Differential responses to CD40 ligation among Burkitt lymphoma lines that are uniformly responsive to Epstein-Barr virus latent membrane protein 1

Ligation of CD40 on the surface of B cells induces multiple phenotypic effects, many of which are mimicked by the EBV latent membrane protein 1 (LMP1) through its interaction with downstream components of the CD40 signaling pathway. Because the effects of LMP1 have been most closely studied in human Burkitt Lymphoma (BL) cell lines retaining a tumor biopsy-like phenotype in vitro, we have examined the response of a panel of such lines to CD40 ligation. Two distinct patterns of response were observed that were unrelated to the surface level of CD40 or to the EBV genome status of the lines. Following exposure to either CD40-specific mAbs or the soluble trimeric ligand (sCD40L), high responder (HR) lines showed rapid aggregation, activation of NF-kappa B, up-regulation of cell surface markers ICAM-1/CD54 and Fas/CD95, and growth inhibition. Aggregation was seen at lower doses than those required to elicit the other effects. By contrast, low responder (LR) lines showed no detectable response to CD40 mAbs, while their responses to sCD40L were limited to activation of NF-kappa B and up-regulation of CD95 only. However, in transfection experiments, LMP1 uniformly induced the full spectrum of phenotypic effects in both HR and LR lines. We conclude that some BL cell lines show a highly restricted response to CD40 ligation but remain fully susceptible to LMP1.

Differential Responses to CD40 Ligation Among Burkitt Lymphoma Lines That Are Uniformly Responsive to Epstein-Barr Virus Latent Membrane Protein 1

Ligation of CD40 on the surface of B cells induces multiple phenotypic effects, many of which are mimicked by the EBV latent membrane protein 1 (LMP1) through its interaction with downstream components of the CD40 signaling pathway. Because the effects of LMP1 have been most closely studied in human Burkitt Lymphoma (BL) cell lines retaining a tumor biopsy-like phenotype in vitro, we have examined the response of a panel of such lines to CD40 ligation. Two distinct patterns of response were observed that were unrelated to the surface level of CD40 or to the EBV genome status of the lines. Following exposure to either CD40-specific mAbs or the soluble trimeric ligand (sCD40L), high responder (HR) lines showed rapid aggregation, activation of NF-κB, up-regulation of cell surface markers ICAM-1/CD54 and Fas/CD95, and growth inhibition. Aggregation was seen at lower doses than those required to elicit the other effects. By contrast, low responder (LR) lines showed no detectable response to CD40 mAbs, while their responses to sCD40L were limited to activation of NF-κB and up-regulation of CD95 only. However, in transfection experiments, LMP1 uniformly induced the full spectrum of phenotypic effects in both HR and LR lines. We conclude that some BL cell lines show a highly restricted response to CD40 ligation but remain fully susceptible to LMP1.

Epstein-Barr virus in Hodgkin's disease

Epstein-Barr virus (EBV), a B-lymphotropic herpesvirus widespread in human populations, is carried by most individuals as an asymptomatic lifelong infection. Much progress has been made in our understanding of virus infection/persistence, and in the role of the cytotoxic T lymphocyte response in control of that infection. This same virus is linked to several malignancies, including endemic Burkitt's lymphoma, post-transplant lymphoproliferative disease and to many cases of Hodgkin's disease (HD). Recent evidence showing that HD, like the other EBV-associated lymphomas, is of B-cell origin suggests that the pathogenesis of these malignancies may share more common ground than previously thought. The biology and cytotoxic T-cell control of primary and persistent EBV infection, and the links between EBV and all three lymphomas are reviewed. The expression of viral antigens in EBV-positive HD raises the possibility of developing tumour immunotherapy, using relevant components of the EBV-specific T-cell response; progress to date, and future prospects for immune control of EBV-positive HD are discussed.

Epstein-barr virus nuclear antigen 1 sequences in endemic and sporadic Burkitt's lymphoma reflect virus strains prevalent in different geographic areas

The Epstein-Barr virus (EBV) nuclear antigen EBNA1 is the only viral protein detectably expressed in virus genome-positive Burkitt's lymphoma (BL); recent work has suggested that viral strains with particular EBNA1 sequence changes are preferentially associated with this tumor and that, within a patient, the tumor-associated variant may have arisen de novo as a rare mutant of the dominant preexisting EBV strain (K. Bhatia, A. Raj, M. J. Gutierrez, J. G. Judde, G. Spangler, H. Venkatesh, and I. T. Magrath, Oncogene 13:177-181, 1996). In the present work we first study 12 BL patients and show that the virus strain in the tumor is identical in EBNA1 sequence and that it is matched at several other polymorphic loci to the dominant strain rescued in vitro from the patient's normal circulating B cells. We then analyze BL-associated virus strains from three different geographic areas (East Africa, Europe, and New Guinea) alongside virus isolates from geographically matched control donors by using sequence changes in two separate regions of the EBNA1 gene (N-terminal codons 1 to 60 and C-terminal codons 460 to 510) to identify the EBNA1 subtype of each virus. Different geographic areas displayed different spectra of EBNA1 subtypes, with only limited overlap between them; even type 2 virus strains, which tended to be more homogeneous than their type 1 counterparts, showed geographic differences at the EBNA1 locus. Most importantly, within any one area the EBNA1 subtypes associated with BL were also found to be prevalent in the general population. We therefore find no evidence that Burkitt lymphomagenesis involves a selection for EBV strains with particular EBNA1 sequence changes.

A re-evaluation of the frequency of CD8+ T cells specific for EBV in healthy virus carriers

EBV is a gammaherpesvirus that can establish both nonproductive (latent) and productive (lytic) infections within the cells of its host. Although T cell responses to EBV latent proteins have been well characterized, little is known about the importance of responses to lytic proteins in long term virus carriers. Here we have compared the frequencies of CD8+ T cells specific for EBV latent and lytic Ags in healthy virus carriers, using three techniques: limiting dilution analysis, enzyme-linked immunospot assay, and FACS staining with tetrameric MHC-peptide complexes. T cells specific for EBV lytic protein epitopes were readily detectable in all donors and were usually more abundant than those specific for latent epitopes. We infer that direct T cell control of viral replicative lesions is maintained in long term carriers of EBV and is an important component of the immune response to this virus. Estimates of CD8+ T cell frequencies varied considerably according to methodology; values obtained from MHC-peptide tetramer staining were, on the average, 4.4-fold higher than those obtained from enzyme-linked immunospot assays, which were, in turn, on the average, 5.3-fold higher than those obtained from limiting dilution analysis. Tetramer staining showed that as many as 5.5% circulating CD8+ T cells in a virus carrier were specific for a single EBV lytic protein epitope. Such values are much greater than previously imagined and illustrate how antigenic challenge from a persistent herpesvirus can influence the composition of the host's CD8+ T cell pool.

A Re-Evaluation of the Frequency of CD8+ T Cells Specific for EBV in Healthy Virus Carriers

EBV is a gammaherpesvirus that can establish both nonproductive (latent) and productive (lytic) infections within the cells of its host. Although T cell responses to EBV latent proteins have been well characterized, little is known about the importance of responses to lytic proteins in long term virus carriers. Here we have compared the frequencies of CD8+ T cells specific for EBV latent and lytic Ags in healthy virus carriers, using three techniques: limiting dilution analysis, enzyme-linked immunospot assay, and FACS staining with tetrameric MHC-peptide complexes. T cells specific for EBV lytic protein epitopes were readily detectable in all donors and were usually more abundant than those specific for latent epitopes. We infer that direct T cell control of viral replicative lesions is maintained in long term carriers of EBV and is an important component of the immune response to this virus. Estimates of CD8+ T cell frequencies varied considerably according to methodology; values obtained from MHC-peptide tetramer staining were, on the average, 4.4-fold higher than those obtained from enzyme-linked immunospot assays, which were, in turn, on the average, 5.3-fold higher than those obtained from limiting dilution analysis. Tetramer staining showed that as many as 5.5% circulating CD8+ T cells in a virus carrier were specific for a single EBV lytic protein epitope. Such values are much greater than previously imagined and illustrate how antigenic challenge from a persistent herpesvirus can influence the composition of the host’s CD8+ T cell pool.

