Study Explains How Epstein-Barr Virus Infection Could Cause MS
Similarities between an Epstein-Barr virus (EBV) protein and a brain protein called GlialCAM may explain why EBV infection increases the risk of multiple sclerosis (MS), results from a new study show.
Due to the structural resemblance of the two proteins, B-cells — a type of immune cell that is critical to the development of MS — might accidentally end up attacking a person’s own healthy tissue while trying to fight off EBV.
The findings begin to unravel the molecular mechanisms behind the long-known association between EBV infection and MS and may pave the way to the development of novel therapies for the neurodegenerative disease.
Results were published in Nature in the study “Clonally Expanded B Cells in Multiple Sclerosis Bind EBV EBNA1 and GlialCAM,” which was made available early due to the importance of the research.
A primer on EBV and B-cells
Epstein-Barr virus infects most people at some point in their lives. Most of the time, EBV infections don’t cause noteworthy symptoms, though the virus can cause infectious mononucleosis (colloquially known as “mono”).
An accumulating body of research has shown a connection between EBV infection and MS. Most recently, a study of more than 10 million U.S. military members showed that infection with EBV increased the risk of developing MS by more than 30 times, placing EBV as the leading cause of MS.
“EBV may be the only risk factor required to develop MS, given essentially 100 percent of people living with MS have been infected with EBV,” Lawrence Steinman, MD, a professor at Stanford University and co-author of the study, said in a press release.
Despite this known connection, it was unclear what series of biochemical events could lead from EBV to MS.
The new study shows that this connection may lie in immune cells called B-cells, which are best known for producing infection-fighting antibodies. A given antibody is able to bind tightly to a specific molecular target — called an antigen, which may be, for example, a piece of a virus — which triggers the immune system to attack.
B-cells basically float around the body with antibodies primed on their surface. If their antibody binds to its antigen, the B-cell becomes activated, rapidly dividing and spewing out pro-inflammatory signaling molecules to sound the alarm to the rest of the immune system.
As an activated B-cell divides to produce more cells, the cells undergo a process called somatic hypermutation (SHM). Basically, this involves making small changes to the antibody structure that aim to increase the antibody’s ability to stick to its specific antigen.
What this study found
Here, scientists analyzed B-cells collected from nine patients — five with a first onset of clinically isolated syndrome (CIS) and four with a relapse of relapsing-remitting MS (RRMS). From analyses of thousands of B-cells, the researchers showed evidence that many of these cells were behaving as though they were attacking an antigen.
The team then selected 148 unique B-cells from the cerebrospinal fluid (the liquid surrounding the brain and spinal cord) for further analysis, looking at the antigens that these B-cells’ antibodies were targeting. The team noted that six of the nine patients had B-cells that targeted an EBV protein called EBNA1, and they performed detailed structural analyses of a particular antibody, called mAb MS39p2w174, that bound to a particular region of EBNA1 referred to as AA365-425.
The scientists then noticed that the AA365-425 region of the EBNA1 protein is structurally quite similar to a part of a human protein called GlialCAM. The GlialCAM protein is expressed by certain cells in the brain, including notably oligodendrocytes — the cells chiefly responsible for producing myelin, the fatty sheath around nerve fibers that is the target of the autoimmune attack in MS.
Tests showed that the mAb MS39p2w174 antibody was able to bind to GlialCAM in a similar manner to how it binds to the viral EBNA1 protein. But the two antibody regions that were important for binding to GlialCAM had been altered via SHM: The germline antibody (that is, prior to undergoing SHM) was able to bind to EBNA1, but had little affinity for GlialCAM.
Further testing of the remaining 147 antibodies revealed 10 antibodies from seven patients that bound to the same region of GlialCAM, plus seven antibodies from four patients that bound to other regions of the protein.
The team then looked at blood samples from 36 MS patients and 20 people without MS (controls). While most people had antibodies against EBNA1 — consistent with the widespread prevalence of EBV — MS patients had markedly higher levels of antibodies that targeted GlialCAM.
Additional analyses of two independent groups — one with 71 MS patients and 50 controls, another with 67 patients and 31 controls — also found higher levels of antibodies that target GlialCAM in MS patients.
Finally, the scientists conducted a series of experiments using mice with experimental autoimmune encephalitis, a model frequently used to study MS. The team found that injecting the mice with EBNA1 (to trigger an immune response against the protein) markedly worsened myelin loss and increased immune cell levels in the brain, indicating a worsening of MS-like disease. Additionally, mice immunized with EBNA1 “generated robust antibody responses to both [EBNA1] and GlialCAM,” the researchers reported.
Putting it all together
From all these data, the researchers constructed a conceptual model for how EBV might spur the development of MS.
First, patrolling B-cells would come in contact with the infecting virus, and some B-cells with antibodies that target the EBNA1 protein would become activated. In the course of activating, these cells undergo SHM, which could inadvertently increase the antibody’s ability to target GlialCAM.
The antibodies then bind to GlialCAM that is expressed by healthy cells in the nervous system, which triggers the immune system to attack — ultimately leading to the development of an autoimmune disease.
“Until now, we didn’t have a step-by-step account of how this drives the immune system to attack a person’s own myelin sheath. This new research fills in those gaps and provides clarity into how EBV infection can cause MS,” Steinman said.
The results imply that targeting EBV infections might be a useful strategy for treating MS — and indeed, Steinman noted, this idea is already being explored. For example, Atara Biotherapeutics is sponsoring a Phase 2 clinical trial called EMBOLD (NCT03283826) to test ATA188, an experimental therapy that aims to kill EBV-infected cells in progressive forms of MS.
“These new data further link MS to EBV-infected B cells and plasma cells, highlighting the role of EBV antigens, including EBNA1 protein, in the development of the disease,” said AJ Joshi, MD, Atara’s chief medical officer.
“Importantly ATA188, Atara’s investigational MS therapy, targets key epitopes of these antigens, including EBNA1, with the hope of ultimately delivering a new treatment option for the millions of people currently living with MS,” Joshi added. “The actively enrolling Phase 2 EMBOLD study, with a formal interim analysis planned for [the second quarter of] this year, will be a major step toward that direction.”