New Brain Lymphatic Vessels Drain Old Concepts

The brain has traditionally been considered an immune privileged organ, in part due to the lack of evidence for lymphatic vasculature (Galea et al., 2007). Two centuries ago, the existence of lymphatic vessels on the surface of the human brain was proposed but it has since been widely dismissed (Lukic et al., 2003). Therefore, while T cells leave all other organs via the lymphatic system to reach nearby lymph nodes, the prevailing view has been that infiltrated T cells exit the brain via venous blood circulation, circumventing the lymph nodes (Ransohoff and Engelhardt, 2012). The recent evidence for the existence of a lymphatic vascular system in the brain's meninges provided independently by two labs, first by the Kipnis lab (Louveau et al., 2015) and then by the Alitalo lab (Aspelund et al., 2015), challenges this view and raises the intriguing possibility of an alternative gateway for T cells to egress the brain. The key for identifying these meningeal lymphatic vessels by both labs was the use of a whole-mount preparation where the meninges were fixed while still attached to the skull, thus enabling the preservation of finemeningeal vessels. Kipnis' lab identified a rather simple network of narrow vessels with a distinct lumen that ran along the blood vessels in the superior sagittal and transverse sinuses on the top of the brain and importantly, expressed multiple molecular hallmarks of lymphatic endothelial cells including lymphatic vessel endothelial hyaluronan receptor 1 (Lyve-1) and vascular endothelial growth factor receptor 3 (VEGFR3). Alitalo's lab additionally described that this lymphatic network becomes more extensive at the base of the brain, where it contains lymphatic valves, and exits the skull alongside the cranial nerves. The anatomical and cellular classification of these meningeal vessels was strengthened by complimentary experiments from both labs demonstrating functional implications of VEGFR3. Injections of a VEGFR3 ligand, VEGF-c, into the cerebrospinal fluid (CSF) resulted in an increased diameter of themeningeal lymphatic vessels. Conversely, mice with impaired VEGFR3-VEGF-c signaling showed a complete absence of meningeal lymphatic vessels indicating that these vessels, like peripheral lymphatic vessels, are functionally regulated by VEGFR3. An essential functional characteristic of lymphatic vasculature is permissiveness to fluid and drainage to lymph nodes. In line with this, dyes injected into the CSF were detected both in the lumen of Lyve-1expressing vessels and in the deep cervical lymphnodes (dcLN),whereas dyes administered in the blood were not. Ligation of the lymphatic