A team of researchers at University of Utah Health have shown the Epstein-Barr virus – which causes mononucleosis and is linked to development of several cancers – uses a novel strategy to survive.
The virus takes the reins of its host’s cellular machinery to make copies of itself and to prioritize the production of its own proteins over those of the host cell.
The researchers hope to exploit this knowledge to develop a new kind of treatment for infection by the Epstein-Barr virus.
Epstein–Barr virus (EBV) is a human herpesvirus found in 95% of the human population. Like other herpesviruses, EBV can be spread from person to person.
However, the virus generally remains latent – that is, it lingers inside of cells without actively replicating – for the lifetime of the host and rarely causes disease beyond the initial infection.
In some cases, though, environmental triggers, including stress or coincident infections and immune suppression, create conditions in which the virus can thrive, occasionally sparking a rare type of lymphoma or other cancer.
Four years ago, the U of U Health research team found that spironolactone, a medicine routinely used to treat heart failure, has an unexpected antiviral activity against EBV.
They discovered the drug targets an EBV protein, called SM, that the Swaminathan lab and others previously showed is essential for EBV replication.
“We were puzzled as to how spironolactone, a drug thought to work on a completely unrelated pathway involving sodium flux in the kidney, would have an effect on the virus or on SM function,” says senior author Sankar Swaminathan, M.D., division chief of infectious disease at U of U Health and professor of internal medicine.
Now, they’ve figured out more about how it works and have published their findings in the Proceedings of the National Academy of Sciences.
Several years ago, a group of French researchers found that spironolactone degrades a host protein called XPB, which plays an important role in cellular transcription – one of the first steps in gene expression.
Following up on this lead, Swaminathan and his colleague Dinesh Verma conducted a series of experiments to explore the potential link between the two proteins that the drug interacts with, XPB and SM.
The researchers first used a molecular biology technique to specifically lower the amount of XPB in host cells. The result was that the virus failed to reactivate and acted as if it no longer had functional SM. Next, the research team showed that SM ferries the XPB protein directly to viral DNA.
Finally, the researchers used a technique that involved chemically tagging uridine, one of the four building blocks of the RNA alphabet, to study replication of the virus. Using this technique, they demonstrated that knocking down XPB resulted in lower levels of messenger RNAs for 15 specific viral proteins whose production SM facilitated, while expression of other EBV genes was not affected.
“We showed that SM surprisingly plays a role in activating transcription and co-opts this one cellular protein to do this,” says Dinesh Verma, Ph.D., research assistant professor of internal medicine.
According to Swaminathan, these 15 proteins perform functions that allow the virus to replicate in healthy people. “The virus has evolved to make these proteins at just the right time to keep the infected cells from getting killed just long enough to make some copies of the virus and maybe infect a couple more cells
before the immune system kicks in and takes care of it,” Swaminathan says. “As is often the case with viruses, this solution is both very specific and highly clever.”
In patients whose immune systems are compromised, these very same properties – keeping the infected cells alive and helping them evade the immune system – can lead to unchecked proliferation, a common characteristic of cancer.
The researchers are now trying to find new drugs that target XPB to prevent reactivation of EBV and other human herpesviruses in transplant patients, HIV patients, and other immunocompromised patients.
“The long-term idea is that we would be able to develop drugs that would keep the virus completely latent and that this would help decrease the risk of the development of cancers related to EBV,” Swaminathan says.
“Epstein–Barr virus co-opts TFIIH component XPB to specifically activate essential viral lytic promoters” by Dinesh Verma, Trenton Mel Church and Sankar Swaminathan has been published online in PNAS.
How the virus spreads
EBV is spread mainly via the transfer of saliva between individuals, which is the reason that glandular fever has been dubbed the ‘kissing disease’. It can also be spread by sharing utensils/toothbrushes etc, through blood (transfusions, organ transplants) and rarely via semen from sexual contact. People infected with the Epstein-Barr virus will retain it for life, but it may not make them sick. In fact, the virus infects almost everyone in developing countries and more than 95 per cent of people in developed countries.
Most people are infected with the virus during childhood, probably by their mothers, and are usually not noticeably affected. On the other hand, people infected for the first time during or after adolescence (10‑20 per cent of people living in developed countries) have a 50 per cent chance of contracting glandular fever. They may be ill for several weeks or months before their immune system kicks in to action.
Symptoms of EBV
The symptoms of EBV can include:
- fever/chills
- fatigue
- inflamed throat
- swollen lymph nodes in the neck
- swollen liver or spleen
- rash
- loss of appetite
- minor aches and pains
Often the symptoms of EBV are not distinguishable from other general colds and flu. Those who do experience symptoms generally recover within 2 to 4 weeks, however full recovery can take months. It is important to note however that while your symptoms may have disappeared, the virus has not. Instead it becomes inactive—essentially hiding away inside the body’s B-cells. It is possible for the EBV to ‘reactivate’ later in life, though this is rare.
