A second Epstein-Barr virus early antigen gene in BamHI fragment M encodes a 48- to 50-kilodalton nuclear protein

Association of Human Leucocyte Antigen Class II, with viral load and immune response to Epstein-Barr virus in adult and pediatric Systemic lupus erythematosus patients

OBJECTIVE

Systemic lupus erythematosus (SLE) is a multisystem autoimmune disease, which is known to be associated with HLA-DRB1 and Epstein-Barr virus (EBV) infection. In the Indian subcontinent where there is high seroendemicity of EBV, we postulated that the association of this virus in adult SLE (aSLE) and pediatric SLE (pSLE) patients would be different and differentially associate with the HLA-DRB1 susceptibility and protective genes.

METHODS

A total of 109 aSLE, 52 pSLE, 215 adult healthy and 63 pediatric healthy controls were recruited. HLA-DRB1 genotyping by PCR-SSP, EBV load estimation by real-time PCR and antibody profiling (IgG & IgM) to EBV antigens by line blot assay were performed.

RESULTS

DRB1*15 was found predominant in pSLE patients and DRB1*03 in aSLE patients. DRB1*15/X heterozygous was predominant in overall SLE patients, although disease severity, like hypocomplementemia, higher autoantibody levels and more organ involvement was observed in *15/*15 homozygous state. EBV strongly associated with pSLE patients showing higher percent of EA-D IgG (p < 0.0001) and p22 IgG (p = 0.035) along with higher viral load (p = 0.001) as compared to healthy controls. In addition, the higher EBV DNA load significantly associated with anti-EA-D IgG (p = 0.013) and DRB1*15/*15 (p = 0.007) in pSLE patients as compared to aSLE patients.

CONCLUSIONS

This study therefore indicates that different HLA-DRB1 allotypes confer susceptibility to SLE in children and adults and disease may be triggered by increased EBV reactivation.

Human herpesvirus 6A U27 plays an essential role for the virus propagation

Human herpesvirus 6A (HHV-6A) is a member of the genus Roseolovirus and the subfamily Betaherpesvirinae. It is similar to and human cytomegalovirus (HCMV). HHV-6A encodes a 41 kDa nuclear phosphoprotein, U27, which acts as a processivity factor in the replication of the viral DNA. HHV-6A U27 has 43% amino acid sequence homology with HCMV UL44, which is important for DNA replication. A previous study on HHV-6A U27 revealed that it greatly increases the in vitro DNA synthesis activity of HHV-6A DNA polymerase. However, the role of U27 during the HHV-6A virus replication process remains unclear. In this study, we constructed a U27-deficient HHV-6A mutant (HHV-6ABACU27mut) with a frameshift insertion at the U27 gene using an HHV-6A bacterial artificial chromosome (BAC) system. Viral reconstitution from the mutant BAC DNA was not detected, in contrast to the wild type and the revertant from the U27 mutant. This suggests that U27 plays a critical role in the life cycle of HHV-6A.

Epstein-Barr virus genome packaging factors accumulate in BMRF1-cores within viral replication compartments

Productive replication of Epstein-Barr virus (EBV) during the lytic cycle occurs in discrete sites within nuclei, termed replication compartments. We previously proposed that replication compartments consist of two subnuclear domains: "ongoing replication foci" and "BMRF1-cores". Viral genome replication takes place in ongoing replication foci, which are enriched with viral replication proteins, such as BALF5 and BALF2. Amplified DNA and BMRF1 protein accumulate in BMRF1-cores, which are surrounded by ongoing replication foci. We here determined the locations of procapsid and genome-packaging proteins of EBV via three-dimensional (3D) surface reconstruction and correlative fluorescence microscopy-electron microscopy (FM-EM). The results revealed that viral factors required for DNA packaging, such as BGLF1, BVRF1, and BFLF1 proteins, are located in the innermost subdomains of the BMRF1-cores. In contrast, capsid structural proteins, such as BBRF1, BORF1, BDLF1, and BVRF2, were found both outside and inside the BMRF1-cores. Based on these observations, we propose a model in which viral procapsids are assembled outside the BMRF1-cores and subsequently migrate therein, where viral DNA encapsidation occurs. To our knowledge, this is the first report describing capsid assembly sites in relation to EBV replication compartments.

Molecular diversity of IgG responses to Epstein-Barr virus proteins in asymptomatic Epstein-Barr virus carriers

The Epstein-Barr virus (EBV) is a ubiquitous pathogen that infects over 90 % of adults. EBV is the primary etiological agent of infectious mononucleosis and is closely associated with nasopharyngeal carcinoma, gastric carcinoma, Hodgkin lymphoma and Burkitt lymphoma. Clinical serological assays for EBV diagnosis only survey a small portion of the viral proteome, which does not represent the total antigenic breadth presented to the immune system during viral infection. In this study, we have generated an expression library containing the majority of EBV ORFs, and have systematically evaluated IgG responses to those EBV proteins in sera from EBV carriers. In addition to confirming previously recognized dominant EBV antigens, this study has identified additional immunodominant antigens, and has revealed a more expansive antigenic profile of the humoral responses to EBV in asymptomatic carriers. This EBV expression library will be deposited in a public repository with the goal of disseminating this new research tool for the application of identifying potential new biomarkers for EBV-associated diseases.

Epstein-Barr Virus-Specific Humoral Immune Responses in Health and Disease

Epstein-Barr virus (EBV) is widely distributed in the world and associated with a still increasing number of acute, chronic, malignant and autoimmune disease syndromes. Humoral immune responses to EBV have been studied for diagnostic, pathogenic and protective (vaccine) purposes. These studies use a range of methodologies, from cell-based immunofluorescence testing to antibody-diversity analysis using immunoblot and epitope analysis using recombinant or synthetic peptide-scanning. First, the individual EBV antigen complexes (VCA , MA, EA(D), EA(R) and EBNA) are defined at cellular and molecular levels, providing a historic overview. The characteristic antibody responses to these complexes in health and disease are described, and differences are highlighted by clinical examples. Options for EBV vaccination are briefly addressed. For a selected number of immunodominant proteins, in particular EBNA1, the interaction with human antibodies is further detailed at the epitope level, revealing interesting insights for structure, function and immunological aspects, not considered previously. Humoral immune responses against EBV-encoded tumour antigens LMP1, LMP2 and BARF1 are addressed, which provide novel options for targeted immunotherapy. Finally, some considerations on EBV-linked autoimmune diseases are given, and mechanisms of antigen mimicry are briefly discussed. Further analysis of humoral immune responses against EBV in health and disease in carefully selected patient cohorts will open new options for understanding pathogenesis of individual EBV-linked diseases and developing targeted diagnostic and therapeutic approaches.

The Rad6/18 ubiquitin complex interacts with the Epstein-Barr virus deubiquitinating enzyme, BPLF1, and contributes to virus infectivity

Epstein-Barr virus (EBV) encodes BPLF1, a lytic cycle protein with deubiquitinating activity that is contained in its N-terminal domain and conserved across the Herpesviridae. EBV replication is associated with cellular DNA replication and repair factors, and initiation of EBV lytic replication induces a DNA damage response, which can be regulated at least in part by BPLF1. The cellular DNA repair pathway, translesion synthesis (TLS), is disrupted by BPLF1, which deubiquitinates the DNA processivity factor, PCNA, and inhibits the recruitment of the TLS polymerase, polymerase eta (Pol eta), after damage to DNA by UV irradiation. Here we showed that the E3 ubiquitin ligase, which activates TLS repair by monoubiquitination of PCNA, is also affected by BPLF1 deubiquitinating activity. First, BPLF1 interacts directly with Rad18, and overexpression of BPLF1 results in increased levels of the Rad18 protein, suggesting that it stabilizes Rad18. Next, expression of functionally active BPLF1 caused relocalization of Rad18 into nuclear foci, which is consistent with sites of cellular DNA replication that occur during S phase. Also, levels of Rad18 remain constant during lytic reactivation of wild-type virus, but reactivation of BPLF1 knockout virus resulted in decreased levels of Rad18. Finally, the contribution of Rad18 levels to infectious virus production was examined with small interfering RNA (siRNA) targeting Rad18. Results demonstrated that reducing levels of Rad18 decreased production of infectious virus, and infectious titers of BPLF1 knockout virus were partially restored by overexpression of Rad18. Thus, BPLF1 interacts with and maintains Rad18 at high levels during lytic replication, which assists in production of infectious virus.

