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glymphatic pathway, and (iii) the meningeal pathway. Each of these
pathways are discussed in the succeeding texts.
The perivascular pathway
The Weller-Carare group [2] has outlined the perivascular path-
way of CNS lymphatic clearance using tracer studies. CNS lym-
phatics are responsible for draining two types of extracellular
fluid (ECF): cerebrospinal fluid (CSF) and ISF; these each drain
via distinct routes from the brain [3] (Fig. 1). The lymphatic drainage
of CSF represents only a minor fraction; studies involving the injec-
tion of radiolabeled albumin into rats traced approximately 15%
penetrating into the CSF, the other 85% draining out with ISF [4].
Most of the CSF drainage in the adult human brain returns back
into the venous blood through arachnoid villi via vacuolar channels
that pass through the endothelium, from the abluminal side to the
veins [5–7]. Only a minor portion of CSF drains through the lym-
phatics in humans, which is significantly different from results in
experimental animals, where up to 50% of the CSF drains into the
lymph nodes [6]. Most recent data from Proulx’s group suggest that
Cervical
lymph node
ISF
CSF
Carotid
artery
Cervical
lymph node
ISF
CSF
Carotid
artery
FIG. 1 Drainage of extracellular fluid
by the CNS lymphatics. The CNS lym-
phatics are responsible for draining two
types of extracellular fluid (ECF): cerebro-
spinal fluid (CSF, shown in blue) and inter-
stitial fluid (ISF, shown in purple). The
drainage of these fluids is via distinct
routes from the brain [3].
94 5. CNS lymphatics in health and disease
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lymphatic vessels are predominantly responsible for CSF outflow in
mice, and this clearance reduces significantly with increasing age
[8]. Injection of Indian ink into the subarachnoid space of rats
revealed drainage pathways along the olfactory nerves within nasal
lymphatic vessels, which ultimately emptied into the superficial and
deep cervical lymph nodes [9]. Similar experiments have revealed
CSF lymphatic drainage occurring through nasal lymphatic vessels
in several animals including mice, cats, dogs, and rabbits [10–12].
Injection of radioactive albumin into the CSF of nonhuman primates
resulted in an accumulation of tracer in the cervical lymph nodes,
supporting CSF lymphatic drainage in nonhuman primates [13].
Direct evidence of CSF lymphatic flow in human was demonstrated
using Microfil injection into recently deceased cadavers that delin-
eated a remarkably similar pattern to that seen in mouse [5].
Lymphatic drainage of ISF differs not only from that used for CSF
but also significantly from lymphatic drainage of other systems.
Mouse studies involving the injection of various tracers revealed a
pathway for ISF drainage within intracranial arterial walls into the
cervical lymph node (LN) in the neck [2, 4]. In particular, ISF and
solutes, but not cells, were found to drain along 100–150-nm-wide
basement membranes within the walls of these arteries and capil-
laries [2]. Interestingly, this lymphatic flow of ISF is described to
occur in the reverse direction from blood flow in these arteries and
capillaries, at a rate similar to lymphatic drainage of other tissues
(0.1–0.3uL/min) [14, 15], and may depend upon rhythmic arterial
blood pulsations to provide propulsion through these channels.
The glymphati c pathway
A separate lymphatic clearance system has been proposed by the
Nedergaard group, based on in vivo two-photonimaging and ex vivo
fluorescence imaging with various tracers [16]. They showed that a
large proportionof subarachnoid CSF (but not ventricular CSF) enters
into the brain parenchyma. This movement was demonstrated to be
rapid and size dependent, the size restriction being dictated by astro-
cytic end-feet that are thought to serve as a size exclusion. This bulk
flow of CSF is believed to be responsible for the clearance of fluid and
solutes from the brain, and the CSF exchanges with the ISF. The water
channel protein aquaporin-4 (AQP4), which is highly polarized to the
end-feet of the astroglia, is critical in facilitating the bulk flux of CSF
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and therefore clearance from the brain parenchyma. Contrast-
enhanced magnetic resonance imaging (MRI) was used to observe
live rat brains and to identify the key nodes of influx at the pineal
and pituitary recesses. [17]. Owing to the importance of AQP4 in glial
cells in this system, it is referred to as the “glymphatic” system. AQP4
antibody-mediated autoimmunity has also been implicated in the
pathogenesis of neuromyelitis optica (NMO), an inflammatory disor-
der of the CNS [18], indicating NMO may potentially be associated
with glymphatic system dysfunction.
