Abstract 3983: Antigen-presenting cancer-associated fibroblasts provide an alternate mechanism for CD4 T cell activation in murine pancreatic ductal adenocarcinoma

Abstract Despite the advent of immune checkpoint inhibitors (ICI) that have markedly changed the treatment landscape for select tumors, pancreatic ductal adenocarcinoma (PDAC) remains unresponsive to such an approach. Features of the PDAC tumor microenvironment that contribute to treatment resistance, including the influence of cancer-associated fibroblasts (CAF) on immune and tumor cells, remain poorly understood. Antigen-presenting CAFs (apCAFs) express MHC-II and activate CD4 T cells in genetically engineered PDAC mouse models, but are rarely found in human PDAC. In a human PDAC atlas consisting of six published single-cell RNA-sequencing datasets, we identified 370 apCAFs out of 8736 total CAFs, the majority of which were from one dataset. Understanding apCAF regulation can reveal how to promote their differentiation and enhance T cell activating capacity in PDAC. Methods: Two KPC clones that differ in sensitivity to ICIs, sKPC (sensitive) and rKPC (resistant), derived from the autochthonous KPC murine model of PDAC, were implanted orthotopically into the pancreas or subcutaneously into the flank of syngeneic C57BL/6 mice. Tumors were harvested at 14-28 days post implantation for quantification of T cells, dendritic cells, and CAFs. To evaluate function, apCAFs were sorted and CD11c+ and CD4+ T cells were isolated from tumors. Naïve CD4+ T cells were also isolated from OT-II mice. apCAFs and CD11c+ cells were pulsed with OVA and co-cultured with OT-II CD4+ T cells, or co-cultured with sKPC tumor infiltrating CD4+ T cells in the presence of endogenous peptides. Results: Total CAFs are equivalent between tumors, but apCAFs are significantly more abundant in sKPC tumors. Furthermore, the apCAF to dendritic cell ratio within these tumors ranged from 5:1 to 50:1. CD11c+ dendritic cells sorted from sKPC tumors and pulsed with OVA peptide robustly activate naïve OT-II CD4+ T cells, as expected from professional APCs; sorted apCAFs also activate naïve OT-II CD4+ T cells, although to a lesser degree. When apCAFs or CD11c+ dendritic cells are cultured with tumor-infiltrating CD4+ T cells, however, they upregulate CD4 activation markers to a similar extent. Conclusions: apCAFs are more abundant in tumors derived from ICI-sensitive KPC clones and greatly outnumber dendritic cells. Functionally, apCAFs from sKPC tumors stimulate tumor-derived CD4 T cells to the same degree as CD11c+ dendritic cells, implicating a role in activation of the immune response in ICI-sensitive tumors. Although a small proportion of the tumor microenvironment, the apCAF subset may be enhanced in number and function by CAF-targeted modulators to affect the antitumor response; transcriptomics analyses are currently underway to identify regulators of apCAFs. Such a strategy could provide a solution to the currently unmet need for new and more effective treatments for PDAC patients. Citation Format: Saumya Maru, Lalitya Andaloori, Kathryn Howe, Kabeer Munjal, James Leatherman, Edward J. Pearce, Elizabeth M. Jaffee. Antigen-presenting cancer-associated fibroblasts provide an alternate mechanism for CD4 T cell activation in murine pancreatic ductal adenocarcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 3983.