Accessing Epstein-Barr virus-specific T-cell memory with peptide-loaded dendritic cells

The conventional means of studying Epstein-Barr virus (EBV)-induced cytotoxic T-lymphocyte (CTL) memory, by in vitro stimulation with the latently infected autologous lymphoblastoid cell line (LCL), has important limitations. First, it gives no information on memory to lytic cycle antigens; second, it preferentially amplifies the dominant components of latent antigen-specific memory at the expense of key subdominant reactivities. Here we describe an alternative approach, based on in vitro stimulation with epitope peptide-loaded dendritic cells (DCs), which allows one to probe the CTL repertoire for any individual reactivity of choice; this method proved significantly more efficient than stimulation with peptide alone. Using this approach we first show that reactivities to the immunodominant and subdominant lytic cycle epitopes identified by T cells during primary EBV infection are regularly detectable in the CTL memory of virus carriers; this implies that in such carriers chronic virus replication remains under direct T-cell control. We further show that subdominant latent cycle reactivities to epitopes in the latent membrane protein LMP2, though rarely undetectable in LCL-stimulated populations, can be reactivated by DC stimulation and selectively expanded as polyclonal CTL lines; the adoptive transfer of such preparations may be of value in targeting certain EBV-positive malignancies.

T cell selection during the evolution of CD8+ T cell memory in vivo

Memory T cell responses are frequently highly restricted in terms of receptor usage. How and when such clonotypic dominance is established remains poorly understood. Here we have investigated the evolution of the T cell responses to an epitope from Epstein-Barr virus (EBV), (FLRGRAYGL), by analyzing TCR use of clones specific for this epitope, derived from peripheral blood mononuclear cells taken from individuals early during primary EBV infection and up to 3 years later. We show that, in a given individual, particular T cell clonotypes are selected early during the primary response to this epitope and that the same clonotypes dominate the late memory response. In one individual direct analysis of HLA-B8-restricted FLRGRAYGL-specific T cells, isolated from peripheral blood lymphocytes taken during primary EBV infection using a tetrameric MHC-peptide complex, confirmed the early selection of the dominant clonotypes.

HLA-B27 Subtype Polymorphism and CTL Epitope Choice: Studies with EBV Peptides Link Immunogenicity with Stability of the B27:Peptide Complex

HLA-B27-restricted CTL responses to EBV are principally directed against two of the EBV nuclear Ags, EBNAs 3B and 3C. We have previously described a target epitope derived from EBNA 3C (residues 258–266, sequence RRIYDLIEL) that is immunodominant in the context of at least three different B27 subtypes, including B*2705 and B*2702. In this study, we show that this peptide binds well to B*2705 and B*2702 in a cell surface binding assay, and that the two B27:peptide complexes are relatively stable, with t1/2 of 20 and 37 h, respectively. We now identify another B27-restricted epitope derived from EBNA 3B (residues 243–253, sequence RRARSLSAERY), which again accords well with the B*2705/B*2702 consensus motifs, having an arginine residue at position 2 and a tyrosine residue at the carboxyl terminus. In this case, five of five B*2702-positive donors respond to the epitope, whereas there was no response in any B*2705-positive donor studied. This peptide binds at least as well to B*2705 as to its restriction element B*2702; however, the two class I:peptide complexes show marked differences in stability, with t1/2 of 9 and 42 h, respectively. Thus, the stability of B27:peptide complexes can vary markedly between different B27 subtypes in ways that may not be apparent from cell surface binding assays and cannot be predicted from currently known peptide consensus motifs, yet which may critically influence CTL epitope choice.

HLA-B27 subtype polymorphism and CTL epitope choice: studies with EBV peptides link immunogenicity with stability of the B27:peptide complex

HLA-B27-restricted CTL responses to EBV are principally directed against two of the EBV nuclear Ags, EBNAs 3B and 3C. We have previously described a target epitope derived from EBNA 3C (residues 258-266, sequence RRIYDLIEL) that is immunodominant in the context of at least three different B27 subtypes, including B*2705 and B*2702. In this study, we show that this peptide binds well to B*2705 and B*2702 in a cell surface binding assay, and that the two B27:peptide complexes are relatively stable, with t1/2 of 20 and 37 h, respectively. We now identify another B27-restricted epitope derived from EBNA 3B (residues 243-253, sequence RRARSLSAERY), which again accords well with the B*2705/B*2702 consensus motifs, having an arginine residue at position 2 and a tyrosine residue at the carboxyl terminus. In this case, five of five B*2702-positive donors respond to the epitope, whereas there was no response in any B*2705-positive donor studied. This peptide binds at least as well to B*2705 as to its restriction element B*2702; however, the two class I:peptide complexes show marked differences in stability, with t1/2 of 9 and 42 h, respectively. Thus, the stability of B27:peptide complexes can vary markedly between different B27 subtypes in ways that may not be apparent from cell surface binding assays and cannot be predicted from currently known peptide consensus motifs, yet which may critically influence CTL epitope choice.

Antigen presenting phenotype of Hodgkin Reed-Sternberg cells: analysis of the HLA class I processing pathway and the effects of interleukin-10 on epstein-barr virus-specific cytotoxic T-cell recognition

Approximately 40% of Hodgkin's disease (HD) cases in Western countries carry Epstein-Barr virus (EBV) in the malignant Hodgkin-Reed-Sternberg (H-RS) cells. HLA class I-restricted cytotoxic T lymphocytes (CTLs) with specificity for viral antigens expressed in H-RS cells therefore have therapeutic potential. However, a prerequisite for CTL therapy is that the tumor target be capable of processing and presenting endogenously expressed antigens via the transporter associated with antigen processing (TAP)-dependent HLA class I pathway. We have assessed the antigen-presenting phenotype of H-RS cells in two ways. First, immunohistochemical analysis of 38 HD biopsies showed that H-RS cells were uniformly TAP1/TAP2-positive and expressed HLA class I in the majority (18 of 24, 75%) of EBV-positive cases compared with only 4 of 14 (29%) of EBV-negative cases. Second, using a panel of 5 H-RS cell lines, we showed that 4 of 5 could process and present EBV proteins to HLA class I-restricted EBV-specific CTL clones. Others have reported that human interleukin-10 (IL-10), which is expressed by H-RS cells in the majority of EBV-positive HD cases, can abrogate CTL recognition in some circumstances. However, IL-10 pretreatment of the H-RS lines or of the EBV-specific CTLs had no such effect in this system. These results support the possibility that EBV-specific CTLs may be used to treat virus-positive HD.

Direct visualization of antigen-specific CD8+ T cells during the primary immune response to Epstein-Barr virus In vivo

Primary infection with virus can stimulate a vigorous cytotoxic T cell response. The magnitude of the antigen-specific component versus the bystander component of a primary T cell response remains controversial. In this study, we have used tetrameric major histocompatibility complex-peptide complexes to directly visualize antigen-specific cluster of differentration (CD)8+ T cells during the primary immune response to Epstein-Barr virus (EBV) infection in humans. We show that massive expansion of activated, antigen-specific T cells occurs during the primary response to this virus. In one individual, T cells specific for a single EBV epitope comprised 44% of the total CD8+ T cells within peripheral blood. The majority of the antigen-specific cells had an activated/memory phenotype, with expression of human histocompatibility leukocyte antigen (HLA) DR, CD38, and CD45RO, downregulation of CD62 leukocyte (CD62L), and low levels of expression of CD45RA. After recovery from AIM, the frequency of antigen-specific T cells fell in most donors studied, although populations of antigen-specific cells continued to be easily detectable for at least 3 yr.

Epidemiology of infection with Epstein-Barr virus types 1 and 2: lessons from the study of a T-cell-immunocompromised hemophilic cohort

In apparent contrast to earlier work on Epstein-Barr virus (EBV) carriage in the general Caucasian population, in vitro virus isolations from human immunodeficiency virus (HIV)-positive male homosexual cohorts have shown frequent examples of multiple EBV infection and an overall prevalence of type 2 EBV strains exceeding 30%. Here we ask to what extent these findings might hold true in another T-cell-immunocompromised cohort, HIV-positive hemophilic patients. Resident EBV strains were rescued within lymphoblastoid cell lines derived from the blood and throat washings of 39 such individuals, using the same in vitro protocols of virus isolation as for the homosexual cohort. A mean of 19 independent cell lines was made per patient, and in each case the resident virus was characterized by PCR-based viral genomic analysis and by immunoblotting to reveal the viral "EBNAprint." By these criteria a significant proportion (14 of 39) of the hemophilic cohort carried more than one EBV strain, suggesting that T-cell impairment does indeed sensitize virus carriers to reinfection with new strains of exogenously transmitted virus. However, the overall incidence of type 2 EBV infection was 10%, which is close to that observed in the earlier work with healthy carriers and substantially lower than that seen in HIV-positive homosexuals. We infer that type 2 EBV is relatively rare in the general Caucasian population but has become endemic in the homosexual community.

