Processing of a multiple membrane spanning Epstein-Barr virus protein for CD8(+) T cell recognition reveals a proteasome-dependent, transporter associated with antigen processing-independent pathway

Epstein-Barr virus (EBV) latent membrane protein (LMP)2 is a multiple membr ane spanning molecule which lacks ectodomains projecting into the lumen of the endoplasmic reticulum (ER). Human CD8(+) cytotoxic T lymphocytes (CTL)s recognize a number of epitopes within LMP2. Assays with epitope-specific C TLs in two different cell backgrounds lacking the transporter associated wi th antigen processing (TAP) consistently show that some, but not all, LMP2 epitopes are presented in a TAP-independent manner. However, unlike publish ed examples of TAP-independent processing from endogenously expressed antig ens, presentation of TAP-independent LMP2 epitopes was abrogated by inhibit ion of proteasomal activity. We found a clear correlation between hydrophob icity of the LMP2 epitope sequence and TAP independence, and experiments wi th vaccinia minigene constructs expressing cytosolic epitope peptides confi rmed that these more hydrophobic peptides were selectively able to access t he HLA class I pathway in TAP-negative cells. Furthermore, the TAP-independ ent phenotype of particular epitope sequences did not require membrane loca tion of the source antigen since (i) TAP-independent LMP2 epitopes inserted into an EBV nuclear antigen and (ii) hydrophobic epitope sequences native to EBV nuclear antigens were both presented in TAP-negative cells. We infer that there is a proteasome-dependent, TAP-independent pathway of antigen p resentation which hydrophobic epitopes can selectively access.