PROJECT SUMMARY Cardiac complications occur in 20-60% of patients with peripheral blood hypereosinophilia irrespective of the cause—hypereosinophilic syndrome (HES), eosinophilic granulomatosis with polyangiitis (EGPA, formerly known as Churg-Strauss syndrome), drug reaction, or parasitic infection. Importantly, heart disease is the main cause of morbidity and mortality in this diverse group of patients. There is a paucity of models that adequately replicate cardiac disease in hypereosinophilia, which hinders mechanistic research that would identify therapeutic targets and advance clinical practice. To address this gap, we recently developed a mouse model of eosinophilic myocarditis that recapitulates many of the features of the disease including eosinophilia with heart involvement leading to early death. Our long-term goal is to understand the cellular and molecular mechanisms of eosinophil-mediated tissue damage. The objective of this grant is to use this mouse model to address critical questions regarding the pathophysiology of eosinophil-mediated cardiac disease. Our central hypothesis is that eosinophils and specific types of eosinophil cell death have a pathogenic role in eosinophilic myocarditis. We propose two specific aims: 1) define the role of eosinophils and their regulated necrosis in eosinophilic myocarditis; and 2) test for evidence of regulated necrosis and anti-eosinophil autoantibodies in patients with eosinophil-associated diseases. These contributions are significant because heart disease is the major cause of morbidity and mortality in patients with hypereosinophilic diseases; the studies in this grant application will enable directly testing mechanistic hypotheses in a model of eosinophilic myocarditis with features reminiscent of those seen in patients with heart disease as a consequence of hypereosinophilia. This project is innovative because it proposes to test innovative hypotheses, including the role of a recently described mode of biochemically regulated and thus targetable eosinophil cell death. Furthermore, the approach is innovative in that we use a new mouse model of spontaneous eosinophilic myocarditis. Understanding more about cardiac disease in a hypereosinophilic setting will provide mechanistic insights that may be generalizable to other conditions of eosinophil-associated heart disease.