Shedding light on MHC class I antigen loading: A UV-labile peptide ligand approach

CD8+ T-lymphocyte recognition and destruction of infected and tumour cells is of critical importance in protective immunity and relies on the surface presentation of altered self- or foreign peptide antigens by major histocompatibility complex class-I (MHC-I) molecules. MHC-I molecules acquire antigenic peptide ligands in the endoplasmic reticulum (ER), before being transported to the cell surface. Loading of MHC-I with an optimal peptide is essential for efficient and effective antigen presentation and is subject to quality control involving a panel of ER chaperones.To enable detailed characterisation of the empty MHC-I, as well as the structural and dynamic processes associated with peptide binding and optimisation, we are making use of a previously characterised UV-labile peptide technology. Using two human MHC-I alleles loaded with UV-labile peptide ligands, we have carried out solution-based and X-ray crystallographic analysis of the global and local conformational changes associated with the transition of MHC-I to a sub-optimally loaded/peptide-receptive state. Our data to date indicate that changes to the fully loaded conformation of the peptide-binding site of MHC-I are not required for peptide association, but may facilitate dissociation and exchange of sub-optimal peptides for higher-affinity candidates.