Exposure to IL-6 reduces chemotherapy-induced apoptosis. The AML cell lines THP-1 (A) and NB4 (B) had less mitoxantrone-induced apoptosis when pretreated for 24 hours with IL-6 and sIL-6Rα at 50 ng/mL and 100 ng/mL, respectively (red), than vehicle-treated cells (blue) (THP-1, n = 5; NB4, n = 8; *P < .05, analysis of variance). (C) Representative primary sample demonstrating decreased mitoxantrone-induced apoptosis with IL-6 pretreatment, and (D) mean apoptosis rates of 7 primary AML samples with vs without IL-6 pretreatment. Duration of mitoxantrone exposure was 24 hours for cell lines and 18 hours for primary samples. Values shown are mean ± standard error of the mean; P < .05 by Wilcoxon signed-rank test. FITC, fluorescein isothiocyanate.