Myeloid-derived suppressor cells (MDSC) play a major role in cancer-related immune suppression, yet the nature of this suppression remains controversial. In this study, we evaluated the ability of MDSCs to elicit CD4⁺ T-cell tolerance in different mouse tumor models. In contrast to CD8⁺ T-cell tolerance, which could be induced by MDSCs in all the tumor models tested, CD4⁺ T-cell tolerance could be elicited in only one of the models (MC38) in which a substantial level of MHC class II was expressed on MDSCs compared with control myeloid cells. Mechanistic investigations revealed that MDSCs deficient in MHC class II could induce tolerance to CD8⁺ T cells but not to CD4⁺ T cells. Unexpectedly, antigen-specific CD4⁺ T cells (but not CD8⁺ T cells) could dramatically enhance the immune suppressive activity of MDSCs by converting them into powerful nonspecific suppressor cells. This striking effect was mediated by direct cell–cell contact through cross-linking of MHC class II on MDSCs. We also implicated an Ets-1 transcription factor–regulated increase in expression of Cox-2 and prostaglandin E2 in MDSCs in mediating this effect. Together, our findings suggest that activated CD4⁺ T cells that are antigen specific may enhance the immune suppressive activity of MDSCs, a mechanism that might serve normally as a negative feedback loop to control immune responses that becomes dysregulated in cancer. Cancer Res; 72(4); 928–38. ©2012 AACR.