Epstein-Barr virus: LMP1 masquerades as an active receptor

The Epstein-Barr virus protein LMP1 is essential for transformation of resting B cells by the virus, but how it works is unclear. Recent results suggest that LMP1 acts as a constitutively active receptor that shares certain characteristics with members of the tumour necrosis factor receptor superfamily.

Relationship between peptide binding and T cell epitope selection: a study with subtypes of HLA-B27

The effect of HLA-B27 polymorphism on antigen presentation was analysed by comparing the binding of three Epstein-Barr virus-derived peptide epitopes to HLA-B27 subtypes with their immunogenicity and antigenicity in the context of these subtypes. The effect of altering the major anchor residue Arg2 on binding or on recognition by peptide-specific cytotoxic T lymphocytes (CTL) was also examined. The three peptides bound significantly to all the B*2701-B*2706 subtypes. This did not correlate with the peptides being immunogenic or recognized by specific CTL in the context of only particular subtypes. In addition, of the three viral epitopes tested, those that were immunogenic in B*2702- or B*2705-restricted responses bound to these subtypes less efficiently than one peptide that was immunogenic only in the B*2704 context. Thus, among several potentially immunogenic peptides from the same virus, the antiviral response is not necessarily directed against the one that binds best to the restricting subtype. These results indicate that HLA-B27 polymorphism influences antigen presentation in ways other than simply peptide affinity. Synthetic analogues lacking the canonical Arg2 motif of HLA-B27-bound peptides, even when binding much worse to the restricting subtype, were recognized equally by CTL specific for the parental peptide. This indicates that Arg2 is not required to maintain the structure of the epitope. The implications of these results for pathogenetic models of HLA-B27-associated disease are discussed.

Immediate early and early lytic cycle proteins are frequent targets of the Epstein-Barr virus-induced cytotoxic T cell response

Epstein-Barr virus (EBV), a human gamma-herpesvirus, can establish both nonproductive (latent) and productive (lytic) infections. Although the CD8+ cytotoxic T lymphocyte (CTL) response to latently infected cells is well characterized, very little is known about T cell controls over lytic infection; this imbalance in our understanding belies the importance of virus-replicative lesions in several aspects of EBV disease pathogenesis. The present work shows that the primary CD8+ CTL response to EBV in infectious mononucleosis patients contains multiple lytic antigen-specific reactivities at levels at least as high as those seen against latent antigens; similar reactivities are also detectable in CTL memory. Clonal analysis revealed individual responses to the two immediate early proteins BZLF1 and BRLF1, and to three (BMLF1, BMRF1, and BALF2) of the six early proteins tested. In several cases, the peptide epitope and HLA-restricting determinant recognized by these CTLs has been defined, one unusual feature being the number of responses restricted through HLA-C alleles. The work strongly suggests that EBV-replicative lesions are subject to direct CTL control in vivo and that immediate early and early proteins are frequently the immunodominant targets. This contrasts with findings in alpha- and beta-herpesvirus systems (herpes simplex, cytomegalovirus) where viral interference with the antigen-processing pathway during lytic infection renders immediate early and early proteins much less immunogenic. The unique capacity of gamma-herpesvirus to amplify the viral load in vivo through a latent growth-transforming infection may have rendered these agents less dependent upon viral replication as a means of successfully colonizing their hosts.

Conserved CTL epitopes within EBV latent membrane protein 2: a potential target for CTL-based tumor therapy

In healthy virus carriers, EBV is subject to strong CTL responses that principally target the EBV nuclear Ag (EBNA) 3A, 3B, 3C subset of virus proteins. In vitro-reactivated CTLs of this kind have proved very effective in treating EBV-positive immunoblastic lymphoma, a malignancy that expresses the full range of virus proteins. However, targeting other EBV-positive tumors will require CTLs that recognize some of the subdominant viral Ags since in nasopharyngeal carcinoma and EBV-positive Hodgkin's disease, EBNA1, latent membrane protein (LMP) 1, and LMP2 are the only virus proteins present. Studying healthy virus carriers (Caucasian and Chinese), we identified five CTL target epitopes in LMP2 restricted through HLA alleles particularly common in the southern Chinese population, which is most at risk for nasopharyngeal carcinoma (HLA-A2, 50%; A11, 50%; A24, 30%; and B40, 32%). Furthermore, we analyzed the effect of HLA subtype polymorphism, especially in the context of A2 for which four subtypes are present at significant frequency in the Chinese population. As to virus polymorphism, LMP2 epitope sequences (in contrast to EBNA 3A, 3B, and 3C epitopes) were shown to be antigenically conserved among EBV isolates from different world populations, including viruses present in nasopharyngeal carcinoma and Hodgkin's disease biopsy samples. Thus, nasopharyngeal carcinoma and Hodgkin's disease are predicted to express LMP2 proteins that contain conserved CTL target epitopes restricted through common HLA alleles; boosting responses to these epitopes could form the basis of a CTL-based therapy for these malignancies.

Conserved CTL epitopes within EBV latent membrane protein 2: a potential target for CTL-based tumor therapy

In healthy virus carriers, EBV is subject to strong CTL responses that principally target the EBV nuclear Ag (EBNA) 3A, 3B, 3C subset of virus proteins. In vitro-reactivated CTLs of this kind have proved very effective in treating EBV-positive immunoblastic lymphoma, a malignancy that expresses the full range of virus proteins. However, targeting other EBV-positive tumors will require CTLs that recognize some of the subdominant viral Ags since in nasopharyngeal carcinoma and EBV-positive Hodgkin's disease, EBNA1, latent membrane protein (LMP) 1, and LMP2 are the only virus proteins present. Studying healthy virus carriers (Caucasian and Chinese), we identified five CTL target epitopes in LMP2 restricted through HLA alleles particularly common in the southern Chinese population, which is most at risk for nasopharyngeal carcinoma (HLA-A2, 50%; A11, 50%; A24, 30%; and B40, 32%). Furthermore, we analyzed the effect of HLA subtype polymorphism, especially in the context of A2 for which four subtypes are present at significant frequency in the Chinese population. As to virus polymorphism, LMP2 epitope sequences (in contrast to EBNA 3A, 3B, and 3C epitopes) were shown to be antigenically conserved among EBV isolates from different world populations, including viruses present in nasopharyngeal carcinoma and Hodgkin's disease biopsy samples. Thus, nasopharyngeal carcinoma and Hodgkin's disease are predicted to express LMP2 proteins that contain conserved CTL target epitopes restricted through common HLA alleles; boosting responses to these epitopes could form the basis of a CTL-based therapy for these malignancies.

Human cytotoxic T lymphocyte responses to Epstein-Barr virus infection

Epstein-Barr virus (EBV) provides one of the most informative systems with which to study cytotoxic T lymphocyte (CTL) responses in humans. The virus establishes a highly immunogenic growth-transforming infection of B lymphocytes, associated with the coordinate expression of six virus-coded nuclear antigens (EBNAs 1, 2, 3A, 3B, 3C, -LP) and two latent membrane proteins (LMPs 1 and 2). This elicits both primary and memory CT8+ CTL responses that are markedly skewed toward HLA allele-specific epitopes drawn from the EBNA3A, 3B, 3C subset of latent proteins, with reactivities to other antigens being generally much less frequent. This hierarchy of immunodominance among the different latent proteins may at least partly reflect their differential accessibility to the HLA class I-processing pathway. Furthermore, CTLs to some of the immunodominant epitopes involve highly conserved T cell receptor (TCR) usage, a level of focusing which evidence suggests could have immunopathological consequences from cross-reactive recognition of other target structures. EBV is associated with a range of human tumors, and there is increasing interest in the possibility of targeting such malignancies using virus-specific CTLs. The dramatic reversal of EBV-driven lymphoproliferations in bone marrow transplant patients following CTL infusion demonstrates the potential of this approach, and here we discuss prospects for its extension to other EBV-positive tumors in which the immunodominant EBNA3A, 3B, 3C proteins are not expressed.

