Epstein-Barr virus-specific cytotoxic T-cell recognition of transfectants expressing the virus-coded latent membrane protein LMP

Host cell and EBNA-2 regulation of Epstein-Barr virus latent-cycle promoter activity in B lymphocytes

The six latent-cycle nuclear antigens (EBNAs) of Epstein-Barr virus (EBV), whose genes share 5' leader exons and two promoters (Cp and Wp), are differentially expressed by cells of the B lineage. To examine the possibility that EBNA gene expression is regulated through selective use of Cp and Wp, we monitored the activity of promoter-chloramphenicol acetyltransferase (CAT) gene constructs transfected into EBV-positive and EBV-negative B lymphocytes and Burkitt's lymphoma cells. Wp was a much stronger promoter than Cp in EBV genome-negative B-cell lines and was used exclusively in primary B cells. When B cells were infected with transforming EBV, Cp became the stronger promoter. This switch was not observed when B cells were infected with an immortalization-deficient virus, P3HR-1, which lacks the EBNA-2 open reading frame and expresses a mutant leader protein (EBNA-LP). Cp function was transactivated when EBV-negative or P3HR-1-infected B cells were cotransfected with Cp and a 12-kb fragment of DNA (BamHI-WWYH) that spanned the P3HR-1 deletion. This activity was mapped to the EBNA-2 gene within WWYH; constructs expressing EBNA-LP did not induce Cp function, and the deletion of 405 bp from the EBNA-2 open reading frame abolished transactivation. This research demonstrates host cell and EBNA-2 regulation of latent-cycle promoter activity in B lymphocytes, a mechanism with implications for persistence of EBV-infected lymphoid cells in vivo.

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.

Stimulation with allogeneic Epstein-Barr virus-transformed lymphoblastoid cell lines generates HLA class I-specific CTLs with different target cell avidity

Epstein-Barr virus (EBV) transformed lymphoblastoid cell lines (LCL) are potent inducers of cytotoxic T-lymphocytes (CTL) in allogeneic mixed lymphocyte cultures (MLC). The contribution of EBV antigens to the induction of cytotoxic responses was investigated by comparing CTL clones derived from allogeneic MLCs of lymphocytes from one EBV seropositive and one seronegative donor for their capacity to lyse paired EBV positive and negative targets. The majority of the clones showed a conventional "HLA-specific" cytotoxicity and lysed equally well HLA-matched LCLs and mitogen-induced T- or B-blasts. A minority of the clones from both donors exhibited an "LCL-selective" killing potential as they lysed poorly T- and B-blasts. The LCL-selective clones did not recognize EBV antigens because they could not discriminate between EBV negative Burkitt lymphoma (BL) lines and their in vitro EBV-converted sublines. MAbs to CD3, CD8, and MHC class I antigens blocked the lysis of LCLs by HLA-specific and LCL-selective CTLs with comparable efficiency suggesting that the two effector types express T-cell receptors of similar affinity. T-blasts were unable to inhibit the lysis of LCLs in cross competition assays. This correlated with a significantly lower expression of the cell adhesion molecules ICAM-1 and LFA-3. The results suggest that stimulation with allogeneic LCLs activates HLA class I-specific CTLs with variable target cell avidity. Only CTLs that act independently of the enhancing effect of cell adhesion molecules are able to lyse mitogen-induced T- and B-blasts.

Immunovirology of transforming viruses

The paradigmatic shift in cellular immunology, initiated by the realization that cytotoxic T lymphocytes recognize major histocompatibility complex class I antigens that carry endogenous (including virally encoded) protein-derived peptides in a special groove, has made a strong impact on the immunopathology of virus-associated tumors.

