TRANSPORTER (TAP)-INDEPENDENT PROCESSING OF A MULTIPLE MEMBRANE-SPANNING PROTEIN, THE EPSTEIN-BARR-VIRUS LATENT MEMBRANE-PROTEIN-2

Antigen presentation to CD8(+) cytotoxic T lymphocytes (CTL) usually i nvolves proteolytic cleavage of antigen in the cytosol and the deliver y of epitope peptides onto major histocompatibility complex class I mo lecules in the endoplasmic reticulum (ER) via the heterodimeric peptid e transporter TAP1/TAP2. In the few exceptional cases where TAP-indepe ndent presentation of an endogenously expressed protein has been obser ved, the epitope-containing domain of the protein either has naturally accessed or has been directed into the ER lumen where it is thought t o become susceptible to ER proteases. Here, we describe a novel exampl e of TAP-independent processing involving the Epstein-Barr virus (EBV) latent membrane protein LMP2, a multiple membrane-spanning protein wi th minimal projection into the ER. Expression of LMP2 in the TAP(-) T2 cell line, whether from the resident EBV genome or from a recombinant vaccinia virus vector vacc-LMP2, rendered the cells sensitive to reco gnition by CTL clones specific for two HLA-A2.1-restricted peptide epi topes, LMP2 329-337 or 426-434. Vacc-LMP2-mediated sensitization to ly sis required expression of the antigen de novo in T2 cells and was blo cked by brefeldin A. In the same experiments, two other EBV-specific C TL epitopes, one derived from LMP2 but restricted through a different HLA allele (All), the other restricted through A2.1 but derived from a different viral protein (BMLF1), did not display TAP-independent proc essing. The results are discussed in relation to the unusual topology of LMP2 in the membrane and the position of the epitope peptides withi n that structure.