Characterization of Multiple CD34 + Cell Populations in Cord Blood

Abstract

Unlike granulocyte colony-stimulating factor (G-CSF)-mobilized peripheral blood, which show a single homogeneous population of CD34⁺ cells, umbilical cord blood (CB) CD34⁺ cells are present as multiple populations, CD34^regular and CD34^bright (the latter comprising 7.0-58.2% of the total CD34⁺ cells), using the ProCOUNT™ procedure or with anti-CD34 labeling of immunoselected cells. The CD34^regular population contains cells with high forward scatter (CD34^regularFSC^high) and with low forward scatter (CD34^regular FSC^low). Immunomagnetically selected CD34⁺ cells, sorted into CD34^regular, CD34^regular FSC^high, CD34^regularFSC^low, and CD34^bright cell populations, were used in in vitro assays: only the CD34^regularFSC^high population transmigrated and showed growth of colony-forming unit (CFU) and long-term culture initiating cells (LTC-IC) colonies. The absolute number of CD34⁺ cells in CB samples was determined by ProCOUNT™ and Stem Kit™ enumeration protocols. In liquid stored CB units, ProCOUNT™ and Stem Kit™ count differences are accounted for by the enumeration of CD34^bright cells. Differences between ProCOUNT™ and Stem Kit™ counts using cryopreserved/thawed samples are accounted for by increased CD34^regular FSC^low cell numbers (2.0 ± 1.4% in liquid stored and 27.8 ± 14.6% in cryopreserved/thawed samples). The ProCOUNT™ assay includes the nonfunctional CD34^bright and CD34^regularFSC^low cells as part of the CD34⁺ cell count, thereby elevating the absolute number of CD34⁺ cells. Using the Stem Kit™ assay method of gating, CD34^bright and CD34^regularFSC^low cells are not counted. Our data indicate that the CD34^regularFSC^high cell population has functional characteristics based on the in vitro assays and a more accurate count of these cells can be achieved using the Stem Kit™ assay.