Maintenance of serum immunoglobulin G antibodies to Epstein-Barr virus (EBV) nuclear antigen 2 in healthy individuals from different age groups in a Japanese population with a high childhood incidence of asymptomatic primary EBV infection

Altered Antibody Response to Epstein-Barr Virus in Patients With Rheumatoid Arthritis and Healthy Subjects Predisposed to the Disease. A Twin Study

Objectives: To study Epstein-Barr virus (EBV) antibody patterns in twin individuals with rheumatoid arthritis (RA) and their healthy co-twins, and to determine the heritability of antibody responses against the EBV encoded EBNA1 protein.

Methods: Isotypes of EBNA1 antibodies were measured in 137 RA affected- and 150 healthy twin pairs. We estimated the effect of RA and RA predisposition, anti-citrullinated antibodies (ACPA), IgM rheumatoid factor (RF), the shared epitope (SE) and the PTPN22-T allele (PTPN22) on the level of EBNA1 antibodies. We also determined the heritability of EBNA1 antibody levels.

Results: IgA-EBNA1 antibody levels were increased in twins from RA discordant twin pairs irrespective of RA, ACPA or IgM-RF status. The IgG-EBNA1 antibody level was elevated in healthy co-twins from RA discordant twin pairs but not in RA affected twins. The IgM-EBNA1 antibody level was elevated in both RA twins and their healthy co-twins. The effect of RA on the IgA-EBNA1 antibody level was reversed when SE was present and with no effect of PTPN22. The heritability of IgA-, IgG- and IgM-EBNA1 antibody level was 40.6, 65.5, and 54.3%, with no effect of environment shared by the twins.

Conclusion: EBNA1 antibody levels are distinctively different between patients with RA and healthy subjects but also between relatives of RA strongly predisposed to RA and healthy subjects. The high level of IgA EBNA1 antibodies associated with RA and a family predisposition to RA is attributable to both genetics incl. the shared epitope and environmental variation.

Reappraisal of Epstein-Barr virus (EBV) in diffuse large B-cell lymphoma (DLBCL): comparative analysis between EBV-positive and EBV-negative DLBCL with EBV-positive bystander cells

AIMS

Epstein-Barr virus (EBV)-positive diffuse large B-cell lymphoma (DLBCL) not otherwise specified is defined as monoclonal EBV+ B-cell proliferation affecting patients without any known immunosuppression. Non-neoplastic EBV+ cells proliferating in or adjacent to EBV- DLBCL were reported recently, but their clinical significance is unclear. Thus, the aim of this study was to investigate the prognostic impact of EBV+ cells in DLBCL.

METHODS AND RESULTS

We compared the clinicopathological characteristics of 30 EBV+ DLBCL patients and 29 and 604 EBV- DLBCL patients with and without EBV+ bystander cells (median age of onset 71, 67 and 62 years, respectively). Both EBV+ DLBCL patients and EBV- DLBCL patients with EBV+ bystander cells tended to have high and high-intermediate International Prognostic Index scores (60% and 59%, respectively), as compared with only 46% of EBV- DLBCL patients without EBV+ bystander cells. EBV- DLBCL patients with EBV+ bystander cells showed a significantly higher incidence of lung involvement than those without EBV+ bystander cells (10% versus 2%, P < 0.05). Furthermore, EBV+ DLBCL patients and EBV- DLBCL patients with EBV+ bystander cells had a poorer prognosis than patients without any detectable EBV+ cells [median overall survival (OS) of 100 months and 40 months versus not reached, P < 0.01]. Notably, EBV+ DLBCL patients and EBV- DLBCL patients with EBV+ bystander cells treated with rituximab showed overlapping survival curves (OS, P = 0.77; progression-free survival, P = 1.0).

CONCLUSIONS

EBV- DLBCL with bystander EBV+ cells has similar clinical characteristics to EBV+ DLBCL. DLBCL with EBV+ bystander cells may be related to both age-related and microenvironment-related immunological deterioration.

