Abstract PO1-25-01: LMTK3 promotes macrophage M2 polarisation and suppresses monocyte infiltration in breast cancer by altering the proteomic cargo of exosomes

Abstract Background: Breast cancer (BC) is a leading cause of cancer death and despite numerous treatment advancements, metastatic BC is incurable. Extracellular vesicles (EVs) are lipid-delimited particles released from cells which contain bioactive protein and nucleic acid cargo. Numerous studies have shown their importance in facilitating communication between cancer cells and the tumour microenvironment, implicating them as mediators of many aspects of carcinogenesis. EV-mediated communication between BC and the innate immune system plays a crucial role in BC progression, metastasis and the maintenance of an immunosuppressive microenvironment. LMTK3 overexpression in BC promotes therapy resistance, disease progression and invasion and metastasis, leading to a poorer prognosis in patients. Analysis of deconvoluted TCGA data indicated that LMTK3 overexpression also correlates with exclusion of M1 macrophages from the tumour microenvironment. This study aimed to explore the role of LMTK3 in EV-mediated communication between BC and immune cells. Methods EVs were collected from conditioned media of wild-type and LMTK3-overexpressing BC cells through differential ultracentrifugation, then characterised by transmission electron microscopy, nanoparticle tracking analysis, quantitative mass spectrometry and Western blotting. Monocytes were treated with EVs and proteomic and phosphoproteomic changes were analysed. 3D coculture models of BC and immune cells were used to assess monocyte and macrophage infiltration, polarisation and anti-cancer activity. Immunofluorescence and flow cytometry was used to assess macrophage polarisation. Bioinformatic analysis of TCGA datasets validated the findings in a clinical setting and the data was also validated in vivo, using syngeneic immunocompetent mouse models. Results LMTK3 overexpression in BC cells promotes the upregulation of a subpopulation of large (80-150nm) CD81+ EVs which are significantly enriched in oncogenic and immunoregulatory proteins, including LDHB, ARHGEF15, PSAT1 and HIC2. EVs from LMTK3-overexpressing but not control BC cells strongly reduce the ability of monocytes to infiltrate BC tumour spheroids and migrate through Matrigel. Proteomics analysis of the monocytes showed an enrichment in M2 polarisation proteins and a downregulation of proteins essential for migration, including S100A8, KLF13 and TRPC3. LMTK3 overexpression in BC also promotes M2 polarisation and exclusion of M1 macrophages in vivo. Summary LMTK3 overexpression in BC correlates with poor prognosis and reduced overall survival. We hypothesise that this is, in part, due to monocyte M2 polarisation and exclusion of M1 macrophages from the TME, mediated by LMTK3-dependent alterations in EV cargo, contributing to tumour immune evasion. Citation Format: Mark Samuels, William Jones, Charlotte Turner, Georgios Giamas. LMTK3 promotes macrophage M2 polarisation and suppresses monocyte infiltration in breast cancer by altering the proteomic cargo of exosomes abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO1-25-01.