CD8 T-cell heterogeneity during T-cell exhaustion and PD-1-targeted immunotherapy

Persistent antigenic stimulation results in loss of effector function or physical deletion of antigen-specific CD8 T cells. This T-cell state is called T-cell exhaustion and occurs during chronic infection and cancer. Antigen-specific CD8 T cells during T-cell exhaustion express the inhibitory receptor PD-1, the expression of which plays a major role in T-cell dysfunction. PD-1 blockade re-invigorates CD8 T-cell immunity and has been proven effective against many different types of human cancer. To further improve the efficacy of PD-1-targeted immunotherapy in cancer patients, a better understanding of T-cell exhaustion is required. Recent studies have revealed that antigen-specific CD8 T cells during T-cell exhaustion are heterogeneous and have also uncovered the detailed mechanisms for PD-1-targeted immunotherapy. Here, we review the CD8 T-cell subsets that arise during T-cell exhaustion, the lineage relationship among these individual subsets and the role of each subset in PD-1 blockade. Also, we discuss potential strategies to enhance the efficacy of PD-1-targeted immunotherapy.