Allogeneic transplant procurement in the times of COVID-19: Quality report from the central European cryopreservation site

Abstract

Background: Because of limitations of transportation imposed by the COVID-19 pandemic, current recommendation calls for cryopreservation of allogeneic stem cell transplants before patient conditioning. A single cell therapy laboratory was selected to function as the central cryopreservation hub for all European registry donor transplants intended for the Australian-Pacific region. We examined properties of these transplants to ascertain how quality is maintained.

Methods: We analyzed 100 pandemic-related allogeneic mobilized blood-derived stem cell apheresis products generated at 30 collection sites throughout Europe, shipped to and cryopreserved at our center between April and November of 2020. Products were shipped in the cool, subsequently frozen with DMSO as cryoprotectant. Irrespective of origin, all products were frozen within the prescribed shelf-life of 72 h.

Results: Prior to cryopreservation, viable stem cell and leukocyte count according to the collection site and our reference laboratory were highly concordant (r² = 0.96 and 0.93, respectively) and viability was > 90% in all instances. Median nominal post-thaw recovery of viable CD34+ cells was 42%. Weakly associated with poorer CD34+ cell recovery was higher leukocyte concentration, but not time lag between apheresis or addition of cryopreservant, respectively, and start of freezing. The correlation between pre- and post-thaw CD34+ cell dose was high (r² = 0.85), hence predictable. Neutrophil and platelet engraftment were prompt with no evidence of dose dependency within the range of administered cell doses (1.31-15.56 × 10⁶ CD34+ cells/kg).

Conclusions: General cryopreservation of allogeneic stem cell transplants is feasible. While more than half of the CD34+ cell content is lost, the remaining stem cells ensure timely engraftment.

Keywords: CD34+ cell enumeration; Engraftment; Graft; Graft quality; Hematopoietic stem cell; Post-thaw recovery; Quality control; Stem cell dose; Stem cell enumeration; Transplant logistics.

Fig. 1

Effect of apheresis product age or duration of pre-freeze storage in DMSO-containing medium on post-thaw recovery. No correlation between post-thaw recovery and the age of the graft or the time lag between DMSO addition and start of cryopreservation. Each dot represents one apheresis product. Statistical measures are depicted in each graph

Fig. 2

Post-thaw recovery as a function of apheresis product WBC concentration. Recovery of viable CD34+ cells correlates negatively but weakly with leukocyte concentration (a). WBC recovery is independent of its concentration (b). CD34+ recovery is associated with WBC recovery (c). Each dot represents one apheresis product. Statistical measures are depicted in each graph

Fig. 3

Correlation of pre- and post-thaw CD34+ dose. Post thaw CD34+ dose strongly correlates with CD34+ dose before cryopreservation. On average 42% of the CD34+ cells are recovered post-thaw, with a narrow margin of error. B is the magnified section of the graph in A. Each dot represents one apheresis product. Statistical measures are depicted in each graph

Fig. 4

Engraftment kinetics as a function of CD34+ cell dose: Time to engraftment for neutrophils (a) or thrombocytes (b) was independent from the post-thaw dose of CD34+ cells. Neutrophil engraftment was defined as the first of three consecutive days with neutrophil counts > 500/µL, platelet engraftment as the first of three successive days with > 20,000/µL platelets without substitution

Fig. 5

Correlation of CD34+ dose assessment in apheresis center and CPC: Nearly perfect accuracy and precision of CD34+ cell (a) and WBC (b) enumeration between 29 HSPC collection centers and the CPC indicates high robustness of current enumeration protocols. Incidentally, they also illustrate the high stability of the products over the observation time. Data from our own apheresis center were omitted for this analysis. Each dot represents one apheresis product. Statistical measures are depicted in each graph