Brain-cervical lymph node crosstalk contributes to brain injury-induced by subarachnoid hemorrhage in mice

The neuroinflammation and peripheral immune response following subarachnoid hemorrhage (SAH) aggravates early brain injury and lead to poor prognosis. Cross-talk between brain and cervical lymph nodes (CLNs) plays a crucial role in neuroinflammation and neurological diseases-associated brain injury. However, whether CLNs participate in brain damage and how injured brain activates immune system in SAH remain unclear. Previously, we reported that extravasated erythrocytes are drained into CLNs early following SAH. Here, we demonstrated that the removal of CLNs attenuated neuroinflammation and neurological deficits in 2 mouse models of SAH. Interestingly, we found that extravasated erythrocytes in CLNs following SAH were significantly engulfed by LYVE-1+ lymphatic endothelial cells (LECs) in both mandibular and deep CLNs. Single-cell RNA sequencing of mandibular LNs revealed that the medullary LEC is the primary LEC subset to degrade erythrocytes via lysosome protease cathepsin S. LECs treated by erythrocytes in vitro leaded to increasing the expression of genes related to inflammatory response, chemokines and lysosome protease cathepsin S. Importantly, deficiency of cathepsin S specifically in LECs using transgenic mice or blockage of cathepsin S by its inhibitor reduced SAH-induced brain damage. These results revealed a mechanism of how extravasated erythrocytes triggers a CLN-mediated inflammatory response and the aggravation of brain injury in SAH, and target this process may become effective therapeutic strategies to alleviate SAH pathology. At 24 hours post SAH ( induced by autologous blood infusion) or sham operation, mandibular lymph nodes were harvest from 20 mice of each group. We performed single-cell RNA sequencing (scRNA-seq) of LECs (CD45-CD31+Podoplanin+) that were isolated from mandibular LNs.