Abstract 4978: MHC class 1 expression and distribution of multiple immune cells in chemotherapy-naïve triple-negative breast cancers: lack of antigen-presentation can bring high TILs and good prognosis, possibly via decreased regulatory T-cell compos

Abstract Triple-negative breast cancers (TNBCs) have not been candidates for targeted therapies because they are negative for ER, PgR, and HER2. In addition, the aggressive nature of TNBCs makes their treatment challenging. However, recent advances include immune checkpoint inhibitors (ICIs), which are effective against some TNBCs. The abundance of tumor-infiltrating lymphocytes (TILs) in the cancer microenvironment correlates with better prognosis, and improved efficacy of chemotherapy and ICIs. We don’t know yet in detail what makes the difference, high or low TIL, and wanted to investigate cancer immunity cycles. We retrospectively recruited 153 consecutive TNBC patients from a single institution, all of whom had primary curative surgery and standard adjuvant chemotherapy. Representative surgical samples were immunohistochemically stained for MHC class 1 and evaluated by a pathologist in a score of 3 (diffusely positive) to 0 (almost negative). Heterogeneous staining was scored as 2 (positive area>50%) or 1 (positive area<50%). TILs were estimated after hematoxylin & eosin staining, and relapse-free survival was recorded. Thirty cases with different levels of MHC expression were stained for CD3, CD4, CD8, CD20, CD56, CD204, FoxP3, and pan-cytokeratin using a multi-fluorescence technique. Six-hundred small areas of interest were selected and assigned to one of four categories according to tumoral MHC (positive/negative) and stromal TILs (high/low). Staining signals were calculated on the images, and clustered to determine the distribution of cells in areas of tumor or stroma. TIL abundance was an independent prognostic factor (HR: 2.52, p=0.013). MHC class 1 expression was diffusely positive in many cases (score 3 in 88 of 153 cases), and four cases were negative (score 0). MHC positivity correlated with higher TILs; nevertheless, the four cases with an MHC score of 0 had high TILs (42.5% on average) and showed no recurrence or death during the mean follow-up period of 14.5 years. Detailed composition of immune cells in areas of high TIL showed infiltration by many CD3+ and CD20+ cells, while CD204+ cells were numerous in areas of low TIL. MHC class 1-negative areas had fewer FoxP3+ cells than positive areas, even in areas of high TIL. There was no significant difference in CD56+ cell number between MHC-positive and -negative areas. Cluster analysis of immune cells revealed CD204 to be a major divider of the population. MHC class 1-negative TNBCs contained relatively high TILs, the main components of which were CD3+ and CD20+ cells. The presence of FoxP3+ cells seems to require HMC class 1, and the lack of negative regulation by FoxP3 might have resulted in high TIL and better prognosis. Our findings indicate a missing link between antigen presentation and T-cell recruitment. Citation Format: Makiko Yamashita, Akira I. Hida, Naomi Gondo, Yasuyo Ohi, Shigehisa Kitano. MHC class 1 expression and distribution of multiple immune cells in chemotherapy-naïve triple-negative breast cancers: lack of antigen-presentation can bring high TILs and good prognosis, possibly via decreased regulatory T-cell composition [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 4978.