IMPORTANCE

Characterization of EBV BPLF1's deubiquitinating activity and identification of its targets and subsequent functional effects remain little studied. All members of the Herpesviridae contain BPLF1 homologs with conserved enzymatic activity, and findings discovered with EBV BPLF1 are likely applicable to other members of the family. Discovery of new targets of BPLF1 will point to cellular pathways and viral processes regulated by the enzymatic activity of the EBV-encoded deubiquitinating enzyme. Here we determined the importance of the cellular ubiquitin ligase Rad18 in these processes and how it is affected by BPLF1. Our findings demonstrate that EBV can co-opt Rad18 as a novel accessory factor in the production of infectious virus.

Different distributions of Epstein-Barr virus early and late gene transcripts within viral replication compartments

Productive replication of the Epstein-Barr virus (EBV) occurs in discrete sites in nuclei, called replication compartments, where viral genome DNA synthesis and transcription take place. The replication compartments include subnuclear domains, designated BMRF1 cores, which are highly enriched in the BMRF1 protein. During viral lytic replication, newly synthesized viral DNA genomes are organized around and then stored inside BMRF1 cores. Here, we examined spatial distribution of viral early and late gene mRNAs within replication compartments using confocal laser scanning microscopy and three-dimensional surface reconstruction imaging. EBV early mRNAs were mainly located outside the BMRF1 cores, while viral late mRNAs were identified inside, corresponding well with the fact that late gene transcription is dependent on viral DNA replication. From these results, we speculate that sites for viral early and late gene transcription are separated with reference to BMRF1 cores.

Nuclear transport of Epstein-Barr virus DNA polymerase is dependent on the BMRF1 polymerase processivity factor and molecular chaperone Hsp90

Epstein-Barr virus (EBV) replication proteins are transported into the nucleus to synthesize viral genomes. We here report molecular mechanisms for nuclear transport of EBV DNA polymerase. The EBV DNA polymerase catalytic subunit BALF5 was found to accumulate in the cytoplasm when expressed alone, while the EBV DNA polymerase processivity factor BMRF1 moved into the nucleus by itself. Coexpression of both proteins, however, resulted in efficient nuclear transport of BALF5. Deletion of the nuclear localization signal of BMRF1 diminished the proteins' nuclear transport, although both proteins can still interact. These results suggest that BALF5 interacts with BMRF1 to effect transport into the nucleus. Interestingly, we found that Hsp90 inhibitors or knockdown of Hsp90β with short hairpin RNA prevented the BALF5 nuclear transport, even in the presence of BMRF1, both in transfection assays and in the context of lytic replication. Immunoprecipitation analyses suggested that the molecular chaperone Hsp90 interacts with BALF5. Treatment with Hsp90 inhibitors blocked viral DNA replication almost completely during lytic infection, and knockdown of Hsp90β reduced viral genome synthesis. Collectively, we speculate that Hsp90 interacts with BALF5 in the cytoplasm to assist complex formation with BMRF1, leading to nuclear transport. Hsp90 inhibitors may be useful for therapy for EBV-associated diseases in the future.

Epstein-Barr virus and systemic lupus erythematosus

The etiology of SLE is not fully established. SLE is a disease with periods of waning disease activity and intermittent flares. This fits well in theory to a latent virus infection, which occasionally switches to lytic cycle, and EBV infection has for long been suspected to be involved. This paper reviews EBV immunobiology and how this is related to SLE pathogenesis by illustrating uncontrolled reactivation of EBV as a disease mechanism for SLE. Studies on EBV in SLE patients show enlarged viral load, abnormal expression of viral lytic genes, impaired EBV-specific T-cell response, and increased levels of EBV-directed antibodies. These results suggest a role for reactivation of EBV infection in SLE. The increased level of EBV antibodies especially comprises an elevated titre of IgA antibodies, and the total number of EBV-reacting antibody isotypes is also enlarged. As EBV is known to be controlled by cell-mediated immunity, the reduced EBV-specific T-cell response in SLE patients may result in defective control of EBV causing frequent reactivation and expression of lytic cycle antigens. This gives rise to enhanced apoptosis and amplified cellular waste load resulting in activation of an immune response and development of EBV-directed antibodies and autoantibodies to cellular antigens.

The influence of Epstein-Barr virus reactivation in patients with Graves' disease

In Graves' disease, the IgG class autoantibody against thyrotropin receptor (TRAb) is produced excessively and induces hyperthyroidism. Epstein-Barr virus (EBV) is one of the human herpesviruses that persists for life, mainly in B lymphocytes, and is occasionally reactivated. Therefore, EBV may affect the antibody production of B lymphocytes that would normally produce TRAb. The purpose of the present study was to evaluate the association of EBV reactivation with the etiology of Graves' disease. Serum levels of EBV antibodies and IgE were determined by ELISA. TRAb levels were determined by radioreceptor assay. We performed in-situ hybridization (ISH) of EBV-encoded small RNA (EBER)1 on the thyroid tissue of one of our patients. In Graves' disease patients with TRAb levels ≥ 10%, EA antibody levels, which indicate EBV reactivation, were moderately but significantly correlated with the levels of TRAb, and weakly but significantly correlated with IgE. EBER1-ISH revealed that one of our patients had EBV-infected lymphocytes infiltrating the thyroid gland. EBV reactivation may stimulate antibody-producing B lymphocytes predisposed to make TRAb, and this may contribute to or exacerbate the disease.

Spatiotemporally different DNA repair systems participate in Epstein-Barr virus genome maturation

Productive replication of Epstein-Barr virus occurs in discrete sites in nuclei, called replication compartments, where viral DNA replication proteins and host homologous recombinational repair (HRR) and mismatch repair (MMR) factors are recruited. Three-dimensional (3D) surface reconstruction imaging clarified the spatial arrangements of these factors within the replication compartments. Subnuclear domains, designated BMRF1 cores, which were highly enriched in viral polymerase processivity factor BMRF1 could be identified inside the replication compartments. Pulse-chase experiments revealed that newly synthesized viral genomes organized around the BMRF1 cores were transferred inward. HRR factors could be demonstrated mainly outside BMRF1 cores, where de novo synthesis of viral DNA was ongoing, whereas MMR factors were found predominantly inside. These results imply that de novo synthesis of viral DNA is coupled with HRR outside the cores, followed by MMR inside cores for quality control of replicated viral genomes. Thus, our approach unveiled a viral genome manufacturing plant.

Tetrameric ring formation of Epstein-Barr virus polymerase processivity factor is crucial for viral replication

The Epstein-Barr virus BMRF1 DNA polymerase processivity factor, which is essential for viral genome replication, exists mainly as a C-shaped head-to-head homodimer but partly forms a ring-shaped tetramer through tail-to-tail association. Based on its molecular structure, several BMRF1 mutant viruses were constructed to examine their influence on viral replication. The R256E virus, which has a severely impaired capacity for DNA binding and polymerase processivity, failed to form replication compartments, resulting in interference of viral replication, while the C95E mutation, which impairs head-to-head contact in vitro, unexpectedly hardly affected the viral replication. Also, surprisingly, replication of the C206E virus, which is expected to have impairment of tail-to-tail contact, was severely restricted, although the mutant protein possesses the same in vitro biochemical activities as the wild type. Since the tail-to-tail contact surface is smaller than that of the head-to-head contact area, its contribution to ring formation might be essential for viral replication.

