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Recent discoveries of the glymphatic system and of meningeal lymphatic vessels have generated a lot of excitement, along with some degree of skepticism. Here, we summarize the state of the field and point out the gaps of knowledge that should be filled through further research. We discuss the glymphatic system as a system that allows CNS perfusion by the cerebrospinal fluid (CSF) and interstitial fluid (ISF). We also describe the recently characterized meningeal lymphatic vessels and their role in drainage of the brain ISF, CSF, CNS-derived molecules, and immune cells from the CNS and meninges to the peripheral (CNS-draining) lymph nodes. We speculate on the relationship between the two systems and their malfunction that may underlie some neurological diseases. Although much remains to be investigated, these new discoveries have changed our understanding of mechanisms underlying CNS immune privilege and CNS drainage. Future studies should explore the communications between the glymphatic system and meningeal lymphatics in CNS disorders and develop new therapeutic modalities targeting these systems., Perivascular pathways: pseudolymphatic vessels of the brain? In peripheral organs, colloids are extravasated across a fenestrated capillary bed, and these proteins, as well as excess tissue fluid, are returned to [...]

The central nervous system undergoes constant immune surveillance, but the route that immune cells take to exit the brain has been unclear as it had been thought to lack a classical lymphatic drainage system; here functional lymphatic vessels able to carry both fluid and immune cells from the cerebrospinal fluid are shown to be located in the brain meninges. A lymphatic system for the brain The central nervous system is under constant immune surveillance, but the exit route for immune cells has been unclear as the brain was thought to lack a classical lymphatic drainage system. Jonathan Kipnis and colleagues now show that the brain does indeed possess functional lymphatic vessels, located in the meninges, and that these vessels are able to carry both fluid and immune cells from the cerebrospinal fluid. The presence of a classical lymphatic system in the central nervous system suggests that current thinking on brain tolerance and the immune privilege of the brain should be revisited. Malfunction of the meningeal lymphatic vessels could be a root cause of a variety of neuroimmunological disorders. One of the characteristics of the central nervous system is the lack of a classical lymphatic drainage system. Although it is now accepted that the central nervous system undergoes constant immune surveillance that takes place within the meningeal compartment.sup.1,2,3, the mechanisms governing the entrance and exit of immune cells from the central nervous system remain poorly understood.sup.4,5,6. In searching for T-cell gateways into and out of the meninges, we discovered functional lymphatic vessels lining the dural sinuses. These structures express all of the molecular hallmarks of lymphatic endothelial cells, are able to carry both fluid and immune cells from the cerebrospinal fluid, and are connected to the deep cervical lymph nodes. The unique location of these vessels may have impeded their discovery to date, thereby contributing to the long-held concept of the absence of lymphatic vasculature in the central nervous system. The discovery of the central nervous system lymphatic system may call for a reassessment of basic assumptions in neuroimmunology and sheds new light on the aetiology of neuroinflammatory and neurodegenerative diseases associated with immune system dysfunction., Author(s): Antoine Louveau [sup.1] [sup.2] , Igor Smirnov [sup.1] [sup.2] , Timothy J. Keyes [sup.1] [sup.2] , Jacob D. Eccles [sup.3] [sup.4] [sup.5] , Sherin J. Rouhani [sup.3] [sup.4] [sup.6] [...]

Author(s): Antoine Louveau; Igor Smirnov; Timothy J. Keyes; Jacob D. Eccles; Sherin J. Rouhani; J. David Peske; Noel C. Derecki; David Castle; James W. Mandell; Kevin S. Lee; Tajie H. [...]

Although macrophages in the meningeal compartments of the central nervous system (CNS) have been comprehensively characterized under steady state, studying their contribution to physiological and pathological processes has been hindered by the lack of specific targeting tools in vivo. Recent findings have shown that the dural sinus and its adjacent lymphatic vessels act as a neuroimmune interface. However, the cellular and functional heterogeneity of extrasinusoidal dural macrophages outside this immune hub is not fully understood. Therefore, we comprehensively characterized these cells using single-cell transcriptomics, fate mapping, confocal imaging, clonal analysis, and transgenic mouse lines. Extrasinusoidal dural macrophages were distinct from leptomeningeal and CNS parenchymal macrophages in terms of their origin, expansion kinetics, and transcriptional profiles. During autoimmune neuroinflammation, extrasinusoidal dural macrophages performed efferocytosis of apoptotic granulocytes. Our results highlight a previously unappreciated myeloid cell diversity and provide insights into the brain's innate immune system.

The study of meningeal lymphatic vessels of the central nervous system (CNS) has recently gathered momentum, with several papers dissecting their role in draining solutes from cerebrospinal fluid and brain (Louveau et al., Nature 523(7560):337-341, 2015; Antila et al., J Exp Med 214(12):3645-3667, 2017; Aspelund et al., J Exp Med 212(7):991-999, 2015). Methodological capabilities, however, have been limited to few laboratories due to difficulties reproducibly visualizing these rare cell subsets in the meninges. To explore meningeal lymphatics fundamental role during homeostasis and how they may contribute to human pathology, the field has begun to require purification and characterization of lymphatic endothelial cells. Here, modern cell biological techniques involving a combination of histological, flow-cytometric, and functional drainage assays are applied to brain and spinal cord meninges and detailed stepwise procedures used for successful in vivo and ex vivo characterization of meningeal lymphatic vessels.