Activation in vitro by BUdR of a productive EB virus infection in the epithelial cells of nasopharyngeal carcinoma

Reflections on Epstein-Barr virus: some recently resolved old uncertainties

The change over recent decades in perceptions of the role of viruses in human cancer-causation is illustrated by the reception given to the discovery of Epstein-Barr virus (EBV) in 1964 compared to that of Kaposi's sarcoma herpesvirus (KSHV or HHV-8) in 1994. Very new data on EBV-like agents in New World monkeys is considered in relation to the antiquity of the association of proto-EBV with early anthropoids. Although the finding that individuals without B lymphocytes do not seem to be infected with EBV appears to have resolved the controversy regarding the permissive cell type producing infectious virus in the oropharynx, the presence of EBV in certain squamous and other epithelial cells raises continuing problems which are discussed. Among many recent successes of molecular biology applied to EBV, new information from such investigations on the genetic defect in X-linked lymphoproliferative syndrome now explains the cause of the disastrous pathological changes underlying the disease.Finally, current progress with vaccines against EBV is reviewed.

Historical background

The persisting ancient view of cancer as a contagious disease ended with 19th century scientific investigations which seemed to show it was not. The resulting dogma against an infectious cause for cancer produced great prejudice in the scientific community against the first report of an oncogenic virus by Rous early in the 20th century and, even in the 1950s, against Gross's finding of a murine leukaemia virus and a murine virus causing solid tumours. The Lucké frog renal carcinoma virus was the first cancer-associated herpesvirus. Intriguingly, an environmental factor, ambient temperature, determines virus genome expression in the poikilothermic frog cells. Although an alpha-herpesvirus, Marek's disease virus of chickens shares some aspects of biological behaviour with Epstein-Barr virus (EBV) of man. Very significantly, its lymphomas are the first naturally occurring malignancy to be controlled by an antiviral vaccine, with implications for human virus-associated cancers. The circumstances and climate of opinion in which successive gamma-herpesviruses were discovered are described. The identification of EBV involved two unconventionalities: its finding in cultured Burkitt's lymphoma cells when no human lymphoid cell had ever been maintained in vitro, and its recognition in the absence of biological activity by the then new technique of electron microscopy. These factors engendered hostility to its acceptance as a new human tumour-associated virus. The EBV-like agents of Old World apes and monkeys and the T-lymphotropic gamma-herpesviruses of New World monkeys were found at about the same time, not long after the discovery of EBV. For many years these were thought to be the only gamma-herpesviruses of non-human primates; however, very recently B-lymphotropic EBV-like agents have been identified in New World species as well. Mouse herpesvirus 68 came to light by chance during a search for arboviruses and has become important as a laboratory model because of its close genetic relatedness to EBV and its comparable biological behaviour. The discovery of Kaposi's sarcoma-associated herpesvirus six years ago was made using unconventional new methods, but, unlike with EBV 30 years before, this did not hinder its acceptance. This contrast is discussed in the context of the great progress in human tumour virology which has been made in recent years.

Molecular virology of Epstein-Barr virus

Epstein-Barr virus (EBV) interacts with its host in three distinct ways in a highly regulated fashion: (i) EBV infects human B lymphocytes and induces proliferation of the infected cells, (ii) it enters into a latent phase in vivo that follows the proliferative phase, and (iii) it can be reactivated giving rise to the production of infectious progeny for reinfection of cells of the same type or transmission of the virus to another individual. In healthy people, these processes take place simultaneously in different anatomical and functional compartments and are linked to each other in a highly dynamic steady-state equilibrium. The development of a genetic system has paved the way for the dissection of those processes at a molecular level that can be studied in vitro, i.e. B-cell immortalization and the lytic cycle leading to production of infectious progeny. Polymerase chain reaction analyses coupled to fluorescent-activated cell sorting has on the other hand allowed a descriptive analysis of the virus-host interaction in peripheral blood cells as well as in tonsillar B cells in vivo. This paper is aimed at compiling our present knowledge on the process of B-cell immortalization in vitro as well as in vivo latency, and attempts to integrate this knowledge into the framework of the viral life cycle in vivo.

