The carbonyl nucleobase adduct MAde is a potent antigen for adaptive polyclonal MR1-restricted T cells.

The major histocompatibility complex (MHC) class I-related molecule MHC-class-I-related protein 1 (MR1) presents metabolites to distinct MR1-restricted T cell subsets, including mucosal-associated invariant T (MAIT) and MR1T cells. However, self-reactive MR1T cells and the nature of recognized antigens remain underexplored. Here, we report a cell endogenous carbonyl adduct of adenine (8-(9H-purin-6-yl)-2-oxa-8-azabicyclo[3.3.1]nona-3,6-diene-4,6-dicarbaldehyde [MAde]) sequestered in the A' pocket of MR1. MAde induced in vitro MR1-mediated stimulation of MR1T cell clones that bound MR1-MAde tetramers. MR1-MAde tetramers identified heterogeneous MR1-reactive T cells ex vivo in healthy donors, individuals with acute myeloid leukemia, and tumor-infiltrating lymphocytes from non-small cell lung adenocarcinoma and hepatocarcinoma. These cells displayed phenotypic, transcriptional, and functional diversity at distinct differentiation stages, indicating their adaptive nature. They were also polyclonal, with some preferential T cell receptor (TCRαβ) pair usage. Thus, MAde is an MR1-presented self-metabolite that enables stimulation and tracking of human-MR1T cells from blood and tissue, aiding our understanding of their roles in health and disease.