Enhancing Th17 cells drainage through meningeal lymphatic vessels alleviate neuroinflammation after subarachnoid hemorrhage.

Background: Subarachnoid hemorrhage (SAH) is a severe cerebrovascular disorder primarily caused by the rupture of aneurysm, which results in a high mortality rate and consequently imposes a significant burden on society. The occurrence of SAH initiates an immune response that further exacerbates brain damage. The acute inflammatory reaction subsequent to SAH plays a crucial role in determining the prognosis. Th17 cells, a subset of T cells, are related to the brain injury following SAH, and it is unclear how Th17 cells are cleared in the brain. Meningeal lymphatic vessels are a newly discovered intracranial fluid transport system that has been shown to drain large molecules and immune cells to deep cervical lymph nodes. There is limited understanding of the role of the meningeal lymphatic system in SAH. The objective of this research is to explore the impact and underlying mechanism of drainage Th17 cells by meningeal lymphatics on SAH.

Methods: Treatments to manipulate meningeal lymphatic function and the CCR7-CCL21 pathway were administered, including laser ablation, injection of VEGF-C geneknockout, and protein injection. Mouse behavior was assessed using the balance beam experiment and the modified Garcia scoring system. Flow cytometry, enzyme-linked immunosorbent assays (ELISA), and immunofluorescence staining were used to study the impact of meningeal lymphatic on SAH drainage. Select patients with unruptured and ruptured aneurysms in our hospital as the control group and the SAH group, with 7 cases in each group. Peripheral blood and cerebrospinal fluid (CSF) samples were assessed by ELISA and flow cytometry.

Results: Mice with SAH showed substantial behavioral abnormalities and brain damage in which immune cells accumulated in the brain. Laser ablation of the meningeal lymphatic system or knockout of the CCR7 gene leads to Th17 cell aggregation in the meninges, resulting in a decreased neurological function score and increased levels of inflammatory factors. Injection of VEGF-C or CCL21 protein promotes Th17 cell drainage to lymph nodes, an increased neurological function score, and decreased levels of inflammatory factors. Clinical blood and CSF results showed that inflammatory factors in SAH group were significantly increased. The number of Th17 cells in the SAH group was significantly higher than the control group. Clinical results confirmed Th17 cells aggravated the level of neuroinflammation after SAH.

Conclusion: This study shows that improving the drainage of Th17 cells by meningeal lymphatics via the CCR7-CCL21 pathway can reduce brain damage and improve behavior in the SAH mouse model. This could lead to new treatment options for SAH.