Transcription of the Epstein-Barr virus genome during latency in growth-transformed lymphocytes

Molecular basis of latency in pathogenic human viruses

Several human viruses are able to latently infect specific target cell populations in vivo. Analysis of the replication cycles of herpes simplex virus, Epstein-Barr virus, and human immunodeficiency virus suggests that the latent infections established by these human pathogens primarily result from a lack of host factors critical for the expression of viral early gene products. The subsequent activation of specific cellular transcription factors in response to extracellular stimuli can induce the expression of these viral regulatory proteins and lead to a burst of lytic viral replication. Latency in these eukaryotic viruses therefore contrasts with latency in bacteriophage, which is maintained primarily by the expression of virally encoded repressors of lytic replication.

BHRF1, the Epstein-Barr virus gene with homology to Bc12, is dispensable for B-lymphocyte transformation and virus replication

The Epstein-Barr virus (EBV) BHRF1 open reading frame is abundantly expressed early in the lytic replication cycle. BHRF1 is also transiently expressed in some latently infected cell lines in the absence of expression of other lytic cycle proteins. BHRF1 shares distant, but significant, colinear primary amino acid sequence homology to Bc12, a cellular gene strongly implicated in the evolution of follicular lymphoma. The experiments reported here used a molecular genetic approach to examine the role of BHRF1 in EBV infection. Isogenic EBV recombinants having either wild-type BHRF1 or a null mutation due to a translational stop signal in place of the 24th BHRF1 codon were used to infect primary B lymphocytes. The BHRF1 mutant recombinants did not differ from the wild type in their ability to infect and transform the growth of primary B lymphocytes, to replicate in the resultant lymphoblastoid cell lines, or to initiate a second round of primary cell transformation. Deletion of the entire BHRF1 open reading frame did not destroy the ability of the mutant virus to maintain cell growth transformation. The significance of these findings with regard to the role of BHRF1 in EBV infection is discussed.

Restricted Epstein-Barr virus protein expression in Burkitt lymphoma is due to a different Epstein-Barr nuclear antigen 1 transcriptional initiation site

Epstein-Barr virus (EBV) expresses six nuclear antigens (EBNAs) and three integral latent membrane proteins (LMPs) in latently infected growth-transformed B lymphoblastoid cell lines (LCLs). In contrast, EBV protein expression in Burkitt lymphoma tissue or in newly established Burkitt lymphoma cell lines is frequently restricted to the EBV genome maintenance protein, EBNA-1. EBNA-1 expression in the absence of other EBNAs and LMP-1 has been an enigma since, in LCLs, all EBNA mRNAs are processed from a single transcript. We now show that the basis for restricted EBV expression in Burkitt lymphoma cells is selective EBNA-1 mRNA transcription from a hitherto unrecognized promoter that is 50 kb closer to the EBNA-1-encoding exon than previously described EBNA-1 promoters. Infected cells with EBNA-1-restricted expression could preferentially persist in vivo in the face of EBV-immune T-cell responses, which are frequently directed against other EBNAs and are also dependent on LMP-1 expression.

The prototypical Epstein-Barr virus-transformed lymphoblastoid cell line IB4 is an unusual variant containing integrated but no episomal viral DNA

IB4 is a prototype, latently Epstein-Barr virus (EBV)-infected, lymphoblastoid cell line. We show here that IB4 contains only integrated EBV genomes. Episomal EBV DNA is not detected by Gardella gel analysis or in situ hybridization. Restriction enzyme mapping indicates that the EBV genomes first circularized and then integrated into and deleted part of the BamHI C fragment. IB4 is therefore the only lymphoblastoid cell line described to date that lacks episomal EBV and has integrated EBV genomes with joined ends. Thus, the detection of joined EBV termini on Southern blots is not as reliable as the Gardella gel system for detecting episomal EBV DNA, and IB4 is not an ideal prototype cell line for the study of latent infection by EBV.

Early events in Epstein-Barr virus infection of human B lymphocytes

The sequence of Epstein-Barr virus (EBV) and B lymphocyte changes in the 3 days following acute infection was analyzed. By 16 hr the average infected lymphocyte had 1 EBV episome. Nuclear protein-2 (EBNA-2) and EBNA-leader protein (-LP) were detected by 12 hr, and by 32 hr were at the levels of stable EBV infection in lymphoblastoid cell lines (LCLs). At 12 hr, all EBNA-LP and EBNA-2 RNAs were initiated from the Pw promoter. By 36 hr a significant EBNA-LP and EBNA-2 RNA fraction initiated from the upstream Pc promoter. Throughout acute infection, a similar fraction of potentially bicistronic EBNA-LP mRNAs had first exon splices which would result in EBNA-LP translation. By 36 hr c-myc RNA was transiently induced, and CD21 and CD23 RNAs were beginning to increase. This coincided with low-level EBNA-1, EBNA-3A, B, and C, and latent membrane protein-1 (LMP-1) expression. By 46 hr, EBNA-1, the EBNA-3s, and LMP-1 were near the levels ordinarily found in LCLs and a substantial fraction of lymphocytes were in S phase. These results are compatible with a key role for EBNA-2 (or EBNA-LP) in regulating virus and cell gene expression. High-level expression of the EBV-encoded small RNAs, EBERs, was delayed beyond 36 hr and may, therefore, be activated by other virus or cell genes. A 65-kDa virion protein persisted in acutely infected cells. This protein could be a mediator of virus or cell gene expression.

