IL-3Rβ_c subunit expression detected by QRT-PCR predicts cytotoxicity of DT₃₈₈IL3 against AML progenitors which engraft in β₂-microglobin-deficient NOD-SCID mice. (A) AML cells from 3 patients (nos. 1, 16, and 13 from Table 1) expressing low levels of IL-3Rβ_c and (B) 4 patients (nos. 8, 17, 10, and 11 from Table 1) who had high levels of IL-3Rβ_c expression were incubated for 24 hours with or without DT₃₈₈IL-3 at 50 and 250 ng/mL and then injected intravenously into sublethally irradiated mice as described in “Patients, materials, and methods.” Data are shown as the mean level of engraftment of human AML cells in cohorts of mice receiving treated or untreated cells 8 to 12 weeks previously as detected by FACS analysis of mouse BM cells for CD45⁺ human cells. ▪ indicates untreated cells; , cells treated with 50 ng/mL DT₃₈₈IL-3; and □, cells treated with 250 ng/mL DT₃₈₈IL-3. Error bars indicate SEM level engraftment for all mice in the cohort. The level of engraftment was significantly (P < .05; Student t test) less than that obtained for the untreated control cells for the 4 patient samples treated with both doses of DT₃₃₈IL3 shown in panel B.