Dendritic cell therapies in transplantation revisited: deletion of recipient DCs deters the effect of therapeutic DCs.

A critical goal in transplantation is the achievement of donor-specific tolerance, minimizing the use of immunosuppressants. Dendritic cells (DCs) are antigen (Ag) presenting cells (APCs) with capability to promote immunity or tolerance. The immune-regulatory properties of DCs have been exploited for generation of tolerogenic/immunosuppressive (IS) DCs that, when transfer systemically, prolong allograft survival in murine models. Surprisingly, the in vivo mechanisms of therapies based on (donor- or recipient-derived) ISDCs in transplantation remain unknown, given that previous studies investigated their effects in vitro, or ex vivo after transplantation. Since once injected, ISDCs are short-lived and transfer Ag to recipient APCs, we assessed the role of recipient DCs by depleting them at the time of ISDC-therapy in a mouse model of cardiac transplantation. The results indicate that, contrary to the accepted paradigm, systemically administered ISDCs reduce the alloresponse and prolong allograft survival, not by themselves, but through conventional DCs (cDCs) of the recipient. These findings raise doubts on the advantages of the currently used ISDC-therapies, since the immune-regulatory properties of the injected ISDC do not seem to be functionally relevant in vivo, and the quiescent/pro-tolerogenic status of cDCs may be compromised in patients with end-stage diseases that require transplantation.