Dendritic cells (DCs) are pivotal antigen-presenting cells and serve a unique role in initiating immunity. To test the hypothesis that pre-immunization of recipient with certain DC subsets of donor origin can influence graft outcome, we have studied the effects of immunization with allogeneic CD4(+)CD8(-)CD11c(+) dendritic cell (CD4(+)DC) and CD4(-)CD8(+)CD11c(+) dendritic cell (CD8(+)DC) on the allograft response. Although both immature CD4(+)DC and CD8(+)DC subsets from DBA/2 were able to prime naive allogeneic C57BL/6 (B6) T cells in mixed lymphocyte reaction (MLR), CD8(+)DC exerted more vigorous alloimmune responses than CD4(+)DC did. Also, CD4(+)DC-driven allogeneic T cell response was attenuated more significantly by anti-CD154 mAb than CD8(+)DC-driven response. Consistent with the MLR results, combined pre-treatment with CD4(+)DC, but not CD8(+)DC, plus anti-CD154 mAb produced donor strain-specific long-term graft survival and induced tolerance while treatment with CD8(+)DC plus anti-CD154 mAb created minimal prolongation of allograft survival in a pancreas islet transplant model (DBA/2-->B6). The beneficial effects exerted by CD4(+)DC and anti-CD154 mAb pre-treatment were correlated with T(h)1 to T(h)2 immune deviation and with the amplified donor-specific suppressive capacity by recipient CD4(+)CD25(+) T cells. These findings highlight the capacity of CD4(+)DC to modulate alloimmune responses, and suggest therapeutic approaches for the induction of donor-specific tolerance.