Co-Transplantation of MSC Improves the Engraftment of HSC after Auto-iBMT in Non-Human Primates.

[Background] Mesenchymal stem cells (MSC) have been shown to play critical roles in various in vivo phenomena including osteoblastic differentiation. It has been suggested that, in the bone marrow, hematopoietic stem cells (HSC) reside in osteoblastic niche, which consists of osteoblasts derived from MSC. In mice, previous studies have demonstrated that co-transplantation of MSC improves the engraftment of HSC, especially after transplantation of the cells into the bone marrow cavity directly, namely intra-bone marrow transplantation (iBMT). However, neither the efficacy nor the dynamics such as migration and homing of HSC after iBMT with MSC have been determined in large animals. Here, using non-human primates, we have investigated the effects of co-transplantation of MSC on the engraftment of HSC after autologus iBMT.

[Methods] Auto-iBMT of cynomolgus monkeys was performed, using bone marrow stromal cells (as MSC) and CD34-positive cells (as HSC). The latter were divided into two equal aliquots, each of which was genetically marked with a different retroviral vector, G1Na or LNL6. Conditioning of iBMT, TBI or administration of busulfan, was followed by hemi-iBMT; that is, the bone marrow of one side (right or left) of the body was transplanted with one HSC aliquot together with MSC, whereas the other side of the identical body was transplanted with the other HSC aliquot alone. Engraftment of each HSC aliquot was evaluated by colony PCR of bone marrow, as well as by PCR of the genomic DNA obtained from peripheral blood or bone marrow of humerus, femur, and ilium. Both PCR could distinguish the dual markings derived from the two HSC aliquots.

[Results] In the first monkey transplanted, we found that the engraftment derived from the co-transplantation aliquot was 4.4-times higher than that derived from the HSC alone aliquot as assessed by colony PCR (48% versus 11%) using the bone marrow samples obtained from the ilium at day 46 post-iBMT. In the second monkey, when the peripheral WBC recovered to 2500–3000/μl after day 28 post-iBMT, 2% of the cells were positive with the retroviral marking derived from the co-transplantation aliquot, although none of them were positive with that derived from the HSC alone aliquot. In addition, colony PCR of the humerus and femur of both sides at day 39 post-iBMT revealed that the engraftment derived from the co-transplantation aliquot was 6.0-times higher than that derived from the HSC alone aliquot. Notably, colony-forming units (CFU) derived from the cotransplantation aliquot were detected in the bone marrow of the opposite side, suggesting that HSC injected into the bone marrow might migrate and achieve homing in the distant bone marrow.

[Conclusion] Taken together, these results indicate that, in auto-iBMT of cynomolgus monkeys, co-transplantation of MSC improves the engraftment of HSC, the efficacy of which might be attributable to additional osteoblastic niche, presumably created from co-transplanted MSC.