Epitope focusing in the primary cytotoxic T cell response to Epstein-Barr virus and its relationship to T cell memory

The relationship between primary and memory cytotoxic T lymphocyte (CTL) responses, and the factors influencing entry into memory, are poorly understood. Here we address this in the context of Epstein-Barr virus (EBV), a persistent human herpesvirus in which memory CTL responses in long-term virus carriers are highly focused on epitopes preferentially drawn from just three of the eight available virus latent proteins, EBNAs 3A, 3B, and 3C. To determine whether this unusual level of focusing is a consequence of long-term virus challenge, we carried out a detailed analysis of EBV antigen/epitope specificities in the primary virus-induced CTL response in 10 infectious mononucleosis (IM) patients of different HLA types. Primary effectors, studied in ex vivo assays and by limiting dilution cloning in vitro, were again highly skewed toward a small number of viral epitopes, almost all derived from the EBNA3 proteins, with CTL to the immunodominant epitope accounting for at least 1% of the circulating CD8+ IM T cell pool. This is the first unequivocal demonstration of an EBV-specific CD8+ CTL response in IM. Prospective studies on individual patients showed that, whereas all of the EBV reactivities found in CTL memory had been detectable earlier during primary infection, the memory population was not simply a scaled down version of the primary response. In particular (a) differences in the relative frequencies of CTL to immunodominant versus subdominant epitopes appeared to be much less marked in memory than in primary populations, and (b) we found at least one clear example in which a significant virus-specific reactivity within the primary response was never detectable in memory.

VH and VL gene analysis in sporadic Burkitt's lymphoma shows somatic hypermutation, intraclonal heterogeneity, and a role for antigen selection

Tumor cell lines and one tumor biopsy from seven cases of Epstein-Barr virus (EBV) genome-negative sporadic Burkitt's lymphoma (BL) have been investigated for usage and mutational pattern of Ig variable region genes. The VH genes were derived from the VH 3 (one) and VH4 (six) families and both the IgM-positive (six) and the IgA-positive (one) were all mutated from their germline counterparts. The VL genes were derived from V kappa 1 (one), V kappa 3 (one), V lambda 1 (four), and V lambda 2 (one) families and were also somatically hypermutated. Biopsy material from one of the IgM-positive cases showed VH and VL sequences that matched the derived cell line, with additional intraclonal sequence heterogeneity, indicating that the tumor cells had undergone posttranformation somatic mutation. Mutational patterns in V(H) genes did not show a conventional role for antigen in selecting tumor cell sequences. In contrast, patterns in VL sequences were consistent with a role for antigen in five of seven cases. The pattern of extensive scattered somatic hypermutation and intraclonal variation is similar to that in VH sequences of EBV genome-positive endemic BL, although the degree of mutational activity is less. These common features indicate that B cells involved in the two variants of BL may share a common clonal history.

Analysis of Epstein-Barr virus gene polymorphisms in normal donors and in virus-associated tumors from different geographic locations

While Epstein-Barr virus (EBV) infection is associated with the development of certain lymphoid and epithelial tumors, the role of the virus in the pathogenesis of these malignancies remains unknown. It has been suggested that EBV strain variation may contribute to tumor development. Two major strains of EBV, type 1 and type 2, have been identified on the basis of genetic polymorphisms and other minor genetic variations give rise to distinct EBV isolates. We analyzed EBV strain variation in healthy individuals and compared them with EBV isolates present in lymphoid and epithelial neoplasms from the same geographic regions. In particular, the incidence of the 30-bp latent membrane protein (LMP1) gene deletion, recently implicated in the development of more aggressive forms of virus-positive lymphomas and Hodgkin's disease [HD], was examined in the normal population. While a preferential association of the LMP1 deletion with the type 2 strain of EBV was observed, there was no increased incidence of virus isolates carrying this deletion in HD, Burkitt's lymphoma, or virus-associated carcinomas compared with the appropriate normal population. A polymorphism in the BamHI F region of the EBV genome, previously identified in Chinese populations, was found at increased incidence in European HD biopsies. Overall, we found that most of the EBV gene polymorphisms detected in EBV isolates from healthy virus carriers occurred with similar frequency in virus-associated tumors from the same geographical region.

Cytostatic effect of Epstein-Barr virus latent membrane protein-1 analyzed using tetracycline-regulated expression in B cell lines

Tetracycline-regulated vectors were used to obtain inducible expression in stable transfected B cell lines of two Epstein-Barr virus (EBV) latent genes, LMP1 and EBNA2. The transfected genes were tightly repressed by low, nontoxic concentrations of tetracycline (< or = 1 microgram/ml) and, following removal of tetracycline, were induced to levels comparable to or up to 3x that of EBV-transformed normal lymphoblastoid cell lines. In transfected DG75 cells, induced expression of LMP1, but not of EBNA2, led to the expected upregulation of various cell surface markers, including: CD40, CD54, CD58, and HLA class I.A novel observation was that both LMP1 and EBNA2 independently caused the downregulation of surface IgM, an effect mirrored in EBV-positive Burkitt lymphoma lines undergoing phenotypic drift during the transition from latency I to latency III in which both LMP1 and EBNA2 are upregulated. Most remarkably, induced LMP1 expression almost completely inhibited cell growth for 4 to 5 days, after which the cells recovered a limited proliferative capacity. The cytostatic effect of LMP1 was observed in all three B cell lines studied: DG75, BJAB, and Akata. Further analysis showed that induction of LMP1 coincided with a reduction in the levels of c-myc, and that the cytostatic effect was due to an accumulation of cells at the G2/M phase of the cell cycle. These data suggest a novel function for the LMP1 oncogene in controlling the proliferation of EBV-infected cells by regulating progress through G2/M phase.

Isolation of intertypic recombinants of Epstein-Barr virus from T-cell-immunocompromised individuals

All wild-type isolates of Epstein-Barr virus (EBV) analyzed to date for allelic polymorphisms of the nuclear antigen EBNA2 gene (in the BamHI YH region of the genome) and of the EBNA3A,-3B, -3C genes (tandemly arranged in the BamHI E region) have proved either uniformly type 1 or uniformly type 2 at all four loci. The absence of detectable intertypic recombination in the wild probably reflects the rarity with which individual carriers, and certainly individual target cells, become coinfected with both virus types. Studying a group of human immunodeficiency virus-positive T-cell-immunocompromised patients known to be at enhanced risk of multiple EBV infections, we have isolated intertypic EBV recombinants from 2 of 40 patients analyzed. These recombinants, whose in vitro transforming capacity appeared at least equal to that of type 1 strains, carried a type 1 EBNA2 allele and type 2 EBNA3A,-3B, and -3C alleles. This was clearly demonstrable at the DNA level by PCR amplification using type-specific primer-probe combinations and was confirmed at the protein level (for EBNA2 and EBNA3C) by immunoblotting with type-specific antibodies. In one patient, the recombinant appeared to be the predominant strain, being the virus most commonly rescued by in vitro transformation both from the blood and from the throat washings on two separate occasions 20 months apart. A regular type 1 virus strain was also present in this individual, but this was not related to the recombinant since the two viruses carried type 1 EBNA2 genes with different patterns of variance from the B95.8 prototype sequence. In the other patient, recombinants were isolated on one occasion from the blood and on a separate occasion, 21 months later, from the throat; these recombinants were almost certainly related, being identical at several genomic polymorphisms and differing only in one facet of the "EBNAprint," the size of the EBNA1 protein. Three different type 1 viruses were also isolated from this patient, two of which carried EBNA2 genes with the same pattern of sequence variation from B95.8 as the recombinant; however, since this is a fairly common pattern of variance, the relationship of these viruses to the recombinant remains an open question. We infer that intertypic recombinants of EBV are not uncommon in HIV-positive T-cell-immunocompromised patients, that they arise in such individuals as a consequence of their increased frequency of mixed-type infections, and that they will prove capable of efficient transmission in the human population.

Transporter (TAP)-independent processing of a multiple membrane-spanning protein, the Epstein-Barr virus latent membrane protein 2

Antigen presentation to CD8+ cytotoxic T lymphocytes (CTL) usually involves proteolytic cleavage of antigen in the cytosol and the delivery of epitope peptides onto major histocompatibility complex class I molecules in the endoplasmic reticulum (ER) via the heterodimeric peptide transporter TAP1/TAP2. In the few exceptional cases where TAP-independent presentation of an endogenously expressed protein has been observed, the epitope-containing domain of the protein either has naturally accessed or has been directed into the ER lumen where it is thought to become susceptible to ER proteases. Here, we describe a novel example of TAP-independent processing involving the Epstein-Barr virus (EBV) latent membrane protein LMP2, a multiple membrane-spanning protein with minimal projection into the ER. Expression of LMP2 in the TAP-T2 cell line, whether from the resident EBV genome or from a recombinant vaccinia virus vector vacc-LMP2, rendered the cells sensitive to recognition by CTL clones specific for two HLA-A2.1-restricted peptide epitopes, LMP2 329-337 or 426-434. Vacc-LMP2-mediated sensitization to lysis required expression of the antigen de novo in T2 cells and was blocked by brefeldin A. In the same experiments, two other EBV-specific CTL epitopes, one derived from LMP2 but restricted through a different HLA allele (A11), the other restricted through A2.1 but derived from a different viral protein (BMLF1), did not display TAP-independent processing. The results are discussed in relation to the unusual topology of LMP2 in the membrane and the position of the epitope peptides within that structure.