Maribavir inhibits epstein-barr virus transcription in addition to viral DNA replication

Although many drugs inhibit the replication of Epstein-Barr virus (EBV) in cell culture systems, there is still no drug that is effective and approved for use in primary EBV infection. More recently, maribavir (MBV), an l-ribofuranoside benzimidazole, has been shown to be a potent and nontoxic inhibitor of EBV replication and to have a mode of action quite distinct from that of acyclic nucleoside analogs such as acyclovir (ACV) that is based primarily on MBV's ability to block the phosphorylation of target proteins by EBV and human cytomegalovirus protein kinases. However, since the antiviral mechanisms of the drug are complex, we have carried out a comprehensive analysis of the effects of MBV on the RNA expression levels of all EBV genes with a quantitative real-time reverse transcription-PCR-based array. We show that in comparisons with ACV, the RNA expression profiles produced by the two drugs are entirely different, with MBV causing a pronounced inhibition of multiple viral mRNAs and with ACV causing virtually none. The results emphasize the different modes of action of the two drugs and suggest that the action of MBV may be linked to indirect effects on the transcription of EBV genes through the interaction of BGLF4 with multiple viral proteins.

Epstein-Barr virus polymerase processivity factor enhances BALF2 promoter transcription as a coactivator for the BZLF1 immediate-early protein

The Epstein-Barr virus (EBV) BMRF1 protein is an essential replication protein acting at viral replication forks as a viral DNA polymerase processivity factor, whereas the BALF2 protein is a single-stranded DNA-binding protein that also acts at replication forks and is most abundantly expressed during viral productive replication. Here we document that the BMRF1 protein evidently enhances viral BZLF1 transcription factor-mediated transactivation of the BALF2 gene promoter. Mutagenesis and electrophoretic mobility shift assays demonstrated the BALF2 promoter to harbor two BZLF1 protein-binding sites (BZLF1-responsive elements). Direct binding of the BZLF1 protein to BZLF1-responsive elements and physical interaction between BZLF1 and BMRF1 proteins are prerequisite for the BMRF1 protein up-regulation of the BALF2 gene promoter. A monomeric mutant, C95E, which is defective in homodimerization, could still interact and enhance BZLF1-mediated transactivation. Furthermore although EBV protein kinase phosphorylates BMRF1 protein extensively, it turned out that phosphorylation of the protein by the kinase is inhibitory to the enhancement of the BZLF1-mediated transactivation of BALF2 promoter. Exogenous expression of BMRF1 protein augmented BALF2 expression in HEK293 cells harboring the EBV genome but lacking BMRF1 and BALF5 genes, demonstrating functions as a transcriptional regulator in the context of viral infection. Overall the BMRF1 protein is a multifunctional protein that cannot only act as a DNA polymerase processivity factor but also enhances BALF2 promoter transcription as a coactivator for the BZLF1 protein, regulating the expression level of viral single-stranded DNA-binding protein.

BFRF1 of Epstein-Barr virus is essential for efficient primary viral envelopment and egress

The molecular mechanisms that underlie maturation and egress of Epstein-Barr virus (EBV) virions are only partially characterized. We have recently shown that the BFRF1 gene, the EBV positional homolog of herpes simplex virus type 1 and pseudorabies virus UL34, is expressed early during EBV lytic replication and that it is found predominantly on the nuclear membrane (A. Farina, R. Santarelli, R. Gonnella, R. Bei, R. Muraro, G. Cardinali, S. Uccini, G. Ragona, L. Frati, A. Faggioni, and A. Angeloni, J. Virol. 74:3235-3244, 2000). These data suggest that the BFRF1 protein might be involved in viral primary envelopment. To precisely determine the function of this protein, we have constructed an EBV mutant devoid of the BFRF1 gene (BFRF1-KO). 293 cells carrying BFRF1-KO showed no differences in comparison with wild-type EBV in terms of DNA lytic replication or expression of late viral proteins upon induction of the lytic cycle. However, binding assays and infection experiments using cell lines or human cord blood lymphocytes showed a clear reduction in the viral mutant titers. Complementation experiments with BFRF1-KO and a BFRF1 expression vector restored viral titers to levels similar to those for the wild-type control, showing that the modifications that we introduced were limited to the BFRF1 gene. Electron microscopic observations showed that the reduction in viral titers was due to sequestration of EBV nucleocapsids in the nuclei of lytically induced cells. This suggests that BFRF1 is involved in transport of the maturing virion across the nuclear membrane. This hypothesis was further supported by the observation that BFRF1 is present in maturing intracellular virions but not in their extracellular counterparts. This implies that BFRF1 is a key protein for EBV maturation.

Murine gammaherpesvirus 68 Rta-dependent activation of the gene 57 promoter

The Rta homolog encoded by murine gammaherpesvirus 68 (gammaHV68) gene 50 is essential for virus replication and is capable of driving virus reactivation from the S11 latently infected B lymphoma cell line. Here we characterize Rta activation of gammaHV68 gene 57, which is abundantly transcribed during the early phase of virus replication. Infection of murine fibroblasts with an Rta null virus demonstrated that transcription of gene 57 is dependent on Rta expression. Analysis of the gene 57 promoter identified 2 distinct regions that are Rta responsive, either in the context of the gene 57 promoter or when cloned upstream of a heterologous promoter. Sequence analysis of these regions revealed homology to known Rta-responsive cis-elements in the closely related Kaposi's sarcoma-associated viral (KSHV) genome. In addition, two candidate binding sites for the cellular transcription factor RBP-Jkappa/CBF1 were also identified in one of the Rta-responsive regions, which may play a role in mediating Rta transactivation similar to that observed in some KSHV Rta-responsive genes. Overall, analysis of the gammaHV68 gene 57 promoter suggests that mechanisms of Rta activation are conserved among gamma2-herpesviruses.

Expression and localization of the Epstein-Barr virus-encoded protein kinase

The protein kinase (PK) encoded by the Epstein-Barr Virus (EBV) BGLF4 gene is the only EBV protein kinase. The expression pattern of EBV PK during the reactivation of the viral lytic cycle and the subcellular localization of the protein were analyzed with a polyclonal antiserum raised against a peptide corresponding to the N terminus of EBV PK. Based on previously published data (E. Gershburg and J. S. Pagano, J. Virol. 76:998-1003, 2002) and the expression pattern described here, we conclude that EBV PK is an early protein that requires viral-DNA replication for maximum expression. By biochemical fractionation, the protein could be detected mainly in the nuclear fraction 4 h after viral reactivation in Akata cells. Nuclear localization could be visualized by indirect immunofluorescence in HeLa cells transiently expressing EBV BGLF4 in the absence of other viral products. Transient expression of 3'-terminal deletion mutants of EBV BGLF4 resulted in cytoplasmic localization, confirming the presence of a nuclear localization site in the C-terminal region of the protein. In contrast to the wild-type EBV PK, all of the mutants were unable to hyperphosphorylate EA-D during coexpression or to phosphorylate ganciclovir, as measured by an in-cell activity assay. Thus, the results demonstrate that the nuclear localization, as well as the kinase activity, of BGFL4 is dependent on an intact C-terminal region.

Tetramer-assisted identification and characterization of epitopes recognized by HLA A*2402-restricted Epstein-Barr virus-specific CD8+ T cells

We determined cytotoxic T lymphocyte (CTL) epitopes through screening with a computer-assisted algorithm and an enzyme-linked immunospot (ELISPOT) assay using in vitro-reactivated polyclonal Epstein-Barr virus (EBV)-specific CD8(+) T cells as responders. In addition, to confirm that the epitopes were generated after endogenous processing and presentation of the EBV proteins, a novel T-cell receptor (TCR) down-regulation assay was introduced, in which a fluorescent tetrameric major histocompatibility complex (MHC)/peptide complex was employed for detecting TCR down-regulation after stimulation with the epitope presented on antigen-presenting cells. Through such screening, 3 HLA A*2402-restricted epitopes were identified: IYVLVMLVL, TYPVLEEMF, and DYNFVKQLF, derived from LMP2, BRLF1, and BMLF1 proteins, respectively. TCR down-regulation assays disclosed that, in contrast to the other 2 epitopes, IYVLVMLVL was not presented on HLA A24-positive fibroblast cells infected with recombinant vaccinia viruses expressing LMP2. Furthermore, ELISPOT assays with an epitope-specific CTL clone demonstrated that the presentation was partially restored by pretreatment of the fibroblast cells with interferon-gamma. The epitope was presented on transporters associated with antigen processing (TAP)-negative T2 cells transfected with plasmids encoding HLA A*2402 and the minimal epitope, indicating that the presentation is TAP independent. In conclusion, the 3 epitopes thus defined could be useful for studying EBV-specific CD8(+) T-cell responses among populations positive for HLA A*2402.