Detection of Epstein-Barr Virus DNA in well and poorly differentiated nasopharyngeal carcinoma cell lines

Undifferentiated and poorly differentiated nasopharyngeal carcinoma (NPC) were known to be tightly associated with Epstein-Barr Virus (EBV). Its association with well differentiated NPC was also reported. In the present study, the presence of EBV was investigated by nucleic acid hybridization, Polymerase Chain Reaction (PCR), Immunoblot and in situ hybridization in two well differentiated NPC cell lines (CNE-1 and HK-1) and two other poorly differentiated NPC cell line (CNE-2 and CNE-3). Contrary to previous report indicating the absence of EBV in these cell lines, EBV DNA and proteins were present in all cell lines. The detection of EBV became more easily when the investigation was carried out on the nude mice tumor induced by transplantation of each NPC epithelial cell line. The EBV latent membrane protein (LMP1) was found by in situ hybridization to be integrated partly in the chromosomal DNA of these cell lines. The observations indicate that EBV could persist for a long time in the carcinoma cells established directly from well and poorly differentiated tumor biopsies and from transplantable NPC tumor in nude mice.

The human nasopharynx as a reservoir for Epstein-Barr virus

Nineteen human nasopharyngeal tissue specimens biopsied from patients with suspected nasopharyngeal carcinoma (NPC) and 15 adenoid tissue specimens obtained by surgery were successfully grown in culture without bacterial and fungal contamination. Fibroblastic growth developed in all of the cultures. In some cases, epithelial growth was also seen among the fibroblastic growth. The cells became round and floated in the culture fluid between 19th and 57th day of culture in 6 out of 10 NPC tissue cultures, 3 out of 9 tissue cultures of apparently normal nasopharyngeal mucosa of patients referred for exclusion of NPC, and 4 out of 15 adenoid tissue cultures. The floating cells consisted of B-lymphoblastoid cells and were positive Epstein-Barr virus (EBV)-associated nuclear antigen (EBNA). The data suggest that a small number of B-lymphocytes in the human nasopharynx serve as a reservoir for EBV.

Characterization of two related Epstein-Barr virus-encoded membrane proteins that are differentially expressed in Burkitt lymphoma and in vitro-transformed cell lines

Two related but differentially expressed potential membrane proteins of Epstein-Barr virus are encoded by the same reading frame in the EcoRI D het region of the viral genome. Potential antigenic sites in the amino acid sequence of these proteins were selected by computer-aided prediction of the secondary structure and two oligopeptides corresponding to regions located in different parts of the proteins were synthesized chemically. Rabbit antisera to these peptides were used for immunoprecipitation of the native viral proteins from Epstein-Barr virus-positive cell lines from various sources. Both predicted membrane proteins could be precipitated from cell lines that had been transformed in vitro with EBV or from cell lines derived from infectious mononucleosis patients. In cell lines established from Burkitt lymphoma, only the smaller polypeptide, which lacks 138 amino acids from the amino terminus, could be identified. Using the synthetic peptides as antigens in ELISA, we detected elevated antibody titers in sera from patients with infectious mononucleosis and nasopharyngeal carcinoma.

Immunofluorescence and electron microscopic investigations of epithelial hybrid cells derived from nasopharyngeal carcinoma (NPC-KT)

The present study was made to investigate some characteristics of the epithelial hybrid cells derived from nasopharyngeal carcinoma (NPC-KT cells) both in vivo and in vitro, using immunofluorescence and electron microscopic techniques. Immunofluorescence and electron microscopic studies have shown that the appearance of Epstein-Barr virus (EBV)-related early antigens, EB-viral capsid antigens and virus particles in nude-mouse-grown-tumour cells were rather repressed, in contrast to, in vitro culture of the NPC-KT cells. The tumours after transplantation of the NPC-KT cells to nude mice showed pathological pictures of poorly differentiated carcinoma with EBV-associated nuclear antigen and derived from the NPC-KT cells by means of cytogenetic studies. More importantly, we have detected EBV-related membrane antigens (MA) on the epithelial NPC-KT cells. To our knowledge, the presence of MA on the malignant epithelial cells of the nasopharynx have never been demonstrated. The results reported here show for the first time the presence of MA on nasopharyngeal carcinoma cells.

