Dural Macrophages in The Regulation of Meningeal Lymphatic Vessel Structure and Brain Lymph Drainage

Abstract

Meningeal lymphatic vessels (mLVs) are the primary route for cerebrospinal fluid (CSF) outflow from the brain. Importantly, dysfunction of mLVs in mice has been shown to increase toxic protein accumulation, resulting in a decline in neurocognitive processing. However, the exact cell types needed to maintain mLVs under homeostatic conditions remain unknown. Because dural macrophages display a unique distribution pattern along mLVs, we hypothesized dural macrophages are required to maintain mLV morphology and function. As such, we show that dural macrophage depletion reduced mLV density and impaired brain lymph drainage 7 days after intra-cisterna magna treatment with clodronate liposomes (CLL). This method of dural macrophage depletion did not impact lymphangiogenic growth factors but significantly reduced matrix metalloproteinase-9 (MMP-9). Subsequent recovery of mLVs density occurred 14 days post-treatment, along with the recovery of LYVE-1+ peri-lymphatic dural macrophages at 21 days post-treatment. These results indicated that dural macrophages maintain mLVs via extracellular matrix (ECM) remodeling. Next, since dural macrophages are replenished by bone marrow-derived cells (BMDCs), we characterized dural macrophage and BMDC dynamics during mLV postnatal development. We observed a delay in mLV development in BMDC reporter mice (LysMgfp/gfp), which have a green fluorescent protein (GFP) knock-in at the Lysozyme M locus. Crossing the LysMgfp/gfp strain with wild-type mice maintained one functional LysM allele (LysMgfp/+), which rescued postnatal mLV development. Our results suggest that a lack of LysM expression caused a delay in mLV development, possibly due to dysfunction in either BMDCs or dural macrophages. Finally, since inflammatory bowel diseases (IBD) are strongly associated with progressive neurocognitive degeneration and can potentially manipulate CNS macrophage populations remotely, we investigated acute DSS-induced colitis in mice and found mLV dysfunction associated with reduced dural MMP transcripts. Taken together, my results demonstrate the importance of dural macrophages and BMDCs in maintaining the structure and function of mLVs, likely through ECM remodeling along the mLVs.