CD137 expression on NK cells is induced in an Fc-dependent fashion. GG mAb was digested into separate Fab and Fc fragments using papain. Digests were passed over protein L or protein A columns, and single Fab fragments or Fc fragments were collected, respectively. IL-2–stimulated NK cells were cultured in the presence of immobilized chimeric mAb or immobilized fragments as indicated in the figure. (A) After 24 hours, CD137 expression was determined by flow cytometry. Gates were set around 7-AAD⁻/annexinV⁻ cells. Numbers in the dot plots indicate the percentage of CD56⁺ (NK) cells expressing CD137 (i). Ratios represent the number of live NK cells and are calculated based on the total number of 7-AAD⁻/annexinV⁻ cells within the total number of acquired cells (ii). (B) CD137 expressed on NK cells is recognized not only by commercial available mouse anti-CD137 mAb but also by both chimeric anti-CD137 mAbs, GA and GG. Experiment shown represents 1 of 3 individual experiments. Statistical analysis is based on 6 (i-GG and i-GA) and 3 (i-GG/Fc and i-GG/Fab) experiments; CD137 expression levels are significantly increased in NK-cell cultures with i-GG and i-GG/Fc compared with i-GA and i-GG/Fab (P = .001). HuIgG1 indicates polyclonal human IgG1; GA, aglycosylated chimeric anti-CD137 mAb; and GG, glycosylated chimeric anti-CD137 mAb.