Cardiac Resident Macrophage-derived Legumain Improves Cardiac Repair via Promoting Clearance and Degradation of Apoptotic Cardiomyocytes after Myocardial Infarction.

Background: Cardiac resident macrophages are self-maintaining that originate from embryonic hematopoiesis. After myocardial infarction (MI), cardiac resident macrophages are responsible for the efficient clearance and degradation of apoptotic cardiomyocytes (efferocytosis). This process is required for inflammation resolution and tissue repair; however, the underlying molecular mechanisms remain unknown. Therefore, we aimed to identify the mechanisms of the continued clearance and degradation of phagolysosomal cargo by cardiac resident macrophages during MI. Methods: Multiple transgenic mice such us Lgmn-/-, Lgmn F/F; LysMCre, LgmnF/F; Cx3cr1CreER, LgmnF/F; LyveCre, and cardiac macrophage Lgmn overexpression by adenovirus gene transfer were used to determine the functional significance of Lgmn in MI. Immune cell filtration and inflammation were examined by flow cytometry and quantitative real-time polymerase chain reaction (qPCR). Moreover, Lgmn expression was analyzed by immunohistochemistry and qPCR in the cardiac tissues of patients with ischemic cardiomyopathy and healthy controls. Results: We identified legumain (Lgmn) as a gene specifically expressed by cardiac resident macrophages. Lgmn deficiency resulted in a considerable exacerbation in cardiac function, accompanied with the accumulation of apoptotic cardiomyocytes and a reduced index of in vivo efferocytosis in the border area. It also led to decreased cytosolic calcium due to defective intracellular calcium mobilization. Furthermore, the formation of LC3-II-dependent phagosome around secondary-encountered apoptotic cardiomyocytes was disabled. In addition, Lgmn deficiency increased infiltration of MHC-IIhigh CCR2+ macrophages and the enhanced recruitment of MHC-IIlow CCR2+ monocytes with downregulation of anti-inflammatory mediators, IL-10 and TGF-β; and upregulation proinflammatory mediators, IL-1β, TNF-α, IL-6, and IFN-γ. Conclusions: Our results directly link efferocytosis to wound healing in the heart and identify Lgmn as a significant link between acute inflammation resolution and organ function.