The Epstein-Barr virus:host balance in acute infectious mononucleosis patients receiving acyclovir anti-viral therapy

Gammaherpesvirus Colonization of the Spleen Requires Lytic Replication in B Cells

Gammaherpesviruses infect lymphocytes and cause lymphocytic cancers. Murid herpesvirus-4 (MuHV-4), Epstein-Barr virus, and Kaposi's sarcoma-associated herpesvirus all infect B cells. Latent infection can spread by B cell recirculation and proliferation, but whether this alone achieves systemic infection is unclear. To test the need of MuHV-4 for lytic infection in B cells, we flanked its essential ORF50 lytic transactivator with loxP sites and then infected mice expressing B cell-specific Cre (CD19-Cre). The floxed virus replicated normally in Cre^- mice. In CD19-Cre mice, nasal and lymph node infections were maintained; but there was little splenomegaly, and splenic virus loads remained low. Cre-mediated removal of other essential lytic genes gave a similar phenotype. CD19-Cre spleen infection by intraperitoneal virus was also impaired. Therefore, MuHV-4 had to emerge lytically from B cells to colonize the spleen. An important role for B cell lytic infection in host colonization is consistent with the large CD8⁺ T cell responses made to gammaherpesvirus lytic antigens during infectious mononucleosis and suggests that vaccine-induced immunity capable of suppressing B cell lytic infection might reduce long-term virus loads.IMPORTANCE Gammaherpesviruses cause B cell cancers. Most models of host colonization derive from cell cultures with continuous, virus-driven B cell proliferation. However, vaccines based on these models have worked poorly. To test whether proliferating B cells suffice for host colonization, we inactivated the capacity of MuHV-4, a gammaherpesvirus of mice, to reemerge from B cells. The modified virus was able to colonize a first wave of B cells in lymph nodes but spread poorly to B cells in secondary sites such as the spleen. Consequently, viral loads remained low. These results were consistent with virus-driven B cell proliferation exploiting normal host pathways and thus having to transfer lytically to new B cells for new proliferation. We conclude that viral lytic infection is a potential target to reduce B cell proliferation.

The Effect of Acyclovir on the Acute and Latent Murine Gammaherpesvirus-68 Infection of Mice

Mice inoculated intranasally with murine gammaherpesvirus-68 were used to evaluate the efficacy of acyclovir (ACV) in the treatment of acute and latent infections. Effectiveness was measured by infectious virus assay of the lung (site of active replication) and infectious centre assay of spleen cells (site of latency). Intraperitoneal administration of ACV at 6-h intervals starting soon after inoculation was more effective in reducing infectious virus in the lung than was treatment with 12-hourly injections commencing 3 days post-infection. Further, ACV treatment during acute infection resulted in an approximately 10-fold reduction in the number of infectious centres in the spleen as compared to placebo-treated animals. However, once latency was established, ACV treatment was not effective in reducing the number of infectious centres in the spleen. This is the first report demonstrating that ACV can be used to minimize the replication of murine gammaherpesvirus in mice at the site of primary infection, resulting in a reduction in the number of latently infected spleen lymphocytes.

Host entry by gamma-herpesviruses--lessons from animal viruses?

The oncogenicity of gamma-herpesviruses (γHVs) motivates efforts to control them and their persistence makes early events key targets for intervention. Human γHVs are often assumed to enter naive hosts orally and infect B cells directly. However, neither assumption is supported by direct evidence, and vaccination with the Epstein-Barr virus (EBV) gp350, to block virion binding to B cells, failed to reduce infection rates. Thus, there is a need to re-evaluate assumptions about γHV host entry. Given the difficulty of analysing early human infections, potentially much can be learned from animal models. Genomic comparisons argue that γHVs colonized mammals long before humans speciation, and so that human γHVs are unlikely to differ dramatically in behaviour from those of other mammals. Murid Herpesvirus-4 (MuHV-4), which like EBV and the Kaposi's Sarcoma-associated Herpesvirus (KSHV) persists in memory B cells, enters new hosts via olfactory neurons and exploits myeloid cells to spread. Integrating these data with existing knowledge of human and veterinary γHVs suggests a new model of host entry, with potentially important implications for infection control.

