HLFExpression Defines the Human Hematopoietic Stem Cell State

Hematopoietic stem cells (HSCs) sustain blood cell homeostasis throughout life and are able to regenerate all blood lineages following transplantation. Despite this clear functional definition, highly enriched isolation of human HSCs can currently only be achieved through combinatorial assessment of multiple surface antigens. While a number of transgenic HSC reporter mouse strains have been described, no analogous approach to prospectively isolate human HSCs has been reported. To identify genes with the most selective expression in human HSCs, we profiled population- and single-cell transcriptomes of fresh and ex vivo cultured cord blood derived HSPCs as well as peripheral blood, adult bone marrow and fetal liver. Based on these analyses, we propose the master transcription factor HLF ( Hepatic Leukemia Factor ) as one of the most specific HSC marker genes. To directly track its expression in human hematopoietic cells, we developed a genomic HLF reporter strategy, capable of selectively labeling the most immature blood cells on the basis of a single engineered parameter. Most importantly, HLF -expressing cells comprise all of the stem cell activity in culture and in vivo during serial transplantation. Taken together, these results experimentally establish HLF as a defining gene of the human hematopoietic stem cell state and outline a new approach to continuously mark these cells with high fidelity. Key Points In the human blood system, HLF expression is specific to stem cell populations in primary anatomical sites and during ex vivo expansion. CRISPR/rAAV6-mediated integration of a genomic HLF -reporter allows selective and stable genetic labeling of human HSCs ex vivo and in vivo .