Epstein-Barr virus-associated primary central nervous system lymphoma in the Japanese population

The incidence of Epstein-Barr virus (EBV)-associated primary central nervous system (CNS) lymphoma in Japan was assessed using in situ hybridization of EBV-encoded small ribonucleic acid-1 (EBER-1) to identify the presence of EBV in 22 cases of formalin-fixed and paraffin-embedded primary CNS lymphoma. All cases were B-cell lymphoma. EBER-1 expression was observed in the nuclei of 3 of 22 primary CNS lymphoma cases (13.6%). The incidence of EBV-positive lymphoma in Japanese cases is higher than previously reported from Western countries. Patients with EBV-positive primary CNS lymphoma showed shorter survival than those with negative tumors (median 4 months vs. 26 months). EBER-1 in situ hybridization for the detection of EBV infection is rapid and reliable. Infrequent association suggests a different pathogenetic mechanism in the evolution of these tumors. Geographical differences in the incidence of EBV-associated primary CNS lymphoma may reflect epidemiological factors.

Evaluation of Epstein-Barr virus antibodies, anti-VCA avidity by immunofluorescence and immunoblot assays for assessment of Epstein-Barr virus immunologic state

Three hundred twenty two serum samples were evaluated by indirect immunofluorescence assay (IFA) and immunoblotting for the assessment of the immunologic status of Epstein-Barr virus (EBV). Serological profiles of 322 serum samples were classified by the IgM and IgG antibodies by the immunofluorescence assay as seronegative, acute infection, reactivation and past infection. Higher rates of seropositivity were determined in serum samples belonging to old age groups. Routine tests used for EBV diagnosis were efficient.

Age-related Epstein-Barr virus-associated B-cell lymphoproliferative disorders: special references to lymphomas surrounding this newly recognized clinicopathologic disease

Epstein-Barr virus (EBV) is associated with some disease entities of malignant lymphomas, including Burkitt lymphoma, Hodgkin lymphoma, immunodeficiency-associated lymphoproliferative disorders (LPD), and a part of diffuse large B-cell lymphoma. We have recently identified a series of elderly patients with EBV-associated (or EBV(+)) B-cell LPD (B-LPD) showing similarities in many respects to immunodeficiency-associated LPD, although no evidence of underlying immunodeficiency was found. Therefore, the nosological category of senile or age-related EBV(+) B-LPD has been proposed for those patients. A larger series of patients with this disease revealed that the relative ratios of such EBV(+) B-LPD to all diffuse large B-cell lymphoma cases were higher with increasing with age, reaching a peak (20-30%) at > or =90 years of age, with a median of 71 years, providing additional evidence for our assertion that this disease may be related to immunological deterioration as a result of the aging process. This new disease entity is characterized pathologically by centroblasts, immunoblasts, and Hodgkin and Reed-Sternberg-like giant cells with a varying degree of reactive components, often posing therapeutic and diagnostic problems for hematologists and pathologists, respectively. The aim of the present review is to briefly summarize the overall clinicopathological profile of this newly recognized age-related (also called 'senile') EBV(+) B-LPD and EBV(+) Hodgkin lymphoma for a practical diagnostic approach.

Age-related EBV-associated B-cell lymphoproliferative disorders constitute a distinct clinicopathologic group: a study of 96 patients

PURPOSE

We have recently reported EBV+ B-cell lymphoproliferative disorders (LPD) occurring predominantly in elderly patients, which shared features of EBV+ B-cell neoplasms arising in the immunologically deteriorated patients despite no predisposing immunodeficiency and were named as senile or age-related EBV+ B-cell LPDs. To further characterize this disease, age-related EBV+ B-cell LPDs were compared with EBV-negative diffuse large B-cell lymphomas (DLBCL).

EXPERIMENTAL DESIGN

Among 1,792 large B-cell LPD cases, 96 EBV+ cases with available clinical data set were enrolled for the present study. For the control group, 107 patients aged over 40 years with EBV-negative DLBCL were selected. We compared clinicopathologic data between two groups and determined prognostic factors by univariate and multivariate analysis.

RESULTS

Patients with age-related EBV+ B-cell LPDs showed a higher age distribution and aggressive clinical features or parameters than EBV-negative DLBCLs: 44% with performance status >1, 58% with serum lactate dehydrogenase level higher than normal, 49% with B symptoms, and higher involvement of skin and lung. Overall survival was thus significantly inferior in age-related EBV+ group than in DLBCLs. Univariate and multivariate analyses further identified two factors, B symptoms and age older than 70 years, independently predictive for survival. A prognostic model using these two variables well defined three risk groups: low risk (no adverse factors), intermediate risk (one factor), and high risk (two factors).

CONCLUSIONS

These findings suggest that age-related EBV+ B-cell LPDs constitute a distinct group, and innovative therapeutic strategies such as EBV-targeted T-cell therapy should be developed for this uncommon disease.