Herpesvirus delivery to the murine respiratory tract

Herpesvirus transmission is sporadic, and infection may be asymptomatic or present only with secondary lesions after dissemination. Consequently host entry remains ill-understood. Experimental infections can be informative, but depend on inoculations that are inherently artificial and so need validation. Mice are a widely used experimental host. Alert mice inhale readily small (5 μl) liquid volumes, and Indian ink, luciferase or radiolabel delivered thus distributed to the nasopharynx and oropharynx. Murid Herpesvirus-4 or Herpes simplex virus type 1 delivered thus infected only the nose, arguing that host entry is nasal rather than oral. Marker or virus delivery to the lung depended on general anesthesia and a large inoculum volume (30 μl), and so needs further validation of physiological relevance. While lungs could be infected at lower doses than the upper respiratory tract, tracking experiments showed that nasal inocula pass mostly into the oropharynx, even when restricted to 1 μl. Thus, the relative inefficiency of experimental upper respiratory tract infection was attributable to limited liquid retention in this site. Nonetheless low volume intranasal delivery to alert mice provides a convenient way to model experimentally an apparently natural mode of herpesvirus host entry.

Antibody limits in vivo murid herpesvirus-4 replication by IgG Fc receptor-dependent functions

Antibody is an important antiviral defence. However, it is considered to do little against human gamma-herpesviruses, which establish predominantly latent infections regulated by T cells. One limitation on analysing these infections has been that latency is already well-established at clinical presentation; early infection may still be accessible to antibody. Here, using murid herpesvirus-4 (MuHV-4), we tested the impact of adoptively transferred antibody on early gamma-herpesvirus infection. Immune sera and neutralizing and non-neutralizing monoclonal antibodies (mAbs) all reduced acute lytic MuHV-4 replication. The reductions, even by neutralizing mAbs, were largely or completely dependent on host IgG Fc receptors. Therefore, passive antibody can blunt acute gamma-herpesvirus lytic infection, and does this principally by IgG Fc-dependent functions rather than by neutralization.

Immune control of mammalian gamma-herpesviruses: lessons from murid herpesvirus-4

Many acute viral infections can be controlled by vaccination; however, vaccinating against persistent infections remains problematic. Herpesviruses are a classic example. Here, we discuss their immune control, particularly that of gamma-herpesviruses, relating the animal model provided by murid herpesvirus-4 (MuHV-4) to human infections. The following points emerge: (i) CD8(+) T-cell evasion by herpesviruses confers a prominent role in host defence on CD4(+) T cells. CD4(+) T cells inhibit MuHV-4 lytic gene expression via gamma-interferon (IFN-gamma). By reducing the lytic secretion of immune evasion proteins, they may also help CD8(+) T cells to control virus-driven lymphoproliferation in mixed lytic/latent lesions. Similarly, CD4(+) T cells specific for Epstein-Barr virus lytic antigens could improve the impact of adoptively transferred, latent antigen-specific CD8(+) T cells. (ii) In general, viral immune evasion necessitates multiple host effectors for optimal control. Thus, subunit vaccines, which tend to prime single effectors, have proved less successful than attenuated virus mutants, which prime multiple effectors. Latency-deficient mutants could make safe and effective gamma-herpesvirus vaccines. (iii) The antibody response to MuHV-4 infection helps to prevent disease but is suboptimal for neutralization. Vaccinating virus carriers with virion fusion complex components improves their neutralization titres. Reducing the infectivity of herpesvirus carriers in this way could be a useful adjunct to vaccinating naive individuals with attenuated mutants.

CD4+ T cells specific for a model latency-associated antigen fail to control a gammaherpesvirusin vivo

CD4(+) T cells play a major role in containing herpesvirus infections. However, their cellular targets remain poorly defined. In vitro CD4(+) T cells have been reported to kill B cells that harbor a latent gammaherpesvirus. We used the B cell-tropic murine gammaherpesvirus-68 (MHV-68) to test whether this also occurred in vivo. MHV-68 that expressed cytoplasmic ovalbumin (OVA) in tandem with its episome maintenance protein, ORF73, stimulated CD8(+) T cells specific for the H2-K(b)-restricted OVA epitope SIINFEKL and was rapidly eliminated from C57BL/6 (H2(b)) mice. However, the same virus failed to stimulate CD4(+) T cells specific for the I-A(d)/I-A(b)-restricted OVA(323-339) epitope. We overcame any barrier to the MHC class II-restricted presentation of an endogenous epitope by substituting OVA(323-339) for the CLIP peptide of the invariant chain (ORF73-IRES-Ii-OVA), again expressed in tandem with ORF73. This virus presented OVA(323-339) but showed little or no latency deficit in either BALB/c (H2(d)) or C57BL/6 mice. Latent antigen-specific CD4(+) T cells therefore either failed to recognize key virus-infected cell populations in vivo or lacked the effector functions required to control them.

