Extracellular ATP (eATP), the most abundant among nucleotides, can act as a mediator during inflammatory responses by binding to plasmamembrane P2 purinergic receptors, which are widely expressed on cells of the immune system. eATP is generally considered as a classical danger signal, which stimulates immune responses in the presence of tissue damage. Converging evidence from several studies using murine models of chronic inflammation have supported this hypothesis; however, the role of eATP in the regulation of human immune function appears to be more complex. Chronic stimulation with micromolar eATP concentrations inhibits the proliferation of T and NK lymphocytes and enhances the capacity of dendritic cells to promote tolerance. The effect of eATP depends on multiple factors, such as the extent of stimulation, eATP concentration, presence/absence of other mediators in the microenvironment, and pattern of P2 receptor engagement.Small but significant differences in the pattern of P2 receptor expression in mice and humans confer the diverse capacities of ATP in regulating the immune response. Such diversity, which is often overlooked, should therefore be carefully considered when evaluating the role of eATP in human inflammatory and autoimmune diseases. (Blood. 2012;120(3): 511-518)
Sources of eATPNucleotides are the constituents of nucleic acids, represent the energy store of the cell, and are involved in intracellular signaling as well as in cell-to-cell communication. 1 During the past 2 decades, increasing evidence has shown that extracellular ATP (eATP) is an important immune modulator. 2 ATP is present at relatively high levels in the cytoplasm of cells, where its concentration ranges from 1 to 10mM. In the extracellular space, its physiologic concentration is considerably lower, ranging between 1 and 10nM. Because of the steep concentration gradient as well as its small size and high mobility, ATP can be rapidly released together with other cellular components after mechanical stress, cell damage, or death. Increased ATP concentration in the extracellular milieu is therefore closely associated with tissue stress or damage. [3][4][5][6] However, nonlytic nucleotide release may occur in many cell types under a variety of conditions. Living cells in the steady-state release ATP through passive leakage. 7,8 Activated platelets represent a relevant source of ATP and release the nucleotide concomitantly with several inflammatory mediators during clot formation. 9 T lymphocytes release ATP during the early stages of activation through pannexin 1 (panx1) channels. 10 In addition, ATP is released during exercise from skeletal muscle as well as from vascular endothelial cells during conditions of increased blood flow or on mechanical stimulus. 11-13 Moreover, nonlytic ATP secretion from endothelial cells and leukocytes may be induced by pathogen-associated molecules, such as lipopolysaccharide. 14-17 ATP is also increased in draining lymph nodes during a contact hypersensitivity reaction. 18 ATP is released during ...