Blood regeneration upon transplantation relies on the activity of long-term repopulating hematopoietic stem cells (LT-HSCs). One of the major controversies in hematopoiesis relates to the apparently different properties that HSCs have in transplantation versus unperturbed settings. In unperturbed steady state hematopoiesis, the most potent HSCs appear to be mostly dormant, and only producing platelet-lineage cells. In turn, upon transplant, even a single transplanted HSC can actively divide and regenerate hundreds of millions of blood progenitors of all lineages. It would thus appear that HSCs have different fundamental properties in each study system.
However, most transplantation studies have only tracked the lineage output of the transplanted HSC clones, and rarely the regeneration of the HSC compartment itself. In addition, clonal assays have not been performed at sufficient resolution to fully capture the diversity and clonal complexity of the regenerated HSC compartment. Here, we have used expressible barcodes, which can be sequenced in conventional single cell RNAseq assays, to simultaneously record the functional outcomes and transcriptional states of thousands of HSCs.
Our analysis revealed multiple clonal HSC behaviors following transplantation that drastically differ in their differentiation activity, lineage-bias and self-renewal. Surprisingly, we witnessed a large fraction of clones that efficiently repopulate the HSC compartment but show limited contribution to differentiated progeny. Furthermore, these inactive clones have increased competitive multilineage serial repopulating capacity, implying that shortly after transplant a subset of clones reestablishes the native-like LT-HSC behaviors. Our results also argue that this clonal distribution of labor is controlled by cell autonomous, heritable properties (i.e. the epigenetic cell state).
Then, using only our clonal readouts to segregate single HSC transcriptomes, we unveiled the transcriptional signatures that associated with unique HSC outcomes (platelet bias, clonal expansion, dormancy, etc.) and unraveled, for the first time, a gene signature for functional long-term serially repopulating clones. We interrogated the drivers of this cell state using an in vivo inducible CRISPR screening and identified 5 novel regulators that are required to regenerate the HSC compartment in a cell autonomous fashion.
In conclusion, we demonstrate that functional LT-HSCs share more similar properties in native and transplantation hematopoiesis than previously expected. Consequently, we unveil a definition of the essential, common functional properties of HSCs and the molecular programs that control them.
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Disclosures
No relevant conflicts of interest to declare.
Cell lineage and cell cycling analyses of the 4d micromere using live imaging in the marine annelid Platynereis dumerilii
