Free IL-18 determines susceptibility to more severe TLR9-induced MAS. Mice of the indicated genotypes were repeatedly injected with phosphate-buffered saline or the TLR9-stimulus CpG and were assessed for (A) serum total and free IL-18; (B) relative spleen size, aspartate aminotransferase, and platelet count; and (C) serum IL-10, CXCL9, and IFN-γ. *Adjusted P < .01, **adjusted P < .001, 2-way ANOVA with Tukey posttest. Significance is only shown for comparisons where adjusted P < .05. Results are representative of at least 2 independent experiments.