Project Summary Scientific Abstract of proposed research project Immune checkpoint blocker therapy has revolutionized our clinical approach in cancer therapy. However, the overall response rate still has room for improvement and varies greatly in different cancer types. The redundant but unique role of immune checkpoints in T cell immunity propels us to further study the role of novel immune checkpoints in cancer therapy. The BigLEN/GPR171 interaction is a newly identified GPCR pathway that has been reported to regulate food uptake and anxiety. Though GPR171 is commonly used as a T cell signature gene, its potential role in T cell immunity has not been explored. Our recent studies have implicated that the GPR171/BigLEN axis is a new T cell checkpoint pathway that can be modulated for cancer immunotherapy. We found that GPR171 is transcribed in T cells and its protein expression is induced upon antigen stimulation. The neuropeptide ligand BigLEN interacts with GPR171 to suppress T cell receptor- mediated signaling pathways and to inhibit T cell proliferation. Loss of GPR171 in T cells leads to hyperactivity to antigen stimulation and GPR171-deficient mice exhibit enhanced antitumor immunity. Blockade of GPR171 signaling by an antagonist promotes antitumor T cell immunity in various mouse tumor models. Our preliminary data further implicate that GPR171 can be an inhibitory receptor on T cell for cannabinoids. In the proposed study, we will dissect the role of GPR171 ligands in GPR171-mediated antitumor suppression. We will further determine the molecular mechanisms that GPR171 signaling inhibits anticancer T cell response. The approach of GPR171 blockade for cancer therapy will be tested in clinical tumor models, including chimeric antigen receptor T-cell therapy and humanized mouse model together with anti- PD-1 therapy. By the completion of these studies, our discovery that GPR171 is a receptor for cannabinoids will help to better understand the impact of cannabis usage in cancer treatment, more importantly, provide new strategies of cancer immunotherapy.