Flow cytometry to estimate circulating hematopoietic progenitors for autologous transplantation: comparative analysis of different CD34 monoclonal antibodies.

We report that the 2 fluorescein (FITC) conjugated CD34 monoclonal antibodies available to date, namely FITC-8G12 and FITC-QBEND10, exhibit different capabilities of detecting circulating hematopoietic progenitors (CHP) as cells expressing the CD34 antigen (CD34+) by direct immunofluorescence flow cytometry. Mean fluorescence intensity conferred by FITC-QBEND10 to CD34+ CHP is 2.8-4.3 times lower than that conferred by FITC-8G12. By indirect immunofluorescence, native unconjugated QBEND10 and 8G12 antibodies detect CD34+ CHP in a comparable manner, thus indicating that the decrease in QBEND10 affinity is due to FITC-conjugation. Utilization of FITC-QBEND10 instead of FICT-8G12 to estimate infrequent (usually less than or equal to 5%) CD34+ CHP for clinical decision-making in autologous transplantation of CHP exposes clinicians to the risk of either overestimating (false positive) or underestimating (false negative) these cells in peripheral blood. Recently published guidelines for large-scale collection of CHP for autologous transplantation in cancer patients were based on data obtained with FITC-8G12. The same guidelines cannot be considered valid if FITC-QBEND10 is employed. We recommend FITC-8G12 as the optimal reagent to date for standardizing results of autologous CHP transplantation in cancer therapy.