Inhibition of human dendritic cell maturation and function by the novel immunosuppressant FK778

FK778, a derivative of the active leflunomide-metabolite, A77 1726, has been shown to be a powerful immunosuppressant in several transplantation models, particularly efficient in the prevention of chronic allograft rejection. However, the cellular and molecular mechanisms underlying these effects of FK778 have not been investigated yet in detail. Because dendritic cells (DCs) are the most potent antigen-presenting cells (APCs) and are essential for the initiation of immune responses including acute and chronic allograft rejection, we investigated whether FK778 affects this particular cell type.

Allogeneic T cell stimulation by FK778-treated human monocyte-derived DCs was determined by mixed leukocyte cultures. Surface molecule expression was analyzed by flow-cytometric analysis and cytokine production by ELISA from culture supernatants. Activation of NF-kappaB in DCs was assessed by electrophoretic mobility shift assays.

Treatment of DCs with FK778 inhibited their potency to stimulate allogeneic T cells. In line, LPS- and CD40L-induced upregulation of DC surface activation markers and production of IL-12 was significantly inhibited, irrespective of whether cells were treated during or after the monocyte to DC differentiation period. The effects of FK778 on DCs were not reversible by exogenous uridine indicating that FK778 acts independently of its action as an inhibitor of pyrimidine synthesis. On the signaling level, activation of NF-kappaB, the essential transcription factor involved in DC maturation and function, was markedly inhibited by FK778.

Inhibition of activation and function of DCs as the central APCs may significantly contribute to the immunosuppressive profile of FK778 when applied after allogeneic organ transplantation.