Tumor-expressed GPNMB orchestrates Siglec-9 ⁺ TAM polarization and EMT to promote metastasis in triple-negative breast cancer

Metastasis remains the leading cause of cancer-related mortality, driven by complex interactions within the tumor microenvironment (TME). Tumor-associated macrophages (TAMs) play a pivotal role in metastatic progression, yet their molecular diversity and upstream regulators remain poorly defined. Glycoprotein nonmetastatic melanoma protein B (GPNMB), overexpressed in subsets of tumors including triple-negative breast cancer (TNBC), is implicated in epithelial-mesenchymal transition (EMT) and cancer stemness. Recent single-cell RNA sequencing (scRNA-seq) identified GPNMB as a marker of immunosuppressive TAMs associated with poor prognosis, but its mechanistic role in TNBC has remained unclear. Coculturing monocytic cells with three-dimensional TNBC spheres induced GPNMB⁺ TAMs expressing sialic acid-binding immunoglobulin-like lectin 9 (Siglec-9). Tumor-expressed GPNMB promotes monocyte-to-TAM polarization by inducing secondary GPNMB expression in monocytes, establishing a feed-forward amplification loop. GPNMB knockdown in TNBC cells inhibited immunosuppressive TAM subsets, including Siglec-9⁺ and EMT-associated populations, as determined by deconvolution of bulk RNA-seq data using a custom TAM signature matrix derived from publicly available TNBC scRNA-seq datasets. TNBC-derived GPNMB carried α2,3-sialylation, whereas macrophage-derived GPNMB carried α2,6-sialylation, enabling differential Siglec-9 recognition. Elevated GPNMB and Siglec-9 correlated with poor prognosis in TNBCcohorts. Importantly, dual inhibition of Siglec-E (murine Siglec-9 ortholog) and PD-1 reduced tumor stemness, suppressed IL-6-dependent EMT, and limited lung metastasis in vivo. The GPNMB-Siglec-9 axis thus represents a critical glyco-immunological checkpoint driving TAM-mediated metastasis, providing a promising therapeutic target in TNBC.