Induction of PRDM1 in responsive CLL samples correlates with increased differentiation. (A) Secreted IgM levels after 5 days of treatment of responsive (R) and nonresponsive (NR) CLL samples with IL-21. (B) Morphology of representative responsive and nonresponsive CLL samples (n = 3 each) examined directly ex vivo (T0), after 5 days in culture medium alone (untreated), or after 5 days of stimulation by IL-21 with and without co-stimuli (anti-IgM, CD40L, and IL-2) (×100 magnification; Nikon ACT-1 v2.63 microscope software Giemsa staining).