Structural insight into MR1-mediated recognition of the mucosal associated invariant T cell receptor

Mucosal-associated invariant T (MAIT) cells express a semiinvariant alpha beta T cell receptor (TCR) that binds MHC class I-like molecule (MR1). However, the molecular basis for MAIT TCR recognition by MR1 is unknown. In this study, we present the crystal structure of a human V alpha 7.2J alpha 33-V beta 2 MAIT TCR. Mutagenesis revealed highly conserved requirements for the MAIT TCR-MR1 interaction across different human MAIT TCRs stimulated by distinct microbial sources. Individual residues within the MAIT TCR beta chain were dispensable for the interaction with MR1, whereas the invariant MAIT TCR alpha chain controlled specificity through a small number of residues, which are conserved across species and located within the V alpha-J alpha regions. Mutagenesis of MR1 showed that only two residues, which were centrally positioned and on opposing sides of the antigen-binding cleft of MR1, were essential for MAIT cell activation. The mutagenesis data are consistent with a centrally located MAIT TCR-MR1 docking that was dominated by the alpha chain of the MAIT TCR. This candidate docking mode contrasts with that of the NKT TCR-CD1d-antigen interaction, in which both the alpha and beta chain of the NKT TCR is required for ligation above the F'-pocket of CD1d.