SCF and IL-2/15 lead to modest synergistic proliferation of NK cells. For panels A-C, the y-axis represents the mean percentage proliferation (± SEM) above that measured for untreated controls as measured by the MTS assay. (A) Proliferation of the NK-cell line DERL-7 in response to stimulation with SCF, IL-2, or SCF + IL-2. Cells were starved in SFM for 48 hours, washed, and stimulated in triplicate wells with SCF (100 ng/mL), IL-2 (150 IU/mL), or SCF + IL-2 for 48 hours. (B) IL-15 (20 ng/mL) is substituted for IL-2 and leads to similar findings as seen in panel A. (C) Modest synergistic proliferation of primary, human CD56^bright NK cells is seen in response to SCF + IL-2 compared with SCF (250 ng/mL) or IL-2 (150 IU/mL) alone as originally described.⁶  All pairwise comparisons are statistically significant between treatment groups (P < .05) in panels A-C, and each figure shows combined data from at least 3 independent experiments.