SnapShot: T Cell Exhaustion

Exhaustion is a state of T cell differentiation common during chronic infection and cancer that results in T cell dysfunction. Exhausted T cells (TEX) progressively lose effector functions and develop defects in memory T cell properties due to exposure to persistent antigen and inflammation. Exhaustion can be reversed, at least partially, by blocking inhibitory pathways such as PD-1, resulting in improved pathogen or tumor control. To view this SnapShot, open or download the PDF

Genetic absence of PD-1 promotes accumulation of terminally differentiated exhausted CD8+ T cells

Programmed Death-1 (PD-1) has received considerable attention as a key regulator of CD8+ T cell exhaustion during chronic infection and cancer because blockade of this pathway partially reverses T cell dysfunction. Although the PD-1 pathway is critical in regulating established “exhausted” CD8+ T cells (TEX cells), it is unclear whether PD-1 directly causes T cell exhaustion. We show that PD-1 is not required for the induction of exhaustion in mice with chronic lymphocytic choriomeningitis virus (LCMV) infection. In fact, some aspects of exhaustion are more severe with genetic deletion of PD-1 from the onset of infection. Increased proliferation between days 8 and 14 postinfection is associated with subsequent decreased CD8+ T cell survival and disruption of a critical proliferative hierarchy necessary to maintain exhausted populations long term. Ultimately, the absence of PD-1 leads to the accumulation of more cytotoxic, but terminally differentiated, CD8+ TEX cells. These results demonstrate that CD8+ T cell exhaustion can occur in the absence of PD-1. They also highlight a novel role for PD-1 in preserving TEX cell populations from overstimulation, excessive proliferation, and terminal differentiation

Heterologous Vaccination and Checkpoint Blockade Synergize to Induce Antileukemia Immunity

Checkpoint blockade-based immunotherapies are effective in cancers with high numbers of nonsynonymous mutations. In contrast, current paradigms suggest that such approaches will be ineffective in cancers with few nonsynonymous mutations. To examine this issue, we made use of a murine model of BCR-ABL+ B-lineage acute lymphoblastic leukemia. Using a principal component analysis, we found that robust MHC class II expression, coupled with appropriate costimulation, correlated with lower leukemic burden. We next assessed whether checkpoint blockade or therapeutic vaccination could improve survival in mice with pre-established leukemia. Consistent with the low mutation load in our leukemia model, we found that checkpoint blockade alone had only modest effects on survival. In contrast, robust heterologous vaccination with a peptide derived from the BCR-ABL fusion (BAp), a key driver mutation, generated a small population of mice that survived long-term. Checkpoint blockade strongly synergized with heterologous vaccination to enhance overall survival in mice with leukemia. Enhanced survival did not correlate with numbers of BAp:I-Ab–specific T cells, but rather with increased expression of IL-10, IL-17, and granzyme B and decreased expression of programmed death 1 on these cells. Our findings demonstrate that vaccination to key driver mutations cooperates with checkpoint blockade and allows for immune control of cancers with low nonsynonymous mutation loads