Expansion of CFC (A) and LTC-IC (B) in M2-10B4 contact cultures initiated with PB CD34⁺HLA-DR⁺ (n = 6) and CD34⁺HLA-DR⁻ (CFC n = 10; LTC-IC n = 9) cells. Twelve thousand CD34⁺HLA-DR⁺ or CD34⁺HLA-DR⁻ cells were cultured for 2 to 5 weeks in LTC medium without cytokines in contact with irradiated M2-10B4 feeders. Cells were collected at 2 and 5 weeks and 1/6 of the progeny was replated in methylcellulose assay to enumerate the number of CFC (equivalent of 2,000 day 0 cells) and 5/6 of the progeny in LDA onto M2-10B4 feeders (equivalent of 10,000 day 0 cells) for 5 weeks to enumerate the number of LTC-IC. The number of CFC present at 2 and 5 weeks was divided by the number of CFC per 2,000 freshly sorted cells to calculate CFC expansion. The number of LTC-IC at 2 and 5 weeks was divided by the number of LTC-IC in freshly sorted cells to calculate LTC-IC expansion. Results represent mean ± SEM. (A) The number of CFC on day 0 was 482 ± 45 CFC/2,000 CD34⁺HLA-DR⁺ cells and 52 ± 15 CFC/2,000 CD34⁺HLA-DR⁻ cells; *P < .01 comparison between CFC expansion in cultures initiated with CD34⁺HLA-DR⁺ and CD34⁺HLA-DR⁻ cells. (B) The absolute number of LTC-IC on day 0 was 124 ± 23 LTC-IC/10,000 CD34⁺HLA-DR⁺ cells and 46 ± 24 LTC-IC/10,000 CD34⁺HLA-DR⁻ cells; *P < .01 comparison between LTC-IC maintenance in cultures initiated with CD34⁺HLA-DR⁺ and CD34⁺HLA-DR⁻ cells.