ABOVE: The meningeal lymphatic system of a mouse’s brain, shown here in purple and pink

Adamaged drainage system in the brain might be behind the spotty performance of some Alzheimer’s therapies, according to a study published April 28 in Nature. Mice modeling the neurodegenerative disorder that received plaque-busting antibodies along with a treatment to stimulate the growth of lymphatic vessels in the brain saw many of their symptoms reversed. Mice with damaged lymphatics, on the other hand, didn’t respond as well to the antibodies. This suggests that dysfunctional lymphatics might hinder the performance of antibody-based immunotherapy, an approach that has had mixed results in clinical trials among Alzheimer’s patients. 

“Whenever a paper provides us with a novel way to look at Alzheimer’s, such as this one does . . . it opens up a world of possibilities,” says Gabrielle Britton, a neuroscientist at the Instituto...

The buildup of amyloid-β plaques in the brain is a hallmark of the disease, and one of the most promising immunotherapies has been a monoclonal antibody called aducanumab that breaks them up. But two clinical trials were discontinued after they yielded contradictory results, and scientists have been working ever since to figure out why as the companies continue to move forward with new trials of the therapy. The working hypothesis, Britton tells The Scientist, is that the discrepancy stems from some unexplained variation among participants.

The latest study in mice points to the brain’s lymphatic system as a potential source of that variation.

See “Brain’s Fluid Drains via Lymphatic Vessels at the Base of the Skull

In 2015, Washington University School of Medicine neuroimmunologist Jonathan Kipnis was one of several researchers who discovered an extension of the lymphatic system, named the meningeal lymphatic system, that surrounds the brain and works in conjunction with the blood-brain barrier (BBB) and the cerebrospinal fluid (CSF) to whisk away waste. Kipnis and his team, some of whom receive funding from the developers of aducanumab or otherwise hold patents or stake in companies involved in treating Alzheimer’s, described in 2018 how this lymphatic system changes as mice age: it becomes progressively dysfunctional. The group also found that those changes affect not only what material makes it out of the brain, but how efficiently molecules such as drugs can be transported inside.

Right now, all the experiments we have done are not refuting the initial hypothesis that lymphatics, microglia, and [the] BBB are all functioning together as one unit.

—Jonathan Kipnis, Washington University School of Medicine

Kipnis says he wondered whether the health of a person’s lymphatic system might explain the trials’ conflicting results. If the lymphatic system is the sink, then a clog might affect the efficacy of treatments. Poor drainage might mean the antibodies aren’t making it into the brain, or if they are, maybe there is an issue getting waste out once the antibodies do their job. “We know we can successfully dissolve plaques, but then you have all these toxic secondary compounds floating around,” Kipnis says. “The lymphatic system may be boring and uninteresting to some, but it has to work.”

There are no direct ways to image the lymphatic system using MRI or PET scans to determine if it’s healthy or not—meaning there’s no way to retroactively analyze participants in the clinical trials. And while scientists can study the system in mice, the procedure is often lethal. Kipnis says he thought about which cells could be studied as an indirect way of assessing lymphatic health and settled on microglia, immune sentinels that are common throughout in the brain. If the brain were building up toxic compounds it couldn’t otherwise flush, these microglial cells would most certainly be activated as part of its inflammatory response. 

To test his hypothesis regarding the health of the lymphatic system and the outcome of immunotherapies, Kipnis and his team used a mouse model with several mutations found in early-onset familial Alzheimer’s disease. They damaged the meningeal lymphatic tissue in half the mice, mimicking the effects of age, and gave a subset of each group the mouse version of one of two antibody therapies, aducanumab or BAN2401. Control mice either did not have their lymphatic system damaged or did not receive a drug. 

See “Infographic: How Cytokines Flow into and out of the Brain

Mice that had their lymphatic vessels ablated developed plaques more quickly, responded more poorly to the antibody therapies, and performed worse on cognitive tests such as finding a platform submerged in water, compared with mice that had their lymphatic systems intact. Using tracers, the team showed that a damaged lymphatic system both limited the number of antibodies making it into the brain from the CSF and mucked up the drainage of toxic breakdown products to the lymph nodes. The mice’s poor performance on a battery of cognitive tests suggests that faulty drainage may partially explain the mental decline that marks the disease, Kipnis tells The Scientist.

A glial link to humans

Kipnis also looked at gene expression patterns in microglia and brain endothelial cells (BECs), components of the brain’s vasculature that might work in tandem with microglial cells to regulate an immune response. In mice with damaged lymphatic vessels, microglia entered a disease-associated state characterized by an upregulation of genes involved in neuroinflammatory pathways that were more likely to promote neurodegeneration, and BECs likewise entered a proinflammatory state. A poorly functioning lymphatic system, Kipnis says, seems to impair these other aspects of brain physiology that are involved in the removal of waste too. “Right now, all the experiments we have done are not refuting the initial hypothesis that lymphatics, microglia, and [the] BBB are all functioning together as one unit.” 

When the team compared gene expression profiles of mouse microglia to those from postmortem human brains of people who had Alzheimer’s disease, the signatures looked similar. This finding “provides some reassurance that they are actually looking at phenomena that relates to humans,” says Stanley Rockson, a vascular biologist at Stanford University who studies lymphatic diseases but was not involved in the research. 

Rockson notes that these findings add to a growing sense of the importance of glial cells such as microglia in signaling and immunology, including a role in the immunological response to neurodegenerative diseases such as Alzheimer’s. “Glial cells, like the lymphatics themselves, represent part of this semi-invisible substrate that maintains normal function of the more prominent cellular types that they’re supporting,” he tells The Scientist. “We haven’t done a good job of understanding the ways in which that support is delivered and how inadequate support promotes disease, but this is a great example.”

See “A Tweak to Immune Cells Reverses Aging in Mice

Lymphatic boost for Alzheimer’s immunotherapy

The clear question is whether treatments aimed at improving the functioning of the brain’s lymphatic system could work in tandem with Alzheimer’s immunotherapies. Currently, there are no approved human treatments targeting lymphatics’ function, but a protein called vascular endothelial growth factor-C (VEGF-C) has been shown to increase the growth of lymphatic vessels in mice. In the latest study,   when Kipnis administered VEGF-C along with murine aducanumab, the effect was synergistic—the two treatments together were much better at removing plaques than the antibodies by themselves. 

These results hint that combining treatments in humans might have a similar effect, but that will be the domain of future studies. “As much as we rely upon mouse experiments, the translation to human disease is often fraught,” Rockson says. Because VEGF-C is not an approved human treatment, “that’s one aspect where the translation may have some holes in it.”

Moving forward, Kipnis and his team are planning to further refine their metrics for assessing the health of the lymphatic system. If they could develop a set of biomarkers or gene expression profiles, for example, they might be able to screen and stratify patients by how likely they are to respond to treatment. “The scientific community will probably be able to figure this out,” Kipnis tells The Scientist.

See “Biogen Presents Data on Efficacy of Alzheimer’s Drug

S. Da Mesquita et al., “Meningeal lymphatics affect microglia responses and anti-Aβ immunotherapy,” Nature, doi:10.1038/s41586-021-03489-0, 2021.

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A microscopy image of a mouse brain that shows the lymphatic system in bright purple and pink

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