3365 Research Article Introduction Macrophages are responsible for the maintenance of tissue homeostasis and for orchestrating defence responses and healing processes in adult tissues (Hume, 2006). One of the major functions of macrophages is the clearance of exogenous non-self as well as unwanted-self components present in the extracellular environment. The selectivity of ligand recognition is a result of a highly specific pattern of scavenger receptor expression by resident tissue macrophages and by newly recruited monocytes undergoing differentiation to macrophages (Gordon, 2002). A distinction between apoptotic cells (unwanted-self) and microbial pathogens (non-self) must be made because the clearance of apoptotic cells by phagocytes occurs in the absence of inflammatory responses (Savill et al., 2002). Various subpopulations of alternatively activated (M2) macrophages express molecular repertoires leading to tolerance and the resolution of inflammation, and display a high phagocytic capacity (Gratchev et al., 2005). Using our in vitro model for alternatively activated macrophages we demonstrated that combination of interleukin-4 (IL-4) and dexamethasone stimulates the phagocytic capacity of macrophages and induces expression of scavenger receptors including CD163, MARCO, and stabilin-1 (Gratchev et al., 2005; Kzhyshkowska et al., 2006c; Politz et al., 2002). In vivo stabilin-1 is expressed by cells specialized in the clearance of unwanted-self and in the maintenance of tissue homeostasis, including different subsets of tissue macrophages as well as non-continuous endothelial cells in lymph nodes, liver, spleen and bone marrow (Goerdt et al., 1993; Goerdt and Orfanos, 1999; Goerdt et al., 1999; Martens et al., 2006; Politz et al., 2002). Stabilin-1 (also called FEEL-1 and CLEVER-1) and stabilin-2 (also called FEEL-2 and HARE) are type I transmembrane receptors with multiple functions (Kzhyshkowska et al., 2006a). They were initially identified as high molecular weight proteins produced by sinusoidal endothelial cells and have been reported to function as scavenger receptors. These proteins affect the endocytosis of acetylated low-density lipoprotein (acLDL) and advanced glycation end products (AGE) and have binding activities with Gram- negative and Gram-positive bacteria (Adachi and Tsujimoto, 2002; Kzhyshkowska et al., 2005; Tamura et al., 2003). They have also been reported to contribute to the endothelial adhesion of lymphocytes (Irjala et al., 2003; Jung et al., 2007; Salmi et al., 2004). Although stabilin-1 shares common features with stabilin-2, its function appears to be significantly different from that of stabilin- 2. For example, while stabilin-2 efficiently mediates the uptake of hyaluronic acid (HA) and AGE-modified proteins, stabilin-1 does not bind HA and binds AGE-modified proteins with a lower affinity. A recent study reported that stabilin-1 mediates the internalization Stabilin-1 is specifically expressed in alternatively activated macrophages. These macrophages participate in anti- inflammatory and healing processes, and display a high phagocytic capacity. In this study, we provide evidence that stabilin-1 is a membrane receptor that performs a crucial function in the clearance of cell corpses. Stabilin-1 is expressed on the cell surface of alternatively activated macrophages and is recruited to the sites of recognition and engulfment of apoptotic bodies, as well as to early phagosomes. Blocking stabilin-1 in macrophages results in defective engulfment of aged red blood cells. Ectopic expression of stabilin-1 induces the binding and engulfment of aged cells in mouse fibroblast L cells. The binding and phagocytosis are dependent on phosphatidylserine (PS), which is well known as an engulfing ligand. Furthermore, using PS-coated beads, we demonstrate that PS directly interacts with stabilin-1 and is sufficient for stabilin-1-mediated phagocytosis. EGF-like domain repeat in stabilin-1 is responsible for PS recognition and binding. Thus, our results demonstrate that stabilin-1, found on alternatively activated macrophages, is a phagocytic receptor mediating the clearance of apoptotic cells in a PS-dependent manner. Therefore, this protein might play an important role in the maintenance of tissue homeostasis and prevention of autoimmunity. Supplementary material available online at http://jcs.biologists.org/cgi/content/full/122/18/3365/DC1 Key words: Stabilin-1, Phagocytosis, Aged RBC, Apoptotic cells, Macrophages Summary Stabilin-1 mediates phosphatidylserine-dependent clearance of cell corpses in alternatively activated macrophages Seung-Yoon Park 2 , Mi-Yeon Jung 1 , Sung-Jin Lee 1 , Kae-Bok Kang 1 , Alexei Gratchev 3 , Vladimir Riabov 3 , Julia Kzhyshkowska 3,4, * and In-San Kim 1, * 1 Department of Biochemistry and Cell Biology, Cell and Matrix Research Institute, School of Medicine, Kyungpook National University, Daegu 700-422, Korea 2 Department of Biochemistry, School of Medicine, Dongguk University, Kyungju 780-714, Korea 3 Department of Dermatology, University Medical Centre Mannheim, Ruprecht-Karls University of Heidelberg, Mannheim, Germany 4 Institute of General Pathology and Pathophysiology, Russian Academy of Medical Sciences, Moscow, Russia *Authors for correspondence (julia.kzhyshkowska@haut.ma.uni-heidelberg.de; iskim@knu.ac.kr) Accepted 10 July 2009 Journal of Cell Science 122, 3365-3373 Published by The Company of Biologists 2009 doi:10.1242/jcs.049569 Journal of Cell Science