Mesenchymal Stem Cells Promote the Engraftment of Cord Blood CD34⁺ Cells but Do Not Accelarate Platelet Recovery NOD/SCID Mice.

Aim: Delayed platelet reconstitution in the peripheral blood (PB) remains a problem in transplantation with umbilical cord blood (CB)-derived stem cells. Previously, we have shown that transplantation with ex-vivo expanded CB CD34⁺ cells (CD34^exp) with thrombopoietin for 10 days, results in an accelerated platelet reconstitution in NOD/SCID mice. It has been shown that mesenchymal stem cells (MSC) are able to enhance the overall engraftment when co-transplanted with CB CD34⁺ cells. Therefore, we investigated whether co-transplantation of MSC with CD34⁺ cells or CD34^exp cells may have an additive effect in shortening the time to platelet recovery and on the total number of platelets in the PB at 6 weeks after transplantation.

Methods: To evaluate the time to platelet recovery and the total number of platelets at 6 weeks after transplantation, we used 4 groups of irradiated NOD/SCID mice, divided according to the transplant received: 1) CD34⁺ 2) MSC+CD34⁺ 3) CD34^exp 4) MSC+CD34^exp. Human platelet recovery was measured twice a week for the first three weeks and once a week thereafter, using an assay that reliably detects 1x10⁶plt/L. The percentage of human CD45⁺ cells in the bone marrow (BM) was evaluated at 6 weeks after transplantation.

Results: In accordance with previous experiments, platelet recovery started earlier in mice transplanted with CD34^exp cells compared to CD34⁺ cells (Table 1). Co-transplantation of MSC with CD34⁺ cells did not result in an accelerated platelet recovery during the first 2 weeks after transplantation, as was observed for expanded cells. However, co-transplantation of MSC did enhance the number of platelets at 6 weeks after transplantation (454.2±264.5 plt/μ l for MSC+CD34⁺ vs. 101.9±78.4 plt/μ l for CD34⁺). MSC had no affect on either the time to platelet recovery nor the total number of human platelets at 6 weeks after transplantation when co-transplanted with CD34^exp cells. To assess the overall efficacy of the MSC on the engraftment of human CB cells, we evaluated the percentage of human CD45⁺ cells in the BM of the NOD/SCID mice at 6 weeks after transplantation. In mice transplanted with MSC+CD34⁺, the percentage of human CD45⁺ cells was higher compared to controls transplanted with CD34⁺ cells only (30.4% for MSC+CD34⁺ vs. 17.8% for CD34⁺). No further engraftment enhancing effect of MSC was observed following transplantation of CD34^exp cells only (32.1% for CD34^exp vs. 35.7% for MSC+CD34^exp).

Conclusion: Our results show that transplantation with CD34^exp cells results in an accelerated platelet recovery in NOD/SCID mice, an effect that can not be achieved by co-transplantation of MSC+CD34⁺ cells. However, at 6 weeks after transplantation co-transplantation with MSC+CD34⁺ cells results in a higher number of platelets in the PB. In addition, the level of engraftment of human CD45⁺ cells in the BM of NOD/SCID mice is increased by co-transplantation of MSC+CD34⁺ cells. In contrast, MSC did not affect the time to platelet recovery, the number of human platelets at 6 weeks after transplantation, or the engraftment of human CD45⁺ cells in the BM when co-transplanted with CD34^exp.