Reduced differentiation capacity in CLL samples is not overcome by additional co-stimuli. (A) Induction of PRDM1 protein in normal B cells (CD19⁺CD27⁺ [memory normal B] and total CD19⁺ [normal B]) after 2 days of treatment with IL-21, compared with levels seen in representative responsive and nonresponsive CLL samples (n = 3). (B) PRDM1 expression in a representative nonresponsive CLL clone and normal B-cell (CD19⁺) sample after 2 days of treatment with IL-21 ± anti-IgM ± CD40L. The lysate of a responsive CLL clone treated for 2 days with IL-21 ± anti-IgM is included in each western blot for comparison (right hand lanes) (n = 4). (C) PRDM1 induced in representative responsive and nonresponsive CLL samples after 2 days of treatment with CpG-ODN ± anti-IgM (n = 4).