Abstract 1535: Differential immune microenvironments in triple-negative breast cancer and hormone receptor-positive HER2-negative breast cancer: A spatial analysis via multiplex immunofluorescence

Abstract Objective: To elucidate differences in immune cell (IC) composition and their spatial distribution between triple-negative breast cancer (TNBC) and hormone receptor-positive HER2-negative breast cancer (HR+HER2-BC), and to evaluate the effect of PD-L1 status on these parameters. Methods: We selected 32 patients with ≥60% tumor-infiltrating lymphocytes (TIL) (TNBC, n=18; HR+HER2- BC, n=14). Multiplex immunofluorescence was employed on resected tumor tissues. Using markers such as CK, CD20, CD8, CD4, and FOXP3, IC types and tumor cells (TC) were identified in both stromal and tumor areas. We analyzed and compared cellular density and proportional distribution of IC, and spatial interactions (both IC-TC and IC-IC) based on tumor subtype and PD-L1 status. Results: TNBC exhibited a unique IC composition with a higher proportion of CD8+IC (stroma: 27% vs 17%, p0. 001; tumor: 54% vs 31%, p0. 001) and CD4+FOXP3+IC (stroma: 3. 9% vs 3. 0%, p=0. 036) than HR+HER2-BC. Among PD-L1 positive cases, TNBC had a denser stromal population of CD4+FOXP3+IC (146. 4±67. 1/mm2 vs 114. 3±146. 9/mm2, p=0. 036). Both tumor subtypes displayed varied IC compositions based on PD-L1 status. Density plots revealed a pronounced clustering of IC near TC in TNBC, especially in PD-L1 positive cases. Analysis of cell-cell interactions showed diverse adjacency patterns between TC and various IC combinations with frequencies differing by tumor subtype and PD-L1 status. Conclusion: The immune landscape, typified by IC composition and spatial distribution, varies between TNBC and HR+HER2- BC, with PD-L1 status further modulating these patterns. Citation Format: Yoon Jin Cha, Inho Park, Su-Jin Shin. Differential immune microenvironments in triple-negative breast cancer and hormone receptor-positive HER2-negative breast cancer: A spatial analysis via multiplex immunofluorescence 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 1535.