Epstein-Barr virus-related reactive lymphoproliferative disorders in middle-aged or elderly patients presenting with atypical features. A clinicopathological study of six cases

We report six cases of Epstein-Barr virus (EBV)-associated lymphoproliferative disorders (LPDs) in middle-aged and elderly patients exhibiting atypical clinical findings. The patients, two males and four females, ranged in age from 52 to 74 years, with a median age of 64 years. Clinically, they were characterized by tonsillar tumor, cervical lymphadenopathy, and absence of atypical lymphocytosis of the peripheral blood. "B"symptoms were recorded in only two cases. Pancytopenia was recorded in one case during the disease course. The clinical course was self-limited. Histologically, all lesions were characterized by effacement of the follicles and expansion of the interfollicular area with proliferation of small vessels, indicating atypical lymphoid proliferation. In the interfollicular area, there was a mixed infiltrate, including small-to-medium-sized lymphocytes, plasma cells, and T-and B-immunoblasts. Immunoblasts resembling Reed-Sternberg cells were observed in four lesions. Three lesions contained numerous mature plasma cells, plasmacytoid cells, and immature plasma cells in some areas. In situ hybridization demonstrated a varying number of EBV-infected lymphocytes in the interfollicular area. The overall histomorphologic findings of the present six cases were similar to those of infectious mononucleosis (IM) in younger patients. However, the clinical findings were quite different from those of IM in the younger age population. To avoid overdiagnosis and overtreatment, we emphasize the need to be aware of the atypical clinical presentation of EBV-related LPDs in middle-aged or elderly patients, and to turn careful attention to these clinical and laboratory findings as well as to the morphologic features.

Contrasting geographic distribution profiles of the herpes simplex virus type 1 BgOL and BgKL variants in Japan suggest dispersion and replacement

Thelifelong latent infection-reactivation mode of infection of herpes simplex virus type 1 (HSV-1) transmitted by close contact has allowed a diversity of restriction fragment length polymorphism (RFLP) variations to accumulate in human populations. Whether and how the variants of the HSV-1 that is ubiquitous worldwide spread to different human populations is not clear. In our previous study the geographically gradient distribution of the HSV-1 BgK(L) variant, which is a good marker for the BgK(L):SaCFJ(M):SaGH(M):SaD/E(L):KpM(S) variant, suggested that BgK(L) dispersed geographically. Southern hybridization analyses showed that in BgK(L) the BglII cleavage site between the BglII K and small "Q/#13" fragments is lost, the SalI cleavage sites between the SalI J and C and between SalI F and J fragments are lost, and the SalI E fragment is abnormally large (SaE(L) variation). The RFLP and geographic distribution of one more HSV-1 RFLP variant, BgO(L), were comparatively analyzed. The BglII cleavage site between the BglII O and Q/#13 fragments is lost in BgOL. BgO(L) clinical isolates were not associated with any of the SaCFJ(M), SaE(L), SaGH(M), or KpM(S) variations, whereas one-fourth of the non-BgO(L):non-BgK(L) isolates was associated with SaCFJ(M) and SaGH(M), indicating that BgK(L) and BgO(L) are distant in terms of diversification. BgO(L) is distributed highly in the northeastern region and the southwestern island of Kyushu but is rare between the two regions in Japan, in a remarkable contrast to BgK(L). These are the first epidemiologic data to show contrasting geographic distribution profiles of two HSV-1 variants and suggest the gradual dispersion and replacement of HSV-1 variants.

Epstein-Barr virus-associated enteritis with multiple ulcers after stem cell transplantation: first histologically confirmed case

The present case involves unique enteritis forming multiple ulcers associated with Epstein-Barr virus (EBV). A 57-year-old man had undergone a reduced intensity allogeneic stem cell transplantation for a relapse of multiple myeloma following sequential autologous peripheral blood stem cell transplantation. The ileum, resected for massive melena, showed multiple irregular ulcers with occasional cobblestone-like appearance. There was inflammation including numerous plasma cells in the ulcer bases and surrounding areas, where many EBV-infected plasma cells were detected by double staining with EBV-encoded small RNA-1 (EBER-1) in situ hybridization and CD79a, while EBV-infected epithelial cells were not noted. The number of EBER-1-positive cells in the ileum (mucosa, 1451 cells/mm(2); submucosa, 465 cells/mm(2)) was much larger than in control samples (malignant lymphoma or leukemia after allogeneic stem cell transplantation, n = 4, range 0-113 cells/mm(2); malignant lymphoma after chemotherapy, n = 14, range 0-0.89 cells/mm(2); colon cancer, n = 12, range 0-3.5 cells/mm(2)). In the mucosa near the ulcers, EBER-1-positive cells often surrounded and involved the glandular epithelium, forming lymphoepithelial-like lesions. The histological findings differ from post-transplant lymphoproliferative disorders or intestinal thrombotic microangiopathy, and this is the first case of EBV-associated enteritis with ulcers characterized by numerous plasma cells and lymphoepithelial-like lesions after stem cell transplantation.

