Background/Aims
Bone marrow mesenchymal stromal cells (BM-MSCs) exert diverse functions, including supporting alcohol detoxification and providing a niche for monocyte development. However, their role in regulating monocytes during alcohol-related liver disease (ALD) remains unclear. This study investigates how BM-MSCs orchestrate the egress of anti-inflammatory monocytes from BM to the liver in ALD.

Methods
Wild-type, leptin receptor (LepR)⁺ BM-MSC-specific Slc7a11 knockout, and natural killer (NK) cell-specific Grm5 knockout mice were fed an ethanol diet for 8 weeks. Tissue analyses were performed using single-cell RNA sequencing (scRNA-seq), immunostaining, and flow cytometry. Blood and liver samples from ALD patients were examined.

Results
scRNA-seq revealed a distinct population of BM-derived Ly6C^low hepatic macrophages expressing interleukin-1 receptor 2 (IL-1R2), an IL-1β decoy receptor, in ethanol-fed mice. In the BM, alcohol exposure upregulated the gene expression of alcohol-metabolizing enzymes (Adh1, Aldh2), xCT (Slc7a11), and chemokines (Cxcl9, Cxcl10) in LepR+ BM-MSCs, promoting NK cell recruitment and interferon-γ (IFN-γ) production via metabotropic glutamate receptor 5 (mGluR5) activation. Subsequently, IFN-γ enhanced IL-1R2 expression and suppressed CX₃CR1 in neighboring Ly6C^low BM monocytes, facilitating their hepatic migration. LepR⁺ BM-MSC-specific xCT and NK cell-specific mGluR5 knockout mice exhibited exacerbated liver injury and elevated blood IL-1β levels, while recombinant IL-1R2 administration improved ameliorated ALD in wild-type mice. Consistently, increased IL-1R2 levels were observed in plasma, CD14⁺CD16⁺ blood monocytes, and liver tissues of ALD patients.

Conclusions
We identified a BM-liver axis in which glutamate released by BM-MSCs activates mGluR5 in BM NK cells, driving IL-1R2⁺ monocyte migration to the liver and attenuating ALD progression.