Addition of the 4-1BB costimulatory molecule to the chimeric receptors augments their capacity to induce NK cytotoxicity against NK-resistant leukemic cells. (A) Expanded primary NK cells expressing chimeric receptors were incubated for 4 hours with the B-lineage ALL cell lines 380 and RS4;11 at the indicated E/T ratios. Each data point represents the mean (± SD; n = 4) percentage of ALL cell killing after culture as compared to that of parallel cultures without NK cells. At all E/T ratios, cytotoxicity of NK cells expressing chimeric receptors containing 4-1BB was significantly higher than that induced by receptors without 4-1BB (P < .001). (B) Flow cytometric dot plots show staining with anti-CD56 and anti-CD22 after a 4-hour coculture of NK cells (CD56⁺) and ALL cells (380; CD22⁺) at a 2:1 ratio. The percentage of cell killing obtained with NK cells expressing different chimeric receptors (% kill) was calculated by comparing the number of viable CD22⁺ ALL cells recovered after the test culture to that of parallel cultures without NK cells.