Non-redundant immune checkpoints direct therapeutic resistance to chemoimmunotherapy in pancreatic ductal adenocarcinoma

Pancreatic ductal adenocarcinoma (PDA) is defined by a myeloid-enriched microenvironment and has shown remarkable resistance to immune checkpoint blockade (e.g., PD-1 and CTLA-4). Here, we sought to define the role of myeloid immunosuppression in immune resistance in PDA. We report that depletion of CSF1R+ myeloid cells in combination with anti-PD-1 and chemotherapy triggers T cell infiltration into PDA but causes compensatory remodeling of the myeloid compartment with limited tumor control. Unexpectedly, combined therapy against multiple myeloid targets including CSF1R, CCR2/5 and CXCR2 was insufficient to overcome treatment resistance. High-dimensional single cell analyses performed on T cell infiltrates in human and mouse PDA revealed upregulation of multiple immune checkpoint molecules, including PD-1, LAG-3, and CTLA-4. Combinatorial blockade of PD-1, LAG-3, and CTLA-4 along with chemotherapy and anti-CSF1R was necessary to trigger activation of peripheral CD4+ and CD8+ T cells and led to deep, durable, and complete tumor responses, with each immune checkpoint blockade agent contributing to efficacy. Our findings indicate that a comprehensive approach targeting both negative regulatory signals controlling T cell function and the myeloid compartment will be fundamental to unveiling the potential of immunotherapy in PDA.