Abstract PR6: Prevention of pancreatic intra-epithelial neoplasm progression by a Listeria monocytogenes vaccine targeting mutated Kras, an early genetic event in pancreatic tumor development

Human pancreatic ductal adenocarcinoma (PDA) remains a fatal disease. Biologically, it is naturally resistant to therapy and characterized by an intense desmoplastic reaction to the tumor. Genetically-engineered mouse models of spontaneously developing PDA tumors mimic the multistep progression from pre-cancer to invasive cancer in humans, and are ideal for studying the tumor microenvironment. We investigated the cellular and molecular signals underlying pancreatic intra-epithelial neoplasm (PanIN) progression, and examined the changes in these signals, PanIN progression, and survival following immunotherapy. The KrasG12D/+; Trp53R172H/+; Pdx-1-Cre mouse model of pancreatic ductal adenocarcinoma combines a Kras dominant active mutation with a p53 loss of function mutation, specifically expressed in the pancreas under the control of the Pdx-1 promoter. Mice begin developing PanINs at several weeks old, progressing to PDA and accompanied by the characteristic desmoplastic reaction seen in human PDA. We evaluated the effects of an attenuated Listeria monocytogenes expressing mutated Kras (LM-Kras) vaccine capable of inducing a Kras-specific T cell responses together with regulatory T cell (Treg)-depleting therapy (Cyclophosphamide and anti-CD25 monoclonal antibody). Combination therapy given to mice with early stage PanINs increases survival and delays PanIN progression, whereas intervention in older mice or with single agent immunotherapy fails to alter the course of tumor progression. Combination treatment administered during early PanINs correlated with enhanced CD8+ T cell infiltration and activation. In addition, combination treatment induced a Th1/Th17 cytokine expression pattern in CD4+ T cells and decreased infiltration by CD4+Foxp3+ Tregs. Local, rather than systemic, changes in T cell activation correlated with delay in PanIN progression. Immunohistochemistry revealed granulocytes, myeloid cells, and macrophages infiltrating untreated PanINs and PDA. Analysis of upregulated genes in laser capture micro-dissected specimens comparing advanced PanIN stages and invasive PDA included those associated with recruitment of MDSCs, induction of a suppressive phenotype in these cells, and mediators secreted by cells with a M2 or N2 phenotype. We are further investigating the effect of combination immunotherapy on the granulocyte and myeloid populations with the hypothesis that these signals will be altered with treatment. Thus, vaccination targeting an early genetic event in PDA can slow progression from early PanINs to invasive cancer when Tregs are also altered. Additional studies are ongoing to determine whether PanIN progression can be delayed when vaccination is administered at a time of advanced PanIN development if additional suppressive populations of immune cells within the tumor microenvironment are also inhibited. This abstract is also presented as Poster A20. Citation Format: Bridget Keenan, Yvonne Saenger, Michel Kafrouni, Ashley Leubner, Peter Lauer, Anirban Maitra, Andrew Gunderson, Lisa Coussens, Todd Armstrong, Elizabeth Jaffee. Prevention of pancreatic intra-epithelial neoplasm progression by a Listeria monocytogenes vaccine targeting mutated Kras, an early genetic event in pancreatic tumor development. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology: Multidisciplinary Science Driving Basic and Clinical Advances; Dec 2-5, 2012; Miami, FL. Philadelphia (PA): AACR; Cancer Res 2013;73(1 Suppl):Abstract nr PR6.