This manuscript defines the surface phenotype of different hematopoietic stem and progenitor cell subpopulations with higher resolution than achieved previously. With this further refinement, the investigators reveal more plasticity within the hematopoietic differentiation hierarchy than has been previously recognized.
Prior excellent studies have shown enrichment for murine hematopoietic stem cells (HSCs) and various partially committed progenitor cells, such as common lymphoid progenitors and common myeloid progenitors. These investigators further purify these populations using additional cell surface proteins, including CD105 (endoglin) and CD150 (signaling lymphocytic activation molecule family member 1, SLAM1), which play roles in signal transduction. HSCs, previously known to be Lin-Kit+Sca+, were recently shown by Sean Morrison’s group to be further enriched by selection for CD150 expression. To test the hypothesis that CD105 and CD150 might also be differentially expressed within hematopoietic progenitor subsets, the investigators used flow cytometry to sort subpopulations based on surface expression of lineage markers Sca, Kit, CD150, CD105, and CD41. They extensively analyzed each subpopulation for morphology, in vitro and in vivo function, and gene expression profile.
Among the important findings presented, they show that the Lin-Kit+Sca1- population, which was previously known to be enriched for bipotent megakaryocytic erythroid precursors (MEPs), is CD41- and is comprised of at least three subpopulations based on CD105 and CD150 expression. The CD105+CD150- and CD105+CD150+ cells within this population are already erythroid-committed, and the CD105-CD150+ cells are truly biphenotypic with the ability to differentiate down the erythroid and megakaryocytic lineages. This CD105-CD150+ population represents less than 20 percent of the cells previously called MEP and is referred to as PreMegE. In vitro assays of colonies expanded from single PreMegE cells, a highly rigorous and labor-intensive approach, demonstrated that they were biphenotypic megakaryocytic and erythroid precursors. It remains to be seen whether the corresponding human hematopoietic subpopulations share the phenotypes identified.
Additional analysis of the different blood cell types that differentiated from single cells of the different sorted subpopulations adds to the complexity of current models of the hematopoietic hierarchy. For example, Lin-Kit+Sca+CD150+ cells, which are highly enriched for HSC, would be expected to be polyploid, and thus give rise to multiple different cell types. However, 25 percent of the time, these cells differentiated exclusively into megakaryocytes. This doesn’t fit with our current understanding of hematopoietic lineage steps and suggests that megakaryocytes may differentiate from HSCs without going through multiple intermediate stages.
Gene expression analyses were consistent with the lineage commitments observed in vitro. However, one surprising observation was that common myeloid and common lymphoid progenitors had many genes in common, which could underlie the lineage plasticity that has been found during initial phases of lineage commitment.
In Brief
The improved ability to identify and purify specific hematopoietic subpopulations will help investigators to better understand hematopoietic differentiation in general, and myelopoiesis specifically. The identification of successive lineage-restricted progenitors can be used to study the mechanisms of proliferation and maturation down specific lineages. For example, the highly purified biphenotypic PreMegE population can be purified to study lineage fate decisions and to design approaches for enhancing growth and differentiation of erythrocytes and/or megakaryocytes for in vivo enhancement of these lineages or for transfusion therapy.
Competing Interests
Dr. Krause indicated no relevant conflicts of interest.
