A paradigm shift in the understanding of the exhausted CD8+ T cell (Tex) lineage is underway. Originally thought to be a uniform population that progressively loses effector function in response to persistent antigen, single-cell analysis has now revealed that CD8+ Tex is composed of multiple interconnected subpopulations. The heterogeneity within the CD8+ Tex lineage is comprised of immune checkpoint blockade (ICB) permissive and refractory subsets termed stem-like and terminally differentiated cells, respectively. These populations occupy distinct peripheral and intratumoral niches and are characterized by transcriptional processes that govern transitions between cell states. This review presents key findings in the field to construct an updated view of the spatial, transcriptional, and functional heterogeneity of anti-tumoral CD8+ Tex. These emerging insights broadly call for (re-)focusing cancer immunotherapies to center on the driver mechanism(s) underlying the CD8+ Tex developmental continuum aimed at stabilizing functional subsets.

Material Supplementary 2.DC1http://www.jimmunol.org/content/suppl/2010/01/29/jimmunol.090136

Anti–PD-1/PD-L1 immune checkpoint blockade (ICB) therapy has revolutionized the treatment of many types of cancer over the past decade. The initial therapeutic hypothesis underlying the mechanism of anti–PD-1/PD-L1 ICB was built around the premise that it acts locally in the tumor, reversing the exhaustion of PD-1hiCD8+ T cells by “releasing the brakes.” However, recent studies have provided unprecedented insight into the complexity within the CD8+ T-cell pool in the tumor microenvironment (TME). Single-cell RNA sequencing and epigenetic profiling studies have identified novel cell surface markers, revealing heterogeneity within CD8+ T-cell states classified as unique. Moreover, these studies highlighted that following ICB, CD8+ T-cell states within and outside the TME possess a differential capacity to respond, mobilize to the TME, and seed an effective antitumor immune response. In aggregate, these recent developments have led to a reevaluation of our understanding of both the underlying mechanisms and the sites of action of ICB therapy. Here, we discuss the evidence for the reversibility of CD8+ T-cell exhaustion after ICB treatment and its implication for the further development of cancer immunotherapy.

Effective clinical application of antiviral immunotherapies necessitates enhancing the functional state of natural killer (NK) and CD8+ T cells. An important mechanism for the establishment of viral persistence in the liver is the activation of the PD-1/PD-L1 inhibitory pathway. To examine the role of hepatic myeloid PD-L1 expression during viral infection, we determined the magnitude and quality of antiviral immune responses by administering PD-L1 short-interfering RNA (siRNA) encapsulated in lipidoid nanoparticles (LNP) in mice. Our studies indicate that Kupffer cells (KC) preferentially engulfed PD-L1 LNP within a short period of time and silenced Pdl1 during adenovirus and MCMV infection leading to enhanced NK and CD8+ T cell intrahepatic accumulation, effector function (interferon (IFN)-γ and granzyme B (GrB) production), CD8+ T cell–mediated viral clearance, and memory. Our results demonstrate that PD-L1 knockdown on KCs is central in determining the outcome of liver viral infections, and they represent a new class of gene therapy.