Systemic inflammation modulates the levels of EBI2 and 7α,25-OHC synthesizing (CH25H, CYP7B1) and degrading (HSD3B7) enzymes in brain microvascular cells

Abstract

The EBI2 receptor is a major modulator of innate immunity and, together with its ligand, oxysterol 7α,25OHC, has been implicated in several neuroinflammatory and autoimmune diseases. 7alpha,25OHC is synthesised from cholesterol with CH25H and CYP7B1 enzymes and degraded with HSD3B7. The concentration of 7alpha,25OHC in the brain increases in the early phases of the murine model of multiple sclerosis, leading to an enhanced central nervous system (CNS) infiltration with EBI2-expressing lymphocytes. Here, we aimed to investigate whether the enzymes involved in the synthesis and degradation of 7alpha,25OHC are expressed directly in the mouse brain microvascular cells and whether systemic inflammation modulates their levels in these cells. Normal mouse brain capillaries were isolated and immunostained for EBI2, CH25H, CYP7B1 and HSD3B7. Subsequently, mice were challenged with lipopolysaccharide and the mRNA expression in whole brain homogenates was measured. Changes in the receptor and enzyme levels were quantified directly in endothelial cells (ECs), pericytes and astrocytes. The data indicated high levels of EBI2 in the brain microvascular ECs, pericytes and astrocytes with the highest co-localisation in pericytes. CH25H was detected in ECs and astrocytes with low levels in pericytes. CYP7B1 was moderately expressed in ECs, astrocytes and pericytes. The 7alpha,25OHC degrading enzyme HSD3B7 was the least detected in astrocytes. Moreover, the data indicated that systemic inflammation downregulated the mRNA levels of Ebi2 and upregulated Ch25h expression in the whole brain. Specifically in each cell type, EBI2 was not induced in ECs but increased in astrocytes and pericytes. CH25H levels increased in astrocytes and pericytes and CYP7B1 in ECs and astrocytes. The degrading enzyme, HSD3B7, was least affected by systemic inflammation. Taken together, we here demonstrate that EBI2 and the enzymes regulating its ligand levels are differentially expressed in mouse brain microvessels and are highly modulated by systemic inflammation. Upregulated concentration of 7alpha,25OHC in the brain during inflammation may lead to an increased migration of EBI2-expressing immune cells into the CNS during infection or neuroinflammatory disease. Modulation of the EBI2/oxysterol system in the brain or directly in the brain blood vessels may thus provide a new approach to treating neuroinflammatory diseases including multiple sclerosis.