Expression of interleukin-3 receptor subunits on defined subpopulations of acute myeloid leukemia blasts predicts the cytotoxicity of diphtheria toxin interleukin-3 fusion protein against malignant progenitors that engraft in immunodeficient mice

The interleukin-3 receptor (IL-3R) subunits are overexpressed on acute myeloid leukemia (AML) blasts compared with normal hematopoietic cells and are thus potential targets for novel therapeutic agents. Both fluorescence-activated cell sorter (FACS) analysis and quantitative real-time reverse transcription-polymerase chain reaction (QRT-PCR) were used to quantify expression of the IL-3Rα and β_c subunits on AML cells. QRT-PCR for both subunits was most predictive of killing of AML colony-forming cells (AML-CFCs) by diphtheria toxin-IL-3 fusion protein (DT₃₈₈IL3). Among 19 patient samples, the relative level of the IL-3Rα was higher than the IL-3Rβ_c and highest in CD34⁺CD38 ^-CD71^- cells, enriched for candidate leukemia stem cells, compared with cell fractions depleted of such progenitors. Overall, the amount of IL-3Rβ_c subunit did not vary among sorted subpopulations. However, expression of both subunits varied by more than 10-fold among different AML samples for all subpopulations studied. The level of IL- 3Rβ_c expression versus glyceraldehyde-3-phosphate dehydrogenase (GAPDH) (set at 1000) ranged from 0.14 to 13.56 in CD34⁺CD38 ^-CD71^- cells from different samples; this value was correlated (r = .76, P = .05) with the ability of DT₃₈₈IL3 to kill AML progenitors that engraft in β₂-microglobin-deficient nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice (n = 7). Thus, quantification of IL-3R subunit expression on AML blasts predicts the effectiveness IL-3R-targeted therapy in killing primitive leukemic progenitors.