Abstract 489: Prevention of pancreatic intra-epithelial neoplasm progression by a Listeria monocytogenes vaccine targeting mutated Kras, an early genetic event in pancreatic tumor development.
Authors: Keenan Bridget , Saenger Yvonne , Kafrouni Michel , Leubner Ashley , Lauer Peter , Maitra Anirban , Gunderson Andrew , Coussens Lisa , Armstrong Todd , Jaffee Elizabeth 
Published: August 2014

Abstract
Human pancreatic ductal adenocarcinoma (PDA) remains a fatal and highly treatment-resistant disease, characterized by an intense desmoplastic reaction to the tumor. Genetically-engineered mouse models of spontaneously developing PDA mimic the multistep progression from pre-cancer to invasive cancer in humans, and are ideal for studying the tumor microenvironment. 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 effect of an attenuated Listeria monocytogenes expressing mutated Kras (LM-Kras) vaccine capable of inducing a Kras-specific T cell response together with regulatory T cell (Treg)-depleting therapy (low dose cyclophosphamide and an anti-CD25 monoclonal antibody) on PanIN progression and survival. Combination immunotherapy given to mice with early stage PanINs increased survival and delayed PanIN progression, whereas intervention in older mice or with single agent immunotherapy failed to alter the course of tumor progression. Local, rather than systemic, changes in T cell activation correlated with delay in PanIN 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. Immunohistochemistry revealed granulocytes, myeloid cells, and macrophages infiltrating untreated PanINs and PDA. Analysis of upregulated genes in laser capture micro-dissected specimens of PanINs and invasive PDA included those associated with recruitment of MDSCs, induction of a suppressive phenotype in these cells, and mediators secreted by M2 or N2-type cells. 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.
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 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 489. doi:10.1158/1538-7445.AM2013-489