HLA-A0201 AND HLA-B7 BINDING PEPTIDES IN THE EBV-ENCODED EBNA-1, EBNA-2 AND BZLF-1 PROTEINS DETECTED IN THE MHC CLASS-I STABILIZATION ASSAY- LOW PROPORTION OF BINDING MOTIFS FOR SEVERAL HLA CLASS-I ALLELES INEBNA-1

B lymphocytes immortalized with EBV in vitro, lymphoblastoid cell line s (LCL), express eight EBV-encoded proteins, E8V nuclear antigens -1 t o -6 (EBNA-1 to -6), and latent membrane proteins 1 and 2 (LMP 1 and 2 ). After appropriate stimulations of blood lymphocytes from seropositi ve individuals, MHC-restricted cytotoxic T cells (CTL), which lyse LCL cells, can be generated in vitro. Such CTLs can recognize EBNA-2 to - 6, and LMP 1 and 2, but not EBNA-1-derived peptides presented on the c ell surfaces. We posed the question whether this exceptional feature o f EBNA-1 is due to lack of MHC class I binding peptides. A computer se arch for 11 human leukocyte antigen (HLA) alleles showed that EBNA-1 h as a lower number and lower proportion of relevant binding motifs to s everal alleles than EBNA-2 to -6 and LMP 1 and 2. The low motif number s in EBNA-1 is in line with its apparent failure to generate a CTL res ponse, and it may be the consequence of immunoselection allowing the e xistence of EBV genome-carrying B cells in the immunocompetent hosts. The binding capacities of synthetic peptides of EBNA-1 and -2 and of t he immediate early lytic cycle protein BZLF-1 to HLA-A0201 (A2) and HL A-B7 molecules were tested in an MHC stabilization assay. The peptide transporter-deficient T2 line, which expresses a low level of HLA-A2 a nd its HLA-B7 transfectant subline, were used for this purpose because specifically bound peptides elevate the surface expression of these M HC molecules. Of five synthetic nonamer EBNA-1 peptides which include the relevant binding motif, four bound to A2. In a series of 20-amino acid-long overlapping EBNA-1 peptides none showed binding to A2, while eight peptides bound to B7. Two 20-amino acid-long EBNA-2 and seven B ZLF-1 peptides were identified as A2 binders, and four EBNA-2 and eigh t BZLF-1 peptides bound to 87. Thus, we have excluded the possibility that the inability of the EBNA-1 protein to generate HLA-restricted CT Ls could be due to the lack of HLA class I binding peptides in its seq uence. The finding that several EBNA-1 peptides could bind to these tw o HLA molecules does not, however, necessarily reflect the natural sit uation because the peptides may not be processed and/or transported to the cell surfaces. We have stimulated lymphocytes of healthy donors w ith relevant HLA types with the autologous LCL. The resulting auto-LCL -reactive culture did not lyse autologous phytohaemagglutinin blasts c oated with the binder peptides, and we could not generate auto-LCL-spe cific CTLs by stimulation of lymphocytes with these peptides.