IL-7-secreting Salmonella synergizes with PD-L1 blockade to induce durable antitumor immune memory

Background: Interleukin-7 (IL-7) is a key cytokine that maintains the survival and homeostasis of T cells and induces antitumor immune responses. This study investigated the antitumor effects of attenuated Salmonella genetically engineered to secrete IL-7 against cancer by activating immune cell responses. Methods: Mouse colorectal cancer cell line CT26 and S. typhimurium were used. The S. typhimurium was engineered to secrete IL-7 based on an arabinose induction system. The anti-tumor effect of the engineered Salmonella was examined in vitro and in vivo , both alone and in combination with anti-PD-L1 treatment. Flow cytometry was performed to analyze immune cell populations in tumors and spleens to elucidate the underlying mechanisms. Results: The genetically engineered Salmonella secreted IL-7 upon arabinose induction without affecting its growth. IL-7 secretion in response to arabinose was confirmed via ELISA. In in vivo experiments, the arabinose-induced group exhibited enhanced antitumor effects and improved survival compared to the non-induced group, with body weight recovery observed within five days post- Salmonella injection. TUNEL analysis showed increased apoptosis in tumor tissues following arabinose induction, along with a significant reduction in Ki67 expression. On days 3 and 7 post-induction, flow cytometry revealed an increased T cell population. The activation of cytotoxic T cells was confirmed by elevated levels of granzyme B, perforin, and IFN-gamma. Co-administration of IL-7-secreting Salmonella with anti-PD-L1 antibody resulted in significantly greater tumor suppression compared to Salmonella secreting IL-7 alone. Notably, in the arabinose-induced group, complete tumor regression was observed in 4 out of 5 mice. When tumor cells were re-challenged on the opposite flank 60 days later, no tumor recurrence was detected, indicating the establishment of long-term antitumor immunity. Subsequent flow cytometric analysis of splenocytes confirmed an increased population of memory T cells, supporting the development of durable immune memory following IL-7 secretion. Biodistribution analysis showed that Salmonella was cleared from other organs over time, with predominant accumulation in tumor tissue. Biochemical analyses of liver and kidney function revealed no abnormalities in the arabinose-treated group. The biological safety of the Salmonella strain was confirmed through biodistribution studies and H&E staining. Conclusion: The Salmonella secreting IL-7 enhances antitumor activity by inducing T cell-mediated immune responses, particularly showing potential as an immunotherapy strategy for solid tumors when combined with anti-PD-L1 antibodies. Further research is needed to evaluate its efficacy in various tumor models, long-term safety, and the potential for combination with other immune checkpoint inhibitors.