PROJECT SUMMARY / ABSTRACT Severe lung disease is an increasingly recognized life-threatening complication of systemic juvenile idiopathic arthritis (SJIA-LD), representing a distinct and highly inflammatory interstitial lung disease that affects as many as 1 in 20 children with SJIA. While the etiology of SJIA-LD is unknown, it is strongly linked to macrophage activation syndrome, (MAS), episodic systemic hyperinflammation with SJIA that is driven by interferon gamma (IFNγ). We recently reported that SJIA-LD and MAS share prominent features of IFNγ activation, supporting a key role for this pathway in the pathogenesis of SJIA-LD. However, the mechanisms by which IFNγ activation drives pulmonary inflammation in SJIA-LD is unexplored. In addition, the widespread use of anti-IL-1 therapy for SJIA has been linked to markedly increased incidence of SJIA-LD including distinct clinical features and development of pulmonary alveolar proteinosis (PAP). Our proposed studies provide a critical step to identify the mechanistic causes of SJIA-LD, a necessary step towards developing targeted treatment strategies for and ultimately to prevent SJIA-LD. The objective of this application is to define the mechanisms by which MAS and persistent IFNγ drive the pathogenesis of SJIA-LD. Accordingly, our central hypothesis to be tested is that persistent IFNγ pathway activation leads to alveolar macrophage dysfunction and lung inflammation in SJIA-LD and is amplified by anti-IL-1 biologic therapy. To study the mechanisms of SJIA-LD, we will utilize overlapping approaches in our established mouse model system to directly test effects of persistent IFNγ on pulmonary inflammation, and the requirement of IL-1 signaling for alveolar macrophage functional phenotypes. In parallel, we will define IFNγ-driven functional polarization phenotypes of alveolar macrophages in children with SJIA-LD. Specific Aim 1 will determine whether persistent IFNγ activation is the key driver of lung inflammation during MAS. We hypothesize that persistent IFNγ activation during chronic/recurrent MAS leads to the development of lung disease in mice. Specific Aim 2 will identify mechanisms of alveolar macrophage reprogramming in experimental MAS. We hypothesize that IL-1 blockade in the setting of persistent IFNγ activation during MAS reprograms alveolar macrophages towards inflammatory phenotypes and inhibits anti-inflammatory/resolution and homeostatic functions. Specific Aim 3 will define IFNγ-driven alveolar macrophage populations in children with SJIA-LD. We hypothesize that alveolar macrophage subsets in SJIA-LD display an IFNγ-driven inflammatory phenotype that prevents normal homeostatic functions. We anticipate that the proposed experiments will define the function of persistent IFNγ activation and IL-1 blockade as drivers of lung inflammation and alveolar macrophage dysfunction in MAS and SJIA-LD. Together these studies will advance our long-term goal of identifying the causes of and deve...