Extensive early apoptosis in frozen-thawed CD34-positive stem cells decreases threshold doses for haematological recovery after autologous peripheral blood progenitor cell transplantation

Stem cell doses necessary for engraftment after myeloablative therapy as defined for fresh transplants vary largely. Loss of CD34⁺ cell quality after cryopreservation might contribute to this variation. With a new early apoptosis assay including the vital stain Syto16, together with the permeability marker 7-AAD, CD34⁺ cell viability in leucapheresis samples of 49 lymphoma patients receiving a BEAM regimen was analysed. After freeze-thawing large numbers of non-viable, early apoptotic cells appeared, leading to only 42% viability compared to 72% using 7-AAD only. Based on this Syto16 staining in the frozen-thawed grafts, threshold numbers for adequate haematological recovery of 2.8-3.0 × 10⁶ CD34⁺ cells/kg body weight determined for fresh grafts, now decreased to 1.2-1.3 × 10⁶ CD34⁺ cells/kg. In whole blood transplantation of lymphoma patients (n = 45) receiving a BEAM-like regimen, low doses of CD34⁺ cells were sufficient for recovery (0.3-0.4 × 10⁶ CD34⁺ cells/kg). In contrast to freeze-thawing of leucapheresis material, a high viability of CD34⁺ cells was preserved during storage for 3 days at 4°C, leaving threshold doses for recovery unchanged. In conclusion, the Syto16 assay reveals the presence of many more non-functional stem cells in frozen-thawed transplants than presumed thus far. This led to a factor 2.3-fold adjustment downward of viable CD34⁺ threshold doses for haematological recovery.