Epstein-Barr Virus and Multiple Sclerosis : Epidemiology (original) (raw)
Considerable support exists for the hypothesis that multiple sclerosis (MS) is an autoimmune response to infection with common virus(es), 1–4 and that the risk of disease is higher if the infection occurs in late childhood or adolescence. 5–7 Among the several viruses that have been implicated is the Epstein-Barr virus (EBV). 8–10 Epidemiologic evidence of its involvement include higher prevalences of anti-EBV seropositivity (virtually 100%) in MS patients compared with matched controls, 10,11 higher concentrations of serum antibodies against both the EBV viral capsid antigen (VCA) and nuclear antigens (EBNA-1), 10,12 and more frequent history of infectious mononucleosis, a marker of late age at infection with EBV. 13–16 A causal relation would be biologically plausible. EBV has the ability to interfere with the immunological mechanisms that underlie the development of tolerance for self antigens, such as apoptosis of B-lymphocytes, 17 and selection of the T-cell receptor repertoire, 18,19 and to produce a lifetime continuous antigenic stimulation that elicits both cellular and antibody autoimmune responses. 20,21 Molecular mimicry or deregulation of the immune system has been invoked to explain the associations of the EBV virus with several autoimmune diseases, including systemic lupus, 22,23 rheumatoid arthritis, 24–26 Sjögren’s syndrome, 27 and autoimmune hepatitis. 28
Although the serological results have been interpreted by some as supportive of a role of EBV in the pathogenesis of MS, 10,29 previous analyses were mostly focused on antibodies titers, and could not exclude the possibility that the higher levels in MS patients were due to a nonspecific immune activation rather than to a causal relation. To address this issue, we have re-analyzed the data of case-control studies on EBV infection and MS, and estimated the relative risk of MS among EBV-infected vs non-infected subjects.
Materials And Methods
Relevant published articles were identified by conducting a broad MEDLINE search using medical subject headings in addition to free text words and their truncations. 30 Furthermore, we reviewed the references of the retrieved articles to identify papers not indexed in MEDLINE. We found 12 studies comparing anti-EBV antibodies in MS patients and controls. We excluded two of these studies because the prevalence of seropositivity in cases and controls was not reported. 31,32 We excluded a third study because the report did not indicate whether the only seronegative individual in 20 case-control pairs was a case or a control, 33 and a fourth because the case series included patients with optic neuritis or isolated demyelinating lesions in addition to clinical definite MS. 34 Eight studies remained. 10–12,29,35–38 Six of these studies used indirect immunofluorescence against the VCA as a marker of past infection with EBV, one used ELISA to detect antibodies against early antigen (EA) and EBNA 1, and one a combination of IFA and ELISA for different antibodies. 37 We conducted stratified analyses with each study as a single stratum, and estimated the relative risk of MS for seropositive compared with seronegative subjects using the Mantel-Haenszel odds ratio. 39 We obtained exact confidence intervals using Mehta formulas 40 and the software program Epiinfo. 41 Although analyses based on the matched sets would be most appropriate, the raw data needed for this purpose are no longer available for the four studies that employed individual matching (Patrick F. Bray and Ciro Valent Sumaya, personal communications, 1998). For these studies, which all used one-to-one matching, we calculated the lowest possible value of the matched-pair odds ratio.
Results
The main characteristics of the studies included in the analysis are shown in Table 1 and the results in Table 2. Overall, only 8 of 1,005 cases were classified as seronegative (<1%), vs 103 of the 1,060 controls (10%). The estimated odds ratio is 13.5 (95% confidence interval = 6.3–31.4). A one-to-one matched design was used in studies 3, 4, 5, and 7 (see Table 1); the lowest possible matched pair odds ratio in these studies would be equal to 36/1 = 36 (the lowest ratio of discordant pairs). In the one study for which the raw data are available, 10 the matched odds ratio was 14. Since prevalence of EBV infection in adults is high, if the observed odds ratio reflected a causal relation, most cases of MS in the population would be attributable to EBV infection. For a prevalence of EBV infection of 90%, the population attributable fraction would be close to 90%. 39
Characteristics of Studies Relating EBV Seropositivity to MS
Seropositivity among MS Patients Compared with Controls by Study
Discussion
In this systematic review of case-control studies we found a strong association between the presence of antibodies against EBV and risk of MS; although publication bias cannot be excluded, it is highly improbable that it would account for such a strong association.
In our analyses, we have used the presence of antibodies as a marker of infection with EBV. Although highly sensitive and specific, the tests used to determine seropositivity are imperfect, and differential misclassification of cases and controls is possible. Two artifacts could have caused a spurious association between EBV and MS. The first is a higher rate of false positive EBV serologies among cases than among controls, as could occur because of the increased levels of antibodies against several other viruses in MS patients. 4 The second is a higher rate of false-negative EBV serologies among controls, because of their lower levels of circulating anti-EBV antibodies. More specifically, assuming that 90% of both cases and controls were in fact infected by EBV, the results observed could be obtained if the specificity of the diagnostic test was 10% or less among cases, and close to 100% among controls. Alternatively, under the assumption that both cases and controls were all infected, the results could be obtained if sensitivity was 90% or less among controls and 100% among cases. Although the second scenario may appear more plausible, sensitivity of anti-VCA IFA is very high in healthy subjects, as attested by rates of seropositivity close to 100% in several populations 42 and by the high incidence of infectious mononucleosis in seronegative subjects. 42 Thus, bias from this source seems unlikely to explain the observed association completely, although it cannot be ruled out.
