Molecular Imaging of Mechanisms of Impaired Cardiac Repair After Macrophage Depletion in Mice: A Longitudinal Multitracer Imaging Study

Macrophage depletion before myocardial infarction (MI) in mice disrupts the inflammatory healing response and leads to left ventricular thrombus formation and tissue calcification. Here, we aimed to determine the effect of clodronate-loaded liposome-mediated macrophage depletion on C-C chemokine receptor 2 (CCR2)–expressing immune cells after MI in male and female mice. Further, we sought to discern the relationship of clodronate liposomes with thrombus calcification and functional outcome using multitracer imaging. <b>Methods:</b> Male and female C57BL/6N mice underwent 60-min ischemia/reperfusion MI or sham surgery after a single or repeated injection of clodronate liposomes for macrophage depletion or injection with liposomes containing phosphate-buffered saline. Macrophage depletion was verified in peripheral blood. PET imaging was performed to assess infarct CCR2 signal at 2 or 4 d after MI and fibroblast activation protein expression at 7 d after MI. Microcalcification was assessed by ¹⁸F-NaF PET/CT over 4 wk. Cardiac function was assessed at 6 wk by MRI. Histopathology validated inflammation and calcification in the heart. <b>Results:</b> Macrophage depletion significantly increased susceptibility to early death after MI, especially in male mice. Molecular imaging revealed an early increase in cardiac CCR2 content after MI compared with sham, which persisted despite macrophage depletion. Histology verified reduced CD68 cell content but sustained CCR2 signal that partially overlapped CD11c, a marker of dendritic cells among other cell types. Ly6G-positive cell content increased in the myocardium of macrophage-depleted mice after MI but did not overlap with CCR2. All macrophage-depleted male mice and 64% of macrophage-depleted female mice developed a dense intracavity thrombus overlying endocardial damage, with gradual microcalcification starting from 2 wk and reaching a maximum at 4 wk after MI. Macrophage depletion did not impact infarct size or contractile function at 6 wk after MI in surviving mice. <b>Conclusion:</b> Macrophage depletion before MI elevated the risk of acute death, particularly in male mice. A persistent CCR2 PET signal suggested that non–CD68-positive immune cells contribute to tracer substrate and altered proportions of other immune cell subtypes during healing. These findings suggest that extreme antiinflammatory interventions after MI may have unintended consequences for scar formation and remodeling, which may be monitored by molecular imaging.