Inflammatory signalling at the blood-brain barrier: Contributions of pericytes and endothelia

Abstract

The blood-brain barrier (BBB) is an important protective structure that restricts the free passage of blood components and circulating immune cells into the brain. The BBB is primarily composed of endothelial cells that line the inside of the vessel, and is surrounded by pericytes that perform critical roles in its formation and maintenance. In this project, I aimed to examine aspects of inflammatory signalling at the BBB using primary human brain pericytes and endothelia. Initially, pericytes were characterised in human brain tissue to develop a panel of markers for pericytes in vitro. Characterisation studies validated that in vitro cultures of pericytes recapitulated their in situ phenotype, as well as identifying two novel markers of vascular smooth muscle cell phenotype in situ and in vitro. A protocol for the isolation of primary human brain endothelia was then developed to study the interactions of pericytes and endothelia in vitro, and these cultures were shown to retain their unique brain endothelial phenotype. Using cultures of pericytes and endothelia, differences in the inflammatory response of these two cell types were identified, including unique secretions, and differential sensitivity to inflammatory stimuli. Subsequently, the role of platelet-derived growth factor-BB (PDGF-BB) was investigated in greater detail. PDGF-BB was found to cause a proliferative response in brain pericytes, as has been found previously, but also induced the expression of inflammation-related secretions. The pathways through which this inflammatory response was mediated were dissected using genetic and pharmacological inhibitors. Interestingly, it was found that PDGF-BB altered pericyteendothelial interactions, stopping pericytes from strengthening the endothelial barrier. The role of PDGF-BB was investigated further in co-cultures, incorporating astrocytes, microglia, endothelia, and pericytes. It was observed that PDGF-BB improves the formation of blood vessel-like endothelial cords in vitro due to reduced cell death and increased expression of angiogenesis-associated molecules, reminiscent of its effects in vivo. The BBB is a critical structure in the brain, but has not yet been targeted to treat disease. Together these findings provide novel insights into cell-cell signalling at the BBB, and how these interactions are modified during inflammatory conditions. Because BBB damage and neuroinflammation are critical disease processes in neurological diseases, the factors that govern neuroinflammation at the BBB present potential drug targets to prevent the progression of AD.