LXRα Regulates oxLDL-Induced Trained Immunity in Macrophages.

Reprogramming of metabolic pathways in monocytes and macrophages can induce a proatherosclerotic inflammatory memory called trained innate immunity. Here, we have analyzed the role of the Liver X receptor (LXR), a crucial regulator of metabolism and inflammation, in oxidized low-density lipoprotein (oxLDL)-induced trained innate immunity. Human monocytes were incubated with LXR agonists, antagonists, and oxLDL for 24 h. After five days of resting time, cells were restimulated with the TLR-2 agonist Pam3cys. OxLDL priming induced the expression of LXRα but not LXRß. Pharmacologic LXR activation was enhanced, while LXR inhibition prevented the oxLDL-induced inflammatory response. Furthermore, LXR inhibition blocked the metabolic changes necessary for epigenetic reprogramming associated with trained immunity. In fact, enrichment of activating histone marks at the IL-6 and TNFα promotor was reduced following LXR inhibition. Based on the differential expression of the LXR isoforms, we inhibited LXRα and LXRß genes using siRNA in THP1 cells. As expected, siRNA-mediated knock-down of LXRα blocked the oxLDL-induced inflammatory response, while knock-down of LXRß had no effect. We demonstrate a specific and novel role of the LXRα isoform in the regulation of oxLDL-induced trained immunity. Our data reveal important aspects of LXR signaling in innate immunity with relevance to atherosclerosis formation.

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.