Characterization of the role of macrophages in the progression of non-alcoholic steatohepatitis

Non-alcoholic fatty liver disease (NAFLD) is becoming the most common form of liver injury worldwide in relation to the diffusion of overweight and obesity. In about 15-20% of patients with NAFLD the disease evolves to non-alcoholic steatohepatitis (NASH), characterized by hepatocellular damage and lobular inflammation that often evolves to hepatic fibrosis and cirrhosis. In my doctoral project, I have investigated some aspect of the inflammation associated to NASH in order to get more insides on the mechanisms responsible for the progression to fibrosis. As experimental model, we used C57BL/6 and Balb/c mice in which NASH was induced by feeding a methionine and choline deficient (MCD) diet up to eight weeks. In these animals, we have observed that the evolution of NASH is associated with a progressive increase in hepatic macrophages that changes their function and morphology. In fact, in the early phase of the disease the onset of inflammation is characterized by a prominent M1 activation that account for inter-strain difference in the susceptibility to NASH between C57BL/6 and Balb/c mice. With the progression of the disease toward fibrosis macrophages show a decline in M1 responses in relation with an increased production by the macrophages them-selves of the anti-inflammatory protein Annexin A1 (AnxA1). Using AnxA1-null mice and recombinant AnxA1 it has been possible to determine that AnxA1 acts on macrophages by down-modulating M1 polarization through stimulation of IL-10 production. Furthermore, AnxA1 has also a functional role in controlling fibrogenesis by the regulation of galectin-3 production. Although the advanced phases of experimental NASH are characterized by a decline in macrophage M1 responses, this does not parallel with a lowering of hepatic inflammation. Experiments performed with NF-kB1-null mice have shown that this phenomenon can be ascribed to an enhanced liver recruitment of osteopontin-producing NKT cells. Indeed, also in wild type animals the number of liver NKT cells changes during the evolution of NASH declining in the early phases of the disease and expanding again in the advanced phases in relation to an increased hepatic production of IL-15, a cytokine involved in controlling T-cell survival. Collectively, these results indicate that NASH progression involves multiple interactions between macrophages and NKT cells and they represent a starting point to investigate whether genetic differences in NF-kB1 and Annexin A1 may account for the inter-individual variability in the evolution of NASH as well as to test AnnexinA1 analogues as possible novel treatments to control NASH evolution.