Functional differences between type 1 and type 2 Epstein-Barr virus EBNA-2

Epstein-Barr virus (EBV) establishes a lifelong latent infection in the human host. In vitro EBV immortalises primary B lymphocytes giving rise to latently infected lymphoblastoid cell lines (LCLs). EBV strains are classified as type 1 or type 2 according to the sequence of the EBNA-2 gene, which expresses a transcription factor that induces viral and cell genes important for B cell proliferation. Type 1 EBV transforms primary B lymphocytes into LCLs much more efficiently than type 2 EBV, a difference previously mapped to the EBNA-2 locus. In this study, the greater transforming activity of type 1 EBV was found to correlate with a stronger and more rapid induction of the viral oncogene LMP-1 and the cell gene CXCR7 (which are both required for proliferation of EBV-LCLs) during infection of primary B cells with EBVBAC recombinant viruses. The enhanced ability of type 1 EBNA-2 to induce LMP-1 and CXCR7 genes was also confirmed in Burkitt’s Lymphoma cell lines. Although the major sequence differences between type 1 and type 2 EBNA-2 lie in N-terminal parts of the protein, the superior ability of type 1 EBNA-2 to sustain proliferation of EBV-LCLs is mostly determined by the C-terminal region of the protein. Substitution of the C-terminal third of type 1 EBNA-2 into the type 2 protein is sufficient to confer type 1 growth phenotype and type 1 expression levels of LMP-1 and CXCR7 in an EREB2.5 cell growth assay. Within this region, the RG, CR7 and TAD domains are the minimum type 1 sequences required. Sequencing the C-terminal part of EBNA-2 from additional EBV isolates showed high sequence identity within type 1 isolates or within type 2 isolates, indicating that the functional differences mapped are typical of EBV type sequences