Effect of temporary storage of cryopreserved cellular therapy products at −80⁰ celsius on cell recovery and viability

Cellular therapy (CT) involving the transplantation of hematopoietic progenitor cells (HPC) is a treatment modality for both benign and malignant disorders. All autologous products require cryopreservation while allogeneic product cryopreservation became more common during the Coronavirus disease 2019 pandemic. Cells are stored in liquid nitrogen (LN₂) freezers which can malfunction and products may have to be temporarily stored in a mechanical −80 °C freezer if additional LN₂ freezer space is not available. The practice of temporary short-term −80 °C storage is present but there is no study to show that the product is unaffected by the temporary storage at a significantly warmer temperature. In this study, we identified previously collected CT products that were cryopreserved for now-deceased recipients that had remaining cryovials with aliquots of products for quality control purposes. Vials from 20 collections were split into 4 groups of 5 in with one vial placed in temporary storage at −80 °C for 2–5 weeks before returning to LN₂ storage while another vial remained in LN₂ storage for the entire duration of the study. The vials were then simultaneously thawed, processed, and evaluated for total nucleated cell (TNC) and CD34 + cell count and TNC and CD34 + cell viability to determine if there were any differences induced by temporary −80 °C storage. No statistically significant differences were seen after 4 weeks of −80 °C storage; however, after 5 weeks, a statistically significant decrease in TNC viability and viable TNC count, but not CD34 + cell viability and viable CD34 + cell count was observed. These results provide some reassurance to CT processing labs that if there is a failure in their LN₂ storage for cryopreserved products, these products may be safely stored at −80 °C for up to 4 weeks and returned to LN₂ storage without compromising CD34 + cell viability.