Exclusive expression of Epstein-Barr virus nuclear antigen 1 in Burkitt lymphoma arises from a third promoter, distinct from the promoters used in latently infected lymphocytes

Epstein-Barr virus transformation of human B lymphocytes in vitro results in the expression of six viral nuclear antigens (EBNAs) and three viral membrane proteins. However, examination of viral gene expression in fresh Burkitt lymphoma isolates has revealed expression of only one of the nuclear antigens, EBNA-1. Previous transcriptional analyses of the EBNA-encoding genes demonstrated that all these genes are driven from one of two distal promoters located near the left end of the viral genome, raising the question of how exclusive expression of EBNA-1 occurs in Burkitt lymphoma tumors. Although most established Burkitt lymphoma cell lines (group 3) exhibit the full-expression pattern of viral antigens seen in lymphoblastoid cell lines, a few cell lines have been established that retain the restricted pattern of viral gene expression (group 1). In this paper we characterize transcription of the EBNA-1 gene in a group 1 Burkitt lymphoma cell line and show that (i) neither Cp nor Wp, the promoters involved in driving EBNA gene expression in lymphoblastoid cell lines, are active in this cell line; (ii) treatment of this cell line with 5-azacytidine, previously shown to induce expression of all EBNA genes, induced Cp and Wp activity; (iii) sizes of the EBNA-1 transcripts detected in two group 1 Burkitt lymphoma cell lines correlated with each other and were distinct from the size of the EBNA-1 transcript seen in lymphoblastoid cell lines; (iv) the EBNA-1 transcripts in the group 1 Burkitt lymphoma cell lines do not hybridize to a probe containing the common 5' exons present in all the EBNA transcripts from lymphoblastoid cell lines; and (v) anchored-PCR cloning the 5' region of the EBNA-1 transcript from one of the group 1 cell lines identified two exons, FQ and U, upstream of the EBNA-1 coding exon. The FQ exon lies just downstream of a TATAA box, which may represent the promoter for transcription of EBNA-1 in these cells. It is particularly noteworthy that an incomplete EBNA-1 cDNA clone from a nasopharyngeal carcinoma tumor line that expresses EBNA-1, but not the other EBNAs, has been characterized; this EBNA-1 transcript also contains the FQ/U splice junction, suggesting that the organization of exons upstream of the EBNA-1 coding exon is the same and that this organization may reflect a viral program for exclusive EBNA-1 expression.

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 epstein-Barr-virus-encoded membrane protein LMP but not the nuclear antigen EBNA-1 induces rejection of transfected murine mammary carcinoma cells

The EBV-encoded membrane protein (LMP) and the nuclear antigen EBNA-1 were compared for their capacity to induce rejection of transfected murine mammary carcinoma cells in syngeneic hosts. The tumorigenic potential of stable LMP and EBNA-1 expressing sublines of the ACA (H-2f)-derived mammary carcinoma line S6C was tested in pre-immunized syngeneic and semi-syngeneic animals. LMP expressing S6C cells elicited a strong rejection response as demonstrated by the lower tumor take and slower growth in immunized vs. control mice. In contrast, EBNA-1-expressing cells were non-immunogenic in syngeneic hosts and in one semi-syngeneic F1-hybrid. Rejection in 2 additional F1-hybrids did not appear to be due to EBNA-1-specific immune responses. Our findings support the hypothesis that the escape of EBV carrying tumor cells from EBV-specific immune surveillance may be facilitated by the fact that viral gene expression is limited to EBNA-1.

The latent membrane protein-1 in Epstein-Barr virus-transformed lymphoblastoid cells is found with ubiquitin-protein conjugates and heat-shock protein 70 in lysosomes oriented around the microtubule organizing centre

Immunofluorescence studies on Epstein-Barr virus (EBV)-transformed lymphoblastoid cells have previously shown that the latent membrane transforming protein (LMP-1) is found in patch-like inclusions which also immunostain for vimentin. We now show that EBV transformation causes a major reorganization of intermediate filaments, microtubules, mitochondria, and lysosomal elements, which generally become oriented around the microtubule organizing centre. Immunogold electron microscopy shows that LMP-1 is primarily concentrated in secondary lysosomes together with ubiquitin-protein conjugates and heat-shock protein 70. Intermediate filament inclusion formation with the above characteristics may be a general response triggered by other membrane glycoproteins; as seen, for example, in major human neurodegenerative diseases such as diffuse Lewy body disease.

