Identification of a naturally processed HLA-DR-restricted T-helper epitope in Epstein-Barr virus nuclear antigen type 1

Epstein-Barr virus nuclear antigen type 1 (EBNA1), the only viral protein that is unequivocally expressed in all Epstein-Barr virus (EBV)-associated malignant diseases, is essential for viral DNA replication and maintenance of the viral episome in infected cells. A glycine-alanine repeat domain inhibits antigen processing through the ubiquitin-proteasome pathway for presentation on human leukocyte antigen (HLA) class I molecules. EBNA1 is not protected from the HLA class 11 processing pathway, and CD4(+) HLA class II-restricted T cells recognize the antigen. CD4(+) T-helper (Th) cells play critical roles in initiating, regulating, and maintaining immune responses against viral infections and tumors, so that inclusion of EBNA I as a target antigen may improve immunotherapy for EBV-associated cancers. In this study, the authors used the TEPITOPE software program to predict promiscuous class 11 epitope candidates. After several HLA-DR-restricted peptides were identified by in vitro analysis of the T-cell response to synthetic peptides, a T-cell clone was established that was specific for one of the peptides. Functional studies were performed with this clone. The CD4(+) T helper cells specific for the HLA-DR15-restricted peptide EBNAl482 (AEGLRAL-LARSHVER) recognized naturally processed EBNA1 protein. This epitope was presented by several HLA-DR alleles, including DR4, DR7, and DR11. The inclusion of the promiscuous, naturally processed EBNAl482 epitope in vaccine constructs could enhance immune responses against EBV-positive cancers.