Canonically, anatomical descriptions of the central nervous system (CNS) do not include any lymphatic vessels. The absence of lymphatic structures was hypothesized to safeguard the brain against damage from foreign molecules and cells, much like the blood-barrier isolates the brain from circulating molecules in the blood. However, deeper characterization of the cell types present in the CNS has repeatedly demonstrated that immune cells are present, are actively searching for pathogens and infected cells, and play significant roles in a number of neurological diseases and conditions. Many theories have been developed to explain how these cells travel into and out of the CNS, but to date, none have been confirmed.
In a recent study in Nature, a team led by Dr. Jonathan Kipnis at the University of Virginia performed highly detailed examinations of mouse brains and the immune cell types present in meninges. Their investigations reveal previously overlooked vascular structures lining the dural sinuses and expressing all the characteristic markers of lymphatic endothelial cells. Moreover, they connect to the peripheral lymphatic system. This discovery clarifies many long-standing questions regarding neuroanatomy and the movement of immune cells within the CNS, and may provide new avenues for the development of therapeutics for neurological and neurodegenerative diseases.
With a goal of identifying ways that immune cells might enter or leave the meninges, Dr. Kipnis’ team performed extensive sectioning, immunofluorescent staining, and imaging of brains from C57BL/6J mice (Stock# 000664). Lymphatic structures were visualized following staining for the lymphatic endothelial marker, Lyve-1, or by observing red fluorescence from the Prox1 promoter in the lymphatic vessel reporter strain C57BL/6-Tg(Prox1-tdTomato)12Nrud/J (Stock # 022766). Lymphatic vessel-like structures lining the dural sinuses correlated with higher numbers of nearby T cells and MHC class II-expressing cells, suggesting that these structures transport these immune cells and likely are a portal for their entry into the CNS. These vessels originate near the eyes and pass over the olfactory bulb, where they then parallel the sinuses. Further characterization of these vessels revealed that they possess many hallmarks of peripheral lymphatic vessels, including a non-continuous basement membrane, absence of smooth muscle cells, and expression of CCL21, an immune cell chemoattractant.
The vessels join with the deep cervical lymph nodes, connecting them with the entirety of the lymphatic system. The researchers validated that immune cells can pass from the meninges to peripheral vessels by demonstrating that T cells accumulated in the meningeal spaces when the animals’ cervical lymph nodes were ligated.
The identification of these elusive structures was facilitated by advanced microscopy techniques and improved immunofluorescent tools. The connection of the CNS to the lymphatic system answers many long-standing questions regarding the role of immune surveillance in these tissues. Future exploration of the functional role of these vessels in normal and diseased brains may provide important insight into neurological disease mechanisms and help in the development of improved therapies.