Importance of cell-mediated immunity in Marek's disease and other viral tumor diseases

Cell-mediated immune (CMI) responses to viral tumor diseases are often used as examples of the importance of antiviral and antitumor immunity in chickens. Especially, reticuloendotheliosis virus (REV) and Marek's disease herpesvirus (MDV) are used as models to study the development of cytotoxic T-lymphocytes against viral and tumor antigens and activation of natural killer (NK) cells. Major histocompatibility complex Class I-restricted, antiviral cytotoxic T-lymphocytes expressing CD4-/CD8+ markers are induced after infection with REV. Thus far, this is the only example of Class I-restricted cytotoxic T-lymphocytes in chickens. Antiviral cytotoxic T-lymphocytes may be induced by infection with MDV or by vaccination, but conclusive evidence has not yet been provided. Antitumor responses have not been demonstrated against REV-induced tumors. Although Marek's disease is often used as an example for the importance of antitumor immunity, there is a lack of convincing data demonstrating antitumor immunity mediated by cytotoxic T-lymphocytes. Activation of NK cells by MDV infection or vaccination is probably an important part of CMI responses against Marek's disease viral antigens but not against tumor antigens.

Expression of Epstein-Barr virus latent gene products in tumour cells of Hodgkin's disease

The Epstein-Barr virus (EBV)-encoded latent gene products, latent membrane protein (LMP) and EBV nuclear antigen 2 (EBNA 2), seem to have important roles in EBV-induced cell transformation in vitro, and have been implicated as important effector molecules in EBV-associated lymphomagenesis. Because up to 35% of Hodgkin's disease (HD) samples have been reported to contain EBV genomes, the expression of LMP and EBNA 2 in these tumours was investigated. 84 cases of HD were studied with monoclonal antibodies and immunohistochemical labelling of acetone-fixed cryostat sections. LMP, but not EBNA 2, was demonstrated in Reed-Sternberg (RS) cells of 40 cases (48%); the two proteins were easily detected in transformed lymphocytes of positive control acute infectious mononucleosis tonsils. LMP expression in RS cells varied according to the histological subtype of HD (1/10 cases [10%] of lymphocyte predominance subtype, 16/50 cases [32%] of nodular sclerosis, 23/24 [96%] cases of mixed cellularity type). That the LMP antibodies showed no substantial cross-reactivity with negative control tissues shows that they are useful probes for the diagnosis of latent EBV infection in tissue sections. The findings suggest that EBV is associated with more cases of HD than was previously recognised, that in positive cases RS cells express a latent infection protein phenotype (LMP+, EBNA 2-) which differs from that of other EBV-associated lymphomas, and that LMP expression is related to histologically aggressive subtypes of HD.

Expression of Epstein-Barr virus-encoded growth-transformation-associated proteins in lymphoproliferations of bone-marrow transplant recipients

The expression of Epstein-Barr virus (EBV)-encoded, growth-transformation-associated proteins was studied in lymphoproliferations of 9 allogeneic bone-marrow transplant (BMT) recipients. Immunoblots of cell lysates were probed with polyspecific and monospecific antisera directed against EBNA 1, 2, 3 and 6, and the membrane protein LMP. All tumors expressed EBNA 1 and LMP. EBNA 2 was detected in the tumors of 8 patients, and EBNA 3 and 6 in the tumors of 5 patients. The LMP regulatory sequences, 5' of the LMP gene, were totally unmethylated in all 7 cases, while the coding sequences of LMP and EBNA 2 were more methylated in CpG dinucleotides. EBV-transformed lymphoblastoid cell lines (LCL) express EBNA 1 to 6 and LMP; in contrast, Burkitt lymphomas express only EBNA 1. In vitro experiments have shown that EBNA 2, 3 and LMP can generate targets for cytotoxic T cells (CTL). These combined observations are consistent with the hypothesis that the EBV-associated lymphoproliferative disease of the BMT recipients escapes CTL-mediated rejection due to the failure of host immunosurveillance rather than to the down-regulation of immunogenic EBV-encoded antigens.

