MHC Class I-related protein 1 (MR1) is a non-classical antigen presenting molecule which presents vitamin B-related metabolite antigens to mucosal associated invariant T (MAIT) cells. MAIT cells are a subset of innate-like T cells that are abundant in humans, accounting for up to 10% of blood T cells. The MR1-MAIT cell axis is highly conserved among mammals and has been implicated in immunity to pathogens, cancer immunosurveillance, and tissue repair.
MAIT cells require both MR1 and microbial metabolite antigens for their development in the thymus, expansion, and establishment in peripheral tissues. MR1 can present a diverse range of antigens, with the most potent forming a covalent Schiff base bond with a lysine residue (K43) in its antigen binding cleft. It is currently unknown whether covalently bound antigens are essential for MAIT cell development and activation in vivo. Mutating the K43 residue to alanine (K43A) allows MR1K43A expression on the cell surface devoid of Schiff base-bound antigens, and investigations into the function of MR1 with non-covalently bound ligands, or no ligand, to MAIT cell biology.
We generated knock-in mice expressing MR1K43A and investigated MAIT cell development and peripheral expansion. We found that mutant MR1K43A molecules were expressed in thymocytes at higher levels than wild-type MR1, though the MR1K43A mice lacked detectable MAIT cells in peripheral tissues. However, small numbers of CD44+ cells that recognise the MAIT cell-specific tetramer were detected in the thymus of MR1K43A mice that were not present in MR1-knockout mice. This implies that the early stages of MAIT cell development may not require presentation of Schiff base-forming antigens, but presentation of such antigens is required for complete MAIT cell development, expansion, and maintenance in peripheral tissues.
Our results demonstrate the importance of covalent antigen attachment to MR1 for MAIT cell development and selection in vivo.