Proliferation of KIR-negative and KIR-positive NK cells. Peripheral blood mononuclear cells were stained with CFSE (5-carboxyfluorescein diacetate succinimidyl ester) and natural killer (NK) cells were purified by magnetic beads. The CFSE-positive, killer cell immunoglobulin-like receptor (KIR)-negative fraction was purified by fluorescence-activated cell sorting before culture for 7 days with IL-2 or IL-15 in the presence or absence of IL-21. The grey dots represent the KIR-negative fraction and the black dots represent the KIR-positive fraction after 7 days of culture (left panels). The percentage indicates the fraction KIR-positive and KIR-negative NK cells after 7 days of culture. The number of cells undergoing division in the KIR negative (in grey) and the KIR-positive (in black) fraction were analyzed after 7 days of culture on CD56+/KIR- (in grey) or CD56+/KIR+ (black) gated cells (right panel).