[Development of immune tolerance in liver transplantation]

The liver is a privileged organ and has a lower incidence of rejection than other organs. However, immunosuppressive regimens are still required to control the alloreactive T-lymphocyte response after transplantation. These treatments may lead to severe complications, such as infectious diseases, cancers, cardiovascular diseases and chronic renal insufficiency. In clinical transplantation there is increasing evidence that some liver transplant recipients who cease taking immunosuppressive (IS) drugs maintain allograft function, suggesting that tolerance is already present. This strategy is feasible in 25-33% of liver transplant recipients. A series of experimental and clinical observations indicates that liver allografts can even provide "tolerogenic" properties for other organ grafts. In the clinical setting, clinical operational tolerance (COT) is defined as the absence of acute and chronic rejection and graft survival with normal function and histology in an IS-free, fully immunocompetent host, usually as an end result of a successful attempt at IS withdrawal. The exact mechanisms involved in achieving transplant tolerance remain unknown, although animal models suggest a possible role for regulatory T cells (Treg). Recent data have demonstrated an increase in the frequency of CD4+ CD25(high) T cells and FoxP3 transcripts during IS withdrawal in operationally tolerant liver transplant recipients. The data obtained from transcriptional profiling of the peripheral blood of IS-free liver transplant recipients suggest that there is a molecular signature of tolerance that could be employed to identify tolerant liver transplant recipients and that innate immune cells are likely to play a major role in the maintenance of COT after liver transplantation.