Abstract B014: Immuno-modulation of the tumor microenvironment of pancreatic adenocarcinoma following isotoxic high-dose stereotactic body radiotherapy (iHD-SBRT)
Authors: Bouchart Christelle , Senar Oier Azurmendi , Navez Julie , Verset Laurine , Boisson Anaïs , Hein Matthieu , D’Haene Nicky , Van Gestel Dirk , Moretti Luigi , Demetter Pieter , Bachet Jean-Baptiste , Willard-Gallo Karen , Nicolle Rémy , Arsenijevic Tatjana , Van Laethem Jean-Luc 
Published: January 2024

INTRODUCTION: Pancreatic ductal adenocarcinoma (PDAC) remains one of the deadliest tumors. Unlike other tumors, the progress of systemic therapies has been very slow, mainly due to the peculiar and resistant tumor microenvironment (TME) of PDAC. The addition of ablative stereotactic body radiation therapy (SBRT) in a total neoadjuvant strategy is promising for the treatment of localized PDAC and is currently being explored in several clinical trials. However, if radiation therapy possesses the ability to modulate the TME, the impact of high-dose SBRT is still poorly known in PDAC. Here, we aim to characterize by immunohistochemistry (IHC) and RNA sequencing (RNAseq) analysis the immuno-modulations of the TME following isotoxic high-dose stereotactic body radiotherapy (iHD-SBRT). MATERIAL: Paraffin-embedded residual tumoral tissues of 50 localized PDAC resected between 2011 and 2020 were used: seventeen patients had surgery first, seventeen received an induction chemotherapy with FOLFIRINOX (FFX) only and sixteen with FFX followed by an iHD-SBRT designed to individually maximize the dose prescribed to the tumor and vessels interfaces up to Dmax(0.5cc)&lt;53Gy in 5 fractions. RESULTS: We found that although collagen deposition increases significantly after iHD-SBRT (COL1: 83.27 vs 78.60 vs 68.04%, p&lt;0.001 for iHD-SBRT, FFX and treatment naïve cohort respectively), the intra-tumoral lymphocyte infiltration is globally not altered, including for cytotoxic CD8+ lymphocytes. Only the IHC expression of CD4+ T helper population is significantly decreased after iHD-SBRT. Using gene set enrichment analysis (GSEA) of the KEGG pathway, the IL-17 signaling pathway related to the T17 cells is particularly significantly increased after iHD-SBRT compared to FFX only. After iHD-SBRT, the IHC expression levels of FOXP3+ cells and PD-L1 are significantly increased. No difference is observed for CD68+ expression however, cell type enrichment analysis from our RNA-seq data using xCell algorithm shows that the M2-polarized CD68+ cells are increased after iHD-SBRT compared to FFX alone. PD-1 expression is significantly decreased after iHD-SBRT. Furthermore, RNAseq data revealed that iHD-SBRT is associated with significant enrichment in basaloid cells (p&lt;0.0024), a subtype of basal–like cells associated with immunomodulatory stroma and better clinical outcomes. GSEA canonical pathway analysis further revealed metabolism-related pathways such as oxidative phosphorylation and gluthathione metabolism significantly enriched in iHD-SBRT treated group, suggesting the potential of mitochondrial inhibitors as candidates for combination therapy. CONCLUSIONS: iHD-SBRT is able to durably immuno-modulate the TME of PDAC. If the overall T-cell infiltration is preserved, we have identified some increased pro-tumoral cells and pathways after iHD-SBRT that could improve the development of better-oriented combination trials involving high-dose SBRT.

Citation Format: Christelle Bouchart, Oier Azurmendi Senar, Julie Navez, Laurine Verset, Anaïs Boisson, Matthieu Hein, Nicky D’Haene, Dirk Van Gestel, Luigi Moretti, Pieter Demetter, Jean-Baptiste Bachet, Karen Willard-Gallo, Rémy Nicolle, Tatjana Arsenijevic, Jean-Luc Van Laethem. Immuno-modulation of the tumor microenvironment of pancreatic adenocarcinoma following isotoxic high-dose stereotactic body radiotherapy (iHD-SBRT) [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Pancreatic Cancer; 2023 Sep 27-30; Boston, Massachusetts. Philadelphia (PA): AACR; Cancer Res 2024;84(2 Suppl):Abstract nr B014.