A spurious positive relation between EBV infection and MS could also result from preferential inclusion of seropositive cases or seronegative controls. Evidence against this explanation is provided by the prevalence of seropositivity among controls, which is similar to that reported in population surveys, 42 and by the inclusion in the studies of all the available cases meeting standard diagnostic criteria. Although over-selection of seropositive cases could still have occurred if MS in seronegative subjects manifested itself without the typical signs required for the diagnosis, or if MS in individuals not infected with EBV was rapidly fatal (a possibility because of the inclusion in all studies of prevalent cases), there are no data to support either of these hypotheses. Moreover, MS is more common in individuals of higher socio-economic status 43 who have lower expected rates of infection with EBV, 42 making it unlikely that the control groups had lower rates of EBV infection than the population that generated the cases. The consistency of the finding across eight independent investigations conducted in five different countries and with different methods of control selection reduces the probability of a systematic selection bias even further. Finally, the strength of the association and the nature of the exposure make confounding by unmeasured factors—such as genetic predisposition to both EBV infection and MS 44 —an unlikely explanation.
An obvious question is whether similar results would be obtained for seropositivity against other viruses. Unfortunately, prevalence of seropositivity between cases and controls were not compared in most investigations, and a comprehensive analysis as that presented here for EBV is not possible. Comparisons of antibody titers for several viruses that were examined have produced inconsistent results. Null results have been published for herpes viruses I and II, 37,45 herpes zoster, 37,45 cytomegalovirus, 37,45 rubella, 45 measles, 33 and mumps. 45 In a recent trial of acyclovir among 57 patients with MS, 46 seropositivity to herpesvirus 2 at baseline was only 26%, suggesting that most cases of MS occur in individuals not infected with this virus (in the same patients, seropositivity for EBV was 100%). Although there is some support for a role of herpesvirus 6 in MS, 47 evidence is far from being conclusive. 48–50 Most importantly, a causal role of many of these viruses would not by itself explain the epidemiology of MS. For example, the lack of a decline in MS incidence following massive immunization against measles, 51 makes a primary etiologic role of this virus unlikely. Nevertheless, a role of other viruses in combination with EBV remains an attractive hypothesis.
A causal relation between EBV and MS, accentuated when infection occurs around puberty, would fit a wealth of epidemiologic data, including the geographic distribution of the disease, the earlier onset of MS among women, 1,42 the decreased rates of MS among individuals migrating during childhood from high to low risk areas, 52 the lack of increased risk among migrants in the opposite direction, the observation that patients with MS had lived closer to each other than expected at ages 13–20, 21 the social-class gradient, 43,53,54 and the association of MS with history of infectious mononucleosis. 13–16 A single subtype of EBV virus in members of a MS cluster has recently been reported. 55 The biological plausibility is also strong, and it has been recently reviewed. 20 The ability of EBV to immortalize and activate the antibody producing cells 17 and to interfere with the normal process of T-cell repertoire definition 18,19 is likely to predispose to autoimmune reactions. Cross-reactions of anti-EBNA antibodies with epitopes of a neuroglial antigen have also been reported. 21 Although there is still much that we do not know about the molecular steps that may relate EBV infection to MS, a full understanding of these mechanism is not necessary to establish a causal association.
Well-designed case-control studies in populations with relatively low prevalence of EBV, such as young adults in the United States or Europe, and preferably enrolling recently diagnosed cases, should be conducted. Ideally, these investigations should be nested within cohorts with prospectively collected blood samples, to reduce the spurious effect of the aspecific immune activation in MS patients. If these studies confirm the previous results, the epidemiologic and biological studies taken together would provide compelling evidence for a primary causal role of EBV in the development of MS. This conclusion would have far-reaching implications for both the prevention and treatment of MS. More challenging, but nevertheless possible by using longitudinal data with repeated serological assessments, will be to obtain accurate estimates of the effect of age at infection on risk. This last step may be critical for prevention. Even if an effective vaccine will be developed soon, 17 failure to provide long-term sterilizing immunity may have the unintended effect of delaying the age at infection and be potentially harmful. If early age at infection were proved to be protective, exposure to EBV itself early in life may be the best solution. On the treatment side, a causal link between EBV and MS will motivate a search for new antiviral treatments or immunological interventions aimed at interfering with the virus-host relation in ways that may reduce the autoimmune response.
Acknowledgments
We thank Walter Willett and Nancy Mueller for their expert advice and Jill Arnold for technical assistance.
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Keywords:
multiple sclerosis; Epstein-Barr Virus; viral infections; blood antibodies; case-control studies; meta-analysis
© 2000 Lippincott Williams & Wilkins, Inc.