CD137 expression on human NK cells associates with phenotypic markers of activation and proinflammatory cytokine secretion. Gates were set around CD56⁺ (NK) cells and numbers in the dot plots indicate the percentage of NK cells expressing indicated surface marker. CD137 expression on IL-2–stimulated NK cells associates with increased levels of CD69 (A) and CD54^bright (B) expression in cultures with immobilized huIgG1 and GG mAb. Proinflammatory cytokines were determined by ELISA in supernatants after 24 hours of culture. (C) TNF-α levels were significantly (*P = .047) increased in cultures with immobilized GG mAb. (D) IFN-γ levels were significantly (*P = .048) increased in cultures with immobilized GG mAb. Immobilized mAb culture conditions and soluble mAb culture conditions are indicated with i- and s-, respectively. Experiment shown represents 1 of 6 individual experiments. Statistical analysis is based on 6 experiments; CD69 expression levels are significantly increased in NK-cell cultures with i-GG (P = .036) and CD54^bright expression levels are significantly increased in NK-cell cultures with i-huIgG1 (P = .019) and i-GG (P = .004) compared with i-GA. HuIgG1 indicates polyclonal human IgG1; GA, aglycosylated chimeric anti-CD137 mAb; GG, glycosylated chimeric anti-CD137 mAb; and CTM, cetuximab. Error bars represent SD.