Multiplex immunohistochemistry of chronic active multiple sclerosis lesions links fibroblast-associated vessels with immune cell cuffs

Background: Multiple sclerosis (MS) is a chronic neurodegenerative disorder in which inflammatory demyelinating lesions affect the brain, optic nerve and spinal cord. MS lesion formation is accompanied by profound changes to blood vessels, including the density of PDGFR{beta}+ mural cells, historically identified as pericytes. Intriguingly, in recent years, single-cell and lineage tracing studies have shown that the PDGFR{beta}+ cell population is heterogeneous, comprising both pericytes and perivascular fibroblasts. Yet, due to their overlapping expression profiles, the spatial distribution of these cell populations in MS lesions remains poorly understood. Methods: We employed multiplex immunohistochemistry for endothelial cells (CD31), basement membrane (laminin), fibroblasts (PDGFR{beta}, COL1A1, SMA), pericytes (PDGFR{beta}, SLC6A12) and immune cells (CD45, CD68) to characterize the spatial localization of fibroblasts and pericytes in MS lesions, and how this relates to perivascular space enlargement and immune cell presence. Results: We analysed 17633 individual vessels across 5 control white matter, 5 normal-appearing white matter, 4 active and 4 chronic active MS lesions. By carefully delineating endothelium and perivascular compartments, we find that perivascular space area but not number of vessels is increased in MS lesions. Through mining of publicly available sequencing datasets, we confirm COL1A1 and SLC6A12 as fibroblast and pericyte markers, respectively, in the human brain. COL1A1+ and SLCA12+ vessels were largely distinct of one another. Unsupervised clustering of the expression profile of PDGFR{beta}, COL1A1 and SLC6A12 in individual vessels distinguished three partially overlapping vessel clusters. Of these, the fibroblast-associated vessel type (COL1A1 high, SLC6A12 low) was increased in chronic active lesion rim and center. Importantly, fibroblast-associated vessels were related to increased perivascular space enlargement and more accumulation of immune cells. Conclusion: We identify distinct fibroblast- and pericyte-associated vascular phenotypes in human white matter. Notably, fibroblast-associated vessels are increased in chronic active lesions, where they are related to immune cell cuffs. These findings provide a spatial link between perivascular fibroblasts and chronic inflammation in MS.