(NewsNation) — A new study suggests that the way immune system cells respond inside the protective covering around the brain may contribute to cognitive decline in people with high blood pressure.

Funded by the National Institutes of Health and published in Nature Neuroscience, the study examined cognitive decline in those with chronic hypertension, which affects more than one billion people worldwide. While a hypertension-induced stroke can lead to cognitive decline, patients who have not had a stroke may also experience a decline in cognitive function.

Researchers found mice with high blood pressure had abnormally high levels of a chemical called interleukin-17 (IL-17) in cerebral spinal fluid and the brain. IL-17 is normally released to activate the immune system, and previous research showed that a diet high in salt increased IL-17 in the gut.

Once in the brain, researchers found IL-17 activated immune cells called macrophages which are responsible for increasing inflammation and fighting off infection. Mice where the receptor for IL-17 was deleted in macrophages and mice that had macrophages deleted did not show signs of either high blood pressure or cognitive decline.

Researchers believe hypertension increases IL-17 in the dura mater of the brain and disrupts the protective barrier around the brain, allowing IL-17 to enter cerebral spinal fluid.

A drug used to prevent immune system T cells moving from the lymph nodes into the barrier around the brain and an antibody used to block T cell activity around the brain barrier both restored cognitive function in the mice.

According to researchers, this could suggest new treatment options for patients that target overactive T cells in the brain.