Unique aspects of rejection and tolerance in liver transplantation.

Spontaneous acceptance of liver allografts occurs in several species. However, tolerance is rare in human transplant patients even though rejection is relatively easily reversed. Histological features of acute rejection in liver transplantation are similar to those in other organs. Nevertheless, mechanisms of rejection of liver transplants may differ in degrees and cellular involvement. Liver-specific cell populations, such as Kupffer cells (KCs), liver sinusoidal epithelial cells (LSECs), and hepatic stellate cells (HSCs), may contribute to liver tolerogenicity. Other mechanisms, such as microchimerism, soluble major histocompatibility complex (MHC molecules), donor human leukocyte antigen (HLA)-C genotype, and regulatory T cells, may participate in inducing tolerance. The low incidence of hyperacute or antibody-mediated rejection in liver might be linked to the infrequency of chronic rejection of liver transplants. Understanding the mechanisms of liver transplant rejection/tolerance and the availability of better immune monitoring could help develop strategies to recognize tolerance and reduce rejection.