Effect of cryopreservation methodology on neutrophil and platelet engraftment in unrelated donor cord blood transplantation

Cord blood (CB) engraftment is known to be dependent on many factors including conditioning regimen, HLA matching and total nucleated cell (TNC) or viable CD34 cell dose. Techniques used to process CB are similar across most banks, but cryopreservation can be performed via a controlled rate freezer (CRF) or −80°C passive freeze (PF) method. This study aimed to determine whether the cryopreservation method affects CB engraftment.

Two AusCord Cord Blood Banks provided data for all single CB transplants performed prior to 2015 (n = 560). Cases were excluded from analysis if <4/6 HLA match with recipient (n = 4) or no transplant data received (n = 24), and censored at 70 days for neutrophil and 150 days for platelet engraftment. Of the 532 CB transplants analysed, 49.8% were cryopreserved using a CRF with cooling rate of 1–3°C/min until <−40°C. HLA matching level, proportion of patients with non-malignant conditions, and infused TNC or viable CD34 doses were similar between the CRF and PF groups, however the PF patient group were 4 years younger. Multivariable regression models were fitted with patient age, non-malignant status, HLA match, TNC and CD34 doses.

As expected, neutrophil and platelet engraftment were more rapid with better HLA matching and higher TNC or CD34 doses. The method of cryopreservation did not significantly affect the median time to attain neutrophil engraftment (CRF = 22 [95%CI: 21–23 days, PF=23 [95%CI: 22–25], p = 0.2) or the proportion of patients that attained neutrophil engraftment by 70 days (CRF=88% [95%CI:84–92%], PF = 84% [95%CI:80–89%], p = 0.2). However, platelet engraftment was significantly faster in the CRF group (median 46 days [95% CI: 43–49] vs 53 [95% CI: 48–61] for PF, p = 0.002) and a higher proportion of patients attained platelet engraftment by day 150 (77% [95%CI: 71–82] vs 68% [95%CI: 62–73], p = 0.02). In the Cox proportional hazard multivariable regression model, the CRF group were 1.45 times more likely to achieve platelet engraftment than the PF group (95% CI: 1.17–1.81, p = 0.0007).

This study indicates that cryopreservation methodology can affect the platelet engraftment potential of CB. The demonstrated benefit of the CRF method is likely related to consistent attainment of the optimal 1–3°C/min cooling rate during the critical early phase of cryopreservation. This study emphasizes the importance of establishing an optimal freezing rate in cryopreservation of haemopoietic progenitor cells for transplantation.