Determination of antiviral efficacy against lymphotropic herpesviruses utilizing flow cytometry

Epstein-Barr virus (EBV), human herpesvirus type 6 (HHV-6), and human herpesvirus type 8 (HHV-8) comprise a group of lymphotropic herpesviruses which are responsible for a wide range of diseases, including lymphoproliferative disorders and tumors. We have developed several flow cytometric assay (FACS) systems to evaluate antiviral efficacy against EBV, HHV-6 and HHV-8. Assays using either EBV or HHV-8, members of the gammaherpesvirus subfamily, have shown that while EBV responds well to acyclovir (ACV), HHV-8 was most sensitive to cidofovir (CDV). Since HHV-6 strains are divided into two sub-groups, A and B, we evaluated antiviral efficacy for strains from each group. The group A strain, HHV-6(GS), was inhibited by foscarnet (PFA), CDV and ganciclovir (GCV) in both Sup-T1 and HSB-2 cell lines. HHV-6(Z-29), a representative group B virus, was inhibited by GCV and CDV but not by PFA. Our findings indicate that flow cytometry can be utilized to efficiently evaluate new antiviral agents against lymphotropic herpesviruses and that the results are comparable to those obtained by other methods such as immunofluorescence.

Treatment of Epstein-Barr virus-associated malignancies with specific T cells

Latent Epstein-Barr virus (EBV) infection is associated with a heterogeneous group of malignancies, including Burkitt's lymphoma, Hodgkin's disease, nasopharyngeal carcinoma, and lymphoproliferative disease (LPD). The development of adoptive immunotherapies for these malignancies is being fueled by the successful generation of allogeneic donor derived EBV-specific cytotoxic T cells (CTL) for the prevention and treatment of EBV-LPD after hematopoietic stem cell transplantation. This approach is being extended to EBV-LPD after solid organ transplantation by use of autologous and haploidentical EBV-specific CTL. For other EBV-associated malignancies, there is only limited clinical experience with EBV-specific CTL. With few exceptions, only patients with recurrent Hodgkin's disease have been treated with autologous EBV-specific CTL, and although the results have been promising, they do not include cures. Lack of CTL efficacy may reflect either down-regulation of immunodominant EBV proteins, which are major CTL targets, or the presence of inhibitory cytokines. Further improvement of EBV-specific CTL therapy for Hodgkin's disease will require improved methods to activate and expand CTL specific for the latent EBV genes expressed in Hodgkin's disease and to genetically modify the expanded CTL to render them resistant to inhibitory cytokines. If effective, such strategies could be applied not only to other EBV-associated malignancies, but also to a broad range of human tumors with defined tumor antigens and similar immune evasion strategies.

Comparative analysis of the transforming mechanisms of Epstein-Barr virus, Kaposi's sarcoma-associated herpesvirus, and Herpesvirus saimiri

Members of the gamma herpesvirus family include the lymphocryptoviruses (gamma-1 herpesviruses) and the rhadinoviruses (gamma-2 herpesviruses). Gammaherpesvirinae uniformly establish long-term, latent, reactivatable infection of lymphocytes, and several members of the gamma herpesviruses are associated with lymphoproliferative diseases. Epstein-Barr virus is a lymphocryptovirus, whereas Kaposi sarcoma-associated herpesvirus and Herpesvirus saimiri are members of the rhadinovirus family. Genes encoded by these viruses are involved in a diverse array of cellular signaling pathways. This review attempts to cover our understanding of how viral proteins deregulate cellular signaling pathways that ultimately contribute to the conversion of normal cells to cancerous cells.

Epstein-Barr virus (EBV) in infectious mononucleosis: detection of the virus in tonsillar B lymphocytes but not in desquamated oropharyngeal epithelial cells

AIMS

Despite its well established tropism for B cells, the nature of the cellular compartment(s) mediating primary and persistent Epstein-Barr virus (EBV) infection is still a matter of controversy. In view of the association of EBV with several lymphoid and epithelial malignancies, resolution of this issue is important.