The Leeuwenhoek lecture, 1983. A prototype vaccine to prevent Epstein-Barr virus-associated tumours

EB virus is a herpesvirus that infects all human communities. The infection is life-long and usually asymptomatic. Excessive reaction to primary infection leads to infectious mononucleosis while immunological failures give fatal lymphoproliferative diseases. The virus is associated with endemic Burkitt's lymphoma and undifferentiated nasopharyngeal carcinoma. In world cancer terms Burkitt's lymphoma is insignificant, but nasopharyngeal carcinoma has a high incidence in certain important populations. By analogy with herpesvirus-induced animal cancers, prevention of infection should greatly reduce subsequent development of tumours. A prototype vaccine has therefore been produced based on the virus-determined antigen (MA gp340) that elicits virus-neutralizing antibodies. A sensitive assay has permitted the elaboration of an efficient antigen preparation method and the product has been rendered highly immunogenic, as tested in mice and rabbits, by incorporation in liposomes. The only animal suitable for experimental EB virus infection is the little-known cottontop tamarin; a breeding colony has been successfully established and protection against virus challenge assessed in immunized tamarins. The overall structure of the antigen has been determined in preparation for future production by synthesis or rDNA technology.

Epstein-Barr virus activation of human epithelial-nasopharyngeal carcinoma hybrid cells

The authors used indirect immunofluorescence and electron microscopic techniques to study the effects of 5-iododeoxyuridine (IUDR) and the tumor promotor 12-O-tetradecanoylphorbol-13-acetate (TPA) on the epithelial hybrid cell line (NPC-KT) derived from nasopharyngeal carcinoma. The hybrid cells contained Epstein-Barr virus (EBV)-induced early antigens at a very low level but not Epstein-Barr viral capsid antigens in spontaneous production; the hybrid cells responded significantly to both IUDR and TPA induction. The chemicals induced both early antigens and viral capsid antigens in the NPC-KT cells. Moreover, the virus particles were detected in the altered NPC-KT cells by IUDR treatment. The data suggest that the EBV-induced agents activate the EBV productive cycle in a proportion of nasopharyngeal carcinoma cells.

Epstein-Barr virus transcription in nasopharyngeal carcinoma

Sequences which encode Epstein-Barr virus (EBV) RNA in nasopharyngeal carcinoma (NPC) tissue have been identified. We utilized human biopsy material directly as well as NPC grown in nude mice. Total RNA was extracted from the tumor material and separated into polyadenylated and nonpolyadenylated fractions by oligodeoxythymidylate-cellulose chromatography. This material was used as template to construct 32P-labeled cDNA. The labeled cDNAs were hybridized to Southern blots of recombinant EBV DNA fragments. Three of the biopsies, F, 49, and 55, contained polyadenylated RNA homologous to the EBV BamHI fragments V and K, and EcoRI-DIJhet. These same fragments encode the most abundant polyribosomal RNAs in latently infected lymphoblastoid cell lines. The sequences which encoded nonpolyadenylated RNA in NPC tumor 49 were more extensive and included BamHI fragments C, V, B, E, and K, and EcoRI fragments DIJhet, E, F, and G1, a result that indicates selective polyadenylation in EBV RNA processing. A fourth biopsy, NPC tumor 18, contained polyadenylated RNA homologous to the BamHI fragments H, B, K, Y, B1, I1, and A and EcoRI fragments F and G2. A similar pattern of transcription was identified in three tumor specimens from nude mice, 4, 5, and 8. Transformation of lymphocytes did not occur after cocultivation in vitro with explants from these nude mice tumors. This transcriptional pattern may represent an activated state of the EBV genome, formerly not detected in tumor tissue, which is analogous to the state of abortive infection identified in induced in vitro cell systems.

