Parenchymal Border Macrophages May Help Clear Brain Toxic Waste

Many neurodegenerative diseases, such as Parkinson’s, Alzheimer’s, and others, are characterized by damaged groups of proteins in the brain. Although they have made significant efforts, scientists have only partially succeeded in finding treatments for these conditions by wiping away these toxic clusters.

Now, scientists at Washington University School of Medicine in St. Louis have discovered a novel strategy to enhance waste clearance from the brain, potentially treating or even preventing neurodegenerative diseases. They showed that the immune cells that surround the brain affect how well waste is removed from the brain, and that these immune cells aren’t working as well in old mice, as well as in people with Alzheimer’s disease and in mice with the disease.

Also, they found that giving an immune-stimulating substance to old mice renews immune cells and makes it easier for the brain to get rid of waste. The results point to a fresh strategy for slowing down some of the negative effects of aging on the brain.

Potential Neurodegenerative Disease Treatment Approach

While the death of neurons has long been the focus of research into Alzheimer’s, other cells, such as macrophages, important in the maintenance of homeostasis, on the surface of the brain, may also be involved in the disease.

“It doesn’t look likely that we will be able to revive dead or dying neurons, but the immune cells that sit on the borders of the brain are a feasible target for treating age-related brain diseases. They’re more accessible, and could be drugged or replaced. In this study, we treated aged mice with a molecule that can activate aged immune cells, and it worked in improving fluid flow and waste clearance from the brain. This holds promise as an approach to treating neurodegenerative diseases,”

said senior author Jonathan Kipnis, Ph.D.

Kipnis is a leading expert in the new field of neuroimmunology, which looks at how the immune system affects the brain both when it is healthy and when it is sick. He found a network of vessels in 2015 that transports fluid, immune cells, and small molecules from the brain into the lymph nodes, which are home to a large number of immune system cells.

He and his coworkers demonstrated last year that certain experimental Alzheimer’s therapies work better in mice when combined with a regimen designed to enhance the removal of fluid and debris from the brain. In another 2021 study, Kipnis and his team found that immune cells located in the outer layer of the meninges — tissue that covers the brain – monitor cerebrospinal fluid as it exits the brain.

Parenchymal Border Macrophages

Parenchymal Border Macrophages
PBMs are distinct from microglia and sample CSF and ISF.
(a) CD206+ PBMs (cyan) are easily distinguishable from IBA1+ microglia (yellow) and are located at the vicinity of i.v. lectin+ large blood vessels (red). Scale bar, 100 μm.
(b) Parenchymal Border Macrophagess are located outside of the brain vasculature, in the perivascular space.
(c) Quantification of whole brain sections showing spatial distribution of PBMs through both perivascular space (PVS) and leptomeninges (LM). Scale bar, 20 μm. n = 5 mice.
(d) Gating strategy for Parenchymal Border Macrophages detection. PBMs were defined as DAPI−CD45+TCRb−CD19−CD11b+CD64hiF4/80hiCD206+ cells. Parenchymal Border Macrophagess can be divided in subtypes using MHCII and CD38.
(e) WT mice received an i.c.m. injection of Alexa-647 conjugated ovalbumin (OVA; 45 kDa; 1 mg/ml; 5 μl). One hour after OVA injection, mice received an i.v. injection of Alexa-594 conjugated lectin (30 μl) and were perfused five minutes later. Maximum projection image obtained by light sheet microscopy from a cleared mouse brain showing brain OVA (magenta) distribution at the vicinity of i.v.-injected lectin+ blood vessels (cyan). Scale bar, 1mm.
(f) WT mice received an i.c.m. injection of Alexa-647 conjugated ovalbumin (OVA; 45 kDa; 1 mg/ml; 5 μl). Mice were perfused one hour after OVA injection. Representative stereomicroscopy images showing whole brain OVA distribution from the distal part of the middle cerebral artery (MCA), and quantification of both perivascular and cellular OVA distribution. Scale bars, 1mm and 200 μm (inset). n = 6 mice.
(g) Experimental schematic: WT mice received an i.c.m. injection of FITC Dextran (FITCDex; 4 kDa; 10 mg/ml; 5 μl) and brain were harvested one hour later. Brain coronal sections were stained for anti-CD206 (cyan) and DAPI. Scale bars, 2 mm and 50 μm (insets).
(h) Experimental schematic: WT mice received an intrastriatal (i.s.) injection of a cocktail containing 0.5 μl of FITC-Dex (10mg/ml; green) and 0.5 μl of OVA (1 mg/ml; magenta) and brains were harvested one hour later. Brains were then stained for anti-CD206 (cyan). Scale bars, 2 mm and 50 μm (insets).
(i) Mice received an i.s. injection of A488-OVA (green) and an i.c.m. injection of A647-OVA (magenta) one hour later. Mice were perfused one hour later (two hours after the i.s. injection). Some cells sampled both i.s. and i.c.m. OVAs. Scale bars, 2 mm and 100 μm (inset). All data are presented as mean values +/− SEM.
Credit: Nature (2022). DOI: 10.1038/s41586-022-05397-3

Kipnis and Antoine Drieu, Ph.D., a postdoctoral researcher and the paper’s first author, wanted to figure out what immune cells do in the leptomeninges, the tissues that cover the brain and spinal cord, and along the brain’s blood vessels. The fact that these cells are located at the boundary between brain tissue and cerebrospinal fluid led researchers to name them parenchymal border macrophages.

Using mouse studies, Kipnis, Drieu, and colleagues found that these macrophages control the movement of blood vessels, which in turn regulates the flow of lubricating fluid through the brain. The brain accumulated debris when these macrophages were lacking or ineffective.

In many neurodegenerative diseases, like Alzheimer’s, stroke, Parkinson’s, and multiple sclerosis, the flow of cerebrospinal fluid is reduced.

“If we can restore fluid flow through the brain just by boosting these macrophages, maybe we can slow the progression of these diseases. It’s a dream, but who knows? It might work,”

Drieu said.

Reduced Brain Blood Flow

Further research showed that the parenchymal border macrophages of people with Alzheimer’s disease and mice with a disease similar to Alzheimer’s are different: Immune cells are less able to take in and get rid of waste, and they can’t control the flow of fluids as well.

As part of the normal human aging process, brain fluid flow begins to decline around the age of 50. The same phenomenon occurs in older mice.

The type of border macrophage that is crucial for waste clearance and fluid flow is rare in older mice, according to research by Kipnis, Drieu, and colleagues.

Old mice were given a protein that increases macrophage activity, and the border macrophages began acting more like those from younger mice as a result. The treatment also helped get rid of waste from the mice’s brains and increased fluid flow.

Overall, the results show that parenchymal border macrophages could be targeted with drugs to treat brain clearance problems caused by getting older or having Alzheimer’s.

“I am discussing with colleagues how we can replace or rejuvenate those cells in aging brains and as a treatment for Alzheimer’s. I hope that one day we will be able to slow down or delay the development of age-related brain diseases with this approach,”

Kipnis, a professor of neurology, neuroscience, and neurosurgery, stated.

Reference:

Drieu, A., Du, S., Storck, S.E. et al. Parenchymal border macrophages regulate the flow dynamics of the cerebrospinal fluid. Nature (2022). doi: 10.1038/s41586-022-05397-3