METHODS

Desquamated oropharyngeal epithelial cells from 10 patients with acute infectious mononucleosis and from seven chronic virus carriers were studied for evidence of EBV infection using in situ hybridisation for the detection of the small EBV encoded RNAs (EBERs) and of the viral genome. In addition, immunocytochemistry was used to detect the BZLF1 transactivator protein of EBV.

RESULTS

There was no evidence of latent or replicative EBV infection in oropharyngeal epithelial cells in any of the samples. In contrast, EBV infected B cells were readily identified in a tonsil from a patient with infectious mononucleosis.

CONCLUSIONS

The results suggest that oropharyngeal epithelial cells are not a major site of EBV infection and provide further support for the notion that B cells mediate primary and persistent EBV infection.

2'-Deoxy-5-ethyl-beta-4'-thiouridine inhibits replication of murine gammaherpesvirus and delays the onset of virus latency

The antiviral thionucleoside analogue 2'-deoxy-5-ethyl-beta-4'-thiouridine (4'-S-EtdU) was shown to be a more potent inhibitor of gammaherpesvirus infection than acyclovir. This compound inhibits replication of murine herpesvirus (MHV)-68 in the lungs of mice when given 3 days post-infection. However, as with other nucleoside analogues, it was unable to prevent the establishment of latency, despite delaying the onset of latent infection in the spleen. In contrast, virus persistence in the lung was inhibited following drug treatment, although persistence was re-established in mice when treatment was suspended after 12 days. These data suggest that 4'-S-EtdU is a highly effective inhibitor of murine gammaherpesvirus replication and as such provides a powerful tool to study the pathogenesis of this virus in vivo.

Preventing acute rejection, Epstein-Barr virus infection, and posttransplant lymphoproliferative disorders after kidney transplantation: use of aciclovir and mycophenolate mofetil in a steroid-free immunosuppressive protocol

BACKGROUND

A widely held view is that any increase in the potency of an immunosuppressive agent will lead to an increase in infection and malignancy, such as life-threatening Epstein-Barr virus (EBV) induced posttransplant lymphoproliferative disorders (PTLD). We tested this paradigm by studying the effect of adding mofetil to a steroid-free protocol under cover of high-dose aciclovir prophylaxis on the number of acute rejections, EBV infections and PTLDs after kidney transplantation.

METHODS

EBV serology was performed in 267 consecutive renal transplantations (1990-1997). All were treated with cyclosporine with an initial 10-day antilymphocyte globulin course, supplemented from September 1995 with MMF. In 208 consecutive transplantations after June 1992 aciclovir 3200 mg/day was given for 3 months posttransplantation.

RESULTS

After an observation period of up to 7 years we found that: (1) primary or reactivated EBV infection (PREBV) was correlated to acute rejection (treated with OKT3; P<0.00005) and to the incidence of PTLD (P=0.03; P=0.01, if Hodgkin's disease is included); (2) aciclovir protected against PREBV (P<0.00005) and (3) adding mofetil to the immunosuppressive protocol reduced PREBV further (P=0.0001), (4) in 78 transplantations treated with cyclosporine/antilymphocyte globulin/mofetil we observed only 10 acute rejections (P=0.0001), 10 PREBVs (P<0.00005), and no PTLDs compared with the cyclosporine/antilymphocyte globulin group (P=0.04).

CONCLUSIONS

Supplemental immunosuppression with mofetil protects against acute rejection. In combination with aciclovir, there is also a reduction in the number of PREBVs, apparently as a result of both direct viral prophylaxis and better rejection control, and in the incidence of EBV-induced PTLD.

X-Linked agammaglobulinemia patients are not infected with Epstein-Barr virus: implications for the biology of the virus

Epstein-Barr virus (EBV) infects both B lymphocytes and squamous epithelial cells in vitro, but the cell type(s) required to establish primary and persistent infection in vivo has not been definitively elucidated. The aim of this study was to investigate a group of individuals who lack mature B lymphocytes due to the rare heritable disorder X-linked agammaglobulinemia in order to determine the role of the B cell in the infection process. The results show that none of these individuals harbored EBV in their blood or throat washings. Furthermore, no EBV-specific memory cytotoxic T lymphocytes were found, suggesting that they had not undergone infection in the past. In contrast, 50% of individuals were found to carry human herpesvirus 6, showing that they are infectible by another lymphotropic herpesvirus. These results add weight to the theory that B lymphocytes, and not oropharyngeal epithelial cells, may be required for primary infection with EBV.

