Optimum storage conditions of cord blood-derived hematopoietic progenitor cells before isolation remain unknown. We therefore evaluated CD34 þ cells isolated from cord blood units (n ¼ 57) within 1 h after collection and following storage for 24, 48 and 72 h at either room temperature (RT) or 4 1C. Isolated CD34 þ cells were analyzed for their cell count, immunophenotype, apoptosis rate, clonogenicity and transmigration capacity in response to stromaderived factor 1a using direct-paired comparisons (n ¼ 27). CD34 þ , CD133 þ and CD45 þ positivity after isolation remained the same under all conditions. After 24 h, CD34 þ cell counts and numbers of CFU-GM colonies dropped regardless of the storage temperature. After 48 h, the number of CD34 þ cells increased compared to 24 h, if the cord blood had been stored at RT resulting in almost three times more CD34 þ cells than at 4 1C. These cells had a lower early apoptosis rate and formed four times more BFU-E than those stored at 4 1C with equivalent plating efficiencies. CD34 þ cells kept at RT for 48 h had the highest transmigration capacities, which paralleled an increased CXCR-4 expression. Cord blood should be stored at RT before CD34 þ isolation and a storage time for 48 h should be preferred to 24 h.

Purpose. Different opinions regarding the storage conditions of cord blood before cryopreservation exist. Most experts recommend the storage at room temperature and the cyropreservation within the first 24 hours after delivery of the baby. However, the optimum storage temperature is unknown. Methods. Cord blood units (n=46) were stored for 24, 48 and 72 hours at room temperature (RT) and 4°C. After storage, CD34(+) cells were isolated and analyzed for their cell count, flow cytometry profile, apoptosis rate, colony-forming capacity by methylcellulose assays, and transmigration capacity in response to stroma-derived factor 1. If possible, a direct-paired comparison was performed (n=15). Results. The cell count of peripheral blood mononuclear cells was generally 10fold higher in cord blood units stored at RT than in cord blood units stored at 4°C, leading to a 10fold higher number of isolated CD34(+) cells. There was no difference in the frequency of CD34(+) cells after immunomagnetic isolation (>95%) or in the frequency of CD133(+) and CD45(+) cells. At 48 hours, the apoptosis rate was lower at RT than at 4°C (37.7±7.4 vs 49.9±4.2%). The plating efficiencies (PE) were highest after the units had been stored for 48 hours (RT 14.7±3.6%, 4°C 16.3±4%) independent of the storage temperature. The cells stored at RT for 48 hours had the highest transmigration capacity (54±11.9% vs 4°C 10.7±2%). Storage times of 24 and 72 hours resulted in PEs of on average below 10%. CD34(+)cells stored for 24 hours migrated less than the cells stored at 48 hours. Their transmigration capacity showed no temperature-dependent difference (RT 24.6±7.1 vs 4°C 36.1±10.2%). Conclusion. Our data imply that CD34(+) cells from cord-blood units stored for 48 hours at room temperature have the highest plating efficiencies and transmigration capacities. A storage time for 48 hours before cryopreservation seems tolerable, since there was no increase of the apoptosis rate.