Cord blood cryobiology: Impact on transplant outcomes

Abstract

The therapeutic potential of cord blood (CB) for hematopoietic stem/progenitor cell (HSC) transplantation is primarily limited by the number (dose) of functionally viable HSC administered to the transplant recipient. The process of cryopreservation and thawing of CB products is the single greatest and most variable source of HSC loss from collection to infusion and the effects of cryopreservation and thaw on CB HSC function are inherent in all published comparisons of CB versus BM or PBSC transplant outcome. We performed a series of studies to examine the effects of cooling rate (CR) and type of cryoprotectant (DMSO, propanediol) on CB HSC cell number (TNC and CD34), viability (trypan blue, 7-AAD) and function (CFU assay) post-thaw. Varying the CR from −1 to −20 deg. C/min. had little effect on recovery (%) of CB HSC number and trypan blue viability (%), however, CB HSC function (CFU recovery (%)) decreased markedly at CR above −5 deg. C/min. Monitoring of the CR of cord blood units is subject to artifacts arising from alterations in thermistor probe placement necessitating careful review of cooling curves and on-site validation of the cryopreservation system. Frozen CB units placed in liquid N2 must occasionally be transferred to other locations or temporarily removed from liquid N2 for retrieval of attached segments exposing the unit to potential warming effects (PWE). Transfer of CB units in Styrofoam containers with liquid N2 or segment removal in liquid N2 vapor does not cause any detectable loss of HSC number, viability or function. Similarly, repeated exposure of frozen CB units to 10 consecutive 1 min. intervals at room temperature also had no significant effect on HSC number, viability or function. We conclude that transient warming during CB unit transfer or segment removal does not result in measurable PWE.