PD-L1 Engagement on T cells Promotes Self-tolerance and Suppression of Neighboring Macrophages and Effector T cells in Cancer

PD-1 ligation delimits immunogenic responses in T cells. However, the consequences of PD-L1 ligation in T cells are uncertain. We found that T cell expression of PD-L1 in cancer was regulated by tumor-antigen, microbial signals, and sterile inflammatory cues. PD-L1+ T cells exerted tumor-promoting tolerance via three distinct mechanisms: (i) Binding of PD-L1 induced STAT3-dependent back-signaling in CD4+ T cells preventing activation, reducing Th1-polarization, and directing Th17-differentiation. PD-L1 signaling also induced an anergic Tbet-IFNg- phenotype in CD8+ T cells and was equally suppressive compared to PD-1 signaling. (ii) PD-L1+ T cells restrained effector T cells via the canonical PD-L1-PD-1 axis and were sufficient to accelerate tumorigenesis even in the absence of endogenous PD-L1. (iii) PD-L1+ T cells engaged PD-1+ macrophages inducing an alternative M2-like program, which had crippling effects on adaptive anti-tumor immunity. Collectively, we demonstrate that PD-L1+ T cells have diverse tolerogenic effects on tumor immunity. For RNA-Seq experiments, mouse PDL1 knockout and wild-type T cells were stimulated in vitro using anti-CD3 and anti-CD28. Alternatively, CD45+ leukocytes were FACS-sorted from mouse pancreatic tumors 3 weeks after orthotopic implantation.