Low IL-12 concentrations sustained human NK cell survival and induced NK cell proliferation during IL-2 or IL-15 and NKp30 co-stimulation. Purified human peripheral blood NK cells were stimulated for 6 d. Cytokines were supplemented at the beginning of the cell culture in U-shaped 96-well plates. n = number of donors. Mean ± SD. Paired t test, parametric; *, P < 0.05; **, < P < 0.01. IL-12^low = 2.5 ng/ml, IL-2^low = 3 U/ml, and IL-15^low = 1 ng/ml. (A) Purified human peripheral blood NK cells were stimulated with increasing concentrations of IL-2 (U/ml) or IL-15 (ng/ml), cell proliferation was measured using cell trace violet dilution, and cell viability was measured by fixable near-infrared viability dye. n = number of donors. Mean ± SD (two independent experiments). (B) Proliferation (CTV-dilution) assay induced by IL-12^low (2.5 ng/ml), IL-2^low (3 U/ml), IL-15^low (1 ng/ml), and mIgG1 isotype-matched control-coated (B.i), or anti-NKp30–coated beads (B.ii). Left, CTV histograms; middle, CTV geometric MFI (gMFI); right, relative cell viability (>2 independent experiments).