Novel immunosuppressive effect of FK506 by upregulation of PD-L1 via FKBP51 in heart transplantation

The calcineurin inhibitor-FK506-is a first-line immunosuppressant that regulates T cell secretion of IL-2 and other cytokines. However, the mechanism of its protective effect on target cells and its role on tumour recurrence and interaction with anti-tumour immune checkpoint inhibitors, such as PD-L1 blocking, are still unclear. Here, in a murine heart transplantation model, we observed the upregulation of programmed death-ligand 1 (PD-L1) expression by FK506 in both dendritic cells (DCs) and allografts. Blocking PD-L1 during FK506 treatment increased IFN-γ and TNF-α expression, enhanced CD4⁺ and CD8⁺ T cell proliferation, and suppressed Treg differentiation. Moreover, PD-L1 decreased T cell infiltration and induced T cell Apoptosis in both the spleen and graft. PD-L1 was not only required in FK506-mediated immunosuppression but also upregulated by FK506. Treatment with SAFit2, a FKBP51 selective inhibitor, reduced the expression of PD-L1 on DCs and the grafts and interfered with the immunosuppressive effect of FK506, suggesting that the mechanism depends on FK506-binding protein (FKBP) 51 expression. Overall, our results add new insights into the role of FK506, not only on T cell cytokine secretion but also on co-inhibitory molecular regulation and target cell immune privilege.