The meningeal pathway
More recently, specific molecular markers for lymphatic endothe-
lial cells have become available and have allowed two independent
studies to identify anatomical lymphatic vessels in the brain’s
meninges: first by Louveau et al. and then Aspelund et al. [19,
20]. Louveau et al. observed a simple network of narrow lymphatic
vessels in the meninges underlying the bony parts of the skull
(Fig. 2). These vessels, with a distinct lumen, run parallel to the dural
sinuses. Meningeal lymphatics, identified in adult mice, express
most of the molecular hallmarks of lymphatic endothelial cells,
including the lymphatic vascular endothelial hyaluronic acid recep-
tor (LYVE-1), prospero homeobox protein-1 (Prox1), podoplanin,
and vascular endothelial growth factor receptor (VEGFR)-3. These
meningeal lymphatics have been shown to not belong to the
“cardiovasculature” [20] and are devoid of smooth muscle cells,
maintaining only anatomical and molecular features characteristic
of “initial lymphatics.” Similar to initial peripheral lymphatics, men-
ingeal lymphatics display a discontinuous basement membrane sur-
rounded by anchoring filaments without intraluminal (secondary)
valves [20] (Fig. 3). Aspelund et al. further identified meningeal lym-
phatics, although relatively scarce, in the superior parts of the skull,
becoming more extensive at the skull base [19]. Lymphatics at the
base of the skull have intraluminal valves and exit the skull along-
side arteries, veins, and cranial nerves. The valves show evidence for
the presence of smooth muscle cells in these vessels, suggesting a
transition from initial to collecting lymphatics as these vessels exit
the brain [19]. Further studies are still needed to examine if these
vessels demonstrate collecting vessel structure or display active lym-
phangion pumping function. Brain ISF and CSF are absorbed into
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the meningeal lymphatics from the subarachnoid space and are
transported downstream of deep cervical LNs. Additional studies
are also necessary to examine the extent of meningeal lymphatic net-
works, their roles in CSF clearance, and ultimately how these find-
ings can be translated into clinical applications.
FIG. 2 Schematic representation of the pattern of intracranial cerebral spinal fluid
flow. (A) Cerebral spinal fluid (CSF) is produced by the choroid plexus, flows through
the lateral and third ventricles, and exits through the foramina of Luschka and
Magendie, to reach the subarachnoid space over the convexities. (B) Movement of CSF
and interstitial fluid (ISF) to and from the subarachnoid space. CSF is also able to diffuse
in and out of the brain parenchyma along the perivascular space. From Louveau A, Smirnov
I, Keyes TJ, Eccles JD, Rouhani SJ, Peske JD, et al. Structural and functional features of central
nervous system lymphatic vessels. Nature 2015;523(7560):337–341, with permission.
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The role of the lymphatic system in diseases of the CNS
Although the significance of CNS lymphatic drainage has yet to
be fully explored, it is likely to play an important part in the CNS
physiology, and its disturbances may lead to a number of neuroim-
munological diseases, for example, Alzheimer’s disease (AD),
Blood flow
Perivascular flow of ISF
CSF mixes with ISF
BC<
4
Brain
parenchyma
Glymphatics para-arterial inflow of CSF
Blood flow
End-feet of
astrocytes
Subarachnoid space
Dura matter
Glymphatics para-venous outflow of CSF-ISF
Cervical
lymph node
Perivascular flow of ISF
Meningeal
pathway
ISF and CSF flo
w
FIG. 3 Schematic displaying meningeal lymphatics. The meningeal lymphatics dis-
play a discontinuous basement membrane surrounded by anchoring filaments without
intraluminal (secondary) valves.
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Parkinson’s disease (PD), and multiple sclerosis (MS). Lymphatic
drainage in the brain is especially critical because intracranial pres-
sure is extremely sensitive to the volume and CSF-ISF balance and
intracranial pressure therefore elevate rapidly with CSF or ISF
imbalance. Impaired CSF drainage can also cause hydrocephalus,
where CSF penetrating into white matter causes interstitial edema
[21]. Furthermore, elevated intracranial pressure also impairs cere-
bral blood flow, which can lead to widespread cerebrovascular, neu-
ral, and metabolic distress [6].
Alzheimer’s disease
Alzheimer’s disease (AD) is characterized by intraneuronal accu-
mulations of tau and the extracellular accumulation of insoluble
amyloid-beta plaques in the brain [22]. Amyloid-beta is produced
in the brain itself and can serve as an endogenous “tracer” of
CNS lymphatic drainage [23]. AD brains typically exhibit increased
fibrous thickening, arterial stiffening, and diminished pulsatility,
which may in turn limit periarterial lymphatic transport [24]. When
lymphatic drainage is impaired, cerebral vessels exhibit increased
insoluble amyloid-beta deposits in the arteries and microvessels
and may further limit perivascular lymphatic drainage; this is espe-
cially evident in cerebral amyloid angiopathy (CAA) [23, 24].
