Macrophage Recognition and Phagocytosis of Apoptotic Fibroblasts Is Critically Dependent on Fibroblast-Derived Thrombospondin 1 and CD36

The induction of fibroblast apoptosis and their clearance by phagocytes is essential for normal wound healing and prevention of scarring. However, little is known about the clearance of apoptotic fibroblasts and whether apoptotic cells are active participants in the recruitment and activation of phagocytes. In this study, we provide the first evidence that apoptotic fibroblasts actively release increased amounts of thrombospondin (TSP1) to actively recruit macrophages. Expression of TSP1 and its receptor CD36 was increased on the surface of apoptotic fibroblasts. By chemical cross-linking and immunoprecipitation we show that TSP1 and CD36 were directly associated. This was confirmed by confocal microscopy. Blockade of either CD36 or TSP1 on apoptotic fibroblasts inhibited phagocytosis. Blockade of alpha v beta 3 integrins as well as CD36 and TSP1 on macrophages inhibited phagocytosis. In contrast, phosphatidylserine or lectins were not involved. These findings suggest that apoptotic fibroblasts release TSP1 as a signal to recruit macrophages while the up-regulated expression of the CD36/TSP1 complex on their cell surface may form a ligand bridging the fibroblast to a complex consisting of alpha v beta 3/CD36/TSP1 on macrophages. These results establish fundamental mechanisms for the clearance of apoptotic fibroblasts and may provide insights into the processes involved in normal wound repair.