HUMAN CORD BLOOD-DERIVED PRIMITIVE PROGENITORS ARE ENRICHED IN CD34(-KIT(-) CELLS - CORRELATION BETWEEN LONG-TERM CULTURE-INITIATING CELLS AND TELOMERASE EXPRESSION()C)

We studied the functional characteristics of subpopulations of cord bl ood-derived CD34(+) cells expressing different levels of CD38 and c-ki t antigens, using clonal cell culture and long-term culture with allog eneic bone marrow stromal cells or the MS-5 murine stromal cell line t o assay long-term culture-initiating cells (LTC-IC) in each subpopulat ion. To investigate the capacity for replication, proliferation, and d ifferentiation 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 o f LTC-IC was low in their antigen-positive counterparts (one per 84 CD 34(+)CD38(+) cells, one per 90 CD34(+)c-kit(low) cells, and very low a mong CD34(+)c-kit(high) cells). It was noteworthy that some LTC-IC der ived from CD34(+)CD38(-) as well as CD34(+)c-kit(-) cells generated co lony-forming cells (CFCs) after up to 9 weeks of coculture. Telomerase activity was consistently low in CD34(+)CD38(-) and CD34(+)c-kit(-) c ells compared to CD38(+) or c-kit(high or low) cells, suggesting that CD34(+)CD38(-) or c-kit(-) cells are likely to be more quiescent. Thes e results suggest that the CD34(+)CD38(-) and CD34(+)c-kit(-) cell pop ulations are primitive stem/progenitor cells, and that the telomerase activity of these cells correlates with their proliferative capacity a s well as their stage of differentiation.