Ilca et al. determined that exogenous delivery of plasma membrane-targeted TAPBPR (a peptide editor typically expressed intracellularly) or exogenous soluble TAPBPR to target cells in vitro can catalyze the exchange of specific added peptides onto cell surface-expressed MHC-I molecules, allowing for the rapid and efficient (low peptide concentration) loading of high-affinity immunogenic peptides of choice onto MHC-I, bypassing typical intracellular antigen processing. Tumor cells loaded with peptides via TAPBPR were able to engage with TCRs on peptide-specific CD8+ T cells, inducing IFNγ secretion and enhancing T cell-mediated killing of target cells.

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-gamma 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.

Author Info: (1) Department of Pathology, University of Cambridge, CB2 1QP Cambridge, United Kingdom. (2) Department of Pathology, University of Cambridge, CB2 1QP Cambridge, United Kingdom. (3

Author Info: (1) Department of Pathology, University of Cambridge, CB2 1QP Cambridge, United Kingdom. (2) Department of Pathology, University of Cambridge, CB2 1QP Cambridge, United Kingdom. (3) Department of Medicine, University of Cambridge, Addenbrookes Hospital, CB2 0QQ Cambridge, United Kingdom. (4) Cancer Research UK Cambridge Institute, University of Cambridge, Li Ka Shing Centre, CB2 0RE Cambridge, United Kingdom. (5) Department of Pathology, University of Cambridge, CB2 1QP Cambridge, United Kingdom; lhb22@cam.ac.uk.