Steatosis Drives monocyte-derived Macrophage Accumulation in Human Metabolic Dysfunction-Associated Fatty Liver Disease
Background & Aims: Metabolic dysfunction-associated fatty liver disease (MAFLD) is a common complication of obesity with a hallmark feature of hepatic steatosis. Recent data from animal models of MALFD have demonstrated substantial changes in macrophage composition in the fatty liver. In humans, the relationship between liver macrophage heterogeneity and liver steatosis is less clear. Methods: Liver tissue from 21 participants was collected at time of bariatric surgery and analyzed using flow cytometry, immunofluorescence, and H&E microscopy. Single-cell RNA sequencing was also conducted on a subset of samples (n=3). Intrahepatic triglyceride content was assessed via MRI and tissue histology. Mouse models of hepatic steatosis were used to investigate observations made from human liver tissue. Results: We observed variable degrees of liver steatosis with minimal fibrosis in our participants. Single-cell RNA sequencing revealed four macrophage clusters that exist in the human fatty liver encompassing Kupffer cells (KC) and monocyte-derived macrophages (MdM). The genes expressed in these macrophage subsets were similar to those observed in mouse models of MAFLD.  Hepatic CD14+ monocytes/macrophage number correlated with the degree of steatosis. Using mouse models of early liver steatosis we demonstrate recruitment of MdMs precedes KC loss and liver damage and that MdMs may serve a role in lipid uptake during MAFLD. Conclusions: The human liver in MAFLD contains macrophage subsets that align well with those that appear in mouse models of fatty liver disease. Recruited myeloid cells correlate well with the degree of liver steatosis in humans and this occurs prior to changes in KC number.  MdMs appear to have a role in lipid uptake during early stages of MALFD. Overall design: CD45 cells were freshly isolated from human liver tissue collected at the time of laproscopic gastric bypass surgery.