Although the immune-brain connection has been studied and hypothesi...
Lymphatic vessels are responsible for conducting lymph between diff...
The immune connection to neurological diseases has long been hypoth...
A lymph vessel is a thin tube that carries lymph (lymphatic flu...
The blood–brain barrier (BBB) is a highly selective semipermeable m...
Paolo Mascagni (January 25, 1755 – October 19, 1815) was an Italian...
*For correspondence: reichds@
ninds.nih.gov
†
These authors contributed
equally to this work
Competing interest:
See
page 13
Funding: See page 12
Received: 19 June 2017
Accepted: 01 September 2017
Published: 03 October 2017
Reviewing editor: Heidi
Johansen-Berg, University of
Oxford, United Kingdom
This is an open-access article,
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Human and nonhuman primate meninges
harbor lymphatic vessels that can be
visualized noninvasively by MRI
Martina Absinta
1†
, Seung-Kwon Ha
1†
, Govind Nair
1
, Pascal Sati
1
,
Nicholas J Luciano
1
, Maryknoll Palisoc
2
, Antoine Louveau
3
, Kareem A Zaghloul
4
,
Stefania Pittaluga
2
, Jonathan Kipnis
3
, Daniel S Reich
1
*
1
Translational Neuroradiology Section, National Institute of Neurological Disorders
and Stroke, National Institutes of Health, Bethesda, United States;
2
Hematopathology Section, Laboratory of Pathology, National Cancer Institute,
National Institutes of Health, Bethesda, United States;
3
Center for Brain
Immunology and Glia, Department of Neuroscience, School of Medicine, University
of Virginia, Charlottesville, United States;
4
Surgical Neurology Branch, National
Institute of Neurological Disorders and Stroke, National Institutes of Health,
Bethesda, United States
Abstract Here, we report the existence of meningeal lymphatic vessels in human and nonhuman
primates (common marmoset monkeys) and the feasibility of noninvasively imaging and mapping
them in vivo with high-resolution, clinical MRI. On T2-FLAIR and T1-weighted black-blood imaging,
lymphatic vessels enhance with gadobutrol, a gadolinium-based contrast agent with high
propensity to extravasate across a permeable capillary endothelial barrier, but not with
gadofosveset, a blood-pool contrast agent. The topography of these vessels, running alongside
dural venous sinuses, recapitulates the meningeal lymphatic system of rodents. In primates,
meningeal lymphatics display a typical panel of lymphatic endothelial markers by
immunohistochemistry. This discovery holds promise for better understanding the normal
physiology of lymphatic drainage from the central nervous system and potential aberrations in
neurological diseases.
DOI: https://doi.org/10.7554/eLife.29738.001
Introduction
Recent reports (Aspelund et al., 2015; Louveau et al., 2015b) described the existence of a network
of true lymphatic vessels within the mammalian dura mater that runs alongside blood vessels, nota-
bly the superior sagittal and transverse sinuses. The dural lymphatic vessels display typical immuno-
histochemical markers that identify lymphatic vessels elsewhere in the body. They provide an
alternate conduit for drainage of immune cells and cerebrospinal fluid (CSF) from the brain, beyond
previously described pathways of flow: via arachnoid granulations into the dural venous sinuses, and
via the cribriform plate into the ethmoid region (
Weller et al., 2009). Although early reports, based
on injections of India ink into the cisterna magna of the rat, suggested that the dural pathway
accounts for only a minority of the drainage (
Kida et al., 1993), the more recent studies
(
Aspelund et al., 2015; Louveau et al., 2015b), which are based on injections of fluorescent tracers
and in vivo microscopy, indicate that the dural system may be substantially more important for drain-
age of macromolecules and immune cells than previously realized.
