Self-Regulation of Memory CD8 T Cell Metabolism through Extracellular ATP Signaling.

Following activation, CD8 T cells transition from reliance on mitochondrial respiration to increasing utilization of aerobic glycolysis. After the effector phase, however, reversion to mitochondrial metabolism is pivotal generating memory CD8 T cells. We recently showed that sensing of extracellular ATP (eATP) through the receptor P2RX7 is crucial for both production and the long-term survival of memory CD8 T cells, evidently through promoting mitochondrial maintenance. Unexpectedly, these results indicated that sustained P2RX7 activation is required for memory CD8 T cell homeostasis, suggesting constant exposure to eATP, in contrast with the proposed role of eATP as an acute "danger" signal released by dying cells. Active release through transmembrane channels is another path for eATP export. Indeed, CD8 T cells express Pannexin 1 (Panx1) which has a reported eATP release function in vitro and is itself induced by P2RX7 and/or TCR engagement. Such a role for Panx1 could potentially provide a feed-forward mechanism for cell-autonomous P2RX7 signaling. This model envisages that memory CD8 T cells maintain themselves at the cost of reduced intracellular ATP levels, which at first glance would seem to be detrimental for sustained T cell maintenance. On the other hand, the need to tightly regulate levels of intracellular ATP may be critical for the durability and adaptability of memory CD8 T cells, hence engagement of the P2RX7/Panx1 axis may allow these cells to fine tune their metabolic status to meet changing demands. In this Perspective, we discuss how this pathway may influence memory T cell maintenance.