Regulatory T cells (Tregs) are essential for immune tolerance and are under active development as cell therapy in transplantation. However, the widespread use of the calcineurin inhibitor tacrolimus may inadvertently suppress Treg proliferation and activation, undermining their therapeutic potential. Tacrolimus binds to the FKBP12 protein in T cells, forming a complex that blocks calcineurin-NFAT signaling and suppresses IL-2 gene transcription, thereby inhibiting T cell activation. In this study, we investigated whether deleting FKBP12 in human Tregs could prevent tacrolimus-mediated suppression. Using CRISPR-Cas9 gene editing, FKBP12 was knocked out inexpanded human Tregs, which were then cultured for seven days with tacrolimus (10 ng/mL) or control, under varying IL-2 concentrations (100-500 IU/mL). We observed that tacrolimus significantly reduced the proliferation of control Tregs, even in conditions with 500 IU/mL IL-2, whereas FKBP12-knockout Tregs maintained robust proliferation comparable to untreated cells. We found no discernible changes in Treg phenotype or stability following FKBP12 deletion or tacrolimus exposure: edited Tregs retained normal expression of the lineage-defining marker FOXP3, displayed a global transcriptomic profile nearly indistinguishable from controls, and were similarly suppressive, indicating that they remainedTregs. These findings demonstrate that the antiproliferative effect of tacrolimus on Tregs is critically dependent on FKBP12, mirroring its mechanism in conventional T cells. By genetically uncoupling tacrolimus from its target in Tregs, this approach suggests a strategy to preserve Treg numbers during tacrolimus-based immunosuppression in transplant recipients, potentially enhancing Treg-based therapies for transplantation tolerance. ex vivo bona fide
