Distinct Polymorphisms in HLA Class I Molecules Govern Their Susceptibility to Peptide Editing by TAPBPR.

Understanding how peptide selection is controlled on different major histocompatibility complex class I (MHC I) molecules is pivotal for determining how variations in these proteins influence our predisposition to infectious diseases, cancer, and autoinflammatory conditions. Although the intracellular chaperone TAPBPR edits MHC I peptides, it is unclear which allotypes are subjected to TAPBPR-mediated peptide editing. Here, we examine the ability of 97 different human leukocyte antigen (HLA) class I allotypes to interact with TAPBPR. We reveal a striking preference of TAPBPR for HLA-A, particularly for supertypes A2 and A24, over HLA-B and -C molecules. We demonstrate that the increased propensity of these HLA-A molecules to undergo TAPBPR-mediated peptide editing is determined by molecular features of the HLA-A F pocket, specifically residues H114 and Y116. This work reveals that specific polymorphisms in MHC I strongly influence their susceptibility to chaperone-mediated peptide editing, which may play a significant role in disease predisposition.

TAPBPR mediates peptide dissociation from MHC class I using a leucine lever.

Tapasin and TAPBPR are known to perform peptide editing on major histocompatibility complex class I (MHC I) molecules; however, the precise molecular mechanism(s) involved in this process remain largely enigmatic. Here, using immunopeptidomics in combination with novel cell-based assays that assess TAPBPR-mediated peptide exchange, we reveal a critical role for the K22-D35 loop of TAPBPR in mediating peptide exchange on MHC I. We identify a specific leucine within this loop that enables TAPBPR to facilitate peptide dissociation from MHC I. Moreover, we delineate the molecular features of the MHC I F pocket required for TAPBPR to promote peptide dissociation in a loop-dependent manner. These data reveal that chaperone-mediated peptide editing on MHC I can occur by different mechanisms dependent on the C-terminal residue that the MHC I accommodates in its F pocket and provide novel insights that may inform the therapeutic potential of TAPBPR manipulation to increase tumour immunogenicity.

Utilizing TAPBPR to promote exogenous peptide loading onto cell surface MHC I molecules.

The repertoire of peptides displayed at the cell surface by MHC I molecules is shaped by two intracellular peptide editors, tapasin and TAPBPR. While cell-free assays have proven extremely useful in identifying the function of both of these proteins, here we explored whether a more physiological system could be developed to assess TAPBPR-mediated peptide editing on MHC I. We reveal that membrane-associated TAPBPR targeted to the plasma membrane retains its ability to function as a peptide editor and efficiently catalyzes peptide exchange on surface-expressed MHC I molecules. Additionally, we show that soluble TAPBPR, consisting of the luminal domain alone, added to intact cells, also functions as an effective peptide editor on surface MHC I molecules. Thus, we have established two systems in which TAPBPR-mediated peptide exchange on MHC class I can be interrogated. Furthermore, we could use both plasma membrane-targeted and exogenous soluble TAPBPR to display immunogenic peptides on surface MHC I molecules and consequently induce T cell receptor engagement, IFN-γ secretion, and T cell-mediated killing of target cells. Thus, we have developed an efficient way to by-pass the natural antigen presentation pathway of cells and load immunogenic peptides of choice onto cells. Our findings highlight a potential therapeutic use for TAPBPR in increasing the immunogenicity of tumors in the future.