Monocytic Clearance of Dying Cardiomyocytes by CD36 following Myocardial Infarction

Background. During Myocardial Infarction (MI), occlusion of a cardiac artery causes ischemic damage and cardiomyocyte death. Efficient clearance of myocardial apoptosis is a prerequisite for inflammation resolution and tissue repair after MI. Infarction triggers the early recruitment of Ly6CHI monocytes, which transition into resolving Ly6CLO macrophages. Previous studies have linked key apoptotic cell receptors, such as MerTK on cardiac Ly6cLO macrophages, to be required for clearance of dying cardiomyocytes. The contribution and molecular identities of clearance receptors on precursor Ly6CHI monocytes, as well as the factors that regulate Ly6CHI to LO transition, remain unknown. Methods. Candidate phagocytic receptors were phenotyped on infiltrating cardiac monocytes during experimental MI. Causal requirements of the clearance receptor CD36 on infract size, Ly6CLO macrophage differentiation and inflammation resolution was tested via bone marrow transplant from CD36 deficient donors. Results. Flow cytometric analysis of candidate engulfment receptors revealed the scavenger receptor CD36 to be elevated on recruited cardiac Ly6CHI monocytes day 1 post MI. In vivo, comparisons between WT vs CD36 -/- bone marrow recipients revealed myeloid specific requirements for CD36 on limiting infarct size, specifically during the Ly6CHI-stage of cardiac inflammation. Monocyte CD36 was also required for phagocytosis of dying mCherry+ cardiomyocytes (CMs). Decreased cardiomyocyte uptake was associated with increased ratio of Ly6CHI:Ly6CLO cells during the reparative phase (3-7 days post MI) in CD36 knockouts. In these cells, we also found decreased expression of the master regulator of Ly6CLO differentiation, Nr4a1. Nr4a1 was also shown to be a direct regulator of MerTK in macrophages in vitro and in vivo. Finally, to study the total impact of efferocytosis in the heart, CD36/MerTK double knockouts had compromised phagocytic clearance in both Ly6cHI and Ly6cLO phases of cardiac inflammation with an increased incidence of myocardial rupture. Conclusions. These data link CD36-mediated phagocytosis of dying cardiomyocytes to myocardial repair. Additionally, we identified monocytic efferocytosis to be a novel initiating signal for the transition to Ly6CLO-mediated macrophage cardiac repair.