Abstract A62: Regulatory T-cell depletion promotes oncogenic Kras-driven pancreatic carcinogenesis
Authors: Zhang Yaqing , Lazarus Jenny , Steele Nina , Yan Wei , Lee Ho-Joon , Halbrook Christopher J. , Menjivar Rosa , Kemp Samantha B. , Sirihorachai Veerin , Carpenter Eileen S. , Nevison Anna C. , Vinta Alekya , Anderson Michelle A. , Crawford Howard C. , Lyssiotis Costas A. , Frankel Timothy L. , Bednar Filip , Magliano Marina Pasca di 
Published: March 2020

Abstract
Objective: Pancreatic cancer is a deadly malignancy, and little progress has been made in the survival of patients during the past 40 years. Harnessing the immune system to elicit anticancer response is an attractive therapeutic option, alone or in combination with antitumor agents. Targeting regulatory T cell (Treg), which acts to suppress immune responses, holds promise as one potential approach to relieve the immune suppression of pancreatic cancer, but functional studies investigating whether Treg depletion at different stages of pancreatic cancer has therapeutic benefit are needed.
Methods: We are using KC; Foxp3DTR mice generated by crossing KC (p48-Cre; LSL-KrasG12D) with Foxp3DTR (B6.129(Cg)-Foxp3tm3(DTR/GFP)Ayr/J, Jackson Laboratory) to determine the requirement of Tregs during oncogenic Kras-induced pancreatic cancer precursor lesion formation and progression. We depleted Foxp3-expressing Tregs by Diphtheria Toxin (DT) injection at different time points—either before or after PanIN formation. Multiplex immunohistochemistry and Cytometry by Time-of-Flight (CyTOF) were performed for tumor immunophenotyping; RNA sequencing was used for transcriptomic analysis of the tumor epithelial cells, fibroblasts, and myeloid cells.
Results: Surprisingly, instead of blocking tumorigenesis, depletion of Tregs during these early stages led to accelerated tumor progression. We show that Tregs are a key source of TGFb, and accordingly their depletion alters fibroblast population, exemplified by loss of Smooth Muscle Actin expression. Treg depletion further resulted in a compensatory influx of suppressive myeloid cells, thereby preventing an antitumor immune response. Mechanistically, we show that this myeloid recruitment was driven through the CCR1 axis, with the receptor expressed on tumor-associated myeloid cells and several ligands upregulated in epithelial cells and fibroblasts upon Treg depletion. Finally, blockade of CCR1 signaling successfully inhibits tumor progression upon Treg-depletion.
Conclusion: Our study revealed an immunosuppressive network of interactions between different components within the pancreatic tumor microenvironment, including Tregs, fibroblasts, tumor cells, and macrophages. Depleting one component (Tregs) can lead to the compensation of immunosuppression from the rest of components within the network. Thus, a thorough understanding of the immunosuppressive network in pancreatic neoplasia is necessary to optimize our approaches to treatment and identify opportunities for combination therapy and prevent potential feedback mechanisms resulting in tumor resistance and relapse.
Citation Format: Yaqing Zhang, Jenny Lazarus, Nina Steele, Wei Yan, Ho-Joon Lee, Christopher J. Halbrook, Rosa Menjivar, Samantha B. Kemp, Veerin Sirihorachai, Eileen S. Carpenter, Anna C. Nevison, Alekya Vinta, Michelle A. Anderson, Howard C. Crawford, Costas A. Lyssiotis, Timothy L. Frankel, Filip Bednar, Marina Pasca di Magliano. Regulatory T-cell depletion promotes oncogenic Kras-driven pancreatic carcinogenesis [abstract]. In: Proceedings of the AACR Special Conference on Pancreatic Cancer: Advances in Science and Clinical Care; 2019 Sept 6-9; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2019;79(24 Suppl):Abstract nr A62.