Mesenchymal Stem Cells Promote the Engraftment of Cord Blood CD34+ Cells but Do Not Accelarate Platelet Recovery NOD/SCID Mice.
Authors: van Hensbergen Yvette , de Boer Helen , Slot Manon C. , Schipper Laurus F. , Brand Anneke , Fibbe Willem E. 
Published: April 2020

Abstract
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 (CD34exp) 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 CD34exp 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) CD34exp 4) MSC+CD34exp. 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 CD34exp 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 CD34exp 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 CD34exp cells only (32.1% for CD34exp vs. 35.7% for MSC+CD34exp).
Conclusion: Our results show that transplantation with CD34exp 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 CD34exp.
Table 1: % of mice with ≥ 1x106 platelets/L in the PB Days post transplantation 6 9 13 16 CD34+ 0% 20% 67% 100% MSC+CD34+ 20% 0% 80% 100% CD34exp 83% 100% 100% 100% MSC+CD34exp 60% 100% 100% 100%