The Epstein-Barr Virus BamHI C Promoter Is Not Essential for B Cell Immortalization In Vitro , but It Greatly Enhances B Cell Growth Transformation

Abstract

ABSTRACT Epstein-Barr virus (EBV) infection of B cells leads to the sequential activation of two viral promoters, Wp and Cp, resulting in the expression of six EBV nuclear antigens (EBNAs) and the viral Bcl2 homologue BHRF1. The viral transactivator EBNA2 is required for this switch from Wp to Cp usage during the initial stages of infection. EBNA2-dependent Cp transcription is mediated by the EBNA2 response element (E2RE), a region that contains at least two binding sites for cellular factors; one of these sites, CBF1, interacts with RBP-JK, which then recruits EBNA2 to the transcription initiation complex. Here we demonstrate that the B cell-specific transcription factor BSAP/Pax5 binds to a second site, CBF2, in the E2RE. Deletion of the E2RE in the context of a recombinant virus greatly diminished levels of Cp-initiated transcripts during the initial stages of infection but did not affect the levels of Wp-initiated transcripts or EBNA mRNAs. Consistent with this finding, viruses deleted for the E2RE were not markedly impaired in their ability to induce B cell transformation in vitro . In contrast, a larger deletion of the entire Cp region did reduce EBNA mRNA levels early after infection and subsequently almost completely ablated lymphoblastoid cell line (LCL) outgrowth. Notably, however, rare LCLs could be established following infection with Cp-deleted viruses, and these were indistinguishable from wild-type-derived LCLs in terms of steady-state EBV gene transcription. These data indicate that, unlike Wp, Cp is dispensable for the virus' growth-transforming activity. IMPORTANCE Epstein-Barr virus (EBV), a B lymphotropic herpesvirus etiologically linked to several B cell malignancies, efficiently induces B cell proliferation leading to the outgrowth of lymphoblastoid cell lines (LCLs). The initial stages of this growth-transforming infection are characterized by the sequential activation of two viral promoters, Wp and Cp, both of which appear to be preferentially active in target B cells. In this work, we have investigated the importance of Cp activity in initiating B cell proliferation and maintaining LCL growth. Using recombinant viruses, we demonstrate that while Cp is not essential for LCL outgrowth in vitro , it enhances transformation efficiency by >100-fold. We also show that Cp, like Wp, interacts with the B cell-specific activator protein BSAP/Pax5. We suggest that EBV has evolved this two-promoter system to ensure efficient colonization of the host B cell system in vivo .