Nucleotides released by apoptotic cells act as a find-me signal to promote phagocytic clearance

Apoptosis: how cells become targets Apoptosis occurs in essentially all tissues as part of normal development and homeostasis. Yet even in tissues with high cellular turnover, apoptotic cells are rarely seen; this has been attributed to the ability of apoptotic cells to advertise their presence via release of 'find-me' signals to recruit phagocytes and initiate prompt clearance. It has been unclear, however, what type of find-me signals are released by apoptotic cells and how these are sensed by phagocytes. In this paper apoptotic cells are shown to release ATP and UTP that act as a 'find me ' signal and chemoattractant for phagocytes expressing the P2Y 2 ATP/UTP receptor. The efficient removal of apoptotic cells in vivo is thought to be due to the release of 'find-me' signals by apoptotic cells that recruit motile phagocytes. Here, the caspase-dependent release of ATP and UTP during the early stages of apoptosis is demonstrated. ATP and UTP are found to act as chemoattractants in a process mediated through the ATP/UTP receptor P2Y 2 , which is present on monocytes and macrophages. Phagocytic removal of apoptotic cells occurs efficiently in vivo such that even in tissues with significant apoptosis, very few apoptotic cells are detectable 1 . This is thought to be due to the release of ‘find-me’ signals by apoptotic cells that recruit motile phagocytes such as monocytes, macrophages and dendritic cells, leading to the prompt clearance of the dying cells 2 . However, the identity and in vivo relevance of such find-me signals are not well understood. Here, through several lines of evidence, we identify extracellular nucleotides as a critical apoptotic cell find-me signal. We demonstrate the caspase-dependent release of ATP and UTP (in equimolar quantities) during the early stages of apoptosis by primary thymocytes and cell lines. Purified nucleotides at these concentrations were sufficient to induce monocyte recruitment comparable to that of apoptotic cell supernatants. Enzymatic removal of ATP and UTP (by apyrase or the expression of ectopic CD39) abrogated the ability of apoptotic cell supernatants to recruit monocytes in vitro and in vivo . We then identified the ATP/UTP receptor P2Y 2 as a critical sensor of nucleotides released by apoptotic cells using RNA interference-mediated depletion studies in monocytes, and macrophages from P2Y 2 -null mice 3 . The relevance of nucleotides in apoptotic cell clearance in vivo was revealed by two approaches. First, i