CD137 blockade enhances NK-cell reactivity against CD137L-expressing AML cells. Allogenic polyclonal NK cells were cultured with or without CD137L-expressing or CD137L⁻ AML cells of patients with more than 80% blast count in the presence or absence of 10 μg/mL mouse IgG1 or blocking CD137 mAb. (A) Granule mobilization after 3 hours was analyzed by FACS for CD107a expression. Numbers in top right quadrants indicate the percentage of CD107a⁺ NK cells. (B) Cytotoxicity of NK cells alone (♦) or in the presence of mouse IgG1 (■) or CD137 mAb (▲) was evaluated by 2-hour Europium assays. (C) IFN-γ levels in culture supernatants were determined after 24 hours by ELISA. Means of triplicates with SDs of 1 representative experiment each of a total of at least 3 with similar results are shown. *Statistically significant differences.