Naive CD8⁺, CD4⁺T cells, and NK cells require different amounts of IL-15 to expand. UCBMCs were labeled with CFSE and cultured in the presence of variable concentrations of IL-15 for 7 days, followed by FACS analysis. NK cells (♦) were gated on CD3^-CD16⁺CD56⁺, CD4⁺ T cells (▴) on CD3⁺CD4⁺, and CD8⁺ T cells (▪) on CD3⁺CD8⁺. The percentage of cells in the initial population that had undergone one or more rounds of division after IL-15 stimulation was calculated as described in He et al.19 Data depicted are the mean ± SD from 3 experiments performed.