Systemic inflammatory regulation of blood-brain barrier inflammation
The endothelial cells of the blood-brain barrier are essential for the maintenance of homeostasis in the central nervous system. These cells act as a physical and transport barrier, restricting the movement of systemic immune cells and blood components into the brain. The physical barrier is mediated by the formation of tight and adherens junctions, and the redistribution of these in disease states and systemic inflammation, and the consequent extravasation of immune cells provide valuable targets to reduce neuroinflammation. Multiple Sclerosis is an autoimmune disease in which autoreactive T cells mediate the destruction of the neuronal myelin sheath, leading to a reduction in the speed of neuronal processing, and numerous debilitating symptoms. Blood-brain barrier breakdown is essential to the development of MS, and endothelial cell morphology is vastly different within sites of active lesions and normal appearing white matter. This study attempted to determine the in vitro effect of MS patient serum on the integrity of the endothelial cell barrier and to identify barrier altering components. Individual cytokines which altered the endothelial cell barrier were further analysed to determine their effect on junctional protein expression and the pro-inflammatory secretome. Additionally, the effects of ligands of the toll-like receptor family, a family of innate immune pattern recognition receptors were also analysed, to simulate the effect of systemic bacterial or viral infection at the blood-brain barrier. Interestingly, it was found that the FDA-approved TLR7 ligand imiquimod could increase the endothelial barrier integrity and attenuate the effects of pro-inflammatory stimuli in a TLR7-independent manner. The pathways by which imiquimod was mediating these effects were interrogated, and it was observed that both a cAMP-dependent selective protein kinase A-activator and adenosine A2B receptor agonist could recreate these effects. However, an A2B receptor antagonist did not block the imiquimod-mediated effects. The endothelial cells of the blood-brain barrier are an essential interface between the systemic circulation and the brain. These findings provide novel insights into the effect of pro-inflammatory mediators at this interface and identify potential targets for the modulation of blood-brain barrier permeability and the attenuation of pro-inflammatory insults.
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