Jiangshui-derived Lactobacillus plantarum GR-4 enhances the therapeutic efficacy of PD-1/PD-L1 inhibitors against gastric cancer

Objective To investigate whether the Jiangshui-derived probiotic Lactobacillus plantarum GR-4 can enhance gastric cancer sensitivity to PD-1 inhibitors and reshape the tumor immune microenvironment. Methods A mouse gastric cancer model was established. Mice received oral GR-4 monotherapy, PD-1 blocking antibody monotherapy, or combined GR-4 and PD-1 antibody treatment. In vivo antitumor efficacy was evaluated by measuring tumor weight. We assessed CD8 + T-cell infiltration, cytokines related to T-cell activation and differentiation, and the expression of the activation marker CD69 on CD3 + /CD8 + T cells within tumor tissues. 16S rRNA sequencing of gut microbiota and fecal metabolomic analysis were performed to explore the regulatory effects of GR-4 on gut microbiota composition and metabolic profiles. In vitro , human gastric cancer BGC-823 cells were treated with GR-4 combined with PD-1 intervention, and transcriptome sequencing was applied to further validate the underlying molecular regulatory mechanisms. Results In vivo results demonstrated that combined GR-4 and PD-1 antibody treatment exerted prominent synergistic antitumor activity, effectively suppressing tumor growth while markedly mitigating PD-1-associated body weight loss (p < 0.01). Further assays revealed that GR-4 synergistically elevated tumor-infiltrating CD3 + /CD8 + T-cell levels and upregulated CD69 expression on these cells, thereby remodelling the tumor immune microenvironment. Additionally, GR-4, whether administered alone or combined with PD-1 antibody, optimized gut microbiota structure, increased the abundance of probiotic genera including Ligilactobacillus and Bacteroides , reversed intestinal metabolic disturbances, and raised the levels of short-chain fatty-acid-related metabolites such as ethoxyacetic acid and 2,5-dihydroxybenzoic acid. Transcriptomic sequencing showed significant enrichment of the PD-1 checkpoint pathway in the GR-4 group versus Ctrl (p=0.01), whereas no significant enrichment was observed between the PD-1 inhibitor group and GR-4/PD-1 inhibitor group (p=0.14). Conclusion This study is the first to confirm that Lactobacillus plantarum GR-4 synergizes with PD-1 inhibitors to restrain gastric cancer progression via T-cell activation and elevated intratumoral T-cell infiltration. These findings provide novel theoretical evidence and a potential combinatorial immunotherapeutic strategy for gastric cancer.