Antigen presenting cells of the mononuclear phagocyte system in the human liver.

Abstract

The antigen presenting cells of mononuclear phagocytic system (APC), comprised of monocytes, macrophages, and dendritic cells (DC), are essential in tissue homeostasis and in determining the balance of the immune response. APC are believed to play a central role in the liver’s unique immunological state by which the immune system in the liver tolerates and clears large amounts of otherwise pro-inflammatory bacteria and bacterial by-products from the gut. We sought to provide further insights into the nature of liver APC both in terms of their phenotype, function and how these aspects are related to their function within the liver lobule. A systematic review of the literature highlighted that APC in the human liver have been poorly described, the literature has focused on macrophages but not described heterogeneity or the function of these cells. We developed a technique to perform multicolor flow cytometry on liver tissue and found that liver APC were similar to those found in the blood. We also identified a population of cells that are likely to represent macrophages. We then developed an association/phagocytosis assay and confirmed that these cells were the most prominent at associating-with/phagocytosing and producing pro-inflammatory cytokines in response to whole enteric bacteria (Escherichia Coli – E.coli) and the pathogen associated molecular pattern (PAMP) - lipopolysaccharide (LPS). In order to characterize APC in situ we were first required to describe the vascular supply and structural aspects of the liver lobule. We used a mixture of stromal, endothelial and epithelial cell markers to reveal an acinar distribution of liver sinusoidal endothelial cells (LSEC) that may be representative of different microenvironments within the lobule. We described a novel distribution of liver stellate cells and identified central venous endothelial and portal structures for which a framework to assess APC could be applied. We then characterized the distribution of APC in the lobule. Macrophages were found to also follow an acinar pattern, with larger more scavenger-like cells, that are located around Zone- 1 (Z1) of the liver acinus. The portal tracts (PT) contained unique macrophages and a concentration of DC. Monocytes were found in the sinusoids. Finally, we assessed APC populations in archival liver biopsies to determine if there were differentiating features between hepatitis c recurrence (HCVr) and acute cellular rejection (ACR) in patients who had developed graft dysfunction following liver transplantation (LT) for hepatitis c induced liver failure. Both conditions showed increased numbers of APC, however the major distinguishing features appeared to be as a result of the proliferative nature of ACR. The work presented in this thesis describes new methodological techniques and findings that utilize the power of flow cytometry in unravelling the complexity of human liver APC. The detail provided by immunofluorescence microscopy highlight the importance of the liver lobule’s acinar structure and improves our understanding of vascular and supporting cells in the lobule. These findings facilitate an anatomical and functional appreciation of human liver LSEC and APC and will enable further studies into the normal function of these cells, how they change and how to potentially target them in therapy to treat disease.