Scientists have long suspected — but failed to prove — a link between certain viral infections and the development of multiple sclerosis (MS). Now, a study led by Stanford Medicine researchers has proved that the Epstein-Barr virus, a common type of herpes virus, triggers multiple sclerosis by priming the immune system to attack the body’s own nervous system, according to a news release from the university.
The study, published in Nature, shows that approximately 20% to 25% of patients with multiple sclerosis have antibodies in their blood that bind tightly to both a protein from the Epstein-Barr virus, called EBNA1, and a protein made in the brain and spinal cord, called the glial cell adhesion molecule, or GlialCAM.
“Part of the EBV protein mimics your own host protein — in this case, GlialCAM, found in the insulating sheath on nerves,” said William Robinson, MD, PhD, Professor of Immunology and Rheumatology at Stanford. “This means that when the immune system attacks EBV to clear the virus, it also ends up targeting GlialCAM in the myelin.”
Myelin forms the protective coating around nerve cells, and when it’s damaged, electrical impulses can no longer jump efficiently from one nerve to the next, resulting in the numbness, muscle weakness and severe fatigue of multiple sclerosis.
MS and viruses: an elusive connection
Previous research has shown that multiple sclerosis patients have increased antibodies to a variety of common viruses, including measles, mumps, varicella-zoster, and Epstein-Barr virus. In fact, more than 99% of MS patients have EBV antibodies in their blood, indicating a prior infection, compared with 94% of healthy individuals. But despite this epidemiologic correlation, scientists have struggled to prove a causal connection.
To search for this mechanistic link, the researchers started by examining the antibodies produced by immune cells in the blood and spinal fluid of nine MS patients. Unlike in healthy individuals, the immune cells of MS patients' traffic to the brain and spinal cord, where they produce large amounts of a few types of antibodies. Patterns of these antibody proteins, called oligoclonal bands, are found during analysis of the spinal fluid and are part of the diagnostic criteria for MS.
“No one knows exactly what those antibodies bind to or where they’re from,” Robinson said. “So, the first thing we did was analyze the antibodies from the oligoclonal bands and showed that they come from B cells in the spinal fluid.”
B cells are a type of white blood cell made in the bone marrow, and the technology to sequence these cells individually was developed by the Robinson lab about eight years ago. “In the past, researchers would take serum and spinal fluid from MS patients and test them on planar arrays or throw them on histology slides to see what sticks,” Lanz said. “What we did was a different approach: We took B cells from the spinal fluid, single-cell sorted them and sequenced each one separately. In a single-cell format and at the scale of tens to hundreds of B cells per patient, that had not been done before.”
Once the researchers determined that the oligoclonal bands in MS are produced by the sorted B cells in the spinal fluid, they expressed individual antibodies from these cells and tested them for reactivity against hundreds of different antigens.
Six of the nine MS patients had antibodies that bound to the EBV protein EBNA1, and eight of nine had antibodies to some fragment of EBNA1. The researchers focused on one antibody that binds EBNA1 in a region known to elicit high reactivity in MS patients. They were then able to solve the crystal structure of the antibody-antigen complex, to determine which parts were most important for binding.
Next, the researchers tested the same antibody on a microarray containing more than 16,000 human proteins. When they discovered that the antibody also bound with high affinity to GlialCAM, they knew they’d found a specific mechanism for how EBV infection could trigger multiple sclerosis.
To find out what percentage of MS might be caused by this so-called “molecular mimicry” between EBNA1 and GlialCAM, the researchers looked at a broader sample of MS patients and found elevated reactivity to the EBNA1 protein and GlialCAM in 20% to 25% of blood samples in three separate MS cohorts.
A study of 801 MS cases from more than 10 million active-duty military personnel over 20 years found that EBV infection was present in all but one case at the time of MS onset. A paper describing that study, published this month in Science, found that of 35 people who were initially EBV-negative, all but one became infected with EBV before the onset of MS. In addition, this separate group of researchers identified the same EBNA1 region as a major antibody target in MS patients. Together with the discovery of EBNA1/GlialCAM cross-reactivity, this data provides compelling evidence that EBV is the trigger for the vast majority of MS cases, as Robinson and Steinman point out in a Science Perspective.
The discovery of how EBV triggers multiple sclerosis could also have ramifications for research into other autoimmune diseases, such as lupus and rheumatoid arthritis, which, like MS, have been significantly associated with EBV infection in epidemiologic studies.
