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Viruses are like pirates winning territory by capturing cells in the body and enslaving their inner workings to produce more virus particles. Basically, the immune system responds with inflammation and toxins that may be fatal for infected and healthy cells alike. 

But inside the brain, the immune system has an extra layer, its bulwark, which in response to any attack from herpes virus blocks the assault without initiating any inflammation, and thus without damaging brain tissue. 

This was the novel discovery made by an international team of researchers led by Professor Søren Riis Paludan of Aarhus University.

The newly discovered defence mechanism consists of a protein inside the cell membrane known as TMEFF1, which keeps viruses out. Herpes virus penetrates the neurons by first binding to one specific protein and then to a second. TMEFF1 binds to both of the two proteins, thereby preventing virus from entering the cell. 

To return to the pirates analogy; the virus is unable to make it over the railings to board the ship. 

The discovery is described in an article in the prestigious scientific journal Nature, and is quite a breakthrough, Paludan explains: 

“This is a unique discovery, but we don’t believe it’s a one-off. We have already made discoveries in the laboratory indicating that it’s just one of several kinds of mechanism. So our hope is that we have now discovered the first in a long line of antiviral mechanisms, not only in the brain, but in other tissue, too.” 

In other words, this discovery potentially opens a new door within immunology, and may benefit disease research and the development of novel medicinal therapies. 

From mystification to hypothesis

Paludan’s quest to identify antiviral mechanisms in the brain was born out of his mystification with prevailing theories on the workings of the immune system. Because he was unable to square them with the brain. 

“All the known immune system mechanisms have the potential to damage healthy tissue by inflammation when activated by infections. In the body, cells regenerate, but the neurons in the brain cannot be replaced. And it simply can’t be right that the brain damages itself in trying to fight off an infection,” says Paludan.

He researches disease prevention, the immune system, and specifically brain defence mechanisms at the Department of Biomedicine, Aarhus University. 

And with his team, the professor has formulated a hypothesis that the brain has some special defence mechanisms. 

“We believe that established theories about the immune system are overly simplistic,” he says.

“There has to be an extra layer in the brain’s immune system to protect the neurons against attack without inducing inflammation; meaning some mechanisms that specifically block virus replication. And we believe that evolution has promoted these protective mechanisms in the brain in order to minimise damage to the brain during an infection.”

The hypothesis was formulated in an article in 2021. In that same year, Paludan received the Lundbeck Foundation’s LF Professorship, a research grant worth DKK 28,232,500, to investigate these defence mechanisms.

Now, he and his team of researchers have discovered one of the mechanisms. TMEFF1 is the first-ever discovery of a factor that specifically protects neurons against herpes virus – without inducing inflammation, and thus helps support the hypothesis.

The brain is the porcelain

The discovery took eight years. Associate Professor Manja Idorn led the experimental arm of the research project, while studies and analyses were conducted in collaboration with a large team of international researchers at the Center for Immunology of Viral infections (CiViA) in Aarhus.  CiViA was established by the Danish National Research Foundation for the purpose of identifying novel mechanisms in antiviral defences. 

The Lundbeck Foundation grant has made it possible for Paludan’s team to concentrate exclusively on the brain. 

“I’m particularly interested in the brain because this is presumably the organ that is most sensitive to immune-mediated tissue damage. So, it really is a fragile organ. It’s the porcelain. So the immunological mechanisms have to be very specific. Or they have to be active for a very short time,” he explains. 

Herpes virus, or more precisely, herpes simplex virus (HSV), was selected for the study because it is an example of a virus that spreads easily in the population and which can infect nerve cells, just not those in the brain. While up to 80 per cent of the world’s population have latent herpes virus and some of them will have outbreaks of sores on their lips or genitals, in only around 1 in 250,000 cases does the virus cause life-threatening brain inflammation called encephalitis. This means that something in the brain must be blocking viral activity.

To find out what protects the neurons, the researchers started with genetic screening of neurons.

“You take a row of cells, maybe 10-50 million, and in every single one of them you knock out one single gene, so each cell is missing a gene. You then infect the cells with virus, and the vast majority become infected. Some cells may have a higher viral load. The idea is that the gene which was knocked out in those cells must be one that blocks virus replication,” explains Paludan. 

Knockout testing was the technique used by the researchers to identify the gene that controls production of the TMEFF1 protein. 

The next step was to test the promising candidate on a battery of neurons in petri dishes. In some of the neurons, the investigators knocked out the gene so the whole battery was exposed to virus. It turned out that those missing the TMEFF1 gene were the ones most susceptible to infection. 

The third step was testing in mice to find out whether there was an effect in a living organism. And there was. 

“When we saw that effect on brain infection in mice we were sure that this was important,” says Paludan, adding that this finding spurred on the entire research team. 

The last and most difficult step was to investigate how TMEFF1 actually works.

In this step, the researchers identified the two protein substances that TMEFF1 binds to in order to keep out virus. 

Brain resilience may be immune system-dependent

The researchers believe that the previously unknown defence mechanisms subdue the inflammation during an infection, lessening its effect and duration. Because of this, healthy neurons suffer less harm.

Paludan and his team will now be following up the study by looking for more candidates for the extra immune system layer.

The team will also be investigating if the mechanisms influence brain resilience.

“Perhaps the brain becomes more resilient when inflammatory responses are suppressed by these mechanisms. Or conversely, it’s conceivable that age-related inhibition of some of the defence mechanisms may render the brain vulnerable,” says Paludan.

The professor is planning to investigate a possible link, as demonstrated by other studies, between the immune system’s response to herpes virus and neurodegenerative diseases like Alzheimer’s and Parkinson’s. 

Paludan and his team are not alone in their interest in brain defence against virus. An American team of researchers from Rockefeller University in New York have made a discovery that underpins the significance of TMEFF1. In their genetic database, they found two patients with encephalitis caused by HSV infection, both of whom completely lack TMEFF1.k. Their discovery is published alongside the article from Aarhus.