Macrophage depletion delays infiltration of inflammatory macrophages into the infarct area after myocardial infarction in mice

Abstract Background/Introduction Myeloid-derived inflammatory macrophages expressing CCR2 are crucial for scar formation and repair after myocardial infarction (MI). Interference with macrophage dynamics impairs healing and promotes intraventricular thrombus formation, but the precise dynamics of CCR2 macrophages in the left ventricle (LV) under these conditions remain unclear. Purpose We investigated the impact of macrophage depletion on infarct CCR2 expression, microcalcification, and cardiac functional outcome in mice after reperfused MI. Methods Male and female C57BL/6N mice underwent 60 min ischemia/reperfusion MI (n=24) or sham (n=8) 24h after injection of clodronate-liposomes (n=8m, 8f) for macrophage depletion, or PBS-liposomes (n=8m, 8f). Infarct CCR2 expression was measured on MI+4d by 68Ga-ECL1i PET. Microcalcification was assessed by 18F-NaF PET/CT at 4wks. MRI and 99mTc-tetrofosmin SPECT calculated LV function and infarct sizes at 6wks. Results Acute mortality was markedly higher after macrophage depletion within 7d of MI compared to PBS (%, 37.5 vs 0). Infarct CCR2 expression was significantly increased in macrophage depletion MI vs sham on d4 (%injected dose (ID)/g; 0.39±0.13 vs 0.25±0.05; p=0.005), but similar between PBS MI and sham (ID/g; 0.31±0.06, p=0.1), indicating rapid repopulation of peripheral monocytes after single time macrophage depletion, and delayed infiltration into injured myocardium. There were no sex dependent differences in infarct CCR2 expression. Infarct size after MI was 25.9±9.3 %LV with no difference between treatments. LV ejection fraction at 6wks was significantly reduced after MI vs sham, but comparable between treatments (%; clodronate: 33±9 vs PBS: 37±8 vs sham 66±5, p<0.001). Heart weight at 6wks was significantly increased only in macrophage depleted MI compared to sham (heart weight/body weight (mg/g); 5.5±0.8 vs 4.6±0.4, p=0.02). Male macrophage-depleted mice and 50% of female mice exhibited a dense intracavity thrombus adherent to the infarct wall after MI, which showed significantly higher 18F-NaF uptake at 4wks compared to PBS MI (%ID/g; 6.0±4.9 vs 0.4±0.3; p<0.001), highlighting necessity of timely infiltration of the infarct with inflammatory macrophages for adequate cardiac repair. Conclusion By 4d after reperfused MI, the myocardium exhibits signs of inflammation resolution and decline of CCR2+ cell content in mice. Single time macrophage depletion prior to MI delays but does not prevent infiltration of CCR2+ inflammatory macrophages into infarct territories. Male mice are at higher risk of LV thrombus formation, demonstrating sex dependent differences in cardiac repair, which seem to be independent from the amount of CCR+ macrophages.