Frequency of multiple Epstein-Barr virus infections in T-cell-immunocompromised individuals

The Epstein-Barr virus (EBV) carrier state is characterized by latent infection of the general B-cell pool and by chronic virus replication at oropharyngeal sites. In Caucasian populations, most healthy carriers seem to harbor one dominant transforming virus strain, usually of type I rather than type 2, which persists over time and is detectable both in the blood and in the throat. This finding implies that once the virus carrier state is established, both viral reservoirs are largely if not completely protected from infection with additional strains. However, it is not known which facets of the immune response offer that protection. Here we address this question by a detailed study of EBV carriage in patients T-cell immunocompromised as a result of chronic human immunodeficiency virus (HIV) infection. Resident EBV strains were rescued from blood and from throat washings by using an in vitro transformation assay which aims to minimize bias toward faster-growing transformants; in this way, a mean of 16 independent isolations were made from each of 35 HIV-positive (predominantly male homosexual) patients. These virus isolates were characterized first at the DNA level by PCR amplification across type-specific polymorphisms in the EBNA2 and EBNA3C genes and across the 30-bp deletion and 33-bp repeat loci in the LMP1 gene and then at the protein level by immunoblotting for the strain-specific "EBNAprint" of EBNA1, -2, and -3C molecular weights. By these criteria, 18 of 35 patients harbored only one detectable EBV strain, usually of type 1, as do healthy carriers. However, the other 17 patients showed clear evidence of multiple infection with different EBV strains. In eight cases these strains were of the same type, again usually type 1, and were more often found coresident in throat washings than in the blood. By contrast, a further nine patients gave evidence of coinfection with type 1 and type 2 strains, and in these cases both virus types were detectable in the blood as well as in the throat. Immunological assays on these HIV-positive patients as a group showed a marked impairment of T-cell responses, reflected in reduced levels of EBV-specific cytotoxic T-cell memory, but an elevation of humoral responses, reflected in raised antibody titers to the EBV envelope glycoprotein gp340 and by the maintenance of virus neutralizing antibodies in serum. We infer that selective impairment of the T-cell system predisposes the host to infection with additional exogenously transmitted EBV strains.

Large clonal expansions of CD8+ T cells in acute infectious mononucleosis

Primary infection with Epstein-Barr virus often results in the clinical syndrome of acute infectious mononucleosis (glandular fever). This illness is characterized by a striking lymphocytosis, the nature of which has been controversial. We show that large monoclonal or oligoclonal populations of CD8+ T cells account for a significant proportion of the lymphocytosis and provide molecular evidence that these populations have been driven by antigen. The results suggest that the selective and massive expansion of a few dominant clones of CD8+ T cells is an important feature of the primary response to this virus.

Cytotoxic T lymphocyte responses to Epstein-Barr virus

Epstein-Barr virus induces a potent cytotoxic T lymphocyte response in man that is preferentially directed towards a particular subset of the virus latent cycle antigens; the immunodominance of these proteins, apparent in both primary and memory responses, may reflect some differential access to the HLA class I processing pathway. Effector cells recognizing these immunodominant antigens can reverse the growth of virus-induced lymphoproliferative lesions in immunosuppressed patients; however, responses to some of the subdominant latent proteins will be needed to target other virus-positive tumours.

A recombinant adenovirus expressing an Epstein-Barr virus (EBV) target antigen can selectively reactivate rare components of EBV cytotoxic T-lymphocyte memory in vitro

While the bulk of a virus-induced cytotoxic T-lymphocyte (CTL) response may focus on a few immunodominant viral antigens, in certain tumor virus systems the detectability of clones recognizing other, subdominant antigens can assume particular importance. By using the human CTL response to Epstein-Barr virus (EBV) as a model system, here we show that even rare components of virus-specific memory can be selectively reactivated in vitro when the relevant target antigen is expressed in autologous stimulator cells from a recombinant adenovirus (RAd) vector. We generated a replication-deficient adenovirus, RAd-E3C, which in skin fibroblast cultures expressed the EBV nuclear antigen EBNA3C at a 10- to 100-fold-higher level than that naturally present in EBV-transformed lymphoblastoid cell lines (LCLs). Initial experiments with a donor whose polyclonal CTL response to LCL stimulation contained a strong EBNA3C-specific component showed that these CTLs could be efficiently reactivated by in vitro stimulation either with RAd-E3C-infected fibroblasts or with RAd-E3C-infected peripheral blood mononuclear cells. Then we studied donors whose responses to LCL stimulation contained little if any detectable EBNA3C reactivity but were dominated by clones recognizing other EBV target antigens; in vitro stimulation with RAd-E3C-infected peripheral blood mononuclear cells selectively reactivated EBNA3C-specific CTL clones from these individuals, with the epitope specificities of responses subsequently identified at the peptide level. This RAd-based approach could be applied more generally to screen for human CTL responses against any candidate target antigen expressed by tumor cells.

In vitro production of stable Epstein-Barr virus-positive epithelial cell clones which resemble the virus:cell interaction observed in nasopharyngeal carcinoma

The interaction of Epstein-Barr virus (EBV) with epithelial cells and the consequent role of the virus in the aetiology of undifferentiated nasopharyngeal carcinoma (NPC) is poorly understood. One important obstacle to work in this area has been the lack of an epithelial cell culture system in which EBV is stably maintained. Using a previously described approach in which CR2-transfected epithelial cells (SVK-CR2) are rendered susceptible to transient EBV infection (Li et al., Nature 356, 347, 1992), we now demonstrate that the pattern of EBV latent gene transcription in these acutely infected epithelial cells differs from that observed in virus-infected primary B cells. In addition, some of these epithelial cells spontaneously entered the EBV lytic cycle. By cloning Akata virus-infected SVK-CR2 cells we generated two stable lines which remained EBV positive for more than 1.5 years at which time further subclones were isolated. These cloned lines carry the EBV genome as an episome and exclusively use the FQp promoter for driving EBNA1 transcription, display no Cp/Wp promoter activity, and express low levels of the LMP mRNAs. Unlike acutely infected SVK-CR2 cells, the cloned lines responded poorly to suspension-induced terminal differentiation and were impaired in their ability to enter the virus lytic cycle. These results, showing similarities between the cloned EBV-positive SVK-CR2 lines and NPC tumour cells, suggest that stable maintenance of EBV in epithelial cells may require an undifferentiated cellular environment.

The Epstein-Barr virus gene BHRF1, a homologue of the cellular oncogene Bcl-2, inhibits apoptosis induced by gamma radiation and chemotherapeutic drugs

Analysis of apoptosis, active and controllable cell death, has demonstrated that the size of a cell population can be regulated by changes in the cell death rate as well as in the rates of proliferation and differentiation. Factors which alter the rate of cell death, such as expression of the proto-oncogene bcl-2, can therefore directly affect the number of cells within a population. Bcl-2 has been shown to suppress apoptosis in response to a variety of stimuli and to act as a complementary survival signal for the random acquisition of other oncogenic mutations, such as deregulated c-myc. The Epstein Barr virus (EBV) gene BHRF1 was the first of a family of bcl-2 homologues now being identified. BHRF1 and bcl-2 share 25% primary amino acid sequence homology. Here we show that gamma radiation and several cytotoxic anticancer agents induce apoptosis in Burkitt's lymphoma (BL) cell lines, as has been found in several other systems. Using gene transfection studies we have also shown that expression of either BHRF1 or bcl-2 in BL cell lines significantly suppresses apoptosis in response to a variety of anticancer treatment. This has confirmed that BHRF1 is functionally homologous to bcl-2 in B-cells and suggests that BHRF1 may act to prevent apoptosis during EBV infection, maximising virus particle production, as has been suggested for other human and insect viral genes. Suppression of chemotherapeutic drug induced cell death by bcl-2 and BHRF1 as demonstrated in this cell system, results in resistance to a variety of different agents and may represent an alternative mechanism by which multidrug resistance arises during chemotherapy.

