BackgroundPancreatic cancer is characterised by the accumulation of a fibro-inflammatory stroma. Within this stromal reaction, myeloid cells are a predominant population. Distinct myeloid subsets have been correlated with tumour promotion and unmasking of anti-tumour immunity.ObjectiveThe goal of this study was to determine the effect of myeloid cell depletion on the onset and progression of pancreatic cancer and to understand the relationship between myeloid cells and T cell-mediated immunity within the pancreatic cancer microenvironment.MethodsPrimary mouse pancreatic cancer cells were transplanted into CD11b-diphtheria toxin receptor (DTR) mice. Alternatively, the iKras* mouse model of pancreatic cancer was crossed into CD11b-DTR mice. CD11b+ cells (mostly myeloid cell population) were depleted by diphtheria toxin treatment during tumour initiation or in established tumours.ResultsDepletion of myeloid cells prevented KrasG12D-driven pancreatic cancer initiation. In pre-established tumours, myeloid cell depletion arrested tumour growth and in some cases, induced tumour regressions that were dependent on CD8+ T cells. We found that myeloid cells inhibited CD8+ T-cell anti-tumour activity by inducing the expression of programmed cell death-ligand 1 (PD-L1) in tumour cells in an epidermal growth factor receptor (EGFR)/mitogen-activated protein kinases (MAPK)-dependent manner.ConclusionOur results show that myeloid cells support immune evasion in pancreatic cancer through EGFR/MAPK-dependent regulation of PD-L1 expression on tumour cells. Derailing this crosstalk between myeloid cells and tumour cells is sufficient to restore anti-tumour immunity mediated by CD8+ T cells, a finding with implications for the design of immune therapies for pancreatic cancer.

Summary Pancreatic cancer, a hypovascular and highly desmoplastic cancer, is characterized by tumor expression of Hedgehog (HH) ligands which signal to fibroblasts in the surrounding stroma that in turn promote tumor survival and growth. However, the mechanisms and consequences of stromal HH pathway activation are not well understood. Here we show that the HH co-receptors GAS1, BOC, and CDON are expressed in cancer-associated fibroblasts. Deletion of two co-receptors (Gas1 and Boc) in fibroblasts reduces HH-responsiveness. Strikingly, these fibroblasts promote greater tumor growth in vivo that correlates with increased tumor-associated vascularity. In contrast, deletion of all three co-receptors (Gas1, Boc and Cdon) results in the near complete abrogation of HH signaling and a corresponding failure to promote tumorigenesis and angiogenesis. Collectively, these data identify a novel role for HH-dosage in pancreatic cancer promotion and may explain the clinical failure of HH pathway blockade as a therapeutic approach in pancreatic cancer.

Pancreatic cancer is characterized by an extensive desmoplastic stroma, the functional relevance of which is poorly understood. Activated fibroblasts are a prevalent component of the stroma, and traditionally, these cells have been considered as a homogenous population derived from pancreatic stellate cells. In this study, we highlight a previously unappreciated heterogeneity of the fibroblast population within the stroma. In particular, a subset of stromal fibroblasts has characteristics of mesenchymal stem cells (MSCs). MSCs are present in the normal pancreas as well as in the carcinomatous pancreas (CA-MSCs). Here, we determine that CA-MSCs have increased tumor-promoting function compared with MSCs in normal pancreas. This ability to promote tumor growth is associated with CA-MSCs’ unique ability to promote alternative macrophage polarization. Thus, our study identifies a previously uncharacterized cell population within the stroma and sheds light on tumor-promoting interactions between different components of the stroma.SignificanceTargeting the stroma is emerging as a new paradigm in pancreatic cancer; however, efforts to that effect are hampered by our limited understanding of the nature and function of stromal components. Here, we uncover previously unappreciated heterogeneity within the stroma and identify interactions among stromal components that promote tumor growth and could be targeted therapeutically.

Pancreatic cancer, one of the deadliest human malignancies, is associated with oncogenic Kras and is most commonly preceded by precursor lesions known as pancreatic intraepithelial neoplasias (PanIN). PanIN formation is accompanied by the establishment of an immunotolerant microenvironment. However, the immune contribution to the initiation of pancreatic cancer is currently poorly understood. Here, we genetically eliminate CD4+ T cells in the iKras* mouse model of pancreatic cancer, in the context of pancreatitis, to determine the functional role of CD4+ T cells during mutant Kras-driven pancreatic carcinogenesis. We show that oncogenic Kras-expressing epithelial cells drive the establishment of an immunosuppressive microenvironment through the recruitment and activity of CD4+ T cells. Furthermore, we show that CD4+ T cells functionally repress the activity of CD8+ cells. Elimination of CD4+ T cells uncovers the antineoplastic function of CD8+ cells and blocks the onset of pancreatic carcinogenesis. Thus, our studies uncover essential and opposing roles of immune cells during PanIN formation and provide a rationale to explore immunomodulatory approaches in pancreatic cancer.