Abstract PS3-12-18: Exploratory Analysis of Immune Cell Subsets in Luminal B HER2-negative Breast Cancer and Their Association with Pathologic Response

Abstract Background: Although tumor-infiltrating lymphocytes (TILs) are associated with better outcomes in triple-negative and HER2-positive breast cancer, their prognostic value in luminal HER2-negative subtype remains unclear. This study aimed to evaluate TIL subpopulations in luminal breast cancer and their association with pathologic complete response. Materials and methods: This prospective trial included 72 pts. with ER+ (10%), HER2 negative breast cancer II-III stages. Both stromal and intratumoral subpopulations of TILs were assessed with flow cytometry on a tumor sample obtained by core-biopsy directly before the start of neoadjuvant chemotherapy (NACT). We analyzed CD3-CD8+, CD3+HLA- DR+, CD3-HLA-DR+, HLA-DR+, CD8+CD28+, CD8+CD28-, CD8+CD28+/CD8+CD28-, CD4+/CD8+, 11b+28-, 11b+28+, 11b- 28-, 11b-28+, CD8+CD4+, CD4+CD152+ minor subpopulations and other linear subpopulations. All the pts received NACT with 4 cycles of dose-dense AC every 2 weeks then 4 cycles of docetaxel every 3 weeks in a N.N. Blokhin National Medical Research from 2018 to 2023. Results: The mean value of TIL is 2.58% (median 0.9%; range 0-30.6%). Stage II was diagnosed in 7 patients (9.7%), and stage III in 65 patients (90.3%). G2 tumors were found in 58 patients (80.6%), G3 in 14 (19.4%). The mean age was 46 ± 9 years (range 25-70), with most patients being premenopausal (72.2%). No significant differences in TIL levels were observed across age groups (median 0.9% in patients ≤50 years vs. 0.8% in 50 years; p = 0.450). Nevertheless, older patients showed a numerical increase in CD16+ and CD8+CD279+(PD-1+) cells, along with a decrease in CD4+CD25+ lymphocytes, consistent with previous reports. There was no significant difference in mean value of total TIL levels between patients with good (RCB 0-1) and poor (RCB 2-3) pathologic response (3.92% vs. 2.09%, p = 0.137). Most analyzed subpopulations (including CD3+CD4+, CD3+CD8+, CD4+CD25highCD127-/low, and others) showed no significant association with treatment response. However, CD4+CD25+ (11.2% vs. 7.5%; p = 0.013) and CD8+PD-1+ lymphocytes (9.5% vs. 16.3%; p = 0.041) significantly differed between the RCB 0-1 and RCB 2-3 groups. Higher levels of CD4+CD25+ (more than median) lymphocytes were significantly associated with favorable response (RCB 0-1 - 57.1% vs. 8.3%, p = 0.001), while lower levels of CD8+PD-1+ lymphocytes were also associated with a better outcome (RCB0-1 46.2% vs. 12.0%, p = 0.007). Univariate regression analysis identified the following as predictors of better pathologic response: absence of lymph node involvement (N0), G3 tumor grade, age ≤50 years, progesterone receptor expression 20%, low CD8+PD-1+, and high CD4+CD25+ levels. However, none of these factors retained independent predictive value in multivariate analysis. Conclusion: These findings suggest a potential prognostic relevance of specific TIL subpopulations, particularly CD4+CD25+ and CD8+PD-1+ lymphocytes, in luminal breast cancer. Further studies with larger cohorts are needed to validate these observations. Citation Format: M. Khoroshilov, E. Kovalenko, Z. Kadagidze, T. Zabotina, E. Evdokimova, Y. Zhulikov, A. Petrovskiy, I. Vorotnikov, M. Kiselevskiy, E. Artamonova. Exploratory Analysis of Immune Cell Subsets in Luminal B HER2-negative Breast Cancer and Their Association with Pathologic Response abstract. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS3-12-18.