Impaired CNS lymphatic function results in the loss of normal
homeostasis in the brain, for example, insufficient removal of Ab
and other metabolites. Such accumulation of Ab can result in the for-
mation of toxic forms of Ab that causes neuronal injury [25]. This
evidence suggests the importance of CNS lymphatic function in
amyloid-beta clearance; an improved understanding of the physio-
logical responses could provide possible targets for drug discovery
programs for therapy.
Multiple sclerosis
The CNS is considered to be relatively “immunologically
privileged.” Nevertheless, immune reactions do occur in the CNS,
and its associated lymphatics drain into cervical LN and influence
neuroimmunity. Antigen injection into the brain parenchyma (or
CSF) has been shown to trigger the formation of antibodies, and
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similarly, myelin antigens have been detected in cervical LNs in MS
patients and in experimental autoimmune encephalomyelitis (EAE),
“an experimental mouse model for MS” [26, 27]. The pathogenesis of
MS and EAE involves T-cell-mediated immune reactions in the
brain. These T cells enter the CNS via interactions between specific
integrins on the T cells (alpha4beta1 and alpha4beta7) and vascular
adhesion molecule (VCAM) and mucosal addressin cell adhesion
molecule-1 (MAdCAM-1) on brain endothelial cells [28]. Cervical
lymphadenectomy has been shown to reduce T cell-mediated
immune reactions in the brain of EAE and consequently EAE sever-
ity, leading to the conclusion that the T lymphocytes may come from
cervical LNs [29]. MS pathogenesis involves the drainage of soluble
antigens to LN from the CNS leading to the activation of T cells for
CNS-specific proteins [30]. Another possibility is molecular mimicry
resulting in the activation of T cells for CNS proteins in the periph-
ery [3]. Activated T cells then migrate to cervical LN where they
interact with antigens drained from the CNS, which drive autoim-
munity [3, 28, 31].
Cerebral ischemic injury and microinfarcts
The role of the CNS lymphatic system in the pathogenesis of
stroke has been investigated using the cervical lymphatic blockade
(CLB) model, in which the cervical lymph nodes are surgically
removed after ligating the input and output vessels. This model
therefore blocks the cerebral lymphatic drainage, causing excess
water and waste retention that can lead to brain edema and intracra-
nial hypertension [1]. The importance of cerebral lymphatic function
was demonstrated in the ischemic stroke model in which the middle
cerebral artery is occluded; the resulting phenotype (such as edema,
oxidative stress injury, and neuronal damage) was dramatically
worsened with cervical lymph node blockade [32], and CLB has also
been shown to cause an expansion of infarct volume in rats [33].
In terms of secondary injury from ischemia, oxidative injury signif-
icantly worsened with CLB, indicated by the increased activity of
lactate dehydrogenase in the serum and the decreased activity of
superoxide dismutase in brain tissues, supporting the role of CNS
lymphatic drainage in the protection of the brain, especially from
secondary oxidative injury [34].
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Aging
Aging plays a considerable role in both central and peripheral
immune responses, incorporating both the glymphatic system and
brain lymphatic vasculature. As we age, immunosenescence (or a
decline in immune cell functions) sets in [35, 36] leading to increased
cytokine and chemokine production, for example, IL-1beta, IL-6,
IL-18, and TNF-alpha suggestive of links between CNS hyperim-
mune activation and advancing age and neuroinflammation. This
increased low-grade chronic inflammatory milieu in aged brains
is especially relevant and present in neurodegenerative disorders.
Several studies have shown that when senescent cells were removed
from aged mice artificially, the animals lived longer and were
healthier [37, 38]. These findings further support the important role
that the glymphatic system and brain lymphatic vasculature play in
trying to maintain a healthy environment for brain-resident cells,
which prolongs cognitions and preserves brain structure.
Aging also increases cancer risk, and with respect to the brain,
aging is a strong risk factor for the most aggressive adult brain can-
cer, glioma. While the mechanisms underlying how aging signifi-
cantly contributes to the occurrence of gliomas are unclear, recent
studies suggest that rather than undergoing proliferation and self-
renewal in response to growth signals, some dysregulated neuronal
progenitor cells undergo senescence instead [39, 40].
