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COVID-19 Associated Delays Impact Quality of Cryopreserved CD34+ Products

Oct 20, 2020 10:07:00 AM / by Nancy Andon, MSc

Blood bags in cooler-1Healthy CD34 cells are used to replace stem cells in people whose normal blood cells have been damaged by cancer or other serious autoimmune diseases. Because patients often do not have enough healthy cells for an effective treatment dose, the starting material for these transplants often comes from healthy donors.

Unfortunately, the COVID-19 pandemic poses risks to stem cell donors traveling to hospitals and clinics. To mitigate these risks, health professionals have recommended that CD34 cell products be cryopreserved following collection, before being transported to the patient administration site. However, little is known about the impacts of processing and transport delays on these cryopreserved products. Since the viability and recovery of cryopreserved CD34 cells directly impact transplant success, it is crucial to understand how best to preserve the stability of these products.

A recent collaborative study involving a group of 9 medical laboratories across Australia sought to determine the effects of variable CD34 cell recovery on stem cell transplants. [1] The group were particularly interested in investigating how transit time between initial cell collection and cryopreservation affected the post-thaw quality of cryopreserved products.

Like all cell therapy products, CD34 cells are highly-temperature sensitive. Even a short amount of time spent outside of the optimal storage temperature range can lower cell viability and functional recovery. Because starting material cryopreservation is often done offsite from the collection facility, CD34 cell products are now being exposed to increased sample handling and longer transit times…both of which are known to impact product quality. On top of that, the variable conditions under which starting material is prepared and cryopreserved adds to variability of the final product — again increasing patient risk and the risk of failed manufacturing runs.

Both post-thaw viability and post-thaw recovery correlated with transit time and more complex sample handling. In this publication’s study of 305 mobilized blood collections, the median viability of freshly isolated CD34 cells was 99%. By contrast, the same CD34 cell collections had a median post-thaw viability of 87%, with an alarming range of between 6-99% viability. Generally, poorer recovery was associated with longer duration of liquid storage. For CD34 cells collected >48 hours before cryopreservation, a highly significant 43% of the products had <50% recovery rates.

Despite the poor recovery rates, the good news is that of the products that have so far been thawed and infused into patients, even those with <50% CD34 cell recovery achieved adequate neutrophil recovery, a marker for successful engraftment.

Cryopreservation is and will continue to be a practical necessity for cell therapy, even without the added risks brought about by the COVID-19 pandemic. HemaCare’s state-of-the-art collection facility includes GMP-compliant on-site processing capabilities, eliminating the need for transport of starting materials to pre-processing and cryopreservation facilities. Instead, CD34 cells are isolated and cryopreserved on-site immediately post collection, lowering the risk of losses in viability and function.

Transplantation, collection, and processing centers should be aware that cryopreserved CD34 cell recovery and viability are variable post-thaw, and work toward reducing the risk that variability entails.

Visit our website today to learn more about our on-site processing facility!


  1. Purtill D., et al. Variable CD341 recovery of cryopreserved allogeneic HPC products transplant implications during the COVID-19 pandemic. Blood Advances. 4(17); 4147-4150. Sep 2020.
  2. Juliano L. et al. The Importance of Collection, Processing and Biopreservation Best Practices in Determining CAR-T Starting Material Quality. Cell and Gene Therapy Insights. May 2018.

Topics: Stem Cells

Nancy Andon, MSc

Written by Nancy Andon, MSc

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