Class I major histocompatibility complex-restricted cytotoxic T lymphocytes specific for Epstein-Barr virus (EBV)-transformed B lymphoblastoid cell lines against which they were raised

We have raised CD8+ cytotoxic T lymphocytes (CTL) from three Epstein-Barr virus-seropositive donors by incubating peripheral blood lymphocytes with irradiated autologous B95.8-strain EBV-transformed B lymphoblastoid cells (LCL). However, to detect lysis in a standard 51Cr release assay of the LCL against which these CTL were raised, superinfection with recombinant vaccinia expressing the appropriate EBV protein or incubation with the peptide epitope was necessary. The untreated LCL were not lysed, even though Western blotting demonstrated that they expressed the EBV antigens containing the CTL epitopes. We have found CTL of this phenotype that are restricted by human leukocyte antigen-A2, -A3, -B7, or -B39, and which recognize the EBV latent proteins, EBV nuclear antigen (EBNA)-3A, EBNA-3C, or terminal protein. During these experiments, we identified a new human leukocyte antigen-A3-restricted EBNA-3A epitope, residues 603-611, RLRAEAGVK. We raised a spontaneous LCL, transformed by endogenous EBV, from one donor, but this was also not lysed. For at least one of the epitopes, CTL from another donor lysed the LCL without superinfection or addition of peptides. We conclude that the CTL were unable to achieve a high enough avidity of interaction with untreated LCL to trigger effector function, although the LCL were able to stimulate them to grow in vitro for up to 4 mo. To assess whether a small percentage of the LCL might possess a higher antigen density, we used an assay of tumor necrosis factor release from a CTL clone, which was able to detect antigen-bearing cells representing only 1% of a stimulating LCL population. Nevertheless, the untreated autologous LCL line failed to stimulate tumor necrosis factor release.

Analysis of VH genes used by neoplastic B cells in endemic Burkitt's lymphoma shows somatic hypermutation and intraclonal heterogeneity

Tumor cell lines from six typical cases of endemic Epstein-Barr virus (EBV) genome-positive Burkitt's lymphoma (BL) have been investigated for usage and mutational pattern of Ig VH genes. The neoplastic cells all had a t(8;14) (q24;q32) translocation involving the c-myc protooncogene. The VH genes were derived from VH1, VH3 and VH4, and both the IgM-positive (four cases) and IgG-positive (two cases) were extensively mutated from germline sequence. In two cases, early and late passage tumor cells were available, and the VH nucleotide sequences were identical, indicating that mutations had not accumulated in vitro. In a further case, there was evidence of sequence heterogeneity, which appeared to have been generated in vivo, indicating that the tumor cell VH gene was able to undergo posttranslocation somatic hypermutation. Analysis of the relatively nonpolymorphic VH4 genes for the pattern of replacement or silent mutations did not show a role for antigen selection in the expressed sequences.

Epstein-Barr virus isolates with the major HLA B35.01-restricted cytotoxic T lymphocyte epitope are prevalent in a highly B35.01-positive African population

An influence of cytotoxic T lymphocyte (CTL) response over Epstein-Barr virus (EBV) evolution was first suggested by the finding that virus isolates from highly HLA-A11-positive Oriental populations were specifically mutated in two immunodominant A11-restricted CTL epitopes. Here we turn to a second HLA allele, B35.01 and show that B35.01-restricted CTL responses in Caucasian donors reproducibly map to a single peptide epitope, YPLHEQHGM, representing residues 458-466 of the type 1 EBV nuclear antigen 3A protein (B95.8 strain). In this case, however, most EBV isolates from a highly B35.01-positive population (in The Gambia) either retained the CTL epitope sequence or carried a mutation (P-->S at position 2) which conserved antigenicity; changes leading to reduced antigenicity (Y-->N at position 1) were found in only a minority of cases. Furthermore, CTL recognizing the YPLHEQHGM epitope could be reactivated from the blood of some B35.01-positive Gambian donors by in vitro stimulation with the synthetic peptide, indicating that epitope-specific immunity does exist in this population. Possible differences between the A11-based and B35.01-based studies are discussed.

Immune intervention against virus-associated human cancers

BACKGROUND

A number of viruses have been shown to be carcinogenic in humans, including Epstein-Barr virus (EBV), hepatitis B virus (HBV), human papillomavirus (HPV) types 16 and 18, and human T-lymphotrophic virus (HTLV) 1. Cancer results from viral transformation of a single progenitor cell; the pathogenesis is complex, and viral infection is only one of many factors involved. Taking EBV-associated tumours as an example, a number of potential immune interventions, aimed at preventing viral infection or targeting virus-positive tumour cells, have been investigated. ENVELOPE GLYCOPROTEIN-BASED VACCINES: The gp340 glycoprotein is the principal target of the neutralizing antibody response to EBV. A vaccine based on purified gp340 has been shown to protect against EBV-associated lymphoproliferative disease (B-cell lymphoma) in an animal model. Phase I clinical trials are being established. CYTOTOXIC T-LYMPHOCYTE(CTL) EPITOPE-BASED VACCINES: EBV infection provokes a powerful CTL-mediated immune response that is directed primarily against the EBNA 3A, 3B, 3C subset of lal viral antigens. Clinical trials are investigating the effect of immunization with synthetic peptides representing EBNA3-derived CTL epitopes. CTL-BASED IMMUNOTHERAPY: Administration of activated T-cells has been shown to reverse lymphoproliferative disease in patients undergoing bone marrow transplantation. This approach may also be useful in other forms of cancer.

Epstein-Barr virus latency in blood mononuclear cells: analysis of viral gene transcription during primary infection and in the carrier state

Epstein-Barr virus (EBV) can display different forms of latent infection in B-cell lines in vitro; however, the types of infection normally established by the virus in vivo remain largely unexplored. Here we have approached this question by analyzing the types of viral RNAs present in mononuclear cells freshly isolated from the blood of 14 infectious mononucleosis patients undergoing primary EBV infection and 6 long-term virus carriers. Reverse transcription-PCR amplifications were carried out with a panel of oligonucleotide primers and probes which specifically detect (i) the EBER1 RNA common to all forms of latency, (ii) transcripts either from the Cp and Wp promoters generating all six nuclear antigen (EBNA1, -2, -3A, -3B, -3C, -LP) mRNAs or from the Fp promoter generating a uniquely spliced EBNA1 mRNA, (iii) the latent membrane protein (LMP1 and 2A) mRNAs, and (iv) the BZLF1 mRNA, an immediate-early marker of lytic cycle. Viral transcription in infectious mononucleosis mononuclear cells (and in the B-cell-enriched fraction) regularly included the full spectrum of latent RNAs seen during EBV-induced B-cell growth transformation in vitro, i.e., EBER1, Cp/Wp-initiated EBNA mRNAs, and LMP1/LMP2 mRNAs, in the absence of lytic BZLF1 transcripts. In addition, transcripts with the splice pattern of Fp-initiated EBNA1 mRNA, hitherto seen only in vivo in certain EBV-positive tumors, were frequently detected. In long-term virus carriers, the mononuclear cells were again positive for latent (EBER1) and negative for lytic (BZLF1) markers; Cp/Wp-initiated RNAs were not detected in these samples, but in several individuals it was possible to amplify both Fp-initiated EBNA1 mRNA and LMP2A mRNA signals. We suggest (i) that primary infection is associated with a transient virus-driven expansion of the infected B-cell pool through a program of virus gene expression like that seen in in vitro-transformed cells and (ii) that long-term virus carriage is associated with a switch from Cp/Wp to Fp usage and thus to a more restricted form of latent protein expression that may render the infected cells less susceptible to recognition by the virus-specific cytotoxic T-cell response.

Upregulation of bcl-2 by the Epstein-Barr virus latent membrane protein LMP1: a B-cell-specific response that is delayed relative to NF-kappa B activation and to induction of cell surface markers

An ability of the Epstein-Barr virus latent membrane protein LMP1 to enhance the survival of infected B cells through upregulation of the bcl-2 oncogene was first suggested by experiments involving gene transfection and the selection of stable LMP1+ clones (S. Henderson, M. Rowe, C. Gregory, F. Wang, E. Kieff, and A. Rickinson, Cell 65:1107-1115, 1991). However, it was not possible to ascertain whether Bcl-2 upregulation was a specific consequence of LMP1 expression or an artifact of the selection procedure whereby rare Bcl-2+ cells already present in the starting population might best be able to tolerate the potentially toxic effects of LMP1. We therefore reexamined this issue by using two different experimental approaches that allowed LMP1-induced effects to be monitored immediately following expression of the viral protein and in the absence of selective pressures; activation of the NF-kappa B transcription factor and upregulation of the cell adhesion molecule ICAM-1 were used as early indices of LMP1 function. In the first approach, stable clones of two B-cell lines carrying an LMP1 gene under the control of an inducible metallothionein promoter were induced to express LMP1 in all cells. Activation of NK-kappa B and upregulation of ICAM-1 occurred within 24 h and were followed at 48 to 72 h by upregulation of Bcl-2. In the second approach, we tested the generality of this phenomenon by transiently expressing LMP1 from a strong constitutively active promoter in a range of different cell types. All six B-cell lines tested showed NF-kappa B activation in response to LMP1 expression, and this was followed in five of six lines by expression of ICAM-1 and Bcl-2. In the same experiments, all three non-B-cell lines showed NF-kappa B activation and ICAM-1 upregulation but never any effect upon Bcl-2. We therefore conclude that Bcl-2 upregulation is part of the panoply of cellular changes induced by LMP1 but that the effect is cell type specific. Our data also suggest that whilst NF-kappa B may be an essential component of LMP1 signal transduction, other cell-specific factors may be required to effect some functions of the viral protein.