Whether a similar network of dural lymphatics is present in primates remains unknown. Moreover,
noninvasive visualization of the dural lymphatics – a necessary first step to understanding their
Absinta et al. eLife 2017;6:e29738. DOI: https://doi.org/10.7554/eLife.29738 1 of 15
SHORT REPORT
normal physiology and potential aberrations in neurological diseases – has not been reported. We
therefore verified pathologically the existence of a dural lymphatic network in human and nonhuman
primates (common marmoset monkeys) and evaluated two magnetic resonance imaging (MRI) tech-
niques that might enable its visualization in vivo. First, the T2-weighted fluid-attenuation inversion
recovery (T2-FLAIR) pulse sequence, which is the clinical standard for detecting lesions within the
brain parenchyma, is highly sensitive to the presence of gadolinium-based contrast agents in the
CSF (
Mamourian et al., 2000; Mathews et al., 1999; Absinta et al., 2015). Second, ‘black-blood’
imaging sequences, which are typically used for measurement of vascular wall thickness or detection
of atherosclerotic plaque, are tuned to darken the contents of blood vessels (even when they contain
a gadolinium-based contrast agent), but in the process the images may highlight vessels with other
contents and flow properties (
Mandell et al., 2017). For comparison, we also acquired a postcon-
trast T1-weighted Magnetization Prepared Rapid Acquisition of Gradient Echoes (MPRAGE) MRI
sequence, which is widely implemented for structural brain imaging and depicts avid enhancement
of dura mater and blood vessels, but which would not be expected to discriminate lymphatic
vessels.
Results and discussion
Cerebral blood vessels have a highly regulated blood-brain barrier, protecting the neuropil from
many contents of the circulating blood. Under physiological conditions, the blood-brain barrier pre-
vents gadolinium-based chelates in standard clinical use from passing into the Virchow-Robin peri-
vascular spaces and parenchyma, so that these structures do not enhance on MRI. On the other
hand, dural blood vessels lack a blood-meningeal barrier, enabling leakage of circulating fluids and
small substances, including gadolinium-based compounds. This explains the thin, though often
incomplete, dural enhancement that is seen on conventional T1-weighted MRI scans under
eLife digest How does the brain rid itself of waste products? Other organs in the body achieve
this via a system called the lymphatic system. A network of lymphatic vessels extends throughout
the body in a pattern similar to that of blood vessels. Waste products from cells, plus bacteria,
viruses and excess fluids drain out of the body’s tissues into lymphatic vessels, which transfer them
to the bloodstream. Blood vessels then carry the waste products to the kidneys, which filter them
out for excretion. Lymphatic vessels are also a highway for circulation of white blood cells, which
fight infections, and are therefore an important part of the immune system.
Unlike other organs, the brain does not contain lymphatic vessels. So how does it remove waste?
Some of the brain’s waste products enter the fluid that bathes and protects the brain – the
cerebrospinal fluid – before being disposed of via the bloodstream. However, recent studies in
rodents have also shown the presence of lymphatic vessels inside the outer membrane surrounding
the brain, the dura mater.
Absinta, Ha et al. now show that the dura mater of people and marmoset monkeys contains
lymphatic vessels too. Spotting lymphatic vessels is challenging because they resemble blood
vessels, which are much more numerous. In addition, Absinta, Ha et al. found a way to visualize the
lymphatic vessels in the dura mater using brain magnetic resonance imaging, and could confirm that
lymphatic vessels are present in autopsy tissue using special staining methods.
For magnetic resonance imaging, monkeys and human volunteers received an injection of a dye-
like substance called gadolinium, which travels via the bloodstream to the brain. In the dura mater,
gadolinium leaks out of blood vessels and collects inside lymphatic vessels, which show up as bright
white areas on brain scans. To confirm that the white areas were lymphatic vessels, the experiment
was repeated using a different dye that does not leak out of blood vessels. As expected, the signals
observed in the previous brain scans did not appear.
By visualizing the lymphatic system, this technique makes it possible to study how the brain
removes waste products and circulates white blood cells, and to examine whether this process is
impaired in aging or disease.
DOI: https://doi.org/10.7554/eLife.29738.002
Absinta et al. eLife 2017;6:e29738. DOI: https://doi.org/10.7554/eLife.29738 2 of 15
Short report Human Biology and Medicine Neuroscience