Meningeal lymphatic vessels

The meningeal lymphatic vessels (or meningeal lymphatics) are a recently discovered network of conventional lymphatic vessels located parallel to the dural sinuses and meningeal arteries of the mammalian central nervous system (CNS). As a part of the lymphatic system, the meningeal lymphatics are responsible for draining immune cells, small molecules, and excess fluid from the CNS and into the deep cervical lymph nodes.^[1][2]

While it was initially believed that both the brain and meninges were devoid of lymphatic vasculature, recent studies, the first by Antoine Louveau and Jonathan Kipnis at the University of Virginia, submitted in October 2014, and another confirming the discovery by Aleksanteri Aspelund and Kari Alitalo at the University of Helsinki submitted in December 2014, identified and described the basic biology of the meningeal lymphatics using a combination of histological, live-imaging, and genetic tools.^[1][2] In general, their work is thought to extend that of the Danish neuroscientist Maiken Nedergaard in identifying the pathway connecting the glymphatic system to the meningeal compartment.

Currently, the role that the meningeal lymphatics play in neurological disease is yet to be explored. However, there is some speculation that they may contribute to autoimmune and inflammatory diseases of the CNS due to their role in connecting the immune and nervous systems.

Background

In peripheral organs, lymphatic vessels are responsible for conducting lymph between different parts of the body. In general, lymphatic drainage is important for maintaining fluid homeostasis as well as providing a means for immune cells to traffic into draining lymph nodes from other parts of the body, allowing for immune surveillance of bodily tissues.

For years, it was thought that the mammalian CNS did not contain a lymphatic system and thus relied upon alternative routes such as the glymphatic system,^[3] a cerebrospinal fluid (CSF) drainage pathway under the cribriform plate and into the lymphatics of the nasal mucosa,^[4] and arachnoid granulations to clear itself of excess protein, fluid, and metabolic waste products. Furthermore, the presumed absence of CNS lymphatics was an important pillar in the long-held dogma that the CNS is an immune-privileged tissue to which immune cells have incredibly restricted access under normal physiological conditions.

Discovery of the meningeal lymphatic vascular system

The discovery of the meningeal lymphatic vascular system was reported in two independent studies conducted in 2015 by Louveau et al. and Aspelund et al., which made the discovery using different methods. Louveau et al. noticed an unusual alignment of immune cells along the dural sinus using a meningeal whole-mount technique. Using lymphatic endothelial cell-specific markers and electron microscopy, the authors found that the immune cells were not inside blood vessels, but rather were organized inside lymphatic vessels.^[1]

Aspelund et al. had previously discovered that in the eye, another immune-privileged organ, the Schlemm's canal is a lymphatic-like vessel.^[5] As Schlemm's canal was previously considered to be a venous sinus, the authors subsequently hypothesized that similar vessels may also be found in the brain due to its similarly immune-privileged status.^[2]

Hidden vessels

In an interview with Ira Flatow on NPR’s Science Friday, Kipnis described the meningeal lymphatics as “well-hidden” when asked how, unlike the rest of the lymphatic system, they had remained unmapped into the 21st century.^[6] While many scientists study the brain parenchyma proper, Kipnis explained, his lab is relatively unique in studying the meninges, a system of membranes that envelop the brain and spinal cord.

“We are among the few labs who are interested in this very unique area of the brain: the coverings of the brain - the so-called ‘meninges.’ We’ve been looking into this area for a few years now,” Kipnis said. “I was lucky to have a phenomenal post-doctoral fellow in my lab, Dr. Antoine Louveau, who developed a very unique technique of mounting this entire covering as a whole-mount. I think this is what allowed us to find those vessels.”^[6]

Visualization of meningeal lymphatic vessels

Example of a meningeal whole-mount taken from an adult mouse.^[7] Laying the whole-mount on a glass slide allows for histological analysis of the entire dura, including the superior sagittal and transverse sinuses.

