OxLDL-mediated immunologic memory in endothelial cells.

Trained innate immunity describes the metabolic reprogramming and long-term proinflammatory activation of innate immune cells in response to different pathogen or damage associated molecular patterns, such as oxidized low-density lipoprotein (oxLDL). Here, we have investigated whether the regulatory networks of trained innate immunity also control endothelial cell activation following oxLDL treatment. Human aortic endothelial cells (HAECs) were primed with oxLDL for 24 h. After a resting time of 4 days, cells were restimulated with the TLR2-agonist PAM3cys4. OxLDL priming induced a proinflammatory memory with increased production of inflammatory cytokines such as IL-6, IL-8 and MCP-1 in response to PAM3cys4 restimulation. This memory formation was dependent on TLR2 activation. Furthermore, oxLDL priming of HAECs caused characteristic metabolic and epigenetic reprogramming, including activation of mTOR-HIF1α-signaling with increases in glucose consumption and lactate production, as well as epigenetic modifications in inflammatory gene promoters. Inhibition of mTOR-HIF1α-signaling or histone methyltransferases blocked the observed phenotype. Furthermore, primed HAECs showed epigenetic activation of ICAM-1 and increased ICAM-1 expression in a HIF1α-dependent manner. Accordingly, live cell imaging revealed increased monocyte adhesion and transmigration following oxLDL priming. In summary, we demonstrate that oxLDL-mediated endothelial cell activation represents an immunologic event, which triggers metabolic and epigenetic reprogramming. Molecular mechanisms regulating trained innate immunity in innate immune cells also regulate this sustained proinflammatory phenotype in HAECs with enhanced atheroprone cell functions. Further research is necessary to elucidate the detailed metabolic regulation and the functional relevance for atherosclerosis formation in vivo.

LXR Activation Induces a Proinflammatory Trained Innate Immunity-Phenotype in Human Monocytes.

Objectives: The concept of trained innate immunity describes a long-term proinflammatory memory in innate immune cells. Trained innate immunity is regulated through reprogramming of cellular metabolic pathways including cholesterol and fatty acid synthesis. Here, we have analyzed the role of Liver X Receptor (LXR), a key regulator of cholesterol and fatty acid homeostasis, in trained innate immunity. Methods and Results: Human monocytes were isolated and incubated with different stimuli for 24 h, including LXR agonists, antagonists and Bacillus Calmette-Guerin (BCG) vaccine. After 5 days resting time, cells were restimulated with the TLR2-agonist Pam3cys. LXR activation did not only increase BCG trained immunity, but also induced a long-term inflammatory activation by itself. This inflammatory activation by LXR agonists was accompanied by characteristic features of trained innate immunity, such as activating histone marks on inflammatory gene promoters and metabolic reprogramming with increased lactate production and decreased oxygen consumption rate. Mechanistically, LXR priming increased cellular acetyl-CoA levels and was dependent on the activation of the mevalonate pathway and IL-1ß signaling. In contrast to mevalonate pathway inhibition, blocking fatty acid synthesis further increased proinflammatory priming by LXR. Conclusion: We demonstrate that LXR activation induces a proinflammatory trained immunity phenotype in human monocytes through epigenetic and metabolic reprogramming. Our data reveal important novel aspects of LXR signaling in innate immunity.