Lymphocyte apoptosis and cell replacement in human liver allografts

Background. Apoptosis of graft-infiltrating T cells has been described after rodent liver transplantation. The aim of this study was to assess lymphocyte apoptosis in human allografts. Additionally, kinetics of leukocyte turnover were studied to determine whether apoptotic cells were likely to be of donor or recipient origin. Methods. Liver biopsy specimens (n=36) taken between days 3 and 1855 were stained with terminal deoxynucleotidyl transferase dUTP nick end-labeling and anti-CD3 to detect apoptotic lymphocytes. Renal allograft and hepatitis C biopsy specimens served as controls. Donor cell turnover was studied in sex-mismatched grafts using Y-chromosome in situ hybridization to detect recipient cells and double immunostaining for leukocyte phenotyping. Results. T-cell apoptosis was prominent in hepatic sinusoids (72% of biopsy specimens) as early as day 3. It ranged from 0% to 18.2% of CD3(+) cells (mean 5.28 +/- 0.82%) and persisted for > 14 days, including time points > 1 year. There was no difference between biopsy specimens with or without rejection (6.34 +/- 1.14% and 4.61 +/- 1.13%, P=NS). Apoptotic cells in portal tracts were less frequent (33% of biopsy specimens) and less abundant (1.13 +/- 0.36%, P<0.0001). No lymphocyte apoptosis was seen in renal allograft biopsy specimens or hepatitis C biopsy specimens, indicating that it is a distinctive feature of the liver allograft. Persisting lymphocyte apoptosis even after donor lymphocytes had been replaced suggests that recipient lymphocyte deletion must occur. Donor Kupffer cells persisted for many months. Conclusions. Our results suggest that the sinusoidal microenvironment promotes recipient lymphocyte apoptosis, which may account for the improved outcome of liver grafts compared with other organ allografts.