Abstract 1967: Plexin-B2 mediates tumor cell clustering with monocyte interactions in breast cancer metastasis

Abstract The dogma of single tumor cell-mediated metastasis has been challenged by the discovery of multicellular circulating tumor cells (CTCs) clusters which are 20-50 times more likely to produce viable metastasis than single CTCs. However, the molecular mechanisms underlying CTC clusters and CTC interactions with immune cells are largely unknown. Combining integrated proteomic ranking, targeted analyses, and functional determination, we identified the transmembrane protein Plexin-B2 (PLXNB2) as one of the top hits whose expression in human primary tumors is associated with unfavorable distant metastasis-free survival. PLXNB2 expression is also enriched in CTC clusters compared to single CTCs from patients with advanced breast cancer. Loss of PLXNB2 reduces the formation of homotypic tumor cell clusters and heterotypic tumor-myeloid cell clusters in triple-negative breast cancer. Interactions between PLXNB2 and its ligand Sema4C on tumor cells mediate homotypic cluster formation, and PLXNB2 binding with Sema4A on myeloid cells (monocytes) promotes heterotypic CTC cluster formation, thereby driving lung metastasis in breast cancer. Depletion of Plxnb2 in mouse breast tumor cells also blocked CTC-myeloid cell clustering and spontaneous metastasis in immune-competent mice. Additionally, using global proteomic analysis, we identified novel downstream effectors of the PLXNB2 pathway associated with tumor cell clustering in breast cancer. Thus, PLXNB2 is a novel therapeutic target for preventing new metastasis in triple-negative breast cancer. Citation Format: Emma Schuster, Nurmaa Dashzeveg, Fangjia Tong, Yuzhi Jia, Lamiaa El-Shennawy, Tong Zhang, Andrew Hoffmann, Kibria Golam, Youbin Zhang, Joshua Squires, Erika Ramos, Rokana Taftaf, David Scholten, Hannah Almubarak, Valery Adorno-Cruz, David Sullivan, Carolina Reduzzi, Allegra Minor, William Purev-Ochir, Rong Xu, Kalliopi Siziopikou, Leonidas Plantanias, Ami Shah, William Muller, William Gradishar, Chia-Feng Tsai, Tujin Shi, Huiping Liu. Plexin-B2 mediates tumor cell clustering with monocyte interactions in breast cancer metastasis. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 1967.