T cell activation is a highly regulated process involving peptide-MHC engagement of the T cell receptor and positive costimulatory signals. Upon activation, coinhibitory 'checkpoints', including programmed cell death protein 1 (PD1), become induced to regulate T cells. PD1 has an essential role in balancing protective immunity and immunopathology, homeostasis and tolerance. However, during responses to chronic pathogens and tumours, PD1 expression can limit protective immunity. Recently developed PD1 pathway inhibitors have revolutionized cancer treatment for some patients, but the majority of patients do not show complete responses, and adverse events have been noted. This Review discusses the diverse roles of the PD1 pathway in regulating immune responses and how this knowledge can improve cancer immunotherapy as well as restore and/or maintain tolerance during autoimmunity and transplantation.
Follicular regulatory T (TFR) cells are a newly defined regulatory T cell (Treg) subset that suppresses follicular helper T cell-mediated B cell responses in the germinal center reaction. The precise costimulatory signal requirements for proper TFR cell differentiation and function are still not known. Using conditional knockout strategies of CD28, we previously demonstrated that loss of CD28 signaling in Tregs caused autoimmunity in mice (termed CD28-ΔTreg mice), characterized by lymphadenopathy, accumulation of activated T cells, and cell-mediated inflammation of the skin and lung. In this study, we show that CD28 signaling is required for TFR cell differentiation. Treg-specific deletion of CD28 caused a reduction in TFR cell numbers and function, which resulted in increased germinal center B cells and Ab production. Moreover, residual CD28-deficient TFR cells showed a diminished suppressive capacity as assessed by their ability to inhibit Ab responses in vitro. Surprisingly, genetic deletion of B cells in CD28-ΔTreg mice prevented the development of lymphadenopathy and CD4+ T cell activation, and autoimmunity that mainly targeted skin and lung tissues. Thus, autoimmunity occurring in mice with CD28-deficient Tregs appears to be driven primarily by loss of TFR cell differentiation and function with resulting B cell-driven inflammation.
Programmed cell death-1 (PD-1)-targeted therapies enhance T cell responses and show efficacy in multiple cancers, but the role of costimulatory molecules in this T cell rescue remains elusive. Here, we demonstrate that the CD28/B7 costimulatory pathway is essential for effective PD-1 therapy during chronic viral infection. Conditional gene deletion showed a cell-intrinsic requirement of CD28 for CD8 T cell proliferation after PD-1 blockade. B7-costimulation was also necessary for effective PD-1 therapy in tumor-bearing mice. In addition, we found that CD8 T cells proliferating in blood after PD-1 therapy of lung cancer patients were predominantly CD28-positive. Taken together, these data demonstrate CD28-costimulation requirement for CD8 T cell rescue and suggest an important role for the CD28/B7 pathway in PD-1 therapy of cancer patients.
It is unclear whether PD-L1 on tumor cells is sufficient for tumor immune evasion or simply correlates with an inflamed tumor microenvironment. We used three mouse tumor models sensitive to PD-1 blockade to evaluate the significance of PD-L1 on tumor versus nontumor cells. PD-L1 on nontumor cells is critical for inhibiting antitumor immunity in B16 melanoma and a genetically engineered melanoma. In contrast, PD-L1 on MC38 colorectal adenocarcinoma cells is sufficient to suppress antitumor immunity, as deletion of PD-L1 on highly immunogenic MC38 tumor cells allows effective antitumor immunity. MC38-derived PD-L1 potently inhibited CD8(+) T cell cytotoxicity. Wild-type MC38 cells outcompeted PD-L1-deleted MC38 cells in vivo, demonstrating tumor PD-L1 confers a selective advantage. Thus, both tumor- and host-derived PD-L1 can play critical roles in immunosuppression. Differences in tumor immunogenicity appear to underlie their relative importance. Our findings establish reduced cytotoxicity as a key mechanism by which tumor PD-L1 suppresses antitumor immunity and demonstrate that tumor PD-L1 is not just a marker of suppressed antitumor immunity.