Laura M. McLane

Exhausted CD8 T (Tex) cells are a distinct cell lineage that arise during chronic infections and cancers in animal models and humans. Tex cells are characterized by progressive loss of effector functions, high and sustained inhibitory receptor expression, metabolic dysregulation, poor memory recall and homeostatic self-renewal, and distinct transcriptional and epigenetic programs. The ability to reinvigorate Tex cells through inhibitory receptor blockade, such as αPD-1, highlights the therapeutic potential of targeting this population. Emerging insights into the mechanisms of exhaustion are informing immunotherapies for cancer and chronic infections. However, like other immune cells, Tex cells are heterogeneous and include progenitor and terminal subsets with unique characteristics and responses to checkpoint blockade. Here, we review our current understanding of Tex cell biology, including the developmental paths, transcriptional and epigenetic features, and cell intrinsic and extrinsic factors contributing to exhaustion and how this knowledge may inform therapeutic targeting of Tex cells in chronic infections, autoimmunity, and cancer.

The T-box transcription factors T-bet and Eomesodermin (Eomes) have been well defined as key drivers of immune cell development and cytolytic function. While the majority of studies have defined the roles of these factors in the context of murine T-cells, recent results have revealed that T-bet, and possibly Eomes, are expressed in other immune cell subsets. To date, the expression patterns of these factors in subsets of human peripheral blood mononuclear cells beyond T-cells remain relatively uncharacterized. In this study, we used multiparametric flow cytometry to characterize T-bet and Eomes expression in major human blood cell subsets, including total CD4(+) and CD8(+) T-cells, γδ T-cells, invariant NKT cells, natural killer cells, B-cells, and dendritic cells. Our studies identified novel cell subsets that express T-bet and Eomes and raise implications for their possible functions in the context of other human immune cell subsets besides their well-known roles in T-cells.

In mice, two T-box transcription factors, T-box expressed in T cells (T-bet) and eomesodermin (Eomes), drive the differentiation of CD8 T cell lineages; however, little is known regarding their role in human CD8 T cell differentiation. In this study, we characterized T-bet and Eomes expression and localization within human CD8 memory T cell populations. We find that T-bet and Eomes are broadly expressed in human memory CD8 T cells, with increasing levels of T-bet and Eomes strongly correlating with differentiation from central memory to effector memory and effector subpopulations. In resting T cells, T-bet levels directly correlate to subcellular localization, with a higher propensity for nuclear expression of T-bet within T-bet(hi) cells and predominantly cytoplasmic expression in T-bet(lo) cells. In addition, Eomes is also localized to either the nucleus or the cytoplasm. Upon TCR stimulation, the percentage of T cells that express T-bet dramatically increases, whereas the percentage of cells expressing Eomes remains largely unchanged across all memory populations. Of interest, T-bet, but not Eomes, relocalizes to the nucleus in the majority of cells across all populations within 24 h post stimulation. These data indicate that T-bet and Eomes are likely regulated at the level of subcellular localization, potentially via different mechanisms. Together, these findings suggest a novel model for CD8 T cell differentiation in humans that is based on the localization of T-bet and Eomes.