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Based on strong preclinical data showing that regulatory dendritic cells (DCreg) of donor origin can promote long-term graft survival in rodents and suppress allograft rejection in non-human primates, we have embarked on a first-in-human phase I/II safety and preliminary efficacy trial of donor-derived DCreg in living donor liver transplantation (LTx).In the study protocol, DCreg are generated from elutriated monocytes isolated from the leukapheresed donor,14-28 days (d) before transplant. The donor-derived DCreg are infused iv (target range 2.5-10.106/kg) 7 d before transplant (d −7), while a half dose of MPA is administered from d −7 to d 0. Standard-of-care immunosuppression (steroid, MPA, tacrolimus) is administered post-LTx, and in those patients that meet eligibility criteria for weaning, immunosuppressive therapy is gradually withdrawn, starting with MPA reduction 6 mths post-LTx. Sequential immunological analyses are conducted on recipient blood and tissue samples to track the donor-derived DCreg, evaluate anti-donor reactivity and examine potential mechanisms of immune hyporesponsiveness. In the first LTx recipient (HLA-B12+) to receive donor-derived (HLA-A3+) DCreg, peripheral blood samples were obtained before and after cell infusion; native liver tissue was obtained at the time of graft implantation.Donor-derived DCreg generated for infusion were HLA-DR+, CD1c−, CD11c+, CD83−, IRF4lo and expressed a high programed death ligand-1 (PD-L1):CD86 ratio. In parallel small scale in vitro functional analyses, the DCreg failed to respond to LPS, and induced donor-specific hyporesponsiveness of recipient CD4 and CD8 T cells. The donor-derived DCreg (HLA-A3+) displaying a similar phenotype to that exhibited pre-infusion, were detected in whole blood immediately after completion of the cell infusion that comprised 5.106 DCreg/kg (450.106 total DCreg). No infusion reaction or cytokine release response was observed. Three days post-infusion (d −4), HLA-A3+ donor-derived DC could no longer be detected in blood, although a small population of DC co-expressing both recipient (HLA-B12) and donor MHC (HLA-A3), ie cross-dressed DC, were evident in the circulation. At the time of transplant, before graft implantation, cross-dressed (HLA-A3+B12+) DC could also be detected in non-parenchymal cell populations isolated from the diseased native liver. Using a 14-color panel of mAbs, these tissue-resident cross-dressed DC were PD-L1hi, IRF4lo and Foxp3hiAfter safe infusion of donor-derived DCreg into a prospective living donor LTx recipient, cross-dressed DC were detected in the peripheral blood within a few days and in the native diseased liver at the time of graft implantation. These cross-dressed DC, that appear to have acquired donor MHC from the infused DCreg, exhibit low levels of the DC transcription/maturation factor IRF4 and high levels of PD-L1 and Foxp3, suggesting possible in vivo regulatory function.