Co-delivery of chemokine CXCL9 and costimulatory ligand TNFSF9 by mesenchymal stem cells reprograms the immune microenvironment for triple-negative breast cancer

Triple-negative breast cancer (TNBC) is a particularly aggressive subtype of breast cancer with a high risk of relapse and metastasis. Due to limited tumor immune infiltration in TNBC, the effectiveness of immunotherapy is constrained. In this study, analysis of human datasets (TCGA-BRCA and Kim cohorts) revealed that tumor necrosis factor superfamily member 9 (TNFSF9) and C-X-C motif chemokine ligand (CXCL9) expressions correlate with improved prognosis and enhanced immune cell infiltration. To exploit this, we engineered human umbilical cord mesenchymal stem cells (hUC-MSCs) to co-express TNFSF9 and CXCL9 (MSC-T9C9). In murine TNBC models, the engineered MSC-T9C9 recruits CD8⁺ T cells and natural killer (NK) cells to the tumor site, thereby increasing immune infiltration and remodeling the tumor immune microenvironment through activating CD8⁺ T cells and NK cells. This therapeutic strategy proved both effective and well-tolerated. Moreover, MSC-T9C9 enhanced the prognosis and therapeutic efficacy of anti-PD-1 immunotherapy in vivo. These findings demonstrate that the engineered MSC co-expressing chemokine CXCL9 and costimulatory ligand TNFSF9 effectively suppresses TNBC growth by reprogramming the intra-tumoral immune landscape, which offers a promising and safe immunotherapeutic strategy for TNBC treatment.