Brain endothelial tricellular junctions as novel sites for T cell diapedesis across the blood–brain barrier

Abstract

The migration of activated T cells across the blood–brain barrier (BBB) is a critical step in central nervous system (CNS) immune surveillance and inflammation. Whereas T cell diapedesis across the intact BBB seems to occur preferentially through the BBB cellular junctions, impaired BBB integrity during neuroinflammation is accompanied by increased transcellular T cell diapedesis. The underlying mechanisms directing T cells to paracellular versus transcellular sites of diapedesis across the BBB remain to be explored. By combining in vitro live-cell imaging of T cell migration across primary mouse brain microvascular endothelial cells (pMBMECs) under physiological flow with serial block-face scanning electron microscopy (SBF-SEM), we have identified BBB tricellular junctions as novel sites for T cell diapedesis across the BBB. Downregulated expression of tricellular junctional proteins or protein-based targeting of their interactions in pMBMEC monolayers correlated with enhanced transcellular T cell diapedesis, and abluminal presence of chemokines increased T cell diapedesis through tricellular junctions. Our observations assign an entirely novel role to BBB tricellular junctions in regulating T cell entry into the CNS. This article has an associated First Person interview with the first author of the paper.

Tight Junctions of the Neurovascular Unit

“…Alterations in claudin-5 expression have been implicated in a number of neurological conditions including schizophrenia, depression, epilepsy and traumatic brain injury (Doherty et al, 2016;Menard et al, 2017;Greene et al, 2018Greene et al, , 2020Farrell et al, 2019). In addition, claudin-5 remodeling occurs at sites of leukocyte transmigration in both physiological and pathological conditions such as Multiple Sclerosis (Paul et al, 2013;Winger et al, 2014;Castro Dias et al, 2021) and mislocalization of claudin-5 occurs in a mouse model of oxygen induced retinopathy (Luo et al, 2011). Recent work has found the inner retinal blood vessels to be highly dynamic with claudin-5 expression regulated in a circadian-manner and claudin-5 changes being a key mediator in initiating dry agerelated macular degeneration like pathology .…”

Alzheimer’s-Like Pathology at the Crossroads of HIV-Associated Neurological Disorders

“…Future comparative studies assessing AD patients and HIV-positive individuals are needed in order to understand the intersection and overlap between these two diseases and also to better understand how we can alleviate the inflammatory symptoms of these processes. Finally, neuroinflammation is known to affect the permeability of the BBB, allowing increased and destructive T cell migration via the downregulation of integral structure proteins [ 171 ]. Examining cells such as pericytes in the context of HIV and AD pathology is, therefore, highly important and is becoming an area of increased investigation.…”

Enhancing T Cell Chemotaxis and Infiltration in Glioblastoma

“…The endothelial cell signature also changes from CNS specific to mirroring the periphery, thereby promoting immune trafficking from the blood (preprint [94]). The endothelial cells of the BBB are sealed by adherens junctions, a continuous series of complex tight junctions, and recently discovered tricellular junctions [95]. In the inflammatory state, these tricellular junctions have been suggested to be the primary site of cellular migration, through the downregulation of proteins (tricellulin and angulins) which normally maintain their morphology [95,96].…”

“…The endothelial cells of the BBB are sealed by adherens junctions, a continuous series of complex tight junctions, and recently discovered tricellular junctions [95]. In the inflammatory state, these tricellular junctions have been suggested to be the primary site of cellular migration, through the downregulation of proteins (tricellulin and angulins) which normally maintain their morphology [95,96]. Interestingly, recombinant CCL2 and CCL5 administration was demonstrated in vitro to enhance T cell diapedesis through tricellular junctions.…”

“…Interestingly, recombinant CCL2 and CCL5 administration was demonstrated in vitro to enhance T cell diapedesis through tricellular junctions. This may therefore offer a therapeutic strategy specifically to enhance paracellular crossing at the BBB, although their effect on recruiting regulatory T cells must also be considered [95,96].…”