Dr Duncan Purtill

Cryopreservation of allogeneic hematopoietic progenitor cell (HPC) products can lead to variable CD34+ cell recovery and viability, according to Australian research.

The study, published in Blood Advances, says that cryopreservation of the products before recipients commence conditioning is now recommended by haematological societies including BMTSANZ and centres, to offset travel and other risks due to the COVID-19 pandemic.

However it appears the freezing process can decrease the post-thaw graft quality, particularly if the cells were manipulated before being preserved, were stored for a long period of time before freezing or had high white cell content.

The study analysed 305 samples of allogeneic bone marrow or G-CSF–mobilised peripheral blood products cryopreserved at nine Australian cell processing labs between 2015 and 2019.

The study found a median post-thaw CD34+ cell recovery of 74% but it ranged widely from 6% to 122%. About 15% of products had <50% recovery and just 1% had <20% recovery.

It found longer transit time before cryopreservation, higher initial white cell count, and product manipulation such as plasma reduction were significantly associated with poorer CD34+ cell recovery.

“Although the magnitude of each of these effects was small in our analysis, they were incremental, such that the likelihood of poor CD34+ cell recovery in a manipulated product with long liquid storage time was relatively high,” the study said.

The study found that transit time of the product and initial white cell count were also associated with post-thaw cell viability.

“As of 20 March 2020, 111 products (36%) had been thawed and infused. Of 80 products infused alone as HPCs for transplantation, neutrophil recovery occurred in 75 (94%) at a median of 19 days after infusion (range, 12-33 days). All 5 infused products with <50% CD34+ cell recovery achieved neutrophil recovery (median, 16 days; range, 12-33 days). At the time of this writing, only 1 of 9 products with <20% recovery had been infused, as donor lymphocytes.”

However there was the potential that the significant cell loss after thawing may mean that the remaining cells may be too few, or too damaged, to achieve timely bone marrow recovery in the patient after infusion.

“Our observations support the conclusion that transplant centres should request a higher CD34+ cell dose from donors whose products are to be cryopreserved and suggest that optimisation of precryopreservation factors, such as transit time and WCC, during storage may improve postthaw yield,” the study concluded.

Lead investigator Dr Duncan Purtill, medical director of the Bone Marrow Transplant Laboratory at WA’s Fiona Stanley Hospital, said in a statement that donor and transplant systems were well coordinated and effective prior to COVID-19.

“Now, with irregular flights and closed borders, travel and transportation are not assured,” he said.

“Our findings could be a note of caution for transplant centres to not take for granted that the frozen product they have received will show perfect recovery once thawed.”

“I hope centres will insist on receiving a pilot vial which has been frozen and transported in the same way. They can assess the pilot vial to determine its viability before they use the full product and start chemotherapy for the patient.”