A Stable Isopterin Mimic of a Potent Bacterial Stimulant of T Cells

ABSTRACT

Mucosal‐associated invariant T cells (MAIT cells) are innate‐like immune cells (T lymphocytes) that protect against bacterial infections. They are potently activated by the microbial metabolite 5‐(2‐oxopropylideneamino)‐

‐ribitylaminouracil (5‐OP‐RU), which bonds covalently to an amine of a specific lysine residue of the antigen‐presenting protein MR1 to form an imine (Schiff base). However, 5‐OP‐RU undergoes rapid degradative intramolecular cyclisation in water, and more robust analogues are needed for therapeutic applications. Here, inspired by the structural similarities between 5‐OP‐RU and 6‐formylpterin, a water‐stable covalent MR1 ligand that does not activate MAIT cells, we describe the design and synthesis of a novel isopterin analogue of this microbial natural product, and show that it is both water‐stable and activates reporter cells expressing the MAIT T cell receptor. Further, we present a protein‐bound crystal structure and molecular dynamics simulations of this stable analogue to show that it mimics the MR1‐bound conformation of 5‐OP‐RU. This study demonstrates the potential of this bicyclic scaffold for stabilising 5‐OP‐RU mimetics, advances our understanding of the molecular requirements for MR1 binding and MAIT cell activation, and informs the design and synthesis of future MAIT cell antigens as potential immunotherapeutics.