Potent Immunomodulators Developed from an Unstable Bacterial Metabolite of Vitamin B2 Biosynthesis.

Bacterial synthesis of vitamin B2 generates a by-product, 5-(2-oxopropylideneamino)-d-ribityl-aminouracil (5-OP-RU), with potent immunological properties in mammals, but rapid inactivation in water limits practical uses. This natural product covalently bonds to immunological protein MR1 in antigen presenting cells (APCs), enabling MR1 to traffic to the cell surface, where it interacts with T cell receptors (TCRs) on mucosal associated invariant T lymphocytes (MAIT cells), activating their immunological and antimicrobial properties. Here, we develop several new series of water-stable compounds tailored for powerful and distinctive immunological functions. We report their water stability, capacity to bind MR1 and traffic it to the cell surface (EC50 17 nM), potent activation (EC50 56 pM) or inhibition (IC50 80 nM) of interacting MAIT cells, and develop compounds with diazirine-alkyne, biotin, or fluorophore labels for studying cellular MR1. Computer modelling casts new light on the molecular mechanism of activation, revealing that activators are first captured in MR1 via  pi-interactions and H-bonds, before tighter covalent bonding to Lys43 in MR1. This chemical study advances our molecular understanding of how bacterial metabolites are captured by MR1, influence cell surface expression of MR1, interact and modify human T cells; offering new clues for developing novel vaccine adjuvants, immunotherapeutics, and cancer drugs.