Up regulation of the Epstein-Barr virus (EBV)-encoded membrane protein LMP in the Burkitt's lymphoma line Daudi after exposure to n-butyrate and after EBV superinfection

The Burkitt's lymphoma line Daudi carries a nontransforming Epstein-Barr virus (EBV) strain that has a deletion in the BamHI WYH region of the genome coding for the EBV nuclear antigen 2 (EBNA-2). Daudi cells fail to express the EBV-encoded latent membrane protein (LMP) (D. Ghosh and E. Kieff, J. Virol. 64:1855-1858, 1990). We show that LMP expression can be up regulated by exposure to n-butyrate and by superinfection with the B95-8 (B virus)- and P3HR1 (P virus)-derived EBV strains. Two LMP polypeptides of 60 and 48 kilodaltons (kDa) were detected in immunoblots of Daudi cells that had been exposed to 3 mM n-butyrate for 24 h. The intensity of the 48-kDa LMP increased during 72 h, in parallel with the appearance of early antigen-positive cells. The 60-kDa LMP was expressed at a low level and remained constant. Superinfection of Daudi cells with B and P virus induced the 60-kDa LMP within 3 h. In addition, P virus induced the 48-kDa LMP at a low level. The B virus-encoded EBNA-2 and EBNA-5 were detected 12 h after superinfection. The B virus-encoded 63-kDa LMP was coexpressed with the endogenous LMP after 48 h. Inactivation of the virus by UV illumination abolished the expression of the B virus-encoded antigens but did not affect the induction of the endogenous LMP. The B-cell activation marker CD23 was up regulated by B virus superinfection but not by n-butyrate exposure. CD23 was also expressed at a higher level in a stable B virus-converted subline, E95A-Daudi, that was EBNA-2 positive and coexpressed the Daudi virus- and B virus-encoded LMP. The results suggest that LMP expression is regulated by the interaction of cellular and viral factors. Binding of the virus to its membrane receptor might be involved in the triggering of cellular control mechanisms. Viral gene products are not directly involved in this function but may contribute to create a permissive cellular environment for LMP expression.

Expression of the Epstein-Barr virus (EBV)-encoded membrane antigen (LMP) increases the stimulatory capacity of EBV-negative B lymphoma lines in allogeneic mixed lymphocyte cultures

Epstein-Barr virus (EBV)-negative Burkitt lymphoma (BL) lines are poor stimulators in allogeneic mixed lymphocyte cultures compared to EBV-transformed lymphoblastoid cell lines derived from the same individuals. We have previously shown that the stimulatory capacity of the tumor cells is increased after EBV conversion (Avila-Carino et al., Int. J. Cancer 1987. 40: 691). As a first step towards the identification of the viral gene product responsible for this change we have studied the influence of the EBV latent membrane protein (LMP) on the stimulatory capacity of the EBV-negative BL lines BL41 and DG75 and the B lymphoma line BJAB. Four LMP-transfected sublines of BL41, four DG75 LMP transfectants and one LMP-transfected subline of BJAB showed a significantly stronger stimulatory capacity than the original line. The effect was directly proportional to the amount of LMP detected in each transfectant but was not due to reactivation of LMP-specific memory cells since lymphocytes from EBV-seropositive and -seronegative individuals responded equally. In order to define the relation between LMP expression and induction of stimulatory capacity, DG75 was transfected with constructs containing the LMP gene under the control of an heat-shock promoter. The peak of LMP expression in heat shock-treated cells preceded the appearance of stimulatory capacity by 6-12 h suggesting that critical amounts of the protein may be required to induce the phenotypic change recognized by the T cells. LMP influenced in a dose-dependent manner the expression of the adhesion molecules LFA-1, LFA-3 and ICAM-1 and B cell activation markers CD23 and CD39 in transfected sublines of BL41, but did not affect the expression of these markers in the DG75 and BJAB cell line. All LMP-expressing transfectants showed an increased capacity to form conjugates with unprimed allogeneic lymphocytes.

