Abstract 5847: The splanchnic mesenchyme is the main tissue origin of fibroblasts in the pancreas during homeostasis and tumorigenesis
Authors: Han Lu , Wu Yongxia , Fang Kun , Sweeney Sean , Roesner Ulyss , Parrish Melodie , Patel Khushbu , Walter Tom , Piermattei Julia , Trimboli Anthony , Lefler Julia , Timmers Cynthia , Yu Xue-Zhong , Jin Victor , Zimmermann Michael , Mathison Angela , Urrutia Raul , Ostrowski Michael , Leone Gustavo 
Published: April 2023

Abstract
In pancreatic ductal adenocarcinoma (PDAC), cancer associated fibroblasts (CAFs) play critical and complex roles in the tumor microenvironment. CAFs are also a major cell type in the desmoplastic stroma in PDAC and may account for half of the entire tumor tissue. Multiple subtypes of CAFs have been suggested, but the tissue origin(s) of CAF subtypes are unknown and genetic tools to robustly target them in vivo are lacking. Here we aimed to examine three potential tissue sources of CAFs: the pancreatic epithelium (through epithelium-to-mesenchyme transition), the bone marrow (through circulation), and the pancreatic mesenchyme or tissue resident fibroblasts (TRFs) in the normal pancreas (through proliferation). We utilized a genetically engineered mouse model of PDAC, where Kras and p53 mutations were engineered in the pancreatic epithelium using an Flp-Frt system. To determine whether the pancreatic epithelium gives rise to CAFs, we permanently labeled the pancreatic epithelium with a GFP reporter and traced their cell descendants by GFP expression. Despite robust GFP labeling of the epithelium, GFP expression was rarely identified in CAFs. To determine whether the bone marrow gives rise to CAFs, we transplanted donor bone marrow carrying a ubiquitously expressed GFP reporter to GFP-negative recipient mice. We found that minimal proportion of pancreatic CAFs were tagged with GFP. Lastly, to determine whether pancreatic TRFs give rise to CAFs, we used an inducible CreER-LoxP system to allow for permanent Tomato labeling in TRF progenitors, the splanchnic mesenchyme, during mid-gestation. Lineage tracing in PDAC showed that the vast majority of CAFs were labeled with Tomato expression, suggesting their splanchnic origin. Furthermore, certain splanchnic gene expression signatures persisted in subsets of CAFs in both the PDAC mouse model and human patient samples. Deletion of one of the splanchnic genes, Gata6, in CAFs resulted in increased tumor burden in the pancreas, suggesting a tumor-restraining role of Gata6 in CAFs. In summary, we found that the pancreatic epithelium and bone marrow contributes to a minimal proportion of CAFs in PDAC. Meanwhile, pancreatic TRFs are derived from the splanchnic mesenchyme during fetal development and they expand to contribute to the vast majority of CAFs in PDAC. Moreover, the persistence of splanchnic signature defines subtypes of CAFs, with a potential tumor-suppressing function. This study provides genetic approaches to robustly target CAFs in vivo, and novel insights into CAF origin, heterogeneity and function in PDAC.
Citation Format: Lu Han, Yongxia Wu, Kun Fang, Sean Sweeney, Ulyss Roesner, Melodie Parrish, Khushbu Patel, Tom Walter, Julia Piermattei, Anthony Trimboli, Julia Lefler, Cynthia Timmers, Xue-Zhong Yu, Victor Jin, Michael Zimmermann, Angela Mathison, Raul Urrutia, Michael Ostrowski, Gustavo Leone. The splanchnic mesenchyme is the main tissue origin of fibroblasts in the pancreas during homeostasis and tumorigenesis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5847.