ABSTRACT Adoptive transfer of hematopoietic stem and progenitor cells (HSPC) can provide effective treatment for non- malignant disorders in the form of allogeneic hematopoietic stem cell transplantation (HSCT) or as a platform for autologous gene therapy. Bone marrow (BM) niches are the operationally-defined core units that sustain hematopoietic function, where signals from non-hematopoietic support cells adapt HSPC output to organismal demand. HSPC occupancy in the BM, however, is a competitive process, and HSCT patients typically require conditioning to eliminate endogenous hematopoiesis and “create space”, thereby promoting homing and engraftment in highly vascularized niches. Secretory activity is a fundamental HSPC property, but the impact of signals that emerge from HSPC, and their cellular targets in the niche are not well understood. The gap in knowledge regarding the crosstalk between HSPC and vascular niche endothelial cells specifically, may hold important biological insight and opportunities to improve competitive HSPC niche fitness, and a potentially novel strategy to offset the toxicity and long-term side effects from HSCT conditioning treatments. The secretion of nanometer sized extracellular vesicles (EVs) that traffic protein and RNA cargo is a constitutive cellular function. Intriguingly, mobilized CD34+ HSPC -which provide known advantages to engraftment after HSCT- have been shown to release EVs (EVCD34) that are highly abundant in microRNA-126, an “angiomiR” that regulates Akt/mTOR signaling, which in turn is central to EC hematopoiesis support. Others showed that adoptively transferred EVCD34, rich in miR-126, enhance EC proliferation and function. Our principal hypothesis is that EVHSPC trafficking of miR-126 actively shapes EC function in the vascular BM niche to enhance occupancy and improve repopulation. Three aims will test our hypothesis. In Aim 1 we will determine the cell-autonomous impact of EVHSPC secretion on self-renewal and repopulation potency under conditions of homeostasis and stress. Aim 2 will test the role of endothelial cells as predominant cellular targets for EV trafficking in the BM niche. Aim 3 will dissect the specific role of miR-126 and in regulating BM endothelial cell fates and the involved signaling pathways. As a proof of principle study, this proposal places an experimental and mechanistic focus on EVHSPC, but insight into the biologic role of the HSPC secretome in the BM niche will be broad. Translationally, our long-term goal is the development of new, non-toxic approaches to improve outcomes after allogeneic HSCT and autologous gene therapy.