TLR9 decreases FOXO3 expression to prevent excessive inflammation in macrophage activation syndrome

Adult-onset Still's disease (AOSD) is characterized by an overwhelming inflammatory response and hyperactivation of monocytes/macrophages, which may cause macrophage activation syndrome (MAS). Here, we demonstrate the forkhead box protein O3 (FOXO3), a transcriptional factor downregulated by interferon, as an important regulator of inflammation in AOSD-MAS. FOXO3 expression is downregulated in monocytes/PBMCs from patients with AOSD, especially in those developing MAS. A negative correlation between FOXO3 expression with disease activity and inflammatory level is identified. FOXO3 downregulation can be induced by TLR9 activation both in the murine MAS model and TLR9 agonist-stimulated macrophages in vitro, through transcriptional regulation and phosphorylation by AKT. Depletion of Foxo3 protected mice from hyperinflammatory response and organ damage in MAS, and mechanistically, alleviated NLRP3 inflammasome activation in macrophages. Our study reveals mechanisms of FOXO3 in facilitating AOSD-MAS development and identifies the critical role of FOXO3 in the self-negative regulation of inflammation in AOSD through suppression by TLR9 signaling.