Background There has been a growing emphasis on the investigation of gut microbiota in enhancing the effectiveness of PD-1 inhibitors. However, there are many technical and ethical challenges associated with directly utilizing live bacteria in the clinical translation process. Therefore, this study aims to explore the potential for enhancing the effectiveness of PD-1 inhibitors through the use of pasteurized bacteria and to clarify its mechanism of action.

Methods In this study, we used subcutaneous tumor-bearing mice to validate the impact of pasteurized B. thetaiotaomicron on the efficacy of PD-1 inhibitors against MSI-L colorectal cancer. Subsequently, we assessed the infiltration of immune cells in tumor microenvironment (TME) and the expression of PD-L1 on dendritic cells (DCs) in tumor-draining lymph nodes (TDLN) by flow cytometry and analyzed the alterations in the expression and concentration of chemokines in tumor cells by RT-PCR and ELISA experiments. Additionally, we verified the effect of pasteurized B. thetaiotaomicron on the migration of T cells into the TME through transwell experiments and animal experiments.

Results We found that pasteurized B. thetaiotaomicron could enhance the efficacy of PD-1 inhibitors by increasing the number of CD4⁺ T cells and CD8⁺ T cells in the TME. Meanwhile, we observed that pasteurized B. thetaiotaomicron could increase the expression of PD-L1 on DCs, particularly CD103⁺ DCs in TDLN. In addition, we found that pasteurized B. thetaiotaomicron can promote the expression and secretion of chemokines (CXCL9, CXCL11) in tumor cells, which is conducive to promoting the migration of CD4⁺ T cells and CD8⁺ T cells to TME and enhancing the efficacy of PD-1 inhibitors against MSI-L colorectal cancer.

Conclusions Pasteurized B. thetaiotaomicron upregulates the expression of PD-L1 on DCs, leading to the suppression of T-cell activation. Blocking the PD-1/PD-L1 signaling pathway between DCs and T cells can reactivate T cells. Moreover, pasteurized B. thetaiotaomicron also induces tumor cells to secrete chemokines such as CXCL9 and CXCL11, promoting the migration of reactivated T cells to the tumor microenvironment and enhancing the efficacy of PD-1 inhibitors.