Trafficking of monocytes and their differentiation to dendritic cells and macrophages in the human liver

Most liver diseases occur as a consequence of uncontrolled inflammation mediated by white blood cells recruited into the liver from the blood. White blood cells enter the liver via channels or sinusoids that are lined by specialised cells called endothelial cells which the white cells must bind to and migrate between before they can enter liver tissue. We propose that this process will a) determine the nature of the cells recruited and b) determine how those cells become activated and thereby the outcome of liver injury. We have developed complex test tube models in which combinations of human liver cells are grown under particular conditions that mimic the environment within the diseased liver sinusoids. We will use these models to investigate how white cells are recruited and how interactions with sinusoidal cells can define the activation status of the cells during their recruitment. Understanding this process will elucidate why in some circumstances white blood cells can promote the repair of the damaged liver whereas in other circumstances they amplify liver injury. We hope to use this information to develop new therapies to manipulate white blood cells in the liver in favour of repair rather than continuing liver damage and thereby to reverse inflammatory liver disease and prevent liver cirrhosis.

Circulating monocytes gives rise to tissue-resident macrophages and dendritic cells which are critical regulators of inflammatory and immune-mediated responses. The liver contains functionally distinct macrophages and dendritic cells (DC) that contribute to local immune responses by activating immune responses or promoting inflammatory resolution. Thus activated DCs induce immune responses to pathogens whereas resting DCs induce tolerance; and although some activation pathways result in macrophages that drive immune-mediated liver injury others result in reparative macrophages that promote resolution. The balance of intrahepatic monocyte/macrophages activation will thus be a critical determinant in the outcome of liver injury. At least two phenotypically distinct subsets of monocytes circulate in blood which can be distinguished by their expression of specific cell surface markers. These cells enter the liver by migrating across sinusoidal endothelium and through the subendothelial matrix where they interact with other cells including fibroblasts and hepatocytes. We propose that the activation and differentiation of monocytes into liver DCs and macrophages will be shaped by interactions with sinusoidal endothelial cells during transmigration and then with matrix and parenchymal cells once the monocytes are in tissue. We have developed co-culture models using primary human liver cells that recapitulate the sinusoidal cellular environment in vitro. We will use these models to determine 1) the molecules involved in recruiting different subsets of monocytes from the blood into liver 2) the ability of underlying hepatocytes or fibroblasts to modulate this process in the presence of inflammatory stimuli 3) the effect of interactions with endothelium, subendothelial matrix and other liver cells on subsequent monocyte differentiation and activation. We hypothesise that migration through the multi-cellular hepatic sinusoids will confer on monocytes a distinct phenotype and function that will determine how these cells behave within liver tissue. Modulation of these processes may suggest ways in which monocyte recruitment and activation can be manipulated to favour resolution rather that persistence of inflammation in chronic liver disease.