Abstract PR001: The splanchnic mesenchyme during fetal development is the major source of pancreatic cancer associated fibroblasts

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, suggesting little contribution of epithelium to the CAF pool. To determine whether the bone marrow gives rise to CAFs, we transplanted donor bone marrow carrying a ubiquitously expressed GFP reporter allele to GFP-negative recipient mice. We found that only a small proportion of pancreatic CAFs were tagged with GFP, suggesting their bone marrow origin. Lastly, to determine whether pancreatic TRFs give rise to CAFs, we used an inducible CreER-LoxP system to allow for permanent Tomato labeling in TRFs 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 were persistent in subsets of CAFs in both the PDAC mouse model and human patient samples. In summary, we found that bone marrow contributes to a small proportion of CAFs in PDAC, and the pancreatic epithelium contributes even less. 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. This study provides approaches to robustly target CAFs in vivo and novel insights into CAF heterogeneity in PDAC. Citation Format: Lu Han, Yongxia Xu, Sean Sweeney, Ulyss Roesner, Melodie Parrish, Khushbu Patel, Xuezhong Yu, Michael Ostrowski, Gustavo Leone. The splanchnic mesenchyme during fetal development is the major source of pancreatic cancer associated fibroblasts [abstract]. In: Proceedings of the AACR Virtual Special Conference on the Evolving Tumor Microenvironment in Cancer Progression: Mechanisms and Emerging Therapeutic Opportunities; in association with the Tumor Microenvironment (TME) Working Group; 2021 Jan 11-12. Philadelphia (PA): AACR; Cancer Res 2021;81(5 Suppl):Abstract nr PR001.