Mechanisms of Donor‐Specific Tolerance in Recipients of Haploidentical Combined Bone Marrow/Kidney Transplantation

We recently reported long‐term organ allograft survival without ongoing immunosuppression in four of five patients receiving combined kidney and bone marrow transplantation from haploidentical donors following nonmyeloablative conditioning. In vitro assays up to 18 months revealed donor‐specific unresponsiveness. We now demonstrate that T cell recovery is gradual and is characterized by memory‐type cell predominance and an increased proportion of CD4+CD25+CD127−FOXP3+ Treg during the lymphopenic period. Complete donor‐specific unresponsiveness in proliferative and cytotoxic assays, and in limiting dilution analyses of IL‐2‐producing and cytotoxic cells, developed and persisted for the 3‐year follow‐up in all patients, and extended to donor renal tubular epithelial cells. Assays in two of four patients were consistent with a role for a suppressive tolerance mechanism at 6 months to 1 year, but later (≥18 months) studies on all four patients provided no evidence for a suppressive mechanism. Our studies demonstrate, for the first time, long‐term, systemic donor‐specific unresponsiveness in patients with HLA‐mismatched allograft tolerance. While regulatory cells may play an early role, long‐term tolerance appears to be maintained by a deletion or anergy mechanism. Patients rendered tolerant to renal allografts by combined kidney and bone marrow transplantation demonstrate long‐term, specific and complete T cell unresponsiveness to their donors by mechanisms that may include clonal deletion.