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Most developing thymocytes (95%) undergo apoptosis, but in the steady state only 1-2% are detectable as apoptotic (4,5). It has been speculated that dying thymocytes secrete soluble factors that attract [...], 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.sub.2] as a critical sensor of nucleotides released by apoptotic cells using RNA interference-mediated depletion studies in monocytes, and macrophages from [P2Y.sub.2]-null mice (3). The relevance of nucleotides in apoptotic cell clearance in vivo was revealed by two approaches. First, in a murine air-pouch model, apoptotic cell supernatants induced a threefold greater recruitment of monocytes and macrophages than supernatants from healthy cells did; this recruitment was abolished by depletion of nucleotides and was significantly decreased in [P2Y.sub.2.sup.-/-] (also known as [P2ry.sub.2.sup.-/-]) mice. Second, clearance of apoptotic thymocytes was significantly impaired by either depletion of nucleotides or interference with P2Y receptor function (by pharmacological inhibition or in [P2Y.sub.2.sup.-/-] mice). These results identify nucleotides as a critical find-me cue released by apoptotic cells to promote P2Y2-dependent recruitment of phagocytes, and provide evidence for a clear relationship between a find-me signal and efficient corpse clearance in vivo.

A study conducted on polarized human nasal epithelial (HNE) cells shows that the release of intracellular Ca2+ is controlled by the stimulation of the P2 purinoceptors on the ipsilateral membrane. Treatment of HNE with ATP stimulates the P2 receptors, which are linked to phospholipase C, on the ipsilateral but not on the contralateral side of the membrane. The results suggest that the release of Ca2+ in response to receptor stimulation is not controlled by inositol (1,4,5) trisphosphate.