Isolation and expression of three open reading frames from ovine herpesvirus-2

Ovine herpesvirus-2 (OvHV-2), a member of the gammaherpesviruses (genus Rhadinovirus), asymptomatically infects its natural host, the sheep, but causes malignant catarrhal fever (MCF) in susceptible hosts, such as cattle, deer and pigs. A permissive cell culture system for virus replication has not been identified but viral DNA is present within lymphoblastoid cell lines (LCLs) established from cases of MCF. During this study, a cDNA expression library generated from LCLs was screened with sheep sera and two cDNAs were isolated. One cDNA contained two open reading frames (ORFs) that show similarity to ORFs 58 and 59 of alcelaphine herpesvirus-1 (AlHV-1), a closely related gammaherpesvirus that also causes MCF. Both ORFs 58 and 59 are conserved throughout the gammaherpesviruses. ORF 58 is predicted to be a membrane protein, while ORF 59 has been shown to be an early lytic gene that functions as a DNA polymerase processivity factor. The second cDNA clone contained a partial ORF showing limited similarity to AlHV-1 ORF 73, a homologue of the latency-associated nuclear antigen of human herpesvirus-8, which is associated with latent infections. The full-length OvHV-2 ORF 73 was cloned subsequently by PCR. The ORFs isolated from the library were cloned into a bacterial expression vector and the recombinant proteins tested for their reactivity to sera from OvHV-2-infected animals. An ORF 59 fusion protein was recognized specifically by sera from OvHV-2-infected cattle and will be used to develop a sero-diagnostic test.

Coupled particle light scattering: a new technique for serodiagnosis of Epstein-Barr virus infection

The Coupled Particle Light Scattering technique was evaluated for serological diagnosis of Epstein-Barr Virus (EBV) infection. Two hundred ninety-six patient sera selected from several clinical categories (acute infection, non-primary infection, interfering non-EBV infection, non-infected) were tested for IgM and IgG antibodies (anti-VCA, anti-EBNA and anti-EA). Determination of EBV IgG with Copalis multiplex was accurate when compared with Enzygnost Anti-EBV/IgG ELISA. Although the sensitivity of Copalis IgM for acute infections was 100% a positive IgM result did not always indicate an acute infection. Strong reactivity to IgG EA (ratio 3, 1) and IgG VCA (ratio 13, 3) correlated with persistent infection or reactivation. The CopalisI has many advantages over the existing methods, such as the possibility to measure three semi-quantitative IgG responses to three different EBV antigens simultaneously.

Regulation of Epstein-Barr virus promoters in oral epithelial cells and lymphocytes

Hairy leukoplakia (HL) is a proliferative lesion of the tongue that supports abundant Epstein-Barr virus (EBV) replication. Previous work showed high-level expression of the EBV BMRF2 gene in HL. To characterize the regulation of BMRF2 expression in HL, we mapped the 5' ends of the BMRF1 and BMRF2 transcripts and showed that BMRF2 is expressed from a novel internal promoter within the BMRF1 coding region. Mechanisms of BMRF2 regulation were compared in oral epithelial cells and B lymphocytes, as were those of BMRF1 and BDLF3, early and late EBV transcripts, respectively, that are also known to be expressed in HL. Basal activity of the putative BMRF2 promoter was 10-fold higher in HSC-3 epithelial cells than in B lymphocytes. The BMRF2 and the BDLF3 promoters were responsive to induction by phorbol ester, but unlike the BMRF1 promoter, they were not responsive to BZLF1 transactivation. By mutational analysis, the major activity of the BMRF2 promoter mapped to a 50-bp region, which includes a TATA-like element and a GC box. The BMRF2 promoter may be regulated differentially from the BMRF1 promoter and more closely resembles that of BDLF3. This novel BMRF2 promoter likely belongs to a class of viral promoters that is more responsive to mechanisms known to induce epithelial cell differentiation, consistent with its high level of expression in HL.

Human cytomegalovirus UL69 protein induces cells to accumulate in G1 phase of the cell cycle

Earlier studies have revealed that human cytomegalovirus rapidly inhibits the growth of fibroblasts, blocking cell cycle progression at multiple points, including the G1-to-S-phase transition. The present study demonstrates that the UL69 protein, a virus-encoded constituent of the virion, is able to arrest cell cycle progression when introduced into uninfected cells. Expression of the UL69 protein causes U2 OS cells and primary human fibroblasts to accumulate within the G1 compartment of the cell cycle, and serum fails to induce the progression of quiescent human fibroblasts into the S phase when the protein is present. Therefore, the UL69 protein is at least partially responsible for the cell cycle block that is instituted after infection of permissive cells with human cytomegalovirus.

Epstein-Barr virus DNase contains two nuclear localization signals, which are different in sensitivity to the hydrophobic regions

The DNase of Epstein-Barr virus (EBV) is a 470-amino-acid protein which possesses both endonuclease and exonuclease activities and accepts both double-stranded DNA and single-stranded DNA as substrates. It has been reported that this protein may be found in the nucleus and/or cytoplasm of infected cells. In this study, using cell fractionation and immunoblotting to determine the distribution of EBV DNase in Akata cells stimulated with anti-human immunoglobulin G antibody (anti-IgG), the DNase was found to be located predominantly in the nucleus. To map the signals in DNase which mediate its nuclear localization, we monitored the nuclear transport of fusion proteins consisting of various fragments of EBV DNase linked to a cytoplasmic protein, beta-galactosidase (beta-Gal). The results demonstrated that two regions of the DNase with nuclear localization signal (NLS) activity, designated NLS-A (amino acids 239-266) and NLS-B (amino acids 291-306), were able independently to localize the beta-Gal to the nuclei of HEp-2 and HeLa cells. Five basic residues (R or K) were found in each NLS and distributed differently in primary structure. The basic domains and flanking residues of NLS-A and NLS-B are 250YKRPCKRSFIRFI262 and 294LKDVRKRKLGPGH306, respectively. Further examination of these sequences revealed that NLS-A contains bulky aromatic amino acids (Y and F) which may diminish its capacity to act as a strong NLS and lacks the typical proline and glycine helix-breakers. However, NLS-B contains typical proline and glycine helix-breakers and the histidine residue at amino acid 306 is required for NLS activity. In addition, two hydrophobic regions within the DNase were found to inhibit the function of NLS-A but not NLS-B, suggesting that these two domains are different types of NLSs and differ in their sensitivity to hydrophobic regions in the context of protein structure.

Cloning and functional analysis of Kaposi's sarcoma-associated herpesvirus DNA polymerase and its processivity factor

Kaposi's sarcoma-associated herpesvirus (KSHV), or human herpesvirus 8, is a newly identified virus with tumorigenic potential. Here, we cloned and expressed the DNA polymerase (Pol-8) of KSHV and its processivity factor (PF-8). Pol-8 bound specifically to PF-8 in vitro. Moreover, the DNA synthesis activity of Pol-8 was shown in vitro to be strongly dependent on PF-8. Addition of PF-8 to Pol-8 allowed efficient synthesis of fully extended DNA products corresponding to the full-length M13 template (7,249 nucleotides), whereas Pol-8 alone could incorporate only several nucleotides. The specificity of PF-8 and Pol-8 for each other was demonstrated by their inability to be functionally replaced by the DNA polymerases and processivity factors of herpes simplex virus 1 and human herpesvirus 6.

