Low concentrations of IL-2 acting through the high-affinity IL-2 receptor costimulate CD56^bright NK cell secretion of IFN-γ.

(A) Sorted CD56^bright NK cells were cultured for 72 hours with medium alone, IL-2 (10 pM, 2.3 IU/mL), IL-12 (10 U/mL), or IL-2 plus IL-12, and cell culture supernatants were then assayed for human IFN-γ protein production (top panel). Prior to the addition of cytokines, cells were preincubated for one hour with saturating concentrations of an anti-CD25 mAb (selectively blocking the α subunit of the high-affinity IL-2R) or an isotype control mAb (bottom panel). As a negative control, CD56^dim NK cells do not express the high-affinity IL-2R and fail to produce IFN-γ. (B) Sorted CD56^bright NK cells were cultured with various concentrations of IL-2 with or without 10 U/mL IL-12 (top panel) or various concentrations of IL-12 with or without 10 pM IL-2 (bottom panel). Cell culture supernatants were harvested at 72 hours and assayed for IFN-γ protein by ELISA. Results represent the means ± SEMs of replicate wells, expressed in pg/mL, and are representative of at least 3 separate experiments.