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Proteins use immune cells to cause brain disease

Controlling the number or function of immune cells could protect against deadly brain diseases, a review study concludes.

Disturbed distribution of immune cells in a Peyer’s patch during worm infection.
Inflammation, ageing and co-infection play a role in the outcome of prion diseases.

Changes to the immune system could buffer the effects of deadly brain infections called prion diseases, a review of research studies suggests.

Prions, which cause infection and neurodegeneration in people and animals, exploit certain tissues and immune cells to establish disease in the brain, scientists from the Roslin Institute have found.

Factors such as ageing, inflammation and co-infection with viruses, bacteria or parasites influence the outcome of these diseases, the review concluded.

Variation in people’s immune responses could explain why prion infection results in illness in some individuals and not in others, the review suggests.

Identifying key factors at play could be crucial in the development of management strategies for prion infection.

Limiting immune cells

Researchers focused on prion diseases such as natural sheep scrapie, chronic wasting disease in deer and bovine spongiform encephalopathy (BSE) in cattle.

They were able to manipulate the likelihood of disease development and protect against rapid development through changes to the number or function of certain immune cells.

Rather than protecting against prion infection, the spleen, which has a key role in supporting immunity, and other elements of the immune system may instead help prion diseases become established, researchers have found.

Removal of the spleen in studies in mice led to a reduction in the spread of disease, suggesting that lowering the number of immune cells slowed the infection rate.

Factors influencing disease

Stimulating the immune system to bring about inflammation was found to speed the development of disease, while the use of anti-inflammatory drugs that can supress the immune system reduced the susceptibility to disease.

The strong inflammatory responses in patients with severe Covid-19 disease could accelerate the neurodegeneration caused by prion disease or other related brain disorders such as Alzheimer’s disease.

The effects of inflammation on immune cells in the gut may have influenced an individual risk of developing prion disease after exposure to contaminated food during the BSE outbreak in the 1990s, researchers suggest.

Disease occurred mainly in younger people, the researchers noted. Ageing causes a reduction in immunity and defence against diseases, which was also shown to impede prion establishment, and may explain why only a few cases of disease occurred in the elderly.

Infection of mice with intestinal worms caused inflammation which accelerated the onset of prion disease in the brain. A range of ongoing studies are examining the implications that co-infections, especially viruses, might have in humans.

To date there is no effective treatment for prion diseases. An insight into how the immune system could be responsible for differing outcomes of infection and examining how factors such as inflammation, ageing and multiple infection can affect the spread of disease will be essential for the development of new interventions and possible treatments.

Professor Neil MabbottRoslin Institute

The research is published in the International Journal of Molecular Sciences and was funded by the Biotechnology and Biological Sciences Research Council, part of UK Research and Innovation.

**The Roslin Institute is part of the University of Edinburgh’s Royal (Dick) School of Veterinary Studies. **

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