Diagnosing EBV
It can be difficult for doctors to diagnose EBV because its symptoms so closely resemble a range of other illnesses. However, a blood test can confirm the presence of EBV antibodies in the system, whether someone is susceptible to the virus, or has had a recent or past infection. Find out more about the laboratory testing of EBV.
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Most people are infected with Epstein-Barr virus (EBV) during childhood or adolescence, resulting in a persistent, mostly latent EBV infection. The primary EBV infection often manifests as infectious mononucleosis (IM), especially in adolescence [1,2].
Globally EBV is causally linked to nearly 200000 incident cancers and 18000 deaths from multiple sclerosis annually [3,4], with IM elevating the risk of Hodgkin lymphoma and multiple sclerosis for unknown reasons [5–7].
Functions of EBV antibody levels as predictors of disease risk is an active field of research, see [8] and references therein.
At the same time it is unclear why upon primary EBV infection some individuals present with IM, while others do not [9]. Disease severity and duration correlate much better with e.g. CD8+ cell counts than with the viral kinetics itself and the expansion of the CD8+ cell count is controlled in asymptomatic EBV infection despite virus loads similar to those experienced in symptomatic EBV infection [1,2,10–14].
Hence current understanding suggests that IM is caused by overreaction by the immune system, rather than EBV infection per se (viremia or B-cell expansion). There now seems to be broad agreement that a massive expansion of the number of EBV-specific CD8+ cells is a characteristic of IM, while changes in the proportions of other cell populations seem less well-established [1,2,10–13,15,16].
Clinically, IM is typically characterized by fever, pharyngitis, lymphadenopathy and fatigue. The IM symptoms are believed to be caused mostly, if not entirely by the exaggerated CD8+ response [2,15,16].
Presumably IM is the same disease in teenagers as in children, because the immunological response to EBV infection is recognizably the same [10,17].
The way IM occurrence depends on age and sex is incompletely described and hard to interpret etiologically. The age distribution of incident IM is dominated by a distinct peak in the middle of the teenage years [18,19].
However, as an etiological clue this is not particularly useful because the depicted rate is not really a rate, i.e. a number of IM cases divided by the time at risk of those who have not seroconverted. Rather it is a product of the prevalence of EBV-naϊve persons, the hazard rate of seroconverting and the attack rate, i.e. the fraction of primary EBV infections that is accompanied by IM.
Attack rates have only been estimated in young adults [20–23], and estimated sero-conversion rates are practically non-existent too.
It would therefore be valuable to devise and fit a mathematically coherent model, projecting what would be the age- and sex-specific seroconversion rate and attack rate in a hypothetical population where the observed age- and sex-specific EBV-prevalence and IM occurrence in the target population apply.
Such a model could quantify e.g. how much of the IM teenage peak is due to changed behavior (changing hazard of seroconversion), and how much to changed susceptibility to IM (changing attack rate) in teenagers compared with pre-adolescents.
As proof of concept we therefore fitted such a model based on a few large data sets, with Danes age 0–29 years in 2006–2011 as our target population.
Background
Infectious mononucleosis (IM) is a common adverse presentation of primary infection with Epstein-Barr virus (EBV) in adolescence and later, but is rarely recognized in early childhood where primary EBV infection commonly occurs.
It is not known what triggers IM, and also not why IM risk upon primary EBV infection (IM attack rate) seemingly varies between children and adolescents. IM symptoms may be severe and persist for a long time. IM also markedly elevates the risk of Hodgkin lymphoma and multiple sclerosis for unknown reasons.
The way IM occurrence depends on age and sex is incompletely described and hard to interpret etiologically, because it depends on three quantities that are not readily observable: the prevalence of EBV-naϊve persons, the hazard rate of seroconverting and the attack rate, i.e. the fraction of primary EBV infections that is accompanied by IM.
We therefore aimed to provide these quantities indirectly, to obtain epidemiologically interpretable measures of the dynamics of IM occurrence to provide etiological clues.
Methods and findings
We used joint modeling of EBV prevalence and IM occurrence data to provide detailed sex- and age-specific EBV infection rates and IM attack rates and derivatives thereof for a target population of all Danes age 0–29 years in 2006–2011.
We demonstrate for the first time that IM attack rates increase dramatically rather precisely in conjunction to typical ages of puberty onset. The shape of the seroconversion hazard rate for children and teenagers confirmed a priori expectations and underlined the importance of what happens at age 0–2 years.
The cumulative risk of IM before age 30 years was 13.3% for males and 22.4% for females. IM is likely to become more common through delaying EBV infection in years to come.
Conclusions
The change in attack rate at typical ages of puberty onset suggests that the immunologic response to EBV drastically changes over a relatively short age-span. We speculate that these changes are an integrated part of normal sexual maturation.
Our findings may inform further etiologic research into EBV-related diseases and vaccine design. Our methodology is applicable to the epidemiological study of any infectious agent that establishes a persistent infection in the host and the sequelae thereof.
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More information: Dinesh Verma et al. Epstein–Barr virus co-opts TFIIH component XPB to specifically activate essential viral lytic promoters, Proceedings of the National Academy of Sciences (2020). DOI: 10.1073/pnas.2000625117
