Neutrophil-like monocytes (NeuMos) with high levels of CXCL2 expression orchestrate systemic granulopoiesis and drive immunotherapy resistance in TNBC

Triple-negative breast cancer (TNBC) is characterized by early metastatic spread and poor response to immunotherapy, yet the systemic immune mechanisms underlying this aggressive behavior remain poorly defined. Using a comprehensive multi-omics approach that integrates single-cell RNA sequencing (scRNA-seq), high-dimensional mass cytometry (CyTOF), flow cytometry analyses, in vivo genetic perturbations, and multiple syngeneic TNBC models, our preliminary data suggested that recently introduced neutrophil-like monocyte population (NeuMo) emerges selectively within highly metastatic tumors. NeuMos exhibit a dynamic monocyte-granulocyte transcriptional profile and initiate a pro-inflammatory cascade driven by CXCL2, and G-CSF signaling. This axis induces robust emergency granulopoiesis in the spleen, markedly expanding immature CD11b⁺Ly6G⁺CXCR2⁺ granulocytic cells that infiltrate tumors and peripheral tissues establishing systemic immunosuppression. In contrast, non-metastatic EMT6 tumors maintain balanced adaptive and innate immune compartments with minimal granulocytic activation, highlighting the specificity of NeuMo-driven inflammation to aggressive TNBC states. Functional experiments demonstrate that deletion of S100A9, an upstream amplifier of this inflammatory circuit, disrupts granulocytic expansion, reduces splenic hypertrophy, diminishes lung metastasis, and significantly enhances the therapeutic efficacy of PD-L1 blockade in both AT3 and E0771 TNBC models. These findings put S100A9 as a critical mediator of immunotherapy resistance. Analysis of human TNBC datasets, including TCGA, reveals early induction of S100A8/A9, G-CSF, and CXCL1/2 gene programs associated with high-grade tumors and poor survival, underscoring the clinical relevance of this pathway. Collectively, our work identifies NeuMo-driven inflammatory mediator as a central mechanism linking TNBC tumor-intrinsic programs to systemic immune remodeling, metastasis, and checkpoint resistance. These results not only define a novel myeloid-based biomarker signature for aggressive TNBC but also nominate the S100A9/CXCL2/G-CSF axis as an actionable therapeutic target to improve immunotherapy outcomes.