Microbial Metabolites Potentiate MAIT Cell Anti-Tumor Immunity Against Solid Tumors

Mucosal-associated invariant T (MAIT) cells sense riboflavin metabolites through the monomorphic antigen-presenting molecule MR1, providing a unique opportunity to therapeutically mobilize a broadly shared T cell population without genetic engineering. Here, we show that the highly potent microbial metabolites, 5-OP-RU and 5-OE-RU, can be exploited as pharmacologic precision immune activators to drive human MAIT cell responses against solid tumors. Ligand stimulation in human co-culture systems elicited robust MAIT cell cytotoxicity, inflammatory cytokine secretion, and transcriptional states transformation. In vivo administration of riboflavin ligands 5-OP-RU significantly suppressed tumor growth in xenograft liver cancer models. Metabolite-driven MAIT activation also reprogrammed the local immune landscape, enhancing effector function and overcoming features of the immunosuppressive niche by markedly eliminating tumor-associated macrophages (TAMs) and myeloid-derived suppressor cells (MDSCs) within the tumor microenvironment (TME). These findings reveal that microbial riboflavin metabolites can power and redirect MAIT cells to solid tumors, establishing MR1–metabolite signaling as a tractable therapeutic axis for liver cancer and other solid malignancies.