Functional mapping of the Epstein-Barr virus genome: identification of sites coding for the restricted early antigen, the diffuse early antigen, and the nuclear antigen

Attempts were made to functionally map antigenic expression of the Epstein-Barr virus (EBV) to specific regions on the EBV genome, using the B95-8 strain. Experiments were performed to map the expression of early antigen (EA), both restricted and diffuse (R and D, respectively), and the EBV nuclear antigen (EBNA), using intact B95-8 DNA, cloned BamHI fragments or Charon 4A fragments. DNA preparations were microinjected into two EBV genome-negative epithelial tumor cell lines. Expression of EBV antigens was monitored using precharacterized human sera, as well as monoclonal antibodies to EA-R and EA-D. The data suggest that EA-R maps to the BamHI H fragment, and EA-D maps to the Charon 4A fragment 7. A previous report that BamHI K is associated with the expression of a nuclear neoantigen tentatively identified as EBNA (W.P. Summers, E.A. Grogan, D. Shedd, M. Robert, C.R. Liu, and G. Miller, Proc. Nat. Acad. Sci. USA 79, 5688-5692, 1982) was also confirmed.

A culture method giving substantial yields of normal nasopharyngeal epithelial cells for work with Epstein-Barr virus

A culture method, utilising a feeder layer of lethally irradiated 3T3 fibroblasts and medium supplemented with hydrocortisone, cholera toxin, and epidermal growth factor, has been elaborated for the in vitro growth of normal human nasopharyngeal epithelial cells. This method allowed the cells to be grown in vitro for periods of up to 146 days, very considerably longer than in previously reported studies, and ensured that the cultures remained largely free from contaminating human fibroblasts. It was found possible to subculture the nasopharyngeal epithelial cells through numerous passages both by dispersing monolayers into single cell suspensions and by transferring coverslip monolayers of the cells to individual Petri dishes. By combining these two methods, at least 50 replicate epithelial cultures could be produced from each tissue sample, thus providing for the first time cultured nasopharyngeal epithelial cells in quantities suitable for extensive experiments with Epstein-Barr virus.

Morphological evidence of viruses in nasopharyngeal carcinoma?

Human tumor tissue from patients with nasopharyngeal carcinoma was investigated electron microscopically before and after transplantation to nude mice. Although we were unable to find distinct herpes type-C particles in human tissue, these particles could be seen following tissue transplantation to nude mice in two cases. The findings are discussed as to whether those particles are either endogenous in nude mice or expressed by the tumor tissue as a consequence of transplantation.

Hybridization between a human epithelial line, infectable by Epstein-Barr virus, and Burkitt lymphoma lines: membrane properties, superinfectability, inducibility and tumorigenicity

The human epithelial line U, which is partially infectable with EBV, was hybridized with the EBV-genome carrying Burkitt lymphoma lines P3HR-1 and Daudi. Authenticity of the hybrids U-Put and U-Dut was established by isoenzyme studies. Although the two hybrids carried the EBV genome derived from the lymphoma parent, being 100% positive for Epstein-Barr-virus-associated nuclear antigen (EBNA), they resembled the U parent in many respects: they were deficient for membrane immunoglobulins and Fc receptors, and had a lower concentration of EBV-C3 receptors than either parent. Unlike the P3HR-1 parent, U-Put hybrid was nonpermissive for both the EBV cycle antigens, early antigen (EA) and viral capsid antigen (VCA). The inducing agent 12-O-tetra-decanoyl-phorbol-13-acetate (TPA) caused distinct viral early antigen synthesis (EA) in U-Put, lower, however, than that of the parental P3HR-1. U-Dut was completely nonpermissive and noninducible for early and viral capsid antigens. Thus, even an epithelial parent infectable by EBV restricted, although not completely, expression of EBV antigens, with the exception of EBNA. It has been suggested that EBNA is an autonomous function of the viral genome, independent of host cell control; the latter regulates expression of antigens related to viral cycle. The hybrids U-Put and U-Dut resembled the U parent also in regard to growth in soft agar and tumorigenicity in nude mice, although in this respect the lymphoma parental properties were not completely eclipsed.

