Human cord blood-derived primitive progenitors are enriched in CD34⁺c-kit⁻ cells: correlation between long-term culture-initiating cells and telomerase expression

Abstract

We studied the functional characteristics of subpopulations of cord blood-derived CD34⁺ cells expressing different levels of CD38 and c-kit antigens, using clonal cell culture and long-term culture with allogeneic bone marrow stromal cells or the MS-5 murine stromal cell line to assay long-term culture-initiating cells (LTC-IC) in each subpopulation. To investigate the capacity for replication, proliferation, and differentiation of each subpopulation of CD34⁺ cells, we also studied the correlation between LTC-IC and telomerase activity. After 5 weeks of coculture, LTC-IC accounted for one out of 32 CD34⁺CD38⁻ cells and one out of 33 CD34⁺c-kit⁻ cells. In contrast, the frequency of LTC-IC was low in their antigen-positive counterparts (one per 84 CD34⁺CD38⁺ cells, one per 90 CD34⁺c-kit^low cells, and very low among CD34⁺c-kit^high cells). It was noteworthy that some LTC-IC derived from CD34⁺CD38⁻ as well as CD34⁺c-kit⁻ cells generated colony-forming cells (CFCs) after up to 9 weeks of coculture. Telomerase activity was consistently low in CD34⁺CD38⁻ and CD34⁺c-kit⁻ cells compared to CD38⁺ or c-kit^{high or low} cells, suggesting that CD34⁺CD38⁻ or c-kit⁻ cells are likely to be more quiescent. These results suggest that the CD34⁺CD38⁻ and CD34⁺c-kit⁻ cell populations are primitive stem/progenitor cells, and that the telomerase activity of these cells correlates with their proliferative capacity as well as their stage of differentiation.