Contribution of innate-activating receptors to IL-12 mediated NK cell proliferation. Purified human peripheral blood NK cells were primed with IL-2^high (300 U/ml) for 3 d before IL-12^low (2.5 ng/ml) stimulation. Cytokines were added only at the beginning of the culture. IL-2 was washed out before the subsequent stimulation. n = number of donors. Mean ± SD. mIgG1-coated beads were used as a negative control (≥2 independent experiments). (A) Intracellular IFNγ, IL-2, and IL-15 levels in IL-2–primed NK cells after 24 h of the indicated stimulation. (B) Proliferation (CTV) assay: CTV gMFI levels of IL-2–primed NK cells from 2 donors with a NKG2C^high adaptive NK cell subset (>30%) following the indicated innate-activating receptor stimulation with or without IL-12^low. IL-2^high (300 U/ml) was used as a positive control. Media without cytokines was used as a negative control. Right: NK activating receptor expression levels ex vivo (black) or after 3 d of IL-2 priming (red).