Genetic Evidence of a Novel Blood Differentiation Pathway from Lympho-Myeloid Hematopoietic Stem/Progenitor Cells, Independent of Common Myeloid Progenitors.

We have recently identified three novel subsets of multipotent hematopoietic stem/progenitor cells (HSCs) in the Lin⁻Sca-1⁺c-Kit^hi (LSK) compartment of adult murine bone marrow based on differential expression of CD34 and the cytokine tyrosine kinase receptor Flt3. Long-term HSCs (LT-HSCs) lack CD34 and Flt3 expression (LSKCD34-flt3-), whereas short-term HSCs (ST-HSCs) are LSKCD34⁺flt3^−. A third LSK population is characterized by co-expression of CD34 and Flt3 (LSKCD34⁺flt3⁺) and possess a combined myeloid (granulocyte and monocyte) and lymphoid (B and T cell) differentiation potential, but surprisingly lack megakaryocytic (Mk) and erythroid (E) potential in vitro and in vivo. These findings implicate an alternative road map for blood lineage development distinct from the classical model in which the first lineage commitment step of HSCs is thought to result in a strict separation into myelopoiesis and lymphopoiesis. In the current study we sought genetic evidence in further support of this new model through genetic profiling of these three HSC subpopulations, using affymetrix chips, quantitative (Q)-PCR and single cell PCR. In contrast to the pluripotent LSKCD34⁻Flt3⁻ LT-HSCs and LSKCD34⁺Flt3⁻ ST-HSCs, LSKCD34⁺Flt3⁺ cells downregulated or turned of genes critically involved in promoting Mk and E lineage development, such as the Epo and Tpo receptors as well as the transcription factor Gata-1. In contrast, the gene for Il-7rα, critically involved in early B and T cell development was upregulated in LSKFlt3⁺ cells, but absent in LT-HSCs and ST-HSCs. However, in agreement with their sustained ability to produce granulocytes and monocytes, G-CSFR and Pu.1 expression was sustained from LT-HSC through the LSKCD34⁺flt3⁺ stage Particularly noteworthy, single cell PCR demonstrated that a fraction of single LSKCD34⁺Flt3⁺ cells upregulating Il-7rα gene expression, sustained and co-expressed G-CSFR expression. Thus, genetic profiling at the single cell level provide further and compelling evidence for a novel road map for blood lineage development, independent of the common myeloid progenitor (CMP). Our biological and gene expression data rather supports the existence of a pathway in which HSCs lose their lineage potentials one by one, starting with the Mk and E lineages.