Induction of Epstein-Barr virus nuclear antigen and DNA synthesis in a human epithelial cell line after Epstein-Barr virus infection

The association of Epstein-Barr virus (EBV) with nasopharyngeal carcinoma is supported by the presence of EBV genomes in the epithelial elements of the tumor and by elevated antibody titers to EBV-specific antigens in the patients; the levels of these titers are related to the clinical course of the disease. However, since most laboratory data suggest that EBV is a B-lymphotropic virus, it is unclear how the virus becomes associated with the epithelial elements of the nasopharynx. The purpose of the present work was to find a human model system to study this association. A human epithelial line (U) was found that could be directly infected by EBV, and viral functions, the induction of EBV nuclear antigen and cellular DNA synthesis, were demonstrated. The U line was established in 1957 by the late H. J. Van Kooten (Kok-Doorschodt at the University of Utrecht), and although it is no longer diploid, it exhibits density inhibition. When U cells were infected with EBV, EBV nuclear antigen was expressed in 6 to 16% of the cells, 1 and 2 days after infection with B95-8 virus, but not with the P3HR-1 strain. No evidence for virus replication was obtained; immunofluorescence staining for early antigens and virus capsid antigens gave negative results. Quantitative adsorption experiments for EBV indicated that the adsorption capacity of U cells is significant (60% of Raji cells). The present results also demonstrated that infection with the virus overcomes block(s) in cellular DNA synthesis caused by 5-fluorodeoxyuridine. The induction of DNA synthesis was determined by increased incorporation of [3H]thymidine into the cells. The highest level of isotope incorporation was observed at about 15 h after infection and thereafter decreased. Analysis of the induced DNA indicated that it was of cellular origin.

[Comparative morphology of lymphoepithelial carcinoma in primary tumor, metastasis, and xenograft (author's transl)]

Tissue of lymphoepithelial tumor metastasis was transplanted subcutaneously to thymus-aplastic "nude" mice. After 75 days a pea-size tumor was developing which again was transplanted to further murine passages. During the second passage the tumor was growing faster and material was taken for light and electron microscopy. The results were compared with the datas obtained from the primary nasopharyngeal carcinoma and metastasis. The tumor cells of the metastasis and of the xenograft showed all morphological criteria of a non-keratinizing squamous epithelial carcinoma of undifferentiated type. The ultrastructural criteria of the human NPC-tumor cells were all present including cytoplasmic deposits of Corona viruses and Herpes virus-like particles within the nuclei and the cytoplasm.

Site of Epstein-Barr virus replication in the oropharynx

Cells and cell-free material in saliva, parotid secretions, and throat washings from patients with acute infectious mononucleosis and patients undergoing tonsillectomy were assayed for the presence of infectious Epstein-Barr (EBV) virus. The agent was invariably present in cell-free form in saliva; neither infectious virus nor viral antigens were found in the cells. Tonsillar lymphocytes from eight patients were also free of EBV. In 10 of 40 patients virus was recovered in secretions from parotid-gland orifices or ducts. These findings suggest that the salivary glands are the site of EBV production in the oropharynx.

