Allergens may alter the brains of allergy sufferers

Scientists found that when allergic mice were exposed to grass pollen, the mice had an allergic reaction, as expected. But two unexpected results also happened in the brains of the mice—microglia were deactivated and new neurons were formed, both of which occurred in the hippocampus, according a study published in Frontiers in Cellular Neuroscience.

Accumulating research indicates that the immune system can affect central nervous system (CNS) functions like learning and memory. For this study, researchers in Austria investigated the effects of allergic lung inflammation on the hippocampus—a region known for cellular plasticity and cognitive functions in the adult brain.

The investigators sensitized one group of mice to seasonal allergies and used another group of mice as healthy controls. When the researchers exposed both groups of mice to timothy grass pollen, the allergic mice showed a typical allergic immune response (with high levels of allergen-specific IgE and IgG1 and low levels of IgG2c).

But the allergic immune response also affected the mice’s immune cells in the hippocampus in a surprising way—allergy led to a “deactivation” of microglia in the hippocampal neurogenic niche, the authors noted. Microglia are the tissue macrophages of the CNS and are responsible for CNS immune surveillance, which includes regulating the neurogenic niche—an area known for its central role in cognitive functions.

“It was highly unexpected to see the deactivation of microglia in the hippocampus [due to allergic reaction], partly because other studies have shown the reverse effect on microglia following bacterial infection,” explained Barbara Klein, PhD, one of the study’s authors and a researcher at the Institute of Molecular Regenerative Medicine at Paracelsus Medical University, in Salzburg, Austria.

“We know that the response of the immune system in the body is different in the case of an allergic reaction vs. a bacterial infection,” Dr. Klein said. “What this tells us is that the effect on the brain depends on the type of immune reaction in the body.”

Why are microglia deactivated in the hippocampus of allergic mice? At this point, the researchers can only speculate. “However, with the current experimental set-up, we do not know if this microglial deactivation is transient or persists for longer periods,” they wrote. “If immune surveillance in the hippocampus is down-regulated for extended periods, this may have detrimental consequences.”

Because the experiment involved young and healthy adult mice, the allergy-induced changes in hippocampal microglia may actually have been rather subtle. “Therefore, it would be highly interesting to investigate what allergy does to microglia in the aged CNS, which might already be somewhat primed for a pro-inflammatory activation,” the researchers noted.

The allergic reaction also caused an increase in neurogenesis in the hippocampus. Could this mean that allergy has an effect on cognitive functions?

“So far, we can only hypothesize that the observed increase in hippocampal neurogenesis may also have functional consequences on long-term potentiation or learning and memory. For this, further studies including electrophysiological analysis and behavioral tests are needed,” the researchers concluded.