Linking Tumor-Infiltrating Immune Cells to Molecular Markers in Endometrial Cancer

Background: The composition of tumor-infiltrating lymphocytes (TILs) in endometrial cancer may be linked to both clinicopathologic factors and underlying molecular alterations. We sought to characterize TIL subsets recovered from fresh tumor tissue and peritoneal washing cytology and to determine how these immune profiles relate to routinely used immunohistochemical markers. Methods: Fresh surgical specimens and peritoneal washings were obtained from patients with endometrial cancer who provided informed consent. Mononuclear cells were isolated and analyzed by flow cytometry to quantify CD3-CD19+ B cells, CD3-CD16+CD56+ natural killer (NK) cells, CD4+CD25+ regulatory T cells (Tregs), and CD3+CD8+ cytotoxic T cells. These populations were compared according to histopathologic features and immunohistochemical results, including lymphovascular space invasion (LVSI), PD-L1, estrogen receptor (ER), p53, HER2, mismatch repair proteins (MLH1, MSH2, MSH6, PMS2), and microsatellite instability (MSI) status. Results: Sixty immune profiles derived from endometrial cancer specimens were evaluable. Higher proportions of CD3-CD19+ B cells were observed in LVSI-positive tumors, whereas ER-positive tumors showed lower levels of this subset (both p<0.05). CD3-CD16+CD56+ NK cells were more prevalent in cases with deficient mismatch repair (dMMR). CD4+CD25+ Tregs were reduced in ER-positive tumors (p<0.05). CD3+CD8+ cytotoxic T cells were less frequent in HER2-positive tumors but increased in dMMR cases. No additional significant associations were detected between immune subsets and the other immunohistochemical markers examined. Conclusions: Immune cell profiles obtained from endometrial tumor tissue and peritoneal washing cytology appear to capture key clinicopathologic and molecular features, particularly LVSI, ER, HER2, and mismatch repair status. Validation in larger cohorts with integrated oncologic outcome data is needed to clarify the prognostic and potential predictive relevance of these immune signatures. Citation Format: Seungmee Lee, Jin-Young Kim, Shin-Wha Lee, Eun Ji Nam, Ji Hae Seo, Sojin Shin. Linking tumor-infiltrating immune cells from tissue and peritoneal cytology to molecular markers in endometrial cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 813.