Macrophage activation syndrome (MAS) is a potentially fatal complication of rheumatic diseases. It is most commonly associated with systemic juvenile idiopathic arthritis (sJIA). White blood cells (WBCs) have been identified as critical to the pathogenesis of MAS. Monocytes, a type of WBC, are found in blood and are critically important to MAS pathogenesis. In this project, I will define changes in monocytes from children with MAS in comparison to children with sJIA without MAS and healthy children. To identify differences, we are using RNA sequencing, which provides unprecedented insight into the function and response of these cells to the MAS environment. I will also perform additional studies to understand the basis for the inflammatory environment in MAS. Through these studies, we aim to define the contribution and response of monocytes during MAS, providing greater insight into the mechanisms of disease and potentially identifying novel targets for therapy.