Myocarditis is a rare but potentially devastating inflammatory heart disease characterized by myocardial inflammation. Up to 30% of myocarditis patients go on to develop dilated cardiomyopathy (DCM), which is a major cause of heart failure in children and young adults. The proposed project, when completed, will significantly improve the understanding of the immunologic phenomenon leading to myocardial remodeling, fibrosis and heart failure. The time window between the inflammatory phase and development of DCM and heart failure is variable, but usually broad enough to allow secondary preventive treatment if specific targets are found. That knowledge might lead to development of novel diagnostic approaches and targeted treatment to prevent dilated cardiomyopathy. Furthermore, according to our preliminary data, those phenomena might be common to other heart inflammatory diseases like myocardial infarction. We have discovered a novel pathway leading to DCM development in experimental autoimmune myocarditis (EAM) mouse model. IL-17A signaling induces myeloid cytokines, such as Granulocyte macrophage colony-stimulating factor (GM-CSF), and CCL2 (MCP1) expression from cardiac fibroblasts (CF). GM-CSF then promotes differentiation of cardiac infiltrating monocytes toward Ly6Chi inflammatory monocytes, which promote DCM. The novelty of our approach is the focus on the immune modulatory role of cardiac fibroblasts. We propose that CD45?CD31?CD29+PDGFR?+Sca-1+ CF subset (Sca-1+ CFs) is the main producer of myeloid cytokines in EAM, Coxsackievirus B3-induced viral myocarditis, and myocardial infarction mouse models. Moreover, Sca1+CFs are plastic and able to switch their cytokine profiles based on microenvironments. Furthermore, we detected the same population of CFs expressing GM-CSF in frozen endomyocardial biopsy samples from patients with chronic ischemic cardiomyopathy. Our overall hypothesis is that Sca-1+ cardiac fibroblasts contribute to the disease progression from myocarditis to dilated cardiomyopathy through IL-17A-induced GM-CSF production. First, in Aim 1, we will evaluate whether the specific deletion of IL-17RA on Sca-1+ CFs will protect the mice from DCM. Next, we will block the GM-CSF production in Sca-1+ CFs by Adenoviral-associated vector (AAV)-delivered shRNA, targeting GM-CSF under Sca-1+ CF specific promoter. In Aim 2, we will investigate the plasticity of Sca-1+ CF cytokine profile using CCL2-expressing (mCherry+) cells to trace the cytokine production of the same cell following the Th microenvironment change. Next, we will determine the necessity of NF-?B/NFAT signaling pathway in IL-17A- driven DCM through specific ablation of p65 in Sca-1+ CFs. Finally, in Aim 3, we will examine the human cardiac fibroblasts plasticity and signaling pathways involved in GM-CSF production using human primary cardiac fibroblasts and frozen endomyocardial biopsy samples from myocarditis and cardiomyopathy patients.