IL21 and B-cell chronic lymphocytic leukemia: Whereas IL-21 induced apoptosis of the CLL cells outside of their protective niche,20 CD40L-stimulated CLL cells may be induced to proliferate in its presence.21 Despite intersample variation at earlier time points, up to 20% of CLL cells proliferated after a continuous 72 h exposure to either 1.5 μM CpG-ODN or 25 ng/ml IL-21 (Figure 1a).