Abstract 1463: Ionic immune suppression within the tumor microenvironment limits T cell effector function
Authors: Eil Robert L. , Roychoudhuri Rahul , Sukumar Madhu , Clever David , Pan Jenny H. , Patel Shashank , Palmer Douglas C. , Restifo Nicholas P. 
Published: July 2016

Tumors progress in immunocompetent hosts despite the local infiltration of tumor-specific effector T cells. Recent advances have identified ion transport as important for T cell activation and function. We report that the concentration of potassium is markedly elevated within murine and human tumors in comparison to other tissues while other ions are minimally perturbed. Additionally, ex vivo stimulation of effector T cells in hyperkalemic conditions led to profound suppression of T cell activation, cytokine production, and glycolytic metabolism without affecting viability - suggesting a role for potassium in tumor immune evasion. Characterization of this phenomenon with whole-transcriptome RNA-Sequencing revealed that elevated potassium specifically repressed TCR induced transcripts with the effector cytokine interferon gamma (IFNγ) being among the most depressed. Moreover, hyperkalemia limited TCR induced glucose uptake and consumption, metabolic changes required for full effector function.

Prior investigations into the role of potassium transport in cellular physiology have centered on the presumption that the ion's sole function is to maintain cellular membrane potential and calcium influx. However, we find that elevated potassium had no effect on TCR induced calcium signaling. Although hyperkalemia increased the cytoplasmic membrane potential of effector T cells as expected, other methods to similarly depolarize the cell without the addition of extracellular potassium did not produce the inhibitory phenomenon. Therefore, we conclude that the immunosuppressive effect induced by hyperkalemia is independent of potassium's previously understood role in the regulation of cytoplasmic membrane potential and calcium signaling. Causatively, elevated potassium led to blunted serine/threonine phosphorylation within the Akt/mTOR signaling cascade following TCR ligation, while pharmacologic or genetic means to restore Akt signaling rescued effector function.

Mechanistically, the suppressive effect of elevated extracellular potassium was directly related to an increase in intracellular levels, and depletion of intracellular potassium restored full T cell activation and cytokine production. T cells genetically reprogrammed to express the potassium efflux channel Kcna3 were resistant to potassium mediated suppression - displaying lower intracellular potassium levels and increased cytokine production. Finally, tumor-specific Kcna3 expressing cells exhibited augmented in vivo intratumoral IFNγ production, Akt/mTOR signaling, and enhanced anti-tumor activity against established melanoma in tumor bearing mice. These results uncover a novel axis of tumor-induced immune suppression via an ‘ionic’ checkpoint. The manipulation of cellular ion transport represents an entirely new approach for cancer immunotherapy.

Citation Format: Robert L. Eil, Rahul Roychoudhuri, Madhu Sukumar, David Clever, Jenny H. Pan, Shashank Patel, Douglas C. Palmer, Nicholas P. Restifo. Ionic immune suppression within the tumor microenvironment limits T cell effector function. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1463.