Deletional and regulatory mechanisms coalesce to drive transplantation tolerance through mixed chimerism

Establishing donor-specific immunological tolerance could improve long-term outcome by obviating the need for immunosuppressive drug therapy, which is currently required to control alloreactivity after organ transplantation. Mixed chimerism is defined as the engraftment of donor hematopoietic stem cells in the recipient, leading to viable coexistence of both donor and recipient leukocytes. In numerous experimental models, cotransplantation of donor bone marrow (BM) into preconditioned (e.g., through irradiation or cytotoxic drugs) recipients leads to transplantation tolerance through (mixed) chimerism. Mixed chimerism offers immunological advantages for clinical translation; pilot trials have established proof of concept by deliberately inducing tolerance in humans. Widespread clinical application is prevented, however, by the harsh preconditioning currently necessary for permitting BM engraftment. Recently, the immunological mechanisms inducing and maintaining tolerance in experimental mixed chimerism have been defined, revealing a more prominent role for regulation than historically assumed. The evidence from murine models suggests that both deletional and regulatory mechanisms are critical in promoting complete tolerance, encompassing also the minor histocompatibility antigens. Here, we review the current understanding of tolerance through mixed chimerism and provide an outlook on how to realize widespread clinical translation based on mechanistic insights gained from chimerism protocols, including cell therapy with polyclonal regulatory T cells.