Abstract 6752: Mapping the spatial organisation of B cells and immune signalling in TNBC to characterise response to immunotherapy

Abstract In recent years, B cells and tertiary lymphoid structures (TLS) have emerged as key predictors of immunotherapy response across multiple cancers. B cells exist in various states of spatial organisation within the tumour microenvironment (TME), likely supporting many of their functional roles. However, the landscape of B cell spatial organisation in cancer, and specifically in triple-negative breast cancer (TNBC), is poorly characterised. Organisation and function are processes that are likely orchestrated by a diverse set of immune signalling, but the interplay between organisation and its underlying signalling remains largely uncharacterised in cancer, underscoring the need for a comprehensive and systematic investigation. To characterise the spatial landscape of B cells and immune signalling, we implemented a novel approach for the simultaneous detection of protein and RNA in situ using imaging mass cytometry. We applied this assay to a randomised immunotherapy clinical trial, NeoTRIP, where patients with TNBC were treated with neoadjuvant chemotherapy with or without atezolizumab. Samples were collected from 268 patients at three timepoints: baseline, on-treatment and post-treatment. This study identified that germinal centre B, B and plasma cell densities were significantly associated with response to immunotherapy on-treatment. We identified distinct B and plasma cell niches in TNBC, and showed that B activated niche cell densities were also strong predictors of response to immunotherapy. Additionally, we found that many B and plasma to TME interactions were enriched among immunotherapy responders. These findings were complemented by strong treatment-induced cellular dynamics. Strikingly, the most marked increase in TME cell phenotype abundance in immunotherapy responders was observed in B cells - particularly naïve B cells and broader B cell populations, suggesting that effective immunotherapy induces their expansion following treatment. We subsequently showed that cytokines likely behave in a spatially coordinated manner in the TME, being significantly enriched in the vicinity of the observed B and plasma cell niches. Cytokine expression mirrored the observed immune expansion on-treatment and was also associated with immunotherapy response. Specifically, cytokine-enriched niches appeared to be a feature of immunotherapy responders, suggesting that together organisation and signalling play a central role in treatment responses. Together, these findings highlight a previously unexplored diversity of B cell spatial organisation and its associated immune signalling in TNBC, suggesting that B cell features may serve as on-treatment predictors of immunotherapy response. Citation Format: Lubna Ahmad, Xiao Qian Wang, Neus Masqué-Soler, Ellen Schrader, Chiun-Sheng Huang, Daniel Egle, Maurizio Callari, Matteo Dugo, Begoña Bermejo, Claudio Zamagni, Marc Thill, Antonio Antón, Stefania Russo, Elena Sevillano, Eva Maria Ciruelos, Richard Greil, Balázs Győrffy, Vladimir Semiglazov, Marco Colleoni, Catherine M. Kelly, Lucia Del Mastro, Gabriella Mariani, Guiseppe Viale, Luca Gianni, Giampaolo Bianchini, Raza Ali. Mapping the spatial organisation of B cells and immune signalling in TNBC to characterise response to immunotherapy 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 6752.