A major DNA binding protein encoded by BALF2 open reading frame of Epstein-Barr virus (EBV) forms a complex with other EBV DNA-binding proteins: DNAase, EA-D, and DNA polymerase

A major 135-kDa DNA binding protein (mDBP) encoded by the BALF2 open reading frame of Epstein-Barr Virus (EBV) is known to be an essential protein for the induction of the lytic cycle. The present investigation was carried out to know whether this protein forms a complex in vivo with other viral DNA binding proteins (DBP) involved in DNA replication: DNA polymerase, EA-D (diffused early antigen), and DNAase. Immunoprecipitation assays followed by mono- and two-dimensional electrophoresis showed that mDBP forms a complex with these three DBP. Other complexes were also found such as EA-D/DNAase, DNA polymerase/DNAase, and DNA polymerase/EA-D. The complexed forms already exist in the early stage of EBV cycle before DNA synthesis is induced in the EBV producer P3HR-1 cell line. The exonuclease activity encoded by DNAase was found to be inhibited when this enzyme complexed with mDBP, while the EBV DNA polymerase retained its activity in the complexed form with mDBP. Our results suggest that these complexes already present before DNA synthesis are necessary for EBV DNA synthesis.

Transcriptional patterns of the pCD41 (U27) locus of human herpesvirus 6

Human herpesvirus 6 (HHV-6) is a lymphotropic herpesvirus, and in vitro, it can productively infect many of the same cell types that human immunodeficiency virus (HIV) infects. Simultaneous infection of T cells by HIV and HHV-6 can lead to both activation of the HIV promoter and acceleration of the cytopathic effects. Several HHV-6 genes have been demonstrated to activate HIV promoter expression. Among them is a cDNA clone, pCD41 (U27), which codes for the HHV-6 DNA polymerase accessory protein. We have now further characterized the transcription pattern in the pCD41 locus and identified at least six RNA species, ranging in size from 1.2 to 4.5 kb. Northern (RNA) blot analyses showed no significant difference in RNA patterns between the HHV-6 variant A (GS) and variant B (Z29) viruses. All the RNA species detected by pCD41 are polyadenylated and polyribosome associated, suggesting that they may be actively engaged in protein synthesis. Cycloheximide and phosphonoacetic acid inhibition assay results indicate that all the pCD41 RNA species belong to the herpesviral early-late family. Using primer extension and S1 mapping techniques, the 5' and 3' ends of each transcript were mapped to different positions, and no splicing was observed.

Alteration of a single serine in the basic domain of the Epstein-Barr virus ZEBRA protein separates its functions of transcriptional activation and disruption of latency

The ZEBRA protein from Epstein-Barr virus (EBV) activates a switch from the latent to the lytic expression program of the virus. ZEBRA, a member of the bZIP family of DNA-binding proteins, is a transcriptional activator capable of inducing expression from viral lytic cycle promoters. It had previously been thought that ZEBRA's capacity to disrupt EBV latency resided primarily in its ability to activate transcription of genes that encode products required for lytic replication. We generated a point mutant of ZEBRA, Z(S186A), that was not impaired in its ability to activate transcription; however, this mutation abolished its ability to initiate the viral lytic cascade. The mutant, containing a serine-to-alanine substitution in the DNA-binding domain of the protein, bound to several known ZEBRA-binding sites and activated transcription from reporters bearing known ZEBRA-responsive promoters but did not disrupt latency in EBV-infected cell lines. Therefore, initiation of the EBV lytic cycle by the ZEBRA protein requires a function in addition to transcriptional activation; a change of serine 186 to alanine in the DNA-binding domain of ZEBRA abolished this additional function and uncovered a new role for the ZEBRA protein in disruption of EBV latency. The additional function that is required for initiation of the lytic viral life cycle is likely to require phosphorylation of serine 186 of the ZEBRA protein, which may influence either DNA recognition or transcriptional activation of lytic viral promoters in a chromatinized viral episome.

The Epstein-Barr virus (EBV) DNA polymerase accessory protein, BMRF1, activates the essential downstream component of the EBV oriLyt

The EBV DNA polymerase accessory protein, BMRF1, is an essential component of the viral DNA polymerase and is required for lytic EBV replication. In addition to its polymerase accessory protein function, we have recently reported that BMRF1 is a transcriptional activator, inducing expression of the essential oriLyt promoter, BHLF1. Here we have precisely mapped the BMRF1-response element in the BHLF1 promoter. We demonstrate that a region of oriLyt (the "downstream component"), previously shown to be one of two domains absolutely essential for oriLyt replication, is required for BMRF1-induced activation of the BHLF1 promoter. Furthermore, the downstream component of oriLyt is sufficient to confer BMRF1-responsiveness to a heterologous promoter. The downstream component contains Sp1 binding sites, and confers Sp1-responsiveness to a heterologous promoter. A series of plasmids containing various protions of the oriLyt downstream component were constructed and analyzed for their ability to respond to the BMRF1 versus Sp1 transactivators. Although the BMRF1-responsive region of the downstream component overlaps the Sp1-responsive element, certain oriLyt sequences required for maximal BMRF1-responsiveness were not required for maximal Sp1-responsiveness. In particular, a site-directed mutation altering the downstream component sequence GATGG (located from -588 to -592 relative to the BHLF1 transcription initiation site) did not affect Sp1-responsiveness, but reduced BMRF-1-responsiveness by 75% and abolished oriLyt replication. Although BMRF1 possesses nonspecific DNA binding activity, were unable to demonstrate specific BMRF1 binding to the downstream component of oriLyt. Our results suggest that BMRF1-induced activation of the essential downstream component of oriLyt may play an important role in oriLyt replication.

Evidence that the UL84 gene product of human cytomegalovirus is essential for promoting oriLyt-dependent DNA replication and formation of replication compartments in cotransfection assays

The protein products of 11 viral genomic loci cooperate in a transient cotransfection assay to mediate lytic-phase DNA replication of oriLyt, the human cytomegalovirus (HCMV) origin of replication. Six of these genes have homology with the well-characterized herpes simplex virus replication genes and encode core replication machinery proteins that are typically essential for DNA synthesis. The remaining five HCMV gene loci, initially referred to as auxiliary components, include several known immediate-early (IE) transcriptional regulatory proteins as well as genes encoding functionally uncharacterized polypeptides. Some or all of the auxiliary components may be necessary in trans to replicate the HCMV oriLyt only because they are required for efficient expression or transactivation of the native early promoters and 3' processing elements included in the genomic clones. Therefore, we reassessed the requirements for the auxiliary components by adding constitutive heterologous promoters and control signals to the coding regions and carrying out transient DpnI replication assays in cotransfected Vero cells. The results revealed that in the presence of the UL69 posttranscriptional activator and the remaining auxiliary polypeptides, UL84 was the only auxiliary component that could not be omitted to obtain oriLyt-dependent DNA replication. Nevertheless, in human diploid fibroblasts, some additional auxiliary loci as well as UL84 were critical. There was also an obligatory requirement for UL84, in cooperation with two other auxiliary factors, UL112-113 and IE2, and the core machinery, to constitute the minimal HCMV proteins necessary to direct oriLyt-dependent DNA amplification. However, the Epstein-Barr virus core replication genes could substitute for the HCMV core genes, and in these circumstances, UL84 alone directed amplification of HCMV oriLyt. Moreover, there was also an absolute requirement for UL84 along with the core and other auxiliary factors for the formation of intranuclear replication compartments as assayed by immunofluorescence in transient DNA cotransfection assays. These compartments were typical of those associated with active viral DNA replication in HCMV-infected cells, they incorporated pulse-labeled bromodeoxyuridine, and their formation was both phosphonoacetic acid sensitive and oriLyt dependent. These results demonstrate that UL84 is obligatory for both intranuclear replication compartment formation and origin-dependent DNA amplification and suggest that it is a key viral component in promoting the initiation of HCMV oriLyt-directed DNA replication.

