Introduction Chronic liver inflammation is a leading cause of morbidity and mortality world wide, characterised by a dysregulated tissue repair driven by uncontrolled inflammation that leads to fibrosis, cirrhosis and hepatocellular carcinoma. We have investigated the role of different monocyte subsets: classical (CD14++CD16−/Mon1), intermediate (CD14++CD16+/Mon2) and non-classical (CD14+CD16++/Mon3) monocytes in human liver disease.
Methods Liver-infiltrating and peripheral blood mononuclear cells (MNC) were isolated from normal individuals or patients with liver disease (ALD/NASH/PSC/AIH) and sorted into phenotypic subsets that were studied for their differentiation in response to Th1/Th2 cytokines by flow cytometry, migration across TNFα/IFNγ stimulated hepatic sinusoidal endothelial cells (HSEC) under physiological flow and phagocytic activity of zymosan bioparticles. The ability of different monocytes to activate hepatic stellate cells (HSC) was assessed using QRT-PCR (aSMA and COL1α1 gene expression changes). In this study monocyte subsets are defined as Mon1 (CD14++CD16−), Mon2 (CD14++CD16++) and Mon3 (CD14+CD16++). In functional experiments Mon2 and Mon3 were studied together and are defined as total CD16+ monocytes.
Results Mon1 comprised 80% of MNC in the blood of healthy subjects and patients with liver disease but only 50% of MNC in both normal and diseased liver. Mon2 comprised 9% and 14% of MNC in normal and diseased blood respectively, but were significantly increased in normal and diseased livers (42 and 30% of MNC, respectively). Transmigration of total CD16+ monocytes across inflamed HSEC was 2.3-fold higher compared to CD14+ respectively. In vitro stimulation of Mon1 with TGFβ1 or IL-10 for 5 days induced 16- and 20-fold increases in CD16 expression. Liver infiltrating Mon2 expressed higher levels of CD163 and HLA-DR compared to Mon1, colocalized with CD68 and demonstrated high phagocytic activity, indicative of a macrophage phenotype. Diseased-liver-derived total CD16+ monocytes secreted higher levels of CCL2, IL-6, IL-8 and IL-13 and induced a twofold increase in aSMA and COL1a1 expression in co-cultured HSC.
Conclusion Compared with normal livers, diseased livers harbour fewer CD14+ but significantly more CD16+ monocytes that secrete profibrotic cytokines and are able to activate HSC. CD16+ monocyte accumulation in the liver is the result of enhanced recruitment from blood and also local differentiation from CD14+ in response to TGFβ1 and IL-10 present in the fibrotic microenvironment.
Competing interests None declared.