Geographical distribution of the herpes simplex virus type 1 BgKL variant in Japan suggests gradual dispersion of the virus from Shikoku Island to the other Islands

Restriction endonuclease fragment length polymorphism (RFLP) is useful for the epidemiological study of herpes simplex virus type 1 (HSV-1). We report here the identification of a major BglII RFLP variant of HSV-1, designated BgKL, found in 27.0% of 636 HSV-1 clinical isolates. We have also established its geographic distribution in Japan. BgKL has an unusually large BglII K fragment. SalI cleavage analyses showed that 97% of BgKL variant isolates lack both the SalI C-J and the F-J cleavage sites and have an unusually large SalI D or E fragment, and 91% of the BgKL variants lack both SalI G and H fragments. Furthermore, 96% of BgKL isolates have an unusually small KpnI M fragment. Therefore, BgKL is a marker for these five mutations in most HSV-1 isolates and is a useful HSV-1 RFLP marker. The BgKL variant was found in 59% of HSV-1 isolates from Shikoku Island, 44% of HSV-1 isolates from the Chugoku region of Honshu Island, 31% of HSV-1 isolates from Kyushu Island, 0% of HSV-1 isolates from Okinawa Island, 49% of HSV-1 isolates from Osaka, 27% of HSV-1 isolates from Shiga, 13% of HSV-1 isolates from the Chubu Region, and 9% of HSV-1 isolates from the Tohoku Region of Honshu Island. Differences in the frequency of BgKL between the Shikoku-Chugoku-Osaka area (49%) and Kyushu, between Kyushu and Okinawa, between the Shikoku-Chugoku-Osaka area and Shiga, and between Shiga and Tohoku are all statistically significant. The BgKL frequency decreases in a geographical gradient suggest that this HSV-1 variant was dispersed from Shikoku to the surrounding regions and then to more distant regions. The BgKL frequency in Tokyo was similar to the nationwide average. These are the first data to suggest a geographic and demographic dispersion pattern of HSV-1. Implications for the epidemiology and diversification of HSV-1 are discussed.

Quantitative analysis of herpesvirus load in the lymph nodes of patients with histiocytic necrotizing lymphadenitis using a real-time PCR assay

The cause of histiocytic necrotizing lymphadenitis (HNL) has been ascribed to viral infection, but its pathogenesis still remains unknown. Real-time PCR assays are useful not only for their sensitivity of detection but also for the quantitation of viral DNA with a wide linear range. We accordingly used this technique to estimate for each patient the viral load of the following members of the herpesvirus family: Epstein-Barr virus (EBV), cytomegalovirus (CMV), and human herpesvirus (HHV) types 6, 7, and 8. Samples of patients diagnosed as reactive lymphadenopathy (RL) were included for control. Thirty percent (6/20 cases) and 63% (12/19 cases) of the HNL and RL patients were positive for EBV, and the mean of the detectable EBV viral load of the HNL and that of the RL patients were 463 and 355 (copies/mug DNA), respectively. By in situ hybridization, EBV-encoded RNA could be detected in the lymph tissue samples with more than 14.3 copies/mug of EBV DNA. No significant difference was detected between the number of HNL patients with HHV6 DNA (3/20, 15%) or HHV7 DNA (2/20, 10%) and RL controls. CMV and HHV8 were not detected in the DNA from any patient. In this study, we were unable to definitively identify the causative herpesvirus for HNL; however, 1 HNL case had an extremely large copy number of HHV6-DNA and displayed positive immunostaining for the HHV6 early/late antigen in lesional areas of the node, suggesting that HHV6 infection may be associated with some cases of HNL.