Immunization of common marmosets with Epstein-Barr virus (EBV) envelope glycoprotein gp340: effect on viral shedding following EBV challenge

Epstein-Barr virus (EBV), the cause of infectious mononucleosis, is involved in the pathogenesis of several human cancers, the highest frequency of association being found in undifferentiated nasopharyngeal carcinoma and endemic Burkitt's lymphoma. The development of animal models in which potential vaccines can be tested is important. EBV infection of the common marmoset, using the M81 strain originally derived from a patient with nasopharyngeal carcinoma, induces a carrier state in this animal. Persistent infection is characterized by the production of antibodies to viral antigens, and the secretion of EBV DNA into buccal fluids. Following immunization with envelope glycoprotein gp340 derived from a bovine papilloma virus expression vector, prior to EBV infection, viral DNA was detected significantly less frequently in the buccal fluids of immunized, than of nonimmunized, infected animals, indicating that although the carrier state had not been abolished, it had been altered. A reduction in virus load was also observed when offspring of seronegative, and on occasion seropositive, parents were immunized neonatally, before EBV challenge.

Epstein-Barr virus expression within keratinizing nasopharyngeal carcinoma

Three stages of maturation can be seen in keratinizing nasopharyngeal carcinomas. These stages are similar morphologically to basal cells, intermediate and superficial squamous cells seen in normal squamous epithelium. Taking advantage of such a diverse tumour cell population, 10 keratinizing nasopharyngeal carcinoma (NPC) were examined by in situ hybridization for the presence of latent Epstein-Barr Virus (EBV) using EBV encoded RNAs (EBERs) and by immunohistology for the presence of EBV early antigen-diffuse (EA-D) and the 350/220 kd membrane glycoprotein of the EBV. The basal cell-like tumour cells are mainly infected latently with the virus; viral replication was found in isolated intermediate squamous cells, whilst superficial squamous cells are largely depleted of all the viral markers. We used a control series of nonkeratinizing nasopharyngeal carcinomas composed of undifferentiated and poorly differentiated tumour cells and EBV latency was present in these tumours. Viral replication was detected by RT-PCR, in the undifferentiated tumours but viral replication was not seen by immunohistology. The possible relationship between EBV life cycle in these tumours and tumour cell differentiation is discussed in the light of these findings.

Epstein-Barr virus latent infection in vivo

Epstein-Barr virus (EBV) is an exclusively human herpes virus which is recognised as the causative agent of infectious mononucleosis and which is implicated in the aetiology of several cancers. However, it is particularly remarkable that this virus is harboured without causing symptoms for the lifetime of most immunocompetent adults. Virus and host have co-evolved over millions of years, achieving a balance between viral persistence and immune control. It is this dynamic equilibrium which is the focus of this review. The main site of viral persistence is within latently infected lymphocytes, although infectious virus is also released into saliva from productively infected cells in the oropharynx. In vitro, EBV efficiently transforms resting B cells to activated, perpetually dividing lymphoblasts. These express a repertoire of eight viral antigens, several of which have been found to be targets for cytotoxic T cell (CTL) responses in healthy carriers. Transformed lymphoblasts are susceptible to immune control in vivo, and are abundant only during primary infection or in individuals with impaired cell mediated immunity. Other types of viral latent infection have been identified in malignant cell lines, in which EBV expresses a more restricted range of antigens. These also may have their in vivo equivalents during natural infection in healthy carriers. It is likely that the virus evades elimination by the immune system by establishing infection in non-activated, relatively non immunogenic B cells, in which the main CTL target antigens are not expressed. Copyright 1997 John Wiley & Sons, Ltd.

Epstein-Barr virus (EBV) infection in infectious mononucleosis: virus latency, replication and phenotype of EBV-infected cells

