Passive freezing is equivalent to controlled-rate freezing for hematopoietic progenitor cell engraftment

There are different methods for cryopreserving hematopoietic progenitor cells (HPCs) for long term storage in a liquid nitrogen freezer at temperatures <-150°C. Controlled-rate freezing (CRF) has been considered the gold standard but uncontrolled-rate freezing/passive freezing (PF) using a -80°C mechanical freezer has gained acceptance. This retrospective study of 50 HPC products was performed to compare the effects of CRF and PF on total nucleated cell (TNC) viability, CD34+ cell viability, and engraftment. Though the mean TNC viability post-thaw was greater for HPCs frozen in a CRF compared to PF (74.2% ± 9.9% CRF group (N = 25) vs 68.4% ± 9.4% PF group (N = 25), P = 0.038), there was no significant difference in CD34+ cell viability post-thaw between the groups (77.1% ± 11.3% CRF group (N = 13) vs 78.5% ± 8.0% PF group (N = 25), P = 0.664). The number of days to neutrophil engraftment (12.4 ± 5.0 CRF group (N = 12) vs 15.0 ± 7.7 PF group (N = 16), P = 0.324) and platelet engraftment (21.5 ± 9.1 CRF group (N = 12) vs 22.3 ± 22.8 PF group (N = 16), P = 0.915) were also similar between the two groups. Cryopreservation outcomes using CRF or PF are comparable so PF is an acceptable alternative to CRF for initial cryopreservation before long term storage in a liquid nitrogen freezer.