IL-18BP^−/−mice display a severe form of MAS after repeated CpG injections. Female WT and IL-18BP^−/− mice received CpG injections on days 0, 2, and 4 and were killed on day 7. (A) Variations from baseline body weight are represented as mean ± SD for WT (n = 11) and IL-18BP^−/− (n = 12) littermates. Circulating (Bi) white blood cell, (Bii) red blood cell, and (Biii) platelet counts and (C) plasma ferritin levels of WT and IL-18BP^−/− mice obtained before (baseline) and after CpG injections are represented as individual values for each mouse. Data were pooled from 2 independent experiments. (D) Hemophagocytes (arrowhead) were observed on May-Grünwald Giemsa–stained bone marrow smears only in CpG-challenged IL-18BP^−/− mice (original magnification ×1000). The Mann-Whitney test was used to compare WT and IL-18BP^−/− mice at 1 time point and to compare naive and CpG-challenged mice. The Wilcoxon signed-rank test was used to compare baseline and day 7 values within each genotype when data were paired. *P < .05; **P < .01; ***P < .005.