Project Summary/Abstract The goal of this research is to investigate the development of inflammatory macrophages and disease in Macrophage Activation Syndrome (MAS). MAS is a serious and potentially fatal complication of rheumatic disease or viral infection. MAS is characterized by the development of cytopenias, including anemia and thromobocytopenia, and accumulation of hemophagocytes— activated macrophages that phagocytose red blood cells (RBCs). I will investigate spontaneous MAS-like disease in a mouse model of Systemic Lupus Erythematosus. In the TLR7.1 model, the overexpression of and constitutively active signaling through the endosomal single-stranded RNA sensor Toll-like receptor 7 (TLR7) drives chronic inflammation and subsequent MAS disease. TLR7.1 mice develop thrombocytopenia, anemia, and a novel population of hemophagocytes, inflammatory hemophagocytes (iHPCs), that spontaneously differentiate from Ly6Chi monocytes. Further, in this model, the development of anemia is positively correlated with iHPC phagocytosis of RBCs indicating that iHPC activity may be driving disease. In humans, SNPs in the gene encoding the transcription factor interferon regulatory factor 5 (IRF5) are associated with MAS development. IRF5 signaling is critical for the development of inflammatory macrophages in the context several inflammatory diseases. Previous work in our lab showed that IRF5 ablation in vivo ameliorates iHPC differentiation downstream of acute TLR7 signaling. However, in the context of in vivo TLR7- driven MAS, the role of IRF5 signaling in iHPC production and in MAS disease is unclear. In preliminary data, I show in TLR7.1 mice that signaling through IRF5 is critical for iHPC development, iHPC RBC phagocytosis, and anemia and thrombocytopenia indicative of MAS disease development. Based on these findings, the goal of my proposal is to determine the role of IRF5 signaling, specifically in Ly6Chi monocytes, in TLR7- driven iHPC differentiation and MAS disease (AIM 1), and to determine what other signals synergize with TLR7 to drive iHPC differentiation and MAS disease, with a specific focus on heme and type I interferons (AIM 2). Overall, completion of the proposed research along with my additional training described here will allow me to pursue my goals of becoming an independent investigator and faculty member with a focus on education.