Neutrophil extracellular traps promote lung colonization of triple-negative breast cancer by inducing mesenchymal to epithelial transition.

Metastatic recurrence is still a major problem for triple-negative breast cancer (TNBC). Neutrophil extracellular traps (NETs) in the pulmonary microenvironment are known to play key roles in the metastatic colonization and outgrowth of TNBC. However, the exact role that NETs play in determining the phenotypic plasticity and reactivation of disseminated tumor cells (DTCs) during acute relapse remains unclear. Here, we utilized an orthotopic, acute recurrent metastasis mouse model by transplanting 4T1 cells into wild-type or NET-deficient (Padi4-/- ) mice, followed by primary tumor resection. Although Padi4 deficiency had no effect on the growth of the primary tumors, it significantly reduced lung metastasis and prolonged survival. 4T1 cells re-established from WT lungs displayed a highly proliferative epithelial phenotype, whereas those from Padi4-/- mice appeared to maintain a quiescent mesenchymal state. Crucially, tissue-adapted NET-forming pulmonary neutrophils could facilitate a mesenchymal-to-epithelial transition (MET)-like switch by robustly upregulating transforming growth factor-beta (TGF-β) negative regulators, thereby attenuating autocrine signaling. Collectively, our findings suggest that tissue-adapted NETs serve as crucial microenvironmental cues that support the exit from quiescence and drive metastatic outgrowth, highlighting a promising perioperative therapeutic target for TNBC.