The present study aimed to investigate pathways that contribute to uptake and transcytosis of high‐density lipoproteins (HDLs) and HDL‐associated α‐tocopherol (αTocH) across an in vitro model of the blood–brain barrier (BBB). In primary porcine brain capillary endothelial cells HDL‐associated αTocH was taken up in 10‐fold excess of HDL holoparticles, indicating efficient selective uptake, a pathway mediated by scavenger receptor class B, type I (SR‐BI). SR‐BI was present in caveolae of brain capillary endothelial cells and expressed almost exclusively at the apical membrane. Disruption of caveolae with methyl‐β‐cyclodextrin (CDX) resulted in (mis)sorting of SR‐BI to the basolateral membrane. Immunohistochemistry of porcine brain cryosections revealed SR‐BI expression on brain capillary endothelial cells and presumably astrocytic endfeet. HDL‐associated [¹⁴C]αTocH taken up by brain capillary endothelial cells was recovered in sucrose gradient fractions containing the majority of cellular caveolin‐1, the major caveolae‐associated protein. During mass transfer studies using αTocH‐enriched HDL, approximately 50% of cellular αTocH was recovered with the bulk of cellular caveolin‐1 and SR‐BI. Efflux experiments revealed that a substantial amount of cell‐associated [¹⁴C]αTocH could be mobilized into the culture medium. In addition, apical‐to‐basolateral transport of HDL holoparticles and HDL‐associated αTocH was saturable. Results from the present study suggest that part of cerebral apolipoprotein A‐I and αTocH originates from plasma HDL transcytosed across the BBB and that caveolae‐located SR‐BI facilitates selective uptake of HDL‐associated αTocH at the BBB.

The objective of the present study was to investigate the involvement of key players in reverse cholesterol/24(S)OH-cholesterol transport in primary porcine brain capillary endothelial cells (pBCEC) that constitute the BBB. We identified that, in addition to scavenger receptor class B, type I (SR-BI), pBCEC express ABCA1 and apolipoprotein A-I (apoA-I) mRNA and protein. Studies on the regulation of ABCA1 by the liver X receptor agonist 24(S)OH-cholesterol revealed increased ABCA1 expression and apoA-I-dependent [³H]cholesterol efflux from pBCEC. In unpolarized pBCEC, high density lipoprotein, subclass 3 (HDL₃)-dependent [³H]cholesterol efflux, was unaffected by 24(S)OH-cholesterol treatment but was enhanced 5-fold in SR-BI overexpressing pBCEC. Efflux of cellular 24(S)-[³H]OH-cholesterol was highly efficient, independent of ABCA1, and correlated with SR-BI expression. Polarized pBCEC were cultured on porous membrane filters that allow separate access to the apical and the basolateral compartment. Addition of cholesterol acceptors to the apical compartment resulted in preferential [³H]cholesterol efflux to the basolateral compartment. HDL₃ was a better promoter of basolateral [³H]cholesterol efflux than lipid-free apoA-I. Basolateral pretreatment with 24(S)OH-cholesterol enhanced apoA-I-dependent basolateral cholesterol efflux up to 2-fold along with the induction of ABCA1 at the basolateral membrane. Secretion of apoA-I also occurred preferentially to the basolateral compartment, where the majority of apoA-I was recovered in an HDL-like density range. In contrast, 24(S)-[³H]OH-cholesterol was mobilized efficiently to the apical compartment of the in vitro BBB by HDL₃, low density lipoprotein, and serum. These results suggest the existence of an autoregulatory mechanism for removal of potentially neurotoxic 24(S)OH-cholesterol. In conclusion, the apoA-I/ABCA1- and HDL/SR-BI-dependent pathways modulate polarized sterol mobilization at the BBB.

