MHC CLASS II-RESTRICTED PRESENTATION OF ENDOGENOUSLY SYNTHESIZED ANTIGEN - EPSTEIN - BARR VIRUS-TRANSFORMED B-CELL LINES CAN PRESENT THE VIRAL GLYCOPROTEIN GP340 BY 2 DISTINCT PATHWAYS

Epstein-Barr virus (EBV) transformed B lymphoblastoid cell lines (LCL) efficiently process exogenous antigens for MHC class II-restricted pr esentation via the chloroquine-sensitive endosomal pathway. Using MHC class II-restricted T cell clones specific for EBV structural proteins , however, we frequently observed significant responses to autologous LCL cells without the addition of exogenous virus. Such responses were reduced by pre-treating the LCL with acyclovir (ACV), a drug blocking productive EBV infection. This suggested T cell recognition of antige n synthesized by LCL cells spontaneously entering virus productive cyc le, and led us to question by what route(s) MHC class II-restricted pr esentation of endogenously synthesized virion proteins was occurring. Cell sorting experiments, using the viral envelope glycoprotein gp340 as a surface marker of productively-infected cells, confirmed that sti mulatory activity lay within the gp340-positive fraction. However, clo ser analysis revealed that most of these cells were not productively-i nfected but were EBV receptor-positive and had bound released virus. W e infer that receptor-mediated delivery of released virus into the end osomal pathway is one route whereby an LCL can present endogenously sy nthesized EBV proteins on MHC class II molecules. To ask whether anoth er, more direct, route of processing was possible, we used a recombina nt vaccinia viral vector to express gp340 de novo in ACV-treated LCLs. Significantly, these cells presented the endogenously synthesized ant igen to autologous gp340-specific T cell clones via a chloroquine-resi stant pathway. In the same experiments, vaccinia-mediated expression o f a signal peptide-deleted form of gp340 did not lead to T cell stimul ation, suggesting that this second route of processing required entry of endogenously synthesized antigen into the endoplasmic reticulum.