Immortalizing genes of Epstein-Barr virus

EBV immortalizes human B lymphocytes efficiently. Ten of its approximately 100 genes are expressed in these proliferating lymphoblasts and are candidates for mediating the changes central to the immortalization of the cell. Enough has been learned now about three of these viral genes to indicate that they are likely to be required for immortalization. As more is learned, additional genes of EBV will probably be found to support the process of immortalization of the host cell. EBNA-2 has been shown genetically to be required for EBV to immortalize an infected B lymphocyte. The biochemical activities of EBNA-2 that constitute this requirement have not been identified. Many experiments indicate that EBNA-2 affects the accumulation of specific viral and cellular RNAs. These effects, however, can be detected only in certain EBV-negative B-lymphoblastoid cells. It is, therefore, not clear that the known effects of EBNA-2 adequately explain its ubiquitous requirement in the immortalization of primary human B lymphocytes. LMP is likely to be required for immortalization because it can affect the growth properties of established human lymphoid and epithelial cells and can transform at least two established rodent cells to proliferate in an anchorage-independent manner. The structure of this viral protein, its position in the plasma membrane, many of its biochemical properties, as well as studies of its mutant derivatives are consistent with its acting as a growth factor receptor or affecting the activity of such a receptor. However, no biochemical activity has been assigned directly to LMP, and both its mechanism of action and its possible contribution to immortalization by EBV remain enigmatic. EBNA-1 presumably is required for EBV to immortalize a B lymphocyte because it is essential for the initiation of plasmid DNA replication by EBV. Circumstantial observations indicate also that EBNA-1 is probably necessary for sustaining viral DNA replication in the proliferating cell population. EBNA-1 may well affect the regulation of transcription of viral genes that themselves are required for immortalization. These roles of EBNA-1 are performed in part by its site-specific binding to the elements of oriP required in cis for the replication of EBV plasmid DNAs. It is probable that EBNA-1 also binds both to a set of cellular proteins that function in transcription and to a nonidentical set of cellular proteins that function in replication. EBV effects a fascinating phenotypic change in B lymphocytes it infects. It does so by using several viral genes that alter the physiology of the cell by different means.(ABSTRACT TRUNCATED AT 400 WORDS)

Epstein-Barr virus nuclear antigen 2 transactivates latent membrane protein LMP1

Several lines of evidence are compatible with the hypothesis that Epstein-Barr virus (EBV) nuclear antigen 2 (EBNA-2) or leader protein (EBNA-LP) affects expression of the EBV latent infection membrane protein LMP1. We now demonstrate the following. (i) Acute transfection and expression of EBNA-2 under control of simian virus 40 or Moloney murine leukemia virus promoters resulted in increased LMP1 expression in P3HR-1-infected Burkitt's lymphoma cells and the P3HR-1 or Daudi cell line. (ii) Transfection and expression of EBNA-LP alone had no effect on LMP1 expression and did not act synergistically with EBNA-2 to affect LMP1 expression. (iii) LMP1 expression in Daudi and P3HR-1-infected cells was controlled at the mRNA level, and EBNA-2 expression in Daudi cells increased LMP1 mRNA. (iv) No other EBV genes were required for EBNA-2 transactivation of LMP1 since cotransfection of recombinant EBNA-2 expression vectors and genomic LMP1 DNA fragments enhanced LMP1 expression in the EBV-negative B-lymphoma cell lines BJAB, Louckes, and BL30. (v) An EBNA-2-responsive element was found within the -512 to +40 LMP1 DNA since this DNA linked to a chloramphenicol acetyltransferase reporter gene was transactivated by cotransfection with an EBNA-2 expression vector. (vi) The EBV type 2 EBNA-2 transactivated LMP1 as well as the EBV type 1 EBNA-2. (vii) Two deletions within the EBNA-2 gene which rendered EBV transformation incompetent did not transactivate LMP1, whereas a transformation-competent EBNA-2 deletion mutant did transactivate LMP1. LMP1 is a potent effector of B-lymphocyte activation and can act synergistically with EBNA-2 to induce cellular CD23 gene expression. Thus, EBNA-2 transactivation of LMP1 amplifies the biological impact of EBNA-2 and underscores its central role in EBV-induced growth transformation.

cis-acting regulatory elements near the Epstein-Barr virus latent-infection membrane protein transcriptional start site

Epstein-Barr virus (EBV) latent-infection membrane protein (LMP) gene cis-acting regulatory sequences were assayed in human B lymphocytes by using chloramphenicol acetyltransferase (cat) gene expression as a reporter. The activities of progressively longer upstream elements from bases -55 to -2350 were compared. At least two positive cis-activating regulatory components (-155 to -147 and -234 to -205) upstream of the LMP promoter were defined. LMP promoter cat gene constructs were more active in a Burkitt's lymphoma cell line latently infected with the B95 EBV strain than in the same cells latently infected with the P3HR1 EBV strain. Since the P3HR1- and B95-infected cells differ in EBNA-2 and EBNA-LP expression, EBNA-2 or EBNA-LP is a likely transactivator of the LMP promoter. Probable cognate sequences for known transcription factors in the LMP promoter are discussed.