Functional and physical interactions between the Epstein-Barr virus (EBV) proteins BZLF1 and BMRF1: Effects on EBV transcription and lytic replication

The Epstein-Barr virus (EBV) proteins BZLF1 and BMRF1 are both essential for lytic EBV replication. BZLF1 is a transcriptional activator which binds directly to the lytic origin of replication (oriLyt) and plays a critical role in the disruption of viral latency. The BMRF1 protein is required for viral polymerase processivity. Here we demonstrate that the BMRF1 gene product functions as a transcriptional activator and has direct (as well as indirect) interactions with the BZLF1 gene product. The BMRF1 gene product activates an essential oriLyt promoter, BHLF1, but does not activate two other early EBV promoters (BMRF1 and BHRF1). Direct interaction between the BMRF1 and BZLF1 gene products requires the first 45 amino acids of BMRF1 and the bZip domain of BZLF1. The effect of the BZLF1-BMRF1 interaction on early EBV transcription is complex and is promoter specific. The oriLyt BHLF1 promoter is activated by either the BZLF1 or BMRF1 gene product alone and is further activated by the combination of the BZLF1 and BMRF1 gene products. Enhanced activation of BHLF1 transcription by the BMRF1-BZLF1 combination does not require direct interaction between these proteins. In contrast, BZLF1-induced activation of the BMRF1 promoter is inhibited in the presence of the BMRF1 gene product. A point mutation in the BZLF1 protein (amino acid 200), which prevents in vitro interaction with the BMRF1 protein but which does not reduce BZLF1 transactivator function, allows the BZLF1 protein to activate the BMRF1 promoter equally well in the presence or absence of the BMRF1 gene product. Therefore, direct interaction between the BZLF1 and BMRF1 proteins may inhibit BZLF1-induced transcription of the BMRF1 promoter. BZLF1 mutated at amino acid 200 is as efficient as wild-type BZLF1 in promoting replication of an oriLyt plasmid. However, this mutation reduces the ability of BZLF1 to induce lytic replication of the endogenous viral genome in D98/HE-R-1 cells. Our results indicate that functional and physical interactions between the BMRF1 and BZLF1 proteins may modulate the efficiency of lytic EBV infection. The BMRF1 gene product clearly has a transcriptional, as well as replicative, role during lytic EBV infection.

UL69 of human cytomegalovirus, an open reading frame with homology to ICP27 of herpes simplex virus, encodes a transactivator of gene expression

The UL69 open reading frame of human cytomegalovirus (HCMV) is homologous to the immediate-early protein ICP27 of herpes simplex virus, an essential viral regulatory protein involved in the transition from early to late gene expression. Genes with homology to ICP27 have been detected in all subclasses of herpesviruses so far. While the respective proteins in alpha- and gammaherpesviruses have been defined as trans-regulatory molecules, nothing is known about these genes in betaherpesviruses. This study was therefore undertaken in order to investigate expression from the UL69 gene locus of HCMV. Northern (RNA) blot experiments revealed a complex pattern of transcripts that changed during the time course of the HCMV replicative cycle: two transcripts of 2.7 and 3.5 kb that were regulated differentially could be detected as early as 7 h after infection. However, these transcripts could not be detected in the presence of cycloheximide. Additional, larger transcripts were present exclusively at late times after infection. To analyze protein expression from the UL69 gene region, the UL69 open reading frame was expressed as a histidine-tagged protein in Escherichia coli. A specific antiserum was generated and used to detect the UL69 protein in HCMV-infected cells which revealed its localization within the intranuclear inclusions that are characteristic for HCMV infection. In cotransfection experiments, an HCMV true late promoter could not be activated by UL69, whereas an early promoter and several heterologous promoters were stimulated about 10-fold. Complementation studies showed that the UL69 protein cannot substitute for ICP27 in the context of the HSV infection, suggesting functional differences between these two proteins. In summary, these experiments define a novel regulatory protein encoded by HCMV that is expressed as an early-late gene and appears to exert a broad stimulatory effect on gene expression.

Gene mapping and expression of two immunodominant Epstein-Barr virus capsid proteins

The genomic localization of two immunodominant genes encoding two proteins of the Epstein-Barr virus capsid antigen (VCA) complex, VCA-p18 and VCA-p40, has been identified. For that purpose, lambda gt11-based cDNA libraries were constructed from HH514.c16 cells induced for virus production. The libraries were screened with a monoclonal antibody, EBV.OT41A, directed against VCA-p40 or with affinity-purified human antibodies against VCA-p18. Sequencing of the inserts of positive plaques showed that VCA-p18 and VCA-p40 are encoded within open reading frames (ORFs) BFRF3 and BdRF1, respectively. Peptide scanning analysis of the predicted protein of ORF BdRF1 resulted in defining the epitope of monoclonal antibody EBV.OT41A at the C-terminal region. The dominant VCA-p18 reactivity of human sera can be completely inhibited by preadsorption with Escherichia coli-expressed BFRF3-beta-galactosidase. Serum of a rabbit immunized with BFRF3-beta galactosidase reacts with a VCA-specific protein of 18 kDa. In addition, BFRF3-beta-galactosidase affinity-purified antibodies react with VCA-p18 of virus-producing cells (HH514.c16). Complete inhibition of viral DNA polymerase activity by phosphonoacetic acid is associated with the absence of RNAs and protein products of both ORFs, indicating that VCA-p18 and VCA-p40 are true late antigens.

Purification and characterization of the DNA-binding activity of the Epstein-Barr virus DNA polymerase accessory protein BMRF1 gene products, as expressed in insect cells by using the baculovirus system

A recombinant baculovirus containing the complete sequence for the Epstein-Barr virus (EBV) BMRF1 gene product, the EBV DNA polymerase accessory protein, under the control of the polyhedrin promoter was constructed. Insect cells infected with the recombinant virus produced two phosphoproteins of 52 and 50 kDa and one unphosphorylated protein of 48 kDa, recognized by anti-BMRF1 protein-specific monoclonal antibody. The major protein bands were 50 and 48 kDa. The expressed BMRF1 gene products were purified to near homogeneity from the nuclear extract of the recombinant baculovirus-infected insect cells by double-stranded DNA-cellulose column chromatography followed by heparin-agarose column chromatography. The purified BMRF1 gene products exhibited higher binding affinity for double-stranded DNA than for single-stranded DNA without ATP hydrolysis. The protein-DNA interaction did not necessarily require a primer terminus. The present system will open the way for the biochemical characterization of the EBV DNA polymerase accessory protein.

Identification of a protein encoded in the EB-viral open reading frame BMRF2

Using monospecific rabbit sera against a peptide derived from a potential antigenic region of the Epstein-Barr viral amino acid sequence encoded in the open reading frame BMRF2 we could identify a protein-complex of 53/55 kDa in chemically induced B95-8, P3HR1 and Raji cell lines. This protein could be shown to be membrane-associated, as predicted by previous computer analysis of the secondary structure and hydrophilicity pattern, and may be a member of EBV-induced membrane proteins in lytically infected cells.

Formation of intranuclear replication compartments of Epstein-Barr virus with redistribution of BZLF1 and BMRF1 gene products

The localizations of the Epstein-Barr virus immediate-early transcriptional activator BZLF1 protein ZEBRA, of the BMRF1 early antigen diffuse component (EA-D), and of viral DNA replication were studied in the Burkitt's lymphoma cell line Akata treated with anti-human immunoglobulin antibodies. Prompt and sequential appearance of ZEBRA, EA-D, and viral DNA was observed in about 70% of the cells. At early times after activation, ZEBRA had a diffuse intranuclear distribution, but later it was concentrated in globular regions within the nucleus. EA-D appeared first in a finely stippled pattern and then in a diffuse pattern. At late times, EA-D concentrated in globular regions similar to those with ZEBRA. Double staining for ZEBRA and EA-D revealed that ZEBRA followed the morphological changes of EA-D with a 1-2 hr delay and that both finally coalesced in the same structures, where in situ hybridization localized replicating viral DNA. The redistribution of both ZEBRA and EA-D to these compartments depended upon the replication of lytic viral DNA. These findings indicate that these globular regions are sites for viral replication and that transcription of EBV late genes may be regulated in these structures.

