Impaired NK cell maturation and function in patients with SCN. Total NK cell (CD3⁻CD56⁺; A) and percentages of CD56^bright NK cells (B) in peripheral blood of healthy controls (black squares), SCN patients not treated with G-CSF (open circles) or SCN patient treated with G-CSF (black diamonds). (C-F) PBMCs from healthy control individuals (black squares), SCN patients not treated with G-CSF (open circles), or SCN patient treated with G-CSF (black diamonds) were stimulated for 4 h with K562 target cells (C–E) or antibody-coated cells (ADCC; D-F) at an effector/target ratio of 5:2 and 50:1, respectively. Frequencies of IFN-γ–producing NK cells (C and D) and NK cell cytotoxic activity (E and F) were determined by FACS. Statistical significance was determined in a Mann-Whitney test. *, P < 0.05; **, P < 0.01; ***, P < 0.005.