CD8⁺ T Cells Keep Their (K⁺)urrency for Function

Abstract

CD8⁺ T-cell responses are influenced by ion abundance, which can widely vary within the tumor microenvironment. In this issue, Collier and colleagues investigated how intracellular versus extracellular potassium ion (K⁺) regulates intratumoral CD8⁺ Tcells. They show that,while excessive extracellular K⁺ induces exhaustion, intracellular K⁺ is needed for protection from dysfunction. This work shows additional evidence that the regulation of CD8⁺ T-cell responses depends on a fine balance between intracellular and extracellular metabolite levels.

The advent of cytotoxic CD8⁺ T cell–based modern immunotherapies has revolutionized the landscape for cancer treatment, leading to improved prognosis for many different cancer types. However, a considerable proportion of patients do not develop long-term disease control. One of the main reasons behind this lack of durable protection may lie with intratumoral CD8⁺ T cells. CD8⁺ T cells can infiltrate tumors, but their long-term maintenance within the tumor microenvironment is only found in a small proportion of patients. Therefore, understanding the factors promoting the long-term function and survival of these cells is crucial to define how to improve anticancer therapies. Among these factors, exposure to extracellular metabolites such as ions has been increasingly appreciated as a crucial regulatory element for CD8⁺ T cells (1). Extracellular ion concentrations, however, are often tied with fluctuations in their intracellular concentrations.

In this issue, Collier and colleagues used a combination of mouse models and human cell in vitro studies to define how intracellular versus extracellular concentrations of one ion, potassium (K⁺), would affect the functional state of CD8⁺ T cells (2). The authors had previously found that increased intratumoral levels of extracellular K⁺ suppress CD8⁺ T-cell responses, through inhibition of T cell receptor (TCR)–associated downstream signaling pathways (3). They confirmed these findings in this current work. Because of the immunosuppressive effect of extracellular K⁺, a reasonable assumption would be that lowering intracellular K⁺ would boost CD8⁺ T-cell function. To test this hypothesis, genetic ablation of the ATPase Na⁺/K⁺ transporter (through targeting Atp1a1) was done in CD8⁺ T cells—because those transporters are abundantly expressed in intratumoral CD8⁺ T cells. Unexpectedly, lowering intracellular K⁺ through Atp1a1-KO led to accelerated terminal differentiation and dysfunction. This was associated with tonic hyperactivity in multiple downstream pathways associated with T-cell priming. As a result, Atp1a1-KO CD8⁺ T cells had constitutively higher accumulation of intracellular reactive oxygen species (ROS), and a subsequent higher susceptibility to cell death and lower effector function. Indeed, CD8⁺ T cells lacking Atp1a1 were less efficient in surviving in the tumor microenvironment and in protecting against tumor growth. In addition, the authors found that reverting the Atp1a1KO–induced phenotype with in vitro addition of exogenous K⁺ was sufficient to induce protection from dysfunction, with the same being observed in the presence of antioxidant treatments such as N-acetyl cysteine (NAC). Surprisingly, though, inhibition of ROS accumulation was not sufficient to restore the functionality of intratumoral T cells in vivo. As the authors pointed out, this may suggest that the major function of intracellular K⁺ is to prevent tonic TCR signaling. Further exploring a possible divergence of effects of K⁺ on ROS levels versus TCR signaling may shed light on context-dependent effects of K⁺ maintenance. It will also be important to define how the decay in intracellular K⁺ influences the epigenetic changes necessary for the acquisition of the exhaustion program (4).

Thus, overall, Collier and colleagues provide evidence that intracellular K⁺ levels can directly impact intracellular ROS accumulation. It will be interesting, in future studies, to assess whether intracellular K⁺ levels must decrease to favor effector function of T cells. Similar studies in reactivated memory T cells will also be research worthy. Another noteworthy observation is that Na⁺/K⁺ ATPases modulate the intracellular environment beyond simply importing K⁺. Therefore, the effects of Atp1a1-KO may be related to these other changes.

This work indicates that maintenance of stemness and resistance to exhaustion are directly linked to restraint of downstream TCR signaling pathways and that intracellular ion concentrations (K⁺) can contribute to this negative regulation. It also provides evidence that the fine balance between extracellular and intracellular metabolites is crucial to define the outcome of CD8⁺ T-cell responses. Moreover, it shows the complexity of such interactions, as an example of a metabolite where the effects of extracellular versus intracellular location are opposite.