NK cells mediate preventive efficacy of intravenous BCG against lung metastasis in mice

Lung metastases frequently arise from primary tumors, including bladder cancer, and represent a critical negative prognostic factor. Natural Killer (NK) cells have shown to play a vital role in controlling metastasis. Consequently, tumor cells have evolved specific mechanisms to evade NK cell-mediated immune surveillance, promoting metastasis and resistance to immunotherapy. In this study, we investigated the prophylactic and therapeutic potential of intravenous Bacillus Calmette–Guérin (BCG) in preventing lung metastases from bladder cancer cells using a murine model. We demonstrated that prophylactic BCG administration significantly reduced tumor burden and prolonged survival, largely through NK cell activation. However, BCG treatment was ineffective when administered over established tumors, likely due to tumor-driven immune evasion mechanisms. Our results revealed the contribution of interferon-gamma (IFN-γ) to tumor resistance. Tumor cells exposed to IFN-γ upregulated were more resistant to BCG in vivo , which correlated with the overexpression of immune checkpoint molecules, whereas disruption of the IFN-γ signaling pathway in tumor cells partially restored the therapeutic efficacy of BCG. Our findings highlight the importance of understanding tumor immune escape mechanisms and suggest that BCG could be a promising treatment for preventing lung metastases in bladder cancer.