The tumor microenvironment is characterized by an immunosuppressive state. Although PD-1/PD-L1 blockade therapy activates the immune system against tumors, it has limited long-term efficacy, prompting the development of combination therapies with targeted treatments to improve cancer treatment outcomes. Recent advancements have revitalized interest in using attenuated Salmonella strains as cancer therapeutics that target tumors, induce immune responses, and promote tumor cell death, although complete tumor suppression remains challenging. We aimed to induce antitumor effects by activating the suppressed immune system within the tumor microenvironment using Salmonella-mediated secretion of interleukin-21 (IL-21). We used the tumor-targeting ability of Salmonella and its flagellar type-3 secretion system (FT3SS) to induce the secretion of IL-21 into the tumor microenvironment via the flagellar system and evaluated the local immune response. We also evaluated the efficacy of combining Salmonella-mediated IL-21 delivery and anti-PD-L1 therapy in a mouse model. IL-21 secretion promoted the recruitment of CD4⁺ and CD8⁺ T cells and enhanced the expression of cytotoxicity-related molecules. Tumor-bearing mice treated with the combination therapy with anti-PD-L1 antibodies showed improved survival rates and enhanced tumor growth inhibition. This study demonstrates the tumor-targeting capability and in vivo safety of Salmonella, highlighting its potential as a powerful cancer therapy platform.

Engineering Salmonella  /  Interleukin-21  /  Immunotherapy  /  Immune checkpoint inhibitor  /  Flagellar type-3 secretion system  /  Tumor microenvironment  /  Combination therapy  /  Anti-tumor effect