Multiple Sclerosis Is Likely Caused by a Virus, Finds Study of 10 Million Military Personnel
Multiple Sclerosis Is Likely Caused by a Virus, Finds Study of 10 Million Military Personnel
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The demyelinating neurodegenerative disease multiple sclerosis (MS) is a complication of infection by the Epstein-Barr virus (EBV), suggests a new study published in Science by researchers at Harvard Medical School.
The breakthrough findings, powered by a gargantuan dataset taken from two decades of US military personnel, represent the culmination of years of circumstantial evidence showing links between EBV, an endemic and latent infection found in up to 95% of the population, and the onset of MS.
MS – a complication of viral infection
“The key finding is that MS is a complication of infection with EBV,” says study senior author Alberto Ascherio, a professor of epidemiology and nutrition at Harvard’s T.H. Chan School of Public Health. MS has been considered for many years an autoimmune disease of unknown etiology. I think this study establishes that this immune process that leads to brain damage is driven by infection with EBV.”
The link between MS and EBV has been suspected for many years, without any killer study able to establish a solid causal link. While the great majority of healthy adults show prior EBV infection, the number rises even further for MS patients – 99.5% of this group test positive. Ascherio says that this new study firmly establishes that connection. “It was quite striking how black and white the results are. I think the results are very solid and leave very little doubt [about the causal link],” he says.
What is EBV?
The Epstein-Barr virus (EBV) is a herpesvirus that causes the childhood disease infectious mononucleosis. Primarily transmitting through saliva, it persists as a latent infection in human B-cells.
Efforts to establish causal confirmation of previous studies hinting at a MS-EBV link have, until now, faced a major challenge. With so many people infected with EBV by even early adulthood, producing a sample population substantial enough to find EBV-negative individuals that would then go on to develop MS would require a unique study design with an unprecedented sample size.
“There is no comparable population in the world”
Ascherio and his team found that exceptional sample in the records of the Department of Defense Serum Repository (DoDSR). All active US military personnel are required to submit a blood serum sample at the start and then after every two years of service. Primarily designed to detect HIV, the DoDSR has built up to a vast cornucopia of biological data made up of over 62 million serum samples taken from over 10 million individuals.
“There is no comparable population in the world,” says Ascherio. Using it, his team were then able to identify personnel that showed antibodies against EBV, an indicator of infection, at the time of their first sample donation. In a demonstration of the ubiquitous nature of EBV, just 5.3% of the individuals examined were uninfected. Separately, they looked for individuals that developed MS during their service period, noting 801 such cases that had EBV status records.
800 of 801 MS cases occurred in individuals who had previously tested positive for EBV. 35 of those individuals had tested negative upon their first donation, and all but one then became positive (a process called seroconversion) prior to their MS symptoms emerging, corresponding to a 97% seroconversion rate. By contrast, just 57% of an initially EBV-negative control population who didn’t later develop MS showed seroconversion. Becoming EBV-positive resulted in a 32-fold increased risk of later developing MS as opposed to remaining EBV-negative. The next-strongest known risk factor for MS is having a set of genes that encode for proteins found of the surface of certain immune cells. People with a particular set of these immune cells, who have a homozygous genotype for the HLA-DR15 allele, have a threefold increased risk of MS.
Put simply, apart from one case, every single MS patient documented in the study became EBV-positive prior to their symptoms developing. The median time from seroconversion to MS diagnosis was 7.5 years, with a wide range seen from 2 to 15 years.
Ruling out confounders
In the absence of a randomized controlled trial, the gold-standard for assessing cause and effect in disease onset, it was possible that the team’s findings could be explained by a confounding variable or by reverse causation. To rule these out, the team conducted further analysis of the serum data.
Could there be another factor that simply increased the risk of both getting EBV and MS? The huge risk factor associated with EBV made that unlikely, but Ascherio’s team examined antibodies against cytomegalovirus, another saliva-borne virus that has also infected most of the world’s population, to serve as a negative control. Individuals who were CMV-negative at their first sample showed no increased risk of MS if they later became CMV-positive. CMV positivity was actually associated with a lower risk of MS.
MS is thought to have a long prodromal phase, meaning the disease could affect the immune system years before symptom onset and diagnosis. Could the EBV-MS relationship be reversed? Perhaps people who have MS but don’t yet show symptoms are more likely to develop an infection such as EBV. To rule this out, Ascherio’s team looked at 30 MS patients, and 30 healthy controls. They used a search tool called VirScan that enables the detection of any antibody raised against any protein in any of the ~200 viruses known to infect humans. The only virus to show significantly increased presence in MS cases was EBV.
These findings could potentially reshape the direction of MS research. But EBV cannot be the whole story. The study suggests that EBV seropositivity is necessary to develop MS, but it is clearly not sufficient, or the disease would affect 95% of the world’s population, as opposed to only the few million cases of MS thought to exist worldwide.
It is likely that genetic and environmental factors instead contribute to the disease onset, meaning that once an individual has been infected with EBV, their own unique genotype and phenotype will go on to determine whether they will later develop MS. A companion perspectives piece published in the same issue of Science by Stanford University’s William H. Robinson and Lawrence Steinman suggested that molecular mimicry may be involved. This would imply that immune cells targeting EBV are accidentally attacking the protective myelin sheath around axons, leading to the neurodegeneration and motor symptoms of MS. Supporting this theory, prior study has identified EBV-infected B cells in the brains of MS patients.
One final question from Ascherio’s data concerns the lone EBV-negative MS patient in the study. The paper’s discussion section identifies infection in between sampling or failure to seroconvert as potential explanations, but another reason, Ascherio suggests, may be due to the limitations of diagnosis.
“Before the polio vaccine, say 99% of poliomyelitis cases were caused by the polio virus, but, unless you define the disease based on the infectious agent, for any disease, there will be occasional cases that are not caused by that that agent,” says Ascherio. For example, acute flaccid paralysis – which was virtually eradicated after the success of the polio vaccination program, still is seen in rare cases. “There is no reason,” says Ascherio, “that MS should be an exception, because, even if MS is a complication of EBV infection, when you describe the disease clinically and radiologically, there will always be an occasional case that is due to something else.”
Can we eradicate MS?
Interest will now also move to the prospect of developing vaccines and antivirals against EBV with the aim of eradicating MS. No vaccine currently exists, but Ascherio says its development is realistic. The more interesting treatment option now, he points out, could be adapted from immunosuppressive therapy. Currently, he says, the most effective treatment is anti-CD20 immunotherapy. “This depletes the B cells,” says Ascherio, “in which EBV establishes infection. Most likely, the reason the anti-CD20 is so effective is because by getting rid of the cells, we also get rid of the virus, at least temporarily.”
A more effective treatment, he says, could instead target EBV directly with antivirals, while leaving the B cells unharmed. Again, those treatments do not yet exist. But now, says Ascherio, research at least knows the questions it wants to ask, rather than dealing with MS’s previously unknown etiology. “Once you establish the causal connection, I think that it's a question of providing sufficient rationale for research on antivirals, specifically for EBV, that could help the 2.8 million people with MS in the world,” Ascherio concludes.
Bjornevik, K, Cortese, M, Healy, BC, et al. Longitudinal analysis reveals high prevalence of Epstein-Barr Virus associated with multiple sclerosis. Science. 2022. doi: 10.1126/science.abj8222