The latent membrane protein 2 gene of Epstein-Barr virus is important for efficient B cell immortalization

The viral latent membrane proteins 2 (LMP2) of Epstein-Barr virus (EBV) were analysed genetically to evaluate their role in B cell immortalization. LMP2 is transcribed as two differently spliced mRNAs which code for the LMP2A and -B proteins, also called terminal protein-1 and -2. LMP2A and -B are found in latently infected, growth-transformed B lymphocytes in vitro, in different human tumours, and in latently infected B cells in vivo. Two different approaches were used to generate EBV mutants in which the second, third and part of the fourth exon of the LMP2 gene were deleted by insertion of a marker gene. Initially, conventional homologous recombination in a Burkitt's lymphoma cell line (P3HR1) between the endogenous EBV genome and an introduced plasmid was used to generate EBV mutants. This experiment identified LMP2 as dispensable for B cell immortalization as has been reported. In a second approach, the same LMP2 mutant gene was analysed in the context of a mini-EBV plasmid. These are E. coli constructs that are sufficient when packaged into an EBV coat both to initiate and to maintain proliferation of infected B cells. In comparison with a fully competent mini-EBV, LMP2- mini-EBVs were found to be greatly reduced in their capacity to yield immortalized B cell clones. This finding confirmed the initially observed bias against LMP2- B cell clones, most of which were found to be coinfected with complementing P3HR1 virus. These results indicate that LMP2 contributes to the efficiency of B cell immortalization and that the LMP2s phenotype is auxiliary in nature.