Breast tumor microbiome regulates anti-tumor immunity and T cell-associated metabolites.

The breast tumor microbiome has emerged as a potential regulator of tumor immunity, yet its interactions with intratumoral lymphocytes and metabolites remain poorly defined. Here, we investigated relationships among CD8+ tumor-infiltrating lymphocytes (TILs), the breast tumor microbiome, and tumor metabolome. In a cohort of 46 breast cancer patients, Staphylococcus was the only bacterial genus whose intratumoral abundance positively correlated with cytotoxic CD8+ T cell markers and innate-like T cell signatures, including multiple KLR-family receptors. Several metabolites were significantly associated with CD8+ TILs, among which NADH, γ-glutamyltryptophan, and γ-glutamylglutamate differed between Staphylococcus-positive and Staphylococcus-negative tumors. Analysis of an independent cohort of 314 treatment-naïve patients further showed that the association between intratumoral Staphylococcus, heightened CD8+ T cell activity, and the KLR-associated innate-like T cell program was specific to triple-negative breast cancer (TNBC). In TNBC mouse models, direct intratumoral injection of Staphylococcus aureus depleted intratumoral NAD metabolites and suppressed tumor growth by activating CD8+ TILs. Together, these findings identify a link between low-biomass intratumoral bacteria and local anti-tumor immunity, and highlight Staphylococcus and TIL-associated metabolites as potential biomarkers and therapeutic targets for breast cancer immunotherapy.