CpG methylation of viral DNA in EBV-associated tumours

In EBV-immortalized lymphoblastoid cell lines (LCLs) a small number of "latent" proteins are expressed. These are the EBV nuclear antigens, EBNAs 1-6, and a latent membrane protein, LMP. We have investigated the expression of these proteins in a variety of EBV-associated tumours and cell lines. Whereas transplant and B-cell lymphomas from cotton-top tamarins appear to express the full range of antigens found in LCLs, we and others have found that in Burkitt's lymphomas (BL) and a nasopharyngeal carcinoma (NPC) isolate, EBNA expression is restricted to EBNA-I. (In NPC, but not in BL, LMP may also be expressed). In order to ask what restricts the expression of EBNA 2-6 in NPC and BL cells it seemed reasonable to consider the possibility that the DNA sequences normally regulating expression of these antigens could be chemically modified. In this analysis, a tight inverse correlation between methylation of CpG dinucleotides in the 5' flanking region of the EBNA-2 gene and the expression of EBNAs 2-6 has been revealed. In the NPC tumour, CpG methylation within the gene is also observed, as is specific methylation over the EBNA-I region I and II binding sites (in oriP). The significance of these observations is considered.

Identification of a glucocorticoid-responsive element in Epstein-Barr virus

Immortalization of B lymphocytes by Epstein-Barr virus (EBV) is complex and poorly understood. However, some evidence suggests that glucocorticoids influence this process. We identified a glucocorticoid-responsive element in the BamHI C fragment of EBV which we call ES-1. In glucocorticoid-treated cells, ES-1 enhanced chloramphenicol acetyltransferase gene expression from the herpes simplex virus thymidine kinase promoter, as well as the EBV Bam-C promoter, from which several latent viral gene products are transcribed. By Northern blot analysis, glucocorticoid treatment enhanced transcription from the Bam-C promoter in Jijoye cells, a Burkitt's lymphoma cell line. In addition, the DNA-binding domain of the glucocorticoid receptor bound specifically to the ES-1 region. These glucocorticoid effects on the Bam-C promoter region may provide some insight into the process of EBV immortalization.

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.

Stable transfection of Epstein-Barr virus (EBV) nuclear antigen 2 in lymphoma cells containing the EBV P3HR1 genome induces expression of B-cell activation molecules CD21 and CD23

A set of B-cell activation molecules, including the Epstein-Barr virus (EBV) receptor CR2 (CD21) and the B-cell activation antigen CD23 (Blast2/Fc epsilon RII), is turned on by infecting EBV-negative B-lymphoma cell lines with immortalizing strains of the viruslike B95-8 (BL/B95 cells). This up regulation may represent one of the mechanisms involved in EBV-mediated B-cell immortalization. The P3HR1 nonimmortalizing strain of the virus, which is deleted for the entire Epstein-Barr nuclear antigen 2 (EBNA2) protein open reading frame, is incapable of inducing the expression of CR2 and CD23, suggesting a crucial role for EBNA2 in the activation of these molecules. In addition, lymphoma cells containing the P3HR1 genome (BL/P3HR1 cells) do not express the viral latent membrane protein (LMP), which is regularly expressed in cells infected with immortalizing viral strains. Using electroporation, we have transfected the EBNA2 gene cloned in an episomal vector into BL/P3HR1 cells and have obtained cell clones that stably express the EBNA2 protein. In these clones, EBNA2 expression was associated with an increased amount of CR2 and CD23 steady-state RNAs. Of the three species of CD23 mRNAs described, the Fc epsilon RIIa species was preferentially expressed in these EBNA2-expressing clones. An increased cell surface expression of CR2 but not of CD23 was observed, and the soluble form of CD23 molecule (SCD23) was released. We were, however, not able to detect any expression of LMP in these cell clones. These data demonstrate that EBNA2 gene is able to complement P3HR1 virus latent functions to induce the activation of CR2 and CD23 expression, and they emphasize the role of EBNA2 protein in the modulation of cellular gene implicated in B-cell proliferation and hence in EBV-mediated B-cell immortalization. Nevertheless, EBNA2 expression in BL/P3HR1 cells is not able to restore the level of CR2 and CD23 expression observed in BL/B95 cells, suggesting that other cellular or viral proteins may also have an important role in the activation of these molecules: the viral LMP seems to be a good candidate.

