EPCR and TM Guide Hematopoetic Stem Cell Homing to the Bone Marrow without Niche Clearance

Bone marrow (BM) recognition, homing and lodgment of long term repopulating hematopoietic stem cells (LT HSCs) are essential first steps for durable blood cell production during embryonic development and in clinical stem cell transplantation. Rare, BM retained LT HSCs endowed with the highest repopulation potential highly express EPCR (endothelial protein c receptor) and PAR1, which control LT HSC retention and chemotherapy resistance by limiting nitric oxide levels. We found that transplanted EPCR+ LT HSCs preferentially home to the BM, in contrast to immature progenitors. Intriguingly, EPCR+ LT HSCs can engraft the BM of non-irradiated recipients, while remaining quiescent, without giving rise to differentiated progeny. Strikingly, the quiescent homed EPCR+ LT HSCs were awakened by treating engrafted hosts with nitric oxide donor or with low dose chemotherapy, revealing that preconditioning and clearance of occupied HSC niches are not required.Importantly, we have treated human cord blood CD34+ stem and progenitor cells with a PAR1 mimicking peptide in vitro for 2 hours  which reduced their nitric oxide levels and doubled their homing to the bone marrow of transplanted immune deficient mice. Our study provides mechanistic insights concerning LT HSC homing, which may lead to improved BM transplantation protocols and be applied to prevent chemotherapy resistance of EPCR-expressing cancer stem cell