The proapoptotic effect of IL-21 and CpG ODN on B-CLL cells is synergistic. PBMCs from 3 patients with B-CLL were cultured with CpG ODN and IL-21 at different concentration ratios for 4 days, and the percentage of apoptotic cells was determined. (A) Three individual experiments demonstrating a synergistic interaction between CpG ODN and IL-21 are shown. A horizontal line indicates an additive interaction, while the observed convex curve demonstrates synergy. (B) The effect of varying concentrations of CpG ODN and IL-21 demonstrated that both agents induced greater apoptosis than either agent alone, even at concentrations that give peak effects for each individual agent. Plotted are the mean cell-survival rates (n = 3). Error bars indicate SEM.