Molecular basis of TAPBPR-mediated peptide editing on MHC class I molecules

Abstract

Major histocompatibility complex (MHC) class I molecules present fragments of the cellular proteome, in the form of short peptides, to the cell surface for the inspection by cytotoxic T cells. This process is a crucial immunosurveillance mechanism used to induce appropriate immune responses against intracellular pathogens and cancer. In order to generate optimal T cell-mediated immune responses, prior to their export to the cell surface, MHC class I molecules undergo a process known as peptide selection. Optimal peptide selection is facilitated by two intracellular peptide editors, tapasin and TAPBPR. TAPBPR was shown to shape the peptide repertoire presented on MHC class I at the cell surface, either by directly catalysing peptide exchange on MHC class I molecules or by associating with the quality control enzyme UDP-glycoprotein glucosyltransferase 1 (UGT1), which selects optimally-loaded MHC class I molecules for export to the cell surface. Given that unlike tapasin, TAPBPR could catalyse peptide editing on MHC class I on its own in solution, I sought to test whether TAPBPR could also function as a peptide exchange catalyst on MHC class I molecules present on the surface of cells. By examining the artefactual expression of TAPBPR at the cell surface upon over-expression, I developed two novel cellular assays which allowed me to explore the function of TAPBPR as a peptide exchange catalyst on plasma membrane-expressed MHC class I molecules. I showed that, when given access the cell surface, TAPBPR can promote efficient peptide exchange on surface expressed MHC class I molecules. These assays allowed me to demonstrate that the 22-35 loop of TAPBPR was essential for its peptide exchange function. Moreover, I revealed that residue L30 within the loop was both necessary and sufficient for the efficient ability of TAPBPR to dissociate peptides from MHC class I molecules that typically accommodate hydrophobic anchor residues in their F pocket. This enabled me to propose a new mechanistic model for TAPBPR-mediated peptide editing. I further addressed the molecular basis governing the compatibility between TAPBPR and MHC class I molecules, by screening a wide panel of human leukocyte antigen (HLA) class I allotypes for their relative propensities to undergo peptide editing by TAPBPR. TAPBPR displayed a clear functional preference for HLA-A molecules, particularly for members of the A2 and A24 supertypes, over HLA-B and -C molecules. This preference appears to be driven by specific molecular features of the MHC class I F pocket, in particular residues H114 and Y116. Finally, I explored the potential translational applications of using TAPBPR as a peptide exchange catalyst on surface-expressed MHC class I molecules. I demonstrated that recombinant TAPBPR can be utilised to load immunogenic peptides of choice directly onto plasma-membrane expression MHC class I, thus overriding the internal antigen presentation pathway. Subsequently, I revealed that, TAPBPR can be used to induce T cell-mediated killing of tumour cells. These findings highlight a potential therapeutic application of TAPBPR in increasing the immune recognition of tumours.