Leukocyte trafficking in transplantation tolerance: Home sweet home

Efficient leukocyte trafficking through blood, secondary lymphoid organs, lymphatics, and tissue is critical during the development of the immune response to transplant antigens. Understanding trafficking and migration is important for determining the anatomic domains in which lymphocytes interact with alloantigen presenting cells during the establishment of tolerance. Leukocyte trafficking into the lymph nodes is a process generally known as lymph node homing, and is an important event that determines the outcome of the immune response. However, identifying the precise lymphoid organs where T lymphocytes interact with alloantigen-presenting dendritic cells (DC) under systemic immunosuppression is also necessary for manipulating the immune response towards the development of tolerance. This review summarizes recent findings with regard to leukocyte trafficking to distinct anatomic sites that are responsible for the induction and maintenance of tolerance, and compare these findings to studies that analyze leukocyte migration during priming and rejection in organ transplantation.