The prevailing idea regarding the mechanism(s) by which therapeutic immunosuppressive dendritic cells (DCs) restrain alloimmunity is based on the concept that they interact directly with antidonor T cells, inducing anergy, deletion, and/or regulation. However, this idea has not been tested in vivo. Using prototypic in vitro-generated maturation-resistant (MR) DCs, we demonstrate that once MR-DCs carrying donor antigen (Ag) are administered intravenously, they decrease the direct and indirect pathway T-cell responses and prolong heart allograft survival but fail to directly regulate T cells in vivo. Rather, injected MR-DCs are short-lived and reprocessed by recipient DCs for presentation to indirect pathway CD4(+) T cells, resulting in abortive activation and deletion without detrimental effect on the number of indirect CD4(+) FoxP3(+) T cells, thus increasing the regulatory to effector T cell relative percentage. The effect on the antidonor response was independent of the method used to generate therapeutic DCs or their viability; and in accordance with the idea that recipient Ag-presenting cells mediate the effects of therapeutic DCs in transplantation, prolongation of allograft survival was achieved using donor apoptotic MR-DCs or those lacking surface major histocompatibility complex molecules. We therefore conclude that therapeutic DCs function as Ag-transporting cells rather than Ag-presenting cells to prolong allograft survival.