Antibody responses against polypeptide components of Epstein-Barr virus-induced early diffuse antigen in patients with connective tissue diseases

The specific humoral response against polypeptide components of Epstein-Barr virus (EBV), the induced early diffuse antigen (EA-D), in patients with connective tissue diseases, including systemic lupus erythematosus (SLE) and mixed connective tissue disease (MCTD), was investigated by using the immunoblotting technique. The EA(D)-positive sera from patients with infectious mononucleosis (IM), nasopharyngeal carcinoma (NPC), immunocompromised patients (renal transplant recipients and patients with AIDS) as well as the EA(D)-negative sera from patients with Burkitt's lymphoma and from clinically healthy subjects served as controls. Seven major antigenic polypeptides with molecular weights of 33 kDa, 35 kDa, 52 kDa, 54 kDa, 56 kDa, 58 kDa, and 134 kDa were detected reproducibly by the EA(D)-positive reference sera and, in particular, by each of the NPC sera tested. The EA(D)-positive sera from the other groups showed various combinations of detection patterns and few samples reacted with all the major EA(D) polypeptides. Seventy-three percent of sera from SLE and 47% of sera from MCTD were found to react with EA(D). Sixty-one percent of sera from SLE vs. 5% from MCTD detected all the EA(D) polypeptides. These results could either reflect perturbations of the immune response linked to the autoimmune disease or suggest a possible pathogenic role of EBV.

Epstein-Barr virus (EBV) infection of murine L cells expressing recombinant human EBV/C3d receptor

The normal host range of Epstein-Barr virus (EBV) is limited to primate B lymphocytes and certain epithelial cells that express the C3d/EBV receptor [complement receptor 2 (CR2, CD21)]. In the present study, expansion of the tissue tropism of EBV has been accomplished by stably transfecting the murine fibroblast L cell line with pMT.CR2. neo.1, a eukaryotic expression vector promoting the transcription of a complementary DNA insert encoding human CR2. High CR2-expressing transfected L cells were selected by fluorescence-activated cell sorting. The recombinant CR2 was shown to have the same molecular weight as wild-type CR2 from Raji cells and to mediate the binding by the transfectants of particles bearing the iC3b and C3d fragments of the third component of complement. All CR2-expressing L cells, but not nontransfected controls, also bound EBV, as assessed by indirect immunofluorescence. After a 60-hr culture, approximately 0.5% of the CR2-expressing cells preincubated with EBV demonstrated immunofluorescent staining of EBV nuclear antigen with serum from a patient with nasopharyngeal carcinoma. No fluorescent staining of cells was seen with monoclonal antibodies to the early antigen complex or to gp350/220, indicating that the infection was predominantly latent. Infected cells cultured for up to 4 weeks remained EBV nuclear antigen-positive. The capacity of recombinant human CR2 to confer on murine L cells susceptibility to stable latent infection by EBV indicates that this receptor is a primary determinant of the tissue tropism of EBV and may facilitate studies of cell-specific factors that regulate the viral growth cycle.

Both the rightward and the leftward open reading frames within the BamHI M DNA fragment of Epstein-Barr virus act as trans-activators of gene expression

The BamHI M DNA fragment of Epstein-Barr virus was shown to activate transcription of the cotransfected chloramphenicol acetyltransferase gene under the control of the simian virus 40 early promoter. Both the BamHI-BglII and the HindIII-BamHI subfragments of the BamHI M fragment, corresponding to the rightward reading frame BMRF1 and the leftward reading frame BMLF1, respectively, had the ability to activate transcription from the simian virus 40 promoter. The trans-activating function was well correlated with the expression of nuclear early antigens, which suggests that early antigens encoded by BMRF1 and BMLF1 are responsible for trans-activation and possibly play a role in regulated expression of virus genomes.

Expression of an early Epstein-Barr virus antigen (EA-D) in E. coli. Brief report

The 1.34 kb BcII-BgIII-fragment of the BamHI-M region of Epstein-Barr virus genome, comprising the complete BMRF1 open reading frame, was cloned into the tryptophan regulated E. coli expression vector pATH1. The resulting fusion protein, having a molecular weight of 80 kd, is recognized not only by anti-early antigen (EA)-positive human sera but also by the monoclonal antibody R3 directed against the diffuse component of EA (EA-D). A possible use for this fusion protein as an indicator protein in diagnosis of IgA antibodies against EA-D is presented.

Characterization of a cDNA clone corresponding to a transcript from the Epstein-Barr virus BamHI M fragment: evidence for overlapping mRNAs

A 1.95-kilobase cDNA clone was isolated by screening a size-selected lambda gt10 cDNA library prepared from an Epstein-Barr virus-transformed B-cell line, IB4, with the Epstein-Barr virus BamHI M fragment. Sequence analysis revealed that this clone contains about 75% of the BMRF1 and the complete BMRF2 open reading frames. The transcript is not spliced, and the polyadenylation signal at base pair 2641 of the BamHI M fragment is used. Northern blots (RNA blots) indicate that this polyadenylation signal is used for three overlapping mRNAs. The sizes of these transcripts are 3.5, 2.6, and 1.5 kilobases.

Identification of an Epstein-Barr virus early gene encoding a second component of the restricted early antigen complex

When the latent Epstein-Barr virus (EBV) genome in B95-8 cells is induced into a replicative phase, two abundant early RNAs are transcribed rightward from the EBV BamHI H DNA fragment into BamHI F. Analysis of cDNA clones prepared from the RNA of cells replicating EBV revealed that both RNAs contain the BHRF1 open reading frame. Part of BHRF1, cloned into a prokaryotic fusion protein expression vector, expressed a fusion protein in Escherichia coli and the purified fusion protein was used to generate a monoclonal antibody against BHRF1. This antibody was then employed to characterize the protein encoded by BHRF1 in cells replicating EBV. The monoclonal antibody reacted with a 17-kDa protein component of the restricted early antigen (EA) complex. The distribution of the protein in cells was similar to that noted when sera from patients with African Burkitt's lymphoma were used to stain these cells. The protein was synthesized before the major 47-56 kDa protein associated with the diffuse component of EA in superinfected Raji cells. All human sera containing antibodies to EA as determined by immunofluorescence (IF) reacted with the protein as did some sera determined to be anti-VCA positive and anti-EA negative by IF. The predicted amino acid sequence of the protein has characteristics which suggest that it is a membrane protein. It also has significant homology with both the anchor region of polyoma middle T antigen and with the predicted protein product of the bcl-2 mRNA activated by the 14/18 chromosome translocation characteristic of follicular lymphomas. This latter homology is extensive and colinear, suggesting common evolution and function. However, neither a mRNA which could efficiently translate the BHRF1 protein nor the BHRF1 protein could be detected in latently infected cells. Thus, the bcl-2 predicted protein is similar to an EBV protein synthesized in the early phase of virus infection.

Characterization of the Epstein-Barr virus-induced early polypeptide complex p50/58 EA-D using rabbit antisera, a monoclonal antibody, and human antibodies

A polypeptide complex (p52) belonging to the D-subspecificity of the EBV-induced early antigens (EA-D) was purified from chemically induced P3HR-1 cells. Rabbit antisera raised against the isolated polypeptides reacted with components of EA-D as could be shown by indirect immunofluorescence and immunoperoxidase staining of IdU-induced EA positive Raji cells, ELISA, and immunoblotting. In one-dimensional immunoblots the rabbit antisera detected a predominant polypeptide complex of 52 kDa. Two-dimensional immunoblots prepared with proteins from IdU-induced Raji cells showed that the rabbit sera detect three series of polypeptides of 52 kDa (pl 8.5-6.2), 55-58 kDa (pl 6.2-4.5), and 48-50 kDa (pl 6.0-4.5). These three groups of polypeptides could also be identified by 50 high titered anti-EA-D positive human sera and a specific monoclonal antibody (R3) as being the main components of EA-D in Raji and B95-8 cells. All polypeptides of the p50/58 complex showed DNA binding properties either by themselves or by an interaction with other proteins. When TPA or IdU-induced Raji cells were labeled with 2Pi, two phosphorylated polypeptides pp50 and pp58 could be immunoprecipitated with the rabbit sera and a high anti-EA titered human serum. The time course of the synthesis of polypeptides associated with the EA-D complex was studied by 2-D immunoblots: EA polypeptides of 52 kDa appeared as early as 6 hr after the addition of IdU to Raji cells in culture, polypeptides of 55-58 and 48-50 kDa after 18 and 25 hr, respectively. The coordinated appearance of these groups of polypeptides and their similar size and reactivity with human sera and rabbit antisera produced against the isolated p52 as well as with a monoclonal antibody (R3) suggested that most of these polypeptides are derived from post-translational modifications of one or a few initially synthesized polypeptides, possibly p52. Phosphorylation seems to be at least one possibility of post-translational modification.

