The restoration of myeloid-derived suppressor cells as functional antigen-presenting cells by NKT cell help and all-trans-retinoic acid treatment.

Myeloid-derived suppressor cells (MDSCs), which accumulate during tumor progression, have been shown to function as important suppressor cells. In a previous study, we showed that immunosuppressive MDSCs could function as immunogenic antigen-presenting cells (APCs) with the help of activated natural killer T (NKT) cells. In the current study, however, we found that MDSCs harvested at a late time point after tumor injection (late MDSCs) were poorly immunogenic even when stimulated with activated NKT cells. As tumor growth progressed, the expression of MHC and costimulatory molecules on MDSCs was gradually down-regulated. Late MDSCs also had innate defects in activation and differentiation mediated by cytokine stimuli. Although late MDSCs treated only with all-trans-retinoic acid (ATRA), a stimulating agent for MDSC differentiation, could not become immunogenic, NKT ligand-loaded, ATRA-treated late MDSCs could be converted into immunogenic APCs to induce incremental immune responses. Furthermore, these effects were mediated by NKT cells secreting IFNγ, and ATRA-mediated increases in glutathione (GSH) levels. Thus, combined treatment with differentiating and activating agents is a prerequisite for the conversion of late MDSCs into immunogenic APCs. Collectively, these results suggest that combined treatments are required for the differentiation and activation of late MDSCs in late stage cancer.