Beyond the long-lasting use of BCG as a vaccine against TB, it is an approved immunotherapy for high-risk, non-muscle invasive bladder cancer (NMIBC) patients, used for preventing relapse in carcinoma in situ (CIS) of the bladder. About 75% of the bladder cancer (BC) patients are diagnosed with NMIBC and one third are treated with a BCG instillation protocol of which about two-thirds of the patients stay disease-free [10,11]. Following completion of the BCG treatment with the repeated instillation protocol, about two-thirds of the patients remain disease-free. While the exact mechanism underlying the protective effect of BCG is still unknown, several studies have suggested that it stimulates the Th1 response that leads to cytokine secretion (e.g., IL-2, IL-8, IL-18, and TNFα), which activates a cytotoxic response toward the remaining cancer cells [10,12].

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8151667/