Long-term gemcitabine treatment reshapes the pancreatic tumor microenvironment and sensitizes murine carcinoma to combination immunotherapy

Pancreatic ductal adenocarcinoma (PDAC) is a leading cause of cancer-related death with a median survival time of 6-12 months. Most patients present with disseminated disease and the majority are offered palliative chemotherapy. With no approved treatment modalities for patients who progress on chemotherapy, we explored the effects of long-term gemcitabine administration on the tumor microenvironment to identify potential therapeutic options for chemorefractory PDAC. Using a combination of mouse models, primary cell line-derived xenografts, and established tumor cell lines, we first evaluated chemotherapy-induced alterations in the tumor secretome and immune surface proteins by high throughput proteomic arrays. In addition to enhancing antigen presentation and immune checkpoint expression, gemcitabine consistently increased the synthesis of CCL/CXCL chemokines and TGFb-associated signals. These secreted factors altered the composition of the tumor stroma, conferring gemcitabine resistance to cancer-associated fibroblasts in vitro and further enhancing TGFb1 bio-synthesis. Combined gemcitabine and anti-PD-1 treatment in transgenic models of murine PDAC failed to alter disease course unless mice also underwent genetic or pharmacologic ablation of TGFb signaling. In the setting of TGFb signaling deficiency, gemcitabine and anti-PD-1 led to a robust CD8^þ T-cell response and decrease in tumor burden, markedly enhancing overall survival. These results suggest that gemcitabine successfully primes PDAC tumors for immune checkpoint inhibition by enhancing antigen presentation only following disruption of the immunosuppressive cytokine barrier. Given the current lack of third-line treatment options, this approach warrants consideration in the clinical management of gemcitabine-refractory PDAC.