To visualize the dura using immunohistochemistry, Louveau et al. utilized a method of dissecting, preparing, and staining meningeal tissue from adult mice as a whole-mount.^[7] Briefly, the technique entails cutting around the base of the skull (inferior to the post-tympanic hook), removing the lower portion of the skull and brain, and fixing the dura within the skullcap. Following fixation, the dura can be dissected out of the skullcap as a single piece of tissue that can be utilized for histological analysis.^[7]

Aspelund et al. utilized a combination of Prox1-GFP and Vegfr3-LacZ reporters, which enable the visualization of lymphatic vessels by fluorescent microscopy or after X-gal staining, respectively. Using these techniques, Aspelund et al. were able to trace the course of the vessels to the base of the skull.^[2]

Basic biology of the meningeal lymphatics

Anatomy and route of drainage

After noticing an unusual alignment of immune cells along the dural sinus using the meningeal whole-mount technique, Louveau et al. found a network of vessels in the dura that expressed lymphatic endothelial cell marker proteins, including PROX1, LYVE1, and PDPN.^[1] In follow-up experiments, the vessels were also shown to extend along the length of both the superior sagittal and transverse sinuses, and (after the performance of anatomical tracing experiments with Evans blue dye) to directly connect to the deep cervical lymph nodes.^[1]

Using a combination of imaging technologies, Aspelund et al. found that meningeal lymphatic vessels drain down (and eventually out of) the skull along the dural venous sinuses and meningeal arteries. Meningeal lymphatic vessels also drained out of the skull alongside cranial nerves and through the cribriform plate. Molecular profiling indicated that the vessels are by all standards conventional lymphatic vessels: they express high levels of PROX1, LYVE1, PDPN and VEGFR3, but low levels of PECAM1. Subsequent experiments revealed that meningeal lymphatic vessels absorb cerebrospinal fluid and drain into the deep cervical lymph nodes.^[2]

Importantly, the meningeal lymphatics present several unique attributes that differentiate them from lymphatic vessels in peripheral organs. Compared to peripheral lymphatic vessels, the meningeal lymphatic network is markedly less complex (with far less tissue coverage and lymphatic branching). Furthermore, meningeal lymphatic vessels are generally smaller than those in the periphery and display an interesting structural homogeneity along the dural sinuses, remaining thinner and mostly unbranched along the superior sagittal sinus while growing larger and more branched along the transverse sinuses.^[1] Interestingly, the meningeal lymphatic vessels are also unique for their scarcity of valves, which prevent back-flow of lymph. While the vessels in the superior parts of the skull were mostly devoid of valves, the larger lymphatic vessels of the basal parts only contain scattered valves.^[2]

Physiological roles in draining CSF and immune cells

Confocal micrograph of meningeal lymphatic vessels and trafficking immune cells. LYVE1 (green), CD3e (red), and DAPI (blue) are shown.

Like peripheral lymphatic vessels, the meningeal lymphatics were shown to serve both the tissue drainage and immune cell trafficking functions of the lymphatic system. First, multiphoton live imaging experiments performed on anesthetized mice demonstrated that the meningeal lymphatics are capable of draining fluorescent dyes injected intracisternally into the CSF, indicating that the meningeal lymphatics are capable of draining fluid from their surrounding environment. In addition, histological analysis revealed that the meningeal lymphatics constitutively contain T cells, B cells, and MHC class II-expressing myeloid cells, demonstrating that meningeal lymphatic vessels are capable of carrying immune cells.^[1]

Additional studies also showed that the meningeal lymphatics respond to recombinant vascular endothelial growth factor C (VEGFC) exposure by increasing in diameter^[1] and completely fail to develop when VEGFC and VEGFD signaling is inhibited,^[2] providing evidence that the meningeal lymphatics share developmental characteristics with those in the periphery.

Furthermore, tracing the outflow of compounds injected into the brain parenchyma indicated that meningeal lymphatics function downstream of the glymphatic system. Genetically engineered mice that lack the meningeal lymphatic vessels demonstrated attenuated clearance of macromolecules from the brain. The uptake of tracers from the brain into deep cervical lymph nodes was completely abrogated. However, brain interstitial fluid pressure and water content were unaffected. These data suggested that meningeal lymphatic vessels are important for the clearance of macromolecules from the brain parenchyma, but in physiological settings the brain can compensate in solute clearance^[2]

Implications for neurological disease

The role, if any, that the meningeal lymphatics play in diseases of the nervous system is an area of active research - particularly with regard to neurological disorders in which immunity is a fundamental player such as multiple sclerosis, Alzheimer’s Disease, amyotrophic lateral sclerosis, Hennekam syndrome, and Prader-Willi syndrome. Importantly, however, preliminary data suggest that there seem to be identifiable meningeal lymphatic vessels in post-mortem human tissue, suggesting the possibility that they contribute to human disease.^[1]

Further reading

Work characterizing the meningeal lymphatics has been covered in a variety of news outlets, including Time Magazine,^[8] The Guardian,^[9] The Huffington Post,^[10] NPR,^[6] and several science blogs.^[11]

References