Cardioprotective function of cardiac macrophages

The heart is composed of several cell types including cardiomyocytes, cardiac fibroblasts, endothelial, and smooth muscle cells. In addition to these major cell types, cardiac macrophages are also present in small numbers under physiological conditions. Recently, the resident macrophage is considered to have vital functions in the maintenance of tissues and homeostasis in many organs, including brain, liver, adipose tissue, lymphatic tissue, and intestinal tract. However, detailed functions of the cardiac resident macrophage are not fully understood. Although the removal of debris arising from damaged cardiomyocytes and pro-inflammatory effects after heart injuries are conventional tasks of cardiac macrophages (classically activated macrophage or M1 macrophage), novel functions like anti-inflammatory roles, adaptive response, and tissue maintenance have also been reported in recent years. Macrophages that possess these novel functions are generally so-called M2 macrophages, which are alternatively activated and show anti-inflammatory phenotype under pathological conditions. In this review, we focus on the cardioprotective function of the cardiac macrophage and discuss in light of unveiled fundamental functions of macrophages that have been also found in other organs.