Die phänotypische Diversität von Monozyten und Makrophagen in der nicht-alkoholischen Fettlebererkrankung

Non-alcoholic fatty liver disease (NAFLD) is one major clinical burden in modern western societies. Monocytes and monocyte-derived macrophages (MoMF) have been shown to drive inflammation and thereby aggravate fatty liver disease. In this study, the inflammatory phenotype of both bone marrow monocytes and their tissue resident successors was investigated during NAFLD progression. Therefore, wildtype (WT) and chemokine (C-C-motif) receptor 2 deficient (Ccr2-/-) mice were fed a high-fat, high-sugar and high-cholesterol Western diet for up to 16 weeks. NAFLD phenotype was assessed in the first step by measuring serum trans-aminase levels, liver triglycerides and influx of leukocytes into the liver tissue. Next, the inflammatory polarization of myeloid leukocytes from bone marrow and liver was investigated by multicolor flow cytometry, single cell RNA sequencing and in vitro polarization assays. Furthermore, the inflammatory polarization of monocytes and MoMF was also investigated during acute liver injury in healthy controls and Western diet fed obese mice. Acute liver injury was induced by a non-lethal overdose of acet-aminophen.It could be shown that during NAFLD progression, bone marrow monocytes infiltrate into the liver. These liver infiltrating monocytes contribute to NAFLD progression, as Ccr2-/- mice, lacking infiltrating inflammatory monocytes, showed significantly reduced liver steatosis compared to equally treated WT mice. Single cell RNA sequencing and flow cytometry analysis of myeloid bone marrow and liver leukocytes showed that monocytes, MoMF as well as dendritic cells showed a reduced gene expression of the inflammatory marker calprotectin (S100A8/A9 heterodimer). On the other hand, the anti-inflammatory marker CD301 was shown to be highly expressed by MoMF during NAFLD progression. In vitro experiments using bone marrow derived macro-phages could confirm that treatment with free fatty acids induced a similar inflammatory polarization as observed in monocytes and MoMF following Western diet feeding. Furthermore, the anti-inflammatory phenotype in bone marrow monocytes which was induced during NAFLD progression was shown to be persistent during acute liver injury following an overdose of acetaminophen. Taken together, these data suggest that bone marrow monocytes respond to changes in dietary composition during NAFLD progression, which induces an adaptive reduction of their inflammatory capacity