Peripheral blood and intrahepatic subsets of T lymphocyte activation and function in liver allograft rejection and drug-induced tolerance in rats

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T lymphocyte kinetics in liver allograft rejection were studied by measuring levels of activated (CD25+ and class II MHC+) T lymphocytes (CD5+) and T cell subsets (CD4 and CD8) in peripheral blood and the livers of allogeneic DA (RT1 a) to LEW (RT1′) and syngeneic LEW to LEW orthotopic rat liver transplants. Median survival was 10 days in untreated allogeneic rats ( n = 17). Mean (±SEM) T lymphocyte class II MHC expression increased from 3.4 ± 0.44% (day 2 3 ) to 4.9 ± 1.1% (day 7) ( p = 0.01). Complete sequential data were available for nine animals over the period of rejection confirming the increase in class II MHC expression ( p = 0.05) and showing a decrease in CD25 expression ( P = 0.05). There was a significant fall in CD4:CD8 ratio from day 2 3 to day 7 ( p = 0.002). CD25 and class II MHC molecule expression and the CD4:CD8 ratio remained unchanged over the comparable period in the syngeneic LEW to LEW control model ( n = 5, p > 0.3 for all comparisons). Cyclosporin A (5 mg/kg/day) was given orally for 17 days and then withdrawn to induce allograft tolerance in a further nine DA to LEW rats (median survival > 100 days). Samples taken at 2 3 , 7, 17, 30, 40 and 100 days showed that T cell activation marker expression remained low during cyclosporin treatment (e.g. class II MHC expression 2.32 ±0.35%; CD25 expression 3.53±0.44% on day 7) but increased thereafter (e.g. class II MHC expression 8.19±0.65%, CD25 13.25±0.95% on day 100). There was a fall in CD4:CD8 ratio throughout ( p < 0.001). Intrahepatic mononuclear cells were harvested from six normal livers, four allogeneic livers (at 10 days), five syngeneic livers (at 10 days) and five tolerant allografts (at 100 days). Rejecting grafts showed the highest proportion of T lymphocytes (66.8±4.0% vs 18.5±3.6% in controls, p = 0.01). T cell activation was higher in both rejecting and tolerant grafts versus normal control livers ( p = 0.05 for CD25 and p < 0.01 for class II MHC expression). CD8+ lymphocytes predominanted in the hepatic infiltrate in all models , although the majority of these cells were both CD5 and αβ T cell receptor negative. There was a higher proportion of T cells in tolerant allografts (44.5±5.2%, p < 0.01) compared with control animals. Serial changes in peripheral T lymphocyte subsets may be useful in monitoring experimental acute rejection. In peripheral blood, the increase in T cell activation and loss of CD4+ lymphocytes, along with evidence of increased intragraft infiltration by this subset, implies that it has a primary role in the development of tolerance in this model.