Macrophage depletion impairs adequate cardiac repair in mouse models of myocardial infarction with variable transmurality - insights from multimodality molecular imaging

Abstract

\n \n \n Type of funding sources: Public grant(s) – National budget only. Main funding source(s): Deutsche Forschungsgemeinschaft (DFG)\n \n \n \n Balanced myocardial tissue inflammation following acute myocardial infarction (MI) is needed for optimal cardiac repair. Macrophages contribute to wound healing, but may also be deleterious.\n \n \n \n We investigated the impact of macrophage depletion on early cardiac inflammation and later functional outcome in two models of MI with variable transmurality.\n \n \n \n C57BL/6N mice received clodronate-liposomes for macrophage depletion (n\u2009=\u200949) or control PBS-liposomes (n\u2009=\u200923). After 24h, mice underwent permanent occlusion (PO) or transient ischemia-reperfusion (I/R, 60min) of the left coronary artery or sham surgery. Cardiac inflammation was assessed on MI\u2009+\u20091d, 3d, and 7d by CXCR4-targeted PET/CT using 68Ga-pentixafor. 99mTc-sestamibi SPECT/CT and cardiac magnetic resonance (CMR) calculated infarct sizes and left ventricular (LV) function at 1wk and 6wks. 18F-NaF PET/CT measured tissue microcalcification at 4wks. Imaging signals were validated by ex vivo autoradiography and immunohistochemistry.\n \n \n \n Clodronate macrophage depletion did not affect infarct size compared to PBS, but perfusion defects at 6wks were significantly larger after PO compared to I/R (%LV, 32\u2009±\u200911 vs 14\u2009±\u200910, p\u2009=\u20090.01). In both models, infarct CXCR4 expression was higher after macrophage depletion vs PBS at all timepoints (%injected dose (ID)/g; d3: PO: 1.4\u2009±\u20090.2 vs 0.9\u2009±\u20090.1; I/R: 1.4\u2009±\u20090.2 vs 1.0\u2009±\u20090.02; p\u2009<\u20090.05), and confirmed by ex vivo autoradiography. Immunostaining demonstrated fewer macrophages and higher neutrophil content within the myocardium after macrophage depletion vs PBS at 1d, 3d, and 7d post-MI. Acute LV rupture after PO was more frequent in macrophage-depleted than PBS mice (37% vs 17%). Conversely, surviving PO mice showed a similarly impaired ejection fraction (EF) after macrophage depletion vs PBS at 6wks (%, 32\u2009±\u20099 vs 32\u2009±\u200911, p\u2009=\u20090.84). No acute LV rupture was observed after I/R, but macrophage depletion led to worse EF (%, 42\u2009±\u200911 vs 54\u2009±\u20093, p\u2009=\u20090.1). Macrophage-depleted mice exhibited a dense intracavity thrombus adherent to the infarct wall after either injury, as visualized on CMR at 1wk. 18F-NaF PET identified active calcification localized to the thrombus region 4wks after MI, which was colocalized to CT opaque regions at 6wks.\n \n \n \n Macrophage depletion impairs cardiac repair via several mechanisms including neutrophil-dominated inflammation, LV thrombus formation and tissue calcification. This observation underscores the requirement of macrophages for effective healing and may explain adverse response to broad anti-inflammatory therapy in myocardial ischemia.\n