For decades, the lymphatic system that removes waste and transports white blood cells around the body was considered absent from the mammalian brain. Then in 2015, groups in Finland and the United States independently reported the existence of an active lymphatic system in the outermost layer of brain covering, the dura mater. That was in mice. Now, a paper in the October 3 eLife extends the work to people and marmoset monkeys. Researchers led by Daniel Reich at the National Institute of Neurological Disorders and Stroke, Bethesda, Maryland, tweaked MRI protocols to allow the scans to light up lymphatic vessels in the dura. Immunostaining on postmortem brains confirmed that these structures were indeed lymphatic and not blood vessels. The system may represent a previously unknown clearance route for immune cells and large molecules from the human brain, although that remains to be demonstrated, Reich said. If so, the discovery would alter fundamental assumptions. “The communication between the immune system and the human brain may not be as different from the rest of the body as we initially thought,” he told Alzforum.
Other researchers welcomed the new ability to image these vessels in living people. “The work reported in this paper is a major step forward in the study of dural lymphatics … In vivo imaging in patients would greatly enhance our knowledge of the pathophysiology of intractable diseases such as multiple sclerosis and dementia, and may aid in their management and therapy,” Roy Weller and Roxana Carare at the University of Southampton, England, U.K., wrote to Alzforum.
The presence of these lymphatic vessels in the membranes, i.e., meninges, that wrap the central nervous system had been noted before, with the Italian anatomist Paolo Mascagni, credited with discovery of the lymph system, first reporting their existence in 1787 (Bucchieri et al., 2015; Mascagni 1787). More recent studies also described structures in the human dura that could be lymphatic vessels (Li et al., 1996). Others dismissed these as blood vessels, which are easily confused with lymph vessels.
Specific markers for lymphatic vessels were identified only recently, and the tough, leathery dura mater resists staining, Reich noted. Still, interest in the topic of brain lymph surged after the mouse studies revealed a functional lymphatic system in mammalian meninges. In mice, the system cleared cerebrospinal fluid, large molecules, and immune cells from the brain and transported them to cervical lymph nodes (Aspelund et al., 2015; Louveau et al., 2015).
Those findings inspired Reich to look for lymphatic vessels in human brains using MRI. To visualize meningeal vessels, joint first authors Martina Absinta and Seung-Kwon Ha used the gadolinium-based dye gadobutrol. Because blood vessels in the dura lack a blood-brain barrier, contrast agents such as gadobutrol can leak out and be taken up by adjacent lymphatic vessels. However, the signal from any such vessels is normally swamped by the signal from blood, which produces a bright smear throughout the dura on MRI scans. To get around this, the researchers tuned the scanner to subtract the blood signal, in effect rendering these vessels invisible. This is possible because blood flows faster and affects the magnetic properties of gadolinium differently than lymph, Reich noted. With the bright blood vessels blotted out, small surrounding vessels became visible (see image above). These measured about 1 millimeter in diameter, and were present in all five people examined. The vessels ran parallel to the dural venous sinuses, which drain blood from the brain, and matched the lymphatic arrangement reported in mice.
The researchers also scanned three marmosets, and saw similar vessels light up with gadobutrol. On pathological examination of the marmoset brains, the vessels expressed lymphatic markers. The authors immunostained three human postmortem brains for lymphatic markers, as well. The technique revealed a branched network of lymphatic vessels in the dura, including many small vessels only a few microns in diameter that were invisible on MRI. Reich noted that it took some time for his group to find the right staining conditions for the dura, perhaps explaining why these lymphatic vessels had not been identified before.
Reich’s study did not examine how the lymphatic vessels worked. Costantino Iadecola at Weill Cornell Medical College in New York noted that it remains to be seen if they drain fluid and solutes from the brain parenchyma, as the mouse vessels do. It is also unclear how this lymphatic system interacts with the “glymphatic” drainage system identified in the human brain, in which astrocytes help drive fluid flow through brain parenchyma, carrying away soluble Aβ and other waste products (Iliff et al., 2012; Iliff et al., 2013; Zeppenfeld et al., 2017). Reich believes that the glymphatic fluid that helps rinse the brain parenchyma may eventually end up in the lymphatic channels of the dura, although more research will be needed to find out how these systems connect.
Iadecola was particularly intrigued by the implications of this lymphatic system for brain immune function. Researchers used to think the brain had but a limited immune response. That’s because white blood cells can enter the brain and be exposed to antigens there, but there was no known pathway for them to leave. If these cells slip out through lymphatic channels, however, then they could travel to lymphoid organs, proliferate, and return to the brain in force to trigger a dramatic immune response. Iadecola wondered if this could be a factor in multiple sclerosis, where immune cells attack the white matter of the brain. He noted that scientists now know the meninges are loaded with lymphocytes, which can enter the brain after injury (Kim et al., 2009; Roth et al., 2014). “The meninges are emerging as almost a lymphoid organ, so it would make sense that there is this lymphatic pathway in the meninges to maintain homeostasis,” Iadecola said.
Others speculated on the applications for neurodegenerative disease. “The meningeal lymphatic system could also play a role in removing metabolic waste and proteinaceous toxic accumulates from brain. It would be very interesting to determine whether there is any dysfunction in lymphatic drainage in neurological diseases, especially neurodegenerative diseases which have aberrant protein accumulation,” Berislav Zlokovic and colleagues at the University of Southern California in Los Angeles wrote to Alzforum.
Reich plans to study whether meningeal lymphatic vessels malfunction in various neurological and neurodegenerative diseases. He noted that the MRI methods used in this study are variations on ones used routinely in clinical practice, and easily could be applied to disease studies.
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