Primary Epstein-Barr virus (EBV) infection may manifest itself as a benign lymphoproliferative disorder, infections mononucleosis (IM). EBV infection has been characterized in lymphoreticular tissues from nine patients with IM using the abundantly expressed EBV-encoded nuclear RNAs (EBERs) as a marker of latent infection. Expression of the virus-encoded nuclear antigen (EBNA) 2 and of the latent membrane protein (LMP) 1 was seen in variable proportions of cells in all cases. Double labelling revealed heterogeneous expression patterns of these proteins. Thus, in addition to cells revealing phenotypes consistent with latencies I (EBNA2-/LMP1-) and III (EBNA2+/LMP1+), cells displaying a latency II pattern (EBNA2-/LMP1+) were observed. Cells expressing EBNA2 but not LMP1 were also detected; whilst this may represent a transitory phenomenon, the exact significance of this observation is at present uncertain. EBER-specific in situ hybridization in conjunction with immunohistochemistry revealed expression of the EBERs mainly in B-lymphocytes, many of which showed features of plasma cell differentiation. By contrast, convincing evidence of latent EBV infection was not found in T-cells, epithelial or endothelial cells. Double-labelling immunohistochemistry revealed expression of the replication-associated BZLF1 protein in small lymphoid cells, often showing plasmacytoid differentiation. There was no unambiguous expression of this protein in other cell types. These results suggest that B-cells are the primary target of EBV infection and that plasma cells may be a source of infectious virus found in the saliva of IM patients.

Epstein-Barr virus: biology and disease

Epstein-Barr virus is a human herpes virus which, whilst found as a widespread asymptomatic infection, is also associated with certain tumours of lymphoid and epithelial origin including Burkitt's lymphoma (BL), immunoblastic lymphoma (IBL), Hodgkin's Disease (HD) and nasopharyngeal carcinoma (NPC). A unique characteristic of EBV is its ability to infect and transform primary resting B lymphocytes in vitro into permanently growing lymphoblastoid cell lines (LCLs); this effect is associated with constitutive expression of a limited set of viral genes. Interestingly, the pattern of EBV gene expression observed in LCLs in vitro is also a feature of IBLs, a tumour associated with immunosuppression. The other EBV associated tumours display a more restricted pattern of EBV latent protein expression. B cell lines can be activated in vitro into the virus replicative cycle, where a large number of viral genes associated with EBV DNA replication and virus assembly are synthesised. Whilst EBV can be detected in throat washings from seropositive individuals, the only in vivo situation where full virus replication can be reliably observed in hairy leukoplakia (HL), a benign lesion of lingual epithelium frequently found in AIDS patients. Thus, the relative contribution of lymphoid cells and epithelial cells to latent EBV infection/persistence vs replication in vivo remains controversial. Recent studies suggest that HL represents a focus of EBV replication in the absence of a truly latent infection and this supports the contention that EBV persistence resides in the lymphoid compartment. These aspects together with the role of EBV in oral diseases and the effect of certain EBV genes on the control of epithelial cell growth and differentiation will be discussed.

The ecology and pathology of Epstein-Barr virus

Epstein-Barr virus achieves its ubiquitous and uniform epidemiological distribution by a dual strategy of latency to guarantee lifelong persistence and intermittent replication to guarantee transmission. These two functions appear to dictate residence in different cell types: latency in B lymphocytes and replication in epithelial cells. Both of these cell compartments are potential sites for EBV-associated malignancies.

Substrate specificity of Epstein-Barr virus thymidine kinase

Purified recombinant protein encoded by the BXLF-I open reading frame of the Epstein-Barr virus genome has thymidine kinase activity. The substrate behaviors of various nucleosides toward this enzyme were tested. Halogenated deoxyuridines, zidovudine, and bromovinyldeoxyuridine are efficient substrates, while acyclovir and dihydroxypropylmethylguanine are relatively poor substrates for the Epstein-Barr virus thymidine kinase.

Epstein-Barr virus persistence and virus-associated tumours

The Epstein-Barr virus (EBV) has been implicated in the aetiology of many human lymphoid and epithelial malignancies. Although EBV is B lymphotropic in vitro, it has been hypothesised that oropharyngeal epithelium is important in primary EBV infection, replication, and persistence in vivo, and that infection of B lymphocytes is secondary. This hypothesis has been challenged by several recent studies. On the basis of current evidence, we propose that primary EBV infection and virus persistence are mediated through B lymphocytes, and that latent infection of epithelial cells is accidental and irrelevant to virus persistence, although important in the development of certain carcinomas. To what extent T cells are involved in EBV persistence remains uncertain. Clarification of the possible part played by EBV in the development of virus-associated tumours requires a better understanding of the mode of EBV persistence and the identification of the stage in the carcinogenic process at which EBV infection occurs.