Biological and biochemical observations on isolates of EB virus from the malignant epithelial cells of two nasopharyngeal carcinomas

Foetal, adult seronegative, and cotton-top marmoset lymphocytes have been transformed into cell lines by EB virus from the malignant epithelial cells of two nasopharyngeal carcinomas. The nature of the cell lines was checked by karyotyping and by light and electron microscopy, and the presence of the EB virus genome was demonstrated by immunofluorescence tests. Immunofluorescence also showed the incidence of EB virus-producing cells in each line and this was checked by electron microscopy and the use of a lymphocyte transformation assay. Two foetal-derived lines did not produce virus spontaneously, could not be activated with various inducers, and were found by DNA reassociation kinetics to carry only small numbers of genome copies per cell. An adult-derived line produced virus, could be activated to produce more, and provided enough infectivity to transform marmoset cells. The resulting marmoset line made profuse transforming virus; it thus provides the first abundant source of NPC-derived infectious EB virus for comparative studies. The results are discussed in relation to the interactions with comparable target cells of EB virus from normal individuals and from patients with other diseases.

Identification of murine endogenous xenotropic retrovirus in cultured multicellular tumour spheroids from nude-mouse-passaged nasopharyngeal carcinoma

After nude-mouse-passage, a carcinoma of the nasopharynx was found to contain a few scattered C-type retrovirus particles. Culture of this nude-mouse-grown material over solid agar allowed the human tumour cells to grow as multicellular spheroids which increased in number by budding and could be subcultured for up to 6 months. Within the spheroids the human tumour cells expressed increased retrovirus replication and large numbers of C-type particles were observed. Treatment with a halogenated pyrimidine further enhanced the virus production. This tissue culture system provided sufficient virus production for the identification of the retrovirus as an endogenous xenotropic murine leukaemia virus and not a human agent.

In vitro immune responses to PPD, extracts from Raji cells and nasopharyngeal carcinoma biopsies in NPC leucocytes

Peripheral leucocytes from nasopharyngeal carcinoma (NPC) patients and control subjects, which included healthy subjects and patients with other cancers, have been tested against PPD and a panel of extracts from Raji cells and pooled NPC biopsies, using the blast transformation and the macrophage migration inhibition assays. The results of both assays indicated that the in vitro cell-mediated immune (CMI) responses to the Raji-cell extracts and NPC-biopsy extracts were associated with NPC. However, the peripheral leucocytes from NPC patients and control subjects responded similarly to PPD. These results are in general accord with the skin tests reported by Levine et al. (1976) and Ho, Ng and Kwan (1977b). The antigenic specificity of the NPC-associated CMI responses remains, however, to be resolved, as the extracts used in these or in the in vivo CMI studies were heterogeneous mixtures.

Isolation of infectious EB virus from the epithelial tumour cells of nasopharyngeal carcinoma

Evidence of herpesvirus replication has been found by light and electron microscopy in the malignant epithelial cells of two out of six nasopharyngeal carcinomas (NPC) examined directly after growth in nude mice to eliminate non-malignant infiltrating cells. The agent has been identified as EB virus by immunofluorescence tests for EB virus capsid antigen, and has been shown to be biologically active by its ability to infect and transform foetal cord blood lymphocytes. Lymphoblastoid cell lines which express the EB virus nuclear antigen have been established from the transformed foetal lymphocytes, and thus carry the first isolate of the virus from the actual epithelial tumour cells of NPC, in a form suitable for further investigation. The results are discussed in terms of the relationship of EB virus to NPC epithelial cells.

Neutralizing EBV-specific IgA in throat washings of nasopharyngeal carcinoma (NPC) patients

Throat washings from 26 nasopharyngeal carcinoma (NPC) patients from Hong Kong and Tunisia were studied for the presence of transforming EBV. Only six (23%) were found positive which led to the hypothesis of a neutralizing factor in such salivas. The search for EBV-specific antibodies showed that NPC saliva contained neutralizing VCA and EA IgA (54 and 27% respectively) and VCA and EA IgG (73 AND 54% respectively). Both transforming and non-transforming throat washings contained virus particles as visualized by electron microscopy, but in non-transforming salivas (containing IgA and IgG) the particles were found to be clumped. Comparative study of throat washings from patients with Burkitt's lymphoma (BL); infectious mononucleosis (IM), immunodeficiencies, other cancers, and healthy subjects showed that IgA were restricted to NPC cases.