Abstract 1096: Single cell transcriptomic sequencing of pancreatic ductal adenocarcinoma reveals an enrichment of immunosuppressive pathways in patients at high risk for early local progression following stereotactic body radiation therapy
Authors: Bernard Vincent , Elhammali Adnan , Lin Daniel , Tzeng Ching-Wei , Bhutani Manoop S. , Taniguchi Cullen 
Published: April 2023

Abstract
Patients with locally advanced pancreatic ductal adenocarcinoma (PDAC) have limited therapeutic options with their disease being defined as neither metastatic nor amenable to surgical resection. Although stereotactic body radiation therapy (SBRT) is a viable treatment option in this context, disease progression remains far too common in PDAC. Understanding the interaction between radiation, cancer cells, and the tumor microenvironment (TME) may lead to an understanding of novel resistance pathways that can be exploited.
In order to identify these potential nodes of therapeutic actionability, we leveraged a phase I/II dose escalation clinical trial of SBRT with a radiomodulating agent, GC4419, undertaken in patients with locally advanced PDAC. In this trial, core biopsies were obtained following neoadjuvant chemotherapy, but before and after SBRT with 50-60Gy in 5 fractions. A total of 16 biopsies underwent single cell RNA sequencing with 7 patients with matched samples before and after radiation.
Of the 7 matched patients, a total of 3 experienced early local progression following SBRT, and were deemed to be nonresponders. We observed a significant increase in NK-like cells and a decrease in B-cells and Mast cells following SBRT in nonresponders. On the other hand, we also observed a significant enrichment in pathways related to downregulation of innate immunity within macrophage populations following SBRT suggesting that an immunosuppressive microenvironment may prevail in these patients. Subsequently, we observed functionally distinct cancer-associated fibroblasts (CAFs) subtypes, including myofibroblastic, inflammatory, and antigen presenting CAFs (apCAF). ApCAFs are believed to be involved in antigen presentation to CD4 T cells, but lack the costimulatory molecules to activate immune response, and are thus presumed to contribute to an immunosuppressive microenvironment through anergy. Interestingly, the population of apCAFs decreased significantly following SBRT in non-responders, and increased in those patients deemed to be responders. Thus whether apCAFs have an additional inflammatory role in the setting of patients receiving radiation remains to be further understood.
We have shown how single cell RNA sequencing can be used to profile the kinetics of cell populations following SBRT. This has revealed an interplay between members of the tumor microenvironment whereby immunosuppressive pathways appear to prevail over immunostimulatory signals in nonresponders. Understanding which pathways are enriched in those patients at highest risk for local recurrence may provide insight into novel therapeutic strategies that can be implemented concurrently with radiation therapy.
Citation Format: Vincent Bernard, Adnan Elhammali, Daniel Lin, Ching-Wei Tzeng, Manoop S. Bhutani, Cullen Taniguchi. Single cell transcriptomic sequencing of pancreatic ductal adenocarcinoma reveals an enrichment of immunosuppressive pathways in patients at high risk for early local progression following stereotactic body radiation therapy [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 1096.