Administration of exogenous interleukin-2 abrogates spontaneous rat liver allograft acceptance but does not affect long-term established graft survival

Background. Spontaneous tolerance to the orthotopic liver allograft uniformly occurs in the DA (RT1(a)) to PVG (RT1(c)) rat combination despite a fully allogeneic barrier. Methods. To assess whether spontaneous acceptance might be the consequence of a T cell help deficit at the time of the first exposure of alloantigens to the host, we studied the effect of exogenous interleukin (IL)-2 injections at the time of liver transplantation and during long-term follow-up. Results. Although spontaneous acceptance of the liver allograft constantly ensued in the DA to PVG combination, a daily injection of recombinant IL-2 (3x10(5) U) uniformly provoked acute cellular rejection of the liver allograft and consequently the death of animals by postoperative day 5-6. Simultaneous to the graft loss, hepatic enzymes (alanine aminotransferase) increased more than 50-fold in IL-2-treated recipients, whereas similar IL-2 treatment did not produce any hepatic dysfunction in syngeneic animals. By immunohistology, the expression of the cw chain of the IL-2 receptor, usually undetectable in untreated animals, was evident on CD4 and CD8 lymphocytes infiltrating the liver graft. In contrast, a similar IL-2 regimen and even higher IL-2 doses (1x10(6) U) did not abrogate the liver allograft survival during long-term follow-up. Conclusions. Our results demonstrate that spontaneous rat liver allograft acceptance may be abolished by exogenous IL-2 injections, which suggests that an ''inherent T cell help deficit'' might be implicated in the spontaneous acceptance mechanisms of DA to PVG liver allografts.