Expansion of NK cells after 1 week of culture with genetically modified K562 cells. Peripheral blood mononuclear cells from 7 healthy individuals (represented by different symbols) were cultured with various preparations of K562 at a 1:1 ratio in the presence of low-dose (10 U/mL) IL-2. Percentages of CD56⁺CD3^- NK cells and CD3⁺ T lymphocytes after 7 days of culture relative to the number of input cells are shown. Each data point represents the average of 2 measurements; bars correspond to the median expansion in each group. K562 cells expressing both membrane-bound IL-15 and 4-1BBL (K562-mb15-41BBL) induced a markedly superior expansion of NK cells (P < .001 by the Tukey honest significant difference test) without inducing T-cell proliferation; there were no significant differences among other pairwise comparisons of NK expansions obtained with K562, K562-41BBL, and K562-mb15.