# Active LXR signaling, coupled with elevated mitochondrial and glycolytic metabolism contributes to GM-CSF–induced trained immunity

## TL;DR

GM-CSF boosts trained immunity by increasing cell metabolism and activating LXR signaling, which could help treat chronic inflammation.

## Contribution

This study reveals that LXR signaling and metabolic changes are essential for GM-CSF-induced trained immunity.

## Key findings

- GM-CSF enhances trained immunity through increased glycolysis, mitochondrial activity, and fatty acid oxidation.
- LXR signaling, possibly via PPARγ, mediates GM-CSF-induced trained immunity by regulating glycolytic flux and histone acetylation.
- Pharmacological inhibition of LXR reduces trained immunity, suggesting it as a potential therapeutic target.

## Abstract

Granulocyte-macrophage colony-stimulating factor (GM-CSF) contributes to the host defense and the pathogenesis of inflammatory diseases at least in part through inducing trained immunity (TI), however, the mechanism remains poorly characterized. In this paper, we systematically investigated the associated metabolic and epigenetic reprogramming, with a particular focus on the role of liver X receptors (LXRs) in this process. We employed a comprehensive experimental approach, including in vitro isolation and purification of human monocytes from healthy donors, cytokine assays, quantitative PCR, Seahorse metabolic analysis, flow cytometry, and chromatin immunoprecipitation (ChIP), shotgun lipidomics, as well as transcriptomic data analysis to investigate GM-CSF–induced trained immunity. Our results demonstrate that GM-CSF induces TI by enhancing cellular metabolism, as evidenced by increased glycolysis, mitochondrial activity, fatty acid oxidation, and pyruvate metabolism. Lipidomics and RNA sequencing analyses revealed upregulation of lipid synthesis, high triglyceride storage, and acetyl-CoA–producing pathways, leading to increased histone acetylation in GM-CSF–trained cells. Furthermore, glycolysis and mitochondrial metabolism are essential for establishing TI in these cells. Notably, pharmacological inhibition of GM-CSF activated LXR signaling, which potentially mediated via PPARγ, attenuated GM-CSF–induced TI via reducing glycolytic flux and histone acetylation while activation of LXR amplified these effects. Together, these results highlight the role of LXR in linking cellular metabolism with epigenetic reprogramming and demonstrate that elevated metabolic activity and active LXR signaling both are essential for GM-CSF–induced trained immunity. Importantly, these pathways may represent therapeutic targets for modulating GM-CSF–driven maladaptive inflammation in chronic inflammatory diseases.

Schematic diagram illustrating GM-CSF–induced trained immunity. Upon GM-CSF binding to its receptor, intracellular signaling pathways involving PI3K, AKT, HIF-1α, and c-MYC are activated. In parallel, genes regulating lipid metabolism, including PPARγ, are induced, leading to activation of liver X receptor (LXR) signaling and its downstream target genes. GM-CSF signaling also promotes metabolic reprogramming characterized by increased glucose uptake, enhanced glycolysis through upregulation of GLUT1, HK2, and PFKFB3, and elevated mitochondrial fatty acid oxidation. Together, these metabolic changes increase acetyl-CoA production, resulting in enhanced histone acetylation and stable epigenetic remodeling that supports trained immunity.