IL-21 increases NK-cell–mediated lysis of MCL cells. Enriched human NK cells from peripheral blood were incubated with ⁵¹Cr-labeled (A) Jeko-1 and (B) MCL primary tumor cells coated with IL-21 at various effector/target ratios as described in “Materials and methods.” In (C), ⁵¹Cr-labeled MCL primary tumor cells were coated with rituximab alone or in the presence of IL-21, followed by incubation with NK cells at variable effector/target ratios. Percentage of specific lysis was derived from ⁵¹Cr released by target cells. Data are representative of 3 independent experiments. Error bars represent SD between triplicate wells. *P < .05; **P < .01; ***P < .0005; ****P < .0001.