The mechanism by which CD1d-restricted Valpha14 natural killer T (NKT) cells participate in transplant tolerance has yet to be completely clarified. Recently, we showed that repeated activation of NKT cells by their specific glycolipid ligand, alpha-galactosylceramide, leads to a change in function to an immune regulatory role with IL-10 production. Moreover, these cells were shown to be able to induce regulatory dendritic cells (DCs). In this study, we showed that NKT cells from transplant tolerant recipients of cardiac allograft produced higher levels of IL-10, which is required for the maintenance of tolerance; this was proved by adoptive transfer experiments. In addition, DCs from wild-type (WT) tolerant recipients but not NKT cell-deficient recipients showed a higher IL-10-producing profile, a more immature phenotype, and tolerogenic capability. CD4 T cells from WT tolerant recipients but not NKT cell-deficient recipients also produced higher levels of IL-10 upon alloantigen stimulation and showed lower proliferative activity that was reversed by blocking the IL-10 receptor. These data indicate the existence of IL-10-dependent immune regulatory interplay among NKT cells, DCs, and CD4 T cells, even in the absence of artificial stimulation of NKT cells with synthetic glicolipids, which is required for the maintenance of transplant tolerance.