Effect of IL-18 on spleen cell proliferation. Splenocytes from WT (•) or ICE-deficient (○) mice were incubated with the indicated concentrations of IL-18. Proliferation was assessed after 72 hours of incubation. Data are the mean ± SEM of 3 mice per group and are expressed as the percentage of change in MTS absorbance over unstimulated cells (100%).