Influence of storage conditions on viability of hematopoietic stem cells and leukocyte subpopulations in fresh and cryopreserved umbilical cord blood samples.

Umbilical cord blood (UCB) units are cryopreserved for long-term storage, but it is still not fully investigated how cryopreservation and freezing affect the viability of different cell types. This prospective study evaluated the stability of fresh and cryopreserved UCB samples in various storage conditions using the 7-AAD/annexin V method. UCBs were collected "ex-utero" and processed according to the institutional standard operating procedures. In the stability study, fresh UCB buffy coat (BC) samples were stored at +4 °C and room temperature (RT) for up to 24 h, while thawed samples for up to one and 2 h. After a defined time period, cells were labelled again and analysed. Early apoptosis was most prevalent in CD34+ cells and least in T lymphocytes in both fresh and thawed samples. The highest post-thaw recovery was observed for T lymphocytes. The total CD19+ (P = 0.001) and CD16+/56+ (P = 0.004) cell counts were statistically significantly reduced when fresh samples were stored at RT. Total NK cell counts were also reduced when samples were stored at +4 °C (P = 0.036). In cryopreserved samples, there was statistically significant differences in total cell counts for all cell populations when samples were stored at RT, and under these storage conditions, early apoptosis of B and NK cells also occurred. The results showed different post-thaw recoveries of leukocyte subpopulations in the samples of cryopreserved UCB units. Cell exposure to the cryoprotective solution post-thaw does not affect total cell count or further development of apoptosis when UCB samples are stored at +4 °C for up to 2 h.