Myeloid-derived suppressor cells (MDSCs), which are known to be accumulated in the blood, spleen, and bone marrow of tumor-bearing mice and cancer patients, were tested as APCs for a cellular vaccine because they have phenotypical similarity with inflammatory monocytes and may be differentiated from the same precursors as monocytes. Although MDSCs have immunosuppressive properties, in vivo transferred MDSCs, which present tumor Ag and NKT cell ligand (alpha-galactosylceramide), significantly prolonged survival time in metastatic tumor-bearing mice in a CD8(+) cell-, NK cell-, and NKT cell-dependent manner vs a CD4(+) T cell- and host dendritic cell-independent manner. Major concerns about using MDSCs as APCs in a vaccine are their suppression of CTLs and their induction of Foxp3(+) regulatory T cells. However, alpha-galactosylceramide-loaded MDSCs did not suppress CD4(+) and CD8(+) T cells and allowed for the generation of Ag-specific CTL immunity without increasing the generation of regulatory T cells. Furthermore, stimulation with activated NKT cells induced changes on MDSCs in phenotypical or maturation markers, including CD11b, CD11c, and CD86. Taken together, these findings suggest that NKT cells facilitate the conversion of immunosuppressive MDSCs into immunogenic APCs, eliciting successful antitumor immunity and providing the basis for alternative cell-based vaccines.