Abstract
We recently identified a novel Lin−Sca-1+c-kithiCD34+Flt3hi (LSKCD34+Flt3hi) lymphoid-primed multipotent progenitor (LMPP) in adult mouse bone marrow which, although possessing a combined lymphoid (B and T cell) and myeloid (granulocyte-monocyte; GM) differentiation potential, have little or no ability to adopt erythroid (E) and megakaryocyte (MK) lineage fates (Adolfsson et al, Cell121:295, 2005). The identification of this lineage restricted lymphomyeloid progenitor implicates the existence of alternative roadmaps for lineage commitment of pluripotent hematopoietic stem cells (HSCs), distinct from the classical model suggesting that the first HSC commitment step results in a strict separation into common lymphoid and myeloid progenitors. Herein we provide further, genetic evidence for such a model. Affymetrix global gene profiling, quantitative PCR, and multiplex single cell PCR analysis of LSKCD34−Flt3− long-term (LT)-HSCs, LSKCD34+Flt3− short-term (ST)-HSCs and LSKCD34+Flt3hi LMPPs, demonstrate that LMPPs in contrast to LT-HSCs and ST-HSCs down-regulate or turn off a number of genes critically involved in MkE lineage development, including GATA-1 and the receptors for erythropoietin and thrombopoietin. In contrast, a number of genes specific for early lymphoid development, including Rag-1, sterile Ig and IL-7 receptor are upregulated in LMPPs but absent in LT-HSCs and ST-HSCs. Importantly, within the LMPP, these lymphoid genes are typically co-expressed with a number of GM associated genes such as G-CSF receptor and MPO, but virtually never co-expressed with MkE associated genes. Investigating fetal liver day 14.5 we also provide evidence for existence of the LSKCD34+Flt3hi LMPPs at this early stage of development, and using a single cell clonal assay promoting combined B, T and myeloid lineage development, we demonstrate that a large fraction of fetal LMPPs lacking MkE potential possess a combined GM, B and T cell potential. Thus, evaluation at the single cell level of combined lineage potentials and multilineage gene expression provide compelling evidence for lymphoid-priming within the HSC compartment being preceeded by a loss of MkE potential, but occurring prior to loss of GM potential.
Reconstructing Boolean network ensembles from single-cell data for unraveling dynamics in the aging of human hematopoietic stem cells