Abstract 1579: Macrophage infiltration and polarization in pancreatic ductal adenocarcinoma following stereotactic body radiation therapy

Abstract Introduction: Stereotactic body radiation therapy (SBRT) has emerged as a promising treatment modality for pancreatic ductal adenocarcinoma (PDAC) due to its precise delivery and potential to improve local control. Beyond its direct impact on tumor cells, accumulating evidence suggests that SBRT may also modulate the tumor microenvironment (TME), influencing immune responses. Macrophages play a crucial role in the TME with their polarization into pro-inflammatory (M1) or pro-tumoral (M2) phenotypes significantly impacting cancer progression. Studying the TME and developing immune therapies may be helpful for metastatic PDAC, which currently only has non-curative treatment. The TME, in particular the polarization of macrophages, has not been well studied following radiation treatment. Therefore, our work aims to study how SBRT affects the infiltration and polarization of macrophages in the PDAC TME in a time-dependent manner. Methods: To establish and study PDAC C57BL/6J mice were orthotopically injected with a luciferase-labeled KPC tumor cell line. SBRT was administered to tumor-bearing mice following a schedule of 10Gy radiation per fraction in total of 5 fractions on days 10-14 post tumor implantation. Mice were euthanized on day 1, 3 and 7 after irradiation. Tumor tissues were collected for immunohistochemistry (IHC) staining and flow cytometry with antibodies targeting CD68, CD11b, CD86 (M1 marker), and CD206 (M2 marker). Images were analyzed and quantified using ImageJ and GraphPad Prism. Results: Via the use of IHC staining, we found no significant difference in the number of cells stained with the CD68 antibody (macrophage marker) when compared throughout groups. Additionally, we found no significant difference in the number of macrophages stained with the M2 marker. However, we did observe a significant increase in the number of macrophages stained with the M1 marker in the group receiving SBRT. Through flow cytometry of the tumor tissues we observed an increasing trend, as time passed, in the number of macrophages expressing the M1 marker. The same trend was observed in macrophages expressing both the M1 and M2 markers. However, a change in the total number of macrophages or those expressing only the M2 marker was not observed. Conclusions: SBRT does not affect the total number of infiltrated macrophages in the TME or the amount of M2 macrophages. However, we did observe that the percentage of M1 macrophage increases significantly as time passes after SBRT. We theorize that the increase in M1 macrophages may be due to an M2 to M1 transition in the M2 population. Our study provides preliminary rationale to use macrophage-targeted immune therapy to potentiate the efficacy of SBRT for PDAC treatment, though we understand that further follow-up studies with a larger sample size and longer time course are required to obtain significant and meaningful results. Citation Format: Emil D. Gonzalez Marrero, Liang Wang, Benjamin R. Schrank, Albert C. Koong. Macrophage infiltration and polarization in pancreatic ductal adenocarcinoma following stereotactic body radiation therapy 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 (6Suppl): Abstract nr 1579.