PD-1-mediated attrition of polyfunctional memory CD8+ T cells in chronic toxoplasma infection.

We reported earlier that during chronic toxoplasmosis CD8(+) T cells become functionally exhausted with concomitant PD-1 upregulation, leading to eventual host mortality. However, how immune exhaustion specifically mediates attrition of CD8 polyfunctionality, a hallmark of potent T-cell response, during persistent infections has not been addressed. In this study, we demonstrate that PD-1 is preferentially expressed on polyfunctional memory CD8(+) T cells, which renders them susceptible to apoptosis. In vitro blockade of the PD-1-PD-L1 pathway dramatically reduces apoptosis of polyfunctional and interferon γ(+)/granzyme B(-) memory but not effector CD8(+) T cells. In summary, the present report underscores the critical role of the PD-1-PD-L1 pathway in mediating attrition of this important CD8(+) T-cell subset and addresses the mechanistic basis of how αPD-L1 therapy reinvigorates polyfunctional CD8 response during chronic infections. The conclusions of this study can have profound immunotherapeutic implications in combating recrudescent toxoplasmosis as well other chronic infections.