Role of the hepatic parenchyma in liver transplant tolerance: a paradigm revisited.

Unlike other solid organs, liver transplants are spontaneously accepted in a wide range of animal models. In the clinic, transplanted livers also display privileged immunological properties allowing weaning of immunosuppression therapy in up to 20% of selected patients. To explain this phenomenon, many studies have focused on the role of donor-derived 'passenger' leukocytes that are thought to induce antigen-specific tolerance by migrating from the graft into recipient secondary lymphoid tissues. Although convincing evidence exists that these cells are able to elicit antiallograft T cell hyporesponsiveness, several studies argue against an exclusive role for this cell population and even question whether it is critical in conferring donor MHC-specific tolerance. Instead, these studies suggest that the hepatic parenchyma plays a more critical role in this phenomenon. In this review we will reinterpret the results of old and more recent literature in light of recent advances in the field of liver immunology to explain the contribution of both passenger leukocytes and liver tissue in the liver tolerance effect.