Epstein-Barr virus and carcinomas

The Epstein-Barr virus, a human B lymphotropic herpes virus, is strongly associated with undifferentiated nasopharyngeal carcinoma, which is endemic in Southern China and North Africa. More recently, an association of the virus with certain other epithelial malignancies has been described, some of which are more common in Western countries. Also, the observation that oral hairy leukoplakia, an acquired immunodeficiency syndrome-related lesion of the tongue, supports Epstein-Barr virus replication in the epithelial cell compartment has further strengthened the notion that Epstein-Barr virus infection of epithelial cells may be an important phenomenon in vivo. The purpose of this article is to review the association of Epstein-Barr virus with nasopharyngeal carcinomas, to examine the evidence suggesting an association of the virus with other epithelial neoplasias and to discuss Epstein-Barr virus infection of non-neoplastic epithelial cells.

Analysis of Epstein-Barr virus infection in nasopharyngeal biopsies from a group at high risk of nasopharyngeal carcinoma

Although Epstein-Barr virus (EBV) is consistently associated with the epithelial malignancy nasopharyngeal carcinoma (NPC), it is not clear to what extent the normal virus carrier state involves infection of nasopharyngeal epithelium. We attempted to examine this question by screening 26 nasopharyngeal punch biopsies from EBV-carrying Chinese Malaysians who had presented with clinical symptoms possibly indicative of NPC, but in whom histological analysis of an adjacent biopsy had revealed no evidence of tumour. Assays included (i) in situ hybridization with 35S-labelled riboprobes specific for EBERs (rather than with BamHI W DNA probes which can give false-positive results); (ii) cDNA amplification across defined splice junctions of the EBNA1 and BamHI A transcripts expressed in latently-infected NPC cells and of the BHRF1 lytic-cycle transcript; and (iii) immunostaining for the immediate early lytic-cycle protein BZLF1. Of the 26 biopsies examined, all 23 showing normal nasopharyngeal histology were consistently negative for both latent and lytic-cycle markers. The other 3 cases were all positive for EBNA1 and BamHI A transcripts; these RNAs were almost certainly of tumour rather than normal-cell origin since these particular biopsies were the only ones to reveal localized foci of EBER-positive NPC cells; such biopsies were again negative for lytic-cycle markers. We provisionally conclude that EBV infection of the normal nasopharynx is not a regular feature of the virus carrier state and that screening nasopharyngeal biopsies for viral RNA markers of the latent cycle could be useful in NPC diagnosis.

Spontaneous outgrowth of Epstein-Barr virus-positive B-cell lines from circulating human B cells of different buoyant densities

Epstein-Barr virus (EBV) has potent cell-growth-transforming activity for human B lymphocytes in vitro, yet appears to persist in the circulating B-cell pool of virus carriers in vivo as a largely asymptomatic (i.e., non-growth-transforming) infection. The true nature of this infection, and the identity of the cells involved, remain to be determined. Studies of Lewin et al. (1987) have suggested (i) that the frequency of virus-infected cells in the circulating B-cell pool differs in different buoyant density fractions, being most abundant in the low-density population, and (ii) that rare virus-infected cells with the capacity for direct in vitro outgrowth to EBV-transformed cell lines are segregated within the high-density population. We have repeated this work using B-cell fractions from a much larger panel of asymptomatic virus carriers and find (i) that the incidence of virus-infected B cells is not significantly different between high- and low-density fractions, and (ii) that virus-infected cells from both fractions give rise to EBV-transformed cell lines in culture predominantly through a 2-step mechanism of virus replication and secondary infection rather than by direct outgrowth.

Epstein-Barr virus-infected B cells persist in the circulation of acyclovir-treated virus carriers

In this study, infectious Epstein-Barr virus (EBV) shedding in the oropharynx and numbers of virus-infected B cells in the blood have been monitored in long-term virus carriers receiving acyclovir (ACV) therapy for herpes zoster. Eleven patients on oral ACV were followed prospectively before, during and for 2 weeks after treatment. As expected, the low levels of EBV shedding in these virus carriers (measured as cord-blood lymphocyte transforming activity in throat washings) were eliminated during the period of ACV treatment and returned at later times. Over the same period, however, the frequency of virus-infected B cells in the blood (measured by spontaneous transformation in limiting dilution assay) remained completely unchanged. Regression assays showed that these same patients had normal levels of EBV-specific cytotoxic T-cell immunity, so that the in vivo persistence of virus-infected B cells could not be ascribed to a defect in T-cell surveillance. We infer that the in vivo half-life of the virus-infected B-cell pool in long-term virus carriers is measured in months rather than days. We further suggest that such persistence requires a novel form of virus:B-cell interaction distinct from the type of "latent" infection displayed by in vitro-transformed cells.