Pancreatic ductal adenocarcinoma (PDAC) is projected to become the second deadliest cancer in the US by 2025 1, with 5-year survival at less than 10% 2. In other recalcitrant cancers, immunotherapy has shown unprecedented response rates, including durable remissions after drug discontinuation. However, responses to immunotherapy in PDAC are rare. Accumulating evidence in mice and humans suggests that this remarkable resistance is linked to the complex, dueling role of the immune system in simultaneously promoting and restraining PDAC.