Knockdown of BCL-X_L in combination with agents that suppress BCL2A1 completely resensitizes CLL cells to ABT-737. CLL cells were transfected with negative control siRNA or BCL-X siRNA and immediately after nucleofection cultured on parental or CD154-expressing cells in the presence of IL-4 (10 ng/mL) for 24 hours. (A,B) After removal from parental or CD154-expressing cells, CLL cells were exposed to 50 μM seliciclib for 20 hours, and protein expression was analyzed by Western blotting (A). Seliciclib treatment was followed by exposure to different concentrations of ABT-737 for 4 hours, and apoptosis was assessed as PS externalization and Annexin V/FITC binding (solid lines indicate control; dotted lines, seliciclib; B, n = 4). (C,D) After removal from CD154-expressing cells, CLL cells were exposed to 500 nM TSA for 20 hours, and protein expression was analyzed by Western blotting (C; *nonspecific band). TSA treatment was followed by exposure to different concentrations of ABT-737 for 4 hours, and apoptosis was assessed as PS externalization and Annexin V/FITC binding (solid lines indicate control; dotted lines, TSA; D, n = 3). (B,D) Statistics were done using one-way ANOVA followed by Dunnett multiple comparison test using parental as the control group. Reversal of resistance as indicated by no significant difference to the control group is indicated with † (P > .05).