PROJECT SUMMARY Myocarditis caused by viral infections is a leading cause of sudden death in young adults and can progress to dilated cardiomyopathy (DCM) and the need for a heart transplant. There is a clinical need to better understand the mechanisms of the innate immune response underlying myocarditis in order to improve diagnosis and prevent sudden death. Sex differences exist for most autoimmune and cardiovascular diseases. Men have a higher incidence and severity of cardiovascular diseases including myocarditis/DCM than women. In a proteomics study we found that serum vitamin D binding protein (DBP) and complement components were upregulated in men with myocarditis compared to women, suggesting that sex differences in both DBP and complement may increase cardiac inflammation in men. The activity and mechanism of DBP in innate immune cells in myocarditis has not been previously investigated. In a highly translational preclinical animal model we found that male mice deficient in DBP have significantly reduced inflammation and complement components including C3 and C5/C5a compared to wild type controls indicating that DBP increases myocarditis in male mice. This study examines whether sex differences in plasma DBP levels in patients with myocarditis correlate to measures of heart failure like New York Heart Association Class, heart failure biomarkers, sex hormones and/or complement components. We will further define the mechanisms whereby DBP increases the innate immune system leading to increased cardiac inflammation using DBP deficient mice, C5a receptor antagonist and C5a recombinant protein. We will additionally examine the effect of sex hormones on DBP and complement components during viral myocarditis by looking at correlations between DBP, complement and sex hormone levels. The successful completion of the study will significantly impact the field of myocarditis by providing a mechanistic understanding of circulating DBP as a novel biomarker that may be used to predict the risk of heart failure from myocarditis and determine whether therapies targeting DBP could reduce innate immune cell recruitment and/or prevent myocarditis and sudden death.