PD-1 endocytosis unleashes the cytolytic potential of check-point blockade in tumor immunity

Summary PD-1 immune checkpoint blockade (ICB) is now a promising first-line treatment for many cancers. While the steric blockade of PD-1 binding to its ligand plays a role, the role of internalisation in promoting the efficacy of ICB has not been explored. In this study, we show that PD-1 internalisation also contributes by unlocking the full cytolytic potential of ICB in cancer immunotherapy. We found that anti-mouse and human PD-1 downregulate a subset of PD-1 surface receptors on T-cells with high-density surface PD-1 leaving T-cells with intermediate expression resistant to further internalisation. Down regulation was seen on both CD4 and CD8 cells but was maximally effective on CD8 effector cells. In human T-cells, nivolumab outperformed pembrolizumab in terms of rate and efficacy. We also found that PD-1 internalisation depended on bivalent antibody (Ab)-induced crosslinking, while monovalent Ab sterically blocked PD-1 without inducing endocytosis. Immunologically, while both monovalent and bivalent Ab limited B16-PD-L1 tumor growth, bivalent Ab was significantly more effective. In molecular terms, while both antibodies increased granzyme B (GZMB) expression in CD8+ cytolytic T-cells, the induction of the second key cytolytic pore-forming mediator, perforin, was dependent on the blockade and internalisation mediated by bilavent anti-PD-1. Our findings unveil a novel mechanism in checkpoint blockade where steric blockade combined with the removal of PD-1 from the cell surface by endocytosis can complement and optimize therapy. The targeting of PD-1 internalisation holds promise for enhancing anti-tumor immunity and improving PD-1 checkpoint blockade therapy. Graphical Abstract In brief Ben Saad et al define the mechanism of PD-1 inhibitory endocytosis and show that the removal of surface PD-1 by endocytosis plays a role in complementing and optimizing checkpoint blockade. Targeting PD-1 internalisation holds promise for enhancing anti-tumor immunity and improving the efficacy of PD-1 checkpoint blockade therapy.