BACKGROUND: Although dendritic cells (DCs) are the most influential antigen presenting cells maturation of DC is the critical control point toward either activation or regulation of immunity. We hypothesized that pretreatment with donor DCs, if which were maturation-resistant in vivo, could enhance engraftment by inducing inactivated state for allo- reactive T cell clones. METHODS: Immature DCs were prepared by 6- day culture of BM cells and we used paraformaldehyde for locking the DCs as immature phenotypes. We did in vitro and in vivo MLR to evaluate the effect of maturation resistant DCs on alloreactive T cells and we confirmed the effect of DCs in MHC full mismatched skin and islet transplantation model. RESULTS: Fixed DCs in immature state were resistant to maturation stimuli and weak stimulator for allo-reactive T cells (CB6F1-->C3H). In contrast, fixed DCs in mature state stimulated allogeneic T cell proliferation effectively. Splenocytes isolated from mice 2 weeks after maturation resistant DC injection could not be reactivated and maintained naive phenotype when cocultured with allogeneic splenocytes (BALB/c-->C57BL6). Consistent with this finding maturation resistant DC treatment suppressed MLR-driven T cell division (CB6F1-->C3H) as assessed by CFSE analysis. But, CD25+ T cells depletion by treatment with anti-CD25 prior to DCs transfer attenuated this regulatory effect of DCs. In a MHC mismatched transplantation model (CB6F1-->C3H), treatment with maturation-resistant DCs 2 weeks before operation, markedly prolonged skin and islet graft survival. But C3H mice pretreated with CB6F1 DCs rejected DBA1 (H-2q) skin graft within 14 days. CONCLUSION: These findings suggest the maintenance of immaturity of DCs is a key factor in modulating alloimmune responses through dendritic cells.