Epicardium-Derived Cells Formed After Myocardial Injury Display Phagocytic Activity Permitting In Vivo Labeling and Tracking.

Epicardium-derived cells (EPDCs) cover the heart surface and can function as a source of both progenitor cells and trophic factors for cardiac repair. Currently, EPDCs cannot be conveniently labeled in vivo to permit imaging and cell tracking. EPDCs formed after myocardial infarction (MI) preferentially take up a perfluorocarbon-containing nanoemulsion (PFC-NE; 130 +/- 32 nm) injected 3 days after injury, as measured by F-19-magnetic resonance imaging (F-19-MRI). Flow cytometry, immune electron microscopy, and green fluorescent protein (GFP)-transgenic rats (only immune cells, but not epicardial cells, are GFP(+)) demonstrated that PFC-containing EPDCs are nonhematopoietic (CD45(-)/CD11b(-)) but stain positive for markers of mesenchymal stem cells such as platelet-derived growth factor receptor alpha (PDGFR-alpha) CD73, CD105, and CD90. When rhodamine-coupled PFC-NE was used, we found that rho(+) vessel-like structures formed within the infarcted myocardium, comprising approximately 10% of a Marge vessels positive for smooth muscle actin (SM-actin). The epicardial cell layer, positive for Wilms' tumor 1 (WT-1), PDGFR-alpha, or KI-67, was shown to be well capillarized (293 +/- 78 capillaries per mm(2)), including fenestrated endothelium. Freshly isolated EPDCs were positive for WT-1, GATA-4, KI-67, and FLK-1 (75%), PDGFR-alpha (50%), and SM-actin (28%) and also exhibited a high capacity for nanoparticle and cell debris uptake. This study demonstrates that EPDCs formed after MI display strong endocytic activity to take up i. v.-injected labeled nanoemulsions. This feature permitted in vivo labeling and tracking of EPDCs, demonstrating their role in myo- and vasculogenesis. The newly discovered endocytic activity permits in vivo imaging of EPDCs with F-19-MRI and may be used for the liposomal delivery of substances to further study their reparative potential.