Cross-recognition of a mouse H-2-peptide complex by human HLA-restricted cytotoxic T cells

Epstein-Barr virus (EBV)-specific human cytotoxic T lymphocyte (CTL) lines from a virus-immune HLA-A11+ donor CMc produced significant lysis of transfected mouse P815 target cells (H-2d background) expressing an introduced HLA-A11 heavy chain gene and the EBV gene encoding latent membrane protein (LMP). To identify the target epitope being recognized, HLA-A11+ transfectants of P815/A11) were pre-exposed to synthetic peptides corresponding to fragments of the LMP sequence and the cells then tested for lysis by CMc effectors; untransfected P815 parent cells were included in these assays as control recipients of the same peptides. Unexpectedly, we identified a peptide preparation (corresponding to LMP residues 124-137) which specifically sensitized not only P815/A11 cells but also the parental P815 cells to lysis. Four independent anti-EBV CTL lines derived from donor CMc gave the same result and single-cell cloning confirmed that lysis was mediated by a subset of the HLA-A11-restricted EBV-specific CTL clones which dominate CMc effector populations. Two lines of evidence indicated that peptide presentation to the human CTL was occurring through H-2Kd molecules on the surface of P815/A11 and P815 cells: (a) a monoclonal antibody reactive with H-2Kd specifically blocked the lysis of both P815/A11 and P815 peptide-treated targets, and (b) on an extended target cell panel, peptide-mediated sensitization was also obtained with a second H-2d mouse line A20.2J and with an H-2Kd transfectant of the H-2k line R1E. This work has therefore identified EBV-specific HLA-A11-restricted human CTL which show fortuitous cross-recognition of a synthetic peptide in the context of a mouse H-2Kd molecule. Such potential for xenogeneic cross-reactivity needs to be borne in mind in situations where the target specificity of human CTL is being analyzed on HLA gene-transfected murine target lines.

Expression of Epstein-Barr virus transformation-associated genes in tissues of patients with EBV lymphoproliferative disease

Epstein-Barr virus (EBV) has been associated with serious or fatal lymphoproliferative disease in immunocompromised patients. EBV nuclear protein 2 and latent membrane protein are characteristically expressed in B lymphocytes proliferating in vitro in response to growth transformation by EBV. These two proteins are thought to be effectors of lymphocyte growth since they increase the expression of B-lymphocyte activation (CD23) and cell-adhesion (LFA 3 and ICAM 1) molecules in vitro. Using monoclonal antibody-immune microscopy, we have demonstrated that these two EBV proteins and their associated B-lymphocyte activation or adhesion molecules are expressed in the infiltrating B lymphocytes in immunocompromised patients with EBV lymphoproliferative disease. These monoclonal antibodies should be useful in the early diagnosis of EBV lymphoproliferative disease and in distinguishing it from other B-lymphocyte cancers associated with EBV, such as Burkitt's lymphoma. The finding of EBV nuclear protein 2 and latent membrane protein and their associated activation or adhesion molecules provides a further pathophysiologic link between EBV and the proliferation of B lymphocytes in immunocompromised patients.

