Flow cytometric CD34+ determination in stem cell transplantation: before or after cryopreservation of grafts?

Various attempts have been made to standardize and improve the reproducibility of flow cytometric determination of CD34+ hematopoietic progenitor cells. It is still not clear, however, whether the quantification of CD34+ cells in a stem cell graft should be done before or after cryopreservation. To address this issue, we investigated 78 unselected and 32 immunomagnetically selected autologous and allogeneic leukapheresis products (LA) before and after cryopreservation using pilot vials. Cell numbers were quantified within a Neubauer chamber, and CD34+ content was determined by flow cytometry; propidium iodide staining was used to exclude dead cells from analysis. Before freezing, the mean viable CD34 cell content in the unselected samples was 1.22% and increased after thawing to a mean of 2.16% of viable cells. Taking into account cell loss and cell death, the overall recovery of viable cells was 64.5%; all CD34+ cells could be recovered. Mean purity in the CD34-selected cell fraction was 85% (48-97) before and 91.3% (67-99) after thawing. The number of viable cells was 86.8% before and 86.1% after freezing with a 93.9% recovery of total cells. This leads to a mean 93.7% (SD +/- 23.1) recovery of viable cells and 100% (SD +/- 22.3) recovery of viable CD34+ cells. There was no significant difference in tolerance to freeze/thaw stress between cells from heavily pretreated autologous patients and healthy allogeneic donors. Our data show that freezing significantly increases the percentage of CD34(+) cells in unmanipulated LA, probably due to the death of granulocytes and mononuclear cells (MNCs). Nevertheless, the overall number of viable CD34+ cells in unselected as well as selected samples remains unchanged. Thus, CD34 data from different laboratories, for example, within multicenter trials, should be comparable independent of the different time points of acquisition.