Gut Lactobacillus contribute to the progression of breast cancer by affecting the anti-tumor activities of immune cells in the TME of tumor-bearing mice.

• Gut microbiota dysbiosis influences the progression of breast cancer. • Less Lactobacillus may be beneficial for anti-tumor immune responses in the TME. • Lactobacillus-mediated pro-tumor effect is related to the production of lactic acid. • High concentration of lactic acid might diminish the anti-tumor immunity. Studies have proven that gut microbiota dysbiosis may influence the carcinogenesis and outcomes of multiple cancers. However, it is still unclear whether gut microbiota dysbiosis affect the progression of breast cancer, especially triple-negative breast cancer. In the present study, by using gut microbiota dysbiosis murine model established by treatment of mice with streptomycin, we found Lactobacillus and the metabolite-lactic acid are the pivotal factors for 4T1 tumor progression. In fact, streptomycin-treated mice exhibited slower tumor growth, in parallel with less abundance of Lactobacillus in the gut. Supplementation with Lactobacillus resulted in a rapid tumor growth, following a decrease in the expression of mRNAs for anti-tumor-related factors but an increase in the M2 polarization. The elevated percentages of IFN-γ-producing CD4+T cells and CD8+T cells in the tumor microenvironment of streptomycin-treated tumor-bearing mice may be vanished by supplementation of Lactobacillus. It seems likely that lactobacillus-mediated pro-tumor effect is related to the production of lactic acid. A decrease in the levels of lactic acid in the cecal feces and tumor tissues were observed in streptomycin-treated tumor bearing mice. However, supplementation of Lactobacillus can restore streptomycin-reduced concentration of lactic acid in the tumor tissues, suggesting that gut Lactobacillus are the source of lactic acid. Bioinformatics analysis result suggests high concentration of lactic acid in tumor sites may be related to the diminished anti-tumor immunity in the TME. This study reveals a correlation between gut Lactobacillus and tumor progression in a murine 4T1 tumor model, providing experimental evidence for clinical treatment of breast cancer. [ABSTRACT FROM AUTHOR]