Ionic immune suppression within the tumour microenvironment limits T cell effector function

Tumours progress despite being infiltrated by effector T cells. Tumour necrosis is associated with poor survival in a variety of cancers. Here, we report that that necrosis causes release of an intracellular ion, potassium, into the extracellular fluid of human and mouse tumours. Surprisingly, elevated extracellular potassium ([K+]e) was sufficient to profoundly suppress mouse and human T cell anti-tumour function. Elevations in [K+]e acted to acutely impair T cell receptor (TCR) dependent Akt-mTOR phosphorylation and effector function. Potassium mediated suppression of Akt-mTOR signalling and T cell effector function required intact activity of PP2A, a serine/threonine phosphatase. The suppressive effect mediated by elevated [K+]e required a T cell-intrinsic increase in intracellular potassium ([K+]i) and was independent of changes in plasma membrane potential (Vm). Finally, ionic reprogramming of tumour-specific T cells via over-expression of the voltage-gated potassium channel Kv1.3 lowered [K+]i and improved effector functions in vitro and in vivo, with this gain of function being dependent on intact channel function. Consequently, Kv1.3 T cell expression enhanced tumour clearance and the survival of melanoma-bearing mice. These results uncover a previously undescribed ionic checkpoint against T cell function within tumours and identify new strategies for cancer immunotherapy. Overall design: RNA expression was measured by RNA-Seq on day 5 of cultures, maintained in individual biologial triplicates which were stimulated with immobilized anti-CD3/28 antibodies or kept in complete media (no stim) - with equivalent conditions treated with isotonic media containing elevated potassium.