Distinct Role of Dural and Leptomeningeal Macrophages in Maintaining Cerebrospinal Fluid Drainage to Meningeal Lymphatic Vessels

Cerebrospinal fluid (CSF) drains along the perivascular space, known as the glymphatic system, to the meninges, where meningeal lymphatic vessels (MLVs) remove toxic products with excess CSF from the brain. Macrophages are widely present in the leptomeninges and dura mater of meninges. However, whether leptomeningeal and dural macrophages play the same or distinct roles in maintaining optimal CSF drainage remains unclear. By intra-cisterna magna (i.c.m.) injection of clodronate liposomes (CLL), we observed a comprehensive depletion of leptomeningeal macrophages, a selective reduction in dural sinus-associated macrophages, a decreased density of MLVs, and disrupted CSF drainage. Further studies showed that macrophage depletion was associated with the infiltration of monocytes and the recovery of monocyte-derived macrophages. By day 14 post-CLL, although both dural macrophages and MLVs had recovered, leptomeningeal macrophages and CSF drainage had not been restored. Furthermore, our study showed that intraperitoneal (i.p.) injection of anti-colony stimulating factor 1 receptor (aCSF1R) antibody selectively depleted macrophages in the dura mater but not in the leptomeninges, without affecting MLVs or CSF drainage. Our study suggests that leptomeningeal macrophages, distinct from the dural macrophages, are essential for CSF drainage to the MLVs.