Secondary CD34⁺ and CD34⁻ cells. (A) Telomerase activity at baseline and weeks 1 to 4 of ex vivo expansion in primary CD34⁺ (•), secondary CD34⁺ (▪), and CD34⁻ () cells is shown ([A] depicts the relative telomerase activity in percentage of the SK-N-SH neuroblastoma cell line control). After 1 week of culture, telomerase activity was upregulated in primary CD34⁺ and CD34⁻ cells. In primary CD34⁺ cells, telomerase activity increased 20-fold (median) compared with baseline levels. Secondary CD34⁺ cells isolated at week 1 of culture displayed a 1.4-fold telomerase increase compared with primary CD34⁺ cells, but a sharp decline after 1 week of expansion and no upregulation of telomerase (week 1 → week 2). There was no upregulation of telomerase activity in primary or in secondary CD34⁺ cells in the following weeks (weeks 2 to 4) of culture. In CD34⁻ cells, telomerase was fivefold to 10-fold lower in the first 2 weeks of expansion compared with primary CD34⁺ cells and undetectable in weeks 3 and 4. (B) Cumulative population doublings in primary CD34⁺, secondary CD34⁺, and CD34⁻ cells are shown. Similarly to the reduced upregulation of telomerase activity in secondary CD34⁺ cells, cellular expansion was also reduced.