Metabolomics reveals nucleoside analogs for regulating mucosal-associated invariant T cell responses

Although mucosal-associated invariant T (MAIT) cells recognize riboflavin-like metabolites from Gram-negative bacteria, MAIT cell stimulation by broad bacterial families and mammalian cells suggests the existence of novel ligands from different biological sources. Here we established a comparative platform of functional metabolomics and used Mycobacterium tuberculosis as a model to characterize novel metabolites for MAIT cell activation. We extracted and fractionated small metabolites of M. tuberculosis using high-performance liquid chromatography, showing a different MAIT cell stimulation pattern of M. tuberculosis metabolite fractions in comparison with Escherichia coli fractions. Mass profiling predicted multiple nucleoside analogs enriched in a biologically active fraction of M. tuberculosis. Whereas the synthetic forms of these predicted M. tuberculosis nucleosides were unavailable, structural-based autodocking of analogous nucleosides conserved in mammals supported potential binding with MR1 protein. Indeed, functional assays of these conserved nucleosides demonstrated guanosine as a stimulator and deoxyformyluridine as an inhibitor of MAIT cell activation. Identification of bioactive nucleoside metabolites broadly conserved in bacterial and mammalian systems will facilitate an understanding of the regulatory roles of MAIT cells in infectious and inflammatory conditions.