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Journal of Clinical Microbiology, January 2006, p. 47-50, Vol. 44, No. 1
0095-1137/06/$08.00+0 doi:10.1128/JCM.44.1.47-50.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.
Laboratoire de Génétique Humaine des Maladies Infectieuses, Faculté Necker, Université de Paris René Descartes INSERM U550, Paris, France,1 Service d'Hématologie, Immunologie biologiques, Hôpital Bicêtre, Le Kremlin-Bicêtre, France,2 Laboratoire de Virologie, Hôpital Tenon, Paris, France,3 Service d'Onco-hématologie, Hôpital Saint-Louis, Paris, France,4 Département d'Hématologie, Institut Gustave Roussy, Paris, France,5 Service d'Hématologie adultes, Hôpital Necker, Paris, France6
Received 7 July 2005/ Returned for modification 22 September 2005/ Accepted 8 October 2005
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The EBV antigens classically used for serological testing include EBV nuclear antigen type 1 (EBNA-1), viral capsid antigen (VCA), and early antigen (EA). After primary infection, anti-VCA and anti-EBNA-1 antibodies persist for life. EBNA-1 is expressed during latent infection, whereas VCA and EA, like the recently described EBV transactivator protein (ZEBRA) (4), are expressed during lytic infection. High anti-VCA, anti-EA, and anti-ZEBRA immunoglobulin G (IgG) titers are therefore considered to be markers of EBV reactivation (8). Remarkably, anti-VCA titers are high at the time of diagnosis of HL and BL and also years before and after diagnosis in both diseases (3, 13). A role of EBV reactivation in HL is also suggested by the detection of anti-ZEBRA antibodies in patients at diagnosis of EBV-positive HL (4).
In addition to serological testing, the EBV viral loads (VLo) can be measured in peripheral blood mononuclear cells (PBMCs), serum, or whole blood. In healthy subjects, VLo is frequently detected at low levels in PBMCs, and it remains steady over time in each individual (10). This makes it possible to evaluate the number of circulating EBV-infected memory B cells. As EBV does not replicate in these cells in nonpathological conditions (1), it is not excreted into the serum of healthy subjects. In contrast, high VLos in the PBMCs and/or serum are observed in patients at diagnosis of EBV-related lymphoid malignancies (HL, BL, PTLD) (5, 6, 9, 14). The physiological significance of positive VLo in healthy subjects remains unclear, but high VLo in individuals who have undergone transplants has been shown to be predictive of PTLD (14).
The relationships between anti-EBV serological titers and VLo have rarely been studied. Only one small longitudinal study of healthy subjects has reported that an increase in VLo is frequently followed by an increase in anti-EBV titers (12). In human immunodeficiency virus (HIV)-infected patients without lymphoma, high VLo in whole blood has been found to be associated with high plasma anti-VCA titers (16). In children receiving solid-organ transplants, VLo in blood has also been found to be correlated with anti-EA and anti-VCA IgG titers (15). We recruited HL patients in remission and their families for a genetic study. We report here the significant correlations observed between plasma anti-VCA antibody titers and VLo in the PBMCs of HL patients and their relatives.
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EBV measures. Anti-EBNA-1 IgG, anti-VCA IgM, anti-EA IgG, and anti-VCA IgG titers were determined with enzyme-linked immunosorbent assay kits (DiaSorin, Stillwater, OK) (ImmunoWELL bmd s.a for IgG anti-VCA) and are expressed in units per milliliter. A positive serology for EBV was defined as a positive result for anti-VCA antibody.
PBMCs were isolated from blood samples on Ficoll-Paque (Pharmacia Biotech), and pellets of 106 cells were frozen at 80°C. DNA was extracted from cell pellets by use of a QIAmp blood kit (QIAGEN Inc., Courtaboeuf, France). VLo in PBMCs was determined by real-time quantitative PCR with a fluorogenic probe (2). The PCR primers were selected to amplify the thymidine kinase gene. The forward and reverse primers were 5'-GACAACTCGGCCGTGATGGA-3' and 5'-TGAAGTTGGAGGCGGACGA-3', respectively. The fluorogenic TaqMan probe was 5'-6-carboxyfluorescein-TGACCTTTGGCGCGGCCTCCTGC-6-carboxytetramethylrhodamine-3'. For preparation of the standard curve, the 121-bp PCR product was directly inserted into a pcDNA 3.1 vector (Invitrogen, Groningen, The Netherlands) containing one copy of the EBV PCR target region. Tenfold serial dilutions in water (106 to 10 copies) were prepared. The same preparations were used in each test. Real-time quantitative PCR for EBV and albumin DNA quantification were carried out simultaneously to determine the amount of cellular DNA in each sample (2).
Statistical analysis.
We analyzed the data for patients and their relatives separately. Antibody titers were analyzed as a quantitative variable. VLo was considered as a binary variable (virus detectable or undetectable) and as an ordinal variable (undetectable, 10 to 99, 100 to 299, or
300 copies/106 PBMCs). Age was considered as a four-class variable (15 to 29, 30 to 44, 45 to 59, or
60 years). Statistical analyses were conducted with SAS software (version 8.2; SAS Institute, Cary, NC). Categorical variables (e.g., VLo according to patient status) were compared using chi-square tests (Proc Freq). Antibody titers were compared according to binary and categorical variables by distribution-free analysis of variance (ANOVA) based on ranks (Proc Rank and Proc Anova). We also tested the effect of VLo as an ordinal variable on antibody titers with the nonparametric Jonckheere-Terpstra test for trend (Proc Freq) (7).
