Monoclonal Antibodies to the Latent Membrane Protein of Epstein-Barr Virus Reveal Heterogeneity of the Protein and Inducible Expression in Virus-transformed Cells

Monoclonal antibodies specific for the 'latent membrane protein' (LMP) of Epstein-Barr virus (EBV), one of the effector proteins of EBV-induced B cell transformation, have been generated from mice immunized with a beta-galactosidase fusion protein containing the carboxyl half of the B95.8 strain LMP sequence. Four monoclonal IgG1 antibodies, designated CS.1, CS.2, CS.3 and CS.4, which together recognized at least three different epitopes on the molecule, were used to examine various aspects of LMP expression in B cell lines transformed in vitro. The pooled CS.1 to 4 reagent detected the LMPs encoded by each of 20 geographically distinct EBV isolates, despite a degree of inter-isolate heterogeneity in the size and antigenicity of the protein. In cell lines carrying the prototype B95.8 virus strain, particularly if these were virus producers, an additional lower molecular weight LMP was also detected; this appeared to correspond to the truncated form of the protein already predicted to exist from the analysis of B95.8 lytic cycle mRNAs. Attempts were made to identify an analogous truncated form of LMP in cell lines carrying other virus isolates after treatment with phorbol ester and/or sodium butyrate to induce virus production. Surprisingly these experiments showed that expression of the full length LMP molecule was itself strongly inducible by these agents; when monitored at the single cell level, this was a generalized response and was not restricted to cells entering a lytic cycle. Expression of LMP in EBV-transformed B cells therefore appears to be subject to a distinct type of regulation.