Recovery of cord blood hematopoietic progenitors after successive freezing and thawing procedures

Cord blood (CB) is a valuable source of stem cells. Most CB units are still cryopreserved in single bags in the world's CB banks. Thawing a single CB unit, dividing it into two parts, expanding the smaller one and refreezing the other would optimize ex vivo expansion of CB progenitors prior to transplantation: expanded and unexpanded cells could be infused together to accelerate early engraftment.The feasibility of refreezing CB samples was investigated by evaluating the effect of 3 successive cryopreservation procedures in 9 CB units. The number and viability of WBC, BFU-E, CFU-GM, CFU-MIX, LTC-IC, and the absolute CD34+ cell count were assessed at time 0 and after each thawing. The percentage of CD34 cells expressing CD38, L-selectin, VLA-4, VLA-5, H-CAM, LFA-1 and CXCR4 was also evaluated.After three freezing and thawing procedures, WBC counts decreased, while lymphocytes were unchanged. Viability was 90% of basal values after the first thawing and did not change. BFU-E decreased significantly only after the third thawing. CFU-GM and CFU-MIX did not change significantly, nor did LTC-IC, CD34+ cell counts and CAM and CXCR4 expression on CD34+/ CD38-- cells.These data show that two successive freeze-thaw procedures do not significantly affect the clonogenic potential and CAM expression of cord blood progenitors. This information could be exploited to devise new options in ex vivo expansion procedures and quality controls prior to transplantation.