*Purpose: Our purpose was to ascertain the fate of ex vivo-generated donor-derived DCreg and their influence on host T cells prior to transplant in a first-in-human phase I/II feasibility, safety and preliminary efficacy trial of donor-derived DCreg in living-donor liver transplantation (LTx) being conducted in conjunction with withdrawal of immunosuppression (ClinicalTrials.gov Identifier: NCT03164265).

*Methods: DCreg generated from 14 living donors’ elutriated monocytes were infused 7 days before LTx. Sequential (pre-, post-infusion, day -3 to -1, and day of Tx) immunological analyses were conducted on recipient blood and lymph nodes (day 0) to track donor-derived DCreg (by flow cytometry, image-stream, and confocal microscopy using mAbs to mis-matched MHC class I ) and to evaluate CD8⁺T cells and T regulatory cell (Treg) populations in peripheral blood.

*Results: Intact donor DCreg were detected in whole blood of 3 out of 8 recipients that could be analyzed, immediately after infusion, and in one patient 3 days post-infusion. DC co-expressing both recipient and donor MHC, ie cross-dressed DC, were identified in blood and lymph nodes of the majority of recipients by flow and image-stream at all time points tested. In selected patients, cross-dressing was also visualized by confocal microscopy. Cross-dressing of host monocytes was also detected, predominantly in the intermediate (CD14⁺⁺CD16⁺ inflammatory) subset, rarely in the classical (CD14⁺⁺CD16^–) subset, and not in the non-classical (CD14^dimCD16⁺⁺ patrolling) subset. On the day of LTx, a significant decrease in Tbet, Eomes and memory T (T_mem) expression by recipient CD8⁺ T cells in 10 out of 14, 7 out of 14, and 10 out of 14 patients, respectively, was detected, paralleled by an increase in the incidence and absolute numbers of CD25^hiFoxP3⁺CD127^– Treg in 10 out of 12 patients compared to pre-infusion levels, resulting in significantly elevated Treg/Tbet, Treg/Eomes and Treg/T_mem ratios.

*Conclusions: Both intact donor-derived DCreg and cross-dressed recipient -DC and -monocytes can be detected in peripheral blood and lymph nodes following their infusion into prospective LTx recipients. DCreg infusion is associated with modifications in host memory CD8⁺ T cells and Treg, that suggest their possible in vivo immunoregulatory function.

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