Mertk Receptor Mutation Reduces Efferocytosis Efficiency and Promotes Apoptotic Cell Accumulation and Plaque Necrosis in Atherosclerotic Lesions of Apoe −/− Mice

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Abstract
Objective— Atherosclerotic plaques that are prone to disruption and acute thrombotic vascular events are characterized by large necrotic cores. Necrotic cores result from the combination of macrophage apoptosis and defective phagocytic clearance (efferocytosis) of these apoptotic cells. We previously showed that macrophages with tyrosine kinase-defective Mertk receptor (Mertk KD ) have a defect in phagocytic clearance of apoptotic macrophages in vitro. Herein we test the hypothesis that the Mertk KD mutation would result in increased accumulation of apoptotic cells and promote necrotic core expansion in a mouse model of advanced atherosclerosis. Methods and Results— Mertk KD ; Apoe −/− mice and control Apoe −/− mice were fed a Western-type diet for 10 or 16 weeks, and aortic root lesions were analyzed for apoptosis and plaque necrosis. We found that the plaques of the Mertk KD ; Apoe −/− mice had a significant increase in terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL)-positive apoptotic cells. Most importantly, there were more non-macrophage-associated apoptotic cells in the Mertk KD lesions, consistent with defective efferocytosis. The more advanced (16-week) Mertk KD ; Apoe −/− plaques were more necrotic, consistent with a progression from apoptotic cell accumulation to plaque necrosis in the setting of a defective efferocytosis receptor. Conclusion— In a mouse model of advanced atherosclerosis, mutation of the phagocytic Mertk receptor promotes the accumulation of apoptotic cells and the formation of necrotic plaques. These data are consistent with the notion that a defect in an efferocytosis receptor can accelerate the progression of atherosclerosis and suggest a novel therapeutic target to prevent advanced plaque progression and its clinical consequences.