IDENTIFICATION OF A CYTOTOXIC T-LYMPHOCYTE RESPONSE TO THE NOVEL BARF0 PROTEIN OF EPSTEIN-BARR-VIRUS - A CRITICAL ROLE FOR ANTIGEN EXPRESSION

The Epstein-Barr virus (EBV)-encoded BARF0 open reading frame gene pro ducts are consistently expressed in EBV-positive Burkitt's lymphoma (B L) cell lines, nasopharyngeal carcinoma cell lines, and lymphoblastoid cell lines (LCLs). Here we show that the BARF0 sequence includes an H LA A0201-restricted cytotoxic T-lymphocyte (CTL) epitope. By using th eoretically predicted HLA A2 binding motifs and peptide loaded antigen presentation-deficient T2 cells, polyclonal BARF0-specific CD8(+) CTL s were isolated from four different healthy EBV-seropositive donors bu t not from two seronegative donors. These CTL lines recognized the pep tide epitope LLWAARPRL, which was found to be conserved in 33 of 34 vi rus strains originating from Caucasian, African, and Asian individuals . The BARF0-specific CTL lines could lyse EBV-negative BL cells stably transfected with the BARF0 gene but did not kill HLA A2-matched EBV-p ositive BL cells and LCLs in a standard Cr-51 release assay. Reverse t ranscriptase PCR analysis demonstrated that these EBV-positive cell li nes expressed significantly lower levels of BARF0 mRNA than transfecte d cells. This data indicated that the BARF0 epitope could be endogenou sly processed; however, antigen levels in the target cell were a limit ing factor for the effective interaction between BARF0-expressing cell s and CTLs. The limited expression of BARF0 antigen in EBV-infected BL cells and LCLs might contribute to the escape of immune recognition f rom virus-specific CTLs present in the host.