Background
Pancreatic ductal adenocarcinoma (PDAC) has little response to immune checkpoint inhibitors. An in-depth understanding of the immune microenvironment from a comprehensive and dynamic perspective is critical to generate effective therapeutic strategies for PDAC.
Methods
Using mass cytometry and immunohistochemistry, we explored the dynamic changes of tumor-infiltrating immune cells during the development of PDAC in a genetically engineered mouse model (KrasG12D/+; Trp53R172H/+; Pdx1-cre) and human specimens. PD-L1-/- mice were crossed with KrasG12D/+; TgfβR2flox/flox; Ptf1a-cre mice to achieve early depletion of PD-L1 in pancreatic cancer. Combination therapy of Arginase-1 (Arg-1) inhibitor and anti-PD-1 mAb was validated in syngeneic mouse models. Findings: Two different stages of immunosuppression with unique features were observed in both mouse model and human specimens. Early stage of immunosuppression featured highly abundant Tregs during acinar-to-ductal metaplasia, despite of a prominent and continuous presence of effector lymphocytes. The differentiation/activation branch of Ly-6C+ monocytes changed from a BST2+/MHC-II+ phenotype to an Arg-1+ phenotype over time during PDAC development. The late stage of immunosuppression thus featured the presence of a large number of myeloid suppressive cells together with a significant reduction of effector lymphocytes. Removal of PD-L1 from the beginning efficiently triggered anti-tumor immunity and significantly prolonged survival in PDAC-developing mice. Targeting Arg1+ macrophages with an Arg-1 inhibitor synergized with anti-PD-1 immunotherapy and led to PDAC-specific immune memory. Interpretation: By demonstrating the coevolution of histopathology and immunology in PDAC, this study highlights the necessity and value of early intervention and combinational approach in leveraging immunotherapy to treat pancreatic cancer. Funding: A full list of funding bodies that contributed to this study can be found in the Acknowledgements section.