TLR9 and Splenomegaly: The best described mouse model of MAS involves repeated administration of the TLR9 agonist CpG, leading to clinical features that closely mimic the “subclinical” MAS observed in nearly one-third of SJIA patients (Supplemental Figure 1; supplemental material available online with this article; https://doi.org/10.1172/jci.insight.147593DS1) (11), including anemia, lymphopenia, thrombocytopenia, massive splenomegaly, and hyperferritinemia (Supplemental Figure 2).