ABSTRACT

The role of bone marrow cells in repairing ectodermal tissue, such as skin epidermis, is not clear. To explore this process further, this study examined a particular form of cutaneous repair, skin grafting. Grafting of full thickness wild-type mouse skin onto mice that had received a green fluorescent protein-bone marrow transplant after whole body irradiation led to an abundance of bone marrow-derived epithelial cells in follicular and interfollicular epidermis that persisted for at least 5 mo. The source of the epithelial progenitors was the nonhematopoietic, platelet-derived growth factor receptor α-positive (Lin − /PDGFRα + ) bone marrow cell population. Skin grafts release high mobility group box 1 (HMGB1) in vitro and in vivo, which can mobilize the Lin − /PDGFRα + cells from bone marrow to target the engrafted skin. These data provide unique insight into how skin grafts facilitate tissue repair and identify strategies germane to regenerative medicine for skin and, perhaps, other ectodermal defects or diseases.

IntroductionBone marrow transplantation (BMT) is an established therapeutic modality for both malignant and nonmalignant disorders of hematopoietic stem cells. After wide recognition that bone marrow also contains progenitors of bone, [1][2][3][4] we postulated that BMT should be applicable to the treatment of osteopoietic as well as hematopoietic disorders. 5 Nilsson et al demonstrated that transplantation of whole bone marrow leads to donor-derived osteopoiesis in mice, 6 while Pereira et al showed that systemically infused murine mesenchymal stromal cells (MSCs), which are plastic adherent in vitro, 7 engrafted in bone. 3 We showed that BMT in children with osteogenesis imperfecta (OI), a genetic disorder of collagen type I, the major structural protein in bone, leads to donor-derived osteopoiesis and consequent improvement in the microscopic structure of bone 5 and in the clinical manifestations of OI. 8 Recently, BMT in a murine model of OI has corroborated our early human studies. 9 Taken together, these data validate the functional competence of donor-derived osteopoietic cells, providing the necessary proof to move forward with the development of marrow cell-based treatments for disorders of bone.Despite this progress, the cellular mechanism(s) by which BMT gives rise to robust osteopoietic activity remains unproven. Pereira et al reported that systemically infused murine MSCs engrafted in the bone of a murine model of OI, and generated a small but statistically significant increase in collagen, 10 supporting the prevailing view that BMT-associated donor-derived osteopoiesis was attributable to the engraftment and differentiation of MSCs. Thus, we reasoned that a decrease in the rate of clinical improvement in our OI patients after BMT 8 might be corrected with a boost of donor-derived MSCs, which in fact led to a second wave of accelerated growth velocity in all 5 evaluable patients. 11 This result suggested that MSCs isolated on the basis of their adherence to plastic may provide adequate therapy for patients with OI or other bone disorders. However, the issue is complicated by work showing that so-called nonadherent bone marrow cells (NABMCs) have measurable osteoprogenitor activity, 12-14 raising questions as to the developmental origin of the transplantable marrow osteoprogenitors that give rise to donor-derived osteopoiesis and hence to the marrow population most likely to yield clinical improvement in patients.Here we show that NABMCs are significantly more robust transplantable osteoprogenitors than MSCs in mice, suggesting NABMC would be effective cell therapy for bone disorders. Translating this laboratory observation to a pilot clinical trial, T cell-depleted marrow mononuclear cells, comprising Ͻ 0.01% MSCs, engraft in bone after intravenous infusion and lead to a remarkable acceleration of growth in some OI patients, suggesting vigorous osteoprogenitor activity in humans as predicted by our animal model. Finally, we demonstrate that NABMCs produce their clinical activity by engrafting in bon...

• After radioablative conditioning, host megakaryocytes promote endosteal HSC niche expansion and donor stem cell engraftment.• Thrombopoietin administration before radiation and bone marrow transplant enhances megakaryocyte promotion of HSC engraftment.Successful hematopoietic stem cell (HSC) transplantation requires donor HSC engraftment within specialized bone marrow microenvironments known as HSC niches. We have previously reported a profound remodeling of the endosteal osteoblastic HSC niche after total body irradiation (TBI), defined as relocalization of surviving megakaryocytes to the niche site and marked expansion of endosteal osteoblasts. We now demonstrate that host megakaryocytes function critically in expansion of the endosteal niche after preparative radioablation and in the engraftment of donor HSC. We show that TBI-induced migration of megakaryocytes to the endosteal niche depends on thrombopoietin signaling through the c-MPL receptor on megakaryocytes, as well as CD41 integrin-mediated adhesion. Moreover, niche osteoblast proliferation post-TBI required megakaryocyte-secreted platelet-derived growth factor-BB. Furthermore, blockade of c-MPL-dependent megakaryocyte migration and function after TBI resulted in a significant decrease in donor HSC engraftment in primary and competitive secondary transplantation assays. Finally, we administered thrombopoietin to mice beginning 5 days before marrow radioablation and ending 24 hours before transplant to enhance megakaryocyte function post-TBI, and found that this strategy significantly enhanced donor HSC engraftment, providing a rationale for improving hematopoietic recovery and perhaps overall outcome after clinical HSC transplantation. (Blood. 2013;121(26):5238-5249)