Introduction: Identification and understanding of the cells and processes that can generate, sustain and influence the hematopoieitic stem cell (HSC) niche and hematopoiesis are critical for the development of a more comprehensive knowledge of normal hematopoiesis, stem cell homing, trafficking, differentiation, hematopoietic pathology and more effective and less toxic clinical interventions. Growth and renewal in many tissues are initiated by stem cells, supported by the microenvironment (niche) in which they reside. While recent work has begun to describe functional interactions between stem cells and their niches, little is known about the formation of stem cell niches, nor the ability to directly manipulate them.
Methods & Results: We found that, aside from immune barriers, donor HSC engraftment is restricted by occupancy of appropriate niches by host HSCs. In a murine model, we demonstrated that administration of ACK2, an antibody that blocks c-kit, leads to the highly efficient removal of host HSCs. Subsequent transplantation with donor HSCs led to high levels of donor chimerism levels of up to 90%. Extrapolation of these methods to humans may enable mild but effective conditioning regimens for transplantation.
Furthermore we established a functional, in vivo assay (via implantation of cells under the renal capsule) to isolate the determinants of HSC niche formation and activity. Using this novel assay, we found that a population of progenitor cells (CD45-Tie2-aV+CD105+Thy1.1-; CD105+Thy1-) sorted from 15.5 dpc fetal limbs and transplanted under the adult mouse renal capsule recruit host-derived vasculatures in a VEGF dependent manner, produce donor-derived ectopic bones through endochondral ossification, and generate a marrow cavity populated by host-derived long term reconstituting HSC. In contrast, CD45-Tie2-aV+CD105+Thy1a+ (CD105+Thy1+) progenitors form bone that does not contain a marrow cavity. Suppression of factors involved in endochondral ossification, such as osterix and VEGF, inhibited niche generation.
Conclusions: A novel antibody based approach to target and remove host stem cells may lead to less toxic transplant regimens for donor stem cell transplantation. Additionally, our identification of the limb-derived skeletal progenitor cells capable of endochondral ossification and the basic mechanisms of HSC niche initiation, provides a functional framework by which future studies on HSC-niche interactions at the cellular level can be carried out.