Epstein-Barr virus (EBV) latent membrane protein 1 increases HLA class II expression in an EBV-negative B cell line

Transformation of normal resting B cells by the Epstein-Barr virus (EBV) leads to the establishment of permanent lymphoblastoid cell lines (LCL) which express high levels of HLA antigens and which are highly efficient in antigen presentation. Certain features of the LCL phenotype can be reproduced by transfecting EBV-negative B lymphoma (BL) cell lines with individual EBV latent genes under heterologous promoters. In this work we have analyzed a series of subclones derived from the EBV-negative cell line Louckes, stably transfected with constructs encoding EBV latent genes for their expression of HLA class II molecules. Louckes parental cells and control transfectants expressed detectable levels of HLA-DR, DQ and DP antigens on the cellular surface by cytofluorometry, but these levels were significantly increased in transfectants expressing the virus-coded latent membrane protein 1 (LMP-1). Northern blotting for the individual alpha and beta chain mRNA at each of the three HLA class II loci indicated correspondingly increased levels of HLA class II transcripts in the LMP-1 transfectants. Transfectants expressing the virus-coded nuclear antigens EBNA-1, EBNA-2 or EBNA-LP showed no significant changes in these parameters. These observations indicate that up-regulation of HLA-class II molecules can be a part of the changes induced by LMP-1 in B cells.

T cell responses and virus evolution: loss of HLA A11-restricted CTL epitopes in Epstein-Barr virus isolates from highly A11-positive populations by selective mutation of anchor residues

Epstein-Barr virus (EBV) is a B lymphotropic herpesvirus of humans that elicits strong HLA class I-restricted cytotoxic T lymphocyte (CTL) responses. An influence of such responses on virus evolution was first suggested by our finding that EBV isolates from the highly HLA A11-positive Papua New Guinea (PNG) population carried a lys-thr mutation at residue 424 of the nuclear antigen EBV-encoded nuclear antigen (EBNA4) that destroyed the immunodominant target epitope for A11-restricted CTL recognition. Here we turn to a much larger population, Southern Chinese, where the A11 allele is again present in over 50% of the individuals. Each of 23 EBV isolates analyzed from this population were also mutated in the EBNA4 416-424 epitope, the mutations selectively involving one of the two anchor residues in positions 2 (417 val-leu) or 9 (424 lys-asp, -arg or -thr) that are critical for A11-peptide interaction. The majority of the Chinese isolates and all 10 PNG isolates also carried mutations affecting positions 1 and 2 of the next most immunodominant A11-restricted epitope, EBNA4 residues 399-408. These changes clearly affected antigenicity since A11-positive lymphoblastoid cell lines (LCLs) carrying these mutant EBV strains were not recognized by A11-restricted CTLs raised against the prototype B95.8 virus. Furthermore, Chinese donors naturally infected with these mutant viruses did not mount detectable A11-restricted CTL responses on in vitro stimulation with autologous LCL cells carrying either the B95.8 or their endogenous EBV strain. In two different highly A11-positive populations, therefore, immune pressure appears to have selected for resident EBV strains lacking immunodominant A11-restricted CTL epitopes.

HLA A2.1-restricted cytotoxic T cells recognizing a range of Epstein-Barr virus isolates through a defined epitope in latent membrane protein LMP2

Cytotoxic T-lymphocyte (CTL) responses induced by persistent Epstein-Barr virus (EBV) infection in normal B-lymphoid tissues could potentially be directed against EBV-positive malignancies if expression of the relevant viral target proteins is maintained in tumor cells. For malignancies such as nasopharyngeal carcinoma and Hodgkin's disease, this will require CTL targeting against the nuclear antigen EBNA1 or the latent membrane proteins LMP1 and LMP2. Here we analyze in detail a B95.8 EBV-reactivated CTL response which is specific for LMP2 and restricted through a common HLA allele, A2.1. We found that in vitro-reactivated CTL preparations from several A2.1-positive virus-immune donors contained detectable reactivity against A2.1-bearing target cells expressing either LMP2A or the smaller LMP2B protein from recombinant vaccinia virus vectors. Peptide sensitization experiments then mapped the A2.1-restricted response to a single epitope, the nonamer CLGGLLTMV (LMP2A residues 426 to 434), whose sequence accords well with the proposed peptide binding motif for A2.1. Most Caucasian and African virus isolates (whether of type 1 or type 2) were identical in sequence to B95.8 across this LMP2 epitope region, although 2 of 12 such isolates encoded a Leu-->Ile change at epitope position 6. In contrast, most Southeast Asian and New Guinean isolates (whether of type 1 or type 2) constituted a different virus group with a Cys-->Ser mutation at epitope position 1. CTLs raised against the B95.8-encoded epitope were nevertheless able to recognize these variant epitope sequences in the context of A2.1 whether they were provided exogenously as synthetic peptides or generated endogenously in B cells transformed with the variant viruses. A CTL response of this kind could have therapeutic potential in that it is directed against a protein expressed in many EBV-positive malignancies, is reactive across a range of virus isolates, and is restricted through a relatively common HLA allele.

Different HLA-B27 subtypes present the same immunodominant Epstein-Barr virus peptide

An immunological basis has been postulated for the strong association between at least five subtypes of the HLA-B27 allele (B27.01, .02, .04, .05, and .06) and ankylosing spondylitis, namely that cytotoxic T lymphocyte (CTL) responses are induced against an "arthritogenic" peptide that these different subtypes can all present. This requires a degree of overlap between the peptide binding repertoires of different B27 molecules. The present work, using CTL responses to Epstein-Barr virus (EBV) as a model system in which to identify B27-restricted epitopes, provides the first direct evidence that different disease-related alleles can present the same immunodominant peptide. We first noted that EBV-specific CTL clones, whether from B27.05-, B27.02-, or B27.04-positive donors, were largely subtype-specific in their restriction, recognizing only EBV-transformed B cell lines of the relevant B27 subtype. However, when tested against targets expressing individual EBV proteins from recombinant vaccinia virus vectors, all B27.05-restricted, all B27.02-restricted, and a proportion of B27.04-restricted clones were reactive to the same viral nuclear antigen, Epstein-Barr nuclear antigen (EBNA)3C. In subsequent peptide sensitization assays, all the EBNA3C-specific clones tested and also the EBNA3C-specific component within polyclonal CTL preparations from B27.05-, B27.02-, or B27.04-positive donors recognized the same immunodominant viral peptide RRIYDLIEL (EBNA3C residues 258-266). This sequence accords well with the proposed B27.05 peptide motif and clearly must be accommodated within the different peptide binding grooves of B27.05, B27.02, and B27.04 molecules. Clonal analysis revealed a second component of the B27.04-restricted response that was not shared with other subtypes. This was directed against an EBV latent membrane protein LMP2 epitope whose sequence RRRWRRLTV satisfies some but not all requirements of the B27.05 peptide motif. We conclude that there is indeed a degree of functional overlap between different B27 subtypes in their selection and presentation of CTL epitopes.

Transcripts from the Epstein-Barr virus BamHI A fragment are detectable in all three forms of virus latency

An unexpected feature of the latency II form of Epstein-Barr virus (EBV) infection seen in the epithelial tumor nasopharyngeal carcinoma (NPC) is the presence of spliced polyadenylated RNAs encoded from the BamHI A fragment of the viral genome and running in the opposite orientation to several BamHI-A lytic cycle genes. The importance of these BamHI-A transcripts and the specificity of their association with NPC remain to be determined. In this study, we examined the extent to which such RNAs are present in other transcriptionally distinct forms of EBV latency seen in B cells. Two independent assays of BamHI-A transcription were employed: amplification across defined splice junctions in cDNAs, using the polymerase chain reaction, and in situ hybridization with a radiolabeled riboprobe specific for a putative open reading frame downstream of these splice junctions. Such methods, which easily detected BamHI-A RNAs in fresh NPC biopsies and transplantable NPC lines, also revealed consistent expression of these transcripts in all EBV-positive Burkitt's lymphoma cell lines displaying the highly restricted latency I form of infection (BamHI-F promoter usage) as well as in all EBV-transformed lymphoblastoid cell lines (LCLs) displaying the latency III form of infection (BamHI-C/W promoter usage). Expression in established LCLs, occurring irrespective of virus producer status, was not a consequence of continued in vitro passage; thus, appropriately spliced BamHI-A transcripts could be amplified from normal B cells within 1 day of their experimental infection in vitro, along with BamHI-C/W promoter-initiated but not BamHI-F promoter-initiated mRNAs. In situ hybridization both on Burkitt's lymphoma cell lines and on LCLs showed that essentially every cell contained BamHI-A transcripts, although at levels apparently lower than those observed in NPC. We conclude that expression of the BamHI-A RNAs is a consistent feature shared by all known forms of latent EBV infection.

