Prospective identification and lineage tracing of top-level hematopoietic stem cells that sustain adult hematopoiesis

Hematopoietic stem cells (HSC) sustain long-term reconstitution of hematopoiesis in primary transplantation recipients. Few HSC can serially reconstitute secondary recipients, and their identity and contribution to normal hematopoiesis remain moot. We directed transgene expression to a distinct fraction of HSC in the adult bone marrow. Epxression of the reporter transgene segregated with reconstituting activity during secondary transplantations. The labeled cells had an undifferentiated phenotype and expression profile, were slow-cycling and localized to the vascular niche. Inducible genetic labeling showed the transgene-expressing HSC gave rise to other cells within the HSC populations, confirming their top position in the differentiation hierarchy. Importantly, labeled HSC gave rise to more than two-thirds of all myeloid cells and platelets in adult mice, and this contribution could be further accelerated by interferon response. Thus, the rare "top-level" HSC with serial reconstitution capacity also serve as the major source of endogenous hematopoiesis in adult animals. Overall design: Sorted LSK CD48- CD150+ Map17-GFP+ and Map17-GFP- HSCs and LSK CD48+ CD150- Map17-GFP-MPPs were sequenced for mRNA profiling.