How inconspicuous cells in a little-known organ fight inflammation in the brain

Deep in the brain, layers of tissue known as the choroid plexus produce cerebrospinal fluid (CSF) and act as a protective barrier between the brain and the CSF. But the lab of Dr. Maria Lehtinen at Boston Children’s Hospital has shown that the little-known choroid plexus can do much more than that. For example, it secretes factors that promote brain and CSF health and influences the brain as it develops.

A study led by Dr. Huixin Xu in Lehtinen’s lab, which is part of the Department of Pathology and affiliated with the FM Kirby Neurobiology Center, now shows in great detail how the choroid plexus overcomes brain inflammation by working with immune cells.

The team’s observations, published in cellcould be the basis for future treatments of neurodegenerative and neurodevelopmental disorders in which inflammation plays a role.

A real-time window to the choroid plexus

Xu and his colleagues studied the response to inflammation in a mouse model that mimics meningitis infection. To create a window into the brain, they carefully replaced part of the skull with a piece of clear Plexiglas.

Using live two-photon imaging, they were able to observe the progression of inflammation in the choroid plexus in real time – in three dimensions.

In addition, they used single-cell RNA sequencing to analyze what each cell type in the cerebrospinal fluid and choroid plexus does during the course of an infection. José Ordovás-Montañés, Ph.D., and graduate student Peter Lotfy, both in the Division of Gastroenterology, were key contributors to this analysis.

What they saw was a complicated dance.

“The choroid plexus is considered a gateway for immune cells, but there is not much evidence for this,” says Xu. “Through imaging, we were able to observe how new immune cells from the blood entered the cerebrospinal fluid through the choroid plexus. We were also able to see how cells from the spinal fluid entered the choroid plexus – the cells migrated in two directions.”

At the center of the action were epithelial cells in the choroid plexus, at the border with the cerebrospinal fluid. While epithelial cells are sometimes portrayed as a passive physical barrier, these cells were powerful actors that coordinated the tissue response from start to finish.

Tracking the immune response

At the beginning of the mock infection, epithelial cells facilitated the breakdown of the dense matrix that holds them together, creating an opening through which immune cells could pass. Neutrophils – the first responders – were observed to overcome the choroid plexus barrier and enter the cerebrospinal fluid (CSF).

After 48 hours, the majority of monocytes and macrophages were still entering the bloodstream, which is typical of tissue inflammation. However, another group of monocytes and macrophages were entering the choroid plexus in the opposite direction – from the cerebrospinal fluid.

The epithelial cells sent signals to recruit them while simultaneously secreting sticky adhesion molecules that helped the immune cells dock and accumulate at the choroid plexus, as well as growth factors that nourished them and helped them mature.

When the infection finally subsided, the mature macrophages eliminated the neutrophils, helped re-seal the choroid plexus barrier, and disappeared from the scene. The inflammation resolved.

“The choroid plexus is like a conductor that orchestrates this symphony of a response,” says Lehtinen. “I never thought that epithelial cells would be able to organize these things. Technological advances have allowed us to uncover this exciting biology that nobody would have expected.”

Is the choroid plexus used specifically to treat diseases?

Because of all its functions, Lehtinen is convinced that the choroid plexus is a true immune organ. She and Xu are interested in exploring its role in various disease constellations.

Inflammation following infection is associated with a growing number of lifelong neurological diseases, ranging from microcephaly due to congenital Zika infections to cognitive impairment associated with Long COVID. Brain inflammation is also a feature of neurodegenerative diseases such as Alzheimer’s.

“This study gives us a pretty clear and concise way to show the steps of inflammation,” says Xu. “It gives us a framework that we can then apply to see what happens in neurodegenerative and neurodevelopmental disorders.”

Lehtinen envisions a treatment that uses the choroid plexus and/or cerebrospinal fluid to protect the brain. This could, for example, prevent the neurotoxic effects of chemotherapy or protect the brains of babies in the womb from maternal inflammation.

“Epithelial cells are very secretory – could we use that to reduce inflammation?” asks Lehtinen. “Could we intervene with gene therapies? There are a million questions.”