Chimeric receptors bearing 4-1BB induce a superior NK-cell activation. (A) Expanded primary NK cells expressing chimeric receptors were incubated for 24 hours with the ALL cell line RS4;11 at a 1:1 ratio. Flow cytometric dot plots illustrated CD25 expression (in the y-axes) and side scatter (SSC; in the x-axes) of GFP⁺ cells before and after culture. The percentages of CD25⁺ NK cells are indicated. (B) Production of IFN-γ and GM-CSF by NK cells expressing different chimeric receptors after 24 hours of culture with 697 cells at a 1:1 ratio (mean ± SD of 3 measurements). The 4-1BB receptors elicited a significantly higher production of both cytokines (P < .005).