DCs (dendritic cells), play an important role in allo-recognition, but there is no direct evidence that DCs trigger and control allo-responses. In this study, we investigated the role of DCs by using a self-created MyD88 (myeloid differentiation factor 88) inhibitor and a minor antigen-mismatched (HY-mismatched) skin transplantation model in mice.An HY-mismatched skin allograft model (BALB/c ♂ → BALB/c ♀) using MyD88-knockout mice was employed in this study. WT (Wild-type) BALB/c immature BMDCs (bone marrow dendritic cells) were co-cultured with or without TJ-M2010 in vitro for 12 hours before transfusion of DCs (2× 10(7)/mouse) into recipients of skin transplantations. Wild-type BALB/c BMDCs were stimulated by the TLR9 (Toll-like receptor 9) agonist, CpG (cytidine-phosphate-guanosine), in the presence of TJ-M2010 or not for 12 hours in vitro. Costimulatory molecules CD80/CD86 were analyzed by flow cytometry. WT C57BL/6 naïve T cells stained with CFSE (carboxyfluorescein diacetate succinimidyl ester) were co-cultured with WT BALB/c DCs for 3 days in the presence or absence of TJ-M2010. CD3(+)/CFSE(+) cells were analyzed by flow cytometry.TJ-M2010 inhibited DC maturation and T cell proliferation. Permanent survival of the skin allografts was observed in MyD88 knockout (MyD88- KO) mice. However, skin allografts were rejected by MyD88-KO mice infused with wild-type (WT) DCs. More interestingly, after treating the WT DCs with TJ-M2010, the DC infusion could not reverse the tolerance to skin allografts.This study provided evidence that DCs play an essential role in alloresponses.