Central tolerance promoted by cell chimerism.

Historically, successful allotransplantation was only achieved by utilizing powerful immunosuppressive drugs that were exposing the patient to severe opportunistic infections. The thymus of the transplant recipient renders such therapy obligatory as it constitutively blocks self-reactive T cells while allowing alloreactive T cells to mature and populate the periphery. In 1992, a follow-up study revealed the presence of donor leukocytes in long-term transplant survivors. The stable persistence of recipient and donor leukocytes in the transplanted patient, referred to as "chimerism", was considered the reason why in some cases it was even possible to stop immunosuppressive treatment without damaging the transplanted organ. Unfortunately, it quickly became evident that stable, persistent allogeneic chimerism was not easily achievable by design. Recently, a novel approach has been identified to help address this clinical gap in knowledge: Cotransplantation of a donor graft with a thymic organoid populated with donor precursor cells generates stable, long-term chimerism in the recipient. In humanized mice, the implantation of thymic organoids, populated with human donor inducible pluripotent stem cell (iPSC)-derived thymic epithelial cells (TECs) and the same donor CD34+ bone marrow precursors, induces tolerance to human leukocyte antigen (HLA)-matched donor tissues/organs. This technology will allow successful allotransplantation of cells/organs even between Major Histocompatibility Complex (MHC)-noncompatible individuals and allow getting rid of immunosuppressive treatments reducing recipient morbidity.