Hematopoietic stem cell transplantation (HSCT) is increasingly used as a curative therapy for a variety of disorders of the hematopoietic and immune systems. Recipients of HSCT frequently have iron overload resulting from chronic transfusion therapy for anemia, especially in patients with myelodysplastic syndromes and thalassemia. In this study, we established an iron overload NOD/SCID model to create a bone marrow microenvironment in which the hematopoietic niche was impaired. In these iron overload NOD/SCID mice, IBM co-transplantation of UC-MSC and UCB-MNC significantly promotes the engraftment of UC-MNC when compared with the group receiving IBM transplantation of only UC-MNC. To our knowledge, this is the first study reporting the novel approach of IBM co-transplantation of UC-MSC and UCB-MNC and showing the improved engraftment efficacy. Since the UC-MSC is easy to be obtained without invasive procedures and could be used with high compatibility due to its weak antigenicity, this study provided an insight on the feasibility and benefits of UC-MSC co-transplantation in HSCT.
In addition, IBM injection showed remarkable effects on the homing of HSC. Six weeks after the transplantation of DiR-labeled UC-MNC, the in vivo fluorescence imaging was performed. The IBM injection (IBM) group showed the retention of transplanted DiR-labeled UC-MNC in the bone marrow, whereas the intravenous (IV) group showed the transplanted cells were mainly accumulated in the spleen. The results suggested that IBM injection might be able to reduce the loss of HSC in the reticuloendothelial system and promote the homing of HSC. This finding was inconsistent with previous reports showing IBM is an effective approach for HSCT compared to traditional IV transplantation [10, 12, 13]. However, we found there was no difference between IBM and IV groups regarding the engraftment rate of UCB-MNC when using the iron overload model. The reason causing this discrepancy might be that the impairment of the hematopoietic niche by iron overload might affect the engraftment rate of HSCT conducted by different methods. Nevertheless, here we suggested even in an iron overloaded condition, IBM co-transplantation of MSCs might be a feasible approach to overcome the compromised hematopoietic microenvironment to achieve successful HSCT.
The debilitating effect of iron overload has been indicated by the increasing clinical evidence that iron overload has a suppressive effect on hematopoiesis in myelodysplastic syndromes or anemia patients and that iron chelation therapy could improve this situation[14, 15]. Iron overload markedly decreased the ratio and clonogenic function of murine HSC progenitors by the elevation of reactive oxygen species (ROS). Reducing the ROS level improved the long-term and multi-lineage engraftment of HSC after transplantation. Iron accumulation in the bone marrow niche of thalassemia patients which might increase ROS production to impair the most primitive MSC pool as well as MSC stemness. A weakened antioxidative response and diminished expression of bone marrow niche-associated genes in the MSC were found. This caused a functional impairment in MSC hematopoietic supportive capacity in vitro and in vivo. Since the beneficial effects of MSC were hampered by iron overload, we, therefore, replenished the MSC pool by IBM co-transplantation of MSC with HSC. In agreement, this approach is successful in overcoming the iron overload-mediated impairment of hematopoiesis capacity. Whether ROS involves the beneficial effects is worth of further investigation.
MSCs are important in the regulation of hematopoietic stem cell niche due to their self-renewal and differentiation capacity into tissues of mesodermal origin. These MSC derivatives support hematopoiesis through release various molecules that play a crucial role in migration, homing, self-renewal, proliferation, and differentiation of HSC. For example, MSC could differentiate to osteoblasts which are closely related to the successful engraftment of stem cells [19, 20]. MSC also secrete extracellular matrices such as aminoglycan and proteoglycan and maintain the hematopoietic microenvironment by releasing soluble factors for the interaction between cells[21, 22]. Different subtypes of MSC interact with hematopoietic stem cells in specific areas around blood vessels. CD271+ and CD271+/CD146−/low MSCs are described as bone lining cells associated with long-term hematopoietic stem cells in the hypoxic region of bone marrow. CD146+ and CD271+/CD146+ MSCs are located in the perivascular regions of the bone marrow sinus and related to the activation and proliferation of hematopoietic progenitor stem cells. Specific subsets of MSC might be considered for the co-transplantation therapy of MSC and HSC to further improve the engraftment efficacy.
Mechanistically, our study found co-transplantation with UC-MSC significantly increased the expression of VEGF-a, OPN, and SDF-1a in the bone marrow, which might be closely related to the paracrine effect of MSC. VEGF-a, a critical factor for angiogenesis, also mediates the proliferation and migration of osteoblasts. Previous studies found that the beneficial effects of VEGF-a on stem cells might be via inhibition of irradiation-induced apoptosis and facilitation of cell survival by paracrine effect[25, 26]. Bone marrow morphogen osteopontin (OPN), which is abundantly present in the bone marrow extracellular matrix, can maintain hematopoietic stem cell reconstitution potential and the progenitor pool in the bone marrow[27, 28]. SDF-1/CXCR4 axis is indispensable to the homing of HSC[29, 30]. The collective effects of increased expression of VEGF-a, OPN, and SDF-1a induced by co-transplantation of UC-MSC might be multifactorial and beneficial for the reconstitution of bone marrow hematopoietic niche. In the bone marrow sections of the IBM MSC + group, the higher cellularity of hematopoietic cells and fewer lipid vacuoles also indicated the role of UC-MSC in tissue regeneration and repair for iron overloaded bone marrow microenvironment. This study provided a feasible therapeutic strategy as well as an experimental model that could be used for further study on finding a better treatment for the iron overloaded patients with hematopoietic dysfunction.