Vitamin B2 byproducts may help certain immune cells recognize and destroy solid tumors, pointing to a possible drug-based way to activate the body’s own defenses without genetic engineering.
Microbial metabolites derived from riboflavin, or vitamin B2, can potently activate mucosal-associated invariant T cells, known as MAIT cells, enabling them to kill a range of solid tumor cells and clear immunosuppressive cells that protect tumors, researchers report in a July 25 bioRxiv preprint.
“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,” the researchers write in the paper.
Solid tumors such as liver, ovarian, lung, breast and colorectal cancers often resist conventional immunotherapies because their microenvironments suppress immune attack. MAIT cells already circulate in human blood and are especially abundant in the liver and other barrier tissues, yet inside tumors they frequently sit inactive. The new findings, researchers say, suggest the main barrier is not a lack of killing power but a shortage of the right chemical signal.
MAIT cells detect small molecules produced during bacterial vitamin B2 synthesis. These molecules, notably 5-OP-RU and the related 5-OE-RU, are presented on a protein called MR1 that is found on many cell types. Because MR1 is nearly identical across people, the same ligands can theoretically activate MAIT cells in almost anyone.
The team first showed that adding 5-OP-RU to ordinary blood samples from healthy donors or from patients with liver or ovarian cancer selectively expanded the MAIT population, sometimes raising their share of T cells from under 10 percent to as high as 60 percent. Parallel experiments with 5-OE-RU produced similar expansion.
When the activated MAIT cells were mixed with a panel of twelve human tumor cell lines representing liver, ovarian, melanoma, lung, breast and colorectal cancers, they killed far more target cells than unstimulated MAIT cells or ordinary T cells. Tumor cells engineered to lack MR1 largely escaped killing, while those forced to over-express MR1 became more vulnerable. The same enhancement occurred against primary tumor cells freshly isolated from liver-cancer patients.
Beyond direct tumor killing, the activated MAIT cells preferentially eliminated two major immunosuppressive populations—tumor-associated macrophages and myeloid-derived suppressor cells—that express high levels of MR1. In three-dimensional organoid cultures that mixed liver tumor cells with immunosuppressive macrophages, metabolite-stimulated MAIT cells restored tumor killing that ordinary T cells could not achieve.
In living mice carrying human liver-tumor cells, weekly doses of 5-OP-RU together with transferred human MAIT cells produced rapid and sustained tumor clearance in both orthotopic liver and subcutaneous models, while MAIT cells alone or ordinary T cells had only modest effects. Analysis of the tumor microenvironment showed that the metabolite treatment also depleted the human myeloid cells that had been introduced to mimic an immunosuppressive niche.
Single-cell RNA sequencing revealed that exposure to the metabolite drove nearly the entire MAIT population into a highly cytotoxic transcriptional state within 24 hours, accompanied by metabolic rewiring that supports effector function.
“In this study, we systematically define microbial metabolite–driven MR1 signaling as a tractable strategy to mobilize human MAIT cells for anti-tumor immunity across solid tumors,” the authors state. They further note that “the principal limitation of MAIT cells in cancer is not intrinsic effector capacity but insufficient antigenic licensing within tumor environments.”
Journal Reference: bioRxiv. DOI: 10.64898/2026.07.24.740456
