Down-regulation of endothelial TLR4 signalling after apo A-I gene transfer contributes to improved survival in an experimental model of lipopolysaccharide-induced inflammation

The protective effects of high-density lipoprotein (HDL) under lipopolysaccharide (LPS) conditions have been well documented. Here, we investigated whether an effect of HDL on Toll-like receptor 4 (TLR4) expression and signalling may contribute to its endothelial-protective effects and to improved survival in a mouse model of LPS-induced inflammation and lethality. HDL cholesterol increased 1.7-fold (p < 0.005) and lung endothelial TLR4 expression decreased 8.4-fold (p < 0.005) 2 weeks after apolipoprotein (apo) A-I gene transfer. Following LPS administration in apo A-I gene transfer mice, lung TLR4 and lung MyD88 mRNA expression, reflecting TLR4 signalling, were 3.0-fold (p < 0.05) and 2.1-fold (p < 0.05) lower, respectively, than in LPS control mice. Concomitantly, LPS-induced lung neutrophil infiltration, lung oedema and mortality were significantly attenuated following apo A–I transfer. In vitro, supplementation of HDL or apo A–I to human microvascular endothelial cells-1 24 h before LPS administration reduced TLR4 expression, as assessed by fluorescent-activated cell sorting, and decreased the LPS-induced MyD88 mRNA expression and NF-κB activity, independently of LPS binding. In conclusion, HDL reduces TLR4 expression and signalling in endothelial cells, which may contribute significantly to the protective effects of HDL in LPS-induced inflammation and lethality

Apolipoprotein C-I is an <it>APOE</it> genotype-dependent suppressor of glial activation

<p>Abstract</p> <p>Background</p> <p>Inheritance of the human ϵ4 allele of the apolipoprotein (apo) E gene (<it>APOE</it>) significantly increases the risk of developing Alzheimer’s disease (AD), in addition to adversely influencing clinical outcomes of other neurologic diseases. While apoE isoforms differentially interact with amyloid β (Aβ), a pleiotropic neurotoxin key to AD etiology, more recent work has focused on immune regulation in AD pathogenesis and on the mechanisms of innate immunomodulatory effects associated with inheritance of different <it>APOE</it> alleles. <it>APOE</it> genotype modulates expression of proximal genes including <it>APOC1</it>, which encodes a small apolipoprotein that is associated with Aβ plaques. Here we tested the hypothesis that <it>APOE</it>-genotype dependent innate immunomodulation may be mediated in part by apoC-I.</p> <p>Methods</p> <p>ApoC-I concentration in cerebrospinal fluid from control subjects of differing <it>APOE</it> genotypes was quantified by ELISA. Real-time PCR and ELISA were used to analyze apoC-I mRNA and protein expression, respectively, in liver, serum, cerebral cortex, and cultured primary astrocytes derived from mice with targeted replacement of murine APOE for human <it>APOE</it> ϵ3 or ϵ4. ApoC-I direct modulation of innate immune activity was investigated in cultured murine primary microglia and astrocytes, as well as human differentiated macrophages, using specific toll-like receptor agonists LPS and PIC as well as Aβ.</p> <p>Results</p> <p>ApoC-I levels varied with <it>APOE</it> genotype in humans and in <it>APOE</it> targeted replacement mice, with ϵ4 carriers showing significantly less apoC-I in both species. ApoC-I potently reduced pro-inflammatory cytokine secretion from primary murine microglia and astrocytes, and human macrophages, stimulated with LPS, PIC, or Aβ.</p> <p>Conclusions</p> <p>ApoC-I is immunosuppressive. Our results illuminate a novel potential mechanism for <it>APOE</it> genotype risk for AD; one in which patients with an ϵ4 allele have decreased expression of apoC-I resulting in increased innate immune activity.</p