By carrying out experiments on mice, American researchers have found that immune cells surrounding the brain produce a molecule which, when absorbed by neurons, acts on the behavior of rodents, making them less anxious.
- The immune cells surrounding the brain produce an IL-17 molecule which, when inhibited, has the effect of reducing the feeling of anxiety.
- The role of this molecule could be to discourage behaviors that increase the risk of infection or predation, the researchers believe.
There is a connection between the body and the mind, and the immune system is perhaps one of the central elements.
This is the discovery made by researchers at Washington University School of Medicine in St. Louis, Missouri (USA). In a study published in the journal Nature Immunology, they explain having discovered in mice the existence of a molecule called IL-17 and which would have an effect on their behavior. Produced by the immune cells surrounding the brain, it would have the effect of inhibiting anxiety in rodents when it is absorbed by neurons.
“The brain and the body are not as separate as people think”, explains Professor Jonathan Kipnis, lead author of the work. According to him, this immune molecule could “affecting brain function through interactions with neurons to influence anxiety-like behaviors in mice”. It now remains to be determined whether the IL-17 molecule has the same effect on humans.
Decreased alertness and anxiety
IL-17 is a cytokine, that is, a signaling molecule that orchestrates the immune response to infection by activating and directing immune cells.
Already linked to autism spectrum syndromes in animals and suspected of playing a role in human depression, the IL-17 molecule could influence brain disorders. Indeed, the tissues surrounding the brain are full of immune cells, including so-called gamma-delta T cells, which produce the molecule IL-17. The researchers therefore sought to find out whether these gamma delta T cells had an impact on the behavior of the mice.
They therefore subjected mice to tests of memory, social behavior, food search and anxiety. It turned out that mice lacking gamma-delta T cells or the IL-17 molecule were indistinguishable from mice with normal immune systems on all counts except anxiety.
In the wild, open fields leave mice exposed to predators such as owls and hawks, and so they have developed a fear of open spaces. The researchers performed two separate tests that involved giving the mice the chance to enter the exposed areas. While mice with normal amounts of gamma-delta T cells and IL-17 levels mostly stood at the more protective edges and closed areas during testing, mice without gamma-delta T cells or IL-17 ventured into open areas. For the authors of the study, this lack of vigilance is an indicator of a decrease in anxiety.
Extra protection against infections
What is the role played by the IL-17 molecule and by gamma delta cells in the immune response? To find out, the researchers injected the mice with a lipopolysaccharide, which is a bacterial product that causes a strong immune response. Gamma delta T cells in the tissues surrounding the mice’s brains produced more IL-17 in response to the injection. However, when the animals were treated with antibiotics, the amount of IL-17 was reduced: this suggests that the gamma delta T cells could detect the presence of normal bacteria such as those that make up the intestinal microbiota, but also the pathogenic bacteria, and respond appropriately to regulate behavior.
The result would be increased vigilance, which could help rodents survive infection by discouraging behaviors that increase the risk of re-infection or predation when they are weakened, the researchers suggest.
“The immune system and the brain most likely evolved together, says Professor Kalil Alves de Lima, who participated in the study. Selecting special molecules to protect us both immunologically and behaviorally is a smart way to protect us against infections. This is a good example of how cytokines, which essentially evolved to fight pathogens, also act on the brain and modulate behavior.”
Researchers are now studying how gamma delta T cells in the meninges detect bacterial signals from other parts of the body. They are also studying how signaling of the IL-17 molecule in neurons translates into behavioral changes.
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