Elucidating Mechanisms of Immune Suppression in Mouse and Human Pancreatic Cancer

Pancreatic ductal adenocarcinoma (PDA) is one of the most lethal human malignancies. Currently, PDA has a dismal 5-year survival rate of 10% and is projected to become the second leading cause of cancer related deaths by 2030. PDA is overwhelming resistant to chemotherapy, radiotherapy, and immunotherapy appraoches. The only cure is surgical resection that only 20% of patients are eligible to receive. Unfortunately, 80% of patients who undergo surgery ultimately relapse with metastatic disease. It is critical to understand the biology underlying PDA for progress to be made. PDA tumors are characterized by a robust fibroinflammatory stroma that constitutes the bulk of the tumor volume. This rich tumor microenvironment (TME) is comprised of vasculature, nerves, extracellular matrix (ECM) components, fibroblasts and immune cells. The immune infiltration in PDA is immunosuppressive in nature. While immune checkpoint blockade has proven beneficial in other cancers, that benefit has not extended to PDA, due to this robust immune suppression. This dissertation will focus on mechanisms for immune suppression in both mouse and human PDA. First, I discuss T cell mediated mechanisms for immune suppression in human PDA. We used a multimodal approach of multiplex immunohistochemistry, mass cytometry, and single cell RNA sequencing to map the immune landscape in human PDA. With this approach we provided further evidence that the T cell infiltration and further the expression of immune checkpoints in PDA is heterogenous. We discovered the previously underappreciated immune checkpoint TIGIT is elevated in human PDA tumors and correlated with exhausted T cells compared to effector T cells. This work highlighted the importance of evaluating TIGIT as a potential therapeutic target and presented the importance of precision medicine when considering immune therapy. Secondly, this dissertation focuses on myeloid-mediated mechanisms of immune suppression. This dissertation has two chapters on using mouse and human models to elucidate the role of myeloid cells in PDA immune suppression. I first discuss the role of Apolipoprotein E (ApoE) in mediating immune suppression via NF-kB signaling. Using our human single cell RNA sequencing dataset, we show ApoE is expressed highly in macrophages and serum levels stratify survival. Mechanistically, loss of ApoE results in smaller tumors and increased T cell infiltration. We determined our T cell phenotype was mediated by CXCL1 secretion via NF-kB signaling. We next evaluated myeloid cells systemically in PDA. We performed single cell RNA sequencing on biomaterial scaffolds, that serve as a synthetic metastatic niche, mouse and human primary tumors, human liver metastases and human peripheral blood to obtain a comprehensive evaluation of the systemic immune system changes in response to a primary PDA tumor. In this work, we discovered a population of monocytes/macrophages that highly expressed complement genes (C1QA, C1QB) that is elevated in PDA. Taken together, this dissertation uses both mouse and human systems to map the immune landscape in PDA. We have identified novel proteins of interest in mediating immune suppression. Further work is necessary to evaluate functional implications as well as translational potential.