The role of macrophages and apoptotic cell clearance in the maintenance of liver tolerance

Abstract

Phagocytosis is a process that contributes to the maintenance of homeostatic conditions in the body via the recognition and engulfment of apoptotic cells generated under physiological conditions or as result of tissue damage. Macrophages are professional phagocytic cells adorned with a variety of phagocytic receptors which guarantee the proper clearance of dying cells. However, dysregulation in the phagocytic receptor expression can lead to failure in the process of dead cell clearance and is associated with the exacerbation of several autoimmune diseases. Phagocytosis of apoptotic cells by macrophages constitutes a critical immune signal, not only for dying cells and debris, but, most importantly, for regulating macrophage function and the re-establishment of homeostasis upon different types of tissue damage. The liver is an immunologically tolerant organ in which macrophages are defined as a heterogenous population of immune cells that account for the largest non-parenchymal cell population. Changes in the regulation of the functional activity of liver macrophages are associated with chronic inflammation and several liver diseases, including NASH, liver fibrosis and hepatocellular carcinoma. The present study demonstrates that an alteration in the phagocytic capacity of macrophages occurs in the liver of Mdr2-/- mice, a model of primary sclerosing cholangitis (PSC), a cholestatic autoimmune disease characterised by progressive inflammation and fibrosis. PSC patients are reported to have altered levels and composition of bile acids, which characterise the course of the disease. Here, a connection is formed between the dysregulated bile acid pool and the reduction in the phagocytic capacity of liver macrophages. Furthermore, the bile acid profile of Mdr2-/- mice was investigated and an innovative phagocytosis-dependent mechanism of bile acid accumulation inside macrophages was demonstrated. The data in this thesis also demonstrate that the accumulation of bile acids inside macrophages significantly decreased their ability to further uptake apoptotic cells, thereby contributing to the presence of unremoved apoptotic bodies in the liver and sustaining the inflammation. In addition, in order to validate the data obtained in the mouse model in a human context, PBMCs were isolated from PSC patients and healthy donors as controls, and the phagocytic receptor expression and phagocytic capacity was assessed in the monocyte subset. However, no differences were found in the frequency of phagocytic receptors in monocytes from PSC patients. Accordingly, the normal expression of phagocytic receptors correlates with a functional phagocytic capability in circulation, suggesting the alteration in the macrophage phagocytosis is liver-related. In conclusion, the present data indicate that in the liver, a bile acid-mediated alteration in the phagocytic machinery operated by macrophages occurs during the course of PSC. This results in defective apoptotic cell clearance, thereby contributing to progressive liver inflammation. These findings reveal the importance of proper phagocytosis in the maintenance of liver tolerance and lay the foundations for a deeper analysis on the role of macrophages during cholestatic diseases.