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Anti-EBNA-1 and anti-EA IgG titers did not differ between subjects with detectable and undetectable virus (Table 1). In contrast, subjects with detectable virus had significantly higher anti-VCA IgG titers than those in whom the virus was undetectable (P = 0.02). In analysis of VLo as a four-class variable, significant differences in anti-VCA IgG titers were observed between classes (P = 6.103). The test for trend confirmed that there was a significant positive correlation between anti-VCA IgG titers and VLo (P = 3.103). Mean (SEM) anti-VCA titers increased from 4,278 (344) U/ml for subjects with undetectable virus to 6,520 (752) U/ml for subjects with a VLo of 300 or more copies/106 PBMCs (Fig. 1).
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TABLE 1. Distribution of EBV antibody titersa according to VLo as a binary variable (undetectable versus detectable)
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FIG. 1. Anti-VCA titers distributions according to VLo ordinal classes. Mean titers with standard SEMs are shown for each ordinal class of VLo for patients (squares, full lines) and their relatives (diamonds, dotted lines). Sample sizes for each VLo class are presented in the bottom line.
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There was no significant difference between subjects with and without detectable virus in anti-EBNA-1 or anti-EA IgG titers (Table 1). As seen with HL patients, subjects with detectable virus had significantly higher anti-VCA IgG titers than those in whom the virus was not detected (P = 3.103). As anti-VCA IgG titers increased slightly with age in relatives (P = 0.03), we redid the analysis, adjusting for age; similar results were obtained (P = 2.103). When VLo was considered as a four-class variable, significant differences in anti-VCA IgG titers between classes were found after adjustment for age (P = 6.103). Similar results were obtained when family was included as a covariate in the ANOVA, indicating that the familial relationships did not affect our findings (data not shown). The test for trend, which is not adjusted for age, confirmed that there was a significant positive correlation between VLo and anti-VCA IgG titers (P = 6.104) (Fig. 1). Anti-VCA IgG titers increased from 3,870 (234) U/ml for subjects in whom the virus was undetectable to 6,400 (754) for subjects with VLo of 300 or more copies/106 PBMCs.
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The lack of association between VLo in blood and anti-EBNA-1 titers is consistent with previous studies of HIV-infected and posttransplant patients (16). In contrast, anti-EA antibody titers and VLo have been found to be correlated in children followed up after receiving solid-organ transplants (15). This finding contrasts with our results for an adult population of HL patients in remission and healthy subjects. However, in the study of children (15), an increase in anti-EA titers occurred in the context of primary EBV infection, as most children were EBV negative at the time of transplantation. The apparent lack of consistency between the results obtained in these two studies may be accounted for by the known transient nature of anti-EA antibody production in primary infection (3, 13).
A positive correlation between anti-VCA titers and VLo has been reported for HIV-infected subjects. Plasma anti-VCA titers were found to be significantly higher in subjects with a high VLo in blood than in those with a low VLo in blood in a population of 99 HIV-infected subjects (16). Drouet et al. studied patients with EBV-associated HL and also reported that the detection of virus in the serum was correlated with high anti-VCA titers and positive anti-ZEBRA IgG in 30 patients at the time of diagnosis (4). VLo in PBMCs was not determined in this last study. The only study to investigate this relationship in healthy subjects reported the kinetics of serum anti-EBV titers (determined with a mixture of lytic and latent EBV proteins) and VLo in PBMCs for 22 subjects (12). An increase in VLo or the persistence of virus detection was followed by an increase in anti-EBV titers in some subjects (12). Our results showing that there is a strong correlation between anti-VCA IgG titers and VLo in PBMCs are consistent with these previous reports of studies performed in different contexts. Further studies of infectious mononucleosis patients are needed to investigate whether or not the same pattern of correlations is observed in the context of primary EBV infection.
The long-term EBV reservoir consists of EBV-infected memory B cells, which account for a substantial proportion of PBMCs. EBV does not replicate in these cells. Instead, EBV reactivation has been shown to be initiated following the differentiation of EBV-infected memory B cells into plasma cells (11) in the germinal center of lymph nodes or tonsils. The signals driving this differentiation remain to be determined. Virions produced following differentiation may initiate the infection of new memory B cells, replenish the B-cell reservoir, and increase VLo in PBMCs (18). As VCA antigens are expressed during viral replication, virion production is likely to lead to an increase in anti-VCA IgG titers. An increase in viral replication could then lead to increases in both EBV VLo in PBMCs and plasma anti-VCA titers. Our results suggest that anti-VCA IgG titers and VLo may be considered as markers of EBV reactivation.
Financial support was provided by Association pour la Recherche contre le Cancer, Ligue contre le Cancer, Fondation de France, Fondation Schlumberger, and Fondation BNP Paribas. C.B. is supported by Fondation de France.
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