Abstract 220: Critical Dependence on Timing of Cardiac Injection for Engraftment of Endothelial Progenitor Cells and Functional Benefit Post-Myocardial Infarction
BACKGROUND: Treatment of acute myocardial infarction (MI) with endothelial progenitor cells (EPC) has previously been shown to lead to improved cardiac function. However, cell retention and effects on cardiac function in relation to the timing of injection following MI remains largely undefined.
METHODS: Female F344 rats were subjected to LAD ligation and male bone marrow-derived early growth EPCs (1x106 cells) were injected into the peri-infarct zone at specific time points post-MI (PMI 1 hr, 7, 21, 35 days). Animals were sacrificed at 7 days post-cell injection (post-I) and cell engraftment was assessed by real-time, quantitative PCR using a Sry3 (Y chromosome) Taqman probe. Fluorescently-labelled EPCs injected into the peri-infarct zone (1 hr, 7d PMI) were visualized by whole organ imaging (Kodak 2000MM) to assess cell fate (15 min and 24 hr post-I).
RESULTS: Cardiac cell retention was maximal when delivered at 7d PMI (33% ±10 of injected cells, 7d post-I). Both earlier and later injection time points (i.e. 1 hr, 21, 35d PMI) resulted in minimal LV cell retention (0.52%± 0.06, 0.09% ± 0.05, 0.1% ± 0.04, respectively). Similarly, hemodynamic and echocardiographic assessment showed improvement only with injection at 7d PMI: LVEDP, 17±3 vs. 28±2 mmHg, p<0.05 and transmitral E/A ratio, 5.5±1 vs. 6.2±2, p=0.07 relative to control. Fluorescent imaging data revealed rapid redistribution of cells injected at 1 hr PMI to the lung within 15 min post-I (heart, 15% ±5.03 of injected cells; lung, 65% ±25), although a subsequent late homing of EPCs to the heart was seen at 24 hrs (heart, 24% ±2, p<0.05 relative to 15 min; lung 36% ±5). Relative to 1 hr PMI, cell injection 7d PMI resulted in higher cardiac retention and less lung redistribution at 15 min (81% ±15% and 32% ±6, respectively, p<0.05) with persistently higher cardiac retention but comparable lung accumulation at 24 hr post-I (32±4%, p<0.05; 52% ±16, p=NS, respectively).
CONCLUSIONS: The magnitude of EPC engraftment and improvement in cardiac function were critically dependent on injection timing subsequent to MI. These data demonstrate that the optimal time point of injection needs to be clearly defined for clinical trials of cell therapy following MI.