Abrogating Regulatory T Cells Overcomes Tumor-Specific T cell Exhaustion and Prevents Metastatic Pancreatic Cancer

Abstract Pancreatic ductal adenocarcinoma (PDA) is the 4th leading cause of cancer related deaths and has a dismal 5-year survival rate of 10 percent. PDA lethality is attributed to late diagnosis, early metastasis, and therapeutic resistance. Metastasis can occur before the development of histologically detectable tumors and is a leading cause of cancer-related deaths. We identified that pancreatic tumor cells derived from mice that resist immunotherapy (e.g., tumor escape variants, TEV) and re-implanted into the pancreas of syngeneic and immunocompetent mice rapidly metastasize, reflecting the pathogenesis of human PDA. We show that TEVs retain the targeted tumor antigen, and despite a defect in IFNgamma-inducible MHC class I upregulation, TEVs remain sensitive to tumor antigen specific T cell-mediated lysis in vitro, suggesting TEVs may confer unique qualities in vivo to resist T cell killing. Using a peptide:MHC tetramer to identify the tumor-specific CD8 T cells, we identified that intratumoral T cells in EV tumors have increased Granzyme B production and a reduction in prototypical exhaustion markers PD1, Lag3, and Tox, perhaps due to reduced MHC class I signaling. Notably, primary tumors from TEVs were significantly enriched for Foxp3+ Tregs as compared to parental tumors. Using a genetic model, we identified that Treg depletion resulted in a drastic reduction in tumor burden and metastasis and improved tumor-specific T cell function in TEV tumors. Tumor-cell intrinsic changes driving Treg accumulation in TEVs will be discussed. In summary, Tregs are key drivers of both T cell exhaustion and immunosuppression in pancreatic cancer and may prove a valuable clinical target for tumors that evade immune checkpoint blockade. Supported by grants from NIH (R01 CA249393)