Hematopoietic Stem Cells Differentiate Through A Multipotent Progenitor Before Generating All Mature Hematopoietic Cells

Hematopoietic stem cells (HSC) are able to both self-renew indefinitely and differentiate into all the various mature hematopoietic cell types. The developmental pathways taken by hematopoietic stem cells as well as the point at which hematopoietic stem cells begin to commit to specific cell fates is highly controversial. We present evidence that the multipotent progenitor (MPP) is a critical developmental intermediate between HSC and mature hematopoietic cells using three different but complementary approaches. First, using a lineage tracing mouse model containing a Flk2-Cre transgene in conjunction with a dual-color fluorescent reporter we determined that all hematopoietic lineages develop through the MPP during steady-state hematopoiesis, upon hematopoietic stress, and upon hematopoietic stem cell transplantation. Second, we found the impaired propagation of the MPP stage in Flk2-/- mice negatively impacted the formation of all hematopoietic lineages. These two complementary approaches solidify a developmental pathway between HSC and mature hematopoietic cell types that requires the MPP as a developmental intermediate. Next, to assess lineage bias we quantified mature cell production from transplanted hematopoietic stem and progenitor cells, as well as performed single-cell in vivo assays to assess functional heterogeneity within these populations. The MPP produced mature cells in the same order of abundance as transplanted HSC, and as present during steady-state hematopoiesis. MPP also displayed multilineage reconstitution at the single-cell level, providing strong evidence that lineage commitment occurs after the loss of self-renewal and the subsequent formation of the MPP