Here, we utilized spontaneous models of pancreatic and lung cancer to examine how neoantigenicity shapes tumor immunity and progression. As expected, neoantigen expression during lung adenocarcinoma development leads to T cell-mediated immunity and disease restraint. By contrast, neoantigen expression in pancreatic ductal adenocarcinoma (PDAC) results in exacerbation of a fibro-inflammatory microenvironment that drives disease progression and metastasis. Pathogenic T_H17 responses are responsible for this neoantigen-induced tumor progression in PDAC. Underlying these divergent T cell responses in pancreas and lung cancer are differences in infiltrating conventional dendritic cells (cDCs). Overcoming cDC deficiency in early-stage PDAC leads to disease restraint, while restoration of cDC function in advanced PDAC restores tumor-restraining immunity and enhances responsiveness to radiation therapy.