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After myocardial infarction (MI), cardiac fibroblasts differentiate into myofibroblasts. Myodifferentiation leads to excessive extracellular matrix proteins secretion within the myocardium, resulting in fibrosis which holds a prominent role in adverse left ventricular (LV) remodeling and heart failure development. TGFβ1 is the main mediator of myodifferentiation and is abundantly produced by platelets. Moreover, platelets are involved in TGFβ1 activation via the Glycoprotein A Repetitions Predominant (GARP) present on their surface. Although platelets are known to infiltrate the infarcted myocardium, their role as a source of active TGFβ1 in post-MI cardiac remodeling has never been investigated. The current proposal sought to determine the contribution of platelet GARP in the regulation of fibrosis and LV remodeling after MI. We generated a new Cre transgenic mouse strain that allows megakaryocyte/platelet specific invalidation of GARP (GpIba-Cre x GARPfl/fl). Platelet function and serum production of active TGFβ1 was assessed by FACS and ELISA, respectively. WT and pGARP KO mice underwent MI via permanent ligation of the left anterior descending coronary artery. Inflammation and cardiac fibrosis were evaluated by immunohistochemistry, histology or RT-qPCR. Cardiac function was measured by echocardiography. GARP exposure at platelet surface increased upon thrombin and CRP stimulation. Interestingly, GARP deficiency does not impair platelet function but dramatically reduces serum production of active TGFβ1. Following MI, platelets infiltrate the myocardium between 1 and 3 days, compared to the sham-operated mice. As expected, a decrease in collagen I and III mRNA content were measured in pGARP KO infarcts, at 7 days post-MI. We also revealed an increased in inflammatory cytokines and MMPs expression in pGARP KO infarcts, at 1-day post-infarction. Moreover, preliminary echocardiography data highlights the presence of LV pseudoaneurysm and increased LV dilation in the pGARP KO infarcted hearts. Our data confirms that platelet GARP protein is crucial for TGFβ1 activation. The early platelet infiltration into the infarcted myocardium seems to be associated with a modulation of the inflammatory process, which might impact the healing process. [ABSTRACT FROM AUTHOR]