*Purpose: The induction of tolerance to kidney transplants after development of specific unresponsiveness of recipient immune cells to donor alloantigens allows elimination of maintenance immunosuppressive (IS) drugs while preventing graft loss due to rejection. We have previously reported successful induction of mixed chimerism and tolerance in recipients of fully HLA-matched living donor kidney and hematopoietic cell (HCT) transplants. This work continues in HLA-mismatched transplants, wherein achieving persistent mixed chimerism is more challenging.

*Methods: Living donors underwent hematopoietic stem cell mobilization and leukopheresis 6 weeks before kidney donation. The recipient conditioning regimen for the HCT consisted of 10 doses of total lymphoid irradiation (TLI) and 5 doses of anti-thymocyte globulin (ATG) following kidney transplant, with HCT on day 11. The HCT consisted of CD34+ hematopoietic progenitor cells and CD3+ cells. Twenty-nine patients received fully HLA-matched living donor transplants. Twenty-five patients received HLA-mismatched (1-haplotype matched) living donor transplants with an escalating dose of CD3+ cells to enhance HCT engraftment. Six patients recently received HLA-mismatched living donor transplants with substitution of a dose of total body irradiation (TBI) for the final dose of TLI.

*Results: Of the 29 fully HLA-matched living donor transplants, complete IS drug withdrawal was achieved in 24, all of whom had at least 6 months of persistent mixed chimerism. Of the 24, 17 are alive, well and have been off IS drugs from 4 to 14 years without evidence of rejection. Two off IS drugs died due to cardiovascular events at 2 and 3 years. Two returned to IS drugs after loss of chimerism at 1 year followed by acute rejection at 4 years off IS drugs, and 3 after relapse of kidney disease at 1, 5 and 6 years off IS drugs. In the 25 1-haplotype matched living donor transplants, mixed whole blood chimerism persisted in 10 (mean 20% chimerism) at the end of the 1st year posttransplant while immunosuppression was reduced to tacrolimus monotherapy. Chimerism was lost during the 2nd year in the 5 who were withdrawn from monotherapy; they returned to IS drug(s). Mixed chimerism without rejection has persisted in 3 maintained on IS monotherapy, for up to 5 years. The recent addition of a dose of TBI markedly increased levels and stability of mixed chimerism during the 1st year posttransplant.

*Conclusions: The loss of chimerism during withdrawal of IS drug monotherapy in the HLA-mismatched transplants may have been due to the low level of whole blood chimerism, a contrast to the HLA-matched. A very low single dose of TBI added to the TLI/ATG conditioning regimen increased early chimerism. The stability of high levels of mixed chimerism after complete IS drug withdrawal during the 2nd year posttransplant is the subject of continuing investigation.

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