Inhibition of antigen receptor-dependent Ca2+ signals and NF-AT activation by P2X7 receptors in human B lymphocytes

Abstract

One of the first intracellular signals after antigen binding by the antigen receptor of B lymphocytes is the increased intracellular Ca2+ concentration ([Ca2+]i), which is followed by several intracellular signaling events like the nuclear translocation of the transcription factor NF-AT controlling the fate of B lymphocytes after their activation. Extracellular ATP, which is released from cells under several pathological conditions, is considered a danger-associated signal serving as an immunomodulator. We investigated the interaction of antigen receptor (BCR) and P2X7 receptor (P2X7R) activation on [Ca2+]i signaling and on nuclear translocation of the transcription factor NF-AT in human B lymphocytes. Although the P2X7R is an ATP-gated Ca2+-permeable ion channel, P2X7R activation inhibits the BCR-mediated [Ca2+]i responses. This effect is mimicked by cell membrane depolarization induced by an increase in the extracellular K+ concentration or by application of the Na+ ionophore gramicidin, but is abolished by stabilization of the membrane potential using the K+ ionophore valinomycin, by extracellular Mg2+, which is known to inhibit P2X7R-dependent effects, or by replacing Na+ by the less P2X7R-permeable Tris+ ion. Furthermore, P2X7R activation by ATP inhibits the BCR-dependent translocation of the transcription factor NF-ATc1 to the nucleus. We therefore conclude that extracellular ATP via the P2X7R mediates inhibitory effects on B cell activation. This may be of relevance for understanding of the activation of the BCR under pathological conditions and for the development of therapeutic strategies targeting human B lymphocytes or P2X7 receptors.