Bone Regeneration by Transplantation of Human Mesenchymal Stromal Cells in a Rabbit Mandibular Distraction Osteogenesis Model

Ex vivo expanded mesenchymal stromal cells (MSCs) represent a potential cell population for tissue regeneration strategy. Xenogeneic transplantation using human MSCs (hMSCs) can be an approach to reveal what hMSCs guide in bone regeneration with distinguishable gene expression from a host animal. In this study, we investigated the regenerating effect of hMSCs varying injection time point in a rabbit distraction osteogenesis model. Undifferentiated hMSCs (2 x 10(6) cells) were injected transcutaneously into the osteotomy site of one side of the mandible 1 day before the onset of distraction (Group 1) or after distraction (Group 2). The contralateral side of the mandible, which was subjected to distraction, but no hMSC injection, was used as the control in each group. hMSCs showed lack of major histocompatibility complex class II expression and suppression of xenogeneic lymphocyte proliferation stimulated by a proinflammatory cytokine. A microcomputed tomography-based evaluation showed a significant increase in new bone volume in the distracted callus in Group 1 compared to the contralateral side. Injection of hMSCs increased the bone mineral density (BMD) of the regenerated bone in both Group 1 and 2, although the former had a higher BMD than the latter. hMSCs of Group 1 subjected to distraction after injection expressed insulin-like growth factor-1 (IGF-1) and fibronectin (FN), while the expression of most osteoblast differentiation-related markers and growth factors was negligible. These results demonstrated that hMSCs exerted immune suppressive behavior in rabbit T cells in vitro, and hMSC transplantation into the distracted callus of a rabbit model provided osteogenic benefits that were more pronounced when the hMSCs were injected just before distraction than at the end of distraction. The beneficial effect of hMSCs might be mediated, partly by the expression of matrix proteins or IGF-1, which are known to favor bone formation.