A new reporter strategy reveals a subset of nonmigratory hematopoietic stem cells located in a dynamic bone marrow niche.

Interactions of hematopoietic stem cells (HSCs) with the bone marrow microenvironment are critical to regulate stem cell function in homeostasis, emergency hematopoiesis and ageing. The dynamic behavior of endogenous HSCs in their niches has been challenging to study due to the complexity of markers required to define pure HSCs. Multiple recently developed reporter strategies advanced our capacity to identify HSCs in situ. Yet, they provide different levels of HSC enrichment and are frequently not stable in situations of perturbed hematopoiesis, leading to contradictory observations of HSC localization and migratory behavior. Here we employed a new double reporter strategy by combining the Hoxb5-mKO2 and Vwf-GFP reporters, that are stably expressed in homeostasis, stress and ageing. Hoxb5+Vwf+ cells represent a subset of highly pure and potent long-term HSCs with a platelet-biased pattern of differentiation, that are included in the populations identified by other reporter strategies. Intravital microscopy revealed Hoxb5+Vwf+ cells to be non-migratory in homeostasis, ageing, or following acute thrombocytopenia when these cells are actively proliferating, indicating that this non-migratory behavior is independent of cell cycle status. We found Hoxb5+Vwf+ HSCs in perivascular niches with a higher coverage by LepR+ mesenchymal cells but not preferentially located closer to megakaryocytes, when compared to Hoxb5+Vwf- cells. Nevertheless, stress-induced megakaryopoiesis following acute thrombocytopenia bring megakaryocytes into closer proximity of Hoxb5+Vwf+ HSCs, revealing previously unrecognized niche dynamics in HSC regulation during regeneration.