- The immune cells surrounding the brain produce an IL-17 molecule which, when inhibited, has the effect of reducing feelings 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 body and mind, and the immune system may be one of the central elements.
This is the discovery made by researchers at the Faculty of Medicine at Washington University in St. Louis, Missouri (United States). In a study published in the journal Nature Immunology, they explain having discovered in mice the existence of a molecule called IL-17 which would have an effect on their behavior. Produced by immune cells surrounding the brain, it is believed to inhibit anxiety in rodents when absorbed by neurons.
“The brain and the body are not as separate as people think”, explains Professor Jonathan Kipnis, main author of the work. According to him, this immune molecule could “affect brain function through interactions with neurons to influence anxiety-like behaviors in mice”. It remains to be determined whether the IL-17 molecule has the same effect on humans.
Decreased vigilance and anxiety
IL-17 is a cytokine, which 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. In fact, the tissues surrounding the brain are full of immune cells, including so-called gamma-delta T cells, which produce the IL-17 molecule. The researchers therefore investigated whether these gamma delta T cells had an impact on the behavior of mice.
So they subjected mice to tests for memory, social behavior, foraging, and anxiety. It turned out that mice lacking gamma-delta T cells or IL-17 molecules were indistinguishable from mice with normal immune systems in all areas except anxiety.
In the wild, open fields leave mice exposed to predators such as owls and hawks, and they have therefore developed a fear of open spaces. The researchers performed two separate tests which involved giving the mice the opportunity to enter exposed areas. While mice with normal amounts of gamma-delta T cells and IL-17 levels mostly stood on the more protective edges and in 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.
Additional protection against infections
What role does the IL-17 molecule and gamma delta cells play in the immune response? To find out, the researchers injected the mice with a lipopolysaccharide, which is a bacterial product that elicits a strong immune response. Gamma delta T cells in the tissues surrounding the brain of mice 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 gamma delta T cells could detect the presence of normal bacteria such as those that make up the gut microbiota, but also pathogenic bacteria, and react appropriately to regulate behavior.
This would result in increased vigilance, which could help rodents survive infection by discouraging behaviors that increase the risk of a new infection or predation when they are weakened, say the researchers.
“The immune system and the brain most likely evolved together, says Prof. Kalil Alves from Lima, who participated in the study. Selecting special molecules to protect us both immunologically and behaviorally is a smart way to protect ourselves against infections. It’s 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 the signaling of the IL-17 molecule in neurons results in changes in behavior.