Epstein Barr Virus (EBV) is a herpes virus which is spread by saliva and infects around 95% of us during our lifetimes. In the majority of people EBV infection does not cause prominent symptoms - it may cause a ‘flu-like’ illness which gets better on its own, or it may go completely unnoticed.
Some people who are infected with EBV experience a more severe illness - termed glandular fever or infectious mononucleosis - characterised by fatigue, sore throat, lymph node enlargement, fevers, and a mild hepatitis (liver inflammation). EBV is also associated with the development of various cancers, including lymphoma and head and neck cancer.
There is lots of evidence to suggest a link between EBV infection and the risk of developing MS:
The high levels of antibodies (suggesting a specific immune response) against EBV in pwMS;
The slightly higher rates of EBV infection in pwMS (I.e probably 100% of pwMS are infected vs 95% of the general population);
The increased risk of developing MS after glandular fever (around 2.5x more likely);
The hypothesis that EBV infection is an important step in developing MS makes sense a priori as well: we know that EBV evades the immune system by infecting B cells, driving them towards dormancy, and then promoting their long-term survival. EBV has involved an ingenious arsenal of tricks to promote its own survival by fashioning a long-term niche for itself inside B cells. This dormant period is termed ‘latent EBV infection’. A minority of EBV-infected cells will enter the ‘lytic phase’ of infection, in which the virus rapidly replicates, kills the host cells, and is shed to infect other cells. Given the relatively recent slew of data suggesting that B cells are key drivers of the disease, any factor which promotes survival of B cells and hijacks the normal pathways of B cell maturation could conceivably predispose people to developing the disease.
So a crucial question is whether EBV is present in the brains of people with MS. This has been slightly controversial - some people have said yes, and others have been unable to replicate those results.
This study used a combination of biopsy and post-mortem brain tissue to see whether EBV-infected B cells could be detected in the brain. Participants were 17 pwMS and 9 controls without neurological diseases.
Latent EBV infection was detected using a marker called Latent Membrane Protein 1 (LMP1), and lytic infection was detected using a marker called BZLF1. LMP1 was detected in chronic lesions both with and without active inflammation. Importantly, LMP1 was also present in the brains of healthy individuals. BZLF1 was present in both control brains and brains from pwMS. Interestingly, BZLF1 was not present in chronic active lesions - this is a surprising finding which implies that lytic infection is present in healthy brains, in chronic inactive lesions, but not in lesions with acute inflammation.
To validate these findings, the authors used a different technique – in situ hybridisation - to detect EBV infection in a subset of cases (7 pwMS and 4 controls). They used a marker called EBER-1, a small RNA molecule which is produced by EBV during latency. EBER-1 was detected in 6/7 pwMS and 2/4 controls.
However, when the authors tried to quantify their findings, they found that there were no statistically significant differences between MS and control brains in terms of the number of EBV-infected cells, using any of the three markers.
For me this paper does not answer the killer question it set out to answer. It highlights the difficulties of detecting EBV infection - to name a few: the discrepancy between different techniques and markers, the nonspecific nature of these markers, and the low absolute numbers of infected cells.
I am not sure how to interpret these results. EBV-infected B cells appear to be present in both control and ‘MS’ brains, and may be slightly more common in MS. However, the data do not support any stronger claims about the presence or quantity of EBV in the brains of pwMS. The main issue I have with the study is the incredible heterogeneity of the participants: of the 17 pwMS, no clinical details were available for 9, 3 had tumefactive MS - a rare, highly aggressive form - and the others had a mix of primary and secondary progressive disease. This crucially limits the conclusions that can be drawn and is probably a major reason that the results were so noisy. It is also not clear which results are from biopsies and which from post-mortem - although the authors say there was little difference between these two, it would be reassuring to see some results to support this statement.
In short - this study is an attempt to put to bed the question of whether EBV infection is present in the brains of people with MS. In my view it does not settle the debate. The best evidence to date does suggest that EBV is present in both MS and control brains, but is more common in pwMS (90% vs 24%).
Clearly, the presence of EBV-infected B cells in the brain is not sufficient to drive disease activity. Further work needs to be done to understand why some people are perfectly healthy despite having EBV in their brain, whereas others go on to develop MS. Deeper understanding of how EBV is linked to MS is an essential first step towards developing safe treatments (and vaccines) which target EBV. EBV-targeted treatments are already in early stages of development.
Objective We sought to confirm the presence and frequency of B cells and Epstein-Barr virus (EBV) (latent and lytic phase) antigens in archived MS and non-MS brain tissue by immunohistochemistry.
Methods We quantified the type and location of B-cell subsets within active and chronic MS brain lesions in relation to viral antigen expression. The presence of EBV-infected cells was further confirmed by in situ hybridization to detect the EBV RNA transcript, EBV-encoded RNA-1 (EBER-1).
Results We report the presence of EBV latent membrane protein 1 (LMP-1) in 93% of MS and 78% of control brains, with a greater percentage of MS brains containing CD138+ plasma cells and LMP-1–rich populations. Notably, 78% of chronic MS lesions and 33.3% of non-MS brains contained parenchymal CD138+ plasma cells. EBV early lytic protein, EBV immediate-early lytic gene (BZLF1), was also observed in 46% of MS, primarily in association with chronic lesions and 44% of non-MS brain tissue. Furthermore, 85% of MS brains revealed frequent EBER-positive cells, whereas non-MS brains seldom contained EBER-positive cells. EBV infection was detectable, by immunohistochemistry and by in situ hybridization, in both MS and non-MS brains, although latent virus was more prevalent in MS brains, while lytic virus was restricted to chronic MS lesions.
Conclusions Together, our observations suggest an uncharacterized link between the EBV virus life cycle and MS pathogenesis.