Total body irradiation is permissive for mesenchymal stem cell-mediated new bone formation following local transplantation

Skeletal injury is a major clinical challenge accentuated by the decrease of bone marrow-derived mesenchymal stem/stromal cells (BMSCs) with age or disease. Numerous experimental and clinical studies have revealed that BMSCs hold great promise for regenerative therapies due to their direct osteogenic potential and indirect trophic/paracrine actions. Increasing evidence suggests that stromal cell-derived factor-1 (SDF-1) is involved in modulating the host response to the injury. Common problems with BMSC therapy include poor cell engraftment, which can be addressed by total body irradiation (TBI) prior to transplantation. In this study, we tested the hypothesis that direct tibial transplantation of BMSCs drives endogenous bone formation in a dose-dependent manner, which is enhanced by TBI, and investigated the potential role of SDF-1 in facilitating these events. We found that TBI is permissive for transplanted BMSCs to engraft and contribute to new bone formation. Bone marrow (BM) interstitial fluid analysis revealed no differences of SDF-1 splice variants in irradiated animals compared to controls, despite the increased mRNA and protein levels expressed in whole BM cells. This correlated with increased dipeptidyl peptidase IV activity and the failure to induce chemotaxis of BMSCs in vitro. We found increased mRNA expression levels of the major SDF-1-cleaving proteases in whole BM cells from irradiated animals suggesting distinct spatial differences within the BM in which SDF-1 may play different autocrine and paracrine signaling roles beyond the immediate cell surface microenvironment.