Inducible secretion of LIGHT by engineered probiotics enables localized cytokine therapy and robust antitumor immunity

Triple-negative breast Cancer (TNBC), exemplified by the 4T1 model, exhibits a highly immunosuppressive tumor microenvironment (TME) and strong metastatic potential, resulting in poor responses to current immunotherapies. TNFSF14 (LIGHT) is a potent immunostimulatory cytokine capable of remodeling the TME through the HVEM and LTβR signaling. However, its systemic administration is limited by dose-dependent toxicity. Here, we developed a tumor microenvironment-responsive engineered E. coli system for targeted LIGHT delivery. LIGHT expression was controlled by a lactic acid-inducible promoter and fused with pelB for periplasmic secretion, ensuring selective activation within lactic acid-rich tumor cores. In BALB/c mice bearing 4T1 subcutaneous tumors and experimental lung metastases, intravenously administered bacteria were evaluated for biodistribution, antitumor efficacy, and immune modulation. The engineered strain selectively colonized tumors, achieving strong intratumoral LIGHT expression with minimal systemic exposure. Compared with vector controls, LIGHT-expressing bacteria significantly suppressed primary tumor growth and markedly reduced lung metastatic lesions. Mechanistically, this treatment increased intratumoral CD8⁺ T-cell infiltration, enhanced dendritic cell maturation, and shifted the TME toward an immune-activated state. Thus, this lactic acid-responsive Bacterial platform enables safe, localized cytokine delivery and represents a promising therapeutic strategy for refractory TNBC.