Fetuin/α2-HS glycoprotein enhances phagocytosis of apoptotic cells and macropinocytosis by human macrophages

Inflammatory diseases are associated with reduced serum concentrations of α₂-HS glycoprotein (the human homologue of bovine fetuin), but the role of fetuin in inflammation is poorly understood. We hypothesized that fetuin may influence the resolution of inflammation by modulating the phagocytosis of apoptotic cells by macrophages. Using an in vitro flow cytometry-based phagocytosis assay, we investigated the role of fetuin in apoptotic cell clearance. Bovine fetuin and human α₂-HS glycoprotein significantly augmented the phagocytosis of apoptotic cells by human peripheral blood monocyte-derived macrophages, whereas the control proteins BSA, sialylated BSA and asialofetuin were ineffective. The enhancement of phagocytosis was concentration-dependent, and required the presence of intact fetuin at the time of interaction between macrophages and apoptotic cells. Fetuin also substantially increased the uptake of labelled dextran 70000 by macrophages, which occurs by macropinocytosis, suggesting that this may be one of the mechanisms utilized for apoptotic cell uptake.