Proteasome inhibitors induce the presentation of an Epstein-Barr virus nuclear antigen 1-derived cytotoxic T lymphocyte epitope in Burkitt's lymphoma cells.

The Epstein-Barr virus (EBV) nuclear antigen 1 (EBNA1) is generally expressed in all EBV-associated tumours and is therefore an interesting target for immunotherapy. However, evidence for the recognition and elimination of EBV-transformed and Burkitt's lymphoma (BL) cells by cytotoxic T lymphocytes (CTLs) specific for endogenously presented EBNA1-derived epitopes remains elusive. We confirm here that CTLs specific for the HLA-B35/B53-presented EBNA1-derived HPVGEADYFEY (HPV) epitope are detectable in the majority of HLA-B35 individuals, and recognize EBV-transformed B lymphocytes, thereby demonstrating that the GAr domain does not fully inhibit the class I presentation of the HPV epitope. In contrast, BL cells are not recognized by HPV-specific CTLs, suggesting that other mechanisms contribute to providing a full protection from EBNA1-specific CTL-mediated lysis. One of the major differences between BL cells and lymphoplastoid cell lines (LCLs) is the proteasome; indeed, proteasomes from BL cells demonstrate far lower chymotryptic and tryptic-like activities compared with proteasomes from LCLs. Hence, inefficient proteasomal processing is likely to be the main reason for the poor presentation of this epitope in BL cells. Interestingly, we show that treatments with proteasome inhibitors partially restore the capacity of BL cells to present the HPV epitope. This indicates that proteasomes from BL cells, although less efficient in degrading reference substrates than proteasomes from LCLs, are able to destroy the HPV epitope, which can, however, be generated and presented after partial inhibition of the proteasome. These findings suggest the use of proteasome inhibitors, alone or in combination with other drugs, as a strategy for the treatment of EBNA1-carrying tumours.