Abstract A096: Myeloid cells are required for pancreatic carcinogenesis and PD-1/PD-L1 checkpoint activation

Myeloid cells, including both macrophages and immature myeloid cells/myeloid derived suppressor cells (MDSCs), accumulate during the progression of pancreatic cancer. The goal of this study was to determine the effect of myeloid cell depletion on the onset and progression of pancreatic cancer, and to understand the relationship between myeloid cells and T cell-mediated immunity within the pancreatic cancer microenvironment. Primary mouse pancreatic cancer cells were transplanted into CD11b-DTR mice. Alternatively, the iKras* mouse model of pancreatic cancer was crossed into CD11b-DTR mice. CD11b+ cells were depleted by Diphtheria Toxin treatment during tumor initiation or in established tumors. Depletion of myeloid cells prevented KrasG12D driven pancreatic cancer initiation. Myeloid cells are required for sustained MAPK signaling in pancreatic epithelial cells during the onset of carcinogenesis, notwithstanding the expression of oncogenic Kras. In pre-established tumors, myeloid cell depletion arrested tumor growth and in some cases, induced tumor regressions that were dependent on CD8+ T cells. We found that myeloid cells inhibited CD8+ T cell anti-tumor activity by inducing the expression of Programmed cell death-ligand 1 (PD-L1) in tumor cells in an EGFR/MAPK dependent manner. Treatment with MEK inhibitors lowers the intratumoral expression of PD-L1 and renders the tumor susceptible to PD-1 blockade. Our results show that myeloid cells support immune evasion in pancreatic cancer through EGFR/MAPK dependent regulation of PD-L1 expression on tumor cells. Derailing this cross-talk between myeloid cells and tumor cells is sufficient to restore anti-tumor immunity mediated by CD8+ T cells, a finding with implications for the design of immune therapies for pancreatic cancer. Note: This abstract was not presented at the conference. Citation Format: Yaqing Zhang, Ashley Velez-Delgado, Esha Mathew, Dongjun Li, Flor M. Mendez, Kevin Flannagan, Andrew D. Rhim, Diane M. Simeone, Gregory L. Beatty, Marina Pasca di Magliano. Myeloid cells are required for pancreatic carcinogenesis and PD-1/PD-L1 checkpoint activation [abstract]. In: Proceedings of the Second CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; 2016 Sept 25-28; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(11 Suppl):Abstract nr A096.