Sensitivity of the Enzygnost anti-EBV/IgG for the determination of the Epstein-Barr virus immune status in pediatric patients

The Enzygnost Anti-EBV/IgG shows good performance for populations not defined by age. Samples from 349 pediatric outpatients were screened with the assay and with immunofluorescence assay (IFA) as the gold standard. The Enzygnost assay showed a sensitivity of 85%. Ten and one-tenth percent of EBV seropositive samples were classified as intermediate, and 4.9% were falsely determined as seronegative when compared to the standard IgG anti-virus capsid antigen (VCA) IFA. Our data suggest a limited sensitivity of the assay with samples from pediatric patients.

Nuclear import of Epstein-Barr virus nuclear antigen 1 mediated by NPI-1 (Importin alpha5) is up- and down-regulated by phosphorylation of the nuclear localization signal for which Lys379 and Arg380 are essential

Epstein-Barr virus (EBV) nuclear antigen 1 (EBNA-1) is essential for replication of episomal EBV DNAs and maintenance of latency. Multifunctional EBNA-1 is phosphorylated, but the significance of EBNA-1 phosphorylation is not known. Here, we examined the effects on nuclear translocation of Ser phosphorylation of the EBNA-1 nuclear localization signal (NLS) sequence, 379Lys-Arg-Pro-Arg-Ser-Pro-Ser-Ser386. We found that Lys379Ala and Arg380Ala substitutions greatly reduced nuclear transport and steady-state levels of green fluorescent protein (GFP)-EBNA1, whereas Pro381Ala, Arg382Ala, Pro384Ala, and Glu378Ala substitutions did not. Microinjection of modified EBNA-1 NLS peptide-inserted proteins and NLS peptides cross-linked to bovine serum albumin (BSA) showed that Ala substitution for three NLS Ser residues reduced the efficiency of nuclear import. Similar microinjection analyses demonstrated that phosphorylation of Ser385 accelerated the rate of nuclear import, but phosphorylation of Ser383 and Ser386 reduced it. However, transfection analyses of GFP-EBNA1 mutants with the Ser-to-Ala substitution causing reduced nuclear import efficiency did not result in a decrease in the nuclear accumulation level of EBNA-1. The results suggest dynamic nuclear transport control of phosphorylated EBNA-1 proteins, although the nuclear localization level of EBNA-1 that binds to cellular chromosomes and chromatin seems unchanged. The karyopherin alpha NPI-1 (importin alpha5), a nuclear import adaptor, bound more strongly to Ser385-phosphorylated NLS than to any other phosphorylated or nonphosphorylated forms. Rch1 (importin alpha1) bound only weakly and Qip1 (importin alpha3) did not bind to the Ser385-phosphorylated NLS. These findings suggest that the amino-terminal 379Lys-Arg380 is essential for the EBNA-1 NLS and that Ser385 phosphorylation up-regulates nuclear transport efficiency of EBNA-1 by increasing its binding affinity to NPI-1, while phosphorylation of Ser386 and Ser383 down-regulates it.

Telomere size and telomerase activity in Epstein-Barr virus (EBV)-positive and EBV-negative Burkitt’s lymphoma cell lines

The telomere repeat lengths of BL cell lines were quantified by measuring terminal restriction fragment (TRF). Epstein-Barr virus (EBV)-positive Namalwa, Raji, and EB-3 cell lines have long telomeres, i.e. TRFs 10-19 kbp, whereas the Daudi cell line, producing a transformation-defective EBV mutant, has TRFs approximately 2.2 kbp. EBV-negative BJAB and DG75 cell lines have short TRFs 3.9-5.4 kbp, shorter than the approximately 12 kbp TRFs in PBLs. Telomerase activities of these BL cell lines are similar. TRFs of non-BL lymphoma cell lines are 2.3-5.5 kbp. Fluorescent in situ hybridization (FISH) studies of these cell lines showed remarkable heterogeneity of telomere size in chromosomes in the same BL cell. These results suggest that EBV-positive and EBV-negative BL cell lines have experienced various telomere dynamics.

