Cytokine production capacity of IL-15/HC–activated CD56⁺ cells. Purified CD56⁺ cells cultured for 20 days with IL-15 in the absence or presence of HC, at concentrations ranging from 10^–8 M to 10^–4 M, were tested for their capacity to produce cytokines, after an additional 48-hour incubation with IL-12 and IL-18. The data represent the mean plus or minus SD from 3 independent experiments, using different healthy donors.