Apoptosis as a mechanism of cell death in liver allograft rejection

It is generally recognized that there are two mechanisms of cell death, apoptosis and necrosis. Apoptosis--programmed cell death--is involved in numerous states of physiological cell deletion. Recent studies have demonstrated that hepatocytes, under certain conditions, undergo apoptosis. The purpose of this work was to determine if apoptotic cell death is involved in liver allograft rejection. Groups of Lewis (RT1l) rats underwent orthotopic liver transplantation (OLT) from disparate DA (RT1a) or syngeneic Lewis rats. Liver samples were harvested at 1, 2, 3, 4, and 7 days posttransplant and analyzed for apoptotic cell death. Since the characteristics of apoptosis are difficult to discern using routine hematoxylin and eosin staining, we utilized a novel method that detects the classic indicator of apoptosis, nonrandom DNA degradation. Paraffin-embedded tissue sections were end-labeled with nonradioactive dUTP and detection of apoptotic bodies accomplished by immunoassay. The incidence of apoptotic cells increased steadily over time in allografts, in contrast to syngeneic grafts. In this study apoptotic cell death paralleled standard indicators of liver allograft rejection including pathology, mononuclear cell infiltration, and increases in liver enzymes. Moreover, increased expression of TGF-beta 1 correlated with apoptosis in liver allografts, supporting the previously described role for this cytokine in hepatocyte apoptosis. Our results demonstrate, for the first time, that apoptosis may be a mechanism of cell death in liver allograft rejection.