CD137L expression in AML cells. (A) PBMCs from patients with AML and CD34⁺ cells of healthy donors were analyzed by FACS for CD137L and HLA class I expression using specific mAb (shaded peaks) and isotype control (open peaks). Malignant cells were selected as described in “Methods.” (B) RNA from the PBMCs of patients with AML was extracted, reverse transcribed, and investigated for CD137L expression by RT-PCR analysis of equal mRNA levels; 18S rRNA served as control. (C) CD137L and HLA class I surface expression on the indicated AML cell lines was investigated as described in panel A. (D) CD137L mRNA expression by the indicated AML cell lines was investigated as described in panel B. (E) SFI levels of CD137L surface expression on patient AML cells were determined by FACS. Patients were then subdivided according to FAB types. Association with specific subtypes was calculated by 1-way ANOVA. Horizontal bars indicate means within each group. *Statistically significant differences.