The current state and future of T-cell exhaustion research

Abstract

'Exhaustion' is a term used to describe a state of native and redirected T-cell hypo-responsiveness resulting from persistent antigen exposure during chronic viral infections or cancer. Although a well-established phenotype across mice and humans, exhaustion at the molecular level remains poorly defined and inconsistent across the literature. This is, in part, due to an overreliance on surface receptors to define these cells and explain exhaustive behaviours, an incomplete understanding of how exhaustion arises, and a lack of clarity over whether exhaustion is the same across contexts, e.g. chronic viral infections versus cancer. With the development of systems-based genetic approaches such as single-cell RNA-seq and CRISPR screens applied to in vivo data, we are moving closer to a consensus view of exhaustion, although understanding how it arises remains challenging given the difficulty in manipulating the in vivo setting. Accordingly, producing and studying exhausted T-cells ex vivo are burgeoning, allowing experiments to be conducted at scale up and with high throughput. Here, we first review what is currently known about T-cell exhaustion and how it's being studied. We then discuss how improvements in their method of isolation/production and examining the impact of different microenvironmental signals and cell interactions have now become an active area of research. Finally, we discuss what the future holds for the analysis of this physiological condition and, given the diversity of ways in which exhausted cells are now being generated, propose the adoption of a unified approach to clearly defining exhaustion using a set of metabolic-, epigenetic-, transcriptional-, and activation-based phenotypic markers, that we call 'M.E.T.A'.

Keywords: T-cell activation; T-cell exhaustion; immunotherapy; in vitro exhaustion; inhibitory receptors.

Figure 1.

The most common properties used to define exhausted T cells. The expression of inhibitory receptors, their effector functions, expression of specific transcription factors, metabolic activity and their epigenetic status are used to define exhaustion of T cells.

Figure 2.

Summary of the different approaches to generate/isolate exhausted T cells. The methods can be broadly divided into ex vivo and in vitro approaches. In an ex vivo approach exhausted T cells are either isolated from well-established mouse models of exhaustion or from human patients whereas in an in vitro approach, exhausted cells are generated through different activation regiments as summarized.