Project Summary To date, the only curative option for Sickle Cell Disease (SCD) is transplantation of allogeneic HSC. Recently, clinical trials utilizing gene corrected autologous HSC have been initiated and hold much promise. However significant limitations and challenges remain for HSC transplantation in SCD patients. In humanized SCD mice we showed for the first time the pathologic impact of SCD on bone marrow (BM) vascular and perivascular niches that are deemed critical to HSC transplantation and steady state hematopoiesis. Specifically, the studies uncovered a disorganized and structurally abnormal BM neovascular network of increased numbers of highly tortuous arterioles occupying the majority of the BM cavity, as well as fragmented sinusoidal vessels filled with aggregates of erythroid and myeloid cells. Additionally, the number of CXCL12 producing perivascular mesenchymal niche cells, was reduced thus further supporting a defective hematopoietic niche in SCD mice. Further investigations provided a model of SCD BM, where slow RBC flow and vaso-occlusion further diminish local oxygen availability in the physiologically hypoxic BM cavity. These events trigger an angiogenic milieu conducive to aberrant vessel growth. Remarkably, the distorted neovascular network was completely reversed by a 6 weeks of blood transfusion highlighting the plasticity of the vascular niche. Additional, unpublished preliminary data indicate that long-term (16 weeks) HSC reconstitution when transplanted into SCD mice is significantly compromised. Collectively, these studies have led us to hypothesize that impaired engraftment of HSPCs into SCD BM is caused by structural and functional abnormalities of the hematopoietic vascular and/or mesenchymal niche; and that correction of key vascular niche cell defects will improve HSPC engraftment. Stem cell mobilization may also be affected by the BM niche and we will seek to understand how stem cells egress from the BM to the blood in SCD. Thus three aims are proposed to test these hypotheses: 1. to define the SCD niche defects at the molecular and cellular level in humanized SCD mouse 2. to determine which niche defects are normalized by blood transfusion and define the BM niche defects in patients; and 3. to ascertain the link between BM niche defects and impaired HSCP engraftment. Collectively, these investigations will help optimize approaches towards efficient and long-term hematopoietic engraftment in the context of curative therapies.