Mucosal-associated invariant T cells recognize a tumor-derived metabolite in the DNA synthesis pathway.

Mucosal-associated invariant T (MAIT) cells recognize metabolite-derived ligands, but the identity of such ligands in the context of cancer remains poorly defined. Herein, we demonstrate that the tumor-derived metabolite 5-formyl tetrahydrofolate (5-formyl THF), which is an intermediate of the folate metabolism pathway, induces T cell receptor (TCR)-dependent activation of mouse mucosal-associated invariant T (MAIT) cells. The activity was weaker than that of a potent MAIT cell agonist 5-(2-oxopropylideneamino)-6-d-ribitylaminouracil (5-OP-RU). Amino methyltransferase (Amt), an enzyme that is necessary for the generation of 5-formyl THF from tumor cells, was essential for the activation of mouse MAIT cells. Genetic deletion of Amt in the tumor resulted in impaired activation of mouse MAIT cells. In contrast, Amt overexpression in the tumor led to an enhancement in the agonistic activity of the MAIT cells. The introduction of mouse MAIT TCRs to primary T cells from MAIT cell-deficient mice also conferred reactivity to 5-formyl THF. Using single-cell TCR sequencing analysis, a 5-formyl THF-reactive MAIT cell clonotype was identified in humans. These results demonstrate that 5-formyl THF is the first tumor-derived ligand recognized by MAIT cells in both humans and mice. However, the functional relevance of this pathway in vivo remains to be determined.