loc-Kitlo fraction (Kondo et al., 1997Kondo M. Weissman I.L. Akashi K. Cell. 1997; 91: 661-672Abstract Full Text Full Text PDF PubMed Scopus (1655) Google Scholar). Based on the existence of these defined progenitor populations, the first lineage commitment steps had been considered to occur downstream of ST-HSCs, with the myeloid and lymphoid developmental pathways being largely independent (Figure 1A). Recently, several studies have suggested that lineage commitment may occur at the level of ST-HSCs and precede a simple bifurcation of myeloid and lymphoid fates. Spangrude et al. have reported that Thy-1− ST LSK cells failed to reconstitute the erythroid lineage efficiently, and that this population gradually lost erythroid potential in vitro (Slayton et al., 2001Slayton W.B. Mojica M.P. Pierce L.J. Spangrude G.J. Ann. N Y Acad. Sci. 2001; 938: 157-165Crossref PubMed Scopus (7) Google Scholar). Igarashi et al. reported that ST-HSCs contain a fraction of cells that express rag-1, a lymphoid-specific gene, and that these RAG-1+ LSK cells, termed early lymphoid progenitors (ELPs), differentiated mainly into lymphoid lineages but retained weak granulocyte/monocyte (G/M) potential (Igarashi et al., 2002Igarashi H. Gregory S.C. Yokota T. Sakaguchi N. Kincade P.W. Immunity. 2002; 17: 117-130Abstract Full Text Full Text PDF PubMed Scopus (372) Google Scholar). Thus, the loss of G/M potential could be the final event in lymphoid commitment as ST-HSCs transit to lymphoid-restricted progenitors. Adolfsson et al., 2005Adolfsson J. Mansson R. Buza-Vidas N. Hultquist A. Liuba K. Jensen C.T. Bryder D. Yang L. Borge O.-J. Thoren L. et al.Cell. 2005; 121 (this issue): 295-306Abstract Full Text Full Text PDF PubMed Scopus (916) Google Scholar have now identified a Flt3+ population within the ST LSK fraction that features robust lymphoid potential and the ability to generate G/M lineages. Whereas previous studies by this (Adolfsson et al., 2001Adolfsson J. Borge O.J. Bryder D. Theilgaard-Monch K. Astrand-Grundstrom I. Sitnicka E. Sasaki Y. Jacobsen S.E. Immunity. 2001; 15: 659-669Abstract Full Text Full Text PDF PubMed Scopus (531) Google Scholar) group have demonstrated that upregulation of Flt3 by LT-HSCs marks loss of self-renewal potential, this new work is the first demonstration that this loss is coupled functionally with a loss of erythrocyte/megakaryocyte (E/Meg) potential. LSK CD34+Flt-3+ cells gave rise to G/M, T, and B cells in vitro but rarely formed megakaryocyte or erythroid colonies. After transplantation, the contribution of LSK CD34+Flt-3+ cells to erythroid development was weak when compared to their Flt-3− counterparts. Thus, they propose that Flt3 expression within ST-HSCs marks a new committed progenitor that is able to both generate lymphoid progeny and bypass the CMP stage in generating G/M progeny. The most interesting finding of this study is that E/Meg potential is lost first as HSCs commit to differentiation. Rather than a mutually exclusive bifurcation of myeloid and lymphoid potential, as previously proposed (Akashi et al., 2000Akashi K. Traver D. Miyamoto T. Weissman I.L. Nature. 2000; 404: 193-197Crossref PubMed Scopus (1906) Google Scholar), this work demonstrates that the E/Meg differentiation program can be shut down before G/M potential is lost. A major question remaining is whether these findings represent a common physiological differentiation pathway for both myeloid and lymphoid lineages or whether these findings simply reflect an ordered loss of myeloid potentials as ST-HSCs commit to lymphoid fates via the CLP. Importantly, lineage relationship experiments were previously performed that demonstrated the CLP is a direct descendant of Flt3+ LSK cells (Adolfsson et al., 2001Adolfsson J. Borge O.J. Bryder D. Theilgaard-Monch K. Astrand-Grundstrom I. Sitnicka E. Sasaki Y. Jacobsen S.E. Immunity. 2001; 15: 659-669Abstract Full Text Full Text PDF PubMed Scopus (531) Google Scholar). If this new progenitor subset represents a new G/M pathway as proposed, it would be expected that GMPs would also be downstream of this cell type. This experiment remains to be performed, as do studies aimed at determining the relative contribution of this fraction to normal, steady-state G/M production. Molecular analyses demonstrated that only 7% of Flt3+ LSK cells coexpressed lymphoid and myeloid genes, suggesting that the noted bipotentiality may reflect heterogeneity within this fraction. This expression profile is in contrast to other bipotent populations such as the CMP, where more than 60% of single cells coexpressed G/M and E/Meg genes. Further work is thus required to ascertain the role of Flt3+ LSK cells in normal myeloid cell development. Previous reports have shown that, while Flt3 ligand is dispensable for G/M development, it is critical for the generation of CLPs from Flt3+ LSK cells (Sitnicka et al., 2002Sitnicka E. Bryder D. Theilgaard-Monch K. Buza-Vidas N. Adolfsson J. Jacobsen S.E. Immunity. 2002; 17: 463-472Abstract Full Text Full Text PDF PubMed Scopus (213) Google Scholar). Recent studies have also suggested that the CMP population can be subdivided by Flt3 expression. Interestingly, the minor B lymphoid potential described within this fraction, as well as most of the dendritic cell potential, was contained within the Flt3+ subset (D'Amico and Wu, 2003D'Amico A.D. Wu L. J. Exp. Med. 2003; 198: 293-303Crossref PubMed Scopus (304) Google Scholar). Together, these results further support an important role of Flt3 signaling in lymphoid commitment and warrant more precise studies examining the lineage relationships of these new progenitor cell subsets with those previously identified. In addition to improved lineage studies, an issue that many recent studies have raised is a failure to compare populations that are identical to those published by other groups. For example, both Adolfsson et al., 2005Adolfsson J. Mansson R. Buza-Vidas N. Hultquist A. Liuba K. Jensen C.T. Bryder D. Yang L. Borge O.-J. Thoren L. et al.Cell. 2005; 121 (this issue): 295-306Abstract Full Text Full Text PDF PubMed Scopus (916) Google Scholar and D'Amico and Wu, 2003D'Amico A.D. Wu L. J. Exp. Med. 2003; 198: 293-303Crossref PubMed Scopus (304) Google Scholar claim to subdivide the CMP population by Flt3 expression. Both groups however, due to apparent technical limitations, substituted visualization of Flt3 for an important CMP marker. Failure to study Flt3 expression in the context of the conventional myeloid progenitor isolation protocol results in contamination of the CMP subset with other cell types, yet isolated fractions were termed Flt3+ or Flt3− "CMPs." This has become a common problem in the stem and progenitor cell field and is not acceptable if one truly wishes to compare identical populations. Improvements in flow cytometry, such as the inclusion of additional antibodies coupled to additional fluorochromes, need to be achieved by the field to enable meaningful comparisons between laboratories. Prospective isolation strategies largely rely upon differences in cell surface markers that correlate with cell fate commitments. These commitment events are generally associated with changes in transcriptional regulation. Continued examination of current and yet-to-be-discovered stem and progenitor cell subsets will be important in determining the molecular mechanisms underlying hematopoietic fate commitment and in creating high-resolution maps for all blood cell lineages.