TARGETING EPSTEIN-BARR-VIRUS NUCLEAR ANTIGEN-1 (EBNA1) THROUGH THE CLASS-II PATHWAY RESTORES IMMUNE RECOGNITION BY EBNA1-SPECIFIC CYTOTOXICT-LYMPHOCYTES - EVIDENCE FOR HLA-DM-INDEPENDENT PROCESSING

Epstein-Barr virus (EBV) nuclear antigen 1 (EBNA1) is the only viral p rotein consistently expressed in all malignancies associated with EBV and there is now convincing evidence to suggest that EBNA1 is not reco gnized by MHC class I-restricted cytotoxic T lymphocytes (CTL), The la ck of recognition of EBNA1 has been attributed to a cis-acting inhibit ory effect of glycine-alanine repetitive (G-Ar) sequences on the endog enous processing of this antigen through the class I pathway, In the p resent study we have explored the possibility of targeting EBNA1 throu gh an alternative mechanism using the MHC class II pathway. Using puri fied EBNA1 protein, we demonstrate here that CD4(+) CTL can efficientl y recognize EBV-transformed a cells and Burkitt's lymphoma cells follo wing exogenous sensitization with this antigen, and this immune recogn ition is not affected by the G-Ar domain within EBNA1. Analysis of the processing mechanism revealed that intracellular loading of class II molecules with an EBNA1 epitope occurs through an HLA-DM-independent p athway, These results highlight a novel mechanism for immune recogniti on of EBNA1 and also demonstrate that the G-Ar-mediated protection fro m processing can be overridden if this antigen is presented through th e class II pathway.