Microglia contribute to the glia limitans around arteries, capillaries and veins under physiological conditions, in a model of neuroinflammation and in human brain tissue

Microglia represent resident immune cells of the central nervous system (CNS), which have been shown to be involved in the pathophysiology of practically every neuropathology. As microglia were described to participate in the formation of the astroglial glia limitans around CNS vessels, they are part of the neurovascular unit (NVU). Since the NVU is a highly specialized structure, being functionally and morphologically adapted to differing demands in the arterial, capillary, and venous segments, the present study was aimed to systematically investigate the microglial contribution to the glia limitans along the vascular tree. Thereby, the microglial participation in the glia limitans was demonstrated for arteries, capillaries, and veins by immunoelectron microscopy in wild-type mice. Furthermore, analysis by confocal laser scanning microscopy revealed the highest density of microglial endfeet contacting the glial basement membrane around capillaries, with significantly lower densities around arteries and veins. Importantly, this pattern appeared to be unaltered in the setting of experimental autoimmune encephalomyelitis (EAE) in CX3CR1(CreERT2):R26-Tomato reporter mice, although perivascular infiltrates of blood-borne leukocytes predominantly occur at the level of post-capillary venules. However, EAE animals exhibited significantly increased contact sizes of individual microglial endfeet around arteries and veins. Noteworthy, under EAE conditions, the upregulation of MHC-II was not limited to microglia of the glia limitans of veins showing infiltrates of leukocytes, but also appeared at the capillary level. As a microglial contribution to the glia limitans was also observed in human brain tissue, these findings may help characterizing microglial alterations within the NVU in various neuropathologies.