EBV peptide epitope sensitization restores human cytotoxic T cell recognition of Burkitt's lymphoma cells. Evidence for a critical role for ICAM-2

The pathogenesis of EBV+ Burkitt's lymphoma (BL) suggests evasion of the CTL response against EBV. Two important features of this tumor have been previously suggested to explain this immune evasion, (a) absence/low expression of cellular adhesion molecules and (b) restricted expression of EBV latent Ag. To determine the relative importance of these features in relation to evasion of EBV-specific CTL, a group of BL cell lines with variable expression of the aforementioned phenotypic characteristics were assayed for specific CTL lysis after exogenous addition of EBV peptide epitopes. In spite of down-regulated expression of the adhesion molecules LFA-1, LFA-3, and/or ICAM-1, peptide-sensitized BL cells were recognized and lysed by EBV-specific CTL. Moreover, there was no significant difference between the CTL lysis of the BL cells and that of adhesion molecule-positive control cells over a wide range of peptide epitope concentrations. Blocking experiments with mAb to individual adhesion molecules suggested that virus-specific CTL recognition of lymphoblastoid cell lines was dependent on an intact LFA-3/CD2 pathway. In contrast, the CTL recognition of peptide-sensitized BL cells was critically dependent on the LFA-1/ICAM pathway, with an insignificant contribution by CD2/LFA-3. The consistently high expression of ICAM-2 on all BL cell lines suggests that the accessory function in CTL recognition of these cells is mediated by the LFA-1/ICAM-2 pathway. Thus, down-regulation of LFA-1, LFA-3, and/or ICAM-1 expression on BL cells does not provide an absolute barrier to tumor cell recognition by virus-specific CTL. The ability of virus-specific CTL to recognize peptide epitope-sensitized BL cells as efficiently as normal cells has demonstrated the importance of latent Ag expression in the CTL control of EBV+ tumors.

EBV peptide epitope sensitization restores human cytotoxic T cell recognition of Burkitt's lymphoma cells. Evidence for a critical role for ICAM-2

The pathogenesis of EBV+ Burkitt's lymphoma (BL) suggests evasion of the CTL response against EBV. Two important features of this tumor have been previously suggested to explain this immune evasion, (a) absence/low expression of cellular adhesion molecules and (b) restricted expression of EBV latent Ag. To determine the relative importance of these features in relation to evasion of EBV-specific CTL, a group of BL cell lines with variable expression of the aforementioned phenotypic characteristics were assayed for specific CTL lysis after exogenous addition of EBV peptide epitopes. In spite of down-regulated expression of the adhesion molecules LFA-1, LFA-3, and/or ICAM-1, peptide-sensitized BL cells were recognized and lysed by EBV-specific CTL. Moreover, there was no significant difference between the CTL lysis of the BL cells and that of adhesion molecule-positive control cells over a wide range of peptide epitope concentrations. Blocking experiments with mAb to individual adhesion molecules suggested that virus-specific CTL recognition of lymphoblastoid cell lines was dependent on an intact LFA-3/CD2 pathway. In contrast, the CTL recognition of peptide-sensitized BL cells was critically dependent on the LFA-1/ICAM pathway, with an insignificant contribution by CD2/LFA-3. The consistently high expression of ICAM-2 on all BL cell lines suggests that the accessory function in CTL recognition of these cells is mediated by the LFA-1/ICAM-2 pathway. Thus, down-regulation of LFA-1, LFA-3, and/or ICAM-1 expression on BL cells does not provide an absolute barrier to tumor cell recognition by virus-specific CTL. The ability of virus-specific CTL to recognize peptide epitope-sensitized BL cells as efficiently as normal cells has demonstrated the importance of latent Ag expression in the CTL control of EBV+ tumors.

MHC class II-restricted presentation of endogenously synthesized antigen: Epstein-Barr virus transformed B cell lines can present the viral glycoprotein gp340 by two distinct pathways

Epstein-Barr virus (EBV) transformed B lymphoblastoid cell lines (LCL) efficiently process exogenous antigens for MHC class II-restricted presentation via the chloroquine-sensitive endosomal pathway. Using MHC class II-restricted T cell clones specific for EBV structural proteins, however, we frequently observed significant responses to autologous LCL cells without the addition of exogenous virus. Such responses were reduced by pre-treating the LCL with acyclovir (ACV), a drug blocking productive EBV infection. This suggested T cell recognition of antigen synthesized by LCL cells spontaneously entering virus productive cycle, and led us to question by what route(s) MHC class II-restricted presentation of endogenously synthesized virion proteins was occurring. Cell sorting experiments, using the viral envelope glycoprotein gp340 as a surface marker of productively-infected cells, confirmed that stimulatory activity lay within the gp340-positive fraction. However, closer analysis revealed that most of these cells were not productively-infected but were EBV receptor-positive and had bound released virus. We infer that receptor-mediated delivery of released virus into the endosomal pathway is one route whereby an LCL can present endogenously synthesized EBV proteins on MHC class II molecules. To ask whether another, more direct, route of processing was possible, we used a recombinant vaccinia viral vector to express gp340 de novo in ACV-treated LCLs. Significantly, these cells presented the endogenously synthesized antigen to autologous gp340-specific T cell clones via a chloroquine-resistant pathway. In the same experiments, vaccinia-mediated expression of a signal peptide-deleted form of gp340 did not lead to T cell stimulation, suggesting that this second route of processing required entry of endogenously synthesized antigen into the endoplasmic reticulum.

HLA-A11 epitope loss isolates of Epstein-Barr virus from a highly A11+ population

Cytotoxic T lymphocytes (CTLs) control viral infections by recognizing viral peptides presented by major histocompatibility complex (MHC) class I molecules. Human leukocyte antigen (HLA)-A11-restricted CTLs that recognize peptide residues 416 to 424 of the Epstein-Barr virus (EBV) nuclear antigen-4 frequently dominate EBV-induced responses in A11+ Caucasian donors. This epitope is conserved in type A EBV strains from Caucasians and central African populations, where A11 is relatively infrequent. However, strains from highly A11+ populations in New Guinea carry a lysine-to-threonine mutation at residue 424 that abrogates CTL recognition and binding of the peptide to nascent A11 molecules. The results suggest that evolution of a widespread and genetically stable virus such as EBV is influenced by pressure from MHC-restricted CTL responses.

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.

Multiple HLA A11-restricted cytotoxic T-lymphocyte epitopes of different immunogenicities in the Epstein-Barr virus-encoded nuclear antigen 4

Epstein-Barr virus (EBV), a ubiquitous herpesvirus, induces potent HLA class I-restricted cytotoxic T-lymphocyte (CTL) responses. Analyses of target antigen choice have shown that the very strong CTL responses which are often observed through the HLA A11 allele map are due almost entirely to a single transformation-associated EBV protein, the nuclear antigen EBNA4. Here, we sought to determine the number and relative immunogenicities of HLA A11-restricted epitopes within this 938-amino-acid protein. An initial screening with a series of recombinant vaccinia virus vectors encoding progressively truncated forms of EBNA4 was followed by peptide sensitization experiments using overlapping 14- or 15-mers from the entire sequence. These two approaches allowed the identification of five epitope regions located between residues 101 and 115, 416 and 429, 396 and 410, 481 and 495, and 551 and 564 of the EBNA4 molecule. CTL preparations from all seven HLA A11-positive donors tested had demonstrable reactivities against the 416-to-429 peptide, whereas reactivities against the other epitopes either tended to be lost on serial passage or, for some of the donors, were never detected. The immunodominance of the 416-to-429 epitope was further supported by peptide dilution assays using polyclonal effectors and by CTL cloning experiments. Analysis of the 416-to-429 region identified the nanomer 416-424 (IVTDFSVIK) as the cognate peptide. This peptide was able to sensitize targets to lysis by A11-restricted CTL clones at concentrations as low as 5 x 10(-14) M.