Following transplantation, the liver is relatively privileged compared with other solid organ allografts. Acute rejection is readily reversed, chronic rejection is uncommon, and cotransplantation of liver with another organ confers these benefits to the second graft. Additionally, orthotopic liver transplantation (OLT) in rodents is often associated with spontaneous graft tolerance. Despite this, there remain problems in clinical liver transplantation. Typical acute rejection (AR) is well understood, but increasingly there are later changes in some allografts that cause diagnostic confusion. It is unclear whether these changes are related to rejection, infection or other causes. Another late complication, chronic rejection (CR), .has an unpredictable course but may be reversible. The factors determining why progressive disease develops are only now being determined.
The aims of the work described in this thesis were twofold: first, to examine immune events in liver grafts as a way to understand why they are better accepted and second, to re-examine poorly understood late graft dysfunction in an attempt to refine pathogenetic mechanisms and improve diagnostic criteria.
In the initial experiments, lymphocyte apoptosis was assessed by staining clinical liver biopsy specimens using terminal deoxynucleotidyl transferase dUTP nick end-labelling (TUNEL) and costaining with the T-cell marker CD3. It was found that persistent levels of lymphocyte apoptosis were present at various time points from three days to over one year, most commonly in the liver sinusoids. Since both host and graft lymphocytes could potentially be dying, the kinetics of cellular replacement were determined in a different set of biopsies. Using sex-mismatched graft biopsies, and staining for male DNA, rapid replacement of graft T cells by host lymphocytes was found, suggesting that the increased lymphocyte apoptosis was due, at least in part, to accelerated death of host T cells.
Other experiments focussed on cytokine expression in rejecting and non-rejecting liver grafts. Although it was hypothesised at the time that rejection would be the outcome of a Thi response, analysis of cytokine expression showed no distinctive pattern apart from a higher frequency of interleukin (IL)-4 expression during AR. To extend these studies, several approaches were attempted to determine the cellular sources of cytokines, but no procedure could be developed that allowed sensitive and specific localisation of cytokine expression in fixed tissue samples.
Later studies were more applied. A particularly difficult histological lesion to assess in late post-OLT biopsies is centrilobular necrosis. It was present in 11% of our patients and was studied using a clinicopathological approach. In an analysis of 54 patients, the histological spectrum and distinguishing features of groups with acute rejection, chronic rejection, transient or chronic hepatitis and other diagnoses were determined. The third group, with hepatitis, included recurrent and de novo autoimmune hepatitis. The exact relationship between the primary and graft diseases warrants further study.
Finally, because of its unpredictable development, fibrosis in CR was investigated. Progressive fibrosis is an important prognostic feature in CR and it was hypothesised that venous occlusion of both portal and hepatic venular branches could be more important than graft arteriopathy. This proved correct, and end-stage grafts with CR demonstrated a close relationship between increased frequency and severity of venous lesions and the stage of fibrosis. There was also a time-dependent contribution from bile duct proliferation.
In summary, the studies suggested that lymphocyte deletion rather than cytokine-mediated immune deviation could be more important in causing the 'liver effect' after OLT. The mechanism causing lymphocyte deletion remains undefined in humans. In applied clinical studies, a previously undescribed lesion of chronic allograft hepatitis with centrilobular necrosis was described, and further analysis is needed to determine the precise nature of this change, and its relationship to autoimmune liver disease. Finally, because of the importance of venous sclerosis to CR progression, further dissection of the immunological events potentiating this vascular lesion could be of benefit to salvage more grafts with early, potentially reversible CR.