CXCL9 and macrophage activation syndrome: Consistent with previous studies, SD was associated with marked elevation of plasma IL-18 levels (median 17,220 pg/mL), while the MAS group showed even higher IL-18 levels (median 101,546 pg/mL) as well as increased IFN-γ production, as reflected by high levels of the IFN-γ–inducible chemokine CXCL9 (median 3,889 pg/mL versus 366 pg/mL in the SD group; Supplemental Table 1) (14, 15, 20).