Until recently, the concept that the brain is an ‘immune privileged’ organ was widely accepted. This concept was built from two main morphological concepts, 1) the impenetrability of the blood brain barrier (BBB) by large molecules such as antibodies or cells; and 2) absence of an intra-cranial lymphatic system. Now, however, a study by A. Louveau et al. published in Nature magazine challenges this ‘dogma’, and may overturn decades of textbook teaching.
The existence of fluid in the cerebral ventricles was observed with skepticism by the first anatomists. Besides reports of cerebral fluid existence made by Vesalius, Glisson, Haller, Vidus-Vidius, Valsalva and Varolius in the 17thand 18thcenturies, it wasn’t until 1770 when Cotugno  first demonstrated the presence of liquid (and not vapor) in the ventricles of living animals. In three notable reports published in the “Journal de Physiologie Expérimentale et Pathologique”, François Magendie confirmed the connection between the subarachnoid space and the cerebral ventricles, coining the term “cerebrospinal fluid” (CSF) in 1825 .
In the following years, Key & Retzius (1876)  elegantly showed that the CSF is essentially absorbed by arachnoid granulations (then called Pacchionian granulations) into the venous sinuses (Figure 1). Curiously, the authors also reported the existence of an accessory drainage system that was slower and less important than the venous sinus. Indeed, by the end of the 19th and early 20th centuries, many studies corroborated this view, showing the anatomical and physiological pathways of cerebrospinal fluid absorption to the venous system through arachnoid granulations. Also reported was a slower accessory drainage system to the lymphatics and cervical lymph nodes, via the nasal cribroid plate [4-7].
Figure 1. Formation of the arachnoid granulations. Protrusions containing CFS from the subarachnoid cavity empty in the superior sagittal sinus, following the systemic circulation. Credit:upright-health.com.
But one particular issue was not matching. How could the CSF flow to the cervical lymph node if the brain has no lymphatic vessels? It was largely known that the arachnoid membrane can spread out for a short distance along the cranial and spinal nerves, forming a prolongation ending in a cul-de-sac morphology observed after the injection of air or dyes into the subarachnoid space [1,3,7]. Of note, when dyes are injected into the subarachnoid space, the perineural space of the first cranial nerve, the olfactory, showed large dye deposits, just above the cribriform plate, labeling even the nasal mucous membrane. At the microscopic level, dye granules were found beneath epithelial cells of the mucous surface in firm adherence to fiber strands lined by endothelial cells but without connective tissue: i.e. lymphatic channels . Then, CSF can reach the nasal cul-de-sac to be absorbed by the local lymphatic channels. In fact, more recent works with humans and non-humans animals elegantly showed the passage of the CSF beneath the olfactory bulbs to reach the cervical lymph nodes [8-11], (Figure 2).
Figure 2. Localization of the nasal lymphatic system and its connections. Credit:openi.nlm.nih.gov.
Antigens injected into the brain ventricles can be found in the cervical lymph nodes [12, 13], and lead to antibody formation . The same migration pattern is also observed after the injection of bone marrow-derived dendritic cells (DCs), macrophages or T-cells into brain ventricles,but not in brain parenchyma [15-17]. However, not all cells injected into the central nervous system follow the nasal lymphatic vessels pathway. Studies showed the expression of adhesion molecules by the epithelium of the choroid plexus and DC migration through the ependymal layer [16, 18,19]. Also, the accessory nasal pathway plays only a small part in the absorption of CSF [7,20].
In the new elegant Nature study, Louveau and colleagues described the location of lymphatic vessels along the dural sinuses, adding a new pathway of how the interstitial fluid can leave the brain parenchyma to the cervical lymph nodes.
Importantly, the work of Louveu and colleagues, to the delight of neuroanatomists, showed the exact position of the brain lymphatic channels. The lymphatic channels converge in long vessels lying in each side of the dural sinuses (the sagittal and transverse, see Figure 3) that, accompanying the jugular veins, drain to the deep cervical lymph node. Those meningeal lymphatic vessels to the delight of immunologists also carry leukocytes. Approximately 24% of all sinusal T cells and ~12% of all sinusal MHCII+(major histocompatibility complex class II) cells could be found in these lymphatic vessels, including CD11c+ (dendritic cells) and B220+ (B lymphocytes) cells.
Figure 3. Connection between the glymphatic system and the meningeal lymphatic system. A schematic representation of a connection between the glymphatic system, responsible for collecting of the interstitial fluids from within the central nervous system parenchyma to cerebrospinal fluid, and the newly identified meningeal lymphatic system. Credit:nature.com.
However, an important question still remains: which lymphatic pathway is critical for the immune response, the nasal or the new meningeal? After injecting Evans blue (a dye that does not cross the BBB) intracerebroventricularly, a blue coloration was firstly detected in the meningeal lymphatic vessels within 30 minutes. At later time points, a progressive intensification of the blue coloration could be observed in the deep cervical lymph nodes. This effect was not observed after Evans blue injection into the nasal mucosa, concluding that the meningeal vessels, and not the nasal lymphatic channels are the primary route for CSF drainage to lymph nodes.
The findings by A. Louveau et al. may stimulate a paradigm shift and some fundamental changes the way scientists look at the brain-immune system’s interactions. The somehow unexpected resultand conclusion that the brain is like every other tissue connected to the peripheral immune system may also change some major immunology concepts.As stated by the authors “current dogmas regarding brain tolerance and the immune privilege of the brain should be revisited”.
Of note, this paradigm shift may also have important fundamental and clinical implications for brain inflammatory diseases such as multiple sclerosis, Alzheimer’s disease, autism and beyond.
Gabriel Shimizu Bassi – Department of Immunology, Ribeirão Preto Medical School, University of São Paulo (USP), Ribeirão Preto – São Paulo, Brazil. Email: firstname.lastname@example.org
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