Human herpesviruses: a consideration of the latent state

Images

Identification and characterization of a human cytomegalovirus gene coding for a membrane protein that is conserved among human herpesviruses

A rabbit antiserum was raised against envelope material from purified human cytomegalovirus strain AD169. The serum recognized polypeptides 200, 170, 160, 75, 58, and 45 kilodaltons in size. It was used to screen a cDNA library constructed from poly(A)+ RNA from human cytomegalovirus-infected cells in the expression vector lambda gt11. A recombinant bacteriophage expressing cytomegalovirus-specific sequences was identified, and the corresponding gene was mapped to the HindIII R fragment. The gene is transcribed into a late 1.5-kilobase RNA. The nucleotide sequence of the coding region was determined. Computer analysis of the gene product revealed a polypeptide containing multiple potential membrane-spanning domains, representing a type of protein not identified in the envelope of herpesviruses before. The protein shows homology on the amino acid level to hypothetical proteins from reading frames BBRF3 of Epstein-Barr virus, UL10 of herpes simplex virus type 1, and ORF50 of varicella-zoster virus. By using an antiserum raised against procaryote-expressed parts of the cytomegalovirus membrane protein, a 45-kilodalton structural component of the virus was identified as the gene product.

Host cell-dependent regulation of growth transformation-associated Epstein-Barr virus antigens in somatic cell hybrids

We have analyzed the expression of the three major known growth transformation-associated Epstein-Barr virus (EBV) proteins, EBNA-1, EBNA-2, and latent membrane protein (LMP), in a series of somatic cell hybrids derived from the fusion of EBV-carrying Burkitt lymphoma (BL) lines with EBV-positive or EBV-negative B-cell lines. Independently of the cell phenotype, EBNA-1 was invariably coexpressed in all EBV-carrying hybrids. In hybrids between EBV-carrying, LMP-positive and LMP-negative Burkitt lymphoma lines, LMP was expressed, indicating positive control. Two EBV-negative lymphoma lines, Ramos and BJAB, differed in their ability to express LMP after B95-8 virus-induced conversion and after hybridization with Raji cells. BJAB was permissive while Ramos was nonpermissive for LMP, although both expressed EBNA-2. The EBNA-2-deleted P3HR-1 virus gave the same pattern of LMP expression in these two cells. Our findings indicate that the expression of EBNA-1, EBNA-2, and LMP is regulated by independent mechanisms.

Epstein-Barr virus latent infection membrane protein alters the human B-lymphocyte phenotype: deletion of the amino terminus abolishes activity

A latent infection membrane protein (LMP) encoded by the Epstein-Barr virus (EBV) genome in latently infected, growth-transformed lymphocytes alters the phenotype of a human EBV-negative B-lymphoma cell line (Louckes) when introduced by gene transfer. These LMP-expressing cells exhibit increased homotypic adhesion due to increased expression of the adhesion molecules LFA-1 and ICAM-1. Increased homotypic adhesion could foster B-cell growth by facilitating autocrine growth factor effects. LFA-3 expression is also induced. The induction of LFA-3 and ICAM-1 results in increased heterotypic adhesion to T lymphocytes. This could result in more effective T-cell immune surveillance. Since LMP is expressed in EBV-transformed lymphocytes and has been demonstrated to transform rodent fibroblasts in vitro, a wide range of possible effects on B-lymphoma cell growth were assayed. In the Louckes B-lymphoma cell line, EBV LMP causes increased cell size, acid production, plasma membrane ruffling, and villous projections. Although cell proliferation rate was not greatly affected, the steady-state intracellular free calcium level, transforming growth factor beta responsiveness, and expression of the lymphocyte activation markers (CD23 and transferrin receptor) were increased. Thus, LMP appears to be a mediator of EBV effects on B-cell transformation. In transfected lymphoma cells, LMP localizes to patches at the cell periphery and associates with the cytoskeleton as it does in EBV-transformed B lymphocytes or in rodent fibroblasts. A partially deleted form of LMP (D1LMP) does not aggregate in patches or associate with the cytoskeleton and had little effect on B-cell growth. Thus, cytoskeletal association may be integral to LMP activity.