Deletion mutants in the gene encoding the herpes simplex virus type 1 immediate-early protein ICP0 exhibit impaired growth in cell culture

We report the construction and characterization of deletion mutants in the herpes simplex virus type 1 gene encoding the immediate-early protein ICP0. In the event that ICP0 proved to play an essential role in virus replication, ICP0-transformed Vero cells were generated to serve as permissive hosts for such mutants. Two mutants, dlX0.7 and dlX3.1, were isolated in these cells by a marker rescue-transfer procedure involving the rescue of an ICP4 deletion mutant and the simultaneous insertion of a linked deletion in the ICP0 gene. Mutant dlX0.7 contained a 700-base-pair deletion in both copies of ICP0. The deletion lay entirely within the transcript specified by the gene. dlX0.7 induced the synthesis of an ICP0-specific mRNA that was approximately 0.7 kilobases smaller than the corresponding mRNA specified by wild-type virus. The 3.1-kilobase deletion in both copies of the ICP0 gene in mutant dlX3.1 removed the majority of the transcriptional-regulatory signals and coding sequences, retaining only sequences at the 3' end of the gene. As expected, no ICP0-specific mRNA was detected in dlX3.1-infected Nero cells (G418-resistant Vero cells). Both mutants grew in all cells tested, although their burst sizes were 10- to 100-fold lower than that of wild-type virus. Although the plaque sizes of dlX0.7 and dlX3.1 were equally small on Nero and ICP0-transformed cells, the plating efficiency of the mutants was 15- to 50-fold greater on ICP0-transformed cells than on Nero cells. The mutants exhibited modest interference with the growth of wild-type virus in mixed infections, an effect that was abolished by UV irradiation of the mutants, implying that interference required viral gene expression. Polypeptide profiles generated by the mutants in Nero cells were qualitatively similar to that of wild-type virus. Quantitatively, only slight reductions in the levels of certain late viral polypeptides were observed, a phenomenon also borne out by analysis of viral glycoproteins. Both mutants induced the synthesis of significant, although reduced, levels of viral DNA relative to wild-type virus. Taken together, the results demonstrate that ICP0 is not essential for productive infection in cell culture but that this protein plays a significant role in viral growth, as indicated by the impaired abilities of the mutants to replicate.

Identification and mapping of Epstein-Barr virus early antigens and demonstration of a viral gene activator that functions in trans

The BamHI M DNA fragment of the Epstein-Barr virus (EBV) genome was inserted in two orientations into a simian virus 40-based expression vector, and the EBV-specific proteins produced in COS-7 monkey cells were examined. In one orientation, termed BamHI-M rightward reading frame 1 (BMRF1), a set of phosphoproteins ranging in size from 47,000 to 54,000 daltons was synthesized. These proteins reacted with monoclonal and polyclonal antisera, defining them as components of the EBV early antigen diffuse set of proteins (EA-D). The BamHI M DNA fragment in the opposite orientation, termed BamHI-M leftward reading frame 1 (BMLF1), directed the synthesis of a nuclear antigen detected by antibodies in serum from a patient with nasopharyngeal carcinoma. The BMLF1 antigen was not detected by monoclonal or polyclonal antibodies directed against the EA-D complex. A series of deletion mutants were constructed in the BamHI M DNA fragment, and the EA-D complex and BMLF1 antigen were mapped to discrete open reading frames in this DNA fragment. A test for several possible functions of these antigens showed that the BMLF1 antigen had the ability to activate or enhance, in trans, the level of expression of a gene under the control of the adenovirus early region 3 promoter or the simian virus 40 early promoter in the absence of its cis-acting enhancer. These experiments demonstrate a new gene function, encoded by EBV, that may be important in the positive regulation of viral or cellular genes.

Localization of the coding region for an Epstein-Barr virus early antigen and inducible expression of this 60-kilodalton nuclear protein in transfected fibroblast cell lines

Expression of a component of the Epstein-Barr virus early antigen (EA) complex has been studied in fibroblast cells transfected with both wild-type and P3HR-1 defective DNA fragments covering the BamHI-M-S region of the Epstein-Barr virus genome. Baby hamster kidney (BHK) cells transfected with the BglII-J fragment and stained with human serum that was positive for the diffuse component of EA [EA(D)] in an indirect immunofluorescence assay exhibited positive nuclear staining in 5% of the cell population. Cleavage of BglII-J before transfection with the restriction enzyme BglII, StuI, HindIII, or PvuII did not affect EA expression, whereas prior cleavage with BamHI or EcoRI reduced or eliminated synthesis of EA. These observations were confirmed by using individual cloned subfragments. A Bal 31 deletion clone (pTS1) in which the HindIII and StuI sites were eliminated retained activity, whereas a clone (pTS5) in which the deletion extended closer to the EcoRI site had greatly reduced activity. Transfection of the individual BamHI-M or BamHI-S fragments, which span BglII-J, also resulted in little or no EA expression. The 2.1-kilobase biologically active region defined by these experiments corresponds precisely to the BMLF1 open reading frame. Immunoblot analyses of BHK cells transfected with either P3HR-1 defective DNA clones or the BglII-J wild-type fragment identified the product of this EA(D) coding region as a family of polypeptides consisting of a major 60-kilodalton product and minor 45- and 50-kilodalton species. In latently Epstein-Barr virus-infected lymphocytes these early antigens are not expressed, but can be induced by treatment of the cultures with sodium butyrate or phorbol esters. Using the BglII-J and pTS6 clones that were positive in transient assays, we also established Neor coselected BHK and Vero cell lines which showed similar regulated expression of the 60-kilodalton EA(D) protein. In these cell lines constitutive expression of EA(D) was limited (0.1% positive by indirect immunofluorescence and undetectable by immunoblot analysis). However, expression of EA(D) could be induced by treatment with sodium butyrate. In the induced cultures, up to 30% of the cells were EA(D) positive by immunofluorescence, and there was a concomitant appearance of the 60-kilodalton EA(D) polypeptide.

Sequence-specific DNA binding of the Epstein-Barr virus nuclear antigen (EBNA-1) to clustered sites in the plasmid maintenance region

Latently infected B lymphocytes continuously express an Epstein-Barr Virus nuclear antigen (EBNA-1) required in trans for maintenance of the plasmid state of the EBV genome. Filter binding assays and DNAase I footprinting analyses revealed that the carboxy-terminal domain of EBNA-1 protects binding sites at three different loci in the 172,000 bp EBV genome. Two of these loci correspond to essential elements within an 1800 bp segment defined as the minimal region required for plasmid maintenance (ori-P). Binding to each of 20 X 30 bp tandem repeats in the "sink" locus protects 25 bp centered over a 12 bp palindromic consensus sequence TAGCATATGCTA. The nearby dyad symmetry "origin" locus contains two 46 bp protected regions each encompassing two paired core binding sites. The demonstration of sequence-specific binding at multiple loci suggests that EBNA-1 has pleiotropic functions, which may include control of copy number and segregation of the EBV plasmids as well as initiation of replication.