MS-linked Molecule IL-17 Impaired Immune Cells in Mice’s Brains

Marta Figueiredo, PhD avatar

by Marta Figueiredo, PhD |

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Sustained, excessive levels of interleukin 17A (IL-17A) ā€” a pro-inflammatory molecule linked to multiple sclerosisĀ (MS) and other autoimmune diseases ā€” reduce the number and activity of the brainā€™s immune cells in the dentate gyrus, according to a study in mice.

The dentate gyrus is part of the hippocampus, a brain region with a well-established role in memory.

However, no other significant changes were observed in hippocampal cells and function in the mice with chronically high IL-17 levels.

Further studies are needed to better characterize the consequences of IL-17A-induced chronic inflammation and to clarify the underlying mechanism of the drop in immune cellsā€™ activity in the brain, the researchers noted.

ā€œThese mutant mice can be used in future studies as a model for chronic IL-17A-related inflammation,ā€ Yosuke Takei, MD, PhD, said in a press release. Takei is the studyā€™s senior author and professor at the University of Tsukuba in Japan.

ā€œFurther neuronal and behavioral testing will help us begin to understand IL-17Aā€™s role in a range of debilitating neurological disorders,ā€ Takei said.

The study, ā€œEffects of RORĪ³t overexpression on the murine central nervous system,ā€ was published in the journal Neuropsychopharmacology Reports.

T-helper 17 (Th17) cells are a population of Th cells mainly located in the gut that produce the pro-inflammatory molecule IL-17A. Both Th17 cells and IL-17A are considered to play a key role in MS and other autoimmune and neurological disorders.

Tetsuya Sasaki, PhD, an author of the study and assistant professor at the University of Tsukuba, said that in addition to MS, ā€œrecent reports show that IL-17A is also a factor in Alzheimerā€™s disease, schizophrenia, and autism spectrum disorder.ā€

Notably, a previous study showed that a gut Th17-derivedĀ increase in ILā€17A levels in the blood leads to cognitive dysfunction in mice, but how excessive IL-17A affects the brain remains unclear.

Now, Takeiā€™s team evaluated how chronically high levels of IL-17A in the blood affected the brain’s cells and function by using mice genetically modified to overly produce the retinoic acid receptor-related-orphan-receptor-gamma t (ROR-gamma t), a key molecule for Th17 cells maturation and IL-17A production.

ā€œOur strategy,ā€ said Sasaki, ā€œwas to induce more generic helper T-cells to become the kind that produce IL-17A.ā€

The researchers found that their strategy worked, as higher ROR-gamma t levels increased the production of IL-17A in the miceā€™s gut and excessive IL-17A levels in the blood, compared with normal mice.

The team then analyzed the effects of these high levels of IL-17A in microglia (brainā€™s immune cells) and astrocytes (nerve cell-supporting cells) in the hippocampus, a brain area involved in learning and memory and that is affected in MS.

Results showed that mice with excessive IL-17A had reduced activity and numbers of microglia in a specific region of the hippocampus (the dentate gyrus) compared with normal mice. However, no changes in astrocytes were observed between the two types of mice.

Further analyses in the hippocampus revealed that high IL-17A levels did not affect nerve cell formation and levels of molecules involved in nerve cell communication.

Notably, mice with chronic IL-17A-derived inflammation had no major defects in spatial memory, compared with normal mice.

ā€œThese results suggest that the level of inflammation induced by elevation of ILā€17A was not enough to cause robust changes in [nerve cell communication] in the hippocampus,ā€ the researchers wrote.

The team also hypothesized that the IL-17A-associated drop in microgliaā€™s activity may be due to immunoregulatory mechanisms. Th17 cells are known to also produce high levels of the pro-inflammatory molecule TNF that activates regulatory T-cells, a type of immune cell that works to dampen immune and inflammatory responses.