Although maternal–fetal cholesterol transfer may serve to compensate for insufficient fetal cholesterol biosynthesis under pathological conditions, it may have detrimental consequences under conditions of maternal hypercholesterolemia leading to preatherosclerotic lesion development in fetal aortas. Maternal cholesterol may enter fetal circulation by traversing syncytiotrophoblast and endothelial layers of the placenta. We hypothesized that endothelial cells (ECs) of the fetoplacental vasculature display a high and tightly regulated capacity for cholesterol release. Using ECs isolated from human term placenta (HPECs), we investigated cholesterol release capacity and examined transporters involved in cholesterol efflux pathways controlled by liver-X-receptors (LXRs). HPECs demonstrated 2.5-fold higher cholesterol release to lipid-free apolipoprotein (apo)A-I than human umbilical vein ECs (HUVECs), whereas both cell types showed similar cholesterol efflux to high-density lipoproteins (HDLs). Interestingly, treatment of HPECs with LXR activators increased cholesterol efflux to both types of acceptors, whereas no such response could be observed for HUVECs. In line with enhanced cholesterol efflux, LXR activation in HPECs increased expression of ATP-binding cassette transporters ABCA1 and ABCG1, while not altering expression of ABCG4 and scavenger receptor class B type I (SR-BI). Inhibition of ABCA1 or silencing of ABCG1 decreased cholesterol efflux to apoA-I (−70%) and HDL ₃ (−57%), respectively. Immunohistochemistry localized both transporters predominantly to the apical membranes of placental ECs in situ. Thus, ECs of human term placenta exhibit unique, efficient and LXR-regulated cholesterol efflux mechanisms. We propose a sequential pathway mediated by ABCA1 and ABCG1, respectively, by which HPECs participate in forming mature HDL in the fetal blood.

Recently, we demonstrated a net blood-to-brain passage of the oxysterol 27-hydroxycholesterol corresponding to 4–5 mg/day. As the steady-state levels of this sterol are only 1–2 μg/g brain tissue, we hypothesized that it is metabolized and subsequently eliminated from the brain. To explore this concept, we first measured the capacity of in vitro systems representing the major cell populations found in the brain to metabolize 27-hydroxycholesterol. We show here that 27-hydroxycholesterol is metabolized into the known C₂₇ steroidal acid 7α-hydroxy-3-oxo-4-cholestenoic acid by neuronal cell models only. Using an in vitro model of the blood-brain barrier, we demonstrate that 7α-hydroxy-3-oxo-4-cholestenoic acid is efficiently transferred across monolayers of primary brain microvascular endothelial cells. Finally, we measured the concentration of 7α-hydroxy-3-oxo-4-cholestenoic acid in plasma from the internal jugular vein and brachial artery of healthy volunteers. Calculation of the arteriovenous concentration difference revealed a significant in vivo flux of this steroid from the brain into the circulation in human. Together, these studies identify a novel metabolic route for the elimination of 27-hydroxylated sterols from the brain. Given the emerging connections between cholesterol and neurodegeneration, this pathway may be of importance for the development of these conditions.

Vaccination has a significant impact on morbidity and mortality. High vaccination coverage rates are required to achieve herd protection against vaccine-preventable diseases. However, limited vaccine access and hesitancy among specific communities represent significant obstacles to this goal. This review provides an overview of critical factors associated with vaccination among disadvantaged groups in World Health Organisation European countries. Initial searches yielded 18,109 publications from four databases, and 104 studies from 19 out of 53 countries reporting 22 vaccine-preventable diseases were included. Nine groups representing the populations of interest were identified, and most of the studies focused on asylum seekers, refugees, migrants and deprived communities. Recall of previous vaccinations received was poor, and serology was conducted in some cases to confirm protection for those who received prior vaccinations. Vaccination coverage was lower among study populations compared to the general population or national average. Factors that influenced uptake, which presented differently at different population levels, included health service accessibility, language and vaccine literacy, including risk perception, disease severity and vaccination benefits. Strategies that could be implemented in vaccination policy and programs were also identified. Overall, interventions specific to target communities are vital to improving uptake. More innovative strategies need to be deployed to improve vaccination coverage among disadvantaged groups.