PROJECT SUMMARY We are requesting an Administrative Supplement for the INCLUDE Project for R01 AR076242 “IRF5 and Macrophage Activation Syndrome in systemic Juvenile Idiopathic Arthritis”. The proposed studies for the in this Supplement are within the scope of the active parent grant, focused on Down syndrome (DS) and are aligned with the goals of the overall INCLUDE Project—Component 1: Targeted high risk – high reward basic science studies highly relevant to DS. Our studies take advantage of a growing registry and biorepository of DS individuals that has been established at the Benaroya Research Institute by Drs. Bernard Khor and Jane Buckner. The objectives of this administrative supplement are to better understand innate immune alterations in DS. Individuals with DS have immune dysfunction evidenced by decreased vaccine responses, increased severity of viral infections and increased incidence of certain autoimmune diseases, including autoimmune thyroiditis, type 1 diabetes and juvenile idiopathic arthritis (JIA). DS individuals also have increased type 1 IFN responses and increased amounts of some inflammatory cytokines and specific alterations in T and B cells. However, how these alterations lead to the overall changes in immunity in DS is not well-understood. Importantly, there has been much focus on adaptive immune responses in DS, with less focus on cells of the innate immune system, including monocytes and macrophages, important in the pathogenesis of many autoimmune diseases, including (JIA). This supplement is within the scope of the Parent R01 as it: 1) investigates monocytes and TLR responses in these cells, which are the focus of the parent R01, 2) relates to a form of juvenile arthritis seen more frequently in individuals with DS, as the parent grant focuses on another form of juvenile arthritis (systemic juvenile idiopathic arthritis (SJIA)), and 3) will leverage our existing single cell RNA-Seq (scRNA-Seq) data funded by the parent R01. Data generated from this supplement will inform our parent R01 by giving us additional monocyte scRNA-Seq datasets to compare with our already generated scRNA-Seq datasets from children with SJIA-associated MAS and matched controls. This supplement is within NIAMS INCLUDE Program priorities to better understand arthritic disorders in DS across the lifepan. Because monocytes and macrophages are key players in pathogenesis of all forms of arthritis, understanding whether these cells and their responses are dysregulated in DS will lead to a better understanding of their contributions to the increased incidence of arthritis in children with DS.

PROJECT NARRATIVE The parent R01 focuses on understanding the role of monocytes, a type of white blood cell important in inflammatory responses, in Macrophage Activation Syndrome (MAS) is an acute complication associated with rheumatic diseases, most commonly seen in patients with systemic juvenile idiopathic arthritis (SJIA). In this administrative supplement for the INCLUDE program we will investigate how monocytes may contribute to the immune dysregulation seen individuals with Down syndrome and compare this to the information we have learned about SJIA and MAS. Together, this work will shed light onto mechanisms of inflammatory arthritis including that seen in children with Down syndrome.