Jagged-1+ damage-resistant myeloid progenitors initiate the recovery of the bone marrow endothelial-niche Free

Combined chemotherapy and irradiation is a well-established preconditioning treatment to ensure the engraftment of healthy hematopoietic stem cells (HSCs) for HSC transplantation (HSCT). HSCT is routinely prescribed but mostly reserved as a last resort treatment for many hematological malignancies. However, these myeloablative treatments inflict severe damage to the bone marrow (BM) endothelium, thus reducing long-term transplantation efficiency and potentially contributing to cases of non-relapse mortality (NRM). Notch signaling plays an essential role in facilitating cell-to-cell interactions to promote endothelial cell (EC) survival and HSC regeneration in the bone marrow niche following myeloablation. However, how ECs interact with hematopoietic cells following myeloablation in the context of Notch signaling is yet to be mechanistically defined. We show that transgenic deletion of Notch ligand Jagged-1 (Jag1) from the surface of hematopoietic cells (Jag1^-/-VavCre⁺) causes severe hematopoietic and endothelial damage and poor overall survival following sub-lethal irradiation and 5-FU treatment. Additionally, vascular permeability is significantly compromised in Jag1 mutants, leading to a leaky endothelial barrier and profound reduction in HSC retention. We found a novel damage-resistant myeloid-derived progenitor (DR-MDP) population that upregulates Jag1 ligand during myeloablation. When transfused, DR-MDPs enhance survival, endothelial cell viability, and HSC recovery by promoting active Notch signaling in the bone marrow ECs. Improved viability by reduction of EC apoptosis is highlighted in a dose-dependent manner, with as little as 10,000 DR-MDPs sufficient for ideal BM niche recovery. To visualize this effect, we utilized whole-mount sternum imaging and observed a significant improvement in vascular organization after irradiation. Single-cell RNA sequencing of our DR-MDPs reveals subsets of actively dividing and chemotactic populations that contribute to this process, which are deficient in Jag1 mutants. Specifically, we've shown that the Ccr2/Ccl2 chemotactic signaling plays a facilitative role in recruiting DR-MDPs to the endothelium in vitro and in vivo. Furthermore, we've identified a subset of cKit⁺DR-MDPs that exhibit enhanced proliferative myeloid potential and can accelerate recovery of the endothelium when compared to cKit^-counterparts. We've also employed a cell-free method to re-establish vascular recovery utilizing non-fusogenic nanoparticles coated with Jag1 to enhance hematopoietic and endothelial viability following irradiation. Thus, DR-MDPs are a novel myeloid population that relies on active Jag1 and Ccr2/Ccl2 signaling to provide robust recovery to the BM endothelium following myeloablative treatments.