Transient warming events occurring after freezing impairs umbilical cord-derived mesenchymal stromal cells functionality

Background. Mesenchymal stromal cells (MSCs) have shown promising results for the treatment of refractory acute graft-versus-host disease. While safety of MSC infusion has been demonstrated, the use of cryopreserved MSCs in clinical trials has raised concerns regarding the retention of their functional activity. This has led to the recommendation by experts in the field to use freshly harvested MSCs, even though this approach is much less practical from a logistic point of view. In the present study, we revisited the impact of cryopreservation on MSC functionality and addressed the possibility that warming events on frozen cells rather than cryopreservation per se could impact MSC functionality. Methods. Following controlled-rate freezing to -130 degrees C, umbilical cord-derived MSCs were left at room temperature (RT) for 2-10 min or on dry ice for 10 min, before being transferred into liquid nitrogen (LqN(2)). MSCs of each group were subsequently tested (viability, functionality and cellular damage) and compared with their freshly harvested counterparts. Results. We demonstrated that freshly harvested MSCs as well as cryopreserved MSCs that were left on dry ice following step-down freezing have comparable viability, functionality and integrity. In contrast, cryopreserved MSCs that were left at RT before being transferred into LqN2 were functionally impaired and showed cellular damage upon thawing even though they exhibited high viability. Discussion. Warming events after freezing and not cryopreservation per se significantly impair MSC functionality, indicating that cryopreserved MSCs can be an advantageous alternative to freshly harvested cells for therapeutic purposes.