Targeting the fibroblast-immune cell crosstalk to relieve immune suppression in the pancreatic cancer microenvironment

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ABSTRACT Pancreatic cancer is characterized by an extensive fibroinflammatory reaction, or tumor stroma. While immune cells are abundant within the stroma, they are largely immune suppressive, and therefore pancreatic cancer is largely unresponsive to immunotherapy. Genetically engineered mouse models of pancreatic cancer recapitulate the stepwise progression of pancreatic cancer, and are ideal to study precursor lesions, such as pancreatic intraepithelial neoplasia (PanIN). Analysis of the immune infiltrates at the PanIN stage revealed that immune suppression is established very early on and precedes malignant progression. The mechanisms underlying the establishment of the immune suppression in pancreatic cancer remain unknown and understanding them is of fundamental importance to design new chemotherapy approaches for pancreatic cancer. We previously used single cell RNA sequencing to characterize gene expression profiles in the human pancreatic cancer immune infiltrate. We then mapped potential cell-cell interactions within the microenvironment based on reciprocal expression of ligands and receptors. Among predicted interactions, we identified WNT signaling activation in the T cell compartment of pancreatic cancer, driven by ligands expressed by tumor cells and cancer associated fibroblasts (CAFs). We and others have previously associated inappropriate activation of embryonic signaling pathways, including WNT signaling, as a characteristic of pancreatic cancer. WNT signaling is one of the core pathways activated in pancreatic cancer. We previously showed that ablation of epithelial WNT signaling inhibits the onset of pancreatic carcinogenesis, but the potential role of WNT in the pancreatic cancer microenvironment and specifically in immune cells is unknown. CAFs express several ligands of the WNT family; to ablate their expression, we inactivated PORCN (PORCUPINE), a transmembrane enzyme required for acylation and secretion of WNT ligands, in pancreatic fibroblasts. We then transplanted pancreatic cancer cells and observed reduced growth. To determine whether T cell activation of WNT signaling was important for this effect, we generated mice where the transcription factors TCF7, encoding for the protein TCF1, was inactivated in CD4+ T cells. In these animals, we observed reduced growth of transplanted tumors, alterations in the CAF phenotype, and increased activation of anti-tumor immunity. In this proposal, we plan to build on our preliminary data to dissect the mechanism of WNT signaling driven immune suppression in pancreatic cancer. The long term goal is to design targeting approaches that might reverse immune suppression in this disease.