Studies of expression and possible functional role of purinergic receptors in cell-mediated immunity: Experimental approaches, controls, and caveats

Abstract

The expression of purinergic receptors for the physiologically abundant molecules of extracellular ATP (eATP) and e adenosine (Ado) in lymphoid cells of T-cell lineage has been demonstrated convincingly in short-term biochemical and pharmacologic assays. T-cell differentiation-related expression of P2X 7 receptors for ATP was demonstrated in murine T-cells, whereas A 2A adenosine receptors were shown to be responsible for observed increases in cAMP during incubation of T-cells with adenosine. The possibility of a feed-back regulation mechanism of T-cell functions with eATP is suggested by patterns of expression of purinergic receptors, which may follow the immediate early response genes. The functioning of adenosine receptors was suggested to be involved in the pathogenesis of human disease, adenosine deaminase severe combined immunodeficiency. Extracellular ATP and adenosine affect T-cell differentiation and effector functions in vitro; however, convincing evidence of purinergic modulation of immune response in vivo or in long-term immunoassays in vitro is still lacking. An explanation of the effects of eATP and eAdo on T-lymphocytes' differentiation and effector functions must include considerations of (1) possible intracellular effects of ATP, adenosine, and/or their metabolites; 2) the effects of eAdo- and eATP-triggered transmembrane signaling through P1 and P2 classes of purinergic receptors, respectively; and (3) the reversible phosphorylation of extracellular domains of functionally important cell surface proteins by ATP. The use of existing agonists and antagonists of purinergic receptors and of ATP and of adenosine-hydrolyzing enzymes has some limited mechanistic implications, but it may lead to artifacts due to uncertainties between extracellular and intracellular effects of these reagents and the possible presence of contaminants in commercial preparations of enzymes.