Objective Bone marrow-derived myeloid cells accumulate in the liver as monocytes and macrophages during the progression of obesity-related non-alcoholic fatty liver disease (NAFLD) to steatohepatitis (NASH). Myeloid cells comprise heterogeneous subsets, and dietary overnutrition may affect macrophages in the liver and bone marrow. We therefore aimed at characterising in depth the functional adaptations of myeloid cells in fatty liver.
Design We employed single-cell RNA sequencing to comprehensively assess the heterogeneity of myeloid cells in the liver and bone marrow during NAFLD, by analysing C57BL/6 mice fed with a high-fat, high-sugar, high-cholesterol ‘Western diet’ for 16 weeks. We also characterised NAFLD-driven functional adaptations of macrophages in vitro and their functional relevance during steatohepatitis in vivo.
Results Single-cell RNA sequencing identified distinct myeloid cell clusters in the liver and bone marrow. In both compartments, monocyte-derived populations were largely expanded in NASH-affected mice. Importantly, the liver myeloid compartment adapted a unique inflammatory phenotype during NAFLD progression, exemplarily characterised by downregulated inflammatory calprotectin (S100A8/A9) in macrophage and dendritic cell subsets. This distinctive gene signature was also found in their bone marrow precursors. The NASH myeloid phenotype was principally recapitulated by in vitro exposure of bone marrow-derived macrophages with fatty acids, depended on toll-like receptor 4 signalling and defined a characteristic response pattern to lipopolysaccharide stimulation. This imprinted and stable NASH myeloid immune phenotype functionally determined inflammatory responses following acute liver injury (acetaminophen poisoning) in vivo.
Conclusion Liver myeloid leucocytes and their bone marrow precursors adapt a common and functionally relevant inflammatory signature during NAFLD progression.
- fatty acids
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Contributors FT guided the research. OK designed, performed and analysed the animal and single-cell RNA sequencing experiments. JH performed the in vitro studies. ATA, DPPB and IGC performed the RNA sequencing analysis. JCM, MK, TR, TP, TL and CT contributed to research design and/or conducted the experiments. FT and OK wrote the manuscript. All authors reviewed and approved the manuscript.
Funding This work was supported by the German Research Foundation (DFG; Ta434/5-1 and SFB/TRR57). The study sponsor had no role in the study design or in the collection, analysis and interpretation of data.
Competing interests Work in the lab of FT has received research funding from Allergan, Galapagos, Inventiva and Bristol Myers Squibb.
Ethics approval All in vivo experiments were performed under conditions approved by the appropriate institutional and governmental authorities according to German legal requirements.
Provenance and peer review Not commissioned; externally peer reviewed.
Patient consent for publication Not required.