Abstract A049: CD8+ T cell decline from regression to recurrence in ovarian cancer is linked to expansion of suppressive macrophages

Abstract High grade serous ovarian cancer (HGSOC) is the most common and deadliest form of ovarian cancer. Recurrence after chemotherapy remains a major challenge, particularly in homologous recombination proficient (HRP) patients. However, the interval of low tumor burden between therapy-induced remission and recurrence presents an ideal window to engage the immune system. Our work aims to characterize the immune microenvironment of HRP HGSOC throughout post-chemotherapy recurrence to understand mechanisms of suppression and identify potential targets for immunotherapy. To study immune responses in pre- and post-treatment HRP HGSOC, we utilized syngeneic murine models. These models were derived from the CPAK.SIY cell line, driven by Ccne1 OE , p53 R172H , Akt2 OE , and Kras G12V and engineered to express the model antigen SIY. To model pre- and post-chemotherapy states, we in vivo passaged CPAK.SIY in the presence of carboplatin/paclitaxel chemotherapy or vehicle. The resulting derivatives are termed CPAK.SIYP1 (chemotherapy-sensitive) and CPAK.SIYCHEM (chemotherapy-resistant). Upon chemotherapy treatment, CPAK.SIYP1 recapitulates the initial regression observed following frontline therapy, while CPAK.SIYCHEM models resistance. We utilized OVASEEK, a whole-organ ex vivo imaging platform, to detect and isolate early tumor lesions followed by immunofluorescence to profile post-treatment spatial dynamics. We observed CD8+ T cell infiltration in CPAK.SIYP1 tumors decreased upon chemotherapy treatment. CD8+ T cells that remained in the tumor after chemotherapy expressed higher levels of CD25 and Granzyme B, as measured by flow cytometry, displaying a potential for increased effector function. Interestingly, CPAK.SIYCHEM tumors, modeling a resistant state, did not show increased immunity. In fact, CD8+ T cells in CPAK.SIYCHEM tumors expressed strikingly low levels of Granzyme B. This indicates that CD8+ T cell responses mounted during initial chemotherapy may not persist as recurrent disease progresses. To mechanistically understand how initial chemotherapy mediates an immune evasive state, we performed single-cell RNA sequencing (scRNAseq) on CPAK.SIYP1 and CPAK.SIYCHEM tumors with and without chemotherapy treatment. Analysis of scRNAseq data indicated that in all tumors, the immune compartment was dominated by macrophages. Most macrophages scored highly for suppressive tumor-associated macrophage (TAM) signatures. Notably, the two macrophage populations that appeared most suppressive were enriched in chemotherapy-treated tumors, implicating a role for these TAMs in shaping the tumor microenvironment and inhibiting CD8+ T cell activation during disease recurrence. Together, these findings suggest that while chemotherapy initially enhances CD8+ effector T cells, the resulting tumor microenvironment evolves to become highly suppressive, contributing to immune evasion. Specifically, the enrichment of highly suppressive TAM populations during this transition highlights them as key drivers of post-treatment immune dysfunction and promising targets for immunotherapy. Citation Format: Grace Wolczanski, Emma Schuler, Margaret Billingsley, Mae Pryor, Samuel Freeman, Neelkanth Bardhan, Vivek Rastogi, Fiona Chatterjee, Richard Van, Angela Belcher, Sohrab Shah, Paula Hammond, Stefani Spranger. CD8+ T cell decline from regression to recurrence in ovarian cancer is linked to expansion of suppressive macrophages [abstract]. In: Proceedings of the AACR Immuno-Oncology Conference (AACR IO): Discovery and Innovation in Cancer Immunology: Revolutionizing Treatment through Immunotherapy; 2026 Feb 18-21; Los Angeles, CA. Philadelphia (PA): AACR; Cancer Immunol Res 2026;14(2 Suppl):Abstract nr A049.