Inflammatory pseudotumor of the liver associated with malignant disease: report of two cases and a review of the literature

Two cases of inflammatory pseudotumor (IPT) of the liver associated with gastrointestinal tract cancer are reported. In addition, the etiological correlation between IPT and abscesses of the liver in cancer patients is discussed. The first patient was a 63-year-old woman who underwent distal gastrectomy and partial hepatectomy under a diagnosis of stomach cancer with liver metastasis. The second patient was a 66-year-old man who had undergone surgery for rectal cancer 6 years previously and underwent partial hepatectomy under a diagnosis of metastasis of rectal cancer to the liver. The gastric cancer was a papillary adenocarcinoma limited to the mucosa, and the rectal cancer was a moderately differentiated adenocarcinoma limited to the subserosa. The resected liver tumor in the first case measured 5.5 x 5.0 x 4.0 cm and was 2.5 x 1.9 x 1.6 cm in the second. The cut surface showed that both masses were well circumscribed and divided into lobules by fibrous tissue. They were yellowish white in color and there was no evidence of necrosis or hemorrhage. Histologically the masses consisted of fibrous areas and cellular areas, and the cellular areas consisted of fascicles of plump spindle cells mingled with varying numbers of plasma cells, lymphocytes, and histiocytes. The masses were diagnosed as IPTs. Obliterating phlebitis suggesting infection via the portal vein was seen in the adjacent liver tissue in both cases. According to previous cases reported in the literature, there are three types of cancers associated with hepatic IPT: gastrointestinal tract cancer, biliary tract cancer, and cancers that need strong systemic chemotherapy. The underlying cancer types of IPT of the liver are almost similar to those associated with pyogenic liver abscesses suggesting the etiological correlation between IPT and abscesses of the liver.

Elevated immunoglobulin G antibodies to the proline-rich amino-terminal region of Epstein-Barr virus nuclear antigen-2 in sera from patients with systemic connective tissue diseases and from a subgroup of Sjögren's syndrome patients with pulmonary involvements

Associations of Epstein-Barr virus (EBV) and autoimmune diseases have been hypothesized. We have analysed IgG antibodies to EBV nuclear antigen (EBNA)-2 in sera from Japanese patients with autoimmune systemic connective tissue diseases (CTD), exemplified by systemic lupus erythematosus (SLE), primary Sjogren's syndrome (SS), rheumatoid arthritis (RA), systemic sclerosis (SSc) and secondary SS (classical CTDs complicated with SS). An enzyme-linked immunosorbent assay (ELISA) which uses glutathione-S-transferase polypeptides fused to EBV nuclear antigen (EBNA)-2 and EBNA-1 was developed. Ratios of IgG antibody reactivity to whole IgG concentrations of sera were calculated to normalize EBNA-2 and EBNA-1 antibody levels to the hypergammaglobulinaemia that occurs in CTD. The ELISA optical density OD(450) readings of IgG antibodies to both the amino-terminal aa 1-116 of EBNA-2 and carboxyl-terminal aa 451-641 of EBNA-1 were elevated significantly in patients with SLE, primary SS, RA, SSc and secondary SS when compared to EBNA-1. The OD readings were divided by serum IgG concentrations to normalize for the hypergammaglobulinaemia. The specific levels of IgG antibodies to the amino-terminal region of EBNA-2 were elevated in patients with SLE, primary SS or RA, as well as those with secondary SS complicated with SLE or RA. The EBNA-2 amino-terminal region contains a polyproline tract and a proline-rich sequence and has considerable amino acid sequence homology with many cellular proline-rich proteins. High ratios of EBNA-2 aa 1-116 to EBNA-1 aa 451-641 IgG antibody levels which probably suggest reactivation of EBV latent infection were associated significantly with pulmonary involvement in SS patients. These results are consistent with the hypothesis that the sequence similarity between the amino-terminal region of EBNA-2 and proline-rich cellular proteins is associated with pathogenesis in a subpopulation of CTD patients, possibly by the molecular mimicry-epitope shift mechanism.

Monoclonal antibodies against regions topologically surrounding the homodimeric beta-barrel interface of Epstein-Barr virus nuclear antigen-1

Epstein-Barr virus (EBV) nuclear antigen-1 (EBNA-1) is essential for maintenance of EBV latency. Four mouse monoclonal antibodies (mAbs) against the part of the EBNA-1 sequence (amino acids 451-641) containing the domain that forms a homodimeric eight-stranded beta-barrel were generated and characterized, examined for immunocytochemical staining, immunoblotting and isoelectric focusing of EBNA-1 proteins, and used to examine interactions between EBNA-1 polypeptides by far-Western blot assays. Far-Western blot analyses using the mAbs suggest that both the beta-strand (aa 593-604) and alpha helix (aa 568-582) are essential for EBNA-1 dimerization, consistent with yeast two-hybrid studies of mutant EBNA-1 polypeptides. These mAbs should be useful for studies on the structure and function of EBNA-1 proteins.