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Background Semi-mature dendritic cells (DCs) may induce tolerance rather than immunity.However,little is known about the regulatory mechanism by which these DCs induce transplant tolerance.Myeloid differentiation factor 88 (MyD88) is a key adaptor of Toil-like receptor signaling,which plays a critical role in DC maturation.Activation of MyD88-silenced immature DCs results in the generation of semi-mature DCs.We explored the possibility of using these DCs to induce intestinal transplant tolerance in rats.Methods MyD88 expression was silenced in bone marrow DCs (F344 rats) using small interfering RNAs for 24 hours,at which point,lipopolysaccharide (LPS) was added to the culture for another 48 hours.These cells were analyzed for their in vitro and in vivo tolerizing capacities.Results Semi-mature DCs expressing moderate levels of MHC class Ⅱ and low levels of co-stimulatory molecules were found to produce interleukin (IL)-10,while IL-12 production was decreased.In vitro co-culture with completely allogeneic T cells from Wistar rats led to a significant decrease in alloreactive T-cell responses.In vivo,the transfer of semi-mature DCs (1×106 ceils) followed by the transplantation of fully mismatched intestinal grafts (F344 rats) led to significantly prolonged survival compared to rats receiving immature and mature DCs.Serum from semi-mature DC-treated rats contained lower concentrations of the pro-inflammatory cytokines IL-2 and interferon-Y 5 days after transplantation.Conclusion Semi-mature DCs may promote inducible allograft tolerance and this study suggests a new strategy by which to facilitate the induction of transplant tolerance.