Bone marrow chimerism and transplantation tolerance

Engraftment of allogeneic or xenogeneic pluripotent hematopoietic stem cells into nonmyeloablated but immunodepleted (preconditioned) recipients can produce a state of immunological tolerance to donor and host. Host and donor hematopoietic cells entering the thymus ensure deletion of both donor- and host-reactive thymocytes. Additional mechanisms are involved in tolerance induced in recipients that are not immunodepleted. Grafting of donor thymic tissue to thymectomized recipients is an alternative approach for inducing central T cell tolerance without the requirement for engraftment of donor hematopoietic stem cells. During the past year, advances have been made in understanding both the requirements for preconditioning and the mechanisms of tolerance induction in the above transplantation models.