Functional characterization of TPO-expanded CD34+ cord blood cells identifies CD34- CD61- cells as platelet-producing cells early after transplantation in NOD/SCID mice and rCD34+ cells as CAFC colony-forming cells.

Transplantation of thrombopoietin (TPO)-expanded cord blood CD34(+) cells accelerates human platelet recovery in NOD/SCID mice. It is unknown which subpopulations of the TPO-expanded cells mediate accelerated platelet recovery and bone marrow (BM) engraftment. In this study, the contribution of these subpopulations to human platelet appearance in the blood and BM engraftment was studied in NOD/SCID mice. Following transplantation of CD34(-) /CD61(-)/lineage(-) cells (Lin(-)), human platelets were detected in the blood of recipient mice from day 4. Both time to platelet recovery and blood platelet counts at 6 weeks after transplantation showed Lin(-) dose dependence. The Lin(-) population was virtually negative for lineage marker expression and lacked CD42b expression but was heterogeneous with regard to CD36 and CD38 expression, reflecting a population in transit but not fully committed toward the megakaryocyte (MK) lineage. Although no definitive phenotype could be established of the cells generating prompt platelet production and cells generating platelets 6 weeks after transplantation, this relatively heterogeneous Lin(-) population is prerequisite to accelerate platelet recovery in vivo. The interval to platelet recovery after transplantation of the CD34(+) cells remaining after expansion (rCD34(+)) was similar to mice transplanted with nonexpanded CD34(+) cells, although the total platelet counts and the engraftment levels in the BM were lower. Cobblestone area-forming cell colony-forming cells resided mostly in the rCD34(+) population. The pro-MK CD61(+) cells did not contribute to human platelet recovery or engraftment in the BM. Our study shows that not all expanded cells appear critical for transplantation. These data support that functional characterization of the expanded cell populations is warranted to make future expansion protocols suitable for clinical application.