CD137 expression on human NK cells. NK cells were incubated in the presence or absence of 50 U of IL-2 or 10 ng/mL IL-15. (A) Up-regulation of CD137 and CD69 after the indicated times was analyzed by FACS using specific mAb and mouse IgG1 as isotype control. Numbers in dot plots indicate the percentage of CD137⁺ or CD69⁺ CD56⁺CD3⁻ NK cells. (B) CD137 mRNA expression in freshly isolated, IL-15–activated (48 hours) and polyclonal NK cells was investigated by RT-PCR analysis of equal mRNA levels; 18S rRNA served as control. (C) Expression of CD137 and CD69 on polyclonal NK cells was analyzed by FACS as described in panel A. (D) Total RNA of freshly isolated NK cells after culture for the indicated times in the absence or presence of IL-15 was isolated and reverse transcribed. Relative copy numbers of CD137 were determined by quantitative PCR and normalized with 18S rRNA expression. (E) The percentage of CD137- and CD69-expressing CD56⁺CD3⁻ NK cells among the PBMCs of 10 healthy donors and 20 patients with AML as obtained by FACS is shown for each investigated donor/patient.