Exploiting the CNS lymphatics for drug discovery
As our knowledge of the lymphatic system has expanded, so has
the design of lymphatic delivery systems for drug delivery. Initially,
these were simple systems that relied on passive lymphatic access;
however, rapid progress in the lymphatic field now means that lym-
phatic delivery systems are now much more complex involving the
use of nanotechnology to mimic endogenous macromolecules and
lipid conjugates that “hitchhike” onto lipid transport processes [37].
Additionally, it is now appreciated that the lymphatic system is
involved in a number of processes and diseases, which have com-
pounded the need and optimization of drug delivery systems to
treat diseases such as inflammatory diseases (e.g., rheumatoid
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arthritis and inflammatory bowel disease [41, 42]), cancer and met-
astatic disease [43, 44], infections, and autoimmune diseases—such
as HIV, hepatitis, and Ebola virus [45–48]—and also issues associ-
ated with solid organ transplant rejections [49, 50].
Maximizing drug delivery into the lymphatics
Several major advantages for drug discovery programs have
focused on routes of lymphatic delivery based on the ability to
increase drug exposure and/or enhance therapeutic efficacy (i.e.,
increased efficacy and lower toxicity) in immunotherapy, vaccina-
tion, viral therapy, and cancer metastasis (by targeting lymph
nodes) [37]. Rates of flow into the blood capillaries is approximately
100–500-fold higher than lymph flow, and as such, drainage of
orally or parenterally delivered small molecules and drugs typically
occurs via this route into the interstitial space (of importance, intes-
tinal lymphatic transport bypasses the hepatic first-pass metabo-
lism). In the case of larger molecules such as proteins or large
peptides, drainage into capillaries is not an option due to size,
and as such, they are always transported directly into the lym-
phatics. In addition, ALL injected drugs enter the lymphatics first.
Thus, drug delivery programs have focused on targeting the lym-
phatic system for effective drug treatments. Recent developments
and advancements in the fields of materials and pharmaceutical sci-
ences have driven the movement into finding additional methods of
delivery to facilitate the movement of low-molecular-mass thera-
peutic directly into the lymphatics; these include the use of synthetic
macromolecular carriers such as liposomes, in situ association with
endogenous macromolecular constructs such as lipoproteins, and
also the use of cells with inherent lymphotropic properties such
as leukocytes [37, 51]. Other approaches include targeting of the
lymph nodes to deliver effective therapy: for example, Liu et al.
[52] elegantly used the albumin hitchhiking approach to molecular
vaccines, via the synthesis of amphiphiles (amph-vaccines) [52].Inso
doing, they were able to successfully deliver optimized CpG-DNA/
peptide amph-vaccines to mice increasing T-cell priming and anti-
tumor efficacy with reduced toxicity.
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The breakthrough in discovering the glymphatic and lymphatic
systems in the brain has opened up the window of opportunity to
identify new therapeutic targets for the treatment of neurovascular
and neurodegenerative disorders. These newly identified cerebral
systems are both able to facilitate the transport and clearance of
fluid, drug-like molecules, macromolecules, proteins, and immune
cells from the brain [53]. These events have huge implications in
drug discovery programs, particularly in the context of clearance
of a drug, which may be much less efficient in aging or in neurovas-
cular and neurodegenerative disorders. These are exciting times,
and certainly, the drug pipeline (with new or repurposed drugs)
will continue to expand as targeting of the glymphatic and lym-
phatic system continues.
Summary
In this chapter, we have reviewed that recent developments in
lymphatic vascular biology have led to the understanding that the
brain does indeed have an efficient waste disposal system that uses
both the glymphatic system and the meningeal lymphatic vessels.
These previously unknown mechanisms have spearheaded greater
opportunities for drug discovery programs (focusing on drugs, pro-
drugs, vaccines, and delivery systems) targeting the lymphatics for
the treatment of a number of neurovascular and neurodegenerative
disorders. Much work is still needed, but the pace progress is
increasing and will lead to many future therapies and refinements
that target these systems.
Acknowledgments
The authors would like to thank Ms. G. Kaur for her help with Figs. 1 and 3.
References
[1] Sun BL, Wang LH, Yang T, Sun JY, Mao LL, Yang MF, et al. Lymphatic drainage
system of the brain: a novel target for intervention of neurological diseases.
Prog Neurobiol 2018;163-164:118–43.
[2] Carare RO, Bernardes-Silva M, Newman TA, Page AM, Nicoll JA, Perry VH,
et al. Solutes, but not cells, drain from the brain parenchyma along
103References
https://t.me/medicina_free
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